394 research outputs found
Dark states in the magnetotransport through triple quantum dots
We consider the transport through a system of three coupled quantum dots in a
perpendicular magnetic field. At zero field, destructive interference can trap
an electron in a dark state -- a coherent superposition of dot states that
completely blocks current flow. The magnetic field can disrupt this
interference giving rise to oscillations in the current and its higher-order
statistics as the field is increased. These oscillations have a period of
either the flux-quantum or half the flux-quantum, depending on the dot
geometry. We give results for the stationary current and for the shotnoise and
skewness at zero and finite frequency.Comment: 7 pages, 7 figure
Influence of e-e scattering on the temperature dependence of the resistance of a classical ballistic point contact in a two-dimensional electron system
We experimentally investigate the temperature (T) dependence of the
resistance of a classical ballistic point contact (PC) in a two-dimensional
electron system (2DES). The split-gate PC is realized in a high-quality
AlGaAs/GaAs heterostructure. The PC resistance is found to drop by more than
10% as T is raised from 0.5 K to 4.2 K. In the absence of a magnetic field, the
T dependence is roughly linear below 2 K and tends to saturate at higher T.
Perpendicular magnetic fields on the order of a few 10 mT suppress the
T-dependent contribution dR. This effect is more pronounced at lower
temperatures, causing a crossover to a nearly parabolic T dependence in a
magnetic field. The normalized magnetic field dependencies dR(B) permit an
empiric single parameter scaling in a wide range of PC gate voltages. These
observations give strong evidence for the influence of electron-electron (e-e)
scattering on the resistance of ballistic PCs. Our results are in qualitative
agreement with a recent theory of the e-e scattering based T dependence of the
conductance of classical ballistic PCs [ Phys. Rev. Lett. 101 216807 (2008) and
Phys. Rev. B 81 125316 (2010)].Comment: as publishe
Calculations of spin induced transport in ferromagnets
Based on first-principles density functional calculations, a general approach
for determining and analyzing the degree of spin polarization (P) in
ferromagnets is presented. The approach employs the so-called tetrahedron
method to evaluate the Fermi surface integrations of P in both ballistic and
diffusive regimes. The validity of the method is examined by comparing the
calculated P values for Fe and Ni with the experiment. The method is shown to
yield highly accurate results with minimal computational effort. Within our
approach, it is also possible to systematically analyze the contributions of
various types of electronic states to the spin induced transport. As a case
study, the transport properties of the soft-ferromagnet CeMnNi4 are
investigated in order to explain the origin of the existing difference between
the experimental and theoretical values of P in this intermetallic compound.Comment: 6 pages, 4 figures; to appear in Physical Review B 75 (2007
Tunneling of Bloch electrons through vacuum barrier
Tunneling of Bloch electrons through a vacuum barrier introduces new physical
effects in comparison with the textbook case of free (plane wave) electrons.
For the latter, the exponential decay rate in the vacuum is minimal for
electrons with the parallel component of momentum , and
the prefactor is defined by the electron momentum component in the normal to
the surface direction. However, the decay rate of Bloch electrons may be
minimal at an arbitrary (``hot spots''), and the prefactor
is determined by the electron's group velocity, rather than by its
quasimomentum.Comment: 4 pages, no fig
Zero-bias anomalies in electrochemically fabricated nanojunctions
A streamlined technique for the electrochemical fabrication of metal
nanojunctions (MNJs) between lithographically defined electrodes is presented.
The first low-temperature transport measurements in such structures reveal
suppression of the conductance near zero-bias. The size of the zero-bias
anomaly (ZBA) depends strongly on the fabrication electrochemistry and the
dimensions of the resulting MNJ. We present evidence that the nonperturbative
ZBA in atomic-scale junctions is due to a density of states suppression in the
leads.Comment: 4 pages, 4 figure
Anomalous Aharonov-Bohm conductance oscillations from topological insulator surface states
We study transport properties of a topological insulator nanowire when a
magnetic field is applied along its length. We predict that with strong surface
disorder, a characteristic signature of the band topology is revealed in
Aharonov Bohm (AB) oscillations of the conductance. These oscillations have a
component with anomalous period , and with conductance maxima at
odd multiples of , i.e. when the AB phase for surface electrons
is . This is intimately connected to the band topology and a surface
curvature induced Berry phase, special to topological insulator surfaces. We
discuss similarities and differences from recent experiments on BiSe
nanoribbons, and optimal conditions for observing this effect.Comment: 7 pages, 2 figure
Nonlinear magnetoconductance of a classical ballistic system
We study nonlinear transport through a classical ballistic system accounting
for the Coulomb interaction between electrons. The joint effect of the applied
bias and magnetic field on the electron trajectories results in a
component of the non-linear current which lacks the
symmetry: . At zero temperature the magnitude of
is of the same order as that arising from the quantum
interference mechanism. At higher temperatures the classical mechanism is
expected to dominate due to its relatively weak temperature dependence.Comment: 5 pages, 1 figur
Dynamical mean field theory for strongly correlated inhomogeneous multilayered nanostructures
Dynamical mean field theory is employed to calculate the properties of
multilayered inhomogeneous devices composed of semi-infinite metallic lead
layers coupled via barrier planes that are made from a strongly correlated
material (and can be tuned through the metal-insulator Mott transition). We
find that the Friedel oscillations in the metallic leads are immediately frozen
in and don't change as the thickness of the barrier increases from one to
eighty planes. We also identify a generalization of the Thouless energy that
describes the crossover from tunneling to incoherent Ohmic transport in the
insulating barrier. We qualitatively compare the results of these
self-consistent many-body calculations with the assumptions of
non-self-consistent Landauer-based approaches to shed light on when such
approaches are likely to yield good results for the transport.Comment: 15 pages, 12 figures, submitted to Phys. Rev.
Interaction of a CO molecule with a Pt monatomic wire: electronic structure and ballistic conductance
We carry out a first-principles density functional study of the interaction
between a monatomic Pt wire and a CO molecule, comparing the energy of
different adsorption configurations (bridge, on top, substitutional, and tilted
bridge) and discussing the effects of spin-orbit (SO) coupling on the
electronic structure and on the ballistic conductance of two of these systems
(bridge and substitutional). We find that, when the wire is unstrained, the
bridge configuration is energetically favored, while the substitutional
geometry becomes possible only after the breaking of the Pt-Pt bond next to CO.
The interaction can be described by a donation/back-donation process similar to
that occurring when CO adsorbs on transition-metal surfaces, a picture which
remains valid also in presence of SO coupling. The ballistic conductance of the
(tipless) nanowire is not much reduced by the adsorption of the molecule on the
bridge and on-top sites, but shows a significant drop in the substitutional
case. The differences in the electronic structure due to the SO coupling
influence the transmission only at energies far away from the Fermi level so
that fully- and scalar-relativistic conductances do not differ significantly.Comment: 12 pages, 12 figures; figure misplacement and minor syntax issues
fixed, some references updated and correcte
Spin-polarized electron transport in ferromagnet/semiconductor heterostructures: Unification of ballistic and diffusive transport
A theory of spin-polarized electron transport in ferromagnet/semiconductor
heterostructures, based on a unified semiclassical description of ballistic and
diffusive transport in semiconductor structures, is developed. The aim is to
provide a framework for studying the interplay of spin relaxation and transport
mechanism in spintronic devices. A key element of the unified description of
transport inside a (nondegenerate) semiconductor is the thermoballistic current
consisting of electrons which move ballistically in the electric field arising
from internal and external electrostatic potentials, and which are thermalized
at randomly distributed equilibration points. The ballistic component in the
unified description gives rise to discontinuities in the chemical potential at
the boundaries of the semiconductor, which are related to the Sharvin interface
conductance. By allowing spin relaxation to occur during the ballistic motion
between the equilibration points, a thermoballistic spin-polarized current and
density are constructed in terms of a spin transport function. An integral
equation for this function is derived for arbitrary values of the momentum and
spin relaxation lengths. For field-driven transport in a homogeneous
semiconductor, the integral equation can be converted into a second-order
differential equation that generalizes the standard spin drift-diffusion
equation. The spin polarization in ferromagnet/semiconductor heterostructures
is obtained by invoking continuity of the current spin polarization and
matching the spin-resolved chemical potentials on the ferromagnet sides of the
interfaces. Allowance is made for spin-selective interface resistances.
Examples are considered which illustrate the effects of transport mechanism and
electric field.Comment: 23 pages, 8 figures, REVTEX 4; minor corrections introduced; to
appear in Phys. Rev.
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