246 research outputs found
The anisotropic Heisenberg model in a transverse magnetic field
One dimensional spin-1/2 model in a transverse magnetic field is
studied. It is shown that the field induces the gap in the spectrum of the
model with easy-plain anisotropy. Using conformal invariance the field
dependence of the gap at small fields is found. The ground state phase diagram
is obtained. It contains four phases with different types of the long range
order (LRO) and a disordered one. These phases are separated by critical lines,
where the gap and the long range order vanish. Using scaling estimations and a
mean-field approach as well as numerical calculations in the vicinity of all
critical lines we found the critical exponents of the gap and the LRO. It is
shown that transition line between the ordered and disordered phases belongs to
the universality class of the transverse Ising model.Comment: 10 pages, 3 figures, LaTe
Influence of spin structures and nesting on Fermi surface and a pseudogap anisotropy in t-t'-U Hubbard model
Influence of two type of spin structures on the form of the Fermi surface
(FS) and a photoemission intensity map is studied for t-t'-U Hubbard model.
Mean field calculations are done for the stripe phase and for the spiral spin
structure. It is shown, that unlike a case of electron doping, the hole-doped
models are unstable with respect to formation of such structures. The pseudogap
anisotropies are different for h- and e- doping. In accordance with ARPES data
for La2SrxCuO4 the stripe phase is characterized by quasi-one-dimensional
segments of FS at k=(\pi,0) and by suppression of spectral weight in diagonal
direction. It is shown that spiral structures display the polarisation
anisotropy: different segments of FS correspond to electros with different spin
polarisations.Comment: 12 pages, 4 figure
Low-temperature thermodynamics of the classical frustrated ferromagnetic chain in magnetic field
Low-temperature magnetization curves of the classical frustrated
ferromagnetic chain in the external magnetic field near the transition point
between the ferromagnetic and the helical phases is studied. It is shown that
the calculation of the partition function in the scaling limit reduces to the
solution of the Schr\"{o}dinger equation of the special form for the quantum
particle. It is proposed that the magnetization of the classical model in the
ferromagnetic part of the phase diagram including the transition point defines
the universal scaling function which is valid for quantum model as well.
Explicit analytical formulae for the magnetization are given in the limiting
cases of low and high magnetic fields. The influence of the easy-axis
anisotropy on the magnetic properties of the model is studied. It is shown that
even small anisotropy essentially changes the behavior of the susceptibility in
the vicinity of the transition point.Comment: 14 pages, 5 figure
Multi-magnon bound states in easy-axis ferromagnetic zigzag spin chain
The frustrated spin-1/2 chain with weakly anisotropic ferromagnetic
nearest-neighbor and antiferromagnetic next-nearest-neighbor exchanges is
studied. We focus on the excitation spectrum and the low-temperature
thermodynamics in the ferromagnetic region of the ground state phase diagram.
It is shown that the excitation spectrum of the model is characterized by the
existence of the multi-magnon bound states. These excitations determine the
low-temperature magnetic susceptibility. The energy of the bound magnon
complexes is found using the scaling estimates of the perturbation theory and
numerical calculations. The relation of the considered model to the
edge-sharing cuprate is discussed.Comment: 18 pages, 6 figure
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