64 research outputs found
Enhancement of the Kondo temperature of magnetic impurities in metallic point contacts due to the fluctuations of the local density of states
The effect of local density of states (LDOS) fluctuations on the dynamics of
a Kondo impurities in a small metallic point contact (PC) is studied. To
estimate the spatial and energy dependent LDOS fluctuations we investigate a
model PC by means of a transfer matrix formalism. For small PC's in the
nanometer scale we find that near to the orifice strong LDOS fluctuations
develop. These fluctuations may shift the Kondo temperature by several orders
of magnitude, and result in a strong broadening of the PC Kondo peak in
agreement with the results of recent measurements.Comment: 11 pages, uuencoded gz-compressed tar-files, latex, figures in a
PostScript fil
Theory of Manganese-Manganese interaction in GaMnAs
We investigate the interaction of two Mn ions in the dilute magnetic
semiconductor GaMnAs using the variational envelope wave function approach
within the framework of six band model of the valence band. We find that the
effective interaction between the Mn core spins at a typical separation d is
strongly anisotropic for active Mn concentrations less than x = 1.3%, but it is
almost isotropic for shorter distances (d < 13A). As a result, in unannealed
and strongly compensated samples strong frustration effects must be present. We
also verify that an effective Hamiltonian description can be used in the dilute
limit, x < 1.3%, and extract the parameters of this effective Hamiltonian.Comment: 11 pages, 8 figure
Perturbative theory of the non-equilibrium singlet-triplet transition
We study equilibrium and non-equilibrium properties of a two-level quantum
dot close to the singlet-triplet transition. We treat the on-site Coulomb
interaction and Hund's rule coupling perturbatively within the Keldysh
formalism. We compute the spectral functions and the differential conductance
of the dot. For moderate interactions our perturbative approach captures the
Kondo effect and many of the experimentally observed properties.Comment: Contribution to the proceedings of ICM2009 (International Conference
on Magnetism
Dissipative quantum phase transition in a quantum dot
We study the transport properties of a quantum dot (QD) with highly resistive
gate electrodes, and show that the QD displays a quantum phase transition
analogous to the famous dissipative phase transition first identified by S.
Chakravarty [Phys. Rev. Lett. {\bf 49}, 681 (1982)]; for a review see [A. J.
Leggett {\em et al.}, Rev. Mod. Phys. {\bf 59}, 1 (1987)]. At temperature T=0,
the charge on the central island of a conventional QD changes smoothly as a
function of gate voltage, due to quantum fluctuations. However, for
sufficiently large gate resistance charge fluctuations on the island can freeze
out even at the degeneracy point, causing the charge on the island to change in
sharp steps as a function of gate voltage. For the steps remain
smeared out by quantum fluctuations. The Coulomb blockade peaks in conductance
display anomalous scaling at intermediate temperatures, and at very low
temperatures a sharp step develops in the QD conductance.Comment: 4+ pages, 3 figure
Theory of optical conductivity for dilute GaMnAs
We construct a semi-microscopic theory, to describe the optical conductivity
of GaMnAs in the dilute limit, x = 1%. We construct an effective Hamiltonian
that captures inside-impurity band optical transitions as well as transitions
between the valence band and the impurity band. All parameters of the
Hamiltonian are computed from microscopic variational calculations. We find a
metal-insulator transition within the impurity band in the concentration range,
x = 0.2 -0.3 for uncompensated and x = 1-3% for compensated samples, in good
agreement with the experiments. We find an optical mass m_opt = m_e, which is
almost independent of the impurity concentration excepting in the vicinity of
the metal-insulator transition, where it reaches values as large as m_opt = 10
m_e. We also reproduce a mid-infrared peak at \hbar \omega = 200 meV, which
redshifts upon doping, in quantitative agreement with the experiments.Comment: 17 pages, 16 figures, submitted to Physical Review
Theory of frequency-dependent spin current noise through correlated quantum dots
We analyze the equilibrium and non-equilibrium frequency-dependent spin
current noise and spin conductance through a quantum dot in the local moment
regime. Spin current correlations are shown to behave markedly differently from
charge correlations: Equilibrium spin cross-correlations are suppressed at
frequencies below the Kondo scale, and are characterized by a universal
function that we determine numerically for zero temperature. For asymmetrical
quantum dots dynamical spin accumulation resonance is found for frequencies of
the order of the Kondo energy. At higher temperatures surprising low-frequency
anomalies related to overall spin conservation appear
Comment on ``Point-Contact Study of Fast and Slow Two-Level Fluctuators in Metallic Glasses'' by Keijsers, Shklyarevskii and van Kempen
We point out that a recent experiment by Keijsers, Shklyarevskii and van
Kempen [Phys. Rev. Lett. 77, 3411 (1996)] on metallic glass point contacts
containing two-level systems (TLS) effectively measures for the first time the
conductance contributions of individual TLSs (previous experiments averaged
over a distribution of TLSs). For this case the predictions of the Kozub-Kulik
theory for the TLS-electron interaction differ from those of the
Vladar-Zawadowski theory, the latter giving much better agreement with
experiment.Comment: 1 page Revtex, 1 figure, to appear in Phys. Rev. Let
Non-equilibrium time evolution and rephasing in the quantum sine-Gordon model
We discuss the non-equilibrium time evolution of the phase field in the
sine-Gordon model using two very different approaches: the truncated Wigner
approximation and the truncated conformal space approach. We demonstrate that
the two approaches agree for a period covering the first few oscillations,
thereby giving a solid theoretical prediction in the framework of sine-Gordon
model, which is thought to describe the dynamics of two bosonic condensates in
quasi-one-dimensional traps coupled via a Josephson tunneling term. We
conclude, however, that the recently observed phase-locking behavior cannot be
explained in terms of homogeneous sine-Gordon dynamics, which hints at the role
of other degrees of freedom or inhomogeneity in the experimental system.Comment: 34 pages, 9 figure
Theory of inelastic scattering from magnetic impurities
We use the numerical renormalization group method tocalculate the single
particle matrix elements of the many body -matrix of the conduction
electrons scattered by a magnetic impurity at T=0 temperature. Since
determines both the total and the elastic, spin-diagonal scattering cross
sections, we are able to compute the full energy-, spin- and magnetic field
dependence of the inelastic scattering cross section, . We find an almost linear frequency dependence of below the Kondo temperature, , which crosses over to a
behavior only at extremely low energies. Our method can be
generalized to other quantum impurity models.Comment: 4 page
Non-equilibrium frequency-dependent noise through a quantum dot: A real time functional renormalization group approach
We construct a real time current-conserving functional renormalization group
(RG) scheme on the Keldysh contour to study frequency-dependent transport and
noise through a quantum dot in the local moment regime. We find that the
current vertex develops a non-trivial non-local structure in time, governed by
a new set of RG equations. Solving these RG equations, we compute the complete
frequency and temperature-dependence of the noise spectrum. For voltages large
compared to the Kondo temperature, , two sharp anti-resonances
are found in the noise spectrum at frequencies , and
correspondingly, two peaks in the ac conductance through the dot.Comment: 4 pages, 4 figure
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