Copper Oxide Planes Research Articles

Negative lattice expansion from the superconductivity–antiferromagnetism crossover in ruthenium copper oxides

The mechanism of high-transition-temperature (high-T(c)) superconductivity in doped copper oxides is an enduring problem. Antiferromagnetism is established as the competing order, but the relationship between the two states in the intervening 'pseudogap' regime has become a central puzzle. The role of the crystal lattice, which is important in conventional superconductors, also remains unclear. Here we report an anomalous increase of the distance between copper oxide planes on cooling, which results in negative thermal volume expansion, for layered ruthenium copper oxides that have been doped to the boundary of antiferromagnetism and superconductivity. We propose that a crossover between these states is driven by spin ordering in the ruthenium oxide layers, revealing a novel mechanism for negative lattice expansion in solids. The differences in volume and lattice strain between the distinct superconducting and antiferromagnetic states can account for the phase segregation phenomena found extensively in low-doped copper oxides, and show that Cooper pair formation is coupled to the lattice. Unusually large variations of resistivity with magnetic field are found in these ruthenium copper oxides at low temperatures through coupling between the ordered Ru and Cu spins.

The electronic contribution to the specific heat of NdBa $\mathsf{_2}\mathsf{Cu_3}\mathsf{O_{6 + x}}$

From measurements of the specific heat of $\mathrm{NdBa_2Cu_3O_{6 + \mathnormal{x}}}$ in the temperature range between 20 K and 300 K the electronic contribution C e (T)/T has been derived. The results depend strongly on the assumptions made for the normal-state reference, especially the phonon contribution. Taking into account entropy conservation between the superconductor and a hypothetical normal-state reference, we found a temperature independent electronic contribution of this normal-state reference without any sign of a pseudogap for both optimum doped and underdoped samples. For oxygen concentrations between x = 0.79 and x = 0.89 a broad hump in C e (T)/T is observed around 120 K, which we ascribe to pair formation above T c . The dependence of the hole concentration n h in the copper oxide planes on the oxygen concentration x in the copper oxide chains was calculated by means of bond-valence sums. We found that the optimum doping of the copper oxide planes is n h,opt = 0.24 for $\mathrm{\mathnormal{R}Ba_2Cu_3O_{6 + \mathnormal{x}}}$ (R = Nd, Y) irrespective of the element on the rare-earth site.

Victor John Emery

Victor John Emery

Dispersive Excitations in the High-Temperature SuperconductorLa2−xSrxCuO4

High-resolution neutron scattering experiments on optimally doped La2-xSrxCuO4 (x=0.16) reveal that the magnetic excitations are dispersive. The dispersion is the same as in YBa2Cu3O6.85, and is quantitatively related to that observed with charge sensitive probes. The associated velocity in La2-xSrxCuO4 is only weakly dependent on doping with a value close to the spin-wave velocity of the insulating (x=0) parent compound. In contrast with the insulator, the excitations broaden rapidly with increasing energy, forming a continuum at higher energy and bear a remarkable resemblance to multiparticle excitations observed in 1D S=1/2 antiferromagnets. The magnetic correlations are 2D, and so rule out the simplest scenarios where the copper oxide planes are subdivided into weakly interacting 1D magnets.

Vortex fluctuations in high-Tcthin films close to the resistive transition

Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and I–V measurements.In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs.The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa2Cu3O7 (YBCO) and MgB2 thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length.In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices.The I–V characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.

A pure Hubbard model with demonstrable pairing adjacent to the Mott-insulating phase

We introduce a Hubbard model on a particular class of geometries, and consider the effect of doping the highly spin-degenerate Mott-insulating state with a microscopic number of holes in the extreme strong-coupling limit. The geometry is quite general, with pairs of atomic sites at each superlattice vertex, and a highly frustrated inter-atomic connectivity: the one dimensional realization is a chain of edge-sharing tetrahedra. The sole model parameter is the ratio of intra-pair to inter-pair hopping matrix elements. If the intra-pair hopping is negligible then introducing a microscopic number of holes results in a ferromagnetic Nagaoka groundstate. Conversely, if the intra-pair hopping is comparable with the inter-pair hopping then the groundstate is low spin with short-ranged spin correlations. We exactly solve the correlated motion of a pair of holes in such a state and find that, in 1-d and 2-d, they form a bound pair on a length scale that increases with diminishing binding energy. This result is pertinent to the long-standing problem of hole motion in the copper-oxide planes of the high-temperature superconductors: we have rigorously shown that, on our frustrated geometry, the holes pair up and a short-ranged low-spin state is generated by hole motion alone.

Chemical control of hole-doped superconductivity and magnetism inGd2−xCexRuSr2Cu2O10−δ

Three Gd 2 - x Ce x RuSr 2 Cu 2 O 1 0 - δ samples (x=0.5, as prepared and after high pressure oxygenation and x =0.7) have been investigated by synchrotron powder x-ray diffraction and magnetization measurements. Precise coordinates and site occupancies for the oxygen atoms have been refined from the x-ray experiments. Estimates of the hole doping of the copper oxide planes based on the bond lengths, via the bond valence sum method, are found to be inaccurate. However, doping estimates based on the refined oxygen contents are in good agreement with the variation of superconductivity, and show that chemical doping, rather Cu/Ru band overlap, is the doping mechanism. The magnetic ordering temperatures of the Ru moments are not a simple function of the doping concentration, but depend upon both the Gd/Ce ratio and the oxygen content.

Growth of single unit-cell superconducting La 2− xSr xCuO 4 films

We have developed an approach to grow high quality ultra-thin films of La 2− x Sr x CuO 4 with molecular beam epitaxy, by adding a homoepitaxial buffer layer in order to minimize the degradation of the film structure at the interface. The advantage of this method is to enable a further reduction of the minimal thickness of a superconducting La 1.9Sr 0.1CuO 4 film. The main result of our work is that a single unit cell (only two copper oxide planes) grown on a SrLaAlO 4 substrate exhibits a superconducting transition at 12.5 K (zero resistance) and an in-plane magnetic penetration depth λ ab (0)=535 nm.

Inelastic neutron scattering study of Tl 2Ba 2CuO 6+ δ

We present inelastic neutron scattering data on optimally doped Tl 2Ba 2CuO 6+ δ (T c =92 K), a single-layer copper oxide with undistorted copper oxide planes. The data indicate a magnetic resonant mode below T c, as previously observed in YBa 2Cu 3O 6+ δ and Bi 2Sr 2CaCu 2O 8+ δ . The mode is thus a generic feature of the copper oxide superconductors.

Making high Tc higher: a theoretical proposal

There is considerable evidence that the highest T c obtainable in a copper-oxide plane is limited by the competition between two effects: On the one hand, as the concentration of doped holes, x, is increased, the pairing scale, which is related to the properties of a doped Mott insulator, decreases. On the other hand, the superfluid density, which controls the stiffness of the system to phase fluctuations, vanishes as x→0, and increases with increasing x. Optimal T c is obtained at a crossover from a phase ordering dominated regime at small x to a pairing dominated regime at large x. If this description is valid, then higher T c's can be obtained in an array of coupled planes with different doped hole concentrations, such that a high pairing scale is derived from the underdoped planes and a large phase stiffness from the optimally or overdoped ones.

C-axis optical sum rule in the Josephson-coupled vortex state

Observed violations of the c-axis optical sum rule can give important information on deviations from in-plane Fermi liquid behavior and on the nature of interlayer coupling between adjacent copper oxide planes. Application of a magnetic field perpendicular to these planes is another way to probe in-plane dynamics. We find that the optical sum rule is considerably modified in the presence of the c-axis magnetic field. Interlayer correlation of pancake vortices is involved in the sum-rule modification; however, details of the vortex distribution in the plane are less important.

Thermodynamics ofd-wave superconductors in a magnetic field

We investigate the thermodynamic properties of a two-dimensional d-wave superconductor in the vortex state using a semiclassical approach, and argue that such an approach is valid for the analysis of the experimental data on high-temperature superconductors. We develop a formalism where the spatial average of a physical quantity is written as an integral over the probability density of the Doppler shift, and evaluate this probability density for several model cases. The approach is then used to analyse the behavior of the specific heat and the NMR spin-lattice relaxation rate in a magnetic field. We compare our results with the experimental measurements, and explain the origin of the discrepancy between the results from different groups. We also address the observability of the recently predicted fourfold oscillations of the specific heat for the magnetic field parallel to the copper oxide planes. We consider both the orbital and the Zeeman effects, and conclude that at experimentally relevant temperatures Zeeman splitting does not appreciably reduce the anisotropy, although it does change the field dependence of the anisotropic specific heat. We predict a scaling law for the non-exponentially decaying NMR magnetization, and discuss different approaches to the effective relaxation rate.

Chimie douce route to hetero-structured high-Tc cuprates

Inorganic–inorganic and organic–inorganic hetero-structured high-Tc superconductors were achieved through a chimie douce (mild chemistry) route, in which inorganic or organic components were incorporated into the copper oxide superconductors, Bi2Sr2Cam−1CumOy (m=1, 2, and 3; BSCCO). According to the physico–chemical analyses for these nanohybrid materials, it is suggested that the high-Tc superconductivity is an intrinsic two-dimensional property of the copper oxide plane. All of these hetero-structured nanohybrid materials have an unique structural feature of a superconducting–insulating–superconducting (S–I–S) multilayer with atomically clean interfaces. Especially, the organic–inorganic nanohybrid is expected to be a promising precursor for superconducting colloidal suspension or nanoparticles, which could be applied to the fabrication of superconducting films or wires.

In-plane magnetic field dependence of intrinsic Josephson junctions in Tl-Ba-Ca-Cu-O thin films

We have fabricated intrinsic Josephson bridges using Tl-Ba-Ca-Cu-O thin films grown on vicinal lanthanum aluminate substrates. Measurements of the lock-in transition allow us align a magnetic field of up to 4 T with the copper oxide planes to better than 0.3/spl deg/. This allows us to study the dynamics of Josephson fluxons in the films. For fields above 1.4 T and at low currents we observe Josephson flux creep. Above a field-dependent threshold current we observe free flow of the fluxons with a flux-flow resistance which is proportional to field. The fluxons reach a field-independent velocity before the jump to the quasiparticle branch occurs. Our results suggest that the development of a sub-mm-wave oscillator using these bridges will require sub-micron patterning.

Structural distortions and extension of superconducting region in the Tc– p phase diagram of HgBa 2CuO 4+ δ

In monolayer Hg-based high- T c materials, the T c– p phase diagram follows a relationship different from that followed by many other high- T c materials. It has been observed that an extension of the superconducting region takes place in the overdoped region and a buckling of the Cu–O–Cu bond on the copper–oxide plane is introduced when overdoped materials are made by substituting thallium in the place of mercury. Connection between this buckling and the extension of superconducting region is investigated, and an interpretation of the extension proposed in the light of a quantum paraelectric model.

Combined paramagnetic and diamagnetic response ofYBa2Cu3O7

It has been predicted that the zero frequency density of states of YBCO in the superconducting phase can display interesting anisotropy effects when a magnetic field is applied parallel to the copper-oxide planes, due to the diamagnetic response of the quasiparticles. In this paper we incorporate paramagnetism into the theory and show that it lessens the anisotropy and can even eliminate it altogether. At the same time paramagnetism also changes the scaling with the square root of the magnetic field first deduced by Volovik leading to an experimentally testable prediction. We also map out the analytic structure of the zero frequency density of states as a function of the diamagnetic and paramagnetic energies. At certain critical magnetic field values we predict kinks as we vary the magnetic field. However, these probably lie beyond currently accessible field strengths.

Planar tunneling spectroscopy ofY1−xPrxBa2Cu3O7thin films as a function of crystallographic orientation

We present a systematic study of the charge transport and quasiparticle tunneling properties of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ thin films. Pr doping increases the resistivity along the copper oxide planes and suppresses the superconducting critical temperature ${T}_{c},$ ultimately inducing a superconductor-insulator transition. The tunneling conductance is reproducible and correlated with the crystallographic film orientation. The crystallographic dependence can be divided into two distinct categories: tunneling into (001)-oriented $(c$-axis) films and tunneling into (100)-, (110)-, and (103)-oriented $(ab$-oriented) films. c-axis tunneling data exhibit a conductance dip at zero bias and a broad temperature-dependent peak over $\ensuremath{\sim}15--40$ mV that decreases in magnitude but stays fixed in energy for increasing Pr doping levels. $\mathrm{ab}$-plane tunneling data exhibit a zero-bias conductance peak and a gaplike feature at an energy that scales roughly linearly with ${T}_{c}$ for $x=0.0,$ 0.2, and 0.4. When $x=0.5,$ the resistivity is not linear in temperature and a zero-bias conductance dip is observed. The background conductance that ensures conservation of states in the low-temperature $\mathrm{ab}$-plane data exhibits temperature- and doping-dependent structure over $\ensuremath{\sim}15--40$ mV that is very similar to the peak observed in c-axis tunneling. Finally, analysis of the temperature and magnetic field dependence of the zero bias conductance peak indicates that states are conserved to within $\ensuremath{\sim}20%,$ supporting its interpretation as a feature of a superconducting density of states.

Electronic structure for Zn-doped copper-oxide planes

A microscopical model for the Zn-doped CuO 2 plane is developed within the p- d model with strong correlations. Based on band structure calculations, Zn impurities are considered as vacancies for the d-states on Cu sites. Further reduction to an effective one-band model results in the t- J model with vacancies for the spin 1 2 d- states at the Zn-sites. By employing the T-matrix formalism for the Green functions in terms of the Hubbard operators the density of electronic states (DOS) is calculated. Symmetry analysis of the perturbation matrix shows that in the system with d-type electronic wave functions additional DOS of d-, p- and s-types appear due to the perturbation of local energy levels and the interaction between nearest neighbors around the vacancy. The local and resonant state formation caused by Zn impurities is analysed.

Difference between microscopic and effective overlaps in the copper-oxide planes of high-Tc superconductors

We investigate parametrizations required to reproduce observed Fermi surfaces of overdoped and slightly underdoped LSCO in two different models. One is the standard three-band saddle-point slave boson (SB). The other is a model of CuO4 ‘molecules’ randomly tiled in the plane, emphasizing the distinction between the local overlap, always the full (bare) one, and the effective overlap, connected with the reduced bandwidth due to the random tiling (RT). We conclude this distinction is physically significant for underdoped, but not for overdoped LSCO. This is consistent with the observed open Fermi surfaces being due to correlation effects.

Alternating current Josephson effect in intrinsic Josephson bridges in Tl2Ba2CaCu2O8 thin films

We have performed transport measurements on bridges patterned in misaligned thin films of the superconductor Tl2Ba2CaCu2O8. There is a c-axis component of current flow along the bridge, giving rise to hysteretic Josephson-like current–voltage curves. The temperature dependence of the critical current follows the Ambegaokar–Baratoff theory with IcRN up to 26 mV at 4.2 K. Microwave emission from the Josephson junctions near Tc (≈103 K) has been detected using an X-band detector. We show that 700±15 junctions in the bridge are actively oscillating, confirming that the junctions are “intrinsic” junctions formed by adjacent copper oxide planes in the Tl2Ba2CaCu2O8 crystal structure.