arXiv subjects

Dmitry K. Efimkin

Publications and source records attributed to Dmitry K. Efimkin.

At least 19 recordsLinked to original sources

arXiv2026

Topological hybridisation of plasmons with ferrimagnetic magnons

We study the formation of hybrid plasmon-magnon modes in a heterostructure comprising a monolayer semiconductor with strong Rashba spin-orbit coupling -- specifically, Janus transition-metal dichalcogenides (TMDs) -- and an insulating ferrimagnet, such as yttrium iron garnet-based compounds. Using a combined microscopic-macroscopic framework for plasmon-magnon coupling, we show that plasmons and magnons strongly hybridize over both GHz and THz frequency ranges, enabling experimental access well above cryogenic temperatures. Moreover, the developed approach provides an efficient and natural classification of the topology of the hybrid modes, rooted in the phase winding of the plasmon-magnon coupling induced by spin-momentum locking and the associated chiral winding of the electronic spin along the Fermi contours. Finally, we identify experimentally accessible manifestations of the hybridization, such as topological interface modes and an anomalous thermal Hall response.

cond-mat.mes-hall↗

arXiv2026

Weyl magnetoplasma waves in magnetic Weyl semimetals

Weyl degeneracies in spectra of magnetoplasma waves enable nonreciprocal energy flow and topologically protected modes, yet conventional materials require impractical magnetic fields to operate. Developing an effective Hamiltonian framework for magnetic Weyl semimetals, we show that these systems overcome the limit, hosting Weyl magnetoplasma physics at zero field due to their giant intrinsic anomalous Hall response. The resulting topology supports nonreciprocal modes localized at magnetic domain walls, including a pair of topological "Fermi-arc-like modes and additional bound states. These effects are fully developed across a broad THz window, and we propose feasible experimental routes for their detection.

cond-mat.mes-hall↗

arXiv2025

Anomalous skew scattering of plasmons in a Dirac electron fluid

The Berry phase-related nontrivial electronic band geometries can significantly influence bulk and edge plasmons resulting in their non-reciprocal propagation and opening new opportunities for plasmonics. In the present work, we extend the hydrodynamic framework to describe the scattering of plasmons in a Dirac electron fluid off a circular region with an induced nonzero anomalous Hall response, i.e. a Berry flux target. We demonstrate that the scattering has a giant asymmetry or skewness and exhibits a series of resonances. The latter appears due to a chiral non-topological trapped mode circulating the target. We discuss possible experimental realizations, including the surface of a topological insulator film and graphene irradiated by the circularly polarized beam.

cond-mat.mes-hall↗

arXiv2025

Chiral electronic network within skyrmionic lattice on topological insulator surfaces

We consider a proximity effect between Dirac surface states of a topological insulator and the skyrmion phase of an insulating magnet. A single skyrmion results in the surface states having a chiral gapless mode confined to the perimeter of the skyrmion. For the lattice of skyrmions, the tunneling coupling between confined states leads to the formation of low energy bands delocalized across the whole system. We show that the structure of these bands can be investigated with the help of the phenomenological chiral network model with a kagome lattice geometry. While the network model by itself can be in a chiral Floquet phase unattainable without external periodic driving, we show how to use a procedure known as band reconstruction to obtain the low energy bands of the electrons on the surface of the topological insulator for which there is no external driving. We conclude that band reconstruction is essential for the broad class of network models recently introduced to describe the electronic properties of different nanostructures.

cond-mat.mes-hall↗

arXiv2025

Theory of Band Gap Reduction Due to Conduction Electrons in 2D TMDs: Imaginary Frequency Formalism

Two Dimensional (2D) Transition Metal Dichalcogenides (TMDs) possess a large direct band gap which has been experimentally observed to shrink with increasing charge carrier density (doping). The effect has been the subject of theoretical study in recent years using various approaches and approximations. In this work we develop the theory of bandgap renormalization based on Feynman diagrammatic technique in the imaginary frequency formalism. We consider dynamical screening from conduction band electrons using the random phase approximation (RPA), as well as screening from a metallic gate. While our theory is general for any 2D semiconductor, to be specific we consider MoS$_2$ and WSe$_2$ and compare with available experimental data. In both cases we calculate large band gap renormalization that reaches several hundred meV at relatively low carrier density. This is in good agreement with experimental data.

cond-mat.mes-hall↗

arXiv2025

Valley-mediated singlet- and triplet-polaron interactions and quantum dynamics in a doped WSe$_2$ monolayer

In doped transition metal dichalcogenides, optically created excitons (bound electron-hole pairs) can strongly interact with a Fermi sea of electrons to form Fermi polaron quasiparticles. When there are two distinct Fermi seas, as is the case in WSe$_2$, there are two flavors of lowest-energy (attractive) polarons -- singlet and triplet -- where the exciton is coupled to the Fermi sea in the same or opposite valley, respectively. Using two-dimensional coherent electronic spectroscopy, we analyze how their quantum decoherence evolves with doping density and determine the condition under which stable Fermi polarons form. Because of the large oscillator strength associated with these resonances, intrinsic quantum dynamics of polarons as well as valley coherence between coupled singlet- and triplet polarons occur on sub-picosecond time scales. Surprisingly, we find that a dark-to-bright state conversion process leads to a particularly long-lived singlet polaron valley polarization, persisting up to 200-800 ps. Valley coherence between the singlet- and triplet polaron is correlated with their energy fluctuations. Our finding provides valuable guidance for the electrical and optical control of spin and valley indexes in atomically thin semiconductors.

cond-mat.mes-hall↗

arXiv2024

Rydberg Exciton-Polaritons in a Magnetic Field

We theoretically investigate exciton-polaritons in a two-dimensional (2D) semiconductor heterostructure, where a static magnetic field is applied perpendicular to the plane. To explore the interplay between magnetic field and a strong light-matter coupling, we employ a fully microscopic theory that explicitly incorporates electrons, holes and photons in a semiconductor microcavity. Furthermore, we exploit a mapping between the 2D harmonic oscillator and the 2D hydrogen atom that allows us to efficiently solve the problem numerically for the entire Rydberg series as well as for the ground-state exciton. In contrast to previous approaches, we can readily obtain the real-space exciton wave functions and we show how they shrink in size with increasing magnetic field, which mirrors their increasing interaction energy and oscillator strength. We compare our theory with recent experiments on exciton-polaritons in GaAs heterostructures in an external magnetic field and we find excellent agreement with the measured polariton energies. Crucially, we are able to capture the observed light-induced changes to the exciton in the regime of very strong light-matter coupling where a perturbative coupled oscillator description breaks down. Our work can guide future experimental efforts to engineer and control Rydberg excitons and exciton-polaritons in a range of 2D materials.

cond-mat.mes-hall↗

arXiv2024

Excitons in Atomically Thin TMD in Electric and Magnetic Fields

The magnetic field dependence of photoabsorption provides direct insights into the band structure of semiconductors. It is perhaps surprising that there is a large discrepancy between electron, hole, and reduced mass reported in the recent literature. Motivated by this puzzle we reconsider excitonic magneto-absorption and find that the commonly employed perturbative approach, namely for computing the diamagnetic shift, is inadequate to account for the parameter ranges considered in existing data. In particular, we develop the theory for strong magnetic field and, upon analysis of the data, arrive at the set of exciton parameters different to what has been estimated perturbatively in the literature. Only s-wave excitons are visible in photoluminescence as the spectral weight of p-wave states is too small, this limits the amount of information that can be extracted about the underlying band structure. To overcome this, we propose to study p-wave states by mixing them with s-wave states by external in-plane electric field and show that a moderate DC electric field would provide sufficient mixing to brighten p-wave states. We calculate energies of the p-wave states including the effects of valley-orbital splitting and the orbital Zeeman shift, and show that this provides direct information on the electron-hole mass asymmetry.

cond-mat.mes-hall↗

arXiv2024

Giant resonant skew scattering of plasma waves in a two-dimensional electron gas

Electron skew scattering by impurities is one of the major mechanisms behind the anomalous Hall effect in ferromagnetic nanostructures. It is particularly strong at the surface of topological insulators where electron dynamics is governed by the spin-1/2 Dirac equation. Motivated by recently discovered mappings between hydrodynamics and the spin-1 Dirac equation, we consider the scattering of plasma waves -- propagating charge density oscillations -- excited in graphene off a non-uniform magnetic field created by an adjacent circular micromagnet. The calculated scattering amplitude not only exhibits a giant asymmetry, or skewness, but is resonantly enhanced if the frequency of the incoming wave matches the frequency of the chiral trapped mode circulating the micromagnet in only one direction. Furthermore, if the frequency of the incoming plasma wave is a few times larger than the Larmor frequency, the angular distribution of its forward scattering is almost indistinguishable from that of a Dirac electron at the surface of a topological insulator scattering off a magnetic impurity. The micrometer scale of the proposed setup enables direct investigations of individual skew scattering events previously inaccessible in electronic systems.

cond-mat.mes-hall↗

arXiv2023

Weyl excitations via helicon-phonon mixing in conducting materials

Quasiparticles with Weyl dispersion can display an abundance of novel topological, thermodynamic and transport phenomena, which is why novel Weyl materials and platforms for Weyl physics are being intensively looked for in electronic, magnetic, photonic and acoustic systems. We demonstrate that conducting materials in magnetic fields generically host Weyl excitations due to the hybridisation of phonons with helicons, collective neutral modes of electrons interacting with electromagnetic waves propagating in the material. Such Weyl excitations are, in general, created by the interactions of helicons with longitudinal acoustic phonons. An additional type of Weyl excitation in polar crystals comes from the interaction between helicons and longitudinal optical phonons. Such excitations can be detected in X-ray and Raman scattering experiments. The existence of the Weyl excitations involving optical phonons in the bulk of the materials also leads to the formation of topologically protected surface arc states that can be detected via surface plasmon resonance.

cond-mat.mes-hall↗

arXiv2023

Topological hybrid electron-hole Cooper pairing

We consider electron--hole Cooper pair condensation in a heterostructure formed by a topological insulator film and a quantum well. We argue that the helical nature of the Dirac electronic states at the topological insulator surface results in the presence of two competing degenerate pairing channels. The corresponding paired states have an unconventional symmetry in the order parameter describing the Cooper pair condensate, can be classified by the topological Chern invariant, and are topologically distinct. We discuss possible manifestations of the nontrivial topology, including the formation of chiral states at the domain walls separating two distinct states, quantized anomalous transport phenomena, and connections with chiral topological superconductivity.

cond-mat.mes-hall↗

arXiv2023

Equatorial Waves in Rotating Bubble-Trapped Superfluids

As the Earth rotates, the Coriolis force causes several oceanic and atmospheric waves to be trapped along the equator, including Kelvin, Yanai, Rossby, and Poincaré modes. It has been demonstrated that the mathematical origin of these waves is related to the nontrivial topology of the underlying hydrodynamic equations. Inspired by recent observations of Bose-Einstein condensation (BEC) in bubble-shaped traps in microgravity ultracold quantum gas experiments, we show that equatorial modes are supported by a rapidly rotating condensate in a spherical geometry. Based on a zero-temperature coarse-grained hydrodynamic framework, we reformulate the coupled oscillations of the superfluid and the Abrikosov vortex lattice resulting from rotation by a Schrödinger-like eigenvalue problem. The obtained non-Hermitian Hamiltonian is topologically nontrivial. Furthermore, we solve the hydrodynamic equations for a spherical geometry and find that the rotating superfluid hosts Kelvin, Yanai, and Poincaré equatorial modes, but not the Rossby mode. Our predictions can be tested with state-of-the-art bubble-shaped trapped BEC experiments.

cond-mat.quant-gas↗

arXiv2023

Quantum Dynamics of Attractive and Repulsive Polarons in a Doped MoSe$_2$ Monolayer

When mobile impurities are introduced and coupled to a Fermi sea, new quasiparticles known as Fermi polarons are formed. There are two interesting, yet drastically different regimes of the Fermi polaron problem: (I) the attractive polaron (AP) branch, connected to pairing phenomena spanning the crossover from BCS superfluidity to the Bose-Einstein condensation of molecules; and (II) the repulsive branch (RP), which underlies the physics responsible for Stoner's itinerant ferromagnetism. Here, we study Fermi polarons in two dimensional systems, where many questions and debates regarding their nature persist. The model system we investigate is a doped MoSe$_2$ monolayer. We find the observed AP-RP energy splitting and the quantum dynamics of attractive polarons agree with the predictions of polaron theory. As the doping density increases, the quantum dephasing of the attractive polarons remains constant, indicative of stable quasiparticles, while the repulsive polaron dephasing rate increases nearly quadratically. The dynamics of Fermi polarons are of critical importance for understanding the pairing and magnetic instabilities that lead to the formation of rich quantum phases found in a wide range of physical systems including nuclei, cold atomic gases, and solids.

cond-mat.mes-hall↗

arXiv2022

Exciton-polaron interactions in monolayer WS$_2$

Interactions between quasiparticles are of fundamental importance and ultimately determine the macroscopic properties of quantum matter. A famous example is the phenomenon of superconductivity, which arises from attractive electron-electron interactions that are mediated by phonons or even other more exotic fluctuations in the material. Here we introduce mobile exciton impurities into a two-dimensional electron gas and investigate the interactions between the resulting Fermi polaron quasiparticles. We employ multi-dimensional coherent spectroscopy on monolayer WS$_2$, which provides an ideal platform for determining the nature of polaron-polaron interactions due to the underlying trion fine structure and the valley specific optical selection rules. At low electron doping densities, we find that the dominant interactions are between polaron states that are dressed by the same Fermi sea. In the absence of bound polaron pairs (bipolarons), we show using a minimal microscopic model that these interactions originate from a phase-space filling effect, where excitons compete for the same electrons. We furthermore reveal the existence of a bipolaron bound state with remarkably large binding energy, involving excitons in different valleys cooperatively bound to the same electron. Our work lays the foundation for probing and understanding strong electron correlation effects in two-dimensional layered structures such as moiré superlattices.

cond-mat.mes-hall↗

arXiv2022

Equatorial magnetoplasma waves

Due to its rotation, Earth traps a few equatorial ocean and atmospheric waves, including Kelvin, Yanai, Rossby, and Poincare modes. It has been recently demonstrated that the mathematical origin of equatorial waves is intricately related to the nontrivial topology of hydrodynamic equations describing oceans or the atmosphere. In the present work, we consider plasma oscillations supported by a two-dimensional electron gas confined at the surface of a sphere or a cylinder. We argue that in the presence of a uniform magnetic field, these systems host a set of equatorial magnetoplasma waves that are counterparts to the equatorial waves trapped by Earth. For a spherical geometry, the equatorial modes are well developed only if their penetration length is smaller than the radius of the sphere. For a cylindrical geometry, the spectrum of equatorial modes is weakly dependent on the cylinder radius and overcomes finite-size effects. We argue that this exceptional robustness can be explained by destructive interference effects. We discuss possible experimental setups, including grains and rods composed of topological insulators (e.g., Bi2Se3) or metal-coated dielectrics (e.g., Au2S).

cond-mat.mes-hall↗

arXiv2022

Topological fluctuating electron-hole Cooper pairs in graphene-GaAs heterostructures

Fluctuating Cooper pairs formed by spatially separated electrons and holes are precursors of their equilibrium condensation. Their presence strongly impacts transport phenomena and interlayer tunneling in double-layer systems above the transition temperature. Here, we consider a hybrid graphene/quantum well double-layer system and focus on the dynamics of fluctuating Cooper pairs formed by conventional electrons and Dirac holes. We demonstrate that the chiral nature of Dirac holes is manifested in the presence of two (almost) degenerate competing pairing channels, which are intertwined by effective pseudospin-orbit interactions. We argue that the spectrum of the Ginzburg-Landau Hamiltonian describing the energetics of fluctuating Cooper pairs is geometrically nontrivial and can be characterized by the half-integer topological Chern number. We derive a kinetic equation for fluctuating Cooper pairs and demonstrate that their nontrivial geometries generate two anomalous velocities of distinct geometrical origins. These velocities are intricately connected with the Berry curvature and the quantum metric for the Ginzburg-Landau Hamiltonian, respectively. The resulting anomalous contributions to conductivity are singular at the transition temperature, and we discuss possible setups for their experimental observation.

cond-mat.supr-con↗

arXiv2021

Hybrid dark excitons in monolayer $\hbox{MoS}_2$

Transition metal dichalcogenides have a rich exciton landscape consisting of a variety of bright and dark excitonic states. We consider the lowest-energy dark states in $\hbox{MoS}_2$, which can be referred to as hybrid excitons, as they are formed by a Dirac electron and a Schrödinger hole. The chiral nature of the Dirac electron introduces asymmetry to the excited exciton state spectrum and entangles the relative motion of the electron and hole with the center-of-mass motion. We demonstrate that the entanglement generates an additional contribution to the Berry curvature of hybrid excitons. The additional contribution is substrate-dependent and accounts for almost one quarter of the Berry curvature in suspended samples. The nontrivial geometry of hybrid excitons is manifested in the optical anomalous valley Hall effect, which can be observed via recently developed pump-probe photoemission spectroscopy. We argue that the Hall angle of hybrid excitons is approximately one half of that for bright excitons. Moreover, the anticipated long lifetime of hybrid excitons favors an extended propagation distance and allows the spatial separation of hybrid excitons with different valley indices.

cond-mat.mes-hall↗

arXiv2021

Anomalous drag in electron-hole condensates with granulated order

We explain the strong interlayer drag resistance observed at low temperatures in bilayer electron-hole systems in terms of an interplay between local electron-hole-pair condensation and disorder-induced carrier density variations. Smooth disorder drives the condensate into a granulated phase where interlayer coherence is formed only in well separated and disconnected regions, or grains, and the densities of electrons and holes accidentally match. The drag resistance is then dominated by Andreev scattering of charge carries between layers at the grains that transfers momentum between layers. We show that this scenario can account for the observed dependence of the drag resistivity on temperature, and on the average charge imbalance between layers.

cond-mat.mes-hall↗