arXiv subjects

K. N. Boldyrev

Publications and source records attributed to K. N. Boldyrev.

16 recordsLinked to original sources

arXiv2026

Terahertz optical activity near crystal field transitions of Tm3+ ions in magnetoelectric alumoborates

Crystal field (CF) excitations in the ground multiplet $^3H_6$ of Tm$^{3+}$ ions were investigated using terahertz transmission spectra of magnetoelectric TmAl$_3$(BO$_3$)$_4$ and Tm$_{0.05}$Yb$_{0.1}$Y$_{0.85}$Al$_3$(BO$_3$)$_4$. These excitations were identified as mainly magnetic dipole transitions from the ground singlet A$_1$ to the next excited doublet E, split by the crystal field of the D$_3$ symmetry. The fine structure of the modes was resolved at low temperatures. It manifested differently in lightly doped and in pure Tm borates, consistent with different distortions of the local crystal field with the D$_3$ symmetry. Strong natural optical activity was observed near the CF transitions resulting in a polarization plane rotation up to 25 degrees. The optical activity is quantitatively described by contributions of magnetic and electric dipole transitions to dynamic magnetoelectric susceptibility and taking into account the classification of local distortions.

cond-mat.mtrl-sci↗

arXiv2026

High-resolution spectroscopy of Pr3+ions in YAl3(BO3)4:Pr3+. Crystal-field, hyperfine,and electron-deformation interactions

Optical transmission spectra of YAl3(BO3)4 crystals doped with the Pr3+ ions in concentrations 1 and 2.5 at. % were studied by high-resolution (up to 0.05 cm-1) Fourier spectroscopy, including in magnetic field parallel to the trigonal c axis of the crystal. The g factors of several crystal-field doublets of Pr3+ were determined. The crystal-field calculations performed using the exchange-charge model and high-resolution spectroscopy data allowed us to obtain a physically reasonable set of crystal-field parameters. The observed splitting of a number of doublets in zero magnetic field is explained by the presence of random lattice deformations. Simulation of the profiles of observed deformational doublets was carried out taking into account both hyperfine and electron-deformation interactions. The width of the distribution function of random strains was estimated. The main sources of random strains in YAl3(BO3)4:Pr3+ crystals are discussed.

physics.optics↗

arXiv2024

Lattice dynamics and mixing of polar phonons in the rare-earth orthoferrite TbFeO$_{3}$

Rare-earth orthoferrites are a promising platform for antiferromagnetic spintronics with a rich variety of terahertz spin and lattice dynamics phenomena. For instance, it has been experimentally demonstrated that the light-driven optical phonons can coherently manipulate macroscopic magnetic states via nonlinear magnetophononic effects. Here using TbFeO$_{3}$ as an example, we reveal the origin of the mode mixing between the LO and TO phonons, which is important for understanding of nonlinear phononics. We performed a comprehensive study of the lattice dynamics of the TbFeO$_{3}$ single crystal by polarized infrared and Raman scattering spectroscopic techniques, and experimentally obtained and carefully analyzed the spectra of anisotropic complex dielectric functions in the far-infrared spectral range. This allowed us to reliably identify the symmetries and parameters of most infrared- and Raman-active phonons. Next, the experimental studies were supplemented by the lattice dynamics calculations which allowed us to propose the normal mode assignments. We reveal that the relation between LO and TO polar phonons is complex and does not strictly follow the ``LO-TO rule'' due to the strong mode mixing. We further analyze how displacements of different ions contribute to phonon modes and reveal that magnetic Fe ions are not involved in Raman-active phonons, thus shedding light on a lack of spin phonon coupling for such phonons. The obtained results establish a solid basis for further in-depth experimental research in the field of nonlinear phononics and magnetophononics in rare-earth orthoferrites.

cond-mat.mtrl-sci↗

arXiv2024

Polar phonons and magnetic excitations in the antiferromagnet CoF$_2$

The coupling between antiferromagnetic spins and infrared-active phonons in solids is responsible for many intriguing phenomena and is a field of intense research with extensive potential applications in the modern devices based on antiferromagnetic spintronics and phononics. Insulating rutile antiferromagnetic crystal CoF$_2$ is one of the model materials for studying nonlinear magnetophononics due to the strong spin-lattice coupling as a result of the orbitally degenerate ground state of Co$^{2+}$ ions manifested in the plethora of static and induced piezomagnetic effects. Here we report results on the complete infrared spectroscopy study of lattice and magnetic dynamics in CoF$_2$ in a wide temperature range and their careful analysis. We observed that infrared-active phonons demonstrate frequency shifts at the antiferromagnetic ordering. Furthermore, using first-principles calculations, we examined the lattice dynamics and disclosed that these frequency shifts are rather due to the spin-phonon coupling than geometrical lattice effects. Next we found that the low-frequency dielectric permittivity demonstrates distinct changes at the antiferromagnetic ordering due to the spontaneous magnetodielectric effect caused by the behavior of infrared-active phonons. In addition, we have observed magnetic excitations in the infrared spectra and identified their magnetodipole origin. To strengthen our conclusions, we analyze the theoretical phonon-magnon coupling overall phonons at the $Γ$ point. We conclude that the largest effect comes from the $A_{1g}$ and $B_{2g}$ Raman-active modes. As such, our results establish a solid basis for further investigations and more deeper understanding of the coupling of phonons with spins and magnetic excitations in antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2022

Anomalous behavior of the $E_u^1$ infrared-active phonon mode in a Bi$_{2-x}$Sr$_x$Se$_3$ crystal

We have studied spectral evolution of the $E_u^1$ phonon line of a topological insulator Bi$_{2-x}$Sr$_x$Se$_3$ with temperature. Unlike the Raman-active phonons, the $E_u^1$ mode demonstrates softening upon cooling the crystal, and the corresponding spectral line acquires a pronounced Fano-like shape at temperatures $T\lesssim100$ K. We interpret the latter effect as a signature of specific coupling of the bulk infrared active phonons to surface Dirac electrons. Using coherent resonant excitation of the $E_u^1$ mode as a surface sensitive tool, we have detected softening of the surface counterpart of the bulk $E_u^1$ phonon mode upon strontium doping. This observation can be an evidence of enhanced electron-phonon interaction at the surface of the Bi$_{2-x}$Sr$_x$Se$_3$ crystal.

cond-mat.mtrl-sci↗

arXiv2021

Incipient geometric lattice instability of cubic fluoroperovskites

Inorganic metal halide perovskites are promising materials for next-generation technologies due to a plethora of unique physical properties, many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic. Here, we experimentally show that cubic fluoroperovskites exhibit proximity to incipient ferroelectrics, which manifested in the softening of the low-frequency polar phonons in the Brillouin zone center at cooling. Furthermore, we reveal the coupling between harmonic and anharmonic force constants of the softening phonons and their correlation with the perovskite tolerance factor. Next, using first-principles calculations, we examine the lattice dynamics of the cubic fluoroperovskites and disclose the incipient lattice instability at which the harmonic force constants of low-lying phonons tend to decrease with a reduction of tolerance factor at all high-symmetry points of the Brillouin zone. The correlations with the tolerance factor indicate the geometric origin of observed incipient lattice instability in the cubic fluoroperovskites caused by the steric effect due to the volume filling of the unit cell by different radius ions. These results provide insights into the lattice dynamics and potential ferroelectric properties of inorganic lead-free metal halide perovskites, relevant to further design and synthesis of new multifunctional materials.

cond-mat.mtrl-sci↗

arXiv2019

Spatially controlled fabrication of single NV centers in IIa HPHT diamond

Single NV centers in HPHT IIa diamond are fabricated by helium implantation through lithographic masks. The concentrations of created NV centers in different growth sectors of HPHT are compared quantitatively. It is shown that the purest {001} growth sector (GS) of HPHT diamond allows to create groups of single NV centers in predetermined locations. The {001} GS HPHT diamond is thus considered a good material for applications that involve single NV centers.

physics.app-ph↗

arXiv2018

Coherent phonons in a $Bi_2Se_3$ film generated by an intense single-cycle THz pulse

We report an observation of coherent phonons of $E_g^1$, $E_u^1$, $A_{1g}^1$, and $E_g^2$ symmetry generated in a single-crystal film of $Bi_2Se_3$ by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the $E_g^2$ mode. The generation of $E_g^2$ phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear $E_u^1$ oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of $E_g^2$. The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of $E_g^1$ (1.1 THz) and $A_{1g}^1$ (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in $Bi_2Se_3$.

cond-mat.mtrl-sci↗

arXiv2017

Lattice dynamics and electronic transitions in a structurally-complex layered copper borate Cu$_3$(BO$_3$)$_2$

Copper borate Cu$_3$(BO$_3$)$_2$ is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper Cu$^{2+}$ ions occupy sixteen nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of 3$d^9$ copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low $P\overline{1}$ space group symmetry and a large unit cell with Z=10. Unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 cm$^{-1}$, which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at $T_N$=10 K thus evidencing magnetoelastic interaction. No new phonons were found below $T_N$, which excludes the spin-Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at $\sim$1.8 eV is observed, which we assign to overlapping of transitions between the 3$d^9$ states of Cu$^{2+}$ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around $\sim$2.8-3.0 eV.

cond-mat.mtrl-sci↗

arXiv2016

Spectroscopy of f-f transitions, crystal-field calculations, and magnetic and quadrupole helix chirality in DyFe$_3$(BO$_3$)$_4$

Recently, quadrupole helix chirality and its domain structure was observed in resonant x-ray diffraction experiments on DyFe$_3$(BO$_3$)$_4$ using circularly polarized x rays [T. Usui et al., Nature Materials 13, 611 (2014)]. We show that this effect can be explained quantitatively by calculating the quadrupole moments of the Dy$^{3+}$ ions induced by the low-symmetry ($C_2$) crystal-field (CF) component. In this work, the CF parameters for the Dy$^{3+}$ ions in the $P3_121$ ($P3_221$) phase of DyFe$_3$(BO$_3$)$_4$ are obtained from CF calculations based on the analysis of high-resolution temperature-dependent optical spectroscopy data. We also consider the helix chirality of the single-site magnetic susceptibility tensors of the Dy$^{3+}$ ions in the paramagnetic $P3_121$ ($P3_221$) phase and suggest a neutron diffraction experiment to reveal it.

cond-mat.mtrl-sci↗

arXiv2016

Bifurcations observed in the spectra of coupled electron-phonon modes in multiferroic $PrFe_3(BO_3)_4$ subjected to a magnetic field

We report on bifurcations effect mediated by the electron-phonon coupling in a concentrated rare-earth-containing antiferromagnet, observed in the spectra of coupled $4f$-electron-phonon modes under the influence of an external magnetic field. The effect was observed in the low-temperature far-infrared (terahertz) reflection spectra of a multiferroic easy-axis antiferromagnet $PrFe_3(BO_3)_4$ in magnetic fields $\textbf{B}_{ext}||c$. Both paramagnetic and magnetically ordered phases (including a spin-flop one) were studied in magnetic fields up to 30 T. We show that the field behavior of the coupled modes can be successfully explained and modeled on the base of the equation derived in the frame of the theory of coupled electron-phonon modes, with the same field-independent electron-phonon interaction constant $|W| = 14.8 cm^{-1}$.

cond-mat.mtrl-sci↗

arXiv2016

Bulk graphanes synthesized from benzene and pyridine

We report a high-pressure high-temperature synthesis of several millimeter-sized samples of bulk graphanes with the composition C-H(D) from benzene and graphene-derivative C-H-N$_{0.2}$ from pyridine. X-ray diffraction, transmission electron microscopy, and infrared spectroscopy of new materials reveal relatively large (several nanometers in size) crystalline grains of an sp3-bonded graphane lattice (3-cycle-4-step, the orthorhombic structure with Pbca space group and parameters a = 9.5-9.8, b = 8.9-9.1, c = 17.1-17.3 $Å$). The main hydrogen groups in samples are C-H groups connected by aliphatic bonds. The synthesized graphanes at atmospheric pressure are stable up to 500 C. The macroscopic density of CH samples is 1.5-1.57 g cm$^{-3} $ and the refractive index is 1.78-1.80. The absorption spectra of samples with a high degree of crystallization exhibits a weak absorption maximum at 2.8 eV, which is responsible for the yellow-orange color, large absorption maximum at 4 eV and an absorption edge associated with the width of the optical gap at 5.2 eV. The bulk modulus (30-37 GPa) and shear modulus (15-18 GPa) of the fabricated samples, as well as their hardness (1-1.5 GPa), are about twice as high as the respective values for polycrystalline graphite. The solution of metalorganic complexes in benzene and pyridine makes it possible to obtain doped graphanes, which can have extraordinary electron transport and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2016

Lattice dynamics and a magnetic-structural phase transition in the nickel orthoborate $Ni_{3}(BO_{3})_{2}$

Nickel orthoborate $Ni_{3}(BO_{3})_{2}$ having a complex orthorhombic structure $P_{nnm}$ (#58, Z=2) of the kotoite type is known for quite a long time as an antiferromagnetic material below $T_{N}$ = 46 K, but up to now its physical properties including the lattice dynamics have not been explored. Six magnetic nickel $Ni^{2+}$ ions (S=1) in the unit cell are distributed over the 2a and 4f positions in the centers of distorted $[O_{6}]$ octahedra. The $[NiO_{6}]$ units are linked via rigid $[BO_{3}]$ groups and these structural particularities impose restrictions on the lattice dynamics and spin-phonon interactions. We performed the symmetry analysis of the phonon modes at the center of the Brillouin zone. The structural parameters and phonon modes were calculated using Dmol3 program. We report and analyze results of infrared and Raman studies of phonon spectra measured in all required polarizations. Most of the even and odd phonons predicted on the basis of the symmetry analysis and theoretical calculations were reliably identified in the measured spectra. Absorption measurements in the infrared region showed emergence of several very narrow and weak phonons at the magnetic ordering temperature $T_{N}$. This observation proves the existence of a structural phase transition not reported before which is evidently coupled intrinsically with the magnetic dynamics of $Ni_{3}(BO_{3})_{2}$. A clear evidence of spin-phonon interaction was observed for some particular phonons below $T_{N}$.

cond-mat.mtrl-sci↗

arXiv2015

Luminescent Ge-related centre in high-pressure synthesized diamond

We report on the high-pressure synthesis of novel nano- and microcrystalline high-quality diamonds with luminescent Ge-related centers. Observation of the four-line fine structure in luminescence at 2 eV (602 nm) at temperatures below 80 K manifests a high quality of diamonds. We demonstrate germanium and carbon isotope shifts in the fine structure of luminescence at 602 nm and its vibrational sideband which allows us to unambiguously associate the center with the germanium impurity entering into the diamond lattice. We show that there are two ground-state energy levels with the separation of 0.7 meV and two excited-state levels separated by 4.6 meV in the electronic structure of the center and suggest a split-vacancy structure of this center.

cond-mat.mtrl-sci↗

arXiv2014

Antiferromagnetic dichroism and Davydov splitting of 3d-excitons in a complex multisublattice magnetoelectric CuB2O4

The space and time symmetry breaking at magnetic phase transitions in multiferroics results in a number of strongly pronounced optical effects. Our high-resolution spectroscopic study of 3d- excitons in a complex multi-sublattice magnetoelectric CuB2O4 demonstrates that, among those, a large antiferromagnetic linear dichroism is observed which is highly sublattice-sensitive to subtle changes in the spin subsystems. We prove that the discovered linear dichroism is related microscopically to the magnetic Davydov splitting of the exciton states. We announce a novel magnetic phase transition and argue that an elliptical spiral structure rather than a simple circular helix is realized in the incommensurate phase, these findings being overlooked in previous studies by optical and other techniques. We claim that this spectroscopic method can be effectively applied to other materials for revealing hidden features of magnetic structures and phase transitions.

cond-mat.mtrl-sci↗

arXiv2014

Splitting of a nondegenerate phonon mode in PrFe3(BO3)4: 4f crystal-field level between TO and LO phonon frequencies

A new effect originating from the crystal-field-excitation - phonon coupling was observed (in the far infrared spectra of a multiferroic PrFe3(BO3)4). The reststrahlen band corresponding to the A2 symmetry nondegenerate phonon mode near 50 cm-1 (1.5 THz) splits into two bands at about 100 K, well above TN = 32 K. These bands shift and narrow progressively with further lowering the temperature, demonstrating pronounced peculiarities at TN. The observed effects were explained by an interaction of the A2 phonon mode with the 4f crystal-field electronic excitation of Pr3+ whose frequency falls into the TO - LO frequency region of the phonon mode. Inversion of the TO and LO frequencies for the electronic excitation and a formation of coupled electron-phonon modes are discussed. Fitting of the TO frequency vs temperature experimental plots by theoretical curves revealed the value 14.6 cm-1 for the electron-phonon coupling constant.

cond-mat.mtrl-sci↗