Resume

• 2001

  University of California

  San Diego

  Boston College

  University of Toronto

  Los Alamos Laboratory

  Associate Professor

  Boston College

  University of California

  San Diego

  PhD

  Physics

• 2000

  UCSA Politics Committee

  Los Alamos Laboratory

  UCSA Politics Committee

  Boston College

  Chestnut Hill

  MA

  Assistant Professor

  University of Toronto

  UCSA Politics Committee

• 1997

  BS

  Physics

• 1991

  St. Lukes

  Friends Seminary

• Experimentation

  AFM

  Physics

  Nanotechnology

  Scanning Electron Microscopy

  Superconductors

  Materials Science

  Graphene

  Spectroscopy

  Experimental Design

  Optical Spectroscopy

  Optics

  Characterization

  Nanomaterials

  Nanofabrication

  Raman Microscopy

  Stability of exfoliated Bi2Sr2DyxCa1−xCu2O8+δ studied by Raman microscopy

  Nearly nanometer-thick cuprates are an appealing platform for devices as well as exploring the roles of dimensionality

  disorder

  and free carrier density in these compounds. To this end we have produced exfoliated crystals of Bi2Sr2DyxCa1−xCu2O8+δ on oxidized silicon substrates. The exfoliated crystals were characterized via atomic force and polarized Raman microscopies. Proper procedures for production

  handling

  and monitoring of these thin oxides are described. We observe a significant change in the effective exchange constant J of these exfoliated crystals.

  Stability of exfoliated Bi2Sr2DyxCa1−xCu2O8+δ studied by Raman microscopy

  Near room temperature

  MnAs films align into two phases

  one ferromagnetic and the other paramagnetic. These phases take the intriguing form of nanoscale wires. We have exploited this phase coexistence to form linear arrays of magnetite nanoparticles without the need for photolithography. This is confirmed via extensive scanning probe microscopy.

  One-dimensional alignment of nanoparticles via magnetic sorting

  Interest in the superconducting proximity effect has been reinvigorated recently by novel optoelectronic applications as well as by the possible emergence of the elusive Majorana fermion at the interface between topological insulators and superconductors. Here we produce high-temperature superconductivity in Bi2Se3 and Bi2Te3 via proximity to Bi2Sr2CaCu2O8+δ

  to access higher temperature and energy scales for this phenomenon. This was achieved by a new mechanical bonding technique that we developed

  enabling the fabrication of high-quality junctions between materials

  unobtainable by conventional approaches. We observe proximity-induced superconductivity in Bi2Se3 and Bi2Te3 persisting up to at least 80 K—a temperature an order of magnitude higher than any previous observations. Moreover

  the induced superconducting gap in our devices reaches values of 10 mV

  significantly enhancing the relevant energy scales. Our results open new directions for fundamental studies in condensed matter physics and enable a wide range of applications in spintronics and quantum computing.

  Proximity-induced high-temperature superconductivity in the topological insulators Bi2Se3 and Bi2Te3

  Optical investigation of thermoelectric topological crystalline insulator Pb0.77Sn0.23Se

  Pb0.77Sn0.23Se is a promising thermoelectric alloy that exhibits a temperature dependent band inversion below 300 K. Recent work has shown that this band inversion also coincides with a trivial to nontrivial topological phase transition. To understand how the properties critical to thermoelectric efficiency are affected by the band inversion

  we measured the broadband optical response of Pb0.77Sn0.23Se as a function of temperature. We find clear optical signatures suggesting the band inversion occurs at 160±15 K

  and use the extended Drude model to accurately determine a T3/2 dependence of the bulk carrier lifetime

  associated with electron-acoustic phonon scattering. Due to the high bulk carrier doping level

  no discriminating signatures of the topological surface states are found

  although their presence cannot be excluded from our data.

  Optical investigation of thermoelectric topological crystalline insulator Pb0.77Sn0.23Se

  In the recently discovered class of materials known as topological insulators

  the presence of strong spin-orbit coupling causes certain topological invariants in the bulk to differ from their values in vacuum. The sudden change in invariants at the interface results in metallic

  time reversal invariant surface states whose properties are useful for applications in spintronics and quantum computation. However

  a key challenge is to fabricate these materials on the nanoscale appropriate for devices and probing the surface. To this end we have produced 2 nm thick nanocrystals of the topological insulator Bi2Se3 via mechanical exfoliation. For crystals thinner than 10 nm we observe the emergence of an additional mode in the Raman spectrum. The emergent mode intensity together with the other results presented here provide a recipe for production and thickness characterization of Bi2Se3 nanocrystals.

  Fabrication and characterization of topological insulator Bi2Se3 nanocrystals

  David D. Awschalom

  Dimitri N. Basov

  The band structure of a prototypical dilute magnetic semiconductor (DMS)

  Ga1-xMnxAs

  is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (EF) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [σ1(ω)] with carrier density are only consistent with EF lying in an IB. Furthermore

  the large effective mass (m*) of the carriers inferred from our analysis of σ1(ω) supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of Ga1-xMnxAs.

  Impurity band conduction in a high temperature ferromagnetic semiconductor

  In bismuth ferrite (BiFeO3)

  antiferromagnetic and ferroelectric order coexist at room temperature

  making it of particular interest for studying magnetoelectric coupling. The mutual control of magnetic and electric properties is very useful for a wide variety of applications. This has led to an enormous amount of research into the properties of BiFeO3. Nonetheless

  one of the most fundamental aspects of this material

  namely the symmetries of the lattice vibrations

  remains controversial. We present a comprehensive Raman study of BiFeO3 single crystals with the approach of monitoring the Raman spectra while rotating the polarization direction of the excitation laser. Our method results in unambiguous assignment of the phonon symmetries and explains the origin of the controversy in the literature. Furthermore

  it provides access to the Raman tensor elements enabling direct comparison with theoretical calculations. Hence

  this allows the study of symmetry breaking and coupling mechanisms in a wide range of complex materials and may lead to a noninvasive

  all-optical method to determine the orientation and magnitude of the ferroelectric polarization.

  Raman study of the phonon symmetries in BiFeO3 single crystals

  Burch