4,463 research outputs found
Mesoscopics in Spintronics: Quantum Interference Effects in Spin-Polarized Electron Transport
We generalize a Landauer-type formula, using a realspin-space Green
function technique, to treat spin-dependent transport in quantum-coherent
conductors attached to two ferromagnetic contacts. The formalism is employed to
study the properties of components of an exact zero-temperature conductance
matrix , as well as their mesoscopic fluctuations, describing
injection and detection of a spin-polarized current in a two-dimensional system
where electrons exhibit an interplay between Rashba spin-orbit (SO) coupling
and phase-coherent propagation through a disordered medium. Strong Rashba
coupling leads to a dramatic reduction of localization effects on the
conductances and their fluctuations, whose features depend on the
spin-polarization of injected electrons. In the limit of weak Rashba
interaction antilocalization vanishes (i.e., the sum of the matrix elements of
is almost independent of the SO coupling), but the partial
spin-resolved conductances can still be non-zero. Besides spin-resolved
conductance fluctuations and antilocalization, unusual quantum interference
effects are revealed in this system leading to a negative difference between
the partial conductances for a parallel and an antiparallel orientation of the
contact magnetization, in a range of disorder strengths and for a particular
spin-polarization of incoming electron with respect to the direction of Rashba
electric field.Comment: 12 pages, 13 embedded EPS figures, substantially enlarged version
with some new results and calculational detail
Negative differential resistance in graphene-nanoribbon/carbon-nanotube crossbars: A first-principles multiterminal quantum transport study
We simulate quantum transport between a graphene nanoribbon (GNR) and a
single-walled carbon nanotube (CNT) where electrons traverse vacuum gap between
them. The GNR covers CNT over a nanoscale region while their relative rotation
is 90 degrees, thereby forming a four-terminal crossbar where the bias voltage
is applied between CNT and GNR terminals. The CNT and GNR are chosen as either
semiconducting (s) or metallic (m) based on whether their two-terminal
conductance exhibits a gap as a function of the Fermi energy or not,
respectively. We find nonlinear current-voltage (I-V) characteristics in all
three investigated devices---mGNR-sCNT, sGNR-sCNT and mGNR-mCNT
crossbars---which are asymmetric with respect to changing the bias voltage from
positive to negative. Furthermore, the I-V characteristics of mGNR-sCNT
crossbar exhibits negative differential resistance (NDR) with low onset voltage
V and peak-to-valley current ratio .
The overlap region of the crossbars contains only carbon and
hydrogen atoms which paves the way for nanoelectronic devices ultrascaled well
below the smallest horizontal length scale envisioned by the international
technology roadmap for semiconductors. Our analysis is based on the
nonequilibrium Green function formalism combined with density functional theory
(NEGF-DFT), where we also provide an overview of recent extensions of NEGF-DFT
framework (originally developed for two-terminal devices) to multiterminal
devices.Comment: PDFLaTeX, 17 pages, 6 color figures; prepared for the special issue
of the Journal of Computational Electronics on "Multiscale and Multiphysics
Modeling of Nanostructures and Devices
How to construct the proper gauge-invariant density matrix in steady-state nonequilibrium: Applications to spin-transfer and spin-orbit torques
Experiments observing spin density and spin currents (responsible for, e.g.,
spin-transfer torque) in spintronic devices measure only the nonequilibrium
contributions to these quantities, typically driven by injecting unpolarized
charge current or by applying external time-dependent fields. On the other
hand, theoretical approaches to calculate them operate with both the
nonequilibrium (carried by electrons around the Fermi surface) and the
equilibrium (carried by the Fermi sea electrons) contributions. Thus, an
unambiguous procedure should remove the equilibrium contributions, thereby
rendering the nonequilibrium ones which are measurable and satisfy the
gauge-invariant condition according to which expectation values of physical
quantities should not change when electric potential everywhere is shifted by a
constant amount. Using the framework of nonequilibrium Green functions, we
delineate such procedure which yields the proper gauge-invariant nonequilibrium
density matrix in the linear-response and elastic transport regime for
current-carrying steady state of an open quantum system connected to two
macroscopic reservoirs. Its usage is illustrated by computing: (i) conventional
spin-transfer torque (STT) in asymmetric F/I/F magnetic tunnel junctions
(MTJs); (ii) unconventional STT in asymmetric N/I/F semi-MTJs with the strong
Rashba spin-orbit coupling (SOC) at the I/F interface and injected current
perpendicular to that plane; and (iii) current-driven spin density within a
clean ferromagnetic Rashba spin-split two-dimensional electron gas (2DEG) which
generates SO torque in laterally patterned N/F/I heterostructures when such
2DEG is located at the N/F interface and injected charge current flows parallel
to the plane.Comment: 18 pages, 5 color EPS figures; mini-review prepared for SPIN (World
Scientific); typos corrected and references added in v
Spin-memory loss due to spin-orbit coupling at ferromagnet/heavy-metal interfaces: Ab initio spin-density matrix approach
Spin-memory loss (SML) of electrons traversing
ferromagnetic-metal/heavy-metal (FM/HM), FM/normal-metal (FM/NM) and HM/NM
interfaces is a fundamental phenomenon that must be invoked to explain
consistently large number of spintronic experiments. However, its strength
extracted by fitting experimental data to phenomenological semiclassical
theory, which replaces each interface by a fictitious bulk diffusive layer, is
poorly understood from a microscopic quantum framework and/or materials
properties. Here we describe an ensemble of flowing spin quantum states using
spin-density matrix, so that SML is measured like any decoherence process by
the decay of its off-diagonal elements or, equivalently, by the reduction of
the magnitude of polarization vector. By combining this framework with density
functional theory (DFT), we examine how all three components of the
polarization vector change at Co/Ta, Co/Pt, Co/Cu, Pt/Cu and Pt/Au interfaces
embedded within Cu/FM/HM/Cu vertical heterostructures. In addition, we use ab
initio Green's functions to compute spectral functions and spin textures over
FM, HM and NM monolayers around these interfaces which quantify interfacial
spin-orbit coupling and explain the microscopic origin of SML in long-standing
puzzles, such as why it is nonzero at Co/Cu interface; why it is very large at
Pt/Cu interface; and why it occurs even in the absence of disorder, intermixing
and magnons at the interface.Comment: 6 pages, 4 figures, PDFLaTeX; published versio
Controlling Decoherence of Transported Quantum Spin Information in Semiconductor Spintronics
We investigate quantum coherence of electron spin transported through a
semiconductor spintronic device, where spins are envisaged to be controlled by
electrical means via spin-orbit interactions. To quantify the degree of spin
coherence, which can be diminished by an intrinsic mechanism where spin and
orbital degrees of freedom become entangled in the course of transport
involving spin-orbit interaction and scattering, we study the decay of the
off-diagonal elements of the spin density matrix extracted directly from the
Landauer transmission matrix of quantum transport. This technique is applied to
understand how to preserve quantum interference effects of fragile
superpositions of spin states in ballistic and non-ballistic multichannel
semiconductor spintronic devices.Comment: 7 pages, 3 color EPS figures, prepared for Proceedings of
International Symposium on Mesoscopic Superconductivity and Spintronics 2004
(Atsugi, Japan, March 1-4, 2004
Time-retarded damping and magnetic inertia in the Landau-Lifshitz-Gilbert equation self-consistently coupled to electronic time-dependent nonequilibrium Green functions
The conventional Landau-Lifshitz-Gilbert (LLG) equation is a widely used tool
to describe dynamics of local magnetic moments, viewed as classical vectors of
fixed length, with their change assumed to take place simultaneously with the
cause. Here we demonstrate that recently developed [M. D. Petrovi\'{c} {\em et
al.}, {\tt arXiv:1802.05682}] self-consistent coupling of the LLG equation to
time-dependent quantum-mechanical description of electrons microscopically
generates time-retarded damping in the LLG equation described by a memory
kernel which is also spatially dependent. For sufficiently slow dynamics of
local magnetic moments, the memory kernel can be expanded to extract the
Gilbert damping (proportional to first time derivative of magnetization) and
magnetic inertia (proportional to second time derivative of magnetization)
terms whose parameters, however, are time-dependent in contrast to
time-independent parameters used in the conventional LLG equation. We use
examples of single or multiple magnetic moments precessing in an external
magnetic field, as well as field-driven motion of a magnetic domain wall (DW),
to quantify the difference in their time evolution computed from conventional
LLG equation vs. TDNEGF+LLG quantum-classical hybrid approach. The faster DW
motion predicted by TDNEGF+LLG approach reveals that important quantum effects,
stemming from finite amount of time which it takes for conduction electron spin
to react to the motion of classical local magnetic moments, are missing from
conventional classical micromagnetics simulations. We also demonstrate large
discrepancy between TDNEGF+LLG-computed numerically exact and, therefore,
nonperturbative result for charge current pumped by a moving DW and the same
quantity computed by perturbative spin motive force formula combined with the
conventional LLG equation.Comment: 12 pages; PDFLaTe
Edge currents and nanopore arrays in zigzag and chiral graphene nanoribbons as a route toward high- thermoelectrics
We analyze electronic and phononic quantum transport through zigzag or chiral
graphene nanoribbons (GNRs) perforated with an array of nanopores. Since local
charge current profiles in these GNRs are peaked around their edges, drilling
nanopores in their interior does not affect such edge charge currents while
drastically reducing heat current carried by phonons in sufficiently long
wires. The combination of these two effects can yield highly efficient
thermoelectric devices with maximum at liquid nitrogen
temperature and at room temperature achieved in m
long zigzag GNRs with nanopores of variable diameter and spacing between them.
Our analysis is based on the -orbital tight-binding Hamiltonian with up to
third nearest-neighbor hopping for electronic subsystem, the empirical
fourth-nearest-neighbor model for phononic subsystem, and nonequilibrium Green
function formalism to study quantum transport in both of these models.Comment: 5 pages, 5 figures, PDFLaTe
Reduction of Josephson critical current in short ballistic SNS weak links
We present fully self-consistent calculations of the thermodynamic properties
of three-dimensional clean SNS Josephson junctions, where S is an s-wave
short-coherence-length superconductor and N is a clean normal metal. The
junction is modeled on an infinite cubic lattice such that the transverse width
of the S is the same as that of the N, and its thickness is tuned from the
short to long limit. Both the reduced order parameter near the SN boundary and
the short coherence length depress the critical Josephson current , even
in short junctions. This is contrasted with recent measurements on SNS
junctions finding much smaller products than expected from the
standard (non-self consistent and quasiclassical) predictions. We also find
unusual current-phase relations, a ``phase anti-dipole'' spatial distribution
of the self-consistently determined contribution to the macroscopic phase, and
an ``unexpected'' minigap in the local density of states within the N region.Comment: 5 pages, 4 embedded EPS figure
Equilibrium Properties of Double-Screened-Dipole-Barrier SINIS Josephson Junctions
We report on a self-consistent microscopic study of the DC Josephson effect
in junctions where screened dipole layers at the interfaces
generate a double-barrier multilayered structure. Our approach starts
from a microscopic Hamiltonian defined on a simple cubic lattice, with an
attractive Hubbard term accounting for the short coherence length
superconducting order in the semi-infinite leads, and a spatially extended
charge distribution (screened dipole layer) induced by the difference in Fermi
energies of the superconductor and the clean normal metal interlayer .
By employing the temperature Green function technique, in a continued fraction
representation, the influence of such spatially inhomogeneous barriers on the
proximity effect, current-phase relation, critical supercurrent and normal
state junction resistance, is investigated for different normal interlayer
thicknesses and barrier heights. These results are of relevance for high-
grain boundary junctions, and also reveal one of the mechanisms that can lead
to low critical currents of apparently ballistic junctions while
increasing its normal state resistance in a much weaker fashion. When the
region is a doped semiconductor, we find a substantial change in the dipole
layer (generated by a small Fermi level mismatch) upon crossing the
superconducting critical temperature, which is a new signature of proximity
effect and might be related to recent Raman studies in Nb/InAs bilayers.Comment: 15 pages, 15 EPS embedded figure
Suppression of the "quasiclassical" proximity gap in correlated-metal--superconductor structures
We study the energy and spatial dependence of the local density of states in
a superconductor--correlated-metal--superconductor Josephson junction, where
the correlated metal is a non-Fermi liquid (described by the Falicov-Kimball
model). Many-body correlations are treated with dynamical mean-field theory,
extended to inhomogeneous systems. While quasiclassical theories predict a
minigap in the spectrum of a disordered Fermi liquid which is proximity-coupled
within a mesoscopic junction, we find that increasing electron correlations
destroy any minigap that might be opened in the absence of many-body
correlations.Comment: 5 pages, 3 embedded EPS figures; some issues clarified with new
result presented in the inset of Fig.
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