Monday December 7, 2020 5:30 - 7:00 p.m. (EST)

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Energy Materials Poster Submissions

The I2-II-IV-X4 (I = Li, Ag, Cu; II = Ba, Sr, Pb, Eu; IV = Si, Ge, Sn; X = S, Se) materials family has recently been explored for energy conversion and infrared nonlinear optical applications. These materials are promising photovoltaic and photoelectrochemical absorbers, thermoelectric generation compounds, as well as potential near-infrared frequency doubling candidates that rival industry standards in performance. The high dissimilarity between the I and II ions within these compounds prevents anti-site defects that could be electronically harmful for optoelectronic device applications from forming. In this stoichiometry these semiconductors form in one of five highly related crystal structures and, using the structural similarities, we have derived a pair of tolerance factors to describe the phase stability of these compounds. With these tolerance factors as predictors, we have synthesized five new I2-II-IV-X4-type semiconductors including Cu2PbGeS4, Cu2SrSiS4, Ag2BaSiS4, Ag2SrSiS4, and Ag2SrGeS4. We then characterized the optical and electronic properties of these compounds, demonstrating energy band gaps in ranges relevant for photovoltaic (Cu2PbGeS4), photoelectrochemical (Ag2BaSiS4, Ag2SrSiS4, and Ag2SrGeS4) and optical (Ag2BaSiS4, Ag2SrSiS4, and Cu2SrSiS4) applications. Finally, we establish the potential of Ag2SrSiS4 and Ag2SrGeS4 as nonlinear optical materials showing second harmonic generation from the near infrared region.

Authors: Garrett C. McKeown Wessler, Tianlin Wang, Jon-Paul Sun, Yuheng Liao, Martin Fischer, Volker Blum, & David B. Mitzi

Materials research is generating a wealth of data across a vast community. Specifically, the volume of available data on hybrid organic-inorganic perovskites (HOIPs) and related growth in this area is now immense. Keeping track of data of different origins, sample types or levels of theory, with a diverse set of different relevant observables and discoveries, is a challenging task at best. We here present an open database, “HybriD^3” (Design, discovery and dissemination (D^3) of data related to hybrid materials, https://materials.hybrid3.duke.edu), aiming to collect, curate, and make available materials data related to HOIP. The database is designed to provide a broad set of data, i.e., experimental and computational, related to in principle any materials property of relevance to the community: structure, optical or electronic properties, and more. A key goal is to provide the ability to curate data, that is, identify property information closest to the actual properties of a real material prepared in a specific way (bulk crystalline, powder, thin film, nanocrystalline, …). Importantly, the database is open to the community and designed to accept community input. While the “HybriD^3” database is focused on a particular materials class, the problem of making materials data of all kinds available in a structured, reproducible way is general. The software underlying the HybriD^3 database is thus available as a separate open-source project “MatD^3” (https://github.com/HybriD3-database/MatD3). We also describe this software stack, which can enable materials data at any scale, from small workgroups via focused projects all the way to large and general, open and reproducible materials data collections.

Xiaochen Du[1,2], Raul Laasner[3], Xixi Qin[3], Connor Clayton[4], Svenja Janke[3], Becca Lau[3], Chi Liu[1], Sampreeti Bhattacharya[5], Juliana Mendes[6], Jun Hu[5], Dovletgeldi Seyitliyev[7], Ruyi Song[1], Manoj Jana[1], Matti Ropo[8], Franky So[6], Kenan Gundogdu[7], Wei You[5], Yosuke Kanai[5], David B. Mitzi[3,1], Volker Blum[3,1]

  1. Department of Chemistry, Duke University, Durham, NC, United States.
  2. Department of Computer Science, Duke University, Durham, NC, United States.
  3. Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC, United States.
  4. Department of Materials Science, Carnegie Mellon University, Pittsburgh, PA, United States.
  5. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
  6. Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, United States.
  7. Department of Physics, North Carolina State University, Raleigh, NC, United States.
  8. University of Turku, Turku, Finland.

Sodium-based solid-state batteries constitute a promising alternative to current lithium technology, owing to the high abundance of sodium [1]. However, traditional Na-based solid electrolytes (SE), such as 𝛽-alumina and so-called NASICON compounds, have so far exhibited only limited ionic conductivities. Encouragingly, Na3PS4 and its derivatives offer promising ionic conductivities [2]. Na3PS4 and Na3PSe4 both crystallize in a tetragonal phase (P4̅21𝑐) at low temperature and transform to a cubic phase (I4̅3𝑚) around 530K and 270K, respectively[3-6], which exhibits high ionic conductivity of order mS/cm [3,5]. We have performed the quasielastic (QENS) and inelastic neutron scattering (INS) measurements to probe the Na diffusion dynamics and the potential role of phonons in facilitating the fast diffusion process. The QENS and INS measurements were performed using the BASIS and ARCS spectrometers at the Spallation Neutron Source.
Our neutron scattering measurements, complemented with our ab-initio molecular dynamics (AIMD) simulations and anharmonic first-principles phonon simulations, enable us to determine the jump like diffusion mechanism of Na. We identify strongly anharmonic low-energy phonon modes involving PX4 units, which couple to Na diffusion. The QENS measurements and AIMD simulations show that Na ions hop between octahedral sites (6b Wycoff site), with jump length and residence time djump~3.5 Å and res~500 ps. Large diffusion coefficients are estimated using a Chudley Elliot jump diffusion model and are attributed to a soft lattice, and PX4 liberational modes, in particular in Na3PSe4. The simulations also reveal strongly correlated motions of Na ions along 1D chains in both compounds. Our combined experiments and simulations also enable us to quantify the importance of ionic correlations and coherent scattering effects in QENS analysis.

Authors: M K Gupta, Jingxuan Ding, Zachary Hood,
Naresh Osti, Douglas L. Abernathy, Olivier Delaire

First principles molecular dynamics of liquid water usually suffer from two problems — the accuracy is limited by the first principles model and the statistical convergence is limited by the large computational cost. In this work, to improve the accuracy of the model, we examine two first principles models on the fifth’s rung of Jacob’s ladder of density functionals. To improve the statistical convergence, the neural network force field is used to extend the molecular dynamics trajectories to the statistical convergence regime. The translational, rotational, and vibrational dynamics of liquid water have been examined over the temperature range of 260K-360K. With RPA, the dynamics are in very good agreement with the experiments. However, with MP2, the dynamics are too slow compared to the experiments. The van der Waals correction MP2D doesn’t seem to fix the slow dynamics of MP2. In both models, nuclear quantum effects play a surprisingly small effect on the dynamics of liquid water in both these two models except for the intramolecular vibrational dynamics.

Authors: Yi Yao, Yosuke Kanai

Cu2BaSn(S,Se)4 (CBTSSe) has been gaining attention as a prospective solar absorber, since it employs low-toxicity and abundant metals, while offering low-cost manufacturing options, controllable stoichiometry, high absorption coefficient (>104 cm-1) and bandgap (Eg) tunability (1.5-2.0 eV). Besides these suitable optoelectronic properties, CBTSSe does not suffer from anti-site disorder in contrast with Cu2ZnSn(S,Se)4, as confirmed by Shin et al. with vacuum-deposited films [1]. The current study focuses on solution-deposited nominally stoichiometric CBTSSe films [2] with a bandgap of 1.59 eV and explores the fundamental film properties using several spectroscopic techniques. Temperature- and excitation-dependent photoluminescence studies reveal a dominant defect emission at ~1.5 eV and a second deep defect feature at 1.15 eV. Time-resolved terahertz spectroscopy measurements show few tens of picoseconds (~50 ps) surface and few nanoseconds (~3 ns) bulk lifetimes, as well as ~140 cm2/Vs mobility, the latter of which is in the range of reported values for CZTSSe. The solution-processed CBTSSe films suffer from exacerbated surface recombination at the junction of CdS and CBTSSe, due to cliff-like band alignment with 0.63 eV conduction band offset (as measured by ultraviolet photoemission spectroscopy). Employing these films in an Al-Ni/ITO/ZnO/CdS/CBTSSe/Mo photovoltaic device structure, we report open-circuit voltage (VOC), short-circuit current density, fill factor, and efficiency of 470 mV, 14.3 mA/cm2, 43.6%, and 2.93%, a record performance level among solution-processed CBTSSe devices. Device performance in this study may be mainly limited by interfacial properties and inadequately selected device structure (e.g., poorly matched buffer layer), which both enhance recombination-associated losses in VOC. The physical measurements provided for the nominally stoichiometric solution-processed CBTSSe absorber point to critical areas for future improvement of CBTSSe and related photovoltaic cells in the quest for higher efficiency devices based on earth-abundant metals.
[1] Chem. Mater., 28 (2016), 4771-4780.
[2] Chem. Mater., 30 (2018), 6116-6123.

Authors: Betul Teymur, Sergiu Levcenco, Hannes Hempel, Eric Bergmann, José A. Márquez , Leo Choubrac, Ian G. Hill, Thomas Unold, David B. Mitzi

Thermoelectric materials enable direct conversion of waste heat into electrical energy. The conversion efficiency is inversely proportional to the thermal conductivity, which is generally dominated by phonons in semiconductors. Zintl compounds AMg2X2 constitute a class of new thermoelectric compounds with excellent thermoelectric performance in n-type Mg3(Sb,Bi)2 alloys, with zT values up to 1.6 reported so far. Mg3Sb2 exhibits very low lattice thermal conductivity (~1-1.5 W/m/K at 300K), comparable with PbTe and Bi2Te3, despite a much lighter average ionic mass. We report on neutron scattering and first-principles studies of the lattice dynamics of AMg2X2. Inelastic neutron scattering measurements provided the temperature dependence of the phonon density of states (DOS). Extra peaks and overall softer phonons were found at low frequency in Mg3Sb2 and Mg3Bi2 compared to CaMg2X2 or YbMg2X2. Combined with simulations, we highlight the importance of a specific soft Mg-X chemical bond that suppresses phonon group velocities and drastically enlarges the scattering phase-space, enabling the threefold suppression in thermal conductivity.

Author: Jingxuan Ding, Tyson Lanigan-Atkins, Mario Calderon Cueva, Alexandra Zevalkink, Olivier Delaire

Forming Br/I mixtures is a proven pathway to improve chemical stability and control of the band gap of 3-dimensional (3D) hybrid halide perovskites, such as methylammonium lead iodide (MAPbI3), for photovoltaic applications. However, two-dimensional (2D) hybrid mixed halide perovskites are less well investigated. Here, we applied first-principles total energy calculations to study the configurational space of the alloy structures formed based on phenethylammonium (PEA) lead iodide (PEA)2PbI4 and the corresponding bromide, (PEA)2PbBr4, templates, respectively, since stable pure-iodide (PEA)2PbI4 and pure-bromide (PEA)2PbBr4 have different molecular arrangements. In order to limit the vast overall configurational space of conceivable alloys, we confined the alloy structure space considered to have the same alloy pattern in all the inorganic layers and to 2×2 supercells (i.e., 4 units) of the 2D inorganic planes. Within this space, we determine how the formation energy of the alloy varies with respect to the Br/I ratio. Five Br/I ratios which are 0, 0.25, 0.5, 0.75, and 1 are investigated. Our results show that when Br/I = 0.25, the formation energy are close to zero, or above. At Br/I = 0.5 and 0.75, alloy structures are predicted to have more negative formation energies, i.e., a greater propensity towards forming stable alloys. A miscibility gap around Br/I = 0.25 is consistent with XRD spectra of experimentally synthesized mixed halide (PEA)2Pb(I1-xBrx)¬4 films. On the stable side, the alloy structures with negative formation energies for Br/I =0.75 allow for two kinds of Pb-halide octahedra, possibly explaining a corresponding peak split observed in photoluminescence (PL) spectra of (PEA)2Pb(I1-xBrx)¬4 films.

Authors: Xixi Qin a,e, Niara E. Wright a,b, Junwei Xu c, Steven P. Harvey d, Michael F. Toney c, Adrienne D. Stiff-Roberts a,b , Volker Blum a,e
a University Program in Materials Science and Engineering, Duke University, Durham, NC
b Department of Electrical and Computer Engineering, Duke University, Durham, NC

It has been well established that the electron-phonon coupling in Niobium is responsible for the enhanced damping of its atomic vibrations, making this metal significantly depart from a harmonic crystal. This departure appears as Kohn anomalies, which are pronounced softenings in particular regions of its phonon spectrum. Analysis of these anomalies have thus far been limited to high symmetry directions of the reciprocal lattice of the crystal. By using state of the art neutron inelastic instruments coupled with first principles calculations, we extend the investigation of these anomalies in an entire volume of reciprocal space in aims to rigorously assess current methods that aim to capture its deviation from a harmonic lattice.

Author: Bander Linjawi, Olivier Delaire, Dipnashu Bansal, Doug Abernathy

Bismuth has a long history of importance to condensed matter physics, biological sciences, and materials science. Bi has a Peierls distortion of a simple cubic structure, and the atoms along the body diagonal or trigonal axis are spaced nonequidistantly. It has been demonstrated with experiments that a large softening of the interatomic potentials of Bismuth while in a photoexcited non-equilibrium electronic state (on a picosecond time scale). On the other hand, hydrodynamic heat transport property has been discovered in Bi at temperatures of order 2K. The measured thermal conductivity varies faster than T3 in the temperature range 2K<T<5K., which was identified as a signature of a Poiseuille flow of phonons. Here we report on neutron scattering and first-principles studies of the lattice dynamics of Bi. Inelastic neutron scattering measurements provided the temperature dependence of the phonon dispersion and linewidth. Softening of TA modes in Gamma-T direction and decreasing of phonon lifetime at elevated temperature were detected in experiments and examined by ab initio molecular dynamics. Further we visualized the four-dimensional dynamic structure factor S(Q, E) which depicted the energy isosurface of Bi. These results illustrated the temperature dependent behavior of the lattice dynamics of Bi.

Author: Chengjie Mao

Molecular and materials simulations based on Kohn-Sham density-functional theory (KS-DFT) are the production workhorse for a broad range of applications in physics, chemistry, and materials science. In large-scale KS-DFT simulations, computing the electron density from given Hamiltonian (and possibly overlap) matrices is a key computational bottleneck that limits the tractable system size to roughly several thousand atoms. This poster presents ELSI, an open-source software infrastructure to facilitate the implementation and optimal use of a variety of eigensolvers and density matrix solvers that compute the electron density in KS-DFT. ELSI simplifies the access to these solvers by providing automatic conversion between different matrix storage formats, reasonable default settings, and suggestions on the optimal solver for a given problem. The performance of the solvers supported by ELSI is assessed and rigorously compared by a systematic set of benchmarks performed on the world’s leading supercomputers. The ubiquitous adoption of graphics processing units (GPUs) in high-performance computing opens up new opportunities to accelerate KS-DFT calculations. We have developed and optimized GPU acceleration in the two-stage tridiagonalization eigensolver ELPA2, targeting distributed-memory, GPU-equipped supercomputing architectures. We demonstrate the performance of GPU-accelerated ELPA2 in routine KS-DFT simulations comprising thousands of atoms, for which a couple of GPU-equipped supercomputer nodes reach the throughput of some tens of nodes with conventional central processing units (CPUs). The GPU-accelerated ELPA2 solver can be used through the ELSI interface, smoothly and transparently bringing GPU support to all the KS-DFT codes connected with ELSI. This work is supported by the National Science Foundation under Award No. 1547580 and Award No. 1450280.

Authors: Victor Yu, Carmen Campos, William Dawson, Alberto García, Ville Havu, Ben Hourahine, William Huhn, Mathias Jacquelin, Weile Jia, Murat Keçeli, Pavel Kůs, Raul Laasner, Hermann Lederer, Yingzhou Li, Lin Lin, Jianfeng Lu, Andreas Marek, Peter Messmer, Jose Roman, Álvaro Vázquez-Mayagoitia, Chao Yang, Mina Yoon, Volker Blum

The different compositions, x = 0.02 to x = 0.94, of single crystals of NbxV1-xO2 are grown by using a Chemical Vapor Transport Technique. The different types of crystallographic phase transitions: M1 phase to rutile phase at composition x < 0.09, from 2D-M2 phase to rutile phase at compositions 0.09 ≤ x ≥ 0.24, and no crystallographic phase transition at 0.25 ≤ x ≥ 0.65 are observed. The pure rutile phase at the compositions of x = 0.25 to x = 0.65, the two-dimensional phase from x = 0.76 to x = 0.90, and a body-centered tetragonal phase at x = 0.94 are observed. The metal-insulator transitions and paramagnetic to paramagnetic transitions at lower compositions of x ≤ 0.12 are sharper than their higher compositions transitions, and their electronic transition temperatures correspond to their crystallographic transition temperature. The mechanism of MIT looks like more Mott-Peierls mechanism. Also, the negative θweiss value and Curie tail in magnetic measurements indicate the antiferromagnetic ordering at lower temperatures, but further study will be required to prove it.

Author: Top B. Rawot Chhetri

Translation of structural patterns between different atomic-scale building blocks plays a fundamental role in nature, enabling unique functionalities in contexts ranging from biological systems to synthetic materials. In this work, we demonstrate the transfer of structural asymmetry from layers of chiral molecules to an originally achiral inorganic component in a group of two-dimensional hybrid perovskites with chiral organic cations. Asymmetric hydrogen-bonding interactions, caused by the orientation preference of R- (+)- or S-(−)-1-(1-naphthyl)ethylammonium organic spacer cations, account for the asymmetric distortion pattern of Pb-Br 2D inorganic framework. Spin-orbit coupled hybrid density-functional theory band structure calculations reveal substantial Rashba-Dresselhaus spin-splitting and spin textures of frontier bands associated with the distorted structures, potentially useful for control of spin properties of carriers in devices.

Authors: RUYI SONG, Manoj K. Jana, Haeliang Liu, Dipak R. Khanal, Svenja M. Janke1, Rundong Zhao, Chi Liu, Valy Vardeny, Volker Blum, David B. Mitzi

Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr3 crystals using momentum-resolved neutron and x-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional (2D) sheets of correlated rotations in real space, and could represent precursors to proposed 2D polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide new insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimize their optical and thermal properties.

Authors: T. Lanigan-Atkins, X. He, M. J. Krogstad, D. M. Pajerowski, D. L. Abernathy, Guangyong NMN Xu, Zhijun Xu, D.-Y. Chung, M. G. Kanatzidis, S. Rosenkranz, R. Osborn, and O. Delaire

Tunning the composition of halide elements in mixed halide two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) is a promising approach to control the bandgap for application to optoelectronic devices. However, traditional solution-based thin-film deposition techniques have difficulty in achieving desired mixed halide composition and the post-growth annealing in high temperature are usually needed alongside with these methods, which tend to promotes phase segregation and degradation of mixed halide HOIPs thin film. Resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) as an advanced deposition method can grow high quality thin films without post annealing. In order to describe the film behavior subject to annealing for different mixed halide compositions, optical properties are studied for as-grown and annealed films of (PEA)2Pb(I1-xBrx)4 for x = 0, 0.25, 0.5, 0.75, and 1. PL measurement shows that the mixed halide (PEA)2Pb(I0.75Br0.25)4 has peak split which indicates a phase segregation tendency while for other components x =0, 0.5, 0.75 and 1, there are no such tendency. By using DFT-PBE+TS scheme, we show that (PEA)2Pb(I1-xBrx)4 for x = 0 and x = 1 have different molecule stacking pattern. Assuming the two inorganic layers in the P(2*2) unit cell have the same alloy configuration, we enumerated and relaxed the possible symmetric alloy structures based on (PEA)2PbI4 and (PEA)2PbI4, respectively. The result shows that the formation energies of (PEA)2Pb(I0.75Br0.25)4 are all greater or near zero while other mixed halide compositions ( x = 0.5, 0.75) have alloy structures with significantly negative formation energy, which proves that when x =0.25, the mixed halide composition will tend to form phase segregation, while x = 0.5, and 0.75, there is a possibility to form alloy structures. The theoretical results are consistent with experimental results, which indicate the peak split observed in the PL of (PEA)2Pb(I0.75Br0.25)4 is probably due to phase segregation.

Authors: Niara E. Wright, Junwei Xu, Xixi Qin, Steven P. Harvey, Volker Blum, Michael F. Toney, Adrienne D. Stiff-Roberts

The ability to reversibly store energy in lithium-ion batteries led a technological revolution in mobile electronics, electric vehicles, and grid-scale energy storage. It is important to continue to improve this technology by understanding the mechanisms that can increase both charge storage capacity and kinetics. One concept for doing so is to utilize layered materials with confined interlayer water or solvents as opposed to the vacant/cation filled interlayers of the currently
used active materials. We hypothesize that such materials would exhibit low activation energy barriers for electrochemical charge transfer and fast electrochemical diffusion in the solid state. Molybdenum trioxide (MoO3) and its related layered hydrates are model materials to understand how structural water affects Li+ intercalation kinetics. MoO3∙2H2O is a layered oxide hydrate whose two discrete interlayer water molecules are bound covalently to the MoO3 octahedra and hydrogen bonded, respectively. Dehydration of MoO3∙2H2O leads to layered MoO3∙H2O and MoO3 . The fact that all three materials are layered allows for the decoupling of structural water effects from layered structure effects. Prior electrochemical studies of layered MoO3 hydrates found that Li+ intercalation is possible but mechanistic understanding of their interfacial charge transfer and solid state diffusion are currently unknown. To investigate the hypothesis, MoO3∙2H2O was synthesized via an acid precipitation technique and subsequently dehydrated to form MoO3∙H2O and MoO3. The temperature, pressure, and solvent stability of all three materials were investigated to understand the stability of the interlayer water. The materials were characterized with X-ray diffraction and Raman spectroscopy. Electrochemical characterization of MoO3∙H2O and MoO3 was performed in a non-aqueous Li+ electrolyte to determine the charge storage capacity and kinetics. Overall, this study aims to identify how interlayer structural water affects Li+ intercalation into layered and hydrated MoO3 .

Authors: Matthew Chagnot, James Mitchell, Veronica Augustyn

Metamaterials Poster Submissions

Imaging at any wavelength is often restrained to theoretical and practical limitations. Furthermore, imaging in the terahertz (THz) domain is difficult due to a lack of high-powered sources and efficient detectors. Although progress has been made engineering practical THz imaging devices, these devices remain limited by the Rayleigh diffraction limit and low resolution. Super resolution (SR) reconstruction is a method capable of resolving features beyond these limitations. The method uses a set of low resolution (LR) images to reconstruct a super resolved image. Here we successfully demonstrate super resolution THz imaging with an all dielectric metasurface where we reconstruct a SR image resolving features smaller than the Rayleigh diffraction limit and periodicity of our metasurface.

Authors: Nicolas Lozada-Smith, Kebin Fan, Felix Jin, Sina Farsiu

The design of resonators using supersymmetry (SUSY) allows a method rooted in physics to obtain the refractive index distribution and shape of structures from the desired spectrum. First-order SUSY adapted from quantum physics to optics manipulates the transverse refractive index of guided-wave structures using a nodeless ground state to obtain intended modal content. Second-order SUSY can be implemented using scattering states as a seed function, even with the
presence of nodes. We apply second-order SUSY to the coupled-mode equations by recasting them as the Dirac equation. and design one-dimensional corrugated waveguides to obtain the desired spectral response. Due to the finite length of the grating, inserted states have a finite lifetime. We obtain a bound state in the continuum by the interference of two states at the same frequency decaying in the same waveguide. We demonstrate that degenerate second-order SUSY allows the insertion of two states, which coalesce into Friedrich-Wintgen type bound states in the continuum (BIC) for one-dimensional grating. One-dimensional BIC states can find application as robust high-speed all-optical temporal integrators by lifting restrictions on the length of various sections in the phase-shifted grating.

Author: Nitish Chandra, Natalia M. Litchinitser

In this work, we focus on novel regimes of nonlinear light-matter interactions enabled by engineered photonic quasicrystals. In particular, in the field of nonlinear optics, the advantage of using photonic quasicrystals (PQCs) is the possibility of supporting unlimited combinations of wavevectors that may appear in their reciprocal lattices. We report (i) the nonlinear interactions engineering using quasi-periodic quasi-phase matching; (ii) design and fabrication of PQCs in chalcogenide waveguides; and (iii) experimental demonstration of the predicted new phase-matching regimes enabled by the PQCs. These structures are realized in arsenic trisulfide (As2S3) chalcogenide glass (ChG) that displays a very good nonlinear figure of merit in both the near-infrared and the mid-wave infrared spectral bands. Here, we show that the proposed structures realized in such a highly nonlinear medium may enable a promising platform for both fundamental studies of nonlinear light-matter interactions and applications in wavelength conversion, supercontinuum generation, and development of classical and quantum optical sources.

Authors: Jiannan Gao1, Mikhail Shalaev1, Jesse Frantz2, Jason D. Myers2, Robel Y. Bekele3, Jasbinder S. Sanghera2, and Natalia M. Litchinitser1

We present a theoretical study of the collective quasiparticle excitations responsible for the electromagnetic response of ultrathin plane-parallel homogeneous periodic single-wall carbon nanotube arrays and weakly inhomogeneous single-wall carbon nanotube films. We show that in addition to varying film composition, the collective response can be controlled by varying the film thickness. For single-type nanotube arrays, the real part of the dielectric response shows a broad negative refraction band near a quantum interband transition of the constituent nanotube, whereby the system behaves
as a hyperbolic metamaterial at higher frequencies than those classical plasma oscillations have to offer. By decreasing nanotube diameters it is possible to push this negative refraction into the visible region, and using weakly inhomogeneous multi-type nanotube films broadens its bandwidth.

Authors: Chandra M. Adhikari, Igor V. Bondarev

All-dielectric metasurfaces emerged as prominent platforms to manipulate electromagnetic scattering responses at surfaces, but they require extensive numerical simulations to understand their physical properties. Deep learning approaches have been studied on the accelerated inverse design of metasurfaces, yet more complex geometries remain mostly unexplored. We introduce a recent deep learning method — termed neural-adjoint method — that can accurately identify complex ADM geometry that yields targeted frequency-dependent scatterings. The neural-adjoint method also shows excellent potential in devising active learning algorithms to reduce initial simulation requirements. The neural-adjoint method is not limited to metasurface inverse design and can be applied to other structured material systems.

Authors: Yang Deng, Simiao Ren, Kebin Fan, Jordan M. Malof, Willie J. Padilla.

We simulated an all-dielectric metasurface (ADMs) comprised of free-standing cylindrical structures for a varying asymmetry parameter. We found that several modes are observed and the high-quality factor (HQF) exceeds 10^4. One metasurface design was successfully fabricated and preliminary experiments show the existence of the BIC.

Authors:  Natalie Rozman, Kebin Fan, and Willie Padilla

Scientists in the area of photonics actively study metastructures requiring meta-atoms with a peculiar optical response. High-order multipole excitations lead to different radiation patterns and near-field distributions. We show how a magnetic octupole moment (MOCT) can be resonantly excited by dividing a solid silicon block to an oligomer structure based on nanocubes. In addition, the spectral position of a MOCT resonance can be controlled by changing distance between nanocubes. We demonstrate how magnetic octupole resonance affect properties of single quadrumer scatterers and dielectric metasurfaces based on such meta-atoms. Unusual absorption, magnetic hot-spots and reflection suppression are shown.

The proposed nanostructuring approach allows to overcome diffraction restriction and can be further used to design novel metasurfaces and optical devices. In addition, after being scaled to microwave region, magnetic hot-spots can be exploited to increase a quality of MRI imaging.

Authors:  P. D. Terekhov, A. B. Evlyukhin, A. S. Shalin, A. Karabchevsky

Quantum Materials Poster Submissions

Breathing pyrochlore systems are composed of corner-sharing tetrahedra of different sizes pointing in opposing directions, leading to different intra- and inter-tetrahedra exchange interactions and the emergence of the Dzyaloshinskii-Moriya interaction due to loss of inversion symmetry. They are predicted to host exotic physics including quantum spin ice, quantum spin liquid, and field-tunable topological magnons. Here we will present single-crystal field-dependent unpolarized and polarized inelastic neutron scattering measurements on Yb-based breathing pyrochlore system, as well as a theoretical model that can effectively describe some of our experimental findings.

Authors: Sachith Dissanayake, Zhenzhong Shi, William Steinhardt, Stephen Kuhn, Jeffrey Rau, Nicholas Butch, Matthias Frontzek, Andrey Podlesnyak, Yiming Qiu, Wangchun Chen, David Graf, Tao Hong, Casey Marjerrison, Michel Gingras, Sara Haravifard

Identification, understanding, and manipulation of novel magnetic textures is essential for the discovery of new quantum materials for future spin-based electronic devices. In particular, materials that manifest a large response to external stimuli such as a magnetic field are subject to intense investigation. Here, we study the kagome-net magnet YMn6Sn6 by magnetometry, transport, and neutron diffraction measurements combined with first principles calculations. We identify a number of nontrivial magnetic phases, explain their microscopic nature, and demonstrate that one of them hosts a large topological Hall effect (THE). We propose a new fluctuation-driven mechanism, which leads to the THE at elevated temperatures. This interesting physics comes from parametrically frustrated interplanar exchange interactions that trigger strong magnetic fluctuations. Our results pave a path to new chiral spin textures, promising for novel spintronics.

Authors: D. Connor Jones, Rebecca L. Dally, L. Poudel, D. Michel, N. Thapa Magar, M. Bleuel, Michael A. McGuire, J. S. Jiang, J. F. Mitchell, Jeffrey W. Lynn, I. I. Mazin, and Nirmal J. Ghimire

The Shastry-Sutherland compound SrCu2(BO3)2 features 2D layers of Cu2+ S=1/2 spin dimers which are orthogonal to each other. The ground state of the system is determined by the relative strength of the nearest neighbor and next-nearest neighbor interactions, J and J’ respectively. The ratio of J/J’ can be tuned continuously by application of hydrostatic pressure. The ground state changes from a spin dimer singlet state at ambient pressure to an antiferromagnet state at high pressure. At intermediate pressure a novel 4-spin plaquette singlet state has recently been reported. However, the nature of this plaquette state and how it evolves into other phases remains unclear. Here, we report a comprehensive study of the quantum phase diagram of the plaquette state by tuning temperature, pressure, magnetic field, and chemical doping. We mapped out the evolution of the ground states using complementary techniques such as magnetization measurements and neutron scattering. The results provide insights into the nature of the plaquette state, and also has implications in areas such as studies of deconfined quantum criticality.

Authors: Zhenzhong Shi, Sachith Dissanayake, David E. Graf, Philippe Corboz, Daniel M. Silevitch, Casey Marjerrison, Hanna Dabkowska, Thomas Rosenbaum, Frederic Mila, Sara Haravifard

Breathing Pyrochlore materials have emerged as a promising candidate to study frustrated magnetism and topological magnons. We have initiated design and synthesis of rare-earth based Breathing Pyrochlore compounds. Further, we have been using neutron diffraction and inelastic neutron scattering techniques to probe the static and dynamic properties of these compounds. In this talk, we are going to present our latest experimental results for the Tm-based breathing pyrochlore compound Ba3Tm2Zn5O11.

Authors: Lalit Yadav, Rabindranath Bag , Sachith Dissanayake , Zhenzhong Shi , Guangyong Xu , Craig Brown , Nicholas Butch , Franz Lang , Stephen Blundell, Sara Haravifard

In the last few years YbMgGaO4 and related triangular antiferromagnets emerged as promising quantum spin liquid candidates, and subsequently their ground states have been the subject of ardent debates. Though many experimental and theoretical studies have been devoted to investigating the magnetic properties of these systems at very low temperatures, and exploring a range of possible explanations for the observed spin liquid-like phenomena, a definitive description remains elusive mainly due to chemical disorder. In this presentation we discuss neutron scattering experiments in applied magnetic fields to probe static and dynamic properties of YbZnGaO4.

Authors: William Steinhardt, Sachith Dissanayake, Zhenzhong Shi, Nicholas Butch, David E. Graf, Andrey Podlesnyak, Yaohua Liu, Yang Zhao, Guangyong Xu, Jeffrey Lynn, Casey Marjerrison, Sara Haravifard

Recently, triangular antiferromagnetic materials have attracted attention because competing interactions on the lattice can give rise to exotic phenomena, such as Quantum Spin Liquids. We have initiated systematic efforts to synthesize single crystal samples of a family of rare-earth based triangular antiferromagnet double borates. In particular, focus has been given to one member of this family, Ba3Yb(BO3)3 (BYBO), which features spin-½ ytterbium ions on a triangular lattice. We have conducted neutron scattering studies on both powder and single crystal samples of BYBO to probe the static and dynamic properties of this system. In this talk, we will present the results of our experimental efforts.

Authors: Matthew Ennis, Rabindranath Bag, Sachith Dissanayake, Zhenzhong Shi, Alexander Kolesnikov, Jose A Rodriguez, Nick Butch, Hui Wu, Craig Brown, Sara Haravifard

Important insights into the thermodynamics and mechanism of the VO2 MIT were achieved by probing lattice dynamics using IXS[1] and ultrafast pump-probe x-ray diffraction[2]. But detailed phonon dispersions for the insulating M1 phase have yet to be reported. In addition, abnormally low electronic thermal conductivity in Rutile VO2 requires further examination of phonon anharmonicity and electron-phonon coupling[3]. We report new measurements of phonon dispersions in VO2 and doped crystals, as well as DFT simulations of phonon dispersions and spectral functions. Our results explain the origin of strong phonon damping in Rutile VO2, compared to M1 VO2 and Rutile TiO2, and assess the failure of perturbation theory in predicting accurate phonon linewidths. Our simulations capture the phonon damping behavior beyond perturbation theory, providing critical insights into the unusual lattice thermal conductivity.
1. Budai, J.D., et al., Nature, 2014. 515(7528): p. 535-539.
2. S. Wall*†, S. Yang,* et al., Science 362, pp. 572-576 (2018)
3. Lee, S., et al., Science, 2017. 355(6323): p. 371-374.

Authors: Shan Yang , John D. Budai , Dipanshu Bansal , Xing He , Michael E. Manley , Chen Li , Jiawang Hong , Lynn A. Boatner , Ayman Said , Olivier Delaire

The quantum paraelectric behavior and strongly anharmonic lattice dynamics in SrTiO3 and KTaO3 have attracted interest for decades. By tunning temperature, doping, and external field, quantum paraelectric materials can cross multiple phase transitions and approach a ferroelectric quantum critical point (QCP). Besides the ferroelectric soft mode and phase transition, recent research also revealed unusual thermal transport properties such as thermal Hall effects in SrTiO3 and KTaO3, as well as superconductivity, which motivate detailed studies of lattice dynamics. In particular, understanding phonon anharmonicity near a QCP is crucial to rationalizing the properties of quantum paraelectrics. We used inelastic neutron scattering and first-principle simulations to probe the temperature, electric field, and doping effects on phonons in SrTiO3 and KTaO3. Our experiments reveal striking intensity changes, as well as phonon damping and shifts, reflecting strong acoustic-optic phonon coupling which are reproduced by our calculations. These results provide direct insights into the behaviors of phonon eigenvectors and ionic interactions.

Authors: Xing He, Dipanshu Bansal, Douglas L Abernathy, Barry Winn, Songxue Chi, Lynn Boatner, Olivier Delaire

Geometrically frustrated magnets are considered one of the most interesting topics in condensed matter physics due to a variety of exciting physics such as quantum spin liquid, spin glass and spin ordered state. Rare-earth based borate family having two dimensional (2D) triangular lattice structure is a good candidate to host many exotic ground states. We have successfully synthesized and grown the centimetric size single crystals of a series of borate compounds with different rare earth elements. Low temperature static properties of rare-earth based borates are performed using neutron diffraction while crystalline electric field levels are identified using inelastic neutron scattering technique. In this talk, I will present our latest results achieved from the synthesis and advanced characterization experiments.

Authors: Rabindranath Bag , Matthew Ennis , Sachith Dissanayake , Zhenzhong Shi , Alexander I. Kolesnikov , Hui Wu , Craig M. Brown , David Graf , Sara Haravifard