arXiv subjects

Vadym Apalkov

Publications and source records attributed to Vadym Apalkov.

At least 19 recordsLinked to original sources

arXiv2026

Discernible signatures of fractionally charged anyons in a Pfaffian-Laughlin state

Understanding the nature of quasihole excitations, i.e., anyons that have fractional charge and statistics, has been a challenging problem in condensed matter physics. Our theoretical approach to this problem has been to consider a quantum dot, containing a few charged particles, coupled to the incompressible fluid. It has provided important insights into the energetics of Laughlin quasiholes. Photoluminescence (PL) spectroscopy studies of this system have been able to probe these quasiholes that have confirmed our expectations. Turning to the Pfaffian state, we now observe that such a system is also able to provide valuable information about the Pfaffian quasiholes, viz., the energy dispersion, the charge density distribution and the quasihole creation energy. The energy dispersion of e/4 quasiholes derived here, clearly reflect the interaction between the quantum dot and the incompressible Pfaffian state. PL spectroscopy experiments on the 5/2 Pfaffian-Laughlin state could perhaps shed light on the energetics of these elusive quasiparticles.

cond-mat.str-el↗

arXiv2025

Selective enhancement of Coulomb interactions in planar Weyl fermions

We report on our study of the electron interaction effects in topological two-dimensional (2D) materials placed in a quantizing magnetic field. Taking our cue from a recent experimental report, we consider a particular case of bismuthene monolayer with a strong spin-orbit interaction which can be a Weyl semimetal when placed on a specially tuned substrate. Interestingly, we observe that in some Landau levels of this material, the interaction effects are strongly enhanced compared to those for a conventional 2D system. Such an enhancement of electron-electron interactions in these materials is largely due to an anisotropy present in the materials. Additionally, the interaction effects can be tuned by changing the coupling to the substrate and the strongest inter-electron interactions are observed when the system is a Weyl semimental. The observed enhancement of the interaction effects can therefore be an important signature of the 2D Weyl fermions.

cond-mat.mes-hall↗

arXiv2023

Ultrafast Field-driven Valley Polarization of Transition Metal Dichalcogenide Quantum Dots

We study theoretically the electron dynamics of transition metal dichalcogenide quantum dots in the field of an ultrashort and ultrafast circularly polarized optical pulse. The quantum dots have the shape of a disk and their electron systems are described within an effective model with infinite mass boundary conditions. Similar to transition metal dichalcogenide monolayers, a circularly polarized pulse generates ultrafast valley polarization of such quantum dots. The dependence of the valley polarization on the size of the dot is sensitive to the dot material and, for different materials, show both monotonic increase with the dot radius and nonmonotonic behavior with a local maximum at a finite dot radius.

cond-mat.mes-hall↗

arXiv2022

Interacting Dirac fermions and the rise of Pfaffians in graphene

Fractional Quantum Hall effect (FQHE) is a unique many-body phenomenon, which was discovered in a two-dimensional electron system placed in a strong perpendicular magnetic field. It is entirely due to the electron-electron interactions within a given Landau level. For special filling factors of the Landau level, a many-particle incompressible state with a finite collective gap is formed. Among these states, when the Landau level is half filled, there is a special FQHE state that is described by the Pfaffian function and the state supports charged excitations that obey non-Abelian statistics. Such a $1/2$-FQHE state can be realized only for a special profile of the electron-electron potential. For example, for conventional electron systems, the $1/2$-FQHE state occurs only in the second Landau level, while in a graphene monolayer, no $1/2$-FQHE state can be found in any Landau level. Another type of low-dimensional system is the bilayer graphene, which consists of two graphene monolayers coupled through the inter-layer hopping. The system is quasi-two-dimensional, which makes it possible to tune the inter-electron interaction potential by applying either the bias voltage or the magnetic field that is applied parallel to the bilayer. It so happens that in the bilayer graphene with AB staking, there is one Landau level per valley where the $1/2$-FQHE state can indeed be present. The properties of that $1/2$-FQHE state have a nonmonotonic dependence on the applied magnetic field and this can be even more stable than the one discovered in conventional electron systems.

cond-mat.mes-hall↗

arXiv2021

Three-level Spaser System: a Semi-Classical Analysis

We theoretically study a nanospaser system, which consists of a spherical silver nanoparticle embedded inside a sphere composed of dye molecules. The gain of the system, dye molecules, are described by a three-level model, where the transition frequency between the lowest two energy levels is close to the surface plasmon frequency of the nanosphere. Contrary to a two-level model of spaser, the three-level model takes into account finite relaxation time between the high energy levels of the gain medium. These relaxation processes affect both the spaser threshold and the number of generated plasmons in the continuous wave regime. While for a two-level model of a spaser the number of generated plasmons has a linear dependence on the gain, for the three-level model this dependence becomes quadratic.

physics.optics↗

arXiv2021

Ultrafast valley polarization in bilayer graphene

We study theoretically interaction of a bilayer graphene with a circularly polarized ultrafast optical pulse of a single oscillation at an oblique incidence. The normal component of the pulse breaks the inversion symmetry of the system and opens up a dynamical band-gap, due to which a valley-selective population of the conduction band becomes sensitive to the angle of incident of the pulse. We show that the magnitude of the valley polarization can be controlled by the angle of incidence, the amplitude, and the angle of in-plane polarization of the chiral optical pulse. Subsequently, a sequence of a circularly polarized pulse followed by a linearly polarized femtosecond-long pulse can be used to control the valley polarization created by the preceding pulse. Generally, the linearly polarized pulse depolarizes the system. The magnitude of such a depolarization depends on the amplitude, and the in-plane polarization angle of the linearly polarized pulse. Our protocol provides a favorable platform for applications in valleytronics.

cond-mat.mes-hall↗

arXiv2021

TMDC-based topological nanospaser: single and double threshold behavior

We theoretically study a topological nanospaser, which consists of a silver nanospheroid and MoS$_2$ monolayer ake of a circular shape. The metal nanospheroid acts as a plasmonic nanoresonator that supports two rotating modes, which are coupled to the corresponding valleys of MoS$_2$. We apply external circularly polarized light that selectively pumps only one of the valleys of MoS$_2$. The generated spaser dynamics strongly depends on the size (radius) of the MoS$_2$ nanoflake. For small radius, the system has only one spasing regime when only chirally-matched plasmon mode is generated, while at larger size of MoS$_2$, depending on the pump intensity, there are two regimes. In one regime, only the chirally-matched plasmon mode is generated, while in the other regime both chirally-matched and chirally-mismatched modes exist. Different regimes of spaser operation have also opposite handedness of the far-field radiated of the spaser system. Such topological nanospaser has potential applications in different areas of infrared spectroscopy, sensing, probing, and biomedical treatment.

physics.optics↗

arXiv2020

Femtosecond currents in transition metal dichalcogenides monolayers

We theoretically study the interaction of an ultrafast intense linearly polarized optical pulse with monolayers of transition metal dichalcogenides (TMDCs). Such a strong pulse redistributes electrons between the bands and generates femtosecond currents during the pulse. Due to the large bandwidth of the incident pulse, this process is completely off-resonant. While in TMDCs the time-reversal symmetry is conserved, the inversion symmetry is broken and these monolayers have the axial symmetry along armchair direction but not along the zigzag one. Therefore, the pulse polarized along the asymmetric direction of TMDC monolayer generates both longitudinal, i.e., along the direction of polarization, and transverse, i.e., in the perpendicular direction, currents. Such currents result in charge transfer through the system. We study different TMDC materials and show how the femtosecond transport in TMDC monolayers depend on their parameters, such as lattice constant and bandgap.

cond-mat.mes-hall↗

arXiv2020

Anomalous Ultrafast All-Optical Hall Effect in Gapped Graphene

We propose an ultrafast all-optical anomalous Hall effect in two-dimensional (2D) semiconductors of hexagonal symmetry such as gapped graphene (GG), transition metal dichalcogenides (TMDCs), and hexagonal boron nitride (h-BN). To induce such an effect, the material is subjected to a sequence of two strong-field single-optical-cycle pulses: a chiral pump pulse followed within a few femtoseconds by a probe pulse linearly polarized in the armchair direction of the 2D lattice. Due to the effect of topological resonance, the first (pump) pulse induces a large chirality (valley polarization) in the system, while the second pulse generates a femtosecond pulse of the anomalous Hall current. The proposed effect is the fundamentally the fastest all-optical anomalous Hall effect possible in nature. It can be applied to ultrafast all-optical storage and processing of information, both classical and quantum.

cond-mat.mes-hall↗

arXiv2020

Bilayer graphene in strong ultrafast laser fields

We theoretically investigate the interaction of an ultrastrong femtosecond-long linearly polarized optical pulse with AB-stacked bilayer graphene. The pulse excite electrons from the valence into the conduction band, resulting in finite conduction band population. Such a redistribution of electrons results in the generation of current which can be manipulated by the angle of incidence of the pulse. For the normal incidence, the current along a direction transverse to the polarization of the optical pulse is zero. However, the interlayer symmetry is broken up by a finite angle of incidence which causes induction of electric current in the direction perpendicular to the $x$-$z$ plane of polarization of the pulse. We show that the magnitude and the direction of such a current as well as charge transfer along this direction can be manipulated by tuning the angle of incidence of the laser pulse. Further, the symmetry of the system prohibits the generation of transverse current if the pulse is polarized along $y$-direction.

cond-mat.mes-hall↗

arXiv2020

Topological resonance in Weyl semimetals in circularly-polarized optical pulse

We study theoretically the ultrafast electron dynamics of three-dimensional Weyl semimetals in the field of a laser pulse. For a circularly-polarized pulse, such dynamics is governed by topological resonance, which manifests itself as a specific conduction band population distribution in the vicinity of the Weyl points. The topological resonance is determined by the competition between the topological phase and the dynamic phase and depends on the handedness of a circularly polarized pulse. Also, we show that the conduction band population induced by a circularly-polarized pulse that consists of two oscillations with opposite handedness is highly chiral, which represents the intrinsic chirality of the Weyl points.

physics.optics↗

arXiv2020

Ultrafast strong-field absorption in gapped graphene

We study theoretically the strong-field absorption of an ultrafast optical pulse by a gapped graphene monolayer. At low field amplitudes, the absorbance in the pristine graphene is equal to the universal value of $2.3$ percent. Although the ultrafast optical absorption for low field amplitudes is independent of the polarization, linear or circular, of the applied optical pulse, for high field amplitudes, the absorption strongly depends on the pulse polarization. For a linearly polarized pulse, the optical absorbance is saturated at the value of $\approx 1.4$ percent for the pulse's amplitude of $\geq 0.4~\mathrm{V/Å}$, but no such saturation is observed for a circularly polarized pulse. For the gapped graphene, the absorption of a linearly polarized pulse has a weak dependence on the bandgap, while for a circularly polarized pulse, the absorption is very sensitive to the bandgap. %Opening a bandgap in graphene by placing in on, for example, SiC substrate strongly modify the ultrafast absorption at small field amplitudes.

cond-mat.mes-hall↗

arXiv2019

Topological Nanospaser

We propose a nanospaser made of an achiral plasmonic-metal nanodisk and a two-dimensional chiral gain medium -- a monolayer transition-metal dichalcogenide (TMDC). When one valley of the TMDC is selectively pumped (e.g., by a circular-polarized radiation), the spaser generates a mode carrying a topological charge (chirality) that matches that of the gain valley. The chirally mismatched, time-reversed mode has exactly the same frequency but the opposite topological charge; it is actively suppressed by the gain saturation (population clamping) and never generates leading to a strong topological protection for the generating matched mode. This topological spaser is promising for the use in nanooptics and nanospectroscopy in the near-field especially in applications to biomolecules that are typically chiral. Another potential application is a chiral nanolabel for biomedical applications emitting in the far field an intense circularly-polarized coherent radiation.

cond-mat.mes-hall↗

arXiv2019

Topological Spaser

We theoretically introduce a topological spaser, which consists of a hexagonal array of plasmonic metal nanoshells containing an achiral gain medium in their cores. Such a spaser can generate two mutually time-reversed chiral surface plasmon modes in the $\mathbf K$- and $\mathbf K^\prime$-valleys, which carry the opposite topological charges, $\pm1$, and are described by a two-dimensional $E^{\prime}$ representation of the $D_{3h}$ point symmetry group. Due to the mode competition, this spaser exhibits a bistability: only one of these two modes generates, which is a spontaneous symmetry breaking. Such a spaser can be used for an ultrafast all-optical memory and information processing

cond-mat.mes-hall↗

arXiv2019

Topological resonance and single-optical-cycle valley polarization in gapped graphene

For gapped graphene, we predict that an intense ultrashort (single-oscillation) circularly-polarized optical pulse can induce a large population of the conduction band and a large valley polarization. With an increase in the bandgap, the magnitude of the valley polarization gradually increases from zero (for the native gapless graphene) to a value on the order of unity. The energy bandwidth of the electrons excited into the conduction band can be very large ($\gtrsim 10$ eV for a reasonable pulse amplitude of $\sim 0.5$ $\mathrm{V/Å}$). These phenomena are due to the effect of topological resonance: the matching of the topological (geometric) phase and the dynamic phase. Gapped graphene with tunable bandgap can be used as a convenient generic model of two-dimensional semiconductors with honeycomb generic lattice structures and broken inversion symmetry, such as transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2019

Ultrafast optical currents in gapped graphene

We study theoretically the interaction of ultrashort optical pulses with gapped graphene. Such strong pulse results in finite conduction band population and corresponding electric current both during and after the pulse. Since gapped graphene has broken inversion symmetry, it has an axial symmetry about the $y$-axis but not about the $x$-axis. We show that, in this case, if the linear pulse is polarized along the $x$-axis, the rectified electric current is generated in the $y$ direction. At the same time, the conduction band population distribution in the reciprocal space is symmetric about the $x$-axis. Thus, the rectified current in gapped graphene has inter-band origin, while the intra-band contribution to the rectified current is zero.

cond-mat.mes-hall↗

arXiv2019

Laser pulse waveform control of Dirac fermions in graphene

We theoretically study the Dirac fermion dynamics in a graphene monolayer in the presence of an applied ultrafast laser pulse. The pulse has the duration of a few femtoseconds and the amplitude of ~ 0.1 - 0.5 $\mathrm{V/Å}$. The waveform of the pulse is described by Hermit Gaussian polynomials with varying carrier-envelope phase. We show that the ultrafast dynamics of Dirac fermions strongly depends on the carrier-envelope phase and the frequency of the applied pulse. The ultrafast pulse generates an electric current which results in a finite transferred charge. The ultrafast field-driven current and the corresponding net transferred charge depend on the waveform of the applied pulse. Our results pave the way for the development of ultrafast information processing in the terahertz domain.

cond-mat.mes-hall↗

arXiv2018

Ultrafast topological phenomena in gapped graphene

In the model of gapped graphene, we have shown how the recently predicted topological resonances are solely related to the presence of an energy band gap at the $K$ and $K^\prime$ points of the Brillouin zone. In the field of a strong single-oscillation chiral (circularly-polarized) optical pulse, the topological resonance causes the valley-selective population of the conduction band. This population distribution represents a chiral texture in the reciprocal space that is structured with respect to the pulse separatrix as has earlier been predicted for transition metal dichalcogenides. As the band gap is switched off, this chirality gradually disappears replaced by an achiral distribution characteristic of graphene.

cond-mat.mes-hall↗