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Register a new userHigh-Pressure Synthesis and Superconductivity of Ag-doped Topological Crystalline Insulator SnTe (Sn1-xAgxTe with x = 0-0.5)
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Added by
Yoshikazu Mizuguchi
Publication date
2016
fields
Physics
and research's language is
English
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We have synthesized the single-phase polycrystalline samples of Sn1-xAgxTe, Ag-doped topological crystalline insulator SnTe, with a range of x = 0-0.5 using a high-pressure synthesis method. The crystal structure of Sn1-xAgxTe at room temperature is a cubic NaCl-type structure, which does not vary upon Ag substitution. Bulk superconductivity with a transition temperature of 2.4 K was observed for x = 0.15-0.25, and the optimal Ag content was x = 0.2. The Sn1-xAgxTe superconducting phase will be useful for understanding the superconductivity nature and mechanisms of the carrier-doped SnTe system.
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We report on the impact of hydrostatic pressure on the superconductivity of optimally (Indium) doped SnTe which is established to be derived from a topological crystalline insulating phase. Single crystals of Sn1-xInxTe were synthesized by a modified Bridgman method that exhibited maximum superconducting Tc of 4.4 K for x= 0.5. Hydrostatic pressure upto 2.5 GPa was applied on the crystals of Sn0.5In0.5Te and electrical resistivity as a function of temperature and pressure was measured. We observed decrease in onset superconducting transition temperature from 4.4 K to 2.8 K on increasing pressure from ambient to 2.5 GPa. The normal state resistivity also decreased abruptly by an order of magnitude at 0.5 GPa but for higher pressures, the same decreased marginally. From onset, offset and zero resistivity values, dTc/dP of -0.6K/GPa was confirmed. The low temperature normal state resistivity followed T^2 dependence suggesting Fermi liquid behaviour both for ambient and high pressure data. This increase in metallic characteristics accompanied by normal state Fermi liquid behaviour is in accordance with a dome structure for Tc variation with varying carrier concentration.
We present a neutron scattering study of phonons in single crystals of (Pb$_{0.5}$Sn$_{0.5}$)$_{1-x}$In$_x$Te with $x=0$ (metallic, but nonsuperconducting) and $x=0.2$ (nonmetallic normal state, but superconducting). We map the phonon dispersions (more completely for $x=0$) and find general consistency with theoretical calculations, except for the transverse and longitudinal optical (TO and LO) modes at the Brillouin zone center. At low temperature, both modes are strongly damped but sit at a finite energy ($sim4$ meV in both samples), shifting to higher energy at room temperature. These modes are soft due to a proximate structural instability driven by the sensitivity of Pb-Te and Sn-Te $p$-orbital hybridization to off-center displacements of the metal atoms. The impact of the soft optical modes on the low-energy acoustic modes is inferred from the low thermal conductivity, especially at low temperature. Given that the strongest electron-phonon coupling is predicted for the LO mode, which should be similar for both studied compositions, it is intriguing that only the In-doped crystal is superconducting. In addition, we observe elastic diffuse (Huang) scattering that is qualitatively explained by the difference in Pb-Te and Sn-Te bond lengths within the lattice of randomly distributed Pb and Sn sites. We also confirm the presence of anomalous diffuse low-energy atomic vibrations that we speculatively attribute to local fluctuations of individual Pb atoms between off-center sites.
In this paper, the $Fe$-containing superconductors $Fe_{0.5}Cu_{0.5}Ba_2YCu_2O_{7+delta}$, $Fe_{0.5}Cu_{0.5}BaSrYCu_2O_{7+delta}$ and $Fe_{0.5}Cu_{0.5}Sr_2YCu_2O_{7+delta}$ were successfully prepared by common solid-state reaction followed with a procedure of high pressure synthesis. The structural change and superconducting properties in $(Fe_xCu_{1-x})BaSrYCu_2O_{7+delta}$ ($x$ = 0 $sim$ 1.0) systems were also investigated. Annealing experiments indicate that the occurrence of superconductivity in $Fe_{0.5}Cu_{0.5}(Ba_{1-x}Sr_{x})_2YCu_2O_{7+delta}$ ($x$ = 0, 0.5 and 1) systems is mainly induced by the procedure of high pressure synthesis, which causes the increase of oxygen content and the redistribution of $Fe$ atoms between $Cu(1)$ and $Cu(2)$ sites, but not from possible secondary phase of $YBa_2Cu_3O_{7-delta}$, $YBaSrCu_3O_{7-delta}$ or $YSr_2Cu_3O_{7-delta}$ superconductors.
A new class of materials, Topological Crystalline Insulators (TCIs) have been shown to possess exotic surface state properties that are protected by mirror symmetry. These surface features can be enhanced if the surface-area-to-volume ratio of the material increases, or the signal arising from the bulk of the material can be suppressed. We report the experimental procedures to obtain high quality crystal boules of the TCI, SnTe, from which nanowires and microcrystals can be produced by the vapour-liquid-solid (VLS) technique. Detailed characterisation measurements of the bulk crystals as well as of the nanowires and microcrystals produced are presented. The nanomaterials produced were found to be stoichiometrically similar to the source material used. Electron back-scatter diffraction (EBSD) shows that the majority of the nanocrystals grow in the vicinal {001} direction to the growth normal. The growth conditions to produce the different nanostructures of SnTe have been optimised.
Indium substitution turns the topological crystalline insulator (TCI) Pb$_{0.5}$Sn$_{0.5}$Te into a possible topological superconductor. To investigate the effect of the indium concentration on the crystal structure and superconducting properties of (Pb$_{0.5}$Sn$_{0.5}$)$_{1-x}$In$_{x}$Te, we have grown high-quality single crystals using a modified floating-zone method, and have performed systematic studies for indium content in the range $0leq xleq 0.35$. We find that the single crystals retain the rock salt structure up to the solubility limit of indium ($xsim0.30$). Experimental dependences of the superconducting transition temperature ($T_c$) and the upper critical magnetic field ($H_{c2}$) on the indium content $x$ have been measured. The maximum $T_c$ is determined to be 4.7 K at $x=0.30$, with $mu_0H_{c2}(T=0)approx 5$ T.
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