New Bardeen-Cooper-Schrieffer-type theory at finite temperature with particle-number conservation

We formulate a new Bardeen-Cooper-Schrieffer (BCS)-type theory at finite temperature, by deriving a set of variational equations of the free energy after the particle-number projection. With its broad applicability, this theory can be a useful tool for investigating the pairing phase transition in finite systems with the particle-number conservation. This theory provides effects of the symmetry-restoring fluctuation (SRF) for the pairing phenomena in finite fermionic systems, distinctively from those of additional quantum fluctuations. It is shown by numerical calculations that the phase transition is compatible with the conservation in this theory, and that the SRF shifts up the critical temperature ($T^\mathrm{cr}$). This shift of $T^\mathrm{cr}$ occurs due to reduction of degrees-of-freedom in canonical ensembles, and decreases only slowly as the particle-number increases (or as the level spacing narrows), in contrast to the conventional BCS theory.Comment: 10 pages including 3 figures, to be published in Phys. Rev.

Eternally inflating cosmologies from intersecting spacelike branes

Intersecting spacelike braneworld cosmologies are investigated. The time axis is set on the scale parameter of extra space, which may include more than one timelike metric. Obtained are eternally inflating (i.e. undergoing late-time inflation) Robertson-Walker spacetime and extra space with a constant scale factor. In the case of multibrane solutions, some dimensions are static or shrink. The fact that the largest supersymmetry algebra contains 32 supercharges in 4 dimensions imposes a restriction on the geometry of extra space.Comment: 19 page

Plasmonic nanoparticle enhanced light absorption in GaAs solar cells

We demonstrate an improvement in efficiency of optically thin GaAs solar cells decorated with size-controlled Ag nanoparticles fabricated by masked deposition through anodic aluminum oxide templates. The strong scattering by the interacting surface plasmons in densely formed high aspect-ratio nanoparticles effectively increases the optical path of the incident light in the absorber layers resulting in an 8% increase in the short circuit current density of the cell. The nanoparticle array sheet conductivity also reduces the cell surface sheet resistance evidenced by an improved fill factor. This dual function of plasmonic nanoparticles has potential to enable thinner photovoltaic layers in solar cells

Effects of particle-number conservation on heat capacity of nuclei

By applying the particle-number projection to the finite-temperature BCS theory, the $S$-shaped heat capacity, which has recently been claimed to be a fingerprint of the superfluid-to-normal phase transition in nuclei, is reexamined. It is found that the particle-number (or number-parity) projection gives $S$-shapes in the heat capacity of nuclei which look qualitatively similar to the observed ones. These $S$-shapes are accounted for as effects of the particle-number conservation on the quasiparticle excitations, and occur even when we keep the superfluidity at all temperatures by assuming a constant gap in the BCS theory. The present study illustrates significance of the conservation laws in studying phase transitions of finite systems.Comment: RevTeX4, 12 pages including 5 figures (1 color figure), to be published in PR

High efficiency InGaAs solar cells on Si by InP layer transfer

InP/Si substrates were fabricated through wafer bonding and helium-induced exfoliation of InP, and InGaAs solar cells lattice matched to bulk InP were grown on these substrates using metal-organic chemical-vapor deposition. The photovoltaic characteristics of the InGaAs cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epiready InP substrates, thus providing a demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications

Comparative study of macroscopic quantum tunneling in Bi_2Sr_2CaCu_2O_y intrinsic Josephson junctions with different device structures

We investigated macroscopic quantum tunneling (MQT) of Bi$_2$Sr$_2$CaCu$_2$O$_y$ intrinsic Josephson junctions (IJJs) with two device structures. One is a nanometer-thick small mesa structure with only two or three IJJs and the other is a stack of a few hundreds of IJJs on a narrow bridge structure. Experimental results of switching current distribution for the first switching events from zero-voltage state showed a good agreement with the conventional theory for a single Josephson junction, indicating that a crossover temperature from thermal activation to MQT regime for the former device structure was as high as that for the latter device structure. Together with the observation of multiphoton transitions between quantized energy levels in MQT regime, these results strongly suggest that the observed MQT behavior is intrinsic to a single IJJ in high-$T_c$ cuprates, independent of device structures. The switching current distribution for the second switching events from the first resistive state, which were carefully distinguished from the first switchings, was also compared between two device structures. In spite of the difference in the heat transfer environment, the second switching events for both devices were found to show a similar temperature-independent behavior up to a much higher temperature than the crossover temperature for the first switching. We argue that it cannot be explained in terms of the self-heating owing to dissipative currents after the first switching. As possible candidates, the MQT process for the second switching and the effective increase of electronic temperature due to quasiparticle injection are discussed.Comment: 10pages, 7figures, submitted to Phys. Rev.

Plasmon-enhanced absorption and photocurrent in ultrathin GaAs solar cells with metallic nanostructures

Metallic nanostructures can excite surface plasmons and can dramatically increase the optical path length in thin active photovoltaic layers to enhance overall photoabsorption. This effect has potential for cost and weight reduction with thinned layers and also for efficiency enhancement associated with increased carrier excitation level in the absorber layer

Surface plasmon enhanced photocurrent in thin GaAs solar cells

We report fabrication of size-controlled plasmonic nanoparticle arrays by which optically thin GaAs single junction solar cells are decorated. Ordered Ag and Al nanoparticles with average diameters of 60-150 nm and interparticle spacings of 100-300 nm were templated onto the window layers of the GaAs solar cells using nanoporous anodic aluminum oxide membrane templates. Near the surface plasmon resonances, 60nm-diameter Ag and Al nanoparticles serve as light-absorbers so that non-radiative surface plasmon resonances reduce the photocurrent of the cells, which is improved by increasing the nanoparticle size. Photocurrent enhancements are seen at wavelengths longer than surface plasmon resonance which is maximized near the band gap edge of GaAs. These enhancements can be attributed to the increased optical path in the photovoltaic layers resulting from multi-angle scattering by the nanoparticles, while high scattering efficiency nanoparticles in turn increase the back scattering light out of the cell reducing the photocurrent

Quantum number projection at finite temperature via thermofield dynamics

Applying the thermo field dynamics, we reformulate exact quantum number projection in the finite-temperature Hartree-Fock-Bogoliubov theory. Explicit formulae are derived for the simultaneous projection of particle number and angular momentum, in parallel to the zero-temperature case. We also propose a practical method for the variation-after-projection calculation, by approximating entropy without conflict with the Peierls inequality. The quantum number projection in the finite-temperature mean-field theory will be useful to study effects of quantum fluctuations associated with the conservation laws on thermal properties of nuclei.Comment: 27 pages, using revtex4, to be published in PR