Surface Acoustic Wave Velocity Shift Research Articles

Thermomechanical behavior of surface acoustic waves in ordered arrays of nanodisks studied by near-infrared pump-probe diffraction experiments and finite element simulations

The ultrafast thermal and mechanical dynamics of a two-dimensional lattice of metallic nanodisks has been studied by near-infrared pump-probe diffraction measurements over a temporal range spanning from 100 fs to several nanoseconds. The experiments demonstrate that in these systems a surface acoustic wave (SAW), with a wave vector given by the reciprocal periodicity of the two-dimensional array, can be excited by ̃120 fs Ti:sapphire laser pulses. We unambiguously show that the observed SAW velocity shift originates from the mechanical interaction between the SAWs and the nanodisks, while the correlated SAW damping is due to the energy radiation into the substrate. In order to clarify the interaction between the nanodisks and the substrate, numerical calculations of both the elastic eigenmodes and the time-dynamics of the system, following the impulsive heating excitation by the laser, are performed. Simulations based on finite-elements analysis, together with a wavelet analysis of our experimental data, suggest the opening of a band-gap at the centre of the super-Brillouin zone. The modes at the centre of the super-Brillouin zone are excited following laser excitation, as opposed to thermal population, of the elastic modes.

Surface acoustic wave propagation and inhomogeneities in low-density two-dimensional electron systems near the metal–insulator transition

We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs heterostructure containing a two-dimensional electron system (2DES) in a low-density regime (<10 10 cm −2) at zero magnetic field. The interaction of the surface acoustic wave with the 2DES is not well described by a simple model using low-frequency conductivity measurements. We speculate that this conflict is a result of inhomogeneities in the 2DES, which become very important at low density. This has implications for the putative metal–insulator transition in two dimensions.

The acoustoelectric effect in double layer AlGaAs/GaAs 2D hole systems

The surface acoustic wave velocity shift and acoustoelectric effect have been studied in a double layer GaAs/AlGaAs 2D hole system. The transverse acoustoelectric field exhibited bipolar peaks centred at integer filling factor, in qualitative agreement with the single layer theoretical prediction. Anomalous bipolar peaks have been observed in the longitudinal acoustoelectric effect centred at ν=2 (at 0.6– 0.8 GHz ) and ν=6 and 8 (at 0.8 GHz ).

Surface acoustic wave study of a double layer AlGaAs/GaAs 2D hole system

The temperature and frequency dependence of the surface acoustic wave (SAW) velocity shift has been studied in a double layer 2DHS for 0.34–4.2K and 112–560MHz. The measured velocity shift has been compared to the velocity shift calculated from conventional transport techniques. At ν=1, 2, 4, 6 and 8 maxima in SAW velocity shift have been detected. A hysteretic feature at ν=2 has been observed in both conventional magnetotransport and SAW measurements, which is probably due to slight parallel conduction.

Surface acoustic waves (SAW) interaction with 2DES at spin-splitted Landau levels

Peculiarities associated with spin splitting of the Landau levels were observed during the study of the absorption coefficient and the velocity shift of SAW ( f=30 MHz ), interacting with two-dimensional electron system (2DES) in the GaAs/AlGaAs heterostructures ( n=1.3·10 11– 2.8·10 11 cm −2 ) at T=1.5–4.2 K in magnetic fields up to 7 T. An enhanced Landé factor has been determined: g ∗≃5 . The spin-splitted Landau band width has been assessed.

Theory of magnetoacoustic transport in modulated quantum Hall systems nearν=1/2

Motivated by the experimental results of Willett et al. [Phys. Rev. Lett. $78,$ 4478 (1997)], we develop a magnetotransport theory for the response of a two-dimensional electron gas (2DEG) in the fractional quantum Hall regime near Landau-level filling factor $\ensuremath{\nu}=1/2$ to the surface acoustic wave (SAW) in the presence of an added periodic density modulation. We assume there exists a composite fermion Fermi surface (CF-FS) at $\ensuremath{\nu}=1/2,$ which is a circle, and we show that the deformation of the circular CF-FS due to the density modulation can be at the origin of the observed transport anomalies for the experimental conditions. Our analysis is carried out particularly for the nonlocal case, which corresponds to the SAW experiments. We introduce a concrete model of a deformed CF-FS. The model permits us to explain anomalous features of the response of the modulated 2DEG to the SAW near $\ensuremath{\nu}=1/2:$ namely, the nonlinear wave-vector dependence of the electron conductivity, the appearance of peaks in the SAW velocity shift and attenuation, and the anisotropy of the effect, all of which originate from contributions to the conductivity tensor due to the regions of the originally circular CF-FS, which are flattened by the applied modulation.

Velocity Shift of Surface Acoustic Waves due to Interaction with Composite Fermions in a Modulated Structure

We study the effect of a periodic density modulation on surface acoustic wave (SAW) propagation along a 2D electron gas near Landau level filling $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$. Within the composite fermion theory, the problem is described in terms of fermions subject to a spatially modulated magnetic field and scattered by a random magnetic field. We find that a few percent modulation induces a large peak in the SAW velocity shift, as observed recently by Willett et al. As further support of this theory we find the dc resistivity to be in good agreement with recent data of Smet et al.