We report measurements of noncontact friction between surfaces of NbSe2 and SrTiO3, and a sharp Pt-Ir tip that is oscillated laterally by a quartz tuning fork cantilever. At 4.2 K, the friction coefficients on both the metallic and insulating materials show a giant maximum at the tip-surface distance of several nanometers. The maximum is strongly correlated with an increase in the spring constant of the cantilever. These features can be understood phenomenologically by a distancedependent relaxation mechanism with distributed time scales.PACS numbers: 68.35. Af, 68.35.Ja, 68.37.Ps Friction has been studied for a long time as one of the fundamental subjects in physics. However, the microscopic mechanism of friction is still in dispute [1]. Nanotribology, namely study of friction at nanoscale, is the most important subject not only for understanding friction but also for the development of micro-and nanoelectromechanical devices, which need control of friction at the nanoscale. A significant amount of research effort has been devoted to revealing the mechanism of friction at the nanoscale [2].Interestingly, there is a novel type of friction, so-called noncontact friction, at the nanoscale. In contrast to the ordinary contact friction, noncontact friction occurs when two bodies are not in direct contact. It has been observed in scanning probe microscopy experiments, in which a sharp metal tip oscillates laterally near a flat surface [3][4][5]. Stipe et al. observed the noncontact friction between a Au(111) surface and a Au-coated probe tip attached to a very soft cantilever (spring constant k 0 ∼ 10 −4 N/m) [4]. At temperatures 4< T <300 K, the friction coefficient Γ was approximately 10 −12 kg/s at a tip-sample distance d <10 nm. As a possible mechanism of the noncontact friction, Ohmic losses caused by fluctuating electromagnetic fields were proposed [6,7]. However, the observed friction coefficient is 7 -8 orders of magnitude larger than the values derived by the theories. Some additional mechanisms that could explain the large noncontact friction have been proposed [8,9], but the discrepancy has not been solved.The above-mentioned theories predict that noncontact friction is proportional to the electrical resistivity of samples. It is therefore expected to be higher on insulating materials than on metals [8,9]. It was found experimentally that the noncontact friction coefficients of insulating silica and polymer films were an order of magnitude larger than the value on the Au(111) surface [4,5]. This tendency is qualitatively consistent with the theories, but quantitative contradiction still remains. On the other hand, Karrai and Tiemann (KT) observed a huge Γ, which is estimated to be ∼ 10 −4 kg/s, between a conductive graphite surface and a gold probe tip attached to a hard (k 0 ∼ 10 4 N/m) quartz tuning fork (QTF) at d <10 nm at room temperature [10]. They attributed the origin of the friction to the viscous damping caused by residual adsorbates such as carbon oxide. The friction observed by KT was theref...

Sr2RuO4 is one of the most promising candidates of a topological superconductor with broken time-reversal symmetry, because a number of experiments have revealed evidences for a spin-triplet chiral p-wave superconductivity. In order to clarify the time-reversal symmetry of Sr2RuO4, we introduce a novel test that examines the invariance of the Josephson critical current under the inversion of both the current and magnetic fields, in contrast to the detection of a spontaneous magnetic field employed in past experiments. Analyses of the transport properties of the planar and corner Josephson junctions formed between Sr2RuO4 and Nb reveal the time-reversal invariant superconductivity, most probably helical p-wave, of Sr2RuO4. This state corresponds to a yet-to-be confirmed topologicalcrystallinesuperconductivity that can host two Majorana edge modes at the surface protected by crystalline mirror symmetry. arXiv:1907.03939v2 [cond-mat.supr-con]

Tunneling conductance spectra of normal metal/insulator/superconductor (N/I/S) junctions are calculated to determine the potential of tunneling spectroscopy in investigations of topological superconductivity. Peculiar feature of topological superconductors is the formation of gapless edge states in them. Since the conductance of N/I/S junctions is sensitive to the formation of these edge states, topological superconductivity can be identified through edge-state detection. Herein, the effects of Fermi surface anisotropy and an applied magnetic field on the conductance spectra are analyzed to gather indications that can help to identify the topological nature of actual materials.Comment: 17pages, 1 table, 6 figures, submitted to Physica

Clarifying the chiral domains structure of superconducting Sr2RuO4 has been a long-standing issue in identifying its peculiar topological superconducting state. We evaluated the critical current Ic versus the magnetic field H of Nb/Sr2RuO4 Josephson junctions, changing the junction dimension in expectation of that the number of domains in the junction is controlled. Ic(H) exhibits a recovery from inversion symmetry breaking to invariance when the dimension is reduced to several microns. This inversion invariant behavior indicates the disappearance of domain walls; thus, the size of a single domain is estimated at approximately several microns.

We have developed a process of fabricating high-supercurrent-density contacts to Sr2RuO4 as a key technology for the development of topological quantum computing. To minimize degradation, the niobium contacts were formed on crushed surfaces of Sr2RuO4 crystals by an in situ process. A critical supercurrent density as high as 107 A/m2 was achieved, a value two to three orders of magnitude greater than those reported previously. However, the unexpectedly small magnitude of the product of the critical current and the normal resistance indicates the existence of a fundamental limitation arising from Josephson coupling between spin-singlet and spin-triplet superconductivities.