Competition between Kondo Screening and Quantum Hall Edge Reconstruction.

We report on a Kondo correlated quantum dot connected to two-dimensional leads where we demonstrate the renormalization of the g factor in the pure Zeeman case. i.e., for magnetic fields parallel to the plane of the quantum dot. For the same system, we study the influence of orbital effects by investigating the quantum Hall regime; i.e., a perpendicular magnetic field is applied. In this case an unusual behavior of the suppression of the Kondo effect and of the split zero-bias anomaly is observed. The splitting decreases with magnetic field and shows discontinuous changes that are attributed to the intricate interplay between Kondo screening and the quantum Hall edge structure originating from electrostatic screening. This edge structure, made up of compressible and incompressible stripes, strongly affects the Kondo temperature of the quantum dot and thereby influences the renormalized g factor.

Strong quantum memory at resonant Fermi edges revealed by shot noise.

Studies of non-equilibrium current fluctuations enable assessing correlations involved in quantum transport through nanoscale conductors. They provide additional information to the mean current on charge statistics and the presence of coherence, dissipation, disorder, or entanglement. Shot noise, being a temporal integral of the current autocorrelation function, reveals dynamical information. In particular, it detects presence of non-Markovian dynamics, i.e., memory, within open systems, which has been subject of many current theoretical studies. We report on low-temperature shot noise measurements of electronic transport through InAs quantum dots in the Fermi-edge singularity regime and show that it exhibits strong memory effects caused by quantum correlations between the dot and fermionic reservoirs. Our work, apart from addressing noise in archetypical strongly correlated system of prime interest, discloses generic quantum dynamical mechanism occurring at interacting resonant Fermi edges.

Interaction-induced spin polarization in quantum dots.

The electronic states of lateral many-electron quantum dots in high magnetic fields are analyzed in terms of energy and spin. In a regime with two Landau levels in the dot, several Coulomb-blockade peaks are measured. A zigzag pattern is found as it is known from the Fock-Darwin spectrum. However, only data from Landau level 0 show the typical spin-induced bimodality, whereas features from Landau level 1 cannot be explained with the Fock-Darwin picture. Instead, by including the interaction effects within spin-density-functional theory a good agreement between experiment and theory is obtained. The absence of bimodality on Landau level 1 is found to be due to strong spin polarization.

Universal oscillations in counting statistics.

Noise is a result of stochastic processes that originate from quantum or classical sources. Higher-order cumulants of the probability distribution underlying the stochastic events are believed to contain details that characterize the correlations within a given noise source and its interaction with the environment, but they are often difficult to measure. Here we report measurements of the transient cumulants n(m) of the number n of passed charges to very high orders (up to m = 15) for electron transport through a quantum dot. For large m, the cumulants display striking oscillations as functions of measurement time with magnitudes that grow factorially with m. Using mathematical properties of high-order derivatives in the complex plane we show that the oscillations of the cumulants in fact constitute a universal phenomenon, appearing as functions of almost any parameter, including time in the transient regime. These ubiquitous oscillations and the factorial growth are system-independent and our theory provides a unified interpretation of previous theoretical studies of high-order cumulants as well as our new experimental data.

Prediction of periodontal pathology around third molars using linear mixed effects modeling.

OBJECTIVE: To compare the random intercept multilevel model with other linear mixed effects models in an assessment of the effect of quadrant-, jaw-, and person level covariates on probing depth of asymptomatic third molars. BASIC RESEARCH DESIGN: Five different covariance models were considered: 1) the random intercept multilevel 2) multi-level with unequal jaw variance 3) multi-level with unequal tooth variance 4) multi-level with unequal jaw and side variance and 5) the general linear model for correlated data with unstructured covariance matrix. PARTICIPANTS: 235 subjects with all four third molars erupted were included. Fifty-one percent were female and 75% Caucasian. The average age was 29.1 years (sd = 7.0). RESULTS: The extended multi-level with unequal residual variance was the best fit to the data. Likelihood ratio tests in a stepdown selection approach resulted in a final model for mean probing depth that included one statistically significant three-way interaction (age x gingival inflammation x gender), two statistically significant two-way interactions (jaw x gingival inflammation and jaw x gender) and one significant main effect (ethnicity). CONCLUSIONS: Linear mixed effects modeling is a powerful tool for the analysis of correlated dental data. However, no one covariance structure is appropriate for all purposes.

Noise enhancement due to quantum coherence in coupled quantum dots.

We show that the intriguing observation of noise enhancement in the charge transport through two vertically coupled quantum dots can be explained by the interplay of quantum coherence and strong Coulomb blockade. We demonstrate that this novel mechanism for super-Poissonian charge transfer is very sensitive to decoherence caused by electron-phonon scattering as inferred from the measured temperature dependence.

Multiple transitions of the spin configuration in quantum dots.

Single electron tunneling is studied in a many electron quantum dot in high magnetic fields. For such a system multiple transitions of the spin configuration are theoretically predicted. With a combination of spin blockade and Kondo effect we are able to detect five regions with different spin configurations. Transitions are induced with changing electron numbers.

Enhanced shot noise in tunneling through a stack of coupled quantum dots.

We have investigated the noise properties of the tunneling current through vertically coupled self-assembled InAs quantum dots. We observe super-Poissonian shot noise at low temperatures. For increased temperature this effect is suppressed. The super-Poissonian noise is explained by capacitive coupling between different stacks of quantum dots.

Giant anisotropy of Zeeman splitting of quantum confined acceptors in Si/Ge.

Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are characterized by resonant-tunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light-hole--heavy-hole splitting caused by a large biaxial strain and a strong quantum confinement in the Ge quantum well.

Thermophilic-anaerobic digestion to produce class A biosolids: initial full-scale studies at Hyperion Treatment Plant.

The highest quality of biosolids is called exceptional quality. To qualify for this classification, biosolids must comply with three criteria: (1) metal concentrations, (2) vector-attraction reduction, and (3) the Class A pathogen-density requirements. The City of Los Angeles Bureau of Sanitation Hyperion Treatment Plant (HTP) (Playa del Rey, California) meets the first two requirements. Thus, the objective of this study was to ensure that HTP's biosolids production would meet the Class A pathogen-reduction requirements following the time-temperature regimen for batch processing (U.S. EPA, 1993; Subsection 32, Alternative 1). Because regulations require the pathogen limits to be met at the last point of plant control, biosolids sampling was not limited to immediately after the digesters, i.e., the digester outflows. The sampling extended to several locations in HTP's postdigestion train, in particular, the last points of plant control, i.e., the truck loading facility and the farm for land application. A two-stage, thermophilic-continuous-batch process, consisting of a battery of six egg-shaped digesters, was established in late 2001 for phase I of this study and modified in early 2002 for phase II. As the biosolids were discharged from the second-stage digesters, the Salmonella sp. (pathogen) and fecal-coliform (indicator) densities were well below the limits for Class A biosolids, even though the second-stage-digester temperatures were a few degrees below the temperature required by Alternative 1. Salmonella sp. densities remained below the Class A limit at all postdigestion sampling locations. Fecal-coliform densities were also below the Class A limit at postdigestion-sampling locations, except the truck-loading facility (phases I and II) and the farm for final use of the biosolids (phase II). Although federal regulations require one of the limits for either fecal coliforms or Salmonella sp. to be met, local regulations in Kern County, California, where the biosolids are land-applied, require compliance with both bacterial limits. Additional work identified dewatering, cooling of biosolids after the dewatering centrifuges, and contamination as possible factors in the rise in density of fecal coliforms. These results provided the basis for the full conversion of HTP to the Los Angeles continuous-batch, thermophilic-anaerobic-digestion process. During later phases of testing, this process was demonstrated to produce fully disinfected biosolids at the farm for land application.

Probing a Kondo-correlated quantum dot with spin spectroscopy.

We investigate the Kondo effect and spin blockade observed in a many-electron quantum dot and study the magnetic field dependence. At lower fields, a pronounced Kondo effect is found, which is replaced by the spin blockade at higher fields. In an intermediate regime, both effects are visible. We make use of this combined effect to gain information about the internal spin configuration of our quantum dot. We find that the data cannot be explained assuming regular filling of electronic orbitals. Instead, spin polarized filling seems to be probable.

Spin noise spectroscopy in GaAs.

We observe the noise spectrum of electron spins in bulk GaAs by Faraday-rotation noise spectroscopy. The experimental technique enables the undisturbed measurement of the electron-spin dynamics in semiconductors. We measure exemplarily the electron-spin relaxation time and the electron Landé g factor in -doped GaAs at low temperatures and find good agreement of the measured noise spectrum with a theory based on Poisson distribution probability.

Effects of transient temperature increases on odor production from thermophilic anaerobic digestion.

The City of Los Angeles, Bureau of Sanitation, has implemented thermophilic anaerobic sludge digestion at the Hyperion and Terminal Island Treatment Plants (HTP and TITP). A two-stage continuous-batch process was established at HTP, while a single-stage sequencing batch process was established at TITP. This was to evaluate compliance with the Class A pathogen reduction requirements of U.S. EPA 40 CFR Part 503. A rapid increase of the digester temperature at TITP from 57.5 to 65.5 degrees C caused an increase of the volatile fatty acid to alkalinity ratio, a decline in digester performance, and an elevated production of methyl mercaptan and hydrogen sulfide. A rapid increase of the digester temperature at HTP from 54 to 58 degrees C caused an elevated production of methyl mercaptan, but the effect on the volatile fatty acid to alkalinity ratio and digester performance was insignificant. It is likely that these effects observed at TITP and HTP were transient responses to rapid changes in temperature.

Spin-orbit coupling and anisotropy of spin splitting in quantum dots.

In lateral quantum dots, the combined effect of both Dresselhaus and Bychkov-Rashba spin-orbit coupling is equivalent to an effective magnetic field +/- B(SO) which has the opposite sign for s(z)= +/- 1/2 spin electrons. When the external magnetic field is perpendicular to the planar structure, the field B(SO) generates an additional splitting for electron states as compared to the spin splitting in the in-plane field orientation. The anisotropy of spin splitting has been measured and then analyzed in terms of spin-orbit coupling in several AlGaAs/GaAs quantum dots by means of resonant tunneling spectroscopy. From the measured values and sign of the anisotropy we are able to determine the dominating spin-orbit coupling mechanism.

Observation of an interedge magnetoplasmon mode in a degenerate two-dimensional electron gas.

We study the propagation of edge magnetoplasmons by time-resolved current measurements in a sample which allows for selective detection of edge states in the quantum Hall regime. At filling factors close to nu=3 we observe two decoupled modes of edge excitations, one of which is related to the innermost compressible strip and is identified as an interedge magnetoplasmon mode. From the analysis of the propagation velocities of each mode the internal spatial parameters of the edge structure are derived.

Phonon excitations of composite-fermion landau levels.

Phonon excitations of fractional quantum Hall states at filling factors nu = 1/3, 2/5, 4/7, 3/5, 4/3, and 5/3 are experimentally shown to be based on Landau-level transitions of composite fermions. At filling factor nu = 2/3, however, a linear field dependence of the excitation energy in the high-field regime rather hints towards a spin transition excited by the phonons. We propose to explain this surprising observation by an only partially polarized 2/3 ground state, making the energetically lower lying spin transition also allowed for phonon excitations.

Direct measurement of the g factor of composite fermions.

The activation gap Delta of the fractional quantum Hall states at constant fillings nu=2/3 and 2/5 has been measured as a function of the perpendicular magnetic field B. A linear dependence of Delta on B is observed while approaching the spin-polarization transition. This feature allows a direct measurement of the g factor of composite fermions which appears to be heavily renormalized by interactions and strongly sensitive to the electronic filling factor.

Kondo effect in a few-electron quantum ring.

A small quantum ring with less than ten electrons was studied by transport spectroscopy. For strong coupling to the leads a Kondo effect is observed and used to characterize the spin structure of the system in a wide range of magnetic fields. At small magnetic fields Aharonov-Bohm oscillations influenced by Coulomb interaction appear. They exhibit phase jumps by pi at the Coulomb-blockade resonances. Inside Coulomb-blockade valleys the Aharonov-Bohm oscillations can also be studied due to the finite conductance caused by the Kondo effect. Astonishingly, the maxima of the oscillations show linear shifts with increasing magnetic field and gate voltage.

Influence of high-energy photon beam irradiation on pacemaker operation.

In this paper, we determine the influence of high-energy photon beam irradiation used for external radiotherapy on pacemakers, at different doses and dose rates. Ninety-six pacemakers of various origins and ages underwent in vitro high-energy photon irradiation under technical conditions close to external radiotherapy, using a linear accelerator delivering photons of high energy (18 MV). Various dose levels (up to 200 Gy for certain particularly resistant pacemakers) were delivered to pacemakers located in a water-equivalent phantom with several dose rates (from 0.05 to 8 Gy min(-1)). Observed failures were sorted into eight classes of progressive harmfulness, some of them possibly lethal. One irradiated pacemaker exhibited an important defect at a dose rate of 0.2 Gy min(-1), for a cumulative dose of 0.15 Gy. Two pacemakers showed an important defect at a cumulative dose of 1 Gy, while nine pacemakers failed at a cumulative dose lower than or equal to 2 Gy and 13 failed at a cumulative dose lower than or equal to 5 Gy. The most important failure probability (70% of irradiated pacemakers) is observed for 8 Gy min(-1), whereas no pacemakers failed at a dose rate lower than or equal to 0.2 Gy min(-1). In conclusion, warnings given by manufacturers about the maximum tolerable cumulative radiation doses for safe operation of irradiated pacemakers (5 Gy), even reduced to 2 Gy, are not reliable. The spread of cumulative doses inducing failures is very large since our observations show an important failure at 0.15 Gy, while ten pacemakers withstood more than 140 Gy of cumulative dose. The safe operation of pacemakers under irradiation depends mainly on type and model. It depends also on dose rate. From our observations, for the safe operation of pacemakers, a recommendation of a maximum dose rate of 0.2 Gy min(-1) rejecting direct irradiation of the pacemaker at a standard dose rate for tumour treatment (2 Gy min(-1)) is made.

Hopping conductivity in the quantum Hall effect: revival of universal scaling.

We have measured the temperature dependence of the conductivity sigma(xx) of a two-dimensional electron system deep into the localized regime of the quantum Hall plateau transition. Using variable-range hopping theory we extract directly the localization length xi from this experiment. We use our results to study the scaling behavior of xi as a function of the filling factor distance /deltanu/ to the critical point of the transition. We find for all samples a power-law behavior xi equivalent to /deltanu/(-gamma) in agreement with the theoretically proposed universal exponent gamma = 2.35.