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A. V. Shytov

Publications and source records attributed to A. V. Shytov.

At least 19 recordsLinked to original sources

Magnonic Full Adder Based on 2D Chiral Magnonic Resonators

We use micromagnetic simulations to demonstrate how machine learning can be applied to arrays of chiral magnonic resonators to build a magnonic full adder. The chiral magnonic resonators have form of nano-sized permalloy disks that nonlinearly scatter spin waves propagating in a YIG waveguide. The spin waves are injected from multiple outputs, and the dynamic stray magnetic field of the scattered spin waves is sampled in multiple locations to form several physical output signals. These signals are weighted and combined, either linearly or nonlinearly, to satisfy the logic output of a full adder. The process is known as training and forms the device's output layer. The full adder's performance is evaluated in terms of robustness to input and output noise for a given number of physical output signals and the form of the output layer. When the output layer is linear, as few as three physical signals per logical output are sufficient. When a multilayer perceptron neural network forms the output layer, the number of required output signals is reduced to one, and a nearly perfect classification accuracy is achieved when appropriate preprocessing and augmentation strategies are used.

physics.app-ph↗

Nanoscale Magnonic Neurons

We use micromagnetic simulations to demonstrate neuron functionality of two-dimensional (2D) chiral magnonic resonators. Our design exploits nonlinear resonant scattering of spin waves propagating in a YIG medium from an edge mode of a permalloy nano-element. The reduced frequency and volume of the edge mode facilitate matching it to the YIG modes and give rise to their wide-angle scattering. As the amplitudes of the incident spin waves increase, the edge mode exhibits a positive nonlinear frequency shift. This shift leads to a complex frequency-dependent nonlinear variation of the amplitude and phase of spin waves scattered in different directions. We show that the scattered waves are strong enough to activate secondary neurons. This provides the connectivity required for combining our proposed neurons into 2D magnonic neural networks.

physics.app-ph↗

Controlling acoustic waves using magnetoelastic Fano resonances

We propose and analyze theoretically a class of energy-efficient magneto-elastic devices for analogue signal processing. The signals are carried by transverse acoustic waves while the bias magnetic field controls their scattering from a magneto-elastic slab. By tuning the bias field, one can alter the resonant frequency at which the propagating acoustic waves hybridize with the magnetic modes, and thereby control transmission and reflection coefficients of the acoustic waves. The scattering coefficients exhibit Breit-Wigner/Fano resonant behaviour akin to inelastic scattering in atomic and nuclear physics. Employing oblique incidence geometry, one can effectively enhance the strength of magnetoelastic coupling, and thus countermand the magnetic losses due to the Gilbert damping. We apply our theory to discuss potential benefits and issues in realistic systems and suggest further routes to enhance performance of the proposed devices.

physics.app-ph↗

Hybrid Magnetoacoustic Metamaterials for Ultrasound Control

We propose a class of metamaterials in which propagation of acoustic waves is controlled magnetically through magnetoelastic coupling. The metamaterials are formed by a periodic array of thin magnetic layers ('resonators') embedded in a non-magnetic matrix. Acoustic waves carrying energy through the structure hybridize with the magnetic modes of the resonators ('Fano resonance'). This leads to a rich set of effects, enhanced by Bragg scattering and being most pronounced when the magnetic resonance frequency is close to or lies within acoustic band gaps. The acoustic reflection from the structure exhibits magnetically induced transparency and Borrmann effect. Our analysis shows that the combined effect of the Bragg scattering and Fano resonance may overcome the magnetic damping ubiquitous in realistic systems. This paves a route towards application of such structures in wave computing and signal processing

physics.app-ph↗

Bielectrons in the Dirac sea in graphene: the role of many--body effects

It was shown in PHYSICAL REVIEW B 92, 085409 (2015) that the dynamics of a pair of electrons in graphene can be mapped onto that of a single particle with negative effective mass, leading to bound states of positive energy despite the formally repulsive interaction. However, this conclusion was based on the analysis of the two--particle problem, neglecting the role of the Dirac sea and the many--body effects. The two dominant such effects at zero temperature are screening of the Coulomb interaction by the Dirac sea, and reduction of the available phase space due to Pauli blocking of transitions into the states below the Fermi level. We show that these effects result in strong renormalization of the binding energy, but do not destroy the metastable states. Thus the binding energies are strongly dependent on the chemical potential owing to the combined effects of screening and Pauli blocking. Hence, the quasibound resonances can be tuned by electrostatic doping.

cond-mat.mes-hall↗

Approaching magnetic ordering in graphene materials by FeCl$_3$ intercalation

We show the successful intercalation of large area (1 cm$^2$) epitaxial few-layer graphene grown on 4H-SiC with FeCl$_3$. Upon intercalation the resistivity of this system drops from an average value of $\approx 200 \ Ω/sq$ to $\approx 16 \ Ω/sq$ at room temperature. The magneto-conductance shows a weak localization feature with a temperature dependence typical of graphene Dirac fermions demonstrating the decoupling into parallel hole gases of each carbon layer composing the FeCl$_3$ intercalated structure. The phase coherence length ($\approx 1.2 μ$m at 280 mK) decreases rapidly only for temperatures higher than the 2-D magnetic ordering in the intercalant layer while it tends to saturate for temperatures lower than the antiferromagnetic ordering between the planes of FeCl$_3$ molecules providing the first evidence for magnetic ordering in the extreme two-dimensional limit of graphene.

cond-mat.mes-hall↗

Cloaked Resonant States in Bilayer Graphene

Charge carriers in bilayer graphene occupy two parabolic continua of electron-like and hole-like states which differ by the alignment between carrier pseudospin and its momentum, the property known as chirality. Due to chirality conservation, a strong confining potential can host unusual bound states: electron levels cloaked into the hole continuum. The energy levels and the wave functions of the cloaked states can be obtained by solving the Schrödinger equation for a massive non-chiral particle in the p-wave channel in two dimensions. Eventually, cloaked states slowly decay into the continuum, via trigonal warping effects. We discuss the key properties of cloaked states in circularly symmetric potentials, and show that cloaking should be observable in quantum corral geometries via scanning tunneling probe measurements.

cond-mat.mes-hall↗

Non-linear resistivity and heat dissipation in monolayer graphene

We have experimentally studied the nonlinear nature of electrical conduction in monolayer graphene devices on silica substrates. This nonlinearity manifests itself as a nonmonotonic dependence of the differential resistance on applied DC voltage bias across the sample. At temperatures below ~70K, the differential resistance exhibits a peak near zero bias that can be attributed to self-heating of the charge carriers. We show that the shape of this peak arises from a combination of different energy dissipation mechanisms of the carriers. The energy dissipation at higher carrier temperatures depends critically on the length of the sample. For samples longer than 10um the heat loss is shown to be determined by optical phonons at the silica-graphene interface.

cond-mat.mes-hall↗

Peierls-type Instability and Tunable Band Gap in Functionalized Graphene

Functionalizing graphene was recently shown to have a dramatic effect on the electronic properties of this material. Here we investigate spatial ordering of adatoms driven by the RKKY-type interactions. In the ordered state, which arises via a Peierls-instability-type mechanism, the adatoms reside mainly on one of the two graphene sublattices. Bragg scattering of electron waves induced by sublattice symmetry breaking results in a band gap opening, whereby Dirac fermions acquire a finite mass. The band gap is found to be immune to the adatoms' positional disorder, with only an exponentially small number of localized states residing in the gap. The gapped state is stabilized in a wide range of electron doping. Our findings show that controlled adsorption of adatoms or molecules provides a route to engineering a tunable band gap in graphene.

cond-mat.mes-hall↗

Electron-electron interactions in the conductivity of graphene

The effect of electron-electron interaction on the low-temperature conductivity of graphene is investigated experimentally. Unlike in other two-dimensional systems, the electron-electron interaction correction in graphene is sensitive to the details of disorder. A new temperature regime of the interaction correction is observed where quantum interference is suppressed by intra-valley scattering. We determine the value of the interaction parameter, F_0 ~ -0.1, and show that its small value is due to the chiral nature of interacting electrons.

cond-mat.mes-hall↗

Guided plasmons in graphene p-n junctions

Spatial separation of electrons and holes in graphene gives rise to existence of plasmon waves confined to the boundary region. Theory of such guided plasmon modes within hydrodynamics of electron-hole liquid is developed. For plasmon wavelengths smaller than the size of charged domains plasmon dispersion is found to be ω~ q^(1/4). Frequency, velocity and direction of propagation of guided plasmon modes can be easily controlled by external electric field. In the presence of magnetic field spectrum of additional gapless magnetoplasmon excitations is obtained. Our findings indicate that graphene is a promising material for nanoplasmonics.

cond-mat.mes-hall↗

Klein Backscattering and Fabry-Perot Interference in Graphene Heterojunctions

We present a theory of quantum-coherent transport through a lateral p-n-p structure in graphene, which fully accounts for the interference of forward and backward scattering on the p-n interfaces. The backreflection amplitude changes sign at zero incidence angle because of the Klein phenomenon, adding a phase $π$ to the interference fringes. The contributions of the two p-n interfaces to the phase of the interference cancel with each other at zero magnetic field, but become imbalanced at a finite field. The resulting half a period shift in the Fabry-Perot fringe pattern, induced by a relatively weak magnetic field, can provide a clear signature of Klein scattering in graphene. This effect is shown to be robust in the presence of spatially inhomogeneous potential of moderate strength.

cond-mat.mes-hall↗

Coherent Particle Transfer in an On-Demand Single-Electron Source

Coherent electron transfer from a localized state trapped in a quantum dot into a ballistic conductor, taking place in on-demand electron sources, in general may result in excitation of particle-hole pairs. We consider a simple model for these effects, involving a resonance level with time-dependent energy, and derive Floquet scattering matrix describing inelastic transitions of particles in the Fermi sea. We find that, as the resonance level is driven through the Fermi level, particle transfer may take place completely without particle-hole excitations for certain driving protocols. In particular, such noiseless transfer occurs when the level moves with constant rapidity, its energy changing linearly with time. A detection scheme for studying the coherence of particle transfer is proposed.

cond-mat.mes-hall↗

Quantum Phase Tomography of a Strongly Driven Qubit

The interference between repeated Landau-Zener transitions in a qubit swept through an avoided level crossing results in Stueckelberg oscillations in qubit magnetization. The resulting oscillatory patterns are a hallmark of the coherent strongly-driven regime in qubits, quantum dots and other two-level systems. The two-dimensional Fourier transforms of these patterns are found to exhibit a family of one-dimensional curves in Fourier space, in agreement with recent observations in a superconducting qubit. We interpret these images in terms of time evolution of the quantum phase of qubit state and show that they can be used to probe dephasing mechanisms in the qubit.

cond-mat.mes-hall↗

Transport in Graphene p-n Junctions in Magnetic Field

Ballistic transport in graphene p-n junctions in the presence of magnetic field exhibits two distinct regimes: At low fields, transport is partially suppressed by the field. When the field exceeds a certain critical value, the junction is pinched off by the Landau level formation. Transmission and conductance are found in the entire range of fields using Lorentz boost and mapping to the Landau-Zener problem. We show that perfect transmission occurs at a field-dependent collimation angle, indicating that the chiral dynamics of massless Dirac fermions persists at a finite magnetic field. A current switch, utilizing field-tunable collimation angle, is proposed. With a generalization of the developed approach we study transmission through p-n junctions in graphene bilayer.

cond-mat.mes-hall↗

Atomic Collapse and Quasi-Rydberg States in Graphene

Charge impurities in graphene can host an infinite family of Rydberg-like resonance states of massless Dirac particles. These states, appearing for supercritical charge, are described by Bohr-Sommerfeld quantization of collapsing classical trajectories that descend on point charge, in analogy to Rydberg states relation with planetary orbits. We argue that divalent and trivalent charge impurities in graphene is an ideal system for realization of this atomic collapse regime. Strong coupling of these states to the Dirac continuum via Klein tunneling leads to striking resonance effects with direct signatures in transport, local properties and enhancement of the Kondo effect.

cond-mat.mes-hall↗

Nonlocal Charge Transport Mediated by Spin Diffusion in the Spin-Hall Effect Regime

A nonlocal electric response in the spin-Hall regime, resulting from spin diffusion mediating charge conduction, is predicted. The spin-mediated transport stands out due to its long-range character, and can give dominant contribution to nonlocal resistance. The characteristic range of nonlocality, set by the spin diffusion length, can be large enough to allow detection of this effect in materials such as GaAs despite its small magnitude. The detection is facilitated by a characteristic nonmonotonic dependence of transresistance on the external magnetic field, exhibiting sign changes and decay.

cond-mat.mes-hall↗

Vacuum Polarization and Screening of Supercritical Impurities in Graphene

Screening of charge impurities in graphene is analyzed using the exact solution for vacuum polarization obtained from the massless Dirac-Kepler problem. For the impurity charge below certain critical value no density perturbation is found away from the impurity, in agreement with the linear response theory result. For supercritical charge, however, the polarization distribution is shown to have a power law profile, leading to screening of the excess charge at large distances. The Dirac-Kepler scattering states give rise to standing wave oscillations in the local density of states which appear and become prominent in the supercritical regime.

cond-mat.mes-hall↗