1,806 research outputs found
Electron-magnon coupling and nonlinear tunneling transport in magnetic nanoparticles
We present a theory of single-electron tunneling transport through a
ferromagnetic nanoparticle in which particle-hole excitations are coupled to
spin collective modes. The model employed to describe the interaction between
quasiparticles and collective excitations captures the salient features of a
recent microscopic study. Our analysis of nonlinear quantum transport in the
regime of weak coupling to the external electrodes is based on a rate-equation
formalism for the nonequilibrium occupation probability of the nanoparticle
many-body states. For strong electron-boson coupling, we find that the
tunneling conductance as a function of bias voltage is characterized by a large
and dense set of resonances. Their magnetic field dependence in the large-field
regime is linear, with slopes of the same sign. Both features are in agreement
with recent tunneling experiments.Comment: 4 pages, 2 figure
Positive noise cross-correlations in superconducting hybrids: Roles of interfaces and interactions
Shot noise cross-correlations in normal metal-superconductor-normal metal
structures are discussed at arbitrary interface transparencies using both the
scattering approach of Blonder, Tinkham and Klapwik and a microscopic Green's
function approach. Surprisingly, negative crossed conductance in such set-ups
[R. Melin and D. Feinberg, Phys. Rev. B 70, 174509 (2004)] does not preclude
the possibility of positive noise cross-correlations for almost transparent
contacts. We conclude with a phenomenological discussion of interactions in the
one dimensional leads connected to the superconductor, which induce sign
changes in the noise cross-correlations.Comment: 15 pages, 9 figure
Finite quantum dissipation: the challenge of obtaining specific heat
We consider a free particle coupled with finite strength to a bath and
investigate the evaluation of its specific heat. A harmonic oscillator bath of
Drude type with cutoff frequency omega_D is employed to model an ohmic friction
force with dissipation strength gamma. Two scenarios for obtaining specific
heat are presented. The first one uses the measurement of the kinetic energy of
the free particle while the second one is based on the reduced partition
function. Both descriptions yield results which are consistent with the Third
Law of thermodynamics. Nevertheless, the two methods produce different results
that disagree even in their leading quantum corrections at high temperatures.
We also consider the regime where the cutoff frequency is smaller than the
friction strength, i.e. omega_D<gamma. There, we encounter puzzling results at
low temperatures where the specific heat based on the thermodynamic
prescription becomes negative. This anomaly is rooted in an ill-defined density
of states of the damped free particle which assumes unphysical negative values
when gamma/omega_D>1.Comment: 16 pages, 4 figure
Structural and Magnetic Dynamics in the Magnetic Shape Memory Alloy NiMnGa
Magnetic shape memory Heusler alloys are multiferroics stabilized by the
correlations between electronic, magnetic and structural order. To study these
correlations we use time resolved x-ray diffraction and magneto-optical Kerr
effect experiments to measure the laser induced dynamics in a Heusler alloy
NiMnGa film and reveal a set of timescales intrinsic to the system. We
observe a coherent phonon which we identify as the amplitudon of the modulated
structure and an ultrafast phase transition leading to a quenching of the
incommensurate modulation within 300~fs with a recovery time of a few ps. The
thermally driven martensitic transition to the high temperature cubic phase
proceeds via nucleation within a few ps and domain growth limited by the speed
of sound. The demagnetization time is 320~fs, which is comparable to the
quenching of the structural modulation.Comment: 5 pages, 3 figures. Supplementary materials 5 pages, 5 figure
From non equilibrium quantum Brownian motion to impurity dynamics in 1D quantum liquids
Impurity motion in one dimensional ultra cold quantum liquids confined in an
optical trap has attracted much interest recently. As a step towards its full
understanding, we construct a generating functional from which we derive the
position non equilibrium correlation function of a quantum Brownian particle
with general Gaussian non-factorizing initial conditions. We investigate the
slow dynamics of a particle confined in a harmonic potential after a position
measurement; the rapid relaxation of a particle trapped in a harmonic potential
after a quantum quench realized as a sudden change in the potential parameters;
and the evolution of an impurity in contact with a one dimensional bosonic
quantum gas. We argue that such an impurity-Luttinger liquid system, that has
been recently realized experimentally, admits a simple modeling as quantum
Brownian motion in a super Ohmic bath.Comment: Published version, 23 pages, 2 figure
Quantum nondemolition-like, fast measurement scheme for a superconducting qubit
We present a measurement protocol for a flux qubit coupled to a
dc-Superconducting QUantum Interference Device (SQUID), representative of any
two-state system with a controllable coupling to an harmonic oscillator
quadrature, which consists of two steps. First, the qubit state is imprinted
onto the SQUID via a very short and strong interaction. We show that at the end
of this step the qubit dephases completely, although the perturbation of the
measured qubit observable during this step is weak. In the second step,
information about the qubit is extracted by measuring the SQUID. This step can
have arbitrarily long duration, since it no longer induces qubit errors.Comment: published version, minor correction
Single-electron transistor effect in a two-terminal structure
A peculiarity of the single-electron transistor effect makes it possible to
observe this effect even in structures lacking a gate electrode altogether. The
proposed method can be useful for experimental study of charging effects in
structures with an extremely small central island confined between tunnel
barriers like a nanometer-sized quantum dot or a macromolecule probed with a
tunneling microscope), where it is impossible to provide a gate electrode for
control of the tunnel current.Comment: 5 pages, 2 figure
Fluctuation Dominated Josephson Tunneling with a Scanning Tunneling Microscope
We demonstrate Josephson tunneling in vacuum tunnel junctions formed between
a superconducting scanning tunneling microscope tip and a Pb film, for junction
resistances in the range 50-300 k. We show that the superconducting
phase dynamics is dominated by thermal fluctuations, and that the Josephson
current appears as a peak centered at small finite voltages. In the presence of
microwave fields (f=15.0 GHz) the peak decreases in magnitude and shifts to
higher voltages with increasing rf power, in agreement with theory.Comment: 4 pages, REVTeX, submitted to PR
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