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Dec. 15

Room/TimeRoom 907 / 15:30-17:00
SpeakerKeiju Murata (Keio University)
TitleOut-of-time-order correlators in quantum mechanics  
AbstractThe out-of-time-order correlator (OTOC) is considered as a measure of quantum chaos. We formulate how to calculate the OTOC for quantum mechanics with a general Hamiltonian. We demonstrate explicit calculations of OTOCs for a harmonic oscillator, a particle in a one-dimensional box, a circle billiard and stadium billiards. For the first two cases, OTOCs are periodic in time because of their commensurable energy spectra. For the circle and stadium billiards, they are not recursive but saturate to constant values which are linear in temperature. Although the stadium billiard is a typical example of the classical chaos, an expected exponential growth of the OTOC is not found. We also discuss the classical limit of the OTOC. Analysis of a time evolution of a wavepacket in a box shows that the OTOC can deviate from its classical value at a time much earlier than the Ehrenfest time.

Dec. 5 

Room/TimeRoom 907 / 16:00-17:30
SpeakerNobuyuki Yoshioka (University of Tokyo)
TitleLearning Disordered Topological Phases by Statistical Recovery of Symmetry
AbstractIn machine learning, computational algorithms are constructed and executed to optimize the quantified objective of the problem to be solved. The surging development of the state-of-the-art techniques has led condensed matter physicists to realize the effectiveness of the tools in their own research field, such as phase classification [1,2], solving quantum many-body problem [3,4] and speeding up the Monte Carlo simulation [5]. 
Firstly we start with basic information to understand the classification by the artificial neural network (ANN), which maps out in our work the quantum phase diagram of disordered topological superconductor in class DIII. Given the disorder that keeps the discrete symmetries of the ensemble as a whole, translational symmetry which is broken in the quasiparticle distribution individually is recovered statistically by taking an ensemble average. By using this, we classify the phases by the ANN that learned the quasiparticle distribution in the clean limit and show that the result is totally consistent with the calculation by another independent approach.
If all three phases, namely the Z2, trivial, and the thermal metal phases appear in the clean limit, the machine can classify them with high confidence over the entire phase diagram. If only the former two phases are present, we find that the machine remains confused in the certain region, leading us to conclude the detection of the unknown phase which is eventually identified as the thermal metal phase. 
[1] J. Carrasquilla and R. G. Melko, Nat. Phys. 13, 431 (2017). 
[2] N. Yoshioka, Y. Akagi, and H. Katsura, arXiv:1709.05970. 
[3] G. Carleo and M. Troyer, Science 355, 602 (2017). 
[4] Y. Nomura, A. Darmawan, Y. Yamaji, and M. Imada, arXiv:1709.06475 (2017). 
[5] L. Huang and L. Wang, Phys. Rev. B 95, 035105 (2017).

Nov. 28

Room/TimeRoom 907 / 16:00-17:30
SpeakerHaruki Watanabe (University of Tokyo)
TitleFilling and Symmetry-Based Indicator of Many-Body Chern Number
AbstractThe topology of quantum many-body systems are deeply related to the average number of particles of the system and/or the symmetry representation of the ground state. One of the most established example of this relation is the so-called (Hasings-Oshikawa-) Lieb-Schultz-Mattis theorem, a theorem that applies to a very wide class of Hamiltonian regardless of the interaction strength or the spatial dimension of the system.  In this talk, I will first overview the HOLSM theorem and then discuss recent advances concerning the many-body Chern number.
The talk will be based on
A. Matsugatani, Y. Ishiguro, K. Shiozaki, and H. Watanabe, arXiv:1710.07012.

Nov. 24 

Room/TimeRoom 907 / 16:00-17:30
SpeakerZebin Qiu (University of Tokyo)
TitleElectrodynamics of Chiral Matter
AbstractA many-body system with chiral fermions can exhibit novel transport phenomena that violate parity and time-reversal symmetries, such as the chiral magnetic effect, the anomalous Hall effect, and the anomalous electric charge. We study the electromagnetic and optical properties of such systems by examining the electromagnetic sector of the Chern-Simons theory. Based on the Maxwell-Chern-Simons equations, we derive modified laws for the generation of static electromagnetic fields, and the propagation of electromagnetic waves.

Oct. 20 

Room/TimeRoom 907 / 16:00-17:30
SpeakerArata Yamamoto (University of Tokyo)
TitleCondensed Matter Application of Lattice QCD 

Oct. 6 

Room/TimeRoom 907 / 16:00-17:30
SpeakerToshiaki Fujimori (Keio University)
TitleExact resurgent trans-series and all-order multi-bion contributions in CP^N quantum mechanics
AbstractI will discuss resurgence structures in CP^N quantum mechanics derived from a supersymmetric model with a small SUSY breaking deformation parameter. By using the standard Rayleigh-Schrodinger perturbation theory, we exactly determine how the ground state energy respond to the deformation. The full resurgent trans-series expressions for the expansion coefficients of the ground state energy are found exactly in CP^1 quantum mechanics. We then discuss the semi-classical bion contributions in the complexified path integral formalism and compare them with the exact results. We determine all exact saddle point solutions corresponding to multi-bion configurations in the complexified theory. By evaluating the complexified quasi-moduli integral, we obtain all order multi-bion contributions which are consistent with the exact results.

June 23 

Room/TimeRoom 907 / 16:00-17:30
SpeakerShun Furusawa (RIKEN)
TitleNuclear Equation of State for Core-Collapse Supernovae
AbstractThe core-collapse supernovae are one of the most fascinating phenomena in astrophysics. The mechanism of these events is not clearly understood yet because of their intricacies. One of the underlying problems is uncertainties in the equations of state of hot and dense matter. In this talk, I will give an overview about the nuclear equation of state and its roles in the numerical simulations of supernovae. If time permits, I also introduce a recent study about transitions of hadronic matter to three-flavor quark matter, which may be realized during or after core-collapse supernovae.

June 9 

Room/TimeRoom 907 / 16:00-17:30
SpeakerNoriyuki Sogabe (Keio University)
TitleNew dynamic critical phenomena in nuclear and quark superfluids
AbstractWe study the static and dynamic critical phenomena near the possible high-density QCD critical point in the superfluid phase of nuclear and quark matter. In particular, we find that its dynamic universality class is different from those studied in QCD and condensed matter systems so far. We argue that this novelty stems from the interplay between the chiral criticality and the presence of the superfluid phonon---a feature specific for high-density QCD critical point.

May 8 

Room/TimeRoom 907 / 16:00-17:30
SpeakerDaisuke Satow (Goethe Universität)
TitleTransport coefficients of QGP in strong magnetic fields
AbstractWe compute the transport coefficients in magnetohydrodynamics at finite temperature, in strong magnetic fields (B), which is expected to be generated in heavy ion collision. We use the lowest Landau level approximation, in which the 1-to-2 scattering process is kinematically allowed in contrast to the B=0 case. We find that this effect of the magnetic field is significant in the electrical conductivity and the bulk viscosity: These quantities become sensitive to the current quark mass, and we discuss its physical origin in terms of the chirality conservation and the conformal invariance. This presentation is partially based on Phys. Rev. D 94, 114032 (2016) and arXiv:1610.06839 [hep-ph].

April 7 

Room/TimeRoom 907 / 16:00-17:30
SpeakerTakuya Kanazawa (RIKEN)
TitlePhases of circle-compactified QCD with adjoint fermions at finite density
AbstractWe study chemical-potential dependence of confinement and mass gap in QCD with adjoint fermions in spacetime with one spatial compact direction. By calculating the one-loop effective potential for the Wilson line in the presence of chemical potential, we show that a center-symmetric phase and a center-broken phase alternate when the chemical potential in unit of the compactification scale is increased. In the center-symmetric phase we use semiclassical methods to show that photons in the magnetic bion plasma acquire a mass gap that grows with the chemical potential as a result of anisotropic interactions between monopole-instantons. For the neutral fermionic sector which remains gapless perturbatively, there are two possibilities at non-perturbative level. Either to remain gapless (unbroken global symmetry), or to undergo a novel superfluid transition through a four-fermion interaction (broken global symmetry). If the latter is the case, there exists a new type of BEC-BCS crossover of the diquark pairing across 3 and 4 dimensions.

March 27 

Room/TimeRoom 907 / 16:00-17:30
SpeakerTomoya Hayata (Chuo University)
TitleChiral magnetic effect in multi-Weyl semimatals
AbstractNew topological phase of matters named Weyl semimetal has attracted growing attention in recent years. In a Weyl semimetal, Weyl fermions are realized as low-energy excitations near band touching points called Weyl points. The Weyl points act as monopoles in momentum space, and lead to topological transport phenomena such as the chiral magnetic effect. Since condensed matter system does not have Lorentz and rotational symmetries, there appear Weyl excitations with nonrelativistic dispersion relations, which results in the exotic chiral magnetic effect. In this talk, our recent work on such an exotic chiral magnetic effect in multi-Weyl semimetals is presented [1]. Multi-Weyl semimetal is a Weyl semimetal with multiple-monopole charge. We show that in static magnetic fields, the CME is manifestly topological, and given by the straightforward extension of that for conventional WSMs with unit monopole charge to multiple case. On the other hand, the dynamic CME with time-dependent magnetic field is not manifestly topological, but we found the topological feature hidden there. We propose an experimental setup to measure the multiple monopole charge via the topological nature hidden in the dynamic CME.
[1] Tomoya Hayata, Yuta Kikuchi, Yuya Tanizaki, arXiv:1703.02040 [cond-mat.mes-hall]

March 16 

Room/TimeRoom 907 / 14:00-15:30
SpeakerYork Schroder (UBB Chillan)
TitleFive-loop Beta function and anomalous dimensions in QCD
AbstractI provide an update on a long-term project that aims at evaluating massive vacuum Feynman integrals at the five-loop frontier, with high precision and in various spacetime dimensions. A number of applications are sketched, mainly concerning the determination of anomalous dimensions, for quantum field theories in four, three and two dimensions.