arXiv subjects

Maksym V. Kovalenko

Publications and source records attributed to Maksym V. Kovalenko.

At least 19 recordsLinked to original sources

arXiv2026

Resonant cooling of nuclear spins by optically-oriented holes in MAPbI$_3$ perovskite crystals

Resonant cooling of nuclear spins by photogenerated spin-oriented holes is demonstrated for MAPbI$_3$ perovskite crystals. It is evidenced by Hanle-effect measurements under helicity-modulated excitation with variable frequency. The resonance position in magnetic field shifts toward higher fields with increasing modulation frequency. The invariance of the Hanle curve upon in-plane sample rotation is consistent with the involvement of $^{207}$Pb nuclei with spin $I = 1/2$, which do not exhibit quadrupolar splitting. The shape of the resonance feature in the Hanle curve reveals that the nuclear spins are cooled by carriers with a negative $g$-factor, consistent with holes. The resonance fields associated with the modulation frequencies exceed the half-width of the weakly localized hole contribution to the Hanle curve, indicating that strongly localized holes are the primary carriers responsible for the nuclear spin cooling.

cond-mat.mes-hall↗

arXiv2026

Tuning the optoelectronic properties of wide bandgap perovskites: Data-driven insights from combinatorial synthesis and high-throughput experimentation

The discovery and optimization of wide-bandgap lead halide perovskites (LHPs) is hindered by solution-based workflows with limited scalability. Large compositional parameter spaces present an additional challenge for materials optimization. Here, we establish an integrated, combinatorial workflow based on sequential physical vapor deposition that enables independent tuning of cation (Cs/Pb) and anion (Br/Cl) compositions. Applying automated structural, compositional, and optical characterizations across >500 samples regions of interest are rapidly screened in the quaternary Cs-Pb-Br-Cl space. From the screening, we establish a practical Cs/Pb window of 1.05-1.20 for wide bandgap perovskites, within which elevated PL yields were observed. Through in-depth analysis of the data set, we uncover a high-energy optical transition as a robust determinant for high PL yields. By combining mechanistic insight into the compositional origins of high PL efficiency with a fully integrated, high-throughput screening framework, and by openly releasing the complete multi-modal dataset, this work provides a broadly accessible benchmark to accelerate data-driven discovery of wide-bandgap perovskites.

cond-mat.mtrl-sci↗

arXiv2026

Quantum Back-Action Expands the Excitonic Hilbert Space in a Soft Polar Semiconductor

Electronic excitations in solids are commonly described within a hierarchy in which the excitonic Hamiltonian is defined first and the lattice acts later through renormalization, relaxation, and dephasing. This picture assumes that the optically accessible excitonic manifold is already present at the moment of photoexcitation. Here we show that this assumption fails in a soft polar semiconductor. Using femtosecond coherent multidimensional spectroscopy on lead-halide perovskite nanocrystals, we observe quantum back-action between an electronic excitation and a collective lattice-polarization field that expands the excitonic Hilbert space in real time. The optical pulse first prepares an excitonic polarization, X1. A second configuration, X2, emerges only after the polaron field develops, while coherent X1-X2 coupling appears at later times. State formation and coherence formation are therefore resolved as distinct stages of quasiparticle formation. In contrast, CdSe quantum dots exhibit the conventional limit in which excitonic states and couplings are present at time zero and are only weakly perturbed by phonons. The observed diagonal and anti-diagonal splittings increase with nanocrystal size and correlate with radiative oscillator strength, opposite to expectations from simple quantum confinement. A dynamical polaron-field model describes the lattice polarization as an order parameter that expands the optically accessible manifold and generates time-dependent coherent coupling. These results show that strong system-bath coupling can actively create excitonic states and the coherent manifold in which they evolve.

cond-mat.mes-hall↗

arXiv2026

Layer-dependent Landé $g$-factors of electrons, holes, and excitons in two-dimensional Ruddlesden-Popper lead halide perovskites

Two-dimensional Ruddlesden-Popper lead halide perovskites provide a valuable platform for tailoring charge and spin properties through quantum confinement and reduced symmetry. While the electron and hole Landé $g$-factors in bulk lead halide perovskites exhibit a universal dependence on the band gap energy, their evolution in two-dimensional perovskites has remained largely unexplored. Here, the Zeeman splittings of electrons and holes in (PEA)$_2$MA$_{n-1}$Pb$_n$I$_{3n+1}$ perovskites with the number of inorganic layers ovarying in the range $n=1,...,8$ are measured by means of the spin-flip Raman scattering and time-resolved Kerr rotation magneto-optical techniques. A systematic evolution of the electron and hole $g$-factors with decreasing layer thickness, which deviates from the universal bulk behavior and reveals confinement-driven trends similar to those observed in perovskite nanocrystals, is found. The experimental results are in good qualitative agreement with empirical tight-binding calculations. The exciton $g$-factors are evaluated from the Zeeman splittings of the exciton resonances in reflectivity measured in pulsed magnetic fields up to 55~T. These results provide comprehensive insight into the spin properties of two-dimensional lead halide perovskites and establish them as a tunable platform for engineering spin-dependent phenomena in quantum-confined semiconductors.

cond-mat.other↗

arXiv2026

Millisecond spin relaxation times of distinct electron and hole subensembles in MA$_x$FA$_{1-x}$PbI$_3$ perovskite crystals

The unique combination of outstanding optical quality and attractive spin properties opens new avenues for optical spin control in hybrid organic-inorganic perovskite semiconductors. Using the optically detected magnetic resonance technique, we study the spins of electrons and holes in mixed-cation MA$_x$FA$_{1-x}$PbI$_3$ single crystals with $x = 0.4$ and 0.8. Multiple distinct spin subensembles with $g$-factors spanning from 2.9 to 3.6 for electrons and from 0.5 to 1.2 for holes are resolved, revealing diverse localization environments. We measure the longitudinal spin relaxation times, $T_1$, reaching 2 ms and remaining in the $μ$s range even for weakly localized carriers at the cryogenic temperature of 1.6 K. The magnetic-field dependence of $T_1$ is dominated by the random nuclear (Overhauser) fields with strengths of $\sim 0.4-0.8$ mT for electrons and $\sim 4-12$ mT for holes, corresponding to $μ$s-long correlation times of the hyperfine field determined by carrier hopping between shallow localization sites. The temperature dependence of $T_1$ reveals a weak localization potential of the charge carriers and shows a correlation between $T_1$ and the inhomogeneity of the spin ensemble. These results establish mixed-A-site perovskite single crystals as a promising solid-state platform with long-lived spin states for quantum information applications.

cond-mat.mes-hall↗

arXiv2026

Bright-dark exciton splitting in lead halide perovskite crystals accessed via quantum beats in photon echoes

Understanding the fine structure of excitons is crucial for optoelectronic and quantum photonic applications of lead halide perovskites. It is demonstrated that polarization-sensitive photon echo spectroscopy in magnetic field provides a powerful method to access coherent exciton dynamics and reveal their energy level structure, which is hidden by inhomogeneous broadening. Exciton quantum beats observed in both Faraday and Voigt geometries offer a precise probe of the energy splittings among the four 1$s$ exciton states, enabling determination of the fine structure and bright-dark splittings. Application of this technique to bulk mixed halide perovskite crystals FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ reveals a bright-dark exciton splitting of $Δ_\mathrm{X}=0.46~$meV, along with electron and hole Landé $g$ factors $g_\mathrm{e}=3.38$ and $g_\mathrm{h}=-1.14$, respectively. The quantum beats persist on timescales of 20--50$~$ps, demonstrating remarkably robust spin and optical coherences at cryogenic temperature of 2$~$K. The decay of the quantum beats of the outer doublet is governed by dephasing due to dispersion of the bright-dark splitting of $\sim0.06~$meV caused by localization potential fluctuations, while dephasing in the bright exciton inner doublet originates from a small zero field splitting of $\sim0.035~$meV due to anisotropic potentials.

cond-mat.mes-hall↗

arXiv2026

Hyperfine interaction of electrons and holes with nuclei probed by optical orientation in MAPbI$_3$ perovskite crystals

Optical orientation of electron and hole spins by circularly polarized light is investigated for MAPbI$_3$ single crystals. The Hanle and polarization recovery effects measured in transverse and longitudinal magnetic fields, respectively, evidence the hyperfine interaction with nuclear spins as the main factor determining the spin dynamics of charge carriers at cryogenic temperatures. The parameters of the nuclear spin fluctuations within the carrier localization volume are evaluated. Dynamic polarization of the nuclear spins is demonstrated by the Overhauser field reaching 5 mT for acting on the electrons and -30 mT for acting on the holes.

cond-mat.mes-hall↗

arXiv2026

Millisecond spin coherence of electrons in semiconducting perovskites revealed by spin mode locking

Long spin coherence times of carriers are essential for implementing quantum technologies using semiconductor devices for which, however, a possible obstacle is spin relaxation. For the spin dynamics, decisive features are the band structure, crystal symmetry, and quantum confinement. Perovskite semiconductors recently have come into focus of studies of their spin states, notivated by efficient optical access and potentially long-living coherence. Here, we report an electron spin coherence time $T_2$ of the order of 1 ms, measured for a bulk FA$_{0.95}$Cs$_{0.05}$PbI$_3$ lead halide perovskite crystal. Using periodic laser pulses, we synchronize the electron spin Larmor precession about an external magnetic field in an inhomogeneous ensemble, the effect known as spin mode locking. It appears as a decay of the optically created ensemble spin polarization within the dephasing time $T_2^*$ of up to 20 ns and its revival during the spin coherence time $T_2$ reaching the millisecond range. This exceptionally long spin coherence time in a bulk crystal is complemented by millisecond-long longitudinal spin relaxation times $T_1$ for electrons and holes, measured by optically-detected magnetic resonance. These long-lasting spin dynamics highlight perovskites as promising platform for the quantum devices with all-optical control.

cond-mat.mtrl-sci↗

arXiv2025

Time-Reversed Superfluorescence in a Polaronic Quantum Material

Superfluorescence, the cooperative burst of spontaneous emission from an ensemble of dipoles, arises when microscopic oscillators spontaneously synchronize their phases. Here we show that this process can be reversed in time within quantum materials. Coherent multidimensional spectroscopy of halide perovskite quantum dots reveals a delayed cooperative absorption burst, the mirror image of superfluorescent emission, driven by transient polaron fields that phase-lock unit-cell dipoles within 100 fs. The effect scales systematically with quantum-dot size and halide composition, reaching near-unity coherence fidelity even at 300 K. A microscopic exciton-polaron model reproduces the buildup and decay of the coherent state, identifying lattice polarons as the mediators of synchronization. These results demonstrate that many-body temporal coherence can self-organize and persist at room temperature, opening routes toward engineered collective optical states and superabsorbing quantum devices.

cond-mat.mtrl-sci↗

arXiv2025

Landau Polarons as Generators of Quantum-Coherent States

Since Landau's theory, polarons have been understood as quasiparticles in which charges are dressed by the lattice field, yet decades of transport and spectroscopic studies have yielded only static indirect renormalizations. Whether such dressing can dynamically reorganize electronic spectra to generate new quantum-coherent states has remained unresolved. Here we use femtosecond coherent multidimensional spectroscopy on size and composition controlled perovskite quantum dots to track polaronic field-induced dynamics in real time, revealing their consequences. We observe a delayed condensation into a confined spectrum of coherent states on 50-150 fs timescales, with couplings between these states evolving dynamically on the same timescale. The splittings are robust, exhibit anomalous linear size dependence, exceed single-particle splittings and manifest at 300 K. A Raman-constrained spin-boson Hamiltonian captures both the anomalous scaling and dynamical onset, establishing polarons as generators of coherent manifolds that enable collective quantum phenomena including superradiance, superfluorescence and superabsorption.

cond-mat.mes-hall↗

arXiv2025

The Comparison of Colloidal PbS QD Photoconductors and Hybrid Phototransistors

The simplicity in the fabrication of photoconductors makes them a valuable choice to investigate optoelectronic properties of colloidal quantum dot (cQD) films. Lateral photoconductors generally require a large size, in the mm2, and are limited in operation speed due to the presence of trapping sites. In contrast, hybrid phototransistors are fabricated in the um2 scale and benefit from such trapping sites, allowing the measurement of low light levels in the nW/cm2. The question, however, arises whether high responsivity values are required for the detection of low light levels or the compatible detectivity of photoconductors is sufficient. Here, we directly compare photoconductors and hybrid phototransistors with an identical EDT-treated PbS cQD film. We highlight that a comparable D* is not enough for the purpose of measuring low light levels, as the resulting photocurrents need to be readily accessible. Furthermore, we also showcase temperature-activated photocurrent dynamics resulting in a negative photocurrent (NPC) effect. This NPC simultaneously improves the frequency bandwidth and photocurrent, enabling operation speeds up to 100 kHz.

cond-mat.mtrl-sci↗

arXiv2025

Purcell-enhanced single-photon generation from CsPbBr$_3$ quantum dots in in-situ selected Laguerre-Gaussian modes

Single photons in Laguerre-Gaussian (LG) beams, which carry orbital angular momentum (OAM), could enable more robust and efficient photonic quantum communication and information processing, as well as enhanced sensitivity in quantum metrology and imaging. However, as most implementations are indirect or require additional mode-shaping elements, direct generation of single photons with OAM has received growing interest. Colloidal lead halide perovskite quantum dots (QDs) have recently emerged as a versatile material that can produce indistinguishable single photons quasi-deterministically at high rate. Here, we integrate single CsPbBr$_3$ QDs into an open Fabry-Perot microcavity with a nanofabricated Gaussian-shaped deformation, demonstrating Purcell-enhanced single-photon generation into individual cavity modes with up to 18.1 $\pm$ 0.2 times accelerated decay, down to tens of picoseconds. By in-situ tuning of the cavity resonance, we can selectively couple a single QD to different LG modes carrying OAM and observe the spatial patterns of the generated single-photon beams emitted from the cavity. Our findings open the door to high photon rate sources that directly generate single-photon LG beams for advanced quantum photonic applications.

physics.optics↗

arXiv2025

Radiation-Triggered Superfluorescent Scintillation in Quantum-Ordered Perovskite Nanocrystal Superlattices

Superfluorescence, a cooperative emission phenomenon arising from the coherent coupling of excited dipoles, has historically been observed under optical excitation in carefully engineered quantum systems. Here, we report the first observation of superfluorescence triggered by ionizing radiation in lead-halide perovskite nanocrystal (NC) superlattices. Using CsPbBr3 NC superlattices with long-range structural and electronic order, we demonstrate that secondary electrons generated by high-energy photons can induce efficient cooperative emission bursts characteristic of superfluorescence with unprecedented scintillation lifetime of ~40 ps, thereby introducing a new class of coherent scintillating metamaterials. Side-by-side optical and scintillation measurements reveal a direct analogy between ionizing and intense optical excitation, both leading to high excitonic densities that result in superfluorescent emission, even at mild, technologically accessible cryogenic temperatures. The discovery that incoherent, stochastic ionization cascades can seed coherent many-body optical responses with radiatively accelerated luminescence and large Stokes shifts establishes a pathway toward ultrafast, reabsorption-free, quantum-ordered nanotechnological scintillators, paving the way for the future development of radiation detectors based on quantum technologies for advanced radiation detection applications.

physics.optics↗

arXiv2025

Deterministic Integration of CsPbBr3 Quantum Dots with Plasmonic Ring Microcavities

Perovskite quantum dots hold great promise for quantum information processing as wavelength-tunable single photon sources operable over a broad temperature range. However, their deterministic integration into nanophotonic structures remains a major challenge, limited by their random spatial distribution and non-directional emission. In this work, we employ a two-step electron beam lithography process to deterministically place CsPbBr3 quantum dots within the mode volume of plasmonic ring microcavities. Simulations predict strong field enhancement within the cavity, boosting photon emission rates via the Purcell effect and improving the quantum efficiency of the emitters. Experimentally, coupling ensembles of CsPbBr3 quantum dots to the cavities results in a four-fold enhancement in photoluminescence intensity and a three-fold reduction in fluorescence lifetime at room temperature. Single-emitter coupling is further investigated at cryogenic temperatures, leading to a two-fold reduction in radiative lifetime. These results demonstrate a scalable approach for the integration of perovskite quantum dots into nanophotonic cavities and quantum photonic circuits.

physics.optics↗

arXiv2025

Resonant spin amplification and accumulation in MAPbI$_3$ single crystals

Quantum technologic and spintronic applications require reliable semiconducting materials that enable a significant, long-living spin polarization of electronic excitations and offer the ability to manipulate it optically in an external field. Due to the specifics of band structure and remarkable spin-dependent properties, the lead halide perovskite semiconductors are suitable candidates for that. Here, the carrier spin dynamics in a MAPbI$_3$ (MA = methylammonium) perovskite single crystal with thickness of 20 $μ$m are studied by the time-resolved Kerr ellipticity technique at cryogenic temperatures. Long times of longitudinal electron spin relaxation $T_1 = 30$ ns and transverse electron spin dephasing $T_{2,e}^*=21$ ns are found. The spin dynamics lasting longer than the applied laser pulse repetition period give rise to spin accumulation effects. We exploit them through the resonant spin amplification, polarization recovery, and spin inertia techniques to study the electron and hole spin systems coupled with the nuclear spins. These results establish the lead halide perovskite semiconductors as suitable platform for quantum technologies relying on spin-dependent phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Optical orientation of excitons and charged carriers in MAPbI$_3$ perovskite single crystals in the orthorhombic phase

Optical orientation of exciton and carrier spins by circularly polarized light is the basic phenomenon in the spin physics of semiconductors. Here, we investigate spin orientation in MAPbI3 lead halide perovskite crystals at the cryogenic temperature of 1.6 K, where the material has an orthorhombic crystal structure. The recombination and spin dynamics of excitons and carriers are measured by time-resolved photoluminescence after circularly polarized excitation. The optical orientation of excitons reaches 85\%, which persists within their lifetime of 15-80 ps. This high orientation is maintained for excitation laser detunings from the exciton resonance to higher energies by up to 0.3 eV, then decreases and vanishes above 1.5 eV detuning. This indicates that the Dyakonov-Perel spin relaxation mechanism based on inversion symmetry breaking is inactive in MAPbI3 crystals with orthorhombic symmetry. The optical orientation of localized and spatially-separated electrons and holes results in 40\% circular polarization of their emission. Their contributions can be identified from the complex spin beats dynamics in transverse magnetic field. The dynamics analysis gives values of the Landé g-factors of 2.83 for electrons and 0.54 for holes. Also, the magnetic-field-induced polarization of excitons and carriers is analyzed in magnetic fields up to 6 T, showing that their spin relaxation times are longer than their lifetimes. Namely, for the excitons, the spin relaxation time exceeds the lifetime by a factor of 6. We model the dynamics of optical orientation degree for cumulative contributions of excitons and carriers and show that the exciton recombination dynamics can control these dynamics. The polarized emission of excitons and localized carriers, produced by their polarization on Zeeman-split levels in magnetic fields, is modeled.

cond-mat.mes-hall↗

arXiv2025

Coherent spin dynamics of electrons and holes photogenerated with large kinetic energy in lead halide perovskite crystals

The coherent spin dynamics of electrons and holes are studied in a FA0.9Cs0.1PbI2.8Br0.2 perovskite bulk crystal, using time-resolved Kerr ellipticity in a two-color pump-probe scheme. The probe photon energy is tuned to the exciton resonance, while the pump photon energy is detuned from it up to 0.75 eV to higher energies. The spin-oriented electrons and holes photogenerated with significant excess kinetic energy relax into states in vicinity of the band gap, where they undergo Larmor precession in an external magnetic field. At cryogenic temperatures down to 1.6 K, the spin dephasing time reaches the nanosecond range. During energy relaxation, the carrier spin relaxation is inefficient and only happens when the carriers become localized. In experiments with two pump pulses, all-optical control of the amplitudes and phases of the electron and hole spin signals is achieved in the additive regime by varying the intensities of the pump pulses and the time delay between them.

cond-mat.mes-hall↗

arXiv2024

Ultrafast Superradiant Scintillation from Weakly Confined CsPbBr3 Nanocrystals

Efficiency and emission rate are two traditionally conflicting parameters in radiation detection, and achieving their simultaneous maximization could significantly advance ultrafast time-of-flight (ToF) technologies. In this study, we demonstrate that this goal is attainable by harnessing the giant oscillator strength (GOS) inherent to weakly confined perovskite nanocrystals, which enables superradiant scintillation under mildly cryogenic conditions that align seamlessly with ToF technologies. We show that the radiative acceleration due to GOS encompasses both single and multiple exciton dynamics arising from ionizing interactions, further enhanced by suppressed non-radiative losses and Auger recombination at 80 K. The outcome is ultrafast scintillation with 420 ps lifetime and light yield of ~10'000 photons/MeV for diluted NC solutions, all without non-radiative losses. Temperature-dependent light-guiding experiments on test-bed nanocomposite scintillators finally indicate that the light-transport capability remains unaffected by the accumulation of band-edge oscillator strength due to GOS. These findings suggest a promising pathway toward developing ultrafast nanotech scintillators with optimized light output and timing performance.

physics.optics↗