Unconventional magnetic phase diagram of cuprate superconductor La2-xSrxCuO4 at quantum critical point x = 1/9

We propose a new magnetic phase diagram of La2-xSrxCuO4 around a quantum critical point x = 1/9 based on field-cooled magnetization measurements and critical fittings. A new phase boundary Tm2(H) is discovered which buries deeply below the first order vortex melting line in the vortex solid phase. The coupling between superconductivity and antiferromagnetism is found to be attractive below Tm2(H) while repulsive above. The attractive coupling between superconducting order and static antiferromagnetic order provides compelling experimental evidence that the antiferromagnetism microscopically coexists and collaborates with the high temperature superconductivity in cuprates.Comment: 17 pages, 4 figure

In-plane and Out-of-plane Plasma Resonances in Optimally Doped La1.84Sr0.16CuO4

We addressed the inconsistency between the electron mass anisotropy ratios determined by the far-infrared experiments and DC conductivity measurements. By eliminating possible sources of error and increasing the sensitivity and resolution in the far-infrared reflectivity measurement on the single crystalline and on the polycrystalline La1.84Sr0.16CuO4, we have unambiguously identified that the source of the mass anisotropy problem is in the estimation of the free electron density involved in the charge transport and superconductivity. In this study we found that only 2.8 % of the total doping-induced charge density is itinerant at optimal doping. Our result not only resolves the mass anisotropy puzzle but also points to a novel electronic structure formed by the rest of the electrons that sets the stage for the high temperature superconductivity

Universal scaling of c-axis dc conductivity for the underdoped hightemperature cuprate superconductors

Coexistence of the "metallic-like" in-plane and the "semiconducting-like" out-of-plane (caxis) dc conductivities ({\sigma}c), generating a huge anisotropy in the underdoped hightemperature cuprate superconductors (HTCS), defies our current understanding of metal. In this report we present an intrinsic doping dependence of {\sigma}c. We find that the {\sigma}c for the underdoped HTCS is universally scaled to the {\sigma}c at the optimal doped-hole concentration. The universal scaling behavior suggests that there are three intrinsic processes contribute to {\sigma}c: (i) the doping-dependent-activated gap; (ii) the exponential doping dependences and (iii) the tunneling between adjacent CuO2 block layers. They are the essential underlying characteristics of the c-axis transport for all HTCSs.Comment: 14 pages, 3 figures. Accepted for the publication in Solid State Communication

Anomalous superconducting properties at magic doping levels in under-doped La2-xSrxCuO4 single crystals

A series of high-quality under-doped La2-xSrxCuO4 superconductor crystals with x = 0.063 - 0.125 were prepared by traveling-solvent floating-zone (TSFZ) technique. We found by dc magnetic measurements that, in this series of crystals, the superconducting transition was quite sharp in the vicinity of the hole densities of x = 1/16 and x = 1/9 while it was much broader away from these two "magic numbers", and the Meissner fraction showed a remarkable minimum near x = 1/9. We concluded that these phenomena are reflections of intrinsic properties of this cuprate system. Our observations are discussed in light of recently proposed composite charge model together with charge inhomogeneity and electronic phase separations.Comment: 4 pages, 4 figures; Presented at the 7th Intl. Conf. M2S.(Rio de Janeiro, May 2003