Linear temperature dependence of conductivity in the "insulating" regime of dilute two-dimensional holes in GaAs

The conductivity of extremely high mobility dilute two-dimensional holes in GaAs changes linearly with temperature in the insulating side of the metal-insulator transition. Hopping conduction, characterized by an exponentially decreasing conductivity with decreasing temperature, is not observed when the conductivity is smaller than $e^{2}/h$. We suggest that strong interactions in a regime close to the Wigner crystallization must be playing a role in the unusual transport.Comment: 3 pages, 2 figure

Non-linear magnetoconductivity of the two-dimensional electron fluid and solid on liquid helium

This thesis describes an experimental investigation of the properties of two dimensional electrons above the surface of superfluid "4He. The measurements presented were obtained in the 2D electron solid state below the melting temperature T_m and in the fluid phase above T_m. The longitudinal magnetoconductivity #sigma#_x_x (B) of two dimensional electron sheet on liquid helium was measured using the ac Sommer-Tanner technique, in which an array of six co-planar electrodes was capacitively coupled to the electrons at low frequencies (f < 100 kHz). Precisely defined Corbino disk geometry electrodes were obtained using the optical lithography techniques at Southampton University. Experiments were performed in the temperature range 0.05#<=#T#<=#1.2K, low magnetic fields B#<=#0.5T, frequencies 1#<=#f#<=#100kHz, drive voltages 1#<=#V_0#<=#1000mV for electron densities n #approx# 10"1"2 m"-"2. In the fluid phase at low magnetic fields, Drude behaviour is seen with #sigma#_x_x (B) #propor to# 1/B"2 , where only a small non-linearity was observed. At low temperatures (100-300 mK) a sharp transition in #sigma#_x_x (B) is observed at #GAMMA#_m = 136, which is interpreted as the phase transition to a 2D electron solid (#GAMMA# = e"2 #sq root##pi#n/(4#pi##epsilon#_H_e#epsilon#_0k_BT) is the ratio of the potential to the kinetic energy of the electrons). The melting temperature T_m, was found to be independent of magnetic field (below approximately 1T) at low drive voltages. However at higher magnetic fields (or drive voltages), the transition at T_m is much less pronounced. At low magnetic fields, the magnetoconductivity #sigma#_x_x (B) of a classical 2D electron crystal on superfluid "4He was found to be non-linear. Experimentally, the magnetoconductivity can be expressed as #sigma#_x_x (B) = K f V_0/#nu#_HBh, where K is a constant, h is the helium depth and #nu#_H is the Hall velocity. For increasing V_0, the Hall velocity #nu#_H reaches a limiting value equal to the ripplon velocity v_1 at the first reciprocal lattice vector G_1 of the electron crystal. This strong non-linearity arises from coherent many-electron Bragg-Cherenkov emission of ripplons on the helium surface with wave vector q close to the first reciprocal lattice vector G_1 of the electron solid, as explained by Dykman and Rubo (1997a). At high drive, the drag force on the electrons diverges as #nu#_H approaches #nu#_1. The magnetoconductivity then decreases sharply for #nu#_H > #nu#_1, which gives a sharp increase in the third harmonic current. Shirahama and Kono (1995a) interpreted this as a dynamical transition in which the electrons collectively slide out of the periodic deformation of the helium surface at some threshold value of the excitation voltage, to form a solid without coupled plasmon ripplon (CPR) modes. Theoretically predicted effects (Rubo and Lea, 1998) from the coupling to the diffusive shear waves were not observed experimentally. We conclude that the solid is not rotating rigidly but must be polycrystalline, that slip occurs and that our measurements give the true conductivity #sigma#_x_x (B) at the low experimental frequencies. (author)Available from British Library Document Supply Centre-DSC:DXN038629 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Hall-velocity limited magnetoconductivity in a 2D Wigner solid

The magnetoconductivity s(B) of a classical 2D electron crystal on superfluid4He is non-linear. Experimentally we find a contribution to s(B) which at constant field, gives s(B)8J x, the current density, while at constant current, s(B) 8 1/B. In this region the Hall velocity ¿H slowly approaches the ripplon velocity ¿I at the first reciprocal lattice vector, due to strong electron-ripplon interactions with the helium. The magnetoconductivity decreases sharply for ¿H>¿I. Fluctuations in s(B) are seen above the melting temperature

Hall-velocity limited magnetoconductivity in a classical two-dimensional Wigner crystal

The magnetoconductivity sxx(B) of a classical two-dimensional electron crystal on superfluid 4He is nonlinear. Experimentally, we find a contribution to sxx(B) which, at constant field B, gives sxx(B)¿Jx, the current density, while at constant current, sxx(B)¿1/B. For increasing Jx, the Hall velocity ¿H slowly approaches the ripplon velocity ¿l at the first reciprocal lattice vector, due to strong electron-ripplon interactions with the helium. The magnetoconductivity then decreases sharply for ¿H>¿1

Hall-velocity limited magnetoconductivity in a classical two-dimensional Wigner crystal

The magnetoconductivity sxx(B) of a classical two-dimensional electron crystal on superfluid 4He is nonlinear. Experimentally, we find a contribution to sxx(B) which, at constant field B, gives sxx(B)¿Jx, the current density, while at constant current, sxx(B)¿1/B. For increasing Jx, the Hall velocity ¿H slowly approaches the ripplon velocity ¿l at the first reciprocal lattice vector, due to strong electron-ripplon interactions with the helium. The magnetoconductivity then decreases sharply for ¿H>¿1

Magnetotransport of 2D electrons on liquid helium in the fluid and solid phases

The magnetoconductivity s(B) in the two-dimensional (2D) nondegenerate electron fluid and 2D solid has been analyzed theoretically and investigated experimentally, from 60 mK to 1.3 K in magnetic fieldsB up to 8 Tesla. In the fluid phase, s(B) is described by the Drude model in weak to moderately strong classical fields, including the range ßB»1. At higher fields (depending on the density s(B) is nonmonotonous and diplays a minimum. This behavior is due to many-electron effects, which can be described in terms of cyclotron orbit diffusion controlled by an internal fluctuational electric field. The squared internal field derived from experiments is in good agreement with computer simulations. In the solid phase electron transport becomes strongly non-linear even for weak driving voltagesV 0. Experimentally we determine, from the losses, the effective AC Corbino conductivity at a frequencyf. We find that s(BafV 0/B forV 0 below some threshold voltageV c . In this region the Hall velocity ¿ H approaches the ripplon phase velocityv 1=w(G 1)/G 1 at the first reciprocal lattice vectorG 1 of the electron solid. We suggest that this behaviour is due to to a resonant drag force from the Bragg-Cerenkov radiation of coherent ripplons by the moving crystal