5 research outputs found
Spin relaxation of "upstream" electrons in quantum wires: Failure of the drift diffusion model
The classical drift diffusion (DD) model of spin transport treats spin
relaxation via an empirical parameter known as the ``spin diffusion length''.
According to this model, the ensemble averaged spin of electrons drifting and
diffusing in a solid decays exponentially with distance due to spin dephasing
interactions. The characteristic length scale associated with this decay is the
spin diffusion length. The DD model also predicts that this length is different
for ``upstream'' electrons traveling in a decelerating electric field than for
``downstream'' electrons traveling in an accelerating field. However this
picture ignores energy quantization in confined systems (e.g. quantum wires)
and therefore fails to capture the non-trivial influence of subband structure
on spin relaxation. Here we highlight this influence by simulating upstream
spin transport in a multi-subband quantum wire, in the presence of
D'yakonov-Perel' spin relaxation, using a semi-classical model that accounts
for the subband structure rigorously.
We find that upstream spin transport has a complex dynamics that defies the
simplistic definition of a ``spin diffusion length''.
In fact, spin does not decay exponentially or even monotonically with
distance, and the drift diffusion picture fails to explain the qualitative
behavior, let alone predict quantitative features accurately. Unrelated to spin
transport, we also find that upstream electrons undergo a ``population
inversion'' as a consequence of the energy dependence of the density of states
in a quasi one-dimensional structure.Comment: 13 figures. To appear in Phys. Rev.
The inequality of charge and spin diffusion coefficients
Since spin and charge are both carried by electrons (or holes) in a solid, it is natural to assume that charge and spin diffusion coefficients will be the same. Drift-diffusion models of spin transport typically assume so. Here, we show analytically that the two diffusion coefficients can be vastly different in quantum wires. Although we do not consider quantum wells or bulk systems, it is likely that the two coefficients will be different in those systems as well. Thus, it is important to distinguish between them in transportmodels, particularly those applied to quantum wire based devices
Electron Spin for Classical Information Processing: A Brief Survey of Spin-Based Logic Devices, Gates and Circuits
In electronics, information has been traditionally stored, processed and
communicated using an electron's charge. This paradigm is increasingly turning
out to be energy-inefficient, because movement of charge within an
information-processing device invariably causes current flow and an associated
dissipation. Replacing charge with the "spin" of an electron to encode
information may eliminate much of this dissipation and lead to more
energy-efficient "green electronics". This realization has spurred significant
research in spintronic devices and circuits where spin either directly acts as
the physical variable for hosting information or augments the role of charge.
In this review article, we discuss and elucidate some of these ideas, and
highlight their strengths and weaknesses. Many of them can potentially reduce
energy dissipation significantly, but unfortunately are error-prone and
unreliable. Moreover, there are serious obstacles to their technological
implementation that may be difficult to overcome in the near term.
This review addresses three constructs: (1) single devices or binary switches
that can be constituents of Boolean logic gates for digital information
processing, (2) complete gates that are capable of performing specific Boolean
logic operations, and (3) combinational circuits or architectures (equivalent
to many gates working in unison) that are capable of performing universal
computation.Comment: Topical Revie
Chlorosulfonic Acid Stretched Carbon Nanotube Sheet for Flexible and Low-Voltage Heating Applications
The carbon nanotube (CNT) is celebrated for its electrothermal property, which indicates the capability of a material to transform electrical energy into heat due to the Joule effect. The CNT nanostructure itself, as a one-dimensional material, limits the electron conduction path, thereby creating a unique heating phenomenon. In this work, we explore the possible correlation between CNT alignment in sheets and heating performance. The alignment of carbon nanotubes is induced by immersion and stretching in chlorosulfonic acid (CSA) solution. The developed CSA-stretched CNT sheet demonstrated excellent heating performance with a fast response rate of 6.5 °C/s and reached 180 °C in less than 30 s under a low voltage of 2.5 V. The heating profile of the stretched CNT sheet remained stable after bending and twisting movements, making it a suitable heating material for wearable devices, heatable smart windows, and in de-icing or defogging applications. The specific strength and specific conductance of the CSA-stretched CNT sheet also increased five- and two-fold, respectively, in comparison to the pristine CNT sheet