B. Spivak

MESOSCOPIC MECHANISM OF ADIABATIC CHARGE TRANSPORT

Let us apply an external potential fsr, td, which is changing slowly and periodically in time to a metallic sample. This potential causes finite net charge Q transported across the sample per period. This phenomenon, known as adiabatic charge transport [1], has been investigated in several papers [1‐ 4] for a closed system characterized by its ground state wave function corresponding to the instantaneous value of the external potential fsr, td. However, in real experimental situations exact eigenfunctions of electrons are ill defined: The electron energy levels are broadened due to inelastic processes at T fi 0, and, in the case of an open system, are further broadened due to finite dwell time. Here we present a theory of adiabatic charge transport in “open mesoscopic systems.” We demonstrate that at low T both the magnitude and the sign of Q are sample specific quantities. The typical value of Q in disordered (chaotic) systems turns out to be determined by quantum interference effects. We evaluate this value and find that it is much larger than the one in ballistic systems. This enhancement manifests of the well-known fact that at low temperatures, all electronic characteristics of mesoscopic samples are extremely sensitive to changes in the scattering potential [5‐ 8]. Let us start with a qualitative picture of the mesoscopic adiabatic charge transport. The wave functions of electrons in disordered systems exhibit sample specific spatial fluctuations. Therefore, the spatial electron density profile is changing slowly in time, together with the external potential fsr, td. According to the continuity relation, variation of the charge density in time requires currents in the system. The question arises: What is the condition for a total charge transfer during one period to be nonzero? Let the pumping potential fsr, td be characterized by a finite set of functions gstd › hgastdj, a › 1, . . . , m, which are periodic with the same period t0: fsr, td › fsr, gstdd d › X a

Density of States in Superconductor-Normal Metal-Superconductor Junctions

We consider the χ0-dependence of the density of states inside the normal metal of a superconductor-normal metal-superconductor (SNS) junction. Here χ0is the phase difference of two superconductors of the junction. It is shown that in the absence of electron-electron interaction the energy dependence of the density of states has a gap which decreases as χ0increases and closes at χ0= π. Both the analytical expressions for the χ0-dependence of the density of states and the results of numerical simulations are presented.

Collective Rabi oscillations and solitons in a time-dependent BCS pairing problem.

Motivated by recent efforts to achieve cold fermions pairing, we study the nonadiabatic regime of the Bardeen-Cooper-Schrieffer state formation. After the interaction is turned on, at times shorter than the quasiparticle energy relaxation time, the system oscillates between the superfluid and normal state. The collective nonlinear evolution of the BCS-Bogoliubov amplitudes u(p), v(p), along with the pairing function Delta, is shown to be an integrable dynamical problem which admits single soliton and soliton train solitons. We interpret the collective oscillations as Bloch precession of Anderson pseudospins, where each soliton causes a pseudospin 2pi Rabi rotation.

Hydrodynamic Description of Transport in Strongly Correlated Electron Systems

We develop a hydrodynamic description of the resistivity and magnetoresistance of an electron liquid in a smooth disorder potential. This approach is valid when the electron-electron scattering length is sufficiently short. In a broad range of temperatures, the dissipation is dominated by heat fluxes in the electron fluid, and the resistivity is inversely proportional to the thermal conductivity, κ. This is in striking contrast to the Stokes flow, in which the resistance is independent of κ and proportional to the fluid viscosity. We also identify a new hydrodynamic mechanism of spin magnetoresistance.

Chirality effects in carbon nanotubes

We consider chirality related effects in optical, photogalvanic and electron-transport properties of carbon nanotubes. We show that these properties of chiral nanotubes are determined by terms in the electron effective Hamiltonian describing the coupling between the electron wavevector along the tube principal axis and the orbital momentum around the tube circumference. We develop a theory of photogalvanic effects and a theory of d.c. electric current, which is linear in the magnetic field and quadratic in the bias voltage. Moreover, we present analytic estimations for the natural circular dichroism and magneto-spatial effect in the light absorption.

Colloquium: Transport in strongly correlated two dimensional electron fluids

We present an overview of the measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities, and of some of the theories that have been proposed to account for them. Many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium. Rather, they reflect new physics associated with strong correlation effects, which warrant further study.

Semiconductor high-energy radiation scintillation detector

We propose a new scintillation-type detector in which high-energy radiation generates electron–hole pairs in a direct-gap semiconductor material that subsequently recombine producing infrared light to be registered by a photo-detector. The key issue is how to make the semiconductor essentially transparent to its own infrared light, so that photons generated deep inside the semiconductor could reach its surface without tangible attenuation. We discuss two ways to accomplish this, one based on doping the semiconductor with shallow impurities of one polarity type, preferably donors, the other by heterostructure bandgap engineering. The proposed semiconductor scintillator combines the best properties of currently existing radiation detectors and can be used for both simple radiation monitoring, like a Geiger counter, and for high-resolution spectrography of the high-energy radiation. An important advantage of the proposed detector is its fast response time, about 1 ns, essentially limited only by the recombination time of minority carriers. Notably, the fast response comes without any degradation in brightness. When the scintillator is implemented in a qualified semiconductor material (such as InP or GaAs), the photo-detector and associated circuits can be epitaxially integrated on the scintillator slab and the structure can be stacked-up to achieve virtually any desired absorption capability.

Quantum superconductor-metal transition

We consider a system of superconducting grains embedded in a normal metal. At zero temperature this system exhibits a quantum superconductor-normal metal phase transition. This transition can take place at arbitrarily large conductance of the normal metal.

Phase separation in the two-dimensional electron liquid in MOSFET’s

We show that the existence of an intermediate phase between the Fermi liquid and the Wigner crystal phases is a generic property of the two-dimensional pure electron liqd in MOSFETs at zero temperature. The physical reason for the existence of these phases is a partial separation of the uniform phases. We discuss properties of these phases and a possible explanation of experimental results on transport properties of low density electron gas in Si MOSFETs. We also argue that in certain range of parameters the partial phase separation corresponds to a supersolid phas e discussed in [AndreevLifshitz].

Ferromagnetic correlations in quasi-one-dimensional conducting channels

We propose a model that explains the experimental observation of spontaneous spin polarization of conducting electrons in quasi-one-dimensional