Minimum Resistance Anisotropy of Epitaxial Graphene on SiC.

We report on electronic transport measurements in rotational square probe configuration in combination with scanning tunneling potentiometry of epitaxial graphene monolayers which were fabricated by polymer-assisted sublimation growth on SiC substrates. The absence of bilayer graphene on the ultralow step edges of below 0.75 nm scrutinized by atomic force microscopy and scanning tunneling microscopy result in a not yet observed resistance isotropy of graphene on 4H- and 6H-SiC(0001) substrates as low as 2%. We combine microscopic electronic properties with nanoscale transport experiments and thereby disentangle the underlying microscopic scattering mechanism to explain the remaining resistance anisotropy. Eventually, this can be entirely attributed to the resistance and the number of substrate steps which induce local scattering. Thereby, our data represent the ultimate limit for resistance isotropy of epitaxial graphene on SiC for the given miscut of the substrate.

Semiconductor quantized voltage source.

We realize and investigate an all-semiconductor quantized voltage source which generates quantized output voltages V(out) = f(h/e) linked only to two fundamental constants, the electron's charge e and Planck's constant h, and to an applied excitation frequency f. The device is based on an integrated quantized circuit of a single-electron pump operated at pumping frequency f and a quantum Hall device monolithically integrated in series. Robust output voltages up to several µV are generated, which are expected to be scalable by orders of magnitude using present technology. The device might open a new route towards the closure of the quantum metrology triangle.

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.

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.

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.

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.

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.

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.

Dynamical scaling of the quantum Hall plateau transition.

Using different experimental techniques, we examine the dynamical scaling of the quantum Hall plateau transition in a frequency range f=0.1-55 GHz. We present a scheme that allows for a simultaneous scaling analysis of these experiments and all other data in literature. We observe a universal scaling function with an exponent kappa=0.5+/-0.1, yielding a dynamical exponent z=0.9+/-0.2.

High frequency conductivity in the quantum hall regime.

We have measured the complex conductivity sigma(xx) of a two-dimensional electron system in the quantum Hall regime up to frequencies of 6 GHz at electron temperatures below 100 mK. Using both its imaginary and real part we show that sigma(xx) can be scaled to a single function for different frequencies and several transitions between plateaus in the quantum Hall effect. Additionally, the conductivity in the variable-range hopping regime is used for a direct evaluation of the localization length xi. Even for large filling factor distances deltanu from the critical point we find xi approximately equals deltanu(-gamma) with a scaling exponent gamma = 2.3.

[Determination of trenbolone and testosterone residues in meat by high pressure liquid chromatography (author's transl)]. / Nachweis von Trenbolon- und Testosteron-Rückständen in Fleisch durch Hochdruckflüssigkeitschromatographie.

A new method for the qualitative and quantitative determination of trenbolone and testosterone residues in meat, liver, and kidney is described. The analytical procedure consists of the following steps: homogenisation of the meat sample with tetrahydrofurane; liquid-liquid partition first between acetonitrile and hexane, then between sodium hydroxide and petroleum ether/benzene; purification of a silica gel column. The purified sample is analysed by high pressure liquid chromatography using silica gel packings. Detection and quantitative determination are performed with a fluorescence and/or a spectrophotometric detector. Fluorescence measurements are carried out applying a self-made silica gel packed flow-cell. The detection limit in meat extracts is 50 ppb for testosterone and 5 ppb for trenbolone.

[Detection of trenbolone residues in meat by thin layer chromatography and fluorimetry (author's transl)]. / Nachweis von Trenbolonrückständen in Fleisch durch Dünnschichtchromatographie und Fluorimetrie.

An new method for the qualitative and quantitative determination of trenbolone residues in meat, liver and kidney is described. The analytical procedure consists of the following steps: comminution of the meat sample; homogenisation with tetrahydrofurane; liquid-liquid partition first between acetonitril and hexane then between sodium hydroxide solution and petroleum ether/benzene; chromatography on a silica gel column. The detection on the TLC-plate is performed with 366 nm ultra violet light. The quantitative determination is carried out by means of a remission spectral fluorimeter directly on the plate. The detection limit in meat extracts was 5 ppb.

[Detection of estrogen residues in meat by thin layer chromatography and fluorimetry (author's transl)]. / Nachweis von Ostrogenrückständen in Fleisch durch Dünnschichtchromatographie und Fluorimetrie.

A new method for the qualitative and quantitative determination of six anabolic substances is described. The analytical procedure consists of the following steps: comminution of the meat sample; homogenisation with tetrahydrofurane; liquid-liquid partition between sodium hydroxide solution and petroleum ether/benzene; chromatography of the estrogen containing fraction on Sephadex LH-20; derivatization with Dansyl chloride; and finally thin layer chromatography. The qualitative estimation on the TLC-plate is performed with 366 nm ultra violet light. The quantitative determination is carried out either by fluorimetry after elution of the substances from the plate, or by means of a remission spectral fluorimeter directly on the plate. The detection limit of the qualitative estimation in meat extracts was 5--10 ppb. A minimum concentration of 20--50 ppb is necessary for quantitative determination.