L. J. Sandilands

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

Optical evidence of surface state suppression in Bi-based topological insulators

\alpha

Stability of exfoliated Bi 2 Sr 2 Dy x Ca 1 − x Cu 2 O 8 + δ studied by Raman microscopy

-RuCl3, in which Ru ions in 4d5 configuration form a honeycomb lattice. The measured optical spectra exhibit a clear optical gap and excitations within the t2g orbitals. The spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin orbit coupling and electron correlations. Furthermore, 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. We propose that

Structural study of Bi2Sr2CaCu2O8+δ exfoliated nanocrystals

\alpha

Stability of exfoliated Bi

-RuCl3 is a spin-orbit assisted Mott insulator, and the bond-dependent Kitaev interaction may be relevant for this compound.

_2

A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive Bi2Te2Se, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon mediated coupling between bulk and surface states suppresses surface conductance as temperature rises.

Sr

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 Bi2Sr2CaCu2O8 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. Subtle differences between the exfoliated and bulk crystals are also discussed.

_2

We demonstrate that structural and spectroscopic information can be obtained on exfoliated nanocrystals as thin as 6 nm. This can be achieved by using a combination of micro X-ray fluorescence (μXRF), micro X-ray absorption near-edge spectroscopy (μXANES), and X-ray microdiffraction (μXRD) techniques. Highly focused, tunable X-ray beams available at synchrotron sources enable one to use these non-invasive characterization tools to study exfoliated samples on a variety of substrates. As an example, we focused on exfoliated nanocrystals of the high temperature superconductor Bi2Sr2CaCu2O8+δ. μXRF is used to locate the sample of desired thickness; μXANES and μXRD are used to obtain electronic and structural information, respectively. We find that the “4.7b” structural modulation, characteristic of the bulk crystals, is drastically suppressed for exfoliated crystals thinner than 60 nm.

Dy

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 Bi2Sr2CaCu2O8 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. Subtle differences between the exfoliated and bulk crystals are also discussed.

_x

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 Bi2Sr2CaCu2O8 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. Subtle differences between the exfoliated and bulk crystals are also discussed.