Origin of superconducting carriers in "non-doped" T'- (La,RE)2CuO4 (RE = Sm, Eu, Gd, Tb, Lu, and Y) prepared by molecular beam epitaxy

We have performed a systematic investigation of the variations of the lattice constants with substituent rare-earth element concentration x in the nominally undoped superconductors T'-(La3+)2-x(RE3+)xCuO4 (RE = Sm, Eu, Gd, Tb, Lu, and Y), which we have recently discovered using MBE. The results show both the in-plane and out-of-plane lattice constants (a0 and c0) linearly decrease with x, whose extrapolation to x = 2 agrees well with the reported a0 and c0 values for each T'-RE2CuO4. This behavior is what one would expect simply from the ionic size difference between La3+ and RE3+. The absence of the Cu-O bond stretching due to electron-doping, which is commonly observed in electron-doped T' and infinite-layer superconductors, implies that electron doping via oxygen deficiencies is, at least, not a main source of charge carriers.Comment: proceedings of ISS 200

Ce doping in T-La2CuO4 films: Broken electron-hole symmetry for high-Tc superconductivity

We attempted Ce doping in La2CuO4 with the K2NiF4 (T) structure by molecular beam epitaxy. At low growth temperature and with an appropriate substrate choice, we found that Ce can be incorporated into the K2NiF4 lattice up to x ~ 0.06, which had not yet been realized in bulk synthesis. The doping of Ce makes T-La2-xCexCuO4 more insulating, which is in sharp contrast to Ce doping in La2CuO4 with the Nd2CuO4 structure, which makes the compounds superconducting. The observed smooth increase in resistivity from hole-doped side (T-La2-xSrxCuO4) to electron-doped side (T-La2-xCexCuO4) indicates that electron-hole symmetry is broken in the T-phase materials.Comment: proceedings of ISS 200

Phase control of La2CuO4 in thin-film synthesis

The lanthanum copper oxide, La2CuO4, which is an end member of the prototype high-Tc superconductors (La,Sr)2CuO4 and (La,Ba)2CuO4, crystallizes in the "K2NiF4" structure in high-temperature bulk synthesis. The crystal chemistry, however, predicts that La2CuO4 is at the borderline of the K2NiF4 stability and that it can crystallize in the Nd2CuO4 structure at low synthesis temperatures. In this article we demonstrate that low-temperature thin-film synthesis actually crystallizes La2CuO4 in the Nd2CuO4 structure. We also show that the phase control of "K2NiF4"-type La2CuO4 versus "Nd2CuO4"-type La2CuO4 can be achieved by varying the synthesis temperature and using different substrates.Comment: 4 pages, 5 figures, submitted to PRB, revte

Magnetic Penetration Depth Measurements of Pr2−x_{2-x}Cex_xCuO4−δ_{4-\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$_{4-\delta}$ ($0.115 \leq x \leq 0.152$) superconducting films grown on SrTiO$_3$ (001) substrates coated with a buffer layer of insulating Pr$_{2}$CuO$_{4}$. $\lambda^{-2}(0)$, $T_c$ and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, $0.124\leq x \leq 0.144$, $\lambda^{-2}(T)$ 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 $0.29\leq\Delta_{\rm min}/k_BT_c\leq1.01$.Comment: 4 pages 5 figure

Far-infrared and submillimeter-wave conductivity in electron-doped cuprate La_{2-x}Ce_xCuO_4

We performed far-infrared and submillimeter-wave conductivity experiments in the electron-doped cuprate La_{2-x}Ce_xCuO_4 with x = 0.081 (underdoped regime, T_c = 25 K). The onset of the absorption in the superconducting state is gradual in frequency and is inconsistent with the isotropic s-wave gap. Instead, a narrow quasiparticle peak is observed at zero frequency and a second peak at finite frequencies, clear fingerprints of the conductivity in a d-wave superconductor. A far-infrared conductivity peak can be attributed to 4Delta_0, or to 2Delta_0 + Delta_spin, where Delta_spin is the resonance frequency of the spin-fluctuations. The infrared conductivity as well as the suppression of the quasiparticle scattering rate below T_c are qualitatively similar to the results in the hole-doped cuprates.Comment: 5 pages, 4 figures include