447 research outputs found
Quantum turnstile operation of single-molecule magnets
The time-dependent transport through single-molecule magnets coupled to
magnetic or non-magnetic electrodes is studied in the framework of the
generalized master equation method. We investigate the transient regime induced
by the periodic switching of the source and drain contacts. If the electrodes
have opposite magnetizations the quantum turnstile operation allows the
stepwise writing of intermediate excited states. In turn, the transient
currents provide a way to read these states. Within our approach we take into
account both the uniaxial and transverse anisotropy. The latter may induce
additional quantum tunneling processes which affect the efficiency of the
proposed read-and-write scheme. An equally weighted mixture of molecular spin
states can be prepared if one of the electrodes is ferromagnetic.Comment: 19 pages, 6 figure
Superconductivity and Physical Properties of CaPd2Ge2 Single Crystals
We present the superconducting and normal state properties of CaPd2Ge2 single
crystal investigated by magnetic susceptibility \chi, isothermal magnetization
M, heat capacity C_p, in-plane electrical resistivity \rho and London
penetration depth \lambda versus temperature T and magnetic field H
measurements. Bulk superconductivity is inferred from the \rho(T) and C_p(T)
data. The \rho(T) data exhibit metallic behavior and undergoes a
superconducting transition with T_c onset = 1.98 K and zero resistivity state
at T_c 0 = 1.67 K. The \chi(T) reveal the onset of superconductivity at 2.0 K.
For T>2.0 K, the \chi(T) and M(H) are weakly anisotropic paramagnetic with
\chi_ab > \chi_c. The C_p(T) confirm the bulk superconductivity below T_c =
1.69(3) K. The superconducting state electronic heat capacity is analyzed
within the framework of a single-band \alpha-model of BCS superconductivity and
various normal and superconducting state parameters are estimated. Within the
\alpha-model, the C_p(T) data and the ab plane \lambda(T) data consistently
indicate a moderately anisotropic s-wave gap with \Delta(0)/k_B T_c ~ 1.6,
somewhat smaller than the BCS value of 1.764. The relationship of the heat
capacity jump at T_c and the penetration depth measurement to the anisotropy in
the s-wave gap is discussed.Comment: 12 pages, 9 figures, 2 Tables; Submitted to PR
Wiedemann-Franz law and non-vanishing temperature scale across the field-tuned quantum critical point of YbRh2Si2
The in-plane thermal conductivity kappa(T) and electrical resistivity rho(T)
of the heavy-fermion metal YbRh2Si2 were measured down to 50 mK for magnetic
fields H parallel and perpendicular to the tetragonal c axis, through the
field-tuned quantum critical point, Hc, at which antiferromagnetic order ends.
The thermal and electrical resistivities, w(T) and rho(T), show a linear
temperature dependence below 1 K, typical of the non-Fermi liquid behavior
found near antiferromagnetic quantum critical points, but this dependence does
not persist down to T = 0. Below a characteristic temperature T* ~ 0.35 K,
which depends weakly on H, w(T) and rho(T) both deviate downward and converge
in the T = 0 limit. We propose that T* marks the onset of short-range magnetic
correlations, persisting beyond Hc. By comparing samples of different purity,
we conclude that the Wiedemann-Franz law holds in YbRh2Si2, even at Hc,
implying that no fundamental breakdown of quasiparticle behavior occurs in this
material. The overall phenomenology of heat and charge transport in YbRh2Si2 is
similar to that observed in the heavy-fermion metal CeCoIn5, near its own
field-tuned quantum critical point.Comment: 8 figures, 8 page
Effects of screening on the two-dimensional electron transport properties in modulation doped heterostructures
Cataloged from PDF version of article.The effects of screening on the polar optical phonon scattering rates and on the transport properties of the two-dimensional electron gas in AlGaAs/GaAs modulation doped heterostructures have been investigated through Monte Carlo simulations incorporating the three valleys of the conduction band, size quantization in the Gamma valley and the lowest three subbands in the quantum-well. At typical sheet densities observed in modulation doped field-effect transistors, screening considerably affects the electron transport properties under moderately large fields and at low temperatures, by lowering the intrasubband polar-optical phonon scattering rates especially in the first subband. The results show that screening, which is usually ignored in device Monte Carlo simulations, should be included in the simulation in order to be able to predict the device performance correctly. (C) 1998 Elsevier Science Ltd. All rights reserved
Dynamical screening effects in hot-electron scattering from electron-hole plasma and LO-phonon modes in quantum wires
Cataloged from PDF version of article.We present a fully dynamical and finite temperature study of the hot-electron momentum relaxation rate and the power loss in a coupled system of electron-hole plasma and bulk LO-phonons in a quantum wire structure. Interactions of the scattered electron with neutral plasma components and phonons are treated on an equal footing within the random-phase approximation. Coupled mode effects substantially change the transport properties of the system at low temperatures. Particularly, the ''plasmon-like'' and ''LO-phonon-like'' excitations yield comparable rates which, as a consequence of the singular nature of the 1D density of states, can be large at the threshold. This is in contrast to room temperature results where only the LO-phonon mode contributes significantly to the rate. The density and temperature dependence of the power loss reveals that dynamical screening effects are important, and energy-momentum conservation cannot be satisfied above a certain density for a given initial energy. Copyright (C) 1996 Elsevier Science Lt
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