arXiv subjects

Alexander I. Lebedev

Publications and source records attributed to Alexander I. Lebedev.

At least 19 recordsLinked to original sources

arXiv2026

Ferroelectricity and antiferroelectricity in the BaS-PbS system with the rocksalt structure

The ferroelectric instability in superstructures, superlattices, quantum wires, and disordered solid solutions in the BaS--PbS system with the NaCl structure has been discovered and investigated using first-principles calculations within the density functional theory. The emergence of ferroelectricity in these structures is associated with the instability of TO phonons in linear --Pb--S--Pb--S-- chains, which arises as a result of stretching of the structures upon the introduction of large barium atoms. Additionally, it has been discovered that, alongside ferroelectric phases, the structures also exhibit stable, competing antiferroelectric phases and those with a mixed ferroelectric--antiferroelectric ordering (ferroelectrically polarized one-dimensional Pb--S chains arranged in an ordered or disordered manner in the perpendicular direction). These phases often become the ground state of the studied systems. The closeness of the energies of the ferroelectric, antiferroelectric, and mixed states indicates the emergence of a multi-minimum potential with an infinite number of wells separated by potential barriers in the configuration space. This suggests a possible emergence of nonergodicity in the structures at low temperatures.

cond-mat.mtrl-sci↗

arXiv2026

Spontaneous structural reconstructions and properties of ultrathin triangular ZnSe nanoplatelets

Two-dimensional (2D) materials have revolutionized all areas of development of high-performance electronic devices. In particular, the unique electronic and optical properties of II--VI semiconductor nanoplatelets have been found to be very promising for optoelectronics. However, not all properties of this intriguing class of materials are yet known. A new, previously unknown hexagonal 2D structure of ZnSe nanoplatelets whose energy is lower than the energies of all previously studied systems is found from first-principles calculations. This structure appears as a result of spontaneous reconstruction of the wurtzite structure and differs from it by the stacking order of the bulk and near-surface Zn atomic layers. The phonon spectrum, electronic structure, and band gap of the obtained nanoplatelets are calculated. The phonon spectra of the nanoplatelets are in complete agreement with the spectra observed in experiment and differ strongly from the vibrational spectra of ZnSe nanoclusters. The adsorption of ZnCl$_2$ and $L$-cysteine molecules on the surface of the nanoplatelets is studied and is shown to be accompanied by yet another spontaneous reconstruction of the hexagonal structure into a tetragonal one and a new rearrangement of Zn atoms in the near-surface layers. Calculations of the natural optical activity of nanoplatelets covered with $L$-cysteine reveal an increase in the specific (calculated per chiral molecule) optical activity, which is especially strong for the Janus structures, as compared to the free $L$-cysteine molecule.

cond-mat.mtrl-sci↗

arXiv2024

Spontaneous strain in quasi-two-dimensional Janus CdSe nanoplatelets and its microscopic mechanisms

Spontaneous strain and spontaneous folding of thin nanoplatelets are known phenomena whose microscopic mechanisms are still debating. In this work, first-principles calculations are used to study the mechanical stresses that arise in Janus CdSe nanoplatelets and result in their spontaneous strain. Calculations reveal the existence of three microscopic mechanisms of this phenomenon. Two bulk mechanisms are associated with the inverse piezoelectric effect in an electric field created by the difference in electronegativities of ligands and by the depolarizing field resulting from the difference in the potential jumps in electrical double layers on the surfaces of nanoplatelets. These mechanisms account for 5-25% of the observed effect. The third mechanism is associated with the surface strain of nanoplatelets by bridging bonds, and its influence is predominant. It is shown that the latter mechanism cause spontaneous folding of thin CdSe nanoplatelets and, depending on the values of surface stresses and lateral orientation of nanoplatelets, can result in formation of their experimentally observed structures such as scrolls, spirals, and twisted ribbons.

cond-mat.mtrl-sci↗

arXiv2021

Raman and infrared studies of CdSe/CdS core/shell nanoplatelets

The vibrational spectroscopy of semiconductor nanostructures can provide important information on their structure. In this work, experimental Raman and infrared spectra are compared with vibrational spectra of CdSe/CdS core/shell nanoplatelets calculated from first principles using the density functional theory. The calculations confirm the two-mode behavior of phonon spectra of nanostructures. An analysis of the experimental spectra reveals the absence of modes with a high amplitude of vibrations of surface atoms, which indicates their strong damping. Taking into account the difference in the damping of different modes and their calculated intensities, all bands in the spectra are unambiguously identified. It is found that the frequencies of longitudinal optical modes in heterostructures are close to the frequencies of LO phonons in bulk strained constituents, whereas the frequencies of transverse modes can differ significantly from those of the corresponding TO phonons. It is shown that an anomalous thickness dependence of CdS TO mode is due to a noticeable surface relaxation of the outer Cd layer in the nanostructure.

cond-mat.mtrl-sci↗

arXiv2021

Piezoelectric properties of II-IV/I-V and II-IV/III-III ferroelectric perovskite superlattices

The stability of high-symmetry $P4mm$ polar phase in eleven ferroelectric perovskite superlattices with the polar discontinuity is studied from first principles. In most superlattices, this phase exhibits either the ferroelectric or the antiferrodistortive instability, or both of them. The structure of the ground state and, for a number of systems, also of metastable phases in these superlattices is found. The spontaneous polarization and piezoelectric properties of superlattices are calculated. The appearance of high piezoelectric coefficients (up to 150-270 pC/N) in some superlattices is associated with the strain-induced local rearrangement of certain atomic groups in the primitive cell.

cond-mat.mtrl-sci↗

arXiv2020

Piezoelectric properties of ferroelectric perovskite superlattices with polar discontinuity

The stability of a high-symmetry $P4mm$ polar phase in seventeen short-period ferroelectric perovskite superlattices with polar discontinuity is studied from first principles within the density-functional theory. It is shown that in most superlattices this phase exhibits either the ferroelectric instability or the antiferrodistortive one, or both of them. For each superlattice, the ground-state structure, the structure of possible metastable phases, the spontaneous polarization, and piezoelectric properties are calculated. The properties of superlattices with the polar discontinuity are compared with those of superlattices with broken symmetry and of ordinary superlattices, which have no the polar discontinuity. It is shown that high piezoelectric coefficients (up to 150--270 pC/N) in some superlattices with the polar discontinuity are due to the appearance of strong lattice distortions, whose symmetry follows that of a low-lying polar phonon mode of the ground-state structure, under the influence of external strain.

cond-mat.mtrl-sci↗

arXiv2019

Negative thermal expansion in CdSe quasi-two-dimensional nanoplatelets

The in-plane coefficient of thermal expansion (CTE) for CdSe nanoplatelets with the zinc-blende structure containing from two to five monolayers is calculated from first principles within the quasiharmonic approximation. A comparison of the obtained results with those for bulk CdSe with both the zinc-blende and wurtzite structures finds a significant increase in the magnitude of negative CTE and the temperature range of its observation in nanoplatelets. The main contribution to the negative thermal expansion in CdSe nanoplatelets is given by the out-of-plane flexural ZA mode and in-plane optical $E$ modes that arise from the folding of TA phonon of bulk CdSe.

cond-mat.mtrl-sci↗

arXiv2018

Ferroelectricity and piezoelectricity in monolayers and nanoplatelets of SnS

The ground-state structure of monolayers and nanoplatelets of SnS with a thickness from two to five monolayers is calculated from first principles. It is shown that nanoobjects with only odd number of monolayers are ferroelectric. The ferroelectric, piezoelectric, and elastic properties of these polar structures are calculated. The appearance of polarization in these nanoobjects is explained by an uncompensated polarization that exists in an antiferroelectric structure of bulk SnS. The mechanism of ferroelectricity, in which the ferroelectric distortion is associated with short-range ordering of lone pairs, can be regarded as a way of creating ferroelectrics with high Curie temperature.

cond-mat.mtrl-sci↗

arXiv2017

Lattice dynamics of quasi-two-dimensional CdSe nanoplatelets and their Raman and infrared spectra

Phonon spectra of CdSe nanoplatelets (2-6 ML) with the zinc-blende structure were calculated from first principles within the density-functional theory. It turned out that the Lamb modes in nanoplatelets are in fact optical rather than acoustic vibrations. Phonon spectra of the nanoplatelets show the appearance of a large number of low-frequency modes inherited from TA phonons in bulk CdSe. Calculations of the Raman spectra indicate a need to revise the interpretation of available experimental data. The largest contribution to the Raman spectra is provided by the quasi-Lamb modes with the $A_1$ symmetry. The $B_2$ modes whose frequencies depend on the environment of nanoplatelets and whose properties are closest to the properties of LO phonons explain the results obtained in the "nanoparticle-on-mirror" geometry. The features in Raman spectra previously attributed to surface optical (SO) modes should be interpreted as a manifestation of lower-order quasi-Lamb $A_1$ modes. Calculations of the infrared spectra find, in addition to the TO phonon line, the appearance of intense lines from surface modes originating from terminating F(Cl) atoms on the surface of nanoplatelets and true SO-modes.

cond-mat.mtrl-sci↗

arXiv2017

High-energy exciton transitions in quasi-two-dimensional cadmium chalcogenide nanoplatelets

Semiconductor nanoparticles of cadmium chalcogenides are known to exhibit pronounced thickness-dependent $E_0$ series of exciton transitions at the $Γ$ point of the Brillouin zone (BZ). In this work, we report an experimental evidence for high-energy series of exciton transitions, which originates from BZ points different from the $Γ$ point, in the family of cadmium chalcogenide quasi-2D nanoplatelets (NPLs). Intensive UV absorption bands demonstrating a pronounced size effect are observed for CdTe, CdSe, and CdS NPLs in addition to the $E_0$ exciton bands in the visible region. These new bands are attributed to transitions analogous to the $E_1$, $E_1+Δ_1$, and $E_2$ series observed in bulk crystals. First-principles DFT calculations of the electronic structure and absorption spectra support this explanation and show that the main contribution to these optical transitions comes from $X$ and $M$ points of the 2D BZ, which originate from $L$ and $X$ points of the 3D BZ. At the same time, the $E_0$ series of transitions at the $Γ$ point is well described by the multiband effective-mass model. The observation of the UV exciton bands reveals tunable optical properties of cadmium chalcogenide NPLs in UV spectral region, which may be interesting for practical applications.

cond-mat.mtrl-sci↗

arXiv2016

Dielectric, piezoelectric, and elastic properties of BaTiO$_3$/SrTiO$_3$ ferroelectric superlattices from first principles

The effect of epitaxial strain on the phonon spectra, crystal structure, spontaneous polarization, dielectric, piezoelectric, and elastic properties of (001)-oriented ferroelectric (BaTiO$_3$)$_m$/(SrTiO$_3$)$_n$ superlattices ($m = n = {}$1-4) was studied using the first-principles density-functional theory. The ground state of free-standing superlattices is the monoclinic $Cm$ polar phase. Under the in-plane biaxial compressive strain, it transforms to tetragonal $P4mm$ polar phase, and under the in-plane biaxial tensile strain, it transforms to orthorhombic $Amm2$ polar phase. When changing the in-plane lattice parameter, a softening of several optical and acoustic modes appears at the boundaries between the polar phases, and corresponding components of dielectric, piezoelectric, and elastic tensors diverge critically. The comparison of the mixing enthalpy of disordered Ba$_{0.5}$Sr$_{0.5}$TiO$_3$ solid solution modeled using two special quasirandom structures SQS-4 with the mixing enthalpy of the superlattices reveals a tendency of the BaTiO$_3$-SrTiO$_3$ system to short-range ordering and shows that these superlattices are thermodynamically quite stable.

cond-mat.mtrl-sci↗

arXiv2016

Metastability effects in strained and stressed SrTiO3 films

The sequence of ground states for SrTiO3 film subjected to epitaxial strain as well as to mechanical stress along the [001] and [110] axes is calculated from first principles within the density functional theory. Under the fixed-strain boundary conditions, an increase in the lattice parameter of a substrate results in the $I4cm \to I4/mcm \to Ima2 \to Cm \to Fmm2 \to Ima2$(II) sequence of ground states. Under the fixed-stress boundary conditions, the phase sequence is different and depends on how the stress is applied. It is revealed that the simultaneous presence of competing ferroelectric and antiferrodistortive instabilities in SrTiO3 gives rise to the appearance of metastable phases, whose number increases dramatically under the fixed-stress conditions. In the metastable phases, the octahedral rotation patterns are shown to differ substantially from those in the ground state. It is suggested that in systems with competing instabilities, each polar phase has its optimal octahedral rotation pattern which stabilizes this phase and creates a potential barrier preventing this phase to be transformed into other structures.

cond-mat.mtrl-sci↗

arXiv2015

Ferroelectric phase transition in orthorhombic CdTiO3: First-principles studies

The crystal structures and phonon spectra of orthorhombic cadmium titanate with the $Pbnm$ structure and of its two possible ferroelectrically distorted phases with $Pbn2_1$ and $Pb2_1m$ space groups were calculated from first principles within the density functional theory. The obtained structural parameters and frequencies of Raman- and infrared-active modes are in good agreement with available experimental data for the $Pbnm$ phase. Expansion of the total energy in a Taylor series of two order parameters showed that the ground state of the system corresponds to the $Pbn2_1$ structure into which the $Pbnm$ phase transforms through a second-order phase transition without intermediate phases. A substantial discrepancy between the calculated and experimentally observed lattice distortions and spontaneous polarization in the polar phase was explained by quantum fluctuations as well as by existence of twinning and competing long-period structures.

cond-mat.mtrl-sci↗

arXiv2014

Band offsets in heterojunctions between cubic perovskite oxides

The band offsets for nine heterojunctions between titanates, zirconates, and niobates with the cubic perovskite structure were calculated from first principles. The effect of strain in contacting oxides on their energy structure, many-body corrections to the position of the band edges (calculated in the GW approximation), and the splitting of the conduction band resulting from spin-orbit interaction were consistently taken into account. It was shown that the neglect of the many-body effects can lead to errors in determination of the band offsets up to 0.36 eV. The failure of the transitivity rule, which is often used to determine the band offsets in heterojunctions, was demonstrated and its cause was explained.

cond-mat.mtrl-sci↗

arXiv2013

Structural position and oxidation state of nickel in SrTiO3

The properties of Ni-doped strontium titanate are studied using X-ray diffraction and XAFS spectroscopy. It is shown that regardless of the preparation conditions, the SrTi1-xNixO3 solid solution and the NiTiO3 phase are the most stable phases which can coexist. According to the EXAFS data, in the single-phase sample of SrTi0.97Ni0.03O3, the Ni atoms substitute for the Ti ones and are on-center. The distortion of the oxygen octahedra is not observed. The XANES spectra analysis shows that the oxidation state of nickel in NiTiO3 is 2+, and in the SrTi1-xNixO3 solid solution it is close to 4+. It is shown that the strongest light absorption in doped samples is associated with the presence of tetravalent nickel in the SrTi1-xNixO3 solid solution. This doping seems the most promising one for solar energy converters that exploit the bulk photovoltaic effect.

cond-mat.mtrl-sci↗

arXiv2013

Ab initio calculations of phonon spectra in ATiO3 perovskite crystals (A = Ca, Sr, Ba, Ra, Cd, Zn, Mg, Ge, Sn, Pb)

The phonon spectra of calcium, strontium, barium, radium, cadmium, zinc, magnesium, germanium, tin, and lead titanates with the perovskite structure are calculated from first principles within the density functional theory. By analyzing the unstable modes in the phonon spectra, the possible lattice distortions are determined and the energies of the corresponding phases are calculated. From analyzing the phonon spectra, force constants, and eigenvectors of TO phonons, a conclusion is drawn on the origin of the ferroelectricity in considered crystals. It is shown that the main factors determining the possible off-centering of atoms in the A position are the geometric size and electronic configuration of these atoms.

cond-mat.mtrl-sci↗

arXiv2013

Properties of BaTiO3/BaZrO3 ferroelectric superlattices with competing instabilities

Properties of (BaTiO$_3$)$_1$/(BaZrO$_3$)$_n$ ferroelectric superlattices (SLs) with $n = {}$1--7 grown in the [001] direction are calculated from first principles within the density functional theory. It is revealed that the quasi-two-dimensional ferroelectricity occurs in these SLs in the barium titanate layers with a thickness of one unit cell; the polarization is oriented in the layer plane and weakly interacts with the polarization in neighboring layers. The ferroelectric ordering energy and the height of the barrier separating different orientational states of polarization in these SLs are sufficiently large to provide the formation of an array of independent polarized planes at 300 K. The effect of the structural instability on the properties of SLs is considered. It is shown that the ground state is a result of simultaneous condensation of the $Γ_{15}$ polar phonon and phonons at the $M$ point (for SLs with even period) or at the $A$ point (for SLs with odd period); it is a polar structure with out-of-phase rotations of the octahedra in neighboring layers, in which highly polarized layers are spatially separated from the layers with strong rotations. The competition between the ferroelectric and structural instabilities in biaxially compressed SLs manifests itself in that the switching on of the octahedra rotations leads to an abrupt change of the polarization direction and can cause an improper ferroelectric phase transition to occur. It was shown that the experimentally observed $z$-component of polarization in the SLs can appear only as a result of the mechanical stress relaxation.

cond-mat.mtrl-sci↗

arXiv2013

Crystal structure and properties of barium thorate BaThO$_3$ from first principles

The phonon spectrum of cubic BaThO$_3$ with the perovskite structure is calculated from first principles within the density functional theory. The analysis of unstable modes in the phonon spectrum enables to determine the symmetry of all possible distorted phases, calculate their energies, and show that the ground-state structure of barium thorate is $Pbnm$. For this structure, the static and optical dielectric constants, elastic moduli, heat capacity, Raman spectra, and the energy band gap in the LDA and $GW$ approximations are calculated. The possibility of the structural phase transitions in BaThO$_3$ is also discussed.

cond-mat.mtrl-sci↗