Bismuth Doping Alters Structural Phase Transitions in Methylammonium Lead Tribromide Single Crystals.

We study the effects of bismuth doping on the crystal structure and phase transitions in single crystals of the perovskite semiconductor methylammonium lead tribromide, MAPbBr. By measuring the temperature-dependent specific heat capacity (), we find that as the Bi doping increases, the phase transition assigned to the cubic to tetragonal phase boundary decreases in temperature. Furthermore, after doping we observe one phase transition between 135 and 155 K, in contrast to two transitions observed in the undoped single crystal.

Apparatus design for measuring of the strain dependence of the Seebeck coefficient of single crystals.

We present the design and construction of an apparatus that measures the Seebeck coefficient of single crystals under in situ tunable strain at cryogenic temperatures. A home-built three piezostack apparatus applies uni-axial stress to a single crystalline sample and modulates anisotropic strain up to 0.7%.

Evidence for a strain-tuned topological phase transition in ZrTe.

A phase transition between topologically distinct insulating phases involves closing and reopening the bandgap. Near the topological phase transition, the bulk energy spectrum is characterized by a massive Dirac dispersion, where the bandgap plays the role of mass. We report measurements of strain dependence of electrical transport properties of ZrTe, which is known to host massive Dirac fermions in the bulk due to its proximity to a topological phase transition.