(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.

Ring geometries have fascinated experimental and theoretical physicists over many years. Open rings connected to leads allow the observation of the Aharonov-Bohm effect [1], a paradigm of quantum mechanical phase coherence [2,3]. The phase coherence of transport through a quantum dot embedded in one arm of an open ring has been demonstrated [4]. The energy spectrum of closed rings [5] has only recently been analysed by optical experiments [6,7] and is the basis for the prediction of persistent currents [8] and related experiments [9-11]. Here we report magnetotransport experiments on a ring-shaped semiconductor quantum dot in the Coulomb blockade regime [12]. The measurements allow us to extract the discrete energy levels of a realistic ring, which are found to agree well with theoretical expectations. Such an agreement, so far only found for few-electron quantum dots, is here extended to a many-electron system [13]. In a semiclassical language our results indicate that electron motion is governed by regular rather than chaotic motion, an unexplored regime in many-electron quantum dots.

We study the effect of the edge disorder on the conductance of the graphene nanoribbons (GNRs). We find that only very modest edge disorder is sufficient to induce the conduction energy gap in the otherwise metallic GNRs and to lift any difference in the conductance between nanoribbons of different edge geometry. We relate the formation of the conduction gap to the pronounced edge disorder induced Anderson-type localization which leads to the strongly enhanced density of states at the edges, formation of surface-like states and to blocking of conductive paths through the ribbons.

We consider a model in which positive and negative particles diffuse in an asymmetric, CP -invariant way on a ring. The positive particles hop clockwise, the negative counterclockwise and oppositely-charged adjacent particles may swap positions. Monte-Carlo simulations and analytic calculations suggest that the model has three phases; a "pure" phase in which one has three pinned blocks of only positive, negative particles and vacancies, and in which translational invariance is spontaneously broken, a "mixed" phase with a non-vanishing current in which the three blocks are positive, negative and neutral, and a disordered phase without blocks.cond-mat/9708128 August 1997 ⋄ work done with partial support of the EC TMR programme, grant FMRX-CT96-0012

The fluctuations and the distribution of the conductance peak spacings of a quantum dot in the Coulomb-blockade regime are studied and compared with the predictions of random matrix theory (RMT). The experimental data were obtained in transport measurements performed on a semiconductor quantum dot fabricated in a GaAs-AlGaAs heterostructure. It is found that the fluctuations in the peak spacings are considerably larger than the mean level spacing in the quantum dot. The distribution of the spacings appears Gaussian both for zero and for non-zero magnetic field and deviates strongly from the RMT-predictions.PACS numbers: 73.20. Dx,73.23.Hk,05.45.+b Advanced nanofabrication techniques have made it possible to confine small numbers of electrons electrostatically within the two-dimensional electron gas (2DEG) of a semiconductor heterostructure [1,2]. Both the electric charge and energy of such "quantum dots" are quantised and hence such structures are sometimes referred to as "artificial atoms" [3,4]. In transport measurements the charging of these electron islands with single electrons leads to the observation of periodic conductance oscillations in the Coulomb-blockade regime [1]. These reflect the electrostatic coupling of the quantum dot to its environment and, additionally, they contain information about the eigenenergies and eigenfunctions of the electrons in the dot. Due to irregularities in the electrostatic confinement potential and electron-electron interactions, the corresponding classical motion of the electrons in the quantum dot can be expected to be chaotic (nonintegrable) [5,6,7]. Consequently, recent experiments have considered the peak height distribution [8,9], parametric conductance correlations [9] and level statistics [10] of a quantum dot in the Coulomb-blockade regime to test the concepts developed for the quantum mechanical description of classically chaotic systems ("quantum chaos" [11,12]). In particular random matrix theory (RMT) 1

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.

Coulomb blockade resonances are measured in a GaAs quantum dot in which both shape deformations and interactions are small. The parametric evolution of the Coulomb blockade peaks shows a pronounced pair correlation in both position and amplitude, which is interpreted as spin pairing. As a consequence, the nearest-neighbor distribution of peak spacings can be well approximated by a modified bimodal Wigner surmise, in which interactions are taken into account beyond the constant interaction model.

We investigate the crossover between weak localization and weak antilocalization in InAs nanowires of different diameters ͑75 nm-140 nm-217 nm͒. For a magnetic field applied perpendicularly to the nanowire axis, we extract the spin orbit and coherence lengths using a quasi-one-dimensional model of the conductance. We find a spin-orbit length inversely proportional to the width of the nanowire. When a parallel magnetic field is applied, we observe that the weak-antilocalization contribution is less affected by the magnetic field than in the perpendicular case.