arXiv subjects

Alexander A. Demkov

Publications and source records attributed to Alexander A. Demkov.

14 recordsLinked to original sources

arXiv2026

Hybrid BaTiO3/TiO2 Metasurface for Efficient Gigahertz-Speed Free-Space Electro-Optic Modulation

Free-space electro-optic modulators are key to emerging photonic systems, yet their performance remains limited by trade-offs between modulation efficiency, bandwidth, and device aperture. Here we report a hybrid BaTiO3 (BTO)/TiO2 metasurface for large-aperture, efficient, gigahertz-speed free-space electro-optic modulation. Combining scalable BTO film growth by radio-frequency magnetron sputtering with mature TiO2 nanofabrication, we pattern the metasurface in TiO2 on an unetched BTO layer. The resulting devices support guided-mode resonances with quality factors exceeding 1300 and an optical confinement factor of ~0.8, while the continuous BTO layer makes efficient use of the applied voltage, together maximizing the overlap between the optical and driving fields within the BTO. A device with a 0.3 mm x 0.3 mm metasurface achieves a transmittance modulation efficiency of ~0.020 per volt and a -3 dB electro-optic bandwidth of ~0.8 GHz, with an effective Pockels coefficient of ~151 pm/V for the BTO. This establishes a scalable route to high-performance free-space electro-optic modulators for LiDAR, free-space optical communication, and reconfigurable optical computing.

physics.optics↗

arXiv2026

High Coupling Tunable Acoustic Resonators in Monolithic Barium Titanate

The growing number of wireless communication bands has driven demand for compact, low-loss, and frequency adjustable RF filtering. Tunable acoustic resonators are well suited to address these needs, offering a path toward reconfigurable front ends with reduced component count. In this work, we extend upon previous conference results to investigate epitaxial barium titanate (BTO) grown on silicon as a platform for tunable acoustic resonators. We demonstrate lateral excitation of symmetric Lamb (S0) modes in 120 nm X-cut BTO membranes using a multi-cell electrode architecture that simultaneously achieves high electromechanical coupling and practical impedance levels. Devices are fabricated with laterally patterned electrodes on released BTO membranes. Under applied DC bias, ferroelectric domains align, allowing electrical excitation, frequency tuning, and quality-factor enhancement of acoustic modes. The primary resonance near 700 MHz exhibits a Bode quality factor of 175, electromechanical coupling up to 25.1%, and series and parallel resonance tunability of 2.3% and 5.6%, respectively. Voltage-dependent material parameters, including permittivity, stiffness, and piezoelectric coefficients, are extracted through a combination of modified Butterworth-Van Dyke modeling and finite-element simulation to explain the observed trends. These results highlight monolithic BTO on silicon as a promising material system for laterally excited, tunable acoustic resonators for reconfigurable RF applications.

eess.SP↗

arXiv2026

Tunable Ferroelectric Acoustic Resonators in Monolithic Thin-Film Barium Titanate

The increasing development of wireless communication bands has motivated the development of compact, low-loss, and frequency adjustable RF filtering technologies. Acoustic resonators are the ideal solution to these requirements, and tunable implementations offer a path toward reconfigurable front ends. In this work, we investigate epitaxial barium titanate (BTO) grown on silicon as a platform for tunable acoustic resonators operating in the sub-GHz regime. We demonstrate lateral excitation of symmetric lamb (S0) modes in X-cut BTO membranes, in contrast to prior thickness-defined ferroelectric resonators. Devices are designed using finite-element simulations and fabricated with laterally patterned electrodes that enable overtone coupling to multiple resonant modes. Under applied DC bias, ferroelectric domains align, allowing electrical excitation, frequency tuning, and quality-factor enhancement of acoustic modes. Resonances near 300 MHz and 700 MHz exhibit electromechanical coupling up to 8% and bias-dependent frequency tuning, with a distinct transition in behavior near 20 V. These results highlight monolithic BTO on silicon as a promising material system for laterally excited, tunable acoustic resonators for reconfigurable RF applications.

eess.SY↗

arXiv2019

Ultra-Low-Power Tuning in Hybrid Barium Titanate-Silicon Nitride Electro-Optic Devices on Silicon

As the optical analogue to integrated electronics, integrated photonics has already found widespread use in data centers in the form of optical interconnects. As global network traffic continues its rapid expansion, the power consumption of such circuits becomes a critical consideration. Electrically tunable devices in photonic integrated circuits contribute significantly to the total power budget, as they traditionally rely on inherently power-consuming phenomena such as the plasma dispersion effect or the thermo-optic effect for operation. Here, we demonstrate ultra-low-power refractive index tuning in a hybrid barium titanate (BTO)-silicon nitride (SiN) platform integrated on silicon. We achieve tuning by exploiting the large electric field-driven Pockels effect in ferroelectric BTO thin films of sub-100 nm thickness. The extrapolated power consumption for tuning a free spectral range (FSR) in racetrack resonator devices is only 106 nW/FSR, several orders of magnitude less than many previous reports. We demonstrate the technological potential of our hybrid BTO-SiN technology by compensating thermally induced refractive index variations over a temperature range of 20 °C and by using our platform to fabricate tunable multiresonator optical filters. Our hybrid BTO-SiN technology significantly advances the field of ultra-low-power integrated photonic devices and allows for the realization of next-generation efficient photonic circuits for use in a variety of fields, including communications, sensing, and computing.

physics.app-ph↗

arXiv2015

Quench dynamics of Anderson impurity model using configuration interaction method

We study the quench dynamics of an Anderson impurity model using the configuration interaction (CI) method. In particular, we focus on the relaxation behavior of the impurity occupation. The system is found to behave very differently in the weak-coupling and strong-coupling regimes. In the weak-coupling regime, the impurity occupation relaxes to a time-independent constant quickly after only a few oscillations. In the strong-coupling regime, the impurity occupation develops a fast oscillation, with a much slower relaxation. We show that it is the multi-peak structure in the many-body energy spectrum that separates these two regimes. The characteristic behavior, including the power-law decay and the period of oscillation, can also be related to certain features in the many-body energy spectrum. The respective advantages of several impurity solvers are discussed, and the convergence of different CI truncation schemes is provided.

cond-mat.str-el↗

arXiv2015

Final-state effect on x-ray photoelectron spectrum of nominally $d^1$ and $n$-doped $d^0$ transition metal oxides

We investigate the x-ray photoelectron spectroscopy (XPS) of nominally $d^1$ and $n$-doped $d^0$ transition metal oxides including NbO$_2$, SrVO$_3$, and LaTiO$_3$ (nominally $d^1$), as well as $n$-doped SrTiO$_3$ (nominally $d^0$). In the case of single phase $d^1$ oxides, we find that the XPS spectra (specifically photoelectrons from Nb $3d$, V $2p$, Ti $2p$ core levels) all display at least two, and sometimes three distinct components, which can be consistently identified as $d^0$, $d^1$, and $d^2$ oxidation states (with decreasing order in binding energy). Electron doping increases the $d^2$ component but decreases the $d^0$ component, whereas hole doping reverses this trend; a single $d^1$ peak is never observed, and the $d^0$ peak is always present even in phase-pure samples. In the case of $n$-doped SrTiO$_3$, the $d^1$ component appears as a weak shoulder with respect to the main $d^0$ peak. We argue that these multiple peaks should be understood as being due to the final-state effect and are intrinsic to the materials. Their presence does not necessarily imply the existence of spatially localized ions of different oxidation states nor of separate phases. A simple model is provided to illustrate this interpretation, and several experiments are discussed accordingly. The key parameter to determine the relative importance between the initial-state and final-state effects is also pointed out.

cond-mat.str-el↗

arXiv2013

Electron Correlation in Oxygen Vacancy in SrTiO$_3$

Oxygen vacancies are an important type of defect in transition metal oxides. In SrTiO$_3$ they are believed to be the main donors in an otherwise intrinsic crystal. At the same time, a relatively deep gap state associated with the vacancy is widely reported. To explain this inconsistency we investigate the effect of electron correlation in an oxygen vacancy (OV) in SrTiO$_3$. When taking correlation into account, we find that the OV-induced localized level can at most trap one electron, while the second electron occupies the conduction band. Our results offer a natural explanation of how the OV in SrTiO$_3$ can produce a deep in-gap level (about 1 eV below the conduction band bottom) in photoemission, and at the same time be an electron donor. Our analysis implies an OV in SrTiO$_3$ should be fundamentally regarded as a magnetic impurity, whose deep level is always partially occupied due to the strong Coulomb repulsion. An OV-based Anderson impurity model is derived, and its implications are discussed.

cond-mat.str-el↗

arXiv2013

Lattice distortion effects on topological phases in (LaNiO$_3$)$_2$/(LaAlO$_3$)$_N$ heterostructures grown along the [111] direction

We theoretically investigate the influence of internal and external strain on topological phases in (LaNiO$_3$)$_2$/(LaAlO$_3$)$_N$ heterostructures grown along the [111] direction. At the Hartree-Fock level, topological phases originate from an interaction-generated effective spin-orbit coupling that opens a gap in the band structure. For the unstrained system, there is a quadratic band touching at the $Γ$ point at the Fermi energy for unpolarized electrons and Dirac points at K, K$'$ at the Fermi energy for fully polarized electrons. Using density functional theory we show that the quadratic band touching and Dirac points are remarkably stable to internal strain-induced out-of-plane distortions and rotations of the oxygen octahedra, which we compute. The lack of a gap opening from internal strain implies a robustness to the mean-field predicted topological phases for both the polarized and unpolarized systems. We also consider an external strain imposed along the [001] cubic direction and show this can open a gap at the $Γ$ point but leaves the Dirac points intact. Finally, we compute a phase diagram for parameters relevant to LaNiO$_3$ which shows that strain favors a phase with polarized orbitals and antiferromagnetic spin order, but leaves earlier predictions for a zero-magnetic field topological quantum Hall state essentially unchanged. Taken together, our results suggest that the [111] growth direction in perovskites may lead to thin films that are relatively immune to distortion effects compared to those grown along [001].

cond-mat.str-el↗

arXiv2013

Efficient variational approach to the impurity problem and its application to the dynamical mean-field theory

Within the framework of exact diagonalization (ED), we compute the ground state of Anderson impurity problem using the variational approach based on the configuration interaction (CI) expansion. We demonstrate that an accurate ground state can be obtained by iteratively diagonalizing a matrix with the dimension that is less than 10$%$ of the full Hamiltonian. The efficiency of the CI expansion for different problems is analyzed. By way of example, we apply this method to the single-site dynamical mean field theory using ED as the impurity solver. Specifically, to demonstrate the usefulness of this approach, we solve the attractive Hubbard model in the grand-canonical ensemble, where the s-wave superconducting solution is explicitly obtained.

cond-mat.str-el↗

arXiv2012

Electronic structure of (LaNiO$_3$)$_2$/(LaAlO$_3$)$_N$ heterostructures grown along [111]

The electronic structure of a LaNiO$_3$ bilayer grown along the [111] direction and confined between insulating layers of LaAlO$_3$ is theoretically investigated using a combination of first principle calculations and effective multi-orbital lattice models. The LDA band structure is well reproduced by a tight-binding model for the Ni-$e_g$ orbitals defined on the buckled honeycomb lattice. We highlight peculiar properties of this model which include almost flat bands as well as linear and quadratic band crossing points. The effect of local correlations is discussed within the LDA$+U$ scheme and within the Hartree-Fock approximation for interacting multi-orbital lattice models. Over a wide range of interaction parameters we find that a ferromagnetic phase is energetically favored. We discuss the possibility of additional orbital order which could stabilize a spontaneous Chern insulator with chiral edge modes or a staggered orbital phase with a $\sqrt{3}\times\sqrt{3}$ reconstruction of the unit cell. By studying an interacting nickel-oxygen lattice model we find that the stability of these orbitally ordered phases also depends on the value of the charge-transfer energy. Controlling the charge-transfer energy might therefore be an important step towards engineering exotic electronic phases in certain classes of oxide heterostructures.

cond-mat.str-el↗

arXiv2009

Interfacial Magnetoelectric Coupling in Tri-component Superlattices

Using first-principles density functional theory, we investigate the interfacial magnetoelectric coupling in a tri-component superlattice composed of a ferromagnetic metal (FM), ferroelectric (FE), and normal metal (NM). Using Fe/FE/Pt as a model system, we show that a net and cumulative interfacial magnetization is induced in the FM metal near the FM/FE interface. A carefully analysis of the magnetic moments in Fe reveals that the interfacial magnetization is a consequence of a complex interplay of interfacial charge transfer, chemical bonding, and spin dependent electrostatic screening. The last effect is linear in the FE polarization, is switchable upon its reversal, and yields a substantial interfacial magnetoelectric coupling.

cond-mat.mtrl-sci↗

arXiv2009

Magnetoelectric Coupling and Electric Control of Magnetization in Ferromagnet-Ferroelectric-Metal Superlattices

Ferromagnet-ferroelectric-metal superlattices are proposed to realize the large room-temperature magnetoelectric effect. Spin dependent electron screening is the fundamental mechanism at the microscopic level. We also predict an electric control of magnetization in this structure. The naturally broken inversion symmetry in our tri-component structure introduces a magnetoelectric coupling energy of $P M^2$. Such a magnetoelectric coupling effect is general in ferromagnet-ferroelectric heterostructures, independent of particular chemical or physical bonding, and will play an important role in the field of multiferroics.

cond-mat.mes-hall↗

arXiv2008

Absence of Critical Thickness in an Ultrathin Improper Ferroelectric Film

We study the ferroelectric stability and surface structural properties of an oxygen-terminated hexagonal YMnO$_3$ ultra-thin film using density functional theory. Under an open circuit boundary condition, the ferroelectric state with the spontaneous polarization normal to the (0001) surface, is found to be metastable in a single domain state despite the presence of a depolarizing field. We establish a connection between the result and the role of improper ferroelectric transition. Our results imply that improper ferroelectric ultrathin films can have rather unique properties that are distinctive from those of very thin films of ordinary ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2007

Spin-Filtering Multiferroic-Semiconductor Heterojunctions

We report on the structural and electronic properties of the interface between the multiferoic oxide YMnO$_3$ and wide band-gap semiconductor GaN studied with the Hubbard-corrected local spin density approximation (LSDA+U) to density-functional theory (DFT). We find that the band offsets at the interface between antiferromagnetically ordered YMnO$_3$ and GaN are different for spin-up and spin-down states. This behavior is due to the spin splitting of the valence band induced by the interface. The energy barrier depends on the relative orientation of the electric polarization with respect to the polarization direction of the GaN substrate suggesting an opportunity to create magnetic tunnel junctions in this materials system.

cond-mat.mtrl-sci↗