Layered Cuprate Superconductors Research Articles

La[formula omitted]Ba[formula omitted]CuO[formula omitted] as a superconducting Rosetta Stone

The high-temperature superconductivity in layered cuprates discovered by Bednorz and Müller arrived as a shock. Gradually, the presence of competing orders, such as antiferromagnetism and charge order, were discovered; however, the relationship to the superconductivity has been confusing. It so happens that the original cuprate superconductor family La2−xBaxCuO4 contains all of the relevant phases, with extreme competition among them, and analysis of these phases provides strong clues to the nature of the superconductivity in cuprates.

Superconductivity from Charge Order in Cuprates

Superconductivity in layered cuprates is induced by doping holes into a parent antiferromagnetic insulator. It is now recognized that another common emergent order involves charge stripes, and our understanding of the relationship between charge stripes and superconductivity has been evolving. Here we review studies of 214 cuprate families obtained by doping La$_2$CuO$_4$. Charge-stripe order tends to compete with bulk superconductivity; nevertheless, there is plentiful evidence that it coexists with two-dimensional superconductivity. This has been interpreted in terms of pair-density-wave superconductivity, and the perspective has shifted from competing to intertwined orders. In fact, a new picture of superconductivity based on pairing within charge stripes has been proposed, as we discuss.

Josephson Plasmon Resonance in Tl2Ba2CaCu2O8 High‐Temperature Superconductor Tunable Terahertz Metamaterials

AbstractThe Josephson plasmon resonance (JPR) offers a valuable probe to investigate the superconductivity in layered cuprate superconductors. However, the coupling between free space radiation and JPR in high‐temperature superconductor (HTS) film remains challenging because the excitation of JPR demands the c‐axis oriented electric field. The subwavelength resonators in metamaterials can enhance the localized electric field, which can be utilized to resolve this difficulty. Here, a tunable terahertz (THz) metamaterial made from Tl2Ba2CaCu2O8 (Tl‐2212) HTS film is developed. The spectral response of Tl‐2212 metamaterial has a tunable property at temperatures up to 90 K. The resonant excitation of Josephson plasmon in the metamaterial is observed. Simulation results indicate that the scattering of subwavelength resonators can provide the component of the z‐axis electric field for the resonant excitation. The coupling between JPR and resonance modes of metamaterials is observed and explained using coupled mode theory. The temperature dependence of JPR frequency shows accordance with the experimental results of the pure film. This work provides an avenue to excite the JPR and probe superconducting condensate in the layered superconductor. The development of Josephson plasmonic metamaterials may contribute to tunable and nonlinear THz devices.

Absence of Luther-Emery superconducting phase in the three-band model for cuprate ladders

Correlated-electron theories of superconductivity in layered cuprates often start from the premise of a gapped spin-liquid phase proximate to the superconducting state. This assumption is justified based on analytical and numerical demonstrations of a superconducting Luther-Emery phase in the doped 2-leg one-band Hubbard ladder, and the perceived analogy between coupled ladders and the two-dimensional ${\mathrm{CuO}}_{2}$ layer. We demonstrate from accurate density matrix renormalization group studies the absence of the superconducting Luther-Emery phase in the doped 2-leg three-band ladder consisting of both copper and oxygen, even as the spin gap is large in the undoped three-band ladder. For realistic oxygen-oxygen hopping and Hubbard repulsion on the oxygen atoms, density-density rather than pairing correlations are dominant at long range. This result is equally valid whether or not the oxygens outside the ladder proper, over and above the rung and leg oxygens, are included in the computation. These results demonstrate the critical importance of oxygen orbitals, and raise disturbing questions about the applicability of many of the existing correlated-electron theories of superconductivity.

Influence of pseudo-gap and interlayer coupling on isotope effect in bilayer cuprate superconductors

The present investigations deal with the study of the isotope effect in bilayer cuprate superconductors. We have considered the BCS-effective Hamiltonian that includes the intra and inter-layer kinetic energies and in plane attractive interaction responsible for pairing in these systems. In order to look for pseudo-gap having momentum dependence of the form of d-wave superconducting order parameter, a pseudo-gap order parameter is also introduced in the in-plane quasi-particle energy. Such a model calculation can be applicable for the high Tc cuprates like Bi2Sr2CaCu2O8+x having two CuO2 planes per unit cell and also for several other cuprate systems with a possibility to extend for the multi-layer cuprates. Employing Green's function formalism, we obtain the expressions for the superconducting order parameter and hence the isotope effect coefficient as a function of interlayer coupling, transition temperature and pseudo-gap order parameter. On the basis of numerical computation, we have pointed out that isotope effect increases due to coupling between the planes and decreases with the transition temperature. The presence of phenomenological pseudo-gap parameter in the quasi-particle spectrum enhances the isotope effect. These results are viewed in terms of recent experimental works on isotope effect in layered cuprate superconductors.

Detection of Acoustic Plasmons in Hole-Doped Lanthanum and Bismuth Cuprate Superconductors Using Resonant Inelastic X-Ray Scattering.

High T_{c} superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic x-ray scattering to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors (La_{1.84}Sr_{0.16}CuO_{4} and Bi_{2}Sr_{1.6}La_{0.4}CuO_{6+δ}), crucially completing the picture. Interestingly, in contrast to the quasistatic charge ordering which manifests at both Cu and O sites, the observed acoustic plasmons are predominantly associated with the O sites, revealing a unique dichotomy in the behavior of valence charges in hole-doped cuprates.

Formation of intermediate coupling optical polarons and bipolarons in two-dimensional systems

The formation of the optical polaron and bipolaron in two-dimensional (2D) systems is studied in the intermediate electron–phonon coupling regime. The total energies of the 2D polaron and bipolaron are calculated by using the Buimistrov–Pekar method of canonical transformations. The obtained results are compared with other existing results obtained by using the Feynman path integral method and the modified Lee–Low–Pines unitary transformation method. It is shown that the electron–phonon correlation significantly reduces the total energy of the 2D polaron in comparison with the energy of the strong coupling (adiabatic) polaron. It is found that the polaron formation in 2D systems is possible when the electron–phonon coupling constant α is greater than the critical value αc≃2.94, which is much lower than a critical value of the electron–phonon coupling constant α in three-dimensional (3D) systems. The critical values of the Fröhlich coupling constant α and the ratio η=ε∞/ε0 (where ε∞ and ε0 are the high frequency and static dielectric constants, respectively), which determine the bipolaron stability region in 2D systems, are calculated numerically. It is interesting for application to the layered cuprate superconductors that the (bi)polarons are formed more easily in quasi-2D regions than in the bulk. It is argued that the high-Tc cuprate superconductivity can exist above the bulk superconducting transition temperature Tc as the persisting superfluidity of polaronic (bosonic) Cooper pairs and large bipolarons at quasi-2D grain boundaries or in the CuO2 layers above Tc.

Multiple Electronic Components and Lifshitz Transitions by Oxygen Wires Formation in Layered Cuprates and Nickelates

There is growing compelling experimental evidence that a quantum complex matter scenario made of multiple electronic components and competing quantum phases is needed to grab the key physics of high critical temperature ( T c ) superconductivity in layered cuprates. While it is known that defect self-organization controls T c , the mechanism remains an open issue. Here we focus on the theoretical prediction of the multiband electronic structure and the formation of broken Fermi surfaces generated by the self-organization of oxygen interstitials O i atomic wires in the spacer layers in HgBa 2 CuO 4 ± δ , La 2 CuO 4 ± δ and La 2 NiO 4 ± δ , by means of self-consistent Linear Muffin-Tin Orbital (LMTO) calculations. The electronic structure of a first phase of ordered O i atomic wires and of a second glassy phase made of disordered O i impurities have been studied through supercell calculations. We show the common features of the influence of O i wires in the electronic structure in three types of materials. The ordering of O i into wires leads to a separation of the electronic states between the O i ensemble and the rest of the bulk. The wire formation first produces quantum confined localized states near the wire, which coexist with, Second, delocalized states in the Fermi surface (FS) of doped cuprates. A new scenario emerges for high T c superconductivity, where Kitaev wires with Majorana bound states are proximity-coupled to a 2D d-wave superconductor.

Thermal transport in layered structure of YBa2Cu3O7−δ superconductors

The heat transfer study in YBa2Cu3O[Formula: see text] superconductors structures is focused on the influence of the effect of scattering events in cross-plane and in-plane references. Understanding the mechanism of controlling the thermal conductivity of layered superconductors is an area of interest for nano microelectronics and thermo-electronic technological applications. The model of the thermal conduction, and phonon transport perpendicular and parallel to the layers of YBa2Cu3O[Formula: see text] are developed. It has been justified via numerical estimation and found substantial diminution in thermal conductivities in both in-plane and cross-plane directions of layered cuprate superconductors.

Interrogating the superconductor Ca10(Pt4As8)(Fe2-xPtxAs2)5 Layer-by-layer.

Ever since the discovery of high-Tc superconductivity in layered cuprates, the roles that individual layers play have been debated, due to difficulty in layer-by-layer characterization. While there is similar challenge in many Fe-based layered superconductors, the newly-discovered Ca10(Pt4As8)(Fe2As2)5 provides opportunities to explore superconductivity layer by layer, because it contains both superconducting building blocks (Fe2As2 layers) and intermediate Pt4As8 layers. Cleaving a single crystal under ultra-high vacuum results in multiple terminations: an ordered Pt4As8 layer, two reconstructed Ca layers on the top of a Pt4As8 layer, and disordered Ca layer on the top of Fe2As2 layer. The electronic properties of individual layers are studied using scanning tunneling microscopy/spectroscopy (STM/S), which reveals different spectra for each surface. Remarkably superconducting coherence peaks are seen only on the ordered Ca/Pt4As8 layer. Our results indicate that an ordered structure with proper charge balance is required in order to preserve superconductivity.

Scattering Events and Heat Conductivity of Layered La 2-x Sr x CuO 4 Superconductors

The problem of heat conduction in layered La2-x SrxCuO4 superconductor has been investigated in a new frame work of in-plane and cross-plane concepts with the help of modified Callaway model of thermal conductivity based on relaxation time approximation. Using the many body quantum dynamical theory of the expressions for thermal conductivity in context of in-plane and cross plane have been obtained and results are found in excellent agreement with experimental observations for layered La2-x SrxCuO4 cuprate superconductors. The theory explores the possibility of device fabrication cold in one direction and hot in the other.

Temperature-independent pseudogap and thermally activated c-axis hopping conductivity in layered cuprate superconductors

The carrier localization (confinement) and c-axis charge transport in high-Tc layered cuprates have been investigated in the temperature-independent pseudogap state over a wide doping range from the underdoped to the overdoped regime, focusing on the nanoscale phase separation into two different domains (i.e. carrier-rich CuO2 layers and carrier-poor regions between the CuO2 layers) caused by inherent nanoscale electronic inhomogeneities and on the real-space pairing of polaronic carriers caused by strong electron–phonon interactions in carrier-poor regions. A simple microscopic model for layered cuprate superconductors is proposed to describe the carrier confinement and formation of localized bipolarons in the low-doping regions between the CuO2 layers and to simulate the c-axis hopping transport at the thermal dissociation of such bipolarons. This model is found to describe properly the c-axis charge transport, which is governed by thermally activated hopping of polarons residing in the localized state between the CuO2 layers due to the existence of the temperature-independent bipolaronic pseudogap, and the insulating behavior of the c-axis resistivity ρc(T) observed above Tc in various cuprate superconductors, from the underdoped to the overdoped regime. It is found that the insulating behavior of ρc(T) is changed to metallic behavior with disappearing of bipolaronic pseudogap.

First direct observation of the Van Hove singularity in the tunnelling spectra of cuprates

In two-dimensional (2D) lattices, the electronic levels are unevenly spaced, and the density of states (DOS) displays a logarithmic divergence known as the Van Hove singularity (VHS). This is the case in particular for the layered cuprate superconductors. The scanning tunnelling microscope (STM) probes the DOS, and is therefore the ideal tool to observe the VHS. No STM study of cuprate superconductors has reported such an observation so far giving rise to a debate about the possibility of observing directly the normal state DOS in the tunnelling spectra. In this study, we show for the first time that the VHS is unambiguously observed in STM measurements performed on the cuprate Bi2Sr2CuO6+δ (Bi-2201). Beside closing the debate, our analysis proves the presence of the pseudogap in the overdoped side of the phase diagram of Bi-2201 and discredits the scenario of the pseudogap phase crossing the superconducting dome.

Theoretical Study of Specific Heat, Density of States, Free Energy, and Critical Field of High Temperature Cuprate Superconductors Based on Intra and Interlayer Interactions

The role of attractive interlayer and intralayer interactions in layered high Tc cuprate superconductors have been investigated using a one-band two layer tight binding Hamiltonian. Self-consistent equations for the superconducting order parameter (Δ) and critical temperature (Tc) are derived using double time Green’s functions and equation of motion method. The expression for excitonic type correlation (γc), specific heat, density of states, free energy, and critical field are obtained. The interlayer interactions play an important role in the enhancement of Tc in layered high Tc cuprates. The oxygen isotope effect is also analyzed. The agreement between theoretical and experimental results for the system YBa2−xLaxCu3O7 (0≤x≤0.5) is quite satisfactory.

Phonon-mediated superconducting transitions in layered cuprate superconductors

A phonon-mediated $d$-wave BCS-like model is presented for a homologous series of layered cuprate superconductors. We show that the dependence of both the superconducting transition temperature ${T}_{c}$ and oxygen isotope exponent $\ensuremath{\alpha}$ on the doping level, number of $\mathrm{Cu}{\mathrm{O}}_{2}$ layers, pressure, and cation disorder in the model superconducting series $\mathrm{Hg}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{n\ensuremath{-}1}{\mathrm{Cu}}_{n}{\mathrm{O}}_{2n+2+\ensuremath{\delta}}$ can be well reproduced within one single theoretical framework. We also find that the theoretical model accounts well for the superconducting transitions in other homologous series such as the bismuth- and thallium-based family. Our results suggest that the interlayer coupling plays an important role in the significant enhancement of ${T}_{c}$ and in the systematic reduction of $\ensuremath{\alpha}$ in a layered homologous series.

Binding energy of a Cooper pairs with non-zero center of mass momentum in d-wave superconductors

Abstract The binding energy of Cooper pairs has been calculated for the case of d -wave symmetry of the superconducting gap in layered cuprate superconductors. We assume that Cooper pairs are formed by the short range potential and then derive the binding energy in the form Δ kq = Δ x ( q ) cos k x a + Δ y ( q ) cos k y a + Ω x ( q ) sin k x a + Ω y ( q ) sin k y a , where q is a total momentum of the pair. Numerical solutions of the self-consistent system of the integral equations for quantities Δ x ( q ) , Δ y ( q ) and Ω x ( q ) , Ω y ( q ) along different lines in q x , q y planes have been obtained. Anisotropy of the depairing total momentum (or depairing current) has been calculated.

Magnetic excitations of charge-ordered [formula omitted

The incommensurate magnetic excitations of spin–charge ordered La 2 NiO 4.11 were studied by inelastic neutron scattering. With increasing energy up to ∼ 20 meV the maximum intensity of the spin excitations is observed to shift slightly towards the 2D antiferromagnetic wave vector ( 1 / 2 , 1 / 2 ) . This asymmetry in the magnon dispersion about the incommensurate wave vector is a similar effect, though less marked, to what has been observed in the layered cuprate superconductors.

Superconducting materials for large scale applications

Since the 1960s, Nb-Ti (superconducting transition temperature T/sub c/=9 K) and Nb/sub 3/Sn (T/sub c/=18 K) have been the materials of choice for virtually all superconducting magnets. However, the prospects for the future changed dramatically in 1987 with the discovery of layered cuprate superconductors with T/sub c/ values that now extend up to about 135 K. Fabrication of useful conductors out of the cuprates has been difficult, but a first generation of silver-sheathed composite conductors based on (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ (T/sub c//spl sim/110 K) has already been commercialized. Recent progress on a second generation of biaxially aligned coated conductors using the less anisotropic YBa/sub 2/Cu/sub 3/O/sub 7/ structure has been rapid, suggesting that it too might enter service in the near future. The discovery of superconductivity in MgB/sub 2/ below 39 K in 2001 has brought yet another candidate material to the large-scale applications mix. Two distinct markets for superconductor wires exist-the more classical low-temperature magnet applications such as particle accelerators, nuclear magnetic resonance and magnetic resonance imaging magnets, and plasma-containment magnets for fusion power, and the newer and potentially much larger market for electric power equipment, such as motors, generators, synchronous condensers, power transmission cables, transformers, and fault-current limiters for the electric utility grid. We review key properties and recent progress in these materials and assess their prospects for further development and application.

Role of interlayer coupling on the isotope effect in layered high- Tc cuprate superconductors

The present work deals with the study of the role of interlayer coupling on the isotope effect in high- T c layered cuprate superconductors. For this purpose, we have considered two-layer tight-binding model that includes the intralayer attractive BCS type interactions and interlayer coupling in the form of single particle and Josephson Cooper pairing tunneling. Such a model calculation will be applicable for the high- T c cuprate system like YBa 2Cu 3O 7− x , having two CuO 2 plane per unit cell, and also several other cuprate materials and may be extended for multilayer cuprate systems. To calculate the isotope effect coefficient ( α), we derive the expression for the superconducting order parameter within the BCS mean field. We find that the isotope effect coefficient ( α) increases with single particle tunneling but is independent of the Josephson Cooper tunneling. Finally, we have explained our theoretical results on isotope effect coefficient in the light of existing experimental observations.

Transport properties of (Pb 2Cu)Sr 2Dy xCe n−x° Cu 2O 2 n+6 : n=3, 5 epitaxial films

In-plane resistivity and thermopower for separated pairs of CuO 2 planes in (Pb 2Cu)Sr 2Dy x Ce n−x Cu 2O 2 n+6 : n=3, 5 have been studied. After optimization of the film growth conditions, the hole density estimated from thermopower is within the range for the occurrence of superconductivity in layered cuprates. However, resistivity shows weak localization behavior reflecting the low dimensionality of the electronic system. The electrical conduction within a CuO 2 plane does not deteriorate with the increase of the distance between a pair of CuO 2 planes from o.87 nm ( n=3) to 1.41 nm ( n=5).