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Papers by Andrea Donarini

Physical Review B

Quantum dot spin valves are characterized by exchange fields which induce spin precession and gen... more Quantum dot spin valves are characterized by exchange fields which induce spin precession and generate current spin resonances even in absence of spin splitting. Analogous effects have been studied in double quantum dots, in which the orbital degree of freedom, the pseudospin, replaces the spin in the valve configuration. We generalize, now, this setup to allow for arbitrary spin and orbital polarization of the leads, thus obtaining an even richer variety of current resonances, stemming from the precession dynamics of entangled spin and pseudospin. We observe for both vectors a delicate interplay of decoherence, pumping and precession which can only be understood by also considering the dynamics of the spin-pseudospin correlators. The numerical results are obtained in the framework of a generalized master equation within the cotunneling approximation and are complemented by the analytics of a coherent sequential tunneling model.

Physical Review B, 2021

Recent lightwave-STM experiments have shown space and time resolution of single molecule vibratio... more Recent lightwave-STM experiments have shown space and time resolution of single molecule vibrations directly on their intrinsic length and time scales. We address here theoretically the electronic dynamics of a copper-phthalocynanine in a lightwave-STM, explored within a pump-probe cycle scheme. The spin-orbit interaction in the metallic center induces beatings of the electric charge flowing through the molecule as a function of the delay time between the pump and the probe pulses. Interference between the quasi-degenerate anionic states of the molecule and the intertwined dynamics of the associated spin and pseudospin degrees of freedom are the key aspects of such phenomenon. We study the dynamics directly in the time domain within a generalized master equation approach.

Physical review letters, Jan 4, 2017

The frontier orbital sequence of individual dicyanovinyl-substituted oligothiophene molecules is ... more The frontier orbital sequence of individual dicyanovinyl-substituted oligothiophene molecules is studied by means of scanning tunneling microscopy. On NaCl/Cu(111), the molecules are neutral, and the two lowest unoccupied molecular states are observed in the expected order of increasing energy. On NaCl/Cu(311), where the molecules are negatively charged, the sequence of two observed molecular orbitals is reversed, such that the one with one more nodal plane appears lower in energy. These experimental results, in open contradiction with a single-particle interpretation, are explained by a many-body theory predicting a strongly entangled doubly charged ground state.

Physical Review B, 2017

The Fano factor stability diagram of a C3v symmetric triangular quantum dot is analysed for incre... more The Fano factor stability diagram of a C3v symmetric triangular quantum dot is analysed for increasing electron fillings N. At low filling, conventional Poissonian and sub-Poissonian behavior, caused by the interplay of electron-electron interactions and Fermi statistics, is found. At larger filling, N ≥ 2, super-Poissonian noise and a peculiar bias voltage dependence of the Fano factor are observed at Coulomb and interference blockade. An analysis of the Fano map unravels a nontrivial electron bunching mechanism arising from the presence of degenerate many-body states combined with orbital interference and Coulomb interactions.

Beilstein Journal of Nanotechnology, 2015

The interplay of exchange correlations and spin–orbit interaction (SOI) on the many-body spectrum... more 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 that 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 scanning tunneling microscopy-based magnetoconductance measurements can yield clear signatures of both these SOI-induced effects.

Mathematics in Industry, 2008

Shuttle devices are a class of Nanoelectromechanical systems generically described as movable sin... more Shuttle devices are a class of Nanoelectromechanical systems generically described as movable single electron transistors. They exhibit an electromechanical instability from the standard tunnelling regime to the shuttling regime in which the quantum dot oscillates and transfer one electron per cycle. I present a theory for the device in which both the electrical and mechanical degrees of freedom are quantized. The different operating regimes are detected by analyzing current, noise and Wigner function distributions. The calculation of the stationary solution for the Generalized Master Equation which describes the system dynamics is the starting point for the evaluation of these quantities and represents a numerically challenging problem due to the size of the Hilbert space necessary to capture the tunnelling to shuttling transition.

Physical Review B, 2015

We report on a systematic study of the Coulomb blockade effects in nanofabricated narrow constric... more 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.

Physical Review Letters, 2013

In scanning tunneling experiments on semiconductor surfaces, the energy scale within the tunnelin... more In scanning tunneling experiments on semiconductor surfaces, the energy scale within the tunneling junction is usually unknown due to tip-induced band bending. Here, we experimentally recover the zero point of the energy scale by combining scanning tunneling microscopy with Kelvin probe force spectroscopy. With this technique, we revisit shallow acceptors buried in GaAs. Enhanced acceptor-related conductance is observed in negative, zero, and positive band-bending regimes. An Anderson-Hubbard model is used to rationalize our findings, capturing the crossover between the acceptor state being part of an impurity band for zero band bending, and the acceptor state being split off and localized for strong negative or positive band bending, respectively.

Physical Review B, 2020

We study the transport properties of an Anderson-Holstein model with orbital degeneracies and a t... more We study the transport properties of an Anderson-Holstein model with orbital degeneracies and a tunneling phase that allows for the formation of dark states. The resulting destructive interference yields a characteristic pattern of positive and negative differential conductance features with enhanced shot noise, without further asymmetry requirements in the coupling to the leads. The transport characteristics are strongly influenced by the Lamb-shift renormalization of the system Hamiltonian. Thus, the electron-vibron coupling cannot be extracted by a simple fit of the current steps to a Poisson distribution. For strong vibronic relaxation, a simpler effective model with analytical solution allows for a better understanding and moreover demonstrates the robustness of the described effects.

Physical Review B, 2016

We demonstrate the nonequilibrium tip induced control of the spin state of copper phthalocyanine ... more We demonstrate the nonequilibrium tip induced control of the spin state of copper phthalocyanine on an insulator coated substrate. We find that, under the condition of energetic proximity of many-body neutral excited states to the anionic ground state, 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 scanning tunneling microscopy 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.

Phys Rev B, Oct 14, 2008

Interference effects strongly affect the transport characteristics of a benzene single-electron t... more Interference effects strongly affect the transport characteristics of a benzene single-electron transistor (SET) and for this reason we call it interference SET (I-SET). We focus on the effects of degeneracies between many-body states of the isolated benzene. We show that the particular current blocking and selective conductance suppression occurring in the benzene I-SET are due to interference effects between the orbitally degenerate states. Further we study the impact of reduced symmetry due to anchor groups or potential drop over the molecule. We identify in the quasi-degeneracy of the involved molecular states the necessary condition for the robustness of the results.

A microscopic theory of the transport in a scanning tunneling microscope (STM) setup is introduce... more A microscopic theory of the transport in a scanning tunneling microscope (STM) setup is introduced for π-conjugated molecules on insulating films, based on the density matrix formalism. A key role is played in the theory by the energy dependent tunneling rates which account for the coupling of the molecule to the tip and to the substrate. In particular, we analyze how the geometrical differences between the localized tip and extended substrate are encoded in the tunneling rate and influence the transport characteristics. Finally, using benzene as an example of a planar, rotationally symmetric molecule, we calculate the STM current-voltage characteristics and current maps and analyze them in terms of few relevant angular momentum channels.

We discuss methods for numerically solving the generalized Master equation GME which governs the ... more We discuss methods for numerically solving the generalized Master equation GME which governs the time-evolution of the reduced density matrix of a mechanically movable mesoscopic device in a dissipative environment. As a specific example, we consider the quantum shuttle -- a generic quantum nanoelectromechanical system (NEMS). When expressed in the oscillator basis, the static limit of the GME becomes a

A quantum shuttle is an archetypical nanoelectromechanical device, where the mechanical degree of... more A quantum shuttle is an archetypical nanoelectromechanical device, where the mechanical degree of freedom is quantized. Using a full-scale numerical solution of the generalized master equation describing the shuttle, we have recently shown (Novotny et al., Phys. Rev. Lett. 92, 248302 (2004)) that for certain limits of the shuttle parameters one can distinguish three distinct charge transport mechanisms: (i) an

An ab initio based theoretical approach to describe nonequilibrium many-body effects in molecular... more 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 are 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.