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

Completions of Z/(p)-Tate cohomology of periodic spectra

We construct splittings of some completions of the Z/(p)-Tate cohomology of E(n) and some related spectra. In particular, we split (a completion of) tE(n) as a (completion of) a wedge of E(n-1)'s as a spectrum, where t is shorthand for the fixed points of the Z/(p)-Tate cohomology spectrum (ie Mahowald's inverse limit of P_{-k} smash SE(n)). We also give a multiplicative splitting of tE(n) after a suitable base extension.Comment: 30 pages. Published copy, also available at http://www.maths.warwick.ac.uk/gt/GTVol2/paper8.abs.htm

Anharmonicity and asymmetry of Landau levels for a two-dimensional electron gas

We calculate the density of states of a two dimensional electron gas located at the interface of a GaAlAs/GaAs heterojunction. The disorder potential which is generally created by a single doping layer behind a spacer, is here enhanced by the presence of a second delta doped layer of scatterers which can be repulsive or attractive impurities. We have calculated the density of states by means of the Klauder's approximation, in the presence of a magnetic field of arbitrary strength. At low field either band tails or impurity bands are observed for attractive potentials, depending on the impurity concentration. At higher field, impurity bands are observed for both repulsive and attractive potentials. We discuss the effect of such an asymmetrical density of states on the transport properties in the quantum Hall effect regime.Comment: 22 pages, 12 figures. submitted to Phys. Rev.

Nonlinear Evolution of Anisotropic Cosmological Power

There has been growing interest in the possibility of testing more precisely the assumption of statistical isotropy of primordial density perturbations. If it is to be tested with galaxy surveys at distance scales <~ 10 Mpc, then nonlinear evolution of anisotropic power must be understood. To this end, we calculate the angular dependence of the power spectrum to third order in perturbation theory for a primordial power spectrum with a quadrupole dependence on the wavevector direction. Our results suggest that primordial power anisotropies will be suppressed by <~ 7% in the quasilinear regime. We also show that the skewness in the statistically anisotropic theory differs by no more than 1% from that in the isotropic theory.Comment: 4 pages, 1 figure; accepted for publication in Physical Review Letter

Self-Consistent Electron Subbands of Gaas/Algaas Heterostructure in Magnetic Fields Parallel to the Interface

The effect of strong magnetic fields parallel to GaAs/AlGaAs interface on the subband structure of a 2D electron layer is ivestigated theoretically. The system with two levels occupied in zero magnetic field is considered and the magnetic field induced depletion of the second subband is studied. The confining potential and the electron dispersion relations are calculated self-consistently, the electron- electron interaction is taken into account in the Hartree approximation.Comment: written in LaTeX, 8 pages, 4 figs. available on request from [email protected]

A Tonks Giradeau Gas in the Presence of a Local Potential

The physics of a Tonks-Giradeau Gas in the presence of a local potential is studied. In order to evaluate the single particle density matrix (SPDM) of the many-body ground state, the Wiger-Jordan transformation is used. The eigenvector with the largest eigenvalue of the SPDM corresponds to the "Bose-Einstein Condensate"(BEC) State. We find that the "BEC" state density at the positon of the local potential decreases, as expected, in the case of a repulsive potential. For an attractive potential, it decreases or increases depending on the strength of the potential. The superfluidity of this system is investigated both numerically and perturbatively. An experimental method for detecting the effect of an impurity in a Tonks-Giradueau gas is discussed.Comment: 14 pages, 5 figure

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

Interactions of keV sterile neutrinos with matter

A sterile neutrino with mass of several keV is a well-motivated dark-matter candidate, and it can also explain the observed velocities of pulsars via anisotropic emission of sterile neutrinos from a cooling neutron star. We discuss the interactions of such relic particles with matter and comment on the prospects of future direct detection experiments. A relic sterile neutrino can interact, via sterile-active mixing, with matter fermions by means of electroweak currents, with the final state containing a relativistic active neutrino. The recoil momentum impacted onto a matter fermion is determined by the sterile neutrino mass and is enough to ionize atoms and flip the spins of nuclei. While this suggests a possibility of direct experimental detection, we calculate the rates and show that building a realistic detector of the required size would be a daunting challenge.Comment: 5 pages, 1 figur

Metal-to-insulator crossover and pseudogap in single-layer compound Bi2+x_{2+x}Sr2−x_{2-x}Cu1+y_{1+y}O6+δ_{6+\delta} single crystals in high magnetic fields

The in-plane $\rho_{ab}(H)$ and the out-of-plane $\rho_c(H)$ magneto-transport in magnetic fields up to 28 T has been investigated in a series of high quality, single crystal, hole-doped La-free Bi2201 cuprates for a wide doping range and over a wide range of temperatures down to 40 mK. With decreasing hole concentration going from the overdoped (p=0.2) to the underdoped (p=0.12) regimes, a crossover from a metallic to and insulating behavior of $\rho_{ab}(T)$ is observed in the low temperature normal state, resulting in a disorder induced metal insulator transition. In the zero temperature limit, the normal state ratio $\rho_c(H)/\rho_{ab}(H)$ of the heavily underdoped samples in pure Bi2201 shows an anisotropic 3D behavior, in striking contrast with that observed in La-doped Bi2201 and LSCO systems. Our data strongly support that that the negative out-of-plane magnetoresistance is largely governed by interlayer conduction of quasiparticles in the superconducting state, accompanied by a small contribution of normal state transport associated with the field dependent pseudogap. Both in the optimal and overdoped regimes, the semiconducting behavior of $\rho_c(H)$ persists even for magnetic fields above the pseudogap closing field $H_{pg}$. The method suggested by Shibauchi \textit{et al.} (Phys. Rev. Lett. \textbf{86}, 5763, (2001)) for evaluating $H_{pg}$ is unsuccessful for both under- and overdoped Bi2201 samples. Our findings suggest that the normal state pseudogap is not always a precursor of superconductivity.Comment: 11 pages, 8 figures, published in PRB Nov 200