Hall plateau diagram for the Hofstadter butterfly energy spectrum

We extensively study the localization and the quantum Hall effect in the Hofstadter butterfly, which emerges in a two-dimensional electron system with a weak two-dimensional periodic potential. We numerically calculate the Hall conductivity and the localization length for finite systems with the disorder in general magnetic fields, and estimate the energies of the extended levels in an infinite system. We obtain the Hall plateau diagram on the whole region of the Hofstadter butterfly, and propose a theory for the evolution of the plateau structure with increasing disorder. There we show that a subband with the Hall conductivity $n e^2/h$ has $|n|$ separated bunches of extended levels, at least for an integer $n \leq 2$. We also find that the clusters of the subbands with identical Hall conductivity, which repeatedly appear in the Hofstadter butterfly, have a similar localization property.Comment: 9 pages, 12 figure

Quantum oscillations in a topological insulator Bi_{1-x}Sb_{x}

We have studied transport and magnetic properties of Bi_{1-x}Sb_x, which is believed to be a topological insulator - a new state of matter where an insulating bulk supports an intrinsically metallic surface. In nominally insulating Bi_{0.91}Sb_{0.09} crystals, we observed strong quantum oscillations of the magnetization and the resistivity originating from a Fermi surface which has a clear two-dimensional character. In addition, a three-dimensional Fermi surface is found to coexist, which is possibly due to an unusual coupling of the bulk to the surface. This finding demonstrates that quantum oscillations can be a powerful tool to directly probe the novel electronic states in topological insulators.Comment: 4 pages, 4 figure

Coulomb drag in high Landau levels

Recent experiments on Coulomb drag in the quantum Hall regime have yielded a number of surprises. The most striking observations are that the Coulomb drag can become negative in high Landau levels and that its temperature dependence is non-monotonous. We develop a systematic diagrammatic theory of Coulomb drag in strong magnetic fields explaining these puzzling experiments. The theory is applicable both in the diffusive and the ballistic regimes; we focus on the experimentally relevant ballistic regime (interlayer distance $a$ smaller than the cyclotron radius $R_c$). It is shown that the drag at strong magnetic fields is an interplay of two contributions arising from different sources of particle-hole asymmetry, namely the curvature of the zero-field electron dispersion and the particle-hole asymmetry associated with Landau quantization. The former contribution is positive and governs the high-temperature increase in the drag resistivity. On the other hand, the latter one, which is dominant at low $T$, has an oscillatory sign (depending on the difference in filling factors of the two layers) and gives rise to a sharp peak in the temperature dependence at $T$ of the order of the Landau level width.Comment: 26 pages, 13 figure

16O+16O^{16}{\rm O} + ^{16}{\rm O} nature of the superdeformed band of 32S^{32}{\rm S} and the evolution of the molecular structure

The relation between the superdeformed band of $^{32}{\rm S}$ and $^{16}{\rm O} + ^{16}{\rm O}$ molecular bands is studied by the deformed-base antisymmetrized molecular dynamics with the Gogny D1S force. It is found that the obtained superdeformed band members of $^{32}{\rm S}$ have considerable amount of the $^{16}{\rm O} + ^{16}{\rm O}$ component. Above the superdeformed band, we have obtained two excited rotational bands which have more prominent character of the $^{16}{\rm O} + ^{16}{\rm O}$ molecular band. These three rotational bands are regarded as a series of $^{16}{\rm O} + ^{16}{\rm O}$ molecular bands which were predicted by using the unique $^{16}{\rm O}$ -$^{16}{\rm O}$ optical potentil. As the excitation energy and principal quantum number of the relative motion increase, the $^{16}{\rm O} + ^{16}{\rm O}$ cluster structure becomes more prominent but at the same time, the band members are fragmented into several states

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

Fees in an Imperfect World: An Application to Motor Vehicle Emissions

This paper compares an emissions fee on measured vehicle emissions rates to a mandatory regulation that requires all vehicles to maintain emissions below a minimum standard. We model the motorist’s decision under the fee policy and simulate the fee and regulatory policies using data from an emissions inspection program that includes test and repair information for more than 50,000 vehicles. Under ideal conditions with perfect information and no subsidies, the fee on emissions rates performs substantially better than the regulatory policy. When more realistic modeling of available information and market conditions are included, there is little difference in the cost and effectiveness of the fee and regulatory programs. In particular, we find that the ability of the polluter to assess the emissions and cost outcomes of is critical importance for the performance of the fee policy.pollution fees, emissions control, vehicle pollution, inspection and maintenance

Quantum Hall Effect on the Hofstadter Butterfly

Motivated by recent experimental attempts to detect the Hofstadter butterfly, we numerically calculate the Hall conductivity in a modulated two-dimensional electron system with disorder in the quantum Hall regime. We identify the critical energies where the states are extended for each of butterfly subbands, and obtain the trajectory as a function of the disorder. Remarkably, we find that when the modulation becomes anisotropic, the critical energy branches accompanying a change of the Hall conductivity.Comment: 4 pages, 6 figure