Plasmon dynamics in graphene

arXiv (Cornell University) · 2026-01-15

Plasmon are collective oscillations of mobile electrons with dynamics controlled by their charge stiffness("Drude weight"). Using terahertz spacetime metrology, we probe Plasmon dynamics of mono- and bi-layer graphene. In both systems, the experimentally measured Drude weight systematically exceeds the prediction based on non-interacting electronic system. The relative enhancement increases as the carrier density decreases. We attribute the observed deviation to the interplay of interactions and wave function structure of the Dirac fermions in multi-layer graphene. Our results establish that pseudospin structure of the single-particle electronic wave function can directly influence collective excitations, with implications that extend beyond graphene to a broad class of quantum materials.

Plasmon dynamics in graphene

arXiv (Cornell University) · 2026-01-15

Plasmon are collective oscillations of mobile electrons with dynamics controlled by their charge stiffness("Drude weight"). Using terahertz spacetime metrology, we probe Plasmon dynamics of mono- and bi-layer graphene. In both systems, the experimentally measured Drude weight systematically exceeds the prediction based on non-interacting electronic system. The relative enhancement increases as the carrier density decreases. We attribute the observed deviation to the interplay of interactions and wave function structure of the Dirac fermions in multi-layer graphene. Our results establish that pseudospin structure of the single-particle electronic wave function can directly influence collective excitations, with implications that extend beyond graphene to a broad class of quantum materials.

Application of deep neural networks for computing the renormalization group flow of the two-dimensional <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>ϕ</mml:mi> <mml:mn>4</mml:mn> </mml:msup> </mml:mrow> </mml:math> field theory

Machine Learning Science and Technology · 2026-02-23

Abstract We introduce RGFlow, a deep neural network–based real-space renormalization group (RG) framework tailored for continuum scalar field theories. Leveraging generative capabilities of flow-based neural networks, RGFlow autonomously learns real-space RG transformations from data without prior knowledge of the underlying model. In contrast to conventional approaches, RGFlow is bijective (information-preserving) and is optimized based on the principle of minimal mutual information. We demonstrate the method on two examples. The first one is a one-dimensional Gaussian model, where RGFlow is shown to learn the classical decimation rule. The second is the two-dimensional φ 4 theory, where the network successfully identifies a Wilson–Fisher-like critical point and provides an estimate of the correlation-length critical exponent.

Application of deep neural networks for computing the renormalization group flow of the two-dimensional <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>ϕ</mml:mi> <mml:mn>4</mml:mn> </mml:msup> </mml:mrow> </mml:math> field theory

Machine Learning Science and Technology · 2026-02-23

Abstract We introduce RGFlow, a deep neural network–based real-space renormalization group (RG) framework tailored for continuum scalar field theories. Leveraging generative capabilities of flow-based neural networks, RGFlow autonomously learns real-space RG transformations from data without prior knowledge of the underlying model. In contrast to conventional approaches, RGFlow is bijective (information-preserving) and is optimized based on the principle of minimal mutual information. We demonstrate the method on two examples. The first one is a one-dimensional Gaussian model, where RGFlow is shown to learn the classical decimation rule. The second is the two-dimensional φ 4 theory, where the network successfully identifies a Wilson–Fisher-like critical point and provides an estimate of the correlation-length critical exponent.

Polaritonic quantum matter

Nanophotonics · 2025-05-05 · 11 citations

Abstract Polaritons are quantum mechanical superpositions of photon states with elementary excitations in molecules and solids. The light–matter admixture causes a characteristic frequency‐momentum dispersion shared by all polaritons irrespective of the microscopic nature of material excitations that could entail charge, spin, lattice or orbital effects. Polaritons retain the strong nonlinearities of their matter component and simultaneously inherit ray‐like propagation of light. Polaritons prompt new properties, enable new opportunities for spectroscopy/imaging, empower quantum simulations and give rise to new forms of synthetic quantum matter. Here, we review the emergent effects rooted in polaritonic quasiparticles in a wide variety of their physical implementations. We present a broad portfolio of the physical platforms and phenomena of what we term polaritonic quantum matter. We discuss the unifying aspects of polaritons across different platforms and physical implementations and focus on recent developments in: polaritonic imaging, cavity electrodynamics and cavity materials engineering, topology and nonlinearities, as well as quantum polaritonics.

Current-driven nonequilibrium electrodynamics in graphene revealed by nano-infrared imaging

Nature Communications · 2025-04-24 · 5 citations

Electrons in low-dimensional materials driven out of equilibrium by a strong electric field exhibit intriguing effects that have direct analogues in high-energy physics. In this work we demonstrate that two of these effects can be observed in graphene, leading to relevant implications for light-matter interactions at the nanoscale. For doped graphene, the Cherenkov emission of phonons caused by the fast flow of out-of-equilibrium electrons was found to induce direction-dependent asymmetric plasmon damping and an unexpected generation of photocurrent. For graphene close to charge neutrality, incident infrared photons were found to disrupt the creation-recombination balance of electron-hole pairs enabled by the condensed matter version of the Schwinger effect, resulting in an excess photocurrent that we term Schwinger photocurrent. Both Schwinger and Cherenkov photocurrents are different from other known light-to-current down conversions scenarios and thus expand the family of photoelectric effects in solid state devices. Through nano-infrared imaging methodology, we provide a more comprehensive view of current-driven nonequilibrium electrodynamics in graphene. Here, the authors report the observation of two solid-state analogues of well-known high-energy physics effects in graphene samples irradiated by infrared photons under non-equilibrium conditions. Depending on the carrier density of graphene, they observed asymmetric plasmon damping, and anomalous photocurrents associated with the condensed matter versions of the Cherenkov and Schwinger effects.

Spacetime Mapping of Spatially Sustained Polariton Under Time-Varying Excitation

2025-08-13

Using terahertz near-field nanoscopy, we demonstrate that temporal shaping of plasmon polaritons in graphene effectively suppresses their spatial decay. Our experiments and simulations reveal a universal principle for spacetime wave engineering, broadly applicable in photonic, acoustic, plasmonic, and quantum systems.

Plasmonic Polarization Sensing of Electrostatic Superlattice Potentials

Physical Review X · 2025-01-31 · 7 citations

Plasmon polaritons are formed by coupling light with delocalized electrons. The half-light and half-matter nature of plasmon polaritons endows them with unparalleled tunability via a range of parameters, such as dielectric environments and carrier density. Therefore, plasmon polaritons are expected to be tuned when in proximity to polar materials since the carrier density is tuned by an electrostatic potential; conversely, the plasmon polariton response might enable the sensing of polarization. Here, we use infrared nanoimaging and nanophotocurrent measurements to investigate heterostructures composed of graphene and twisted hexagonal boron nitride (t-BN), with alternating polarization in a triangular network of moiré stacking domains. We observe that the carrier density and the corresponding plasmonic response of graphene are modulated by polar domains in t-BN. In addition, we demonstrate that the nanometer-wide domain walls of graphene moirés superlattices, created by the polar domains of t-BN, provide momenta to assist the plasmonic excitations. Furthermore, our study establishes that the plasmon of graphene could function as a delicate sensor for polarization textures. The evolution of polarization textures in t-BN under uniform electric fields is tomographically examined via plasmonic imaging. Strikingly, no noticeable polarization switching is observed under applied electric fields up to <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>0.21</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi mathvariant="normal">V</a:mi><a:mo stretchy="false">/</a:mo><a:mi>nm</a:mi></a:mrow></a:math>, at variance with transport reports. Our nanoimages unambiguously reveal that t-BN with triangular domains acts like a ferrielectric rather than a ferroelectric as claimed by many previous studies.

Collective Modes in Multilayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>Graphene</mml:mi> <mml:mo>/</mml:mo> <mml:mi>α</mml:mi> <mml:mtext>−</mml:mtext> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>RuCl</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> Heterostructures

Physical Review X · 2025-07-23 · 2 citations

Collective modes in multilayer graphene, such as plasmons and phonons, exhibit sensitivity to displacement fields and interlayer coupling, distinguishing them from their counterparts in single-layer graphene. Here, we engineer collective modes in charge-transfer heterostructures composed of multilayer graphene and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>α</a:mi> <a:mtext>−</a:mtext> <a:msub> <a:mi>RuCl</a:mi> <a:mn>3</a:mn> </a:msub> </a:math> . In heterostructures with a single <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mi>α</c:mi> <c:mtext>−</c:mtext> <c:msub> <c:mrow> <c:mi>RuCl</c:mi> </c:mrow> <c:mrow> <c:mn>3</c:mn> </c:mrow> </c:msub> </c:mrow> </c:math> interface, the charge transfer generates displacement fields up to 7 V/nm at the interface between <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>α</e:mi> <e:mtext>−</e:mtext> <e:msub> <e:mi>RuCl</e:mi> <e:mn>3</e:mn> </e:msub> </e:math> and the adjacent graphene layer—the highest value achieved through charge-transfer methods. As a result of the broken inversion symmetry, we discover enhanced nonlinear optical response and modified phonon selection rules. Conversely, we find that multilayer graphene sandwiched between two <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>α</g:mi> <g:mtext>−</g:mtext> <g:msub> <g:mi>RuCl</g:mi> <g:mn>3</g:mn> </g:msub> </g:math> flakes causes displacement fields to cancel. There, we achieve carrier densities as high as <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mn>8</i:mn> <i:mo>×</i:mo> <i:msup> <i:mn>10</i:mn> <i:mn>13</i:mn> </i:msup> <i:mtext> </i:mtext> <i:mtext> </i:mtext> <i:mrow> <i:msup> <i:mrow> <i:mi>cm</i:mi> </i:mrow> <i:mrow> <i:mo>−</i:mo> <i:mn>2</i:mn> </i:mrow> </i:msup> </i:mrow> </i:math> in multilayer graphene and restore the phonon selection rules to their unperturbed state. Meanwhile, we demonstrate that plasmonic properties derive from the depletion of multiple valence bands. As a result of the quasilinear band dispersion, these “Dirac multiband plasmons” are relatively unaffected by displacement fields. On the other hand, the inverted heterostructure sequence—two multilayer graphene sheets encapsulating <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>α</k:mi> <k:mtext>−</k:mtext> <k:msub> <k:mi>RuCl</k:mi> <k:mn>3</k:mn> </k:msub> </k:math> —activates significant alteration of the plasmons via interlayer plasmon-plasmon coupling. Hence, multilayer graphene and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>α</m:mi> <m:mtext>−</m:mtext> <m:msub> <m:mi>RuCl</m:mi> <m:mn>3</m:mn> </m:msub> </m:math> heterostructures offer a gate-free platform for engineering collective modes derived from inversion symmetry and interlayer coupling.

Mott transition in excitonic Bose polarons

ArXiv.org · 2025-04-09

For a neutral system of positive and negative charges, such as atoms in a crystal, increasing the density causes the Mott transition from bound electrons to free electrons. The density of optically generated electron-hole systems can be controlled in situ by the power of optical excitation that enables the Mott transition from excitons, the bound pairs of electrons and holes, to free electrons and holes with increasing density. These Mott transitions occur in systems of pairs of the same kind, such as atoms or excitons. However, a different type of the Mott transition can occur for Bose polarons. A Bose polaron is a mobile particle of one kind in a Bose gas of particles of another kind. For the Mott transition in polarons, the polaron states vanish with increasing density of the surrounding gas. In this paper, we present the observation of this type of the Mott transition and the measurement of the Mott transition parameter $n_{\rm M}^{1/2} a_{\rm B}$ in 2D excitonic Bose polarons.