A. A. Zvyagin

The open spin chain with impurity: an exact solution

Using a model of an open spin chain that is exactly soluble by means of a Bethe ansatz, we study the effects of a boundary magnetic field and an impurity spin coupled to the chain. An impurity spin only scatters forward, while the boundary is purely a back-scatterer. Two parameters for the impurity and one for the boundary permit us to mimic the effect of real magnetic impurity, with both forward and backward scattering.

On the origin of heavy quasiparticles in LiV2O4

An explanation is provided for the heavy quasiparticle excitations in LiV2O4. It differs considerably from that of other known heavy-fermion systems. Main ingredients of our theory are the cubic spinel structure of the material and strong short-range correlations of the d electrons. The large γ coefficient is shown to result from excitations of Heisenberg spin ½ rings and chains. The required coupling constant is calculated from LDA+U calculations and is found to be of the right size. Also the calculated Sommerfeld-Wilson ratio is reasonably close to the observed one.

Evolution of the Kondo state of YbRh2Si2 probed by high-field ESR.

An electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 for fields up to approximately 8 T reveals a strongly anisotropic signal in the Kondo state below approximately 25 K. A similarity between the T dependence of the ESR parameters and that of the specific heat and the 29Si nuclear magnetic resonance data gives evidence that the ESR response is given by heavy fermions. Tuning the Kondo effect on the 4f states with magnetic fields approximately 2-8 T and temperature 2-25 K yields a gradual change of the ESR g factor and linewidth which reflects the evolution of the Kondo state in this Kondo lattice system.

“Antichiral” exactly solvable effectively two-dimensional quantum spin model

Abstract We present a number of effectively 2D (or quasi-1D) exactly solvable quantum spin models with T and P symmetry breaking. The ground state energies have been found. For the antiferromagnetic case we show the “antichiral” spin character for the models under consideration. The behaviour of the systems in an external magnetic field and their excitations have been investigated.

Bethe ansatz solvable multi-chain quantum systems

In this article we review recent developments in the one-dimensional Bethe ansatz solvable multi-chain quantum models. The algebraic version of the Bethe ansatz (the quantum inverse scattering method) permits us to construct new families of integrable Hamiltonians using simple generalizations of the well known constructions of the single-chain model. First we consider the easiest example (‘basic’ model) of this class of models: the antiferromagnetic two-chain spin- 1 model with the nearest-neighbour and next-nearest-neighbour spin-frustrating interactions (zigzag chain). We show how the algebra of the quantum inverse scattering method works for this model, and what are the important features of the Hamiltonian (which reveal the topological properties of two dimensions together with the one-dimensional properties). We consider the solution of the Bethe ansatz for the ground state (in particular, commensurate–incommensurate quantum phase transitions present due to competing spin-frustrating interactions are discussed) and construct the thermal Bethe ansatz (in the form of the ‘quantum transfer matrix’) for this model. Then possible generalizations of the basic model are considered: an inclusion of a magnetic anisotropy, higher-spin representations (including the important case of a quantum ferrimagnet), the multi-chain case, internal degrees of freedom of particles at each site, etc. We observe the similarities and differences between this class of models and related exactly solvable models: other groups of multi-chain lattice models, quantum field theory models and magnetic impurity (Kondo-like) models. Finally, the behaviour of non-integrable (less constrained) multi-chain quantum models is discussed.

Exact solution for an exchange impurity in a one-dimensional correlated host

The one-dimensional t - J model at the supersymmetric point, J = 2t, with a magnetic impurity of arbitrary spin S is studied exactly using the Bethe ansatz technique. The impurity interacts by spin exchange with the electrons on neighbouring sites of a lattice without destroying the integrability. The discrete Bethe ansatz equations diagonalizing the model, and the thermodynamic equations are derived. The impurity free energy is obtained for arbitrary band filling as a function of temperature and external magnetic field. The impurity can localize up to one itinerant hole, and has in general mixed-valent properties.

Low-temperature behavior of the magnetoelastic characteristics of praseodymium ferroborate

The behavior of the elastic moduli and sound absorption in a PrFe3(BO3)4 single crystal at low temperatures is studied. A transition of the magnetic subsystem into an antiferromagnetically ordered state is manifested in the temperature behavior of the sound velocities and absorption. The characteristic behavior of the elastic properties of PrFe3(BO3)4 in an external magnetic field is observed. A phenomenological theory that gives a qualitative description of the observed features is constructed. It is proposed that a weak magnetic moment exists in the crystal.

New physics in frustrated magnets: Spin ices, monopoles, etc. (Review Article)

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 so-called 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 Diracs mono-poles. In this article we review the main achievements of theory and experiment in this field of physics.

Magnetic phase transitions in the NdFe3(BO3)4 multiferroic

Low-temperature studies of the behavior of the sound velocity and attenuation of acoustic modes have been performed on a single crystal NdFe3(BO3)4 Transitions of the magnetic subsystem to the antiferromagnetically ordered state at TN ≈ 30.6 K have been revealed in the temperature behavior of the elastic characteristics. The features in the temperature behavior of elastic characteristics of the neodymium ferroborate and its behavior in the external magnetic field, applied in the basic plane of the crystal, permit us to suppose that the transition to an incommensurate spiral phase is realized in the system. This phase transition behaves as the first order one. H–T phase diagrams for the cases H ∥ a and H ∥ b have been constructed. The phenomenological theory, which explains observed features, has been developed.

Low-temperature phase transitions in the rare-earth ferroborate Nd0.75Dy0.25Fe3(BO3)4

The low-temperature behavior of the elastic characteristics of a Nd0.75Dy0.25Fe3(BO3)4 single crystal has been studied. Features are found in the temperature and magnetic-field dependences of the velocity and absorption of transverse sound. These features are interpreted as being a manifestation of magnetic phase transitions in the compound studied. The H‐T phase diagram is constructed.