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Seminarium Fizyki Materii Skondensowanej

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Zapraszamy na spotkanie o godzinie 12:15

Banhi Chatterjee (Jozef Stefan Institute, Ljbuljana, Slovenia)

Modeling of correlated electronic systems within DMFT (Online!)

The effect of electronic correlation is important to correctly describe the compounds with partially filled d-f shells; e.g. transition metal oxides, actinides, high Tc superconductors etc. Correlation effects give rise to a host of novel phenomena, e.g. Mott metal to insulator transition, excitonic phase transition, colossal magnetoresistance etc. The density functional theory (DFT) fails to properly account for the correlation effects and hence cannot explain these phenomena. Correlations can be modeled using the DFT+U, where U is the Coulomb interaction, or DFT+DMFT (Dynamical Mean Field Theory). It is shown for the 5f intermetallic compound UGa2, DFT+DMFT can reproduce the experimental X-ray absorption spectra, photo-emission spectra, and magnetic moments better than DFT+U for an optimized choice of spin polarized double counting.As a separate problem, it is discussed how DMFT in a real space (R- DMFT) within some additional approximation can describe Friedel oscillations (FO) in finite lattice systems in the presence of charged impurities for metallic, insulating and Mott insulating regime solving an inhomogeneous one band Hubbard model. Charged impurities are modeled using a single impurity potential, two, multiple discreet impurities, extended inhomogeneties, and a chain of impurities representing a domain wall. In each case, FO is damped with the interactions and disappears at the Mott transition. Interference patterns are seen on FO for two or more impurities and they are dependent on the lattice geometry. These model studies could be useful to understand the role of defects, dopants in transition metal oxides which are the target functional materials for the future Mott transistors. For seminar link contact Pawel Jakubczyk by email.

Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15

Krzysztof Myśliwy (IST Austria)

On the energy-momentum relation of the polaron at strong coupling

The polaron model is a basic model of quantum field theory describing a single particle interacting with a bosonic field. It arises in many physical contexts. We are mostly concerned with models applicable in the context of an impurity atom in a BEC as well as the problem of electrons in polar crystals.We investigate rigorously the ground state of the model at fixed momentum and for large values of the coupling constant. In the strong coupling regime, the bosonic field is expected to behave purely classically. Accordingly, the effective mass of the polaron at strong coupling is conjectured to be asymptotically equal to the one obtained from the semiclassical counterpart of the problem, first studied by Landau and Pekar in the 1940s. For polaron models with regularized form factors and phonon dispersion relations of superfluid type, i.e., linear for small wavenumbers as in the interacting Bose gas, we prove that for a large window of momenta below the radiation threshold, the energy-momentum relation as strong coupling is indeed essentially parabolic with semi-latus rectum equal to the Landau-Pekar effective mass as expected. For the Froehlich polaron describing electrons in polar crystals where the dispersion relation is of the optical type and the form factor is formally UV-singular due to the nature of the point charge-dipole interaction, we are able to give the corresponding upper bound. In contrast to the regular case, this requires the inclusion of the quantum fluctuations of the phonon field, which makes the problem considerably more difficult.During the talk, priority will be given to a healthy balance between rigor and physical intuition.Based on joint works with David Mitrouskas and Robert Seiringer.

Zapraszamy na spotkanie o godzinie 12:15

Jakub Zakrzewski (UJ Cracow)

New phases in non-standard Bose-Hubbard models

I will discuss possible new phases in Bose-Hubbard models extended to long-range interaction. In particular I will discuss the "infinite-range" model as resulting from an effective Hamiltonian describing bosons inside the cavity and pumped by an external laser source.

Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15

Anna Francuz (UJ, Kraków)

Determining topological order with tensor networks

Topologically ordered phases of matter, which fall beyond the Landau paradigm of phases and phase transitions, are especially hard to analyze due to degeneracy of the ground state and no local order parameter. In my talk I will introduce theories aiming at classifying topologically ordered states and novel numerical approaches, tensor networks, to determine both Abelian and non-Abelian topological order starting from a lattice Hamiltonian.

Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15

Felipe Sant'Ana (IFT UW)

Correlation aspects of interacting bosons in one dimension

In this talk I will discuss our developments on understanding the relevant excitations to calculate the one-body correlation function for the Lieb-Liniger model.Also, we'll talk about the harmonically trapped Lieb-Liniger and the achievements on studying the Tan's contact at finite temperatures.

Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15

Bartłomiej Wiendlocha (AGH University of Science and Technology, Krakow)

Effect of the spin-orbit coupling on the electron-phonon interaction in superconductors: first principles studies

Recent widespread interest in topological materials intensified studies on various compounds containing heavy elements, like Pb or Bi. This is of course related to the strong spin-orbit coupling (SOC), which should be present in such materials, and should strongly influence their physical properties. Because some of these materials exhibit superconductivity, a natural question arises what is the spin-orbit coupling effect of the electron-phonon interaction and superconductivity of such materials, containing heavy elements?Thanks to the ongoing development of computational techniques, calculations of the electron-phonon interaction function, taking into account the spin-orbit coupling, became available recently. In this work we present several case studies, where the spin-orbit interaction effects on the electronic structure, phonons, and the electron-phonon coupling (EPC) is investigated using density-functional calculations.As the prime example we will shortly discuss the role of spin-orbit interaction in determining the electronic and phononic properties of the elemental Pb and Pb-Bi alloy, which is probably the strongest coupled electron-phonon superconductor. Next we will discuss the case of a type-I superconductor CaBi2 and explain the mechanism behind the almost twice reduction of EPC constant by SOC. As the last example we will study a noncentrosymmetric ThCoC2, where SOC splits the Fermi surface but surprisingly has a little impact on the electron-phonon interaction.

Zapraszamy na spotkanie o godzinie 12:15

Józef Spałek (UJ, Cracow)

Can quantum particles become distinguishable?

The indistinguishability principle of quantum particles was introduced in physics by W. Natanson in 1911. Its most popular form used to this day has been proposed by P.A.M. Dirac in 1926 through the (anti)symmetry principle of the multiparticle wave function. The same is assumed for the quasiparticles emerging in interacting systems. Here I show that this principle may be violated in correlated systems, in which the effective quasiparticle mass may depend of its spin direction. This happens in the spin-polarized systems, where the indistinguishable particles of the unnpolarized state transform into distinguishable and subsuquently transform back to their undistinguishable correspondants upon reaching the magnetic saturation. General remarks concerning testing of this fundamental principle are planned.
Join Zoom Meeting
https://us02web.zoom.us/j/83686899432?pwd=RXNtZkhtZFpLazVmMVRFY1BtVlI0Zz09
Meeting ID: 836 8689 9432
Passcode: cond-matt

Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15

Mikhail Zvonarev (LPTMS, CNRS, Universite Paris-Saclay)

Mobile impurity in a quantum fluid

I am planning to discuss several recent findings (theoretical andexperimental) in the dynamics of an impurity particle injected into aquantum liquid:(i) The momentum distribution of the impurity in one dimension subjectto a constant external force exhibits characteristic Bragg reflectionsat the edge of an emergent Brillouin zone. As a consequence, theimpurity exhibits periodic dynamics that is interpreted as Blochoscillations, which arise even though the quantum liquid istranslationally invariant.(ii) The impurity injected into a one-dimensional liquid with someinitial momentum sheds only a part of it to the background gas, andforms a correlated state that no longer decays in time; furthermore,if the initial momentum is large enough, the impurity undergoeslong-lived oscillations. The value of the impurity's velocity atinfinite time lies between zero and the speed of sound in the gas, andis determined by the injection protocol. This way, the impurity'sfrictionless motion is a dynamically emergent phenomenon whosedescription goes beyond accounting for the kinematic constraints ofLandau's approach to superfluidity.(iii) The impurity traveling through a weakly interactingthree-dimensional Bose-Einstein condensate (BEC) of ultra-cold atomsshows a phase transition in the ground state and far-from-equilibriumproperties of the system as a function of the impurity's velocity.Above a critical initial velocity, the impurity's final velocityreaches a fundamental kinematic boundary, the BEC's speed of sound.For weak and intermediate impurity-BEC interactions, the critical lineis determined by the effective mass of the polaron. This quantumtransition can be interpreted as a dynamical Cherenkov effect; theimpurity-BEC interaction causes the dynamics of the system to appearas if the bare impurity is propagating through a BEC whose speed ofsound is altered by an effective refractive index.

Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15

Ipsita Mandal (IFJ PAN, Cracow)

Controlled Access to the Low-Energy Physics of Critical Fermi Surfaces

Condensed matter physics is the physics of solids and liquids (condensed phases of matter). It is the study of the complex behaviour of a large number of interacting particles such that their collective behaviour gives rise to emergent properties. In this talk, I will discuss some interesting quantum condensed matter systems with theirintriguing emergent phenomena arising from complexity.I will revisit Landau's Fermi liquid (FL) theory for normal metals, and thereby outline the properties of the non-Fermi liquid (NFL) metals (also called "strange" metals) which cannot be described within the Landau framework, due to the destruction of the Landau quasiparticles. In particular, I will focus on critical Fermi surface states, where there is a well-defined Fermi surface, but no quasiparticles, as a result of strong interactions between the Fermisurface and some emergent massless boson(s). I will outline a framework to extract the low-energy physics of such systems in a controlled approximation, using the tool of dimensional regularization.The seminar will be held online. You may join by following the link: https://us02web.zoom.us/j/83686899432?pwd=RXNtZkhtZFpLazVmMVRFY1BtVlI0Zz09

Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15

Grzegorz Łach (IFT FUW)

Residual entropy of ice-I

Water is known to form ~20 different solid phases. In many of them the orientations of water molecules in crystal lattice are disordered which leads to residual entropy as T -> 0. This configurational entropy has only been calculated exactly for 2D ice model on square lattice (Lieb, 1967). For 3D ice models on hexagonal (ice-Ih) and cubic (ice-Ic) lattices, their residual entropies have been estimated by Pauling (1932) and improved by Nagle and Onsager (1966) but since then there was no noticeable improvement neither in analytic results nor in numerical calculations. This left some unresolved questions, which I will address during the talk.

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