Motivated by observations of extreme magnetoresistance (XMR) in bulk crystals of rare-earth monopnictide (RE-V) compounds and emerging applications in novel spintronic and plasmonic devices based on thin-film semimetals, we have investigated the electronic band structure and transport behavior of epitaxial GdSb thin films grown on III-V semiconductor surfaces. The Gd3+ ion in GdSb has a high spin S=7/2 and no orbital angular momentum, serving as a model system for studying the effects of antiferromagnetic order and strong exchange coupling on the resulting Fermi surface and magnetotransport properties of RE-Vs. We present a surface and structural characterization study mapping the optimal synthesis window of thin epitaxial GdSb films grown on III-V lattice-matched buffer layers via molecular beam epitaxy. To determine the factors limiting XMR in RE-V thin films and provide a benchmark for band structure predictions of topological phases of RE-Vs, the electronic band structure of GdSb thin films is studied, comparing carrier densities extracted from magnetotransport, angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. ARPES shows hole-carrier rich topologically-trivial semi-metallic band structure close to complete electron-hole compensation, with quantum confinement effects in the thin films observed through the presence of quantum well states. DFT predicted Fermi wavevectors are in excellent agreement with values obtained from quantum oscillations observed in magnetic field-dependent resistivity measurements. An electron-rich Hall coefficient is measured despite the higher hole carrier density, attributed to the higher electron Hall mobility. The carrier mobilities are limited by surface and interface scattering, resulting in lower magnetoresistance than that measured for bulk crystals.
Topological materials often exhibit remarkably linear, non-saturating magnetoresistance (LMR), which is both of scientific and technological importance. However, the role of topologically non-trivial states in the emergence of such a behaviour has been difficult to establish in experiments. Here, we show how strong interaction between the topological surface states (TSS) with a positive g-factor and the bulk carriers can lead to a smearing of the Landau levels giving rise to an LMR behavior in a semi-metallic Heusler compound. The role of TSS is established by controllably reducing the surface-bulk coupling by a combination of substitution alloying and the application of high magnetic field, when the LMR behavior transmutes into a quantum Hall phase arising from the TSS. Our work establishes that small changes in the coupling strength between the surface and the bulk carriers can have a profound impact on the magnetotransport behavior in topological materials. In the process, we lay out a strategy to both reveal and manipulate the exotic properties of TSS in compounds with a semi-metallic bulk band structure, as is the case in multi-functional Heusler compounds.
The use of current-generated spin-orbit torques[1] to drive magnetization dynamics is under investigation to enable a new generation of non-volatile, low-power magnetic memory. Previous research has focused on spin-orbit torques generated by heavy metals[2-8], interfaces with strong Rashba interactions[9,10] and topological insulators [11-14]. These families of materials can all be well-described using models with noninteracting-electron bandstructures. Here, we show that electronic interactions within a strongly correlated heavy fermion material, the Kondo lattice system YbAl$_{3}$, can provide a large enhancement in spin-orbit torque. The spin-torque conductivity increases by approximately a factor of 4 as a function of decreasing temperature from room temperature to the coherence temperature of YbAl$_{3}$ ($T^* \approx 37$ K), with a saturation at lower temperatures, achieving a maximum value greater than any heavy metal element. This temperature dependence mimics the increase and saturation at $T^*$ of the density of states at the Fermi level arising from the ytterbium 4$f$-derived heavy bands in the Kondo regime, as measured by angle-resolved photoemission spectroscopy[15]. We therefore identify the many-body Kondo resonance as the source of the large enhancement of spin-orbit torque in YbAl$_{3}$. Our observation reveals new opportunities in spin-orbit torque manipulation of magnetic memories by engineering quantum many-body states.
Topological materials often exhibit remarkably linear, non-saturating magnetoresistance (LMR), which is both of scientific and technological importance. However, the role of topologically non-trivial states in the emergence of such a behaviour has eluded clear demonstration in experiments. Here, by reducing the coupling between the topological surface states (TSS) and the bulk carriers we controllably tune the LMR behavior in Pt1-xAuxLuSb into distinct plateaus in Hall resistance, which we show arise from a quantum Hall phase. This allowed us to reveal how smearing of the Landau levels, which otherwise give rise to a quantum Hall phase, results in an LMR behavior due to strong interaction between the TSS with a positive g-factor and the bulk carriers. We establish that controlling the coupling strength between the surface and the bulk carriers in topological materials can bring about dramatic changes in their magnetotransport behavior. In addition, our work outlines a strategy to reveal macroscopic physical observables of TSS in compounds with a semi-metallic bulk band structure, as is the case in multi-functional Heusler compounds, thereby opening up opportunities for their utilization in hybrid quantum structures.
We present a Vehicle Model (VM) that has 17 degrees of freedom and includes nonlinear tire and powertrain subsystems. Implemented as a relatively small piece of C++ code, the model runs vehicle dynamics 2000 times faster than real time at a simulation time step of $1 \times 10^{-3}, \text {s}$ on a single core of a commodity CPU. When executed on the GPU, one can perform 300000 vehicle simulations in real-time. These properties make the model a good candidate for reinforcement learning, model predictive control, model predictive path integral control, path planning, state estimation, and traffic simulation tasks. The model is expressive in that it can capture the dynamics of vastly different vehicles. This is demonstrated by first calibrating the model to mimic the dynamics of a 1/ $6^{th}$ scale vehicle called the Autonomy Research Testbed (ART) vehicle, which has a mass of approximately 5.8 kg. Subsequently, the model is calibrated to mimic the dynamics of a heavy-duty High Mobility Multipurpose Wheeled Vehicle (HMMWV), which has a mass of 2097 kg. The Bayesian calibration process discussed can $(i)$ handle real-life measurement noise, and $(ii)$ provide model parameter probability distributions. The vehicle model, which is open source and freely available in a public repository, can also be imported into Python owing to SWIG wrapping.
We present high-resolution angle-resolved photoemission spectra of the heavy-fermion superconductor URu2Si2. Detailed measurements as a function of both photon energy and temperature allow us to disentangle a variety of spectral features, revealing the evolution of the low-energy electronic structure across the "hidden order" transition. Above the transition, our measurements reveal the existence of weakly dispersive states that exhibit a large scattering rate and do not appear to shift from above to below the Fermi level, as previously reported. Upon entering the hidden order phase, these states rapidly hybridize with light conduction band states and transform into a coherent heavy fermion liquid, coincident with a dramatic drop in the scattering rate. This evolution is in stark contrast with the gradual crossover expected in Kondo lattice systems, which we attribute to the coupling of the heavy fermion states to the hidden order parameter.
