Electronic Subsystem Research Articles

Properties of materials for resistive RAM based on HfO2 (first principles calculations)

The methods of the theory of the functional of electron density and pseudopotential from the first principles obtained the characteristics of an electronic subsystem of the working layer of RRAM (Resistive Random Access Memory) which is based on the HfO2 It is calculated how the introduction of vacancies into the crystal and the introduction of silver atoms affects the properties of the object. The spatial distributions of the density of valence electrons and their cross sections within the cell, the distribution of the density of electronic states, and electric charges in the vicinity of the Hafnium atoms in different atomic environments are calculated. It is investigated how changes in material structure affect conductive properties.

Polarization-resolved broadband time-resolved optical spectroscopy for complex materials: application to the case of MoTe2 polytypes.

Time-resolved optical spectroscopy (TR-OS) has emerged as a fundamental spectroscopic tool for probing complex materials, to both investigate ground-state-related properties and trigger phase transitions among different states with peculiar electronic and lattice structures. We describe a versatile approach to perform polarization-resolved TR-OS measurements, by combining broadband detection with the capability to simultaneously probe two orthogonal polarization states. This method allows us to probe, with femtoseconds resolution, the frequency-resolved reflectivity or transmittivity variations along two mutually orthogonal directions, matching the principal axis of the crystal structure of the material under scrutiny. We report on the results obtained by acquiring the polarization-dependent transient reflectivity of two polytypes of the MoTe2 compound, with 2H and 1T' crystal structures. We reveal marked anisotropies in the time-resolved reflectivity signal of 1T'-MoTe2, which are connected to the crystal structure of the compound. Polarization- and time- resolved spectroscopic measurements can thus provide information about the nature and dynamics of both the electronic and crystal lattice subsystems, advancing the comprehension of their inter-dependence, in particular in the case of photoinduced phase transitions; in addition, they provide a broadband measurement of transient polarization rotations.

Electron correlations and memory effects in ultrafast electron and hole dynamics in VO2

By applying an approach based on time-dependent density functional theory and dynamical mean-field theory (TDDFT+DMFT) we examine the role of electron correlations in the ultrafast breakdown of the insulating M1 phase in bulk VO2. We consider the case of a spatially homogeneous ultrafast (femtosecond) laser pulse perturbation and present the dynamics of the melting of the insulating state, in particular the time-dependence of the excited charge density. The time-dependence of the chemical potential of the excited electron and hole subsystems shows that even for such short times the dynamics of the system is significantly affected by memory effects—the time-resolved electron–electron interactions. The results pave the way for obtaining a microscopic understanding of the ultrafast dynamics of strongly-correlated materials.

Estimation of the influence of the Nonequilibrium State of the Electronic Subsystem and the Dispersion Properties of a Solid-State Plasma Medium in the conditions of Decay Instability on the reflection of Laser Radiation

Estimation of the influence of the Nonequilibrium State of the Electronic Subsystem and the Dispersion Properties of a Solid-State Plasma Medium in the conditions of Decay Instability on the reflection of Laser Radiation

Amplitude dynamics of the charge density wave in LaTe3 : Theoretical description of pump-probe experiments

We formulate a dynamical model to describe a photo-induced charge density wave (CDW) quench transition and apply it to recent multi-probe experiments on LaTe$_3$ [A. Zong et al., Nat. Phys. 15, 27 (2019)]. Our approach relies on coupled time-dependent Ginzburg-Landau equations tracking two order parameters that represent the modulations of the electronic density and the ionic positions. We aim at describing the amplitude of the order parameters under the assumption that they are homogeneous in space. This description is supplemented by a three-temperature model, which treats separately the electronic temperature, temperature of the lattice phonons with stronger couplings to the electronic subsystem, and temperature of all other phonons. The broad scope of available data for LaTe$_3$ and similar materials as well as the synergy between different time-resolved spectroscopies allow us to extract model parameters. The resulting calculations are in good agreement with ultra-fast electron diffraction experiments, reproducing qualitative and quantitative features of the CDW amplitude evolution during the initial few picoseconds after photoexcitation.

Pb dimerization greatly accelerates charge losses in MAPbI3: Time-domain ab initio analysis.

Metal halide perovskites constitute a new type of semiconducting materials with long charge carrier lifetimes and efficient light-harvesting. The performance of perovskite solar cells and related devices is limited by nonradiative charge and energy losses, facilitated by defects. Combining nonadiabatic molecular dynamics and time-domain density functional theory, we demonstrate that charge losses depend strongly on the defect chemical state. By considering an extra Pb atom in CH3NH3PbI3, which is a common defect in lead halide perovskites, we investigate its influence on charge trapping and recombination. In a chemically inert form as a Pb interstitial, the extra Pb atom has only a mild influence on charge recombination. However, if the extra Pb atom binds to a native Pb atom to form a dimer, the charge trapping and recombination are greatly accelerated because the Pb-dimer creates a localized midgap trap state that couples strongly to the perovskite valence band edge. Holes disappear from the valence band two orders of magnitude faster than in the pristine perovskite and recombine with conduction band electrons one order of magnitude faster. The simulations identify the phonon modes involved in the nonradiative charge trapping and recombination and highlight the importance of rapid decoherence within the electronic subsystem for long carrier lifetimes. The detailed atomistic analysis of the charge trapping and recombination mechanisms enriches the understanding of defect properties and provides theoretical guidance for improving perovskite performance.

Nonlinear Photoacoustic Response on Mechanical Stresses in the Vicinity of a Hole in a D16 Aluminum-Alloy Plate

Characteristics of a photoacoustic signal depending on mechanical stresses in the vicinity of a hole in a D16 aluminum-alloy plate have been analyzed. A new nonlinear photoacoustic effect was found that consists in the occurrence of a nonlinear component in the photoacoustic signal depending on mechanical stresses in the vicinity of a hole. This dependence is detected in the signal on the main modulation frequency of laser radiation. We have proposed a theoretical description of the detected effect of thermoelastic generation of ultrasound in stressed metals based on taking into account the thermal impact of laser radiation on their electron subsystem.

The effect of the domain wall on the entropy and the heat capacity of nickel nanowire

The thermodynamic properties of a cylindrical nickel nanowire with a domain wall (DW) were studied. It was found that the entropy and heat capacity of the DW can be higher than or on the order of the corresponding values of the electron, magnon, and phonon (Debye) subsystems. The conditions at which this effect takes place are determined. The methods of its strengthening are proposed. For instance, an increase in the length of nanowire and the number of DWs by a factor of 10 increases its diameter at which the heat capacity of DW has the same order as the phonon subsystem by 33.5%. An increase in this parameter by 38% occurs upon magnetization of the nanowire to M=1.3·102G.

Ultrafast optically induced spin transfer in ferromagnetic alloys.

The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism.

Triple VTe2/graphene/VTe2 heterostructures as perspective magnetic tunnel junctions

New perspective 1.4 nm thick spin-polarized triple heterostructures based on graphene sandwiched between two vanadium ditelluride monolayers (VTe2/graphene/VTe2) were studied using ab initio DFT technique. Both possible trigonal prismatic (H-VTe2) and octahedral (T-VTe2) VTe2 phases were considered to design and study graphene-based heterostructures. It was shown that the interaction with graphene changes the electronic structure of 2D T-VTe2 from metallic to half-metallic, making T phase perspective to be used for magnetic tunnel junctions. The electronic subsystem of graphene fragment is slightly hole doped. Calculated tunnel magnetoresistance ratio for the favorable heterostructure configuration estimated within the Julliere model is 220%, which opens a way to use VTe2/graphene/VTe2 as prospective magnetic tunnel junction in novel spintronic nanodevices based on tunnel magnetic resistance and spin transfer torque effects.

Interaction of the hydrogen molecule with the environment: stability of the system and the {mathscr{PT}} symmetry breaking

The stability of the hydrogen molecule interacting with the environment according to the balanced gain and loss energy scheme was studied. We determined the properties of the molecule taking into account all electronic interactions, the parameters of the Hamiltonian being computed by the variational method. The interaction of the hydrogen molecule with the environment was modeled parametrically (γ) by means of the non-Hermitian, {mathscr{PT}}-symmetric Hamiltonian. We showed that the hydrogen molecule is dynamically unstable. Its dissociation time (TD) decreases if the γ parameter increases (for γ → 0 we got TD → + ∞). The dynamic instability of the hydrogen molecule is superimposed on the decrease in its static stability as γ increases. Then we can observe the decrease in the dissociation energy value and the existence of the metastable state of the molecule as γMS reaches 0.659374 Ry. The hydrogen molecule is statically unstable when γ > γD = 1.024638 Ry. Moreover, we can also observe the {mathscr{PT}} symmetry breaking effect for the electronic Hamiltonian when {gamma }_{{mathscr{PT}}} = 0.520873 Ry. This effect does not affect such properties of the hydrogen molecule as: the electronic Hamiltonian parameters, the phonon and the rotational energies, and the values of the electron-phonon coupling constants neither it disturbs the dynamics of the electronic subsystem. However, the number of available quantum states goes down to four.

Plasma-Chemical Simulation Fragmentation of Chloroform in the Liquid Phase by Direct Electrical Discharges

The process of chloroform fragmentation under the influence of low-voltage electrical discharges in the liquid phase is modeled. The composition of the low-temperature plasma of chloroform activation products is determined, using a chemical plasma model (CPM). The CPM is based on the principle of minimizing the Gibbs energy, which is supplemented by the concept of conditionally-equilibrium states for describing the non-equilibrium of an electro-contact plasma. The developed model provided an opportunity to characterize the energy-saturated electronic subsystem with a temperature of ≈ 11600 K and its “cold” part (ions, intermediates, synthons, molecules, etc.), with a temperature of about 1000 K. According to the calculation results, the participation of valence unsaturated fragments CCl2, CH, Cl· and molecules HCl, Cl2, C2Cl4, C2Cl2 in the transformation of chloroform.

Нелинейный фотоакустический отклик на механические напряжения вблизи отверстия в пластине из алюминиевого сплава Д16

The features of the behavior of the photoacoustic signal depending on mechanical stresses near the hole in the plate made of aluminum alloy D16 are investigated. A new nonlinear photoacoustic effect was discovered, which consists in the appearance of a nonlinear component of the photoacoustic signal depending on mechanical stresses near the hole. This dependence is recorded in the signal at the modulation frequency of the laser radiation. A theoretical description of the discovered effect of sound generation by the thermoelastic mechanism in stressed metals is proposed basing on the consideration of the thermal effect of laser radiation on their electronic subsystem.

Thermal generation of shift electric current

It is shown that the dissipation of energy in an electron gas confined in a quantum well made of non-centrosymmetric crystal leads to a direct electric current. The current originates from the real-space shift of the wave packets of Bloch electrons at the electron scattering by phonons, which tends to restore thermal equilibrium between the electron and phonon subsystems. We develop a microscopic theory of such a phonogalvanic effect for narrow band gap zinc-blende quantum wells.

Inhomogeneous Electron Density in the Static Field of Nuclei

We consider the thermodynamic potential of a Coulomb system of electrons and nuclei in the adiabatic approximation for the nuclei subsystem. In this approximation, the indirect interaction of nuclei is completely determined by the inhomogeneous density of the electron subsystem in the static field of nuclei. We obtain a decomposition of the inhomogeneous electron density as a sum of “one-center,” “two-center,” etc., group functions of electrons. Based on this, we consider the atomic partition function and the quasineutrality condition in a dilute atomic plasma.

A Cryo-CMOS Voltage Reference in 28-nm FDSOI

The control interface of a large-scale quantum computer will likely require electronic subsystems that operate in close proximity to the qubits, at deep cryogenic temperatures. In this letter, we report low-temperature performance of a custom cryo-CMOS voltage reference circuit fabricated in a 28-nm fully depleted silicon on insulator (FDSOI) CMOS process, dissipating about 15 $\mu \text{W}$ . This MOS-only reference circuit is functional from room temperature down to liquid helium temperature (4 K), showing a temperature coefficient of 0.6 mV/K. The measured supply sensitivity of our reference circuit is better than −50 dB at 4 K temperature. Beyond the specific application as low-power reference, this circuit is an ideal test-vehicle for developing design approaches that mitigate the adverse effects of cryogenic temperatures on circuit performance.

Search for new superconducting compounds with a critical transition temperature Tc close to room temperature under pressure

A new chemical composition of superconducting compounds formed on the basis of elements of the fifth group (semimetals) is proposed within the framework of the quantum Bardin-Cooper-Shriffer quantum theory of superconductivity (BCS-theory) using physical chemistry methods for analyzing equilibrium crystal structures. These compounds satisfy all the conditions for transition to the superconducting state at temperatures close to room temperature and a pressure of ≈107 Pa. As initial chemical elements from which superconducting compounds can be synthesized under pressure, in addition to hydrides, substances that allow the "collectivization" of electrons can be used. The most suitable substances in this sense are the elements of the fifth group of the periodic system or the so-called semimetals, which include Bi, Sb, As, graphite, etc. These elements, by their electrical properties, occupy an intermediate position between metals and semiconductors. They are characterized by a slight overlap of the valence and conduction zones, which leads, on one hand, to the fact that they remain good conductors of electricity up to absolute zero temperature, and on the other hand, they have a significantly lower carrier density compared to metals charge. Moreover, in these substances in a wide temperature range at atmospheric pressure, the stability of the solid phase is maintained and, very importantly, a partial “collectivization” of valence electrons inherent in metals is already realized in the initial state. It is shown that, under the action of pressure p``≈107Pa, semimetals can turn into metals characterized by a specific energy spectrum of electrons. A change in the semimetals structure and in parameters of the electronic subsystem energy spectrum is accompanied by an increase in the electron pairing constant and by the density of electronic states at the Fermi level. In turn, an increase in these parameters makes it possible to transfer semimetals to the superconducting state at temperature ≈300К.

Peculiarities of the Optical and Energy Properties of Thin CdSe Films

The results of experimental and theoretical studies of the optical and energy properties of thin CdSe films obtained by the quasi-closed volume method are presented. The synthesis methodology and the results of structural and optical studies of thin CdSe films deposited on the surface of a quartz substrate of brand KV (GOST (State Standard) 15130-86) are given. The quality of the films is analyzed using the X-ray diffraction, energy dispersive analysis, and scanning electron microscopy techniques. Using the pseudopotential method, the dynamics of changes in the parameters of the electronic subsystem of a CdSe film is theoretically studied. The direct-gap nature of the energy gap of the film is established. Based on the density of states, the genesis of the conduction and valence bands is established. Using the Kramers–Kronig relations, we obtained the spectra of optical dielectric functions and the reflectance, refractive index, and extinction spectra, which satisfactorily correlate with the obtained experimental data.

Особенности оптических и энергетических свойств тонких пленок CdSe

The paper presents the results of experimental and theoretical studies of the optical and energy properties of thin CdSe films obtained by the closed space sublimation. The synthesis method, the results of structural and optical studies of thin CdSe films deposited on the surface of a quartz substrate are presented. The quality of the films was analyzed by x-ray diffraction, energy dispersive analysis and scanning electron microscopy. Within the framework of the pseudopotential method, the dynamics of changes in the parameters of the CdSe films electronic subsystem is theoretically studied. The direct-gap nature of the forbidden gap of the film is established. Based on the density of states, the genesis of the conduction band and the valence band is established. Using the Kramers – Kronig relation, we obtained spectra of optical dielectric functions, optical reflection, refractive and extinction indexes, which satisfactorily correlate with the obtained experimental data.

Strong-field molecular alignment mediated by nonadiabatic charge localization

A new mode of effective interaction of molecular rotational degrees of freedom with an intense, nonresonant, ultrashort laser pulse is explored. Transient nonadiabatic charge redistribution (TNCR) in larger molecules or molecular ions causes impulsive-torque interaction that replaces the traditional mechanism of molecular alignment based on perturbative interaction of the laser field with electronic subsystem as manifested in linear anisotropic polarizability or hyperpolarizability. We explore this new alignment mechanism on a popular generic model of a tight-binding diatomic molecule. We consider the case of rotational wavepacket formation when a molecule is initially in the ground rotational state. The rotational wavepacket emerging from the TNCR interaction consists of states with higher rotational quantum numbers, in comparison with the anisotropic-polarizability case, and the after-pulse alignment oscillations are out-of-phase with those resulting from the traditional interaction. The TNCR interaction mode is expected to play a major role when a strong laser field actually causes extensive nonresonant excitation and/or ionization of a molecule.