Condensed Matter Physics Seminar
2006/2007 | 2007/2008 | 2008/2009 | 2009/2010 | 2010/2011 | 2011/2012 | 2012/2013 | 2013/2014 | 2014/2015 | 2015/2016 | 2016/2017 | 2017/2018 | 2018/2019 | 2019/2020 | 2020/2021 | 2021/2022 | 2022/2023 | 2023/2024 | 2024/2025
2014-06-06 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15 
Marcin Napiórkowski (KMMF WF UW)Phase Diagram of Interacting Bosons within Bogoliubov Theory
In my talk I shall describe the Bogoliubov theory as a variational problemin the class of quasi-free states. After deriving the relevant free energyfunctional, I will discuss properties of its minimizers. In particular, Iwill present the resulting phase diagram.
2014-05-30 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
Prof. Tadeusz Domański (Institute of Physics, M. Curie Sklodowska University, Lublin)Interplay between superconductivity and electron correlationsin the nanoscopic structures
Whenever quantum impurities are present in the bulk superconductors theyusually have a detrimental influence, either breaking the electron pairsor spoiling their long-range coherence. Recently a different kind ofrelationship attracts, however, a considerable amount of interest. Thereare numerous activities addressing the following question 'how can thesuperconducting reservoirs affect the quantum impurities if they arebrought in a contact ?'. In such circumstances the proximity effectspreads electron pairing onto the quantum impurities, converting them into'superconducting grains'. This effect plays a crucial role inhetero-structures, where the quantum dots are embedded between two (ormore) external reservoirs, forming the single electron transistors. Byvarying the gate potential one can then observe the quantum phasetransitions from the BCS-type to the singly occupied configurations.Furthermore, at sufficiently low temperatures also the Kondo physics canbe confronted with the electron pairing in a fully controllable manner [R.Maurand, Ch. Schonenberger, Physics 6, 75 (20013)]. I shall give a surveyon these and similar issues, both with regard tothe recent theoretical studies and experimental realizations.
2014-05-23 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
prof. Zbigniew Adamczyk (Instytut Katalizy i Fizykochemii Powierzchni PAN)Protein Adsorption – Quest for a Universal Approach
Protein adsorption studies are plagued by many confusions andmisinterpretations stemming from the abandonment of basic physics. Therefore, contrary to common views, the thesis of a dominant role ofCoulomb electrostatic interactions in protein adsorption phenomena is formulated. In order to prove this, a combination of theoretical anddirect, in situ experimental methods are implemented. In theoreticalmodeling emphasis is focused on coarse-grained methods, exploiting theefficient bead models of protein molecules. Using this approach basichydrodynamic properties of protein molecules, such as the diffusion tensor,hydrodynamic diameters and intrinsic viscosities are calculated using thelinear Stokes equation. Additionally, the surface blocking functions andjamming coverages for such model shapes are determined using the random sequential adsorption (RSA) simulations. Knowing the jamming coverage and blocking functions boundary conditions for bulk transport equations are formulated. Solutions of these equations for the convection and diffusion-controlled transport are discussed. The theoretical results areexploited for the interpretation of protein adsorption kinetics studied byAFM and in situ electrokinetic methods (streaming potential,microelectrophoresis). Application of such hybrid approaches enabled oneto quantitatively determine protein adsorption mechanisms on varioussubstrates including polymeric microparticles (polystyrene latexes). It is shown that protein adsorption is mainly governed by the discreteelectrostatic interactions among ion pairs with negligible role of otherinteraction types. Anomalous adsorption of proteins at surfaces bearinglike surface charges, where the classical, mean-field theories fail, isexplained in terms of heterogeneous charge distributions on proteinmolecules. By exploiting these experimental data, the validity of thecoarse grained approaches combined with solutions of the continuityequation for quantitatively predicting protein adsorption kinetics isconfirmed.
2014-05-16 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
Łukasz Cywiński (IFPAN)Decoherence of a localized electron spin interacting with a nuclear bath
An electron spin localized in a semiconductor is coupled by contacthyperfine interaction to the nuclei of the atoms of the host material. Thiscoupling is the main reason for decoherence of semiconductor-based spinqubits. Describing the process of decoherence is an interesting problem,because the experimentally relevant parameter regime corresponds to strongqubit-bath coupling, and the bath dynamics is very slow. Both of thesefeatures preclude the use of Born-Markov approximation commonly used fordescription of open quantum systems. I will try to give a survey oftheoretical methods which were used (with varying degree of success) toattack this problem during the last 10 years.
2014-04-11 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
Casper Drukier (University of Frankfurt)Non-analytic corrections in Fermi liquid theory
Recently non-analytic dependences of thermodynamic parameters of Fermiliquids have attracted much attention. In this talk I will discuss thesenon-analyticities which arise due to long range interactions betweenquasi-particles. In particular I will show that, in a weak externalmagnetic field, certain quasi-particle properties exhibit a non-analyticdependence on the magnetic field. The corresponding prefactors will becalculated to leading order in the interaction and it will be shown thatthey are determined fully by low-energy scattering processes. Finally Iwill discuss how the non-analytic terms can be detected in experiments.
2014-04-04 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
Jarosław Piasecki (IFT UW)Stability of fluid phases within the Kirkwood superposition approximation
A short presentation of the Kirkwood superposition approximationleading to a closure of the equilibrium YBG hierarchy will be followedby an analysis of the resulting asymptotic decay of correlations. Itwill be shown that in the case of hard discs, hard spheres, and hardhyperspheres the exponentially damped oscillations characterising thefluid phase become impossible beyond a certain threshold density thusindicating the freezing transition. In an analogous study of a square-well fluid an analytic criterion forthe existence of a liquid-vapor critical point will be presented.However, despite extensive numerical studies the possibility ofsatisfying the derived criterion could not been achieved. The problem hasbeen then solved quite unexpectedly by a simple analytic argument showingthat the generalized compressibility equations ruled out the existence ofa critical point within the Kirkwood superposition approximation.
2014-03-21 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
Jacek Wojtkiewicz (KMMF WF UW)O projektowaniu luminoforow: oddzialywanie eksperymentu i teorii
Luminofory (substancja swiecaca pod wplywem promieniowania UV lubwidzialnego) sa nieodlacznymi skladnikami wiekszosci wspolczesnych zrodelswiatla. Szeroko prowadzi sie prace nad zwiekszeniem ich wydajnosci orazaspektami widmowymi promieniowania emitowanego.W wystapieniu zamierzam opisac zagadke doswiadczalna, ktora pojawila sieprzy badaniu jednego z kandydatow na nowy luminofor: Dlaczego europ wpewnej matrycy fosforanowej swieci nie tak, jak powinien? Powiem rowniez oprobach wyjasnienia teoretycznego przy uzyciu obliczen DFT.
2014-02-28 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
prof. Witold Bardyszewski (IFT UW)Excitonic g-factor in GaN/AlGaN and InGaN/GaN quantum wells
One of the most important parameters characterizing the excitonic spectra in semiconductors in the magnetic field is the effective g-factor which determines the Zeeman splitting of excitonic levels. In the unstrained bulk GaN and InN the top valence subband corresponds to heavy holes, whereas in the bulk AlN, the sequence of levels is inverted with light-hole subband at the top. This inversion may also occur due to built-in tensile strain in properly designed pseudomorfic structures. In particular, in the case of GaN/AlGaN quantum wells (QWs) grown on GaN substrate or InGaN/GaN QWs grown on InGaN virtual buffer layer one obtains structures with tensile strained barriers. By tailoring QW parameters such as well and barrier width and substrate/barriers composition one may achieve the inversion of the valence subband levels and change the value of the g-factor.
2014-01-24 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
N. Gonzalez Szwacki (IFT UW)Magnetyczne właściwości rozcieńczonych półprzewodników magnetycznych III-V
Bardzo szybko rozwijająca się w ostatnich latach nowa dziedzina wiedzy, jaką jest spintronika, jest jedną z głównych przyczyn gwałtownego rozwoju badań nad nowymi materiałami mającymi umożliwić nowatorskie wykorzystanie elektrycznych i magnetycznych stopni swobody do magazynowania i przetwarzania informacji. Jedną z metod poznawczych właściwości tych materiałów jest wykonywanie obliczeń przy użyciu Teorii Funkcjonału Gęstości. W trakcie seminarium przedstawione zostaną właściwości strukturalne, magnetyczne i elektronowe półprzewodników GaAs i GaN domieszkowanych jonami magnetycznymi Mn, Fe i Cr. Dla tych związków badałem zachowanie się jonów magnetycznych (ich zdolność do tworzenia agregatów) zarówno w objętości jak i na powierzchni. Przebadany został również wpływ jednoczesnego domieszkowania atomami niemagnetycznymi (Si, Mg) i jonami metali przejściowymi na własności wspomnianych półprzewodników. Głównym celem moich badań było poznanie mechanizmu tworzenia się agregatów jonów magnetycznych w półprzewodniku, tak aby móc zablokować (lub sterować) ich powstawanie w trakcie otrzymywania tych materiałów. Nadrzędnym celem tych badań jest pogłębienie wiedzy na temat mechanizmu oddziaływania jonów magnetycznych w półprzewodniku magnetycznym, jako potencjalnego materiału ferromagnetycznego z temperaturą Curie powyżej temperatury pokojowej.
2014-01-10 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
dr hab. Jacek Wojtkiewicz (KMMF UW)Lieb-Schupp inequality in application to Heisenberg antiferromagnets
The Lieb-Schupp inequality is a matrix inequality, which allows to relate ground state energies of certain antiferromagnetic systems. In my talk, I will state this inequality and present numerical values for certain Heisenberg antiferromagnetic systems (chains, ladders and quasi-2d systems). I will discuss observed relation of fall-off of spin correlations with saturation of L-S inequality in considered systems. I plan also to mention possible relation of L-S inequality with properties of (analogon of) solvation force in quantum spin systems. (Talk is based on joint work with R. Skolasinski).