Milena Grifoni

Universität Regensburg, Ratisbon, Bavaria, Germany

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Publications (140)453.27 Total impact

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    Benjamin Siegert · Andrea Donarini · Milena Grifoni
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    ABSTRACT: The interplay of exchange correlations and spin-orbit interaction (SOI) on the many-body spectrum of a copper phtalocyanine (CuPc) molecule and their signatures in transport are investigated. We first derive a minimal model Hamiltonian in a basis of frontier orbitals which is able to reproduce experimentally observed singlet-triplet splittings; in a second step SOI effects are included perturbatively. Major consequences of the SOI are the splitting of former degenerate levels and a magnetic anisotropy, which can be captured by an effective low-energy spin Hamiltonian. We show that STM-based magnetoconductance measurements can yield clear signatures of both these SOI induced effects.
    Full-text · Article · Aug 2015 · Beilstein Journal of Nanotechnology
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    ABSTRACT: We investigate the effects of spin-orbit interaction (SOI) and valley mixing on the transport and dynamical properties of a carbon nanotube (CNT) quantum dot in the Kondo regime. As these perturbations break the pseudo-spin symmetry in the CNT spectrum but preserve time-reversal symmetry, they induce a finite splitting $\Delta$ between formerly degenerate Kramers pairs. Correspondingly, a crossover from the SU(4) to the SU(2)-Kondo effect occurs as the strength of these symmetry breaking parameters is varied. Clear signatures of the crossover are discussed both at the level of the spectral function as well as of the conductance. In particular, we demonstrate numerically and support with scaling arguments, that the Kondo temperature scales inversely with the splitting $\Delta$ in the crossover regime. In presence of a finite magnetic field, time reversal symmetry is also broken. We investigate the effects of both parallel and perpendicular fields (with respect to the tube's axis), and discuss the conditions under which Kondo revivals may be achieved.
    Preview · Article · Jul 2015 · Physica E Low-dimensional Systems and Nanostructures
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    Benjamin Siegert · Andrea Donarini · Milena Grifoni
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    ABSTRACT: We demonstrate the tip induced control of the spin state of copper phthalocyanine (CuPc) on an insulator coated substrate. Accounting for electronic correlations, we find that, under the condition of energetic proximity of neutral excited states to the anionic groundstate, the system can undergo a population inversion towards these excited states. The resulting state of the system is accompanied by a change in the total spin quantum number. Experimental signatures of the crossover are the appearance of additional nodal planes in the topographical STM images as well as a strong suppression of the current near the center of the molecule. The robustness of the effect against moderate charge conserving relaxation processes has also been tested.
    Full-text · Article · Jul 2015
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    ABSTRACT: We present measurements of tunneling magnetoresistance (TMR) in single-wall carbon nanotubes attached to ferromagnetic contacts in the Coulomb blockade regime. Strong variations of the TMR with gate voltage over a range of four conductance resonances, including a peculiar double-dip signature, are observed. The data are compared to calculations in the ``dressed second order'' (DSO) framework. In this nonperturbative theory, conductance peak positions and linewidths are affected by charge fluctuations incorporating the properties of the carbon nanotube quantum dot and the ferromagnetic leads. The theory is able to qualitatively reproduce the experimental data.
    No preview · Article · May 2015 · Physical Review B
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    ABSTRACT: Understanding the interplay between many-body phenomena and nonequilibrium in systems with entangled spin and orbital degrees of freedom is a central objective in nanoelectronics. We demonstrate that the combination of Coulomb interaction, spin-orbit coupling, and valley mixing results in a particular selection of the inelastic virtual processes contributing to the Kondo resonance in carbon nanotubes at low temperatures. This effect is dictated by conjugation properties of the underlying carbon nanotube spectrum at zero and finite magnetic field. Our measurements on a clean carbon nanotube are complemented by calculations based on a field-theoretical Keldysh approach to the nonequilibrium Kondo problem which well reproduces the rich experimental observations in Kondo transport.
    No preview · Article · Apr 2015 · Physical Review B
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    ABSTRACT: We report on a systematic study of the Coulomb blockade effects in nanofabricated narrow constrictions in thin (Ga,Mn)As films. Different low-temperature transport regimes have been observed for decreasing constriction sizes: the ohmic, the single electron tunnelling (SET) and a completely insulating regime. In the SET, complex stability diagrams with nested Coulomb diamonds and anomalous conductance suppression in the vicinity of charge degeneracy points have been observed. We rationalize these observations in the SET with a double ferromagnetic island model coupled to ferromagnetic leads. Its transport characteristics are analyzed in terms of a modified orthodox theory of Coulomb blockade which takes into account the energy dependence of the density of states in the metallic islands.
    Full-text · Article · Feb 2015 · Physical Review B
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    ABSTRACT: We present measurements of tunneling magneto-resistance (TMR) in single-wall carbon nanotubes attached to ferromagnetic contacts in the Coulomb blockade regime. Strong variations of the TMR with gate voltage over a range of four conductance resonances, including a peculiar double-dip signature, are observed. The data is compared to calculations in the "dressed second order" (DSO) framework. In this non-perturbative theory, conductance peak positions and linewidths are affected by charge fluctuations incorporating the properties of the CNT quantum dot and the ferromagnetic leads. The theory is able to qualitatively reproduce the experimental data.
    Preview · Article · Feb 2015
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    ABSTRACT: The dissipative dynamics of a quantum bistable system coupled to a Ohmic heat bath is investigated beyond the spin-boson approximation. We propose a novel approximation scheme, based on a real-time path integral approach, which enables to capture the system's dynamics over a wide range of temperatures $T$ and coupling strengths $\gamma$. Exploiting a separation of time scales between the intra-well and inter-well (tunneling) dynamics, a set of coupled integro-differential equations for the populations in a space localized basis is obtained. A phase-diagram in the $(\gamma,T)$ space identifying regions of coherent and incoherent dynamics as well as of bath induced localization is provided.
    Full-text · Article · Dec 2014 · Physical Review E
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    Magdalena Marganska · Piotr Chudzinski · Milena Grifoni
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    ABSTRACT: Electrons in carbon nanotubes (CNTs) possess spin and orbital degrees of freedom. The latter is inherited from the bipartite graphene lattice with two inequivalent Dirac points. The electronic spectra obtained in several transport experiments on CNT quantum dots in parallel magnetic field often show an anticrossing of spectral lines assigned to the opposite Dirac valleys. So far this valley mixing has been attributed to the disorder, with impurity induced scattering. We show that this effect can arise also in ultraclean CNTs of the armchair class and it can be caused solely by the presence of the boundaries. In contrast, in CNTs of the zigzag class it does not occur. These two fundamentally different classes of spectra arise because of different symmetries of the low energy eigenstates of the two types of CNTs. The magnitude of the level splitting depends in a nonmonotonous way on the distance of the involved energy levels from the charge neutrality point.
    Full-text · Article · Dec 2014 · Physical Review B
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    ABSTRACT: We report on nonlinear cotunneling spectroscopy of a carbon nanotube quantum dot coupled to Nb superconducting contacts. Our measurements show rich subgap features in the stability diagram which become more pronounced as the temperature is increased. Applying a transport theory based on the Liouville-von Neumann equation for the density matrix, we show that the transport properties can be attributed to processes involving sequential as well as elastic and inelastic cotunneling of quasiparticles thermally excited across the gap. In particular, we predict thermal replicas of the elastic and inelastic cotunneling peaks, in agreement with our experimental results.
    Full-text · Article · Aug 2014 · New Journal of Physics
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    ABSTRACT: We present electronic transport measurements of a single wall carbon nanotube quantum dot coupled to Nb superconducting contacts. For temperatures comparable to the superconducting gap peculiar transport features are observed inside the Coulomb blockade and superconducting energy gap regions. The observed temperature dependence can be explained in terms of sequential tunneling processes involving thermally excited quasiparticles. In particular, these new channels give rise to two unusual conductance peaks at zero bias in the vicinity of the charge degeneracy point and allow to determine the degeneracy of the ground states involved in transport. The measurements are in good agreement with model calculations.
    Full-text · Article · Mar 2014 · Physical Review B
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    ABSTRACT: Understanding the interplay between many-body correlations and non-equilibrium in systems with entangled spin and orbital degrees of freedom is central for many applications in nano-electronics. Here we demonstrate that hitherto unobserved many-body selection rules govern the Kondo effect in carbon nanotubes where spin and orbital degeneracy is broken by curvature induced spin-orbit coupling and valley mixing. They are dictated by the underlying discrete symmetries of the carbon nanotube spectrum at zero and finite magnetic field. Our measurements on a clean carbon nanotube are complemented by calculations based on a new approach to the non-equilibrium Kondo problem which reproduces the rich experimental observations in Kondo transport in high detail. Our findings open a new route to manipulate transitions between spin-orbital entangled states.
    Full-text · Article · Dec 2013
  • Benjamin Siegert · Andrea Donarini · Milena Grifoni
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    ABSTRACT: We present an STM theory based on the reduced density matrix (RDM) formalism which is able to describe transport proper- ties of an STM junction for conjugated molecules on thin insulating films. It combines a very popular derivation of STM tunneling matrix elements [1], based on Bardeen’s tunneling formalism [2], with a generalized master equation approach for interactingmolecularsystems.Weshowthatthismethodallows the efficient implementation of different tip symmetries in STM simulations. With the example of hydrogen phthalocyanine (H_2 Pc), we study the influence of s- and p-wave tip sym- metries on the constant-height current maps of conjugated molecules.
    No preview · Article · Nov 2013 · physica status solidi (b)
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    Full-text · Dataset · Sep 2013
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    Full-text · Dataset · Sep 2013
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    Sergey Smirnov · Milena Grifoni
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    ABSTRACT: We analyze universal transport properties of a strongly interacting quantum dot in the Kondo regime when the quantum dot is placed in an external magnetic field. The quantum dot is described by the asymmetric Anderson model with the spin degeneracy removed by the magnetic field resulting in the Zeeman splitting. Using an analytical expression for the tunneling density of states found from a Keldysh effective field theory, we obtain in the whole energy range the universal differential conductance and analytically demonstrate its Fermi-liquid and logarithmic behavior at low- and high-energies, respectively, as a function of the magnetic field. We also show results on the zero temperature differential conductance as a function of the bias voltage at different magnetic fields as well as results on finite temperature effects out of equilibrium and at a finite magnetic field. The modern nonequilibrium experimental issues of the critical magnetic field, at which the zero bias maximum of the differential conductance starts to split into two maxima, as well as the distance between these maxima as a function of the magnetic field are also addressed.
    Preview · Article · Apr 2013 · New Journal of Physics
  • Bhaskaran Muralidharan · Milena Grifoni
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    ABSTRACT: We demonstrate the possibility of thermoelectric excitation of single spins dynamics in quantum dots using a non-collinear quantum dot spin valve set up. Many-body exchange fields generated in this set up manifest as effective magnetic fields acting on the single spins inside the quantum dot. We first identify generic conditions by which a Pauli spin blockade in the dot may be thermally created. It is then shown that the resulting spin accumulation may be subject to a dominant field like spin torque due to the effective magnetic field associated with each contact. This spin torque that is generated via a pure thermal gradient may result in long time precession effects due to the prevailing Coulomb blockade conditions. The consequence of this in connection with spin manipulation and the generation of pure spin currents are then discussed briefly.
    No preview · Article · Mar 2013
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    Andrea Donarini · Milena Grifoni

    Full-text · Chapter · Jan 2013
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    Bhaskaran Muralidharan · Milena Grifoni
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    ABSTRACT: We explore thermoelectric spin transport and spin dependent phenomena in a non-collinear quantum dot spin valve set up. Using this set up, we demonstrate the possibility of a thermoelectric excitation of single spin dynamics inside the quantum dot. Many-body exchange fields generated on the single spins in this set up manifest as effective magnetic fields acting on the net spin accumulation in the quantum dot. We first identify generic conditions by which a zero bias spin accumulation in the dot may be created in the thermoelectric regime. The resulting spin accumulation is then shown to be subject to a field-like spin torque due to the effective magnetic field associated with either contact. This spin torque that is generated may yield long-time precession effects due to the prevailing blockade conditions. The implications of these phenomena in connection with single spin manipulation and pure spin current generation are then discussed.
    Full-text · Article · Dec 2012 · Physical review. B, Condensed matter
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    ABSTRACT: An ab initio based theoretical approach to describe nonequilibrium many-body effects in molecular transport is developed. We introduce a basis of localized molecular orbitals and formulate the many-body model in this basis. In particular, the Hubbard-Anderson Hamiltonian is derived for single-molecule junctions with intermediate coupling to the leads. As an example we consider a benzenedithiol junction with gold electrodes. An effective few-level model is obtained, from which spectral and transport properties are computed and analyzed. Electron-electron interaction crucially affects transport and induces multiscale Coulomb blockade at low biases. At large bias, transport through asymmetrically coupled molecular edge states results in the occurrence of "anomalous" conductance features, i.e., of peaks with unexpectedly large/small height or even not located at the expected resonance energies.
    Full-text · Article · Oct 2012 · Physical review. B, Condensed matter

Publication Stats

4k Citations
453.27 Total Impact Points

Institutions

  • 2000-2015
    • Universität Regensburg
      • • Institute of Experimental and Applied Physics
      • • Intitute of Theoretical Physics
      Ratisbon, Bavaria, Germany
  • 2001-2010
    • Delft University of Technology
      • Cosmonanoscience
      Delft, South Holland, Netherlands
    • University of Texas at Austin
      • Department of Physics
      Austin, Texas, United States
  • 2007
    • University of Freiburg
      Freiburg, Baden-Württemberg, Germany
  • 1999
    • Universität Stuttgart
      • Institute of Theoretical Physics
      Stuttgart, Baden-Württemberg, Germany
  • 1995-1998
    • Universität Augsburg
      • Institute of Physics
      Augsberg, Bavaria, Germany
  • 1993-1995
    • Università degli Studi di Genova
      • Department of Physics
      Genova, Liguria, Italy