A. M. Gilbertson

Zero-field spin splitting and spin-dependent broadening in high-mobility InSb/In1−xAlxSb asymmetric quantum well heterostructures

We present high field magneto-transport data from a range of 30nm wide InSb/InAlSb quantum wells. The low temperature carrier mobility of the samples studied ranged from 18.4 to 39.5 m2V-1s-1 with carrier densities between 1.5x1015 and 3.28x1015 m-2. Room temperature mobilities are reported in excess of 6 m2V-1s-1. It is found that the Landau level broadening decreases with carrier density and beating patterns are observed in the magnetoresistance with non-zero node amplitudes in samples with the narrowest broadening despite the presence of a large g-factor. The beating is attributed to Rashba splitting phenomenon and Rashba coupling parameters are extracted from the difference in spin populations for a range of samples and gate biases. The influence of Landau level broadening and spin-dependent scattering rates on the observation of beating in the Shubnikov-de Haas oscillations is investigated by simulations of the magnetoconductance. Data with non-zero beat node amplitudes are accompanied by asymmetric peaks in the Fourier transform, which are successfully reproduced by introducing a spin-dependent broadening in the simulations. It is found that the low-energy (majority) spin up state suffers more scattering than the high-energy (minority) spin down state and that the absence of beating patterns in the majority of (lower density) samples can be attributed to the same effect when the magnitude of the level broadening is large.

Plasmon-Induced Optical Anisotropy in Hybrid Graphene–Metal Nanoparticle Systems

Hybrid plasmonic metal-graphene systems are emerging as a class of optical metamaterials that facilitate strong light-matter interactions and are of potential importance for hot carrier graphene-based light harvesting and active plasmonic applications. Here we use femtosecond pump-probe measurements to study the near-field interaction between graphene and plasmonic gold nanodisk resonators. By selectively probing the plasmon-induced hot carrier dynamics in samples with tailored graphene-gold interfaces, we show that plasmon-induced hot carrier generation in the graphene is dominated by direct photoexcitation with minimal contribution from charge transfer from the gold. The strong near-field interaction manifests as an unexpected and long-lived extrinsic optical anisotropy. The observations are explained by the action of highly localized plasmon-induced hot carriers in the graphene on the subresonant polarizability of the disk resonator. Because localized hot carrier generation in graphene can be exploited to drive electrical currents, plasmonic metal-graphene nanostructures present opportunities for novel hot carrier device concepts.

Experimental determination of the Rashba coefficient in InSb/InAlSb quantum wells at zero magnetic field and elevated temperatures

We report the optical measurement of the spin dynamics at elevated temperatures and in zero magnetic field for two types of degenerately doped n-InSb quantum wells (QWs), one asymmetric (sample A) and one symmetric (sample B) with regards to the electrostatic potential across the QW. Making use of three directly determined experimental parameters: the spin lifetime, τ(s), the sheet carrier concentration, n, and the electron mobility, μ, we directly extract the zero-field spin splitting. For the asymmetric sample where the Rashba interaction is the dominant source of spin splitting, we deduce a room temperature Rashba parameter of α = 0.09 ± 0.1 eV Å which is in good agreement with calculations and we estimate the Rashba coefficient α(0) (a figure of merit for the ease with which electron spins can be modulated via an electric field). We review the merits/limitations of this approach and the implications of our findings for spintronic devices.

A nanoscale Ti/GaAs metal-semiconductor hybrid sensor for room temperature light detection

We report an individually addressable Ti∕GaAs metal-semiconductor hybrid optical nanosensor with positive photoresistance and a sensitivity that increases as the device dimensions shrink. The underlying physics relates to the crossover from ballistic to diffusive transport of the photoinduced carriers and the geometric enhancement of the effect associated with a Schottky-barrier-coupled parallel metal shunt layer. For a 250 nm device under 633 nm illumination we observe a specific detectivity of D(*)=5.06×10(11) cm √Hz∕W with a dynamic response of 40 dB.

Room temperature ballistic transport in InSb quantum well nanodevices.

We report the room temperature observation of significant ballistic electron transport in shallow etched four-terminal mesoscopic devices fabricated on an InSb/AlInSb quantum well (QW) heterostructure with a crucial partitioned growth-buffer scheme. Ballistic electron transport is evidenced by a negative bend resistance signature which is quite clearly observed at 295 K and at current densities in excess of 10(6) A/cm(2). This demonstrates unequivocally that by using effective growth and processing strategies, room temperature ballistic effects can be exploited in InSb/AlInSb QWs at practical device dimensions.

Transport effects in remote-doped InSb/AlxIn1-xSb heterostructures

Low- and high-field magnetotransport measurements on two 30nm -doped InSb/AlInSb quantum wells (QWs) with different doping densities are reported. The QW two-dimensional electron gas (2DEG) carrier densities and mobilities were extracted by analysis of the Hall and quantum Hall data, mobility spectra and Shubnikov-de Haas oscillations. 2DEG channel mobilities of up to 324000cm 2 V 1 s 1 (T = 2K) and 44000cm 2 V 1 s 1 (T = 300K) are extracted. Carrier densities and mobilities for transport parallel to the 2DEG layer are also deduced where observable. The importance of thermally generated carriers in the lower AlInSb barrier material and the role of transport within the -doping plane is considered and the total carrier population as a function of temperature of the two samples is deduced, which is in excellent agreement with experimental observation.

Dimensional crossover and weak localization in a 90 nm n-GaAs thin film.

We report on the magnetotransport in a 90 nm thick n-type GaAs epitaxial thin film in the weak localization (WL) regime. Low temperature (T</=50 K) magnetotransport data are fit with WL theory, from which the phase coherence time, tau(varphi) proportional, variantT(-p) (p=1.22+/-0.01), are extracted. We conclude that the dominant dephasing mechanism at these temperatures is electron-electron (e-e) scattering in the Nyquist limit. Evidence of a crossover from two-dimensional to three-dimensional behavior with respect to both coherent transport (WL) and e-e interactions is observed in the temperature dependence of the zero-field conductivity and tau(varphi), respectively.

Sub-100-nm negative bend resistance ballistic sensors for high spatial resolution magnetic field detection

We report the magnetic field detection properties of ballistic sensors utilizing the negative bend resistance of InSb∕In(1-x)Al(x)Sb quantum well cross junctions as a function of temperature and geometric size. We demonstrate that the maximum responsivity to magnetic field and its linearity increase as the critical device dimension is reduced. This observation deviates from the predictions of the classical billiard ball model unless significant diffuse boundary scattering is included. The smallest device studied has an active sensor area of 35×35 nm(2), with a maximum responsivity of 20 kΩ∕T, and a noise-equivalent field of 0.87μT∕Hz at 100 K.

Evidence for nodal superconductivity in Sr2ScFePO3

Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature, dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7T at 2K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter framework which would be consistent with the recent theoretical model where the height of the P in the Fe-P-Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains.

Quantum well mobility and the effect of gate dielectrics in remote doped InSb/AlxIn1 ? xSb heterostructures

Low- and high-field magnetotransport measurements on 30 nm δ-doped InSb/AlInSb quantum wells with different doping densities are reported. Mobilities over the temperature range 2 to 290 K are described using the relaxation time approximation. Screening by electrons in the doping plane and the temperature variation of the Fermi wave vector and effective mass of the carriers are incorporated into the model. High quality, Shubnikov de Haas oscillations are observed in samples that exhibit single sub-band occupancy. However, higher density samples that show considerable parallel conductance with qualitatively poor ρxx(B) are shown to recover high quality Shubnikov de Haas oscillations by deposition of a surface gate with a SiO2 gate dielectric. We show that the incorporation of this gate dielectric significantly modifies the transport properties and results in an increased mobility over ungated structures with the same carrier density. These observations lead to further insight into the carrier scattering mechanisms present in these InSb/AlInSb structures.