370 research outputs found
The Josephson light-emitting diode
We consider an optical quantum dot where an electron level and a hole level
are coupled to respective superconducting leads. We find that electrons and
holes recombine producing photons at discrete energies as well as a continuous
tail. Further, the spectral lines directly probe the induced superconducting
correlations on the dot. At energies close to the applied bias voltage eV, a
parameter range exists, where radiation proceeds in pairwise emission of
polarization correlated photons. At energies close to 2eV, emitted photons are
associated with Cooper pair transfer and are reminiscent of Josephson
radiation. We discuss how to probe the coherence of these photons in a SQUID
geometry via single photon interference.Comment: Main text: 4 pages, 4 figures, Supplementary material: 8 pages, 4
figure
Measurement of filling factor 5/2 quasiparticle interference: observation of charge e/4 and e/2 period oscillations
A standing problem in low dimensional electron systems is the nature of the
5/2 fractional quantum Hall state: its elementary excitations are a focus for
both elucidating the state's properties and as candidates in methods to perform
topological quantum computation. Interferometric devices may be employed to
manipulate and measure quantum Hall edge excitations. Here we use a small area
edge state interferometer designed to observe quasiparticle interference
effects. Oscillations consistent in detail with the Aharanov-Bohm effect are
observed for integer and fractional quantum Hall states (filling factors 2,
5/3, and 7/3) with periods corresponding to their respective charges and
magnetic field positions. With these as charge calibrations, at 5/2 filling
factor and at lowest temperatures periodic transmission through the device
consistent with quasiparticle charge e/4 is observed. The principal finding of
this work is that in addtion to these e/4 oscillations, periodic structures
corresponding to e/2 are also observed at 5/2 and at lowest temperatures.
Properties of the e/4 and e/2 oscillations are examined with the device
sensitivity sufficient to observe temperature evolution of the 5/2
quasiparticle interference. In the model of quasiparticle interference, this
presence of an effective e/2 period may empirically reflect an e/2
quasiparticle charge, or may reflect multiple passes of the e/4 quasiparticle
around the interferometer. These results are discussed within a picture of e/4
quasiparticle excitations potentially possessing non-Abelian statistics. These
studies demonstrate the capacity to perform interferometry on 5/2 excitations
and reveal properties important for understanding this state and its
excitations.Comment: version 3 contains additional data beyond version 2, 26 pages, 8
figures PNAS 081259910
Thermal spin transport and spin-orbit interaction in ferromagnetic/non-magnetic metals
In this article we extend the currently established diffusion theory of
spin-dependent electrical conduction by including spin-dependent
thermoelectricity and thermal transport. Using this theory, we propose new
experiments aimed at demonstrating novel effects such as the spin-Peltier
effect, the reciprocal of the recently demonstrated thermally driven spin
injection, as well as the magnetic heat valve. We use finite-element methods to
model specific devices in literature to demonstrate our theory. Spin-orbit
effects such as anomalous-Hall, -Nernst, anisotropic magnetoresistance and
spin-Hall are also included in this model
Maximum-entropy theory of steady-state quantum transport
We develop a theoretical framework for describing steady-state quantum transport phenomena, based on the general maximum-entropy principle of nonequilibrium statistical mechanics. The general form of the many-body density matrix is derived, which contains the invariant part of the current operator that guarantees the nonequilibrium and steady-state character of the ensemble. Several examples of the theory are given, demonstrating the relationship of the present treatment to the widely used scattering-state occupation schemes at the level of the self-consistent single-particle approximation. The latter schemes are shown not to maximize the entropy, except in certain limits
Magnetic order of Dy3+ and Fe3+ moments in antiferromagnetic DyFeO3 probed by spin Hall magnetoresistance and spin Seebeck effect
We report on spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE)
in single crystal of the rare-earth antiferromagnet DyFeO with a thin Pt
film contact. The angular shape and symmetry of the SMR at elevated
temperatures reflect the antiferromagnetic order of the Fe moments as
governed by the Zeeman energy, the magnetocrystalline anisotropy and the
Dzyaloshinskii-Moriya interaction. We interpret the observed linear dependence
of the signal on the magnetic field strength as evidence for field-induced
order of the Dy moments up to room temperature. At and below the Morin
temperature of 50K, the SMR monitors the spin-reorientation phase
transition of Fe spins. Below 23K, additional features emerge that
persist below 4K, the ordering temperature of the Dy magnetic
sublattice. We conclude that the combination of SMR and SSE is a simple and
efficient tool to study spin reorientation phase transitions and sublattice
magnetizations
Imaging Inter-Edge State Scattering Centers in the Quantum Hall Regime
We use an atomic force microscope tip as a local gate to study the scattering
between edge channels in a 2D electron gas in the quantum Hall regime. The
scattering is dominated by individual, microscopic scattering centers, which we
directly image here for the first time. The tip voltage dependence of the
scattering indicates that tunneling occurs through weak links and localized
states.Comment: 4 pages, 5 figure
Comparing conductance quantization in quantum wires and Quantum Hall systems
We propose a new calculation of the DC conductance of a 1-dimensional
electron system described by the Luttinger model. Our approach is based on the
ideas of Landauer and B\"{u}ttiker and on the methods of current algebra. We
analyse in detail the way in which the system can be coupled to external
reservoirs. This determines whether the conductance is renormalized or not. We
show that although a quantum wire and a Fractional Quantum Hall system are
described by the same effective theory, their coupling to external reservoirs
is different. As a consequence, the conductance in the wire is quantized in
integer units of per spin orientation whereas the Hall conductance
allows for fractional quantization.Comment: 3 pages, LaTe
Subgap anomaly and above-energy-gap structure in chains of diffusive SNS junctions
We present the results of low-temperature transport measurements on chains of
superconductor--normal-constriction--superconductor (SNS) junctions fabricated
on the basis of superconducting PtSi film. A comparative study of the
properties of the chains, consisting of 3 and 20 SNS junctions in series, and
single SNS junctions reveals essential distinctions in the behavior of the
current-voltage characteristics of the systems: (i) the gradual decrease of the
effective suppression voltage for the excess conductivity observed at zero bias
as the quantity of the SNS junctions increases, (ii) a rich fine structure on
the dependences dV/dI-V at dc bias voltages higher than the superconducting gap
and corresponding to some multiples of 2\Delta/e. A model to explain this
above-energy-gap structure based on energy relaxation of electron via
Cooper-pair-breaking in superconducting island connecting normal metal
electrods is proposed.Comment: RevTex, 5 pages, 4 figure
Voltage-probe-controlled breakdown of the quantum Hall effect
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
Localization fom conductance in few-channel disordered wires
We study localization in two- and three channel quasi-1D systems using
multichain tight-binding Anderson models with nearest-neighbour interchain
hopping. In the three chain case we discuss both the case of free- and that of
periodic boundary conditions between the chains. The finite disordered wires
are connected to ideal leads and the localization length is defined from the
Landauer conductance in terms of the transmission coefficients matrix. The
transmission- and reflection amplitudes in properly defined quantum channels
are obtained from S-matrices constructed from transfer matrices in Bloch wave
bases for the various quasi-1D systems. Our exact analytic expressions for
localization lengths for weak disorder reduce to the Thouless expression for 1D
systems in the limit of vanishing interchain hopping. For weak interchain
hopping the localization length decreases with respect to the 1D value in all
three cases. In the three-channel cases it increases with interchain hopping
over restricted domains of large hopping
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