Events at Physics
Events on Thursday, February 23rd, 2023
- R. G. Herb Condensed Matter Seminar
- VdW heterostructures: a new route to designing quantum matters
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Xiameng Liu, Princeton
- Abstract: From superconductivity to fractionalized particles, fascinating phenomena arise in quantum materials due to the collective behaviors of electrons. These quantum effects challenge our intuition about nature and provide new opportunities for future quantum information technologies. Recently, two-dimensional materials and their heterostructures have become a leading platform for realizing new quantum states of matter. Mechanically assembled layer-by-layer and held together by the van der Waals (vdW) force, vdW heterostructures break through the limitations of traditional material synthesis and offer entirely new ways to create quantum matters. My talk will feature two examples of designing quantum matters with vdW heterostructures. In the first example, I will illustrate how Coulomb interactions across separate atomic layers pair fermions (electrons and holes) into bosons to achieve a superfluid condensate state. Thanks to the tunability of the vdW platform, we can vary the pairing strength and change the nature of this fermion condensate from strong coupling to weak coupling, demonstrating a long-sought paradigm known as the BEC-BCS crossover. In the second example, I will introduce the idea of moiré band engineering, where the interference between two atomic lattices—named the moiré pattern—defines a new periodicity and reforms electronic band structures. In twisted double bilayer graphene, such moiré periodicity creates highly-degenerate bands tunable by a perpendicular electric field. We observed electron correlation effects, including interaction-driven insulation and spontaneous symmetry breaking of spins. Their evolution with the electric fields reveals their close connection with the moiré band features. Finally, I will briefly discuss applications of local probe techniques to uncover hidden quantum properties in vdW platforms and share visions of leveraging rich interplays across atomic interfaces to access major themes in condensed matter physics.
- Host: Victor Brar
- Plasma Physics (Physics/ECE/NE 922) Seminar
- Breaking Barriers in Magnetic Confinement Fusion Using Direct Construction Methods
- Time: 2:30 pm
- Place: B343 Sterling Hall
- Speaker: Rogério Jorge, Instituto Superior Tecnico, Portugal
- Abstract: The pursuit of magnetic confinement fusion demands the implementation of magnetic fields with exceptional properties to maintain high-temperature plasmas, regulate plasma density, fast particle dynamics, and turbulence. In traditional design methods for stellarator machines, magnetic fields and coils were optimized independently, leading to stringent engineering tolerances and a neglect of the impact of turbulence on confinement. Recent advancements in the optimization of stellarator devices, however, have made significant contributions to the field. These innovations include direct near-axis designs, integrated plasma-coil algorithms, and precise quasisymmetric and quasi-isodynamic fields, as well as direct turbulence optimization. These approaches allow for a more comprehensive and efficient optimization process, taking into account the interdependence of magnetic fields and plasma parameters. This presentation will delve into the details of these innovations in magnetic confinement fusion optimization algorithms and their implications for the advancement of fusion devices in plasma physics.
- Astronomy Colloquium
- The next generation of gravitational wave counterpart discovery
- Time: 3:30 pm
- Place: 4421 Sterling Hall
- Speaker: Charlie Kilpatrick, Northwestern University
- Abstract: The promise of multi-messenger astronomy was spectacularly realized in 2017 with the detection of a binary neutron star merger, GW170817, simultaneously localized by LIGO/Virgo in gravitational waves and telescopes observing across the electromagnetic spectrum. This single event led to ground-breaking new discoveries in the physics of compact objects, synthesis of heavy elements, and cosmology. The challenges of extending these discoveries to a large population of electromagnetic counterparts will require new observing and analysis techniques, instrumentation, and collaborations. On the heels of this seminal discovery, I will discuss our results from the most recent LIGO/Virgo observing run to detect new gravitational wave counterparts and the ongoing efforts to rapidly coordinate a global networks of telescopes, identify their multi-wavelength counterparts, and use these observations to further expand our understanding of compact objects and fundamental physics. I will then discuss new facilities and observational programs, where I am leading efforts to extend search and follow up into the X-ray, ultraviolet, and infrared. These include wide-field, multi-wavelength counterpart searches in the ground and space, JWST imaging and spectra for well-localized counterparts, and an ongoing infrared time-domain survey with hundreds of infrared spectra for comparison to candidate gravitational wave counterparts.
- Host: Ke Zhang
- Entropy, molecular motors, and non-thermal equilibrium statistical physics
- Time: 4:00 pm
- Place: 2241 Chamberlin Hall
- Speaker: Steven Chu, Stanford University
- Abstract: The transport of molecular cargos in neuronal cells is analyzed in the context of new developments in statistical physics. We developed bright optical probes which enabled the long-term single tracking of molecular cargos in live neurons for tens of minutes. The number of dynein motors transporting a cargo in a neuron was found to switch stochastically from one to five motors. We were able to resolve individual molecular steps, and new a quantitative chemo-mechanical model where a single step requires the hydrolysis of two ATP molecules.
We also find that the movement approaches a steady-state non-thermal equilibrium with effective temperature, T_eff=〖6×T〗_cell=6×310 K. Also, the minimum “uncertainty principle” limit, ΔQ⋅ϵ^2≥2k_B T_eff, where ΔQ=T_eff ΔS is the heat entropy needed to achieve movement with a normalized precision 〖ϵ(x)〗^2≡((〈x^2 〉-〈x〉^2))⁄〈x〉^2 . This uncertainty limit sets the minimum heat energy needed to achieve a given precision in any physical operation. In the context of intercellular molecular transport, a more uniform motion of the cargo requires a greater expenditure of energy.
- Host: Uwe Bergmann, Alessandro Senes