5 research outputs found
Mesoscopic quantum transport: Resonant tunneling in the presence of strong Coulomb interaction
Coulomb blockade phenomena and quantum fluctuations are studied in mesoscopic
metallic tunnel junctions with high charging energies. If the resistance of the
barriers is large compared to the quantum resistance, transport can be
described by sequential tunneling. Here we study the influence of quantum
fluctuations. They are important when the resistance is small or the
temperature very low. A real-time approach is developed which allows the
diagrammatic classification of ``inelastic resonant tunneling'' processes where
different electrons tunnel coherently back and forth between the leads and the
metallic island. With the help of a nonperturbative resummation technique we
evaluate the spectral density which describes the charge excitations of the
system. From it physical quantities of interest like current and average charge
can be deduced. Our main conclusions are: An energy renormalization leads to a
logarithmic temperature dependence of the renormalized system parameters. A
finite lifetime broadening can change the classical picture drastically. It
gives rise to a strong flattening of the Coulomb oscillations for low
resistances, but in the Coulomb blockade regime inelastic electron cotunneling
persists. The temperature where these effects are important are accessible in
experiments.Comment: 24 pages + 23 figures (available by fax or conventional mail, upon
request) tfp-1994-1
Aharonov-Bohm Interferometry with Interacting Quantum Dots: Spin Configurations, Asymmetric Interference Patterns, Bias-Voltage-Induced Aharonov-Bohm Oscillations, and Symmetries of Transport Coefficients
We study electron transport through multiply-connected mesoscopic geometries
containing interacting quantum dots. Our formulation covers both equilibrium
and non-equilibrium physics. We discuss the relation of coherent transport
channels through the quantum dot to flux-sensitive Aharonov-Bohm oscillations
in the total conductance of the device. Contributions to transport in first and
second order in the intrinsic line width of the dot levels are addressed in
detail. We predict an interaction-induced asymmetry in the amplitude of the
interference signal around resonance peaks as a consequence of incoherence
associated with spin-flip processes. This asymmetry can be used to probe the
total spin of the quantum dot. Such a probe requires less stringent
experimental conditions than the Kondo effect, which provides the same
information. We show that first-order contributions can be partially or even
fully coherent. This contrasts with the sequential-tunneling picture, which
describes first-order transport as a sequence of incoherent tunneling
processes. We predict bias-voltage induced Aharonov-Bohm oscillations of
physical quantities which are independent of flux in the linear-response
regime. Going beyond the Onsager relations we analyze the relations between the
space symmetry group of the setup and the flux-dependent non-linear
conductance.Comment: 22 pages, 11 figure
Topological Defects in the Random-Field XY Model and the Pinned Vortex Lattice to Vortex Glass Transition in Type-II Superconductors
As a simplified model of randomly pinned vortex lattices or charge-density
waves, we study the random-field XY model on square () and simple cubic
() lattices. We verify in Monte Carlo simulations, that the average
spacing between topological defects (vortices) diverges more strongly than the
Imry-Ma pinning length as the random field strength, , is reduced. We
suggest that for the simulation data are consistent with a topological
phase transition at a nonzero critical field, , to a pinned phase that is
defect-free at large length-scales. We also discuss the connection between the
possible existence of this phase transition in the random-field XY model and
the magnetic field driven transition from pinned vortex lattice to vortex glass
in weakly disordered type-II superconductors.Comment: LATEX file; 5 Postscript figures are available from [email protected]