952 research outputs found
Metamagnetism and Lifshitz Transitions in Models for Heavy Fermions
We investigate metamagnetic transitions in models for heavy fermions by
considering the doped Kondo lattice model in two dimensions. Results are
obtained within the framework of dynamical mean field and dynamical cluster
approximations. Universal magnetization curves for different temperatures and
Kondo couplings develop upon scaling with the lattice coherence temperature.
Furthermore, the coupling of the local moments to the magnetic field is varied
to take into account the different Land\'e factors of localized and itinerant
electrons. The competition between the lattice coherence scale and the Zeeman
energy scale allows for two interpretations of the metamagnetism in heavy
fermions: Kondo breakdown or Lifshitz transitions. By tracking the
single-particle residue through the transition, we can uniquely conclude in
favor of the Lifshitz transition scenario. In this scenario, a quasiparticle
band drops below the Fermi energy which leads to a change in topology of the
Fermi surface.Comment: 8 pages, 7 figure
Interactions and magnetic moments near vacancies and resonant impurities in graphene
The effect of electronic interactions in graphene with vacancies or resonant
scatterers is investigated. We apply dynamical mean-field theory in combination
with quantum Monte Carlo simulations, which allow us to treat
non-perturbatively quantum fluctuations beyond Hartree-Fock approximations. The
interactions narrow the width of the resonance and induce a Curie magnetic
susceptibility, signaling the formation of local moments. The absence of
saturation of the susceptibility at low temperatures suggests that the coupling
between the local moment and the conduction electrons is ferromagnetic
Entanglement at the boundary of spin chains near a quantum critical point and in systems with boundary critical points
We analyze the entanglement properties of spins (qubits) attached to the
boundary of spin chains near quantum critical points, or to dissipative
environments, near a boundary critical point, such as Kondo-like systems or the
dissipative two level system. In the first case, we show that the properties of
the entanglement are significantly different from those for bulk spins. The
influence of the proximity to a transition is less marked at the boundary. In
the second case, our results indicate that the entanglement changes abruptly at
the point where coherent quantum oscillations cease to exist. The phase
transition modifies significantly less the entanglement.Comment: 5 pages, 4 figure
Dynamics of Magnetic Defects in Heavy Fermion LiV2O4 from Stretched Exponential 7Li NMR Relaxation
7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small
concentration of magnetic defects within the structure drastically changes the
7Li nuclear magnetization relaxation versus time from a pure exponential as in
pure LiV2O4 to a stretched exponential, indicating glassy behavior of the
magnetic defects. The stretched exponential function is described as arising
from a distribution of 7Li nuclear spin-lattice relaxation rates and we present
a model for the distribution in terms of the dynamics of the magnetic defects.
Our results explain the origin of recent puzzling 7Li NMR literature data on
LiV2O4 and our model is likely applicable to other glassy systems.Comment: Four typeset pages including four figure
Generic susceptibilities of the half-filled Hubbard model in infinite dimensions
Around a metal-to-insulator transition driven by repulsive interaction (Mott
transition) the single particle excitations and the collective excitations are
equally important. Here we present results for the generic susceptibilities at
zero temperature in the half-filled Hubbard model in infinite dimensions.
Profiting from the high resolution of dynamic density-matrix renormalization at
all energies, results for the charge, spin and Cooper-pair susceptibilities in
the metallic and the insulating phase are computed. In the insulating phase, an
almost saturated local magnetic moment appears. In the metallic phase a
pronounced low-energy peak is found in the spin response.Comment: 12 pages, 12 figures; slight changes and one additional figure due to
referees' suggestion
Scaling and Decoherence in the Out-of-Equilibrium Kondo Model
We study the Kondo effect in quantum dots in an out-of-equilibrium state due
to an applied dc-voltage bias. Using the method of infinitesimal unitary
transformations (flow equations), we develop a perturbative scaling picture
that naturally contains both equilibrium coherent and non-equilibrium
decoherence effects. This framework allows one to study the competition between
Kondo effect and current-induced decoherence, and it establishes a large regime
dominated by single-channel Kondo physics for asymmetrically coupled quantum
dots.Comment: 4 pages, 3 figures; v2: minor changes (typos corrected, esp. in Eqs.
(3), (4), references updated, improved layout for figures
Optical study of archetypical valence-fluctuating Eu-systems
We have investigated the optical conductivity of the prominent valence
fluctuating compounds EuIr2Si2 and EuNi2P2 in the infrared energy range to get
new insights into the electronic properties of valence fluctuating systems. For
both compounds we observe upon cooling the formation of a renormalized Drude
response, a partial suppression of the optical conductivity below 100 meV and
the appearance of a mid-infrared peak at 0.15 eV for EuIr2Si2 and at 0.13 eV
for EuNi2P2. Most remarkably, our results show a strong similarity with the
optical spectra reported for many Ce- or Yb-based heavy fermion metals and
intermediate valence systems, although the phase diagrams and the temperature
dependence of the valence differ strongly between Eu- and Ce-/Yb-systems. This
suggests that the hybridization between 4f- and conduction electrons, which is
responsible for the properties of Ce- and Yb-systems, plays an important role
in valence fluctuating Eu-systems
Assisted hopping and interaction effects in impurity models
We study, using Numerical Renormalization Group methods, the generalization
of the Anderson impurity model where the hopping depends on the filling of the
impurity. We show that the model, for sufficiently large values of the assisted
hopping term, shows a regime where local pairing correlations are enhanced.
These correlations involve pairs fluctuating between on site and nearest
neighbor positions
Role of the van Hove Singularity in the Quantum Criticality of the Hubbard Model
A quantum critical point (QCP), separating the non-Fermi liquid region from
the Fermi liquid, exists in the phase diagram of the 2D Hubbard model
[Vidhyadhiraja et. al, Phys. Rev. Lett. 102, 206407 (2009)]. Due to the
vanishing of the critical temperature associated with a phase separation
transition, the QCP is characterized by a vanishing quasiparticle weight. Near
the QCP, the pairing is enhanced since the real part of the bare d-wave p-p
susceptibility exhibits algebraic divergence with decreasing temperature,
replacing the logarithmic divergence found in a Fermi liquid [Yang et. al,
Phys. Rev. Lett. 106, 047004 (2011)]. In this paper we explore the
single-particle and transport properties near the QCP. We focus mainly on a van
Hove singularity (vHS) coming from the relatively flat dispersion that crosses
the Fermi level near the quantum critical filling. The flat part of the
dispersion orthogonal to the antinodal direction remains pinned near the Fermi
level for a range of doping that increases when we include a negative
next-near-neighbor hopping t' in the model. For comparison, we calculate the
bare d-wave pairing susceptibility for non-interacting models with the usual
two-dimensional tight binding dispersion and a hypothetical quartic dispersion.
We find that neither model yields a vHS that completely describes the critical
algebraic behavior of the bare d-wave pairing susceptibility. The resistivity,
thermal conductivity, thermopower, and the Wiedemann-Franz Law are examined in
the Fermi liquid, marginal Fermi liquid, and pseudo-gap doping regions. A
negative next-near-neighbor hopping t' increases the doping region with
marginal Fermi liquid character. Both T and negative t' are relevant variables
for the QCP, and both the transport and the motion of the vHS with filling
suggest that they are qualitatively similar in their effect.Comment: 15 pages, 17 figure
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