7 research outputs found
Observation of oscillating -factor anisotropy arising from strong crystal lattice anisotropy in GaAs spin-3/2 hole quantum point contacts
Many modern spin-based devices rely on the spin-orbit interaction, which is
highly sensitive to the host semiconductor heterostructure and varies
substantially depending on crystal direction, crystal asymmetry (Dresselhaus),
and quantum confinement asymmetry (Rashba). One-dimensional quantum point
contacts are a powerful tool to probe both energy and directional dependence of
spin-orbit interaction through the effect on the hole -factor. In this work
we investigate the role of cubic crystal asymmetry in driving an oscillation in
the in-plane hole -factor anisotropy when the quantum point contact is
rotated with respect to the crystal axes, and we are able to separate
contributions to the Zeeman Hamiltonian arising from Rashba and cubic crystal
asymmetry spin-orbit interactions. The in-plane -factor is found to be
extremely sensitive to the orientation of the quantum point contact, changing
by a factor of when rotated by . This exceptionally strong
crystal lattice anisotropy of the in-plane Zeeman splitting cannot be explained
within axially symmetric theoretical models. Theoretical modelling based on the
combined Luttinger, Rashba and Dresselhaus Hamiltonians that we use here
reveals new spin-orbit contributions to the in-plane hole -factor and
provides an excellent agreement with our experimental data.Comment: 8 pages, 4 figure
Overlapping-gate architecture for silicon Hall bar MOSFET devices in the low electron density and high magnetic field regime
A common issue in low temperature measurements of enhancement-mode
metal-oxide-semiconductor (MOS) field-effect transistors (FETs) in the low
electron density regime is the high contact resistance dominating the device
impedance. In that case a voltage bias applied across the source and drain
contact of a Hall bar MOSFET will mostly fall across the contacts (and not
across the channel) and therefore magneto-transport measurements become
challenging. However, from a physical point of view, the study of MOSFET
nanostructures in the low electron density regime is very interesting (impurity
limited mobility [1], carrier interactions [2,3] and spin-dependent transport
[4]) and it is therefore important to come up with solutions [5,6] that work
around the problem of a high contact resistance in such devices (c.f. Fig. 1
(a)).Comment: 3 page
Geometric control of universal hydrodynamic flow in a two dimensional electron fluid
Fluid dynamics is one of the cornerstones of modern physics and has recently
found applications in the transport of electrons in solids. In most solids
electron transport is dominated by extrinsic factors, such as sample geometry
and scattering from impurities. However in the hydrodynamic regime Coulomb
interactions transform the electron motion from independent particles to the
collective motion of a viscous `electron fluid'. The fluid viscosity is an
intrinsic property of the electron system, determined solely by the
electron-electron interactions. Resolving the universal intrinsic viscosity is
challenging, as it only affects the resistance through interactions with the
sample boundaries, whose roughness is not only unknown but also varies from
device to device. Here we eliminate all unknown parameters by fabricating
samples with smooth sidewalls to achieve the perfect slip boundary condition,
which has been elusive both in molecular fluids and electronic systems. We
engineer the device geometry to create viscous dissipation and reveal the true
intrinsic hydrodynamic properties of a 2D system. We observe a clear transition
from ballistic to hydrodynamic electron motion, driven by both temperature and
magnetic field. We directly measure the viscosity and electron-electron
scattering lifetime (the Fermi quasiparticle lifetime) over a wide temperature
range without fitting parameters, and show they have a strong dependence on
electron density that cannot be explained by conventional theories based on the
Random Phase Approximation
Ballistic transport in one-dimensional p-type GaAs devices
In this thesis we study GaAs one dimensional hole systems with strong spin-orbit interaction effects. The primary focus is the Zeeman splitting of 1D subbands in the two orthogonal in-plane magnetic field directions. We study two types of 1D hole systems based on different (311)A grown heterostructures: a modulation doped GaAs/AlGaAs square quantum well and an undoped induced GaAs/AlGaAs triangular quantum well. The results from the modulation doped 1D wire show enhanced anisotropy of the effective Lande g-factor for the two in-plane field directions (parallel and perpendicular to the wire), compared to that in 2D hole systems. This enhancement is explained by the confinement induced reorientation of the total angular momentum ^ J from perpendicular to the 2D plane to in-plane and parallel to the wire. We use the intrinsic anisotropy of the in-plane g-factors to probe the 0:7 structure and the zero bias anomaly in 1D hole wires. We find that the behaviour of the 0:7 structure and the ZBA are correlated and depend strongly on the orientation of the in-plane field. This result proves the connection between the 0:7 and the ZBA and their relation to spin. We fabricate the first induced hole 1D wire with extremely stable gate characteristics and characterize this device. We also fabricate devices with two orthogonal induced hole wires on one chip, to study the interplay between the confinement, crystallographic anisotropy and spin-orbit coupling and their effect on the Zeeman splitting. We find that the ratios of the g-factors in the two orthogonal field directions for the two wires show opposite behaviour. We compare absolute values of the g-factors relative to the magnetic field direction. For B || [011] the g-factor is large for the wire along [011] and small for the wire along [233]. Whereas for B || [233], the g-factors are large irrespective of the wire direction. The former result can be explained by reorientation of ^ J along the wire, and the latter by an additional off-diagonal Zeeman term, which leads to the out-of-plane component of ^ J when B || [233], and as a result, to enhanced g-factors via increased exchange interactions
Thickness-dependent electronic structure in WTe2 thin films
We study the electronic structure of WTe2 thin films with different thicknesses. High-quality thin-film samples are obtained with carrier mobility up to 5000 cm2 V−1 s−1, which enables us to resolve the four main Fermi pockets from Shubnikov-de Haas (SdH) oscillations. Angle-resolved SdH oscillations show that the WTe2 thin films cross from three-dimensional to two-dimensional electronic systems at a thickness of ∼ 20 nm. Using the field effect, the nature of the Fermi pockets in thin-film WTe2 is identified, and the evolution of SdH oscillation frequencies is traced over different sample thicknesses. It is found that the frequencies dramatically decrease at a thickness of approximately 12 nm, which indicates the onset of finite-size effects on the band structure. Our work pins down two critical length scales of the thickness-dependent electronic structure in WTe2 thin films
Spin–Orbit Interaction in a Two-Dimensional Hole Gas at the Surface of Hydrogenated Diamond
Hydrogenated diamond possesses a
unique surface conductivity as a result of transfer doping by surface
acceptors. Yet, despite being extensively studied for the past two
decades, little is known about the system at low temperature, particularly
whether a two-dimensional hole gas forms at the diamond surface. Here
we report that (100) diamond, when functionalized with hydrogen, supports
a <i>p</i>-type spin-3/2 two-dimensional surface conductivity
with a spin–orbit interaction of 9.74 ± 0.1 meV through
the observation of weak antilocalization effects in magneto-conductivity
measurements at low temperature. Fits to 2D localization theory yield
a spin relaxation length of 30 ± 1 nm and a spin-relaxation time
of ∼0.67 ± 0.02 ps. The existence of a 2D system with
spin orbit coupling at the surface of a wide band gap insulating material
has great potential for future applications in ferromagnet–semiconductor
and superconductor–semiconductor devices
Strong and Tunable Spin–Orbit Coupling in a Two-Dimensional Hole Gas in Ionic-Liquid Gated Diamond Devices
Hydrogen-terminated
diamond possesses due to transfer doping a
quasi-two-dimensional (2D) hole accumulation layer at the surface
with a strong, Rashba-type spin–orbit coupling that arises
from the highly asymmetric confinement potential. By modulating the
hole concentration and thus the potential using an electrostatic gate
with an ionic-liquid dielectric architecture the spin–orbit
splitting can be tuned from 4.6–24.5 meV with a concurrent
spin relaxation length of 33–16 nm and hole sheet densities
of up to 7.23 × 10<sup>13</sup> cm<sup>–2</sup>. This
demonstrates a spin–orbit interaction of unprecedented strength
and tunability for a 2D hole system at the surface of a wide band
gap semiconductor. With a spin relaxation length that is experimentally
accessible using existing nanofabrication techniques, this result
suggests that hydrogen-terminated diamond has great potential for
the study and application of spin transport phenomena