


Forthcoming Seminars at F1
Tuesday 11 Dec 2018 11:15  Lucas Hackl (MPQ Garching)  Low energy spectra from variational tangent planes 

Variational methods are extensively used to study properties of ground states and low excited states. By focusing on a suitable submanifold of states, we can overcome the exponential growth of the Hilbert space with the system size. In this talk, I will introduce a geometric framework to approximate the low lying energy spectrum by studying the linearized Hamiltonian flow on the tangent plane at the ground state. The spectrum is then given by the eigenvalues of the covariant derivative on the variational manifold. Using the paradigmatic BoseHubbard model as an example, I will discuss advantages compared to methods based on directly diagonalizing the Hamiltonian restricted to the tangent plane.
Seminar room for physics (JSI main building) 
Seminars Archive
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 20 Oct 2015 15:00  Daniele Fausti  Witnessing quasiparticles 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 highenergy excitation of electronic origin in cuprates. I will
review some of our recent results in archetypal strongly correlated chargetransfer insulator (La2CuO4) revealing that photoexcitation in correlated materials pilots the formation of itinerant quasiparticles 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 11:00  Ian Affleck  From 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 semiconductor device was tailored to finally provide an indisputable experimental realization of this theory. I will give a nontechnical review of both theory and experiments. 
22 Sep 2015 15:00  Alaska Subedi  Theory of nonlinear phononics for coherent lightcontrol 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 highintensity midinfrared 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 lightinduced insulatortometal transition in manganites observed experimentally. Further more, we predict a new regime in which ultrafast instabilities that break crystal symmetry can be induced. This nonperturbative 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 15:00  Takami Tohyama  A numerical method to compute optical conductivity based on the pumpprobe simulations and its application to the hallfilled onedimensional extended Hubbard model 

V seminarski sobi za fiziko.
Abstract:
A numerical method to calculate optical conductivity based on a pumpprobe setup is discussed [1]. Its validity and limits are demonstrated both in equilibrium and out of
equilibrium. By employing either a steplike or a Gaussianlike 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 probepulse dependence of the optical conductivity in nonequilibrium The
numerical method is applied to nonequilibrium optical responses in the various phases of the hallfilled onedimensional 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 10:30  Felipe Barra  Thermodynamics 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 nonequilibrium physics. Recently it was noted (by R. Kosloff) that in those devices, which are boundarydriven, 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 15:00  Ambroz Kregar  Singlequbit transformations of spincharge 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 spincharge 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 singlequbit transformation on the time scale of 10 ns, allowing a few thousand transformation before spin coherence is lost.

9 Jun 2015 15:00  Jan Kogoj  Nonequilibrium Optical Conductivity of the Holstein Polaron 

3pm, seminarska soba za fiziko
Timeresolved ultrafast 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 nonequilibrium 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 photoexcitation in ultrafast 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 14:15  Taegeun Song  Quantum shuttling with Lorentz backaction 

V torek ob 14:15 v cajni sobi F1
Nanoelectromechanical (NEM) systems are attracting much interest not only because of its divers potential to the application for useful nanodevices but also
due to being an efficient tool for modern nanoelectronics. In the Coulomb block regime, a movable nanosized conductor shows interesting onebyone 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 backaction: Kondo shuttling device[2] and shuttlepromoted current switching device[3]. The
Kondo shuttling device, I will discuss the dynamical properties and possibility of the experimental realization of the dynamical Kondocloud probe by using
high me chanical dissipation tunability. For current switching device, the perspective on practical NEM application based on Lorentz backaction 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 14:30  Federico Becca in Luca Tocchio  Variational wave functions for correlated electron systems 

more... 
27 Mar 2015 14:30  Jacek Herbrych  Laser 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 10:30  Tilen Čadež  Intrinsic 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 spinorbit 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 15:00  Jure Kokalj  Effects 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 10:00  John Jefferson  Interconversion of static and flying electron spin qubits and EPR pair generation 

In this talk I will outline a method of generating fully entangled electronspin pairs using a static quantum dot and a movingquantum dot, realized by the propagation of a surface acoustic wave (SAW) along a onedimensional channel in a semiconductorheterostructure. Results and animations will be presented for loading and unloading the static dot with one or two electrons. Thetwoelectron dot will relax to a singlet ground state and this may be used to demonstrate a fully entangled staticflying (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 15:00  Zala Lenarcic  Charge recombination in excited onedimensional organic Mott insulators 

Seminarska soba za fiziko na IJS (106)
Recent femtosecond pumpprobe experiments on MottHubbard 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 onedimensional organic salt ETF 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 subleading effect of spin background, which is for onedimensional systems decoupled from charge. more... 
3 Jan 2015 15:00  Ziga Osolin  Fine 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 halffilling using the dynamical meanfield theory with numerical renor malization group as the impurity solver, focusing on the detailed structure of the spectral function, selfenergy, 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 inelasticscattering processes. These lead to additional structure inside the bands, in particular asymmetric resonances or dips that become more pronounced in the strongcoupling regime close to the antiferromagnetparamagnetic Kondo insulator quantum phase transition. more... 




7 Dec, 2018 Seminar: Dec 11: Low energy spec... Lucas Hackl (MPQ Garchi...  30 Nov, 2018 Seminar: Dec 4: DCtransport pr... Jan Skolimowski (JSI)...  28 Nov, 2018 Publication: Vidmar L., Hackl L., Bi... Phys. Rev. Lett. 121...  2 Nov, 2018 Seminar: Nov 6: Cesium solitons... Tadej Mežnaršič (F5...  26 Oct, 2018 Publication: Zhang Y., Vidmar L. and... Phys. Rev. A 98...  24 Oct, 2018 Abroad: Invited Talk Nejc Košnik...  24 Oct, 2018 Abroad: Invited Talk Jernej F. Kamenik...  19 Oct, 2018 Seminar: Oct 23: The smothered p... Friedrich Krien... 

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