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Sep 23, 2013
09/13

by
P. Myöhänen; R. Tuovinen; T. Korhonen; G. Stefanucci; R. van Leeuwen

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In this work we investigate the effects of the electron-electron interaction between a molecular junction and the metallic leads in time-dependent quantum transport. We employ the recently developed embedded Kadanoff-Baym method [Phys. Rev. B 80, 115107 (2009)] and show that the molecule-lead interaction changes substantially the transient and steady-state transport properties. We first show that the mean-field Hartree-Fock (HF) approximation does not capture the polarization effects...

Source: http://arxiv.org/abs/1111.6104v1

2
2.0

Jun 30, 2018
06/18

by
N. Säkkinen; Y. Peng; H. Appel; R. van Leeuwen

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We study ground-state properties of a two-site, two-electron Holstein model describing two molecules coupled indirectly via electron-phonon interaction by using both exact diagonalization and self-consistent diagrammatic many-body perturbation theory. The Hartree and self-consistent Born approximations used in the present work are studied at different levels of self-consistency. The governing equations are shown to exhibit multiple solutions when the electron-phonon interaction is sufficiently...

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1403.2968

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3.0

Jun 30, 2018
06/18

by
G. Stefanucci; Y. Pavlyukh; A. -M. Uimonen; R. van Leeuwen

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We present a diagrammatic approach to construct self-energy approximations within many-body perturbation theory with positive spectral properties. The method cures the problem of negative spectral functions which arises from a straightforward inclusion of vertex diagrams beyond the GW approximation. Our approach consists of a two-steps procedure: we first express the approximate many-body self-energy as a product of half-diagrams and then identify the minimal number of half-diagrams to add in...

Topics: Other Condensed Matter, Condensed Matter

Source: http://arxiv.org/abs/1408.6163

3
3.0

Jun 30, 2018
06/18

by
S. E. B. Nielsen; M. Ruggenthaler; R. van Leeuwen

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In this work we focus on a recently introduced method [1] to construct the external potential $v$ that, for a given initial state, produces a prescribed time-dependent density in an interacting quantum many-body system. We show how this method can also be used to perform flexible and efficient quantum control. The simple interpretation of the density (the amount of electrons per volume) allows us to use our physical intuition to consider interesting control problems and to easily restrict the...

Topic: Quantum Physics

Source: http://arxiv.org/abs/1412.3794

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4.0

Jun 30, 2018
06/18

by
K. J. H. Giesbertz; R. van Leeuwen

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Electron correlations in molecules can be divided in short range dynamical correlations, long range Van der Waals type interactions and near degeneracy static correlations. In this work we analyze for a one-dimensional model of a two-electron system how these three types of correlations can be incorporated in a simple wave function of restricted functional form consisting of an orbital product multiplied by a single correlation function $f(r_{12})$ depending on the interelectronic distance...

Topics: Physics, Quantum Physics, Strongly Correlated Electrons, Atomic Physics, Chemical Physics,...

Source: http://arxiv.org/abs/1401.7277

12
12

Jun 28, 2018
06/18

by
E. Perfetto; A. -M. Uimonen; R. van Leeuwen; G. Stefanucci

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We put forward a first-principle NonEquilibrium Green's Function (NEGF) approach to calculate the transient photoabsorption spectrum of optically thin samples. The method can deal with pump fields of arbitrary strength, frequency and duration as well as for overlapping and nonoverlapping pump and probe pulses. The electron-electron repulsion is accounted for by the correlation self-energy, and the resulting numerical scheme deals with matrices that scale quadratically with the system size. Two...

Topics: Mesoscale and Nanoscale Physics, Atomic Physics, Physics, Condensed Matter

Source: http://arxiv.org/abs/1507.05590

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2.0

Jun 30, 2018
06/18

by
R. van Leeuwen; A. Castellanos-Gomez; G. A. Steele; H. S. J. van der Zant; W. J. Venstra

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We measure the energy relaxation rate of single- and few-layer molybdenum disulphide ($\mathrm{MoS_2}$) nanomechanical resonators by detecting the resonator ring-down. Recent experiments on these devices show a remarkably low quality (Q)-factor when taking spectrum measurements at room temperature. The origin of the low spectral Q-factor is an open question, and it has been proposed that besides dissipative processes, frequency fluctuations contribute significantly to the resonance line-width....

Topics: Mesoscale and Nanoscale Physics, Condensed Matter

Source: http://arxiv.org/abs/1405.5666

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Sep 23, 2013
09/13

by
A. Putaja; E. Rasanen; R. van Leeuwen; J. G. Vilhena; M. A. L. Marques

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We derive the semiclassical Kirzhnits expansion of the D-dimensional one-particle density matrix up to the second order in $\hbar$. We focus on the two-dimensional (2D) case and show that all the gradient corrections both to the 2D one-particle density and to the kinetic energy density vanish. However, the 2D Kirzhnits expansion satisfies the consistency criterion of Gross and Proetto [J. Chem. Theory Comput. 5, 844 (2009)] for the functional derivatives of the density and the noninteracting...

Source: http://arxiv.org/abs/1111.6474v1

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4.0

Jun 29, 2018
06/18

by
Y. Pavlyukh; A. -M. Uimonen; G. Stefanucci; R. van Leeuwen

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We present a systematic study of vertex corrections in the homogeneous electron gas at metallic densities. The vertex diagrams are built using a recently proposed positive-definite diagrammatic expansion for the spectral function. The vertex function not only provides corrections to the well known plasmon and particle-hole scatterings, but also gives rise to new physical processes such as generation of two plasmon excitations or the decay of the one-particle state into a two-particles-one-hole...

Topics: Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1607.04309

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Sep 22, 2013
09/13

by
M. Bonitz; K. Balzer; R. van Leeuwen

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It is proven that the center of mass (COM or Kohn) oscillation of a many-body system in a harmonic trap coincides with the motion of a single particle as long as conserving approximations are applied to treat the interactions. The two conditions formulated by Kadanoff and Baym \cite{kb-book} are shown to be sufficient to preserve the COM mode. The result equally applies to zero and finite temperature, as well as to nonequilibrium situations, and to the linear and nonlinear response regimes.

Source: http://arxiv.org/abs/cond-mat/0702633v1

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53

Sep 18, 2013
09/13

by
R. van Leeuwen; G. Stefanucci

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We present a compact and simplified proof of a generalized Wick theorem to calculate the Green's function of bosonic and fermionic systems in an arbitrary initial state. It is shown that the decomposition of the non-interacting $n$-particle Green's function is equivalent to solving a boundary problem for the Martin-Schwinger hierarchy; for non-correlated initial states a one-line proof of the standard Wick theorem is given. Our result leads to new self-energy diagrams and an elegant relation...

Source: http://arxiv.org/abs/1102.4814v2

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Sep 20, 2013
09/13

by
W. J. Venstra; R. van Leeuwen; H. S. J. van der Zant

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We demonstrate strong coupling between the flexural vibration modes of a clamped-clamped micromechanical resonator vibrating at low amplitudes. This coupling enables the direct measurement of the frequency response via amplitude- and phase modulation schemes using the fundamental mode as a mechanical detector. In the linear regime, a frequency shift of $\mathrm{0.8\,Hz}$ is observed for a mode with a line width of $\mathrm{5.8\,Hz}$ in vacuum. The measured response is well-described by the...

Source: http://arxiv.org/abs/1207.1594v2

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12

Jun 27, 2018
06/18

by
A. -M. Uimonen; G. Stefanucci; Y. Pavlyukh; R. van Leeuwen

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In a recent paper [Phys. Rev. B 90, 115134 (2014)] we put forward a diagrammatic expansion for the self-energy which guarantees the positivity of the spectral function. In this work we extend the theory to the density response function. We write the generic diagram for the density-response spectrum as the sum of partitions. In a partition the original diagram is evaluated using time-ordered Green's functions (GF) on the left-half of the diagram, antitime-ordered GF on the right-half of the...

Topics: Other Condensed Matter, Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1503.02158

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17

Jun 28, 2018
06/18

by
N. Säkkinen; Y. Peng; H. Appel; R. van Leeuwen

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We present a Kadanoff-Baym formalism to study time-dependent phenomena for systems of interacting electrons and phonons in the framework of many-body perturbation theory. The formalism takes correctly into account effects of the initial preparation of an equilibrium state, and allows for an explicit time-dependence of both the electronic and phononic degrees of freedom. The method is applied to investigate the charge neutral and non-neutral excitation spectra of a homogeneous, two-site,...

Topics: Mesoscale and Nanoscale Physics, Condensed Matter

Source: http://arxiv.org/abs/1507.04726

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40

Sep 23, 2013
09/13

by
K. Balzer; M. Bonitz; R. van Leeuwen; N. E. Dahlen; A. Stan

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The effect of electron-electron scattering on the equilibrium properties of few-electron quantum dots is investigated by means of nonequilibrium Green's functions theory. The ground and equilibrium state is self-consistently computed from the Matsubara (imaginary time) Green's function for the spatially inhomogeneous quantum dot system whose constituent charge carriers are treated as spin-polarized. To include correlations, the Dyson equation is solved, starting from a Hartree-Fock reference...

Source: http://arxiv.org/abs/0810.2425v1

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27

Sep 18, 2013
09/13

by
M. Ruggenthaler; S. E. B. Nielsen; R. van Leeuwen

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We analytically construct the wave function that, for a given initial state, produces a prescribed density for a quantum ring with two non-interacting particles in a singlet state. In this case the initial state is completely determined by the initial density, the initial time-derivative of the density and a single integer that characterizes the (angular) momentum of the system. We then give an exact analytic expression for the exchange-correlation potential that relates two non-interacting...

Source: http://arxiv.org/abs/1209.2949v3

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44

Sep 21, 2013
09/13

by
A. -M. Uimonen; E. Khosravi; G. Stefanucci; S. Kurth; R. van Leeuwen; E. K. U. Gross

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We study transport through an interacting model system consisting of a central correlated site coupled to finite bandwidth tight-binding leads, which are considered as effectively noninteracting. Its nonequilibrium properties are determined by real-time propagation of the Kadanoff-Baym equations after applying a bias voltage to the system. The electronic interactions on the central site are incorporated by means of self-energy approximations at Hartree-Fock, second Born and GW level. We...

Source: http://arxiv.org/abs/1106.5631v1

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62

Sep 21, 2013
09/13

by
N. Säkkinen; M. Manninen; R. van Leeuwen

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We benchmark many-body perturbation theory by studying neutral, as well as non-neutral, excitations of finite lattice systems. The neutral excitation spectra are obtained by time-propagating the Kadanoff-Baym equations in the Hartree-Fock and second Born approximations. Our method is equivalent to solving the Bethe-Salpeter equation with a high-level kernel while respecting self-consistently, which guarantees the fulfillment of a frequency sum rule. As a result, we find that a time-local...

Source: http://arxiv.org/abs/1108.4106v1

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3.0

Jun 28, 2018
06/18

by
M. Hyrkäs; D. Karlsson; R. van Leeuwen

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We give details on how to calculate spectral functions and Green's functions for finite systems using the Chebyshev polynomial expansion method. We apply the method to a finite Anderson impurity system, and furthermore give details on how to exploit its symmetry to transform the system into smaller orthogonal subsystems. We also consider systems connected to infinite leads, which we study by approximating the unknown self-energy with an exact self-energy for a finite system. As our test case,...

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1511.00962

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17

Jun 26, 2018
06/18

by
C. G. Rocha; R. Tuovinen; R. van Leeuwen; P. Koskinen

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Today graphene nanoribbons and other graphene-based nanostructures can be synthesized with atomic precision. But while investigations have concentrated on straight graphene ribbons of fixed crystal orientation, ribbons with intrinsic curvature have remained mainly unexplored. Here, we investigate electronic transport in intrinsically curved graphene nanoribbons coupled to straight leads, using two computational approaches. Stationary approach shows that while the straight leads govern the...

Topics: Materials Science, Condensed Matter

Source: http://arxiv.org/abs/1501.06934

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9.0

Jun 28, 2018
06/18

by
E. Kramer; D. J. J. van Dorp; R. van Leeuwen; W. J. Venstra

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We investigate the effect of mechanical strain on the dynamics of thin $\mathrm{MoS_2}$ nanodrum resonators. Using a piezoelectric crystal, compressive and tensile biaxial strain is induced in initially flat and buckled devices. In the flat device, we observe a remarkable strain-dependence of the resonance line width, while the change in the resonance frequency is relatively small. In the buckled device, the strain-dependence of the damping is less pronounced, and a clear hysteresis is...

Topics: Mesoscale and Nanoscale Physics, Condensed Matter

Source: http://arxiv.org/abs/1509.03080

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101

Jul 20, 2013
07/13

by
M. Ruggenthaler; K. J. H Giesbertz; M. Penz; R. van Leeuwen

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In a recent letter [Europhys. Lett. 95, 13001 (2011)] the question of whether the density of a time-dependent quantum system determines its external potential was reformulated as a fixed point problem. This idea was used to generalize the existence and uniqueness theorems underlying time-dependent density functional theory. In this work we extend this proof to allow for more general norms and provide a numerical implementation of the fixed-point iteration scheme. We focus on the one-dimensional...

Source: http://arxiv.org/abs/1203.5894v2

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84

Sep 24, 2013
09/13

by
E. Khosravi; A. -M. Uimonen; A. Stan; G. Stefanucci; S. Kurth; R. van Leeuwen; E. K. U. Gross

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The possibility of finding multistability in the density and current of an interacting nanoscale junction coupled to semi-infinite leads is studied at various levels of approximation. The system is driven out of equilibrium by an external bias and the non-equilibrium properties are determined by real-time propagation using both time-dependent density functional theory (TDDFT) and many-body perturbation theory (MBPT). In TDDFT the exchange-correlation effects are described within a recently...

Source: http://arxiv.org/abs/1112.2871v1

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110

Jul 20, 2013
07/13

by
A. -M. Uimonen; E. Khosravi; A. Stan; G. Stefanucci; S. Kurth; R. van Leeuwen; E. K. U. Gross

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We study time-dependent electron transport through an Anderson model. The electronic interactions on the impurity site are included via the self-energy approximations at Hartree-Fock (HF), second Born (2B), GW, and T-Matrix level as well as within a time-dependent density functional (TDDFT) scheme based on the adiabatic Bethe-Ansatz local density approximation (ABALDA) for the exchange correlation potential. The Anderson model is driven out of equilibrium by applying a bias to the leads and its...

Source: http://arxiv.org/abs/1107.0162v2

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246

Oct 5, 2013
10/13

by
R. van Leeuwen and N. E. Dahlen

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Contents: Introduction; Second quantization; The time contour; The Green function; The equations of motion; Conserving approximations; Outlook. Lecture Notes Collection FreeScience.info ID1576 Obtained from http://theochem.chem.rug.nl/research/vanleeuwen/literature/NGF.pdf http://www.freescience.info/go.php?pagename=books&id=1576

Topics: Non-equilibrium systems, "