Cuprate High-temperature Superconductors Research Articles

Macroscopic character of composite high-temperature superconducting wires

The "d-wave" symmetry of the superconducting order in the cuprate high temperature superconductors is a well established fact, and one which identifies them as "unconventional." However, in macroscopic contexts -- including many potential applications ({\it i.e.} superconducting "wires") -- the material is a composite of randomly oriented superconducting grains in a metallic matrix, in which Josephson coupling between grains mediates the onset of long-range phase coherence. Here, we analyze the physics at length scales large compared to the size of such grains, and in particular the macroscopic character of the long-range order that emerges. While XY-glass order and macroscopic d-wave superconductivity may be possible, we show that under many circumstances -- especially when the d-wave superconducting grains are embedded in a metallic matrix -- the most likely order has global s-wave symmetry.

On the quantitative determination of hole-concentration in high-temperature cuprate superconductors

We compared four hole-scales that have been used to determine the hole-concentration in high-temperature cuprate superconductors. We show that the hole-scale, [Formula: see text]-scale, based on the thermoelectric power [T. Honma et al., Phys. Rev. B 70, (2004) 214517.] is quantitatively consistent with spectroscopic probes for many different cuprate materials, while the other hole-scales, based on a well-known dome-shaped [Formula: see text]-curve [M. R. Presland et al., Physica C 176, 95 (1991)], the [Formula: see text]-axis lattice parameter [R. Liang et al., Phys. Rev. B 73, (2006) 180505(R).] and Hall coefficient [Y. Ando et al., Phys. Rev. B 61, (2000) 14956(R).], are not. We show that the quantitatively different hole-scales resulted in opposite conclusion of the same experimental observations. It can also lead to different interpretations of the electronic phase diagram when comparing different physical properties in different high-[Formula: see text] systems. We suggest that the [Formula: see text]-scale is the correct universal scale that works for all high-[Formula: see text] cuprates and it should be used for all quantitative doping dependence studies of cuprates.

Observation of a d-wave gap in electron-doped Sr2IrO4

High temperature superconductivity in cuprates emerges out of a highly enigmatic `pseudogap' metal phase. The mechanism of high temperature superconductivity is likely encrypted in the elusive relationship between the two phases, which spectroscopically is manifested as Fermi arcs---disconnected segments of zero-energy states---collapsing into $d$-wave point nodes upon entering the superconducting phase. Here, we reproduce this distinct cuprate phenomenology in the 5$d$ transition-metal oxide Sr$_2$IrO$_4$. Using angle-resolved photoemission, we show that clean, low-temperature phase of 6-8$\%$ electron-doped Sr$_2$IrO$_4$ has gapless excitations only at four isolated points in the Brillouin zone with a predominant $d$-wave symmetry of the gap. Our work thus establishes a connection between the low-temperature $d$-wave instability and the previously reported high-temperature Fermi arcs in electron-doped Sr$_2$IrO$_4$. Although the physical origin of the $d$-wave gap remains to be understood, Sr$_2$IrO$_4$ is a first non-cuprate material to spectroscopically reproduce the complete phenomenology of the cuprates, thus offering a new material platform to investigate the relationship between the pseudogap and the $d$-wave gap.

Normal-state Nernst effect from bidirectional bond density wave state in high- Tc cuprates

The role of charge order in the phase diagram of high temperature cuprate superconductors has been recently re-emphasized by the experimental discovery of an incipient bi-directional charge density wave (CDW) phase in a class of underdoped cuprates. In a subset of the experiments, the CDW has been found to be accompanied by a d-wave intra-unit-cell form factor, indicating modulation of charge density on the oxygen orbitals sandwiched between neighboring Cu atoms on the CuO planes (the so-called bond-density wave (BDW) phase). Here we take a mean field Q_1=(2\pi/3,0) and Q_2=(0,2\pi/3) bi-directional BDW phase with a d-wave form factor, which closely resembles the experimentally observed charge ordered states in underdoped cuprates, and calculate the Fermi surface topology and the resulting quasiparticle Nernst coefficient as a function of temperature and doping. We establish that, in the appropriate doping ranges where the low temperature phase (in the absence of superconductivity) is a BDW, the Fermi surface consists of an electron and a hole pocket, resulting in a low temperature negative Nernst coefficient as observed in experiments.

Closing the pseudogap quietly

The physical properties of hole-doped cuprate high-temperature superconductors are heavily influenced by an energy gap known as the pseudogap whose origin remains a mystery second only to that of superconductivity itself. A key question is whether the pseudogap closes at a temperature . The absence of a specific heat anomaly, together with persistent entropy losses up to 300 K, have long suggested that the pseudogap does not vanish at . However, amid a growing body of evidence from other techniques pointing to the contrary we revisit this question. Here we investigate if, by adding a temperature dependence to the pseudogap energy and quasiparticle lifetime in the resonating-valence-bond spin-liquid model of Yang, Rice and Zhang, we can close the pseudogap quietly in the specific heat.

Nature of Cooper Instability of Spin Polarons in Cuprate High- $$T_\mathrm{c}$$ T c Superconductors

It is shown that the exchange interaction in the subsystem of localized moments of copper ions can trigger a Cooper instability in the ensemble of spin-polaron quasiparticles in cuprate high-temperature superconductors. It is important that, inherent to cuprates, strong coupling between Fermi and spin degrees of freedom leads on the one hand to formation of spin-polaron quasiparticles and, on the other hand, allows to transfer the interaction between Bose degrees of freedom onto the interaction between Fermi quasiparticles. The solution of the integral equation for the d-wave superconducting order parameter is well consistent with the available experimental data.

Spectral properties near the Mott transition in the two-dimensionalt−Jmodel

The single-particle spectral properties of the two-dimensional t-J model in the parameter regime relevant to cuprate high-temperature superconductors are investigated using cluster perturbation theory. Various anomalous features observed in cuprate high-temperature superconductors are collectively explained in terms of the dominant modes near the Mott transition in this model. Although the behavior of the dominant modes in the low-energy regime is similar to that in the two-dimensional Hubbard model, significant differences appear near the Mott transition for the high-energy electron removal excitations which can be considered to primarily originate from holon modes in one dimension. The overall spectral features are confirmed to remain almost unchanged as the cluster size is increased from 4x4 to 6x6 sites by using a combined method of the non-abelian dynamical density-matrix renormalization group method and cluster perturbation theory.

Superconductivity and superfluidity as universal emergent phenomena

Superconductivity (SC) or superfluidity (SF) is observed across a remarkably broad range of fermionic systems: in BCS, cuprate, iron-based, organic, and heavy-fermion superconductors, and superfluid helium-3 in condensed matter; in a variety of SC/SF phenomena in low-energy nuclear physics; in ultracold, trapped atomic gases; and in various exotic possibilities in neutron stars. The range of physical conditions and differences in microscopic physics defy all attempts to unify this behavior in any conventional picture. Here we propose a unification through the shared symmetry properties of the emergent condensed states, with microscopic differences absorbed into parameters. This, in turn, forces a rethinking of specific occurrences of SC/SF such as cuprate high-temperature superconductivity, which becomes far less mysterious when seen as part of a continuum of behavior shared by a variety of other systems.

Optical gyrotropy and the nonlocal Hall effect in chiral charge-orderedTiSe2

It has been suggested that materials which break spatial inversion symmetry, but not time reversal symmetry, will be optically gyrotropic and display a nonlocal Hall effect. The associated optical rotary power and the suggested possibility of inducing a Kerr effect in such materials, in turn, are central to recent discussions about the nature of the pseudogap phases of various cuprate high-temperature superconductors. In this Rapid Communication, we show that optical gyrotropy and the nonlocal Hall effect provide a sensitive probe of broken inversion symmetry in $1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$. This material was recently found to possess a chiral charge-ordered phase at low temperatures, in which inversion symmetry is spontaneously broken, while time reversal symmetry remains unbroken throughout its phase diagram. We estimate the magnitude of the resulting gyrotropic constant and optical rotary power (the Faraday effect at zero applied field) and suggest that $1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$ may be employed as a model material in the interpretation of measurements on cuprate superconductors.

Berry phase in cuprate superconductors

Geometrical Berry phase is recognized as having profound implications for the properties of electronic systems. Over the last decade, Berry phase has been essential to our understanding of new materials, including graphene and topological insulators. The Berry phase can be accessed via its contribution to the phase mismatch in quantum oscillation experiments, where electrons accumulate a phase as they traverse closed cyclotron orbits in momentum space. The high-temperature cuprate superconductors are a class of materials where the Berry phase is thus far unknown despite the large body of existing quantum oscillations data. In this report we present a systematic Berry phase analysis of Shubnikov - de Haas measurements on the hole-doped cuprates YBa$_2$Cu$_3$O$_{y}$, YBa$_2$Cu$_4$O$_8$, HgBa$_2$CuO$_{4 + \delta}$, and the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$. For the hole-doped materials, a trivial Berry phase of 0 mod $2\pi$ is systematically observed whereas the electron-doped Nd$_{2-x}$Ce$_x$CuO$_4$ exhibits a significant non-zero Berry phase. These observations set constraints on the nature of the high-field normal state of the cuprates and points towards contrasting behaviour between hole-doped and electron-doped materials. We discuss this difference in light of recent developments related to charge density-wave and broken time-reversal symmetry states.

Sr2IrO4: Gateway to cuprate superconductivity?

High temperature superconductivity in cuprates remains a defining challenge in condensed matter physics. Recently, a new set of related compounds based on Ir rather than Cu has been discovered that may be on the verge of superconductivity themselves or be able to shed new light on the underlying interactions responsible for superconductivity in the cuprates.

Colloquium: Theory of intertwined orders in high temperature superconductors

The electronic phase diagrams of many highly correlated systems, and in particular the cuprate high temperature superconductors, are complex, with many different phases appearing with similar-sometimes identical-ordering temperatures even as material properties, such as a dopant concentration, are varied over wide ranges. This complexity is sometimes referred to as "competing orders." However, since the relation is intimate, and can even lead to the existence of new phases of matter such as the putative "pair-density-wave," the general relation is better thought of in terms of "intertwined orders." We selectively analyze some of the experiments in the cuprates which suggest that essential aspects of the physics are reflected in the intertwining of multiple orders-not just in the nature of each order by itself. We also summarize and critique several theoretical ideas concerning the origin and implications of this complexity.

Berry phases and the intrinsic thermal Hall effect in high-temperature cuprate superconductors.

Bogolyubov quasiparticles move in a practically uniform magnetic field in the vortex state of high-temperature cuprate superconductors. When set in motion by an externally applied heat current, the quasiparticles' trajectories may bend, causing a temperature gradient perpendicular to the heat current and the applied magnetic field, resulting in the thermal Hall effect. Here we relate this effect to the Berry curvature of quasiparticle magnetic sub-bands, and calculate the dependence of the intrinsic thermal Hall conductivity on superconductor's temperature, magnetic field and the amplitude of the d-wave pairing. The intrinsic contribution to thermal Hall conductivity displays a rapid onset with increasing temperature, which compares favourably with existing experiments at high magnetic field on the highest purity samples. Because such temperature onset is related to the pairing amplitude, our finding may help to settle a much-debated question of the bulk value of the pairing strength in cuprate superconductors in magnetic field.

Phase diagram of cuprate high-temperature superconductors described by a field theory based on anharmonic oxygen degrees of freedom.

In high temperature superconductors, although some phenomena such as the Mott transition (MT) at low doping are clearly driven by electron correlations, recent experimental data imply that anharmonic oxygen degrees of freedom-characteristic of perovskite materials-are playing a significant role. A key test of the role of anharmonic oxygen is to reproduce the complex cuprate phase diagram from a simple model. Here, we show that a field theory based on nonlinear coupling to anharmonic oxygens, parametrized from ab initio calculations, quantitatively reproduces the cuprate phase diagram for dopings above the MT. Pairing is mediated by renormalized oxygen vibrations transmuted into excitations of the pseudogap. The observed strong dependence of gap to transition temperature ratio on Tc also emerges from this field theory. This work suggests that including vibrational degrees of freedom is key to developing a complete understanding of the cuprates.

Topological superconductivity and unconventional pairing in oxide interfaces.

Pinpointing the microscopic mechanism for superconductivity has proven to be one of the most outstanding challenges in the physics of correlated quantum matter. Thus far, the most direct evidence for an electronic pairing mechanism is the observation of a new symmetry of the order parameter, as done in the cuprate high-temperature superconductors. Alternatively, global, topological invariants allow for a sharp discrimination between states of matter that cannot be transformed into each other adiabatically. Here we propose an unconventional pairing state for the electron fluid in two-dimensional oxide interfaces and establish a direct link to the emergence of non-trivial topological invariants. Topological signatures, in particular Majorana edge states, can then be used to detect the microscopic origin of superconductivity. In addition, we show that also the density wave states that compete with superconductivity sensitively depend on the nature of the pairing interaction.

CDW and Similarity of the Mott Insulator-to-Metal Transition in Cuprates with the Gas-to-Liquid-Liquid Transition in Supercooled Water

New advances in X-ray diffraction, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) using synchrotron radiation have now provided compelling evidence for a short-range charge density wave phase (CDW) called striped phase in the CuO2 plane of all cuprate high-temperature superconductors. The CDW is associated with a bond order wave (BOW) and an orbital density wave (ODW) forming nanoscale puddles which coexist with superconducting puddles below T c . The electronic CDW crystalline phase occurs around the hole doping 0.125 between the Mott charge transfer insulator and the 2D metal. The Van der Waals (VdW) theoretical model for a liquid of anisotropic extended objects proposed for supercooled water is used to describe the following: (a) the underdoped regime as a first spinodal regime of a “slightly doped charge transfer Mott insulator puddles coexisting with the striped polaronic CDW puddles;nd (b) the optimum doping regime as a second spinodal regime where striped polaronic CDW puddles coexist with the normal 2D metal puddles. This complex phase separation with three competing phases depends on the strength of the anisotropic electron-phonon interaction that favours the formation striped polaronic CDW phase.

Charge ordering in the electron-doped superconductor Nd(2-x)Ce(x)CuO₄.

In cuprate high-temperature superconductors, an antiferromagnetic Mott insulating state can be destabilized toward unconventional superconductivity by either hole or electron doping. In hole-doped (p-type) cuprates, a charge ordering (CO) instability competes with superconductivity inside the pseudogap state. We report resonant x-ray scattering measurements that demonstrate the presence of charge ordering in the n-type cuprate Nd(2-x)Ce(x)CuO4 near optimal doping. We find that the CO in Nd(2-x)Ce(x)CuO4 occurs with similar periodicity, and along the same direction, as in p-type cuprates. However, in contrast to the latter, the CO onset in Nd(2-x)Ce(x)CuO4 is higher than the pseudogap temperature, and is in the temperature range where antiferromagnetic fluctuations are first detected. Our discovery opens a parallel path to the study of CO and its relationship to antiferromagnetism and superconductivity.

Quantitative connection between the nanoscale electronic inhomogeneity and the pseudogap of Bi2Sr2CaCu2[formula omitted] superconductors

We have found a quantitative connection between the evolution of the inhomogeneous nanoscale electronic gaps (INSEG) state detected in Bi2Sr2CaCu2O8+δ by scanning tunneling microscopy/spectroscopy (STM/S) and the two universal, the upper and the lower, pseudogaps in high-temperature cuprate superconductors (HTCS). When the doping and temperature dependent INSEG map were analyzed by using our proposed hole-scale, we find that the two pseudogaps are connected to two specific coverages of the CuO2 plane by INSEG: the 50% and 100% coverages of the CuO2 planes by INSEG correspond to the upper and lower pseudogaps, respectively. This quantitative connection to the two pseudogaps indicates that the origin of the measured pseudogap energies and temperatures are intimately related to the geometrical coverage of the CuO2 planes by the INSEG state. We find that INSEG and superconductivity coexist in the underdoped to the overdoped regimes. We suggest that pseudogap states are microscopically inhomogeneous and 100% coverage of the CuO2 planes by the INSEG is a necessary condition for the high-Tc superconductivity.

Spin Fluctuations and High-Temperature Superconductivity in Cuprates

A microscopic theory of spin excitations and superconductivity in strongly correlated electronic systems as cuprates is discussed. We argue that in the Mott-Hubbard regime with two Hubbard subbands, there emerges a new energy scale induced by kinematical interaction of electrons with spin fluctuations of the order of electronic kinetic energy. The spectrum of magnetic excitations reveals a small damping caused by the involvement of spin excitations in the decay process besides the particle-hole continuum. This results in the magnetic resonance mode (RM) with weak temperature dependence as observed in experiments. Electronic spectrum and superconductivity are studied within the extended Hubbard model which includes intersite Coulomb repulsion and electron-phonon interaction. We found the d-wave pairing with high- T c mediated by strong kinematical interaction of electrons with spin fluctuations. Contributions to the d-wave pairing coming from intersite Coulomb repulsion and phonons turned out to be small.

Electronic evidence of an insulator-superconductor crossover in single-layer FeSe/SrTiO3 films.

In high-temperature cuprate superconductors, it is now generally agreed that superconductivity is realized by doping an antiferromagnetic Mott (charge transfer) insulator. The doping-induced insulator-to-superconductor transition has been widely observed in cuprates, which provides important information for understanding the superconductivity mechanism. In the iron-based superconductors, however, the parent compound is mostly antiferromagnetic bad metal, raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture. No evidence of doping-induced insulator-superconductor transition (or crossover) has been reported in the iron-based compounds so far. Here, we report an electronic evidence of an insulator-superconductor crossover observed in the single-layer FeSe film grown on a SrTiO3 substrate. By taking angle-resolved photoemission measurements on the electronic structure and energy gap, we have identified a clear evolution of an insulator to a superconductor with increasing carrier concentration. In particular, the insulator-superconductor crossover in FeSe/SrTiO3 film exhibits similar behaviors to that observed in the cuprate superconductors. Our results suggest that the observed insulator-superconductor crossover may be associated with the two-dimensionality that enhances electron localization or correlation. The reduced dimensionality and the interfacial effect provide a new pathway in searching for new phenomena and novel superconductors with a high transition temperature.