We present a novel technique for measuring the lifetime of quasiparticle excitations of a 2DES by... more We present a novel technique for measuring the lifetime of quasiparticle excitations of a 2DES by investigating the tunnelling into a quantum dot from a 2DES over an extended range of energy from the Fermi energy to the sub-band edge. We ÿnd that the lifetime qp, of a quasihole excitation, caused by removing an electron from a 2DES state with energy below the Fermi energy EF, has the form qp = ˜= , where is a constant of order unity.
Four contact electrical measurements on the anisotropic high-Tc superconductors: voltage reversals
Physica C: Superconductivity, 1996
We describe three mechanisms for the anomalous voltage reversals which are occasionally observed ... more We describe three mechanisms for the anomalous voltage reversals which are occasionally observed during four contact current-voltage measurements on the high temperature superconductors. Voltage reversals have been frequently observed for whiskers of the bismuth strontium calcium copper oxide (BSSCO) compounds, and we describe measurements made on high quality whiskers of the 2:2:1:2 and 2:2:2:3 phases. A simple model based on
The Kosterlitz-Thouless theory for the electrical dissipation in the high temperature superconduc... more The Kosterlitz-Thouless theory for the electrical dissipation in the high temperature superconductors predicts scaling of the current voltage characteristics above the phase transition temperature but not below. We give an expression for the universal scaling curve which provides an excellent fit to data for epitaxial YBa2Cu307 thin films taken in zero applied magnetic field. Surprisingly, the expression also provides an excellent fit to data taken in finite magnetic fields.
Experiments on cryogenically-enhanced levitation and flotation and on magnetic alignment using a 17 T closed-cycle superconducting magnet
hfml.science.ru.nl
... 3. The separation of binary granular mixtures under vibration and differential magnetic levi... more ... 3. The separation of binary granular mixtures under vibration and differential magnetic levitation force, AT Catherall, P. Lopez-Alcaraz, P ... University and National Institute for Materials Science 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, 3-13 Sakura, Tsukuba, Ibaraki 305 ...
ABSTRACT The recent high-precision torsional oscillator experiments of Casey et al. involving thi... more ABSTRACT The recent high-precision torsional oscillator experiments of Casey et al. involving thin films of normal liquid 3He showed that the film decouples from the substrate with a time constant $$\tau_{\mathrm{osc}}$$ which is proportional to T −1 where T is the absolute temperature. We interpret this experiment by adapting a theory due to Meyerovich which was developed for dilute 3He-4He mixtures flowing between two relatively smooth plates. The analysis of the experiment confirms the central idea that $$\tau_{\mathrm{osc}}$$ varies as T −1. The variation of $$\tau _{\mathrm{osc}}$$ with film thickness, d, is affected by the change in the shape of the free surface of the film, due to van der Waals forces, as the film becomes thinner.
Surface stability of granular systems under horizontal and vertical vibration: The applicability of a coefficient of friction
Physical Review E, 2000
We investigate the conditions under which the surface of a granular pile becomes unstable to vibr... more We investigate the conditions under which the surface of a granular pile becomes unstable to vibrations. Three stability boundaries are identified, which depend upon the relative phase of the driving forces and the angle of the prepared slope. The experimental findings can be interpreted within the context of a Coulomb friction model and used to define an effective coefficient of friction. For up-hill motion we find that the coefficient of friction depends strongly on the slope angle and that, in general, it requires less vibration to transport grains uphill than would be otherwise expected.
Liquid oxygen exhibits surface instabilities when subjected to a sufficiently strong magnetic fie... more Liquid oxygen exhibits surface instabilities when subjected to a sufficiently strong magnetic field. A vertically oriented magnetic field gradient both increases the magnetic field value at which the pattern forms and shrinks the length scale of the surface patterning. We show that these effects of the field gradient may be described in terms of an "effective gravity", which in our experiments may be varied from 1g to 360g.
We report time-resolved studies of ballistic phonon absorption in the fractional quantum Hall reg... more We report time-resolved studies of ballistic phonon absorption in the fractional quantum Hall regime at Landau level ÿlling factors of = 1 3 ; 2 5 and 1 2. The technique used can resolve the interaction of the two-dimensional electron system with LA and TA phonons and has been used to measure the temperature variation of the heat capacity of a single layer of electrons at = 1 3. The energy gaps at = 1 3 have also been measured and found to be in good agreement with theory. The roles of compressible and incompressible regions in the phonon absorption process are discussed. Angle resolved measurements at = 2 5 are also in good agreement with theory.
We report time-resolved measurements of phonon absorption in the fractional quantum Hall regime. ... more We report time-resolved measurements of phonon absorption in the fractional quantum Hall regime. Experiments have been conducted on an n-type heterojuncfion grown on a 1.8 mm GaAs wafer. The start of the absorption signal is consistent with the ballistic phonon time-of-flight. The initial rise time is consistent with the lime constant of our device and external circuits. This is followed by unusually slow rise and decay times.
Low-resistance electrical contacts to single crystal BSCCO whiskers
Superconductor Science and Technology, 1995
ABSTRACT We have developed a method for preparing low-resistance contacts to single-crystal whisk... more ABSTRACT We have developed a method for preparing low-resistance contacts to single-crystal whiskers of the 2:2:1:2 phase and of mixed 2:2:1:2/2:2:2:3 phases of BSCCO, and have studied the contact resistivities over the temperature range from 50 K to 300 K. The measurement technique uses a simple three contact geometry requiring a single voltage measurement. The values of the contact resistivities obtained in this work are lower than the great majority of those previously reported for the BSCCO superconductors, making the method suitable for applications which include the nanofabrication of device structures based on micrometre-sized single crystals and single-crystal whiskers. The anomalous values of contact resistivity found in mixed phase whiskers at temperatures between the transition temperatures of the two phases is attributed to spreading resistance and anisotropy.
Phonon spectroscopy of the fractional quantum Hall effect
Physica Scripta, 1996
ABSTRACT The low-lying bulk excitations of two-dimensional electron systems subject to a strong p... more ABSTRACT The low-lying bulk excitations of two-dimensional electron systems subject to a strong perpendicular magnetic field have been studied using phonon absorption. Results reported earlier lead to the conclusion that the magnetoroton minimum energy in the fractional quantum Hall effect can be measured by phonon absorption. Recent time-resolved measurements have demonstrated fundamental differences in phonon absorption between fractional quantum Hall states, non-rational filling factors and filling factors close to 1/2. At non-rational filling factors the response is bolometric, whereas at filling factors corresponding to FQHE states the response also depends on the spectrum of the heater pulse. Around ν = 1/2 the response is largely independent of magnetic field with a weak maximum at ν = 1/2.
Physica E: Low-dimensional Systems and Nanostructures, 2004
In samples used to maintain the US resistance standard the breakdown of the dissipationless integ... more In samples used to maintain the US resistance standard the breakdown of the dissipationless integer quantum Hall effect occurs as a series of dissipative voltage steps. A mechanism for this type of breakdown is proposed, based on the generation of magneto-excitons when the quantum Hall fluid flows past an ionised impurity above a critical velocity. The calculated generation rate gives a voltage step height in good agreement with measurements. We also compare this model to a hydrodynamic description of breakdown.
The current voltage characteristics of a single barrier tunnelling device which contains self-org... more The current voltage characteristics of a single barrier tunnelling device which contains self-organized InAs quantum dots within the barrier are studied in zero and strong magnetic ®eld conditions (both perpendicular and parallel to the interfaces). It is observed that in all cases there are large peaks whenever the energy of the bound state of a dot approaches the chemical potential of the 2d electron system formed at an interface. These are identi®ed as thermally broadened Fermi-edge singularities which arise from many-body contributions to the tunnelling amplitude. We note that the standard theory of such singularities appears to work well in situations when the spin states of the dot are Zeeman split but is qualitatively incorrect in zero ®eld. In the quantum Hall regime the peak height is an oscillatory function of magnetic ®eld in agreement with theoretical predictions.
We have made the first phonon absorption measurements in the fractional quantum Hall regime. Expe... more We have made the first phonon absorption measurements in the fractional quantum Hall regime. Experiments have been conducted on both high mobility n and p-type GaAs/A1GaAs heterojunctions. In the n-type samples used the electron densities were similar whilst the mobilities differed greatly. The energy gaps determined by phonon absorption measurements give almost identical values, demonstrating that this method of measurement is rather insensitive to disorder. The values obtained at v = 2/3 are in good agreement with theoretical estimates of the magnetoroton energy gap. The results on p-type samples again demonstrate the relatively small effect of disorder on the values obtained. The values are much lower than simple theoretical estimates. It is believed this is due to the larger influence of Landau level mixing in these samples.
We report the results of a study of the absorption of non-equilibrium ballistic phonons by a two-... more We report the results of a study of the absorption of non-equilibrium ballistic phonons by a two-dimensional electron system, in strong magnetic ®elds, by measuring its change in resistance. At the even denominator Landau level ®lling factors, m, of 1/2 and 3/2 the electron±phonon interaction is observed to be cut-o above a particular value of the component of the wavevector perpendicular to the low dimensional system. This cut-o is consistent with that due to the ®nite thickness of the 2DES. At fractional quantum Hall eect minima (1/3, 2/5, 2/3) we observe that the cut-o to the interaction depends on the ®lling factor. At m 1/3, the energy gap determined from these experiments is consistent with the theoretical prediction of the magnetoroton energy gap.
Ballistic phonon experiments, in the fractional quantum Hall regime, are described in which the i... more Ballistic phonon experiments, in the fractional quantum Hall regime, are described in which the interactions between the longitudinal (LA) and transverse acoustic (TA) modes and the two-dimensional electron system (2DES) can be resolved. The range of in-plane wave vectors of LA phonons that can interact with the 2DES is restricted and the measured fractional quantum Hall energy gap is found to depend on the in-plane wave vector. Whilst the energies found at Landau level filling factor n ¼ 2 5 are in good agreement with theory, those at n ¼ 2 3 and 1 3 are lower than theoretical predictions. Possible reasons for this discrepancy are discussed.
We present a novel technique for measuring the lifetime of quasiparticle excitations of a 2DES by... more We present a novel technique for measuring the lifetime of quasiparticle excitations of a 2DES by investigating the tunnelling into a quantum dot from a 2DES over an extended range of energy from the Fermi energy to the sub-band edge. We ÿnd that the lifetime qp, of a quasihole excitation, caused by removing an electron from a 2DES state with energy below the Fermi energy EF, has the form qp = ˜= , where is a constant of order unity.
Four contact electrical measurements on the anisotropic high-Tc superconductors: voltage reversals
Physica C: Superconductivity, 1996
We describe three mechanisms for the anomalous voltage reversals which are occasionally observed ... more We describe three mechanisms for the anomalous voltage reversals which are occasionally observed during four contact current-voltage measurements on the high temperature superconductors. Voltage reversals have been frequently observed for whiskers of the bismuth strontium calcium copper oxide (BSSCO) compounds, and we describe measurements made on high quality whiskers of the 2:2:1:2 and 2:2:2:3 phases. A simple model based on
The Kosterlitz-Thouless theory for the electrical dissipation in the high temperature superconduc... more The Kosterlitz-Thouless theory for the electrical dissipation in the high temperature superconductors predicts scaling of the current voltage characteristics above the phase transition temperature but not below. We give an expression for the universal scaling curve which provides an excellent fit to data for epitaxial YBa2Cu307 thin films taken in zero applied magnetic field. Surprisingly, the expression also provides an excellent fit to data taken in finite magnetic fields.
Experiments on cryogenically-enhanced levitation and flotation and on magnetic alignment using a 17 T closed-cycle superconducting magnet
hfml.science.ru.nl
... 3. The separation of binary granular mixtures under vibration and differential magnetic levi... more ... 3. The separation of binary granular mixtures under vibration and differential magnetic levitation force, AT Catherall, P. Lopez-Alcaraz, P ... University and National Institute for Materials Science 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, 3-13 Sakura, Tsukuba, Ibaraki 305 ...
ABSTRACT The recent high-precision torsional oscillator experiments of Casey et al. involving thi... more ABSTRACT The recent high-precision torsional oscillator experiments of Casey et al. involving thin films of normal liquid 3He showed that the film decouples from the substrate with a time constant $$\tau_{\mathrm{osc}}$$ which is proportional to T −1 where T is the absolute temperature. We interpret this experiment by adapting a theory due to Meyerovich which was developed for dilute 3He-4He mixtures flowing between two relatively smooth plates. The analysis of the experiment confirms the central idea that $$\tau_{\mathrm{osc}}$$ varies as T −1. The variation of $$\tau _{\mathrm{osc}}$$ with film thickness, d, is affected by the change in the shape of the free surface of the film, due to van der Waals forces, as the film becomes thinner.
Surface stability of granular systems under horizontal and vertical vibration: The applicability of a coefficient of friction
Physical Review E, 2000
We investigate the conditions under which the surface of a granular pile becomes unstable to vibr... more We investigate the conditions under which the surface of a granular pile becomes unstable to vibrations. Three stability boundaries are identified, which depend upon the relative phase of the driving forces and the angle of the prepared slope. The experimental findings can be interpreted within the context of a Coulomb friction model and used to define an effective coefficient of friction. For up-hill motion we find that the coefficient of friction depends strongly on the slope angle and that, in general, it requires less vibration to transport grains uphill than would be otherwise expected.
Liquid oxygen exhibits surface instabilities when subjected to a sufficiently strong magnetic fie... more Liquid oxygen exhibits surface instabilities when subjected to a sufficiently strong magnetic field. A vertically oriented magnetic field gradient both increases the magnetic field value at which the pattern forms and shrinks the length scale of the surface patterning. We show that these effects of the field gradient may be described in terms of an "effective gravity", which in our experiments may be varied from 1g to 360g.
We report time-resolved studies of ballistic phonon absorption in the fractional quantum Hall reg... more We report time-resolved studies of ballistic phonon absorption in the fractional quantum Hall regime at Landau level ÿlling factors of = 1 3 ; 2 5 and 1 2. The technique used can resolve the interaction of the two-dimensional electron system with LA and TA phonons and has been used to measure the temperature variation of the heat capacity of a single layer of electrons at = 1 3. The energy gaps at = 1 3 have also been measured and found to be in good agreement with theory. The roles of compressible and incompressible regions in the phonon absorption process are discussed. Angle resolved measurements at = 2 5 are also in good agreement with theory.
We report time-resolved measurements of phonon absorption in the fractional quantum Hall regime. ... more We report time-resolved measurements of phonon absorption in the fractional quantum Hall regime. Experiments have been conducted on an n-type heterojuncfion grown on a 1.8 mm GaAs wafer. The start of the absorption signal is consistent with the ballistic phonon time-of-flight. The initial rise time is consistent with the lime constant of our device and external circuits. This is followed by unusually slow rise and decay times.
Low-resistance electrical contacts to single crystal BSCCO whiskers
Superconductor Science and Technology, 1995
ABSTRACT We have developed a method for preparing low-resistance contacts to single-crystal whisk... more ABSTRACT We have developed a method for preparing low-resistance contacts to single-crystal whiskers of the 2:2:1:2 phase and of mixed 2:2:1:2/2:2:2:3 phases of BSCCO, and have studied the contact resistivities over the temperature range from 50 K to 300 K. The measurement technique uses a simple three contact geometry requiring a single voltage measurement. The values of the contact resistivities obtained in this work are lower than the great majority of those previously reported for the BSCCO superconductors, making the method suitable for applications which include the nanofabrication of device structures based on micrometre-sized single crystals and single-crystal whiskers. The anomalous values of contact resistivity found in mixed phase whiskers at temperatures between the transition temperatures of the two phases is attributed to spreading resistance and anisotropy.
Phonon spectroscopy of the fractional quantum Hall effect
Physica Scripta, 1996
ABSTRACT The low-lying bulk excitations of two-dimensional electron systems subject to a strong p... more ABSTRACT The low-lying bulk excitations of two-dimensional electron systems subject to a strong perpendicular magnetic field have been studied using phonon absorption. Results reported earlier lead to the conclusion that the magnetoroton minimum energy in the fractional quantum Hall effect can be measured by phonon absorption. Recent time-resolved measurements have demonstrated fundamental differences in phonon absorption between fractional quantum Hall states, non-rational filling factors and filling factors close to 1/2. At non-rational filling factors the response is bolometric, whereas at filling factors corresponding to FQHE states the response also depends on the spectrum of the heater pulse. Around ν = 1/2 the response is largely independent of magnetic field with a weak maximum at ν = 1/2.
Physica E: Low-dimensional Systems and Nanostructures, 2004
In samples used to maintain the US resistance standard the breakdown of the dissipationless integ... more In samples used to maintain the US resistance standard the breakdown of the dissipationless integer quantum Hall effect occurs as a series of dissipative voltage steps. A mechanism for this type of breakdown is proposed, based on the generation of magneto-excitons when the quantum Hall fluid flows past an ionised impurity above a critical velocity. The calculated generation rate gives a voltage step height in good agreement with measurements. We also compare this model to a hydrodynamic description of breakdown.
The current voltage characteristics of a single barrier tunnelling device which contains self-org... more The current voltage characteristics of a single barrier tunnelling device which contains self-organized InAs quantum dots within the barrier are studied in zero and strong magnetic ®eld conditions (both perpendicular and parallel to the interfaces). It is observed that in all cases there are large peaks whenever the energy of the bound state of a dot approaches the chemical potential of the 2d electron system formed at an interface. These are identi®ed as thermally broadened Fermi-edge singularities which arise from many-body contributions to the tunnelling amplitude. We note that the standard theory of such singularities appears to work well in situations when the spin states of the dot are Zeeman split but is qualitatively incorrect in zero ®eld. In the quantum Hall regime the peak height is an oscillatory function of magnetic ®eld in agreement with theoretical predictions.
We have made the first phonon absorption measurements in the fractional quantum Hall regime. Expe... more We have made the first phonon absorption measurements in the fractional quantum Hall regime. Experiments have been conducted on both high mobility n and p-type GaAs/A1GaAs heterojunctions. In the n-type samples used the electron densities were similar whilst the mobilities differed greatly. The energy gaps determined by phonon absorption measurements give almost identical values, demonstrating that this method of measurement is rather insensitive to disorder. The values obtained at v = 2/3 are in good agreement with theoretical estimates of the magnetoroton energy gap. The results on p-type samples again demonstrate the relatively small effect of disorder on the values obtained. The values are much lower than simple theoretical estimates. It is believed this is due to the larger influence of Landau level mixing in these samples.
We report the results of a study of the absorption of non-equilibrium ballistic phonons by a two-... more We report the results of a study of the absorption of non-equilibrium ballistic phonons by a two-dimensional electron system, in strong magnetic ®elds, by measuring its change in resistance. At the even denominator Landau level ®lling factors, m, of 1/2 and 3/2 the electron±phonon interaction is observed to be cut-o above a particular value of the component of the wavevector perpendicular to the low dimensional system. This cut-o is consistent with that due to the ®nite thickness of the 2DES. At fractional quantum Hall eect minima (1/3, 2/5, 2/3) we observe that the cut-o to the interaction depends on the ®lling factor. At m 1/3, the energy gap determined from these experiments is consistent with the theoretical prediction of the magnetoroton energy gap.
Ballistic phonon experiments, in the fractional quantum Hall regime, are described in which the i... more Ballistic phonon experiments, in the fractional quantum Hall regime, are described in which the interactions between the longitudinal (LA) and transverse acoustic (TA) modes and the two-dimensional electron system (2DES) can be resolved. The range of in-plane wave vectors of LA phonons that can interact with the 2DES is restricted and the measured fractional quantum Hall energy gap is found to depend on the in-plane wave vector. Whilst the energies found at Landau level filling factor n ¼ 2 5 are in good agreement with theory, those at n ¼ 2 3 and 1 3 are lower than theoretical predictions. Possible reasons for this discrepancy are discussed.
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Papers by Keith Benedict