Four efficient n-type dopants have been found for ZrNiSn-based thermoelectric materials. These are Nb or Ta at the zirconium sites, and Sb or Bi at the tin sites. No suitable dopant was found for the nickel sites. In a alloy, a power factor of and a thermal conductivity of were measured at 300 K, resulting in a dimensionless figure of merit ZT = 0.12. These values are increased to and at 700 K.

Edge magnetoplasmons propagating along the smooth boundary of a two-dimensional electron gas are studied by time-resolved magnetotransport experiments. The incident pulse splits up into several pulses due to modal dispersion. The observed delay times are well described by the predictions of Aleiner and Glazman for the fundamental and acoustic modes [Phys. Rev. Lett. 72, 2935], if the quantized Hall conductance is taken into account.[S0031-9007 (96)01655-9] PACS numbers: 73.20.Mf, 73.40.Hm, 73.50.JtPlasma oscillations of a two-dimensional electron gas (2DEG) acquire remarkable properties in the presence of a perpendicular magnetic field [1]. While the excitation of bulk plasmons requires a frequency equal to or larger than the cyclotron frequency v c , the spectrum of edge excitations remains gapless. The direction of allowed k vectors for these low energy excitations is determined by the confining potential and the direction of the Lorentz force acting on the moving carriers. For a given k vector, the edge magnetoplasmon (EMP) frequency decreases with increasing magnetic field.Experimentally, EMPs were discovered as low frequency eigenmodes of a confined 2DEG realized in a semiconductor heterostructure [2] or on the surface of liquid helium [3,4]. The EMP propagation direction was detected by phase sensitive measurements of the induced voltage on small electrodes around the perimeter of the 2DEG [5]. Time-domain measurements demonstrated the unidirectionality of the EMP motion in the quantum Hall effect (QHE) and the fractional QHE regime in a more intuitive way [6]. Using a 2DEG with a screening metallic electrode close by, the propagation of EMP wave packets confined to quantum Hall edge channels was identified in time-domain measurements [7].Recently, additional edge modes were observed, which were characterized by a resonance frequency below the frequency of the conventional EMP mode. An indication of a novel EMP mode was found in the nondegenerate 2DEG on the surface of liquid helium [8], which was explained in terms of oscillations of the boundary. A different set of EMP modes, for which the resonance frequency increased with increasing magnetic field, was observed for positive ions in a circular geometry [9]. These modes were identified with the edge modes predicted by Nazin and Shikin for a circular sheet of electrons on liquid helium with a smooth boundary [10].The theoretical description of plasma oscillations in a 2DEG starts with a seemingly simple set of elementary electrodynamic equations [1,11], which have to be solved self-consistently, in order to obtain the EMP dispersion relation. Since the electric fields are not restricted to the plane of the 2DEG, the problem is truly three dimensional. Analytic solutions could be obtained in a number of limiting cases (for a review, see [1]), and it was realized early that a smooth electron density profile permits additional (so called acoustic) EMP modes [1,10]. For a 2DEG in a semiconductor heterostructure, the dispersion relation for the fundamental mode...

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We investigate the electrodynamic properties of charged edge excitations in the fractional quantum Hall regime by means of time-resolved magnetotransport. In samples with a smooth edge potential and an additional screening electrode, the propagation velocity of high frequency signals is related to the width of fractional edge channels. While the observed edge magnetoplasmons around filling factor 1͞3 hint at a similar electronic edge structure as around filling factor 1, characteristic deviations appear for other fractions, especially around n 2͞3. [S0031-9007(97)04426-8] PACS numbers: 73.40.Hm, 73.20.Mf, 73.50.Jt Although the edge channel (EC) picture [1] is by now well established in the integer quantum Hall (IQH) regime, the electronic structure in the vicinity of the edge of a two-dimensional electron gas (2DEG) under conditions of the fractional quantum Hall (FQH) effect is still under discussion [2][3][4][5]. For a typical 2DEG in a semiconductor heterostructure, the electron density at the edge drops smoothly from its bulk value to zero, the width of the transition region being several hundred nanometers. In a perpendicular magnetic field, the interplay between the smooth confining potential, the Landau quantization, and the electron-electron interaction leads to a phase separation into compressible and incompressible strips near the edge of the sample [2]. In the incompressible regions, the Fermi energy lies in an energy gap arising either from Landau quantization or spin splitting in the IQH regime, or from many-body effects in the FQH regime. In the compressible regions, the so-called edge channels, extended states exist at the Fermi energy. The EC position and width changes as a function of the applied magnetic field, and different methods have been used for a quantitative description [3][4][5][6][7]. It has been found that, in the FQH regime, the charge density profile for a smooth boundary exhibits plateaus at certain fractional filling factors [4,5] similar to the plateaus at integer filling factors, while the edge reconstruction for a steep edge leads to counterflowing edge states [3,4], i.e., the electron density in the vicinity of the edge increases beyond its bulk value, before decreasing to zero.A variety of experiments have directly probed the EC width in the IQH regime by studying the ground state properties and the low-energy edge excitations [8][9][10][11], but experiments in the FQH regime are still scarce. Evidence for a finite EC width in the FQH regime has been given by conventional magnetotransport measurements [12], and low-energy edge excitations have been observed [13,14]. In contrast to the IQH regime [10,11], no direct relationship between their dispersion v͑k͒ and the existence of fractional edge channels could be established.Here, we report on time-resolved magnetotransport measurements of a 2DEG with a smooth edge potential in order to study the dynamic edge excitations in the FQH regime. Investigations of the propagation velocity of an injected voltage pulse show sawtooth o...