1,275 research outputs found
Non-equilibrium conductance of a three-terminal quantum dot in the Kondo regime: Perturbative Renormalization Group
Motivated by recent experiments, we consider a single-electron transistor in
the Kondo regime which is coupled to three leads in the presence of large bias
voltages. Such a steady-state non-equilibrium system is to a large extent
governed by a decoherence rate induced by the current through the dot. As the
two-terminal conductance turns out to be rather insensitive to the decoherence
rate, we study the conductance in a three-terminal device using perturbative
renormalization group and calculate the characteristic splitting of the Kondo
resonance. The interplay between potential biases and anisotropy in coupling to
the three leads determines the decoherence rate and the conditions for strong
coupling.Comment: 4 pages, 4 figure
Giant mass and anomalous mobility of particles in fermionic systems
We calculate the mobility of a heavy particle coupled to a Fermi sea within a
non-perturbative approach valid at all temperatures. The interplay of particle
recoil and of strong coupling effects, leading to the orthogonality catastrophe
for an infinitely heavy particle, is carefully taken into account. We find two
novel types of strong coupling effects: a new low energy scale and
a giant mass renormalization in the case of either near-resonant scattering or
a large transport cross section . The mobility is shown to obey two
different power laws below and above . For ,
where is the Fermi wave length, an exponentially large effective
mass suppresses the mobility.Comment: 4 pages, 4 figure
Wilson chains are not thermal reservoirs
Wilson chains, based on a logarithmic discretization of a continuous
spectrum, are widely used to model an electronic (or bosonic) bath for Kondo
spins and other quantum impurities within the numerical renormalization group
method and other numerical approaches. In this short note we point out that
Wilson chains can not serve as thermal reservoirs as their temperature changes
by a number of order Delta E when a finite amount of energy Delta E is added.
This proves that for a large class of non-equilibrium problems they cannot be
used to predict the long-time behavior.Comment: 2 page
Current induced rotational torques in the skyrmion lattice phase of chiral magnets
In chiral magnets without inversion symmetry, the magnetic structure can form
a lattice of magnetic whirl lines, a two-dimensional skyrmion lattice,
stabilized by spin-orbit interactions in a small range of temperatures and
magnetic fields. The twist of the magnetization within this phase gives rise to
an efficient coupling of macroscopic magnetic domains to spin currents. We
analyze the resulting spin-transfer effects, and, in particular, focus on the
current induced rotation of the magnetic texture by an angle. Such a rotation
can arise from macroscopic temperature gradients in the system as has recently
been shown experimentally and theoretically. Here we investigate an alternative
mechanism, where small distortions of the skyrmion lattice and the transfer of
angular momentum to the underlying atomic lattice play the key role. We employ
the Landau-Lifshitz-Gilbert equation and adapt the Thiele method to derive an
effective equation of motion for the rotational degree of freedom. We discuss
the dependence of the rotation angle on the orientation of the applied magnetic
field and the distance to the phase transition.Comment: 11 pages, 6 figures; minor changes, published versio
Heat transport of clean spin-ladders coupled to phonons: Umklapp scattering and drag
We study the low-temperature heat transport in clean two-leg spin ladder
compounds coupled to three-dimensional phonons. We argue that the very large
heat conductivities observed in such systems can be traced back to the
existence of approximate symmetries and corresponding weakly violated
conservation laws of the effective (gapful) low--energy model, namely
pseudo-momenta. Depending on the ratios of spin gaps and Debye energy and on
the temperature, the magnetic contribution to the heat conductivity can be
positive or negative, and exhibit an activated or anti-activated behavior. In
most regimes, the magnetic heat conductivity is dominated by the spin-phonon
drag: the excitations of the two subsystems have almost the same drift
velocity, and this allows for an estimate of the ratio of the magnetic and
phononic contributions to the heat conductivity.Comment: revised version, 8 pages, 3 figures, added appendi
Quantum chemical modeling of tri-Mn-substituted W-based Keggin polyoxoanions
Using Density Functional Theory (DFT) calculations, we studied the electrochemistry of polyoxome-
talates (POMs), specifically the redox properties of Mn in tri-Mn-substituted W-based Keggin ions. For
direct comparison with recent cyclic voltammetry results [J. Friedl et al. Electrochim. Acta, 141 (2014)
357], we estimated the reversible half-wave potentials of proton- and cation-coupled electron transfer
for Mn(IV/III) and Mn(III/II), respectively. The calculated reversible potentials agree well with
experiment, reproducing the trend with pH for Mn(IV/III). For adequate DFT energies, it is crucial to
apply a reliable description of the electrolyte environment of the POM, accounting also for their rather
high charges, up to
�7 e. To this end, we included the Li+ counterions, required for charge neutralization,
directly in the quantum chemical models which were embedded in a polarizable continuum. We
explored various arrangements of the Li+ ions around the POMs and their effect on both structural
parameters and electrochemical properties of the POMs. Hybrid functionals (TPSSh, B3LYP, PBE0)
overestimate the experimental reduction potentials: the larger the exact-exchange contribution, the
larger the resulting reduction potential. The best agreement with experiment is achieved with the PBE
approach, likely due to fortuitous error cancellation. The results of the present work indicate that a more
sophisticated (atomistic) representation of the electrolyte environment will be beneficial for predicting
redox potentials in better agreement with experiment
Interplay of disorder and spin fluctuations in the resistivity near a quantum critical point
The resistivity in metals near an antiferromagnetic quantum critical point
(QCP) is strongly affected by small amounts of disorder. In a quasi-classical
treatment, we show that an interplay of strongly anisotropic scattering due to
spin fluctuations and isotropic impurity scattering leads to a large regime
where the resistivity varies as T^alpha, with an anomalous exponent, alpha, 1
<= alpha <= 1.5, depending on the amount of disorder. I argue that this
mechanism explains in some detail the anomalous temperature dependence of the
resistivity observed in CePd_2Si_2, CeNi_2Ge_2 and CeIn_3 near the QCP.Comment: 4 pages, 4 eps figures, published version, only small change
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