Abstract:
This perspective discusses the surprising discovery and development of SnSe thermoelectrics. Undoped, hole-doped, and electron-doped SnSe single crystals have successively represented an extraordinarily high thermoelectric figure of merit (ZT) ranging from 2.6 to 2.9, revitalizing efforts on finding new high-performance thermoelectric systems. Their unprecedented performance is mainly attributed to ultralow thermal conductivity arising from the uniquely anisotropic and anharmonic crystal chemistry of SnSe. Soon after the publications on SnSe single crystals, substantial debates were raised on their thermoelectric performance, especially on truth in ultralow thermal conductivity. Very recently, polycrystalline SnSe samples were synthesized, exhibiting lower lattice thermal conductivity and higher ZT than the single crystal samples. This work clearly addressed many questions that have arisen on the intrinsic thermal and charge transport properties of SnSe-based materials. It shows a peak ZT of ~3.1 at 783 K and an average ZT of ~2.0 from 400 to 783 K, which are the record-breaking performances of all bulk thermoelectric materials in any form ever reported.

Our speakers will practice their 10 min talks for APS March Meeting.

Speaker: Daniel Duong
Title: Observation of Superconductivity in Li-intercalated SnSe₂ Single Crystals

Speaker: Govinda Kharal
Title: Experimental and theoretical investigation of magnetoelectric (ME) coupling in aligned multiferroic Janus fibers using second harmonic generation (SHG) polarimetry at different magnetic field orientations

Speaker: Bryan Chavez
Title: Surface and bulk properties of BaMn₂Sb₂

Speaker: Jie Xing
Title: Magnetic properties of layered CsNdSe₂ with triangular lattice

Abstract:
Among the variety of magnetic textures available in nature, antiferromagnetism is one of the most ‘discrete’ because of the exact cancellation of its staggered internal magnetization. Therefore, it is essential to understand the microscopic mechanisms governing antiferromagnetic domains to achieve accurate manipulation and control. Optical second harmonic generation (SHG) is one such effective tool that can be used to probe antiferromagnetic domains and will be the main motivation behind my talk. I will present on how SHG polarimetry and imaging exploit antiferromagnetic (AFM) structure of a parent cuprate Sr2Cu3O4Cl2.

Abstract:
Now that fundamental quantum principles of indeterminacy and measurement have become the basis of new technologies that provide secrecy between two communicating parties, there is a need to provide teaching laboratories that illustrate how these technologies work. In this article, we describe a laboratory exercise in which students perform quantum key distribution with single photons, and see how the secrecy of the communication is ensured by the principles of quantum superposition and state projection. We used a table-top apparatus, similar to those used in correlated-photon undergraduate laboratories, to implement the Bennett-Brassard-84 protocol with polarization-entangled photons. Our experiment shows how the communication between two parties is disrupted by an eavesdropper. We use a simple quartz plate to mimic how an eavesdropper intercepts, measures, and resends the photons used in the communication, and we analyze the state of the light to show how the eavesdropper changes it.

Abstract:
Topological superconductors are an essential component for topologically protected quantum computation and information processing. Although signatures of topological superconductivity have been reported in heterostructures, material realizations of intrinsic topological superconductors are rather rare. For this presentation, I will first give an introduction to some basics of topological superconductors and how they can support Majorana edge modes and then discuss the measurement method used to detect these topological boundary modes and the associated caveats. We will use this information to interpret the data presented in the paper and give a brief assessment of their validity.

Abstract:
Modern-day sensing and imaging applications increasingly rely on accurate measurements of the primary physical quantities associated with light waves: intensity, wavelength, directionality, and polarization. These are convention- ally performed with a series of bulky optical elements, but recently, it has been recognized that optical resonances in nanostructures can be engineered to achieve selective photodetection of light waves with a specific set of predetermined properties. Here, we theoretically illustrate how a thin silicon layer can be patterned into a dislocated nanowire-array that affords detection of circularly polarized light with an efficiency that reaches the theoretical limit for circular dichro- ism of a planar detector in a symmetric external environment. The presence of a periodic arrangement of dislocations is essential in achieving such unparalleled performance as they enable selective excitation of nonlocal, guided-mode resonances for one handedness of light. We also experimentally demonstrate compact, high-performance chiral pho- todetectors created from these dislocated nanowire-arrays. This work highlights the critical role defects can play in enabling new nanophotonic functions and devices.

Abstract: 
Electric currents carrying a net spin polarization are widely used in spintronics, whereas globally spin-neutral currents are expected to play no role in spin-dependent phenomena. Here we show that, in contrast to this common expectation, spin-independent conductance in compensated antiferromagnets and normal metals can be efficiently exploited in spintronics, provided their magnetic space group symmetry supports a non-spin-degenerate Fermi surface. Due to their momentum-dependent spin polarization, such antiferromagnets can be used as active elements in antiferromagnetic tunnel junctions (AFMTJs) and produce a giant tunneling magnetoresistance (TMR) effect. Using RuO2 as a representative compensated antiferromagnet exhibiting spin-independent conductance along the [001] direction but a non-spin-degenerate Fermi surface, we design a RuO2/TiO2/RuO2 (001) AFMTJ, where a globally spin-neutral charge current is controlled by the relative orientation of the Néel vectors of the two RuO2 electrodes, resulting in the TMR effect as large as ~500%. These results are expanded to normal metals which can be used as a counter electrode in AFMTJs with a single antiferromagnetic layer or other elements in spintronic devices. Our work uncovers an unexplored potential of the materials with no global spin polarization for utilizing them in spintronics.

Abstract: 
Exploring THz (1012 Hz) coherent phonon dynamics in two dimensional (2D) materials could advance the development of ultrafast electronic, photonic and phononic devices in atomic-thin platforms. THz coherent phonon dynamics usually only lasts for a few femtoseconds or picoseconds which requires fast probe to explore it. Here we applied the time-resolved pump-probe microscopy to study the excitation and manipulation of the coherent phonon under femtosecond laser excitation in 2D layered materials. The oscillations in transient reflectivity data with frequency around 3.5 THz are attributed to the A1g phonon mode based on our first-principle calculations. Remarkably, a phonon frequency modulation around ~100 GHz is observed when the pump beam polarization is rotated from in-plane to out-of-plane orientation. The phenomenon is repeatable in multiple flake samples. First-principle calculations reveal strong asymmetric in-plane and out-of-plane interactions between atoms in Fe3GeTe2. The control of coherent phonon frequency is attributed to the modification of vibration stiffness/restoring force for A1g phonon mode based on the asymmetric coupling of pump pulses with different orientation of polarization to the anisotropic electron-lattice interaction in Fe3GeTe2 flakes. The finding can be important for development of phononic devices in layered van der Waals material materials and opens new avenues to optically manipulating coherent phonons.