Milena Grifoni

Universität Regensburg, Ratisbon, Bavaria, Germany

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Publications (132)436.3 Total impact

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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.
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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.
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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.
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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.
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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.
    New Journal of Physics 08/2014; 16(12). DOI:10.1088/1367-2630/16/12/123040 · 3.67 Impact Factor
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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.
    Physical Review B 03/2014; 89(24). DOI:10.1103/PhysRevB.89.241405 · 3.66 Impact Factor
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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.
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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.
    physica status solidi (b) 11/2013; 111:216802. DOI:10.1002/pssb.201350002 · 1.61 Impact Factor
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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.
    New Journal of Physics 04/2013; 15(7). DOI:10.1088/1367-2630/15/7/073047 · 3.67 Impact Factor
  • 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.
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    Andrea Donarini, Milena Grifoni
    Quantum Dot Molecules, Lecture Notes in Nanoscale Science and Technology edited by Z. M. Wang and J. Wu, 01/2013: chapter Interference single electron transistors based on quantum dot molecules: pages 51; Springer.
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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.
    Physical review. B, Condensed matter 12/2012; 88(4). DOI:10.1103/PhysRevB.88.045402 · 3.66 Impact Factor
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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.
    Physical review. B, Condensed matter 10/2012; 88(8). DOI:10.1103/PhysRevB.88.085404 · 3.66 Impact Factor
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    Johannes Kern, Milena Grifoni
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    ABSTRACT: We describe linear and nonlinear transport across a single impurity Anderson model quantum dot with intermediate coupling to the leads, i.e., with tunnel coupling of the order of the thermal energy k_B T. The coupling is large enough that sequential tunneling processes alone do not suffice to properly describe the transport characteristics. Upon applying a density matrix approach, the current is expressed in terms of rates obtained by considering a very small class of diagrams which dress the sequential tunneling processes by charge fluctuations. We call this the "dressed second order" (DSO) approximation. One major achievement of the DSO is that, still in the Coulomb blockade regime, it can describe the crossover from thermally broadened to tunneling broadened conductance peaks. When the temperature is decreased even further, the DSO captures "Kondesque" behaviours of the Anderson quantum dot qualitatively: We find a zero bias anomaly of the differential conductance versus applied bias, an enhancement of the conductance with decreasing temperature as well as the onset of universality of the shape of the conductance as function of the temperature. We can address the case of a spin-degenerate level split energetically by a magnetic field and show that, if we assume in addition different capacitive couplings of the two spin-levels to the leads, one of the resonance peaks is vanishing. In case spin-dependent chemical potentials are introduced and only one of the four is varied, the DSO yields in principle only one resonance. This seems to be in agreement with experiments with pseudo-spin. Furthermore, we get qualitative agreement with experimental data showing a cross-over from the Kondo to the empty orbital regime.
    Physics of Condensed Matter 09/2012; 86(9). DOI:10.1140/epjb/e2013-40618-9 · 1.46 Impact Factor
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    ABSTRACT: We present a microscopic theory of transport through quantum dot set-ups coupled to superconducting leads. We derive a master equation for the reduced density matrix to lowest order in the tunneling Hamiltonian and focus on quasiparticle tunneling. For high enough temperatures transport occurs in the subgap region due to thermally excited quasiparticles, which can be used to observe excited states of the system for low bias voltages. On the example of a double quantum dot we show how subgap transport spectroscopy can be done. Moreover, we use the single level quantum dot coupled to a normal and a superconducting lead to give a possible explanation for the subgap features observed in the experiments published in Appl. Phys. Lett. 95, 192103 (2009).
    Physical review. B, Condensed matter 08/2012; 87(15). DOI:10.1103/PhysRevB.87.155439 · 3.66 Impact Factor
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    ABSTRACT: Negative differential conductance (NDC) is a non-linear transport phenomenon ubiquitous in molecular nanojunctions. Its physical origin can be the most diverse. In rotationally symmetric molecules with orbitally degenerate many-body states, it can be ascribed to interference effects. We establish in this paper a criterion to identify the interference blocking scenario by correlating the spectral and the topographical information achievable in an STM single molecule measurement. Simulations of current voltage characteristics and current maps for a Cu-Phthalocyanine (CuPc) on a thin insulating film are presented as experimentally relevant examples.
    Physical review. B, Condensed matter 06/2012; DOI:10.1103/PhysRevB.86.155451 · 3.66 Impact Factor
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    Andrea Donarini, Abdullah Yar, Milena Grifoni
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    ABSTRACT: We investigate bistability and memory effects in a molecular junction weakly coupled to metallic leads with the latter being subject to an adiabatic periodic change of the bias voltage. The system is described by a simple Anderson-Holstein model and its dynamics is calculated via a master equation approach. The controlled electrical switching between the many-body states of the system is achieved due to polaron shift and Franck-Condon blockade in the presence of strong electron-vibron interaction. Particular emphasis is given to the role played by the excited vibronic states in the bistability and hysteretic switching dynamics as a function of the voltage sweeping rates. In general, both the occupation probabilities of the vibronic states and the associated vibron energy show hysteretic behaviour for driving frequencies in a range set by the minimum and maximum lifetimes of the system. The consequences on the transport properties for various driving frequencies and in the limit of DC-bias are also investigated.
    Physics of Condensed Matter 05/2012; 85(9). DOI:10.1140/epjb/e2012-30407-5 · 1.46 Impact Factor
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    Sergey Smirnov, Milena Grifoni
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    ABSTRACT: We present a theory for the Kondo spin-1/2 effect in strongly correlated quantum dots. The theory is applicable at any temperature and voltage. It is based on a quadratic Keldysh effective action parameterized by a universal function. We provide a general analytical form for the tunneling density of states through this universal function for which we propose a simple microscopic model. We apply our theory to the highly asymmetric Anderson model with $U=\infty$ and describe its strong coupling limit, weak coupling limit and crossover region within a single analytical expression. We compare our results with numerical renormalization group in equilibrium and with a real-time renormalization group out of equilibrium and show that the universal shapes of the linear and differential conductance obtained in our theory and in these theories are very close to each other in a wide range of temperatures and voltages. In particular, as in the real-time renormalization group, we predict that at the Kondo voltage the differential conductance is equal to 2/3 of its maximum.
    Physical review. B, Condensed matter 03/2012; 87(12). DOI:10.1103/PhysRevB.87.121302 · 3.66 Impact Factor

Publication Stats

3k Citations
436.30 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
  • 2007
    • University of Freiburg
      Freiburg, Baden-Württemberg, Germany
  • 1999
    • Universität Stuttgart
      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