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    Wyświetlanie 1-6 z 6
    Tytuł:
    Applications of Density Matrix Renormalization Group to Problems in Magnetism
    Autorzy:
    Gehring, G. A.
    Bursill, R. J.
    Xiang, T.
    Powiązania:
    https://bibliotekanauki.pl/articles/1953163.pdf
    Data publikacji:
    1997-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    64.60.-i
    11.10.Gh
    Opis:
    The application of real space renormalization group methods to quantum lattice models has become a topic of great interest following the development of the density matrix renormalization group by White. This method has been used to find the ground and low-lying excited state energies and wave functions of quantum spin models in which the form of the ground state is not clear, for instance because the interactions are frustrated. It has also been applied to fermion problems where the tendency for localization due to the strong Coulomb repulsion is opposed by the lowering of the kinetic energy which occurs as a result of electron transfer. The approach is particularly suitable for one-, or quasi-one-dimensional problems. The method involves truncating the Hilbert space in a systematic and optimised manner. Results for the ground state energy are thus variational bounds. The results for low-lying energies and correlation functions for one-dimensional systems have unprecedented accuracy and the method has become the method of choice for solving one-dimensional quantum spin problems. We review the method and results obtained for the spin-1 chain with biquadratic exchange as well as the spin-1/2 model with competing nearest and next nearest neighbour exchange will be described. More recently, the density matrix renormalization group has been applied to reformulate the coupling constant renormalization group approach which is appropriate for the study of critical properties. This approach has been applied to the anisotropic spin-1/2 Heisenberg chain. Finally, we discuss recent work which has borne promising applications in two dimensions - the Ising model and the two-dimensional Hubbard model.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 1; 105-119
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Quasiparticles in Calcium Doped Yttrium-Iron Garnets
    Autorzy:
    Lehmann-Szweykowska, A.
    Wojciechowski, R. J.
    Gehring, G. A.
    Tobijaszewski, I.
    Powiązania:
    https://bibliotekanauki.pl/articles/1955836.pdf
    Data publikacji:
    1997-02
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    72.20.Jv
    72.80.Ga
    Opis:
    In yttrium-iron garnets an electrical charge is carried by quasiparticles based on compensating holes which move either at oxygen or iron ions of the tetragonal sublattice. The quasiparticles can also get localized because of their coupling to optical phonons on the iron sites. A multi-band model of holes coupled to phonons is applied to find additional energy levels in the gap between the lower of the narrow 3d bands and the wide 2p band, whose occurrence plays a crucial role in a conductivity mechanism. The density of states is considerably modified by the interaction between the holes and local distortion of the lattice.
    Źródło:
    Acta Physica Polonica A; 1997, 91, 2; 423-426
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Characterizations of quasidisks
    Autorzy:
    Gehring, Frederick
    Powiązania:
    https://bibliotekanauki.pl/articles/1341592.pdf
    Data publikacji:
    1999
    Wydawca:
    Polska Akademia Nauk. Instytut Matematyczny PAN
    Źródło:
    Banach Center Publications; 1999, 48, 1; 11-41
    0137-6934
    Pojawia się w:
    Banach Center Publications
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Conductivity and Magnetism of Magnetic Oxides
    Autorzy:
    Gehring, G. A.
    Powiązania:
    https://bibliotekanauki.pl/articles/2012877.pdf
    Data publikacji:
    2000-01
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    75.50.Pp
    75.50.Gg
    Opis:
    In a stoichiometric oxide the energy for the magnetic ordering is due to superexchange. This depends on the virtual transfer of a d electron from the transition ion to the neighbouring oxygen. When the oxide is p-doped there are compensating holes on the oxygen or the transition ion becomes mixed valent. The oxide may then conduct. The same transfer integral enters both the expression for the antiferromagnetic superexchange and the band width of the mobile carriers. Thus materials with a large antiferromagnetic exchange energy will be expected to have a relatively wide conduction band in the doped state and hence to have a high conductivity. In this paper the difference is explored between the materials in which there is true antiferromagnetism and those which are ferrimagnetic. In the antiferromagnets the carriers must destroy the magnetic order as they move. This behaviour is well known from the cuprates. In ferrimagnets the carriers may be able to move entirely on one sublattice. This occurs in Fe$\text{}_{3}$O$\text{}_{4}$ and probably in the doped garnets. In the case where motion is on one sublattice then doping does not destroy the magnetism and there is a relatively small magnetoresistance. An interesting feature is that unlike the cuprates the ferrimagnets do not become good metals at any doping but exhibit hopping conductivity. We explain the apparent paradox that the best conductivity is actually observed in materials where the conduction is only allowed when the antiferromagnetism has been quenched and that the conductivity in ferrimagnets is low.
    Źródło:
    Acta Physica Polonica A; 2000, 97, 1; 175-184
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    The Energy Spectrum of Single-Hole States in Transition Metal Oxides
    Autorzy:
    Lehmann-Szweykowska, A.
    Wojciechowski, R. J.
    Gehring, G. A.
    Powiązania:
    https://bibliotekanauki.pl/articles/2013222.pdf
    Data publikacji:
    2000-03
    Wydawca:
    Polska Akademia Nauk. Instytut Fizyki PAN
    Tematy:
    71.10.Fd
    72.20.-i
    Opis:
    The p-d hybridised single-hole states of the transition-metal-oxygen tetrahedron (TMO$\text{}_{4}$) are collectivised due to the direct p-p hopping between oxygens of different clusters. The lowest-lying energy band is always narrow and fully occupied. The first excited band gets occupied as an effect of valence-uncompensated doping, so it can be almost localised. The possible hole excitations to the two higher energy bands, which are wider, may imply the Mott-like hopping form of charge transport in these systems.
    Źródło:
    Acta Physica Polonica A; 2000, 97, 3; 563-566
    0587-4246
    1898-794X
    Pojawia się w:
    Acta Physica Polonica A
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
      Wyświetlanie 1-6 z 6

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