(24). Single-crystal films are essential for devices based on superconductor, giant magnetoresistance, thermionic, piezoelectric, and ferroelectric metal oxides because the intrinsic properties of the material, rather than its grain boundaries, can be exploited. The most active crystallographic orientation can also be selected. Our results show that epitaxy can be achieved even for systems with very high lattice mismatch, and they provide a method for producing other nonequilibrium phases that cannot be accessed by traditional thermal processing. Golden, ibid. 258, 1918Golden, ibid. 258, (1992. 3. J. A. Switzer et al., ibid. 264, 1573Switzer et al., ibid. 264, (1994 A Hanbury Brown and Twiss experiment for a beam of electrons has been realized in a two-dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter to partition the incident beam into transmitted and reflected partial beams. In the nonequilibrium case the fluctuations in the partial beams are shown to be fully anticorrelated, demonstrating that fermions exclude each other. In equilibrium, the crosscorrelation of current fluctuations at two different contacts is also found to be negative and nonzero, provided that a direct transmission exists between the contacts.

We have experimentally studied shot noise of chaotic cavities defined by two quantum point contacts in series. The cavity noise is determined as ͑1͞4͒2ejIj in agreement with theory and can be well distinguished from other contributions to noise generated at the contacts. Subsequently, we have found that cavity noise decreases if one of the contacts is further opened and reaches nearly zero for a highly asymmetric cavity. Heating inside the cavity due to electron-electron interaction can slightly enhance the noise of large cavities and is also discussed quantitatively.

The surface of shallow Ga[Al]As heterostructures is locally oxidized with an atomic force microscope. The electron gas underneath the oxide is depleted. We demonstrate experimentally that these depleted regions of the two-dimensional electron gas can be made highly resistive at liquid nitrogen temperatures. Thus, local anodic oxidation of high electron mobility transistors with an atomic force microscope provides a novel method to define nanostructures and in-plane gates. Two examples, namely antidots and quantum point contacts as in-plane gate transistors have been fabricated and their performance at low temperatures is discussed.

Using magnetocapacitance data, we directly determine the chemical potential jump at low temperatures of about 25 mK when the filling factor traverses the spin gap at n 1 and the cyclotron gap at n 2. The chemical potential jump for the cyclotron gap is found to increase proportionally to the magnetic field with a slope that is determined by an effective mass 0.071m 0 . The data yield a spin gap that also increases proportionally with magnetic field and is described by an enhanced Landé factor g ഠ 5.2. This result has not been explained by existing theoretical models. [S0031-9007(97)03709-5] PACS numbers: 73.40. Kp, 73.20.Dx It is well known from early magnetotransport studies of Ref.[1] that in a two-dimensional electron system (2DES) the spin gap is much larger than the singleparticle Zeeman energy. The strong enhancement of the gap has been associated with electron-electron interaction effects [2-4] and is often described in terms of an effective "exchange-enhanced" Landé factor. In GaAs effective Landé factors are found that are roughly an order of magnitude larger than the bulk value jgj 0.44. According to the conventional theory [2], the effective Landé factor g should depend on the magnetic field as g~B 21͞2 . Following this model Smith et al. [5] indeed described relatively low-magnetic-field data from Ref.[6], making allowance for considerable overlap of the Landau levels and the finite thickness of the 2DES.More recently it has been shown that taking account of spin-spin correlations gives rise to a reduction of the spin gap that arises from formation of so-called Skyrmions [7,8]-large distortions of the spin field including many flipped spins. These are expected to be important as long as the Zeeman energy is small compared to the Coulomb exchange energy, i.e., in the weak-magneticfield limit. The Skyrmion concept has been tested using different experimental techniques like NMR and transport measurements in tilted magnetic fields and under pressure [9][10][11]. Still, the information about the spin gap, which largely comes from activation energy measurements in odd-integer quantum Hall states, remains restricted and controversial (e.g., [6,10,11]). As a matter of fact, results of such experiments should be treated with care because they yield a "mobility gap" which can, owing to disorder, be very different from the gap in the spectrum.Thus, for determining gaps in the spectrum more direct experimental methods are desirable.In the present paper we directly measure the spin gap in a 2DES by a capacitance technique on samples with field electrodes for controlling the electron density. The energy gap is deduced from the gate voltage dependence of the electron density in the 2DES. Up to a magnetic field of 16 T we find that the spin gap value is proportional to the magnetic field with good accuracy and corresponds to g ഠ 5.2. This result cannot be explained by either the model of exchange-enhanced g factor or the Skyrmion theory.We use AlGaAs-GaAs single-heterojunction samples with a metallic gate on the ...

We report on the experimental realization of a quantum point contact in a semiconductor heterostructure by lithography with an atomic force microscope (AFM). A thin, homogeneous titanium film on top of the chip surface was patterned by local anodic oxidation, induced by a current applied to an n-doped AFM tip. We demonstrate that self-aligned gate structures in the sub-micron regime can be fabricated with this technique.