5,070 research outputs found
Thermodynamic properties of the itinerant-boson ferromagnet
Thermodynamics of a spin-1 Bose gas with ferromagnetic interactions are
investigated via the mean-field theory. It is apparently shown in the specific
heat curve that the system undergoes two phase transitions, the ferromagnetic
transition and the Bose-Einstein condensation, with the Curie point above the
condensation temperature. Above the Curie point, the susceptibility fits the
Curie-Weiss law perfectly. At a fixed temperature, the reciprocal
susceptibility is also in a good linear relationship with the ferromagnetic
interaction.Comment: 5 pages, 5 figure
Fermi Surface Reconstruction without Breakdown of Kondo Screening at Quantum Critical Point
Motivated by recent Hall-effect experiment in YbRhSi, we study ground
state properties of a Kondo lattice model in a two-dimensional square lattice
using variational Monte Carlo method. We show that there are two types of phase
transition, antiferromagnetic transition and topological one (Fermi surface
reconstruction). In a wide region of parameters, these two transitions occur
simultaneously without the breakdown of Kondo screening, accompanied by a
discontinuous change of the Hall coefficient. This result is consistent with
the experiment and gives a novel theoretical picture for the quantum critical
point in heavy fermion systems.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Superconductivity and Pseudogap in Quasi-Two-Dimensional Metals around the Antiferromagnetic Quantum Critical Point
Spin fluctuations (SF) and SF-mediated superconductivity (SC) in
quasi-two-dimensional metals around the antiferrromagnetic (AF) quantum
critical point (QCP) are investigated by using the self-consistent
renormalization theory for SF and the strong coupling theory for SC. We
introduce a parameter y0 as a measure for the distance from the AFQCP which is
approximately proportional to (x-xc), x being the electron (e) or hole (h)
doping concentration to the half-filled band and xc being the value at the
AFQCP. We present phase diagrams in the T-y0 plane including contour maps of
the AF correlation length and AF and SC transition temperatures TN and Tc,
respectively. The Tc curve is dome-shaped with a maximum at around the AFQCP.
The calculated one-electron spectral density shows a pseudogap in the
high-density-of-states region near (pi,0) below around a certain temperature T*
and gives a contour map at the Fermi energy reminiscent of the Fermi arc. These
results are discussed in comparison with e- and h-doped high-Tc cuprates.Comment: 5 pages, 3 figure
Schwinger Boson approach to the fully screened Kondo model
We apply the Schwinger boson scheme to the fully screened Kondo model and
generalize the method to include antiferromagnetic interactions between ions.
Our approach captures the Kondo crossover from local moment behavior to a Fermi
liquid with a non-trivial Wilson ratio. When applied to the two impurity model,
the mean-field theory describes the "Varma Jones" quantum phase transition
between a valence bond state and a heavy Fermi liquid.Comment: 4 pages, 4 figures. Changes to references and text in v
Ferromagnetic Quantum Critical Fluctuations and Anomalous Coexistence of Ferromagnetism and Superconductivity in UCoGe Revealed by Co-NMR and NQR Studies
Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)
studies were performed in the recently discovered UCoGe, in which the
ferromagnetic and superconducting (SC) transitions were reported to occur at
K and K (N. T. Huy {\it et al.}, Phys.
Rev. Lett. {\bf 99} (2007) 067006), in order to investigate the coexistence of
ferromagnetism and superconductivity as well as the normal-state and SC
properties from a microscopic point of view. From the nuclear spin-lattice
relaxation rate and Knight-shift measurements, we confirmed that
ferromagnetic fluctuations which possess a quantum critical character are
present above and the occurrence of ferromagnetic transition at
2.5 K in our polycrystalline sample. The magnetic fluctuations in the normal
state show that UCoGe is an itinerant ferromagnet similar to ZrZn and
YCo. The onset SC transition was identified at K, below
which of 30 % of the volume fraction starts to decrease due to the
opening of the SC gap. This component of , which follows a
dependence in the temperature range of K, coexists with the
magnetic components of showing a dependence below .
From the NQR measurements in the SC state, we suggest that the self-induced
vortex state is realized in UCoGe.Comment: 5 pages, 7 figures. submitted to J. Phys. Soc. Jpn. To appear in J.
Phys. Soc. Jp
Magneto-elastic quantum fluctuations and phase transitions in the iron superconductors
We examine the relevance of magneto-elastic coupling to describe the complex
magnetic and structural behaviour of the different classes of the iron
superconductors. We model the system as a two-dimensional metal whose magnetic
excitations interact with the distortions of the underlying square lattice.
Going beyond mean field we find that quantum fluctuation effects can explain
two unusual features of these materials that have attracted considerable
attention. First, why iron telluride orders magnetically at a non-nesting
wave-vector and not at the nesting wave-vector as
in the iron arsenides, even though the nominal band structures of both these
systems are similar. And second, why the magnetic transition in the
iron arsenides is often preceded by an orthorhombic structural transition.
These are robust properties of the model, independent of microscopic details,
and they emphasize the importance of the magneto-elastic interaction.Comment: 4 pages, 3 figures; minor change
Low-temperature electrical resistivity in paramagnetic spinel LiV2O4
The 3d electron spinel compound LiV2O4 exhibits heavy fermion behaviour below
30K which is related to antiferromagnetic spin fluctuations strongly enhanced
in an extended region of momentum space. This mechanism explains enhanced
thermodynamic quantities and nearly critical NMR relaxation in the framework of
the selfconsistent renormalization (SCR) theory. Here we show that the low-T
Fermi liquid behaviour of the resistivity and a deviation from this behavior
for higher T may also be understood within that context. We calculate the
temperature dependence of the electrical resistivity \rho(T) assuming that two
basic mechanisms of the quasiparticle scattering, resulting from impurities and
spin-fluctuations, operate simultaneously at low temperature. The calculation
is based on the variational principle in the form of a perturbative series
expansion for \rho(T). A peculiar behavior of \rho(T) in LiV2O4 is related to
properties of low-energy spin fluctuations whose T-dependence is obtained from
SCR theory.Comment: 10 pages, 3 figures, to appear in Phys. Rev.
Fermi surfaces in general co-dimension and a new controlled non-trivial fixed point
Traditionally Fermi surfaces for problems in spatial dimensions have
dimensionality , i.e., codimension along which energy varies.
Situations with arise when the gapless fermionic excitations live at
isolated nodal points or lines. For weak short range interactions are
irrelevant at the non-interacting fixed point. Increasing interaction strength
can lead to phase transitions out of this Fermi liquid. We illustrate this by
studying the transition to superconductivity in a controlled
expansion near . The resulting non-trivial fixed point is shown to
describe a scale invariant theory that lives in effective space-time dimension
. Remarkably, the results can be reproduced by the more familiar
Hertz-Millis action for the bosonic superconducting order parameter even though
it lives in different space-time dimensions.Comment: 4 page
Electronic Structure of Nearly Ferromagnetic compound HfZn
The electronic structure of HfZn has been studied based on the density
functional theory within the local-density approximation. The calculation
indicates that HfZn shows ferromagnetic instability. Large enhancement of
the static susceptibility over its non-interacting value is found due to a peak
in the density of states at the Fermi level
Quantum Valence Criticality as Origin of Unconventional Critical Phenomena
It is shown that unconventional critical phenomena commonly observed in
paramagnetic metals YbRh2Si2, YbRh2(Si0.95Ge0.05)2, and beta-YbAlB4 is
naturally explained by the quantum criticality of Yb-valence fluctuations. We
construct the mode coupling theory taking account of local correlation effects
of f electrons and find that unconventional criticality is caused by the
locality of the valence fluctuation mode. We show that measured low-temperature
anomalies such as divergence of uniform spin susceptibility \chi T^{-\zeta)
with giving rise to a huge enhancement of the Wilson ratio and the
emergence of T-linear resistivity are explained in a unified way.Comment: 5 pages, 3 figures, to be published in Physical Review Letter
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