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.     

Nodal order parameter in electron-doped Pr(2-x)CexCuO(4-delta) superconducting films

The London penetration depth, lambda(ab)(T), is reported for thin films of the electron-doped superconductor Pr(2-x)Ce(x)CuO(4-delta) with varying Ce concentration, x=0.13, 0.15, and 0.17. Measurements down to 0.35 K were carried out using a tunnel-diode oscillator with excitation fields applied both perpendicular and parallel to the conducting planes. Films at all three doping levels exhibited power law behavior indicative of d-wave pairing with impurity scattering. These results are fully consistent with previous measurements on single crystals.     

Evidence for a nodeless gap from the superfluid density of optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films

We present measurements of the ab-plane magnetic penetration depth, lambda(T), in five optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films for 1.6 K< or =T < or =T(c) approximately 24 K. Low resistivities, high superfluid densities n(s)(T) proportional, variant lambda(-2)(T), high T(c)'s, and small transition widths are reproducible and indicative of excellent film quality. For all five films, lambda(-2)(T)/lambda(-2)(0) at low T is well fitted by an exponential temperature dependence with a gap, Delta(min), of 0.85k(B)T(c). This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.     

Magnetic-field dependence of the maximum supercurrent of La2-xCexCuO4-y interferometers: evidence for a predominant dx2-y2 superconducting order parameter

We performed a phase-sensitive test of the symmetry of the superconducting order parameter of the electron doped cuprate La(2-x)Ce(x)CuO(4-y) using a superconducting quantum interferometer with spatially distributed Josephson junctions. The studies were made on a thin film grown on a SrTiO3 tetracrystal substrate. The superconducting transition temperature was about 29 K which indicates that the sample is close to optimal doping. The magnetic field dependence of the critical current gives strong evidence for a predominant dx(2)(-y(2)) order parameter symmetry of the sample measured. It also gives upper limits for the s-wave component in a mixed order parameter of the type s+idx(2)(-y(2)).     

Dirty superconductivity in the electron-doped cuprate Pr(2-x)Ce(x)CuO(4-delta): tunneling study

We report a tunneling study between Pr(2-x)Ce(x)CuO(4-delta) and lead as a function of doping, temperature, and magnetic field. The temperature dependence of the gap follows the BCS prediction. Our data fit a nonmonotonic d-wave order parameter for the whole doping range studied. From our data we are able to conclude that the electron-doped cuprate Pr(2-x)Ce(x)CuO(4-delta) is a weak-coupling BCS dirty superconductor.     

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.     

Evidence for a quantum phase transition in Pr2-xCe(x)CuO4-delta from transport measurements

The doping and temperature dependences of the Hall coefficient, R(H), and ab-plane resistivity in the normal state down to 350 mK is reported for oriented films of the electron-doped high-T(c) superconductor Pr(2-x)Ce(x)CuO(4-delta). The doping dependences of beta (rho=rho(0)+ATbeta) and R(H) (at 350 mK) suggest a quantum phase transition at a critical doping near x=0.165.     

Magnetic penetration depth measurements of Pr2-xCexCuO4-delta films on buffered substrates: evidence for a nodeless gap

We report measurements of the inverse squared magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-delta) (0.115< or =x < or =0.152) superconducting films grown on SrTiO3 (001) substrates coated with a buffer layer of insulating Pr2CuO4. lambda(-2)(0), T(c), and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, 0.124< or =x < or =0.144, lambda(-2)(T) at low T is flat: it changes by less than 0.15% over a factor of 3 change in T, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in lambda(-2)(T) produce values of the minimum superconducting gap in the range of 0.29< or =Delta(min)/k(B)T(c)< or =1.01.     

Nonmonotonic d(x(2)-y(2)) superconducting order parameter in Nd2-xCexCuO4

Low energy polarized electronic Raman scattering of the electron-doped superconductor Nd2-x Ce x CuO4 ( x = 0.15, T(c) = 22 K) has revealed a nonmonotonic d(x(2)-y(2)) superconducting order parameter. It has a maximum gap of 4.4k(B)T(c) at Fermi surface intersections with an antiferromagnetic Brillouin zone (the "hot spots") and a smaller gap of 3.3k(B)T(c) at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the hot spots emphasizes the role of antiferromagnetic fluctuations and the similarity in the origin of superconductivity for electron- and hole-doped cuprates.     

Spin correlations and magnetic order in nonsuperconducting Nd(2-x)Ce(x)CuO(4+/-delta)

We report quantitative neutron scattering measurements of the evolution with doping of the Néel temperature, the antiferromagnetic correlations, and the ordered moment of as-grown, nonsuperconducting Nd(2-x)Ce(x)CuO(4+/-delta) (0相似文献   

Field-dependent diamagnetic transition in magnetic superconductor Sm1.85Ce0.15CuO4-y

The magnetic penetration depth of single crystal Sm(1.85)Ce(0.15)CuO(4-y) was measured down to 0.4 K in dc fields up to 7 kOe. For insulating Sm2CuO4, Sm3+ spins order at the Ne el temperature, T(N)=6 K, independent of the applied field. Superconducting Sm(1.85)Ce(0.15)CuO(4-y) (T(c) approximately 23 K) shows a sharp increase in diamagnetic screening below T(*)(H) which varied from 4.0 K (H=0) to 0.5 K (H=7 kOe) for a field along the c axis. If the field was aligned parallel to the conducting planes, T(*) remained unchanged. The unusual field dependence of T(*) indicates a spin-freezing transition that dramatically increases the superfluid density.     

Local measurement of the superfluid density in the pnictide superconductor Ba(Fe(1-x)Co(x))(2)As(2) across the superconducting dome

We measure the penetration depth λab(T) in Ba(Fe(1-x)Co(x))(2)As(2) using local techniques that do not average over the sample. The superfluid density ρs(T) ≡ 1/λab(T)2 has three main features. First, ρs (T = 0) falls sharply on the underdoped side of the dome. Second, λab(T) is flat at low T at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, ρs (T) varies steeply near Tc for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases.     

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-delta

We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr(2-x)Ce(x)CuO(4-delta) using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a d(x2-y2) pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.     

Metal-to-insulator crossover in the low-temperature normal state of Bi(2)Sr(2-x)La(x)CuO(6+delta)

We measure the normal-state in-plane resistivity of Bi(2)Sr(2-x)La(x)CuO(6+delta) single crystals at low temperatures by suppressing superconductivity with 60 T pulsed magnetic fields. With decreasing hole doping, we observe a crossover from a metallic to an insulating behavior in the low-temperature normal state. This crossover is estimated to occur near 1/8 doping, well inside the underdoped regime, and not at optimum doping as reported for other cuprates. The insulating regime is marked by a logarithmic temperature dependence of the resistivity over two decades of temperature, suggesting that a peculiar charge localization is common to the cuprates.     

Unusual Nernst effect suggesting time-reversal violation in the striped cuprate superconductor La(2-x)Ba(x)CuO4

The striped cuprate La(2-x)Ba(x)CuO(4) (x=1/8) undergoes several transitions below the charge-ordering temperature T(co)=54 K. From Nernst experiments, we find that, below T(co), there exists a large, anomalous Nernst signal e(N,even)(H,T) that is symmetric in field H, and remains finite as H→0. The time-reversal violating signal suggests that, below T(co), vortices of one sign are spontaneously created to relieve interlayer phase frustration.     

Field-induced thermal metal-to-insulator transition in underdoped La(2-x)Sr(x)CuO(4+delta)

The transport of heat and charge in cuprates was measured in single crystals of La(2-x)Sr(x)CuO(4+delta) (LSCO) across the doping phase diagram at low temperatures. In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T-->0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied magnetic field, we show that a novel thermal metal-to-insulator transition takes place upon going from the superconducting state to the field-induced normal state.     

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).     

The superconducting transition temperatures of Fe(1+x)Se(1-y), Fe(1+x)Se(1-y)Te(y) and (K/Rb/Cs)(z)Fe(2-x)Se2

In a recent contribution to this journal, it was shown that the transition temperatures of optimal high-T(C) compounds obey the algebraic relation T(C0) = k(-1)(B)/?ζ, where ? is related to the mean spacing between interacting charges in the layers, ζ is the distance between interacting electronic layers, β is a universal constant and k(B) is Boltzmann's constant. The equation was derived assuming pairing based on interlayer Coulomb interactions between physically separated charges. This theory was initially validated for 31 compounds from five different high-T(C) families (within an accuracy of ±1.37 K). Herein we report the addition of Fe(1+x)Se(1-y) and Fe(1+x)Se(1-y)Te(y) (both optimized under pressure) and A(z)Fe(2-x)Se(2) (for A = K, Rb or Cs) to the growing list of Coulomb-mediated superconducting compounds in which T(C0) is determined by the above equation. Doping in these materials is accomplished through the introduction of excess Fe and/or Se deficiency, or a combination of alkali metal and Fe vacancies. Consequently, a very small number of vacancies or interstitials can induce a superconducting state with a substantial transition temperature. The confirmation of the above equation for these Se-based Fe chalcogenides increases to six the number of superconducting families for which the transition temperature can be accurately predicted.     

Interplay of externally doped and thermally activated holes in La(2-x)Sr(x)CuO4 and their impact on the pseudogap crossover

We presented the recent Hall effect data for a number of carriers in La(2-x)Sr(x)CuO4 as the sum of two components: the temperature independent term n0(x), which is due to external doping, and the thermally activated contribution. Their balance determines the crossover temperature T*(x) from the marginal Fermi liquid to pseudogap regime. The activation energy Delta(x) for thermally excited carriers equals the energy between the Fermi surface "arc" and the band bottom, as seen in angle-resolved photoemission spectroscopy experiments. Other implications for the (T, x)-phase diagram of cuprates are also discussed.     

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