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Showing papers by "Andre K. Geim published in 1996"


Journal ArticleDOI
TL;DR: A donor-related resonance is observed in double-barrier resonant tunneling devices with Si donors incorporated in the quantum well in high magnetic fields and indicates unambiguously that the resonance is due to tunneling through the ground state of a shallow donor with two bound electrons (D- level).
Abstract: A donor-related resonance is observed in double-barrier resonant tunneling devices with Si donors incorporated in the quantum well. In high magnetic fields the resonance becomes dominant over the Is resonance associated with the ground state of a single donor. The bias position of the donor resonance, its magnetic field dependence, and large amplitude indicate unambiguously that the resonance is due to tunneling through the ground state of a shallow donor with two bound electrons (D- level).

32 citations


Journal ArticleDOI
TL;DR: A diverging Hc2(T) was observed in overdoped Tl 2Ba 2CuO 6 films, 7 where H c2 increased rapidly with decreasing temperature in the whole temperature range 1.t.001, and in many new superconductors discovered during the last decade exhibit behavior inconsistent with the WHH theory.
Abstract: The theory by Werthamer-Helfand-Hohenberg 1 ~WHH! predicts the universal behavior of the upper critical field Hc2(T) in superconductors with weak electron-phonon coupling. The behavior can be described by a universal function hc2(T) expressed in the reduced variables for magnetic field h5H@Tc(2dHc2 /dT)T5T c # 21 and temperature t5T/Tc . The function hc2(t) has a negative second derivative and saturates to the value hc2’0.7 at t50. The WHH theory has described successfully properties of conventional superconductors but many new superconductors discovered during the last decade exhibit behavior inconsistent with the theory. Amorphous alloys based on transition metals 2 and K 3C 60 ~Ref. 3! shows the linear dependence Hc2(T) down to low temperatures. Some organic superconductors 4 and electrondoped high-temperature superconductors @L 22x Ce x CuO 42y where L5Pr,Sm,Nd ~Refs. 5 and 6!# even have a positive second derivative. Furthermore, in stark contrast to WHH a diverging Hc2(T) was observed in overdoped Tl 2Ba 2CuO 6 films, 7 where Hc2 increased rapidly with decreasing temperature in the whole temperature range 1.t.0.001, and in

27 citations


Journal Article
TL;DR: In this paper, the authors studied magnetoresistance in thin films of amorphous indium oxide with various degrees of oxidation, i.e., with various densities of states in the vicinity of the Fermi level.
Abstract: We have studied magnetoresistance in thin films of amorphous indium oxide with various degrees of oxidation, i. e., with various densities of states in the vicinity of the Fermi level. A large negative magnetoresistance is observed when the films are in the insulating state. The negative field derivative partial derivative R/partial derivative H of the resistance persists up to 20 T, the highest magnetic field in our experiment. The magnetoresistance is described in terms of a field-dependent gap at the Fermi level. Such a gap can arise due either to tunneling in a system of fluctuation-induced superconducting clusters or to the Cooper interaction between electrons localized in shallow minima of the random potential. In the latter case, if the depth of the minima is smaller than the Debye energy, the electrons are virtually delocalized by short-wavelength phonons.

19 citations


Journal ArticleDOI
TL;DR: The interaction of phonons with a two-dimensional electron gas in the fractional quantum Hall regime with phonon drag thermoelectric power (TEP) is investigated and a maximum observed in the TEP is modeled by a weakly diverging effective mass of the composite fermions at this filling factor.
Abstract: We have investigated the interaction of phonons with a two-dimensional electron gas in the fractional quantum Hall regime with phonon drag thermoelectric power (TEP). The TEPs at filling factors with the same even denominator are found to be identical; at other even-denominator filling factors they differ only by a constant factor. Treating these states as composite fermions, the findings are explained by a zero magnetic field theory for phonon drag. A maximum observed in the TEP at nu = 1/2 is modeled by a weakly diverging effective mass of the composite fermions at this filling factor.

17 citations


Journal ArticleDOI
TL;DR: The magnetoresistance measurements of a 2D electron gas subjected to a periodic magnetic field created by the presence of type II superconducting stripes on the surface of the heterostructure observe oscillatory behaviour due to a commensurability effect between the cyclotron radius and the period of the magnetic field.

14 citations


Journal ArticleDOI
TL;DR: It is shown that the SDHD of the heavy-holt: states can be enhanced by increasing the confinement of the Light-hole states through adjustment of the height of the confining barriers.
Abstract: Exact, self-consistently calculated eigenfunctions and eigenvalues of the valence band in degenerate asymmetric double quantum wells are obtained from the full 4x4 Luttinger Hamiltonian for different hole densities. We found the solutions to be extremely sensitive to the charge density. The charge induces an extra asymmetry in the confining potential and leads to a smaller separation of the hole levels in the adjacent wells. This strongly enhances the different leakage of wave functions with opposite magnetic moment, called spin-dependent hole delocalization (SDHD). Furthermore, it is shown that the SDHD of the heavy-holt: states can be enhanced by increasing the confinement of the Light-hole states through adjustment of the height of the confining barriers. The theoretical results are shown to be in excellent agreement with transport and optical experiments, which are proven to be largely determined by space-charge effects.

4 citations


Journal ArticleDOI
TL;DR: In this paper, the motion of two-dimensional electrons in a magnetic field which is periodic on the scale of a few hundred nanometres was investigated and the magnetoresistance oscillations were identified as due to a commensurability effect between the period of the magnetic field and the diameter of the cyclotron orbit.
Abstract: We report on the motion of two-dimensional electrons in a magnetic field which is periodic on the scale of a few hundred nanometres. Such a system exhibits novel magnetoresistance oscillations that we unambiguously identify as due to a commensurability effect between the period of the magnetic field and the diameter of the cyclotron orbit. We find that a semiclassical picture of electron motion can quantitatively describe all of the observed experimental features. We also propose methods of optimizing the effects of the magnetic modulation in hybrid semiconductor-superconductor devices.

3 citations


Journal ArticleDOI
TL;DR: In this paper, the thermopower of high-mobility two-dimensional electron gases at even denominator filling factors v are found to be remarkably similar to each other and also to the thermopsower at zero magnetic field.

2 citations


Journal ArticleDOI
TL;DR: In this article, a superconducting gate on the top of a two-dimensional electron gas (2DEG) was used to partition an external magnetic field into a distribution of submicron flux tubes (vortices) and measure the weak localisation contribution to the 2DEG conductance versus spatial separation of the vortices.

1 citations


Journal ArticleDOI
TL;DR: In this paper, the magnetoresistance of quantum wires, junctions and rings near Landau filling factor v = 1/2 was measured to search for effects due to composite fermions.

1 citations


Journal ArticleDOI
TL;DR: In this paper, magnetometry of individual Al disks with the radiusr from several μm down to 103A was carried out using a novel μ-magnetisation measurement technique which is based on detection of a local magnetic field by submicron Hall probes.
Abstract: We report on magnetometry of individual Al disks with the radiusr from several μm down to 103A. The studies were carried out using a novel μ-magnetisation measurement technique which is based on detection of a local magnetic field by submicron Hall probes. The technique allows non-invasive access to individual superconducting samples of the size comparable with the coherence length ξ(T=0) (0D regime). The superconducting phase diagram of Al disks exhibits an oscillatory behaviour due to the size quantization in the energy spectrum of Cooper pairs, in analogy to the classical Little-Parks experiment. The experiment also reveals a number of new features related to the reduced dimensionality: a) recovery of the type-I phase transition aroundr=ξ(0) while the transition remains type-II in both limitsr >ξ b) mesoscopic correction to the ideal Meissner state due to partial penetration of a vortex and c) metastable superconductivity due to a hysteretic transition between the ground and excited states in the superconducting energy spectrum.

Journal ArticleDOI
TL;DR: In this paper, the interaction between phonons and composite fermions was studied by thermoelectric power (TEP) measurements on the 2DEG and the measurements were carried out on a GaAs-AIGaAs heterostructure.
Abstract: The fractional quantum Hall effect (FQItE) has recently been attributed to quantum oscillations of new quasi-particles, called Composite Fermions (CF) [1, 2]. The CF consist of an electron and an even number of flux quanta. The FQHE of the electrons can be viewed as the integer quantum Hall effect of the CF's. In particular, the magnetic field for which u equals 1/2, appears as zero field for the CF's. We have studied the interaction between phonons and CF's by thermoelectric power (TEP) measurements on the 2DEG. The measurements were carried out on a GaAs-AIGaAs heterostructure with density and mobility in the range of 1.0 - 1.9 x 1011 cm -2 and 0.6 - 1.0 xl06 cm2/Vs, respectively. A known temperature gradient was set up along the sample, which was thermally anchored to the mixing chamber of a dilution refrigerator. 2. RESULTS

Journal ArticleDOI
TL;DR: In this paper, a donor-related resonance at a smaller binding energy is observed in high magnetic fields where it becomes dominant over the 1s resonance, attributed to a D-minus state of a shallow donor.