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):
Nonvolatile photonic switch with magnetic materials on a silicon photonic platform
Speaker: Caleb Duff Date and time: September 8, 11 am Abstract: 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.
Abstract: 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.
Abstract: 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.
Abstract: 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
Abstract: 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.
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
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 
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
Challenge the conventional. Create the exceptional. No Limits.