M. R. Wegewijs

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (52)212.36 Total impact

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    ABSTRACT: By means of electronic transport, we study the transverse magnetic anisotropy of an individual Fe$_4$ single-molecule magnet (SMM) embedded in a three-terminal junction. In particular, we determine in situ the transverse anisotropy of the molecule from the pronounced intensity modulations of the linear conductance, which are observed as a function of applied magnetic field. The proposed technique works at temperatures exceeding the energy scale of the tunnel splittings of the SMM. We deduce that the transverse anisotropy for a single Fe$_4$ molecule captured in a junction is substantially larger than the bulk value.
    07/2014;
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    ABSTRACT: We predict that an excitationless Coulomb-blockaded quantum dot develops a nonequilibrium spin-precession resonance when embedded into a spin valve. At resonance the spin-valve effect is mitigated. The resonance can be detected by stationary dI/dVb spectroscopy and by oscillations in the time-averaged current using a gate-pulsing scheme. The generic noncollinearity of the ferromagnets and junction asymmetry allows for an all-electric determination of spin injection, anisotropy of spin relaxation, and exchange field. We investigate the impact of a nearby superconductor on the resonance position.
    03/2014;
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    R. B. Saptsov, M. R. Wegewijs
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    ABSTRACT: We extend the recently developed causal superfermion approach to the real-time transport theory to time-dependent decay problems.Its usefulness is illustrated for the Anderson model of a quantum dot with tunneling rates depending on spin due to the ferromagnetic electrodes and/or spin polarization of the tunnel junction. We set up a second quantization scheme for density operators in the Liouville-Fock space constructing causal field superoperators using the fundamental physical principles of causality/probability conservation and the fermion-parity superselection (univalence). The time-dependent perturbation series for the time-evolution is renormalized by explicitly performing the wide-band limit on the superoperator level. The short and long-time reservoir correlations are shown to be tightly linked to the occurrence of causal field destruction and creation superoperators, respectively. The effective theory takes as a reference a damped local system, providing an interesting starting point for numerical calculations of memory kernels in real-time. A remarkable feature of this approach is the natural appearance of a measurable fermion-parity protected decay mode. It already can be calculated exactly in the Markovian, infinite temperature limit by leading order perturbation theory, yet persists unaltered for the finite temperature, interaction and tunneling spin polarization. Furthermore, we show how a Liouville-space analog of the Pauli principle directly leads to the exact result in the noninteracting limit: surprisingly, it is obtained in finite (second) order renormalized perturbation theory, both for the self-energy as well as the time-evolution propagator. For this limit we calculate the time-evolution of the full density operator starting from an arbitrary initial state on the quantum dot, including spin and pairing coherences and two-particle correlations.
    11/2013;
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    M. Hell, M. R. Wegewijs, D. P. DiVincenzo
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    ABSTRACT: A sensitive technique for the readout of the state of a qubit is based on the measurement of the conductance through a proximal sensor quantum dot (SQD). Here we theoretically study the coherent backaction of such a measurement on a coupled SQD-charge-qubit system. We derive Markovian kinetic equations for the ensemble-averaged state of the SQD-qubit system, expressed in the coupled dynamics of two charge-state occupations of the SQD and two qubit isospin vectors - one for each SQD charge state. We find that besides introducing dissipation, the detection also renormalizes the coherent evolution of the SQD-qubit system. Basically, if the electron on the detector has time to probe the qubit, then it also has time to fluctuate and thereby renormalize the system parameters. In particular, this induces torques on the qubit isospins, similar to the spin torque generated by the spintronic exchange field in noncollinear spin-valve structures. Secondly, we show that for a consistent description of the detection, one must also include the renormalization effects in the next-to-leading order in the electron tunneling rates, especially at the point of maximal sensitivity of the detector. Although we focus on a charge qubit model, our findings are generic for qubit readout schemes that are based on spin-to-charge conversion using a quantum-dot detector. Furthermore, our study of the stationary current through the SQD - a test measurement verifying that the qubit couples to the detector current - already reveals various significant effects of the isospin torques on the qubit. Our kinetic equations provide a starting point for further studies of the time evolution in charge-based qubit read-out. Finally, we provide a rigorous sum rule that constrains such approximate descriptions of the qubit isospin dynamics and show that it is obeyed by our kinetic equations.
    10/2013;
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    Maciej Misiorny, Michael Hell, Maarten R. Wegewijs
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    ABSTRACT: Superparamagnetism of magnetic adatoms and molecules—preferential alignment of their spins along an easy axis—is a useful effect for nanoscale applications as it prevents undesired spin reversal. The underlying magnetic anisotropy barrier—a quadrupolar energy splitting—originates from spin–orbit interaction and can nowadays be probed by electronic transport measurements. Here we predict that in a much broader class of systems, quantum dots with spins larger than 1/2, superparamagnetism can arise without spin–orbit interaction: by attaching them to ferromagnets, a quadrupolar spintronic exchange field is generated locally. It is observable by means of conductance measurements and leads to enhanced spin filtering even in a state with zero average spin. Analogously to the spintronic dipolar exchange field, giving rise to a local spin torque, the effect is susceptible to electric control and increases with tunnel coupling as well as with spin polarization.
    Nature Physics 10/2013; 9(12):801-805. · 19.35 Impact Factor
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    ABSTRACT: In this article we review aspects of charge and heat transport in interacting quantum dots and molecular junctions under stationary and time-dependent non-equilibrium conditions due to finite electrical and thermal bias. In particular, we discuss how a discrete level spectrum can be beneficial for thermoelectric applications, and investigate the detrimental effects of molecular vibrations on the efficiency of a molecular quantum dot as an energy converter. In addition, we consider the effects of a slow time-dependent modulation of applied voltages on the transport properties of a quantum dot and show how this can be used as a spectroscopic tool complementary to standard dc-measurements. Finally, we combine time-dependent driving with thermoelectrics in a double-quantum dot system - a nanoscale analogue of a cyclic heat engine - and discuss its operation and the main limitations to its performance.
    physica status solidi (b) 06/2013; · 1.49 Impact Factor
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    Michael Hell, Sourin Das, Maarten Rolf Wegewijs
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    ABSTRACT: We revisit the transport of spin-degrees of freedom across an electrically and thermally biased tunnel junction between two ferromagnets with non-collinear magnetizations. Besides the well-known charge and spin currents we show that a non-zero spin-quadrupole current flows between the ferromagnets. This tensor-valued current describes the non-equilibrium transport of spin-anisotropy relating to both local and non-local multi-particle spin correlations of the circuit. This quadratic spin-anisotropy, quantified in terms of the spin-quadrupole moment, is fundamentally a two-electron quantity. In spin-valves with an embedded quantum dot such currents have been shown to result in a quadrupole accumulation that affects the measurable quantum dot spin and charge dynamics. The spin-valve model studied here allows fundamental questions about spin-quadrupole storage and transport to be worked out in detail, while ignoring the detection by a quantum dot. This physical understanding of this particular device is of importance for more complex devices where spin-quadrupole transport can be detected. We demonstrate that, as far as storage and transport are concerned, the spin anisotropy is only partly determined by the spin polarization. In fact, for a thermally biased spin-valve the charge- and spin-current may vanish, while a pure exchange spin-quadrupole current remains, which appears as a fundamental consequence of Pauli's principle. We extend the real-time diagrammatic approach to efficiently calculate the average of multi-particle spin-observables, in particular the spin-quadrupole current. Although the paper addresses only leading order and spin-conserving tunneling we formulate the technique for arbitrary order in an arbitrary, spin-dependent tunnel coupling in a way that lends itself to extension to quantum-dot spin-valve structures.
    Physical review. B, Condensed matter 05/2013; 88(11). · 3.77 Impact Factor
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    ABSTRACT: We investigate charge and spin transport through an adiabatically driven, strongly interacting quantum dot weakly coupled to two metallic contacts with finite bias voltage. Within a kinetic equation approach, we identify coefficients of response to the time-dependent external driving and relate these to the concepts of charge and spin emissivities previously discussed within the time-dependent scattering matrix approach. Expressed in terms of auxiliary vector fields, the response coefficients allow for a straightforward analysis of recently predicted interaction-induced pumping under periodic modulation of the gate and bias voltage [Phys. Rev. Lett. 104, 226803 (2010)]. We perform a detailed study of this effect and the related adiabatic Coulomb blockade spectroscopy, and, in particular, extend it to spin pumping. Analytic formulas for the pumped charge and spin in the regimes of small and large driving amplitude are provided for arbitrary bias. In the absence of a magnetic field, we obtain a striking, simple relation between the pumped charge at zero bias and at bias equal to the Coulomb charging energy. At finite magnetic field, there is a possibility to have interaction-induced pure spin pumping at this finite bias value, and generally, additional features appear in the pumped charge. For large-amplitude adiabatic driving, the magnitude of both the pumped charge and spin at the various resonances saturate at values which are independent of the specific shape of the pumping cycle. Each of these values provide an independent, quantitative measurement of the junction asymmetry.
    Physical review. B, Condensed matter 10/2012; 86(24). · 3.77 Impact Factor
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    ABSTRACT: We present quantum transport measurements of interacting parallel quantum dots formed in the strands of a carbon nanotube rope. In this molecular quantum dot system, transport is dominated by one quantum dot, while additional resonances from parallel side dots appear, which exhibit a weak gate coupling. This differential gating effect provides a tunability of the quantum dot system with only one gate electrode and provides control over the carbon nanotube strand that carries the current. By tuning the system to different states we use quantum transport as a spectroscopic tool to investigate the inter-dot coupling and show a route to distinguish between various side dots. By comparing the experimental data with master equation calculations, we identify conditions for the tunneling rates that are required in order to observe different manifestations of the inter-dot coupling in the transport spectra.
    Physical review. B, Condensed matter 08/2012; 87(3). · 3.77 Impact Factor
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    R. B. Saptsov, M. R. Wegewijs
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    ABSTRACT: We study the transport through a strongly interacting Anderson quantum dot at zero-temperature using the real-time renormalization group (RT-RG) in the framework of a kinetic equation for the reduced density operator. We further develop the general finite temperature real-time transport formalism by introducing field superoperators that obey fermionic statistics. This direct second quantization in Liouville-Fock space strongly simplifies the construction of operators and superoperators which transform irreducibly under the Anderson-model symmetry transformations. The fermionic field superoperators naturally arise from the univalence (fermion-parity) superselection rule for the total system. Expressed in these field superoperators, the causal structure of the perturbation theory for the effective time-evolution superoperator-kernel becomes explicit. The causal structure also implies the existence of a fermion-parity protected eigenvector of the exact Liouvillian, explaining a recently reported result on adiabatic driving [Phys. Rev. B 85, 075301 (2012)] and generalizing it to arbitrary order in the tunnel coupling. Furthermore, in the WBL the causal representation exponentially reduces the number of diagrams for the time-evolution kernel. We perform a complete 2-loop RG analysis at finite voltage and magnetic field, while systematically accounting for the dependence on both the quantum dot and reservoir frequencies. Using the second quantization in Liouville-space and symmetry restrictions we obtain analytical RT-RG equations with an efficient numerical solution and we extensively study the model parameter space, excluding the Kondo regime. The incorporated renormalization effects result in an enhancement of the inelastic cotunneling peak. Moreover, we find a tunnel-induced non-linearity of the stability diagrams at finite voltage, both in the SET and ICT regime.
    Physical review. B, Condensed matter 07/2012; 86(23). · 3.77 Impact Factor
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    ABSTRACT: We propose a scheme for spin-based detection of the bending motion in suspended carbon-nanotubes, using the curvature-induced spin-orbit interaction. We show that the resulting effective spin-phonon coupling can be used to down-convert the high-frequency vibration-modulated spin-orbit field to spin-flip processes at a much lower frequency. This vibration-induced spin-resonance can be controlled with an axial magnetic field. We propose a Pauli spin blockade readout scheme and predict that the leakage current shows pronounced peaks as a function of the external magnetic field. Whereas the resonant peaks allow for frequency readout, the slightly off-resonant current is sensitive to the vibration amplitude.
    Applied Physics Letters 10/2011; · 3.79 Impact Factor
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    ABSTRACT: We show that spin anisotropy can be transferred to an isotropic system by transport of a spin-quadrupole moment. We derive the quadrupole moment current and continuity equation and study a spin-valve structure consisting of two ferromagnets coupled to a quantum dot probing an impurity spin. The quadrupole backaction on their coupled spin results in spin torques and anisotropic spin relaxation which do not follow from standard spin-current considerations. We demonstrate the detection of the impurity spin by charge transport and its manipulation by electric fields.
    Physical Review Letters 08/2011; 107(8):087202. · 7.73 Impact Factor
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    ABSTRACT: Fabrication of single-molecule transistors where electron transport occurs through an individual molecule has become possible due to the recent progress in molecular electronics. Three-terminal configuration allows charging molecules and performing transport spectroscopy in multiple redox states. Single-molecule magnets combining large spin with uniaxial anisotropy are of special interest as appealing candidates for high density memory applications and quantum information processing. We study single-molecule magnets Fe4. Three-terminal junctions are fabricated using electromigration of gold nanowires followed by a self-breaking. High-spin Kondo effect and inelastic cotunneling excitations show up in transport measurements. Several excitations feature the energy close to the energy of zero-field splitting (ZFS) of a ground spin multiplet in bulk. This splitting is caused by the anisotropy and is a hallmark of single-molecule magnets. We observe nonlinear Zeeman effect due to a misalignment of an anisotropy axis and a magnetic field direction. The ZFS energy is increased in oxidized and reduced states of the molecule indicating enhancement of the anisotropy in these states.
    03/2011;
  • Physical Review Letters 01/2011; 106(1). · 7.73 Impact Factor
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    ABSTRACT: We have studied three-terminal electron transport through an individual single-molecule magnet Fe4. Several fingerprints of high spin and magnetic anisotropy have been observed: the Kondo effect in adjacent charge states in an intermediate coupling regime and zero-field splitting in the regime of a weaker coupling. We also observed spin transitions at energies close to the ones reported for bulk Fe4 crystals. Finally, we have demonstrated electric-field control on the magnetic properties of a single molecule.
    Synthetic Metals 01/2011; 161:591-597. · 2.11 Impact Factor
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    Falk May, Maarten R Wegewijs, Walter Hofstetter
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    ABSTRACT: We study electron transport through a single-molecule magnet (SMM) and the interplay of its anisotropic spin with quantized vibrational distortions of the molecule. Based on numerical renormalization group calculations we show that, despite the longitudinal anisotropy barrier and small transverse anisotropy, vibrational fluctuations can induce quantum spin-tunneling (QST) and a QST-Kondo effect. The interplay of spin scattering, QST and molecular vibrations can strongly enhance the Kondo effect and induce an anomalous magnetic field dependence of vibrational Kondo side-bands.
    Beilstein Journal of Nanotechnology 01/2011; 2:693-8. · 2.37 Impact Factor
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    S. Koller, M. Grifoni, M. Leijnse, M. R. Wegewijs
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    ABSTRACT: Various theoretical methods address transport effects in quantum dots beyond single-electron tunneling while accounting for the strong interactions in such systems. In this paper we report a detailed comparison between three prominent approaches to quantum transport: the fourth-order Bloch-Redfield quantum master equation (BR), the real-time diagrammatic technique (RT), and the scattering rate approach based on the T-matrix (TM). Central to the BR and RT is the generalized master equation for the reduced density matrix. We demonstrate the exact equivalence of these two techniques. By accounting for coherences (nondiagonal elements of the density matrix) between nonsecular states, we show how contributions to the transport kernels can be grouped in a physically meaningful way. This not only significantly reduces the numerical cost of evaluating the kernels but also yields expressions similar to those obtained in the TM approach, allowing for a detailed comparison. However, in the TM approach an ad hoc regularization procedure is required to cure spurious divergences in the expressions for the transition rates in the stationary (zero-frequency) limit. We show that these problems derive from incomplete cancellation of reducible contributions and do not occur in the BR and RT techniques, resulting in well-behaved expressions in the latter two cases. Additionally, we show that a standard regularization procedure of the TM rates employed in the literature does not correctly reproduce the BR and RT expressions. All the results apply to general quantum dot models and we present explicit rules for the simplified calculation of the zero-frequency kernels. Although we focus on fourth-order perturbation theory only, the results and implications generalize to higher orders. We illustrate our findings for the single impurity Anderson model with finite Coulomb interaction in a magnetic field.
    Physical review. B, Condensed matter 12/2010; 82(23). · 3.77 Impact Factor
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    ABSTRACT: We demonstrate single electron addition to different strands of a carbon nanotube rope. Anticrossings of anomalous conductance peaks occur in quantum transport measurements through the parallel quantum dots forming on the individual strands. We determine the magnitude and the sign of the hybridization as well as the Coulomb interaction between the carbon nanotube quantum dots, finding that the bonding states dominate the transport. In a magnetic field the hybridization is shown to be selectively suppressed due to spin effects.
    Physical review. B, Condensed matter 11/2010; 83. · 3.77 Impact Factor
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    ABSTRACT: We have measured quantum transport through an individual Fe(4) single-molecule magnet embedded in a three-terminal device geometry. The characteristic zero-field splittings of adjacent charge states and their magnetic field evolution are observed in inelastic tunneling spectroscopy. We demonstrate that the molecule retains its magnetic properties and, moreover, that the magnetic anisotropy is significantly enhanced by reversible electron addition/subtraction controlled with the gate voltage. Single-molecule magnetism can thus be electrically controlled.
    Nano Letters 09/2010; 10(9):3307-11. · 13.03 Impact Factor
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    ABSTRACT: We review recent progress in the theoretical description of correlation and quantum fluctuation phenomena in charge transport through single molecules, quantum dots and quantum wires. Various physical phenomena are addressed, relating to cotunneling, pair-tunneling, adiabatic quantum pumping, charge and spin fluctuations, and inhomogeneous Luttinger liquids. We review theoretical many-body methods to treat correlation effects, quantum fluctuations, non-equilibrium physics, and the time evolution into the stationary state of complex nanoelectronic systems.
    Nanotechnology 07/2010; 21(27):272001. · 3.84 Impact Factor

Publication Stats

730 Citations
212.36 Total Impact Points

Institutions

  • 2008–2013
    • Forschungszentrum Jülich
      Jülich, North Rhine-Westphalia, Germany
  • 2011–2012
    • RWTH Aachen University
      • Institut für Theorie der statistischen Physik A
      Aachen, North Rhine-Westphalia, Germany
  • 2008–2011
    • Goethe-Universität Frankfurt am Main
      • Institut für Theoretische Physik (ITP)
      Frankfurt am Main, Hesse, Germany
  • 2006
    • Ben-Gurion University of the Negev
      • Department of Physics
      Beersheba, Southern District, Israel