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    Wyświetlanie 1-54 z 54
    Tytuł:
    One-Dimensional Falicov-Kimball Model in the Strong Coupling Limit
    Autorzy:
    Farkašovský, P.
    Bat'ko, I.
    Powiązania:
    https://bibliotekanauki.pl/articles/1930516.pdf
    Data publikacji:
    1994-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    Opis:
    We studied the ground state properties of the one-dimensional Falicov-Kimball model in the strong coupling limit for special periodic and aperiodic configurations of ions. The ground state phase diagrams of the model for nearest neighbor and next nearest neighbor hopping are discussed.
    Źródło:
    Acta Physica Polonica A; 1994, 85, 2; 307-310
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Magnetic Properties of R$\text{}_{1-x}^{(1)}$R$\text{}^{(2)}$$\text{}_{x}$Al$\text{}_{2}$ Alloys
    Autorzy:
    Matlak, M.
    Ślebarski, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1887609.pdf
    Data publikacji:
    1991-05
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    Opis:
    Using the multiband s-f model we derive the formulae determining the Curie temperature and magnetic moment as functions of concentration x for R$\text{}_{1-x}^{(1)}$R$\text{}_{x}^{(2)}$Al$\text{}_{2}$ intermetallic alloys, where R$\text{}^{1,2}$ denote magnetic rare earth metals. These formulae, applied to R$\text{}_{1-x}^{(1)}$R$\text{}^{(2)}$ alloys (R$\text{}^{(1)}$ = La, Lu, Y, Zr and R$\text{}^{(2)}$ = Gd) give the linear dependence of the Curie temperature and magnetic moment versus x in full agreement with experimental data.
    Źródło:
    Acta Physica Polonica A; 1991, 79, 5; 721-725
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Frustrated Magnetism in Vanadium Oxides
    Autorzy:
    Thalmeier, P.
    Schmidt, B.
    Yushankhai, V.
    Takimoto, T.
    Powiązania:
    https://bibliotekanauki.pl/articles/1810285.pdf
    Data publikacji:
    2009-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Jm
    75.10.Lp
    Opis:
    The effect of frustration in various localized and itinerant vanadium oxide compounds is discussed within next nearest neighbors Heisenberg and spin fluctuation models, respectively. In the localized moment case the S=1/2 $J_1-J_2$-model on a square lattice exhibits a rich phase diagram with magnetic as well as exotic hidden order phases due to the interplay of frustration and quantum fluctuations. Their signatures in the high field magnetization and in magnetocaloric quantities are surveyed. The possible quantum phase transitions are discussed and applied to layered vanadium oxides of the type $AA'VO(PO_4)_2$ where A, A' = Pb, Zn, Sr, Ba, Cd. In itinerant electron systems magnetic frustration may emerge as a result of electron correlations on a geometrically frustrated lattice. This mechanism causes enhanced spin fluctuations in a large region of momentum space and therefore can lead to a heavy fermion state at low temperatures as in the 3d spinel compound $LiV_2O_4$. The evidence from neutron scattering and NMR experiments is discussed within self-consistent renormalization theory based on local density approximation band structure calculations.
    Źródło:
    Acta Physica Polonica A; 2009, 115, 1; 53-58
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    The Hofstadter Diagram Analyzed in Terms of the Band Structure Parameters for a Planar Metal
    Autorzy:
    Roliński, T.
    Olszewski, S.
    Powiązania:
    https://bibliotekanauki.pl/articles/2046841.pdf
    Data publikacji:
    2006-06
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    75.70.-i
    Opis:
    It is demonstrated that a wing of the Hofstadter diagram calculated for the tightly-bound s-electrons in the simple cubic lattice can be reproduced by the dispersion figure of the wave-vector square considered for the electron states of the same lattice, on the condition that the states having equal energies are taken into account. The dispersion splitting increases systematically with the distance of the states from the Brillouin zone center. A similar wing due to dispersion of the electron momentum is calculated also for the s-electron states in the body-centered cubic lattice.
    Źródło:
    Acta Physica Polonica A; 2006, 109, 6; 753-759
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Pressure Effect on Magnetic Properties of UGa$\text{}_{3}$
    Autorzy:
    Grechnev, G. E.
    Delin, A.
    Eriksson, O.
    Johansson, B.
    Panfilov, A. S.
    Svechkarev, I. V.
    Kaczorowski, D.
    Powiązania:
    https://bibliotekanauki.pl/articles/1964627.pdf
    Data publikacji:
    1997-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    75.20.En
    Opis:
    The magnetic susceptibility, χ, of the itinerant antiferromagnetic compound UGa$\text{}_{3}$ was studied under pressure up to 2 kbar in the temperature range 64-300 K. The measured pressure derivative of the Néel temperature is found to be dT$\text{}_{N}$/dP=-1.1 K/kbar. In order to analyze the experimental magnetovolume effect values, d lnχ/d lnV, the volume dependent electronic structure of UGa$\text{}_{3}$ has been calculated ab initio in a paramagnetic phase by employing a relativistic full-potential linear muffin tin orbital method and including an external magnetic field self-consistently. The calculations revealed a predominance of itinerant uranium f-states at the Fermi energy, as well as a large orbital contribution to χ.
    Źródło:
    Acta Physica Polonica A; 1997, 92, 2; 331-333
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Anomalous Diamagnetism of $YbPb_3$ Compound: Pressure Effects
    Autorzy:
    Baranovskiy, A.
    Grechnev, G.
    Panfilov, A.
    Svechkarev, I.
    Żogał, O.
    Czopnik, A.
    Hackemer, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1813626.pdf
    Data publikacji:
    2008-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    75.80.+q
    Opis:
    The effect of a uniform pressure on the magnetic susceptibility was measured for $YbPb_3$ compound, wherein a degeneracy point of the energy bands is located just below the Fermi level and responsible for the anomalous diamagnetism. Theoretical analysis of the experimental data has revealed that a pronounced increase of diamagnetism with pressure is governed by closing the degeneracy point towards the Fermi energy
    Źródło:
    Acta Physica Polonica A; 2008, 113, 1; 243-246
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Theory of Strongly Correlated Systems in the Limit of Large On-Site Coulomb Repulsion
    Autorzy:
    Izyumov, Yu. A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1860989.pdf
    Data publikacji:
    1994-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    75.30.Cr
    Opis:
    A strongly correlated electronic system is studied within the framework of the t-J model with the help of diagram technique for the Hubbard X-operators. The summing of some series of diagrams is suggested, corresponding to the following three types of approximations: the generalized random phase approximation, the mean field approximation, and the low density approximation. In generalized random phase approximation the dynamic magnetic susceptibility of paramagnetic phase is calculated, and boundaries of stability of this phase are found on the plane of Hamiltonian parameters and electron concentration n. It is shown that at some critical concentration n$\text{}_{c}$ the system undergoes a crossover from the itinerant magnetism (n < $\text{}n_{c}$) to the magnetism with localized magnetic moments (n > $\text{}n_{c}$). Simultaneously, the system transfers from the Fermi-liquid behaviour to the regime of strong electron correlations with nonquasiparticle states. In mean field approximation equations for the order parameters were derived and phase transitions temperatures in a ferromagnetic and antiferromagnetic phases were obtained. In low density approximation a state of the saturated ferromagnetism was investigated and a critical concentration n$\text{}_{s}$ was calculated when the nonsaturated ferromagnetism follows the saturated ferromagnetism.
    Źródło:
    Acta Physica Polonica A; 1994, 85, 1; 103-115
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Theoretical and Mössbauer Effect Study of Magnetic and Electronic Properties of Fe$\text{}_{3}$Al Compound
    Autorzy:
    Deniszczyk, J.
    Frąckowiak, J. E.
    Powiązania:
    https://bibliotekanauki.pl/articles/1956205.pdf
    Data publikacji:
    1997-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.50.Bb
    75.10.Lp
    Opis:
    The electronic properties of Fe$\text{}_{3}$Al were determined experimentally, with the use of the Mossbauer spectroscopy, and theoretically. The band structure of the compounds was investigated applying the self-consistent tight-binding linear muffin tin orbital method. The calculated Fermi contact term of hyperfine fields and the isomer shifts are in good agreement with the values resulting from analysis of experimental data. The different kinds of electron transfer estimated on the base of the proposed "additive model" are also strongly supported by calculations.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 2; 455-458
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Exchange Coupling in GdM Compounds
    Autorzy:
    Buschow, K. H.J.
    Grechnev, G. E.
    Hjelm, A.
    Kasamatsu, Y.
    Panfilov, A. S.
    Svechkarev, I. V.
    Powiązania:
    https://bibliotekanauki.pl/articles/1964560.pdf
    Data publikacji:
    1997-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.20.En
    75.10.Lp
    Opis:
    The magnetic susceptibility of ferromagnetic GdM alloys (M=Cu$\text{}_{1-x}$Ga$\text{}_{x}$, Mg, Zn) has been investigated under helium gas pressure for temperatures above T$\text{}_{C}$. The evaluated pressure derivatives of the paramagnetic Curie temperature, dlnΘ/dP, appeared to be remarkably different for isovalent GdMg and GdZn compounds (-11.2 and -0.1 Mbar$\text{}^{-1}$, respectively). An analysis of the obtained dlnΘ/dP values for GdCu$\text{}_{1-x}$Ga$\text{}_{x}$ alloys and results of ab initio electronic structure calculations have revealed the essential role of 5d electrons as the mediators of exchange coupling in ferromagnetic GdM compounds. The pressure derivatives of T$\text{}_{C}$ were calculated by employing the modern mean-field theory, as well as the spin-fluctuation model. As a result, good agreement is found with the experimental values of dlnΘ/dP.
    Źródło:
    Acta Physica Polonica A; 1997, 92, 2; 267-270
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Phase Separation in Hubbard Model
    Autorzy:
    Samson, J. H.
    Powiązania:
    https://bibliotekanauki.pl/articles/1964683.pdf
    Data publikacji:
    1997-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.25.+z
    75.10.Lp
    Opis:
    The Hartree-Fock ground-state phase diagram of the one-dimensional Hubbard model is calculated in the μ-U plane, restricted to phases with no charge density modulation. This allows antiferromagnetism, saturated ferromagnetism, spiral spin density waves and a collinear structure with unit cell ↑↑↓↓. The spiral phase is unstable against phase separation near quarter-, half- and three-quarter-filling. For large U this occurs at hole (or electron) doping of (3t/π$\text{}^{2}$U)$\text{}^{1}\text{}^{/}\text{}^{3}$ from half-filling.
    Źródło:
    Acta Physica Polonica A; 1997, 92, 2; 363-366
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Ferromagnetism in Hubbard Chains
    Autorzy:
    Cuoco, M.
    Noce, C.
    Romano, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1955828.pdf
    Data publikacji:
    1997-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    75.10.Lp
    Opis:
    We analyze the possible occurrence of ferromagnetism in the Hubbard model, by means of an exact diagonalization study performed on 3- to 8-site chains with periodic boundary conditions. In the case of one hole in the half-filled configuration, we find that the Nagaoka state is reached only in the 3- and in the 4-site case. Ground states characterized by unsaturated ferromagnetism are found when the case of more than one hole is considered.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 2; 387-390
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Ferromagnetic Systems with Hopping Interaction
    Autorzy:
    Górski, G.
    Mizia, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1537008.pdf
    Data publikacji:
    2010-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    75.10.Lp
    Opis:
    We analyze the influence of hopping interaction on magnetic ordering. Scattering scheme of the Hubbard III approximation with included inter-site kinetic electron-electron correlation is used. The hopping interaction and inter-site correlation lead to two spin dependent effects: the band width correction and the band-shift correction. The band-shift correction factor causes an exchange splitting between the spin-up and spin-down spectrum, and its role is similar to the exchange interaction in the classic Stoner model. The spin dependent band width correction enhanced strongly by the inter-site kinetic correlation lowers the kinetic energy of electrons by decreasing the majority spin band width for some electron occupations with respect to the minority spin band width. The results show that in the case of the symmetrical density of states there is only ferromagnetic enhancement. For the strongly asymmetrical density of states there is a ferromagnetic transition.
    Źródło:
    Acta Physica Polonica A; 2010, 118, 2; 336-339
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Electronic Structure and Magnetic Properties of Intermetallic Alloys
    Autorzy:
    Jezierski, A.
    Pugacheva, M.
    Morkowski, J. A.
    Szajek, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1953169.pdf
    Data publikacji:
    1997-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.20.-b
    75.10.Lp
    Opis:
    We present the influence of local ordering on the electronic and magnetic properties of Heusler-type alloys. The band structure and magnetic moments are calculated by ab initio spin-polarized tight binding linear muffin-tin orbital method. The calculated electronic density of states for Pd$\text{}_{2}$TiAl alloy is similar to ultraviolet photoelectron spectroscopy measurements. The self-consistent band calculations showed that the density of states at the Fermi level in Ni$\text{}_{2}$(Nb$\text{}_{(1-x)}$Ti$\text{}_{x}$)Sn and Ni$\text{}_{2}$(Nb$\text{}_{(1-x)}$Ta$\text{}_{x}$)Sn alloys decreased with the increase in Ti or Ta concentration. The total and local magnetic moments in ordered Rh$\text{}_{2}$TMSn (TM = Mn, Fe, Co, Ni, Cu) and Rh$\text{}_{2}$MnX (X = Al, Ga, In, Ge, Sb, Pb) Heusler-type alloys are calculated. The difference between theoretical and experimental results can be connected to the partial disorder in the samples.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 1; 151-159
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Magnetism Within the Limitations of the LDA Band Structure Calculations
    Autorzy:
    Szpunar, B.
    Powiązania:
    https://bibliotekanauki.pl/articles/1929380.pdf
    Data publikacji:
    1993-07
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.25.-s
    75.10.Lp
    Opis:
    We discuss the limitations of the local density approximation for high-temperature superconductors and related to them parent antiferromagnetic insulators. The calculated band-gap accuracy is pointed out for two cases: C$\text{}_{60}$-FCC and YBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{6}$. We also compare the results for parent CaCuO$\text{}_{2}$ and NiO systems, as well as discuss the role of gradient and self-energy (GW) corrections for the Mott insulators.
    Źródło:
    Acta Physica Polonica A; 1993, 84, 1; 21-42
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Strongly Correlated Electrons in a Magnetic Field: Fermi Liquid versus Spin Liquid
    Autorzy:
    Spałek, J.
    Wójcik, W.
    Powiązania:
    https://bibliotekanauki.pl/articles/1930575.pdf
    Data publikacji:
    1994-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.28.-d
    75.10.Lp
    Opis:
    We compare the Fermi liquid (FL) and statistical spin liquid (SSL) representations of the electronic properties for strongly correlated systems. In particular, we discuss the entropy, as well as the magnetization as a function of band filling, temperature and magnetic field. These properties are quite different in the two (FL, SSL) states. Antiferromagnetic state disappears for a small number of holes in the doped Mott insulator.
    Źródło:
    Acta Physica Polonica A; 1994, 85, 2; 357-361
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Phase Diagram of Metal-Insulator Transition in System with Anderson-Hubbard Centers
    Autorzy:
    Skorenkyy, Yu.
    Didukh, L.
    Kramar, O.
    Dovhopyaty, Yu.
    Powiązania:
    https://bibliotekanauki.pl/articles/1418528.pdf
    Data publikacji:
    2012-09
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    71.30.+h
    Opis:
    The model of a strongly correlated system in which periodically spaced Anderson-Hubbard centers are introduced into narrow-band metal is considered. Besides the interactions between localized magnetic moments and strong on-site Coulomb interaction, the model takes into account the hybridization of localized and band states. To study the effect of the lattice deformation on the electrical properties of the system, the phonon term and elastic energy have been taken into account. Green functions for band and localized electrons have been found. On this base, the energy spectrum has been investigated as a function of model parameters, temperature and external pressure. The criterion of the metal-insulator transition for an integer value of electron concentration has been derived and the phase diagram of the metal-insulator transition has been built.
    Źródło:
    Acta Physica Polonica A; 2012, 122, 3; 532-534
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Magnetism in FeRh Alloys
    Autorzy:
    Pugacheva, M.
    Jezierski, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1861254.pdf
    Data publikacji:
    1994-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.25.Pi
    75.10.Lp
    Opis:
    We study the electronic and magnetic properties of FeRh ordered alloys. The electronic structure is calculated by the tight-binding linear muffin-tin orbital method. We observe the decreasing magnetic moment with increase of the ratio c/a during the structural phase transformation from CsCl to CuAu type structure.
    Źródło:
    Acta Physica Polonica A; 1994, 85, 1; 219-221
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Electronic Structure of Pt$\text{}_{1-x}$Mn$\text{}_{x}$ and Pt$\text{}_{1-x}$Cr$\text{}_{x}$ Disordered Alloys
    Autorzy:
    Jezierski, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1924282.pdf
    Data publikacji:
    1992-12
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.25.Pi
    75.10.Lp
    Opis:
    Electronic density of states of disordered fcc Pt$\text{}_{1-x}$Mn$\text{}_{x}$ and Pt$\text{}_{1-x}$Cr$\text{}_{x}$ alloys for 0 < x < 0.35 is calculated by the tight-binding linear muffin-tin orbital (TB LMTO) and the coherent potential approximation (CPA) method. Using the Stoner model it was found that the disordered Pt-Mn and Pt-Cr alloys are paramagnetic.
    Źródło:
    Acta Physica Polonica A; 1992, 82, 6; 983-992
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Band Magnetism with Inter-Site Correlations and Interactions
    Autorzy:
    Mizia, J.
    Górski, G.
    Kucab, K.
    Powiązania:
    https://bibliotekanauki.pl/articles/1374337.pdf
    Data publikacji:
    2014-10
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    75.10.Lp
    75.50.Cc
    Opis:
    We introduce the Hamiltonian to describe narrow band electrons. The physics of driving forces towards ferromagnetism is re-examined. Using different approximations it has been shown that the magnetic moments created by inter-site interaction and inter-site kinetic correlation decrease quickly with temperature. As a result of these interactions and the realistic density of states the Curie temperatures calculated after fitting magnetic moments to their low temperature values are realistic. In the past the Curie temperatures calculated using only the on-site interaction were much higher than the experimental temperatures.
    Źródło:
    Acta Physica Polonica A; 2014, 126, 4a; A-30-A-31
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Modified Equation of Motion Approach for Ferromagnetic Systems
    Autorzy:
    Górski, G.
    Mizia, J.
    Kucab, K.
    Powiązania:
    https://bibliotekanauki.pl/articles/1384824.pdf
    Data publikacji:
    2015-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    75.10.Lp
    75.30.Kz
    Opis:
    The key problem in using the dynamical mean-field theory is finding the appropriate solution to the single impurity Anderson model. We use the modified equation of motion method based on differentiation over two time variables. In such approach we obtain correct description of the Kondo effect not only for systems symmetrical with respect to the half-filled point but also for the systems without symmetry in a half filled point and for broad range of electron concentrations like e.g. the ferromagnetic systems. For the reason of these advantages we investigate in this report dependence of the system magnetic moment on carrier concentration using our modified equation of motion method. We also analyze influence of asymmetric densities of states on ferromagnetic alignment. Our results are compared with dynamical mean-field theory-quantum Monte Carlo calculations and with Hubbard I and III approximations.
    Źródło:
    Acta Physica Polonica A; 2015, 127, 2; 207-209
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Magnetic Aspects of Superconducting Pairing
    Autorzy:
    Acquarone, M.
    Paiusco, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1929387.pdf
    Data publikacji:
    1993-07
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.10.-z
    75.10.Lp
    75.50.Ee
    Opis:
    We discuss some of the recently proposed models dealing with the superconducting state in the presence of strong antiferromagnetic fluctuations, induced by the on-site Coulomb repulsion between electrons (the Hubbard correlation). In the weak coupling limit we treat the influence of correlation effects on the phonon-mediated superconductivity within the BCS-Eliashberg framework. As a non-standard model in this limit we describe next the so-called spin bag model. To discuss strong correlation limit, we describe first the projected Hamiltonian (the t-J Hamiltonian) and the corresponding antiferromagnetic state. The resonating valence bond model is introduced both as a naive version, equivalent to the BCS treatment of the transverse spin fluctuations, as well as a holon-spinon version.
    Źródło:
    Acta Physica Polonica A; 1993, 84, 1; 101-126
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Curie Temperature of Disordered Alloys
    Autorzy:
    Arzhnikov, A. K.
    Dobysheva, L. V.
    Powiązania:
    https://bibliotekanauki.pl/articles/1964783.pdf
    Data publikacji:
    1997-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    75.50.Bb
    75.10.Lp
    Opis:
    The concentrational behavior of the Curie temperature that describes the experiments for disordered alloys Fe-M (M = Al, Si, Ga, Sn) at concentrations of M lower than 10 at.% was obtained in the framework of the Hubbard model. The influence of the spin and charge fluctuations was investigated.
    Źródło:
    Acta Physica Polonica A; 1997, 92, 2; 419-422
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Ground-State Properties and Spin Waves in Ultrathin Fe Films Covered with Magnetic and Nonmagnetic Materials
    Autorzy:
    Świrkowicz, R.
    Powiązania:
    https://bibliotekanauki.pl/articles/1933591.pdf
    Data publikacji:
    1995-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.30.Ds
    75.70.-i
    75.10.Lp
    Opis:
    Properties of ultrathin films consisted of 5 and 7 atomic layers of Fe and covered with magnetic (Co) or nonmagnetic (Cu, Ag) materials are investigated within the framework of the multiband model. Ground-state results consistent with those known from ab initio approaches are obtained. Spin waves are studied in the random phase approximation with the use of the susceptibility method. Amplitudes and dispersion relations are calculated. Two acoustic modes with amplitudes enhanced at the interface are found.
    Źródło:
    Acta Physica Polonica A; 1995, 88, 2; 381-388
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Electronic Properties and Dynamic Susceptibility of MnTe Systems
    Autorzy:
    Wilczyński, M.
    Świrkowicz, R.
    Powiązania:
    https://bibliotekanauki.pl/articles/1968441.pdf
    Data publikacji:
    1997-11
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.50.Pp
    75.10.Lp
    75.70.-i
    Opis:
    Thin MnTe films are investigated. Electronic structure, local density of states and local magnetic moments are calculated for systems with ferromagnetic and antiferromagnetic order. Spin-dependent modifications of the density of states near the surface are results of the changes in p-d hybridization. Magnetic longitudinal and dielectric susceptibilities are calculated.
    Źródło:
    Acta Physica Polonica A; 1997, 92, 5; 1048-1050
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Itinerant Theory of Exchange Constants in Diluted Co-Based Semimagnetic Semiconductors
    Autorzy:
    Mašek, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1888118.pdf
    Data publikacji:
    1991-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    75.30.Ef
    75.50.Pp
    Opis:
    The exchange constants in diluted semimagnetic semiconductors with Co are much larger than in the Mn doped systems. To explain this, we use a semiquantitative, tight binding version of the coherent potential approach and calculate the electronic structure of II-VI mixed semiconductors containing Co and Mn. The exchange constants are deduced from energies of spin reversal processes. The presence of the Co 3d states at the top of the valence band leads to the observed enhancement of the exchange coupling.
    Źródło:
    Acta Physica Polonica A; 1991, 80, 2; 267-270
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Spin Waves and Temperature Dependence of Magnetization in Nickel Thin Films Covered with Iron and Copper
    Autorzy:
    Świrkowicz, R.
    Powiązania:
    https://bibliotekanauki.pl/articles/1929590.pdf
    Data publikacji:
    1993-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    75.30.Ds
    75.70.-i
    Opis:
    Energy spectrum and amplitudes of spin waves, as well as the temperature dependence of the local magnetization in Ni thin films covered with Fe and Cu overlayers are investigated within the framework of the itinerant electron approach. Two acoustic surface modes strongly localized in Fe over-layers are found. Energies of these modes are very close to each other but they are considerably lower than in Ni films with free surfaces. The presence of the strongly localized low-energy modes leads to fast decrease in the surface magnetization as compared to the central layer one. The quasi-linear temperature dependence of the surface magnetization is found whereas the central layer magnetization follows a quasi T$\text{}^{3}\text{}^{/}\text{}^{2}$ law. On the other hand, in films covered with Cu the temperature dependence of the magnetization can be fitted to the Bloch law for all layers. Inside the film the Bloch coefficient weakly depends on the layer index but it rapidly decreases at the surface in the Cu layers.
    Źródło:
    Acta Physica Polonica A; 1993, 84, 2; 353-363
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Onset of Itinerant Ferromagnetism in Cobalt-Nickel Heusler-Type Compounds
    Autorzy:
    Pierre, J.
    Skolozdra, R. V.
    Gorelenko, Yu.k.
    Powiązania:
    https://bibliotekanauki.pl/articles/1860757.pdf
    Data publikacji:
    1994-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.20.En
    75.10.Lp
    75.50.Cc
    Opis:
    The magnetic properties of Co-Ni compounds with MnCu_{2}Al or MgAgAs-type structure (the Heusler phases) were studied. Paramagnetic, ferromagnetic moments, Curie's temperature increase with Co content. The ratio of ordered to paramagnetic moment varies with the Curie temperature following itinerant ferromagnetism models. A metal-insulator transition occurs in the TiCo_{x}Ni_{1-x}Sn series, which causes a more rapid drop of the Curie temperature than in TiCo_{x}Ni_{2-x}Sn solid solutions which remain metallic.
    Źródło:
    Acta Physica Polonica A; 1994, 85, 1; 209-213
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Soft-Mode vs. Localized Moments Regimes of Spin Fluctuations in Itinerant Electron Magnetism
    Autorzy:
    Solontsov, A.
    Lacroix, C.
    Wagner, D.
    Powiązania:
    https://bibliotekanauki.pl/articles/1964668.pdf
    Data publikacji:
    1997-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.40.Gb
    75.10.Lp
    75.50.Ee
    Opis:
    An analysis of different regimes of spin fluctuation behaviour in anharmonic itinerant electron magnets is presented basing on the recently worked out soft-mode theory of spin fluctuations.
    Źródło:
    Acta Physica Polonica A; 1997, 92, 2; 359-362
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Mixed Valence Quasiparticles in CuO$\text{}_{2}$ Planes
    Autorzy:
    Bała, J.
    Oleś, A. M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1955491.pdf
    Data publikacji:
    1997-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.25.Jb
    75.30.Ds
    75.10.Lp
    Opis:
    The motion of a single hole doped to a CuO$\text{}_{2}$ plane of a high temperature superconductor is described by a spin-fermion model which treats explicitly the hybridization with the upper Hubbard band. One finds that sectors of Zhang-Rice bound states coexist with reasonably damped oxygen and copper-like quasiparticles, with a considerable transfer of the spectral weight to the upper Hubbard band. The oxygen states have small dispersions and can be identified in the angle-resolved photoemission experiments for Bi$\text{}_{2}$Sr$\text{}_{2}$CaCu$\text{}_{2}$O$\text{}_{8}$.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 2; 333-336
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Interrelation between the Optical Conductivity and Crystal Field Splitting in Monolayer Manganites
    Autorzy:
    Rościszewski, K.
    Oleś, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1810316.pdf
    Data publikacji:
    2009-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.47.Lx
    75.10.Lp
    63.20.Pw
    Opis:
    We introduce an effective model for correlated $e_g$ electrons which reproduces qualitatively the evolution of magnetic order in monolayer manganites when correlated wave functions are used. Here we address recent optical conductivity measurements suggesting that the splitting between the occupied and empty $e_g$ states is very large in $LaSrMnO_4$, contrary to what is expected for the effective model. We argue that no contradiction was found but several simple-minded one-atom-based expectations concerning crystal-field splitting and optical conductivity are in error.
    Źródło:
    Acta Physica Polonica A; 2009, 115, 1; 107-109
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Theoretical Study of Substitution of Si for Ge in Mn$\text{}_{5}$Ge$\text{}_{3}$
    Autorzy:
    Siberchicot, B.
    Henrion, R.
    Toboła, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1956494.pdf
    Data publikacji:
    1997-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.20.Be
    75.50.Cc
    75.10.Lp
    Opis:
    The substitution of silicon for germanium in Mn$\text{}_{5}$Ge$\text{}_{3}$ leads to a decrease in the magnetization for x < 0.7 and then to phase transition toward a non-collinear antiferromagnetic order for Mn$\text{}_{5}$Si$\text{}_{3}$. The magnetic properties of Mn$\text{}_{5}$Ge$\text{}_{3}$ and Mn$\text{}_{5}$(Ge$\text{}_{0.5}$Si$\text{}_{0.5}$)$\text{}_{3}$ are analyzed by means of band structure calculations.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 2; 467-470
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Electronic Structure of Diluted Semimagnetic Semiconductor (Cd,Co)Se
    Autorzy:
    Cháb, V.
    Mašek, J.
    Orłowski, B. A.
    Kowalski, B. J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1890829.pdf
    Data publikacji:
    1991-09
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    75.50.Pp
    79.60.Eq
    Opis:
    The electronic structure of a new semimagnetic semiconductor (Cd,Co)Se is studied by UPS and calculated by a tight-binding version of the disordered-local-moment theory. The theory accounts for the chemical disorder and for electron interactions within the Co 3d$\text{}^{7}$ shells. Both theory and experiment show Co 3d states deep in the valence band and also at the band edge. The last state seem to be responsible for unique properties of the Co-based semimagnetic semiconductors.
    Źródło:
    Acta Physica Polonica A; 1991, 80, 3; 333-336
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Coexistence of Ferromagnetism with Spin Triplet Superconductivity
    Autorzy:
    Górski, G.
    Kucab, K.
    Mizia, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1810408.pdf
    Data publikacji:
    2009-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.-z
    75.10.Lp
    Opis:
    The experimental results for $ZrZn_2$, URhGe, and in some pressure ranges also for $UGe_2$, showed that the ferromagnetic superconductors are weak itinerant ferromagnets. Guided by these results we describe the phenomenon of coexistence between equal spin triplet pairing superconductivity and ferromagnetism using the extended Stoner model, which includes in Hamiltonian the on-site Coulomb interaction, U, and occupation dependent hopping integral. We use the Hartree-Fock approximation and the Green functions technique. In the Hartree-Fock approximation the on-site Coulomb interaction plays the role of the on-site exchange (Hund's) field. All inter-site interactions will have included the inter-site kinetic correlation, 〈$c_{iσ}^{+}c_{jσ}$〉, within the Hartree-Fock approximation. We introduce the pressure-dependence to the hopping integral. Numerical results are compared with experimental data for $ZrZn_2$. The kinetic correlation creates the superconductivity without help of negative values of the Coulomb interactions. The model can explain stimulation of triplet superconductivity by the weak itinerant ferromagnetism. This effect was observed experimentally in $ZrZn_2$. Numerical analysis also confirms the experimental effect of decrease in critical temperatures (Curie and superconducting) with increasing external pressure.
    Źródło:
    Acta Physica Polonica A; 2009, 115, 1; 141-143
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Phase Separation near Half-Filling Point in Superconducting Compounds
    Autorzy:
    Kucab, K.
    Górski, G.
    Mizia, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1374052.pdf
    Data publikacji:
    2014-10
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    74.20.-z
    75.10.Lp
    Opis:
    We present the model of superconducting ceramics using the single band extended Hubbard Hamiltonian. We investigate the simultaneous presence of antiferromagnetism and d-wave superconductivity in the coherent potential approximation applied to the on-site Coulomb repulsion U. We consider the hopping interaction, Δt, the inter-site charge-charge interaction, V (creating superconductivity), and the single site Hund type exchange interaction, $F_\text{in}$ (creating antiferromagnetism). The influence of these interactions on the separation of superconducting and antiferromagnetic phases near the half-filling point is investigated. Results are compared with the experimental data for YBaCuO and NdCeCuO compounds.
    Źródło:
    Acta Physica Polonica A; 2014, 126, 4a; A-104-A-106
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Partial Localization of Correlated Electrons in an Orbitally Degenerate Narrow Band: Spin Dependent Masses, Saturated Ferromagnetism, and the Effective s-d Model
    Autorzy:
    Jędrak, J.
    Spałek, J.
    Zwicknagl, G.
    Powiązania:
    https://bibliotekanauki.pl/articles/2047325.pdf
    Data publikacji:
    2007-04
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.27.+a
    71.30.+h
    75.10.Lp
    Opis:
    We determined the localization threshold in a partially filled and doubly degenerate model of correlated electrons. Particular emphasis is put on a non-integer band filling n≥1, when the system decomposes into the localized and the itinerant subsystems; this situation is described by an effective s-d model. A simultaneous transition to the ferromagnetic state is discussed as driven by the Hund rule coupling combined with the effective field coming from the correlations. The dependence of the quasiparticle mass on the spin direction appears naturally in the spin-polarized phase and is attributed to the electron correlation effects, as is also a metamagnetic transition in an applied field. Although the main results were obtained within the saddle point slave-boson approach, their qualitative features are discussed in general terms, i.e. as a transition from quantum-mechanical indistinguishability of particles forming the Fermi fluid to a two-component situation.
    Źródło:
    Acta Physica Polonica A; 2007, 111, 4; 619-633
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Magnetic Structure of Electronic Inhomogeneities in Cuprates: Competition between Stripes and Spirals
    Autorzy:
    Seibold, G.
    Markiewicz, R.
    Lorenzana, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1426709.pdf
    Data publikacji:
    2012-05
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.27.+a
    73.22.Gk
    75.10.Lp
    Opis:
    It is shown that the magnetic structure of high-$T_{c}$ superconductors is strongly influenced by the next-nearest neighbor hopping parameter t' which distinguishes different families of cuprates. Our investigations indicate that uniform spirals get favored by a large t'/t ratio but are unstable at small doping towards stripes with spin canting. For large |t'/t| spirals can be stabilized under certain conditions in the overdoped regime which may explain the elastic incommensurate magnetic response recently observed in iron-co-doped Bi2201 materials.
    Źródło:
    Acta Physica Polonica A; 2012, 121, 5-6; 1019-1022
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Coexistence of Spin-Triplet Superconductivity with Antiferromagnetism in Orbitally Degenerate System: Hartree-Fock Approximation
    Autorzy:
    Zegrodnik, M.
    Spałek, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1431255.pdf
    Data publikacji:
    2012-04
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.20.-z
    74.25.Dw
    75.10.Lp
    Opis:
    We consider the coexistence of the Hund's-rule-exchange induced spin-triplet paired state with the antiferromagnetic ordering by starting from the extended Hubbard model for a doubly degenerate band. We use the density of states appropriate for the square lattice and treat the problem in the Hartree-Fock approximation. The temperature dependences of the superconducting gaps, the magnetic moment, and the chemical potential are presented. The free energy in the considered phase is evaluated, as well as the corresponding free energies in four additional phases: paramagnetic, ferromagnetic, superconducting of type A and superconducting of type A1 coexisting with ferromagnetism; they occur in the proper range of parameters: band filling n and the interaction parameters U/W and J/W. The low temperature values of the superconducting gaps and staggered magnetic moment are also analyzed as a function of band filling.
    Źródło:
    Acta Physica Polonica A; 2012, 121, 4; 801-804
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Electron Correlation Phenomena in Selected Transition Metal Oxides
    Autorzy:
    Honig, J. M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1860849.pdf
    Data publikacji:
    1994-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.28.+d
    71.30.+h
    75.10.Lp
    Opis:
    The theory of electron correlation phenomena is reviewed and used to interpret the transport, thermal, or magnetic properties of V_{2}O_{3}, LaNiO_{3} and Fe_{3}O_{4} systems, each of which to a greater or lesser degree displays correlation effects. A rationale is provided for interpreting the various types of phase transitions encountered in the vanadium sesquioxide and magnetite host systems and to show why these are altered by small compositional variations.
    Źródło:
    Acta Physica Polonica A; 1994, 85, 1; 53-77
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Ground States of the Falicov-Kimball Model Extended by Nonlocal Coulomb Interactions
    Autorzy:
    Čenčariková, H.
    Farkašovský, P.
    Žonda, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1813654.pdf
    Data publikacji:
    2008-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    71.27.+a
    71.28.+d
    Opis:
    The small-cluster exact-diagonalizations are used to study the ground states of the Falicov-Kimball model extended by nonlocal Coulomb interactions (the nearest-neighbour interaction $U_{nn}$ and the correlated hopping t'). It is shown that the ground-state phase diagrams found for the conventional Falicov-Kimball model are strongly changed when the nonlocal interactions are added. This is illustrated for two selected values of the on-site Coulomb interaction (U) that represent typical behaviours of the model in the intermediate and strong coupling limit. A number of remarkable results are found. (i) The phase separation takes place for a wide range of $U_{nn}$ and t' in both interaction limits. (ii) New types of inhomogeneous charge ordering are observed for nonzero $U_{nn}$ and t'. (iii) Depending on the values of $U_{nn}$ and t', the model is able to describe both the continuous as well as discontinuous changes of the f-electron occupation number.
    Źródło:
    Acta Physica Polonica A; 2008, 113, 1; 287-290
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Ground States of the Spin-One-Half Falicov-Kimball Model with Hunds Rule Coupling in Two Dimensions
    Autorzy:
    Čenčariková, H.
    Farkašovský, P.
    Tomašovičová, N.
    Žonda, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1810314.pdf
    Data publikacji:
    2009-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    71.27.+a
    71.28.+d
    Opis:
    The spin-one-half Falicov-Kimball model with spin-dependent on-site interaction between localized (f) and itinerant (d) electrons is studied by small-cluster exact-diagonalization calculations and a well-controlled approximative method in two dimensions. The results obtained are used to categorize the ground-state configurations according to common features (charge and spin ordering) for all f and d electron concentrations ($n_f$ and $n_d$) on finite square lattices. It is shown that only a few configuration types form the basic structure of the charge phase diagram in the $n_f-n_d$ plane. In particular, the largest regions of stability correspond to the phase segregated configurations and configurations that can be considered as mixtures of chessboard configurations and the full (empty) lattice. Since the magnetic phase diagram is much richer than the charge phase diagram, the magnetic superstructures are examined only at selected values of f and d electron concentrations.
    Źródło:
    Acta Physica Polonica A; 2009, 115, 1; 101-103
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Thermodynamics of the Generalized Spin-One-Half Falicov-Kimball Model in Two Dimensions
    Autorzy:
    Žonda, M.
    Farkašovský, P.
    Čenčariková, H.
    Powiązania:
    https://bibliotekanauki.pl/articles/1810315.pdf
    Data publikacji:
    2009-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    71.27.+a
    71.28.+d
    Opis:
    The extrapolation of small-cluster exact-diagonalization calculations and the Monte Carlo method is used to study the spin-one-half Falicov-Kimball model extended by the spin-dependent Coulomb interaction (J) between the localized f and itinerant d electrons as well as the on-site Coulomb interaction $(U_{ff})$ between the localized f electrons. It is shown that in the symmetric case the ground-state phase diagram of the model has an extremely simple structure that consists of only two phases, and namely, the charge-density-wave phase and the spin-density-wave phase. The nonzero temperature studies showed that these phases persist also at finite temperatures. The same calculations that we performed for unsymmetric case showed that charge and spin ordering can be destroyed simultaneously or consecutively.
    Źródło:
    Acta Physica Polonica A; 2009, 115, 1; 104-106
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Spin-Triplet Pairing Induced by Hunds Rule Exchange in Orbitally Degenerate Systems: Hartree-Fock Approximation
    Autorzy:
    Zegrodnik, M.
    Spałek, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1426874.pdf
    Data publikacji:
    2012-05
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    74.20.-z
    74.25.Dw
    75.10.Lp
    Opis:
    We discuss the spin-triplet pairing mechanism induced by the Hund rule ferromagnetic exchange. We include explicitly the effect of interband hybridization and treat the problem by starting from an extended Hubbard model for a doubly degenerate band, making the simplest Hartree-Fock approximation for the part involving the pairing and the Hubbard interaction. The conditions of stability of various phases are determined as a function of both band filling and microscopic parameters. The phase diagram contains regions of stability of the spin-triplet superconducting phase coexisting with either saturated or non-saturated ferromagnetism. Phase diagrams for the cases of constant density of states and that of square lattice have been provided. The influence of hybridization on the stability of considered phases, as well as the temperature dependences of magnetic moment and the superconducting gap are also discussed.
    Źródło:
    Acta Physica Polonica A; 2012, 121, 5-6; 1051-1055
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Exchange Interaction as a Source of Real Space Pairing in High Temperature Superconductors and Heavy Fermions: a Brief Overview
    Autorzy:
    Spałek, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1860823.pdf
    Data publikacji:
    1994-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.28.+d
    75.10.Lp
    74.70.Vy
    Opis:
    We characterize the principal difference between the pairing induced by the kinetic exchange interactions for the Fermi-liquid and spin-liquid phases. In the case of hybridized electrons the Kondo-exchange induced pairing is invoked, whereas for a single narrow-band case kinetic exchange is responsible for the spin singlet pairing. The former interaction is applied to heavy-fermion systems, whereas the latter is employed to the high temperature superconductors. The current situation concerning the applicability of our theoretical models is briefly and critically assessed.
    Źródło:
    Acta Physica Polonica A; 1994, 85, 1; 39-52
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Racah Recoupling Matrices for System of Two Free Electrons Confined To Single Star
    Autorzy:
    Lulek, T.
    Kłos, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1945688.pdf
    Data publikacji:
    1996-09
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    02.20.-a
    31.10.+z
    75.10.Lp
    75.20.-g
    Opis:
    Racah-Wigner type of calculus is adapted to the system of two free electrons in a cubic box, confined to a single star p00 of quasimomentum. Various coupling schemes (LS-like, jj-like, and transitive) have been reviewed in analogy to the case of a free atom. Racah recoupling matrices are calculated explicitly, and their quantum-mechanical meaning is exposed in terms of octahedral multipoles.
    Źródło:
    Acta Physica Polonica A; 1996, 90, 3; 483-490
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Ferromagnetic Ground State of Two-Band Hubbard Model with One Electron per Site
    Autorzy:
    Schulenburg, J.
    Richter, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/1955522.pdf
    Data publikacji:
    1997-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.40.Mg
    75.30.Et
    75.50.Dd
    75.10.Lp
    Opis:
    We study the multiband mechanism to favour a ferromagnetic insulating ground state within a two-band Hubbard model. Besides perturbation theory we use exact diagonalization studies to examine the ground state of chains and 2D-lattices. According to second order perturbation theory the exact diagonalization yields a fully polarized ferromagnetic ground state, if the hopping between two ground state orbitals of neighbouring atoms t$\text{}_{gg}$ is small and the hopping between a ground state orbital and an excited orbital t$\text{}_{ge}$ dominates. However, in contrast to the suggestion from the second order perturbation theory this ferromagnetic state is stable only for very small hopping integrals U≫ t$\text{}_{ge}$>t$\text{}_{gg}$. For larger t $\text{}_{ge}$ quantum interference effects lead to complex magnetic structures.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 2; 367-370
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Dynamic Susceptibility in Thin Films with Antiferromagnetic Coupling between Layers
    Autorzy:
    Świrkowicz, R.
    Powiązania:
    https://bibliotekanauki.pl/articles/1952834.pdf
    Data publikacji:
    1996-12
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    75.30.Cr
    75.30.Ds
    75.50.Ee
    Opis:
    The dynamic susceptibility of the system with antiferromagnetic coupling between layers is investigated within the framework of the multiband model using the equation of motion with random phase approximation. Calculations are performed in the mixed Bloch-Wannier representation and a general form for χ is found. The susceptibility can be written in terms of two-particle Green's functions expressed in the local coordinate system with the z axis aligned along the local magnetization. The expression depends on an angle between the magnetization direction in a given layer and the crystal axis. Preliminary numerical calculations are performed for two systems: ultrathin Cr film and Fe/Cr multilayer structure. Imaginary part of the susceptibility corresponding to different layers is calculated and spin waves are discussed.
    Źródło:
    Acta Physica Polonica A; 1996, 90, 6; 1215-1226
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Stripe Phases in Layered Nickelates
    Autorzy:
    Rościszewski, K.
    Oleś, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1426855.pdf
    Data publikacji:
    2012-05
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.25.Dk
    75.47.Lx
    75.10.Lp
    63.20.Pw
    Opis:
    To describe quasi two-dimensional nickelates we introduce an effective Hamiltonian for $e_{g}$ electrons, which includes the kinetic energy, on-site Coulomb interactions, spin-spin and Jahn-Teller (static) terms. The experimental stripe phases are correctly reproduced by the model. The mechanisms responsible for stripe formation are different than those reported in cuprates and manganites.
    Źródło:
    Acta Physica Polonica A; 2012, 121, 5-6; 1048-1050
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Non-Collinear Magnetism in Itinerant-Electron Systems
    Autorzy:
    Kübler, J.
    Powiązania:
    https://bibliotekanauki.pl/articles/2012876.pdf
    Data publikacji:
    2000-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.15.Mb
    75.10.Lp
    75.30.Ds
    75.30.Kz
    Opis:
    The Heisenberg model cannot uncritically be applied to itinerant-electron magnets, including those that show non-collinear order. Density functional theory is therefore generalized to apply to non-collinear itinerant-electron magnets. The appropriate Kohn-Sham equations are discussed and the total energy of spiral magnetic order is used to determine the magnon spectrum in the adiabatic approximation. The energy spectrum of transverse spin fluctuations can also be estimated with the total energy of spiral magnetic order and allows a determination of thermal properties of itinerant-electron magnets. Calculated results for Fe, Co, Ni, and FeCo are discussed and compared with experimental results.
    Źródło:
    Acta Physica Polonica A; 2000, 97, 1; 165-173
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    The Green Function Variational Approximation: Significance of Physical Constraints
    Autorzy:
    Bieniasz, K.
    Berciu, M.
    Oleś, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/1398655.pdf
    Data publikacji:
    2016-08
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.25.Dk
    03.65.Ud
    75.10.Lp
    79.60.-i
    Opis:
    We present a calculation of the spectral properties of a single charge doped at a Cu(3d) site of the Cu-F plane in KCuF₃. The problem is treated by generating the equations of motion for the Green function by means of subsequent Dyson expansions and solving the resulting set of equations. This method, dubbed the variational approximation, is both very dependable and flexible, since it is a systematic expansion with precise control over elementary physical processes. It allows for deep insight into the underlying physics of polaron formation as well as for inclusion of many physical constraints, such as excluding crossing diagrams and double occupation constraint, which are not included in the self-consistent Born approximation. Here we examine the role and importance of such constraints by analyzing various spectral functions obtained in second order variational approximation.
    Źródło:
    Acta Physica Polonica A; 2016, 130, 2; 659-663
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Wigner-Racah Description of the Free-Electron Model
    Autorzy:
    Lulek, T.
    Powiązania:
    https://bibliotekanauki.pl/articles/1891909.pdf
    Data publikacji:
    1991-12
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    02.20.+b
    71.45.-d
    75.10.Lp
    75.20.-g
    Opis:
    A version of Wigner-Racah type of approach is proposed for the model of a free-electron gas in a cubic box. The approach bases on the structure of fibre bundle in description of states of a single electron. Contrary to the tight binding model, the base of the bundle is the reciprocal space rather than the positional one. An elementary quantity of the approach is the star of quasi-momentum. Such a treatment reveals the degeneracy of energy levels of the system of N electrons, in particular of the ground Fermi level, for an uncomplete filling of stars. The associated classification of energy levels and corresponding states can be performed within an atomic-like Wigner-Racah scheme.
    Źródło:
    Acta Physica Polonica A; 1991, 80, 6; 769-779
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Coupled States of the System of Two Free Electrons on a Single Star
    Autorzy:
    Walcerz, S.
    Lulek, T.
    Powiązania:
    https://bibliotekanauki.pl/articles/1891910.pdf
    Data publikacji:
    1991-12
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    02.20.+b
    71.45.-d
    75.10.Lp
    75.20.-g
    Opis:
    A classification scheme of quantum states for the system of two free electrons in a cubic box, confined to a single star of quasi-momentum is proposed within a Racah-Wigner type of approach. Coupling of angular momenta of the atomic case is here substituted by Mackey theorem for transitive representations, which provides a crystalline analogue of orbital angular momentum - the resultant orbit of the geometric symmetry group. The action of the Pauli group is combined with that of the octahedral group which yields the connection between spin (i.e. the singlet or triplet pairing of electrons) and statistic of the positional factor. Resultant singlets and doublets - irreducible representations of the octahedral group, exhibit an ordinary Landau diamagnetic behaviour, whereas triplets are paramagnetic. A relation between the Mackey star and the star of resultant momentum is discussed.
    Źródło:
    Acta Physica Polonica A; 1991, 80, 6; 781-792
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    The Influence of Lattice Defects on the Ground-State Properties of the Falicov-Kimball Model in Two Dimensions
    Autorzy:
    Žonda, M.
    Farkašovský, P.
    Čenčariková, H.
    Powiązania:
    https://bibliotekanauki.pl/articles/1813656.pdf
    Data publikacji:
    2008-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.10.Lp
    71.27.+a
    71.28.+d
    71.30.+h
    Opis:
    The influence of lattice defects (vacancies) on the ground-state properties of the spinless Falicov-Kimball model is studied by a well-controlled numerical method in two dimensions. It is shown that in the presence of vacancies (distributed randomly) the ground states of the Falicov-Kimball model are phase separated for small f-electron concentrations $n_f$ and exhibit the long-range order for $n_f$ near the half-filled band case $n_f$=1/2. In addition, the dependence of average f-orbital occupancy on the concentration of vacancies is calculated for a wide range of model parameters. The resultant behaviours are used to interpret the experimental data obtained for the mixed-valence system $Sm_{1-x}B_6$.
    Źródło:
    Acta Physica Polonica A; 2008, 113, 1; 291-294
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Symmetry Induced Half-Metallic Alkaline Earth Ferromagnets
    Autorzy:
    Adamowicz, L.
    Wierzbicki, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1810473.pdf
    Data publikacji:
    2009-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.15.Mb
    71.20.Dg
    72.25.Ba
    75.10.Lp
    Opis:
    Search for new half-metallic ferromagnetic binary compounds composed of alkaline earth and III (boron) or IV (carbon) elements is reported. Ab initio all-electron density functional theory calculations in the generalised gradient approximation indicate possibility of half-metallic ferromagnetism. This is a new class of theoretically predicted hypothetical materials, without transition metal elements, not as yet discovered experimentally. Ferromagnetism is expected to be induced by suitable crystalline structure with lattice constant above some critical value. The predominant p electrons mechanism seems to be responsible for the formation and interactions of localised magnetic moments on boron or carbon atoms.
    Źródło:
    Acta Physica Polonica A; 2009, 115, 1; 217-219
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Exact Diagonalization Study of an Extended Hubbard Model for a Cubic Cluster at Quarter Filling
    Autorzy:
    Szałowski, K.
    Balcerzak, T.
    Jaščur, M.
    Bobák, A.
    Žukovič, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/1032794.pdf
    Data publikacji:
    2017-04
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.40.Cx
    75.75.-c
    75.10.Lp
    71.27.+a
    65.80.-g
    05.70.Ce
    Opis:
    In the paper the thermodynamics of a cubic cluster with 8 sites at quarter filling is characterized by means of exact diagonalization technique. Particular emphasis is put on the behaviour of such response functions as specific heat and magnetic susceptibility. The system is modelled with extended Hubbard model which includes electron hopping between both first and second nearest neighbours as well as Coulombic interactions, both on-site and between nearest-neighbour sites. The importance of hopping between second nearest neighbours and Coulombic interactions between nearest neighbours for the temperature dependences of thermodynamic response functions is analysed. In particular, the predictions of the Schottky model are compared with the calculations based on the full energy spectrum.
    Źródło:
    Acta Physica Polonica A; 2017, 131, 4; 1012-1014
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
      Wyświetlanie 1-54 z 54

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