Metal-to-Insulator Crossover in the Low-Temperature Normal State of Bi_{2}Sr_{2-x}La_{x}CuO_{6+\delta}

We measure the normal-state in-plane resistivity of La-doped Bi-2201 single crystals at low temperatures by suppressing superconductivity with 60-T pulsed magnetic fields. With decreasing hole doping, we observe a crossover from a metallic to insulating behavior in the low-temperature normal state. This crossover is estimated to occur near 1/8 doping, well inside the underdoped regime, and not at optimum doping as reported for other cuprates. The insulating regime is marked by a logarithmic temperature dependence of the resistivity over two decades of temperature, suggesting that a peculiar charge localization is common to the cuprates.Comment: 4 pages, 5 figures, accepted for publication in PR

High frequency and high wavenumber solar oscillations

We determine the frequencies of solar oscillations covering a wide range of degree (100< l <4000) and frequency (1.5 <\nu<10 mHz) using the ring diagram technique applied to power spectra obtained from MDI (Michelson Doppler Imager) data. The f-mode ridge extends up to degree of approximately 3000, where the line width becomes very large, implying a damping time which is comparable to the time period. The frequencies of high degree f-modes are significantly different from those given by the simple dispersion relation \omega^2=gk. The f-mode peaks in power spectra are distinctly asymmetric and use of asymmetric profile increases the fitted frequency bringing them closer to the frequencies computed for a solar model.Comment: Revised version. 1.2 mHz features identified as artifacts of data analysis. Accepted for publication in Ap

Origin of the large thermoelectric power in oxygen-variable RBaCo_{2}O_{5+x} (R=Gd, Nd)

Thermoelectric properties of GdBaCo_{2}O_{5+x} and NdBaCo_{2}O_{5+x} single crystals have been studied upon continuous doping of CoO_2 planes with either electrons or holes. The thermoelectric response and the resistivity behavior reveal a hopping character of the transport in both compounds, providing the basis for understanding the recently found remarkable divergence of the Seebeck coefficient at x=0.5. The doping dependence of the thermoelectric power evinces that the configurational entropy of charge carriers, enhanced by their spin and orbital degeneracy, plays a key role in the origin of the large thermoelectric response in these correlated oxides.Comment: 5 pages, 4 figures, accepted for publication in PR

Novel anisotropy in the superconducting gap structure of Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} probed by quasiparticle heat transport

Since the nature of pairing interactions is manifested in the superconducting gap symmetry, the exact gap structure, particularly any deviation from the simple d_{x^2-y^2} symmetry, would help elucidating the pairing mechanism in high-T_c cuprates. Anisotropic heat transport measurement in Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} (Bi-2212) reveals that the quasiparticle populations are different for the two nodal directions and thus the gap structure must be uniquely anisotropic, suggesting that pairing is governed by interactions with a rather complicated anisotropy. Intriguingly, it is found that the "plateau" in the magnetic-field dependence of the thermal conductivity is observed only in the b-axis transport.Comment: 4 pages, 5 figures, accepted for publication in Phys. Rev. Let

Extensions of Lieb's concavity theorem

The operator function (A,B)\to\tr f(A,B)(K^*)K, defined on pairs of bounded self-adjoint operators in the domain of a function f of two real variables, is convex for every Hilbert Schmidt operator K, if and only if f is operator convex. As a special case we obtain a new proof of Lieb's concavity theorem for the function (A,B)\to\tr A^pK^*B^{q}K, where p and q are non-negative numbers with sum p+q\le 1. In addition, we prove concavity of the operator function (A,B)\to \tr(A(A+\mu_1)^{-1}K^* B(B+\mu_2)^{-1}K) on its natural domain D_2(\mu_1,\mu_2), cf. Definition 4.1Comment: The format of one reference is changed such that CiteBase can identify i

Thermal conductivity of lightly Sr- and Zn-doped La2_2CuO4_4 single crystals

Both ab-plane and c-axis thermal conductivities ($\kappa_{ab}$ and $\kappa_c$) of lightly doped La$_{2-x}$Sr$_x$CuO$_4$ and La$_2$Cu$_{1-y}$Zn$_y$O$_4$ single crystals ($x$ or $y$ = 0 -- 0.04) are measured from 2 to 300 K. It is found that the low-temperature phonon peak (at 20 -- 25 K) is significantly suppressed upon Sr or Zn doping even at very low doping, though its precise doping dependences show interesting differences between the Sr and Zn dopants, or between the $ab$ plane and the c axis. Most notably, the phonon peak in $\kappa_c$ decreases much more quickly with Sr doping than with Zn doping, while the phonon-peak suppression in $\kappa_{ab}$ shows an opposite trend. It is discussed that the scattering of phonons by stripes is playing an important role in the damping of the phonon heat transport in lightly doped LSCO, in which static spin stripes has been observed by neutron scattering. We also show $\kappa_{ab}$ and $\kappa_c$ data of La$_{1.28}$Nd$_{0.6}$Sr$_{0.12}$CuO$_4$ and La$_{1.68}$Eu$_{0.2}$Sr$_{0.12}$CuO$_4$ single crystals to compare with the data of the lightly doped crystals for the discussion of the role of stripes. At high temperature, the magnon peak (i.e., the peak caused by the spin heat transport near the N\'{e}el temperature) in $\kappa_{ab}(T)$ is found to be rather robust against Zn doping, while it completely disappears with only 1% of Sr doping.Comment: 7 pages, 4 figures, accepted for publication in Phys. Rev.

A Minkowski Type Trace Inequality and Strong Subadditivity of Quantum Entropy II: Convexity and Concavity

We revisit and prove some convexity inequalities for trace functions conjectured in the earlier part I. The main functional considered is \Phi_{p,q}(A_1,A_2,...,A_m) = (trace((\sum_{j=1}^m A_j^p)^{q/p}))^{1/q} for m positive definite operators A_j. In part I we only considered the case q=1 and proved the concavity of \Phi_{p,1} for 0 < p \leq 1 and the convexity for p=2. We conjectured the convexity of \Phi_{p,1} for 1< p < 2. Here we not only settle the unresolved case of joint convexity for 1 \leq p \leq 2, we are also able to include the parameter q\geq 1 and still retain the convexity. Among other things this leads to a definition of an L^q(L^p) norm for operators when 1 \leq p \leq 2 and a Minkowski inequality for operators on a tensor product of three Hilbert spaces -- which leads to another proof of strong subadditivity of entropy. We also prove convexity/concavity properties of some other, related functionals.Comment: Proof of a conjecture in math/0701352. Revised version replaces earlier draft. 18 pages, late

Stability of 1-D Excitons in Carbon Nanotubes under High Laser Excitations

Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide continuum probe, we show that interband exciton peaks in single-walled carbon nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates of the initial densities of excitons from the excitation conditions, combined with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest that their positions do not change at all even near the Mott density. In addition, we found that the presence of lowest-subband excitons broadens all absorption peaks, including those in the second-subband range, which provides a consistent explanation for the complex spectral dependence of pump-probe signals reported for SWNTs.Comment: 4 pages, 4 figure