[Landau ITP Seminars] Friday 28.09.2018

[Serge Krashakov]

Уважаемые сотрудники ИТФ,

На заседании Ученого совета ИТФ в пятницу 28 сентября будет заслушан 
приглашенный доклад:

Victor Yakovenko (University of Maryland)
Superconductivity that breaks time-reversal symmetry and its 
experimental manifestations

Since 2006, it has been found experimentally that superconductivity 
spontaneously breaks time-reversal symmetry (TRS) in certain materials, 
such as Sr2RuO4, UPt3, URu2Si2, and Bi/Ni bilayers. In the latter case, 
we argue that the superconducting order parameter has the winding number 
of +-2 around the Fermi surface, thus making Bi/Ni bilayers a rare 
example of intrinsic 2D topological superconductivity [1]. The 
experimental evidence for TRS breaking comes from the polar Kerr effect, 
which is rotation of polarization of normally incident light upon 
reflection from the sample. Theoretical studies indicate that this 
effect is possible only if a superconductor has more than one band. To 
clarify these conditions, we study a model of chiral TRS-breaking 
superconductivity on the honeycomb lattice [2]. We consider 
superconducting pairing on the neighboring sites belonging to different 
sublattices. The matrix of this superconducting pairing is non-unitary 
and does not commute with the normal-state Hamiltonian. We find that the 
latter condition is necessary for experimental manifestations of the TRS 
breaking. We show that such superconducting pairing generates persistent 
loop currents around each lattice site and opens a topological mass gap 
at the Dirac points with the corresponding chiral edge states, as in 
Haldane's model of the quantum anomalous Hall effect. We calculate the 
intrinsic ac Hall conductivity in the absence of an external magnetic 
field, which determines the polar Kerr effect, and show that it is 
proportional to the loop-current order parameter.
References:
[1] X. Gong, M. Kargarian, A. Stern, D. Yue, H. Zhou, X. Jin, V. M. 
Galitski, V. M. Yakovenko, and J. Xia, Science Advances 3, e1602579 
(2017), arXiv:1609.08538
[2] P. M. R. Brydon, D. S. L. Abergel, D. F. Agterberg, and V. M. 
Yakovenko, arXiv:1802.02280


Кроме того, в 15:00 состоится семинар, на котором будет заслушан 
приглашенный доклад:

Werner Krauth, Ecole normale supérieure, Paris (France)
Irreversible Markov chains: From the TASEP to all-atom Coulomb computations

The Markov chain Monte Carlo method traditionally consists in exploring 
large configuration spaces using a reversible random walk where moves 
are accepted or rejected based on an energy criterion. In this talk, I 
will present recent progress on irreversible Markov chains that 
challenge this picture. In one-dimensional particle systems, the new 
algorithms are related to the TASEP (totally asymmetric simple exclusion 
model). We can rigorously prove that they mix on much shorter time 
scales than the reversible Metropolis algorithms. I will then show how 
these algorithms sample the Boltzmann distribution (and thus explore 
configuration space) without computing the energy. In long-range 
interacting systems, where the computation of the energy is 
time-consuming, this provides a key advantage for the new method. For 
locally charge-neutral systems in three dimensions, we obtain a highly 
efficient algorithm, of N log N complexity in the number N of particles. 
I discuss the main paradox of this method: How is it possible to sample 
the Boltzmann distribution without computing the energy, and then review 
some recent successes as well as prospects and challenges for 
irreversible Markov chains in statistical physics.
References:
S. C. Kapfer, W. Krauth, Physical Review Letters 119, 240603 (2017)
Z. Lei, W. Krauth, arXiv:1806.06786 (2018)
M. F. Faulkner, L. Qin, A. C. Maggs, W. Krauth, arXiv:1804.05795 (2018)

Приглашаются все желающие.