Condensed Matter Physics Seminar
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2021-10-29 (Friday)
room 1.02, Pasteura 5 at 12:15 
Cliò Agrapidis (IFT FUW)The fate of the spin polaron in the 1D t-J model
We study the intrinsic origin of the well-established differences in the motion of a single hole in the 1D and 2D antiferromagnet. To this end, we consider a 1D t-J model, perform the slave fermion transformation to the holon-magnon basis, and solve the obtained model in a numerically exact manner. We explicitly show that the spin polaron quasiparticle, which is well-known from the studies of a single hole in the 2D antiferromagnet, is destroyed in the 1D t-J model by the magnon-magnon interactions. Nevertheless, we observe surprising similarities between the spectra obtained with and without magnon-magnon interactions, indicating that some of the key features of the spin polaron physics are still preserved in 1D.
2021-10-22 (Friday)
room 1.02, Pasteura 5 at 12:15
Stanisław Sołtan (IFT FUW)Weak measurements and quantum Zeno effect
A quantum decaying system can reveal its nonclassical behavior by being noninvasively measured.Correlations of weak measurements in the noninvasive limit violate the classical bound for a universalclass of systems. The violation is related to incompatibility between exponential decay and unitaryevolution, and as such are closely related to the quantum Zeno effect. The phenomenon can be experimentallyobserved by a continuous weak measurement.
2021-10-15 (Friday)
room 1.02, Pasteura 5 at 12:15
Tadeusz Domański (UMCS Lublin)Phase transitions in time-domain: application to superconducting systems
2021-10-08 (Friday)
room 1.02, Pasteura 5 at 12:15
Oleksandr Gamayun (IFT UW)Modeling finite-entropy states with free fermions
The behavior of dynamical correlation functions in one-dimensional quantum systems at zero temperature is now very well understood in terms of linear and non-linear Luttinger models. The "microscopic" justification of these models consists in exactly accounting for the soft-mode excitations around the vacuum state and at most few high-energy excitations. At finite temperature, or more generically for finite entropy states, this direct approach is not strictly applicable due to the different structure of soft excitations. To address these issues we study the asymptotic behavior of correlation functions in one-dimensional free fermion models. On the one hand, we obtain exact answers in terms of Fredholm determinants. On the other hand, based on "microscopic" resummations, we develop a phenomenological approach that introduces the effective form factors and reduces the problem to the zero temperature case. The information about the initial state is transferred into the scattering phase of the effective fermions.I will demonstrate how this works for correlation functions in the XY model, mobile impurity, and the sine-kernel Fredholm determinants.