We present measurements of the dissipation and frequency shift in gold nanomechanical resonators at temperatures down to 10 mK. The resonators were fabricated as doubly clamped beams above a GaAs substrate and actuated magnetomotively. Measurements on beams with frequencies 7.95 and 3.87 MHz revealed that from 30 to 500 mK the dissipation increases with temperature as T 0.5 , with saturation occurring at higher temperatures. The relative frequency shift of the resonators increases logarithmically with temperature up to at least 400 mK. Similarities with the behavior of bulk amorphous solids suggest that the dissipation in our resonators is dominated by two-level systems.

Pure spin currents are measured in micron-wide channels of GaAs two-dimensional electron gas (2DEG). Spins are injected and detected using quantum point contacts, which become spin polarized at high magnetic field. High sensitivity to the spin signal is achieved in a nonlocal measurement geometry, which dramatically reduces spurious signals associated with charge currents. Measured spin relaxation lengths range from 30µm to 50µm, much longer than has been reported in GaAs 2DEG's. The technique developed here provides a flexible tool for the study of spin polarization and spin dynamics in mesoscopic structures defined in 2D semiconductor systems.

We present results from a study of the nonlinear inter-modal coupling between different flexural vibrational modes of a single high-stress, doubly-clamped silicon nitride nanomechanical beam. Using the magnetomotive technique and working at 100 mK we explored the nonlinear behaviour and modal couplings of the first, third and fifth modes of a 25.5 µm long beam. We find very good agreement between our results and a simple analytical model which assumes that the different modes of the resonator are coupled to each other by displacement induced tension in the beam. The small size of our resonator leads to relatively strong nonlinear couplings, for example we find a shift of about 7 Hz in the third mode for a 1 nm displacement in the first mode and frequency shifts ∼20 times larger than the linewidth (130 Hz) are readily observed.

We report a large spin-polarized current injection from a ferromagnetic metal into a nonferromagnetic semiconductor, at a temperature of 100 Kelvin. The modification of the spin-injection process by a nanoscale step edge was observed. On flat gallium arsenide [GaAs(110)] terraces, the injection efficiency was 92%, whereas in a 10-nanometer-wide region around a [111]-oriented step the injection efficiency is reduced by a factor of 6. Alternatively, the spin-relaxation lifetime was reduced by a factor of 12. This reduction is associated with the metallic nature of the step edge. This study advances the realization of using both the charge and spin of the electron in future semiconductor devices.

We measure the thermodynamic magnetization of a low-disordered, strongly correlated twodimensional electron system in silicon in perpendicular magnetic fields. A new, parameter-free method is used to directly determine the spectrum characteristics (Landé g factor and the cyclotron mass) when the Fermi level lies outside the spectral gaps and the interlevel interactions between quasiparticles are avoided. Intralevel interactions are found to strongly modify the magnetization, without affecting the determined g and m . DOI: 10.1103/PhysRevLett.96.046409 PACS numbers: 71.30.+h, 73.40.Qv Magnetization is one of the least studied properties of two-dimensional (2D) electron systems: signals associated with the magnetization of 2D electrons are weak, and measuring them is a challenging experiment. Few experimental observations of the de Haas-van Alphen effect in 2D electron systems were made using SQUID magnetometers [1], pick up coils lithographed above the gate [2], or torque magnetometers [3]. A novel method has recently been used by Prus et al. [4] and Shashkin et al. [5] to measure the spin magnetization of 2D electrons in silicon metaloxide-semiconductor field-effect transistors (MOSFETs). This method entails modulating the magnetic field with an auxiliary coil and measuring the imaginary (out-of-phase) component of the ac current induced between the gate and the 2D electron system, which is proportional to @ =@B (where is the chemical potential). Using the Maxwell relation, @ =@B ÿ@M=@n s , one can then obtain the magnetization M by integrating the induced current over the electron density, n s . Pauli spin susceptibility has been observed to behave critically near the 2D metal-insulator transition, in agreement with previous transport measurements [6,7].Here we apply a similar method to study the thermodynamic magnetization of a low-disordered, strongly correlated 2D electron system in silicon MOSFETs in perpendicular and tilted magnetic fields. By measuring @ =@B at noninteger filling factors, we directly determine the spectrum characteristics without any fitting procedures or parameters. As compared to previously used measuring techniques, the remarkable advantage of the novel method is that it probes the spectrum of the 2D electron system with the Fermi level lying outside the spectral gaps so that the effects of interactions between quasiparticles belonging to different energy levels (interlevel interactions) are avoided. Although intralevel interactions are found to strongly affect the magnetization, the extracted Landé g factor and the cyclotron mass are insensitive to them. Therefore, measured spectrum characteristics are likely to be identical with those of a continuous spectrum. The so-obtained g factor has been found to be weakly enhanced and practically independent of the electron density down to the lowest densities reached ( 1:5 10 11 cm ÿ2 ), while the cyclotron mass becomes strongly enhanced at low n s .Measurements were made in an Oxford dilution refrigerator on clean (100)-silicon samples...