280 research outputs found
Efficiency of Mesoscopic Detectors
We consider a mesoscopic measuring device whose conductance is sensitive to
the state of a two-level system. The detector is described with the help of its
scattering matrix. Its elements can be used to calculate the relaxation and
decoherence time of the system, and determine the characteristic time for a
reliable measurement. We derive conditions needed for an efficient ratio of
decoherence and measurement time. To illustrate the theory we discuss the
distribution function of the efficiency of an ensemble of open chaotic
cavities.Comment: 4 pages, 2 figure
Frequency Scales for Current Statistics of Mesoscopic Conductors
We calculate the third cumulant of current in a chaotic cavity with contacts
of arbitrary transparency as a function of frequency. Its frequency dependence
drastically differs from that of the conventional noise. In addition to a
dispersion at the inverse RC time characteristic of charge relaxation, it has a
low-frequency dispersion at the inverse dwell time of electrons in the cavity.
This effect is suppressed if both contacts have either large or small
transparencies.Comment: 4 page
Local spectroscopy of a proximity superconductor at very low temperature
We performed the local spectroscopy of a Normal-metal--Superconductor (N-S)
junction with the help of a very low temperature (60 mK) Scanning Tunneling
Microscope (STM). The spatial dependence of the local density of states was
probed locally in the vicinity of the N-S interface. We observed spectra with a
fully-developed gap in the regions where a thin normal metal layer caps the
superconductor dot. Close to the S metal edge, a clear pseudo-gap shows up,
which is characteristic of the superconducting proximity effect in the case of
a long normal metal. The experimental results are compared to the predictions
of the quasiclassical theory.Comment: 7 pages, 3 figure
Frustrated three-leg spin tubes: from spin 1/2 with chirality to spin 3/2
Motivated by the recent discovery of the spin tube
[(CuCltachH)Cl]Cl, we investigate the properties of a frustrated
three-leg spin tube with antiferromagnetic intra-ring and inter-ring couplings.
We pay special attention to the evolution of the properties from weak to strong
inter-ring coupling and show on the basis of extensive density matrix
renormalization group and exact diagonalization calculations that the system
undergoes a first-order phase transition between a dimerized gapped phase at
weak coupling that can be described by the usual spin-chirality model and a
gapless critical phase at strong coupling that can be described by an effective
spin-3/2 model. We also show that there is a magnetization plateau at 1/3 in
the gapped phase and slightly beyond. The implications for
[(CuCltachH)Cl]Cl are discussed, with the conclusion that this
system behaves essentially as a spin-3/2 chain.Comment: 8 pages, 9 figures, revised versio
Full counting statistics for voltage and dephasing probes
We present a stochastic path integral method to calculate the full counting
statistics of conductors with energy conserving dephasing probes and
dissipative voltage probes. The approach is explained for the experimentally
important case of a Mach-Zehnder interferometer, but is easily generalized to
more complicated setups. For all geometries where dephasing may be modeled by a
single one-channel dephasing probe we prove that our method yields the same
full counting statistics as phase averaging of the cumulant generating
function.Comment: 4 pages, 2 figure
Noise and Full Counting Statistics of Incoherent Multiple Andreev Reflection
We present a general theory for the full counting statistics of multiple
Andreev reflections in incoherent superconducting-normal-superconducting
contacts. The theory, based on a stochastic path integral approach, is applied
to a superconductor-double barrier system. It is found that all cumulants of
the current show a pronounced subharmonic gap structure at voltages
. For low voltages , the counting statistics
results from diffusion of multiple charges in energy space, giving the th
cumulant , diverging for . We show that this
low-voltage result holds for a large class of incoherent
superconducting-normal-superconducting contacts.Comment: 4 pages, 4 figure
Spectral Features of the Proximity Effect
We calculate the local density of states (LDOS) of a superconductor-normal
metal sandwich at arbitrary impurity concentration. The presence of the
superconductor induces a gap in the normal metal spectrum that is proportional
to the inverse of the elastic mean free path for rather clean systems. For
a mean free path much shorter than the thickness of the normal metal, we find a
gap size proportional to that approaches the behavior predicted by the
Usadel equation (diffusive limit).Comment: LT22 proceeding
Scattering Theory of Mesoscopic Detectors
We consider a two-level system coupled to a mesoscopic two-terminal conductor
that acts as measuring device. As a convenient description of the conductor we
introduce its scattering matrix. We show how its elements can be used to
calculate the relaxation and decoherence rates of the two-level system. Special
emphasis is laid on the charge screening in the conductor that becomes
important in the many-channel limit. Finally we give some examples that
illustrate charge screening in different limits.Comment: contribution to the ECOSS-21 proceedings in a special issue of
SURFACE SCIENC
Statistics of Heat Transfer in Mesoscopic Circuits
A method to calculate the statistics of energy exchange between quantum
systems is presented. The generating function of this statistics is expressed
through a Keldysh path integral. The method is first applied to the problem of
heat dissipation from a biased mesoscopic conductor into the adjacent
reservoirs. We then consider energy dissipation in an electrical circuit around
a mesoscopic conductor. We derive the conditions under which measurements of
the fluctuations of heat dissipation can be used to investigate higher order
cumulants of the charge counting statistics of a mesoscopic conductor.Comment: 9 pages, 6 figure
Slave-boson mean-field theory of the Mott transition in the two-band Hubbard model
Abstract.: We apply the slave-boson approach of Kotliar and Ruckenstein to the two-band Hubbard model with an Ising like Hund's rule coupling and bands of different widths. On the mean-field level of this approach we investigate the Mott transition and observe both separate and joint transitions of the two bands depending on the choice of the inter- and intra-orbital Coulomb interaction parameters. The mean-field calculations allow for a simple physical interpretation and can confirm several aspects of previous work. Beside the case of two individually half-filled bands we also examine what happens if the original metallic bands possess fractional filling either due to finite doping or due to a crystal field which relatively shifts the atomic energy levels of the two orbitals. For appropriate values of the interaction and of the crystal field we can observe a band insulating state and a ferromagnetic meta
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