La2-xSrxCuO4 Single Crystals Research Articles

Precision search for magnetic order in the pseudogap regime of La2−xSrxCuO4by muon spin relaxation

We report a high precision search for orbital-like magnetic order in the pseudogap region of La2-xSrxCuO4 single crystals using zero-field muon spin relaxation (ZF-muSR). In contrast to previous studies of this kind, the effects of the dipolar and quadrupolar interactions of the muon with nearby nuclei are calculated. ZF-muSR spectra with a high number of counts were also recorded to determine whether a magnetically ordered phase exists in dilute regions of the sample. Despite these efforts, we find no evidence for static magnetic order of any kind in the pseudogap region above the hole-doping concentration p = 0.13.

Effect of stripe order strength for the Nernst effect in [formula omitted] single crystals

We have precisely measured the Nernst effect in Nd-doped La2-xSrxCuO4 single crystals with controlling the strength (stability) of the stripe order. We found that the onset temperature Tonset, where the Nernst signal starts increasing, does not change conspicuously in spite of Nd-doping. At low temperatures, on the other hand, the absolute value of the Nernst signal is strongly suppressed in accordance with the strength of the stripe order. These results imply that the fluctuation of (charge) stripe order enhances the Nernst signal below Tonset at high temperatures, and then the stripe order enhanced by Nd-doping suppresses the superconducting fluctuation to reduce the Nernst signal at low temperatures. We also observed an increase of the Nernst signal below the charge order temperature Tch which has been observed in diffraction measurement.

Floating-zone growth and property characterizations of high-quality La2–xSrxCuO4 superconductor crystals

We have grown underdoped (x = 0.11, 0.12) and optimally doped (x = 0.16) La2–xSrxCuO4 single crystals by the traveling-solvent floating-zone technique. In order to prepare good quality cuprate crystals, we have made much effort to optimize the preparation procedures. For example, we haveadopted the sol–gel route to prepare a highly fine and homogeneous La2–xSrxCuO4 precursor powder for fabricating a very dense ceramic feed rod used for the floating-zone growth, and we have also used quite a slow growth rate. The high quality of the grown crystals has been verified by double-crystal x-ray rocking curves, with the full-width-at-half-maximum being only 113–150 arcseconds, which are the best data reported so far for La2–xSrxCuO4 crystals. The superconducting critical temperatures of the grown crystals are 30, 31 and 38.5 K for x = 0.11, 0.12 and 0.16 samples, respectively, according to magnetic measurements.

Doping evolution of the electronic specific heat coefficient in slightly-doped La2-xSrxCuO4 single crystals

Detailed doping dependence of the electronic specific heat coefficient γ is studied for La2-xSrxCuO4 (LSCO) single crystals in the slightly-doped regime. We find that γ systematically increases with doping, and furthermore, even for the samples in the antiferromagnetic (AF) regime, γ already acquires finite value and grows with x. This suggests that finite electronic density of states (DOS) is created in the AF regime where the transport shows strong localization at low temperatures, and this means the system is not a real insulator with a clear gap even though it still keeps long range AF order.

Anomalous enhancement of vortex pinning and microscopic phase separation in the overdoped regime of La2-xSrxCuO4

Our previous study of the magnetic susceptibility, χ, on zero-field and field cooling and magnetization-hysteresis-loop (MHL) for La2-xSrxCuO4 (LSCO) single crystals with x = 0.178 – 0.261 has suggested that a microscopic phase separation into superconducting (SC) and normal-state regions takes place in the overdoped regime of LSCO. In order to clarify the onset hole-concentration where the microscopic phase separation appears, we have carried out χ measurements on zero-field and field cooling and MHL measurements for LSCO single crystals with x = 0.125 and 0.147. From the χ measurements in a low magnetic field on field cooling, absolute values of χ at 2 K, |χ2k|, in x = 0.125 and 0.147 have been found to be smaller than that of x = 0.178, which can be understood in terms of the vortex-pinning effect around twin boundaries in a crystal. Neither two-step SC transition in χ vs- T in a moderate field on zero-field cooling nor second magnetization peak in MHL has been observed in x = 0.125, while both characteristics have been observed in x = 0.147. These results imply no enhancement of the vortex pinning in a moderate field-range and therefore suggest the existence of a homogeneous superconducting state in x = 0.125. Accordingly, it is concluded that the onset hole-concentration of the microscopic phase separation is located between x = 0.125 and 0.147.

Anomalous Criticality in the Electrical Resistivity of La 2– x Sr x CuO 4

The presence or absence of a quantum critical point and its location in the phase diagram of high-temperature superconductors have been subjects of intense scrutiny. Clear evidence for quantum criticality, particularly in the transport properties, has proved elusive because the important low-temperature region is masked by the onset of superconductivity. We present measurements of the low-temperature in-plane resistivity of several highly doped La2-xSrxCuO4 single crystals in which the superconductivity had been stripped away by using high magnetic fields. In contrast to other quantum critical systems, the resistivity varies linearly with temperature over a wide doping range with a gradient that scales monotonically with the superconducting transition temperature. It is maximal at a critical doping level (pc) approximately 0.19 at which superconductivity is most robust. Moreover, its value at pc corresponds to the onset of quasi-particle incoherence along specific momentum directions, implying that the interaction that first promotes high-temperature superconductivity may ultimately destroy the very quasi-particle states involved in the superconducting pairing.

Inhomogeneous superconductivity in both underdoped and overdoped regimes of high-Tc cuprates

Our recent experimental works on the inhomogeneity of superconductivity in both underdoped and overdoped regimes of La2-xSrxCuO4 are reviewed. In order to estimate the superconducting (SC) volume fraction in a sample, bulk-sensitive magnetic-susceptibility and specific-heat measurements have been carried out, using La2-xSrxCuO4 single crystals of good quality with x = 0.05−0.3. As a result, it has been found that the SC volume fraction is not 100% in both underdoped and overdoped regimes, suggesting the occurrence of a phase separation into SC and non-SC regions. Moreover, strong vortex-pinning effects have been observed in moderate magnetic fields in the overdoped regime, suggesting that the phase separation in the overdoped regime is not macroscopic but microscopic.

Magneto-transport in LSCO high-Tc superconducting thin films

We report a crossover from insulating to metallic behaviour near optimal doping in high-Tc thin films of La2−xSrxCuO4 (LSCO) using pulsed magnetic fields to suppress superconductivity and reveal the normal state magneto-transport in the zero temperature limits. The thin film data supports the same key findings reported on LSCO single crystals: (i) that the crossover from insulating to metallic behaviour occurs near optimal doping, and (ii) that the insulating behaviour is characterized by an unusual logarithmic divergence of the resistivity as the temperature approaches zero. A new finding is that the Hall density exhibits a sudden increase at the same Sr doping level at which the insulator-to-metal crossover occurs, possibly indicating a sharp change in the Fermi surface at optimum doping.

Evidence of coherentc-axis charge transport in underdopedLa2−xSrxCuO4superconductors

We have observed the plasma edge in the normal state c-axis far-infrared (far-IR) reflectivity of La2-xSrxCuO4 (LSCO) single crystals at an unlikely Sr-doping concentration x = 0.07 but not in x = 0.09. We find that the c-axis scattering rate (Gc) is surprisingly small, Gc = 13 (+_) 2 cm-1 for x = 0.07 and Gc = 24 (+_) 1 cm-1 for x = 0.09 and nearly temperature independent but increases linearly with doping up to x = 0.16. We suggest that the apparent absence of the normal state plasma edge in the previous measurements is due to the larger Gc than the screened plasma frequency of the coherent free carriers. We conclude that the c-axis charge transport in LSCO in the underdoped regime is intrinsically coherent.

Influence of magnetic field on thermal conductivity of La2－xSrxCuO4 single crystals

We have measured the temperature dependence of thermal conductivity in the ab_plane as well as c_axis of a series of underdoped La2-xSrxCuO4 (x=0063, 0070, 0090, 0110, 0125) single crystals unde r ze ro and 14 T magnetic field. It is found that the thermal conductivity in both ab plane and c-axis are suppressed by the application of the magnetic fiel d parallel to the c-axis. Ｔhe temperature dependence of the thermal conduct ivity suppression is quite similar to the previously reported behavior of the an tiferromagnetic order induced by an applied magnetic field. We think that the fi eld_induced thermal conductivity suppression in ab plane is due to the elect ronic contribution, while along c-axis it is due to the phonon contribution. Both the suppressions of the thermal conductivity in ab plane and along c-axis probably originate from the field-enhanced charge and spin ordering.

Differential Conductance Properties of La2-xSrxCuO4 Intrinsic Josephson Junctions under Magnetic Field

We investigated differential conductance properties of c-axis microbridges fabricated in La2-xSrxCuO4 (LSCO) single crystals. Periodic conductance peaks were observed in both overdoped and underdoped LSCO samples. We applied a magnetic field parallel to the ab-plane and examined the behaviors of differential conductance properties (DCPs). The DCP of the overdoped sample was little affected by magnetic fields up to 4 T, whereas that of the underdoped sample exhibited strong dependence on the magnetic field.

Low-temperature electronic heat transport in La2-xSrxCuO4 single crystals: unusual low-energy physics in the normal and superconducting states.

The thermal conductivity kappa is measured in a series of La2-xSrxCuO4 (0 < or = x < or = 0.22) single crystals down to 90 mK to elucidate the evolution of the residual electronic thermal conductivity kappa(res), which probes the extended quasiparticle states in the d-wave gap. We found that kappa(res)/T grows smoothly, except for a 1/8 anomaly, above x approximately 0.05, and shows no discontinuity at optimum doping, indicating that the behavior of kappa(res)/T is not governed by the metal-insulator crossover in the normal state; as a result, kappa(res)/T is much larger than what the normal-state resistivity would suggest in the underdoped region, which highlights the peculiarities in the low-energy physics in the cuprates.

Same superconducting criticality for underdoped and overdoped La2-xSrxCuO4 single crystals

By measuring the superconducting diamagnetic moments for an underdoped and an overdoped La2-xSrxCuO4 single crystal with equal quality and roughly equal transition temperatures, it is found that the underdoped sample has only one transition which corresponds to H(c2), but the overdoped sample has two transitions with the higher one at H(c2). Further investigation reveals the same upper-critical field H(c2) for both samples although the overall charge densities are very different, indicating the possibility of a very direct and detailed equivalence of the superconducting condensation process in the two doping limits. The second transition for the overdoped sample can be understood as the bulk coupling between the superconducting clusters produced by macroscopic phase separation.

Oxygen-isotope effect on the In-plane penetration depth in underdoped La2-xSrxCuO4 single crystals

We report measurements of the oxygen-isotope effect (OIE) on the in-plane penetration depth lambda(ab)(0) in underdoped La2-xSrxCuO4 single crystals. A highly sensitive magnetic torque sensor with a resolution of Deltatau approximately 10(-12) N m was used for the magnetic measurements on microcrystals with a mass of approximately 10 &mgr;g. The OIE on lambda(-2)(ab)(0) is found to be -10(2)% for x = 0.080 and -8(1)% for x = 0.086. It arises mainly from the oxygen-mass dependence of the in-plane effective mass m(*)(ab). The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.

Intrinsic Josephson effect in La2−xSrxCuO4 mesa junctions with niobium counter electrode

We reveal the intrinsic Josephson effect in a La2−xSrxCuO4 (LSCO) high-Tc superconductor. Nb counterelectrodes are deposited on top of mesas, which are formed on surfaces either parallel or perpendicular to the ab plane of LSCO single crystals. Nb/LSCO interfaces behave as SNS Josephson junctions. For the junctions parallel to the ab plane, we observe many branches in the current–voltage characteristics due to intrinsic Josephson effects. The voltage gaps between the branches are typically 0.1 mV, which is much smaller than the superconducting energy gap 2Δ≅11 mV expected for LSCO from Bardeen–Cooper–Schrieffer theory.

In-plane and out-of-plane magnetoresistance in La2-xSrxCuO4 single crystals.

The magnetoresistance of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ single crystals has been studied extensively over a wide composition range (0.07\ensuremath{\le}x\ensuremath{\le}0.28) using current parallel (in plane) and perpendicular (out of plane) to the ${\mathrm{CuO}}_{2}$ plane. In the underdoped superconducting phase (x\ensuremath{\sim}0.10), the in-plane magnetoconductivity above ${\mathit{T}}_{\mathit{c}}$ is well described as fluctuation conductivity but only with the Aslamasov-Larkin term. The negligibly small Maki-Thompson contribution is suggestive of anisotropic Cooper pairing. We find a pronounced negative and isotropic out-of-plane magnetoresistance at low temperatures in this composition range. In the optimally doped to the overdoped superconducting phases (0.15\ensuremath{\le}x\ensuremath{\le}0.20), a substantial normal-state component is observed in the in-plane magnetoresistance. The classical Kohler's rule appears to break down for the normal-state magnetoresistance, which supports the involvement of two distinct scattering rates ${\mathrm{\ensuremath{\tau}}}_{\mathrm{tr}}$ and ${\mathrm{\ensuremath{\tau}}}_{\mathit{H}}$. In the out-of-plane magnetoresistance, we find an unconventional scaling \ensuremath{\Delta}${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$/${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$\ensuremath{\propto}(H/${\mathrm{\ensuremath{\rho}}}_{\mathit{a}}$${)}^{2}$ for H\ensuremath{\perp}J and (H/T${)}^{2}$ for H\ensuremath{\parallel}J. In contrast to these anomalous behaviors, we find that Kohler's rule holds for both the in-plane and the out-of-plane transverse magnetoresistance in the overdoped normal metal region, implying a conventional anisotropic three-dimensional transport. These findings provide further evidence for the unconventional normal-state transport in the samples which exhibit high-${\mathit{T}}_{\mathit{c}}$ superconductivity. \textcopyright{} 1996 The American Physical Society.

Unconventional lattice stiffening in superconducting La2-xSrxCuO4 single crystals.

Ultrasonic and specific-heat measurements have been performed on single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (LSCO) with x=0.09, 0.14, and 0.19 across the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. We observed jumplike decreases in the longitudinal elastic moduli at ${\mathit{T}}_{\mathit{c}}$, as are seen in conventional superconductors. Anisotropic strain dependence of ${\mathit{T}}_{\mathit{c}}$ was estimated from both the jump in the elastic moduli and specific heat at ${\mathit{T}}_{\mathit{c}}$ via the Ehrenfest relation. With further lowering temperature below ${\mathit{T}}_{\mathit{c}}$, we found a pronounced lattice stiffening in the elastic moduli of LSCO, which contrasts with the continuous softening observed in most of the conventional superconductors. Thermodynamic analysis revealed that the stiffening originates from the change in the superconducting condensation energy under the lattice strain. This unusual lattice stiffening in the superconducting state is possibly a common character of the high-${\mathit{T}}_{\mathit{c}}$ copper oxides.

Homogeneity of Single Crystal of High-Tc Superconductor La2-xSrxCuO4

High-angle double-crystal X-ray diffractometry (HADOX) has been applied to examine the quality of La2-x Srx CuO4 single crystals grown by the travelling-solvent floating-zone method. The two-dimensional intensity distribution in reciprocal space is measured at room temperature for 0014 with CoKα1 radiation. Measurements are made over a range of 7 mm along the axis of growth at intervals of 1 mm. It is found that the crystal contains small part of different orientation with slightly different chemical composition Δx<0.002. In a limited region, the crystal is well orientated and the composition is homogeneous.