Quantum transport properties of two-dimensional systems in disordered magnetic fields with a fixed sign

Quantum transport in disordered magnetic fields is investigated numerically in two-dimensional systems. In particular, the case where the mean and the fluctuation of disordered magnetic fields are of the same order is considered. It is found that in the limit of weak disorder the conductivity exhibits a qualitatively different behavior from that in the conventional random magnetic fields with zero mean. The conductivity is estimated by the equation of motion method and by the two-terminal Landauer formula. It is demonstrated that the conductance stays on the order of $e^2/h$ even in the weak disorder limit. The present behavior can be interpreted in terms of the Drude formula. The Shubnikov-de Haas oscillation is also observed in the weak disorder regime.Comment: 6 pages, 7 figures, to appear in Phys. Rev.

Origin of the anomalous magnetic circular dichroism spectral shape in ferromagnetic (Ga,Mn)As: Impurity bands inside the band gap

The electronic structure of a prototype dilute magnetic semiconductor (DMS), Ga1-xMnxAs, is studied by magnetic circular dichroism (MCD) spectroscopy. We prove that the optical transitions originated from impurity bands cause the strong positive MCD background. The MCD signal due to the E0 transition from the valence band to the conduction band is negative indicating that the p-d exchange interactions between the p-carriers and d-spin is antiferromagnetic. The negative E0 MCD signal also indicates that the hole-doping of the valence band is not so large as previously assumed. The impurity bands seem to play important roles for the ferromagnetism of Ga1-xMnxAs.Comment: 13 pages, 3 figure

Electron Interactions and Scaling Relations for Optical Excitations in Carbon Nanotubes

Recent fluorescence spectroscopy experiments on single wall carbon nanotubes reveal substantial deviations of observed absorption and emission energies from predictions of noninteracting models of the electronic structure. Nonetheless, the data for nearly armchair nanotubes obey a nonlinear scaling relation as a function the tube radius $R$. We show that these effects can be understood in a theory of large radius tubes, derived from the theory of two dimensional graphene where the coulomb interaction leads to a logarithmic correction to the electronic self energy and marginal Fermi liquid behavior. Interactions on length scales larger than the tube circumference lead to strong self energy and excitonic effects that compete and nearly cancel so that the observed optical transitions are dominated by the graphene self energy effects.Comment: 4 page

Parity-violating asymmetry in γd→n⃗p\gamma d \to \vec{n}p with a pionless effective theory

Nuclear parity violation is studied with polarized neutrons in the photodisintegration of the deuteron at low energies. A pionless effective field theory with di-baryon fields is used for the investigation. Hadronic weak interactions are treated by parity-violating di-baryon-nucleon-nucleon vertices, which have undetermined coupling contants. A parity-violating asymmetry in the process is calculated for the incident photon energy up to 30 MeV. If experimental data for the parity-violating asymmetry become available in the future, we will be able to determine the unknown coupling contants in the parity-violating vertices.Comment: 4 pages. A contribution to APFB2011, August 22-26, 2011, Seoul, Kore

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

Conductance of Disordered Wires with Symplectic Symmetry: Comparison between Odd- and Even-Channel Cases

The conductance of disordered wires with symplectic symmetry is studied by numerical simulations on the basis of a tight-binding model on a square lattice consisting of M lattice sites in the transverse direction. If the potential range of scatterers is much larger than the lattice constant, the number N of conducting channels becomes odd (even) when M is odd (even). The average dimensionless conductance g is calculated as a function of system length L. It is shown that when N is odd, the conductance behaves as g --> 1 with increasing L. This indicates the absence of Anderson localization. In the even-channel case, the ordinary localization behavior arises and g decays exponentially with increasing L. It is also shown that the decay of g is much faster in the odd-channel case than in the even-channel case. These numerical results are in qualitative agreement with existing analytic theories.Comment: 4 page

Scanning Tunneling Spectroscopy of Bi2Sr2CuO6+d: New Evidence for the Common Origin of the Pseudogap and Superconductivity

Using scanning tunneling spectroscopy, we investigated the temperature dependence of the quasiparticle density of states of overdoped Bi2Sr2CuO6+δ between 275 mK and 82 K. Below Tc = 10 K, the spectra show a gap with well-defined coherence peaks at ±Δp≃12 meV, which disappear at Tc. Above Tc, the spectra display a clear pseudogap of the same magnitude, gradually filling up and vanishing at T*≃68 K. The comparison with Bi2Sr2CaCu2O8+δ demonstrates that the pseudogap and the superconducting gap scale with each other, providing strong evidence that they have a common origin

Parity-violating nucleon-nucleon interaction from different approaches

Two-pion exchange parity-violating nucleon-nucleon interactions from recent effective field theories and earlier fully covariant approaches are investigated. The potentials are compared with the idea to obtain better insight on the role of low-energy constants appearing in the effective field theory approach and the convergence of this one in terms of a perturbative series. The results are illustrated by considering the longitudinal asymmetry of polarized protons scattering off protons, $\vec{p}+p -> p+p$, and the asymmetry of the photon emission in radiative capture of polarized neutrons by protons, $\vec{n}+p -> d+\gamma$.Comment: 31 page

Enhancement of the spin pumping efficiency by spin-wave mode selection

The spin pumping efficiency of lateral standing spin wave modes in a rectangular YIG/Pt sample has been investigated by means of the inverse spin-Hall effect (ISHE). The standing spin waves drive spin pumping, the generation of spin currents from magnetization precession, into the Pt layer which is converted into a detectable voltage due to the ISHE. We discovered that the spin pumping efficiency is significantly higher for lateral standing surface spin waves rather than for volume spin wave modes. The results suggest that the use of higher-mode surface spin waves allows for the fabrication of an efficient spin-current injector