阿秒光脈沖、磁有序、交變磁體、共振熒光 | 本周物理講座

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報告人：Dr Shengqiang Zhou, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany

時間：4月21日（周二）14:00

單位：中國科學(xué)院物理研究所

地點：M樓249會議室

摘要:

The controlled emergence of unconventional order in low-dimensional quantum materials is a central theme in current condensed-matter research. In this context, the air-stable van der Waals magnetic semiconductor CrSBr provides an attractive platform, combining anisotropic electronic properties with layered magnetism. Below its Néel temperature of 132 K, CrSBr exhibits A-type antiferromagnetic order, consisting of ferromagnetic layers coupled antiferromagnetically along the stacking direction.

In this contribution, I will show how ion irradiation can be used as a tuning knob to drive and control magnetic phase transitions in CrSBr. We observe the emergence of a ferromagnetic phase from the antiferromagnetic ground state at moderate irradiation fluences, followed by its gradual suppression as disorder increases further. By combining structural analysis with density-functional-theory calculations, we identify irradiation-induced interstitial defects, defect-induced magnetic reconstruction, and a transition into a new crystalline phase.

These results establish defect engineering as a powerful route to manipulate symmetry-broken magnetic states in a layered quantum material. More broadly, the results demonstrate how controlled defect landscapes can be exploited to access and spatially pattern emergent magnetic phases in van der Waals quantum materials.

報告人簡介:

周生強博士自2011年擔(dān)任德國亥姆霍茲協(xié)會青年科學(xué)家研究組組長（Helmholtz Young Investigator Group Leader），2018年1月被提聘任為研究所半導(dǎo)體事業(yè)部的主任，工作集中在半導(dǎo)體材料，離子輻照材料改性、離子束分析技術(shù)，以及離子束在磁性功能材料方面的應(yīng)用。獲得17屆國際離子束材料改性青年科學(xué)家獎，培養(yǎng)的博士生分別在國際半導(dǎo)體會議(ICPS)、磁學(xué)會議(ICM)、歐洲材料（EMRS）會議上獲得青年科學(xué)家獎和最佳Poster獎。

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報告人：陳一帆，李政道研究所

時間：4月21日（周二）15:00

單位：中國科學(xué)院理論物理研究所

地點：南樓6420

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報告人：Prof. Anne L'Huillier (Nobel Prize in Physics 2023), Lund University

時間：4月22日（周三）9:00

單位：中國科學(xué)院物理研究所

地點：M樓234報告廳

摘要:

Ultrafast cameras, using ultrashort light flashes, allow the capture of ultrafast motion. In atoms or molecules, attosecond light pulses are needed to capture the motion of electrons (1 as = 10-18 s). This presentation will highlight the physics behind the generation and application of attosecond light pulses (Nobel Prize in Physics 2023).

報告人簡介:

Anne L'Huillier is a Swedish/French researcher in attosecond science. During the first part of her career, she worked at the Commissariat à l'Energie Atomique, in Saclay, France, first as a PhD student until 1986, then as a permanent researcher until 1995. She was postdoc at Chalmers Institute of Technology, Gothenburg. Sweden in 1986, and at the University of Southern California, Los Angeles, USA in 1988. In 1995, she moved to Lund University, Sweden and became full professor in 1997. Her research, both theoretical and experimental, is centered around high-order harmonic generation in gases and its applications, in particular in attosecond science. She was awarded the Nobel Prize in Physics 2023 together with Pierre Agostini and Ferenc Krausz for “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”.

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報告人：Clement Nizak，Sorbonne University

時間：4月22日（周三）14:00

單位：中國科學(xué)院物理研究所

地點：D樓206會議室

摘要:

Most of our understanding of the activity and specificity of enzymes focuses on their active site, which is a small part of their structure in direct interaction with the substrate. Yet, as illustrated by many examples of protein engineering success and failure, enzymatic function often depends crucially on residues away from the active site. Using the well-known example of serine proteases, which display such poorly understood yet pervasive long-range effects, we explore the sequence-function relationship systematically through large-scale experiments and statistical evolutionary analysis. I will present an experimental approach based on droplet microfluidics to analyze the effect of nearly all single mutations of the protease trypsin on its activity towards peptide substrates. The substrate specificity profile of this enzyme is extremely robust to single mutations, as we did not detect any activity towards new substrates in the single mutant library. Substrate specificity can only be fine-tuned towards the initial substrates by single mutations at a handful of residues in and out of the active site. These rare specificity fine-tuning residues are consistent with the analysis of pairwise residue coevolution in the protease family. Further, statistical modeling of pairwise residue coevolution allows engineering trypsin specificity towards new substrates with combinations of mutations.

報告人簡介:

Clement Nizak is a CNRS research director at Sorbonne University, Paris. After completing in 1999 his undergraduate studies in physics and chemistry at ESPCI, Paris, he received his PhD in biophysics in 2003 from Paris VII University. In 2003-2007, he was a postdoctoral associate at Rockefeller University, New York. His current research focuses on large-scale experiments to probe the sequence-function relationship in proteins for downstream statistical modeling, using antibodies and proteases as model systems. His past work includes the development of single-cell phenotyping and sequencing of immune repertoires based on droplet microfluidics, and an experimental physicist perspective on the unicellular/multicellular transition in social microbe populations.

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報告人：Junwei Liu，The Hong Kong University of Science and Technology

時間：4月22日（周三）15:00

單位：北京大學(xué)物理學(xué)院

地點：西563會議室

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報告人：Dr. Marin Bukov，Max Planck Institute for the Physics of Complex Systems

時間：4月23日（周四）15:00

單位：北京大學(xué)物理學(xué)院

地點：物理大樓中212報告廳

摘要:

We derive Floquet theory from quantum geometry. We identify quasienergy folding as a consequence of a broken gauge group of the adiabatic gauge potential U(1)??. Fixing instead the gauge freedom using the parallel-transport gauge uniquely decomposes Floquet dynamics into a purely geometric and a purely dynamical evolution. The dynamical average-energy operator provides an unambiguous sorting of the quasienergy spectrum, identifying a Floquet ground state and suggesting a way to define the filling of Floquet-Bloch bands. We exemplify the features of geometric Floquet theory using an exactly solvable XY model and a non-integrable kicked Ising chain. We elucidate the geometric origin of inherently nonequilibrium effects, like the π-quasienergy splitting in discrete time crystals or π-edge modes in anomalous Floquet topological insulators. The spectrum of the average-energy operator is a susceptible indicator for both heating and spatiotemporal symmetry-breaking transitions. Last, we demonstrate that the periodic lab frame Hamiltonian generates transitionless counterdiabatic driving for Floquet eigenstates. This work directly bridges seemingly unrelated areas of nonequilibrium physics.

報告人簡介:

Dr. Marin Bukov is a theoretical physicist specializing in nonequilibrium quantum many-body systems and quantum control. He obtained his PhD from Boston University under Anatoli Polkovnikov, followed by a Gordon and Betty Moore Foundation postdoctoral fellowship at the University of California, Berkeley. He later led a research group at Sofia University and is currently a group leader at the Max Planck Institute for the Physics of Complex Systems.

His research focuses on out-of-equilibrium dynamics, quantum thermalization, and advanced quantum control techniques such as Floquet engineering and counterdiabatic driving. More recently, he has been pioneering the integration of machine learning with quantum many-body physics.

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報告人：Dong Lai，上海交通大學(xué)李政道研究所

時間：4月23日（周四）15:30

單位：北京大學(xué)物理學(xué)院

地點：KIAA-auditorium

摘要:

I will discuss mechanisms of generating polarized x-rays from magnetars and large eccentricities of exoplanets, and how the underlying "dynamics" is similar to adiabatic evolution of a pendulum.

報告人簡介:

Dong Lai is T.D.Lee Chair Professor and Astrophysics Division head at Tsung-Dao Lee Institute, Shanghai Jiao Tong University. From 1997 to 2025, he was on the faculty at Cornell University, where he was the Benson and Mary Simon Professor in Astrophysics. Dong Lai received B.S. from the University of Science and Technology of China, Ph.D. in theoretical physics from Cornell University in 1994. He was a Prize Postdoctoral Fellow at Caltech, and joined the Cornell Astronomy faculty in 1997. He has made significant contribution to the understanding of the physical processes around compact objects (black holes, neutron stars and white dwarfs), including the QED effect in radiation from magnetic neutron stars, the origin of pulsar kicks, gravitational wave sources, and accrection disks around magnetic stars and compact objects. He has also made important contributions in several areas of exoplanetary dynamics. He has received Sloan Fellowship, Simons Fellowship and distinguished graduate teaching and mentoring award from Cornell. In 2024 he received the Dirk Brouwer Award from the American Astronomical Society, the highest international honor in dynamical astronomy.

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報告人：袁之良，北京量子信息科學(xué)研究院

時間：4月23日（周四）16:00

單位：清華大學(xué)物理系

地點：物理樓W105

摘要:

Resonance fluorescence from a coherently driven two-level emitter is conventionally described using mixed-state field correlations. In this talk, I will present a complementary perspective that emerges through few-photon excitation, in which the emitter and its spontaneous emission are described within a unified quantum-state framework. This picture provides an intuitive understanding of how phase coherence and photon antibunching coexist in resonance fluorescence. I will show how the model accounts for a range of recent observations and leads to experimentally testable predictions, including interference between mutually detuned resonance-fluorescence fields, tunable higher-order coherence through interference with laser light, and the synthesis of nonclassical and entangled photonic states using only passive linear interferometry. More generally, this framework offers a useful way to understand resonance fluorescence and its coherence properties, complementary to the conventional field-correlation approach.

報告人簡介:

袁之良博士，北京量子信息科學(xué)研究院首席科學(xué)家，英國物理學(xué)會會士、美國光學(xué)學(xué)會會士。長期從事光纖量子通信與半導(dǎo)體光量子器件研究，在Nature、Science、PRL等期刊發(fā)表論文150余篇，獲授權(quán)發(fā)明專利70余項，多次受邀在國內(nèi)外重要學(xué)術(shù)會議作大會或邀請報告。

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報告人：Mikhail Skvortsov，Landau Institute for Theoretical Physics, Russian Academy of Sciences

時間：4月24日（周五）15:00、4月27日（周一）15:00

單位：中國科學(xué)院理論物理研究所

地點：南樓6620

摘要:

In various branches of physics, there arises the problem of minimizing a functional F[q(t)] that depends on a t-dependent angular variable q(t), in the presence of a random force f(t). A paradigmatic model for this class of phenomena is provided by balancing an inverted pendulum subjected to a time-dependent horizontal force f(t). The problem is to find an exceptional never-falling trajectory (NFT) q(t) that remains in the upper half-plane for all t. In mathematics, this problem is known as Whitney'?s problem and the existence of the solution has been proved for any f(t). The resulting q(t) is a non-Markovian functional of the force f(t), that prohibits the na?ve use of the Fokker-Planck evolutionary equation.

We have shown that the solution to the Whitney problem, considered over the entire time axis, is unique. This raises the question of describing the statistical properties of this NFT when averaged over an ensemble of random forces. For the white-noise random force, we construct a complete field-theoretical description of the NFT statistics, based in the Parisi-Sourlas supersymmetric representation. After disorder averaging, the action remains local in time that allows reducing the arising functional integral to an effective supersymmetric quantum mechanics. We show that statistics of the NFT is determined by the zero mode of the emerging Hamiltonian. Employing the BRST symmetry, we reduce it to the Fokker-Planch form. The joint probability distribution function of the angle q(t) and its velocity dq(t)/dt is expressed via the Poisson bracket of the zero mode, which is found analytically in the limits of weak and strong noise. We also determine the Lyapunov exponent, describing the divergence of nearby solutions around the NFT. Finally, we discuss the intuitive way of understanding the supersymmetric wave function and relate it to physical observables.

報告人簡介:

Mikhail Skvortsov graduated from the Moscow Institute of Physics and Technology in 1995 and got his PhD in 1998. Since then, he has been continuously employed at the Landau Institute for Theoretical Physics. From 2014 to 2021, he served as an Associate Professor at the Skolkovo Institute of Science and Technology. M. A. Skvortsov is a recognized specialist in the physics of disordered and superconducting systems. His key scientific contributions include explaining the giant fluctuation Nernst effect in superconductors, investigating ergodicity and localization on random regular graphs, developing the Keldysh action approach for disordered superconductors, describing the inhomogeneous state in dirty superconductors, constructing the theory of dynamical localization in quantum dots under periodic driving, characterizing the statistics of the never-falling trajectory in the random Whitney problem.

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