arXiv subjects

A. D. Chepelianskii

Publications and source records attributed to A. D. Chepelianskii.

At least 19 recordsLinked to original sources

arXiv2026

Chern junctions in Moiré-Patterned Graphene/PbI2

Expanding the moire material library continues to unlock novel quantum phases and emergent electronic behaviors. Here, we introduce PbI2 into the moire family and investigate the magnetotransport properties of moire superlattice in a hexagonal boron nitride/graphene/PbI2 heterostructures. In the high-field quantum Hall regime, we observe robust dissipationless transport at the charge neutrality point, indicative of incompressible states at filling factor vh = 0. Additionally, a fractional conductance plateau at 2/3 e2/h emerges, which we attribute to a Chern junction between domains with distinct Chern numbers originating from moire-modulated and conventional integer quantum Hall states. The moire Hofstadter spectrum displays an unconventional flavor sequence, likely influenced by proximity-induced spin-orbit coupling from the PbI2 layer. We also see coherent electronic interference along lines with Chern number vm = -2. These observations provide compelling evidence for the formation of moire domains that nontrivially interrupt incompressible quantum Hall states, reflecting the strong moire potential in the BN/graphene/PbI2 superlattice. We suggest that the moire Hofstadter spectrum coupled with the proximity-induced spin-orbit interaction from PbI2 gives rise to a high magnetic field topological insulator phase explaining ballistic transport at the charge neutrality point in the graphene monolayer.

cond-mat.mes-hall↗

arXiv2025

Analytical approach to the magneto-fluorescence of triplet excitons

The fluorescence of triplet excitons and color-centers is strongly dependent on magnetic field that mixes the zero field spin eigenstates that determine the radiative recombination rates back into the singlet ground state through spin-orbit coupling. For films of molecules, and polycrystalline color-centers samples an average over molecular orientations has to be performed to model the magneto-fluorescence lineshapes. This limits our analytical understanding of the lineshapes and complicates the analysis of the fluorescence dependence on magnetic field. Here, we present a framework that allows to average over triplet molecular orientations analytically. Our approach provides very accurate numerical routines computing precisely the averages matrix elements that appear in magneto-fluorescence and semi-analytical approximations that can be used to model experimental traces.

cond-mat.mes-hall↗

arXiv2025

Spin dependent fluorescence mediated by anti-symmetric exchange in triplet exciton pairs

Singlet fission and triplet-triplet annihilation (TTA) are spin-dependent phenomena critical to optoelectronics. The dynamics of spin populations during geminate triplet pair separation are crucial for controlling fission and TTA rates. We show that the Dzyaloshinskii-Moriya interaction (DMI) induces level crossings between spin manifolds, affecting spin populations and TTA rates in crystalline fission semiconductors. By investigating spin-dependent fluorescence in a triplet exciton pair with the magnetic field aligned along the fine structure tensor, we isolate the effect of DMI, as the triplet wavefunctions remain unaffected by the field. Our results reveal that DMI introduces additional TTA pathways that are forbidden by spin conservation, explaining the observed evolution of optically detected magnetic resonance signals with varying magnetic field. This study highlights the significant impact of DMI on the optical properties of triplet excitons, advancing our understanding of spin dynamics in these systems.

physics.chem-ph↗

arXiv2025

On the optimal relaxation rate for the Metropolis algorithm in one dimension

We study the relaxation of the Metropolis Monte Carlo algorithm corresponding to a single particle trapped in a one-dimensional confining potential, with even jump distributions that ensure that the dynamics verifies detailed balance. Previous work suggested that, for smooth jump distributions, the fastest relaxation rate is obtained as a result of the competition between diffusive and rejection-dominated dynamics. In this work, we show that a new regime comes into play for two-peaked jump distributions, where the relaxation dynamics is neither dominated by diffusion nor rejection: the eigenmodes adopt an oscillatory form, reminiscent of charge density waves (CDW) -- thus we term this new regime the CDW regime. Using a combination of numerical and analytical techniques, the parameter regions corresponding to diffusion, rejection, and CDW are characterised, as well as the transition lines between them -- i.e. a phase diagram is built. The optimal relaxation rate is located at the triple point of phase coexistence, where the transition lines (diffusive-rejection, diffusive-CDW, and CDW-rejection) intersect. Our theoretical framework is checked versus the numerical diagonalisation of the master equation. We also briefly discuss more sophisticated attempts at optimising the relaxation rate to equilibrium.

cond-mat.stat-mech↗

arXiv2024

Cascade of multi-exciton states generated by singlet fission

Identifying multi-exciton states generated from singlet fission is key to understanding the carrier multiplication process, which presents a strategy for improving the efficiency of photovoltaics and bio-imaging. Broadband optically detected magnetic resonance is a sensitive technique to detect multi-exciton states. Here we report a dominant species emerging under intense light excitation corresponding to a weakly exchange coupled triplet pair located on adjacent molecules oriented by nearly 90 degrees, contrasting to the pi-stacked triplet pair under low excitation intensity. The weakly coupled species model precisely reproduces the intricate spin transitions in the Hilbert space of the triplet pair. Combining the magneto photoluminescence and high-magnetic field ODMR, we also identify a strongly exchange-coupled state of three triplet excitons formed by photoexcited V2, which manifests through the magnetic field induced level crossings between its quintet and triplet manifolds. The excellent agreement between the experimental Zeeman fan and the two-triplet spin Hamiltonian highlights the potential of multi-exciton states for quantum information processing.

cond-mat.mes-hall↗

arXiv2023

Quantum Transport and Spectroscopy of Two-dimensional Perovskite/Graphene Interfaces

Quantum transport properties in molecularly thin perovskite/graphene heterostructure are experimentally investigated by Shubnikov-de Hass (SdH) oscillation and photo-resistance spectroscopy. We find an efficient charge transfer between the perovskite nanosheets and graphene, with a high hole concentration in graphene of up to $\rm \sim 2.8 \times 10^{13}\ cm^{-2}$. The perovskite layer also increases Fermi velocity lowering the effective mass of graphene from expected $\rm \sim 0.12\ m_e$ to $\rm \sim 0.08\ m_e$. Combining magneto-resistance and density functional theory calculations, we find that the carrier density in graphene significantly depends on the perovskite termination at the interface, affecting the charge transfer process and leading to a coexistence of regions with different doping. We also investigate the photo-response of the SdH oscillation under illumination. Using photo-resistance spectroscopy, we find evidence of photo-assisted transport across the perovskite layer between two graphene electrodes mediated by hot carriers in perovskite. Our results provide a picture to understand the transport behavior of 2D perovskite/graphene heterostructure and a reference for the controlled design of interfaces in perovskite optoelectronic devices.

cond-mat.mes-hall↗

arXiv2023

Quantum synchronization and entanglement of dissipative qubits coupled to a resonator

We study the properties of a driven cavity coupled to several qubits in the framework of a dissipative Jaynes-Cummings model. We show that the rotating wave approximation (RWA) allows to reduce the description of original driven model to an effective Jaynes-Cummings model with strong coupling between photons and qubits. Two semi-analytical approaches are developed to describe the steady state of this system. We first treat the weak dissipation limit where we derive perturbative series of rate equations that converge to the exact RWA steady-state except near the cavity resonance. This approach exactly describes the multi-photon resonances in the system. Then in the strong dissipation limit we introduce a semiclassical approximation which allows to reproduce the mean spin-projections and cavity state. This approach reproduces the RWA exactly in the strong dissipation limit but provides good qualitative trends even in more quantum regimes. We then focus on quantum synchronization of qubits through their coupling to the cavity. We demonstrate the entangled steady state of a pair of qubits synchronized through their interaction with a driven cavity in presence of dissipation and decoherence. Finally we discuss synchronization of a larger number of qubits.

quant-ph↗

arXiv2023

Spin-dependent recombination mechanisms for quintet bi-excitons generated through singlet fission

We investigate the physical mechanisms for spin-dependent recombination of a strongly bound pair of triplet excitons generated by singlet fission and forming a spin quintet (total spin of two) bi-exciton. For triplet excitons the spin-dependent recombination pathways can involve intersystem crossing or triplet-triplet annihilation back to the singlet ground state. However the modeling of spin-dependent recombination for quintets is still an open question. Here we introduce two theoretical models and compare their predictions with the broadband optically detected magnetic resonance spectrum of a long lived quintet bi-exciton with known molecular structure. This spectrum measures the change in the fluorescence signal induced by microwave excitation of each of the ten possible spin transitions within the quintet manifold as function of the magnetic field. While most of the experimental features can be reproduced for both models, the behavior of some of the transitions is only consistent with the quintet spin-recombination model inspired by triplet intersystem crossing which can reproduce accurately the experimental two-dimensional spectrum with a small number of kinetic parameters. Thus quantitative analysis of the broadband optically detected magnetic resonance signal enables quantitative understanding of the dominant spin-recombination processes and estimation of the out-of equilibrium spin populations.

cond-mat.mes-hall↗

arXiv2021

Coupled pair of one and two dimensional magneto-plasmons on electrons on helium

Electrons on the liquid helium surface form an extremely clean two dimensional system where different plasmon-excitations can coexist. Under a magnetic field time reversal symmetry is broken and all the bulk magneto-plasmons become gaped at frequencies below cyclotron resonance while chiral one dimensional edge magneto-plasmons appear at the system perimeter. We theoretically show that the presence of a homogeneous density gradient in the electron gas leads to the formation of a delocalized magneto-plasmon mode in the same frequency range as the lowest frequency edge-magnetoplasmon mode. We experimentally confirm its existence by measuring the corresponding resonance peak in frequency dependence of the admittance of the electron gas. This allows to realize a prototype system to investigate the coupling between a chiral one-dimensional mode and a single delocalized bulk mode. Such a model system can be important for the understanding of transport properties of topological materials where states of different dimensionality can coexist.

cond-mat.mes-hall↗

arXiv2021

Spin-orbit-enhanced robustness of supercurrent in graphene/WS$_2$ Josephson junctions

We demonstrate enhanced robustness of the supercurrent through graphene-based Josephson junctions in which strong spin-orbit interactions (SOIs) are induced. We compare the persistence of a supercurrent at high magnetic fields between Josephson junctions with graphene on hexagonal boron-nitride and graphene on WS$_2$, where strong SOIs are induced via the proximity effect. We find that in the shortest junctions both systems display signatures of induced superconductivity, characterized by a suppressed differential resistance at a low current, in magnetic fields up to 1 T. In longer junctions however, only graphene on WS$_2$ exhibits induced superconductivity features in such high magnetic fields, and they even persist up to 7 T. We argue that these robust superconducting signatures arise from quasi-ballistic edge states stabilized by the strong SOIs induced in graphene by WS$_2$.

cond-mat.mes-hall↗

arXiv2020

Motional Quantum States of Surface Electrons on Liquid Helium in a Tilted Magnetic Field

The Jaynes-Cummings model (JCM), one of the paradigms of quantum electrodynamics, was introduced to describe interaction between light and a fictitious two-level atom. Recently it was suggested that the JCM Hamiltonian can be invoked to describe the motional states of electrons trapped on the surface of liquid helium and subjected to a constant uniform magnetic field tilted with respect to the surface [Yunusova et al. Phys. Rev. Lett. 122, 176802 (2019)]. In this case, the surface-bound (Rydberg) states of an electron are coupled to the electron cyclotron motion by the in-plane component of tilted field. Here we investigate, both theoretically and experimentally, the spectroscopic properties of surface electrons in a tilted magnetic field and demonstrate that such a system exhibits a variety of phenomena common to the light dressed states of atomic and molecular systems. This shows that electrons on helium realize a prototypical atomic system where interaction between components can be engineered and controlled by simple means and with high accuracy, and which therefore can be potentially used as a new flexible platform for quantum experiments. Our work introduces a pure condensed-matter system of electrons on helium into the context of atomic, molecular and optical physics.

quant-ph↗

arXiv2020

Many-electron system on helium and the color center spectroscopy

Electrons on the helium surface display sharp resonant absorption lines related to the transitions between the subbands of quantized motion transverse to the surface. A magnetic field parallel to the surface strongly affects the absorption spectrum. We show that the effect comes from admixing the out-of-plane motion to the in-plane quantum dynamics of the strongly correlated electron liquid or a Wigner crystal. This is similar to the admixing electron transitions in color centers to phonons. The spectrum permits a direct characterization of the many-electron dynamics and also enables testing the theory of color centers in a system with a controllable coupling.

cond-mat.mes-hall↗

arXiv2020

Spin fine-structure reveals bi-exciton geometry in an organic semiconductor

In organic semiconductors, bi-excitons are key intermediates in carrier-multiplication and exciton annihilation. Their local geometry governs their electronic properties and yet has been challenging to determine. Here, we access the structure of the recently discovered $S=2$ quintet bi-exciton state in an organic semiconductor using broadband optically detected magnetic resonance. We correlate the experimentally extracted spin structure with the molecular crystal geometry to identify the specific molecular pairings on which bi-exciton states reside.

cond-mat.mtrl-sci↗

arXiv2019

Coupling between Rydberg states and Landau levels of electrons trapped on liquid helium

We investigate the coupling between Rydberg states of electrons trapped on a liquid Helium surface and Landau levels induced by a perpendicular magnetic field. We show that this realises a prototype quantum system equivalent to an atom in a cavity, where their coupling strength can be tuned by a parallel magnetic field. We determine experimentally the renormalisation of the atomic transition energies induced by the coupling to the cavity, which can be seen as an analogue of the Lamb shift. When the coupling is sufficiently strong the transition between the ground and first excited Rydberg states splits into two resonances corresponding to dressed states with vacuum and one photon in the cavity. Our results are in quantitative agreement with the energy shifts predicted by the effective atom in a cavity model where all parameters are known with high accuracy.

cond-mat.mes-hall↗

arXiv2018

Spin mixing between subbands and extraordinary Landau levels shift in wide HgTe quantum wells

We present both the experimental and theoretical investigation of a non-trivial electron Landau levels shift in magnetic field in wide ~20 nm HgTe quantum wells: Landau levels split under magnetic fields but become degenerate again when magnetic field increases. We reproduced this behavior qualitatively within an isotropic 6-band Kane model, then using semiclassical calculations we showed this behavior is due to the mixing of the conduction band with total spin 3/2 with the next well subband with spin 1/2 which reduces the average vertical spin from 3/2 to around 1. This change of the average spin changes the Berry phase explaining the evolution of Landau levels under magnetic field.

cond-mat.mes-hall↗

arXiv2018

Can warmer than room temperature electrons levitate above a liquid helium surface ?

We address the problem of overheating of electrons trapped on the liquid helium surface by cyclotron resonance excitation. Previous experiments, suggest that electrons can be heated to temperatures up to 1000K more than three order of magnitude higher than the temperature of the helium bath in the sub-Kelvin range. In this work we attempt to discriminate between a redistribution of thermal origin and other out-of equilibrium mechanisms that would not require so high temperatures like resonant photo-galvanic effects, or negative mobilities. We argue that for a heating scenario the direction of the electron flow under cyclotron resonance can be controlled by the shape of the initial electron density profile, with a dependence that can be modeled accurately within the Poisson-Boltzmann theory framework. This provides an self consistency-check to probe if the redistribution is indeed consistent with a thermal origin. We find that while our experimental results are consistent with the Poisson-Boltzmann theoretical dependence but some deviations suggest that other physical mechanisms can also provide a measurable contribution. Analyzing our results with the heating model we find that the electron temperatures increases with electron density under the same microwave irradiation conditions. This unexpected density dependence calls for a microscopic treatment of the energy relaxation of overheated electrons.

cond-mat.mes-hall↗

arXiv2017

Room temperature magneto-optic effect in silicon light-emitting diodes

In weakly spin-orbit coupled materials, the spin-selective nature of recombination can give rise to large magnetic-field effects, for example on electro-luminescence from molecular semiconductors. While silicon has weak spin-orbit coupling, observing spin-dependent recombination through magneto-electroluminescence is challenging due to the inefficiency of emission due to silicon's indirect band-gap, and to the difficulty in separating spin-dependent phenomena from classical magneto-resistance effects. Here we overcome these challenges to measure magneto-electroluminescence in silicon light-emitting diodes fabricated via gas immersion laser doping. These devices allow us to achieve efficient emission while retaining a well-defined geometry thus suppressing classical magnetoresistance effects to a few percent. We find that electroluminescence can be enhanced by up to 300\% near room temperature in a seven Tesla magnetic field showing that the control of the spin degree of freedom can have a strong impact on the efficiency of silicon LEDs.

cond-mat.mes-hall↗

arXiv2017

Floquet theory of microwave absorption by an impurity in two dimensional electron gas

We investigate the dynamics of a two-dimensional electron gas (2DEG) under circular polarized microwave radiation in presence of dilute localized impurities. Inspired by recent developments on Floquet topological insulators we obtain the Floquet wavefunctions of this system which allow us to predict the microwave absorption and charge density responses of the electron gas, we demonstrate how these properties can be understood from the underlying semiclassical dynamics even for impurities with a size of around a magnetic length. The charge density response takes the form of a rotating charge density vortex around the impurity that can lead to a significant renormalization of the external microwave field which becomes strongly inhomogeneous on the scale of a cyclotron radius around the impurity. We show that this in-homogeneity can suppress the circular polarization dependence which is theoretically expected for MIRO but which was not observed in MIRO experiments on semiconducting 2DEGs. Our explanation, for this so far unexplained polarization independence, has close similarities with the Azbel'-Kaner effect in metals where the interaction length between the microwave field and conduction electrons is much smaller than the cyclotron radius due to skin effect generating harmonics of the cyclotron resonance.

cond-mat.mes-hall↗