# Publications

2020

Dominik M. Juraschek, Derek S. Wang, and Prineha Narang. 11/18/2020. “Sum-frequency excitation of coherent magnons.” arXiv. Publisher's VersionAbstract

Coherent excitation of magnons is conventionally achieved through Raman scattering processes, in which the difference-frequency components of the driving field are resonant with the magnon energy. Here, we describe mechanisms by which the sum-frequency components of the driving field can be used to coherently excite magnons through two-particle absorption processes. We use the Landau-Lifshitz-Gilbert formalism to compare the spin-precession amplitudes that different types of impulsive stimulated and ionic Raman scattering processes and their sum-frequency counterparts induce in an antiferromagnetic model system. We show that sum-frequency mechanisms enabled by linearly polarized driving fields yield excitation efficiencies comparable or larger than established Raman techniques, while elliptical polarizations produce only weak and circularly polarizations no sum-frequency components at all. The mechanisms presented here complete the map for dynamical spin control by the means of Raman-type processes.

David D. Dai, Derek S. Wang, and Prineha Narang. 11/17/2020. “Passive controlled-variable phase gate on photonic qubits via a cascade emitter.” arXiv. Publisher's VersionAbstract

We present a scheme to implement a passive and deterministic controlled-variable phase gate on photonic qubits encoded in the frequency basis. Our gate employs a cascade system with the ground to first excited state interacting with the control photon of a given polarization, and the first to second excited state transition interacting with the target photon of the orthogonal polarization. By controlling the relative detuning between the target photon and the frequency of the transition between the first and second excited states of the cascade emitter, we enable any controlled-phase operation from 0 to π. This gate does not utilize any active control and needs only a single cascade emitter, enabling low-footprint and more efficient decomposition of quantum circuits, especially those rooted in the quantum Fourier transform.

Christopher J. Ciccarino and Prineha Narang. 11/16/2020. “Off balance and over the edge.” Nature Nanotechnology. Publisher's VersionAbstract

Ultrafast spectroscopy measurements present a new direct non-equilibrium energy transfer mechanism across a metal–semiconductor interface, without charge transfer, opening up a new avenue for plasmonic energy conversion.

Georgia T. Papadakis, Christopher J. Ciccarino, Lingling Fan, Meir Orenstein, Prineha Narang, and Shanhui Fan. 11/2/2020. “Deep subwavelength thermal switch via resonant coupling in monolayer hexagonal boron nitride.” arXiv. Publisher's VersionAbstract

Unlike the electrical conductance that can be widely modulated within the same material even in deep nanoscale devices, tuning the thermal conductance within a single material system or nanostructure is extremely challenging and requires a large-scale device. This prohibits the realization of robust ON/OFF states in switching the flow of thermal currents. Here, we present the theory of a thermal switch based on resonant coupling of three photonic resonators, in analogy to the field-effect electronic transistor composed of a source, gate, and drain. As a material platform, we capitalize on the extreme tunability and low-loss resonances observed in the dielectric function of monolayer hexagonal boron nitride (hBN) under controlled strain. We derive the dielectric function of hBN from first principles, including the phonon-polariton linewidths computed by considering phonon isotope and anharmonic phonon-phonon scattering. Subsequently, we propose a strain-controlled hBN-based thermal switch that modulates thermal conductance by more than an order of magnitude, corresponding to an ON/OFF contrast ratio of 98%, in a deep subwavelength nanostructure.

Prineha Narang, Christina A. C. Garcia, and Claudia Felser. 11/2/2020. “The topology of electronic band structures.” Nature Materials. Publisher's VersionAbstract

The study of topology as it relates to physical systems has rapidly accelerated during the past decade. Critical to the realization of new topological phases is an understanding of the materials that exhibit them and precise control of the materials chemistry. The convergence of new theoretical methods using symmetry indicators to identify topological material candidates and the synthesis of high-quality single crystals plays a key role, warranting discussion and context at an accessible level. This Perspective provides a broad introduction to topological phases, their known properties, and material realizations. We focus on recent work in topological Weyl and Dirac semimetals, with a particular emphasis on magnetic Weyl semimetals and emergent fermions in chiral crystals and their extreme responses to excitations, and we highlight areas where the field can continue to make remarkable discoveries. We further examine open questions and directions for the topological materials science community to pursue, including exploration of non-equilibrium properties of Weyl semimetals and cavity-dressed topological materials.

Christopher J. Ciccarino, Johannes Flick, Isaac B. Harris, Matthew E. Trusheim, Dirk R. Englund, and Prineha Narang. 10/30/2020. “Strong spin–orbit quenching via the product Jahn–Teller effect in neutral group IV qubits in diamond.” npj Quantum Materials, 5. Publisher's VersionAbstract

Artificial atom qubits in diamond have emerged as leading candidates for a range of solid-state quantum systems, from quantum sensors to repeater nodes in memory-enhanced quantum communication. Inversion-symmetric group IV vacancy centers, comprised of Si, Ge, Sn and Pb dopants, hold particular promise as their neutrally charged electronic configuration results in a ground-state spin triplet, enabling long spin coherence above cryogenic temperatures. However, despite the tremendous interest in these defects, a theoretical understanding of the electronic and spin structure of these centers remains elusive. In this context, we predict the ground- and excited-state properties of the neutral group IV color centers from first principles. We capture the product Jahn-Teller effect found in the excited state manifold to second order in electron-phonon coupling, and present a non-perturbative treatment of the effect of spin-orbit coupling. Importantly, we find that spin-orbit splitting is strongly quenched due to the dominant Jahn-Teller effect, with the lowest optically-active

^{3}E_{u}state weakly split into m_{s}-resolved states. The predicted complex vibronic spectra of the neutral group IV color centers are essential for their experimental identification and have key implications for use of these systems in quantum information science.Qing-Ge Mu, Dennis Nenno, Yan-Peng Qi, Feng-Ren Fan, Cuiying Pei, Moaz ElGhazali, Johannes Gooth, Claudia Felser, Prineha Narang, and Sergey Medvedev. 10/14/2020. “Suppression of axionic charge density wave and onset of superconductivity in the chiral Weyl semimetal Ta

_{2}Se_{8}I.” arXiv. Publisher's VersionAbstractA Weyl semimetal with strong electron-phonon interaction can show axionic coupling in its insulator state at low temperatures, owing to the formation of a charge density wave (CDW). Such a CDW emerges in the linear chain compound Weyl semimetal Ta

_{2}Se_{8}I below 263 K, resulting in the appearance of the dynamical condensed-matter axion quasiparticle. In this study, we demonstrate that the interchain coupling in Ta_{2}Se_{8}I can be varied to suppress the CDW formation with pressure, while retaining the Weyl semimetal phase at high temperatures. Above 17 GPa, the Weyl semimetal phase does not survive and we induce superconductivity, due to the amorphization of the iodine sub-lattice. Structurally, the one-dimensional Ta-Se-chains remain intact and provide a superconducting channel in one dimension. We highlight that our results show a near-complete suppression of the gap induced by the axionic charge-density wave at pressures inaccessible to previous studies. Including this CDW phase, our experiments and theoretical predictions and analysis reveal the complete topological phase diagram of Ta_{2}Se_{8}I and its relationship to the nearby superconducting state. The results demonstrate Ta_{2}Se_{8}I to be a distinctively versatile platform for exploring correlated topological states.Dominik M. Juraschek, Prineha Narang, and Nicola A. Spaldin. 10/7/2020. “Phono-magnetic analogs to opto-magnetic effects.” Physical Review Research, 2, 4, Pp. 043035. Publisher's VersionAbstract

The magneto-optical and opto-magnetic effects describe the interaction of light with a magnetic medium. The most prominent examples are the Faraday and Cotton-Mouton effects that modify the transmission of light through a medium, and the inverse Faraday and inverse Cotton-Mouton effects that can be used to coherently excite spin waves. Here, we introduce the phenomenology of the analog magneto-phononic and phono-magnetic effects, in which coherently excited vibrational quanta take the place of the light quanta. We show, using a combination of density functional theory and phenomenological modeling, that the effective magnetic fields exerted by these phono-magnetic effects on the spins of antiferromagnetic nickel oxide yield magnitudes comparable or larger than those of the opto-magnetic effects.

Yuxin Yin, Jennifer Coulter, Christopher J. Ciccarino, and Prineha Narang. 10/2/2020. “Theoretical investigation of charge density wave instability in CuS

_{2}.” Physical Review Materials, 4, 10, Pp. 104001. Publisher's VersionAbstractThe existence of a charge density wave (CDW) in transition-metal dichalcogenide (TMDC) CuS

_{2}has remained undetermined since its first experimental synthesis nearly 50 years ago. Despite conflicting experimental literature regarding its low-temperature structure, there exists no theoretical investigation of the phonon properties and lattice stability of this material. By studying the first-principles electronic structure and phonon properties of CuS_{2}at various electronic temperatures, we identify temperature-sensitive soft phonon modes which unveil a previously unreported Kohn anomaly at approximately 100 K. Variation of the electronic temperature shows the presence of two distinct phases, characterized at low electronic temperature by a 2×2×2 periodic charge modulation associated with the motion of the S_{2}dimers. We find this is driven by a slight orbital occupation imbalance of the copper*d*and sulfur*p*orbitals, reminiscent of the Jahn-Teller effect in finite systems. Investigation of the Fermi surface presents a potential Fermi surface nesting vector related to the location of the Kohn anomaly and observed band splittings in the unfolded band structure. The combination of these results suggests a strong possibility of CDW order in CuS_{2}. Further study of CuS_{2}in monolayer form finds no evidence of a CDW phase, as the identified bulk periodic distortions cannot be realized in two dimensions. This behavior sets this material apart from other transition-metal dichalcogenide materials, which exhibit a charge density wave phase down to the two-dimensional limit. As CDW in TMDC materials is considered to compete with superconductivity, the lack of a CDW in monolayer CuS_{2}suggests the possibility of enhanced superconductivity relative to other transition-metal dichalcogenides. Overall, our work identifies CuS_{2}as a previously unrealized candidate to study the interplay of superconductivity, CDW order, and dimensionality.Georgios Varnavides, Adam S. Jermyn, Polina Anikeeva, Claudia Felser, and Prineha Narang. 9/18/2020. “Electron hydrodynamics in anisotropic materials.” Nature Communications, 11, Pp. 4710. Publisher's VersionAbstract

Rotational invariance strongly constrains the viscosity tensor of classical fluids. When this symmetry is broken in anisotropic materials a wide array of novel phenomena become possible. We explore electron fluid behaviors arising from the most general viscosity tensors in two and three dimensions, constrained only thermodynamics and crystal symmetries. We find nontrivial behaviors in both two- and three-dimensional materials, including imprints of the crystal symmetry on the large-scale flow pattern. Breaking time-reversal symmetry introduces a non-dissipative Hall component to the viscosity tensor, and while this vanishes for 3D isotropic systems we show it need not for anisotropic materials. Further, for such systems we find that the electronic fluid stress can couple to the vorticity without breaking time-reversal symmetry. Our work demonstrates the anomalous landscape for electron hydrodynamics in systems beyond graphene, and presents experimental geometries to quantify the effects of electronic viscosity.

Yaxian Wang and Prineha Narang. 9/15/2020. “Anisotropic scattering in the goniopolar metal NaSn

_{2}As_{2}.” Physical Review B, 102, 12, Pp. 125122. Publisher's VersionAbstractRecent experimental discoveries in axis-dependent conduction polarity, or

*goniopolarity*, have observed that the charge carriers can conduct like either electrons or holes depending on the crystallographic direction they travel along in layered compounds such as NaSn_{2}As_{2}. The original theoretical proposal is based on the opposite signs of the carrier effective mass, or the curvature of the Fermi surface, without examining the effect of electron lifetimes, thus leaving a crucial question to address. To elucidate this unusual transport behavior, we present an*ab initio*study of electron scattering in such systems. We study different microscopic scattering mechanisms in NaSn_{2}As_{2}, and we present the electron-phonon scattering time distribution on its Fermi surface in momentum space, the open concave shape of which is proposed to be the origin of the axis-dependent conduction polarity. Further, we obtain the overall anisotropic lifetime tensors in real space at different electron chemical potentials and temperatures, and we discuss how they contribute to the macroscopic thermopower. While we find that the contribution of the in-plane and cross-plane lifetimes exhibits a similar trend, the concave portion of the Fermi surface alters the electron motion significantly in the presence of a magnetic field, thus flipping the conduction polarity as measured via the Hall effect. Our calculations and analysis of NaSn_{2}As_{2}, in comparison with similar systems, also suggest the strong possibility of hydrodynamic electron flow in the system. Finally, our work has implications for anisotropic electron lifetimes in a broad class of goniopolar materials and provides key, general insights into electron scattering on open Fermi surfaces.Uri Vool, Assaf Hamo, Georgios Varnavides, Yaxian Wang, Tony X. Zhou, Nitesh Kumar, Yuliya Dovzhenko, Ziwei Qiu, Christina A. C. Garcia, Andrew T. Pierce, Johannes Gooth, Polina Anikeeva, Claudia Felser, Prineha Narang, and Amir Yacoby. 9/9/2020. “Imaging phonon-mediated hydrodynamic flow in WTe2 with cryogenic quantum magnetometry.” arXiv. Publisher's VersionAbstract

In the presence of strong interactions, electrons in condensed matter systems can behave hydrodynamically thereby exhibiting classical fluid phenomena such as vortices and Poiseuille flow. While in most conductors large screening effects minimize electron-electron interactions, hindering the search for possible hydrodynamic candidate materials, a new class of semimetals has recently been reported to exhibit strong interactions. In this work, we study the current flow in the layered semimetal tungsten ditelluride (WTe2) by imaging the local magnetic field above it using a nitrogen-vacancy (NV) defect in diamond. Our cryogenic scanning magnetometry system allows for temperature-resolved measurement with high sensitivity enabled by the long defect spin coherence. We directly measure the spatial current profile within WTe2 and find it differs substantially from the uniform profile of a Fermi liquid, indicating hydrodynamic flow. Furthermore, our temperature-resolved current profile measurements reveal an unexpected non-monotonic temperature dependence, with hydrodynamic effects strongest at ~20 K. We further elucidate this behavior via ab initio calculations of electron scattering mechanisms, which are used to extract a current profile using the electronic Boltzmann transport equation. These calculations show quantitative agreement with our measurements, capturing the non-monotonic temperature dependence. The combination of experimental and theoretical observations allows us to quantitatively infer the strength of electron-electron interactions in WTe2. We show these strong electron interactions cannot be explained by Coulomb repulsion alone and are predominantly phonon-mediated. This provides a promising avenue in the search for hydrodynamic flow and strong interactions in high carrier density materials.

Stefan Krastanov, Kade Head-Marsden, Sisi Zhou, Steven T. Flammia, Liang Jiang, and Prineha Narang. 9/8/2020. “Unboxing Quantum Black Box Models: Learning Non-Markovian Dynamics.” arXiv. Publisher's VersionAbstract

Characterizing the memory properties of the environment has become critical for the high-fidelity control of qubits and other advanced quantum systems. However, current non-Markovian tomography techniques are either limited to discrete superoperators, or they employ machine learning methods, neither of which provide physical insight into the dynamics of the quantum system. To circumvent this limitation, we design learning architectures that explicitly encode physical constraints like the properties of completely-positive trace-preserving maps in a differential form. This method preserves the versatility of the machine learning approach without sacrificing the efficiency and fidelity of traditional parameter estimation methods. Our approach provides the physical interpretability that machine learning and opaque superoperators lack. Moreover, it is aware of the underlying continuous dynamics typically disregarded by superoperator-based tomography. This paradigm paves the way to noise-aware optimal quantum control and opens a path to exploiting the bath as a control and error mitigation resource.

Johannes Flick and Prineha Narang. 9/4/2020. “

*ab initio*polaritonic potential-energy surfaces for excited-state nonphotonics and polaritonic chemistry.” The Journal of Chemical Physics, 153, Pp. 094116. Publisher's VersionAbstractAdvances in nanophotonics, quantum optics, and low-dimensional materials have enabled precise control of light-matter interactions down to the nanoscale. Combining concepts from each of these fields, there is now an opportunity to create and manipulate photonic matter via strong coupling of molecules to the electromagnetic field. Towards this goal, here we introduce a first principles framework to calculate polaritonic excited-state potential-energy surfaces for strongly coupled light-matter systems. In particular, we demonstrate the applicability of our methodology by calculating the polaritonic excited-state manifold of a Formaldehyde molecule strongly coupled to an optical cavity. This proof-of-concept calculation shows how strong coupling can be exploited to alter photochemical reaction pathways by influencing avoided crossings. Therefore, by introducing an ab initio method to calculate excited-state potential-energy surfaces, our work opens a new avenue for the field of polaritonic chemistry.

Kate Reidy, Georgios Varnavides, Joachim Dahl Thomsen, Abinash Kumar, Thang Pham, Arthur M. Blackburn, Polina Anikeeva, Prineha Narang, James M. LeBeau, and Frances M. Ross. 8/27/2020. “Direct Imaging and Electronic Structure Modulation of Double Moiré Superlattices at the 2D/3D Interface.” arXiv. Publisher's VersionAbstract

The atomic structure at the interface between a two-dimensional (2D) and a three-dimensional (3D) material influences properties such as contact resistance, photo-response, and high-frequency performance. Moiré engineering has yet to be explored for tailoring this 2D/3D interface, despite its success in enabling correlated physics at 2D/2D twisted van der Waals interfaces. Using epitaxially aligned MoS

_{2}/Au{111} as a model system, we apply a geometric convolution technique and four-dimensional scanning transmission electron microscopy (4D STEM) to show that the 3D nature of the Au structure generates two coexisting moiré periods (18 Angstroms and 32 Angstroms) at the 2D/3D interface that are otherwise hidden in conventional electron microscopy imaging. We show, via ab initio electronic structure calculations, that charge density is modulated with the longer of these moiré periods, illustrating the potential for (opto-)electronic modulation via moiré engineering at the 2D/3D interface.Andrew H. Poppe, Yuguang C. Li, Alán Aspuru-Guzik, Curtis P. Berlinguette, Christopher J. Chang, Richard Cogdell, Abigail G. Doyle, Johannes Flick, Nathaniel M. Gabor, Rienk van Grondelle, Sharon Hammes-Schiffer, Shaffiq A. Jaffer, Shana O. Kelley, Mario Leclerc, Karl Leo, Thomas E. Mallouk, Prineha Narang, Gabriela S. Schlau-Cohen, Gregory D. Scholes, Aleksandra Vojvodic, Vivian Wing-Wah Yam, Jenny Y. Yang, and Edward H. Sargent. 8/7/2020. “Bioinspiration in light harvesting and catalysis.” Nature Reviews Materials. Publisher's VersionAbstract

Capturing and converting solar energy into fuels and feedstocks is a global challenge that spans numerous disciplines and fields of research. Billions of years of evolution have allowed natural organisms to hone strategies for harvesting light from the sun and storing energy in the form of carbon–carbon and carbon–hydrogen bonds. Photosynthetic antenna proteins capture solar photons and funnel photoexcitations to reaction centres with high yields, and enzymes catalyze multi-electron reactions, facilitating chemical transformations not yet efficiently implemented using artificially engineered catalysts. Researchers in renewable energy often look to nature to understand the mechanisms at work and, if possible, to explore their translation into artificial systems. Here, we review advances in bioinspiration across the fields of biological light harvesting and chemical energy conversion. We examine how multi-photon and multi-electron reactions in biology can inspire new methods in photoredox chemistry to achieve novel, selective and complex organic transformations; how carbonic-dehydrogenase-inspired design principles enable catalytic reactions such as the conversion of CO

_{2}into useful products such as fuels; and how concepts from photosynthetic antenna complexes and reaction centres can benefit artificial light-harvesting materials. We then consider areas in which bioinspiration could enable advances in the rational design of molecules and materials, the expansion of the synthetic capabilities of catalysts and the valorization of molecular building blocks. We highlight the challenges that must be overcome to realize these advances and propose new directions that may use bioinspiration to achieve them.Chitraleema Chakraborty, Christopher J. Ciccarino, and Prineha Narang. 7/28/2020. “Dynamic modulation of phonon-assisted transitions in quantum defects in monolayer transition-metal dichalcogenide semiconductors.” arXiv. Publisher's VersionAbstract

Quantum localization via atomic point defects in semiconductors is of significant fundamental and technological importance. Quantum defects in monolayer transition-metal dichalcogenide semiconductors have been proposed as stable and scalable optically-addressable spin qubits. Yet, the impact of strong spin-orbit coupling on their dynamical response, for example under optical excitation, has remained elusive. In this context, we study the effect of spin-orbit coupling on the electron-phonon interaction in a single chalcogen vacancy defect in monolayer transition metal dichalcogenides, molybdenum disulfide (MoS

_{2}) and tungsten disulfide (WS_{2}). From ab initio electronic structure theory calculations, we find that spin-orbit interactions tune the magnitude of the electron-phonon coupling in both optical and charge-state transitions of the defect, modulating their respective efficiencies. This observation opens up a promising scheme of dynamically modulating material properties to tune the local behavior of a quantum defect.Ravishankar Sundararaman, Thomas Christensen, Yuan Ping, Nicholas Rivera, John D. Joannopoulos, Marin Soljačić, and Prineha Narang. 7/24/2020. “Plasmonics in Argentene.” Physical Review Materials, 4, 074011. Publisher's VersionAbstract

Merging concepts from the fields of

*ab initio*materials science and nanophotonics, there is now an opportunity to engineer new photonic materials whose optical, transport, and scattering properties are tailored to attain thermodynamic and quantum limits. Here we present first-principles calculations predicting that*Argentene*, a single-crystalline hexagonal close-packed monolayer of Ag, can dramatically surpass the optical properties and electrical conductivity of conventional plasmonic materials. In the low-frequency limit, we show that the scattering rate and resistivity reduce by a factor of 3 compared to the bulk three-dimensional metal. Most importantly, the low scattering rate extends to optical frequencies in sharp contrast to, e.g., graphene, whose scattering rate increase drastically in the near-infrared range due to optical-phonon scattering. Combined with an intrinsically high carrier density, this facilitates highly confined surface plasmons extending to visible frequencies. We evaluate Argentene across three distinct figures of merit, in each outperforming the state-of-the-art, making it a valuable addition to the two-dimensional heterostructure toolkit for quantum optoelectronics.Tomáš Neuman, Derek S. Wang, and Prineha Narang. 7/22/2020. “Nanomagnonic cavities for strong spin-magnon coupling.” arXiv. Publisher's VersionAbstract

We present a theoretical approach to use ferro- or ferrimagnetic nanoparticles as microwave nanomagnonic cavities to concentrate microwave magnetic fields into deeply subwavelength volumes ∼10

^{−13}mm^{3}. We show that the field in such nanocavities can efficiently couple to isolated spin emitters (spin qubits) positioned close to the nanoparticle surface reaching the single magnon-spin strong-coupling regime and mediate efficient long-range quantum state transfer between isolated spin emitters. Nanomagnonic cavities thus pave the way towards magnon-based quantum networks and magnon-mediated quantum gates.Dominik M. Juraschek and Prineha Narang. 7/21/2020. “Giant phonon-induced effective magnetic fields in 4

*f*paramagnets.” arXiv. Publisher's VersionAbstractWe present a mechanism by which circularly driven phonon modes in the rare-earth trihalides generate giant effective magnetic fields acting on the paramagnetic 4

*f*spins. With cerium trichloride (CeCl_{3}) as our model system, we calculate the coherent phonon dynamics in response to the excitation by an ultrashort terahertz pulse using a combination of first-principles calculations and phenomenological modeling. We find that effective magnetic fields of over 100 T can possibly be generated that polarize the spins for experimentally accessible pulse energies. This mechanism potentially creates a way to control the magnetic and electrical order of ferromagnets and ferroelectrics through interfacial coupling with the phonon-induced magnetization in heterostructures.