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.     

电子型超导体Sm2-xCexCuO4(0.00≤x≤0.21)的异常热电势与电阻率

测量了Sm2-xCexCuO4(0.00≤x≤0.21)多晶样品的热电势S和电阻率ρ的温度依赖关系.在缘绝体-金属转变边界处,观测到热电势从绝缘体区明显的弱温度依赖关系到金属区线性温度依赖关系的转变.当Ce的含量由0.09增加到0.21时,高温下S的斜率发生由负到正的转变,这是能带的填充能级发生改变时电子型和空穴型载流子的贡献发生竞争的表现,由电子型向空穴型的过渡发生在x＝0.17处.S和ρ在200K以下的斜率变化是载流子局域化造成的.x＝0.06-0.21的样品在50K处观察到一个正的曳引峰.室温下的热电势S300K和S0(高温区热电势线性外推到0K的值)与Ce含量在绝缘体、欠掺杂和过掺杂区域有不同的依赖关系.过掺杂区域很小的S300K和S0意味着一个宽带的费米液体的贡献,同时ρ满足T2关系,二者相一致.     

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.     

High-field magnetoresistance and Hall effect in Bi2Sr2CuO x single crystals

We investigated the in-plane magnetoresistance and the Hall effect of high-quality Bi₂Sr₂CuO_x single crystals with T _c (midpoint) = 3.7–9.6 K in dc magnetic fields up to 23 T. For T < 10 K, the crystals show the classical positive magnetoresistance. Starting at T ≈ 14 K, an anomalous negative magnetoresistance appears at low magnetic fields; for T ≥ 40 K, the magnetoresistance is negative in the whole studied range of magnetic fields. Temperature and magnetic field dependences of the negative-magnetoresistance single crystals are qualitatively consistent with the electron interaction theory developed for simple semiconductors and disordered metals. As is observed in other cuprate superconductors, the Hall resistivity is negative in the mixed state and changes its sign with increasing field. The linear T-dependence of cotθ_H for the Hall angle in the normal state closely resembles that of the normal-state resistivity as expected for a Fermi liquid picture.     

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.     

Tunneling into the normal state of Pr2-xCexCuO4

The temperature dependence of the tunneling conductance was measured for various doping levels of Pr(2-x)CexCuO4 using planar junctions. A normal state gap is seen at all doping levels studied, x=0.11 to x=0.19. We find it to vanish above a certain temperature T*. T* is greater than T(c) for the underdoped region and it follows T(c) on the overdoped side. This behavior suggests finite pairing amplitude above T(c) on the underdoped side.     

Superconductivity and field-induced magnetism in Pr2-xCexCuO4 single crystals

We report muon-spin rotation and relaxation (muSR) measurements on single crystals of the electron-doped high-T(c) superconductor Pr2-xCexCuO4. In a zero external magnetic field, superconductivity is found to coexist with dilute Cu spins that are static on the muSR time scale. In an applied field, we observe a mu(+)-Knight shift that is primarily due to the magnetic moment induced on the Pr ions. Below the superconducting transition temperature T(c), an additional source of local magnetic field appears throughout the volume of the sample. This finding is shown to be consistent with field-induced antiferromagnetic ordering of the Cu spins. Measurements of the temperature dependence of the in-plane magnetic penetration depth lambda(ab) in the vortex state are also presented.     

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.     

High-field magnetoresistance of polycrystalline ZrZn2

We report magnetoresistance measurements of polycrystalline ZrZn₂ as a function of temperature (4.2–48K) and magnetic field up to 19 T. The results indicate the presence of both positive and negative contributions to the magnetoresistance. The latter is due to spin fluctuations. Below T_c the resistivity varies with temperature like T² over the entire field range (0–19 T). The coefficient of the T² -term decreases with increasing field and fits a $\text{H}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}$ dependence above ～ 10 T, in accordance with theoretical predictions. Complex behaviour of the magnetoresistance is found in the paramagnetic regime above ～ 5 T.     

Phase diagram of the electron-doped La2-xCexCuO4 cuprate superconductor from Andreev bound states at grain boundary junctions

We use quasiparticle tunneling across La2-xCexCuO4 grain boundary junctions to probe the superconducting state and its disappearance with increasing temperature and magnetic field. A zero bias conductance peak due to zero energy surface Andreev bound states is a clear signature of the phase coherence of the superconducting state. Hence, such a peak must disappear at or below the upper critical field Bc2(T). For La2-xCexCuO4 this approach sets a lower bound for Bc2(0) approximately 25 T which is substantially higher than reported previously. The method of probing the superconducting state via Andreev bound states should also be applicable to other cuprate superconductors.     

Neutron-spin resonance in the optimally electron-doped superconductor Nd1.85Ce0.15CuO4-delta

We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped superconductor Nd1.85Ce0.15CuO4-delta above and below its superconducting transition temperature Tc=25 K. In addition to gradually opening a spin pseudogap at the antiferromagnetic ordering wave vector Q=(1/2,1/2,0), the effect of superconductivity is to form a resonance centered also at Q=(1/2,1/2,0) but at energies above the spin pseudogap. The intensity of the resonance develops like a superconducting order parameter, similar to those for hole-doped superconductors and electron-doped Pr0.88LaCe0.12CuO4. The resonance is therefore a general phenomenon of cuprate superconductors, and must be fundamental to the mechanism of high-Tc superconductivity.     

High-field magnetoresistance of noble metals containing rare-earth impurities

We report on magnetoresistance measurements in longitudinal and transverse magnetic fields up to 320 kG for silver and gold containing rare-earth impurities. We focus mainly on the strong anisotropy of the magnetoresistance related to the scattering of conduction electrons by the 4f quadrupoles (non-S ions) and we derive the magnitude of the electron-quadrupole interaction from the analysis of the results. We also consider the isotropic contribution to the magnetoresistance due to exchange scattering. In a number of alloys this contribution is negative in low fields, as this is usually observed in magnetic alloys, but becomes positive in high fields. This change of spin can be ascribed to crystal-field effects.     

Spin-flop transition and the anisotropic magnetoresistance of Pr(1.3-x)La(0.7)CexCuO4: unexpectedly strong spin-charge coupling in the electron-doped cuprates

We use transport and neutron-scattering measurements to show that a magnetic-field-induced transition from noncollinear to collinear spin arrangement in adjacent CuO2 planes of lightly electron-doped Pr(1.3-x)La(0.7)CexCuO4 (x=0.01) crystals affects significantly both the in-plane and out-of-plane resistivity. In the high-field collinear state, the magnetoresistance (MR) does not saturate but exhibits an intriguing fourfold-symmetric angular dependence, oscillating from being positive at B//[100] to being negative at B//[110]. The observed MR of more than 30% at low temperatures induced by a modest modification of the spin structure indicates an unexpectedly strong spin-charge coupling in electron-doped cuprates.