Bulletin of the American Physical Society
Mid-Atlantic Section Fall Meeting 2020
Volume 65, Number 20
Friday–Sunday, December 4–6, 2020; Virtual
Session D04: Quantum Materials & Optics I |
Hide Abstracts |
Chair: Diyar Talbayev, Tulane |
Saturday, December 5, 2020 9:00AM - 9:36AM |
D04.00001: Electrons, Holes, and Excitons in Monolayer Semiconductors: Insights from Optical Spectroscopy in (Really) High Magnetic Fields Invited Speaker: Scott Crooker This talk will discuss recent magneto-optical studies that probe the physics of electrons, holes, and excitons in monolayer ‘transition metal dichalcogenide’ (TMD) semiconductors such as MoS2 and WSe2, as well as the crucial role played by the surrounding dielectric environment. Our first studies focused on revealing fundamental properties relevant for optoelectronics, such as exciton mass, size, binding energy, and dielectric screening. Historically, magneto-optical spectroscopy has played an essential role in determining these properties in semiconductors; however, for TMD monolayers the relevant field scale is substantial (of order 100 tesla!) due to heavy carrier masses and huge exciton binding energies. Fortunately, modern pulsed magnets can achieve this scale. Using exfoliated monolayers affixed to single-mode optical fibers, we performed low-temperature magneto-absorption spectroscopy up to ~90T of all members of the monolayer TMD family. By following the diamagnetic shifts of the exciton’s 1s ground state and its excited Rydberg states, we determined exciton masses, radii, binding energies, dielectric properties, and free-particle bandgaps [1,2]. These data provide essential ingredients for the rational design of optoelectronic van der Waals structures. Separately, WSe2 monolayers were then electrostatically gated and populated with a high density of electrons or holes. Well-resolved sequences of optical transitions were observed in both circular polarizations, which unambiguously and separately indicate the number of filled Landau levels (LLs) in both K and K' valleys. We find that the 2D hole gas becomes unstable against small changes in LL filling and can spontaneously valley polarize [3]. These results cannot be understood within a single-particle picture, highlighting the importance of exchange interactions in determining the ground state of 2D carriers in monolayer semiconductors. [1] M. Goryca et al., Nature Comm. 10, 4172 (2019). [2] A. V. Stier et al., PRL 120, 057405 (2018). [3] J. Li et al., PRL 125, 147602 (2020). [Preview Abstract] |
Saturday, December 5, 2020 9:36AM - 10:12AM |
D04.00002: Magneto-optical Kerr switching properties and spin configurations of magnetic 2D heterostructures Invited Speaker: Alessandro Stroppa We explore the magneto-optical Kerr effect (MOKE) of (CrI$_{\mathrm{3}})_{\mathrm{2}}$ bilayer and (CrBr$_{\mathrm{3}}$/CrI$_{\mathrm{3}})$ mixed bilayers. Starting from CrX$_{\mathrm{3}}$ (X$=$I,Br) monolayers, we considered collinear ferromagnetic (FM) and layered antiferromagnetic (AFM) states for (CrI$_{\mathrm{3}})_{\mathrm{2}}$ and (CrBr$_{\mathrm{3}}$/CrI$_{\mathrm{3}})$ bilayers. The AFM (CrI$_{\mathrm{3}})_{\mathrm{2}}$ bilayer does not show MOKE, consistent with the presence of a symmetry operator combining inversion (I) and time reversal (T) symmetries. The FM state preserves I symmetry but breaks the T symmetry, thus allowing a non-zero Kerr angle, which is reversible by switching the FM spins. The (CrBr$_{\mathrm{3}}$/CrI$_{\mathrm{3}})$ bilayer breaks both the I and T symmetries and thus exhibits MOKE both in the FM and, remarkably, in AFM states. In both FM and AFM configurations, the Kerr angle switches by reversing the spins in both layers. We show that MOKE spectra can help characterize different magnetic configurations in these emerging two-dimensional materials due to a different stacking of the monolayers, even in the AFM case. We propose (CrBr$_{\mathrm{3}}$/CrI$_{\mathrm{3}})$ bilayer as a candidate for AFM spintronics, since the two time-reversed AFM states are associated with opposite Kerr rotation, $i.e.$ they could be used as memory elements. [Preview Abstract] |
Saturday, December 5, 2020 10:12AM - 10:48AM |
D04.00003: Exploring the quantum vacuum with super-intense laser pulses Invited Speaker: Wendell T. Hill, III Multi-petawatt laser pulses of short duration have placed us at the threshold of a new era where novel experimental investigations of nonlinear aspects of electrodynamics -- quantum electrodynamics (QED) -- will be possible. Fundamental tests of QED from the photon side and its intimate coupling to the quantum vacuum are on the horizon. The very essence of the vacuum is pleached with a fundamental tenet of quantum physics -- quantum fluctuation -- virtual particles and antiparticles (e.g., electron-positron pairs) fluctuating into and out of existence. Quantitative measurements of virtual particles not only will challenge calculations from the 1930s, they will set strenuous limits for add-ons to the Standard Model. Photons are unique probes in that they are uncharged and their Bosonic nature allows unlimited numbers of them to be collocated within an arbitrarily-small volume, at least classically. Quantum mechanics makes a different prediction. As the intensity increases, the linear response of light propagating in a physical vacuum, as Maxwell equations demand, gives way to a nonlinear response. Post-Maxwellian theories, such as QED, allow virtual pairs to mediate an interaction between photons that can be viewed, to some extent, as light propagating through material. At high enough intensity the quantum vacuum will break down, inducing real pairs to emerge. The critical intensity ($I_{cr}$) for breakdown, the so-called Schwinger limit, is $\simeq 2 \times 10^{29}$~W/cm$^2$. Even though $I_{cr}$ is beyond current technology, there are fundamental features of the quantum vacuum that can be explored at substantially lower intensities. In this talk we will explore some of these ideas, focusing on the new physics that can be learned, and the tools and conditions required. [Preview Abstract] |
Saturday, December 5, 2020 10:48AM - 11:00AM |
D04.00004: Low Frequency Electrical Resonance in Water Xindong Wang, Qiang Fu We report the observation of sharp electrical resonance of water with width ~2 neV in the low radio frequency range at room temperature. The neV level of the resonant width under room temperature (~25 meV) is consistent with the theory in Wang et al (2020) that predicts a macroscopic long-range coherent quantum mechanical excited states, Majorana fermions, resulting from quantum entanglement of proton hopping at hydrogen bonds. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700