arXiv subjects

R. J. Haug

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

72 records · Page 4Linked to original sources

arXiv2002

Shot noise in self-assembled InAs quantum dots

We investigate the noise properties of a GaAs-AlAs-GaAs tunneling structure with embedded self-assembled InAs quantum dots in the single-electron tunneling regime. We analyze the dependence of the relative noise amplitude of the shot noise on bias voltage. We observe a non-monotonic behaviour of the Fano-factor $α$ with an average value of $α\approx 0.8$ consistent with the asymmetry of the tunneling barriers. Reproducible fluctuations observed in $α$ can be attributed to the successive participation of more and more InAs quantum dots in the tunneling current.

cond-mat.mes-hall↗

arXiv2002

Conductance fluctuations at the quantum Hall plateau transition

We analyze the conductance fluctuations observed in the quantum Hall regime for a bulk two-dimensional electron system in a Corbino geometry. We find that characteristics like the power spectral density and the temperature dependence agree well with simple expectations for universal conductance fluctuations in metals, while the observed amplitude is reduced. In addition, the dephasing length $L_Φ\propto T^{-1/2}$, which governs the temperature dependence of the fluctuations, is surprisingly different from the scaling length $L_{sc}\propto T^{-1}$ governing the width of the quantum Hall plateau transition.

cond-mat.mes-hall↗

arXiv2002

Aharonov-Bohm oscillations of a tunable quantum ring

With an atomic force microscope a ring geometry with self-aligned in-plane gates was directly written into a GaAs/AlGaAs-heterostructure. Transport measurements in the open regime show only one transmitting mode and Aharonov-Bohm oscillations with more than 50% modulation are observed in the conductance. The tuning via in-plane gates allows to study the Aharonov-Bohm effect in the whole range from the open ring to the Coulomb-blockade regime.

cond-mat.mes-hall↗

arXiv2001

Hopping conductivity in the quantum Hall effect -- revival of universal scaling

We have measured the temperature dependence of the conductivity $σ_{xx}$ of a two-dimensional electron system deep into the localized regime of the quantum Hall plateau transition. Using variable-range hopping theory we are able to extract directly the localization length $ξ$ from this experiment. We use our results to study the scaling behavior of $ξ$ as a function of the filling factor distance $|δν|$ to the critical point of the transition. We find for all samples a power-law behavior $ξ\propto|δν|^{-γ}$ with a universal scaling exponent $γ= 2.3$ as proposed theoretically.

cond-mat.mes-hall↗

arXiv2001

Correlation-function spectroscopy of inelastic lifetime in heavily doped GaAs heterostructures

Measurements of resonant tunneling through a localized impurity state are used to probe fluctuations in the local density of states of heavily doped GaAs. The measured differential conductance is analyzed in terms of correlation functions with respect to voltage. A qualitative picture based on the scaling theory of Thouless is developed to relate the observed fluctuations to the statistics of single particle wavefunctions. In a quantitative theory correlation functions are calculated. By comparing the experimental and theoretical correlation functions the effective dimensionality of the emitter is analyzed and the dependence of the inelastic lifetime on energy is extracted.

cond-mat.mes-hall↗

arXiv2000

Energy Dependence of Quasi-Particle Relaxation in a Disordered Fermi Liquid

A spectroscopic method is applied to measure the inelastic quasi-particle relaxation rate in a disordered Fermi liquid. The quasi-particle relaxation rate, $γ$ is deduced from the magnitude of fluctuations in the local density of states, which are probed using resonant tunneling through a localized impurity state. We study its dependence on the excitation energy $E$ measured from the Fermi level. In a disordered metal (heavily doped GaAs) we find $γ\propto E^{3/2}$ within the experimentally accessible energy interval, in agreement with the Altshuler-Aronov theory for electron-electron interactions in diffusive conductors.

cond-mat.mes-hall↗

arXiv2000

High Frequency Conductivity in the Quantum Hall Regime

We have measured the complex conductivity $σ_{xx}$ 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 $σ_{xx}$ can be scaled to a single function for different frequencies and for all investigated 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 $ξ$. Even for large filing factor distances $δν$ from the critical point we find $ξ\propto δν^{-γ}$ with a scaling exponent $γ=2.3$.

cond-mat.mes-hall↗

arXiv2000

High frequency conductivity in the quantum Hall effect

We present high frequency measurements of the diagonal conductivity sigma_xx of a two dimensional electron system in the integer quantum Hall regime. The width of the sigma_xx peaks between QHE minima is analyzed within the framework of scaling theory using both temperature T=100-700 mK and frequency f <= 6 GHz in a two parameter scaling ansatz. For the plateau transition width we find scaling behaviour for both its temperature dependence as well as its frequency dependence. However, the corresponding scaling exponent for temperature kappa=0.42 significantly differs from the one deduced for frequency scaling (c=0.6). Additionally we use the high frequency experiments to suppress the contact resistances that strongly influences DC measurements. We find an intrinsic critical conductivity sigma_c=0.17e^2/h, virtually independent of temperature and filling factor, and deviating significantly from the proposed universal value 0.5e^2/h.

cond-mat.mes-hall↗

arXiv2000

Measurement of the energy dependence of phase relaxation by single electron tunneling

Single electron tunneling through a single impurity level is used to probe the fluctuations of the local density of states in the emitter. The energy dependence of quasi-particle relaxation in the emitter can be extracted from the damping of the fluctuations of the local density of states (LDOS). At larger magnetic fields Zeeman splitting is observed.

cond-mat.mes-hall↗

arXiv2000

Resonant tunneling through zero-dimensional impurity states: Effects of a finite temperature

We have performed temperature dependent tunneling experiments through a single impurity in an asymmetric vertical double barrier tunneling structure. In particular in the charging direction we observe at zero magnetic field a clear shift in the onset voltage of the resonant tunneling current through the impurity. With a magnetic field applied the shift starts to disappear. The experimental observations are explained in terms of resonant tunneling through a spin degenerate impurity level.

cond-mat.mes-hall↗

arXiv2000

Resonant Tunnelling through InAs Quantum Dots in Tilted Magnetic Fields: Experimental Determination of the g-factor Anisotropy

We have determined the Landé factor g* in self-organized InAs quantum dots using resonant-tunnelling experiments. With the magnetic field applied parallel to the growth direction z we find g*_\parallel = 0.75 for the specific dot investigated. When the magnetic field is tilted away by theta from the growth axis, g* gradually increases up to a value g*_\perp = 0.92 when B \perp z. Its angular dependence is found to follow the phenomenological behaviour g* (theta) = \sqrt{(g*_\parallel cos(theta)^2 + (g*_\perp sin(theta)^2}.

cond-mat.mes-hall↗

arXiv2000

Phonon emission and absorption in the fractional quantum Hall effect

We investigate the time dependent thermal relaxation of a two-dimensional electron system in the fractional quantum Hall regime where ballistic phonons are used to heat up the system to a non-equilibrium temperature. The thermal relaxation of a 2DES at $ν=1/2$ can be described in terms of a broad band emission of phonons, with a temperature dependence proportional to $T^4$. In contrast, the relaxation at fractional filling $ν=2/3$ is characterized by phonon emission around a single energy, the magneto-roton gap. This leads to a strongly reduced energy relaxation rate compared to $ν=1/2$ with only a weak temperature dependence for temperatures 150 mK $< T <$ 400 mK.

cond-mat.mes-hall↗

arXiv2000

Experimental evidence for the formation of stripe phases in Si/SiGe

We observe pronounced transport anisotropies in magneto-transport experiments performed in the two-dimensional electron system of a Si/SiGe heterostructure. They occur when an in-plane field is used to tune two Landau levels with opposite spin to energetic coincidence. The observed anisotropies disappear drastically for temperatures above 1 K. We propose that our experimental findings may be caused by the formation of a unidirectional stripe phase oriented perpendicular to the in-plane field.

cond-mat.mes-hall↗

arXiv2000

Magnetic-field-induced singularities in spin dependent tunneling through InAs quantum dots

Current steps attributed to resonant tunneling through individual InAs quantum dots embedded in a GaAs-AlAs-GaAs tunneling device are investigated experimentally in magnetic fields up to 28 T. The steps evolve into strongly enhanced current peaks in high fields. This can be understood as a field-induced Fermi-edge singularity due to the Coulomb interaction between the tunneling electron on the quantum dot and the partly spin polarized Fermi sea in the Landau quantized three-dimensional emitter.

cond-mat.mes-hall↗

arXiv2000

Spin Effects in the Local Density of States of GaAs

We present spin-resolved measurements of the local density of states in Si doped GaAs. Both spin components exhibit strong mesoscopic fluctuations. In the magnetic quantum limit, the main features of the spin-up and spin-down components of the local density of states are found to be identical apart from Zeeman splitting. Based on this observation, we introduce a mesoscopic method to measure the $g$-factor in a material where macroscopic methods are severely restricted by disorder. Differences between the spin-up and spin-down components are discussed in terms of spin relaxation due to spin-orbit coupling.

cond-mat.mes-hall↗

arXiv1998

Quantum Mott transition in a silicon quantum dot

Considering a double-barrier structure formed by a silicon quantum dot covered by natural oxide, we derive simple conditions for the conductance of the dot to become a step-like function of the number of doping atoms inside the dot, with negligible dependence on the actual position of the dopants. The found conditions are feasible in experimentally available structures.

cond-mat.mes-hall↗

arXiv1996

Magneto-optical evidence of the percolation nature of the metal-insulator transition in the 2D electron system

We compare the results of the transport and time-resolved magneto-luminescence measurements in disordered 2D electron systems in GaAs-AlGaAs heterostructures in the extreme quantum limit, in particular, in the vicinity of the metal-insulator transition (MIT). At filling factors $ν<1$, the optical signal has two components: the single-rate exponentially decaying part attributed to a uniform liquid and a power-law long-living tail specific to a microscopically inhomogeneous state of electrons. We interprete this result as a separation of the 2D electron system into a liquid and localized phases, especially because the MIT occurs strikingly close to those filling factors where the liquid occupies ${1\over 2}$ of the sample area (the percollation threshold condition in two-component media).

cond-mat↗

arXiv1995

Spectroscopy of Local Density of States Fluctuations in a Disordered Conductor (Experiment)

The local density of states of a degenerate semiconductor is investigated at low magnetic fields. In order to realize this experiment, we designed a strongly asymmetric double-barrier heterostructure with heavily doped contacts and study the resonant tunneling transport through the lowest localized (zero-dimensional) level in the quantum well. Fine structure in the tunneling current as a function of bias voltage and magnetic field images mesoscopic fluctuations of the local density of states in the emitter contact. Our quantitative analysis demonstrates that the fluctuation pattern originates from quantum interference of diffusive electron waves in the three-dimensional disordered system on the length scale of the mean free path. Since the fluctuations reflect the properties of the electron states below the Fermi level, the observed mesoscopic effect is temperature insensitive.

cond-mat↗