Seminarium Fizyki Materii Skondensowanej
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
2016-06-10 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15 
Wei Chen (ETH Zurich)Statistical aspects of topological phase transitions
The transition from topologically trivial to nontrivial state of a materialusually involves inverting the bulk electronic bands. We show that thefollowing aspects in statistical physics manifest themselves neartopological phase transitions: (1) Despite having no order parameter inthe usual sense, a single-particle correlation function can be defined forany topological lattice models. (2) The correlation length, as well as asusceptibility-like quantity have universal critical exponents thatsatisfy certain scaling laws. (3) The concept of scaling is introduced tojudge topological phase transitions, whose principle is akin to stretchinga messy string to reveal the number of knots it contains.
2016-06-03 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15
Jasper van Wezel (University of Amsterdam)Chiral charge and orbital order
Helical arrangements of spins are common among magnetic phases ofmatter. The first material to harbour a corkscrew pattern of electrondensity on the other hand, was discovered only relatively recently.This difference can be directly attributed to the nature of theinvolved order parameters: whereas rotating a magnetic vector around apropagation direction trivially yields a helical pattern, the purelyscalar charge density cannot straightforwardly support a chiral state.Here, I will show that spiral charge distributions can nonethelessspontaneously form in certain materials, as long as they areaccompanied by a simultaneous emergence of orbital order. Theapplication of this generic theory to two specific materials classeswill be discussed. I will also point out some of its unusualconsequences, like the presence of a nonlocal Hall effect, and thepossibility of an order-out-of-disorder transition.References:[1] J. Ishioka et al., Phys. Rev. Lett., 105, 176401 (2010).[2] J. van Wezel, Europhys. Lett., 96, 67011 (2011).[3] J.P. Castellan et al., Phys. Rev. Lett. 110, 196404 (2013).[4] H. Fukutome, Prog. Theor. Phys. 71, 1 (1984).[5] M. Gradhand et al., Phys. Rev. B, 92, 041111 (2015).
2016-05-20 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15
P. G. Silvestrov (TU Braunschweig, Germany)Noiseless manipulation of helical edge state transport by a quantum magnet
The current through a helical edge state of a quantum-spin-Hallinsulator may be fully transmitted through a magnetically gappedregion due to a combination of spin-transfer torque and spinpumping. Using a scattering approach, we argue that in such asystem the current is effectively carried by electrons withenergies below the magnet-induced gap and well below the Fermienergy. This has striking consequences, such as the absence ofshot noise, an exponential suppression of thermal noise, and anobstruction of thermal transport. For two helical edges covered bythe same quantum magnet, the device can act as robust noiselesscurrent splitter.
2016-05-13 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15
Maciej Maśka (UŚ Katowice)Superfluid insulator: the Kosterlitz-Thouless transition in the phase-fermion model
We consider a coupled boson-fermion model in two dimensions, thatdescribes itinerant fermions hybridizing with localized bosons composed ofpairs of tightly bound opposite-spin fermions. We trace out the fermionicdegrees of freedom and perform a Monte Carlo simulation for the effectiveclassical Hamiltonian of boson phases. We find that the fermions generatean effective long-range temperature-dependent boson-boson coupling that atlow temperature generates a quasi long range order. With increasingtemperature the Kosterlitz-Thouless transition is observed. Theboson-fermion interaction also modifies properties of the fermionsubsystem leading to a disordered bosonic insulator at high temperature inthe strong coupling regime.
2016-04-29 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15
Paweł Bryk (UMCS Lublin)Structure and phase transitions of diblock copolymer monolayers
We use grand canonical Monte Carlo simulation paired with multiplehistogram reweighting, hyperparallel tempering and finite size scaling toinvestigate the structure and phase behaviour of monolayers of diblockcopolymers. The chain molecules are arranged on the square lattice and weconsider both fully flexible and rod-coil polymer models. We investigatehow the length of the polymer and the segment-segment interactions affectthe phase behaviour of the adsorbed molecules. When the interactionsbetween the unlike segments are weaker than the interactions betweenthe segments of the same type we find that the order-disorder phasetransition can be either first-order or continous. The resulting topologyof the phase diagram involves either a triple point, critical end-point ora tricritical point. The mechanism of the order-disorder transitioninvolves the formation of domains resembling those observed during thespinodal decomposition process. The domains subsequently merge and arrangeinto lamellae. These observations are supported by integral geometryanalysis. When the interactions between the unlike segments are enhancedwith respect to the interactions between the segments of the same type,for the rod-coil molecules we observe the formation of a novel hairpinchessboard ordered structure. The resulting phase diagram is of theswan-neck type.
2016-04-15 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15
Michał Matuszewski (IF PAN)Instability of an exciton-polariton condensate
I will describe our work related to the recent observations ofinstabilities of non-equilibrium exciton-polariton condensates. Withoutany free parameters, we find an excellent agreement between theexperimental data and numerical simulations of the open-dissipativeGross-Pitaevskii equation in the case of nonresonant excitation. Thebreakup of the condensate is due to the long-sought reservoir-induceddynamical instability. In the case of resonant excitation, we observed thereal-space collapse of the polariton fluid explained by the effectiveattractive interactions mediated by the lattice phonons.
2016-04-08 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15
Krzysztof Szymański (Wydział Fizyki, Uniwersytet w Białymstoku)Metallic-type conductivity of a system with controlled disorder
A system with metallic type of conductivity and controlled degree ofdisorder was fabricated. The multicomponent and single phase series ofalloys were produced by melt spinning. Problem of perforation will bediscussed. Electronic specific heat and band structure calculationsindicate for metallic properties of the system. Temperature dependence ofspecific resistivity show systematic dependence on degree of structuraldisorder. Strong disorder cause decrease of resistivity with temperature,non typical for metallic systems.
2016-04-01 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15
Tadeusz Domański (Instytut Fizyki, Uniwersytet M. Curie-Skłodowskiej w Lublinie)Realizacja kwazicząstek Majorany w układach nanoskopowych
W referacie zostaną przytoczone fakty empiryczne, wskazujące narealizację kwazicząstek typu Majorany zaindukowanych nietrywialnymnadprzewodnictwem w strukturach nanoskopowych. Objaśnienie charakterutego rodzaju kwazicząstek zostanie przedstawione z perspektywy stanówzwiązanych Shiby/Andreeva, które jako stany brzegowe o zerowej energiipowstają w wyniku oddziaływania spin-orbita i wpływu polamagnetycznego. Doświadczalnym potwierdzeniem obecności stanów o zerowejenergii jest wzmocnienie przewodnictwa prądu tunelowania (zero-biasenhancement). Możliwe są alternatywne interpretacje tego zjawiska, np.jako przejaw zjawiska Kondo w pobliżu kwantowego przejścia fazowego.Nie mniej jednak zdecydowanie przeważają argumenty przemawiające nakorzyść kwazicząstek typu Majorany odpornych na różne zaburzenia,takie jak nieporządek, pola zewnętrzne itp.
2016-03-18 (Piątek)
Zapraszamy do sali 1.02, ul. Pasteura 5 o godzinie 12:15
Bartłomiej Dybiec (UJ)Selected properties of escape processes driven by $\alpha$-stable noises
Using a very simple setup, i.e. escape of a Brownian particle from afinite interval, some selected properties of stochastic dynamics will bepresented. Next, it will be demonstrated how these properties change wheninstead of a Brownian motion one considers more general random walk model,i.e. Levy flights, or when a finite interval is replaced with a half-lineor a point like domain. Finally, properties of first escapes and firstarrivals for Levy flights will be discussed.Levy flights provide natural extension of Brownian motion. Contrary to theBrownian motion trajectories of Levy flights are discontinuous. Discontinuity of trajectories changes conditions of absorption making themnon-local because due to long jumps Levy flight process can leave a domainof motion without hitting a boundary. Analogously, the process can hit atarget without hitting the target's boundary. This in turn makes adistinction between first arrivals and first passages. Moreover, longjumps introduces leapovers because point like boundaries are more likelyto be crossed than hit. As a consequence, properties of models with Levyflights are more intriguing and less intuitive than properties of theirBrownian counterparts.
2016-01-22 (Piątek)
Zapraszamy do sali 0.06, ul. Pasteura 5 o godzinie 12:15
Dieter Vollhardt (University of Augsburg)How does the interaction between electrons influence the lattice structure of a solid?
To answer the fundamental question posed in the title I will first discussthe concept of correlations in electronic systems and then introduce thestate-of-the-art DFT+DMFT scheme, which combines density functional theory(DFT) with the many-body dynamical mean-field theory (DMFT). Then I willexplain how to compute the total energy of correlated materials withinthis approach. One finds that the lattice stability of materials such asFe, FeSe (the simplest iron based superconductor), or V2O3 (the typicalexample of a system with Mott metal-insulator transition) can only beexplained if the local Coulomb repulsion between the electrons is includedexplicitly.