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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Santa Barbara Electronic Theses and Dissertations (2019)

Theoretically proposed by Ettore Majorana in 1937, Majorana fermions are a unique class of particles which are their own anti-particles. This concept is realized in Majorana Zero Modes (MZMs), which are quasi-particles bound to zero energy, with no measurable charge and mass. Arising out of topological states of matter, signatures of MZMs were first experimentally observed in 2012.

These quasi-particles are predicted to exhibit non-abelian braiding statistics, allowing them to “remember” whether they were moved clockwise or counterclockwise around each other, forming a braid in time and position. Being their own anti-particles, fusion or annihilation of a pair of MZMs is expected to lead to a different outcome based on how they were braided, making a pair of MZMs the simplest quantum bit or ‘qubit’, forming the basis of Topological Quantum Computation.

As MZMs are indistinguishable from each other and quantum information is encoded in the exchange of MZMs, it is protected from environmental perturbations (noise), referred to as topological protection. Computation based on these qubits is predicted to be fault tolerant and scalable.

This work focusses on the device heterostructure design, Molecular Beam Epitaxy (MBE) growth and low temperature electrical characterization of superconductor-semiconductor hybrid systems hosting MZMs. Low dimensional electron systems (2D quantum wells, 1D nanowires) in semiconductors with strong spin-orbit interaction (e.g. InAs, InAsxSb1-x and InSb), transparently coupled to a superconductor (e.g. Aluminum), have been investigated.

With an emphasis on improving electron mobility, the first demonstration of an InSb quantum well on InSb substrate, as part of this work, showed a record quantum mobility of 50,000 cm2/Vs. Top and bottom gate control of InSb quantum wells on GaSb substrates has also been demonstrated. MBE of InAs and InAsSb nanowires has also been studied, with demonstration of induced superconductivity in InAsSb nanowires. Recent work focused on MBE of near surface InAsSb quantum wells showing gate-controlled depletion and observation of quantized conductance through a Quantum Point Contact.

Lastly, in-vacuo growth of Aluminum partial shells on InSb nanowires, led to the first observation of quantized Majorana conductance, consistent with predictions. While further proof is necessary, these 1D and 2D hybrid systems, coupled with in-situ selective area evaporation of dielectrics, pave the way for realization of complex topological networks and subsequent demonstration of a fault tolerant topological qubit.

    Cover page: Molecular Beam Epitaxy of Low Dimensional Electron Systems for Topological Quantum Computation
    • Article
    • Peer Reviewed
    UC Santa Barbara Previously Published Works (2017)
    The principal challenge for achieving reconfigurable optical antennas and metasurfaces is the need to generate continuous and large tunability of subwavelength, low-Q resonators. We demonstrate continuous and steady-state refractive index tuning at mid-infrared wavelengths using temperature-dependent control over the low-loss plasma frequency in III-V semiconductors. In doped InSb we demonstrate nearly two-fold increase in the electron effective mass leading to a positive refractive index shift (Δn > 1.5) that is an order of magnitude greater than conventional thermo-optic effects. In undoped films we demonstrate more than 10-fold change in the thermal free-carrier concentration producing a near-unity negative refractive index shift. Exploiting both effects within a single resonator system-intrinsic InSb wires on a heavily doped (epsilon-near-zero) InSb substrate-we demonstrate dynamically steady-state tunable Mie resonances. The observed line-width resonance shifts (Δλ > 1.7 μm) suggest new avenues for highly tunable and steady-state mid-infrared semiconductor antennas.Achieving large tunability of subwavelength resonators is a central challenge in nanophotonics. Here the authors demonstrate refractive index tuning at mid-infrared wavelengths using temperature-dependent control over the low loss plasma frequency in III-V semiconductors.
      Cover page: Ultrawide thermal free-carrier tuning of dielectric antennas coupled to epsilon-near-zero substrates
      • Article
      • Peer Reviewed
      LBL Publications (2022)
      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 a hole-carrier rich, topologically trivial, semimetallic 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 wave vectors 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.
        Cover page: Epitaxial growth, magnetoresistance, and electronic band structure of GdSb magnetic semimetal filmsCreative Commons 'BY-NC' version 4.0 license
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