arXiv subjects

V. Dyakonov

Publications and source records attributed to V. Dyakonov.

At least 19 recordsLinked to original sources

arXiv2026

Structural phase transitions in double perovskite crystals studied by Brillouin light scattering

Inorganic lead-free double perovskites represent particular interest as non-toxic and stable material platform for optoelectronic applications. Here, we employ Brillouin light scattering spectroscopy to investigate the elastic properties and structural phase transitions in single crystals of Cs2AgBiBr6 and Cs2AgBiCl6. A complete set of elastic constants is determined from the Brillouin scattering measurements performed along three different crystallographic directions. Both materials exhibit similar elastic constants and weak elastic anisotropy in the cubic phase. At low temperatures, the lifting of degeneracy of transverse acoustic phonon modes is attributed to a lowering of crystal symmetry. From the temperature dependence of the acoustic phonon frequencies, we determine the structural phase transition temperature of about 43 K for Cs2AgBiCl6, compared to 122 K for the cubic-to-tetragonal phase transition in Cs2AgBiBr6.

cond-mat.mtrl-sci↗

arXiv2023

Ultralong-term high-density data storage with atomic defects in SiC

There is an urgent need to increase the global data storage capacity, as current approaches lag behind the exponential growth of data generation driven by the Internet, social media and cloud technologies. In addition to increasing storage density, new solutions should provide long-term data archiving that goes far beyond traditional magnetic memory, optical disks and solid-state drives. Here, we propose a concept of energy-efficient, ultralong, high-density data archiving based on optically active atomic-size defects in a radiation resistance material, silicon carbide (SiC). The information is written in these defects by focused ion beams and read using photoluminescence or cathodoluminescence. The temperature-dependent deactivation of these defects suggests a retention time minimum over a few generations under ambient conditions. With near-infrared laser excitation, grayscale encoding and multi-layer data storage, the areal density corresponds to that of Blu-ray discs. Furthermore, we demonstrate that the areal density limitation of conventional optical data storage media due to the light diffraction can be overcome by focused electron-beam excitation.

cond-mat.mtrl-sci↗

arXiv2023

Weak dispersion of exciton Landé factor with band gap energy in lead halide perovskites: Approximate compensation of the electron and hole dependences

The photovoltaic and optoelectronic properties of lead halide perovskite semiconductors are controlled by excitons, so that investigation of their fundamental properties is of critical importance. The exciton Landé or g-factor g_X is the key parameter, determining the exciton Zeeman spin splitting in magnetic fields. The exciton, electron and hole carrier g-factors provide information on the band structure, including its anisotropy, and the parameters contributing to the electron and hole effective masses. We measure g_X by reflectivity in magnetic fields up to 60 T for lead halide perovskite crystals. The materials band gap energies at a liquid helium temperature vary widely across the visible spectral range from 1.520 up to 3.213 eV in hybrid organic-inorganic and fully inorganic perovskites with different cations and halogens: FA_{0.9}Cs_{0.1}PbI_{2.8}Br_{0.2], MAPbI_{3}, FAPbBr_{3}, CsPbBr_{3}, and MAPb(Br_{0.05}Cl_{0.95})_{3}. We find the exciton g-factors to be nearly constant, ranging from +2.3 to +2.7. Thus, the strong dependences of the electron and hole g-factors on the band gap roughly compensate each other when combining to the exciton g-factor. The same is true for the anisotropies of the carrier g-factors, resulting in a nearly isotropic exciton g-factor. The experimental data are compared favorably with model calculation results.

cond-mat.mes-hall↗

arXiv2022

Spin dynamics of electrons and holes interacting with nuclei in MAPbI$_3$ perovskite single crystals

Methylammonium lead triiodine (MAPbI$_3$) is a material representative of the hybrid organic-inorganic lead halide perovskites which attract currently great attention due to their photovoltaic efficiency and bright optoelectronic properties. Here, the coherent spin dynamics of charge carriers and spin dependent phenomena induced by the carrier interaction with nuclear spins are studied in MAPbI$_3$ single crystals, using time-resolved Kerr rotation at cryogenic temperatures in magnetic fields up to 3 T. Spin dephasing times up to a few nanoseconds and a longitudinal spin relaxation time of 37 ns are measured. The Larmor spin precession of both resident electrons and holes is identified in the Kerr rotation signals. The Landé factors ($g$-factors) in the orthorhombic crystal phase show a strong anisotropy, ranging for the holes from $-0.28$ to $-0.71$ and for the electrons from $+2.46$ to $+2.98$, while the $g$-factor dispersion of about 1% is rather small. An exciton $g$-factor of $+2.3$ is measured by magneto-reflectivity. A dynamic nuclear polarization by means of spin polarized electrons and holes is achieved in tilted magnetic fields giving access to the carrier-nuclei exchange interaction and the nuclei spin relaxation time exceeding 16 minutes.

cond-mat.mtrl-sci↗

arXiv2021

The Landé factors of electrons and holes in lead halide perovskites: universal dependence on the band gap

The Landé or $g$-factors of charge carriers are decisive for the spin-dependent phenomena in solids and provide also information about the underlying electronic band structure. We present a comprehensive set of experimental data for values and anisotropies of the electron and hole Landé factors in hybrid organic-inorganic (MAPbI$_3$, MAPb(Br$_{0.5}$Cl$_{0.5}$)$_3$, MAPb(Br$_{0.05}$Cl$_{0.95}$)$_3$, FAPbBr$_3$, FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$) and all-inorganic (CsPbBr$_3$) lead halide perovskites, determined by pump-probe Kerr rotation and spin-flip Raman scattering in magnetic fields up to 10~T at cryogenic temperatures. Further, we use first-principles DFT calculations in combination with tight-binding and $\mathbf k \cdot \mathbf p$ approaches to calculate microscopically the Landé factors. The results demonstrate their universal dependence on the band gap energy across the different perovskite material classes, which can be summarized in a universal semi-phenomenological expression, in good agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2021

Photon echo polarimetry of excitons and biexcitons in a CH$_3$NH$_3$PbI$_3$ perovskite single crystal

Lead halide perovskites show remarkable performance when used in photovoltaic and optoelectronic devices. However, the peculiarities of light-matter interactions in these materials in general are far from being fully explored experimentally and theoretically. Here we specifically address the energy level order of optical transitions and demonstrate photon echos in a methylammonium lead triiodide single crystal, thereby determining the optical coherence times $T_2$ for excitons and biexcitons at cryogenic temperature to be 0.79 ps and 0.67 ps, respectively. Most importantly, we have developed an experimental photon-echo polarimetry method that not only identifies the contributions from exciton and biexciton complexes, but also allows accurate determination of the biexciton binding energy of 2.4 meV, even though the period of quantum beats between excitons and biexcitons is much longer than the coherence times of the resonances. Our experimental and theoretical analysis methods contribute to the understanding of the complex mechanism of quasiparticle interactions at moderate pump density and show that even in high-quality perovskite crystals and at very low temperatures, inhomogeneous broadening of excitonic transitions due to local crystal potential fluctuations is a source of optical dephasing.

cond-mat.other↗

arXiv2021

Nongeminate and Geminate Recombination in PTB7:PC$_{71}$BM solar cells

A combination of transient photovoltage (TPV), voltage dependent charge extraction (CE) and time delayed collection field (TDCF) measurements is applied to poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-phenyl-C71-butyric acid (PC$_{71}$BM) bulk heterojunction solar cells to analyze the limitations of photovoltaic performance. Devices are processed from pure chlorobenzene (CB) solution and a subset was optimized with 1,8-diiodooctane (DIO) as co-solvent. The dramatic changes in device performance are discussed with respect to the dominating loss processes. While in the devices processed from CB solution, severe geminate and nongeminate recombination is observed, the use of DIO facilitates efficient polaron pair dissociation and minimizes geminate recombination. Thus, from the determined charge carrier decay rate under open circuit conditions and the voltage dependent charge carrier densities $n(V)$, the nongeminate loss current $j_{loss}$ of the samples with DIO alone enables us to reconstruct the current/voltage ($j/V$) characteristics across the whole operational voltage range. Geminate and nongeminate losses are considered to describe the $j/V$ response of cells prepared without additive, but lead to a clearly overestimated device performance. We attribute the deviation between measured and reconstructed $j/V$ characteristics to trapped charges in isolated domains of pure fullerene phases.

physics.app-ph↗

arXiv2021

Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells

We investigate nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C$_{60}$. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. We apply a combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density. In organic solar cells, charge photogeneration and recombination primarily occur at the donor--acceptor heterointerface. Whereas the BHJ can often be approximated by an effective medium due to rather small scale phase separation, the PHJ has a well defined two-dimensional heterointerface. To study recombination dynamics in PHJ devices most relevant is the charge accumulation at this interface. As from extraction techniques only the spatially averaged carrier concentration can be determined, we derive the charge carrier density at the interface $n_{int}$ from the open circuit voltage. Comparing the experimental results with macroscopic device simulation we discuss the differences of recombination and charge carrier densities in CuPc:C$_{60}$ PHJ and BHJ devices with respect to the device performance. The open circuit voltage of BHJ is larger than for PHJ at low light intensities, but at 0.3 sun the situation is reversed: here, the PHJ can finally take advantage of its generally longer charge carrier lifetimes, as the active recombination region is smaller.

physics.app-ph↗

arXiv2019

Influence of irradiation on defect spin coherence in silicon carbide

Irradiation-induced lattice defects in silicon carbide (SiC) have already exceeded their previous reputation as purely performance-inhibiting. With their remarkable quantum properties, such as long room-temperature spin coherence and the possibility of downscaling to single-photon source level, they have proven to be promising candidates for a multitude of quantum information applications. One of the most crucial parameters of any quantum system is how long its quantum coherence can be preserved. By using the pulsed optically detected magnetic resonance (ODMR) technique, we investigate the spin-lattice relaxation time ($T_1$) and spin coherence time ($T_2$) of silicon vacancies in 4H-SiC created by neutron, electron and proton irradiation in a broad range of fluences. We also examine the effect of irradiation energy and sample annealing. We establish a robustness of the $T_1$ time against all types of irradiation and reveal a universal scaling of the $T_2$ time with the emitter density. Our results can be used to optimize the coherence properties of silicon vacancy qubits in SiC for specific tasks.

cond-mat.mtrl-sci↗

arXiv2019

Removing Leakage and Surface Recombination in Planar Perovskite Solar Cells

Thin-film solar cells suffer from various types of recombination, of which leakage current usually dominates at lower voltages. Herein, we demonstrate first a three-order reduction of the shunt loss mechanism in planar methylammonium lead iodide perovskite solar cells by replacing the commonly used hole transport layer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with a better hole-selective polyarylamine. As a result, these cells exhibit superior operation under reduced light conditions, which we demonstrate for the extreme case of moonlight irradiance, at which open-circuit voltages of 530 mV can still be obtained. By the shunt removal we also observe the VOC to drop to zero after as long as 2 h after the light has been switched off. Second, at higher illumination intensities the dominant losses in the PEDOT:PSS-based cell are ascribed to surface recombination and are also proven to be substantially minimized by instead employing the polyarylamine. We attribute the reduced shunt and surface recombination to the far better suited semiconductor character of the polyarylamine, compared to that of PEDOT:PSS, efficiently blocking electrons from recombining at this electrode.

physics.app-ph↗

arXiv2018

Excitation and coherent control of spin qudit modes with sub-MHz spectral resolution

Quantum bit or qubit is a two-level system, which builds the foundation for quantum computation, simulation, communication and sensing. Quantum states of higher dimension, i.e., qutrits (D = 3) and especially qudits (D = 4 or higher), offer significant advantages. Particularly, they can provide noise-resistant quantum cryptography, simplify quantum logic and improve quantum metrology. Flying and solid-state qudits have been implemented on the basis of photonic chips and superconducting circuits, respectively. However, there is still a lack of room-temperature qudits with long coherence time and high spectral resolution. The silicon vacancy centers in silicon carbide (SiC) with spin S = 3/2 are quite promising in this respect, but until now they were treated as a canonical qubit system. Here, we apply a two-frequency protocol to excite and image multiple qudit modes in a SiC spin ensemble under ambient conditions. Strikingly, their spectral width is about one order of magnitude narrower than the inhomogeneous broadening of the corresponding spin resonance. By applying Ramsey interferometry to these spin qudits, we achieve a spectral selectivity of 600 kHz and a spectral resolution of 30 kHz. As a practical consequence, we demonstrate absolute DC magnetometry insensitive to thermal noise and strain fluctuations.

quant-ph↗

arXiv2017

Highly efficient optical pumping of spin defects in silicon carbide for stimulated microwave emission

We investigate the pump efficiency of silicon vacancy-related spins in silicon carbide. For a crystal inserted into a microwave cavity with a resonance frequency of 9.4 GHz, the spin population inversion factor of 75 with the saturation optical pump power of about 350 mW is achieved at room temperature. At cryogenic temperature, the pump efficiency drastically increases, owing to an exceptionally long spin-lattice relaxation time exceeding one minute. Based on the experimental results, we find realistic conditions under which a silicon carbide maser can operate in continuous-wave mode and serve as a quantum microwave amplifier.

cond-mat.mtrl-sci↗

arXiv2017

Spin and optical properties of silicon vacancies in silicon carbide (a review)

We discuss the fine structure and spin dynamics of spin-3/2 centers associated with silicon vacancies in silicon carbide. The centers have optically addressable spin states which makes them highly promising for quantum technologies. The fine structure of the spin centers turns out to be highly sensitive to mechanical pressure, external magnetic and electric fields, temperature variation, etc., which can be utilized for efficient room-temperature sensing, particularly by purely optical means or through the optically detected magnetic resonance. We discuss the experimental achievements in magnetometry and thermometry based on the spin state mixing at level anticrossings in an external magnetic field and the underlying microscopic mechanisms. We also discuss spin fluctuations in an ensemble of vacancies caused by interaction with environment.

cond-mat.mes-hall↗

arXiv2016

Locking of electron spin coherence over fifty milliseconds in natural silicon carbide

We demonstrate that silicon carbide (SiC) with natural isotope abundance can preserve a coherent spin superposition in silicon vacancies over unexpectedly long time approaching 0.1 seconds. The spin-locked subspace with drastically reduced decoherence rate is attained through the suppression of heteronuclear spin cross-talking by applying a moderate magnetic field in combination with dynamic decoupling from the nuclear spin baths. We identify several phonon-assisted mechanisms of spin-lattice relaxation, ultimately limiting quantum coherence, and find that it can be extremely long at cryogenic temperature, equal or even longer than 8 seconds. Our approach may be extended to other polyatomic compounds and open a path towards improved qubit memory for wafer-scale quantum techmologies.

cond-mat.mtrl-sci↗

arXiv2016

Optical thermometry based on level anticrossing in silicon carbide

We report a giant thermal shift of $2.1 \,$MHz/K related to the excited-state zero-field splitting in the silicon vacancy centers in 4H silicon carbide. It is obtained from the indirect observation of the optically detected magnetic resonance in the excited state using the ground state as an ancilla. Alternatively, relative variations of the zero-field splitting for small temperature differences can be detected without application of radiofrequency fields, by simply monitoring the photoluminescence intensity in the vicinity of the level anticrossing. This effect results in an all-optical thermometry technique with temperature sensitivity of $100 \,$mK/Hz$^{1/2}$ for a detection volume of approximately $10^{-6} \,$mm$^{3}$. In contrast, the zero-field splitting in the ground state does not reveal detectable temperature shift. Using these properties, an integrated magnetic field and temperature sensor can be implemented on the same center.

cond-mat.mtrl-sci↗

arXiv2016

All-optical dc nanotesla magnetometry using silicon vacancy fine structure in isotopically purified silicon carbide

We uncover the fine structure of a silicon vacancy in isotopically purified silicon carbide (4H-$^{28}$SiC) and find extra terms in the spin Hamiltonian, originated from the trigonal pyramidal symmetry of this spin-3/2 color center. These terms give rise to additional spin transitions, which are otherwise forbidden, and lead to a level anticrossing in an external magnetic field. We observe a sharp variation of the photoluminescence intensity in the vicinity of this level anticrossing, which can be used for a purely all-optical sensing of the magnetic field. We achieve dc magnetic field sensitivity of 87 nT Hz$^{-1/2}$ within a volume of $3 \times 10^{-7}$ mm$^{3}$ at room temperature and demonstrate that this contactless method is robust at high temperatures up to at least 500 K. As our approach does not require application of radiofrequency fields, it is scalable to much larger volumes. For an optimized light-trapping waveguide of 3 mm$^{3}$ the projection noise limit is below 100 fT Hz$^{-1/2}$.

cond-mat.mtrl-sci↗

arXiv2015

High precision vector magnetometry with uniaxial quantum centers in silicon carbide

We show that uniaxial color centers in silicon carbide with hexagonal lattice structure can be used to measure not only the strength but also the polar angle of the external magnetic field with respect to the defect axis with high precision. The method is based on the optical detection of multiple spin resonances in the silicon vacancy defect with quadruplet ground state. We achieve a perfect agreement between the experimental and calculated spin resonance spectra without any fitting parameters, providing angle resolution of a few degrees in the magnetic field range up to several millitesla. Our approach is suitable for ensembles as well as for single spin-3/2 color centers, allowing for vector magnetometry on the nanoscale at ambient conditions.

cond-mat.mtrl-sci↗

arXiv2014

Room-temperature near-infrared silicon carbide nanocrystalline emitters based on optically aligned spin defects

Bulk silicon carbide (SiC) is a very promising material system for bio-applications and quantum sensing. However, its optical activity lies beyond the near infrared spectral window for in-vivo imaging and fiber communications due to a large forbidden energy gap. Here, we report the fabrication of SiC nanocrystals and isolation of different nanocrystal fractions ranged from 600 nm down to 60 nm in size. The structural analysis reveals further fragmentation of the smallest nanocrystals into ca. 10-nm-size clusters of high crystalline quality, separated by amorphization areas. We use neutron irradiation to create silicon vacancies, demonstrating near infrared photoluminescence. Finally, we detect, for the first time, room-temperature spin resonances of these silicon vacancies hosted in SiC nanocrystals. This opens intriguing perspectives to use them not only as in-vivo luminescent markers, but also as magnetic field and temperature sensors, allowing for monitoring various physical, chemical and biological processes.

cond-mat.mtrl-sci↗