Field-induced phase transitions of repulsive spin-1 bosons in optical lattices.

We study the phase diagram of repulsively interacting spin-1 bosons in optical lattices at unit filling, showing that an externally induced quadratic Zeeman effect may lead to a rich physics characterized by various phases and phase transitions. We find that the main properties of the system may be described by an effective field model, which provides the precise location of the phase boundaries for any dimension, in excellent agreement with our numerical calculations for one-dimensional (1D) systems. Thus, our work provides a quantitative guide for the experimental analysis of various types of field-induced quantum phase transitions in spin-1 lattice bosons. These transitions, which are precluded in spin-1/2 systems, may be realized by using an externally modified quadratic Zeeman coupling, similar to recent experiments with spinor condensates in the continuum.

Spin nematic and antinematic states in a spin-3/2 isotropic non-Heisenberg magnet.

The ground state phase diagram of a general isotropic spin-3/2 system with nearest-neighbor exchange is shown to contain unconventionally ordered spin nematic and antinematic states, as well as usual ferro- and antiferromagnetic phases. The two nematic phases have spontaneously broken rotational symmetry characterized by the long-range order of the nematic director u, as well as the broken time-reversal symmetry described by the pseudospin vector σ. Nematic phase differs from antinematic one by the type of ordering in σ vectors (uniform versus staggered). The ferromagnet-nematic and antiferromagnet-antinematic phase boundaries exhibit enhanced Sp(4) symmetry and correspond to the recently studied effective theory for spin-3/2 cold gases. We discuss optical properties and topological defects in the nematic phases.

Pairing of solitons in two-dimensional S=1 magnets.

We study a general model of isotropic two-dimensional spin-1 magnet, which is relevant for the physics of ultracold atoms with hyperfine S=1 spins in an optical lattice at odd filling. We demonstrate a novel mechanism of soliton pairing occurring in the vicinity of a special point with an enhanced SU(3) symmetry: upon perturbing the SU(3) symmetry, solitons with odd CP2 topological charge are confined into pairs that remain stable objects.

Circulating current states in bilayer fermionic and bosonic systems.

It is shown that fermionic polar molecules or atoms in a bilayer optical lattice can undergo the transition to a state with circulating currents, which spontaneously breaks the time reversal symmetry. Estimates of relevant temperature scales are given, and experimental signatures of the circulating current phase are identified. Related phenomena in bosonic and spin systems with ring exchange are discussed.

Electron spin resonance in sine-gordon spin chains in the perturbative spinon regime.

We report the low-temperature multifrequency ESR studies of copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g tensor and the Dzyaloshinskii-Moriya interaction, allowing us to test a new theoretical concept proposed recently by Oshikawa and Affleck [Phys. Rev. Lett. 82, 5136 (1999)]. Their theory, based on bosonization and self-energy formalism, can be applied for precise calculation of ESR parameters of S=1/2 antiferromagnetic chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained.

Field-induced disorder in a gapped spin system with nonmagnetic impurities.

We study the combined effect of doping and an external magnetic field on S = 1/2 dimers which are weakly coupled by three-dimensional antiferromagnetic interactions J'. We show that application of an external magnetic field H opposes the long-range Ne el order which is known to result from doping with nonmagnetic impurities and drives the system back to the disordered phase if 3D interactions J' are not too large. We discuss the zero temperature phase diagram in the (H,J') plane and suggest that the reentrant behavior can be experimentally observed.

Excitation hierarchy of the quantum sine-gordon spin chain in a 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 multiparticle excitation modes, are identified.

Zeeman levels with exotic field dependence in the high field phase of an S=1 Heisenberg antiferromagnetic chain.

We have performed electron spin resonance measurements over a wide frequency and magnetic field range on a single crystal of the S=1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C5H14N2)2N3(PF6). We observed gapped excitation branches above the critical field H(c) where the Haldane gap closes. These branches are analyzed by a phenomenological field theory using the complex-field phi(4) model. A satisfactory agreement between experiment and theory is obtained.