Abstract

In this talk, I will discuss the microscopic theory for the attenuation of out-of-plane and in-plane phonons in free-standing and stressed flexible two-dimensional crystalline materials. In the free-standing case, it is possible to find exactly the scaling form of the attenuation, determine its small- and large-frequency asymptotes, and to derive the exact expression for the dynamical exponent of flexural phonons in the long-wave limit. In the stressed case, the presence of nonzero tension strongly reduces the relative magnitude of the phonon's attenuation and, consequently, results in parametrical narrowing of the phonon spectral line due to stress-controlled suppression of the retardation effects in the dynamically screened inter phonon interaction. I will discuss how suppression of the phonon attenuation by nonzero tension might be responsible for high quality factors of mechanical nanoresonators based on flexural two-dimensional materials.

Biography

Prof. Igor Burmistrov received two Ph.D.s in Theoretical Physics (in 2004 from Landau Institute and in 2006 from University of Amsterdam) and a Doctor of Physics and Mathematics degree in 2012 from Landau Institute. Since 2006, he is a permanent member of Landau Institute holding now the position of the deputy director. In 2017–2020, he was also a Humboldt fellow at Karlsruhe Institute of Technology. He is APS Outstanding Referee (2021). In 2022, he was elected as Professor of Russian Academy of Sciences. His research area includes electron transport in low dimensional systems, superconductivity, magnetism, elasticity of flexible two-dimensional materials, physics of open and dissipative systems. He has authored more than 110 peer-reviewed papers.

Abstract

Two-dimensional correlated electronic systems present a fundamental challenge in condensed matter and quantum physics. Thermal tensor networks have emerged as a powerful tool for finite-temperature studies of many-electron systems. In this talk, I will present our recent advances in thermal tensor network methods, focusing on two recent developments: (1) exponential tensor renormalization group (XTRG), which achieves exponential acceleration through density matrix operator squaring, and (2) the efficient tangent-space tensor renormalization group (tanTRG) that incorporates time-dependent variational principle (TDVP) techniques. Our tensor network methods enable accurate low-temperature simulations of fundamental models like the Hubbard and t-J Hamiltonians, as well as material-specific applications such as the t-t'-J model for cuprates and the t-t-J⊥ model for bilayer nickelate superconductors.

Biography

Dr. Wei Li obtained his Ph.D. in Theoretical Physics from CAS in 2012, followed by postdoctoral research at Ludwig-Maximilians-Universität München (2012–2015). He served as an Associate Professor at Beihang University before joining the Institute of Theoretical Physics (ITP-CAS) as a Research Scientist in 2021. His research focuses on strongly correlated quantum systems, including quantum magnetism and correlated electron phenomena, development of novel tensor-network methods, and applications to cutting-edge problems in many-body physics. He has published over 80 papers in leading journals including Nature, Nature Physics, and PRL. He currently leads a CAS-sponsored Youth Innovation Team focused on foundational research in quantum many-body physics and applications.

Abstract

Atmospheric optical turbulence effects on light waves are one of the key limiting factors affecting the efficiency of photoelectric systems. Currently, the characteristics of atmospheric optical turbulence in the upper troposphere and lower stratosphere are still not well understood or thoroughly studied. By integrating ground-based, radiosonde, and in-situ detection methods, observations of atmospheric optical turbulence have been conducted in typical regions of China. Based on these observations, statistical models and parameterized models of upper atmospheric optical turbulence have been established. Combined with a mesoscale meteorological model and a single-column meteorological model based on high-order turbulence closure, prediction models have been developed to derive the temporal and spatial distribution of optical turbulence intensity. This research provides important technical and data support for the demonstration and application of photoelectric systems in the upper atmosphere.

Biography

Dr. Tao Luo received his doctoral degree in space science from University of Science and Technology of China in 2008. He is currently a professor at Hefei Institutes of Physical Science, Chinese Academy of Sciences (CAS), and the deputy director of the Key Laboratory of Atmospheric Optics, CAS. His research area includes atmospheric boundary layer processes, atmospheric turbulence characteristics, and active and passive atmospheric remote sensing. He has authored more than 80 peer-reviewed papers.

Abstract

The talk is devoted to modern development in turbulence in effectively two-dimensional systems, which include thin layers of liquid and the Earth's atmosphere. If the system is limited in size, then in a certain range of parameters, turbulent fluctuations generate a coherent vortex flow. In other words, energy is transferred from chaotic flows to regular ones. It turns out that the structure of the resulting vortices can largely be described analytically. In addition, the very possibility of vortices depends on the ratio between relatively small dissipative constants.

Biography

Prof. Igor V. Kolokolov received his Ph.D. in Theoretical Physics in 1990 and a Doctor of Physics and Mathematics degree in 1998 from the Budker Institute of Nuclear Physics, Novosibirsk. He started his career as a Leading Researcher at the same institute from 1986 to 2003. Between 1993 and 1995, he completed a Postdoctoral Fellowship at INFN, University of Milano, Italy. He joined the Landau Institute of Theoretical Physics in Chernogolovka in 2002 and held positions as deputy director (2003–2018), acting director (2018–2019), and has been director since 2019. He teaches at multiple institutions and has published a textbook "Mathematical Methods of Physics: Problems with Solutions" (Jenny Stanford Publishing, 2024).