Experimental studies of the conductance of open quantum dots show a series of highly regular oscillations at low temperatures as the voltage applied to their defining gates is varied. Simulations of quantum transport through these dots reveal the oscillations to be correlated to the recurrence of specific groups of wave function scars. We furthermore find that nominally identical dots, differing only in the orientation of their input and output contacts, may be used to excite different families of scars, giving rise in turn to measurable transport results. [S0031-9007(99)09319-9] PACS numbers: 73.23. Ad, 85.30.Vw Semiconductor quantum dots, consisting of a submicron sized cavity and quantum point contact leads, are ideally suited for studying the influence of environmental coupling on the discrete level spectrum of quantum systems. Of particular interest here is the nature of electron transport in open dots, whose leads are configured to support a small number of propagating modes. While it is often argued that the level spectrum is continuous in such dots, recent studies have instead emphasized the role that the leads play in selectively exciting discrete dot states during transport [1][2][3][4][5]. In this Letter, we consider the sensitivity of this selection process to the nature of the coupling that is provided between the dot and its external environment. Splitgate dots with different lead orientations are fabricated and their transport properties are measured at low temperatures. When the voltage applied to their defining gates is varied, a series of regular oscillations is observed in the conductance, providing a unique signature of the couplinginduced modifications that arise in the level spectra of the dots. The details of the oscillations are found to be related to the recurrence with gate voltage of strongly scarred wave function states, whose features are sensitive to the lead configuration in the dots. We discuss these results in terms of the influence of the lead openings on electron transport in open dots.Here we summarize the results of studies of more than ten different split-gate dots, whose fabrication has been described elsewhere [6]. We focus, in particular, on the behavior exhibited by two dots with different lead configurations (see Fig. 1). The transport properties of these GaAs͞AlGaAs dots were measured in a dilution refrigerator, at a fridge temperature of 10 mK and using small constant currents with lockin detection [6]. From measurements at this temperature, the wafer mobility and carrier density were determined and were found to be 4 3 10 15 m 22 and 70 m 2 ͞V s, respectively. The effective size of the dots was estimated from the period of AharonovBohm oscillations in the edge state regime [6] and varied from 0.2 0.3 mm, depending on the value of the applied gate voltage. An upper-bound estimate of the number of electrons in the dots is therefore of the order of 100-400, for the same range of gate voltage. Another parameter inferred from magnetotransport studies was the electron ph...

We determine the phase-breaking time v. @ of electrons in ballistic quantum dots, from the aperiodic fluctuations observed in their low-temperature magnetoconductance. Our analysis shows that at temperatures close to a degree Kelvin v. @ scales roughly inversely with temperature, reminiscent of electron-electron scattering in two-dimensional disordered systems. At much lower temperatures, however, a saturation in r& is observed, with the transition between the two regimes occurring once the thermal smearing becomes smaller than the expected level spacing in the dot. We therefore suggest that the saturation results from a transition from twoto zero-dimensional transport, as the discrete level structure of the dot becomes resolved.Electron interference is an important process in determining the electrical properties of mesoscopic conductors, in which phase coherence of the electron wave function is maintained over considerable distances. ' A thorough understanding of the processes that limit phase coherence is therefore crucial to a complete description of transport in these devices. Such an understanding has already been largely achieved in disordered systems, in which electronic motion is diffusive, and in which phase breaking predominantly results from multiple electron-electron scattering at low temperatures.In contrast, the corresponding processes are less well understood in ballistic systems, although experiment has shown that phase breaking can occur via a single scattering event.No well-established theory for phase breaking exists in this regime, which is perhaps somewhat surprising, given the strong current interest in the electrical properties of low-dimensional structures. In particular, a detailed knowledge of the relevant phase-breaking processes is expected to be of importance in assessing the potential of ballistic electron devices for application in future generations of integrated circuitry.An important time scale for characterizing interference is the phase-breaking time 7. &, the time scale over which the phase of electrons is typically conserved. In this paper we therefore discuss an experimental approach for determining & in ballistic quantum dots. In particular, motivated by the studied of Marcus and co-workers, we obtain an estimate for & from the characteristics of the reproducible Auctuations observed in the low-temperature magnetoconductance. ' As is now well understood, the fluctuations result from interference between electrons ballistically confined within the dot, and their properties have recently attracted much interest as a potential probe for studying the effects of quantum chaos. In a previous study, estimates for 7. & were obtained from the Fourier spectra of the low-field fluctuations observed in a stadium-shaped dot. While the authors were able to obtain values for~& in agreement with recent studies in highmobility quantum wires, ' the approach they employ is only expected to be valid under certain restrictive conditions. In particular, the motion of classical particles in the...

Deep-leve transient spectroscopy (DLTS) of bulk traps and interface states in Si MOS diodes are theoretically studied and energy levels, capture cross-sections and spatial and energy density distributions of majority-carrier traps are measured. In P+-implanted unannealed MOS diodes, four bulk traps are measured at Ec-0.18 eV, Ec-0.20 eV, Ec-0.31 eV and Ec-0.45 eV. Their spatial distributions are found to be the same among them within experimental error and thought to be corresponding to the distribution of implanted ions qualitatively. Bulk traps are distinguished from interface states experimentally. The capture cross-section of interface states in non-implanted MOS diodes are measured to be of the order of 10-16 cm2 in the energy range of Ec-0.15 eV to Ec-0.30 eV. The interface state density measured with DLTS is found to be in a reasonable agreement with those detetmined by other methods.

An O(N) algorithm is proposed for calculating linear response functions of non-interacting electrons. This algorithm is simple and suitable to parallel-and vector-computation. Since it avoids O(N 3 ) computational effort of matrix diagonalization, it requires only O(N ) computational efforts where N is the dimension of the statevector. The use of this O(N ) algorithm is very effective since otherwise we have to calculate large number of eigenstates, i.e., the occupied one-electron states up to the Fermi energy and the unoccupied states with higher energy. The advantage of this method compared to the Chebyshev polynomial method recently developed by Wang (L.W. Wang, Phys. Rev. B 49, 10154 (1994);L.W. Wang, Phys. Rev. Lett. 73, 1039Lett. 73, (1994 ) is that our method can calculate linear response functions without any storage of huge statevectors on external storage.

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