Seminars in 2022
* Past seminars in 2021. For the most recent ones, see here.
| 2022/11/29 13:30-15:00 @ Zoom | |
| Masataka Matsumoto (Shanghai U.) | Tricritical phenomena in holographic chiral transitions |
| Tricritical point (TCP) is the end-point of a line of three-phase coexistence (a triple line) at which three coexisting phases simultaneously become identical. A TCP can be observed in various systems, for example, the QCD phase diagram with the chiral limit and a metamagnet such as a FeCl2 crystal. In the AdS/CFT correspondence, a TCP associated with a chiral phase transition has been found in the D3/D7 model [1]. In this talk, I will discuss the recent study [2] of critical phenomena at a tricritical point which emerges in the D3/D7 model in the presence of a finite baryon number density and an external magnetic field. We found all the critical exponents defined in this paper take the mean-field values. I will also compare the results with our previous works about the critical phenomena at the TCP that emerges in the steady state [3,4]. [1] N. Evans, A. Gebauer, K.-Y. Kim, M. Magou, JHEP 03 (2010) 132, arXiv:1002.1885. [2] M. Matsumoto, arXiv:2208.02605. [3] T. Imaizumi, M. Matsumoto, S. Nakamura, Phys. Rev. Lett. 124, 191603 (2020), arXiv:1911.02626. [4] M. Matsumoto, S. Nakamura, Phys. Rev. D 106, 026006 (2022), arXiv:2201.06894. | |
| 2022/10/20 13:30-15:00 @ Zoom | |
| Minoru Eto (Yamagata U.) | Quantum nucleation of topological solitons |
| The chiral soliton lattice is an array of topological solitons realized as ground states of QCD at finite density under strong magnetic fields or rapid rotation, and chiral magnets with an easy-plane anisotropy. In such cases, topological solitons have negative energy due to topological terms originating from the chiral magnetic or vortical effect and the Dzyaloshinskii-Moriya interaction, respectively. We study quantum nucleation of topological solitons in the vacuum through quantum tunneling in 2+1 and 3+1 dimensions, by using a complex ϕ4 (or the axion) model with a topological term proportional to an external field, which is a simplification of low-energy theories of the above systems. In 2+1 dimensions, a pair of a vortex and an anti-vortex is connected by a linear soliton, while in 3+1 dimensions, a vortex is string-like, a soliton is wall-like, and a disk of a soliton wall is bounded by a string loop. Since the tension of solitons can be effectively negative due to the topological term, such a composite configuration of a finite size is created by quantum tunneling and subsequently grows rapidly. We estimate the nucleation probability analytically in the thin-defect approximation and fully calculate it numerically using the relaxation (gradient flow) method. The nucleation probability is maximized when the direction of the soliton is perpendicular to the external field. | |
| 2022/10/5 13:30-15:00 @ Zoom | |
| Yusuke Nishida (Tokyo Tech) | Transport coefficients of a Bose gas in one dimension |
| I will present two of our recent studies on transport coefficients of a Bose gas in one dimension. The first part is on the thermal conductivity [1], which is typically divergent for quantum integrable systems in one dimension. However, it is found to be finite and dominated by an effective three-body interaction that inevitably arises by confining bosons into a tight matter waveguide. The second part is on the bulk viscosity [2], which is computed perturbatively in the high-temperature, weak-coupling, and strong-coupling limits. In particular, the strong-coupling limit is accessible thanks to the Bose-Fermi duality, which is shown for the dynamic bulk viscosity provided by the contact-contact response function. [1] T. Tanaka and Y. Nishida, arXiv:2203.04936 [cond-mat.stat-mech] "Thermal conductivity of a weakly interacting Bose gas by quasi-one dimensionality" [2] T. Tanaka and Y. Nishida, arXiv:2206.07848 [cond-mat.quant-gas] "Bulk viscosity of dual Bose and Fermi gases in one dimension" | |
| 2022/7/25 13:30-15:00 @ Zoom | |
| Hiroyasu Tajima (UEC) | Superconducting-like heat current: Effective cancellation of current-dissipation trade-off by quantum coherence |
| Recent developments in statistical mechanics have revealed a tradeoff between heat current and dissipation [1,2]. In various situations, this current-dissipation tradeoff represents a relationship between thermal energy flow and entropy increase, similar to Joule’s law W=RI^2. On the other hand, the coherence effect on the current-dissipation tradeoff has not been thoroughly analyzed. Here, we systematically analyze how coherence affects the current-dissipation tradeoff [3]. The results can be summarized in the following three rules: 1. Quantum coherence between different energy levels strengthens the trade-off. In other words, the ratio between the square of the heat current and the entropy production ratio corresponding to electrical resistance R (hereafter referred to as "thermal resistance") is increased by the superposition of different energy levels. 2. Coherence between degeneracies weakens the trade-off. That is, thermal resistance is weakened by coherence between degeneracies. 3. With enough coherence between degeneracies, we can cancel the trade-off effectively and make the thermal resistance approximately zero. Then, macroscopic heat flow without entropy increase is realized. These three results directly reveal the coherence effects on heat engine performance. That is, coherence between different energy levels reduces the performance, while coherence between degeneracies increases it. And when there is a sufficient amount of coherence between degeneracies, the efficiency can asymptotically reach the Carnot efficiency (\eta=\eta_{Car}-O(1/N)) while the power is O(N). [1] N. Shiraishi, K. Saito, H. Tasaki, Phys. Rev. Lett. 117, 190601 (2016). [2] A. C. Barato and U. Seifert, Phys. Rev. Lett. 114, 158101 (2015). [3] H. Tajima, K. Funo, Phys. Rev. Lett. 127, 190604 (2021). | |
| 2022/6/15 13:30-15:00 @ Zoom | |
| Yuki Fujimoto (Washington U.) | Non-Abelian vortices in two-flavor dense QCD |
| Recently, the phase of the two-flavor quark matter with the new
pattern of color superconductivity was proposed so that the continuous
crossover from the hadronic to the quark phase is realized [1]; it is
in consonance with the recent observation of neutron stars.
In this talk, I will show the classification of the topological
vortices in this phase. We found that the stable vortices are what we
call the "non-Abelian Alice strings" [2]. They are superfluid
vortices carrying 1/3 quantized circulation and color magnetic fluxes.
I will discuss their properties in comparison to the well-established
CFL vortices in three-flavor symmetric setup, by putting some emphasis
on their peculiarity: the non-Abelian generalization of the Alice
property. I will then discuss in detail the possibility that these
vortices are confined as well as how the vortices in the quark phase
can be connected to those in the hadronic phase [3]. [1] Y. Fujimoto, K. Fukushima, W. Weise, PRD 101, 094009 (2020) [1908.09360]. [2] Y. Fujimoto, M. Nitta, PRD 103, 054002 (2021) [2011.09947]; JHEP 09 (2021) 192 [2103.15185]. [3] Y. Fujimoto, M. Nitta, PRD 103, 114003 (2021) [2102.12928]. | |
| 2022/5/25 13:30-15:00 @ Zoom | |
| Catherine Beauchemin (RIKEN iTHEMS) | Equilibrium or not? Mathematical differences between acute & chronic virus infections |
| The widely acclaimed 1995/1996 papers by Ho, Perelson and others [1,2] demonstrated the important insights that come from mathematical modelling of virus infection kinetics within a person. But there are key dynamical differences between chronic and acute infections, namely whether the infection reaches or maintains some equilibrium or not. In this talk, I will introduce the equations used to describe a virus infection within a person. I will show some of the tricks used by mathematical modellers to extract important rate estimates from measurements in patients infected with chronic diseases, like HIV or Hepatitis C virus. I will explain why it is difficult to extract meaningful information from measurements in patients with an acute infection, like influenza or possibly COVID-19 [3]. I hope to hear from the audience if they have any thoughts about overcoming the issue to extract better rate information from limited data in patients with acute infections. [1] Ho DD, Neumann AU, Perelson AS, Chen W, Leonard JM, Markowitz M., "Rapid turnover of plasma virions and CD4 lymphocytes in HIV-1 infection," Nature. 1995 Jan 12;373(6510):123-6. doi:10.1038/373123a0 [2] Perelson AS, Neumann AU, Markowitz M, Leonard JM, Ho DD., "HIV-1 dynamics in vivo: virion clearance rate, infected cell life-span, and viral generation time," Science. 1996 Mar 15;271(5255):1582-6. doi:10.1126/science.271.5255.1582 [3] Palmer J, Dobrovolny HM, and Beauchemin CAA. The in vivo efficacy of neuraminidase inhibitors cannot be determined from the decay rates of influenza viral titers observed in treated patients. Sci. Rep., 7:40210, 2017. doi:10.1038/srep40210 (* a joint seminar with Biology Study Group at RIKEN iTHEMS) | |
| 2022/4/20 15:30-17:00 @ Zoom | |
| Michael Sentef (MPI Hamburg) | Light-matter control of quantum materials: From light-induced superconductivity to cavity materials |
| In this talk I will discuss recent progress in controlling and inducing materials properties with light [1]. Specifically I will discuss recent experiments showing light-induced superconductivity through phonon driving in an organic kappa salt [2] and its possible theoretical explanation via dynamical Hubbard U [3]. I will then highlight some recent theoretical and experimental progress in cavity quantum materials [4], where the classical laser as a driving field of light-induced properties is replaced by quantum fluctuations of light in confined geometries. Ideas and open questions for future work will be outlined. [1] Colloquium: Nonthermal pathways to ultrafast control in quantum materials, A. de la Torre, D. M. Kennes, M. Claassen, S. Gerber, J. W. McIver, M. A. Sentef, Rev. Mod. Phys. 93, 041002 (2021). [2] Photo-molecular high temperature superconductivity, M. Buzzi, D. Nicoletti, M. Fechner, N. Tancogne-Dejean, M. A. Sentef, A. Georges, M. Dressel, A. Henderson, T. Siegrist, J. A. Schlueter, K. Miyagawa, K. Kanoda, M.-S. Nam, A. Ardavan, J. Coulthard, J. Tindall, F. Schlawin, D. Jaksch, A. Cavalleri, Phys. Rev. X 10, 031028 (2020). [3] Dynamical Superconductivity in a Frustrated Many-Body System, J. Tindall, F. Schlawin, M. Buzzi, D. Nicoletti, J. R. Coulthard, H. Gao, A. Cavalleri, M. A. Sentef, D. Jaksch, Phys. Rev. Lett. 125, 137001 (2020). [4] Cavity Quantum Materials, Frank Schlawin, Dante M. Kennes, Michael A. Sentef, Applied Physics Reviews Reviews 9, 011312 (2022). [OA] arXiv:2112.15018 | |
| 2022/3/30 13:30-15:00 @ Zoom | |
| Gen Tatara (RIKEN) | Hydrodynamic theory of electron and spin transport |
| Electron and spin transports in metals are theoretically studied from a hydrodynamic viewpoint by calculating momentum flux density as a linear response to an applied electric field. Dissipative (ohmic) fluid regime is considered. An angular momentum generation in chiral (Weyl) system and spin motive force (voltage generation) by magnetization-vorticity coupling in anomalous Hall system are discussed. The spin Hall effect is argued from the viewpoint of a spin-vorticity coupling. [1] Funaki, Tatara, Phys. Rev. Res., 3, 023160 (2021). [2] Funaki, Toshio, Tatara, Phys. Rev. Res., 3, 033075 (2021). [3] Tatara, Phys. Rev. B, 104, 184414 (2021). | |
| 2022/3/10 13:30-15:00 @ Zoom | |
| Yuto Ashida (Tokyo) | Nonperturbative cavity/waveguide quantum electrodynamics and dissipative quantum phase transition |
| Strong coupling between matter and quantized electromagnetic modes in cavity or waveguide may offer yet another approach of controlling equilibrium phases or dynamics of many-body systems. Recent developments have realized such strong light-matter interaction in genuinely quantum and nonperturbative regimes, where conventional approximate theoretical methods cannot be applied in general. I will talk about how one can analyze strongly coupled quantum light-matter systems at arbitrary interaction strengths on the basis of an asymptotically disentangling unitary transformation [1,2]. I discuss its application to construction of tight-binding Hamiltonians, dynamics of bound states in the continuum, and revisiting dissipative quantum phase transition in resistively shunted Josephson junctions [3]. [1] Y. Ashida, A. Imamoglu and E. Demler, PRL 126, 153603 (2021). [2] Y. Ashida, A. Imamoglu and E. Demler, arXiv:2105.08833. [3] K. Masuki, H. Sudo, M. Oshikawa and Y. Ashida, arXiv:2111.13710. | |
| 2022/2/15 13:30-15:00 @ Zoom | |
| Takumi Hayashi (Tokyo/RESCEU) | False vacuum decay in the Lorentzian path integral |
| False vacuum decay is a non-perturbative phenomenon in quantum field theory and important quantum process in cosmology. It has relied on the Euclidean formalism developed by Coleman, but there are several subtle issues in cosmological application as a negative mode problem or ambiguity in the definition of the decay rate in the presence of the gravity. Instead of the Euclidean path integral, we directly evaluate the Lorentzian path integral to discuss false vacuum decay and estimate the decay probability. To make the Lorentzian path integral convergent, the deformation of an integral contour is performed on the bassis of the Picard-Lefschetz theory. We show that the nucleation probability of a critical bubble, for which the corresponding bounce action is extremized, has the same exponent as the Euclidean approach. We also extend our computation to the nucleation of a bubble larger or smaller than the critical one to which the Euclidean formalism is not applicable. | |
| 2022/1/20 13:30-15:00 @ Zoom | |
| Tomohiro Tanogami (Kyoto) | A simple XY model for cascade transfer |
| Cascade transfer is the phenomenon that an inviscid conserved quantity, such as energy or enstrophy, is transferred conservatively from large (small) to small (large) scales. As a consequence of this cascade transfer, the distribution of the transferred quantity obeys a universal scaling law independent of the details of large (small) scales. For example, in the energy cascade in fluid turbulence, the energy spectrum follows Kolmogorov's power law [1]. Such behavior is observed even in systems different from ordinary fluids, such as quantum fluid, elastic body, and spin systems. Here, we aim to establish the concept of a universality class for cascade transfer. As a first step toward this end, we propose a simple model representing one universality class [2]. In doing so, we regard cascade transfer as a cooperative phenomenon of unidirectional transport across scales and ask how it emerges from spatially local interactions. The constructed model is a modified XY model with amplitude fluctuations, in which the spin is regarded as the “velocity” of a turbulent field in d dimensions. We show that the model exhibits an inverse energy cascade with the non-Kolmogorov energy spectrum. We also discuss the relation to spin turbulence [3,4] and atmospheric turbulence [5]. [1] U. Frisch, Turbulence (Cambridge university press, 1995) [2] T. Tanogami and S.-i. Sasa, Simple XY Model for Cascade Transfer, arXiv:2106.11670v2 [3] M. Tsubota, Y. Aoki, and K. Fujimoto, Spin-glass-like behavior in the spin turbulence of spinor Bose-Einstein condensates, Phys. Rev. A 88, 061601 (2013) [4] J. F. Rodriguez-Nieva, Turbulent relaxation after a quench in the Heisenberg model, arXiv:2009.11883 [5] G. D. Nastrom, K. S. Gage, and W. H. Jasperson, Kinetic energy spectrum of large-and mesoscale atmospheric processes, Nature 310, 36 (1984) |
Summer Lecture in 2022
| 2022/8/17-19 | 3 day lecture by Jun-ichiro Kishine (Open U.) and Keisuke Fujii (Heidelberg U.). Details can be found here. |