Dmitry Efimkin

• PhD in Science
• Research Associate at University of Texas at Austin

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 th...

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 forma...

The Berry phase-related nontrivial electronic band geometries can significantly influence bulk and edge plasma waves, or 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 plasma waves in a Dirac electron fluid...

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...

Bose polarons are mobile impurities dressed by density fluctuations of a surrounding degenerate Bose gas. These many-body objects have been realized in ultracold atomic gasses and become a subject of intensive studies. In this work, we show that excitons in electron-hole bilayers offer new opportunities for exploring polarons in strongly interactin...

We observed attractive and repulsive Bose-polarons in neutral electron-hole systems in separated electron and hole layers in GaAs heterostructure. The measured energy splitting between the polarons increases with density, in agreement with the theoretical calculations.

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...

We consider electron-hole Cooper pair condensation in a heterostructure formed by a topological insulator (TI) film and a quantum well. We argue that the helical nature of the Dirac electronic states at the TI surface results in the presence of two competing degenerate pairing channels. The corresponding paired states have an unconventional symmetr...

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 unconve...

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 c...

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...

As the Earth rotates, the Coriolis force causes various 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 observatio...

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 ho...

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 int...

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 the magnetic field and strong light-matter coupling, we employ a fully microscopic theory that explicitly incorporates electrons, holes, and ph...

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 int...

Due to its rotation, Earth traps a few equatorial ocean and atmospheric waves, including Kelvin, Yanai, Rossby, and Poincaré 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, w...

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 photo...

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, w...

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 dyna...

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 MoS2, 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...

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 in which interlayer coherence is established only in well...

The surface of a topological insulator hosts Dirac electronic states with the spin-momentum locking, which constrains spin orientation perpendicular to electron momentum. As a result, collective plasma excitations in the interacting Dirac liquid manifest themselves as coupled charge- and spin-waves. Here we demonstrate that the presence of the spin...

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 Schr\"{o}dinger hole. The chiral nature of the Dirac electron introduces as...

Recently, it has been demonstrated that the absorption of moderately doped two-dimensional semiconductors can be described in terms of exciton polarons. In this scenario, attractive and repulsive polaron branches are formed due to interactions between a photoexcited exciton and a Fermi sea of excess charge carriers. These interactions have previous...

The surface of a topological insulator hosts Dirac electronic states with the spin-momentum locking, which constrains spin orientation perpendicular to electron momentum. As a result, collective plasma excitations in the interacting Dirac liquid manifest themselves as coupled charge- and spin-waves. Here we demonstrate that the presence of the spin...

Recently, it has been demonstrated that the absorption of moderately doped two-dimensional semiconductors can be described in terms of exciton-polarons. In this scenario, attractive and repulsive polaron branches are formed due to interactions between a photo-excited exciton and a Fermi sea of excess charge carriers. These interactions have previou...

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 separat...

A strong low-temperature enhancement of the tunneling conductance between graphene bilayers has been reported recently and interpreted as a signature of equilibrium electron-hole pairing, first predicted in bilayers more than 40 years ago but previously unobserved. Here we provide a detailed theory of conductance enhanced by fluctuating electron-ho...

In previous work, we have argued that the optical properties of moderately doped two-dimensional semiconductors can be described in terms of excitons dressed by their interactions with a degenerate Fermi sea of additional charge carriers. These interactions split the bare exciton into attractive and repulsive exciton-polaron branches. The collectiv...

In the presence of a finite interlayer displacement field bilayer graphene has an energy gap that is dependent on stacking and largest for the stable AB and BA stacking arrangements. When the relative orientations between layers are twisted through a small angle to form a moir$\mathrm{\acute{e}}$ pattern, the local stacking arrangement changes slow...

In minimally twisted bilayer graphene, a moir{\'e} pattern consisting of AB and BA stacking regions separated by domain walls forms. These domain walls are predicted to support counterpropogating topologically protected helical (TPH) edge states when the AB and BA regions are gapped. We fabricate designer moir{\'e} crystals with wavelengths longer...

We examine excitons formed in the bulk of a topological insulator as the system is tuned via a parameter between topological and trivial insulating phases, arguing that nontrivial topology has fingerprints in the spectrum of these excitons. The closely related hydrogen atom problem is well known to have a degeneracy due to a hidden symmetry, and th...

We examine excitons formed in the bulk of a topological insulator as the system is tuned via a parameter between topological and trivial insulating phases, arguing that nontrivial topology has fingerprints in the spectrum of these excitons. The closely related hydrogen atom problem is well known to have a degeneracy due to a hidden symmetry, and th...

SmB6 is a mixed valence Kondo insulator that exhibits a sharp increase in resistance following an activated behavior that levels off and saturates below 4K. This behavior can be explained by the proposal of SmB6 representing a new state of matter, a Topological Kondo insulator, in which a Kondo gap is developed and topologically protected surface c...

SmB6 is a mixed valence Kondo insulator that exhibits a sharp increase in resistance following an activated behavior that levels off and saturates below 4K. This behavior can be explained by the proposal of SmB6 representing a new state of matter, a Topological Kondo insulator, in which a Kondo gap is developed and topologically protected surface c...

We study controllable friction in a system consisting of a dark soliton in a one-dimensional Bose-Einstein condensate coupled to a non-interacting Fermi gas. The fermions act as impurity atoms, not part of the original condensate, that scatter off of the soliton. We study semi-classical dynamics of the dark soliton, a particle-like object with nega...

We study controllable friction in a system consisting of a dark soliton in a one-dimensional Bose-Einstein condensate coupled to a non-interacting Fermi gas. The fermions act as impurity atoms, not part of the original condensate, that scatter off of the soliton. We study semi-classical dynamics of the dark soliton, a particle-like object with nega...

Significance Solitons, spatially localized, mobile excitations resulting from an interplay between nonlinearity and dispersion, are ubiquitous in physical systems from water channels and oceans to optical fibers and Bose–Einstein condensates (BECs). From our pulse throbbing at our wrists to rapidly moving tsunamis, solitons appear naturally at a wi...

Recent optical studies of monolayer transition metal dechalcogenides have demonstrated that their excitonic absorption feature splits into two widely separated peaks at finite carrier densities. The additional peak is usually attributed to the presence of trions, bound states of two electrons and a hole or an electron and two holes. Here we argue t...

Radio frequency tunable oscillators are vital electronic components for signal generation, characterization, and processing. They are often constructed with a resonant circuit and a 'negative' resistor, such as a Gunn-diode, involving complex structure and large footprints. Here we report that a piece of SmB6, 100 micron in size, works as a current...

The surface of a 3D topological insulator is described by a helical electron state with the electron's spin and momentum locked together. We show that in the presence of ferromagnetic fluctuations the surface of a topological insulator is unstable towards a superconducting state with unusual pairing, dubbed Amperian pairing. The key idea is that th...

The surface of a 3D topological insulator is described by a helical electron state with the electron's spin and momentum locked together. We show that in the presence of ferromagnetic fluctuations the surface of a topological insulator is unstable towards a superconducting state with unusual pairing, dubbed Amperean pairing. The key idea is that th...

We consider the proximity effect between Dirac states at the surface of a topological insulator and a ferromagnet with easy plane anisotropy, which is described by the \emph{XY}-model and undergoes a Berezinskii-Kosterlitz-Thouless (BKT) phase transition. The surface states of the topological insulator interacting with classical magnetic fluctuatio...

We consider the proximity effect between Dirac states at the surface of a topological insulator and a ferromagnet with easy plane anisotropy, which is described by the \emph{XY}-model and undergoes a Berezinskii-Kosterlitz-Thouless (BKT) phase transition. The surface states of the topological insulator interacting with classical magnetic fluctuatio...

Several recent experiments have reported an anomalous temperature dependence of the Coulomb drag effect in electron-hole bilayers. Motivated by these puzzling data, we study theoretically a low-density electron-hole bilayer, where electrons and holes avoid quantum degeneracy by forming excitonic molecules. We describe the ionization-recombination c...

We explore the quantum dynamics of a bright matter-wave soliton in a quasi-one-dimensional bosonic superfluid with attractive interactions. Specifically, we focus on the dissipative forces experienced by the soliton due to its interaction with Bogoliubov excitations. Using the collective coordinate approach and the Keldysh formalism, a Langevin equ...

We explore the quantum dynamics of a bright matter-wave soliton in a quasi-one-dimensional bosonic superfluid with attractive interactions. Specifically, we focus on the dissipative forces experienced by the soliton due to its interaction with Bogoliubov excitations. Using the collective coordinate approach and the Keldysh formalism, a Langevin equ...

A fully analytical theory of a traveling soliton in a one-dimensional fermionic superfluid is developed within the framework of time-dependent self-consistent Bogoliubov-de Gennes equations, which are solved exactly in the Andreev approximation. The soliton manifests itself in a kink-like profile of the superconducting order parameter and hosts a p...

We consider the interplay between magnetic skyrmions in an insulating thin film and the Dirac surface states of a 3D topological insulator (TI), coupled by proximity effect. The magnetic texture of skyrmions can lead to confinement of Dirac states at the skyrmion radius, where out of plane magnetization vanishes. This confinement can result in char...

When Dirac electrons on the surface of a topological insulator are gapped, the resulting quantum anomalous Hall effect leads to universal magneto-optical Faraday and Kerr effects in the low frequency limit. However, at higher frequencies different excitations can leave their own fingerprints on the magneto-optics and can therefore be probed. In par...

A topological Kondo insulator (TKI) is a strongly-correlated material, where hybridization between the conduction electrons and localized $f$-electrons gives rise to a crossover from a metallic behavior at high temperatures to a topologically non-trivial insulating state at low temperatures. The existing description of the TKIs is based on a slave-...

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities, mediated by Dirac surface states on the surface of a topological insulator, leads to impurities ferromagnetic ordering. We present a self-consistent theory of the ordering, which takes into account a gap in the surface spectrum induced by the exchange field of magneti...

Manifestations in the drag effect of fluctuating Cooper pairs formed by spatially separated electrons and holes populating opposite surfaces of a topological insulator film are considered. We have calculated Aslamazov-Larkin contribution of electron-hole Cooper pair fluctuations to the transresistance of the system. The contribution is the most sin...

Tunneling between opposite surfaces of topological insulator thin film populated by electrons and holes is considered. We predict considerable enhancement of tunneling conductivity by Cooper electron-hole pair fluctuations that are precursor of their Cooper pairing. Cooper pair fluctuations lead to the critical behavior of tunneling conductivity in...

Manifestation of chiral excitons on surfaces of topological insulator thin film with magnetically induced energy gap in the surface spectrum in magnetooptical Kerr and Faraday effects is analyzes. Excitonic contribution to a surface optical conductivity tensor is calculated. Chiral excitons contrary to conventional ones resonantly contribute to Hal...

We consider pairing of massless Dirac electrons and holes located on opposite surfaces of thin film of "strong" three-dimensional topological insulator. Such pairing was predicted to give rise to topological exciton condensate with unusual properties. We estimate quantitatively achievable critical temperature of the pairing with taking into account...

We study collective excitations in a helical electron liquid on a surface of three-dimensional topological insulator. Electron in helical liquid obeys Dirac-like equation for massless particles and direction of its spin is strictly determined by its momentum. Due to this spin-momentum locking, collective excitations in the system manifest themselve...

The drag effect of charge carriers in the system of two graphene layers separated by insulator that is caused by interlayer tunneling is considered. For description of interlayer tunneling the model of randomly positioned tunnel bridges is used. The dependence of transresistivity of the system on the tunnel bridge size for different mutual orientat...

Electron-hole pairing due to the Coulomb interaction in the system of two graphene sheets has been considered. The critical transition temperature has been determined as a function of both the distance between the electron and hole Fermi lines and the triangular distortion of their spectrum. It has been shown that when the distance between Fermi li...

Collective plasmon excitations in a helical electron liquid on the surface of strong three-dimensional topological insulator are considered. The properties and internal structure of these excitations are studied. Due to spin-momentum locking in helical liquid on a surface of topological insulator, the collective excitations should manifest themselv...

We consider disorder effect on electron-hole pairing in the system of two graphene monolayers separated by dielectric barrier. The influence of charged impurities on temperature of phase transition is studied. In spite of large values of mobility of charge carriers in graphene disorder can considerably reduce temperature of electron-hole condensati...