Superconductivity in LaSr(Ba)CuO and related systems

Systematics in the superconducting behaviour of Ln _2−xSr _x(Ba _x)CuO ₄, La _2−xSr _x−yBa _y(Ca _y)CuO ₄, (La _1−xLn _x) _2−ySr _y(Ba _y)CuO ₄ with Ln = Pr, Nd or Y and related systems are presented. The role of oxygen stoichiometry and lattice parameters of these mixed valent copper oxides is indicated. These high T _c superconducting oxides exhibit high-temperature resistivities in the borderline of the metal-nonmetal transition.     

Competition of static stripe and superconducting phases in La(1.48)Nd(0.4)Sr(0.12)CuO(4) controlled by pressure

We have investigated the pressure effect on T(c) and the Hall coefficient in the static stripe-ordered phase of La(1.48)Nd(0.4)Sr(0.12)CuO(4) crystal under hydrostatic pressure. We found a dramatic change of the Hall coefficient and an abrupt increase of T(c) at low pressure of about 0.1 GPa. The results are indicative of a transition from one- to two-dimensional charge transport, associated with the suppression of low-temperature-tetragonal (LTT) phase. From the uniaxial pressure measurements it turns out that the observed critical change is induced primarily due to the in-plane compression of the CuO(2) planes which would make the pinning potential of the LTT lattice distortions weaker.     

Transverse josephson plasma mode in T* cuprate superconductors

A transverse optical plasma mode is observed at far-infrared frequencies within the superconducting gap region by measuring the c-axis optical reflectivity for single crystals of T* cuprate superconductors SmLa0.85Sr0.15CuO4-delta and Nd1.4Sr0.4Ce0.2CuO4-delta. These T* cuprates have two different insulating layers sandwiching the superconducting CuO2 planes, leading to two longitudinal plasmons. Also, the transverse mode is directly observed due to the coupling of the infrared radiation with the current perpendicular to the superconducting layers which are regarded as an alternating array of two inequivalent Josephson junctions.     

Phase-sensitive order parameter symmetry test experiments utilizing Nd(2-x)Ce(x)CuO(4-y)/Nb zigzag junctions

Phase-sensitive order parameter symmetry test experiments are presented on the electron-doped high-T(c) cuprate Nd(2-x)Ce(x)CuO(4-y). These experiments have been conducted using zigzag-shaped thin film Josephson structures, in which the Nd(2-x)Ce(x)CuO(4-y) is connected to the low-T(c) superconductor Nb via an Au barrier layer. For the optimally doped as well as for the overdoped Nd(2-x)Ce(x)CuO(4-y), a clear predominant d(x2-y2)-wave behavior is observed at T=4.2 K. Both compounds were also investigated at T=1.6 K, presenting no indications for a change to a predominant s-wave symmetry with decreasing temperature.     

Enhancement of the superconducting transition temperature of La2-xSrxCuO4 bilayers: role of pairing and phase stiffness

The superconducting transition temperature T(c) of bilayers comprising underdoped La(2-x)Sr(x)CuO(4) films capped by a thin heavily overdoped metallic La(1.65)Sr(0.35)CuO(4) layer, is found to increase with respect to T(c) of the bare underdoped films. The highest T(c) is achieved for x=0.12, close to the "anomalous" 1/8 doping level, and exceeds that of the optimally doped bare film. Our data suggest that the enhanced superconductivity is confined to the interface between the layers. We attribute the effect to a combination of the high pairing scale in the underdoped layer with an enhanced phase stiffness induced by the overdoped film.     

Accurate prediction of large antiferromagnetic interactions in high- T(c) HgBa2Ca(n-1)Cu(n)O(2n+2+delta) ( n = 2,3) superconductor parent compounds

The in-plane nearest-neighbor Heisenberg magnetic coupling constant, J, of La2CuO4, Nd2CuO4, Sr2CuO2Cl2, YBa2Cu3O6, and undoped HgBa(2)Ca(n-1)Cu(n)O(2n+2+delta) ( n = 1,2,3) is calculated from accurate ab initio configuration interaction calculations. For the first four compounds, the theoretical J values are in quantitative agreement with experiment. For the Hg-based compounds the predicted values are -135 meV ( n = 1) and approximately -160 meV ( n = 2,3), the latter being much larger than in previous cases and, for n = 3, increasing with pressure. Nevertheless, the physics governing J in all these layered cuprates appears to be the same. Moreover, calculations suggest a possible relationship between J and T(c).     

Commensurate dynamic magnetic correlations in La2Cu0.9Li0.1O4

When sufficient numbers of holes are introduced into the two-dimensional CuO2 square lattice, dynamic magnetic correlations become incommensurate with underlying lattice in all previously investigated La(2-x)A(x)Cu(1-z)B(z)O(4+y) ( A = Sr or Nd, B = Zn) including high T(c) superconductors and insulators, and in bilayered superconducting YBa2Cu3O6.6 and Bi2Sr2CaCu2O8. Magnetic correlations also become incommensurate in structurally related La2NiO4 when doped with Sr or O. We report an exception to this so-far well-established experimental "rule" in La(2)Cu(1-z)Li(z)O4 in which magnetic correlations remain commensurate.     

Dual nature of the electronic structure of (La(2--x--y)Nd(y)Sr(x))CuO(4) and La(1.85)Sr(0.15)CuO(4).

High resolution angle-resolved photoemission measurements have been carried out on (La(1.4--x)-Nd(0.6)Sr(x))CuO(4), a model system with static one-dimensional (1D) charge ordering (stripe), and (La(1.85)-Sr(0.15))CuO(4), a high temperature superconductor (T(c) = 40 K) with possible dynamic stripes. In addition to the straight segments near ( pi,0) and ( 0,pi) antinodal regions, we have identified the existence of spectral weight along the [1,1] nodal direction in the electronic structure of both systems. This observation of nodal state, together with the straight segments near antinodal regions, reveals the dual nature of the electronic structure of stripes due to the competition of order and disorder.     

Evidence for nodal quasiparticles in electron-doped cuprates from penetration depth measurements

The in-plane magnetic penetration depth, lambda(T), was measured down to 0.4 K in single crystals of electron-doped superconductors, Pr(1.85)Ce(0.15)CuO(4-delta) (PCCO) and Nd(1.85)Ce(0.15)CuO(4-delta) (NCCO). In PCCO, the superfluid density varies as T2 from 0.025 up to roughly 0.3T/T(c) suggestive of a d-wave state with impurities. In NCCO, lambda(T) shows a pronounced upturn for T<4 K due to the paramagnetic contribution of Nd3+ ions. Fits to an s-wave order parameter over the standard BCS range (T/T(c) = 0.32) limit any gap to less than Delta(min)(0)/T(c) = 0.57 in NCCO. For PCCO, the absence of paramagnetism permits a lower temperature fit and yields an upper limit of Delta(min)(0)/T(c) = 0.2.     

Crossing the gap from p- to n-type doping: nature of the states near the chemical potential in La(2)-(x)Sr(x)CuO(4) and Nd(2-x)Ce(x)CuO(4-delta)

We report on an x-ray absorption and resonant photoemission study on single crystals of the high-T(c) cuprates La2-xSrxCuO4 and Nd(2-x)Ce(x)CuO(4-delta). Using an intrinsic energy reference, we find that the chemical potential of La2-xSrxCuO4 lies near the top of the La2CuO4 valence band whereas in Nd(2-x)Ce(x)CuO(4-delta) it is situated near the bottom of the Nd2CuO4 conduction band. The data clearly establish that the introduction of Ce in Nd2CuO4 results in electrons being doped into the CuO2 planes. We infer that the states closest to the chemical potential have a Cu 3d(10) singlet origin in Nd(2-x)Ce(x)CuO(4-delta) and a 3d(9)L singlet origin in La2-xSrxCuO4.     

Mean-field pairing theory for the charge-stripe phase of high-temperature cuprate superconductors

Striped high-T(c) superconductors such as La(2-y-x)Nd(y)Sr(x)CuO(4) and La(2-x)Ba(x)CuO(4) near x = 1/8 show a fascinating competition between spin and charge order and superconductivity. A theory for these systems therefore has to capture both the spin correlations of an antiferromagnet and the pair correlations of a superconductor. For this purpose we present here an effective Hartree-Fock theory incorporating both electron pairing with finite center-of-mass momentum and antiferromagnetism. We show that this theory reproduces the key experimental features such as the formation of the antiferromagnetic stripe patterns at 7/8 band filling or the quasi-one-dimensional electronic structure observed by photoemission spectroscopy.     

Electromagnetic response of static and fluctuating stripes in cuprate superconductors

Using infrared spectroscopy, we found that changes in the in-plane charge dynamics attributable to static stripe order in La(1.275)Nd(0.6)Sr(0.125)CuO(4) or superconductivity in La(1.875)Sr(0.125)CuO(4) are confined to energies smaller than 100 cm(-1). An absorption peak in the low- omega conductivity of the Nd-doped compound is suggestive of localization effects due to the reduced dimensionality of static charge stripes. Neither superconductivity nor static stripe ordering has a noticeable effect on the depression of the scattering rate at omega<1000 cm(-1) characteristic of the pseudogap state in other classes of moderately doped cuprates.     

Strongly correlated s-wave superconductivity in the N-type infinite-layer cuprate

Quasiparticle tunneling spectra of the electron-doped ( n-type) infinite-layer cuprate Sr0.9La0.1CuO2 reveal characteristics that counter a number of common phenomena in the hole-doped ( p-type) cuprates. The optimally doped Sr0.9La0.1CuO2 with T(c) = 43 K exhibits a momentum-independent superconducting gap Delta = 13.0+/-1.0 meV that substantially exceeds the BCS value, and the spectral characteristics indicate insignificant quasiparticle damping by spin fluctuations and the absence of pseudogap. The response to quantum impurities in the Cu sites also differs fundamentally from that of the p-type cuprates with d(x(2)-y(2))-wave pairing symmetry.     

In-plane thermal conductivity of Nd2CuO4: evidence for magnon heat transport

We report the temperature and magnetic field dependence of the in-plane thermal conductivity (kappa(ab)) of high-quality monocrystalline Nd2CuO4. Isothermal measurements of the field dependence of kappa(ab) at low temperatures (2 K相似文献   

Crystal-chemistry aspects and electric-field-gradient (EFG) dispersion in Ca-gallogermanate-type structures studied by Mössbauer spectroscopy

The crystal field disorder in some trigonal germanates of the type X_3-yLn_yFe_2+yGe_4-yO₁₄ (X = Ba, Sr; Ln = La, Nd; y = 0, 1) is studied by ⁵⁷Fe M?ssbauer effect. The dispersion of the electric field gradient (EFG) at the octahedral sites of these compounds is investigated. A correlation of the experimental and calculated EFG data with some crystal-chemistry aspects is presented. Received 3 February 1999     

Evidence of electron fractionalization from photoemission spectra in the high temperature superconductors

In the normal state of the high temperature superconductors Bi(2)Sr(2)CaCu(2)O(8+delta) and La2(-x)Sr(x)CuO4, and in the related "stripe ordered" material, La(1.25)Nd(0.6)Sr(0.15)CuO4, there is sharp structure in the measured single hole spectral function, A<(k-->,omega), considered as a function of k--> at fixed small binding energy omega. At the same time, as a function of omega at fixed k--> on much of the putative Fermi surface, any structure in A<(k-->,omega), other than the Fermi cutoff, is very broad. This is characteristic of the situation in which there are no stable excitations with the quantum numbers of the electron, as is the case in the one-dimensional electron gas.     

Quantitative test of SO(5) symmetry in the vortex state of Nd(1.85)Ce(0.15)CuO4

By numerically solving models with competing superconducting and antiferromagnetic orders, we study the magnetic field dependence of the antiferromagnetic moment in both the weak and strong field regimes. Through a comparison with the neutron scattering results of Kang et al. and Matsuura et al. on Nd(1.85)Ce(0.15)CuO4, we conclude that this system is close to a SO(5) symmetric critical point. We also make a quantitative prediction on increasing the upper critical field B(c2) and the superconducting transition temperature T(c) by applying an in-plane magnetic field.     

Transverse Josephson plasma mode in T* phase SmLa(1-x)Sr(x)CuO(4-delta) single crystals

Far-infrared reflectivity along the c axis in T* phase SmLa(1-x)Sr(x)CuO(4-delta) single crystals is measured down to 8 cm(-1). Below T(c), the conductivity peak is observed at 25 cm(-1) for x = 0.15 ( T(c) = 30 K) along with two reflectivity edges at 13 and 27 cm(-1). The conductivity peak is attributed to the transverse Josephson plasma mode between two longitudinal Josephson plasma modes, while the oscillator strength of the peak is found to be smaller than that calculated using the Josephson-coupled multilayer model. The difference is explained by assuming that only a few junctions at the disordered (La,Sr)(2)O(2-delta) block layer take part in the plasma oscillation with omega(pI(')) = 27 cm(-1).     

Microwave electrodynamics of electron-doped cuprate superconductors

We report microwave cavity perturbation measurements of the temperature dependence of the penetration depth, lambda(T), and conductivity, sigma(T) of Pr(2-x)Ce(x)CuO(4-delta) (PCCO) crystals, as well as parallel-plate resonator measurements of lambda(T) in PCCO thin films. Penetration depth measurements are also presented for a Nd(2-x)Ce(x)CuO(4-delta) (NCCO) crystal. We find that Deltalambda(T) has a power-law behavior for T相似文献   

Very large purely intralayer critical current density in ultrathin cuprate artificial structures

Ultrathin artificial high temperature superconducting structures, consisting of (Ba(0.9)Nd0.1)CuO2+x and CaCuO2 layers, were grown by pulsed laser deposition. Intralayer superconductivity at 60 K was obtained for a structure consisting of a single (CaCuO2) block sandwiched between two (Ba(0.9)Nd0.1)CuO2+x charge reservoir blocks. The purely intralayer critical current density was measured at 4.2 K and resulted to be larger than 10(8) A/cm(2). These findings clearly show that interaction between nearest neighbor (CaCuO2) layers is not essential for high T(c) superconductivity and strongly supports the physical model based on the idea that intralayer interaction alone is responsible for high temperature superconductivity.