Quantized adiabatic quantum pumping due to interference

Recent theoretical calculations, demonstrating that quantized charge transfer due to adiabatically modulated potentials in mesoscopic devices can result purely from the interference of the electron wave functions (without invoking electron-electron interactions) are reviewed: (1) A new formula is derived for the pumped charge Q (per period); It reproduces the Brouwer formula without a bias, and also yields the effect of the modulating potential on the Landauer formula in the presence of a bias. (2) For a turnstile geometry, with time-dependent gate voltages V_L(t) and V_R(t), the magnitude and sign of Q are determined by the relative position and orientation of the closed contour traversed by the system in the {V_L-V_R} plane, relative to the transmission resonances in that plane. Integer values of Q (in units of e) are achieved when a transmission peak falls inside the contour, and are given by the winding number of the contour. (3) When the modulating potential is due to surface acoustic waves, Q exhibits a staircase structure, with integer values, reminiscent of experimental observations.Comment: Invited talk, Localization, Tokyo, August 200

Acoustoelectric current and pumping in a ballistic quantum point contact

The acoustoelectric current induced by a surface acoustic wave (SAW) in a ballistic quantum point contact is considered using a quantum approach. We find that the current is of the "pumping" type and is not related to drag, i.e. to the momentum transfer from the wave to the electron gas. At gate voltages corresponding to the plateaus of the quantized conductance the current is small. It is peaked at the conductance step voltages. The peak current oscillates and decays with increasing SAW wavenumber for short wavelengths. These results contradict previous calculations, based on the classical Boltzmann equation.Comment: 4 pages, 1 figur

Pattern Formation in Semiconductors

In semiconductors, nonlinear generation and recombination processes of free carriers and nonlinear charge transport can give rise to non-equilibrium phase transitions. At low temperatures, the basic nonlinearity is due to the autocatalytic generation of free carriers by impact ionization of shallow impurities. The electric field accelerates free electrons, causing an abrupt increase in free carrier density at a critical electric field. In static electric fields, this nonlinearity is known to yield complex filamentary current patterns bound to electric contacts

Adiabatic transport in nanostructures

A confined system of non-interacting electrons, subject to the combined effect of a time-dependent potential and different external chemical-potentials, is considered. The current flowing through such a system is obtained for arbitrary strengths of the modulating potential, using the adiabatic approximation in an iterative manner. A new formula is derived for the charge pumped through an un-biased system (all external chemical potentials are kept at the same value); It reproduces the Brouwer formula for a two-terminal nanostructure. The formalism presented yields the effect of the chemical potential bias on the pumped charge on one hand, and the modification of the Landauer formula (which gives the current in response to a constant chemical-potential difference) brought about by the modulating potential on the other. Corrections to the adiabatic approximation are derived and discussed.Comment: 8 pages, 2 figure

Operational reliability assessment of the GEOS A spacecraft

Decision theory application to GEOS A spacecraft operational reliability assessmen

Theory and astrophysical consequences of a magnetized torus around a rapidly rotating black hole

(Abbrev.) We analyze the topology, lifetime, and emissions of a torus around a black hole formed in hypernovae and black hole-neutron star coalescence. The torus is ab initio uniformly magnetized, represented by two counter oriented current-rings, and develops a state of suspended accretion against a "magnetic wall" around the black hole. Magnetic stability of the torus gives rise to a new fundamental limit EB/Ek<0.1 for the ratio of poloidal magnetic field energy-to-kinetic energy. The lifetime of rapid spin of the black hole is effectively defined by the timescale of dissipation of black hole-spin energy in the horizon, and satisfies T= 40s (MH/7MSun)(R/6MH)^4(0.03MH/MT) for a black hole of mass MH surrounded by a torus of mass MT and radius R. The torus converts a major fraction Egw/Erot=0.1 into gravitational radiation through a finite number of multipole mass-moments, and a smaller fraction into MeV neutrinos and baryon-rich winds. At a source distance of 100Mpc, these emissions over N=2e4 periods give rise to a characteristic strain amplitude \sqrt{N}hchar=6e-21. We argue that torus winds create an open magnetic flux-tube on the black hole, which carries a minor and standard fraction Ej/Erot=1e-3 in baryon-poor outflows to infinity. We identify this baryon poor output of tens of seconds with GRBs with contemporaneous and strongly correlated emissions in gravitational radiation, conceivably at multiple frequencies. Ultimately, this leaves a black hole binary surrounded by a supernova remnant.Comment: To appear in ApJ (44p

Women in senior post-graduate medicine career roles in the UK: a qualitative study

Objectives:This qualitative study sought to elicit the views, experiences, career journeys and aspirations of women in senior post-graduate medical education roles to identify steps needed to help support career progression.Design:In-depth semi-structured telephone interviews.Setting:UKParticipants:Purposive sample of 12 women in a variety of senior leadership roles in post-graduate medical education in the UK.Main outcome measures:Self reported motivating influences, factors that helped and hindered progress, key branch points, and key educational factors and social support impacting on participants' career in postgraduate medicine.Results:Respondents often reported that career journeys were serendipitous, rather than planned, formal or well structured. Senior women leaders reported having a high internal locus of control, with very high levels of commitment to the NHS. All reported significant levels of drive, although the majority indicated that they were not ambitious in the sense of a strong drive for money, prestige, recognition or power. They perceived that there was an under-representation of women in senior leadership positions and that high-quality female mentorship was particularly important in redressing this imbalance. Social support, such a spouse or other significant family member, was particularly valued as reaffirming and supporting women’s chosen career ambition. Factors that were considered to have hindered career progression included low self-confidence and self-efficacy, the so-called glass ceiling and perceived self-limiting cultural influences. Factors indirectly linked to gender such as part-time versus working full time were reportedly influential in being overlooked for senior leadership roles. Implications of these findings are discussed in the paper.Conclusion:Social support, mentorship and role modelling are all perceived as highly important in redressing perceived gender imbalances in careers in post-graduate medical education