Archive for the ‘2014’ Category

Posted by Kenneth Burch On January - 19 - 2015 Comments Off on Optical investigation of the 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  [ Read More ]

Posted by Kenneth Burch On December - 15 - 2014 Comments Off on Evidence for a new excitation at the interface between a high-Tc superconductor and a topological insulator

High-temperature superconductors exhibit a wide variety of novel excitations. If contacted with a topological insulator, the lifting of spin rotation symmetry in the surface states can lead to the emergence of unconventional superconductivity and novel particles. In pursuit of this possibility, we fabricated high critical-temperature (Tc ∼ 85 K) superconductor/topological insulator (Bi2Sr2CaCu2O8+δ/Bi2Te2Se) junctions. Below 75 K, a zero- bias conductance peak (ZBCP) emerges in the differential conductance spectra of this junction. The magnitude of the ZBCP is suppressed at the same rate for magnetic fields applied parallel or perpendicular to the junction. Furthermore, it can still be observed and does  [ Read More ]

Posted by Kenneth Burch On November - 15 - 2014 Comments Off on Mid-infrared Polaritonic Coupling between Boron Nitride Nanotubes and Graphene

Boron nitride (BN) is considered to be a promising substrate for graphene-based devices in part because its large band gap can serve to insulate graphene in layered heterostructures. At mid- infrared frequencies, graphene supports surface plasmon polaritons (SPPs), whereas hexagonal-BN (h-BN) is found to support surface phonon polaritons (SPhPs). We report on the observation of infrared polaritonic coupling between graphene SPPs and boron nitride nano- tube (BNNT) SPhPs. Infrared scattering type scanning near-field optical microscopy is used to obtain spatial distribution of the two types of polaritons at the nanoscale. The observation suggests that those polaritons interact at the nanoscale in  [ Read More ]

Posted by Kenneth Burch On August - 10 - 2014 Comments Off on Origin of the insulating state in exfoliated high-Tc two-dimensional atomic crystals

We present the results of an optical spectroscopic study of two-dimensional atomic crystals of the high- temperature superconductor Bi2Sr2CaCu2O8+δ. Our measurements reveal a pronounced suppression of the optical conductivity σ1(ω) with thickness. Using an effective medium approximation, we interpret this in terms of an insulating surface layer. The surface layer explains the insulating behavior previously observed in exfoliated Bi2Sr2CaCu2O8+δ and has implications for future studies and potential applications of these materials. Physical Review B Rapid Communications 90, 081402 (2014) 

Posted by Kenneth Burch On August - 3 - 2014 Comments Off on α−RuCl3: A spin-orbit assisted Mott insulator on a honeycomb lattice

We examine the role of spin-orbit coupling in the electronic structure of α−RuCl3, in which Ru ions in 4d5 configuration form a honeycomb lattice. Our x-ray absorption spectroscopy measurements at the Ru L edges exhibit distinct spectral features associated with the presence of substantial spin-orbit coupling, as well as an anomalously large branching ratio. Furthermore the measured optical spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin-orbit coupling and electron correlations. We propose that  α−RuCl3 is a spin-orbit assisted Mott insulator, and that the bond-dependent Kitaev interaction may be important for understanding magnetism of this  [ Read More ]