The behavior in an external magnetic field is studied exactly for a wide class of multichain quantum spin models. It is shown that the magnetic field together with the interchain couplings cause commensurate - incommensurate phase transitions between the gapless phases in the ground state. The conformal limit of these models is studied and it is shown that the low-lying excitations for the incommensurate phases are not independent, because they are governed by the same magnetic field (chemical potential for excitations). A scenario for the transition from one to two space dimensions for the exactly integrable multichain quantum spin models is proposed, and it is shown that the incommensurate phases in an external magnetic field disappear in the limit of an infinite number of coupled spin chains. The similarities in the external field behavior for the quantum multichain spin models and a wide class of quantum field theories are discussed. The scaling exponents for the appearence of the gap in the spectrum of a low-lying excitations of the quantum multichain models due to the relevant perturbations of the integrable theories are calculated.

Індекс рубрикатора НБУВ: В377.3

Рубрики:

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

In this work we consider several exactly solvable models of magnetic impurities in critical quantum antiferromagnetic spin chains and multichannel Kondo impurities. Their ground state properties are studied and the finite set of nonlinear integral equations, which exactly describe the thermodynamics of the models, is constructed. We obtain several analytic low-energy expressions for the temperature, magnetic field, and frequency dependences of important characteristics of exactly solvable disordered quantum spin models and disordered multichannel Kondo impurities with essential many-body interactions. We show that the only low-energy parameter that gets renormalized is the velocity of the low-lying excitations (or the effective crossover scale connected with each impurity); the others appear to be universal. In our study several kinds of strong disorder important for experiments were used. Some of them produce low divergences in certain characteristics of our strongly disordered critical systems (compared with finite values for the homogeneous case or a single impurity). For weak disorder, or for narrow distributions of the local Kondo temperatures, our exact results reveal the presence of Kondo screening of disordered ensembles of magnetic impurities by low-lying excitations of the host. We point out that our results qualitatively coincide with the data of experiments on real disordered quasi-one-dimensional antiferromagnetic systems and with the similar behavior of some heavy metallic alloys.

Індекс рубрикатора НБУВ: В377.1

Рубрики:

Електричні властивості твердих тіл

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

In this Letter we show that a strong disorder in the distribution of exchange couplings between magnetic impurities and hosts in quantum spin chains and non-Fermi-liquid rare earth and actinide compounds can be the reason for magnetic orderings in these systems at low temperatures.

Low-temperature magnetic susceptibility and specific heat of an antiferromagnetic Heisenberg chain with open boundary conditions are calculated with the help of exact Bethe ansatz method. These characteristics behave with temperature in a different way from the ones of a periodic chain.

Several rare-earth compounds, low-dimensional organic conductors, and spin chains exhibit low-temperature divergences of their magnetic susceptibility and specific heat (non-Fermi-liquid behavior). Such divergences are often related to disordered ensembles of magnetic impurities in those systems. In this work the distribution function of the effective characteristic of a single magnetic impurity, the Kondo temperature, is derived. We calculate how distributions of Kondo temperatures depend on the effective dimensionality of the problem and on the concentration of impurities.

Ключ. слова: Q1D quantum spin systems, magnetic impurities, ordering temperature for magnetic systems with impurities
Індекс рубрикатора НБУВ: В377.35

Рубрики:

Антиферомагнетизм твердих тіл

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

Ключ. слова: magnetic sublattice, mean-field approximation, re-entrant phase transitions
Індекс рубрикатора НБУВ: В377.35

Рубрики:

Антиферомагнетизм твердих тіл

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

The degenerate Hubbard-like chain with open boundary conditions is studied ith the help of the Bethe ansatz. The special case of the Bose - Hubbard-like chain is studied in detail. Boundary bound states, which appear as the consequence of the local potential(s), applied to the edge(s) of the open chain are studied in the ground state.

Last years several experiments were performed, which studied the Bose-Einstein condensation of quasi-particles, in particular, magnons in magnetically ordered systems. Recently the Bose-Einstein condensation of magnons was observed at room temperatures in a ferromagnetic film. The theory of the condensation (re-distribution) of magnons under the conditions of pumping, which explains many features of that experiment, is presented. The use of the term "Bose-Einstein condensation of magnons" is discussed.

Індекс рубрикатора НБУВ: В377.35

Рубрики:

Антиферомагнетизм твердих тіл

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

The one-dimensional spin-1/2 model, in which the alternation of the exchange interactions between neighboring spins is accompanied by the next-nearest neighbor (NNN) spin exchange (zig-zag spin ladder with the alternation) is studied. Thermodynamic characteristics of the considered quantum spin chain are obtained in the mean-field like approximation. Depending on the strength of the NNN interactions, the model manifests either the spin-gapped behavior of low-lying excitations at low magnetic fields, or the ferrimagnetic ordering in the ground state with gapless low-lying excitations. The system undergoes second order or first order quantum phase transitions, governed by the external magnetic field, NNN coupling strength, and the degree of the alternation. Hence, NNN spin - spin interactions in a dimerized quantum spin chain can produce a spontaneous magnetization. On the other hand, for quantum spin chains with a spontaneous magnetization, caused by NNN spin - spin couplings, the alternation of nearest-neighbor (NN) exchange interactions can be the reason for destroying of that magnetization and the onset of a spin gap for low-lying excitations. Alternating NN interactions produce a spin gap between two branches of low-energy excitations, and the NNN interactions yield asymmetry of dispersion laws of those excitations, with possible minima, corresponding to incommensurate structures in the spin chain.

The influence of the inhomogeneity on the macroscopic thermal pairwise entanglement for the system of coupled spins 1/2 (qubits) has been studied. The most important effect of the inhomogeneity on the thermal entanglement is in the new role of the external potential (magnetic field), which can produce nonzero entanglement for qubits, situated not far from the inhomogeneity.

Індекс рубрикатора НБУВ: В377.3 + В372.312

Рубрики:

Магнітні властивості твердих тіл. Магнетики

Вакансії і домішки

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

During recent years the interest to frustrated magnets has grown considerably. Such systems reveal very peculiar properties which distinguish them from standard paramagnets, magnetically ordered regular systems (like ferro-, ferri-, and antiferromagnets), or spin glasses. In particular great amount of attention has been devoted to the socalled spin ices, in which magnetic frustration together with the large value of the single-ion magnetic anisotropy of a special kind, yield peculiar behavior. One of the most exciting features of spin ices is related to low-energy emergent excitations, which from many viewpoints can be considered as analogies of Dirac's monopoles. In this article we review the main achievements of theory and experiment in this field of physics.

Індекс рубрикатора НБУВ: В368.313 + В368.33

Рубрики:

Магнітні властивості надпровідників

Кріокристали

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

The effect of spin-orbit interaction in the strongly correlated exactly solvable electron model with magnetic impurity is studied. The considered magnetic impurity reveals the property of a "mobile" one. It is shown that the asymptotics of correlation functions, calculated in the framework of the conformal field theory and finite size corrections of the Bethe ansatz exact solution, are strongly affected by both, the spin-orbit coupling, and by the magnetic impurity.

Spin-lattice effects play an important role in many magnetic materials. In this short review, we give some examples of such effects studied in low-dimensional, frustrated as well as uranium-based antiferromagnets. Utilizing ultrasound measurements at low temperatures and high magnetic fields provides valuable information on the spin-strain interactions. Specifically phase transformations and critical phenomena in magnetic systems with strong spin-lattice interactions are fruitful grounds for sound-velocity and sound-attenuation measurements.

Індекс рубрикатора НБУВ: В377.35

Рубрики:

Антиферомагнетизм твердих тіл

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ