38 research outputs found
Nanoscale tunnel field effect transistor based on a complex oxide lateral heterostructure
We demonstrate a tunnel field effect transistor based on a lateral
heterostructure patterned from an electron gas.
Charge is injected by tunneling from the /
contacts and the current through a narrow channel of insulating
is controlled via an electrostatic side gate. Drain-source
I/V-curves have been measured at low and elevated temperatures. The transistor
shows strong electric-field and temperature-dependent behaviour with a steep
sub-threshold slope %of up to as small as and a
transconductance as high as . A fully
consistent transport model for the drain-source tunneling reproduces the
measured steep sub-threshold slope.Comment: 20 pages, 6 figures, Supplementary material: 4 pages, 2 figure
Kohn Anomalies in Superconductors
I present the detailed behavior of phonon dispersion curves near momenta
which span the electronic Fermi sea in a superconductor. I demonstrate that an
anomaly, similar to the metallic Kohn anomaly, exists in a superconductor's
dispersion curves when the frequency of the phonon spanning the Fermi sea
exceeds twice the superconducting energy gap. This anomaly occurs at
approximately the same momentum but is {\it stronger} than the normal-state
Kohn anomaly. It also survives at finite temperature, unlike the metallic
anomaly. Determination of Fermi surface diameters from the location of these
anomalies, therefore, may be more successful in the superconducting phase than
in the normal state. However, the superconductor's anomaly fades rapidly with
increased phonon frequency and becomes unobservable when the phonon frequency
greatly exceeds the gap. This constraint makes these anomalies useful only in
high-temperature superconductors such as .Comment: 18 pages (revtex) + 11 figures (upon request), NSF-ITP-93-7
Influence of Gap Extrema on the Tunneling Conductance Near an Impurity in an Anisotropic Superconductor
Changes: figures added in postscript form, Eq. (7) and various typos
corrected. We examine the effect of an impurity on the nearby tunneling
conductance in an anisotropically-gapped superconductor. The variation of the
conductance has pronounced spatial dependence which depends strongly on the
Fermi surface location of gap extrema. In particular, different gap symmetries
produce profoundly different spatial features in the conductance. These effects
may be detectable with an STM measurement on the surface of a high-temperature
superconductor.Comment: 12 pages (revtex) + 3 figures (included - postscript), NSF-ITP-93-8
The wave-vector power spectrum of the local tunnelling density of states: ripples in a d-wave sea
A weak scattering potential imposed on a layer of a cuprate
superconductor modulates the local density of states . In recently
reported experimental studies scanning-tunneling maps of have
been Fourier transformed to obtain a wave-vector power spectrum. Here, for the
case of a weak scattering potential, we discuss the structure of this power
spectrum and its relationship to the quasi-particle spectrum and the structure
factor of the scattering potential. Examples of quasi-particle interferences in
normal metals and - and d-wave superconductors are discussed.Comment: 22 pages, 21 figures; enlarged discussion of the d-wave response, to
be published in Physical Review
Spin injection and electric field effect in degenerate semiconductors
We analyze spin-transport in semiconductors in the regime characterized by
(intermediate to degenerate), where is the Fermi
temperature. Such a regime is of great importance since it includes the lightly
doped semiconductor structures used in most experiments; we demonstrate that,
at the same time, it corresponds to the regime in which carrier-carrier
interactions assume a relevant role. Starting from a general formulation of the
drift-diffusion equations, which includes many-body correlation effects, we
perform detailed calculations of the spin injection characteristics of various
heterostructures, and analyze the combined effects of carrier density
variation, applied electric field and Coulomb interaction. We show the
existence of a degenerate regime, peculiar to semiconductors, which strongly
differs, as spin-transport is concerned, from the degenerate regime of metals.Comment: Version accepted for publication in Phys. Rev.
Electric-field dependent spin diffusion and spin injection into semiconductors
We derive a drift-diffusion equation for spin polarization in semiconductors
by consistently taking into account electric-field effects and nondegenerate
electron statistics. We identify a high-field diffusive regime which has no
analogue in metals. In this regime there are two distinct spin diffusion
lengths. Furthermore, spin injection from a ferromagnetic metal into a
semiconductor is enhanced by several orders of magnitude and spins can be
transported over distances much greater than the low-field spin diffusion
length.Comment: 5 pages, 3 eps figure
Method for Measuring the Momentum-Dependent Relative Phase of the Superconducting Gap of High-Temperature Superconductors
The phase variation of the superconducting gap over the (normal) Fermi
surface of the high-temperature superconductors remains a significant
unresolved question. Is the phase of the gap constant, does it change sign, or
is it perhaps complex? A detailed answer to this question would provide
important constraints on various pairing mechanisms. Here we propose a new
method for measuring the relative gap PHASE on the Fermi surface which is
direct, is angle-resolved, and probes the bulk. The required experiments
involve measuring phonon linewidths in the normal and superconducting state,
with resolution available in current facilities. We primarily address the
La_1.85Sr_.15CuO_4 material, but also propose a more detailed study of a
specific phonon in Bi_2Sr_2CaCu_2O_8.Comment: 13 pages (revtex) + 5 figures (postscript-included), NSF-ITP-93-2
Quantum interference between non-magnetic impurities in d_x2-y2-wave superconductors
We study quantum interference of electronic waves that are scattered by
multiple non-magnetic impurities in a d_x2-y2-wave superconductor. We show that
the number of resonance states in the density-of-states (DOS), as well as their
frequency and spatial dependence change significantly as the distance between
the impurities or their orientation relative to the crystal lattice is varied.
Since the latter effect arises from the momentum dependence of the
superconducting gap, we argue that quantum interference is a novel tool to
identify the symmetry of unconventional superconductors.Comment: 4 pages, 4 figure