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This article introduces a few episodes from the author's association with Prof. K. Alex Müller that reflect an aspect of his personality and philosophy toward physics. J. Georg Bednorz and K. Alex Müller's seminal paper on the discovery of high- T c superconductivity starts with a quote from the report of the workshop held in Copper Mountain in 1983. Müller's original copy of the preprint of that report reflects his commitment to the research into superconductivity. Another episode describes how he directed the press meeting at the M 2 S-HTSC III conference in Kanazawa in 1991. [ABSTRACT FROM AUTHOR]

The discoveries of superconductivity in heavily boron-doped diamond in 2004 and silicon in 2006 have renewed the interest in the superconducting state of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a metallic phase from which upon further doping superconductivity can emerge. Recently, we discovered superconductivity in a closely related system: heavily boron-doped silicon carbide. The sample used for that study consisted of cubic and hexagonal SiC phase fractions and hence this led to the question which of them participated in the superconductivity. Here we studied a hexagonal SiC sample, free from cubic SiC phase by means of x-ray diffraction, resistivity, and ac susceptibility. [ABSTRACT FROM AUTHOR]

Enhanced superconductivity in the eutectic system Sr2RuO4–Ru is referred to as the 3-K phase of Sr2RuO4 because of its enhanced superconducting transition temperature Tc of ∼3 K whilst the superconductivity in Sr2RuO4 (Tc of ∼1.5 K) is called the 1.5-K phase. We describe basic properties of the 3-K phase and focus on its field–temperature (H–T) phase diagram. We suggest that the 3-K phase is attributable to surface superconductivity with a two-component order parameter (chiral state) occurring at the interface between Sr2RuO4 and Ru inclusions. [Copyright &y& Elsevier]

Focuses on the superconducting state of the metallic oxide strontium ruthenate. Theories surrounding conventional metallic superconductors; Background of the discovery of superconductivity; Physical and chemical characteristics of strontium ruthenate. INSETS: Box 1. Chiral Superconductivity;Box 2. Vortex Physics.

Nematic states are characterized by rotational symmetry breaking without translational ordering. Recently, nematic superconductivity, in which the superconducting gap spontaneously lifts the rotational symmetry of the lattice, has been discovered. In nematic superconductivity, multiple superconducting domains with different nematic orientations can exist, and these domains can be controlled by a conjugate external stimulus. Domain engineering is quite common in magnets but has not been achieved in superconductors. Here, we report control of the nematic superconductivity and their domains of SrxBi2Se3, through externally-applied uniaxial stress. The suppression of subdomains indicates that it is the Δ4y state that is most favoured under compression along the basal Bi-Bi bonds. This fact allows us to determine the coupling parameter between the nematicity and lattice distortion. These results provide an inevitable step towards microscopic understanding and future utilization of the unique topological nematic superconductivity. Nematic superconductors, exhibiting rotational-symmetry breaking, can form domains; a counterpart of common magnetic domains. Here, the authors report control of nematic superconductivity and their domains of SrxBi2Se3 by uniaxial deformation, a step toward superconductive domain engineering. [ABSTRACT FROM AUTHOR]

Abstract: The superconducting properties of the silver lead oxide Ag5Pb2O6 has been precisely studied through the ac susceptibility measurements. It was concluded that Ag5Pb2O6 is a type-I superconductor. The phase diagram was obtained. The observed small curvature of the H c(T) curve may be attributed to a large anisotropy in the superconducting gap. [Copyright &y& Elsevier]

By means of the magnetocaloric effect, we examine the nature of the superconducting-normal (S-N) transition of Sr2RuO4, a most promising candidate for a spin-triplet superconductor. We provide thermodynamic evidence that the S-N transition of this oxide is of first order below approximately 0.8 K and only for magnetic field directions very close to the conducting plane, in clear contrast to the ordinary type-II superconductors exhibiting second-order S-N transitions. The entropy release across the transition at 0.2 K is 10% of the normal-state entropy. Our result urges an introduction of a new mechanism to break superconductivity by magnetic field. [ABSTRACT FROM AUTHOR]

We will discuss selected issues on our current understanding of the symmetry of order parameter of the unconventional superconductor Sr2RuO4. We point out the importance of the orbital dependence of the superconducting gap in interpreting its probable nodal structure. We also present the effect of impurities which may be used to probe the pairing mechanism. [Copyright &y& Elsevier]

In order to investigate the position of the strontium deficiency in superconductive S r 3 − x SnO, we synthesized and measured X-ray-diffraction patterns of S r 3 − x SnO ( x ∼ 0.5 ). Because no clear peaks originating from superstructures were observed, strontium deficiency is most likely to be randomly distributed. We also performed first-principles band-structure calculations on S r 3 − x SnO ( x = 0, 0.5 ) using two methods: full-potential linearized-augmented plane-wave plus local orbitals method and the Korringa-Kohn-Rostoker Green function method combined with the coherent potential approximation. We revealed that the Fermi energy of S r 3 − x SnO in case of x ∼ 0.5 is about 0.8 eV below the original Fermi energy of the stoichiometric Sr 3 SnO, where the mixing of the valence p and conduction d orbitals are considered to be small. [ABSTRACT FROM AUTHOR]

We have investigated the field–temperature phase diagram of the 3-K phase (Sr2RuO4–Ru eutectic) for two field directions (H∥ab and H∥c) using resistivity data. 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. We propose that these characteristic features can be explained, at least in a qualitative fashion, on the basis of a theory that assumes surface superconductivity with a two-component order parameter at the interface between Sr2RuO4 and Ru inclusions. [Copyright &y& Elsevier]

Abstract: In the Sr3Ru2O7 region of the Sr3Ru2O7–Sr2RuO4 eutectic system, multiple superconducting (SC) transitions are observed with onset temperatures of 1.3 and 1K. Based on various experimental results, the SC transition at 1.3K is attributable to thin Sr2RuO4 layers embedded in the Sr3Ru2O7 region as stacking faults. In this study, in order to clarify the origin of the SC transition at 1K we have investigated its uniaxial pressure effect along the c axis. We revealed that, although Sr3Ru2O7 exhibits ferromagnetism under the uniaxial pressure along the c axis above 0.3GPa, the onset temperature of the SC transition at 1K is almost invariant or increases slightly under the uniaxial pressure up to 0.4GPa. This result suggests that this SC transition is not strongly related to the electronic state of Sr3Ru2O7 and it is consistent with the scenario that the 1-K transition also originates from the embedded thin Sr2RuO4 layers whose thickness is thinner than that of the 1.3-K transition. [ABSTRACT FROM AUTHOR]

Abstract: We review our nuclear-magnetic resonance (NMR) and nuclear-quadrupole-resonance (NQR) studies in superconducting Sr2RuO4, which have been performed in order to investigate the gap structure and the pairing symmetry in the superconducting state and magnetic fluctuations in the normal state. The spin-lattice relaxation rate () of a high-quality sample with shows a sharp decrease without a coherence peak just below , followed by a behavior down to 0.15K. This result indicates that the superconducting gap in pure Sr2RuO4 is a highly anisotropic character with a line-node gap. The Knight shift, which is related to the spin susceptibility, is unchanged in the superconducting state irrespective of the direction of the applied fields and various magnitude of the field. This result strongly suggests that the superconducting pairs are in the spin-triplet state, and the spin direction of the triplet pairs is considered to be changed by small fields of several hundred Oe. [Copyright &y& Elsevier]

We present direct evidence for complex p-wave order parameter symmetry and the presence of dynamicaL chiral order parameter domains of the form Px ± ipy in the ruthenate superconductor Sr2RuO4. The domain structure creates differences in the magnetic field modulation of the critical current of Josephson junctions fabricated on orthogonal faces of Sr2RuO4 single crystals. Transitions between the chiral states of a domain or the motion of domain walls separating them generates telegraph noise in the critical current as a function of magnetic field or time and is responsible for hysteresis observed in field sweeps of the critical current. The presence of such domains confirms the p-wave triplet spin and complex (broken time-reversal symmetry) nature of the superconducting pairing state in Sr2RuO4. [ABSTRACT FROM AUTHOR]

A number of key experiments have provided strong evidence for spin–triplet symmetry in the superconductivity of Sr2RuO4 with Tc=1.5 K. However, there still remain important issues to be clarified before its superconducting state is fully understood. We will focus on two important issues, which appear to be intimately related to each other. First, we present evidence for multiple superconducting phases of Sr2RuO4 in magnetic fields precisely parallel to the RuO2 plane. Second, we show indications that the upper critical field Hc2 is strongly suppressed when the magnetic field is precisely parallel to the RuO2 plane. We discuss the relationship between these two phenomena, which are both characteristic of the parallel field configuration. [Copyright &y& Elsevier]

The lack of thin films is one of the major obstacles in exploring the intriguing quantum properties specific to triplet superconductors. To have a single-domain chiral structure the sample should be made out of thin film, but crystalline imperfections until now have not allowed anybody to succeed in deposition of superconducting thin films of ruthenates. This stops not only general progress in investigating their properties, but in particular forbids practical realization of triplet superconductor qubits. Using the material properties of ruthenates, we have elaborated a method to overcome this problem. This report contains experimental aspects of our recent progress towards triplet superconductor qubits. [Copyright &y& Elsevier]

Transverse ultrasonic measurements have been performed on a single crystal of Sr2RuO4 across the superconducting transition temperature Tc (∼1.40 K). We found an indication of a jump at Tc in transverse elastic modulus C66, which is attributable to the coupling between strains and two-dimensional order parameters (OP) with broken time-reversal symmetry. This leads to additional evidence for the spin-triplet superconductivity with a two-dimensional OP. [Copyright &y& Elsevier]

Sr2RuO4 has become one of the most intensively studied transition metal oxides displaying unconventional superconductivity (Tc=1.5 K). It is known that its eutectic system Sr2RuO4–Ru shows interface superconductivity with an enhanced onset of ∼3 K. It is predicted that in the 3-K phase, a second transition to a chiral state occurs at a temperature between 1.5 and 3 K. The second transition is thought to be observable by ac-susceptibility measurements. We report the low-frequency ac-susceptibility of Sr2RuO4–Ru eutectic measured using a SQUID ac magnetometer. [Copyright &y& Elsevier]

Abstract: In order to explore unconventional superconductivity, we have studied pressure effect on a -electron Mott insulator Ca2RuO4. Pressurisation to Ca2RuO4 above 0.5GPa transforms it from a Mott insulator to a metal with a ferromagnetic ground state. The itinerancy of the ferromagnetic state at 2GPa is quantitatively evidenced by the magnetisation process at 2K. Moreover, the pressure phase diagram of this system suggests the existence of a ferromagnetic quantum phase transition at ∼10GPa. [ABSTRACT FROM AUTHOR]