# Publications

2019
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.
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 CuS2.” arXiv. Publisher's VersionAbstract
The existence of a charge density wave (CDW) in transition metal dichalcogenide CuS2 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 CuS2 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 S2 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 CuS2. Further study of CuS2 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 CuS2 suggests the possibility of enhanced superconductivity relative to other transition metal dichalcogenides. Overall, our work identifies CuS2 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.
Jennifer Coulter, Gavin B. Osterhoudt, Christina A. C. Garcia, Yiping Wang, Vincent Plisson, Bing Shen, Ni Ni, Kenneth S. Burch, and Prineha Narang. 3/18/2019. “Uncovering Electron-Phonon Scattering and Phonon Dynamics in Type-I Weyl Semimetals.” arXiv. Publisher's VersionAbstract
Weyl semimetals are 3D phases of matter with topologically protected states that have remarkable macroscopic transport behaviors. As phonon dynamics and electron-phonon scattering play a critical role in the electrical and thermal transport, we pursue a fundamental understanding of the origin of these effects in type-I Weyl semimetals NbAs and TaAs. In the temperature-dependent Raman spectra of NbAs, we reveal a previously unreported Fano lineshape, a signature stemming from the electron-phonon interaction. Additionally, the temperature dependence of the A1 phonon linewidths in both NbAs and TaAs strongly deviate from the standard model of anharmonic decay. To capture the mechanisms responsible for the observed Fano asymmetry and the atypical phonon linewidth, we present first principles calculations of the phonon self-energy correction due to the electron-phonon interaction. Finally, we investigate the relationship between Fano lineshape, electron-phonon coupling, and locations of the Weyl points in these materials. Through this study of the phonon dynamics and electron-phonon interaction in these Weyl semimetals, we consider specific microscopic pathways which contribute to the nature of their macroscopic transport.
Xuezeng Tian, Dennis S. Kim, Shize Yang, Christopher J. Ciccarino, Yongji Gong, Yongsoo Yang, Yao Yang, Blake Duschatko, Yakun Yuan, Pulickel M. Ajayan, Juan-Carlos Idrobo, Prineha Narang, and Jianwei Miao. 2/26/2019. “Correlating 3D atomic defects and electronic properties of 2D materials with picometer precision.” arXiv. Publisher's VersionAbstract
Two-dimensional (2D) materials and heterostructures exhibit exceptional electronic, optical and chemical properties, promising to find applications ranging from electronics and photovoltaics to quantum information science. However, the exceptional properties of these materials strongly depend on their 3D atomic structure especially crystal defects. Using Re-doped MoS2 as a model, we develop scanning atomic electron tomography (sAET) to determine the atomic positions and crystal defects such as dopants, vacancies and ripples with a 3D precision down to 4 picometers. We measure the full 3D strain tensor and quantify local strains induced by single dopants. By directly providing experimental 3D atomic coordinates to density functional theory (DFT), we obtain more truthful electronic band structures than those derived from conventional DFT calculations relying on relaxed 3D atomic models, which is confirmed by photoluminescence spectra measurements. Furthermore, we observe that the local strain induced by atomic defects along the z-axis is larger than that along the x- and y-axis and thus more strongly affects the electronic property of the 2D material. We anticipate that sAET is not only generally applicable to the determination of the 3D atomic coordinates of 2D materials, heterostructures and thin films, but also could transform ab initio calculations by using experimental atomic coordinates as direct input to reveal more realistic physical, material, chemical and electronic properties.
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.
Fariah Hayee, Leo Yu, Jingyuan Linda Zhang, Christopher J. Ciccarino, Minh Nyugen, Ann F. Marshall, Igor Aharonovich, Jelena Vučković, Prineha Narang, Tony F. Heinz, and Jennifer A Dionne. 1/17/2019. “ Correlated optical and electron microscopy reveal the role of multiple defect species and local strain on quantum emission.” arXiv. Publisher's VersionAbstract
Color-centers in solids have emerged as promising candidates for quantum photonic computing, communications, and sensing applications. Defects in hexagonal boron nitride(hBN) possess high-brightness, room-temperature quantum emission, but their large spectral variability and unknown local structure significantly challenge their technological utility. Here, we directly correlate hBN quantum emission with its local, atomic-scale crystalline structure using correlated photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. Across 20 emitters, we observe zero phonon lines (ZPLs) in PL and CL ranging from 540-720 nm. CL mapping reveals that multiple defects and distinct defect species located within an optically-diffraction-limited region can each contribute to the observed PL spectra. Through high resolution transmission electron imaging, we find that emitters are located in regions with multiple fork-like dislocations. Additionally, local strain maps indicate that strain is not responsible for observed ZPL spectral range, though it can enable spectral tuning of particular emitters. While many emitters have identical ZPLs in CL and PL, others exhibit reversible but distinct CL and PL peaks; density functional calculations indicate that defect complexes and charge-state transitions influence such emission spectra. Our results highlight the sensitivity of defect-driven quantum emission to the surrounding crystallography, providing a foundation for atomic-scale optical characterization.
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 MoS2 and WSe2 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 MoS2. Further, we evaluate the valley polarization times and find that maximum lifetimes at room temperature are of the scale of 1 picosecond for WSe2 in the AB orientation. These results present a pathway to understanding complex heterostructure configurations and ‘magic angle’ physics in TMDCs.
Georgios Varnavides, Adam S. Jermyn, Polina Anikeeva, and Prineha Narang. 11/2/2018. “Non-Equilibrium Phonon Transport Across Nanoscale Interfaces.” arXiv. 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.
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 WP2.” Physical Review B, 98, 11, Pp. 115130–. Publisher's Version
Sharmila 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.
2017
Olga Lozan, Ravishankar Sundararaman, Buntha Ea-Kim, Jean-Michel Rampnoux, Prineha Narang, Stefan Dilhaire, and Philippe Lalanne. 2017. “Increased rise time of electron temperature during adiabatic plasmon focusing.” Nature Communications, 8, 1, Pp. 1656. Publisher's VersionAbstract
Decay of plasmons to hot carriers has recently attracted considerable interest for fundamental studies and applications in quantum plasmonics. Although plasmon-assisted hot carriers in metals have already enabled remarkable physical and chemical phenomena, much remains to be understood to engineer devices. Here, we present an analysis of the spatio-temporal dynamics of hot electrons in an emblematic plasmonic device, the adiabatic nanofocusing surface-plasmon taper. With femtosecond-resolution measurements, we confirm the extraordinary capability of plasmonic tapers to generate hot carriers by slowing down plasmons at the taper apex. The measurements also evidence a substantial increase of the lifetime''of the electron gas temperature at the apex. This interesting effect is interpreted as resulting from an intricate heat flow at the apex. The ability to harness the lifetime''of hot-carrier gases with nanoscale circuits may provide a multitude of applications, such as hot-spot management, nonequilibrium hot-carrier generation, sensing, and photovoltaics.
Bart de Nijs, Felix Benz, Steven J. Barrow, Daniel O. Sigle, Rohit Chikkaraddy, Aniello Palma, Cloudy Carnegie, Marlous Kamp, Ravishankar Sundararaman, Prineha Narang, Oren A. Scherman, and Jeremy J. Baumberg. 2017. “Plasmonic tunnel junctions for single-molecule redox chemistry.” Nature Communications, 8, 1, Pp. 994. Publisher's VersionAbstract
Nanoparticles attached just above a flat metallic surface can trap optical fields in the nanoscale gap. This enables local spectroscopy of a few molecules within each coupled plasmonic hotspot, with near thousand-fold enhancement of the incident fields. As a result of non-radiative relaxation pathways, the plasmons in such sub-nanometre cavities generate hot charge carriers, which can catalyse chemical reactions or induce redox processes in molecules located within the plasmonic hotspots. Here, surface-enhanced Raman spectroscopy allows us to track these hot-electron-induced chemical reduction processes in a series of different aromatic molecules. We demonstrate that by increasing the tunnelling barrier height and the dephasing strength, a transition from coherent to hopping electron transport occurs, enabling observation of redox processes in real time at the single-molecule level.