Formation of Artificial Fermi Surfaces with a Triangular Superlattice on a Conventional Two-Dimensional Electron Gas.

Imposing an external periodic electrostatic potential to the electrons confined in a quantum well makes it possible to engineer synthetic two-dimensional band structures, with electronic properties different from those in the host semiconductor. Here we report the fabrication and study of a tunable triangular artificial lattice on a GaAs/AlGaAs heterostructure where it is possible to transform from the original GaAs band structure and a circular Fermi surface to a new band structure with multiple artificial Fermi surfaces simply by altering a gate bias. For weak electrostatic modulation magnetotransport measurements reveal multiple quantum oscillations and commensurability oscillations due to the electron scattering from the artificial lattice. Increasing the strength of the modulation reveals new commensurability oscillations of the electrons from the artificial Fermi surface scattering from the triangular artificial lattice. These results show that low disorder gate-tunable lateral superlattices can be used to form artificial two-dimensional crystals with designer electronic properties.

Intra-Zero-Energy Landau Level Crossings in Bilayer Graphene at High Electric Fields.

The highly tunable band structure of the zero-energy Landau level (zLL) of bilayer graphene makes it an ideal platform for engineering novel quantum states. However, the zero-energy Landau level at high electric fields has remained largely unexplored. Here we present magnetotransport measurements of bilayer graphene in high transverse electric fields. We observe previously undetected Landau level crossings at filling factors ν = -2, 1, and 3 at high electric fields. These crossings provide constraints for theoretical models of the zero-energy Landau level and show that the orbital, valley, and spin character of the quantum Hall states at high electric fields is very different from low electric fields. At high E, new transitions between states at ν = -2 with different orbital and spin polarization can be controlled by the gate bias, while the transitions between ν = 0 → 1 and ν = 2 → 3 show anomalous behavior.

Silicon-Aluminum Phase-Transformation-Induced Superconducting Rings.

The development of devices that exhibit both superconducting and semiconducting properties is an important endeavor for emerging quantum technologies. We investigate superconducting nanowires fabricated on a silicon-on-insulator (SOI) platform. Aluminum from deposited contact electrodes is found to interdiffuse with Si along the entire length of the nanowire, over micrometer length scales and at temperatures well below the Al-Si eutectic. The phase-transformed material is conformal with the predefined device patterns. The superconducting properties of a transformed mesoscopic ring formed on a SOI platform are investigated. Low-temperature magnetoresistance oscillations, quantized in units of the fluxoid, h/2e, are observed.

Combining n-MOS Charge Sensing with p-MOS Silicon Hole Double Quantum Dots in a CMOS platform.

Holes in silicon quantum dots are receiving attention due to their potential as fast, tunable, and scalable qubits in semiconductor quantum circuits. Despite this, challenges remain in this material system including difficulties using charge sensing to determine the number of holes in a quantum dot, and in controlling the coupling between adjacent quantum dots. We address these problems by fabricating an ambipolar complementary metal-oxide-semiconductor (CMOS) device using multilayer palladium gates. The device consists of an electron charge sensor adjacent to a hole double quantum dot. We demonstrate control of the spin state via electric dipole spin resonance. We achieve smooth control of the interdot coupling rate over 1 order of magnitude and use the charge sensor to perform spin-to-charge conversion to measure the hole singlet-triplet relaxation time of 11 µs for a known hole occupation. These results provide a path toward improving the quality and controllability of hole spin-qubits.

Chester Supersolid of Spatially Indirect Excitons in Double-Layer Semiconductor Heterostructures.

A supersolid, a counterintuitive quantum state in which a rigid lattice of particles flows without resistance, has to date not been unambiguously realized. Here we reveal a supersolid ground state of excitons in a double-layer semiconductor heterostructure over a wide range of layer separations outside the focus of recent experiments. This supersolid conforms to the original Chester supersolid with one exciton per supersolid site, as distinct from the alternative version reported in cold-atom systems of a periodic density modulation or clustering of the superfluid. We provide the phase diagram augmented by the supersolid. This new phase appears at layer separations much smaller than the predicted exciton normal solid, and it persists up to a solid-solid transition where the quantum phase coherence collapses. The ranges of layer separations and exciton densities in our phase diagram are well within reach of the current experimental capabilities.

Pauli Spin Blockade of Heavy Holes in a Silicon Double Quantum Dot.

In this work, we study hole transport in a planar silicon metal-oxide-semiconductor based double quantum dot. We demonstrate Pauli spin blockade in the few hole regime and map the spin relaxation induced leakage current as a function of interdot level spacing and magnetic field. With varied interdot tunnel coupling, we can identify different dominant spin relaxation mechanisms. Application of a strong out-of-plane magnetic field causes an avoided singlet-triplet level crossing, from which the heavy hole g-factor ~0.93 and the strength of spin-orbit interaction ~110 µeV can be obtained. The demonstrated strong spin-orbit interaction of heavy holes promises fast local spin manipulation using only electric fields, which is of great interest for quantum information processing.

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 p-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.

Bottom-up assembly of metallic germanium.

Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material, a light emitting medium in silicon-integrated lasers, and a plasmonic conductor for bio-sensing. Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D). Here we use a bottom-up approach to demonstrate the 3D assembly of atomically sharp doping profiles in germanium by a repeated stacking of two-dimensional (2D) high-density phosphorus layers. This produces high-density (10(19) to 10(20) cm(-3)) low-resistivity (10(-4)Ω · cm) metallic germanium of precisely defined thickness, beyond the capabilities of diffusion-based doping technologies. We demonstrate that free electrons from distinct 2D dopant layers coalesce into a homogeneous 3D conductor using anisotropic quantum interference measurements, atom probe tomography, and density functional theory.