We have clarified how the enhanced superconductivity, often referred to as the 3-K phase superconductivity, develops in the Sr2RuO4-Ru eutectic system.
From the detailed ac and dc susceptibility measurements on well-characterized crystals, we revealed strongly anisotropic shielding, governed by the direction of the screening current dominated within the RuO2 plane rather than by the orientation of the Ru lamellae. The onset temperature of the 3-K phase superconductivity probed by diamagnetic screening is as high as 3.5 K. The temperature dependence of the diamagnetic shielding above around 2 K is well ascribed by the interfacial screening around each Ru lamella. Below around 2 K, the rapid development of the shielding fraction as well as its peculiar esponse to ac and dc magnetic fields are explained by the formation of the Josephson network consisting of inter-lamellar supercurrents.
While the superconducting transition temperature Tc of Sr2RuO4 is 1.5 K, its onset Tc is enhanced as high as 3 K in the Sr2RuO4-Ru eutectic system, which is often referred to as the 3-K phase. We have investigated effects of uniaxial pressure on the non-bulk superconductivity in the 3-K phase. While Tc of pure Sr2RuO4 is known to be suppressed by hydrostatic pressure, a large enhancement of the superconducting volume fraction of the 3-K phase was observed for both out-of-plane and in-plane uniaxial pressures. Especially, under the in-plane pressure, the shielding fraction at 1.8 K of only less…
Superconducting behavior has been observed in the Sr2RuO4-Sr3Ru2O7 eutectic system as grown by the flux-feeding floating zone technique. A supercurrent flows across a single interface between Sr2RuO4 and Sr3Ru2O7 areas at distances that are far beyond those expected in a conventional proximity scenario. The current-voltage characteristics within the Sr3Ru2O7 macrodomain, as extracted from the eutectic, exhibit signatures of superconductivity in the bilayered ruthenate. Detailed microstructural, morphological and compositional analyses address issues on the concentration and the size of Sr2RuO4…
Although Sr3Ru2O7 has not been reported to exhibit superconductivity so far, ac susceptibility measurements revealed multiple superconducting transitions occurring in the Sr3Ru2O7 region cut from Sr3Ru2O7-Sr2RuO4 eutectic crystals. Based on various experimental results, some of us proposed the scenario in which Sr2RuO4 thin slabs with a few layers of the RuO2 plane are embedded in the Sr3Ru2O7 region as stacking faults and multiple superconducting transitions arise from the distribution of the slab thickness. To examine this scenario, we measured the resistivity along the ab plane (rho_ab) usi…
We report superconducting properties of Sr3Ru2O7-Sr2RuO4 eutectic crystals, consisting of the spin-triplet superconductor Sr2RuO4 with a monolayer stacking of RuO2 planes and the metamagnetic normal metal Sr3Ru2O7 with a bilayer stacking. Although Sr3Ru2O7 has not been reported to exhibit superconductivity so far, our AC susceptibility measurements revealed multiple superconducting transitions occurring in the Sr3Ru2O7 region of the eutectic crystals. The diamagnetic shielding essentially reached the full fraction at low AC fields parallel to the c axis. However, both the shielding fraction an…
We report here the structural, electrical and magnetic properties of as-cast V$_{1-x}$Zr$_x$ alloys ($x$ =0 - 0.4) at low temperatures. We observe that all the alloys undergo successive peritectic and eutectic reactions during cooling from the melt which leads to the formation of five phases, namely, a body centred cubic $β$-V phase, two phases with slightly different compositions having face centred cubic ZrV$_2$ structure, a hexagonal closed packed $α$-Zr phase, and the $β$-Zr precipitates. The amount of each phase is found to be dependent on the concentration of zirconium in vanadium. The $…
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 th…
We have investigated the tunnelling properties of the interface between superconducting Sr2RuO4 and a single Ru inclusion in eutectic. By using a micro-fabrication technique, we have made Sr2RuO4/Ru junctions on the eutectic system that consists of Sr2RuO4 and Ru micro-inclusions. Such a eutectic system exhibits surface superconductivity, called the 3-K phase. A zero bias conductance peak (ZBCP) was observed in the 3-K phase. We propose to use the onset of the ZBCP to delineate the phase boundary of a time-reversal symmetry breaking state.
The inhomogeneous 3-Kelvin (3K) phase of the eutectic Sr2RuO4 with Ru inclusions nucleates superconductivity at the interface between Ru and Sr2RuO4. The structure of the interface state and its physical properties are examined here. Two superconducting phases are identified between the transitions to the bulk phase at 1.5K and to the 3K phase. The nucleation of the 3K phase results in a state conserving time reversal symmetry, which generates an intrinsically frustrated superconducting network in samples with many Ru inclusions. At a lower temperature (>1.5K), a discontinuous (first order) tr…
Using micro fabrication techniques, we extracted individual channels of 3-Kelvin (3-K) phase superconductivity in Sr2RuO4-Ru eutectic systems and confirmed odd-parity superconductivity in the 3-K phase, similar to pure Sr2RuO4. Unusual hysteresis in the differential resistance-current and voltage-current characteristics observed below 2 K indicates the internal degrees of freedom of the superconducting state. A possible origin of the hysteresis is current-induced chiral-domain-wall motion due to the chiral p-wave state.
Single crystals of the Sr$_2$RuO$_4$-Ru eutectic system are known to exhibit enhanced superconductivity at 3~K, in addition to the bulk superconductivity of Sr$_2$RuO$_4$ at 1.5~K. The 1.5-K phase is believed to be a spin-triplet, chiral $p$-wave state with the multi-component order parameter, giving rise to chiral domain structure. In contrast, the 3-K phase is attributable to enhanced superconductivity of Sr$_2$RuO$_4$ in the strained interface region between Ru inclusion of a few to tens of micrometers in size and the surrounding Sr$_2$RuO$_4$. We investigate the dynamic behavior of a topol…
Sr$_{2}$RuO$_{4}$ (SRO) is the prime candidate for chiral $p$-wave superconductor with critical temperature $T_{c}(SRO)\sim$1.5 K. Chiral domains with opposite chiralities $p_{x}\pm ip_{y}$ were proposed, but yet to be confirmed. We measure the field dependence of the point contact (PC) resistance between a tungsten tip and the SRO-Ru eutectic crystal, where micrometer-sized Ru inclusions are embedded in SRO with atomic sharp interface. Ruthenium is an $s$-wave superconductor with $T_{c}(Ru)\sim$0.5 K, flux pinned near the Ru inclusions can suppress its superconductivity as reflected from the …
In this talk I outline the general strategy behind the SO(5) theory of high $T_c$ superconductivity. Progress in the direction of exact SO(5) models, numerical exact diagonalization and possible experimental tests are reviewed. I also address the criticisms raised recently against the SO(5) theory and point out directions for future exploration.
The concept of interface superconductivity was introduced over 50 years ago. Some of the greatest physicists of that time wondered whether a quasi-two-dimensional (2D) superconductor can actually exist, what are the peculiarities of 2D superconductivity, and how does the reduced dimensionality affect the critical temperature (Tc). The discovery of high-temperature superconductors, which are composed of coupled 2D superconducting layers, further increased the interest in reduced dimensionality structures. In parallel, the advances in experimental techniques made it possible to grow epitaxial 2D…
Structural and electronic characterization of (Ca0.67Sr0.33)Fe2As2 has been performed as a func- tion of pressure up to 12 GPa using conventional and designer diamond anvil cells. The compound (Ca0.67Sr0.33)Fe2As2 behaves intermediate between its end members-CaFe2As2 and SrFe2As2- displaying a suppression of magnetism and the onset of superconductivity. Like other members of the AEFe2As2 family, (Ca0.67Sr0.33)Fe2As2 undergoes a pressure-induced isostructural volume collapse, which we associate with the development of As-As bonding across the mirror plane of the structure. This collapsed tetrag…
The current status of basic research on the high temperature cuprate superconductors and prospects for technological applications of these materials is discussed. Recent developments concerning other novel superconductors are also briefly described.
We have investigated the magnetic field and temperature dependence of the tunneling spectra of the eutectic system Sr2RuO4-Ru. Electric contacts to individual Ru lamellae embedded in Sr2RuO4 enable the tunneling spectra at the interface between ruthenate and a Ru microinclusion to be measured. A zero bias conductance peak (ZBCP) was observed in the bias voltage dependence of the differential conductance, suggesting that Andreev bound states are present at the interface. The ZBCP starts to appear at a temperature well below the superconducting transition temperature. The onset magnetic field of…
This review presents a summary and evaluations of the superconducting properties of the layered ruthenate Sr2RuO4 as they are known in the autumn of 2011. This paper appends the main progress that has been made since the preceding review by Mackenzie and Maeno was published in 2003. Here, special focus is placed on the critical evaluation of the spin-triplet, odd-parity pairing scenario applied to Sr2RuO4. After an introduction to superconductors with possible odd-parity pairing, accumulated evidence for the pairing symmetry of Sr2RuO4 is examined. Then, significant recent progress on the theo…
We report an in-plane optical spectroscopy study on the iron-selenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_2$. The measurement revealed the development of a sharp reflectance edge below T$_c$ at frequency much smaller than the superconducting energy gap on a relatively incoherent electronic background, a phenomenon which was not seen in any other Fe-based superconductors so far investigated. Furthermore, the feature could be noticeably suppressed and shifted to lower frequency by a moderate magnetic field. Our analysis indicates that this edge structure arises from the development of a Jose…
Single crystals of superconducting OsB_2 [T_c = 2.10(5) K] have been grown using a Cu-B eutectic flux. We confirm that OsB_2 crystallizes in the reported orthorhombic structure (space group Pmmn) at room temperature. Both the normal and superconducting state properties of the crystals are studied using various techniques. Heat capacity versus temperature C(T) measurements yield the normal state electronic specific heat coefficient γ= 1.95(1) mJ/mol K^2 and the Debye temperature Θ_D = 539(2) K. The measured frequencies of Shubnikov-de Haas oscillations are in good agreement with those predicted…
The discovery of high temperature superconductivity in the cuprates in 1986 triggered a spectacular outpouring of creative and innovative scientific inquiry. Much has been learned over the ensuing 28 years about the novel forms of quantum matter that are exhibited in this strongly correlated electron system. This progress has been made possible by improvements in sample quality, coupled with the development and refinement of advanced experimental techniques. In part, avenues of inquiry have been motivated by theoretical developments, and in part new theoretical frameworks have been conceived t…
Recent experimental and theoretical developments in high-temperature superconductivity are reviewed, and the empirically asymmetric behavior between hole-doped and electron-doped cuprates is contrasted. A number of phenomena previously considered as essential for the formation of cuprate superconductivity, such as the pairing symmetry, pseudogap phenomenon, gapped incommensurate spin fluctuations and charged stripes, are found to be non-universal, and are likely the consequence of competing orders. It is suggested that the only ubiquitous properties among all cuprates are the strong electronic…
The review of recent developments in the unconventional superconductivity theory is given. In the fist part I consider the physical origin of the Kerr rotation polarization of light reflected from the surface of superconducting $Sr_2RuO_4$. Then the comparison of magneto-optical responses in superconductors with orbital and spin spontaneous magnetization is presented. The latter result is applied to the estimation of the magneto-optical properties of neutral superfluids with spontaneous magnetization. The second part is devoted to the natural optical activity or gyrotropy properties of noncent…
Room temperature superconductivity has been the most prominent, highly ambitious, but still imaginable, acme of materials physics for half a century. The struggle toward this revolution was foreshadowed by a Victorian novelist and championed,unsuccessfully, by dogged physicists in the 1960s to 1980s who had a workable theory but uncompliant materials. Discovery of superconductivity of H$_3$S at 200 K in the 160-200 GPa pressure range has renewed anticipation of yet higher values of the critical temperature T$_c$. With the several reports of metalization of hydrogen, and theoretical extensions …
The main purpose of the paper is to present an overview of the current situation in the development of understanding of the mechanism of high-Tc superconductivity which arises due to moderately strong, nonlinear electron-phonon interactions and due to magnetic (spin) fluctuations.
Recent experimental discoveries of superconductivity and other exotic electronic states in twisted bilayer graphene (TBG) call for a reconsideration of our traditional theories of these states, usually based on the assumption of the presence of a Fermi surface. Here we show how such developments may even help us finding mechanisms of increasing the critical temperature of superconductivity towards the room temperature.
It is shown that the qualitative model of the high-temperature superconductivity suggested earlier for cuprates and doped picene and based on the idea that the valence electron state depends on the character of the chemical bonds they form and on the Coulomb interaction between the electrons is not only confirmed by the experimental data on iron pnictides but is also improved. From the chemical point of view, the high-temperature superconductivity is associated with additional $π$ bonding along chains of covalently bonded ions via a delocalized $π$ orbital, just like in cuprates. From the phys…
Room temperature superconductivity has been achieved under high pressure in an organically derived carbonaceous sulfur hydride with a critical superconducting transition temperature (Tc) of 288 kelvin. This development is part of a new class of dense, hydrogen rich materials with remarkably high critical temperatures. Metal superhydrides are a subclass of these materials that provide a different and potentially more promising route to very high Tc superconductivity. The most promising binary metal superhydrides contain alkaline or rare earth elements, and recent experimental observations of La…
Thin superconducting films form a unique platform for geometrically-confined, strongly-interacting electrons. They allow an inherent competition between disorder and superconductivity, which in turn enables the intriguing superconducting-to-insulator transition and believed to facilitate the comprehension of high-Tc superconductivity. Furthermore, understanding thin film superconductivity is technologically essential e.g. for photo-detectors, and quantum-computers. Consequently, the absence of an established universal relationships between critical temperature ($T_c$), film thickness ($d$) and…
The discovery of the Meissner effect was a turning point in the history of superconductivity. It demonstrated that superconductivity is an equilibrium state of matter, thus allowing to use thermodynamics for its study. This provided a justification for the two-fluid model of Gorter and Casimir, a seminal thermodynamic theory founded on a postulate of zero entropy of the superconducting (S) component of conduction electrons. It also demonstrated that, apart from zero resistivity, the S phase is also characterized by zero magnetic induction, used as a basic postulate in the theory of Londons und…
In view of recent developments in the investigation on cuprate high-T${}_{\rm c}$ superconductors and the spin-Peierls compound CuGeO${}_{3}$, we study the effect of dilute impurity doping on the spin-Peierls state in quasi-one dimensional systems. We identify a common origin for the emergence of antiferromagnetic order upon the introduction of static vacancies, and superconductivity for mobile holes.
In 2012, we discovered superconductivity in Bi4O4S3 and LaO1-xFxBiS2, whose crystal structures are composed of alternate stacks of a BiS2-type conduction layer and a blocking layer. Since the discovery, many related superconductors and functional materials have been synthesized. Furthermore, the possibility of unconventional superconductivity has been proposed in recent theoretical and experimental studies. In addition, notable correlations between local structure and physical properties have been revealed in the BiS2-based systems. In this review article, we summarize the material development…
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping from a pseudogap state in the underdoped cuprates to a superconducting state at optimal and overdoping reflects an underlying SO(6) instability structure of the (pi,0) saddle points. The pseudogap is probably not associated with superconductivity, but is related to competing nesting instabilities, which are responsible for the stripe phases. We earlier introduced a simple Ansatz of this competition in terms of a pinned Balseiro-Falicov (pBF) model of competing charge density wave and (s-wave) superconductivity. This mod…
The key challenge in superconductivity research is to go beyond the historical mode of discovery-driven research. We put forth a new strategy, which is to combine theoretical developments in the weak-coupling renormalization group approach with the experimental developments in lattice strain driven Fermi surface-engineering. For concreteness we theoretically investigate how superconducting tendencies will be affected by strain engineering of ruthenates' Fermi surface. We first demonstrate that our approach qualitatively reproduces recent experiments under uniaxial strain. We then note that ord…
In contrast to the claim by Yanagi et al. in Ref. [1], the development of orbital fluctuations at $q ~ 0$ due to the "orthorhombic phonon" is unable to explain the high-Tc superconductivity in iron pnictides. The reason is that the orthorhombic-phonon is acoustic with the energy $ω_q \propto q$, although the authors in Ref. [1] treated it as optical phonon ($ω_o=0.02 eV$) inconsistently.
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics properties and could be used in the development of electronics, photonics, spintronics and quantum computing devices. The mechanical exfoliation technique of micro-size TMD flakes has attracted particular interest due to its simplicity and cost effectiveness. However, for most applications, large area and high quality films are preferred. Furthermore, when the thickness of crystalline films is down to the 2D limit (monolayer), exotic properties can be expected due to the quantum confinement and symmetry…
We introduce a McMillan-Ginzburg-Landau theory to describe the cooperative coexistence of charge-density and superconducting order in two-dimensional crystals. With a free-energy that explicitly accounts for the competition between commensurate and incommensurate ground states, we are able to map the transition between these phases and monitor the development of discommensurations in the near-commensurate regime. Attributing the enhancement of superconducting order to density-wave fluctuations, we propose a coupling scheme that yields a phase diagram in qualitative agreement with experiments i…
In several members of the pnictide materials, spin density wave order coexists with superconductivity over a range of dopings and temperature. In this paper we show that odd-frequency superconductivity emerges on the edges of pnictides in such a coexistence phase. In particular, the breaking of spin-rotation symmetry by SDW and translation symmetry by the edge can lead to the development of odd-frequency spin-triplet Cooper pairing on edges of superconducting pnictide samples. In this case, the odd-frequency pairing has even parity components, which are immune to disorder. Our results show tha…
We study the influence of the thickness Df of the plain ferromagnetic (F) film on the electrical resistance of the flux-coupled hybrids, consisting of superconducting (S) Al film and multilayer [Co/Pt] F film with out-of-plain magnetization. The behavior of such hybrids at high and low temperatures is found to be different: the nucleation of superconductivity at high temperatures is governed mainly by the typical lateral dimensions of the magnetic domains, while low temperature properties are determined by topology of the magnetic template. We show that an increase in the Df value leads to a b…
We investigate the development of superconductivity in graphene when the Fermi level becomes close to one of the Van Hove singularities of the electron system. The origin of the pairing instability lies in the strong anisotropy of the e-e scattering at the Van Hove filling, which leads to a channel with attractive coupling when making the projection of the BCS vertex on the symmetry modes with nontrivial angular dependence along the Fermi line. We show that the scale of the superconducting instability may be pushed up to temperatures larger than 10 K, depending on the ability to tune the syste…
Recent development in the physics of high-temperature superconductivity is reviewed, with special emphasis on the studies of the low-energy excitations of cuprate and iron-based superconductors. For cuprate superconductors, a phenomenology based on coexisting competing orders with superconductivity in the ground state of these doped Mott insulators is shown to provide a consistent account for a wide range of experimental findings. In the case of iron-based superconductors, studies of the low-energy excitations reveal interesting similarities and differences when compared with cuprate supercond…
In order to discuss superconductivity in orbital degenerate systems, a microscopic Hamiltonian is introduced. Based on the degenerate model, a strong-coupling theory of superconductivity is developed within the fluctuation exchange (FLEX) approximation where spin and orbital fluctuations, spectra of electron, and superconducting gap function are self-consistently determined. Applying the FLEX approximation to the orbital degenerate model, it is shown that the $d_{x^2-y^2}$-wave superconducting phase is induced by increasing the orbital splitting energy which leads to the development and suppre…
We summarize the recent developments in the model design and computation for a few representative quantum many-body systems, encompassing quantum critical metals beyond the Hertz-Millis-Moriya framework with pseudogap and superconductivity, SYK non-Fermi-liquid with self-tuned quantum criticality and fluctuation induced superconductivity, and the flat-band quantum Moiré lattice models in continuum where the interplay of quantum geometry of flat-band wave function and the long-range Coulomb interactions gives rise to novel insulating phases at integer fillings and superconductivity away from th…
We review the Resonating Valence Bond (RVB) theory of high temperatur e superconductivity using Gutzwiller projected wave functions that incorporate strong correlations. After a general overview of the phenomenon of high temperature superconductivity, we discuss Anderson's RVB picture and its implementation by renormalised mean field theory (RMFT) and variational Monte Carlo (VMC) techniques. We review RMFT and VMC results with an emphasis on recent development s in extending VMC and RMFT techniques to excited states. We compare results obtained from these methods with angle resolved photoemis…
Quasi one-dimensional organic conductors: from Fröhlich conductivity and Peierls insulating state to magnetically-mediated superconductivity, a retrospective
Iron-based superconductors Ba0.7Rb0.3Fe2As2 and CaFe1.92Co0.08As2 of the '122' family have been investigated by means of the 14.41-keV Moessbauer transition in 57Fe versus temperature ranging from the room temperature till 4.2 K. A comparison is made with the previously investigated parent compounds BaFe2As2 and CaFe2As2. It has been found that Moessbauer spectra of these superconductors are composed of the magnetically split component due to development of spin density wave (SDW) and non-magnetic component surviving even at lowest temperatures. The latter component is responsible for supercon…
The Ru doping effect on the Dirac cone states is investigated in iron pnictide superconductors Ba(Fe$_{1-x}$Ru$_x$As)$_2$ using the transverse magnetoresistance (MR) measurements as a function of temperature. The linear development of MR against magnetic field $B$ is observed for $x$ = 0 - 0.244 at low temperatures below the antiferromagnetic transition. The $B$-linear MR is interpreted in terms of the quantum limit of the Dirac cone states by using the model proposed by Abrikosov. An intriguing evidence is shown that the Dirac cone state persists on the electronic phase diagram where the anti…
It is a honor to write a contribution on this memorial for Sandro Massidda. For both of us, at different stages of our life, Sandro was first and foremost a friend. We both admired his humble, playful and profound approach to life and physics. In this contribution we describe the route which permitted to meet a long-standing challenge in solid state physics, i.e. room temperature superconductivity. In less than 20 years the Tc of conventional superconductors, which in the last century had been widely believed to be limited to 25 K, was raised from 40 K in MgB2 to 265 K in LaH10. This discovery…
A systematic muon-spin-relaxation study in Bi-2201 high-Tc cuprates has revealed for the first time that the Cu-spin correlation (CSC) is developed at low temperatures below 2 K in a wide range of hole concentration where superconductivity appears. The CSC tends to become weak gradually with increasing hole-concentration. Moreover, CSC has been enhanced through the 3% substitution of Zn for Cu. These results are quite similar to those observed in La-214 high-Tc cuprates. Accordingly, it has been suggested that the intimate relation between the so-called spin-charge stripe correlations and supe…
We give a broad overview of the history of microwave superconductivity and explore the technological developments that have followed from the unique electrodynamic properties of superconductors. Their low loss properties enable resonators with high quality factors that can nevertheless handle extremely high current densities. This in turn enables superconducting particle accelerators, high-performance filters and analog electronics, including metamaterials, with extreme performance. The macroscopic quantum properties have enabled new generations of ultra-high-speed digital computing and extrao…
The isotope effect provided a crucial key to the development of the BCS (Bardeen-Cooper-Schrieffer) microscopic theory of superconductivity for conventional superconductors. In superconducting cooper oxides (cuprates) showing an unconventional type of superconductivity, the oxygen isotope effect is very peculiar: the exponential coefficient strongly depends on doping level. No consensus has been reached so far on the origin of the isotope effect in the cuprates. Here we show that the oxygen isotope effect in cuprates is in agreement with the bisoliton theory of superconductivity.
