We study the surface state of a doped topological crystalline insulator in the superconducting state. Motivated by Sn$_{1-x}$In$_x$Te, we consider fully gapped pair potentials and calculate the surface spectral function. It is found that mirror-protected zero-energy Andreev bound states appear at the (001) surface and that these states can move along the mirror symmetric line on the surface Brillouin zone. We also show that the surface Andreev bound state changes systematically with doping due to the presence of the Dirac surface state in the normal state.
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 syste…
We report the characterization of the misfit compound (Pb1-xSnxSe2)1.16(TiSe2)2, in which a [100] rocksalt bilayer of Pb1-xSnxSe, which is a topological crystalline insulator in bulk form, alternates with a double layer of the normally non-superconducting layered transition metal dichalcogenide TiSe2. The x dependence of Tc displays a weak dome-like shape with a maximum Tc of 4.5 K at x = 0.2; there is only a subtle change in Tc corresponding to the expected trivial to topological transition in the Pb1-xSnxSe bi-layer. We present a more detailed characterization of the superconductor at x = 0.…
Single-crystalline (Lu, Ca)Ba2Cu3O7-d (Lu(Ca)123) whiskers have been successfully grown using the Te-doping method. X-ray diffraction patterns of Lu(Ca)123 whiskers showed sharp (0 0 l) peaks corresponding to REBa2Cu3O7-d phase (RE = rare earth elements). Transport measurements showed that the superconducting transition occurred at 83 K in the obtained whiskers.
The In-doped topological crystalline insulator Sn$_{1-x}$In$_x$Te is a promising candidate for a topological superconductor, where it is theoretically suggested that In creates an impurity state responsible for superconductivity. We synthesized high purity Sn$_{1-x}$In$_x$Te samples and performed $^{125}$Te-nuclear magnetic resonance (NMR) measurements. The NMR spectra under a magnetic field of $H_0$ = 5 T show a broadening characteristic due to a localized impurity state. The spin-lattice relaxation rate ($1/T_1$) divided by temperature shows a Curie-Weiss like temperature-dependence under $H…
Indium substitution turns the topological crystalline insulator (TCI) Pb$_{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 $0\leq x\leq 0.35$. We find that the single crystals retain the rock salt structure up to the solubility limit of indium ($x\sim0.30$). Experimental depende…
Discovery of exotic phases of matter from the topologically non-trivial systems not only makes the research on topological materials more interesting but also enriches our understanding of the fascinating physics of such materials. Pb$_{0.6}$Sn$_{0.4}$Te was recently shown to be a topological crystalline insulator. Here we show that by forming a mesoscopic point-contact using a normal non-superconducting elemental metal on the surface of Pb$_{0.6}$Sn$_{0.4}$Te a novel superconducting phase is created locally in a confined region under the point-contact. This happens while the bulk of the sampl…
The In-doped topological crystalline insulator Sn_{1-x}In_xTe is a candidate for a topological superconductor, where pseudo-spin-triplet state has been proposed. To clarify the spin symmetry of Sn_{1-x}In_xTe, we perform ^{125}Te-nuclear magnetic resonance (NMR) measurements in polycrystalline samples with 0< x <0.15. The penetration depth calculated from the NMR line width is T-independent below half the superconducting transition temperature (T_c) in polycrystalline Sn_{0.96}In_{0.04}Te, which indicates a fully-opened superconducting gap. In this sample, the spin susceptibility measured by t…
Topological crystalline insulators represent a new state of matter, in which the electronic transport is governed by mirror-symmetry protected Dirac surface states. Due to the helical spin-polarization of these surface states, the proximity of topological crystalline matter to a nearby superconductor is predicted to induce unconventional superconductivity and thus to host Majorana physics. We report on the preparation and characterization of Nb-based superconducting quantum interference devices patterned on top of topological crystalline insulator SnTe thin films. The SnTe films show weak anti…
Disorder driven superconductor to insulator transition via intermediate metallic regime is reported in nano-crystalline thin films of molybdenum. The nano-structured thin films have been deposited at room temperature using DC magnetron sputtering at different argon pressures. The grain size has been tuned using deposition pressure as the sole control parameter. A variation of particle sizes, room temperature resistivity and superconducting transition has been studied as a function of deposition pressure. The nano-crystalline molybdenum thin films are found to have large carrier concentration b…
In this review, we discuss recent progress in the explorations of topological materials beyond topological insulators; specifically, we focus on topological crystalline insulators and bulk topological superconductors. The basic concepts, model Hamiltonians, and novel electronic properties of these new topological materials are explained. The key role of symmetries that underlie their topological properties is elucidated. Key issues in their materials realizations are also discussed.
Superconductors derived from topological insulators and topological crystalline insulators by doping have long been considered to be candidates as topological superconductors. Pb$_{0.5}$Sn$_{0.5}$Te is a topological crystalline insulator with mirror symmetry protected surface states on (001), (011) and (111) oriented surfaces. The superconductor (Pb$_{0.5}$Sn$_{0.5}$)$_{0.7}$In$_{0.3}$Te is induced by In doping in Pb$_{0.5}$Sn$_{0.5}$Te, and is thought to be a topological superconductor. Here we report the first scanning tunneling spectroscopy measurement of the superconducting state as well a…
Superconducting topological crystalline insulators (TCI) are predicted to host new topological phases protected by crystalline symmetries, but available materials are insufficiently suitable for surface studies. To induce superconductivity at the surface of a prototypical TCI SnTe, we use molecular beam epitaxy to grow a heterostructure of SnTe and a high-Tc superconductor Fe(Te,Se), utilizing a 'buffer' layer to bridge the large lattice mismatch between SnTe and Fe(Te,Se). Using low-temperature scanning tunneling microscopy and spectroscopy, we measure a prominent spectral gap on the surface …
We report detailed investigations of the properties of a superconductor obtained by substituting In at the Sn site in the topological crystalline insulator (TCI), SnTe. Transport, magnetization and heat capacity measurements have been performed on crystals of Sn0.6In0.4Te, which is shown to be a bulk superconductor with Tc(onset) at ~4.70(5) K and Tc(zero) at ~3.50(5) K. The upper and lower critical fields are estimated to be μ0Hc2(0) = 1.42(3) T and μ0Hc1(0) = 0.90(3) mT respectively, while κ = 56.4(8) indicates this material is a strongly type II superconductor.
The interplay of time-reversal and $n$-fold rotation symmetries ($n=2,4,6$) is known to bring a new class of topological crystalline insulators (TCIs) having $n$ surface Dirac cones due to surface rotation anomaly. We show that the proximity-induced $s$-wave superconductivity on the surface of these TCIs yields a topological superconducting phase in which two Majorana zero modes are bound to a vortex, and that $n$-fold rotation symmetry ($n=2,4,6$) enriches the topological classification of a superconducting vortex from $\mathbb{Z}_2$ to $\mathbb{Z}_2\times\mathbb{Z}_2$. Using a model of a thr…
The thermal conductivity of the doped topological crystalline insulator, Sn$_{0.6}$In$_{0.4}$Te superconducting single crystal with $T_c =$ 4.1 K, was measured down to 50 mK. It is found that the residual linear term $κ_0/T$ is negligible in zero magnetic field. The $κ_0/T$ shows a slow field dependence at low magnetic field. These results suggest that the superconducting gap is nodeless, unless there exist point nodes with directions perpendicular to the heat current. Due to its high-symmetry fcc crystal structure of Sn$_{0.6}$In$_{0.4}$Te, however, such point nodes can be excluded. Therefore…
Superconducting nanowires can be synthesised in high-throughput chemical routes and hold great promise as a low-dissipative material for superconducting devices. The applicability of superconducting nanowires, however, has been limited due to the lack of an adequate method to fit the nanowires into given electronic circuits. One of the biggest obstacles is to connect metal terminals to superconducting nanowires in order to establish electric contacts. One attractive method to surmount this difficulty is to synthesise superconducting nanowires directly upon metal terminals such that the nanowir…
We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. Current-Voltage characteristic curve shows clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.95) contacts very close to an ideal limit of 1 are formed at the interface between the normal metal (Au) and the superconductor (Al). The very high transparency is ascribed to the single crystallinity of a…
Crystalline topological phases have recently attracted a lot of experimental and theoretical attention. Key advances include the complete elementary band representation analyses of crystalline matter by symmetry indicators and the discovery of higher-order hinge and corner states. However, current classification schemes of such phases are either implicit or limited in scope. We present a new scheme for the explicit classification of crystalline topological insulators and superconductors. These phases are protected by crystallographic point group symmetries and are characterized by bulk topolog…
Topological classification in our previous paper [K. Shiozaki and M. Sato, Phys. Rev. B ${\bf 90}$, 165114 (2014)] is extended to nonsymmorphic crystalline insulators and superconductors. Using the twisted equivariant $K$-theory, we complete the classification of topological crystalline insulators and superconductors in the presence of additional order-two nonsymmorphic space group symmetries. The order-two nonsymmorphic space groups include half lattice translation with $Z_2$ flip, glide, two-fold screw, and their magnetic space groups. We find that the topological periodic table shows modulo…
We complete a classification of topological phases and their topological defects in crystalline insulators and superconductors. We consider topological phases and defects described by non-interacting Bloch and Bogoliubov de Gennes Hamiltonians that support additional order-two spatial symmetry, besides any of ten classes of symmetries defined by time-reversal symmetry and particle-hole symmetry. The additional order-two spatial symmetry we consider is general and it includes $Z_2$ global symmetry, mirror reflection, two-fold rotation, inversion, and their magnetic point group symmetries. We fi…
We formulate topological crystalline materials on the basis of the twisted equivariant $K$-theory. Basic ideas of the twisted equivariant $K$-theory is explained with application to topological phases protected by crystalline symmetries in mind, and systematic methods of topological classification for crystalline materials are presented. Our formulation is applicable to bulk gapful topological crystalline insulators/superconductors and their gapless boundary and defect states, as well as bulk gapless topological materials such as Weyl and Dirac semimetals, and nodal superconductors. As an appl…
Topological crystalline insulators in IV-VI compounds host novel topological surface states consisting of multi-valley massless Dirac fermions at low energy. Here we show that strain generically acts as an effective gauge field on these Dirac fermions and creates pseudo-Landau orbitals without breaking time-reversal symmetry. We predict the realization of this phenomenon in IV-VI semiconductor heterostructures, due to a naturally occurring misfit dislocation array at the interface that produces a periodically varying strain field. Remarkably, the zero-energy Landau orbitals form a flat band in…
