We have investigated the transport properties of one-dimensional (1D) constrictions defined by split-gates in high quality GaAs/AlGaAs heterostructures. In addition to the usual quantized conductance plateaus, the equilibrium conductance shows a structure close to 0.7(2e 2 /h), and in consolidating our previous work [K. J. Thomas et al., Phys. Rev. Lett. 77, 135 (1996)] this 0.7 structure has been investigated in a wide range of samples as a function of temperature, carrier density, in-plane magnetic field B and source-drain voltage V sd . We show that the 0.7 structure is not due to transmission or resonance effects, nor does it arise from the asymmetry of the heterojunction in the growth direction. All the 1D subbands show Zeeman splitting at high B , and in the wide channel limit the g-factor is | g |≈ 0.4, close to that of bulk GaAs. As the channel is progressively narrowed we measure an exchange-enhanced g-factor. The measurements establish that the 0.7 structure is related to spin, and that electron-electron interactions become important for the last few conducting 1D subbands.

We measure the temperature of a 2D electron gas in GaAs from the thermopower of a onedimensional ballistic constriction, using the Mott relation to confirm the calibration from the electrical conductance. Under hot electron conditions, this technique shows that the power loss by the electrons follows a T 5 dependence in the Gruneisen-Bloch regime, as predicted for acoustic phonon emission with a screened piezoelectric interaction. An independent measurement using conventional thermometry based on Shubnikov-de Haas oscillations gives a T 3 loss rate; we discuss reasons for this discrepancy.Accurate electron thermometry is needed in many aspects of low-dimensional semiconductor physics, particularly given the increasing importance of hot electron effects 1 as mesoscopic device dimensions are reduced and electron mobility increases. Surplus heat energy in a two-dimensional electron gas (2DEG) is rapidly shared amongst the carriers through electron-electron interactions, and an effective electron temperature T e is established which may be considerably higher than the crystal lattice temperature T l , to which both external thermometry and refrigeration are coupled. A measurement of the electron temperature is therefore needed to determine how an electron gas thermalizes with its surroundings. Measurements of the thermoelectric response and thermal conductivity of mesoscopic devices are also interesting in their own right, as they provide fundamental information about electronic properties which is not available from electrical transport measurements alone.Although many techniques allow the measurement of bulk lattice temperatures, the weak coupling of the electrons to their surroundings has hampered accurate measurement of the electron temperature T e . Previous techniques have employed the visibility of features in the electrical transport, notably Shubnikov-de Haas (SdH) oscillations 1-6 , but also using the zero field resistance and weak localization corrections. 7,8 Mesoscopic effects such as Coulomb blockade have also been applied as electron thermometers. 9 In this letter, we introduce a novel technique, where the thermoelectric response (thermopower) of a one-dimensional constriction is used to measure the electron temperature. Self-consistent checks confirm the validity of the technique, which we then employ to deduce the energy relaxation rate of heated electrons in a 2DEG in a GaAs/AlGaAs heterostructure, obtaining good agreement with the theory of acoustic phonon emission in the Gruneisen-Bloch regime.A schematic of the device is shown in Fig. 1, and is similar to those used in previous thermopower measurements. 10,11 The area shaded grey shows the etched mesa containing a 2DEG defined at a GaAs/AlGaAs heterojunction, 2770Å below the sur-face of a structure grown by molecular beam epitaxy. 12 From measurements of the low-field SdH oscillations and the zero field resistivity, the 2DEG has an electron density n e = 2.1 × 10 11 cm −2 and mobility of µ = 4.5 × 10 6 cm 2 V −1 s −1 at 1.5 K. Electrons in the he...

A single-particle theory due to Mott predicts a proportionality between the diffusion thermopower and the energy derivative of the logarithm of the conductance. Measurements of a ballistic 1D wire show that the Mott theory remains valid in the presence of a finite current, and that it leads to a direction-sensitive probe of electron transport. We observe an apparent violation of the Mott model at low electron densities, when there is a nonquantized plateau in the conductance at 0.7(2e 2 /h). There is as yet no successful theoretical explanation of this so called 0.7 structure, but the distinctive thermopower signature, which deviates from single-particle predictions, may provide the key to a better understanding.

The surface tension of liquid 4He is determined from the frequencies of micron wavelength capillary waves. The extrapolated zero temperature value, G = 375 + 3 #Jm -2, is in agreement with the pioneering static capillary rise determination but 6Uo higher than the more recent surface tension gravity wave measurements. Flow in the meniscus in this latter experiment is shown to mimic a surface tension correction to the dispersion relation there used which is of the same sign and magnitude as the discrepancy.