Publications (57)242.05 Total impact
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ABSTRACT: We study the transient heat current out of a confined electron system into a weakly coupled electrode in response to a voltage switch. Surprisingly, the decay of the Coulomb interaction energy for this repulsive system exhibits signatures of electronelectron attraction, and is governed by an interactionindependent rate. This can only be understood from a general duality that relates the nonunitary evolution of a quantum state to that of a dual system with inverted energies. Deriving from the fermionparity superselection postulate, the duality applies to a large class of open systems.  [Show abstract] [Hide abstract]
ABSTRACT: We theoretically investigate the backaction of a sensor quantum dot with strong local Coulomb repulsion on the transient dynamics of a qubit that is probed capacitively. We show that the measurement backaction induced by the noise of electron cotunneling through the sensor is surprisingly mitigated by the recently identified coherent backaction [PRB 89, 195405] arising from quantum fluctuations. This renormalization effect is missing in semiclassical stochastic fluctuator models and typically also in BornMarkov approaches, which try to avoid the calculation of the nonstationary, nonequilibrium state of the qubit plus sensor. Technically, we integrate out the currentcarrying electrodes to obtain kinetic equations for the joint, nonequilibrium detectorqubit dynamics. We show that the sensorcurrent response, level renormalization, cotunneling, and leading nonMarkovian corrections always appear together and cannot be turned off individually in an experiment or ignored theoretically. We analyze the backaction on the reduced qubit state  capturing the full nonMarkovian effects imposed by the sensor quantum dot on the qubit  by applying a Liouvillespace decomposition into quasistationary and rapidly decaying modes. Importantly, the sensor cannot be eliminated completely even in the simplest hightemperature, weakmeasurement limit: The qubit state experiences an initial slip that persists over many qubit cycles and depends on the initial preparation of qubit plus sensor quantum dot. A quantumdot sensor can thus not be modeled as a 'black box' without accounting for its dynamical variables. We furthermore find that the Bloch vector relaxes (T1) along an axis that is not orthogonal to the plane in which the Bloch vector dephases (T2), blurring the notions of T1 and T2 times. Finally, the precessional motion of the Bloch vector is distorted into an ellipse in the tilted dephasing plane.  [Show abstract] [Hide abstract]
ABSTRACT: We report transport measurements on a quantum dot in a partly suspended carbon nanotube. Electrostatic tuning allows us to modify and even switch “on” and “off” the coupling to the quantized stretching vibration across several charge states. The magneticfield dependence indicates that only the twoelectron spintriplet excited state couples to the mechanical motion, indicating mechanical coupling to both the valley degree of freedom and the exchange interaction, in contrast to standard models.Keywords: Carbon nanotube; quantum dot; electron transport; nanoelectromechanical systems (NEMS); electronphonon coupling in lowdimensional structures; spin states  [Show abstract] [Hide abstract]
ABSTRACT: We show that charge fluctuation processes are crucial for the nonlinear heat conductance through an interacting nanostructure, even far from a resonance. The often made assumption that offresonant transport proceeds only by virtual occupation of charge states, underlying exchangescattering models of transport, can fail dramatically for heat transport as compared to charge transport. This indicates that nonlinear heat transport spectroscopy may be a very promising experimental tool, in particular when combining energylevel control with recent advances in nanoscale thermometry that allow accurate measurements of heat currents. It provides new qualitative information about relaxation processes which can go unnoticed by the traditional chargecurrent measurements, for instance by strong negative differential heat conductance at positive heat current. 
Article: Probing Transverse Magnetic Anisotropy by Electronic Transport through a SingleMolecule Magnet
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ABSTRACT: By means of electronic transport, we study the transverse magnetic anisotropy of an individual Fe$_4$ singlemolecule magnet (SMM) embedded in a threeterminal 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.  [Show abstract] [Hide abstract]
ABSTRACT: We predict that an excitationless Coulombblockaded quantum dot develops a nonequilibrium spinprecession resonance when embedded into a spin valve. At resonance the spinvalve effect is mitigated. The resonance can be detected by stationary dI/dVb spectroscopy and by oscillations in the timeaveraged current using a gatepulsing scheme. The generic noncollinearity of the ferromagnets and junction asymmetry allows for an allelectric determination of spin injection, anisotropy of spin relaxation, and exchange field. We investigate the impact of a nearby superconductor on the resonance position.  [Show abstract] [Hide abstract]
ABSTRACT: We extend the recently developed causal superfermion approach to the realtime transport theory to timedependent 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 LiouvilleFock space constructing causal field superoperators using the fundamental physical principles of causality/probability conservation and the fermionparity superselection (univalence). The timedependent perturbation series for the timeevolution is renormalized by explicitly performing the wideband limit on the superoperator level. The short and longtime 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 realtime. A remarkable feature of this approach is the natural appearance of a measurable fermionparity 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 Liouvillespace 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 selfenergy as well as the timeevolution propagator. For this limit we calculate the timeevolution of the full density operator starting from an arbitrary initial state on the quantum dot, including spin and pairing coherences and twoparticle correlations.  [Show abstract] [Hide abstract]
ABSTRACT: In this article we review aspects of charge and heat transport in interacting quantum dots and molecular junctions under stationary and timedependent nonequilibrium 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 timedependent 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 dcmeasurements. Finally, we combine timedependent driving with thermoelectrics in a doublequantum dot system  a nanoscale analogue of a cyclic heat engine  and discuss its operation and the main limitations to its performance.  [Show abstract] [Hide abstract]
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 SQDchargequbit system. We derive Markovian kinetic equations for the ensembleaveraged state of the SQDqubit system, expressed in the coupled dynamics of two chargestate 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 SQDqubit 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 spinvalve structures. Secondly, we show that for a consistent description of the detection, one must also include the renormalization effects in the nexttoleading 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 spintocharge conversion using a quantumdot 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 chargebased qubit readout. 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. 
Article: Spintronic magnetic anisotropy
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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.  [Show abstract] [Hide abstract]
ABSTRACT: We revisit the transport of spindegrees of freedom across an electrically and thermally biased tunnel junction between two ferromagnets with noncollinear magnetizations. Besides the wellknown charge and spin currents we show that a nonzero spinquadrupole current flows between the ferromagnets. This tensorvalued current describes the nonequilibrium transport of spinanisotropy relating to both local and nonlocal multiparticle spin correlations of the circuit. This quadratic spinanisotropy, quantified in terms of the spinquadrupole moment, is fundamentally a twoelectron quantity. In spinvalves 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 spinvalve model studied here allows fundamental questions about spinquadrupole 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 spinquadrupole 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 spinvalve the charge and spincurrent may vanish, while a pure exchange spinquadrupole current remains, which appears as a fundamental consequence of Pauli's principle. We extend the realtime diagrammatic approach to efficiently calculate the average of multiparticle spinobservables, in particular the spinquadrupole current. Although the paper addresses only leading order and spinconserving tunneling we formulate the technique for arbitrary order in an arbitrary, spindependent tunnel coupling in a way that lends itself to extension to quantumdot spinvalve structures.  [Show abstract] [Hide abstract]
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 timedependent external driving and relate these to the concepts of charge and spin emissivities previously discussed within the timedependent scattering matrix approach. Expressed in terms of auxiliary vector fields, the response coefficients allow for a straightforward analysis of recently predicted interactioninduced 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 interactioninduced pure spin pumping at this finite bias value, and generally, additional features appear in the pumped charge. For largeamplitude 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.  [Show abstract] [Hide abstract]
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 interdot 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 interdot coupling in the transport spectra.  [Show abstract] [Hide abstract]
ABSTRACT: We study the transport through a strongly interacting Anderson quantum dot at zerotemperature using the realtime renormalization group (RTRG) in the framework of a kinetic equation for the reduced density operator. We further develop the general finite temperature realtime transport formalism by introducing field superoperators that obey fermionic statistics. This direct second quantization in LiouvilleFock space strongly simplifies the construction of operators and superoperators which transform irreducibly under the Andersonmodel symmetry transformations. The fermionic field superoperators naturally arise from the univalence (fermionparity) superselection rule for the total system. Expressed in these field superoperators, the causal structure of the perturbation theory for the effective timeevolution superoperatorkernel becomes explicit. The causal structure also implies the existence of a fermionparity 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 timeevolution kernel. We perform a complete 2loop 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 Liouvillespace and symmetry restrictions we obtain analytical RTRG 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 tunnelinduced nonlinearity of the stability diagrams at finite voltage, both in the SET and ICT regime.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a scheme for spinbased detection of the bending motion in suspended carbonnanotubes, using the curvatureinduced spinorbit interaction. We show that the resulting effective spinphonon coupling can be used to downconvert the highfrequency vibrationmodulated spinorbit field to spinflip processes at a much lower frequency. This vibrationinduced spinresonance 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 offresonant current is sensitive to the vibration amplitude.  [Show abstract] [Hide abstract]
ABSTRACT: We study electron transport through a singlemolecule 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 spintunneling (QST) and a QSTKondo 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 sidebands.  [Show abstract] [Hide abstract]
ABSTRACT: We show that spin anisotropy can be transferred to an isotropic system by transport of a spinquadrupole moment. We derive the quadrupole moment current and continuity equation and study a spinvalve 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 spincurrent considerations. We demonstrate the detection of the impurity spin by charge transport and its manipulation by electric fields. 
Article: Highspin and magnetic anisotropy signatures in threeterminal transport through a single molecule
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ABSTRACT: We have studied threeterminal electron transport through an individual singlemolecule 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 zerofield 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 electricfield control on the magnetic properties of a single molecule.  [Show abstract] [Hide abstract]
ABSTRACT: Fabrication of singlemolecule transistors where electron transport occurs through an individual molecule has become possible due to the recent progress in molecular electronics. Threeterminal configuration allows charging molecules and performing transport spectroscopy in multiple redox states. Singlemolecule 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 singlemolecule magnets Fe4. Threeterminal junctions are fabricated using electromigration of gold nanowires followed by a selfbreaking. Highspin Kondo effect and inelastic cotunneling excitations show up in transport measurements. Several excitations feature the energy close to the energy of zerofield splitting (ZFS) of a ground spin multiplet in bulk. This splitting is caused by the anisotropy and is a hallmark of singlemolecule 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. 
Publication Stats
2k  Citations  
242.05  Total Impact Points  
Top Journals
Institutions

20072015

Forschungszentrum Jülich
 Peter Grünberg Institute (PGI)
Jülich, North RhineWestphalia, Germany


20112014

RWTH Aachen University
 Institut für Theorie der statistischen Physik A
Aachen, North RhineWestphalia, Germany 
GoetheUniversität Frankfurt am Main
 Institute of Theoretical Physics
Frankfurt, Hesse, Germany


2013

University of Delhi
Old Delhi, NCT, India


2009

Universität Regensburg
 Institute of Experimental and Applied Physics
Ratisbon, Bavaria, Germany
