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Spring 2021 Virtual Physics Colloquium

Date:January 27, 2021 (Wednesday)
Time:5:00 p.m.

For the spring semester of 2021, the virtual colloquium will happen on every other Wednesday either at 10-11am or 5-6pm. The detailed schedule and talk information are as below.
For inquiries or suggestions of future speakers, please contact the colloquium working group (Dr. Jane Lixin Dai, Dr. Tran Trung Luu, Dr. Yanjun Tu, Dr. Chenjie Wang, and Dr. Shizhong Zhang).

1. January 27, 2021 (Wednesday) 5:00 p.m.
2. February 10, 2021 (Wednesday) 5:00 p.m.
3. February 24, 2021 (Wednesday) 10:00 a.m.
4. March 10, 2021 (Wednesday) 5:00 p.m.
5. March 24, 2021 (Wednesday)
6. April 7, 2021 (Wednesday) 10:00 a.m.
7. April 21, 2021 (Wednesday)

Charge Separation and Recombination in Next Generation Materials for Solar Energy Conversion

Speaker: Prof. James Durrant
Affiliation: Imperial College London
Date: January 27, 2021 (Wednesday)
Time: 5:00 p.m.

Heat Flow in Quantum Hall States

Speaker: Prof. Moty Heiblum
Affiliation: Weizmann Institute of Science
Date: February 10, 2021 (Wednesday)
Time: 5:00 p.m.
Quantum mechanics sets an upper bound on the amount of charge flow as well as on the amount of heat flow in ballistic one-dimensional channels. The two relevant upper bounds, that combine only fundamental constants, are the quantum of the electrical conductance and the quantum of the thermal conductance. Remarkably, the latter does not depend on particles charge; particles statistics; and even the interaction strength among the particles.
Unlike the relative ease in determining accurately the quantization of the electrical conductance, measuring accurately the thermal conductance is more challenging - as heat flow is not conserved, and accurate and noninvasive temperature measurements are not trivial.
The universality of the thermal conductance was already demonstrated for weakly interacting particles: phonons, photons, and electronic Fermi-liquids. I will describe our work on thermal conductance measurements in the fractional QHE regime. I will concentrate on the method used in the measurements, which were focused on Laughlin’s states and hole-conjugate states – in order to prove the universality of the thermal conductance (which violates the Wiedemann-Franz conjecture). The studies were extended to fractional states in the first-excited Landau level, and in particular on the five half state, which was predicted to be non-abelian. We found a fractional thermal conductance coefficient, indeed proving the non-abelian character of the state. Note our observed topological order of the state (PH-Pfaffian) was not expected in current numerical works (which expect the anti-Pfaffian order). However, our current measurements (using a different method) strengthen the observed topological order.

Speaker: Prof. Claudio Campagnari
Affiliation: University of California at Santa Barbar
Date: February 24, 2021 (Wednesday)
Time: 10:00 a.m.

BCS-BEC Crossover in a 2D Superconductor

Speaker: Prof. Yoshi Iwasa
Affiliation: University of Tokyo
Date: March 10, 2021 (Wednesday)
Time: 5:00 p.m.
The Bardeen-Cooper-Schrieffer (BCS) condensation and the Bose-Einstein condensation (BEC) are the two extreme limits of the ground state of the paired fermion systems, which are theoretically predicted to continuously connected through an intermediate regime [1]. We report the two-dimensional (2D) BCS-BEC realized in a gate-controlled superconductor, electron doped layered material ZrNCl. To observe this phenomenon, we utilized an ionic gating method, which is well known as a powerful tool to control the carrier density in a large scale and induced 2D superconductivity [2].
We have succeeded in controlling the carrier density by nearly two-orders of magnitude, and established an electronic phase diagram through the simultaneous experiments of resistivity and tunneling spectra on the ionic gating devices. We found Tc exhibits dome-like behavior, and more importantly, a wide pseudogap phase was discovered in the low doping regime. In the low carrier density limit, Tc scales as Tc/TF = 0.12, where TF is the Fermi temperature [3], which shows fair agreement with the theoretical prediction in the 2D limit of BEC [4].
[1] M. Randeria and E. Taylor, Annu. Rev. Condens. Matter Phys. 5, 209 (2014).
[2] Y. Saito, T. Nojima and Y. Iwasa, Nat. Rev. Mater. 2, 16094 (2017).
[3] Y. Nakagawa et al., arXiv:2012.05707
[4] S. S. Botelho and C. A. R. Sa´ de Melo, Phys. Rev. Lett. 96, 040404 (2006).

Gravitational Wave Sources at the Hearts of Galaxies

Speaker: Prof. Smadar Naoz
Affiliation: University of California at Los Angeles
Date: April 7, 2021 (Wednesday)
Time: 10:00 a.m.
The recent gravitational-wave detections by LIGO/Virgo revolutionized the way we sense our Universe. However, it remains challenging to explain the formation channels of these sources. Motivated by these challenges, recent studies have emphasized the significant contribution of dynamical formation channels in dense stellar environments to the overall gravitational-wave signals. Focusing on the dense stellar clusters surrounding supermassive black holes at the center of galaxies, I will outline stellar binaries' evolution from birth up to possible gravitational-wave mergers. The supermassive black hole can induce collisions between binary members, while the frequent interactions with the neighbors in this dense environment can sometimes tend to unbind the binary. I will highlight some exotic outcomes, including gravitational-wave emission, for this dynamical evolution channel. I will show how this channel can leave a clear signature on the gravitational-wave signals, allowing differentiation between different merger mechanisms. The Laser Interferometer Space Antenna (LISA) can potentially be used to distinguish between channels.

Anyone interested is welcome to attend.