65 research outputs found
The thermal conductance of a two-channel Kondo model
A theory of thermal transport in a two-channel Kondo system, such as the one
formed by a small quantum dot coupled to two leads and to a larger dot, is
formulated. The interplay of the two screening constants allows an exploration
of the Fermi liquid and non-Fermi liquid regimes. By using analytical, as well
as numerical renormalization group methods, we study the temperature dependence
of the thermal conductance and the Lorentz number. We find that in the low
temperature limit, the Lorentz number attains its universal value, irrespective
of the nature of the ground state.Comment: 4 pages, 4 eps figure
Quantum Noise Measurement of a Carbon Nanotube Quantum Dot in the Kondo Regime
The current emission noise of a carbon nanotube quantum dot in the Kondo
regime is measured at frequencies of the order or higher than the
frequency associated with the Kondo effect , with the Kondo
temperature. The carbon nanotube is coupled via an on-chip resonant circuit to
a quantum noise detector, a superconductor-insulator-superconductor junction.
We find for a Kondo effect related singularity at a
voltage bias , and a strong reduction of this singularity
for , in good agreement with theory. Our experiment
constitutes a new original tool for the investigation of the non-equilibrium
dynamics of many-body phenomena in nanoscale devices.Comment: 6 pages, 4 figure
Measurement of Quantum Noise in a Carbon Nanotube Quantum Dot in the Kondo Regime
The current emission noise of a carbon nanotube quantum dot in the Kondo
regime is measured at frequencies ν of the order or higher than the frequency
associated with the Kondo effect kBTK/h, with TK the Kondo temperature. The
carbon nanotube is coupled via an on-chip resonant circuit to a quantum noise
detector, a superconductor-insulator-superconductor junction. We find for
hν≈kBTK a Kondo effect related singularity at a voltage bias eV≈hν, and a
strong reduction of this singularity for hν≈3kBTK, in good agreement with
theory. Our experiment constitutes a new original tool for the investigation
of the nonequilibrium dynamics of many-body phenomena in nanoscale devices
Positional Disorder, Spin-Orbit Coupling and Frustration in GaMnAs
We study the magnetic properties of metallic GaMnAs. We calculate the
effective RKKY interaction between Mn spins using several realistic models for
the valence band structure of GaAs. We also study the effect of positional
disorder of the Mn on the magnetic properties. We find that the interaction
between two Mn spins is anisotropic due to spin-orbit coupling within both the
so-called spherical approximation and in the more realistic six band model. The
spherical approximation strongly overestimates this anistropy, especially for
short distances between Mn ions. Using the obtained effective Hamiltonian we
carry out Monte Carlo simulations of finite and zero temperature magnetization
and find that, due to orientational frustration of the spins, non-collinear
states appear in both valence band approximations for disordered, uncorrelated
Mn impurities in the small concentration regime. Introducing correlations among
the substitutional Mn positions or increasing the Mn concentration leads to an
increase in the remnant magnetization at zero temperature and an almost fully
polarized ferromagnetic state.Comment: 17 Pages, 13 Figure
Disorder, spin-orbit, and interaction effects in dilute
We derive an effective Hamiltonian for in
the dilute limit, where can be described in
terms of spin polarons hopping between the {\rm Mn} sites and coupled
to the local {\rm Mn} spins. We determine the parameters of our model from
microscopic calculations using both a variational method and an exact
diagonalization within the so-called spherical approximation. Our approach
treats the extremely large Coulomb interaction in a non-perturbative way, and
captures the effects of strong spin-orbit coupling and Mn positional disorder.
We study the effective Hamiltonian in a mean field and variational calculation,
including the effects of interactions between the holes at both zero and finite
temperature. We study the resulting magnetic properties, such as the
magnetization and spin disorder manifest in the generically non-collinear
magnetic state. We find a well formed impurity band fairly well separated from
the valence band up to for which finite size
scaling studies of the participation ratios indicate a localization transition,
even in the presence of strong on-site interactions, where is the fraction of magnetically active Mn. We study the
localization transition as a function of hole concentration, Mn positional
disorder, and interaction strength between the holes.Comment: 15 pages, 12 figure
Renormalization group approach of itinerant electron systems near the Lifshitz point
Using the renormalization approach proposed by Millis for the itinerant
electron systems we calculated the specific heat coefficient for
the magnetic fluctuations with susceptibility near the Lifshitz point. The constant value
obtained for and the logarithmic temperature dependence, specific
for the non-Fermi behavior, have been obtained in agreement with the
experimental dat.Comment: 6 pages, Revte
Electron-fluctuation interaction in a non-Fermi superconductor
We studied the influence of the amplitude fluctuations of a non-Fermi
superconductor on the energy spectrum of the 2D Anderson non-Fermi system. The
classical fluctuations give a temperature dependence in the pseudogap induced
in the fermionic excitations.Comment: revtex fil
Penetration depth anisotropy in two-band superconductors
The anisotropy of the London penetration depth is evaluated for two-band
superconductors with arbitrary inter- and intra-band scattering times. If one
of the bands is clean and the other is dirty in the absence of inter-band
scattering, the anisotropy is dominated by the Fermi surface of the clean band
and is weakly temperature dependent. The inter-band scattering also suppress
the temperature dependence of the anisotropy
Two-Dimensional Electronic Spectroscopy of Chlorophyll a: Solvent Dependent Spectral Evolution
The interaction of the monomeric chlorophyll Q-band electronic transition with solvents of differing physical-chemical properties is investigated through two-dimensional electronic spectroscopy (2DES). Chlorophyll constitutes the key chromophore molecule in light harvesting complexes. It is well-known that the surrounding protein in the light harvesting complex fine-tunes chlorophyll electronic transitions to optimize energy transfer. Therefore, an understanding of the influence of the environment on the monomeric chlorophyll electronic transitions is important. The Q-band 2DES is inhomogeneous at early times, particularly in hydrogen bonding polar solvents, but also in nonpolar solvents like cyclohexane. Interestingly this inhomogeneity persists for long times, even up to the nanosecond time scale in some solvents. The reshaping of the 2DES occurs over multiple time scales and was assigned mainly to spectral diffusion. At early times the reshaping is Gaussian-like, hinting at a strong solvent reorganization effect. The temporal evolution of the 2DES response was analyzed in terms of a Brownian oscillator model. The spectral densities underpinning the Brownian oscillator fitting were recovered for the different solvents. The absorption spectra and Stokes shift were also properly described by this model. The extent and nature of inhomogeneous broadening was a strong function of solvent, being larger in H-bonding and viscous media and smaller in nonpolar solvents. The fastest spectral reshaping components were assigned to solvent dynamics, modified by interactions with the solute
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