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Department of Physics and Astronomy

CMP Seminars and Journal Club

Our meetings are usually on Fridays at 11:00 am in Sumwalt 102. You can find abstracts of the talks at the bottom of this page.
Looking forward to seeing you all!

Upcoming Seminar (Fall 2023):

Physical Properties of LaMnxSb2 Single Crystal

Speaker: Abhinna "Abhi" Rajbanshi
Date and time: September 29, 11 am
Abstract: The Layered manganese AMnPn2 (A = Alkali earth or rare earth atom, Pn = Sb, Bi) compounds are extensively studied for their rich magnetism and electronic structure topology. Magnetization combined with neutron diffraction measurements of LaMn0.86Sb2, a member of the AMnPn2 family, suggests that LaMn0.86Sb2 orders into a G-type antiferromagnetic structure below TN = 146 K. The antiferromagnetic transition is also manifested as a jump in specific heat and a slope change in the resistivity and Seebeck coefficient. With magnetic field applied along c-axis, spin flop transition was observed in LaMn0.86Sb2. At low temperatures, negative magnetoresistance is observed. In general, LaMnxSb2 has Dirac dispersion, confirmed by band structure calculation, and a vacancy tunable magnetism, which could be a fertile ground for studying the interplay of magnetism, charge transport, and possible topological band.

Recent Seminars:

Spin-orbit torque is a very promising effect for switching magnetization in spintronic devices and understanding of effect of different sources of spin moments and mixing them is the key to creating efficient magnetization switching. In this paper Rashba-like Sy, Dresselhaus-like Sx and out-of-plane like Sz spin moments were studied. The current induced out-of-plane spin moment can effectively contribute unconventional out-of-plane damping like torque to switch the mz at zero magnetic field. Similarly, the current induced Dresselhaus-like spin moments can switch mx effectively.

Since the discovery of superconductivity in NdSrNiO2, studying nickel oxide systems has been of great interest. The differences of the mechanisms of superconductivity in nickelates and copper oxide superconductors are not well understood. Akin to its cuprate relative, the Ruddlesden-Popper (RP) compound, La3Ni2O7 contains Ni-O planes that are theorized to possess superconductivity. Prior to this paper, single-crystal studies of La3Ni2O7 had not been done. The researchers report successful growth of La3Ni2O7  single crystals via the high oxygen pressure floating zone technique and by measuring resistivity, susceptibility, and specific heat, they detected anomalies at 110 and 153 K that may indicate the formation of charge and spin density waves, further providing evidence of the similarity of Ni-O systems to known Cu-O superconductors. The technique for synthesizing crystals introduced in this paper for La3Ni2O7  was used to discover superconductivity in the material by Sun et al.

This study investigates the performance of a nonvolatile photonic switch driven by the magneto-optical (MO) effect. Thin-film magnets made of ferromagnetic metals have remanence and maintain the magnetization of the MO garnet. Considering integration on silicon photonic platforms, a thin-film magnet is placed beside the waveguide, and the MO garnet is bonded on the waveguide compatible with the back-end-of-line process. The results obtained demonstrate successfully the nonvolatile MO phase shift and high extinction switching.

In this article, a gold plasmonic teardrop-shaped nanostructure (PTNS) is reported which can be used for the transfer of nanoparticles when the polarization of an excitation beam is switched. The hot spots around PTNS provide efficient optical trapping sites, and their locations are found to be polarization-dependent. An extremely strong local field enhancement is generated by the teardrop tip, enhancing the trap stiffness and the possibility of transferring trapped particles between adjacent PTNSs. By a chain of uniform PTNSs, a nano-optical conveyor belt is constructed that delivers target nanoparticles by having three polarization angles switched periodically. A numerical analysis of optical forces and trap potentials confirms the feasibility of the design for particle trapping and transferring.

Propositional satisfiability problem (SAT) is rep- resented in a conjunctive normal form with multiple clauses, which is an important non-deterministic polynomial-time (NP) complete problem that plays a major role in various applications including artificial intelligence, graph colouring, and circuit analysis. Quantum annealing (QA) is a promising methodology for solving complex SAT problems by exploiting the parallelism of quantum entanglement, where the SAT variables are embedded to the qubits. However, the long embedding time fundamentally limits existing QA-based methods, leading to inefficient hardware implementation and poor scalability.In this paper, we propose HyQSAT, a hybrid approach that integrates QA with the classical Conflict-Driven Clause Learning (CDCL) algorithm to enable end-to-end acceleration for solving SAT problems. Instead of embedding all clauses to QA hardware, we quantitatively estimate the conflict frequency of clauses and apply breadth-first traversal to choose their embedding order. We also consider the hardware topology to maximize the utilization of physical qubits in embedding to QA hardware. Besides, we adjust the embedding coefficients to improve the computation
accuracy under qubit noise. Finally, we present how to interpret the satisfaction probability based on QA energy distribution and use this information to guide the CDCL search. Our experiments demonstrate that HyQSAT can effectively support larger-scale SAT problems that are beyond the capability of existing QA approaches, achieve up to 12.62X end-to-end speedup using D- Wave 2000Q compared to the classic CDCL algorithm on Intel E5 CPU, and considerably reduce the QA embedding time from 17.2s to 15.7μs compared to the D-Wave Minorminer algorithm.

Rare-earth-based triangular antiferromagnets have raised great research interest in frustrated magnetism due to the unusual quantum spin states and transitions. Recently they have been proposed as excellent coolants for sub-Kelvin space applications. For this presentation, I will first introduce basic information about the rare-earth-based triangular antiferromagnets. Then I will show recent studies in KBaRE(BO3)2  for thermodynamic properties and adiabatic demagnetization refrigeration effect. I will also discuss the relation between frustrated magnetism and the performance of adiabatic demagnetization cooling.

It is reported that the noncollinear Weyl semimetal CeAlSi shows sign change of anomalous Hall effect when the anomalous Hall conductivity (σijA) was measured for two different orientations of the magnetic field(B); the magnetic field was applied parallel to the a and c axis of the crystal. It is reported that both the respective Hall conductivities σyzA and σxyA have large values but opposite signs below the Curie temperature (Tc). The value of σyzA  is reported to increase with the rise in temperature and reaches its maximum value at around T= 170K whereas σxyA is observed to have opposite response with the increment in temperature. On the other hand, it is also reported that CeAlSi also shows anomalous Nernest effect where the Nernst conductivity αxyA is measured for B//c. The temperature dependence of σxyA and αxyA/T is studied and hence the properties of the Weyl node is explored.

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

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.

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.

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.

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.

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.

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. 


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