Comment on ``Universal Spin-Flip Transition in Itinerant Antiferromagnets"

In a recent paper [Phys. Rev. Lett. 91, 117201 (2003)], it is argued that an itinerant antiferromagnet in an external magnetic field undergoes a spin-flip transition, in marked contrast with the behavior of a localized antiferromagnet: for a weak magnetic field, the magnetization is parallel to the field, and flips to the perpendicular configuration at a critical value of the field. In this Comment we show that these conclusions are incorrect. The canted state (not considered in the Letter) is the antiferromagnetic ground state of the system up to a critical value of the field where the normal state is restored, in qualitative agreement with the behavior of the magnetization in a localized antiferromagnet.Comment: 1 page, 2 figure

Mean-field theory of a quasi-one-dimensional superconductor in a high magnetic field

At high magnetic field, the semiclassical approximation which underlies the Ginzburg-Landau (GL) theory of the mixed state of type II superconductors breaks down. In a quasi-1D superconductor (weakly coupled chains system) with an {\it open Fermi surface}, a high magnetic field stabilizes a cascade of superconducting phases which ends in a strong reentrance of the superconducting phase. The superconducting state evolves from a triangular Abrikosov vortex lattice in the semiclassical regime towards a Josephson vortex lattice in the reentrant phase. We study the properties of these superconducting phases from a microscopic model in the mean-field approximation. The critical temperature is calculated in the quantum limit approximation (QLA) where only Cooper logarithmic singularities are retained while less divergent terms are ignored. The effects of Pauli pair breaking (PPB) and impurity scattering are taken into account. The Gor'kov equations are solved in the same approximation but ignoring the PPB effect. We derive the GL expansion of the free energy. We obtain the specific heat jump at the transition, the sign of the magnetization and the quasi-particle excitation spectrum. The calculation is extended beyond the QLA taking into account all the pairing channels and the validity of the QLA is discussed in detail.Comment: 35 pages, RevTex, 18 figures available upon reques

Quantum XY criticality in a two-dimensional Bose gas near the Mott transition

We derive the equation of state of a two-dimensional Bose gas in an optical lattice in the framework of the Bose-Hubbard model. We focus on the vicinity of the multicritical points where the quantum phase transition between the Mott insulator and the superfluid phase occurs at fixed density and belongs to the three-dimensional XY model universality class. Using a nonperturbative renormalization-group approach, we compute the pressure $P(\mu,T)$ as a function of chemical potential and temperature. Our results compare favorably with a calculation based on the quantum O(2) model -- we find the same universal scaling function -- and allow us to determine the region of the phase diagram in the vicinity of a quantum multicritical point where the equation of state is universal. We also discuss the possible experimental observation of quantum XY criticality in a ultracold gas in an optical lattice.Comment: v1) 6 pages, 4 figures. v2) Revised versio

Field-induced spin-density-wave phases in TMTSF organic conductors: quantization versus non-quantization

We study the magnetic-field-induced spin-density-wave (FISDW) phases in TMTSF organic conductors in the framework of the quantized nesting model. In agreement with recent suggestions, we find that the SDW wave-vector ${\bf Q}$ deviates from its quantized value near the transition temperature $T_c$ for all phases with quantum numbers $N>0$. Deviations from quantization are more pronounced at low pressure and higher $N$ and may lead to a suppression of the first-order transitions $N+1\to N$ for $N\ge 5$. Below a critical pressure, we find that the N=0 phase invades the entire phase diagram in accordance with earlier experiments. We also show that at T=0, the quantization of ${\bf Q}$ and hence the Hall conductance is always exact. Our results suggest a novel phase transition/crossover at intermediate temperatures between phases with quantized and non-quantized ${\bf Q}$.Comment: 4 pages, 4 figures, Revte