arXiv subjects

T. Heinzel

Publications and source records attributed to T. Heinzel.

At least 37 records · Page 2Linked to original sources

arXiv2008

Edge disorder induced Anderson localization and conduction gap in graphene nanoribbons

We study the effect of the edge disorder on the conductance of the graphene nanoribbons (GNRs). We find that only very modest edge disorder is sufficient to induce the conduction energy gap in the otherwise metallic GNRs and to lift any difference in the conductance between nanoribbons of different edge geometry. We relate the formation of the conduction gap to the pronounced edge disorder induced Anderson-type localization which leads to the strongly enhanced density of states at the edges, formation of surface-like states and to blocking of conductive paths through the ribbons.

cond-mat.mes-hall↗

arXiv2008

Magnetoresistance of antidot lattices with grain boundaries

The magnetotransport properties of antidot lattices containing artificially designed grain boundaries have been measured. We find that the grain boundaries broaden the commensurability resonances and displace them anisotropically. These phenomena are unexpectedly weak but differ characteristically from isotropic, Gaussian disorder in the antidot positions. The observations are interpreted in terms of semiclassical trajectories which tend to localize along the grain boundaries within certain magnetic field intervals. Furthermore, our results indicate how the transport through superlattices generated by self-organizing templates may get influenced by grain boundaries.

cond-mat.mes-hall↗

arXiv2007

Influence of magnetic field offsets on the resistance of magnetic barriers in two-dimensional electron gases

Magnetic barriers in two-dimensional electron gases are shifted in B space by homogeneous, perpendicular magnetic fields. The magnetoresistance across the barrier shows a characteristic asymmetric dip in the regime where the polarity of the homogeneous magnetic field is opposite to that one of the magnetic barrier. The measurements are in quantitative agreement with semiclassical simulations, which reveal that the magnetoresistance originates from the interplay of snake orbits with E x B drift at the edges of the Hall bar and with elastic scattering.

cond-mat.mes-hall↗

arXiv2007

Transport properties of chemically synthesized polypyrrole thin films

The electronic transport in polypyrrole thin films synthesized chemically from the vapor phase is studied as a function of temperature as well as of electric and magnetic fields. We find distinct differences in comparison to the behavior of both polypyrrole films prepared by electrochemical growth as well as of the bulk films obtained from conventional chemical synthesis. For small electric fields F, a transition from Efros-Shklovskii variable range hopping to Arrhenius activated transport is observed at 30 K. High electric fields induce short range hopping. The characteristic hopping distance is found to be proportional to F^(-1/2). The magnetoresistance R(B) is independent of F below a critical magnetic field, above which F counteracts the magnetic field induced localization.

cond-mat.dis-nn↗

arXiv2007

Resonant reflection at magnetic barriers in quantum wires

The conductance of a quantum wire containing a single magnetic barrier is studied numerically by means of the recursive Greens function technique. For sufficiently strong and localized barriers, Fano - type reflection resonances are observed close to the pinch-off regime. They are attributed to a magnetoelectric vortex-type quasibound state inside the magnetic barrier that interferes with an extended mode outside. We furthermore show that disorder can substantially modify the residual conductance around the pinch-off regime.

cond-mat.mes-hall↗

arXiv2007

Effect of edge transmission and elastic scattering on the resistance of magnetic barriers

Strong magnetic barriers are defined in two-dimensional electron gases by magnetizing dysprosium ferromagnetic platelets on top of a Ga[Al]As heterostructure. A small resistance across the barrier is observed even deep inside the closed regime. We have used semiclassical simulations to explain this behavior quantitatively in terms of a combined effect of elastic electron scattering inside the barrier region and E x B drift at the intersection of the magnetic barrier with the edge of the Hall bar.

cond-mat.mes-hall↗

arXiv2005

Commensurability Effects in Hexagonal Antidot Lattices with Large Antidot Diameters

The observation of a novel type of commensurability resonance in two-dimensional, hexagonal antidot lattices is reported. These resonances have a classical character and occur at magnetic fields above the resonance that corresponds to the cyclotron motion around a single antidot. The resonances are visible only for antidots with effective diameters larger than 50 % of the lattice constant. Simulations reveal that they originate from quasi-stable electron trajectories that bounce between three neighboring antidots. This interpretation is backed by the observation of large-period Aharonov-Bohm type oscillations at low temperatures.

cond-mat.mes-hall↗

arXiv2001

Transport signatures of correlated disorder in a two-dimensional electron gas

We report electronic transport measurements on two-dimensional electron gases in a Ga[Al]As heterostructure with an embedded layer of InAs self-assembled quantum dots. At high InAs dot densities, pronounced Altshuler-Aronov-Spivak magnetoresistance oscillations are observed, which indicate short-range ordering of the potential landscape formed by the charged dots and the strain fields. The presence of these oscillations coincides with the observation of a metal-insulator transition, and a maximum in the electron mobility as a function of the electron density. Within a model based on correlated disorder, we establish a relation between these effects.

cond-mat.mes-hall↗

arXiv2001

Energy spectra of quantum rings

Ring geometries have fascinated experimental and theoretical physicists over many years. Open rings connected to leads allow the observation of the Aharonov-Bohm effect, a paradigm of quantum mechanical phase coherence. The phase coherence of transport through a quantum dot embedded in one arm of an open ring has been demonstrated. The energy spectrum of closed rings has only recently been analysed by optical experiments and is the basis for the prediction of persistent currents and related experiments. Here we report magnetotransport experiments on a ring-shaped semiconductor quantum dot in the Coulomb blockade regime. The measurements allow us to extract the discrete energy levels of a realistic ring, which are found to agree well with theoretical expectations. Such an agreement, so far only found for few-electron quantum dots, is here extended to a many-electron system. In a semiclassical language our results indicate that electron motion is governed by regular rather than chaotic motion, an unexplored regime in many-electron quantum dots.

cond-mat.mes-hall↗

arXiv2001

Electronic properties of antidot lattices fabricated by atomic force lithography

Antidot lattices were fabricated by atomic force lithography using local oxidation. High quality finite 20 x20 lattices are demonstrated with periods of 300 nm. The low temperature magnetoresistance shows well developed commensurability oscillations as well as a quenching of the Hall effect around zero magnetic field. In addition, we find B periodic oscillations superimposed on the classical commensurability peaks at temperatures as high as 1.7 K. These observations indicate the high electronic quality of our samples.

cond-mat.mes-hall↗

arXiv2001

Analysis of the resistance in p-SiGe over a wide temperature range

The temperature dependence of a system exhibiting a `metal-insulator transition in two dimensions at zero magnetic field' (MIT) is studied up to 90K. Using a classical scattering model we are able to simulate the non-monotonic temperature dependence of the resistivity in the metallic high density regime. We show that the temperature dependence arises from a complex interplay of metallic and insulating contributions contained in the calculation of the scattering rate $1/\td(E,T)$, each dominating in a limited temperature range.

cond-mat.str-el↗

arXiv2001

The Hanbury Brown and Twiss Experiment with Fermions

We realized an equivalent Hanbury Brown and Twiss experiment for a beam of electrons in a two dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter which is used to partition the incident beam into transmitted and reflected partial beams. The current fluctuations in the reflected and transmitted beam are fully anticorrelated demonstrating that fermions tend to exclude each other (anti-bunching). If the occupation probability of the incident beam is lowered by an additional gate, the anticorrelation is reduced and disappears in the classical limit of a highly diluted beam.

cond-mat.mes-hall↗

arXiv2000

Analysis of the temperature-dependent quantum point contact conductance in view of the metal-insulator transition in two dimensions

The temperature dependence of the conductance of a quantum point contact has been measured. The conductance as a function of the Fermi energy shows temperature-independent fixed points, located at roughly multiple integers of $e^{2}/h$. Around the first fixed point at e$^{2}$/h, the experimental data for different temperatures can been scaled onto a single curve. For pure thermal smearing of the conductance steps, a scaling parameter of one is expected. The measured scaling parameter, however, is significantly larger than 1. The deviations are interpreted as a signature of the potential landscape of the quantum point contact, and of the source-drain bias voltage. We relate our results phenomenologically to the metal-insulator transition in two dimensions.

cond-mat.mes-hall↗

arXiv2000

Single hole transistor in a p-Si/SiGe quantum well

A single hole transistor is patterned in a p-Si/SiGe quantum well by applying voltages to nanostructured top gate electrodes. Gating is achieved by oxidizing the etched semiconductor surface and the mesa walls before evaporation of the top gates. Pronounced Coulomb blockade effects are observed at small coupling of the transistor island to source and drain.

cond-mat.mes-hall↗

arXiv2000

Transport properties of quantum dots with hard walls

Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation with an atomic force microscope. This technique, in combination with top gate voltages, allows us to generate steep walls at the confining edges and small lateral depletion lengths. The confinement is characterized by low-temperature magnetotransport measurements, from which the dots' energy spectrum is reconstructed. We find that in small dots, the addition spectrum can qualitatively be described within a Fock-Darwin model. For a quantitative analysis, however, a hard-wall confinement has to be considered. In large dots, the energy level spectrum deviates even qualitatively from a Fock-Darwin model. The maximum wall steepness achieved is of the order of 0.4 meV/nm.

cond-mat.mes-hall↗

arXiv2000

Shot Noise by Quantum Scattering in Chaotic Cavities

We have experimentally studied shot noise of chaotic cavities defined by two quantum point contacts in series. The cavity noise is determined as 1/4*2e|I| in agreement with theory and can be well distinguished from other contributions to noise generated at the contacts. Subsequently, we have found that cavity noise decreases if one of the contacts is further opened and reaches nearly zero for a highly asymmetric cavity.

cond-mat.mes-hall↗

arXiv2000

Analysis of the Metallic Phase of Two-Dimensional Holes in SiGe in Terms of Temperature Dependent Screening

We find that temperature dependent screening can quantitatively explain the metallic behaviour of the resistivity on the metallic side of the so-called metal-insulator transition in p-SiGe. Interference and interaction effects exhibit the usual insulating behaviour which is expected to overpower the metallic background at sufficiently low temperatures. We find empirically that the concept of a Fermi-liquid describes our data in spite of the large r_s = 8.

cond-mat.str-el↗

arXiv2000

Signatures of spin pairing in a quantum dot in the Coulomb blockade regime

Coulomb blockade resonances are measured in a GaAs quantum dot in which both shape deformations and interactions are small. The parametric evolution of the Coulomb blockade peaks shows a pronounced pair correlation in both position and amplitude, which is interpreted as spin pairing. As a consequence, the nearest-neighbor distribution of peak spacings can be well approximated by a smeared bimodal Wigner surmise, provided that interactions which go beyond the constant interaction model are taken into account.

cond-mat.mes-hall↗