Archive for the ‘Year’ Category

Posted by Kenneth Burch On February - 12 - 2018 0 Comment

We present Raman spectroscopy experiments on exfoliated α-RuCl3, from tens of nm thick down to single layers. Besides unexpectedly finding this material to be air stable, in the thinnest layers we observe the appearance with decreasing temperature of a symmetry-forbidden mode in crossed polarization, along with an anomalous broadening of a mode at 164 cm−1 that is known to couple to a continuum of magnetic excitations. This may be due to an enhancement of magnetic fluctuations and evidence for a distorted honeycomb lattice in single- and bilayer samples. Journal of Physics and Chemistry of Solids 

Posted by Kenneth Burch On May - 6 - 2017 Comments Off on 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 spectra that are consistent with Andreev reflection. This is confirmed by a magnetic field that shifts features via the Doppler effect. This is modeled with a single parameter that accounts for tunneling into a screening supercurrent. Thus the tunneling involves Cooper pairs crossing the heterostructure, showing that the Fermi surface mismatch does not hinder the ability to form transparent interfaces, which is accounted for by the extended Brillouin zone and different lattice  [ Read More ]

Posted by Kenneth Burch On March - 13 - 2017 Comments Off on 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 Bi2 Te3−x Sex series, experimental verification of anharmonicity and its evolution with doping remains elusive. We fill this important gap with high-resolution, temperature-dependent Raman spectroscopy in high-quality single crystals of Bi2Te3, Bi2Te2Se, and Bi2Se3 over the temperature range from 4 to 293 K. Klemens’s model was employed to explain the renormalization of their phonon linewidths. The phonon energies of Bi2Se3 and Bi2Te3 are analyzed in detail from three aspects: lattice  [ Read More ]

Posted by Kenneth Burch On November - 1 - 2016 Comments Off on Automatic Spike Removal Algorithm for Raman Spectra

Raman spectroscopy is a powerful technique, widely used in both academia and industry. In part, the technique’s extensive use stems from its ability to uniquely identify and image various material parameters: composition, strain, temperature, lattice/excitation symmetry, and magnetism in bulk, nano, solid, and organic materials. However, in nanomaterials and samples with low thermal conductivity, these measurements require long acquisition times. On the other hand, charge- coupled device (CCD) detectors used in Raman microscopes are vulnerable to cosmic rays. As a result, many spurious spikes occur in the measured spectra, which can distort the result or require the spectra to be  [ Read More ]

Posted by Kenneth Burch On October - 26 - 2016 Comments Off on Modeling tunneling for the unconventional superconducting proximity effect

Recently there has been reinvigorated interest in the superconducting proximity effect, driven by predictions of the emergence of Majorana fermions. To help guide this search, we have developed a phenomenological model for the tunneling spectra in anisotropic superconductor-normal metal proximity devices. We combine successful approaches used in s-wave proximity and standard d-wave tunneling to reproduce tunneling spectra in d-wave proximity devices, and clarify the origin of various features. Different variations of the pair potential are considered, resulting from the proximity-induced superconductivity. Furthermore, the effective pair potential felt by the quasiparticles is momentum-dependent in contrast to s-wave superconductors. The probabilities of  [ Read More ]