We report on a comprehensive combined experimental and theoretical study of Curie temperature trends in ͑Ga,Mn͒As ferromagnetic semiconductors. Broad agreement between theoretical expectations and measured data allows us to conclude that T c in high-quality metallic samples increases linearly with the number of uncompensated local moments on Mn Ga acceptors, with no sign of saturation. Room temperature ferromagnetism is expected for a 10% concentration of these local moments. Our magnetotransport and magnetization data are consistent with the picture in which Mn impurities incorporated during growth at interstitial Mn I positions act as double-donors and compensate neighboring Mn Ga local moments because of strong nearneighbor Mn Ga u Mn I antiferromagnetic coupling. These defects can be efficiently removed by post-growth annealing. Our analysis suggests that there is no fundamental obstacle to substitutional Mn Ga doping in high-quality materials beyond our current maximum level of 6.8%, although this achievement will require further advances in growth condition control. Modest charge compensation does not limit the maximum Curie temperature possible in ferromagnetic semiconductors based on ͑Ga,Mn͒As.

We present a combined theoretical and experimental study of

PACS. 73.60 -Electronic properties of thin films. PACS. 72.20P -Thermoelectric effects. PACS. 73.40G -Tunnelling: general.Abstract. -The thermopower of a quantum dot, defmed in the two-dimensional electron gas in a GaAs-Alj-Ga! _ ^As heterostructure, is investigated using a current heating technique. At lattice temperatures k^T much smaller than the charging energy e 2 /C, and at small heating currents, sawtoohlike thermopower oscillations are observed äs a function of gate voltage, in agreement with a recent theory. In addition, a remarkable sign reversal of the amplitude of the thermopower oscillations is found in the non-linear regime at large heating currents.Single-electron tunnelling [1] is the dominant mechanism governing the transport properties of a quantum dot that is weakly coupled to reservoirs by tunnel barriers. At temperatures T such that k B T«e 2 /C, with C the capacitance of the dot, it leads the Coulomb-blockade oscillations in the conductance äs a function of the voltage applied to a capacitively coupled gate electrode [2]. Whereas the conductance has been studied extensively, the thermo-electric properties of a quantum dot remain essentially unexplored. Amman et al. have studied theoretically the role of Coulomb interactions on thermo-electric effects in a single mesoscopic tunnel junction, and have used their results to Interpret the thermopower of granulär thin bismuth films [3]. Recently, a theory was developed for the thermopower of a quantum dot in the Coulomb-blockade regime [4]. This theory predicts sawtoothlike oscillations in the thermopower äs a function of the Fermi energy in the reservoirs, with an amplitude that is determined by the charging energy and temperature only.Here, we present an experimental study of the thermo-electric properties of a quantum dot, using the current-heating technique applied previously to study the thermovoltage across a quantum point-contact [5]. At low lattice temperatures and small heating currents,

We show, by SQUID magnetometry, that in (Ga,Mn)As films the in-plane uniaxial magnetic easy axis is consistently associated with particular crystallographic directions and that it can be rotated from the [-110] direction to the [110] direction by low temperature annealing. We show that this behavior is hole-density-dependent and does not originate from surface anisotropy. The presence of uniaxial anisotropy as well its dependence on the hole-concentration and temperature can be explained in terms of the p-d Zener model of the ferromagnetism assuming a small trigonal distortion.Comment: 4 pages, 6 Postscript figures, uses revtex

We report the observation of quantum size efFects in the thermal conductance and Peltier coefficient of a quantum point contact. Our experimental method involves a novel usage of quantum point contacts as local electron gas thermometers.PACS numbers: 73.50.Lw, 72.20.Pa, 73.40.Kp An electrical current I through a conductor is accompanied by a thermal current Q. These currents are driven by the differences in voltage (6V) and temperature (hT) across the conductor. In the regime of linear response, this is conventionally expressed as [1] (1)Here R is the electrical resistance of the conductor, 8 the thermopower, II the Peltier coefficient, and rc the thermal conductance. The cross coefficients S and II are related by the Kelvin-Onsager relation II = ST.(3)In recent years, epitaxial growth and nanofabrication techniques have made it possible to study the regime of quantum ballistic transport [2]. Ideally, in this regime, transport is governed by collisions of charge carriers with the boundaries of the conductor, rather than with impurities or defects. In addition, the lateral size of the conductor can be made comparable to the Fermi wavelength, so that it acts as an electron waveguide.An experimental realization of such an electron waveguide is the quantum point contact (QPC), an electrostatically defined narrow constriction in the twodimensional electron gas (2DEG) of an (Al, Ga)As heterostructure. Electron transport through the point contact is ballistic since the mean free path is much larger than the size of the constriction. In the wider regions on either side of the QPC the transport is diffusive. These regions have a very small resistance (compared to that of the QPC) and may be regarded as electron gas reservoirs [2]. The most striking manifestation of quantum ballistic transport is the quantization in units of 2e /It of the electrical conductance of a QPC [3,4]. This quantization is observed as a series of plateaus in a plot of the conductance versus the voltage on the gate electrodes defining the constriction; the conductance quantum 2e2/It corresponds to a perfectly transmitted one-dimensional (1D) subband or transverse mode in the QPC. Theoretically, it has been argued that quantum size effects in a QPC due to the 1D subband structure should occur as well for the thermal conductance, the thermopower, and the Peltier coefficient [1, 5 -8]. Indeed, quantum oscillations in the thermopower, which line up with the steps between the plateaus in the conductance, have recently been observed experimentally [9]. The conductance and thermopower of a QPC can both be measured, on applying the appropriate current or temperature difference, simply with a voltmeter. A measurement of the Peltier coefficient and the thermal con ductance requires, in addition, a means of detecting the heat flow Q through the constriction. In this Letter, we present a method by which this is possible. We infer Q from the resulting change in the local temperature bT of the electron gas on one or both sides of the QPC.These temperature chan...