# Publications

2019

Georgios Varnavides, Adam S. Jermyn, Polina Anikeeva, and Prineha Narang. 9/3/2019. “Non-Equilibrium Phonon Transport Across Nanoscale Interfaces.” Physical Review B., 100, 115402. Publisher's VersionAbstract

Despite the ubiquity of applications of heat transport across nanoscale interfaces, including integrated circuits, thermoelectrics, and nanotheranostics, an accurate description of phonon transport in these systems remains elusive. Here we present a theoretical and computational framework to describe phonon transport with position, momentum and scattering event resolution. We apply this framework to a single material spherical nanoparticle for which the multidimensional resolution offers insight into the physical origin of phonon thermalization, and length-scale dependent anisotropy of steady-state phonon distributions. We extend the formalism to handle interfaces explicitly and investigate the specific case of semi-coherent materials interfaces by computing the coupling between phonons and interfacial strain resulting from aperiodic array of misfit dislocations. Our framework quantitatively describes the thermal interface resistance within the technologically relevant Si-Ge heterostructures. In future, this formalism could provide new insight into coherent and driven phonon effects in nanoscale materials increasingly accessible via ultrafast, THz and near-field spectroscopies.

Isaac Harris, Christopher J. Ciccarino, Johannes Flick, Dirk R. Englund, and Prineha Narang. 7/29/2019. “Group III Quantum Defects in Diamond are Stable Spin-1 Color Centers.” arXiv. Publisher's VersionAbstract

Color centers in diamond have emerged as leading solid-state artificial atoms for a range of quantum technologies, from quantum sensing to quantum networks. Concerted research activities are now underway to identify new color centers that combine stable spin and optical properties of the nitrogen vacancy (NV

^{−}) with the spectral stability of the silicon vacancy (SiV^{−}) centers in diamond, with recent research identifying other group IV color centers with superior properties. In this Letter, we investigate a new class of diamond quantum emitters from first principles, the group III color centers, which we show to be thermodynamically stable in a spin-1, electric-field insensitive structure. From ab initio electronic structure methods, we characterize the product Jahn-Teller (pJT) effect present in the excited state manifold of these group III color centers, where we capture symmetry-breaking distortions associated with strong electron-phonon coupling. These predictions can guide experimental identification of group III vacancy centers and their use in applications in quantum information science and technology.Siyuan Dai, Wenjing Fang, Nicholas Rivera, Yijing Stehle, Bor-Yuan Jiang, Jialang Shen, Roland Yingjie Tay, Christopher J. Ciccarino, Qiong Ma, Daniel Rodan‐Legrain, Pablo Jarillo‐Herrero, Edwin Hang Tong Teo, Michael M. Fogler, Prineha Narang, Jing Kong, and Dimitri N. Basov. 7/28/2019. “Phonon Polaritons in Monolayers of Hexagonal Boron Nitride.” Advanced Materials, 31, 37. Publisher's VersionAbstract

Phonon polaritons in van der Waals materials reveal significant confinement accompanied with long propagation length: important virtues for tasks pertaining to the control of light and energy flow at the nanoscale. While previous studies of phonon polaritons have relied on relatively thick samples, here reported is the first observation of surface phonon polaritons in single atomic layers and bilayers of hexagonal boron nitride (hBN). Using antenna‐based near‐field microscopy, propagating surface phonon polaritons in mono‐ and bilayer hBN microcrystals are imaged. Phonon polaritons in monolayer hBN are confined in a volume about one million times smaller than the free‐space photons. Both the polariton dispersion and their wavelength–thickness scaling law are altered compared to those of hBN bulk counterparts. These changes are attributed to phonon hardening in monolayer‐thick crystals. The data reported here have bearing on applications of polaritons in metasurfaces and ultrathin optical elements.

Johannes Flick and Prineha Narang. 7/10/2019. “Excited-State Nanophotonic and Polaritonic Chemistry with Ab initio Potential-Energy Surfaces.” arXiv. Publisher's VersionAbstract

Advances 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.

Adam S. Jermyn, Giulia Tagliabue, Harry A. Atwater, William A. Goddard III, Prineha Narang, and Ravishankar Sundararaman. 7/8/2019. “Transport of hot carriers in plasmonic nanostructures.” Physical Review Materials, 3, 075201. Publisher's VersionAbstract

Plasmonic hot carrier devices extract excited carriers from metal nanostructures before equilibration and have the potential to surpass semiconductor light absorbers. However their efficiencies have so far remained well below theoretical limits, which necessitates quantitative prediction of carrier transport and energy loss in plasmonic structures to identify and overcome bottlenecks in carrier harvesting. Here, we present a theoretical and computational framework, nonequilibrium scattering in space and energy (NESSE), to predict the spatial evolution of carrier energy distributions that combines the best features of phase-space (Boltzmann) and particle-based (Monte Carlo) methods. Within the NESSE framework, we bridge first-principles electronic structure predictions of plasmon decay and carrier collision integrals at the atomic scale, with electromagnetic field simulations at the nano- to mesoscale. Finally, we apply NESSE to predict spatially-resolved energy distributions of photoexcited carriers that impact the surface of experimentally realizable plasmonic nanostructures at length scales ranging from tens to several hundreds of nanometers, enabling first-principles design of hot carrier devices.

Nicholas Rivera, Johannes Flick, and Prineha Narang. 5/16/2019. “Variational theory of non-relativistic quantum electrodynamics.” Physical Review Letters, 122, 193603. Publisher's VersionAbstract

The ability to achieve ultrastrong coupling between light and matter promises to bring about new means to control material properties, new concepts for manipulating light at the atomic scale, and new insights into quantum electrodynamics (QED). Thus, there is a need to develop quantitative theories of QED phenomena in complex electronic and photonic systems. In this Letter, we develop a variational theory of general non-relativistic QED systems of coupled light and matter. Essential to our

*Ansatz*is the notion of an effective photonic vacuum whose modes are different than the modes in the absence of light-matter coupling. This variational formulation leads to a set of general equations that can describe the ground state of multielectron systems coupled to many photonic modes in real space. As a first step toward a new*ab initio*approach to ground and excited state energies in QED, we apply our*Ansatz*to describe a multilevel emitter coupled to many optical modes, a system with no analytical solution. We find a compact semianalytical formula which describes ground and excited state energies very well in all regimes of coupling parameters allowed by sum rules. Our formulation provides a nonperturbative theory of Lamb shifts and Casimir-Polder forces, as well as suggest new physical concepts such as the Casimir energy of a single atom in a cavity. Our method thus give rise to highly accurate nonperturbative descriptions of many other phenomena in general QED systems.Toan Trong Tran, Blake Regan, Evgeny A. Ekimov, Zhao Mu, Yu Zhou, Wei-bo Gao, Prineha Narang, Alexander S. Solntsev, Milos Toth, Igor Aharonovich, and Carlo Bradac. 5/3/2019. “Anti-Stokes excitation of solid-state quantum emitters for nanoscale thermometry.” Science Advances, 5, 5, Pp. eaav9180. Publisher's VersionAbstract

Color centers in solids are the fundamental constituents of a plethora of applications such as lasers, light-emitting diodes, and sensors, as well as the foundation of advanced quantum information and communication technologies. Their photoluminescence properties are usually studied under Stokes excitation, in which the emitted photons are at a lower energy than the excitation ones. In this work, we explore the opposite anti-Stokes process, where excitation is performed with lower-energy photons. We report that the process is sufficiently efficient to excite even a single quantum system—namely, the germanium-vacancy center in diamond. Consequently, we leverage the temperature-dependent, phonon-assisted mechanism to realize an all-optical nanoscale thermometry scheme that outperforms any homologous optical method used to date. Our results frame a promising approach for exploring fundamental light-matter interactions in isolated quantum systems and harness it toward the realization of practical nanoscale thermometry and sensing.

Man-Nung Su, Christopher J. Ciccarino, Sushant Kumar, Pratiksha D. Dongare, Seyyed Ali Hosseini Jebeli, David Renard, Yue Zhang, Behnaz Ostovar, Wei-Shun Chang, Peter Nordlander, Naomi J. Halas, Ravishankar Sundararaman, Prineha Narang, and Stephan Link. 4/2/2019. “Ultrafast Electron Dynamics in Single Aluminum Nanostructures.” Nano Letters. Publisher's VersionAbstract

Aluminum nanostructures are a promising alternative material to noble metal nanostructures for several photonic and catalytic applications, but their ultrafast electron dynamics remain elusive. Here, we combine single-particle transient extinction spectroscopy and parameter-free first-principles calculations to investigate the non-equilibrium carrier dynamics in aluminum nanostructures. Unlike gold nanostructures, we find the sub-picosecond optical response of lithographically fabricated aluminum nanodisks to be more sensitive to the lattice temperature than the electron temperature. We assign the rise in the transient transmission to electron–phonon coupling with a pump-power-independent lifetime of 500 ± 100 fs and theoretically confirm this strong electron–phonon coupling behavior. We also measure electron–phonon lifetimes in chemically synthesized aluminum nanocrystals and find them to be even longer (1.0 ± 0.1 ps) than for the nanodisks. We also observe a rise and decay in the transient transmissions with amplitudes that scale with the surface-to-volume ratio of the aluminum nanodisks, implying a possible hot carrier trapping and detrapping at the native oxide shell–metal core interface.

Yuxin Yin, Jennifer Coulter, Christopher J. Ciccarino, and Prineha Narang. 3/21/2019. “A Theoretical Investigation of Charge Density Wave Instability in CuS

_{2}.” arXiv. Publisher's VersionAbstractThe existence of a charge density wave (CDW) in transition metal dichalcogenide 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 study 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 100K. Variation of the electronic temperature shows the presence of two distinct phases, characterized at low temperature by a 2×2×2 periodic charge modulation associated with the motion of the S_{2}dimers. 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 bandstructure. 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 2D. This behavior sets this material apart from other transition metal dichalcogenide materials, which exhibit a charge density wave phase down to the 2D limit. As CDW in TMDC materials is considered to compete with superconductivity, the lack of 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 interplay of superconductivity, CDW order, and dimensionality.Nicholas Rivera, Thomas Christensen, and Prineha Narang. 3/20/2019. “Phonon polaritonics in two-dimensional materials.” Nano Letters, 19, 4, Pp. 2653–2660. Publisher's VersionAbstract

Extreme confinement of electromagnetic energy by phonon polaritons holds the promise of strong and new forms of control over the dynamics of matter. To bring such control to the atomic-scale limit, it is important to consider phonon polaritons in two-dimensional (2D) systems. Recent studies have pointed out that in 2D, splitting between longitudinal and transverse optical (LO and TO) phonons is absent at the Γ point, even for polar materials. Does this lack of LO--TO splitting imply the absence of a phonon polariton in polar monolayers? Here, we derive a first-principles expression for the conductivity of a polar monolayer specified by the wavevector-dependent LO and TO phonon dispersions. In the long-wavelength (local) limit, we find a universal form for the conductivity in terms of the LO phonon frequency at the Γ point, its lifetime, and the group velocity of the LO phonon. Our analysis reveals that the phonon polariton of 2D is simply the LO phonon of the 2D system. For the specific example of hexagonal boron nitride (hBN), we estimate the confinement and propagation losses of the LO phonons, finding that high confinement and reasonable propagation quality factors coincide in regions which may be difficult to detect with current near-field optical microscopy techniques. Finally, we study the interaction of external emitters with two-dimensional hBN nanostructures, finding extreme enhancement of spontaneous emission due to coupling with localized 2D phonon polaritons, and the possibility of multi-mode strong and ultra-strong coupling between an external emitter and hBN phonons. This may lead to the design of new hybrid states of electrons and phonons based on strong coupling.

Matthew E. Trusheim, Noel H. Wan, Kevin C. Chen, Christopher J. Ciccarino, Johannes Flick, Ravishankar Sundararaman, Girish Malladi, Eric Bersin, Michael Walsh, Benjamin Lienhard, Hassaram Bakhru, Prineha Narang, and Dirk Englund. 2/21/2019. “Lead-Related Quantum Emitters in Diamond.” Physical Review B, 99, 7. Publisher's VersionAbstract

We report on quantum emission from Pb-related color centers in diamond following ion implantation and high temperature vacuum annealing. First-principles calculations predict a negatively-charged Pb-vacancy center in a split-vacancy configuration, with a zero-phonon transition around 2.3 eV. Cryogenic photoluminescence measurements performed on emitters in nanofabricated pillars reveal several transitions, including a prominent doublet near 520 nm. The splitting of this doublet, 2 THz, exceeds that reported for other group-IV centers. These observations are consistent with the PbV center, which is expected to have the combination of narrow optical transitions and stable spin states, making it a promising system for quantum network nodes.

2018

Christopher J. Ciccarino, Chitraleema Chakraborty, Dirk R. Englund, and Prineha Narang. 11/26/2018. “Carrier dynamics and spin–valley–layer effects in bilayer transition metal dichalcogenides.” Faraday Discussions. Publisher's VersionAbstract

Transition metal dichalcogenides are an interesting class of low dimensional materials in mono- and few-layer form with diverse applications in valleytronic, optoelectronic and quantum devices. Therefore, the general nature of the band-edges and the interplay with valley dynamics is important from a fundamental and technological standpoint. Bilayers introduce interlayer coupling effects which can have a significant impact on the valley polarization. The combined effect of spin–orbit and interlayer coupling can strongly modify the band structure, phonon interactions and overall carrier dynamics in the material. Here we use first-principles calculations of electron–electron and electron–phonon interactions to investigate bilayer MoS

_{2}and WSe_{2}in both the AA′ and AB stacking configurations. We find that in addition to spin–orbit coupling, interlayer interactions present in the two configurations significantly alter the near-band-edge dynamics. Scattering lifetimes and dynamic behavior are highly material-dependent, despite the similarities and typical trends in TMDCs. Additionally, we capture significant differences in dynamics for the AA′ and AB stacking configurations, with lifetime values differing by up to an order of magnitude between them for MoS_{2}. Further, we evaluate the valley polarization times and find that maximum lifetimes at room temperature are of the scale of 1 picosecond for WSe_{2}in the AB orientation. These results present a pathway to understanding complex heterostructure configurations and ‘magic angle’ physics in TMDCs.Will Finigan, Michael Cubeddu, Thomas Lively, Johannes Flick, and Prineha Narang. 10/18/2018. “Qubit Allocation for Noisy Intermediate-Scale Quantum Computers .” arXiv:1810.08291 [quant-ph]. Publisher's Version

Nicholas Rivera, Jennifer Coulter, Thomas Christensen, and Prineha Narang. 8/31/2018. “Ab initio calculation of phonon polaritons in silicon carbide and boron nitride.” arXiv:1809.00058. Publisher's Version

Ravishankar Sundararaman, Thomas Christensen, Yuan Ping, Nicholas Rivera, John D. Joannopoulos, Marin Soljačić, and Prineha Narang. 6/8/2018. “Plasmonics in Argentene.” arXiv:1806.02672. Publisher's VersionAbstract

Two-dimensional materials exhibit a fascinating range of electronic and photonic properties vital for nanophotonics, quantum optics and emerging quantum information technologies. 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 three 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, spanning the spectrum of typical plasmonic applications; in each, Argentene outperforms the state-of-the-art. This unique combination of properties will make Argentene a valuable addition to the two-dimensional heterostructure toolkit for quantum electronic and photonic technologies.

Johannes Flick and Prineha Narang. 2018. “Cavity-Correlated Electron-Nuclear Dynamics from First Principles.” Physical Review Letters, 121, 11, Pp. 113002–. Publisher's Version

Christopher J. Ciccarino, Thomas Christensen, Ravishankar Sundararaman, and Prineha Narang. 2018. “Dynamics and Spin-Valley Locking Effects in Monolayer Transition Metal Dichalcogenides.” Nano Letters, 18, 9, Pp. 5709–5715. Publisher's Version

Flick Johannes, Rivera Nicholas, and Narang Prineha. 2018. “Strong light-matter coupling in quantum chemistry and quantum photonics.” Nanophotonics, 7, Pp. 1479. Publisher's Version

Jennifer Coulter, Ravishankar Sundararaman, and Prineha Narang. 2018. “Microscopic origins of hydrodynamic transport in the type-II Weyl semimetal WP

_{2}.” Physical Review B, 98, 11, Pp. 115130–. Publisher's VersionSharmila N. Shirodkar, Marios Mattheakis, Paul Cazeaux, Prineha Narang, Marin Soljacic, and Efthimios Kaxiras. 2018. “Quantum plasmons with optical-range frequencies in doped few-layer graphene.” Phys. Rev. B, 97, Pp. 195435. Publisher's VersionAbstract

Although plasmon modes exist in doped graphene, the limited range of doping achieved by gating restricts the plasmon frequencies to a range that does not include the visible and infrared. Here we show, through the use of first-principles calculations, that the high levels of doping achieved by lithium intercalation in bilayer and trilayer graphene shift the plasmon frequencies into the visible range. To obtain physically meaningful results, we introduce a correction of the effect of plasmon interaction across the vacuum separating periodic images of the doped graphene layers, consisting of transparent boundary conditions in the direction perpendicular to the layers; this represents a significant improvement over the exact Coulomb cutoff technique employed in earlier works. The resulting plasmon modes are due to local field effects and the nonlocal response of the material to external electromagnetic fields, requiring a fully quantum mechanical treatment. We describe the features of these quantum plasmons, including the dispersion relation, losses, and field localization. Our findings point to a strategy for fine-tuning the plasmon frequencies in graphene and other two-dimensional materials.