Stripes and Charge Transport Properties of High-T_c Cuprates

Unusual features in the in-plane charge transport in lightly hole-doped La_{2-x}Sr_{x}CuO_{4} single crystals are described. Notably, both the in-plane resistivity and the Hall coefficient show a metallic behavior at moderate temperatures even in the long-range-ordered antiferromagnetic phase, which obviously violates the Mott-Ioffe-Regel criterion for the metallic transport and can hardly be understood without employing the role of charge stripes. Moreover, the mobility of holes in this "metallic" antiferromagnetic state is found to be virtually the same as that in optimally-doped crystals, which strongly suggests that the stripes govern the charge transport in a surprisingly wide doping range up to optimum doping.Comment: Invited paper at ICTP Workshop on Intrinsic Multiscale Structure and Dynamics in Complex Electronic Oxides, Trieste, July 2002. (To be published in the Proceedings of the ICTP Workshop as a World Scientific book.) Some statements are revised and references are added in response to the comments from theorist friend

Topological Crystalline Insulators and Topological Superconductors: From Concepts to Materials

In this review, we discuss recent progress in the explorations of topological materials beyond topological insulators; specifically, we focus on topological crystalline insulators and bulk topological superconductors. The basic concepts, model Hamiltonians, and novel electronic properties of these new topological materials are explained. The key role of symmetries that underlie their topological properties is elucidated. Key issues in their materials realizations are also discussed.Comment: 27 pages, 5 figures, 150 references; invited review article to be published in Annual Review of Condensed Matter Physics, Volume

Comment on "Onset of Boson Mode at the Superconducting Critical Point of Underdoped YBa2Cu3Oy"

In a recent Letter [Phys. Rev. Lett. 97, 207001 (2006)], Doiron-Leyraud et al. reported a study of low-temperature thermal conductivity of underdoped YBa2Cu3Oy across the superconductor-insulator boundary and argued for a novel bosonic mode as well as charge-neutral fermions. In this Comment, we show that both of these claims are dubious.Comment: Accepted for publication in Phys. Rev. Let

Doping n-type carriers by La-substitution for Ba in YBa_2Cu_3O_y system

Thus far, there is no cuprate system where both n-type and p-type charge carriers can be doped without changing the crystallographic structure. For studying the electron-hole symmetry in an identical structure, we try to dope n-type carriers to YBa2Cu3Oy system by reducing oxygen content and substituting La3+ ions for Ba2+. Single crystals of La-doped YBa2Cu3Oy are grown by a flux method with Y2O3 crucibles and it is confirmed that La actually substitutes \~13% of Ba. The oxygen content y can be varied between 6.21 and 6.95 by annealing the crystals in an atmosphere with controlled oxygen partial pressure. The in-plane resistivity rho_ab at room temperature was found to increase with decreasing oxygen content y down to 6.32, but interestingly further decrease in y results in a decrease in rho_ab. The most reduced samples with y = 6.21 show rho_ab of ~30 mOhm cm at room temperature, which is as much as seven orders-of-magnitude smaller than the maximum value at y = 6.32. Furthermore, both the Hall coefficient and the Seebeck coefficient of the y = 6.21 samples are found to be negative at room temperatures. The present results demonstrate that the non-doped Mott-insulating state has been crossed upon reducing y and n-type carriers are successfully doped in this material.Comment: 4 pages, 4 figures, 1 table, accepted for publication in Phys. Rev.