arXiv subjects

George Nichols

Publications and source records attributed to George Nichols.

10 recordsLinked to original sources

arXiv2026

USEQIP: Outcomes and experiences from 17 years of undergraduate summer schools in experimental quantum information science

To grow the quantum information science and technology workforce, opportunities for students to gain experiential learning and build a sense of belonging in the broader community are essential. The Undergraduate School on Experimental Quantum Information Processing (USEQIP) is a two-week summer school for undergraduate students that has been held since 2009 with the goal of introducing undergraduate students from around the world to the tools of quantum information research, paired with a summer internship program. Here we report on the structure, impact, and outlook of the program, including hands-on laboratory activities refined over many iterations of the program. We highlight the career trajectories of program alumni, many of whom have made significant contributions to the quantum field.

physics.ed-ph↗

arXiv2025

Control and Transient Spectroscopy of Engineered Spin-Cavity Back-Action

We present an experimental arrangement that permits engineering of cavity back-action on a mesoscopic spin ensemble. By coupling a superconducting thin-film Nb microstrip resonator to a Trityl OX63 electron spin sample, we access different regimes of spin-cavity dynamics by designing the ensemble size, effective coupling strength, cavity temperature, and spin saturation. We performed transient spectroscopy measurements under continuous microwave drive in the strong radiation damping regime. These measurements exhibit a long-lived plateau response that distinguishes important features of spin-cavity models, such as the radiation damping Bloch equations and Maxwell-Bloch equations. We demonstrate control of the plateau response through adjustment of temperature, microwave drive power, and variable spin saturation. The presented experimental arrangement serves as a robust system to explore the space of spin-cavity dynamics and develop new quantum devices that harness the complexity of mesoscopic spin ensembles coherently interacting with high quality factor cavities.

quant-ph↗

arXiv2024

High transparency induced superconductivity in field effect two-dimensional electron gases in undoped InAs/AlGaSb surface quantum wells

We report on transport characteristics of field effect two-dimensional electron gases (2DEG) in 24 nm wide indium arsenide surface quantum wells. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus are observed to filling factor $\nu=2$ in magnetic fields of up to B = 18 T, at electron densities up to 8$\times 10^{11}$ /cm$^2$. Peak mobility is 11,000 cm$^2$/Vs at 2$\times 10^{12}$ /cm$^2$. Large Rashba spin-orbit coefficients up to 124 meV$\cdot$\r{A} are obtained through weak anti-localization (WAL) measurements. Proximitized superconductivity is demonstrated in Nb-based superconductor-normal-superconductor (SNS) junctions, yielding 78$-$99% interface transparencies from superconducting contacts fabricated ex-situ (post-growth), using two commonly-used experimental techniques for measuring transparencies. These transparencies are on a par with those reported for epitaxially-grown superconductors. These SNS junctions show characteristic voltages $I_c R_{\text{N}}$ up to 870 $\mu$V and critical current densities up to 9.6 $\mu$A/$\mu$m, among the largest values reported for Nb-InAs SNS devices.

cond-mat.mes-hall↗

arXiv2024

Observation of an Abrupt 3D-2D Morphological Transition in Thin Al Layers Grown by MBE on InGaAs surface

Among superconductor/semiconductor hybrid structures, in-situ aluminum (Al) grown on InGaAs/InAs is widely pursued for the experimental realization of Majorana Zero Mode quasiparticles. This is due to the high carrier mobility, low effective mass, and large Landé g-factor of InAs, coupled with the relatively high value of the in-plane critical magnetic field in thin Al films. However, growing a thin, continuous Al layer using the Molecular Beam Epitaxy (MBE) is challenging due to aluminum's high surface mobility and tendency for 3D nucleation on semiconductor surfaces. A study of epitaxial Al thin film growth on In0.75Ga0.25As with MBE is presented, focusing on the effects of the Al growth rate and substrate temperature on the nucleation of Al layers. We find that for low deposition rates, 0.1 Å/s and 0.5 Å/s, the growth continues in 3D mode during the deposition of the nominal 100 Å of Al, resulting in isolated Al islands. However, for growth rates of 1.5 Å/s and above, the 3D growth mode quickly transitions into island coalescence, leading to a uniform 2D Al layer. Moreover, this transition is very abrupt, happening over an Al flux increase of less than 1%. We discuss the growth mechanisms explaining these observations. The results give new insights into the kinetics of Al deposition and show that with sufficiently high Al flux, a 2D growth on substrates at close to room temperature can be achieved already within the first few Al monolayers. This eliminates the need for complex cryogenic substrate cooling and paves the way for the development of high-quality superconductor-semiconductor interfaces in standard MBE systems.

cond-mat.mtrl-sci↗

arXiv2023

Field effect two-dimensional electron gases in modulation-doped InSb surface quantum wells

We report on transport characteristics of field effect two-dimensional electron gases (2DEG) in surface indium antimonide quantum wells. The topmost 5 nm of the 30 nm wide quantum well is doped and shown to promote the formation of reliable, low resistance Ohmic contacts to surface InSb 2DEGs. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus are observed to filling factor $ν=1$ in magnetic fields of up to $B=18$ T. We show that the electron density is gate-tunable, reproducible, and stable from pinch-off to 4$\times 10^{11}$ cm$^{-2}$, and peak mobilities exceed 24,000 cm$^2$/Vs. Large Rashba spin-orbit coefficients up to 110 meV$\cdot$Å are obtained through weak anti-localization measurements. An effective mass of 0.019$m_e$ is determined from temperature-dependent magnetoresistance measurements, and a g-factor of 41 at a density of 3.6$\times 10^{11}$ cm$^{-2}$ is obtained from coincidence measurements in tilted magnetic fields. By comparing two heterostructures with and without a delta-doped layer beneath the quantum well, we find that the carrier density is stable with time when doping in the ternary Al$_{0.1}$In$_{0.9}$Sb barrier is not present. Finally, the effect of modulation doping on structural asymmetry between the two heterostructures is characterized.

cond-mat.mes-hall↗

arXiv2019

Solution to the Gliding Tone Problem

The solution is given to the classical problem of an oscillator driven by a sinusoid of steadily-varying frequency. A closed analytical expression is obtained in the case where the Q-factor of the oscillator is high, equivalent to the rotating wave approximation of atomic physics. In this case all independent variables of the system combine into a single parameter. The results are compared with previous work: series and other approximations, numerical calculations, graphical solutions and analogue simulations. Attention is paid to the distortion of the resonance -- specifically frequency shift and amplitude attenuation -- consequent upon the finite sweep rate. The frequency shift is interpreted as a delay in the appearance of the resonance peak; to leading order this time delay is twice the oscillator's ring-down time. Measurements on a high-Q oscillator are consistent with the mathematical solution.

physics.ins-det↗

arXiv2019

Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides

We conduct a comprehensive study of three different magnetic semiconductors, CrI$_3$, CrBr$_3$, and CrCl$_3$, by incorporating both few- and bi-layer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, as well as magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr$_3$, with transition temperature still close to that of the bulk.

cond-mat.mtrl-sci↗

arXiv2018

Magnetic Field Dependent Microwave Losses in Superconducting Niobium Microstrip Resonators

We describe an experimental protocol to characterize magnetic field dependent microwave losses in superconducting niobium microstrip resonators. Our approach provides a unified view that covers two well-known magnetic field dependent loss mechanisms: quasiparticle generation and vortex motion. We find that quasiparticle generation is the dominant loss mechanism for parallel magnetic fields. For perpendicular fields, the dominant loss mechanism is vortex motion or switches from quasiparticle generation to vortex motion, depending on cooling procedures. In particular, we introduce a plot of the quality factor versus the resonance frequency as a general method for identifying the dominant loss mechanism. We calculate the expected resonance frequency and the quality factor as a function of the magnetic field by modeling the complex resistivity. Key parameters characterizing microwave loss are estimated from comparisons of the observed and expected resonator properties. Based on these key parameters, we find a niobium resonator whose thickness is similar to its penetration depth is the best choice for X-band electron spin resonance applications. Finally, we detect partial release of the Meissner current at the vortex penetration field, suggesting that the interaction between vortices and the Meissner current near the edges is essential to understand the magnetic field dependence of the resonator properties.

cond-mat.supr-con↗

arXiv2018

Origin of magnetoresistance suppression in thin $γ$-MoTe$_2$

We use both classical magnetotransport and quantum oscillation measurements to study the thickness evolution of the extremely large magnetoresistance (XMR) material and type-II Weyl semimetal candidate, $γ$-MoTe$_2$, protected from oxidation. We find that the magnetoresistance is systematically suppressed with reduced thickness. This occurs concomitantly with both a decrease in carrier mobility and increase in electron-hole imbalance. We model the two effects separately and conclude that the XMR effect is more sensitive to the former.

cond-mat.mtrl-sci↗

arXiv2014

Frequency Dependence of the Supersolid Signature in Polycrystalline 4He

We report studies, using a two mode torsional oscillator, of the putative supersolid signature in polycrystalline 4He. Measurements at two frequencies enable us to eliminate the viscoelastic contribution to the signature, and other instrumental effects arising from the temperature dependent shear modulus of the sample. The complex response function of the sample, encoded via its effective moment of inertia, shows an unexpected and unexplained frequency dependence. This cannot be accounted for by glassy dynamics within the sample. The results do not rule out the possibility of supersolidity in bulk solid 4He. ?

cond-mat.other↗