high-Tc Cuprate Superconductors Research Articles

Phonon properties in the simplest cuprate compound SrCuO2

In order to establish macroscopic and microscopic evaluations of phonon properties in cuprate high-Tc superconductors, specific heat and inelastic X-ray scattering (IXS) experiments have been performed in the simplest parent compound SrCuO2. We demonstrate that the phonon density of states (ph-DOS) can be precisely determined from the IXS spectrum. From the ph-DOS spectrum, several fundamental phonon parameters such as the Debye temperature and the average sound velocity have been estimated, the former being crosschecked by the independent measurement of the specific heat.

Microscopic gauge-invariant theory of thec-axis infrared response of bilayer cuprate superconductors and the origin of the superconductivity-induced absorption bands

We report on results of our theoretical study of the c-axis infrared conductivity of bilayer high-Tc cuprate superconductors using a microscopic model involving the bilayer-split (bonding and antibonding) bands. An emphasis is on the gauge-invariance of the theory, which turns out to be essential for the physical understanding of the electrodynamics of these compounds. The description of the optical response involves local (intra-bilayer and inter-bilayer) current densities and local conductivities. The local conductivities are obtained using a microscopic theory, where the quasiparticles of the two bands are coupled to spin fluctuations. The coupling leads to superconductivity and is described at the level of generalized Eliashberg theory. Also addressed is the simpler case of quasiparticles coupled by a separable and nonretarded interaction. The gauge invariance of the theory is achieved by including a suitable class of vertex corrections. The resulting response of the model is studied in detail and an interpretation of two superconductivity-induced peaks in the experimental data of the real part of the c-axis conductivity is proposed. The peak around 400/cm is attributed to a collective mode of the intra-bilayer regions, that is an analogue of the Bogolyubov-Anderson mode playing a crucial role in the theory of the longitudinal response of superconductors. For small values of the bilayer splitting, its nature is similar to that of the transverse plasmon of the phenomenological Josephson superlattice model. The peak around 1000/cm is interpreted as a pair breaking-feature that is related to the electronic coupling through the spacing layers separating the bilayers.

Different carrier compensation effect in La2−xSrxCu0.94Zn0.06O4 and La2−xSrxCu0.94Ni0.06O4 samples

The element substitution for Cu is usually used to investigate the mechanism of the high TC superconductivity in cuprates. However, it is still controversial how substitution suppresses superconductivity. We suggest that the suppression of superconductivity by doping is attributed to the local electrons introduced by the dopant regardless of magnetic or nonmagnetic impurities. Here, the compensation effect has been investigated in the bivalent ions doped La2−xSrxCu0.94Zn0.06O4 (LSCZO) and La2−xSrxCu0.94Ni0.06O4 (LSCNO) systems. It is found that the holes introduced by Sr substitution for La have compensated the local electrons and this results in the reappearance of the superconductivity in LSCNO system, while no trace for the superconductivity is observed in LSCZO system. Interestingly, resistivity for LSCZO system decreases faster with doping level compared to that for LSCNO system. This may be ascribed to the fact that the introduced holes do not compensate the local electrons around Zn impurities but participate in the conductive process. We discuss the possible origin of the different compensation effect between LSCNO and LSCZO systems.

Dispersion of quasi-particles in high T c cuprates

The interplay between superconductivity (SC) and antiferromagnetism (AFM) is studied in strongly correlated systems of high Tc Cuprate superconductors. It is assumed that superconductivity arises due to BCS pairing mechanism in presence of AFM in Cu lattices of Cu-O planes. The total Hamiltonian of the system is mean field one and has been solved exactly by writing the equations of motion for the single particle Green’s functions. Equations for the appropriate single particle co-relation functions are derived and the order parameters corresponding to SC and AFM are determined. It is assumed that the Fermi energy ∈F = 0 and the renormalized localized f energy level coincide with the Fermi level. All the quantities in the final equation for h and Δ are made dimensionless by dividing by 2t, where t is the hopping integral. The temperature dependent values of staggered magnetic field (h) and SC gap (Δ) were determined by solving self-consistent equations for h and Δ. The quasiparticle energy bands are function of AFM gap (h), SC gap (Δ) and hybridization (V). Then the dispersion of quasi-particles are studied at different temperatures by considering temperature dependent values of h and Δ and varying other different model parameters.

Effect of Zn substitution for Cu on Ca2-xNaxCuO2Cl2 near the hole concentration of 18 per Cu

A weakening of superconductivity upon substitution of Cu by Zn (0.5–1%) is observed in a high-Tc cuprate, Ca2-xNaxCuO2Cl2, near the hole concentration of 18 per Cu. The superconducting transition temperature and its volume fraction, estimated by magnetic susceptibility, exhibit a sizable anomaly for x=0.12–0.14, where the slowing down of Cu spin fluctuations below 5 K is demonstrated by muon spin relaxation experiments. These observations are in close resemblance to other typical cuprates including YBa2Cu3O7-δ, and Bi2Sr2Ca1-xYxCu2O8+δ, providing further evidence that Zn-induced “stripe” correlation is a universal feature of high-Tc cuprate superconductors common to that of La2-xAxCuO4 (A=Ba, Sr).

Visualizing pair formation on the atomic scale and the search for the mechanism of superconductivity inhigh-Tc cuprates

We have developed several new experimental techniques, based on the scanningtunneling microscope, to visualize the process of pair formation on the atomicscale and to probe with high precision what controls the strength of pairing inhigh-Tc cuprate superconductor compounds. These new experiments provide evidence that pairingin these exotic superconductors occurs above the bulk transition temperature and innanoscale regions with sizes of 1–3 nm. The high temperature nucleation and proliferationof these nanoscale puddles have a strong connection to the temperature–doping phasediagram of these superconductors. On average we have found that the pairing gapΔ and the temperature at which they first nucleateTp follow thesimple relation: 2Δ/kBTp∼8. Moreover, the variations of the pairing strength on the nanoscale can beexamined to find microscopic clues to the mechanism of pairing. Specifically, wehave found evidence that suggests that strong electronic correlation, as opposedto coupling of electrons to bosons, is responsible for the pairing mechanism inthe cuprates. Surprisingly, we have found that nanoscale measurements ofelectronic correlations in the normal state (at temperatures as high as twiceTc) can be used to predict the strength of the local pairing interaction at low temperatures.

Role of LO phonon on pseudo-gap phase in high-Tc cuprate superconductors

In analyzing the role of optical phonon on transition temperature Tc, the simplified extended BCS-like theory was tried in success and why Coulomb interaction must be large was understood. Oxygen (O) -isotope effect on Tc was explained in good agreement with more than 120 experimental data. The superconductivity appears in itself due to Bose-condensation of bipolarons coupled with surrounding local distortion and the change is discussed basing on the relative movement of light O-atoms in high-Tc oxides. From the present theoretical formula, LO phonon modes are made mainly at O-sites, and the mode couple to electrons changes from Z.B. type (under-doped region) to Z.C. type (over-doped region) at a doped point (x = 1/8 in La2-xBaxCuO4). At optimally doped point, the vibration type of O-atoms exchanges through zero vibration, and therefore, the concentration takes the form of dip point of Tc. In over-doped region, LO-Z.C. mode with negative pressure dependence plays and condenses simultaneously at Tc, whereas in under-doped region, LO-Z.B. mode with positive pressure dependence couples to electrons to make charged boson at pseudo-gap phase, and condensed for T under Tc. By introducing the strong electron correlation in addition to the conventional mechanism of superconductivity, Tc-equation would be derived for d-wave superconductors.

Pressure enhancement of superconductivity in an iron-based layered compound LaFeAsO1-xFx

Electrical resistivity measurements under high pressure were performed on iron-based superconductor LaFeAsO1-xFx. A steep increase in superconducting transition temperature (Tc) was observed for "optimum-doped" LaFeAsO0.89F0.11, whereas "under-doped" LaFeAsO0.95F0.05 shows a moderate increase in Tc with applying pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa. A ratio c/a was observed to decrease with applying pressure, which is similar to the pressure effect on cuprate high-Tc superconductors.

Quasi-particle spectrum around a single vortex in gossamer superconductivity

We have established recently the high-Tc cuprate superconductors in the underdoped region is characterized as gossamer superconductivity (i.e. d-wave superconductivity in the presence of d-spin density wave (dSDW)). Here we first derive the Bogoliubov-de Gennes equation for gossamer superconductivity. Then we study the bound state spectrum around a vortex in the gossamer superconductivity. We find that the presence of the dSDW order parameter eliminate many low energy bound states, which is very consistent with Scanning Tunneling Spectroscopy (STS) data by Pan et al. Also the present model reproduces the checkerboard structure at the core of the vortex as seen by Hoffman et al., though for the precise description of this structure, it is necessary to choose the correct Q-vectors.

Crossover from superconductivity fluctuation to vortex picture in the vortex state of high-Tc cuprate superconductors

In this paper, we introduce our recent results on superconductivity fluctuation measurement of high-Tc cuprate under finite magnetic field both for the hole-doped La2-xSrxCuO4 (LSCO) and the electron-doped La2-xCexCuO4 (LCCO). Under finite magnetic fields, the scaling relation was valid only for weak fields, and for higher fields, aspects as vortices appeared. However, even at low temperatures, just above the first-order phase transition of the vortex lattice, vortex picture alone cannot describe the data satisfactory. Thus, we need a unified theory for the description of a large superconductivity fluctuation under finite magnetic fields for high-Tc cuprates.

Overdoping effect on pair breaking peak energy in the electronic Raman spectra of high-Tc cuprate superconductors

In the superconducting state of cuprates we have measured electronic Raman spectra (ERS) and plotted the pair breaking energy as a function of hole concentration in overdoped regime. The target materials in this study are Ca-substituted YBCO and Tl2201, both of which have similar Tc when it is optimally doped but different number of CuO2 planes in the unit cell. For both materials pair breaking peak energy (Eb) shows decrease with increasing carrier concentration. Interestingly Eb seems to show a rapid drop at a certain carrier concentration, where several measurements suggest the signature of quantum critical point. The similarity of the ERS between the two materials indicates that the behavior is not affected by the number of CuO2 planes in a unit cell, and that electronic state of cuprates may have a remarkable change at the critical hole concentration.

Very High Critical Field and Superior Jc‐Field Performance in NdFeAsO0.82F0.18 with Tc of 51 K

been estimated to be higher than 55 or 63‐65 T in LaFeAsO0.9F0.1, [6,7] 70 T in PrFeAsO0.85F0.15, and over 100 T in SmFeAsO0.85F0.15. [7] The two gap superconductivity proposed for LaFeAsO0.9F0.1 suggests that the Hc2 could be further increased [6] to a significant extent. This is one of the unique features of the FeAs-based new superconductors. In this work, we show that Hc2(48K) ¼13 T, and that the Hc2(0) values can exceed 80‐230 T in a high-pressure (HP) fabricated NdO0.82F0.18FeAs bulk sample with Tc of 51K. We also demonstrate that the supercurrent density in fields from 1 up to 9 Tonly drops by a factor of 2‐6 for T <30K, significantly slower than for MgB2 and high-Tc cuprate superconductors. The very high Hc2 of the sample greatly surpasses those of MgB2 and classic low-temperature superconductors, and the superior Jc-field performance is promising for the use of the new NdFeAsO0.82F0.18 superconductors in highfield applications. The X-ray diffraction (XRD) and refinement results shown in Figure 1 indicate that the NdFeAsO0.82F0.18 sample is nearly single phase, with a tetragonal structure of the P4/nmm

Superconductivity without dependence on valence electron density in Zn doped YBCO systems

This paper reports that the YBa2Cu3—xZnxO7-δ (x = 0–0.4) samples are researched by means of x-ray diffraction, calculations of binding energy, the positron experiments and variations of oxygen content. The results of simulated calculations, positron experiments and variations of oxygen content support the existence of cluster effect. Moreover, it is concluded that the cluster effect is an important factor on suppression of high-Tc cuprate superconductivity and the Tc does not depend on the density of valence electron directly.

A multi-component Fermi surface in the vortex state of an underdoped high-Tc superconductor

To understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high-transition-temperature (high-T(c)) copper oxide superconductors have revealed the existence of a Fermi surface akin to that in normal metals, comprising fermionic carriers that undergo orbital quantization. The unexpectedly small size of the observed carrier pocket, however, leaves open a variety of possibilities for the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we report experiments on quantum oscillations in the magnetization (the de Haas-van Alphen effect) in superconducting YBa(2)Cu(3)O(6.51) that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave model with spiral or related modulated magnetic order, consistent with experimental observations.

Challenges to high-Tc superconductivity in cuprates by exploring condensate properties

In this paper, I introduce recent our results on superconductivity fluctuation measurement of high-Tc cuprate both for the hole-doped La2-xSrxCuO4 (LSCO) and the electron-doped La2-xCexCuO4 (LCCO). In hole doped LSCO, the universality class was found to change twice as a function of doping, starting from the 2D-XY, changing to the 3D XY and another 2D ‘unknown’ behavior. The results favors theoretical interpretations of the phase diagram of high-Tc cuprate which assume the existence of an additional hidden quantum critical point around at the optimum doping. In electron doped cuprate, the superconductivity fluctuation is 3D XY for all samples with different hole doping, which is in sharp contrast to the hole doped cuprate. Thus, the asymmetry of the phase diagram between the hole doped and the electron doped materials is another important key factor to judge the applicability of various theories on high-Tc superconductivity.Under finite magnetic fields, the scaling relation was valid only for weak fields, and for higher fields, aspects as vortices appeared. However, even at low temperatures, just above the first order phase transition, vortex picture alone cannot describe the data satisfactory. Thus, we need a unified theory for the description of a large superconductivity fluctuation under finite magnetic fields for high-Tc cuprates.Finally, I showed that our novel technique of fabricating high-Tc Josephson bridge junction using a small island of Fe was turned out to be very promising.

Inpurity-induced states in inhomogeneous superconductivity in high-Tc cuprates

Quasiparticle states around a nonmagnetic impurity in inhomogeneous superconducting states observed in high-Tc cuprates are studied using a two-dimensional t-J type model. The spatial dependences of the superconducting order parameters are determined within the Bogoliubov-de Gennes theory based on the Gutzwiller approximation. We show that a peak structure is found in the local density of states around the impurity in the large gap regions of the inhomogeneous superconducting states, which has not been experimentally observed yet. Our results suggest that a competing order is realized in the large gap regions in high-Tc cuprates.

Microstrain and polaronic correlation in a model system

We have studied the effect that variations of the local microstrain have on the polaronic correlation function obtained from the exact diagonalization of a simple model Hamiltonian. Although the correlation function exhibits a monotonic behavior as a function of the hole hopping parameter, and thus microstrain, its isotopic shift exhibits a maximum for specific values of t. This result suggests that if there is a coupling between polaronic objects and free carriers, coexisting in high-Tc cuprate superconductors, then this will manifest not only in the superconducting transition, but also on the properties directly related to the polaron formation in these materials.

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.

The ‘magic’ electronic state of high-Tc cuprate superconductors

The available experimental data for 105 distinct families of high-T(c) oxocuprate superconductors point to the presence of a 'magic' intrasheet Cu-O separation of 1.923 A, which yields the highest T(c) values at ambient pressure (Rao 1995). The interpretation with Burdett's model (1993), along with electronic band structure calculations, is centered at a crossing of the O- and Cu-predominated sigma bands and points to the significance of the electron-phonon coupling for these materials.

Rigorous derivation of the mean-field Green functions of the two-band Hubbard model of superconductivity

The Green function (GF) equation of motion technique for solving the effective two-band Hubbard model of high-Tc superconductivity in cuprates (Plakida et al 1995 Phys. Rev. B 51 16599, Plakida et al 2003 JETP 97 331) rests on the Hubbard operator (HO) algebra. We show that, if we take into account the invariance to translations and spin reversal, the HO algebra results in invariance properties of several specific correlation functions. The use of these properties allows rigorous derivation and simplification of the expressions of the frequency matrix (FM) and of the generalized mean-field approximation (GMFA) Green functions (GFs) of the model. For the normal singlet-hopping and anomalous exchange pairing correlation functions which enter the FM and GMFA-GFs, an approximation procedure based on the identification and elimination of exponentially small quantities is described. It secures the reduction of the correlation order to GMFA-GF expressions.