Three-stage nucleation and growth of Ge self-assembled quantum dots (SAQDs) on a relaxed SiGe buffer layer has been studied. Plastic relaxation of the SiGe buffer layer is associated with a network of buried 60° dislocations leading to an undulating strain field. As a result, the surface possesses three different types of sites for the nucleation and growth of Ge SAQDs: over the intersection of two perpendicular buried dislocations, over a single dislocation line, and in the region beyond one diffusion length away from any dislocation. Ge SAQDs are observed to nucleate exclusively over the dislocation intersections first, followed by over single dislocation lines, and finally in the region far away from dislocations. By increasing the Ge coverage at a slow rate, the prenucleation stage at the various sites is observed. It appears that the varying strain field has a significant effect on both the diffusion of Ge adatoms before SAQD nucleation, as well as the shape evolution of the SAQDs after they form. Moreover, two distinctly different self-assembly mechanisms are observed at different sites. There exist denuded zones free of Ge SAQDs adjacent to dislocation lines. The width of the denuded zone can be used to make direct determination of the Ge adatom diffusion lengths. The partially relaxed substrate provides a useful experimental vehicle for the in-depth understanding of the formation mechanism of SAQDs grown epitaxially in the Stranski-Krastanov growth mode. © 2003 The American Physical Society.

We experimentally study the phase diagram of the integer quantized Hall effect, as a function of density and magnetic field. We used a two-dimensional hole system confined in a Ge/SiGe quantum well, where all energy levels are resolved, because the Zeeman splitting is comparable to the cyclotron energy. At low fields and close to the quantum Hall liquid-to-insulator transition, we observe the floating up of the lowest energy level due to the disorder broadening, but no floating of any higher levels, rather a merging of these levels into the insulating state. For a given filling factor, only direct transitions between the insulating phase and higher quantum Hall liquids are observed as a function of density. Finally, we observe a peak in the critical resistivity around a filling factor of 1.

We present experimental results on the quantized Hall insulator in two dimensions. This insulator, with vanishing conductivities, is characterized by the quantization (within experimental accuracy) of the Hall resistance in units of the quantum unit of resistance, h/e2. The measurements were performed in a two-dimensional hole system, confined in a Ge/SiGe quantum well, when the magnetic field is increased above the ν = 1 quantum Hall state. This quantization leads to a nearly perfect semicircle relation for the diagonal and Hall conductivities. Similar results are obtained with a higher-mobility n-type modulation-doped GaAs/AlGaAs sample, when the magnetic field is increased above the ν = 1/3 fractional quantum Hall state.

Quite generally, an insulator is theoretically defined by a vanishing conductivity tensor at the absolute zero of temperature. In classical insulators, such as band insulators, vanishing conductivities lead to diverging resistivities. In other insulators, in particular when a high magnetic field (B) is added, it is possible that while the magneto-resistance diverges, the Hall resistance remains finite, which is known as a Hall insulator. In this letter we demonstrate experimentally the existence of another, more exotic, insulator. This insulator, which terminates the quantum Hall effect series in a two-dimensional electron system, is characterized by a Hall resistance which is approximately quantized in the quantum unit of resistance h/e^2. This insulator is termed a quantized Hall insulator. In addition we show that for the same sample, the insulating state preceding the QHE series, at low-B, is of the HI kind.

We report in this Letter our recent low-temperature transport results in a Si/SiGe quantum well with moderate peak mobility. An apparent metal-insulating transition is observed. Within a small range of densities near the transition, the conductivity σ displays a nonmonotonic temperature dependence. After an initial decrease at high temperatures, σ first increases with decreasing temperature T, showing a metallic behavior. As T continues decreasing, a downturn in σ is observed. This downturn shifts to a lower T at higher densities. More interestingly, the downturn temperature shows a power-law dependence on the mobility at the downturn position, suggesting that a similar downturn is also expected to occur deep in the apparent metallic regime at albeit experimentally inaccessible temperatures. This thus hints that the observed metallic phase in 2D systems might be a finite temperature effect.

We examine the magnetic-field-driven insulator-quantum-Hall-insulator transitions of the two-dimensional hole gas in a Ge/SiGe quantum well. We observe direct transitions between low and high magnetic-field insulators and the (Formula presented) quantum Hall state. With increasing magnetic field, the transitions from insulating to quantum Hall and quantum Hall to insulating are very similar with respect to their transport properties. We address the temperature dependence around the transitions and show that the characteristic energy scale for the high-field transition is larger. © 1997 The American Physical Society.

In this paper, we have experimentally investigated the theoretical predictions of VO-Zni to be a native donor in ZnO. Intrinsically zinc-rich n-type ZnO thin films having ND ∼ 6.23 × 1018 cm-3 grown by molecular beam epitaxy on Si (0 0 1) substrate were annealed in oxygen environment at 500-800 °C, keeping a step of 100 °C for 1 h, each. Room temperature Hall measurements demonstrated that free donor (VO-Zni) concentration decreased exponentially and Arrhenius plot yielded activation energy to be 1.2 ± 0.01 eV. This value is in agreement with theoretically reported activation energy of VO-Zni donor complex in ZnO. We argue; this observation can be explained by two-step process: (i) incoming oxygen fills VO of VO-Zni complex leaving behind Zni; (ii) Zni releases its energy and moves to a lower energy state with respect to the conduction band minima and/or occupies an inactive location. Consequently, Zni-VO complex loses its donor role in the lattice. Our experimental data supported theoretical predictions of VO-Zni to be a native donor. Results from photoluminescence spectroscopy carried out on Zn-rich ZnO additionally justify the existence of VO-Zni complex. © 2013 IOP Publishing Ltd.

We have investigated the valley splitting of two-dimensional electrons in high-quality Si Si1-x Gex heterostructures under tilted magnetic fields. For all the samples in our study, the valley splitting at filling factor ν=3 (Δ3) is significantly different before and after the coincidence angle, at which energy levels cross at the Fermi level. On both sides of the coincidence, a linear dependence of Δ3 on the electron density was observed, while the slope of these two configurations differs by more than a factor of 2. We argue that screening of the Coulomb interaction from the low-lying filled levels, which also explains the observed spin-dependent resistivity, is responsible for the large difference of Δ3 before and after the coincidence. © 2006 The American Physical Society.

The processes ψ(2S) → I π+ π−, γK+ K−, and γpp¯ have been studied using a sample of 3.79 × 106 ψ(2S) decays. We determine the total width of the χc0 to be γtotχc0 = 14.3 ± 2.0 ± 3.0 MeV. We present the first measurement of the branching fraction B(χc0 → pp¯) = (15.9 ± 4.3 ± 5.3) × 10−5, where the first error is statistical and the second one is systematic. Branching fractions of χc0, 2 → π+ π− and K+ K− are also reported. © 1998 American Physical Society.

The Beijing Spectrometer (BES) detector is a general purpose solenoid detector at the Beijing Electron Positron Collider in Beijing, China. The recent upgrade of the initial BES detector to the improved BES II detector is described. A new drift chamber, time-of-flight counter, luminosity monitor compatible with a low-β lattice was built as well as the vertex chamber.