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 ZrTe5, which is known to host massive Dirac fermions in the bulk due to its proximity to a topological phase transition. We observe that the resistivity exhibits a pronounced minimum at a critical strain. We further find that the positive longitudinal magnetoconductance becomes maximal at the critical strain. This nonmonotonic strain dependence is consistent with the switching of sign of the Dirac mass and, hence, a strain-tuned topological phase transition in ZrTe5.

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We study the effects of bismuth doping on the crystal structure and phase transitions of single crystals of the perovskite semiconductor methylammonium lead tribromide, MAPbBr 3 . By measuring the temperature-dependent specific heat capacity (C p ) we find that, as the Bi doping level increases, the temperatures for the bismuth structural phase transitions shift, and the phase boundary assigned to the transition from the cubic to tetragonal phase decreases in temperature. Furthermore, after doping we only observe one phase transition between 135 and 155 K, in contrast to two transitions observed in the undoped single crystal. These results appear strikingly similar to the previously reported effects of mechanical pressure on the perovskite structure. Using X-ray diffraction, we show that, as more Bi is incorporated into the crystal, the lattice constant decreases, as predicted by density functional theory (DFT). Based on the lattice contraction and DFT, we propose that bismuth substitutional doping on the lead site is dominant, resulting in Bi Pb + centers which induce compressive chemical strain that alters the crystalline phase transitions. TOC GRAPHICSKEYWORDS Bismuth doping, MAPbBr 3 perovskite, specific heat capacity, lattice contraction, X-ray diffraction, Halide perovskites have emerged as promising semiconductor materials for applications including solar cells, light-emitting diodes, photodetectors, and lasers. [1][2][3][4] They exhibit unique and tunable optoelectronic properties via facile tailoring of the chemical composition of the structure.In the archetypal perovskite ABX3 crystal structure, A represents a monovalent cation species (A = Cs + , CH3NH3 + (MA + ), or (NH2)2CH3 + (FA + )), B represents a divalent cation (B = Pb +2 , Sn +2 ), and X represents a halide (X = Cl -, Br -, I -). Diverse electronic and structural motifs are thereby accessible by modification of the chemical composition and the dimensionality of the material. 5

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%. An alternating heater system and cernox sensor thermometry measures the Seebeck coefficient along the uniaxial stress direction. To demonstrate the efficacy of this apparatus, we applied uniaxial stress to detwin single crystals of BaFe2As2 in the orthorhombic phase. The obtained Seebeck coefficient anisotropy is in good agreement with previous measurements using a mechanical clamp.