Transport behavior and weak adhesion of quantum confined epitaxial Bi and Bi1-xSbx films

Novel devices, such as those based on principles of spin and magnetization instead of traditional electronic transport, necessitate the development of new materials systems. Bismuth (Bi) is a promising materials systems for these applications due to its high mobility, large spin-orbit interaction and metallic surface states which exhibit Rashba spin-splitting. In addition, Bi exhibits a thickness dependent band gap due to quantum size effects occurring at uniquely long length scales (>100nm) due to its large de Broglie wavelength. This allows the metallic surface states to be isolated from the bulk in the band gap. Alloying Bi with antimony (Sb) to create Bi1-xSbx allows access to topologically non-trivial surface states which have the aforementioned qualities associated with Bi, as well as added protection against backscattering and non-magnetic perturbation. In addition, the non-trivial topology of Bi1-xSbx makes it a contender for topological quantum computing devices using Majorana fermions and braided states. In this thesis, we explore the transport properties of Bi and Bi1-xSbx as the film thicknesses and alloy compositions are varied, providing a basis for custom design of these materials for specific applications. In addition, we report advances in the understanding of Bi and Bi1-xSbx growth on Si (111) through the measurement of their weak adhesion to the Si (111) substrate as well as the identification of (012) oriented crystal growth of Bi1-xSbx and the effect of this crystal structure transition on the transport properties of the films. Finally, we report the development of a dry transfer method which can be used to transfer high quality epitaxial Bi and Bi1-xSbx films to arbitrary substrates which may facilitate their integration into novel spin-based devicesElectrical and Computer Engineerin

Quantum confinement of coherent acoustic phonons in transferred single-crystalline bismuth nanofilms

Coherent acoustic phonon dynamics in single-crystalline bismuth nanofilms transferred to a glass substrate were investigated with ultrafast pump–probe spectroscopy. Coherent phonon signals were substantially enhanced by more than four times when compared with as-grown films on Si (111) substrates. Furthermore, more than 10% reduction of the acoustic phonon velocity was observed when the film thickness decreases to 22 nm, which is attributed to the modified phonon dispersion in extremely thin films from quantum confinement effects.The authors acknowledge support from the National Science Foundation (NASCENT, Grant No. EEC-1160494; CAREER, Grant No. CBET-1351881; No. CBET-1707080; and Center for Dynamics and Control of Materials No. DMR-1720595).Center for Dynamics and Control of Material

Learning-based Calibration of Flux Crosstalk in Transmon Qubit Arrays

Superconducting quantum processors comprising flux-tunable data and coupler qubits are a promising platform for quantum computation. However, magnetic flux crosstalk between the flux-control lines and the constituent qubits impedes precision control of qubit frequencies, presenting a challenge to scaling this platform. In order to implement high-fidelity digital and analog quantum operations, one must characterize the flux crosstalk and compensate for it. In this work, we introduce a learning-based calibration protocol and demonstrate its experimental performance by calibrating an array of 16 flux-tunable transmon qubits. To demonstrate the extensibility of our protocol, we simulate the crosstalk matrix learning procedure for larger arrays of transmon qubits. We observe an empirically linear scaling with system size, while maintaining a median qubit frequency error below $300$ kHz

Spin-Orbit Torque and Nernst Effect in Bi-Sb/Co Heterostructures

Harmonic measurements of the longitudinal and transverse voltages in Bi-Sb/Co bilayers are presented. A large second harmonic voltage signal due to the ordinary Nernst effect is observed. In experiments where a magnetic field is rotated in the film plane, the ordinary Nernst effect shows the same angular dependence in the transverse voltage as the damping-like spin-orbit torque and in the longitudinal voltage as the unidirectional spin-Hall magneto-resistance respectively. Therefore, the ordinary Nernst effect can be a spurious signal in spin-orbit torque measurements, leading to an overestimation of the spin-Hall angle in topological insulators or semimetals.Research was sup- ported by the U.S. Department of Energy under Contract No. DE-AC02-05-CH11231 within the NEMM program (KC2204) and NSF Materials Research Science and En- gineering Center Grant No. DMR-1720595. Device fabri- cation was supported by NSF E3S center at Berkeley and STARNET/LEAST, one of the six centers of the JUMP initiative jointly supported by DARPA/SRC.Center for Dynamics and Control of Material

Hexagonal boron nitride as a low-loss dielectric for superconducting quantum circuits and qubits

Dielectrics with low loss at microwave frequencies are imperative for high-coherence solid-state quantum computing platforms. We study the dielectric loss of hexagonal boron nitride (hBN) thin films in the microwave regime by measuring the quality factor of parallel-plate capacitors (PPCs) made of NbSe$_{2}$-hBN-NbSe$_{2}$ heterostructures integrated into superconducting circuits. The extracted microwave loss tangent of hBN is bounded to be at most in the mid-10$^{-6}$ range in the low temperature, single-photon regime. We integrate hBN PPCs with aluminum Josephson junctions to realize transmon qubits with coherence times reaching 25 $\mu$s, consistent with the hBN loss tangent inferred from resonator measurements. The hBN PPC reduces the qubit feature size by approximately two-orders of magnitude compared to conventional all-aluminum coplanar transmons. Our results establish hBN as a promising dielectric for building high-coherence quantum circuits with substantially reduced footprint and, with a high energy participation that helps to reduce unwanted qubit cross-talk