Showing papers on "High-temperature superconductivity published in 2019"
TL;DR: In this paper, the authors investigated the nature of the electronic interactions that cause pairing and found that superconductivity occurs on cuprates and that pairing occurs on top of the cuprates.
Abstract: Cuprates exhibit exceptionally strong superconductivity. To understand why, it is essential to elucidate the nature of the electronic interactions that cause pairing. Superconductivity occurs on th...
246 citations
TL;DR: A fabrication process is developed that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212), which displays all the fundamental physics of high-Temperature superconductivity and other strongly correlated phenomena in two dimensions.
Abstract: Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity. Here we develop a fabrication process that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212; here, a monolayer refers to a half unit cell that contains two CuO2 planes). The highest superconducting transition temperature of the monolayer is as high as that of optimally doped bulk. The lack of dimensionality effect on the transition temperature defies expectations from the Mermin–Wagner theorem, in contrast to the much-reduced transition temperature in conventional two-dimensional superconductors such as NbSe2. The properties of monolayer Bi-2212 become extremely tunable; our survey of superconductivity, the pseudogap, charge order and the Mott state at various doping concentrations reveals that the phases are indistinguishable from those in the bulk. Monolayer Bi-2212 therefore displays all the fundamental physics of high-temperature superconductivity. Our results establish monolayer copper oxides as a platform for studying high-temperature superconductivity and other strongly correlated phenomena in two dimensions. Transport and scanning tunnelling microscopy studies of freestanding monolayers of an unconventional layered copper oxide establish that the superconducting properties of copper oxides are not changed in the 2D limit.
214 citations
TL;DR: The superconductivity identification of sulfur hydride at pressure is reported, employing an in situ alternating-current magnetic susceptibility technique and determines the superconducting phase diagram, finding thatsuperconductivity suddenly appears at 117 GPa and Tc reaches 183 K at 149 GPa before decreasing monotonically with increasing pressure.
Abstract: The search for high-temperature superconductivity is one of the research frontiers in physics. In the sulfur hydride system, an extremely high Tc (∼200 K) has been recently developed at pressure. However, the Meissner effect measurement above megabar pressures is still a great challenge. Here, we report the superconductivity identification of sulfur hydride at pressure, employing an in situ alternating-current magnetic susceptibility technique. We determine the superconducting phase diagram, finding that superconductivity suddenly appears at 117 GPa and Tc reaches 183 K at 149 GPa before decreasing monotonically with increasing pressure. By means of theoretical calculations, we elucidate the variation of Tc in the low-pressure region in terms of the changing stoichiometry of sulfur hydride and the further decrease in Tc owing to a drop in the electron-phonon interaction parameter λ. This work provides a new insight into clarifying superconducting phenomena and anchoring the superconducting phase diagram in the hydrides.
61 citations
TL;DR: Srir2IrO4, a single-layer member of the Ruddlesden-Popper series iridates, has received much attention as a close analog of cuprate high-temperature superconductors.
Abstract: Over the past few years, Sr2IrO4, a single-layer member of the Ruddlesden–Popper series iridates, has received much attention as a close analog of cuprate high-temperature superconductors. Although...
59 citations
TL;DR: In this article, the authors exploit the intrinsic advantages of hydrogen to strongly enhance phonon-mediated superconductivity in a completely different system, namely, a two-dimensional material with hydrogen adatoms, and find that van Hove singularities in the electronic structure, originating from atomiclike hydrogen states, lead to a strong increase of the electronic density of states at the Fermi level, and thus of the electron-phonon coupling.
Abstract: Hydrogen-based compounds under ultrahigh pressure, such as the polyhydrides ${\mathrm{H}}_{3}\mathrm{S}$ and ${\mathrm{LaH}}_{10}$, superconduct through the conventional electron-phonon coupling mechanism to attain the record critical temperatures known to date. Here we exploit the intrinsic advantages of hydrogen to strongly enhance phonon-mediated superconductivity in a completely different system, namely, a two-dimensional material with hydrogen adatoms. We find that van Hove singularities in the electronic structure, originating from atomiclike hydrogen states, lead to a strong increase of the electronic density of states at the Fermi level, and thus of the electron-phonon coupling. Additionally, the emergence of high-frequency hydrogen-related phonon modes in this system boosts the electron-phonon coupling further. As a concrete example, we demonstrate the effect of hydrogen adatoms on the superconducting properties of monolayer ${\mathrm{MgB}}_{2}$, by solving the fully anisotropic Eliashberg equations, in conjunction with a first-principles description of the electronic and vibrational states, and their coupling. We show that hydrogenation leads to a high critical temperature of 67 K, which can be boosted to over 100 K by biaxial tensile strain.
52 citations
TL;DR: In this paper, nano-whiskers manufactured by a solution blow spinning technique were added to the Y-Ba-Cu-O precursor powders to produce a single grain bulk sample by the buffer-aided top-seeded melt growth (BA-TSMG) processing technique.
32 citations
TL;DR: In this paper, it was shown that p-quinquinquephenyl containing five phenyl rings connected in para position is superconducting when the compound is doped by potassium, with a critical temperature of 7.3K.
31 citations
TL;DR: In this article, the authors used Raman spectroscopy to investigate magnetic ordering in FeSe and further demonstrate that its properties are distinct among the iron-based superconductors.
Abstract: The charge and spin dynamics of the structurally simplest iron-based superconductor, FeSe, may hold the key to understanding the physics of high temperature superconductors in general. Unlike the iron pnictides, FeSe lacks long range magnetic order in spite of a similar structural transition around 90 K. Here, we report results of Raman scattering experiments as a function of temperature and polarization and simulations based on exact diagonalization of a frustrated spin model. Both experiment and theory find a persistent low energy peak close to 500 cm−1 in B1g symmetry, which softens slightly around 100 K, that we assign to spin excitations. By comparing with results from neutron scattering, this study provides evidence for nearly frustrated stripe order in FeSe. The origin of the superconductivity in iron-based superconductors remains elusive and whether a mechanism which describes all members can be found is under constant study. Using Raman spectroscopy the authors investigate magnetic ordering in FeSe, and further demonstrate that its properties are distinct among the iron-based superconductors.
27 citations
03 Jun 2019
TL;DR: In this article, the authors consider theoretically and experimental support to the models of the Fermi-Bose mixtures and the BCS-BEC crossover compared with the strong-coupling approach, which can serve as the cornerstones on the way from hightemperature to room-temperature superconductivity in pressurized metallic hydrides.
Abstract: In this review article we consider theoretically and give experimental support to the models of the Fermi-Bose mixtures and the BCS-BEC (Bardeen Cooper Schrieffer–Bose Einstein) crossover compared with the strong-coupling approach, which can serve as the cornerstones on the way from high-temperature to room-temperature superconductivity in pressurized metallic hydrides. We discuss some key theoretical ideas and mechanisms proposed for unconventional superconductors (cuprates, pnictides, chalcogenides, bismuthates, diborides, heavy-fermions, organics, bilayer graphene, twisted graphene, oxide hetero-structures), superfluids and balanced or imbalanced ultracold Fermi gases in magnetic traps. We build a bridge between unconventional superconductors and recently discovered pressurized hydrides superconductors H3S and LaH10 with the critical temperature close to room temperature. We discuss systems with a line of nodal Dirac points close to the Fermi surface and superconducting shape resonances, and hyperbolic superconducting networks which are very important for the development of novel topological superconductors, for the energetics, for the applications in nano-electronics and quantum computations.
26 citations
TL;DR: In this paper, a systematic structure search in the Ce-H system under pressure reveals novel stable phases with intriguing electronic properties, such as CeH 4, CeH 9, and CeH 10.
Abstract: A systematic structure search in the Ce-H system under pressure reveals novel stable phases with intriguing electronic properties. Several cerium hydrides, CeH 4, CeH 9, and CeH 10, were found to be dynamically stable. Electron-phonon calculations coupled to Bardeen-Cooper-Schrieffer arguments indicate that they might be high- T c superconductors. In particular, the maximum T c values for F 4 ¯ 3 m-CeH 9 and F m 3 ¯ m-CeH 10 are 142 K and 168 K at 94 GPa. These findings may pave the way for achieving room temperature superconductors in dense hydrogen-rich compounds.
25 citations
TL;DR: In this article, the critical temperature of the transition of a translation-invariant bipolaron Bose gas into the superconducting state, its energy, heat capacity and heat of transition were investigated.
Abstract: The properties of a Bose gas of translation-invariant (TI) bipolarons analogous to Cooper pairs are considered. As in the BCS theory, the description of a TI-bipolaron gas is based on the electron-phonon interaction and Froehlich Hamiltonian. As distinct from the BCS theory, when the correlation length greatly exceeds the mean distance between the pairs, here we deal with the opposite case when the correlation length is much less than the distance between the pairs. We calculate the critical temperature of the transition of a TI-bipolaron Bose-gas into the superconducting state, its energy, heat capacity and heat of the transition. The results obtained are used to explain the experiments on high-temperature superconductors. Possible ways of raising the critical temperature of high-temperature superconductors are discussed.
TL;DR: In this paper, a brief review of the basic physics of conventional low-temperature superconductors as well as of the high-treme supercondors are presented with a brief introduction to applications exemplified from high-power to low-power electronic devices.
Abstract: Superconductivity was discovered in 1911 by Kamerlingh Onnes and Holst in mercury at the temperature of liquid helium (4.2 K). It took almost 50 years until in 1957 a microscopic theory of superconductivity, the so-called BCS theory, was developed. Since the discovery a number of superconducting materials were found with transition temperatures up to 23 K. A breakthrough in the field happened in 1986 when Bednorz and Muller discovered a new class of superconductors, the so-called cuprate high-temperature superconductors with transition temperatures as high as 135 K. This surprising discovery initiated new efforts with respect to fundamental physics, material science, and technological applications. In this brief review the basic physics of the conventional low-temperature superconductors as well as of the high-temperature superconductors are presented with a brief introduction to applications exemplified from high-power to low-power electronic devices. Finally, a short outlook and future challenges are presented, finished with possible imaginations for applications of room-temperature superconductivity.
TL;DR: This work achieves protonation of an electron doped high temperature superconductor-La2-xCexCuO4 by gating in the electrochemical regime of the ionic liquid by applying negative voltages.
Abstract: Ion injection controlled by electric field has attracted growing attention due to its tunability over bulk-like materials. Here, we achieve protonation of an electron-doped high-temperature superco...
TL;DR: In this paper, the effect of inhomogeneity on electronic properties of the two-dimensional Hubbard model on a square lattice using dynamical mean-field theory (DMFT) was explored.
Abstract: We explore the effect of inhomogeneity on electronic properties of the two-dimensional Hubbard model on a square lattice using dynamical mean-field theory (DMFT). The inhomogeneity is introduced via modulated lattice hopping such that in the extreme inhomogeneous limit the resulting geometry is a Lieb lattice, which exhibits a flat-band dispersion. The crossover can be observed in the uniform sublattice magnetization which is zero in the homogeneous case and increases with the inhomogeneity. Studying the spatially resolved frequency-dependent local self-energy, we find a crossover from Fermi-liquid to non-Fermi-liquid behavior happening at a moderate value of the inhomogeneity. This emergence of a non-Fermi liquid is concomitant of a quasiflat band. For finite doping the system with small inhomogeneity displays $d$-wave superconductivity coexisting with incommensurate spin-density order, inferred from the presence of oscillatory DMFT solutions. The $d$-wave superconductivity gets suppressed for moderate to large inhomogeneity for any finite doping while the incommensurate spin-density order still exists.
TL;DR: A brief explanation is given of what limits the magnetic field in a tokamak, and why high temperature superconductors (HTSs) are a game changer, not just because of their higher magnetic fields but also for reasons of higher current density and higher operating temperatures.
Abstract: The advantages of high magnetic fields in tokamaks are reviewed, and why they are important in leading to more compact tokamaks. A brief explanation is given of what limits the magnetic field in a ...
TL;DR: In this article, the presence of a topological Dirac nodal line surface state on the [010] faces of the BCS superconductor has been investigated using a combination of angle-resolved photoemission spectroscopy measurements and density-functional theory calculations.
Abstract: Most topological superconductors known to date suffer from low transition temperatures (${T}_{c}$) and/or high fragility to disorder and dopant levels, which is hampering the progress in this promising field. Here, utilizing a combination of angle-resolved photoemission spectroscopy measurements and density-functional theory calculations, we show the presence of a type of topological Dirac nodal line surface state on the [010] faces of the ${T}_{c}=39$ K BCS superconductor ${\mathrm{MgB}}_{2}$. This surface state should be highly tolerant against disorder and inadvertent doping variations and is expected to go superconducting via the proximity effect to the bulk superconductor that this state is intimately connected to. This would represent a form of high-temperature topological superconductivity.
TL;DR: It is theoretically and experimentally demonstrated that critical-current density in high-temperature superconductors can reach unprecedented high values at high fields and temperatures by preventing vortex entry.
Abstract: Superconductors are essential in many present and future technologies, from large-scale devices for medical imaging, accelerators, or fusion experiments to ultra-low-power superconducting electroni...
TL;DR: In this article, the effects of size and aspect ratio of YBCO single grains on these key applied properties have been investigated experimentally and the results compared qualitatively with the predictions of an established theoretical model.
Abstract: Bulk, single grain (RE)Ba 2 Cu 3 O 7-δ [(RE)BCO, where RE is a rare earth element or yttrium] high temperature superconductors exhibit significant potential for use in a variety of engineering applications due to their ability to trap large magnetic fields, which can be up to ten times greater than those generated by conventional, iron-based magnets. Limitations on the maximum size to which single grains can be grown, however, are a major obstacle to the further development of these materials. Indeed, multiple samples are often required to achieve the required superconducting properties in particular applications. The geometry of bulk (RE)BCO single grain samples plays an important role in determining the superconducting properties of a given technical arrangement. In order to gain a better understanding of the full application potential of bulk single grain superconductors, three relatively long, cylindrical YBCO single grains of different diameters were fabricated and their trapped field and total trapped flux measured at 77 K as a function of sample height. The effects of size and aspect ratio of YBCO single grains on these key applied properties have been investigated experimentally and the results compared qualitatively with the predictions of an established theoretical model. Conclusions based on the trapped field measurements on a variety of single grain samples are presented in this study and the possibilities of using assemblies of smaller samples for engineering devices, in particular, are discussed.
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TL;DR: In this paper, a pressure-induced crossover from two-to-three-dimensional superconducting states in the optimally-doped Bi2Sr2CaCu2O8+delta bulk superconductor at a pressure above 2.8 GPa was observed.
Abstract: To decipher the mechanism of high temperature superconductivity, it is important to know how the superconducting pairing emerges from the unusual normal states of cuprate superconductors, including pseudogap, anomalous Fermi liquid and strange metal (SM). A long-standing issue under debate is how the superconducting pairing is formed and condensed in the SM phase because the superconducting transition temperature is the highest in this phase. Here, we report the first experimental observation of a pressure-induced crossover from two- to three-dimensional superconducting states in the optimally-doped Bi2Sr2CaCu2O8+delta bulk superconductor at a pressure above 2.8 GPa, through state-of-the-art in-situ high-pressure measurements of resistance, magnetoresistance and magnetic susceptibility. By analyzing the temperature dependence of resistance, we find that the two-dimensional (2D) superconducting transition exhibits a Berezinski-Kosterlitz-Thouless-like behavior. The emergence of this 2D superconducting transition provides direct and strong evidence that the SM state is predominantly 2D-like. This is important to a thorough understanding of the phase diagram of cuprate superconductors.
TL;DR: In this article, the temperature dependence of the upper critical magnetic field Hc2 in a layered iron-based superconducting (IBS) material of the 11-family, namely the Fe(Se,Te) thin film grown on CaF2 substrate was studied.
Abstract: We study the temperature dependence of the upper critical magnetic field Hc2 in a layered iron-based superconducting (IBS) material of the 11-family, namely the Fe(Se,Te) thin film grown on CaF2 substrate. On the basis of intrinsic anisotropy as well as system dimensionality, it turns useful to make a comparison with an ultrathin film conventional low temperature superconductors (LTS) such as NbN, mostly used for device applications. We compare the anisotropy factors as a function of temperature and magnetic field. Both materials present a peculiar behavior: The LTS behaves as a strong anisotropic system, whereas the IBS shows very weak anisotropic features. The strong NbN character can be directly ascribed to the dimensionality of the ultrathin film employed, thus revealing a geometry effect. The weak Fe(Se,Te) trend should be related to its layered crystallographic structure, thus probing an intrinsic origin of its anisotropy. These characteristics become relevant for the potential application of this material in coated conductor technology.
TL;DR: In this article, the effect of anisotropic strain on the superconducting transition in single crystals of $Ba(Fe(1-x}Co_{x}){2}As_{2}$ through electrical transport, magnetic susceptibility, and x-ray diffraction measurements was investigated.
Abstract: High temperature superconductivity emerges in the vicinity of competing strongly correlated phases. In the iron-based superconductor $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$, the superconducting state shares the composition-temperature phase diagram with an electronic nematic phase and an antiferromagnetic phase that break the crystalline rotational symmetry. Symmetry considerations suggest that anisotropic strain can enhance these competing phases and thus suppress the superconductivity. Here we study the effect of anisotropic strain on the superconducting transition in single crystals of $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ through electrical transport, magnetic susceptibility, and x-ray diffraction measurements. We find that in the underdoped and near-optimally doped regions of the phase diagram, the superconducting critical temperature is rapidly suppressed by both compressive and tensile stress, and in the underdoped case this suppression is enough to induce a strain-tuned superconductor to metal quantum phase transition.
TL;DR: In this paper, a systematic investigation of Au-added YBa2Cu3O7−x (YBCO) samples has been undertaken mainly to understand their structural, magnetic as well as electrical behavior.
Abstract: In this study, a systematic investigation of Au-added YBa2Cu3O7−x (YBCO) samples has been undertaken mainly to understand their structural, magnetic as well as electrical behavior. In order to get high quality of superconducting bulk samples, optimum preparation conditions have been provided. The bulk samples were produced by conventional solid state reaction method at 930 °C under the low oxygen atmosphere. The bulk samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive X-ray, dc electrical resistivity (ρ-T), critical current density and ac magnetic susceptibility measurements. The samples were carried out by means of powder X-ray diffraction for phase analyses, and lattice parameters, SEM for microstructure examinations. From XRD and SEM results, an increase in the intensity of main peaks and average grain size but a increase in the lattice parameter c with addition of Au in comparison with the unadded sample. In addition to this, onset-offset critical transition temperature (
$${T}_{c}^{on}, {T}_{c}^{off}$$
) was determined from dc electrical resistivity (
$$\rho -T$$
) as a function of temperature measurements. Moreover, the transition temperature (Tc) and the critical current density (Jc) are enhanced upon Au addition when compared with unadded YBCO superconductor sample. The increment of Au content decreased the transition temperature and the critical current density values. From the AC susceptibility measurements, inter-grain, intra-grain peak temperature and superconducting transition temperature were determined. The possible reasons for the observed improvements in structural, magnetic and electrical properties due to the Au addition in the YBCO samples were discussed by comparison with other studies about addition of transition metals in high temperature superconductors.
TL;DR: In this paper, the authors argue that experimental results over the past 30 years on electron-doped cuprate materials have provided conclusive answers to these fundamental questions, by establishing that superconductivity originates in pairing of hole carriers in the same band.
Abstract: Since their experimental discovery in 1989, the electron-doped cuprate superconductors have presented both a major challenge and a major opportunity. The major challenge has been to determine whether these materials are fundamentally different from or essentially similar to their hole-doped counterparts; a major opportunity because answering this question would strongly constrain the possible explanations for what is the essential physics that leads to high temperature superconductivity in the cuprates, which is still not agreed upon. Here we argue that experimental results over the past 30 years on electron-doped cuprate materials have provided conclusive answers to these fundamental questions, by establishing that both in hole- and electron-doped cuprates, superconductivity originates in pairing of hole carriers in the same band. We discuss a model to describe this physics that is different from the generally accepted ones, and calculate physical observables that agree with experiment, in particular tunneling characteristics. We argue that our model is simpler, more natural and more compelling than other models. Unlike other models, ours was originally proposed before rather than after many key experiments were performed.
TL;DR: In this article, a holographic Fermionic system coupled with a massive scalar field in an AdS black hole background was studied and two possible scenarios were discussed: 1) when the scalar condenses below a critical temperature, the Fermi surface undergoes a transition from normal phase to pseudo-gap phase.
Abstract: Fermi arcs are disconnected contour of Fermi surface, which can be observed in the pseudo-gap phase of high temperature superconductors. Aiming to understand this pseudo-gap phenomena, we study a holographic Fermionic system coupled with a massive scalar field in an AdS black hole background. Depending on the boundary condition on the scalar field mode, we discuss two possible scenarios. When the scalar condenses below a critical temperature $T_c$, the Fermi surface undergoes a transition from normal phase to pseudo-gap phase. Hence $T_c$ can be the reminiscent of well-known cross over temperature $T^*$ in cuprate superconductor, below which pseudo-gap appears at constant doping. In the second scenario, the bulk scalar develops a non-normalizable profile at arbitrary temperature for non-zero source at the boundary. Therefore, we can tune the Fermi spectrum by tuning a dual source at the boundary. The dual source for this case can be the reminiscent of hole doping in the real cuprate superconductor. For both the cases we have studied Fermi spectrum and observed anisotropic gap in the spectral function depending on the model parameter and studied the properties of Fermi arcs across different phases.
19 Jun 2019
TL;DR: In this article, the authors discuss cuprate high-temperature superconductors and argue that superconductivity of high temperature cuprates is induced by the strong on-site Coulomb interaction.
Abstract: It is very important to elucidate the mechanism of superconductivity for achieving room temperature superconductivity. In the first half of this paper, we give a brief review on mechanisms of superconductivity in many-electron systems. We believe that high-temperature superconductivity may occur in a system with interaction of large-energy scale. Empirically, this is true for superconductors that have been found so far. In the second half of this paper, we discuss cuprate high-temperature superconductors. We argue that superconductivity of high temperature cuprates is induced by the strong on-site Coulomb interaction, that is, the origin of high-temperature superconductivity is the strong electron correlation. We show the results on the ground state of electronic models for high temperature cuprates on the basis of the optimization variational Monte Carlo method. A high-temperature superconducting phase will exist in the strongly correlated region.
TL;DR: In this article, the authors used angle-resolved photoemission spectroscopy (ARPES) to study strongly interacting fermions in an optical lattice using a quantum gas microscope.
Abstract: Angle-resolved photoemission spectroscopy (ARPES) measures the single-particle excitations of a many-body quantum system with both energy and momentum resolution, providing detailed information about strongly interacting materials. ARPES is a direct probe of fermion pairing, and hence a natural technique to study the development of superconductivity in a variety of experimental systems ranging from high temperature superconductors to unitary Fermi gases. In these systems a remnant gap-like feature persists in the normal state, which is referred to as a pseudogap. A quantitative understanding of pseudogap regimes may elucidate details about the pairing mechanisms that lead to superconductivity, but developing this is difficult in real materials partly because the microscopic Hamiltonian is not known. Here we report on the development of ARPES to study strongly interacting fermions in an optical lattice using a quantum gas microscope. We benchmark the technique by measuring the occupied single-particle spectral function of an attractive Fermi-Hubbard system across the BCS-BEC crossover and comparing to quantum Monte Carlo calculations. We find evidence for a pseudogap in our system that opens well above the expected critical temperature for superfluidity. This technique may also be applied to the doped repulsive Hubbard model which is expected to exhibit a pseudogap at temperatures close to those achieved in recent experiments.
TL;DR: In this paper, a short review article on the mechanism of high-temperature superconductivity is presented, where it is shown that the origin of high temperature supercondivity is the strong electron correlation.
Abstract: It is very important to elucidate the mechanism of superconductivity for achieving room temperature superconductivity. This paper is a short review article on the mechanism of high-temperature superconductivity. In the first half of this paper, we give a brief review on mechanisms of superconductivity in many-electron systems. We believe that high-temperature superconductivity may occur in a system with interaction of large-energy scale. Empirically, this is true for superconductors that have been found so far. In the second half of this paper, we discuss cuprate high-temperature superconductors. We argue that superconductivity of high temperature cuprates is induced by the strong on-site Coulomb interaction, that is, the origin of high-temperature superconductivity is the strong electron correlation. We show the results on the ground state of electronic models for high temperature cuprates on the basis of the optimization variational Monte Carlo method. A high-temperature superconducting phase will exist in the strongly correlated region.
TL;DR: In this paper, it was shown that the high temperature superconductivity can only happen when the electrons have a fractional coordination number, in contrast to the low temperature supercondivity where electrons must form pair structures in most cases.
TL;DR: The Pauli-limiting pair-breaking process with a small contribution of the spin-orbit coupling explained the first-order phase transition exhibiting a hysteresis observed at low temperatures.
Abstract: The upper critical field of a cuprate high-temperature superconductor, La1.84Sr0.16CuO4, was investigated by high-frequency self-resonant contactless electrical conductivity measurements in magnetic fields up to 102 T. An irreversible transition was observed at 85 T (T = 4.2 K), defined as the upper critical field. The temperature-dependent upper critical field was argued on the basis of the Werthamer-Helfand-Hohenberg theory. The Pauli-limiting pair-breaking process with a small contribution of the spin-orbit coupling explained the first-order phase transition exhibiting a hysteresis observed at low temperatures.
TL;DR: In this paper, the authors used a high quality factor microwave resonance involving the sample of interest to image the gap nodal structure of superconductors whose pairing symmetry is under debate, which can be used to measure unpatterned samples such as planar thin films and single crystals.
Abstract: We present a new measurement method which can be used to image the gap nodal structure of superconductors whose pairing symmetry is under debate. This technique utilizes a high quality factor microwave resonance involving the sample of interest. While supporting a circularly symmetric standing wave current pattern, the sample is perturbed by a scanned laser beam, creating a photoresponse that was previously shown to reveal the superconducting gap anisotropy. Simulation and the measurement of the photoresponse of an unpatterned Nb film show less than 8% anisotropy, as expected for a superconductor with a nearly isotropic energy gap along with expected systematic uncertainty. On the other hand, measurement of a YBa2Cu3O7−δ thin film shows a clear 4-fold symmetric image with ∼12.5% anisotropy, indicating the well-known 4-fold symmetric dx2−y2 gap nodal structure in the ab-plane. The deduced gap nodal structure can be further cross-checked by low temperature surface impedance data, which are simultaneously measured. The important advantage of the presented method over the previous spiral resonator method is that it does not require a complicated lithographic patterning process which limits one from testing various kinds of materials due to photoresponse arising from patterning defects. This advantage of the presented technique, and the ability to measure unpatterned samples such as planar thin films and single crystals, enables one to survey the pairing symmetry of a wide variety of unconventional superconductors.