321 research outputs found
Single parameter scaling in 1-D localized absorbing systems
Numerical study of the scaling of transmission fluctuations in the 1-D
localization problem in the presence of absorption is carried out. Violations
of single parameter scaling for lossy systems are found and explained on the
basis of a new criterion for different types of scaling behavior derived by
Deych et al [Phys. Rev. Lett., {\bf 84}, 2678 (2000)].Comment: 7 pages, 6 figures, RevTex, submitted to Phys. Rev.
Quantum Pumping in the Magnetic Field: Role of Discrete Symmetries
We consider an effect of the discrete spatial symmetries and magnetic field
on the adiabatic charge pumping in mesoscopic systems. In general case, there
is no symmetry of the pumped charge with respect to the inversion of magnetic
field Q(B) \neq Q(-B). We find that the reflection symmetries give rise to
relations Q(B)=Q(-B) or Q(B)=-Q(-B) depending on the orientation of the
reflection axis. In presence of the center of inversion, Q(B) = 0. Additional
symmetries may arise in the case of bilinear pumping.Comment: 4 page
Statistical properties of phases and delay times of the one-dimensional Anderson model with one open channel
We study the distribution of phases and of Wigner delay times for a
one-dimensional Anderson model with one open channel. Our approach, based on
classical Hamiltonian maps, allows us an analytical treatment. We find that the
distribution of phases depends drastically on the parameter where is the variance of the disorder distribution and
the wavevector. It undergoes a transition from uniformity to singular
behaviour as increases. The distribution of delay times shows
universal power law tails , while the short time behaviour is
- dependent.Comment: 4 pages, 2 figures, Submitted to PR
Ab initio investigation of VOSeO3, a spin gap system with coupled spin dimers
Motivated by an early experimental study of VOSeO3, which suggested that it
is a quasi-2D system of weakly coupled spin dimers with a small spin gap, we
have investigated the electronic structure of this material via
density-functional calculations. These ab initio results indicate that the
system is better thought of as an alternating spin-1/2 chain with moderate
interchain interactions, an analog of (VO)2P2O7. The potential interest of this
system for studies in high magnetic field given the presumably small value of
the spin gap is emphasized.Comment: 4 pages, 5 figure
Influence of typical environments on quantum processes
We present the results of studying the influence of different environmental
states on the coherence of quantum processes. We choose to discuss a simple
model which describe two electronic reservoirs connected through tunneling via
a resonant state. The model could, e.g., serve as an idealization of inelastic
resonant tunneling through a double barrier structure. We develop Schwinger's
closed time path formulation of non-equilibrium quantum statistical mechanics,
and show that the influence of the environment on a coherent quantum process
can be described by the value of a generating functional at a specific force
value, thereby allowing for a unified discussion of destruction of phase
coherence by various environmental states: thermal state, classical noise, time
dependent classical field, and a coherent state. The model allows an extensive
discussion of the influence of dissipation on the coherent quantum process, and
expressions for the transmission coefficient are obtained in the possible
limits.Comment: 46 pages, 11 post script figures. Accepted for publication in
Physical Review
Universal Correlations of Coulomb Blockade Conductance Peaks and the Rotation Scaling in Quantum Dots
We show that the parametric correlations of the conductance peak amplitudes
of a chaotic or weakly disordered quantum dot in the Coulomb blockade regime
become universal upon an appropriate scaling of the parameter. We compute the
universal forms of this correlator for both cases of conserved and broken time
reversal symmetry. For a symmetric dot the correlator is independent of the
details in each lead such as the number of channels and their correlation. We
derive a new scaling, which we call the rotation scaling, that can be computed
directly from the dot's eigenfunction rotation rate or alternatively from the
conductance peak heights, and therefore does not require knowledge of the
spectrum of the dot. The relation of the rotation scaling to the level velocity
scaling is discussed. The exact analytic form of the conductance peak
correlator is derived at short distances. We also calculate the universal
distributions of the average level width velocity for various values of the
scaled parameter. The universality is illustrated in an Anderson model of a
disordered dot.Comment: 35 pages, RevTex, 6 Postscript figure
Resonance approximation and charge loading/unloading in adiabatic quantum pumping
Quantum pumping through mesoscopic quantum dots is known to be enhanced by
resonant transmission. The pumped charge is close to an integer number of
electrons when the pumping contour surrounds a resonance, but the transmission
remains small on the contour. For non-interacting electrons, we give a
quantitative account of the detailed exchange of electrons between the dot and
the leads (to the electron reservoirs) during a pumping cycle. Near isolated
distinct resonances, we use approximate Breit-Wigner expressions for the dot's
Green function to discuss the loading/unloading picture of the pumping: the
fractional charge exchanged between the dot and each lead through a single
resonance point is related to the relative couplings of the dot and the leads
at this resonance. If each resonance point along the pumping contour is
dominated by the coupling to a single lead (which also implies a very small
transmission), then the crossing of each such resonance results in a single
electron exchange between the dot and that lead, ending up with a net quantized
charge. When the resonance approximation is valid, the fractional charges can
also be extracted from the peaks of the transmissions between the various
leads.Comment: 10 pages, 4 figure
Comparison of temperature-dependent calibration methods of an instrument to measure OH and HO₂ radicals using laser-induced fluorescence spectroscopy
Laser-induced fluorescence (LIF) spectroscopy has been widely applied to fieldwork measurements of
OH radicals and HO2, following conversion to OH, over a wide variety of conditions, on different platforms and in simulation chambers. Conventional calibration of HOx (OH + HO2) instruments has mainly relied on a single method, generating known concentrations of HOx from H2O vapour photolysis in a flow of zero air impinging just outside the sample inlet (SHOx = CHOx . [HOx ], where SHOx is
the observed signal and CHOx is the calibration factor). The fluorescence assay by gaseous expansion (FAGE) apparatus designed for HOx measurements in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) at the University of Leeds has been used to examine the sensitivity of
FAGE to external gas temperatures (266–348 K).
The conventional calibration methods give the temperature dependence of COH (relative to the value at 293 K) of (0.0059±0.0015) K−1 and CHO2 of (0.014±0.013) K−1. Errors are 2σ . COH was also determined by observing the decay of hydrocarbons (typically cyclohexane) caused by OH reactions giving COH (again, relative to the value at 293 K) of (0.0038 ± 0.0007) K−1. Additionally, CHO2 was determined based on the second-order kinetics of HO2 recombination with the temperature dependence of CHO2 , relative to 293 K being (0.0064 ± 0.0034) K−1.
The temperature dependence of CHOx depends on the HOx number density, quenching, the relative population of the probed OH rotational level and HOx transmission from the inlet to the detection axis. The first three terms can be calculated and, in combination with the measured values of CHOx, show that HOx transmission increases with temperature. Comparisons with other instruments and the implications of this work are discussed
Weak Localization Effect in Superconductors by Radiation Damage
Large reductions of the superconducting transition temperature and
the accompanying loss of the thermal electrical resistivity (electron-phonon
interaction) due to radiation damage have been observed for several A15
compounds, Chevrel phase and Ternary superconductors, and in
the high fluence regime. We examine these behaviors based on the recent theory
of weak localization effect in superconductors. We find a good fitting to the
experimental data. In particular, weak localization correction to the
phonon-mediated interaction is derived from the density correlation function.
It is shown that weak localization has a strong influence on both the
phonon-mediated interaction and the electron-phonon interaction, which leads to
the universal correlation of and resistance ratio.Comment: 16 pages plus 3 figures, revtex, 76 references, For more information,
Plesse see http://www.fen.bilkent.edu.tr/~yjki
Quantum Effects in Coulomb Blockade
We review the quantum interference effects in a system of interacting
electrons confined to a quantum dot. The review starts with a description of an
isolated quantum dot. We discuss the status of the Random Matrix theory (RMT)
of the one-electron states in the dot, present the universal form of the
interaction Hamiltonian compatible with the RMT, and derive the leading
corrections to the universal interaction Hamiltonian. Next, we discuss a
theoretical description of a dot connected to leads via point contacts. Having
established the theoretical framework to describe such an open system, we
discuss its transport and thermodynamic properties. We review the evolution of
the transport properties with the increase of the contact conductances from
small values to values . In the discussion of transport, the
emphasis is put on mesoscopic fluctuations and the Kondo effect in the
conductance.Comment: 169 pages, 28 figures; several references and footnotes are added,
and noticed typos correcte
- …