An archetypical layered topological insulator Bi 2 Se 3 becomes superconductive upon doping with Sr, Nb or Cu. Superconducting properties of these materials in the presence of in-plane magnetic field demonstrate spontaneous symmetry breaking: 180 • -rotation symmetry of superconductivity versus 120 • -rotation symmetry of the crystal. Such behavior brilliantly confirms nematic topological superconductivity. To what extent this nematicity is due to superconducting pairing in these materials, rather than due to crystal structure distortions? This question remains unanswered, because so far no visible deviations from the 3-fold crystal symmetry were resolved in these materials. To address this question we grow high quality single crystals of Sr x Bi 2 Se 3 , perform detailed x-ray diffraction and magnetotransport studies and reveal that the observed superconducting nematicity direction correlates with the direction of small structural distortions in these samples (∼0.02% elongation in one crystallographic direction). Additional anisotropy comes from orientation of the crystallite axes. 2-fold symmetry of magnetoresistance observed in the most uniform crystals well above the critical temperature demonstrates that these structural distortions are nevertheless strong enough. Our data in combination with strong sample-to-sample variation of the superconductive anisotropy parameter are indicative for significance of the structural factor in the apparent nematic superconductivity in Sr x Bi 2 Se 3 .

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We report molecular beam epitaxy growth of Sr-doped Bi 2 Se 3 films on (111) BaF 2 substrate, aimed to realize unusual superconducting properties inherent to Sr x Bi 2 Se 3 single crystals. Despite wide range of the compositions, we do not achieve superconductivity. To explore the reason for that we study structural, morphological and electronic properties of the films and compare them to the corresponding properties of the single crystals. The dependence of the c-lattice constant in the films on Sr content appears to be more than an order of magnitude stronger than in the crystals. Correspondingly, all other properties also differ substantially, indicating that Sr atoms get different positions in lattices. We argue that these structural discrepancies come from essential differences in growth conditions. Our research calls for more detailed structural studies and novel growth approaches for design of superconducting Sr x Bi 2 Se 3 thin films.PACS numbers:

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We report an observation of coherent phonons of 1 g E , 1 u E , 1 1g A , and 2 g E symmetry generated in a single-crystal film of Bi 2 Se 3 by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the 2 g E mode. The generation of 2 g E phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear 1 u E oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of 2 g E . The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of 1 g E (1.1 THz) and 1 1g A (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in Bi 2 Se 3 .