D. Schuh

Universität Regensburg, Ratisbon, Bavaria, Germany

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Publications (295)1010.16 Total impact

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    ABSTRACT: We investigate the incorporation of manganese into self-catalyzed GaAs nanowires grown in molecular beam epitaxy. Our study reveals that Mn accumulates in the liquid Ga droplet and that no significant incorporation into the nanowire is observed. Using a sequential crystallization of the droplet, we then demonstrate a deterministic and epitaxial growth of MnAs segments at the nanowire tip. This technique may allow the seamless integration of multiple room-temperature ferromagnetic segments into GaAs nanowires with high-crystalline quality.
    No preview · Article · Jan 2016 · Nano Letters
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    ABSTRACT: The control of nuclear spins in quantum dots is essential to explore their many-body dynamics and exploit their prospects for quantum information processing. We present a unique combination of dynamic nuclear spin polarization and electric-dipole-induced spin resonance in an electrostatically defined double quantum dot (DQD) exposed to the strongly inhomogeneous field of two on-chip nanomagnets. Our experiments provide direct and unrivaled access to the nuclear spin polarization distribution and allow us to establish and characterize multiple fixed points. Further, we demonstrate polarization of the DQD environment by nuclear spin diffusion which significantly stabilizes the nuclear spins inside the DQD.
    No preview · Article · Dec 2015 · Physical Review B
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    ABSTRACT: In solids, the high density of charged particles makes many-body interactions a pervasive principle governing optics and electronics. However, Walter Kohn found in 1961 that the cyclotron resonance of Landau-quantized electrons is independent of the seemingly inescapable Coulomb interaction between electrons. Although this surprising theorem has been exploited in sophisticated quantum phenomena, such as ultrastrong light–matter coupling, superradiance and coherent control, the complete absence of nonlinearities excludes many intriguing possibilities, such as quantum-logic protocols. Here, we use intense terahertz pulses to drive the cyclotron response of a two-dimensional electron gas beyond the protective limits of Kohn’s theorem. Anharmonic Landau ladder climbing and distinct terahertz four- and six-wave mixing signatures occur, which our theory links to dynamic Coulomb effects between electrons and the positively charged ion background. This new context for Kohn’s theorem unveils previously inaccessible internal degrees of freedom of Landau electrons, opening up new realms of ultrafast quantum control for electrons.
    No preview · Article · Nov 2015 · Nature Physics
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    ABSTRACT: A method is presented that enables the measurement of spin-orbit coefficients in a diffusive two-dimensional electron gas without the need for processing the sample structure, applying electrical currents or resolving the spatial pattern of the spin mode. It is based on the dependence of the average electron velocity on the spatial distance between local excitation and detection of spin polarization, resulting in a variation of spin precession frequency that in an external magnetic field is linear in the spatial separation. By scanning the relative positions of the exciting and probing spots in a time-resolved Kerr rotation microscope, frequency gradients along the [100] and [010] crystal axes of GaAs/AlGaAs QWs are measured to obtain the Rashba and Dresselhaus spin-orbit coefficients, α and β. This simple method can be applied in a variety of materials with electron diffusion for evaluating spin-orbit coefficients.
    No preview · Article · Oct 2015 · Applied Physics Letters
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    ABSTRACT: We investigate experimentally and theoretically the interference at avoided crossings which are repeatedly traversed as a consequence of an applied ac field. Our model system is a charge qubit in a serial double quantum dot connected to two leads. Our focus lies on effects caused by simultaneous driving with two different frequencies. We work out how the commensurability of the driving frequencies affects the symmetry of the interference patterns both in real space and in Fourier space. For commensurable frequencies, the symmetry depends sensitively on the relative phase between the two modes, whereas for incommensurable frequencies the symmetry of monochromatic driving is always recovered.
    Preview · Article · Oct 2015 · Physical Review B
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    ABSTRACT: Dynamic nuclear spin polarization (DNSP) and electric-dipole-induced spin resonance (EDSR) are major tools to control the nuclear and electron spin dynamics in semiconductor nanostructures. Their local control can be massively enhanced in an inhomogeneous magnetic field. We use two on-chip single-domain nanomagnets to create a static inhomogeneous field across a double quantum dot (DQD) in a GaAs/AlGaAs heterostructure. A unique combination of DNSP and EDSR experiments allows us to establish at least four fixed points (FP) characterized by their individual distributions of nuclear spins. The polarizations inside the DQD can be significantly stabilized using nuclear spin diffusion.
    No preview · Article · Aug 2015
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    ABSTRACT: Understanding the decoherence of electron spins in semiconductors due to their interaction with nuclear spins is of fundamental interest as they realize the central spin model and of practical importance for using electron spins as qubits. Interesting effects arise from the quadrupolar interaction of nuclear spins with electric field gradients, which have been shown to suppress diffusive nuclear spin dynamics. One might thus expect them to enhance electron spin coherence. Here we show experimentally that for gate-defined GaAs quantum dots, quadrupolar broadening of the nuclear Larmor precession can also reduce electron spin coherence due to faster decorrelation of transverse nuclear fields. However, this effect can be eliminated for appropriate field directions. Furthermore, we observe an additional modulation of spin coherence that can be attributed to an anisotropic electronic $g$-tensor. These results complete our understanding of dephasing in gated quantum dots and point to mitigation strategies. They may also help to unravel unexplained behaviour in related systems such as self-assembled quantum dots and III-V nanowires.
    Preview · Article · Aug 2015
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    ABSTRACT: We investigate the angular dependence of the tunneling anisotropic magnetoresistance in (Ga,Mn)As/n-GaAs spin Esaki diodes in the regime where the tunneling process is dominated by the excess current through midgap states in (Ga, Mn) As. We compare it to similar measurements performed in the regime of band-to-band tunneling. Whereas the latter show biaxial symmetry typical for magnetic anisotropy observed in (Ga, Mn) As samples, the former is dominated by uniaxial anisotropy along the < 110 > axes. (C) 2015 AIP Publishing LLC.
    Preview · Article · Jun 2015 · Applied Physics Letters
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    ABSTRACT: We investigate both, experimentally and theoretically, commensurability oscillations in the low-field magnetoresistance of lateral superlattices with broken inversion symmetry. We find that pronounced minima develop in the resistivity &${{\rho }_{xx}}$; when the flat band conditions of several relevant harmonics of the periodic potential nearly coincide.
    No preview · Article · Apr 2015 · New Journal of Physics
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    ABSTRACT: On-chip magnets can be used to implement relatively large local magnetic field gradients in na- noelectronic circuits. Such field gradients provide possibilities for all-electrical control of electron spin-qubits where important coupling constants depend crucially on the detailed field distribution. We present a double quantum dot (QD) hybrid device laterally defined in a GaAs / AlGaAs het- erostructure which incorporates two single domain nanomagnets. They have appreciably different coercive fields which allows us to realize four distinct configurations of the local inhomogeneous field distribution. We perform dc transport spectroscopy in the Pauli-spin blockade regime as well as electric-dipole-induced spin resonance (EDSR) measurements to explore our hybrid nanodevice. Characterizing the two nanomagnets we find excellent agreement with numerical simulations. By comparing the EDSR measurements with a second double QD incorporating just one nanomagnet we reveal an important advantage of having one magnet per QD: It facilitates strong field gradients in each QD and allows to control the electron spins individually for instance in an EDSR experi- ment. With just one single domain nanomagnet and common QD geometries EDSR can likely be performed only in one QD.
    Full-text · Article · Mar 2015 · Physical Review B
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    ABSTRACT: We report on a magneto-photoluminescence (PL) study of Mn modulation-doped InAs/InGaAs/InAlAs quantum wells. Two PL lines corresponding to the radiative recombination of photoelectrons with free and bound-on-Mn holes have been observed. In the presence of a magnetic field applied in the Faraday geometry both lines split into two circularly polarized components. While temperature and magnetic field dependences of the splitting are well described by the Brillouin function, providing an evidence for exchange interaction with spin polarized manganese ions, the value of the splitting exceeds the expected value of the giant Zeeman splitting by two orders of magnitude for a given Mn density. Possible reasons of this striking observation are discussed.
    Full-text · Article · Feb 2015 · Journal of Applied Physics
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    ABSTRACT: We use GaAs-based quantum point contacts as mesoscopic detectors to locally analyze the flow of photogenerated electrons in a two-dimensional electron gas (2DEG) at perpendicular, quantizing magnetic fields. The 2DEG is formed within a quantum well of a doped GaAs/AlGaAs-heterostructure. We find an optoelectronic signal along the lateral boundaries of the 2DEG, which is consistent with an optically induced quantum transport through quantum Hall edge channels. We demonstrate that photogenerated electrons can be directly injected into an edge channel, transported across several tens of micrometers and read-out on-chip by the quantum point contact.
    Preview · Article · Feb 2015 · New Journal of Physics
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    ABSTRACT: When an electric current passes across a potential barrier, the partition process of electrons at the barrier gives rise to the shot noise, reflecting the discrete nature of the electric charge. Here we report the observation of excess shot noise connected with a spin current which is induced by a nonequilibrium spin accumulation in an all-semiconductor lateral spin-valve device. We find that this excess shot noise is proportional to the spin current. Additionally, we determine quantitatively the spin-injection-induced electron temperature by measuring the current noise. Our experiments show that spin accumulation driven shot noise provides a novel means of investigating nonequilibrium spin transport.
    Full-text · Article · Jan 2015 · Physical Review Letters
  • M Oltscher · M Ciorga · M Utz · D Schuh · D Bougeard · D Weiss
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    ABSTRACT: We report on spin injection into a high mobility 2D electron system confined at an (Al,Ga)As/GaAs interface, using (Ga,Mn)As Esaki diode contacts as spin aligners. We measured a clear nonlocal spin valve signal, which varies nonmonotonically with the applied bias voltage. The magnitude of the signal cannot be described by the standard spin drift-diffusion model, because at maximum this would require the spin polarization of the injected current to be much larger than 100%, which is unphysical. A strong correlation of the spin signal with contact width and electron mean free path suggests that ballistic transport in the 2D region below ferromagnetic contacts should be taken into account to fully describe the results.
    No preview · Article · Dec 2014 · Physical Review Letters
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    ABSTRACT: Quantum point contacts (QPCs) and quantum dots (QDs), two elementary building blocks of semiconducting nanodevices, both exhibit famously anomalous conductance features: the 0.7-anomaly in the former case, the Kondo effect in the latter. For both the 0.7-anomaly and the Kondo effect, the conductance shows a remarkably similar low-energy dependence on temperature $T$, source-drain voltage $V_{\rm sd}$ and magnetic field $B$. In a recent publication [F. Bauer et al., Nature, 501, 73 (2013)], we argued that the reason for these similarities is that both a QPC and a KQD feature spin fluctuations that are induced by the sample geometry, confined in a small spatial regime, and enhanced by interactions. Here we further explore this notion experimentally and theoretically by studying the geometric crossover between a QD and a QPC, focussing on the $B$-field dependence of the conductance. We introduce a one-dimensional model that reproduces the essential features of the experiments, including a smooth transition between a Kondo QD and a QPC with 0.7-anomaly. We find that in both cases the anomalously strong negative magnetoconductance goes hand in hand with strongly enhanced local spin fluctuations. Our experimental observations include, in addition to the Kondo effect in a QD and the 0.7-anomaly in a QPC, Fano interference effects in a regime of coexistence between QD and QPC physics, and Fabry-Perot-type resonances on the conductance plateaus of a clean QPC. We argue that Fabry-Perot-type resonances occur generically if the electrostatic potential of the QPC generates a flatter-than-parabolic barrier top.
    Full-text · Article · Sep 2014 · Physical Review B
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    ABSTRACT: Since the prediction of the spin Hall effect more than 40 years ago, significant progress was made in theoretical description as well as in experimental observation, especially in the last decade. In this article, we present three different concepts and measurement geometries for all-electrical detection of the direct and the inverse spin Hall effect in semiconductors. Based on experiments with n- and p-doped GaAs microstructures, we describe our experimental approaches and methods to experimentally identify the spin Hall effect and compare our results to previous experiments and theoretical considerations. Device geometry for the detection of the direct spin Hall effect.
    Full-text · Article · Sep 2014 · physica status solidi (b)
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    Full-text · Dataset · Aug 2014
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    ABSTRACT: Due to its p-like character, the valence band in GaAs-based heterostructures offers rich and complex spin-dependent phenomena. One manifestation is the large anisotropy of Zeeman spin splitting. Using undoped, coupled quantum wells (QWs), we examine this anisotropy by comparing the hole spin dynamics for high- and low-symmetry crystallographic orientations of the QWs. We directly measure the hole $g$ factor via time-resolved Kerr rotation, and for the low-symmetry crystallographic orientations (110) and (113a), we observe a large in-plane anisotropy of the hole $g$ factor, in good agreement with our theoretical calculations. Using resonant spin amplification, we also observe an anisotropy of the hole spin dephasing in the (110)-grown structure, indicating that crystal symmetry may be used to control hole spin dynamics.
    Preview · Article · Aug 2014 · Physical Review B
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    ABSTRACT: When electric current passes across a potential barrier, the partition process of electrons at the barrier gives rise to the shot noise, reflecting the discrete nature of electric charge. Here, beyond this conventional charge shot noise, we bring to light "spin" shot noise associated with spin current that is induced by nonequilibrium spin-dependent chemical potentials in an all-semiconductor lateral spin valve device. We prove that the detected spin shot noise is proportional to the spin current and the resultant Fano factor directly evidences that the spin degree of freedom is preserved in the tunneling process. This demonstrated spin shot noise can serve as a unique probe of spin noise spectroscopy to explore nonequilibrium spin transport.
    Full-text · Article · Jul 2014
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    ABSTRACT: We present an inverted GaAs 2D electron gas with self-assembled InAs quantum dots in close proximity, with the goal of combining quantum transport with quantum optics experiments. We have grown and characterized several wafers -- using transport, AFM and optics -- finding narrow-linewidth optical dots and high-mobility, single subband 2D gases. Despite being buried 500 nm below the surface, the dots are clearly visible on AFM scans, allowing precise localization and paving the way towards a hybrid quantum system integrating optical dots with surface gate-defined nanostructures in the 2D gas.
    Full-text · Article · Mar 2014

Publication Stats

4k Citations
1,010.16 Total Impact Points

Institutions

  • 2001-2015
    • Universität Regensburg
      • • Institute of Experimental and Applied Physics
      • • Fakultät für Physik
      Ratisbon, Bavaria, Germany
  • 2002-2010
    • Technische Universität München
      • Walter Schottky Institut (WSI)
      München, Bavaria, Germany
  • 2005-2006
    • University Hospital Regensburg
      Ratisbon, Bavaria, Germany