54 research outputs found
Large-N limit of a magnetic impurity in unconventional density waves
We investigate the effect of unconventional density wave (UDW) condensate on
an Anderson impurity using large-N technique at T=0. In accordance with
previous treatments of a Kondo impurity in pseudogap phases, we find that Kondo
effect occurs only in a certain range of parameters. The f-electron density of
states reflects the influence of UDW at low energies and around the maximum of
the density wave gap. The static spin susceptibility diverges at the critical
coupling, indicating the transition from strong to weak coupling. In the
dynamic spin susceptibility an additional peak appears showing the presence the
UDW gap. Predictions concerning non-linear density of states are made. Our
results apply to other unconventional condensates such as d-wave
superconductors and d-density waves as well.Comment: 9 pages, 7 figure
Ground State Properties of Anderson Impurity in a Gapless Host
Using the Bethe ansatz method, we study the ground state properties of a
Anderson impurity in a ``gapless'' host, where a density of band
states vanishes at the Fermi level as . As
in metals, the impurity spin is proven to be screened at arbitrary parameters
of the system. However, the impurity occupancy as a function of the bare
impurity energy is shown to acquire novel qualitative features which
demonstrate a nonuniversal behavior of the system. The latter explains why the
Kondo screening is absent (or exists only at quite a large electron-impurity
coupling) in earlier studies based on scaling arguments.Comment: 5 pages, no figure, RevTe
Critical local-moment fluctuations, anomalous exponents, and omega/T scaling in the Kondo problem with a pseudogap
Experiments in heavy-fermion metals and related theoretical work suggest that
critical local-moment fluctuations can play an important role near a
zero-temperature phase transition. We study such fluctuations at the quantum
critical point of a Kondo impurity model in which the density of band states
vanishes as |E|^r at the Fermi energy (E = 0). The local spin response is
described by a set of critical exponents that vary continuously with r. For 0 <
r < 1, the dynamical susceptibility exhibits omega/T scaling with a fractional
exponent, implying that the critical point is interacting.Comment: 4 pages, 3 figures; enhanced figures, expanded discussion of small-r
expansio
Multichannel pseudogap Kondo model: Large-N solution and quantum-critical dynamics
We discuss a multichannel SU(N) Kondo model which displays non-trivial
zero-temperature phase transitions due to a conduction electron density of
states vanishing with a power law at the Fermi level. In a particular large-N
limit, the system is described by coupled integral equations corresponding to a
dynamic saddle point. We exactly determine the universal low-energy behavior of
spectral densities at the scale-invariant fixed points, obtain anomalous
exponents, and compute scaling functions describing the crossover near the
quantum-critical points. We argue that our findings are relevant to recent
experiments on impurity-doped d-wave superconductors.Comment: 4 pages, 3 figs; extended discussion of large-N spin representations,
added references; accepted for publication in PR
Kondo screening in d-wave superconductors in a Zeeman field and implications for STM spectra of Zn-doped cuprates
We consider the screening of an impurity moment in a d-wave superconductor
under the influence of a Zeeman magnetic field. Using the Numerical
Renormalization Group technique, we investigate the resulting pseudogap Kondo
problem, in particular the field-induced crossover behavior in the vicinity of
the zero-field boundary quantum phase transition. The impurity spectral
function and the resulting changes in the local host density of states are
calculated, giving specific predictions for high-field STM measurements on
impurity-doped cuprates.Comment: 5 pages, 4 figs, (v2) remark on c-axis field added, discussion
extended, (v3) final version as publishe
Kondo effect in a quantum critical ferromagnet
We study the Heisenberg ferromagnetic spin chain coupled with a boundary
impurity. Via Bethe ansatz solution, it is found that (i) for J>0, the impurity
spin behaves as a diamagnetic center and is completely screened by 2S bulk
spins in the ground state, no matter how large the impurity spin is; (ii) The
specific heat of the local composite (impurity plus 2S bulk spins which form
bound state with it) shows a simple power law . (iii)For
J<0, the impurity is locked into the critical behavior of the bulk. Possible
phenomena in higher dimensions are discussed.Comment: 6page Revtex, no figure, final version in PRB, Jun 1 issue, 199
Impurity in a d-wave superconductor: Kondo effect and STM spectra
We present a theory for recent STM studies of Zn impurities in the
superconductor BSCCO, using insights from NMR experiments which show that there
is a net S=1/2 moment on the Cu ions near the Zn. We argue that the Kondo spin
dynamics of this moment is the origin of the low bias peak in the differential
conductance, rather than a resonance in a purely potential scattering model.
The spatial and energy dependence of the STM spectra of our model can also fit
the experiments.Comment: 4 pages, 2 color figures. Found improved saddle-point with d-wave
correlations near the impurity; onset of Kondo screening now occurs at a
significantly smaller coupling, but there is little qualitative change in
other features. Noted connection to STM of Kondo impurities in normal metals.
Final version as publishe
Absence of overscreened Kondo effect in ferromagnetic host
We study the low temperature behavior of a boundary magnetic impurity S'=1/2
in an open ferromagnetic Takhatajian-Babujian spin-S chain. For
antiferromagnetic Kondo coupling, it is show via Bethe ansatz solution that the
impurity spin is always locked into the critical behavior the bulk. At low
temperature, a local composite of spin S-1/2 forms near the impurity site and
its contribution to specific heat is of simple power law T^{1/2}. The absence
of overscreened Kondo effect is due to the large correlation length of host
spins which is divergent near the quantum critical point.Comment: 4 pages. to appear in Phys. Rev. B1(R4A)(2000
Localization of quasiparticles in a disordered vortex
We study the diffusive motion of low-energy normal quasiparticles along the
core of a single vortex in a dirty, type-II, s-wave superconductor. The physics
of this system is argued to be described by a one-dimensional supersymmetric
nonlinear sigma model, which differs from the sigma models known for disordered
metallic wires. For an isolated vortex and quasiparticle energies less than the
Thouless energy, we recover the spectral correlations that are predicted by
random matrix theory for the universality class C. We then consider the
transport problem of transmission of quasiparticles through a vortex connected
to particle reservoirs at both ends. The transmittance at zero energy exhibits
a weak localization correction reminiscent of quasi-one-dimensional metallic
systems with symmetry index beta = 1. Weak localization disappears with
increasing energy over a scale set by the Thouless energy. This crossover
should be observable in measurements of the longitudinal heat conductivity of
an ensemble of vortices under mesoscopic conditions. In the regime of strong
localization, the localization length is shown to decrease by a factor of 8 as
the quasiparticle energy goes to zero.Comment: 38 pages, LaTeX2e + epsf, 4 eps figures, one reference adde
Weak localization of disordered quasiparticles in the mixed superconducting state
Starting from a random matrix model, we construct the low-energy effective
field theory for the noninteracting gas of quasiparticles of a disordered
superconductor in the mixed state. The theory is a nonlinear sigma model, with
the order parameter field being a supermatrix whose form is determined solely
on symmetry grounds. The weak localization correction to the field-axis thermal
conductivity is computed for a dilute array of s-wave vortices near the lower
critical field H_c1. We propose that weak localization effects, cut off at low
temperatures by the Zeeman splitting, are responsible for the field dependence
of the thermal conductivity seen in recent high-T_c experiments by Aubin et al.Comment: RevTex, 8 pages, 1 eps figure, typos correcte
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