Transport studies ofLa2−xSrxCuO4near the insulator-metal-superconductor transition
Physical Review B (Condensed Matter)
We have measured the temperature-dependent resistivities of a series of samples of La2 -"-Sr"Cu04 with 0.02~x «0.1 over the temperature range 0.05~T~300 K. We find the onset of superconductivity as x is increased to be correlated with a substantial drop in the magnitude of the normal-state resistivity. We observe no change, however, in the qualitative shape of the resistivity as the superconducting threshold is crossed. We also find that the low-temperature (T~8.0 K) resistivities of the least
... ities of the least concentrated samples can be described by variable range hopping, with a crossover between Coulomb gap and single-particie behavior occurring as x is increased. Since the discovery of high-temperature superconductivity in the La-Ba-Cu-0 system, ' there has been intense interest in the properties of these layered perovskites. Several classes of such materials have been found and superconducting transition temperatures of up to 125 K have been reported. Most of the work on these materials, however, has focused on samples with dopant and oxygen concentrations near that required for an optimal superconducting transition temperature with comparatively little work being done on samples at the crossover to the nonsuperconducting regime. Such studies are of fundamental interest, though, both to increase our understanding of the approach to and the nature of the superconducting state, and also to provide basic information on the normal-state properties of these systems. In order to address these issues, we have carried out transport measurements on La2 "Sr"Cu04 for values of Sr concentration x in the vicinity of the insulator-metalsuperconductor transition. La2 -"Sr"Cu04 was studied in considerable detail3 even before it was first shown to be a high-T, superconductor, and it is now known to be perhaps the simplest such system, possessing only one low-dimensional structure in the form of a single Cu-0 plane per unit cell. The material possesses orthorhombic symmetry at room temperature for x~0.10, switching over to tetragonal for higher concentrations. La2 Sr"Cu04 s is a superconductor in the x 0 limit under certain conditions (probably related to the concentration of oxygen deficiencies b') as well as in the strontium concentration window 0.05%x 50.30. The highest transition temperatures are obtained for x = 0.15. We present here resistivity measurements on a series of samples with 0.02~x~0.10, a range allowing the investigation of the normal, superconducting, and low-temperature insulating regimes of this system. We find the onset of superconductivity as x is varied to be correlated with a large change in the magnitude of the normal-state resistivity. The samples used were sintered ceramic pellets prepared using standard techniques with a Anal thermal history consisting of cooling from 1060 to 550 C in pure 02 and then annealing in 02 at 550'C overnight. The material was stored in dry nitrogen, with no other measures taken to alter or control oxygen concentration. X-ray crystallographic data show single-phase orthorhombic material with no sign of the more concentrated tetragonal structure. The x-ray results, along with the fact that semiquantitative electron-microprobe measurements ' conducted on two of the samples (x 0.05 and 0.055) find no spatial variations in Sr density at the 500-ppm level, make it unlikely that a small volume fraction of more concentrated material is acting as a superconducting short in the x )0. 05 samples. Multiple measurements on a given sample indicated little or no degradation due to repeated temperature cycling. Resistance measurements were made on roughly bar-shaped samples of cross-sectional area 0.05 cm using a four-probe ac technique at 17 Hz. Electrical contacts were made using silver paint, either on the surface of the sample (which was scraped to remove any surface layer) or in grooves cut into it. The absolute accuracy of our resistivities is largely determined by uncertainties in the geometry of these painted contacts and by variations in the densities of the sintered samples, and is estimated to be better than 30%. Data on very low concentration (x & 0.02) samples are not presented as resistivity measurements indicated the existence of regions of superconducting La2Cu04. For low-temperature (s 2 K) measurements, the samples were either top-loaded into or mounted on the mixing chamber of a helium dilution refrigerator. Special care was taken on all low-T data to verify that the contacts had linear I-V characteristics and to avoid Ohmic heating due to the measuring current.