Magnetic phase diagram of a quasi-one-dimensional quantum spin system

We propose an analytical ansatz, using which the ordering temperature of a quasi-one-dimensional (quasi-1D) antiferromagnetic (AF) system (weakly coupled quantum spin-1/2 chains) in the presence of the external magnetic field is calculated. The field dependence of the critical exponents for correlation functions of 1D subsystems plays a very important role. It determines the region of possible re-entrant phase transition, governed by the field. It is shown how the quantum critical point between two phases of the 1D subsystem, caused by spin-frustrating next-nearest neighbor (NNN) and multi-spin ring-like exchanges, affects the field dependence of the ordering temperature. Our results qualitatively agree with the features, observed in experiments on quasi-1D AF systems

Possibility of direct observation of edge Majorana modes in quantum chains

Several scenarios for realization of edge Majorana modes in quantum chain systems: spin chains, chains of Josephson junctions, and chains of coupled cavities in quantum optics, are considered. For all these systems excitations can be presented as superpositions of a spinless fermion and a hole, characteristic for Majorana fermion. We discuss the features of our exact solution with respect to possible experiments, in which edge Majorana fermions can be directly observed when studying magnetic, superconducting, and optical characteristics of such systems.Comment: 8 pages (including supplemental information), 7 figure

New physics in frustrated magnets: Spin ices, monopoles, etc

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 Dirac's monopoles. In this article we review the main achievements of theory and experiment in this field of physics.Comment: review article, 70 pages, 27 figures, accepted for publication in "Low Temperature Physics" ("Fizika Nizkikh Temperatur"

Spin-orbit and impurity scattering in an integrable electron model: Exact results for dynamic correlations

We introduce an integrable model of spin-polarized interacting electrons subject to a spin-conserving spin-orbit interaction. Using Bethe Ansatz and conformal field theory we calculate the exact large-time single-electron and density correlations and find that while the spin-orbit interaction enhances the single-electron Green's function, the density correlations get suppressed. Adding a localized impurity and coupling it to the electrons so that integrability is preserved, the dynamic correlations are found to change significantly after a quantum quench with the impurity interaction switched on suddenly. When the electrons are confined to a periodic structure, the correlations are indifferent to the location of the impurity and only carry an imprint of its intrinsic properties. We conjecture that this unusual feature originates from the integrability of the model.Comment: 8 pages, 1 figur

Excitation hierarchy of the quantum sine-Gordon spin chain in strong magnetic field

The magnetic excitation spectrum of copper pyrimidine dinitrate, a material containing S=1/2 antiferromagnetic chains with alternating g-tensor and the Dzyaloshinskii-Moriya interaction, and exhibiting a field-induced spin gap, is probed using submillimeter wave electron spin resonance spectroscopy. Ten excitation modes are resolved in the low-temperature spectrum, and their frequency-field diagram is systematically studied in magnetic fields up to 25 T. The experimental data are sufficiently detailed to make a very accurate comparison with predictions based on the quantum sine-Gordon field theory. Signatures of three breather branches and a soliton, as well as those of several multi-particle excitation modes are identified.Comment: 4 RevTeX pages, 3 figure

Addendum to ``Multichannel Kondo screening in a one-dimensional correlated electron system''

This is an addendum to our previous work cond-mat/9705048 (published in Europhysics Letters 41, 213 (1998)), clarifying the construction of the two-particle scattering matrices used for studying the magnetic impurity behavior in a multichannel correlated host.Comment: Addendum to cond-mat/9705048 (Europhys. Lett. 41, 213 (1998)

Magnetic excitations in the spin-1 anisotropic antiferromagnet NiCl2−4SC(NH2)2NiCl_2-4SC(NH_2)_2

The spin-1 anisotropic antiferromagnet NiCl_2-4SC(NH2)_2 exhibits a field-induced quantum phase transition that is formally analogous to Bose-Einstein condensation. Here we present results of systematic high-field electron spin resonance (ESR) experimental and theoretical studies of this compound with a special emphasis on single-ion two-magnon bound states. In order to clarify some remaining discrepancies between theory and experiment, the frequency-field dependence of magnetic excitations in this material is reanalyzed. In particular, a more comprehensive interpretation of the experimental signature of single-ion two-magnon bound states is shown to be fully consistent with theoretical results. We also clarify the structure of the ESR spectrum in the so-called intermediate phase.Comment: 9 pages, 10 figure