6,138 research outputs found
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
deviates from its quantized value near the transition temperature for all
phases with quantum numbers . Deviations from quantization are more
pronounced at low pressure and higher and may lead to a suppression of the
first-order transitions for . 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
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 .Comment: 4 pages, 4 figures, Revte
Effective action and collective modes in quasi-one-dimensional spin-density-wave systems
We derive the effective action describing the long-wavelength low-energy
collective modes of quasi-one-dimensional spin-density-wave (SDW) systems,
starting from the Hubbard model within weak coupling approximation. The
effective action for the spin-wave mode corresponds to an anisotropic
non-linear sigma model together with a Berry phase term. We compute the spin
stiffness and the spin-wave velocity. We also obtain the effective action for
the sliding mode (phason) taking into account the density fluctuations from the
outset and in presence of a weak external electromagnetic field. This leads to
coupled equations for the phase of the SDW condensate and the charge density
fluctuations. We also calculate the conductivity and the density-density
correlation function.Comment: 16 pages, Resubmitted to Physical Review B with minor suggested
change
An anomalous magnetic phase transition at 10 K in Nd7Rh3
The compound, Nd7Rh3, crystallizing in Th7Fe3-type hexagonal structure, has
been shown recently by us to exhibit a signature of magnetic phase-coexistence
phenomenon below 10 K after a field cycling, uncharacteristic of stoichiometric
intermetallic compounds, bearing a relevance to the trends in the field of
electronic phase-separation. In order to characterize this compound further, we
have carried out dc magnetic susceptibility (chi), electrical resistivity,
magnetoresistance and heat-capacity measurements as a function temperature (T=
1.8 to 300 K). The results reveal that this compound exhibits another unusual
finding at the 10K-transition in the sense that the plot of chi(T) shows a
sharp increase in the field-cooled cycle, whereas the zero-field-cooled curve
shows a downturn below the transition. In addition, the sign of
magnetoresistance is negative and the magnitude is large over a wide
temperature range in the vicinity of magnetic ordering temperature, with a
sharp variation at 10 K. The results indicate that the transition below 10 K is
first-order in its character.Comment: Appeared in JPCM (Letters) 18 (2006) L40
Mott insulator to superfluid transition of ultracold bosons in an optical lattice near a Feshbach resonance
We study the phase diagram of ultracold bosons in an optical lattice near a
Feshbach resonance. Depending on the boson density, the strength of the optical
lattice potential and the detuning from resonance, the ground state can be a
Mott insulator, a superfluid phase with both an atomic and a molecular
condensate, or a superfluid phase with only a molecular condensate. Mott
insulator to superfluid transitions can be induced either by decreasing the
strength of the optical lattice potential or by varying the detuning from the
Feshbach resonance. Quite generally, we find that for a commensurate density
the ground-state may undergo several insulator-superfluid or
superfluid-insulator transitions as the magnetic field is varied through the
resonance.Comment: 4 pages, 5 figures, RevTex 4; (v2) revised version, to appear in
Europhys. Let
Mott insulator to superfluid transition in the Bose-Hubbard model: a strong-coupling approach
We present a strong-coupling expansion of the Bose-Hubbard model which
describes both the superfluid and the Mott phases of ultracold bosonic atoms in
an optical lattice. By performing two successive Hubbard-Stratonovich
transformations of the intersite hopping term, we derive an effective action
which provides a suitable starting point to study the strong-coupling limit of
the Bose-Hubbard model. This action can be analyzed by taking into account
Gaussian fluctuations about the mean-field approximation as in the Bogoliubov
theory of the weakly interacting Bose gas. In the Mott phase, we reproduce
results of previous mean-field theories and also calculate the momentum
distribution function. In the superfluid phase, we find a gapless spectrum and
compare our results with the Bogoliubov theory.Comment: 8 pages, 6 figures; (v2) Two references adde
Bose-Fermi mixtures in an optical lattice
We study an atomic Bose-Fermi mixture with unpolarized fermions in an optical lattice. We obtain the Mott ground states of such a system in the limit of the deep optical lattice and discuss the effect of quantum fluctuations on these states. We also study the superfluid-insulator transitions of bosons and metal-insulator transition of fermions in such a mixture within a slave-rotor mean-field approximation, and obtain the corresponding phase diagram. We discuss experimental implications of our results
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