D. Schuh

Universität Regensburg, Ratisbon, Bavaria, Germany

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Publications (281)948.8 Total impact

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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.
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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.
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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.
    New Journal of Physics 02/2015; 17(2). DOI:10.1088/1367-2630/17/2/023007 · 3.67 Impact Factor
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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.
    Physical Review Letters 01/2015; 114(1):016601. DOI:10.1103/PhysRevLett.114.016601 · 7.73 Impact Factor
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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.
    Physical Review Letters 12/2014; 113(23):236602. DOI:10.1103/PhysRevLett.113.236602 · 7.73 Impact Factor
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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.
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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.
    physica status solidi (b) 09/2014; DOI:10.1002/pssb.201350206 · 1.61 Impact Factor
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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.
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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.
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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.
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    ABSTRACT: Controlling coherent interaction at avoided crossings and the dynamics there is at the heart of quantum information processing. A particularly intriguing dynamics is observed in the Landau-Zener regime, where periodic passages through the avoided crossing result in an interference pattern carrying information about qubit properties. In this Letter, we demonstrate a straightforward method, based on steady-state experiments, to obtain all relevant information about a qubit, including complex environmental influences. We use a two-electron charge qubit defined in a lateral double quantum dot as test system and demonstrate a long coherence time of T2≃200 ns, which is limited by electron-phonon interaction.
    Physical Review Letters 03/2014; 112(11):116803. DOI:10.1103/PhysRevLett.112.116803 · 7.73 Impact Factor
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    ABSTRACT: We observe a strong negative magnetoresistance at non-quantizing magnetic fields in a high mobility two-dimensional electron gas (2DEG) realized in a GaAs/Al$_{0.25}\,$Ga$_{0.75}\,$As quantum well. The negative magnetoresistance consists of a peak around zero magnetic field and a huge magnetoresistance at larger fields. The peak is attributed to the interplay of smooth disorder and rare strong scatterers. The density of the strong scatterers $n_S$ is determined from the curvature of the peak.
    Physical Review B 01/2014; 90:165434. DOI:10.1103/PhysRevB.90.165434 · 3.66 Impact Factor
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    ABSTRACT: Photoluminescence (PL) and highly circularly-polarized magneto-PL (up to 50% at 6 T) from two-step bandgap InAs/InGaAs/InAlAs quantum wells (QWs) are studied. Bright PL is observed up to room temperature, indicating a high quantum efficiency of the radiative recombination in these QW. The sign of the circular polarization indicates that it stems from the spin polarization of heavy holes caused by the Zeeman effect. Although in magnetic field the PL line are strongly circularly polarized, no energy shift between the counter-polarized PL lines was observed. The results suggest that the electron and the hole g-factor to be of the same sign and close magnitudes.
    Applied Physics Letters 01/2014; 104(10). DOI:10.1063/1.4868644 · 3.52 Impact Factor
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    ABSTRACT: We study the optically induced spin polarization, spin dephasing and diffusion in several high-mobility two-dimensional electron systems, which are embedded in GaAs quantum wells grown on (110)-oriented substrates. The experimental techniques comprise a two-beam magneto-optical spectroscopy system and polarization-resolved photoluminescence. Under weak excitation conditions at liquid-helium temperatures, we observe spin lifetimes above 100 ns in one of our samples, which are reduced with increasing excitation density due to additional, hole-mediated, spin dephasing. The spin dynamic is strongly influenced by the carrier density and the ionization of remote donors, which can be controlled by temperature and above-barrier illumination. The absolute value of the average electron spin polarization in the samples is directly observable in the circular polarization of photoluminescence collected under circularly polarized excitation and reaches values of about 5 percent. Spin diffusion is studied by varying the distance between pump and probe beams in micro-spectroscopy experiments. We observe diffusion lengths above 100 $\mu$m and, at high excitation intensity, a nonmonotonic dependence of the spin polarization on the pump-probe distance.
    Physical Review B 10/2013; 89(7). DOI:10.1103/PhysRevB.89.075424 · 3.66 Impact Factor
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    ABSTRACT: We investigate the role of localized states for electrical spin injection and spin detection in (Ga,Mn)As/GaAs lateral devices. The use of Esaki diodes as spin aligning contacts allows us to tune the relative contribution of direct and two-step tunneling via localized states in the gap of (Ga,Mn)As by changing the bias across the junction. We compare the results of measurements using three-terminal configuration, probing spin accumulation directly beneath the injecting contact, with results from nonlocal measurements, where solely the spin accumulation in GaAs channel is probed. We conclude that double-step tunneling not only generates spin accumulation in localized states but also increases the sensitivity to detect spin accumulation generated in the channel. The latter effect results in a giant enhancement of a detected spin signal, both in 3T and non-local configuration.
    Physical Review B 10/2013; 89(8). DOI:10.1103/PhysRevB.89.081307 · 3.66 Impact Factor
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    ABSTRACT: We report on spin-valve experiments in lateral spin-injection devices with different geometries of (Ga,Mn)As/GaAs spin Esaki diode contacts. We study the influence of the geometry of the contacts, i.e., their widths and the crystallographic orientation, on the magnetization reversal process and the resulting pattern observed in the spin-valve signal. We find that tuning of the magnetic anisotropy of the narrow (Ga,Mn)As stripes by means of lithographically induced anisotropic strain relaxation allows one to realize parallel, antiparallel, and even orthogonal configurations of magnetizations in injector and detector contacts. Understanding of the switching between these configurations during sweeping of the external in-plane magnetic field is crucial for a proper interpretation of the measured nonlocal spin signals.
    Physical Review B 10/2013; 88(15):155308-. DOI:10.1103/PhysRevB.88.155308 · 3.66 Impact Factor
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    ABSTRACT: Quantum point contacts are narrow, one-dimensional constrictions usually patterned in a two-dimensional electron system, for example by applying voltages to local gates. The linear conductance of a point contact, when measured as function of its channel width, is quantized in units of GQ = 2e(2)/h, where e is the electron charge and h is Planck's constant. However, the conductance also has an unexpected shoulder at ∼0.7GQ, known as the '0.7-anomaly', whose origin is still subject to debate. Proposed theoretical explanations have invoked spontaneous spin polarization, ferromagnetic spin coupling, the formation of a quasi-bound state leading to the Kondo effect, Wigner crystallization and various treatments of inelastic scattering. However, explicit calculations that fully reproduce the various experimental observations in the regime of the 0.7-anomaly, including the zero-bias peak that typically accompanies it, are still lacking. Here we offer a detailed microscopic explanation for both the 0.7-anomaly and the zero-bias peak: their common origin is a smeared van Hove singularity in the local density of states at the bottom of the lowest one-dimensional subband of the point contact, which causes an anomalous enhancement in the Hartree potential barrier, the magnetic spin susceptibility and the inelastic scattering rate. We find good qualitative agreement between theoretical calculations and experimental results on the dependence of the conductance on gate voltage, magnetic field, temperature, source-drain voltage (including the zero-bias peak) and interaction strength. We also clarify how the low-energy scale governing the 0.7-anomaly depends on gate voltage and interactions. For low energies, we predict and observe Fermi-liquid behaviour similar to that associated with the Kondo effect in quantum dots. At high energies, however, the similarities between the 0.7-anomaly and the Kondo effect end.
    Nature 08/2013; DOI:10.1038/nature12421 · 42.35 Impact Factor
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    ABSTRACT: In a high mobility two-dimensional electron gas (2DEG) realized in a GaAs / Al0.3Ga0.7As quantum well we observe changes in the Shubnikov-de Haas oscillations (SdHO) and in the Hall resistance for different sample geometries. We observe for each sample geometry a strong negative magnetoresistance around zero magnetic field which consists of a peak around zero magnetic field and of a huge magnetoresistance at larger fields. The peak around zero magnetic field is left unchanged for different geometries.
    Journal of Physics Conference Series 08/2013; 456(1):2003-. DOI:10.1088/1742-6596/456/1/012003
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    ABSTRACT: We present transport measurements on a system of two lateral quantum dots in a perpendicular magnetic field. Due to edge channel formation in an open conducting region, the quantum dots are chirally coupled. When both quantum dots are tuned into the Kondo regime simultaneously, we observe a change in the temperature dependence of the differential conductance. This is explained by the RKKY exchange interaction between the two dots. As a function of bias the differential conductance shows a splitting of the Kondo resonance which changes in the presence of RKKY interaction.
    Journal of Physics Conference Series 08/2013; 456(1):2014-. DOI:10.1088/1742-6596/456/1/012014

Publication Stats

4k Citations
948.80 Total Impact Points

Institutions

  • 2001–2015
    • Universität Regensburg
      • • Institute of Experimental and Applied Physics
      • • Fakultät für Physik
      Ratisbon, Bavaria, Germany
  • 2013
    • Tohoku University
      • Institute for Materials Research
      Japan
  • 2005–2013
    • Ludwig-Maximilians-University of Munich
      • Center for Nanoscience (CeNS)
      München, Bavaria, Germany
  • 2010
    • University Hospital Regensburg
      Ratisbon, Bavaria, Germany
  • 2002–2010
    • Technische Universität München
      • Walter Schottky Institut (WSI)
      München, Bavaria, Germany
  • 2006
    • Ruhr-Universität Bochum
      Bochum, North Rhine-Westphalia, Germany