Condensed Matter Physics Seminar
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2013-01-11 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15 
dr Adam Bednorz (IFT UW)Fundamental violation of time symmetry in quantum noninvasive measurements
Classical direct measurements are noninvasive but quantum are invasive (byprojection). However, for indirect measurements, there exists a speciallimit where both classical and quantum measurements are noninvasive.Surprisingly, classical noninvasive measurements are symmetric withrespect to time reversal but quantum are not. This fundamental paradox iscan be tested in many system e.g. double well, or a quantum dot.
2012-12-14 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
dr hab. Jakub Tworzydło (IFT UW)Thermal metal in topological superconductors
The thermal quantum Hall effect appears in the absence of time-reversalsymmetry in a single layer of a chiral p-wave superconductor. Theinsulator-insulator transition associated with this effect is genericallypresent only for weak disorder. For stronger disorder one finds atransition to a delocalized phase, known as the thermal metal.In contrast, for a helical superconductor in the presence of time-reversalsymmetry the transition between two topologically distinct thermalinsulators occurs via the intervening thermal metal phase, even for anarbitrarily weak disorder.We present a detailed study of a phase diagram and the critical exponentsfor these topologically induced transitions. We also point to an underlingmechanism of Majorana states formation in both chiral and helical systems.
2012-12-07 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Jakub Tworzydło (IFT UW)Topological Anderson insulator
First part of my talk will contain an introduction to the subject of socalled topological insulators. I will then present an effective mediumtheory that explains the disorder-induced transition into a phase ofquantized conductance, discovered in computer simulations of CdHgTequantum wells. The calculated location of the phase boundary at weakdisorder agrees with numerical simulations pointing to the underlyingcrossing of a band edge rather than a mobility edge. The strong disorderphase boundary can be interpreted as an insulator-insulator transition inthe universality class of integer quantum Hall effect.
2012-11-30 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
dr Jan Kunes (Institute of Physics, Academy of Sciences of the Czech Republic, Prague)Towards material specific theory of ordering phenomena: two-particle response in DMFT
Application of the dynamical mean-field theory (DMFT) to multi-band models obtained with ab initio bandstructure methods has lead to betterqualitative and quantitative understanding of correlated-electronmaterials. However, mostly one-particle or local quantities were studiedwithin this approach so far. We will present an efficient numerical methodfor calculation of the non-local two-particle response functions.Computations of this type can be used to search for divergencies, whichidentify instabilities towards long-range ordering. Applications tobenchmark cases such as single-band and two-band Hubbard model will bepresented.
2012-11-23 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Paweł Jakubczyk (IFT UW)Casimir forces in the O(N) models
I present a renormalization-group calculation of the thermal Casimirforces in the O(N) models subject to periodic boundary conditions.
2012-11-16 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Dr Jagoda Slawinska (University of Lodz)Electronic properties of graphene systems studied by first-principles and experimental methods
Graphene's unique properties seem promising for various applications, butin any active device, the layer must interact with the surroundingenvironment which usually perturbs the intrinsic features. On the otherhand, the substrate-induced modifications can be useful to tailor theproperties for desired purposes. For example, combining graphene andtwo-dimensional hexagonal boron nitride (h-BN) allows one to constructmultilayers with electronic properties tunable by electric field effect ina wide range, in particular the transition from exactly linear dispersionto the band-gap opened in the spectrum can be achieved for specificconfiguration. In case of graphene weakly interacting with metallicsurface we have shown, both theoretically and experimentally, that someproperties, such as doping level, can be varied spatially by local changesin the structure of the substrate.
2012-11-09 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
dr Tomasz A. Zaleski (Institute of Low Temperature and Structure Research PAN, Wrocław)Correlations in systems of ultra-cold bosons in optical lattices
Ultra-cold atomic gases in optical lattices are experimental realizationof interacting quantum many-body systems in very clean form. The systemparameters can be precisely controlled and, as a result, strongcorrelation regime can be reached, which shares a lot of similarities withstrongly interacting electronic systems in solid-state physics.Understanding of the behavior of such systems is a challenge fortheoretical approaches. I will present a method being a combination ofBogoliubov theory and quantum rotor model, which makes it possible toanalyze spatial correlations between atoms as a function of latticegeometry and system parameters. This allows me to calculate variousquantities, like time-of-flight absorption images or single-particlespectral functions, also in presence of additional external factors(synthetic magnetic field) and compare them with available experimentalresults.
2012-10-26 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Dr hab. Romuald Lemanski (Institute of Low Temperature Physics, Polish Academy of Sciences, Wroclaw, Poland)Gapless metallic charge-density-wave phase driven by strong electron Correlations
We analyze the transformation from insulator to metal induced by theorder-disorder phase transition within a framework of the spinlessFalicov-Kimball model. Using the Dynamic Mean Field Theory (DMFT)formalism on the Bethe lattice we find rigorously the temperaturedependent Density of States ($DOS$) at half filling in the limit of highdimensions. At zero temperature ($T=0$) the system is ordered to form thecheckerboard pattern and the $DOS$ has the gap $\Delta$ at the Fermi level$E_F=0$, which is proportional to the interaction constant $U$.Having calculated the temperature dependent $DOS$ we study thermodynamicproperties of the system starting from determination of its free energy$F$. Then we find how the order parameter $d$ and the gap $\Delta $ changewith $T$. Finally, we construct the phase diagram in the variables $T$ and$U$, where we display regions of stability of four different phases:ordered insulator, ordered metal, disordered insulator and disorderedmetal.
2012-10-19 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Pawel Jakubczyk (IFT UW)Quantum interface unbinding transitions
We consider interfacial phenomena accompanying bulk quantum phasetransitions in presence of surface fields. We argue that the surfacecontribution to the system free energy involves a singularitycharacteristic of an interfacial phase transition, occurring below thebulk transition temperature Tc. Relying on a phenomenological effectiveinterfacial model we compute the interfacial phase diagram and extract thevalues of the exponents describing the leading interfacial singularitiesin spatial dimensionality d ≥ 3.
2012-10-12 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Prof. dr hab. Jacek Majewski (IFT UW)