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Wyświetlanie 1-15 z 27
Tytuł:
Peculiar thermodynamic properties of the Falicov-Kimball model for small U couplings
Autorzy:
Krawczyk, J.
Lemański, R.
Powiązania:
https://bibliotekanauki.pl/articles/1048506.pdf
Data publikacji:
2018-03
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.20.-b
71.45.Lr
Opis:
We analyse a behaviour of the order parameter and specific heat of the Falicov-Kimball model (FKM) on the Bethe lattice using the Dynamical Mean Field Theory (DMFT) formalism, which provides the exact solution in the limit of large spatial dimensions. In the large U limit, the FKM maps onto the effective Ising model, with the order parameter of the Curie-Weiss form. However, in the small U limit the order parameter takes on unusual shape, with a sharp reduction near T ≈ T_{c}/2. We focus our investigation on a crossover between these two limits, thus we perform our calculations for a set of intermediate and small values of U. We find the overall behaviour of the order parameter and specific heat as a function of temperature to be quite anomalous.
Źródło:
Acta Physica Polonica A; 2018, 133, 3; 369-371
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Analysis of the Finite-Temperature Phase Diagram of the Spinless Falicov-Kimball Model
Autorzy:
Lemański, R.
Powiązania:
https://bibliotekanauki.pl/articles/1398609.pdf
Data publikacji:
2016-08
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.20.-b
71.45.Lr
Opis:
The finite-temperature phase diagram of the spinless Falicov-Kimball model on the Bethe lattice is analyzed using the dynamic mean field theory. Comparing the temperature-dependent density of states at various phases we detected a difference between two phases of the ordered insulator (OI-X and OI-Y) whose stability areas in the phase diagram are separated by the stability area of the ordered conductor. It appears that the difference between OI-X and OI-Y phases is due to the band inversion, consisting in a reversal of those subbands, that lie just above and below the Fermi level and are derived only from one or the other sublattice (+ or -).
Źródło:
Acta Physica Polonica A; 2016, 130, 2; 577-580
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Some Exact Results for the Zero-Bandwidth Extended Hubbard Model with Intersite Charge and Magnetic Interactions
Autorzy:
Kapcia, K.
Kłobus, W.
Robaszkiewicz, S.
Powiązania:
https://bibliotekanauki.pl/articles/1385252.pdf
Data publikacji:
2015-02
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.10.-w
71.10.Hf
71.45.Lr
75.30.Fv
Opis:
The extended Hubbard model in the zero-bandwidth limit is studied. The effective Hamiltonian consists of (i) on-site U interaction, (ii) intersite density-density interaction W, and (iii) Ising-like magnetic exchange interaction J between the nearest-neighbors. We present rigorous (and analytical) results obtained within the transfer-matrix method for 1D chain in two particular cases: (a) W=0 and n=1; (b) U → +∞ and n=1/2 (W ≠ 0, J ≠ 0). We obtain the exact formulae for the partition functions which enables to calculate thermodynamic properties such as entropy, specific heat (c), and double occupancy per site. In both cases the system exhibits an interesting temperature dependence of c involving a characteristic two-peak structure. There are no phase transitions at finite temperatures and the only transitions occur in the ground state.
Źródło:
Acta Physica Polonica A; 2015, 127, 2; 284-286
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Specific Heat Study of Superconductivity in $Cu_{0.061}TiSe_{2}$
Autorzy:
Kačmarčík, J.
Pribulová, Z.
Pal'uchová, V.
Husaníková, P.
Karapetrov, G.
Komanický, V.
Samuely, P.
Powiązania:
https://bibliotekanauki.pl/articles/1371314.pdf
Data publikacji:
2014-07
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
72.80.Ga
74.25.Bt
71.45.Lr
Opis:
In $TiSe_{2}$ a charge-density-waves order is continuously suppressed by copper doping and superconductivity appears. We report a calorimetric study performed on the superconducting single crystal of $Cu_{0.061}TiSe_{2}$ with $T_{c}$=2.8 K, measured by the ac technique at low temperatures down to 0.6 K and magnetic fields up to 2 T. Sharp specific heat anomaly at the superconducting transition is found. The temperature dependence of the electronic specific heat and the angular dependence of the critical temperature, measured at a fixed magnetic field, consistently indicate on the existence of a single s-wave superconducting gap with a coupling strength of 2Δ/$k_{B}T_{c}$=3.6, close to BCS limit.
Źródło:
Acta Physica Polonica A; 2014, 126, 1; 322-323
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Charge Orderings and Phase Separations in Itinerant Fermion Systems at Half Filling
Autorzy:
Czart, W.
Grzybowski, P.
Nogala, M.
Robaszkiewicz, S.
Powiązania:
https://bibliotekanauki.pl/articles/1426750.pdf
Data publikacji:
2012-05
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.30.+h
71.45.Lr
64.75.Gh
Opis:
We analyse the ground state phase diagrams of the charge orderings in narrow band materials using two effective models: (1) the spinless fermion model (t - W) with repulsive intersite interaction ($W_{ij}$ > 0) and (2) the molecular crystal model with the coupling of electrons to intramolecular (crystal field) vibrations. We present results for the case of half filled bands for d = 2 square lattice. The calculations are performed within the (broken symmetry) Hartree-Fock approximation. The study takes into consideration the effects of frustrating next-nearest-neighbour hopping $(t_2)$ on the charge ordered states in these systems. We focus on the two cases: (i) homogeneous phases and phase separations involving checkerboard charge ordering with the nesting vector Q= (π,π) only and (ii) homogeneous phases and phase separations involving two types of charge ordering: (a) checkerboard charge ordering with the nesting vector Q = (π,π), and (b) collinear (CL) charge ordering with Q = (0,π) or Q = (π,0).
Źródło:
Acta Physica Polonica A; 2012, 121, 5-6; 1042-1044
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Effects of Frustrating Hopping on Charge Ordered States in Itinerant Fermion Systems for Arbitrary Concentration in 2D Lattice
Autorzy:
Czart, W.
Grzybowski, P.
Nogala, M.
Robaszkiewicz, S.
Powiązania:
https://bibliotekanauki.pl/articles/1431325.pdf
Data publikacji:
2012-04
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.30.+h
71.45.Lr
64.75.Gh
Opis:
There is ongoing, intense, research in the field of electron charge orderings (CO) and charge density waves phenomena, due to experimental discovery of such phases in numerous important compounds. The aim of this work is to extend recent advances in the field by studying two simple effective paradigmatic models used to describe CO in narrow band materials i.e. (i) a model of correlated electrons: the so-called t-W model of spinless fermions with repulsive interaction W and (ii) the molecular crystal model with the coupling of electrons to intramolecular (crystal field) vibrations in the static limit. The finite temperature phase diagrams are evaluated at arbitrary carriers concentration for several representative cases. Our calculations are performed within the (broken symmetry) HFA for d=2 square lattice and arbitrary carriers concentration. In this contribution we focus on the effects of next-nearest-neighbor hopping on the CO states in these systems and the problem of phase separations involving checkerboard CO with the nesting vector Q=(π,π). The results we show here are an extension of our previous work on the subject.
Źródło:
Acta Physica Polonica A; 2012, 121, 4; 828-831
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Interplay between Charge and Magnetic Orderings in the Zero-Bandwidth Limit of the Extended Hubbard Model for Strong On-Site Repulsion
Autorzy:
Kapcia, K.
Kłobus, W.
Robaszkiewicz, S.
Powiązania:
https://bibliotekanauki.pl/articles/1426720.pdf
Data publikacji:
2012-05
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.45.Lr
75.30.Fv
64.75.Gh
71.10.Hf
Opis:
A simple effective model of charge ordered and (or) magnetically ordered insulators is studied. The tight binding Hamiltonian analyzed consists of (i) the effective on-site interaction U, (ii) the intersite density-density interaction W and (iii) intersite magnetic exchange interaction $J^{z}$ (or $J^{xy}$) between nearest-neighbors. The intersite interaction are treated within the mean-field approximation. One shows that the systems considered can exhibit very interesting multicritical behaviors, including among others bicritical, tricritical, tetracritical and critical end points. The analysis of the model has been performed for an arbitrary electron concentration as well as an arbitrary chemical potential in the limit of strong on-site repulsion (U → +∞). The phase diagrams obtained in such a case are shown to consist of at least 9 different states, including four homogeneous phases: nonordered (NO), ferromagnetic (F), charge ordered (CO), ferrimagnetic (intermediate, I) and five types of phase separation: NO-NO, F-NO, F-F, CO-F, CO-I.
Źródło:
Acta Physica Polonica A; 2012, 121, 5-6; 1032-1034
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Interplay between Correlated Electrons and Quantum Phonons in Charge-Ordered and Mott-Insulating Organic Compounds
Autorzy:
Yonemitsu, K.
Maeshima, N.
Tanaka, Y.
Powiązania:
https://bibliotekanauki.pl/articles/1490009.pdf
Data publikacji:
2012-02
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
78.20.Bh
71.30.+h
71.45.Lr
71.10.Fd
Opis:
At an early stage of the photoinduced transition from an insulator to a metal in quasi-two-dimensional organic conductors, a coherent motion of electrons is observed in a charge-ordered insulator, but not so far in a Mott insulator. The mechanisms of these different photoinduced charge dynamics are theoretically studied by numerical solutions to the time-dependent Schrödinger equation for exact many-electron-phonon wave functions on small clusters of model systems. We use two-dimensional three-quarter-filled extended Holstein-Hubbard models on anisotropic triangular lattices. For a charge-ordered insulator on a lattice simplified from the structure of $α-\text{(BEDT-TTF)}_2I_3$, we indeed find a low-energy collective electronic motion coupled with quantum phonons even if the energy of photoexcitation is away from this energy. For a Mott insulator on a lattice simplified from the structure of $κ-\text{(BEDT-TTF)}_2X$, however, such a collective motion does not appear, and quantum phonons are hardly excited.
Źródło:
Acta Physica Polonica A; 2012, 121, 2; 372-374
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Search for the Photo-Induced Hidden Phase in Inorganic and Organic Systems
Autorzy:
Koshihara, S.
Onda, K.
Okimoto, Y.
Ishikawa, T.
Powiązania:
https://bibliotekanauki.pl/articles/1489970.pdf
Data publikacji:
2012-02
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
78.47.J-
63.20.kd
71.30.+h
71.45.Lr
Opis:
We review the dynamical behavior of strongly electron-lattice (orbital) coupled system induced by photo-excitation of fs laser pulse. By virtue of the femtosecond (fs) spectroscopy and picosecond (ps) time-resolved X-ray diffraction measurements, we demonstrate that the photo-irradiation creates new ordered state so called as "hidden phase" which can never be realized under thermo-equilibrium condition. This exotic phase commonly plays the key role for realizing the gigantic photo-induced changes in optical properties of inorganic and organic electron-lattice strongly coupled systems.
Źródło:
Acta Physica Polonica A; 2012, 121, 2; 328-331
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Stable and Metastable Phases in the Atomic Limit of the Extended Hubbard Model with Intersite Density-Density Interactions
Autorzy:
Kapcia, K.
Robaszkiewicz, S.
Powiązania:
https://bibliotekanauki.pl/articles/1426712.pdf
Data publikacji:
2012-05
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.45.Lr
64.60.My
64.75.Gh
71.10.Hf
Opis:
We have studied a simple effective model of charge ordered insulators. The tight binding Hamiltonian consists of the effective on-site interaction U and the intersite density-density interaction $W_{ij}$ (both: nearest-neighbor and next-nearest-neighbor). In the analysis of the phase diagrams and thermodynamic properties of this model we have adopted the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. Our investigations of the general case (as a function of the electron concentration n) have shown that the system exhibits various critical behaviors including among others bicritical, tricritical, critical-end, and isolated critical points. In this report we concentrate on the metastable phases and transitions between them. One finds that the first- and second order transitions between metastable phases can exist in the system. These transitions occur in the neighborhood of first as well as second order transitions between stable phases. For the case of on-site attraction the regions of metastable homogeneous phases occurrence inside the ranges of phase separated states stability have been also determined.
Źródło:
Acta Physica Polonica A; 2012, 121, 5-6; 1029-1031
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
The Fock Term in the Charge Ordered Phase of the t-W Model
Autorzy:
Bak, M.
Powiązania:
https://bibliotekanauki.pl/articles/1427073.pdf
Data publikacji:
2012-05
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.45.Lr
71.10.Fd
71.27.+a
71.30.+h
Opis:
The paper shows the importance of retaining Hartree and Fock terms, appearing after mean-field decoupling, in the treatment of charge ordered phase of the t-W model, with implications to extended Hubbard model.
Źródło:
Acta Physica Polonica A; 2012, 121, 5-6; 1085-1088
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Charge Ordering and Phase Separations in the Molecular Crystal Model
Autorzy:
Czart, W.
Grzybowski, P.
Robaszkiewicz, S.
Powiązania:
https://bibliotekanauki.pl/articles/1537090.pdf
Data publikacji:
2010-08
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.30.+h
71.45.Lr
64.75.Gh
Opis:
We analyse the ground state phase diagrams and thermodynamic properties of charge orderings in narrow band materials using the molecular crystal model in the static limit. We present results for the hypercubic lattices in dimensions d = 2 and d = ∞. We focus our study on the problem of phase separations involving charge orderings and the effects of next-nearest-neighbor hopping $(t_2)$ on the charge ordered states in these systems. The ground state phase diagrams are evaluated for a few representative cases. Results for the molecular crystal model are compared with those obtained previously for the spinless fermion model with repulsive intersite interaction W.
Źródło:
Acta Physica Polonica A; 2010, 118, 2; 369-372
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Charge Orderings and Phase Separations in the Atomic Limit of the Extended Hubbard Model with Intersite Density-Density Interactions
Autorzy:
Kapcia, K.
Kłobus, W.
Robaszkiewicz, S.
Powiązania:
https://bibliotekanauki.pl/articles/1537054.pdf
Data publikacji:
2010-08
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.45.Lr
64.75.Gh
71.10.Hf
Opis:
A simple effective model of charge ordered insulators is studied. The tight binding Hamiltonian consists of the effective on-site interaction U and the intersite density-density interactions $W_{ij}$ (both: nearest-neighbour and next-nearest neighbour). In the analysis of the phase diagrams we have adopted the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. The phase separated states have not been taken into account in previous analyses. Our investigations of two cases of the on-site interaction: attraction (U/(- $W_{Q}$) = - 10) and repulsion (U/(- $W_{Q}$) = 1.1) show that, depending on the values of the next-nearest neighbour attraction, the system can exhibit not only homogeneous phases: charge ordered and nonordered, but also various phase separated states (charge ordered-nonordered, charge ordered-charge ordered).
Źródło:
Acta Physica Polonica A; 2010, 118, 2; 350-352
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Charge Ordering and Phase Separations in the Spinless Fermion Model with Repulsive Intersite Interaction
Autorzy:
Czart, W.
Robaszkiewicz, S.
Tobijaszewska, B.
Powiązania:
https://bibliotekanauki.pl/articles/1812286.pdf
Data publikacji:
2008-07
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
71.10.Fd
71.30.+h
71.45.Lr
64.75.Gh
Opis:
We study a simple effective model for description of charge orderings in narrow band materials, i.e. the spinless fermion model with repulsive intersite interaction W. The analysis is concentrated on the problem of phase separations and the effects of next-nearest neighbor hopping $t_2$ on the charge ordered states in this system. The cases of d-dimensional (d≥2) hypercubic lattices are considered for arbitrary particle concentration (0 < n < 1). Within the broken symmetry Hartree-Fock approximation the phase diagrams as a function of W and n are evaluated for representative cases. The results for $t_2$ ≠ 0 are compared with those found for the case with nearest neighbor hopping only.
Źródło:
Acta Physica Polonica A; 2008, 114, 1; 129-134
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Pseudogap-Like Phenomena in Cuprates as a Manifestation of Charge-Density Waves
Autorzy:
Ekino, T.
Gabovich, A.
Voitenko, A.
Suan Li, Mai
Szymczak, H.
Pękała, M.
Powiązania:
https://bibliotekanauki.pl/articles/1812272.pdf
Data publikacji:
2008-07
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
73.43.Jn
71.45.Lr
74.50.+r
74.81.-g
Opis:
Differential conductance G as a function of the bias voltage V was measured for break-junctions of superconducting $Bi_2Sr_2CaCu_2O_{8+δ}$ and $YBa_2Cu_3O_{7-δ}$. The dependences G(V) for both materials clearly demonstrate the so-called dip-hump structures outside the gap region. A theory, which suggests the charge-density-wave origin of the dip-hump structures and explains its specific form by intrinsic inhomogeneity of cuprate materials, was developed. The well-known pseudogap features in the tunnel spectra of high-$T_c$ oxides found both below and above the superconducting critical temperature are also described by the theory, which testifies that both the pseudogap and the dip-hump structures have the same origin. Competing theories and various G(V) peculiarities found for a number of superconducting oxides are briefly discussed.
Źródło:
Acta Physica Polonica A; 2008, 114, 1; 59-66
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Wyświetlanie 1-15 z 27

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