Forthcoming Seminars at F-1

Seminars Archive

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20 Oct 2015
Daniele FaustiWitnessing quasi-particles in a strongly correlated electron system
Cajna soba F1 ob 15h In this presentation, I will introduce the field of time domain studies of correlated electron systems focusing on the interaction between phonons and high-energy excitation of electronic origin in cuprates. I will review some of our recent results in archetypal strongly correlated charge-transfer insulator (La2CuO4) revealing that photo-excitation in correlated materials pilots the formation of itinerant quasi-particles which are suddenly dressed (<100 fs) by an ultrafast reaction of the bosonic field. I will introduce our new approach to address correlated electron systems by a full quantum state reconstruction of ultrashort light pulses. I will explain Time Resolved Quantum Tomography and show how this provide a new perspective for addressing by time domain studies the quantum state of low energy boson in complex materials. References Nature Comm. 5, 5112 (2014) New J. Phys. 16 043004 (2014)

2 Oct 2015
Ian AffleckFrom string theory to quantom dot experiments
seminarska soba za fiziko, Part of the string theory revolution of the 1980’s was the development of conformal field theory. While its applicability as a theory of everything is still unclear, it led to solutions of models of conduction electrons interacting with magnetic impurities that predict remarkable behavior. A gated semi-conductor device was tailored to finally provide an indisputable experimental realization of this theory. I will give a non-technical review of both theory and experiments.

22 Sep 2015
Alaska SubediTheory of nonlinear phononics for coherent light-control of solids
Seminarska soba za fiziko. Abstrakt:The use of light to control the structural and electronic properties of solids is an area of great interest for both basic research and potential applications. In this talk, I will present our recent work on a microscopic theory for ultrafast control of solids with high-intensity mid-infrared pulses. Our theory predicts the dynamical path taken by the crystal lattice using first principles calculations of the energy surface and classical equations of motion, as well as symmetry considerations. We identify two classes of dynamics. In the perturbative regime, displacements along the normal mode coordinate of the symmetry preserving Raman active mode can be achieved by cubic anharmonicities. This explains the light-induced insulator-to-metal transition in manganites observed experimentally. Further- more, we predict a new regime in which ultra-fast instabilities that break crystal symmetry can be induced. This non-perturbative effect involves a quartic anhar- monic coupling and occurs above a critical threshold below which the nonlinear dynamics of the driven mode displays softening and dynamical stabilization.

8 Sep 2015
Takami TohyamaA numerical method to compute optical conductivity based on the pump-probe simulations and its application to the hall-filled one-dimensional extended Hubbard model
V seminarski sobi za fiziko. Abstract: A numerical method to calculate optical conductivity based on a pump-probe setup is discussed [1]. Its validity and limits are demonstrated both in equilibrium and out of equilibrium. By employing either a step-like or a Gaussian-like probing vector potential, it is found that in nonequilibrium, the method can be related to the linear response theory [2] or a different generalized Kubo formula [3], respectively. The observation reveals the probe-pulse dependence of the optical conductivity in nonequilibrium The numerical method is applied to nonequilibrium optical responses in the various phases of the hall-filled one-dimensional extended Hubbard model [4,5]. [1] C. Shao, T. Tohyama, H.-G. Luo, and H. Lu, arXiv:1507.01200 [2] Z. Lenarcic, D. Golez, J. Bonca, and P. Prelovsek, Phys. Rev. B 89, 125123 (2014). [3] G. De Filippis, V. Cataudella, E. A. Nowadnick, T. P. Devereaux, A. S. Mishchenko, and N. Nagaosa, Phys. Rev. Lett. 109, 176402 (2012). [4] H. Lu, C. Shao, J. Bonca, D. Manske, and T. Tohyama, Phys. Rev. B 91, 245117 (2015). [5] N. Bittner, T. Tohyama, D. Manske, in preparation.

29 Jun 2015
Felipe BarraThermodynamics of boundary driven open quantum systems
Quantum mechanical devices such as quantum refrigerators or quantum heat en- gines with potential technological applications have been studied in recent years. These devices are also appealing from a fundamental perspective, as they are well suited for the study of non-equilibrium physics. Recently it was noted (by R. Kosloff) that in those devices, which are boundary-driven, there is a problem with the usual definition of the thermodynamic properties. I will briefly review this situation and formulate the thermodynamics of boundary- driven quantum systems in two related and often used scenarios. The first sce- nario is the repeated interaction scheme and the second scenario is the Lindblad dynamics for an open system interacting with the environment in such a way that the Lindblad operators act on the boundaries of the system. I will show that these operations generically require an amount of work neglected in the usual formula- tion of thermodynamics of open quantum systems. I will illustrate these findings in open spin 1/2 chains.

23 Jun 2015
Ambroz KregarSingle-qubit transformations of spin-charge states of electron on a ring in presence of Rashba coupling
Cajna soba F1. Quantum dots in semiconductor heterojunctions are one of the most suitable candidates for realization of quantum computer. Their advantages are long spin coherence times of electrons in semiconductor, well developed technology of their production and simple scalability, to only name a few. Traditionally, magnetic field is used to manipulate spin, but since it is difficult to use it to address single quantum dot, we seek different ways to control it. In our work we theoretically study the possibility of using only electric field to manipulate spin-charge states in quantum dot. The system of interest is an electron in ring shaped quantum dot, entrapped with external gate potential, enabling a controlled movement of the electron around the ring. By considering the potential as parabolic, we can analytically solve the Schrödinger equation with time dependent position of potential and adiabatically changing Rashba coupling. Spin degenerate ground states of harmonic oscillator, interpreted as qubit states, are transformed with unitary transformation, which can be presented as a rotation of points on Bloch sphere. We show that the axis of rotation depends on strength of Rashba coupling while the angle of rotation is controlled by the shift in position of the electron. Proper selection of translations of electron with suitable Rashba coupling enables an arbitrary single-qubit transformation on the time scale of 10 ns, allowing a few thousand transformation before spin coherence is lost.

9 Jun 2015
Jan KogojNon-equilibrium Optical Conductivity of the Holstein Polaron
3pm, seminarska soba za fiziko Time-resolved ultra-fast spectroscopy is a powerful experimental technique, en- abling the study of solids far from equilibrium. Recent advances have pushed the time resolution to extremely short time scales, much shorter than typical phonon oscillation period. It is thus illusory to expect that the system is in (thermal) equilibrium the whole time during probing. It is of utter importance that one knows which theories are applicable at which time scale in order to resolve a dilemma whether one should use strictly non-equilibrium concepts or are (quasi) equilibrium theories justified as well. We follow the time evolution of a Holstein polaron after a quantum quench, im- itating the photo-excitation in ultra-fast spectroscopy. We observe that selected properties indicate relaxation of the system from a highly excited state towards a steady state, dependent only on the total energy of the system. After fairly short times, calculated optical conductivity matches quite well with the thermal form with a well defined effective temperature, however, different initial states with equal total energy result in different effective temperatures. The system un- der investigation therefore does not thermalise in a strict sense despite exhibiting many signatures characteristic of thermalisation.

14 Apr 2015
Taegeun SongQuantum shuttling with Lorentz back-action
V torek ob 14:15 v cajni sobi F-1 Nanoelectromechanical (NEM) systems are attracting much interest not only because of its divers potential to the application for useful nano-devices but also due to being an efficient tool for modern nano-electronics. In the Coulomb block regime, a movable nano-sized conductor shows interesting one-by-one elec- tron transfer phenomenon due to the finite energy pumping originated from the Coulomb interaction, so called, the shuttling process.[1] In this talk, I will introduce two shuttling devices which are governed by Lorentz back-action: Kondo shuttling device[2] and shuttle-promoted current switching device[3]. The Kondo- shuttling device, I will discuss the dynamical properties and possibility of the experimental realization of the dynamical Kondo-cloud probe by using high me- chanical dissipation tunability. For current switching device, the perspective on practical NEM application based on Lorentz back-action will be discussed. [1] R. I. Shekhter, L. Y. Gorelik, I. V. Krive, M. N. Kiselev, A. V. Parafilo, and M. Jonson, Nanoelectromechanical Systems, 1, 1 (2013). [2] T. Song, M. N. Kiselev, K. Kikoin, R. I. Shekhter, and L. Y. Gorelik, New J. of Phys. 16, 033043 (2014). [3] T. Song, L. Y. Gorelik, R. I. Shekhter, M. N. Kiselev, and K. Kikoin. arXiv:1502.07524 (2015). [ Part 2, more...

30 Mar 2015
Federico Becca in Luca TocchioVariational wave functions for correlated electron systems

27 Mar 2015
Jacek HerbrychLaser controlled magnetization within large anisotropy S = 1 chain
Time evolution of the magnetization within large anisotropy S = 1 Heisenberg chain and circularly polarized laser (rotating magnetic filed) is studied numeri- cally and analytically. Results with constant laser frequency Ω = Ω0 are inter- preted in terms of absorption lines of electronic spin resonance spectrum. It is also shown that time dependent laser frequency Ω = Ω(t), the so–called chirping of the laser, is better protocol in order to get larger value of the magnetization or to magnetize the system fast. Both of the protocols yield orders of magnitude larger Mz for Hamiltonian with D > J than for adequate setups for Halden–like systems D < J. Furthermore, comparison of large anisotropy D results with with two–level toy model give satisfactory agreement. more...

26 Mar 2015
Tilen ČadežIntrinsic versus proximity induced zero energy modes in a superconductor with a ferromagnetic adatom chain
Recently an experiment was reported, where localized zero energy modes were observed in the system of a ferromagnetic adatoms on top of a conventional superconductor using scanning tunneling microscope. The results indicate the detection of the Majorana zero energy states. After a short presentation of the experiment I will present a simple 1D system, Kitaev’s chain, where the unpaired Majorana fermions are found at the end of a quantum wire. In the main part I will present the Majorana zero energy modes (MZEM) that occur in a conventional superconductor with a ferromagnetic adatom chain in the presence of Rashba spin-orbit interaction. I will assume a classical adatom magnetic moments and consider both proximity induced and intrinsic superconducting order. By exactly solving Bogoliubov de Gennes (BdG) equations in real space, I identify parametric regimes with the zero energy modes, which occur in both cases. In comparison to proximity induced superconductivity, is the region with MZEM shifted from the band edge to smaller chemical potential. This is connected to the decrease of the average gap function with increasing chemical potential. I will discuss similarities and differences between the MZEM for the two superconducting orders. more...

3 Mar 2015
Jure KokaljEffects of strong electronic correlations on thermal expansion
Many strongly correlated electron systems show poorly understood anomalies in the thermal expansion coefficient as well as in other properties related to lattice softening, e.g., in the sound velocity. In this talk I will present our work towards understanding these properties by focusing on electronic effects in the prototype strongly correlated material; organic charge transfer salts. I will start with the motivation from different experiments and continue with the introduction of a general theoretical framework. Then I will move on to the electronic contribution for strongly correlated electron model (Hubbard on anisotropic triangular lattice) and finish with the discussion of results and future challenges. more...

26 Feb 2015
John JeffersonInterconversion of static and flying electron spin qubits and EPR pair generation
In this talk I will outline a method of generating fully entangled electron-spin pairs using a static quantum dot and a movingquantum dot, realized by the propagation of a surface acoustic wave (SAW) along a one-dimensional channel in a semiconductorheterostructure. Results and animations will be presented for loading and unloading the static dot with one or two electrons. Thetwo-electron dot will relax to a singlet ground state and this may be used to demonstrate a fully entangled static-flying (EPR) pairwhen one of the electrons is ejected by the SAW. I will argue the feasibility of realising these ideas with existing semiconductortechnology and explain how the method may be extended to yield flying or static pairs that are fully entangled and arbitraryinterconversion of static and flying electron spin qubits.

27 Jan 2015
Zala LenarcicCharge recombination in excited one-dimensional organic Mott insulators
Seminarska soba za fiziko na IJS (106) Recent femtosecond pump-probe experiments on Mott-Hubbard insulators reveal charge recombination, which is in picosecond range, much faster than in clean band- gap semiconductors. I will present our proposal for the mechanism that explain the recombination in effectively one-dimensional organic salt ET-F 2TCNQ. I will show that fast recombination processes can be explained even quantitatively assuming that charge energy is transmitted to molecular vibrations. As suggested by experiments it is assumed that effectively positive (holon) and negative charge excitations (doublon) are bound in an exciton. Based on a model that ensure the existence of exciton and couples the charge to vibrations we can express the recombination rate analytically. At a reasonable coupling to vibrations the observed recombination rate is reproduced. I will comment also the sub-leading effect of spin background, which is for one-dimensional systems decoupled from charge. more...

3 Jan 2015
Ziga OsolinFine structure of spectra in antiferromagnetic phase of the Kondo lattice model
Seminarska soba za fiziko. We study the antiferromagnetic phase of the Kondo lattice model on bipartite lattices at half-filling using the dynamical mean-field theory with numerical renor- malization group as the impurity solver, focusing on the detailed structure of the spectral function, self-energy, and optical conductivity. We discuss the de- viations from the simple hybridization picture, which adequately describes the overall band structure of the system (four quasiparticle branches in the reduced Brillouin zone), but neglects all effects of the inelastic-scattering processes. These lead to additional structure inside the bands, in particular asymmetric resonances or dips that become more pronounced in the strong-coupling regime close to the antiferromagnet-paramagnetic Kondo insulator quantum phase transition. more...

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