Postdoctoral Opening

We are looking for new team members to work with our new Cleanroom in a Glovebox with a variety of possible projects: • Fabrication of novel biosensors with 2D materials • Emergent/Topological ...

Andreev Reflection without Fermi Surface Alignment

We address the controversy over the proximity effect between topological materials and high-T csuperconductors. Junctions are produced between Bi2Sr2CaCu2O and materials with different Fermi surfaces (Bi2Te3 and graphite). Both cases reveal tunneling ...

Understanding the evolution of anomalous anharmonicity in Bi2Te3−xSex

The anharmonic effect in thermoelectrics has been a central topic for decades in both condensed matter physics and material science. However, despite the long-believed strong and complex anharmonicity in the ...

Posted by Kenneth Burch On July - 17 - 2016 Comments Off on Magneto-elastic coupling in a potential ferromagnetic 2D atomic crystal

Cr2Ge2Te6 has been of interest for decades, as it is one of only a few naturally forming ferromagnetic semiconductors. Recently, this material has been revisited due to its potential as a two-dimensional semiconducting ferromagnet and a substrate to induce anomalous quantum Hall states in topological insulators. However, many relevant properties of Cr2Ge2Te6 still remain poorly understood, especially the spin-phonon coupling crucial to spintronic, multiferrioc, thermal conductivity, magnetic proximity and the establishment of long range order on the nanoscale. We explore the interplay between the lattice and magnetism through high resolution micro-Raman scattering measurements over the temperature range from 10 to  [ Read More ]

Posted by Kenneth Burch On April - 27 - 2016 Comments Off on Sn-doped Bi1.1Sb0.9Te2S bulk crystal topological insulator with excellent properties

A long-standing issue in topological insulator research has been to find a bulk single crystal material that provides a high-quality platform for characterizing topological surface states without interference from bulk electronic states. This material would ideally be a bulk insulator, have a surface state Dirac point energy well isolated from the bulk valence and conduction bands, display quantum oscillations from the surface state electrons and be growable as large, high-quality bulk single crystals. Here we show that this material obstacle is overcome by bulk crystals of lightly Sn-doped Bi1.1Sb0.9Te2S grown by the vertical Bridgman method. We characterize Sn-BSTS via angle-resolved  [ Read More ]

Posted by Kenneth Burch On April - 5 - 2016 Comments Off on Low vibration high numerical aperture automated variable temperature Raman microscope

Raman micro-spectroscopy is well suited for studying a variety of properties and has been applied to a wide range of areas. Combined with tuneable temperature, Raman spectra can o er even more insights into the properties of materials. However, previous designs of variable temperature Raman microscopes have made it extremely challenging to measure samples with low signal levels due to thermal and positional instabilities as well as low collection e ciencies. Thus contemporary Raman microscope has found limited applicability to probing the subtle physics involved in phase transitions and hysteresis. This paper describes a new design of a closed-cycle, Raman  [ Read More ]

Posted by Kenneth Burch On March - 6 - 2016 Comments Off on Spin-orbit excitations and electronic structure of the putative Kitaev magnet α-RuCl3

Mott insulators with strong spin-orbit coupling have been proposed to host unconventional magnetic states, including the Kitaev quantum spin liquid. The 4d system α-RuCl3 has recently come into view as a candidate Kitaev system, with evidence for unusual spin excitations in magnetic scattering experiments. We apply a combination of optical spectroscopy and Raman scattering to study the electronic structure of this material. Our measurements reveal a series of orbital excitations involving localized total angular momentum states of the Ru ion, implying that strong spin-orbit coupling and electron-electron interactions coexist in this material. Analysis of these features allows us to estimate  [ Read More ]

Posted by Kenneth Burch On January - 28 - 2016 Comments Off on Local phonon mode in thermoelectric Bi2Te2Se from charge neutral anti-sites

Local modes caused by defects play a significant role in the thermal transport properties of thermoelectrics. Of particular interest are charge-neutral defects that suppress thermal conductivity, without significantly reducing electrical transport. Here, we report a temperature dependent Raman study that identifies such a mode in a standard thermoelectric material, Bi2Te2Se. One of the modes observed, whose origin has been debated for decades, was shown most likely to be an antisite defect induced local mode. The anomalous temperature independent broadening of the local mode is ascribed to the random arrangement of Se atoms. The temperature renormalization of all modes is well  [ Read More ]