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Register a new userTopological competition of superconductivity in Pb/Ru/Sr_2RuO_4 junctions
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We devise a new proximity junction configuration where an s-wave superconductivity and the superconductivity of Sr2RuO4 interfere with each other. We reproducibly observe in such a Pb/Ru/Sr2RuO4 junction with a single Pb electrode that the critical current Ic drops sharply just below the bulk Tc of Sr2RuO4 and furthermore increases again below a certain temperature below Tc. In order to explain this extraordinary temperature dependence of Ic, we propose a competition effect involving topologically distinct superconducting phases around Ru inclusions. Thus, such a device may be called a topological superconducting junction.
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The eutectic system Sr2RuO4-Ru is referred to as the 3-K phase of the spin-triplet supeconductor Sr2RuO4 because of its enhanced superconducting transition temperature Tc of ~3 K. We have investigated the field-temperature (H-T) phase diagram of the 3-K phase for fields parallel and perpendicular to the ab-plane of Sr2RuO4, using out-of-plane resistivity measurements. We have found an upturn curvature in the Hc2(T) curve for H // c, and a rather gradual temperature dependence of Hc2 close to Tc for both H // ab and H // c. We have also investigated the dependence of Hc2 on the angle between the field and the ab-plane at several temperatures. Fitting the Ginzburg-Landau effective-mass model apparently fails to reproduce the angle dependence, particularly near H // c and at low temperatures. We propose that all of these charecteric features can be explained, at least in a qualitative fashion, on the basis of a theory by Sigrist and Monien that assumes surface superconductivity with a two-component order parameter occurring at the interface between Sr2RuO4 and Ru inclusions. This provides evidence of the chiral state postulated for the 1.5-K phase by several experiments.
We have fabricated c-axis Josephson junctions on single crystals of (Ba,K)Fe2As2 by using Pb as the counter electrode in two geometries, planar and point contact. Junctions in both geometries show resistively shunted junction I-V curves below the Tc of the counter electrode. Microwave induced steps were observed in the I-V curves, and the critical currents are suppressed with an in-plane magnetic field in a manner consistent with the small junction limit. ICRN products of up to 0.3 mV have been observed in these junctions at 4.2 K. The observation of Josephson coupling along the c-axis between (Ba,K)Fe2As2 and a conventional superconductor suggests the existence of a s-wave superconducting order parameter in this class of iron pnictide superconductors.
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.
Just like insulators can host topological Dirac states at their edges, superconductors can also exhibit topological phases characterized by Majorana edge states. Remarkable zero-energy states have been recently observed at the two ends of proximity induced superconducting wires, and were interpreted as localized Majorana end states in one-dimensional (1D) topological superconductor. By contrast, propagating Majorana states should exist at the 1D edges of two-dimensional (2D) topological superconductors. Here we report the direct observation of dispersive in-gap states surrounding topological superconducting domains made of a single atomic layer of Pb covering magnetic islands of Co/Si(111). We interpret the observed continuous dispersion across the superconducting gap in terms of a spatial topological transition accompanied by a chiral edge mode and residual gaped helical edge states. Our experimental approach enables the engineering and control of a large variety of novel quantum phases. This opens new horizons in the field of quantum materials and quantum electronics where the magnetization of the domains could be used as a control parameter for the manipulation of topological states.
We have performed a systematic angle-resolved photoemission study of as-grown and oxygen-reduced Pr$_{2-x}$Ce$_x$CuO$_4$ and Pr$_{1-x}$LaCe$_{x}$CuO$_4$ electron-doped cuprates. In contrast to the common belief, neither the band filling nor the band parameters are significantly affected by the oxygen reduction process. Instead, we show that the main electronic role of the reduction process is to remove an anisotropic leading edge gap around the Fermi surface. While the nodal leading edge gap is induced by long-range antiferomagnetic order, the origin of the antinodal one remains unclear.
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