460 research outputs found
Ward identities for the Anderson impurity model: derivation via functional methods and the exact renormalization group
Using functional methods and the exact renormalization group we derive Ward
identities for the Anderson impurity model. In particular, we present a
non-perturbative proof of the Yamada-Yosida identities relating certain
coefficients in the low-energy expansion of the self-energy to thermodynamic
particle number and spin susceptibilities of the impurity. Our proof underlines
the relation of the Yamada-Yosida identities to the U(1) x U(1) symmetry
associated with particle number and spin conservation in a magnetic field.Comment: 8 pages, corrected statements about infintite flatband limi
Classical Phase Fluctuations in Incommensurate Peierls Chains
In the pseudogap regime of one-dimensional incommensurate Peierls systems,
fluctuations of the phase of the order parameter prohibit the emergence of
long-range order and generate a finite correlation length. For classical phase
fluctuations, we present exact results for the average electronic density of
states, the mean localization length, the electronic specific heat and the spin
susceptibility at low temperatures. Our results for the susceptibility give a
good fit to experimental data.Comment: 4 Revtex pages, 4 figures, submitted to Phys. Rev. Let
Absence of fermionic quasi-particles in the superfluid state of the attractive Fermi gas
We calculate the effect of order parameter fluctuations on the fermionic
single-particle excitations in the superfluid state of neutral fermions
interacting with short range attractive forces. We show that in dimensions D
\leq 3 the singular effective interaction between the fermions mediated by the
gapless Bogoliubov-Anderson mode prohibits the existence of well-defined
quasi-particles. We explicitly calculate the single-particle spectral function
in the BEC regime in D=3 and show that in this case the quasi-particle residue
and the density of states are logarithmically suppressed.Comment: 4 RevTex pages, 3 figures; title changed, new Figure 1, added
references. We argue that in the entire regime of the BCS-BEC crossover the
quasi-particle picture breaks down in D <=3 for neutral fermions (but NOT for
charged fermions
Renormalization of the BCS-BEC crossover by order parameter fluctuations
We use the functional renormalization group approach with partial
bosonization in the particle-particle channel to study the effect of order
parameter fluctuations on the BCS-BEC crossover of superfluid fermions in three
dimensions. Our approach is based on a new truncation of the vertex expansion
where the renormalization group flow of bosonic two-point functions is closed
by means of Dyson-Schwinger equations and the superfluid order parameter is
related to the single particle gap via a Ward identity. We explicitly calculate
the chemical potential, the single-particle gap, and the superfluid order
parameter at the unitary point and compare our results with experiments and
previous calculations.Comment: 5 pages, 3 figure
Spectral function of the Anderson impurity model at finite temperatures
Using the functional renormalization group (FRG) and the numerical
renormalization group (NRG), we calculate the spectral function of the Anderson
impurity model at zero and finite temperatures. In our FRG scheme spin
fluctuations are treated non-perturbatively via a suitable Hubbard-Stratonovich
field, but vertex corrections are neglected. A comparison with our highly
accurate NRG results shows that this FRG scheme gives a quantitatively good
description of the spectral line-shape at zero and finite temperatures both in
the weak and strong coupling regimes, although at zero temperature the FRG is
not able to reproduce the known exponential narrowing of the Kondo resonance at
strong coupling.Comment: 6 pages, 3 figures; new references adde
Ferromagnetic Luttinger Liquids
We study weak itinerant ferromagnetism in one-dimensional Fermi systems using
perturbation theory and bosonization. We find that longitudinal spin
fluctuations propagate ballistically with velocity v_m << v_F, where v_F is the
Fermi velocity. This leads to a large anomalous dimension in the spin-channel
and strong algebraic singularities in the single-particle spectral function and
in the transverse structure factor for momentum transfers q ~ 2 Delta/v_F,
where 2 Delta is the exchange splitting.Comment: 4 pages, 3 figure
Exact Numerical Calculation of the Density of States of the Fluctuating Gap Model
We develop a powerful numerical algorithm for calculating the density of
states rho(omega) of the fluctuating gap model, which describes the low-energy
physics of disordered Peierls and spin-Peierls chains. We obtain rho(omega)
with unprecedented accuracy from the solution of a simple initial value problem
for a single Riccati equation. Generating Gaussian disorder with large
correlation length xi by means of a simple Markov process, we present a
quantitative study of the behavior of rho (omega) in the pseudogap regime. In
particular, we show that in the commensurate case and in the absence of forward
scattering the pseudogap is overshadowed by a Dyson singularity below a certain
energy scale omega^{ast}, which we explicitly calculate as a function of xi.Comment: 4 revtex pages, 3 figure
Porcine endogenous retroviruses PERV A and A/C recombinant are insensitive to a range of divergent mammalian TRIM5Â proteins including human TRIM5
The potential risk of cross-species transmission of porcine endogenous retroviruses (PERV) to humans has slowed the development of xenotransplantation, using pigs as organ donors. Here, we show that PERVs are insensitive to restriction by divergent TRIM5{alpha} molecules despite the fact that they strongly restrict a variety of divergent lentiviruses. We also show that the human PERV A/C recombinant clone 14/220 reverse transcribes with increased efficiency in human cells, leading to significantly higher infectivity. We conclude that xenotransplantation studies should consider the danger of highly infectious TRIM5{alpha}-insensitive human-tropic PERV recombinants
Algorithm for obtaining the gradient expansion of the local density of states and the free energy of a superconductor
We present an efficient algorithm for obtaining the gauge-invariant gradient
expansion of the local density of states and the free energy of a clean
superconductor. Our method is based on a new mapping of the semiclassical
linearized Gorkov equations onto a pseudo-Schroedinger equation for a
three-component wave-function psi(x), where one component is directly related
to the local density of states. Because psi(x) satisfies a linear equation of
motion, successive terms in the gradient expansion can be obtained by simple
linear iteration. Our method works equally well for real and complex order
parameter, and in the presence of arbitrary external fields. We confirm a
recent calculation of the fourth order correction to the free energy by
Kosztin, Kos, Stone and Leggett [Phys. Rev. B 58, 9365 (1998)], who obtained a
discrepancy with an earlier result by Tewordt [Z. Phys. 180, 385 (1964)]. We
also give the fourth order correction to the local density of states, which has
not been published before.Comment: 12 preprint pages, added remark concerning Eilenberger equation,
accepted for publication in Phys. Rev.
Exact Results for the Crossover from Gaussian to Non-Gaussian Order Parameter Fluctuations in Quasi One-Dimensional Electronic Systems
The physics of quasi one-dimensional Peierls systems is dominated by order
parameter fluctuations. We present an algorithm which allows for the first time
to exactly calculate physical properties of the electrons gas coupled to
classical order parameter fluctuations. The whole range from the Gaussian
regime dominated by amplitude fluctuations to the non-Gaussian regime dominated
by phase fluctuations is accessible. Our results provide insight into the
'pseudogap' phenomenon occurring in underdoped high-temperature
superconductors, quasi one-dimensional organic conductors and liquid metals.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
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