arXiv subjects

R. J. Haug

Publications and source records attributed to R. J. Haug.

At least 19 recordsLinked to original sources

arXiv2017

Strong suppression of shot noise in a feedback-controlled single-electron transistor

Feedback control of quantum mechanical systems is rapidly attracting attention not only due to fundamental questions about quantum measurements but also because of its novel applications in many fields in physics. Quantum control has been studied intensively in quantum optics but recently progress has been made in the control of solid-state qubits as well. In quantum transport only a few active and passive feedback experiments have been realized on the level of single-electrons, though theoretical proposals exist. Here we demonstrate the suppression of shot noise in a single-electron transistor, using an exclusively electronic closed-loop feedback to monitor and adjust the counting statistics. With increasing feedback response we observe a stronger suppression and faster freezing of charge current fluctuations. Our technique is analog to the generation of squeezed light with in-loop photodetection as used in quantum optics. Sub-Poisson single-electron sources will pave the way for high precision measurements in quantum transport similar to its optical or opto-mechanical equivalent.

cond-mat.mes-hall↗

arXiv2016

Strongly temperature dependent resistance of meander-patterned graphene

We have studied the electronic properties of epitaxial graphene devices patterned in a meander shape with the length up to a few centimeters and the width of few tens of microns. These samples show a pronounced dependence of the resistance on temperature. Accurate comparison with theory shows that this temperature dependence originates from the weak localization effect observed over a broad temperature range from 1.5 K up to 77 K. The comparison allows us to estimate the characteristic times related to quantum interference. In addition, a large resistance enhancement with temperature is observed at the quantum Hall regime near the filling factor of 2. Record high resistance and its strong temperature dependence are favorable for the construction of bolometric photodetectors.

cond-mat.mes-hall↗

arXiv2016

Influence of disordered edges on transport properties in graphene

The influence of plasma etched sample edges on electrical transport and doping is studied. Through electrical transport measurements the overall doping and mobility are analyzed for mono- and bilayer graphene samples. As a result the edge contributes strongly to the overall doping of the samples. Furthermore the edge disorder can be found as the main limiting source of the mobility for narrow samples.

cond-mat.mes-hall↗

arXiv2015

Linear magnetoresistance in compensated graphene bilayer

We report a nonsaturating linear magnetoresistance in charge-compensated bilayer graphene in a temperature range from 1.5 to 150 K. The observed linear magnetoresistance disappears away from charge neutrality ruling out the traditional explanation of the effect in terms of the classical random resistor network model. We show that experimental results qualitatively agree with a phenomenological two-fluid model taking into account electron-hole recombination and finite-size sample geometry.

cond-mat.mes-hall↗

arXiv2015

Low-temperature transport properties of a two-dimensional electron gas with additional impurities

We investigate the effect due to background impurities embedded in the region of two-dimensional electron gases to the magnetotransport. These impurities are achieved by homogeneously incorporating Si atoms in single quantum wells of high quality GaAs/AlGaAs heterostructures. Several materials were grown this way with different densities of Si atoms. At low temperature the mobility decreases monotonically with increasing impurity density. In materials with incorporated impurities, effects are observed with increasing temperatures. On the one hand, a minimum in mobility appears, marking a transition between scattering enhancement due to acoustic phonon and mobility enhancement due to impurity driven conductivity. The temperature at which the transition takes place decreases with increasing impurity density. On the other hand, the incorporated impurities induce a non-monotonic temperature dependence of the electron density. The magnitude of this variation rises with increasing impurity density. The theory of electron-electron interaction correction to the conductivity alone fails to explain our results.

cond-mat.mes-hall↗

arXiv2015

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 behaviour 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 which 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.

cond-mat.mes-hall↗

arXiv2015

Current dependence of the huge negative magnetoresistance in high-mobility two-dimensional electron gases

In high-mobility two-dimensional electron gases Landau levels are already formed at very small magnetic field values. Such two-dimensional electron gases show a huge negative magnetoresistance at low temperatures and an unexpected and very strong non-linear behavior with the applied current. This non-linearity depends on carrier concentration and is explained by the subtle interplay of elastic scattering within Landau levels and in between Landau levels.

cond-mat.mes-hall↗

arXiv2015

Magnetoresistance of monolayer graphene with short-range scattering

We present magnetotransport measurements at classical magnetic fields for three graphene monolayers with various levels of disorder. A square root magnetoresistance (SRMR) behavior is observed in one sample which has the characteristic sub-linear conductivity signaling on the presence of short-range disorder in this sample. No square root MR was observed in other samples where short-range scattering is inessential as it is evident from the gate voltage dependences of their conductivities. Comparing our experimental data for the sample with theoretical calculations we found a good qualitative agreement and established the conditions which should be fulfilled in graphene to observe the SRMR experimentally.

cond-mat.mes-hall↗

arXiv2015

Channel Blockade in a Two-Path Triple-Quantum-Dot System

Electronic transport through a two-path triple-quantum-dot system with two source leads and one drain is studied. By separating the conductance of the two double dot paths, we are able to observe double dot and triple dot physics in transport and study the interaction between the paths. We observe channel blockade as a result of inter-channel Coulomb interaction. The experimental results are understood with the help of a theoretical model which calculates the parameters of the system, the stability regions of each state and the full dynamical transport in the triple dot resonances.

cond-mat.mes-hall↗

arXiv2014

Magnetoresistance Induced by Rare Strong Scatterers in a High Mobility 2DEG

We observe a strong negative magnetoresistance at non-quantizing magnetic fields in a high-mobility two-dimensional electron gas (2DEG). This strong negative magnetoresistance consists of a narrow peak around zero magnetic field and a huge magnetoresistance at larger fields. The peak shows parabolic magnetic field dependence and is attributed to the interplay of smooth disorder and rare strong scatterers. We identify the rare strong scatterers as macroscopic defects in the material and determine their density from the peak curvature.

cond-mat.mes-hall↗

arXiv2012

Mixing of Edge States at a Bipolar Graphene Junction

An Atomic Force Microscope is used to locally manipulate a single layer graphene sheet. Transport measurements in this region as well as in the unmanipulated part reveal different charge carrier densities while mobilities stay in the order of 10000 cm^2/(Vs). With a global backgate, the system is tuned from a unipolar n-n' or p-p' junction with different densities to a bipolar p-n junction. Magnetotransport across this junction verifies its nature, showing the expected quantized resistance values as well as the switching with the polarity of the magnetic field. The mixing of edge states at the p-n junction is shown to be supressed at high magnetic fields.

cond-mat.mes-hall↗

arXiv2012

Aharonov-Bohm effect in an electron-hole graphene ring system

Aharonov-Bohm oscillations are observed in a graphene quantum ring with a top gate covering one arm of the ring. As graphene is a gapless semiconductor this geometry allows to study not only the quantum interference of electrons with electrons or holes with holes but also the unique situation of quantum interference between electrons and holes. The period and amplitude of the observed Aharonov-Bohm oscillations are independent of the sign of the applied gate voltage showing the equivalence between unipolar and dipolar interference.

cond-mat.mes-hall↗

arXiv2012

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.

cond-mat.mes-hall↗

arXiv2010

Magnetoresistance in a High Mobility Two-Dimensional Electron Gas

In a high mobility two-dimensional electron gas (2DEG) in a GaAs/AlGaAs quantum well we observe a strong magnetoresistance. In lowering the electron density the magnetoresistance gets more pronounced and reaches values of more than 300%. We observe that the huge magnetoresistance vanishes for increasing the temperature. An additional density dependent factor is introduced to be able to fit the parabolic magnetoresistance to the electron-electron interaction correction.

cond-mat.mes-hall↗

arXiv2010

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 zig-zag 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.

cond-mat.mes-hall↗

arXiv2010

Mobilities and Scattering Times in Decoupled Graphene Monolayers

Folded single layer graphene forms a system of two decoupled monolayers being only a few Angstroms apart. Using magnetotransport measurements we investigate the electronic properties of the two layers conducting in parallel. We show a method to obtain the mobilities for the individual layers despite them being jointly contacted. The mobilities in the upper layer are significantly larger than in the bottom one indicating weaker substrate influence. This is confirmed by larger transport and quantum scattering times in the top layer. Analyzing the temperature dependence of the Shubnikov-de Haas oscillations effective masses and corresponding Fermi velocities are obtained yielding reduced values down to 66 percent in comparison to monolayers.

cond-mat.mes-hall↗

arXiv2009

The three dimensionality of triple quantum dot stability diagrams

We present the full three dimensionality of an electrostatically calculated stability diagram for triple quantum dots. The stability diagram maps out the favored charge configuration of the system as a function of potential shifts due to gate voltages. For triple dots only a three dimensional visualization allows for the complete identification of all its components. Those are most notably the so called quadruple points where four electronic configurations are degenerate, and quantum cellular automata (QCA) processes. The exact positions of these features within the stability diagram are now revealed. Furthermore the influence on transport is studied by comparing the model with a two path triple quantum dot made with local anodic oxidation. The two path setup allows to study the influence of the dots arrangement.

cond-mat.mes-hall↗

arXiv2008

Piezoelectric exciton-acoustic phonon coupling in single quantum dots

Micro-photoluminescence spectroscopy at variable temperature, excitation intensity and energy was performed on a single InAs/AlAs self-assembled quantum dot. The exciton emission line (zero-phonon line, ZPL) exhibits a broad sideband due to exciton-acoustic phonon coupling by the deformation potential mechanism. Additionally, narrow low-energy sidebands at about 0.25 meV of the ZPL are attributed to exciton-acoustic phonon piezoelectric coupling. In lowering the excitation energy or intensity these bands gradually dominate the emission spectrum of the quantum dot while the ZPL disappears. At high excitation intensity the sidebands due to piezoelectric coupling decrease strongly and the ZPL dominates the spectrum as a consequence of screening of the piezoelectric coupling by the photocreated free carriers.

cond-mat.mtrl-sci↗