0000000000956762
AUTHOR
Riku Tuovinen
Time-dependent quantum transport in nanosystems : a nonequilibrium Green's function approach
A time-dependent extension to the Landauer–Büttiker approach to study transient quantum transport in arbitrary junctions composed of leads and conducting devices is developed. The nonequilibrium Green’s function approach is employed for describing the charge and heat transport dynamics. The importance of the developed method is that it provides a closed formula for the time-dependent density matrix in both electronic and phononic systems. In the electronic case the nonequilibrium conditions are due to a switch-on of a bias voltage in the leads or a perturbation in the junction whereas in the phononic case the central region of interest is coupled to reservoirs of di erent temperatures. In b…
Perheenjäsenten yhteinen fanius
Tutkielmassa tarkastellaan perheenjäsenten käsityksiä heidän yhteisestä fa- niudestaan ja se sijoittuu sekä fani- että perhetutkimuksen alueille. Fanitutkimus on tähän asti vahvasti painottunut aikuisten faniuteen, ja tutkimustietoa lasten faniudesta on toistaiseksi vähän. Myös faniuden merkitys perheen kontekstissa on vielä harvinainen tutkimuskohde. Lasten ja aikuisten kulttuurien erkaantu- essa yhä enemmän toisistaan on kuitenkin mielenkiintoista tutkia ilmiötä, joka yhdistää sukupolvia. Käsityksiä selvitettiin ryhmässä ja yksilöllisesti toteutettuina puolistruktu- roituina teemahaastatteluina. Tutkimukseen osallistui lapsia ja aikuisia viidestä perheestä. Saatu aineisto analysoitiin ain…
Curvature in graphene nanoribbons generates temporally and spatially focused electric currents
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 how transport gaps are affected both by the straight leads and by the degree of edge asymmetry in the curved ribbons. An advanced time-dependent approach shows that behind the façade of calm stationary transport the currents run violently: curvatu…
Time-dependent Landauer—Büttiker formalism for superconducting junctions at arbitrary temperatures
We discuss an extension of our earlier work on the time-dependent Landauer– Büttiker formalism for noninteracting electronic transport. The formalism can without complication be extended to superconducting central regions since the Green’s functions in the Nambu representation satisfy the same equations of motion which, in turn, leads to the same closed expression for the equal-time lesser Green’s function, i.e., for the time-dependent reduced one-particle density matrix. We further write the finite-temperature frequency integrals in terms of known special functions thereby considerably speeding up the computation. Simulations in simple normal metal – superconductor – normal metal junctions…
Electronic transport in molecular junctions : The generalized Kadanoff–Baym ansatz with initial contact and correlations
The generalized Kadanoff-Baym ansatz (GKBA) offers a computationally inexpensive approach to simulate out-of-equilibrium quantum systems within the framework of nonequilibrium Green's functions. For finite systems the limitation of neglecting initial correlations in the conventional GKBA approach has recently been overcome [Phys. Rev. B 98, 115148 (2018)]. However, in the context of quantum transport the contacted nature of the initial state, i.e., a junction connected to bulk leads, requires a further extension of the GKBA approach. In this work, we lay down a GKBA scheme which includes initial correlations in a partition-free setting. In practice, this means that the equilibration of the …
Phononic heat transport in the transient regime: An analytic solution
We investigate the time-resolved quantum transport properties of phonons in arbitrary harmonic systems connected to phonon baths at different temperatures. We obtain a closed analytic expression of the time-dependent one-particle reduced density matrix by explicitly solving the equations of motion for the nonequilibrium Green's function. This is achieved through a well-controlled approximation of the frequency-dependent bath self-energy. Our result allows for exploring transient oscillations and relaxation times of local heat currents, and correctly reduces to an earlier known result in the steady-state limit. We apply the formalism to atomic chains, and benchmark the validity of the approx…
Distinguishing Majorana Zero Modes from Impurity States through Time-Resolved Transport
We study time-resolved charge transport in a superconducting nanowire using time-dependent Landauer-B{\"u}ttiker theory. We find that the steady-state Majorana zero-bias conductance peak emerges transiently accompanied by characteristic oscillations after a bias-voltage quench. These oscillations are absent for a trivial impurity state that otherwise shows a very similar steady-state signal as the Majorana zero mode. In addition, we find that Andreev bound states or quasi-Majorana states in the topologically trivial bulk phase can give rise to a zero-bias conductance peak, also retaining the transient properties of the Majorana zero mode. Our results imply that (1) time-resolved transport m…
Time-dependent Landauer-Büttiker formula for transient dynamics
We solve analyti ally the Kadano Baym equations for a nonintera ting jun tion onne ted to an arbitrary number of nonintera ting wide-band terminals. The initial equilibrium state is properly des ribed by the addition of an imaginary tra k to the time ontour. From the solution we obtain the time-dependent ele tron densities and urrents within the jun tion. The nal results are analyti expressions as a fun tion of time, and therefore no time propagation is needed either in transient or in steady-state regimes. We further present and dis uss some appli ations of the obtained formulae. peerReviewed
Many-particle approach to the image-charge effect
Microscopical systems of molecules and atomic chains are studied to obtain information about quantum-transport phenomena. Both exact diagonalization method through second quantization and perturbation theory approach with Feynman diagrams are described, and then the energy spectra for the studied systems are calculated using the two methods.
Image charge dynamics in time-dependent quantum transport
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 responsible for the renormalization of the molecular levels neither in nor out of equilibrium. Furthermore, due to the time-local nature of the HF self-energy there exists a region in parameter space for which …
"Melkein aitoa Star Warsia" : lasten käsityksiä Star Wars -ilmiöstä
Tässä tutkimuksessa selvitettiin lasten käsityksiä Star Wars -ilmiöstä yhdessä esiopetusryhmässä. Tutkimuksen viitekehys koostui lasten kuluttajuustutkimuksen ja fanitutkimuksen tarkastelusta. Aineistonkeruu toteutettiin haastattelemalla lapsia pienryhmissä. Lasten käsitykset Star Warsista luokiteltiin aineistolähtöisellä sisällönanalyysillä ja johtopäätökset tehtiin luokittelun perusteella. Ryhmän pojat korostivat käsityksissään Star Warsin merkitystä leikeissä, sosiaalisissa suhteissa ja erilaisissa materiaaleissa, kuten vaatteissa. Ryhmän tytöt eivät juurikaan esittäneet kiinnostusta Star Warsia kohtaan. Star Wars tarjosi lapsia kiehtovan kehyksen erilaisille leikeille ja se oli lasten y…
Time-dependent Landauer-Büttiker formula: Application to transient dynamics in graphene nanoribbons
In this work we develop a time-dependent extension of the Landauer-B\"uttiker approach to study transient dynamics in time-dependent quantum transport through molecular junctions. A key feature of the approach is that it provides a closed integral expression for the time-dependence of the density matrix of the molecular junction after switch-on of a bias or gate potential which can be evaluated without the necessity of propagating individual single-particle orbitals. This allows for the study of time-dependent transport in large molecular systems coupled to wide band leads. As an application of the formalism we study the transient dynamics of zigzag and armchair graphene nanoribbons of diff…
Quantum interference and the time-dependent radiation of nanojunctions
Using the recently developed time-dependent Landauer-B\"uttiker formalism and Jefimenko's retarded solutions to the Maxwell equations, we show how to compute the time-dependent electromagnetic field produced by the charge and current densities in nanojunctions out of equilibrium. We then apply this formalism to a benzene ring junction, and show that geometry-dependent quantum interference effects can be used to control the magnetic field in the vicinity of the molecule. Then, treating the molecular junction as a quantum emitter, we demonstrate clear signatures of the local molecular geometry in the non-local radiated power.
Time-dependent Landauer-B\"uttiker formalism for superconducting junctions at arbitrary temperatures
We discuss an extension of our earlier work on the time-dependent Landauer--B\"uttiker formalism for noninteracting electronic transport. The formalism can without complication be extended to superconducting central regions since the Green's functions in the Nambu representation satisfy the same equations of motion which, in turn, leads to the same closed expression for the equal-time lesser Green's function, i.e., for the time-dependent reduced one-particle density matrix. We further write the finite-temperature frequency integrals in terms of known special functions thereby considerably speeding up the computation. Numerical simulations in simple normal metal -- superconductor -- normal m…