E. Grivei

QUANTUM TRANSPORT IN A MULTIWALLED CARBON NANOTUBE

We report on electrical resistance measurements of an individual carbon nanotube down to a temperature T = 20 mK. The conductance exhibits a lnT dependence and saturates at low temperature. A magnetic field applied perpendicular to the tube axis increases the conductance and produces aperiodic fluctuations. The data find a global and coherent interpretation in terms of two-dimensional weak localization and universal conductance fluctuations in mesoscopic conductors. The dimensionality of the electronic system is discussed in terms of the peculiar structure of carbon nanotubes.

A silicon-on-insulator quantum wire

Thin, narrow silicon-on-insulator n-channel MOSFETs have been fabricated. The drain current characteristics, when measured as a function of gate voltage at low temperature, exhibit a series of oscillations, which is characteristic of current transport in one-dimensional systems (quantum wires). Theoretical calculation of the current oscillations in the device show reasonable agreement with the experimental characteristics.

Giant low temperature heat capacity of GaAs quantum wells near Landau level filling nu = 1.

We report low temperature (T) heat capacity (C) data on a multiple-quantum-well GaAs/AlGaAs sample in the quantum Hall regime. Relative to its low field magnitude, C exhibits up to similar to 10(5)-fold enhancement near v = 1 where Skyrmions are the ground state of the confined two-dimensional electrons. We attribute the large C to a Skyrmion-induced, strong coupling of the nuclear spin system to the lattice. The data are consistent with the Schottky nuclear heat capacity of Ga and As atoms in the quantum wells, except at very low T where C vs T exhibits a remarkably sharp peak, suggestive of a phase transition in the electronic system.

Specific-heat of Fullerenes Extract

The specific heat of fullerenes extract (C60/C70) has been measured in the temperature range 4 < T < 560 K. The temperature variation of the specific heat exhibits an anomalous behaviour. Below 80 K, it presents a classical temperature variation with a dependence close to a T3 below 10 K. The measurements bring two transitions to the fore : one around 250 K corresponding to the well known orientational phase transition, and the other around 90 K which is attributed to the freezing of the rotational motion of the molecules.

Heat capacity evidence for the suppression of skyrmions at large Zeeman energy.

Measurements on a multilayer two-dimensional electron system (2DES) near Landau-level filling nu = 1 reveal the disappearance of the nuclear spin contribution to the heat capacity as the ratio (g) over tilde between the Zeeman and Coulomb energies exceeds a critical value (g) over tilde(c) approximate to 0.04. This disappearance suggests the vanishing of the Skyrmion-mediated coupling between the lattice and the nuclear spills as the spin excitations of the 2DES make a transition from Skyrmions to single spin flips; above (g) over tilde(c). Our experimental g, is smaller than the calculated (g) over tilde(c) = 0.054 for an ideal 2DES. We discuss possible origins of this discrepancy. [S0031-9007(99)08835-3].

Thermal anomalies and the insulator-metal (I-M) transition in Mn3+/Mn4+ perovskites

We report a comparative study of two ferromagnets-Pr0.7Sr0.1Ca0.2MnO3 and Pr0.85K0.15MnO3-which exhibit the same formal Mn valency, similar T-c and saturated moments at 5 K. While for former sample the magnetic transition is accompanied by the insulator-metal transition at T similar to T-c, the insulator-metal (I-M) transition is not observed in the latter one despite the expectation based on the Goldschmidts tolerance factor. The first order character of the I-M transition in Pr0.7Sr0.1Ca0.2MnO3 is demonstrated by a discontinuity of the unit cell volume, step of the specific heat and huge enhancement of the thermal conductivity. The lack of these anomalies in Pr0.85K0.15MnO3 suggests that the properties of perovskite manganites are critically influenced by both a strong phonon-electron coupling arising from a Jahn-Teller splitting of e(g) orbitals of the Mn3+ ion and the local lattice effects due to size mismatch of the large cations

Thermal and Thermoelectric Properties of Granular Co-ag Solids

We report measurements on the temperature and magnetic field dependences of the thermal conductivity, thermoelectric power, and specific heat of granular Co20Ag80 solids annealed at various temperatures. Giant magnetothermal conductivity and giant magnetothermopower are found to be correlated with the giant magnetoresistance. Thermal conductivity is dominated by its electronic contribution, and the Wiedemann-Franz law is found to be satisfied between 2 and 300 K, indicating that large-angle electron scattering processes dominate. The thermoelectric power is negative and its magnitude is considerably reduced by the annealing process. The contrasting relationship between electrical resistivity and thermoelectric power on annealing and in applied field is curved and discussed. No marked variation of specific heat with magnetic field in relation to the giant magnetoresistance has been observed. At very low temperatures, i.e. below T similar to 1 K, a nuclear contribution to the specific heat appears, whose temperature and magnetic field dependences have been explored.

Magnetization of an incompressible two-dimensional electron gas.

We have measured the magnetization of several single layer, high mobility 2D electron gases, using an ultra-sensitive torque magnetometer. Sharp diamagnetic jumps in the magnetization are observed when an integer number of Landau levels is filled. We relate these jumps to incompressibility gaps in the energy spectrum on the order of the cyclotron energy, in which a small fraction of the total number of states is present. We show that a simple hypothesis for the density of states with almost no Landau level overlap accounts for the observed shape of the magnetization oscillations, The spin-splitting of the Landau levels is observed as well, in particular at v = 1, and gives a measure of the enhancement of the g-factor

Giant heat capacity and nuclear-spin diffusion in GaAs/AlGaAs heterostructures near ν=1

We report low temperature (T) heat capacity (C) data on a multiple-quantum-well GaAs/AlGaAs sample in the quantum-Hall regime. Relative to its low field magnitude, C exhibits up to approximate to 10(5)-fold enhancement near Landau level filling v = 1 where skyrmions are the expected ground state of the confined two-dimensional electrons. We attribute this striking behavior to a skyrmion-induced enhancement of the coupling between the nuclear spin system and the lattice. The data are consistent with the Schottky nuclear heat capacity of Ga and As atoms in the quantum wells, except at very low T where C vs. T exhibits a remarkably sharp peak suggestive of a phase transition in the electronic system. Our quasi-adiabatic experiments provide quantitative evidence that the sharp peak in C vs. T is due to an enhanced nuclear-spin diffusion from the GaAs quantum wells into the AlGaAs barriers. We discuss the physical origin of this enhancement in terms of the possible Skyrme solid-liquid phase transition

The effect of Zeeman energy on heat capacity of GaAs/AlGaAs heterostructures near nu = 1.

By controlling the ratio eta between the Zeeman and Coulomb energies, the heat capacity of a multilayer two-dimensional electron system (2DES) is measured near Landau level filing v = 1 at 60 mK. The data reveal the disappearance of the nuclear spin contribution of GaAs quantum wells to the heat capacity as eta exceeds a critical value eta(c) approximate to 0.03. This result is interpreted as the effect of the transition from Skyrmions to single electron spin flips at large Zeeman energy