We investigate the thermoelectric transport properties of the half-filled lowest Landau level v=1/2 in a gated two-dimensional hole system in a strained Ge/SiGe heterostructure. The electron-diffusion dominated regime is achieved below 600 mK, where the diffusion thermopower Sxxd at v=1/2 shows a linear temperature dependence. In contrast, the diffusion-dominated Nernst signal Sxyd of v=1/2 is found to approach zero, which is independent of the measurement configuration (sweeping magnetic field at a fixed hole density or sweeping the density by a gate at a fixed magnetic field).
The linear energy-momentum dispersion which arises from graphene’s underlying honeycomb lattice gives graphene its unique electronic properties unfound in conventional semiconductors. Theoretically speaking, when an electrostatic potential with hexagonal or honeycomb symmetry is imposed onto a two-dimensional electron/hole system, the band structure is modified in a way that the same linear energy-momentum dispersion could exist. Experimentally, there has not been any evidence from transport demonstrating the so-called “artificial graphene”. In this project, we attempt to create an artificial superlattice potential with hexagonal symmetry for two dimensional carriers in an undoped SiGe heterostructure by patterning a nanoscale hole array in a metallic gate. Using undoped heterostructures allows us to access a very wide density range, which covers the magic densities at which the Dirac points are expected. A process flow for fabricating such field-effect-transistor devices with a lattice constant as small as 90 nm is reported. Magneto-transport measurements performed at 0.3 K show that the superlattice potential in the quantum well in which the two-dimensional system resides is indeed modulated by the gates. However, no signature of the sought-after linear dispersion is observed in the transport data.
Here we present the development of the building blocks of a Josephson parametric amplifier (JPA), namely the superconducting quantum interference device (SQUID) and the inductive pick-up coil that permits current coupling from a quantum dot into the SQUID. We also discuss our efforts in making depletion mode quantum dots using delta doped GaAs quantum wells. Because quantum dot based spin qubits utilize very low-level (~10 - 100pA), short duration (1ms - 1μs) current signals for state preparation and readout, these systems require close proximity cryogenic amplification to prevent signal corruption. Common amplification methods in these semiconductor quantum dots rely on heterojunction bipolar transistors (HBTs) and high electron mobility transistors (HEMTs) to amplify the readout signal from a single qubit. The state of the art for HBTs and HEMTs produce approximately 10µW of power when operating at high bandwidths. For few-qubit systems this level of heat dissipation is acceptable. However, for scaling up the number of qubits to several hundred or a thousand, the heat load produced in a 1 to 1 amplifier to qubit arrangement would overload the cooling capacity of a common dilution refrigerator, which typically has a cooling power of ~100µW at its base temperature. Josephson parametric amplifiers have been shown to dissipate ~1pW of power with current sensitivies on par with HBTs and HEMTs and with bandwidths 30 times that of HBTs and HEMTs, making them attractive for multi-qubit platforms. In this report we describe in detail the fabrication process flow for developing inductive pick-up coils and the fabrication and measurement of NbTiN and A1/A1Ox/A1 SQUIDs.
Quantum materials have long promised to revolutionize everything from energy transmission (high temperature superconductors) to both quantum and classical information systems (topological materials). However, their discovery and application has proceeded in an Edisonian fashion due to both an incomplete theoretical understanding and the difficulty of growing and purifying new materials. This project leverages Sandia's unique atomic precision advanced manufacturing (APAM) capability to design small-scale tunable arrays (designer materials) made of donors in silicon. Their low-energy electronic behavior can mimic quantum materials, and can be tuned by changing the fabrication parameters for the array, thereby enabling the discovery of materials systems which can't yet be synthesized. In this report, we detail three key advances we have made towards development of designer quantum materials. First are advances both in APAM technique and underlying mechanisms required to realize high-yielding donor arrays. Second is the first-ever observation of distinct phases in this material system, manifest in disordered 2D sheets of donors. Finally are advances in modeling the electronic structure of donor clusters and regular structures incorporating them, critical to understanding whether an array is expected to show interesting physics. Combined, these establish the baseline knowledge required to manifest the strongly-correlated phases of the Mott-Hubbard model in donor arrays, the first step to deploying APAM donor arrays as analogues of quantum materials.
We present the fabrication of nano-magnet arrays, comprised of two sets of interleaving SmCo5 and Co nano-magnets, and the subsequent development and implementation of a protocol to program the array to create a one-dimensional rotating magnetic field. We designed the array based on the microstructural and magnetic properties of SmCo5 films annealed under different conditions, also presented here. Leveraging the extremely high contrast in coercivity between SmCo5 and Co, we applied a sequence of external magnetic fields to program the nano-magnet arrays into a configuration with alternating polarization, which based on simulations creates a rotating magnetic field in the vicinity of nano-magnets. Our proof-of-concept demonstration shows that complex, nanoscale magnetic fields can be synthesized through coercivity contrast of constituent magnetic materials and carefully designed sequences of programming magnetic fields.