arXiv subjects

F. V. Kusmartsev

Publications and source records attributed to F. V. Kusmartsev.

At least 37 records · Page 2Linked to original sources

arXiv2016

Modelling a network where the opinion of each unit varies according to a majority ruling of its neighbouring units

The complexity of human behaviour can lead to very unpredictable patterns in social activity and structure. Here we demonstrate the instability of a community network controlled by majority ruling, where an element adopts the most popular opinion of their peers. We modelled a community as a square lattice, and performed sequential time step numerical calculations upon each cell in parallel. Depending on the initial ratio of two opinions, the community can segregate either into separate gangs and cliques, or get dominated by a single opinion. We also note that gangs are separated by neutral or confused groups of individuals, buffering the transition. The behaviors shown by this model can be comfortably applied to many other real life situations, such as neural or ecological networks. The results of this paper have been preliminary published in the Ref. [34].

nlin.CG↗

arXiv2015

Snake states and their symmetries in graphene

Snake states are open trajectories for charged particles propagating in two dimensions under the influence of a spatially varying perpendicular magnetic field. In the quantum limit they are protected edge modes that separate topologically inequivalent ground states and can also occur when the particle density rather than the field is made nonuniform. We examine the correspondence of snake trajectories in single-layer graphene in the quantum limit for two families of domain walls: (a) a uniform doped carrier density in an antisymmetric field profile and (b) antisymmetric carrier distribution in a uniform field. These families support different internal symmetries but the same pattern of boundary and interface currents. We demonstrate that these physically different situations are gauge equivalent when rewritten in a Nambu doubled formulation of the two limiting problems. Using gauge transformations in particle-hole space to connect these problems, we map the protected interfacial modes to the Bogoliubov quasiparticles of an interfacial one-dimensional p-wave paired state. A variational model is introduced to interpret the interfacial solutions of both domain wall problems.

cond-mat.mes-hall↗

arXiv2015

Holographic model for antiferromagnetic quantum phase transition induced by magnetic field

We propose a gravity dual of antiferromagnetic quantum phase transition (QPT) induced by magnetic field and study the critical behavior around the quantum critical point (QCP). It turns out that the boundary critical theory is a strong coupling theory with dynamic exponent $z=2$ and that the hyperscaling law is violated and logarithmic corrections appear near the QCP. Some novel scaling relations are predicated, which can be tested by experiment data in future. We also make some comparison with experimental data on low-dimensional magnets BiCoPO$_5$ and pyrochlores Er$_{2-2x}$Y$_{2x}$Ti$_2$O$_7$.

hep-th↗

arXiv2015

Holographic antiferromganetic quantum criticality and AdS$_2$ scaling limit

A holographic description on antiferromagnetic quantum phase transition (QPT) induced by magnetic field and the criticality in the vicinity of quantum critical point (QCP) have been investigated numerically recently. In this paper, we show that the properties of QPT in this holographic model are governed by a CFT dual to the emergent AdS$_2$ in the IR region, which confirms that the dual boundary theory is a strong coupling theory with dynamic exponent $z=2$ and logarithmic corrections appear. We also compare them with the results from Hertz model by solving RG equation at its upper critical dimension and with some experimental data from pyrochlores Er$_{2-2x}$Y$_{2x}$Ti$_2$O$_7$ and BiCoPO$_5$.

hep-th↗

arXiv2015

Physical Principles of the Amplification of Electromagnetic Radiation Due to Negative Electron Masses in a Semiconductor Superlattice

In a superlattice placed in crossed electric and magnetic fields, under certain conditions, the inversion of electron population can appear at which the average energy of electrons is above the middle of the miniband and the effective mass of the electron is negative. This is the implementation of the negative effective mass amplifier and generator (NEMAG) in the superlattice. It can result in the amplification and generation of terahertz radiation even in the absence of negative differential conductivity.

cond-mat.mes-hall↗

arXiv2014

Physical Properties of Zener Tunnelling Nano-devices in Graphene

By considering the direction of charge carriers and the conservation of probablity current the transmission properties of graphene Zener tunnelling nano-devices were obtained. The scattering properties were then used with an adaptation of the Landauer formalism to calculate an analytical expression for current and conductance. The numerical results of the IV characteristics were then briefly discussed for the graphene step and Zener barrier. A comparison between the theoretical model and experimental results shows the similarities of graphene nanoribbons and infinite sheet graphene.

cond-mat.mtrl-sci↗

arXiv2014

Optical evidence of quantum rotor orbital excitations in orthorhombic manganites

In magnetic compounds with Jahn-Teller (JT) ions (such as Mn3+ or Cu2+), the ordering of the electron or hole orbitals is associated with cooperative lattice distortions. There the role of JT effect, although widely recognised, is still elusive in the ground state properties. We suggest that, in these materials, there exist elementary excitations whose energy spectrum is described in terms of the total angular momentum eigenstates and is quantised as in quantum rotors found in JT centers. We observed features originating from these excitations in the optical spectra of a model compound LaMnO3 using ellipsometry technique. They appear clearly as narrow sidebands accompanying the electron transition between the JT split orbitals on neighbouring Mn3+ ions, strongly influenced by anisotropic spin correlations. We present these results together with new experimental data on photoluminescence and its kinetics found in LaMnO3, which lend additional support to the ellipsometry implying the existence of the quantum rotor orbital excitations. We note that the discovered elementary excitations of quantum rotors may play an important role in many unusual properties observed in these materials upon doping, such as high-temperature superconductivity and colossal magnetoresistance.

cond-mat.str-el↗

arXiv2014

Anomalous multi-order Raman scattering in LaMnO_3: a signature of a quantum lattice effect in a Jahn-Teller crystal

The multi-order Raman scattering is studied up to a fourth order for a detwinned LaMnO_3 crystal. Based on a comprehensive data analysis of the polarisation-dependent Raman spectra, we show that the anomalous features in the multi-order scattering could be the sidebands on the low-energy mode at about 25 cm-1. We suggest that this low-energy mode stems from the tunneling transition between the potential energy minima arising near the Jahn-Teller Mn3+ ion due to the lattice anharmonicity, and the multi-order scattering is activated by this low-energy electronic motion. The sidebands are dominated by the oxygen contribution to the phonon density-of-states, however, there is an admixture of an additional component, which may arise from coupling between the low-energy electronic motion and the vibrational modes.

cond-mat.str-el↗

arXiv2014

Application of Graphene within Optoelectronic Devices and Transistors

Scientists are always yearning for new and exciting ways to unlock graphene's true potential. However, recent reports suggest this two-dimensional material may harbor some unique properties, making it a viable candidate for use in optoelectronic and semiconducting devices. Whereas on one hand, graphene is highly transparent due to its atomic thickness, the material does exhibit a strong interaction with photons. This has clear advantages over existing materials used in photonic devices such as Indium-based compounds. Moreover, the material can be used to 'trap' light and alter the incident wavelength, forming the basis of the plasmonic devices. We also highlight upon graphene's nonlinear optical response to an applied electric field, and the phenomenon of saturable absorption. Within the context of logical devices, graphene has no discernible band-gap. Therefore, generating one will be of utmost importance. Amongst many others, some existing methods to open this band-gap include chemical doping, deformation of the honeycomb structure, or the use of carbon nanotubes (CNTs). We shall also discuss various designs of transistors, including those which incorporate CNTs, and others which exploit the idea of quantum tunneling. A key advantage of the CNT transistor is that ballistic transport occurs throughout the CNT channel, with short channel effects being minimized. We shall also discuss recent developments of the graphene tunneling transistor, with emphasis being placed upon its operational mechanism. Finally, we provide perspective for incorporating graphene within high frequency devices, which do not require a pre-defined band-gap.

cond-mat.mtrl-sci↗

arXiv2014

Cosmic Expansion via Axion-Induced Quintessence

In this paper, dark energy is modelled via a spherically symmetric quintessence scalar field $φ$, the dynamics of which are found to be analogous to a pendulum. This is due to a driving axion potential $V(\left|φ\right|)$, whose origins reside within the study of quantum chromodynamics (QCD). The effect of a cosmological constant $Λ$, introduced to represent the vacuum energy of space, is also investigated. Preliminary results suggest that $Λ$ is analogous to a spring-constant, and thus determining the elasticity of space. Additionally, a cosmological scale-factor $a(t,r)$, notably with an added spatial dependence, is also considered. We propose that due to this inhomogeneous scale-factor, the energy-density is characteristic of temperature fluctuations observed within the cosmic microwave background. Such fluctuations could ultimately lead to a universe composed of filaments and voids; vast expanses of space, separated by regions of localised matter/energy-density. Finally, we provide a means of screening both the cosmological constant and curvature of the universe, to effective values that are more consistent with experimental observation.

gr-qc↗

arXiv2013

Introduction to Graphene Electronics -- A New Era of Digital Transistors and Devices

The speed of silicon-based transistors has reached an impasse in the recent decade, primarily due to scaling techniques and the short-channel effect. Conversely, graphene (a revolutionary new material possessing an atomic thickness) has been shown to exhibit a promising value for electrical conductivity. Graphene would thus appear to alleviate some of the drawbacks associated with silicon-based transistors. It is for this reason why such a material is considered one of the most prominent candidates to replace silicon within nano-scale transistors. The major crux here, is that graphene is intrinsically gapless, and yet, transistors require a band-gap pertaining to a well-defined ON/OFF logical state. Therefore, exactly as to how one would create this band-gap in graphene allotropes is an intensive area of growing research. Existing methods include nano-ribbons, bilayer and multi-layer structures, carbon nanotubes, as well as the usage of the graphene substrates. Graphene transistors can generally be classified according to two working principles. The first is that a single graphene layer, nanoribbon or carbon nanotube can act as a transistor channel, with current being transported along the horizontal axis. The second mechanism is regarded as tunneling, whether this be band-to-band on a single graphene layer, or vertically between adjacent graphene layers. The high-frequency graphene amplifier is another talking point in recent research, since it does not require a clear ON/OFF state, as with logical electronics. This paper reviews both the physical properties and manufacturing methodologies of graphene, as well as graphene-based electronic devices, transistors, and high-frequency amplifiers from past to present studies. Finally, we provide possible perspectives with regards to future developments.

cond-mat.mtrl-sci↗

arXiv2013

Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study

Magnetic materials are usually divided into two classes: those with localised magnetic moments, and those with itinerant charge carriers. We present a comprehensive experimental (spectroscopic ellipsomerty) and theoretical study to demonstrate that these two types of magnetism do not only coexist but complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material the itinerant charge carriers interact with large localised magnetic moments of Tb(4f) states, forming complex magnetic lattices at low temperatures, which we associate with self-organisation of magnetic clusters. The formation of magnetic clusters results in low-energy optical spectral weight shifts, which correspond to opening of the pseudogap in the conduction band of the itinerant charge carriers and development of the low- and high-spin intersite electronic transitions. This phenomenon, driven by self-trapping of electrons by magnetic fluctuations, could be common in correlated metals, including besides Kondo-lattice metals, Fe-based and cuprate superconductors.

cond-mat.str-el↗

arXiv2012

Stripes and superconductivity in the two-dimensional self-consistent model

We found solutions of the Bogoliubov-de Gennes equations for the two-dimensional self-consistent model of superconductors with $d_{x^2-y^2}$ symmetry of the order parameter, taking into account spin and charge distributions. Analytical solutions for spin-charge density wave phases in the absence of the superconductivity ("stripe" and "checkerboard" structures) are presented. Analytical solutions for coexisting superconductivity and stripes are found.

cond-mat.supr-con↗

arXiv2012

Squeezing as the source of inefficiency in the quantum Otto cycle

The availability of controllable macroscopic devices, which maintain quantum coherence over relatively long time intervals, for the first time allows an experimental realization of many effects previously considered only as Gedankenexperiments, such as the operation of quantum heat engines. The theoretical efficiency ηof quantum heat engines is restricted by the same Carnot boundary η_C as for the classical ones: any deviations from quasistatic evolution suppressing ηbelow η_C. Here we investigate an implementation of an analog of the Otto cycle in a tunable quantum coherent circuit and show that the specific source of inefficiency is the quantum squeezing of the thermal state due to the finite speed of compression/expansion of the system.

quant-ph↗

arXiv2010

Extraordinary Magnetoresistance in Hybrid Semiconductor-Metal Systems

We show that extraordinary magnetoresistance (EMR) arises in systems consisting of two components; a semiconducting ring with a metallic inclusion embedded. The im- portant aspect of this discovery is that the system must have a quasi-two-dimensional character. Using the same materials and geometries for the samples as in experiments by Solin et al.[1;2], we show that such systems indeed exhibit a huge magnetoresistance. The magnetoresistance arises due to the switching of electrical current paths passing through the metallic inclusion. Diagrams illustrating the flow of the current density within the samples are utilised in discussion of the mechanism responsible for the magnetoresistance effect. Extensions are then suggested which may be applicable to the silver chalcogenides. Our theory offers an excellent description and explanation of experiments where a huge magnetoresistance has been discovered[2;3].

cond-mat.mtrl-sci↗

arXiv2010

Stability Studies of Vortex State in Superconducting Disks

We are going to analyze the nucleation of vortices in a thin mesoscopic superconducting disk. The Gibbs free energy from London model is formulated. This energy function, with an arbitrary configuration of vortices, is assoicated with the disk's size, applied magnetic field and finite temperature. Then, the optimal solution is obtained by differentiating with respect position r, for fixed applied field and temperature. We also investigate the stability of the different vortex states inside the disk and compare our results with those of other theoretical studies and with available experimental observations. Our results agree with experiments. Besides, we formulate the Gibbs free energy by Ginzburg-Landau (GL) model. Theoretically, the free energy by GL model takes the superconducting density into account but not in London model. Our simulations from both theories show the same quantum states of vortex. We find that the Gibbs free energy by GL model is smaller than by London model.

cond-mat.supr-con↗

arXiv2009

Vortex qubit based on an annular Josephson junction containing a microshort

We report theoretical and experimental work on the development of a vortex qubit based on a microshort in an annular Josephson junction. The microshort creates a potential barrier for the vortex, which produces a double-well potential under the application of an in-plane magnetic field; The field strength tunes the barrier height. A one-dimensional model for this system is presented, from which we calculate the vortex depinning current and attempt frequency as well as the interwell coupling. Implementation of an effective microshort is achieved via a section of insulating barrier that is locally wider in the junction plane. Using a junction with this geometry we demonstrate classical state preparation and readout. The vortex is prepared in a given potential well by sending a series of "shaker" bias current pulses through the junction. Readout is accomplished by measuring the vortex depinning current.

cond-mat.supr-con↗

arXiv2008

Shape waves in 2D Josephson junctions: exact solutions and time dilation

We predict a new class of excitations propagating along a Josephson vortex in two-dimensional Josephson junctions. These excitations are associated with the distortion of a Josephson vortex line and have an analogy with shear waves in solid mechanics. Their shapes can have an arbitrary profile, which is retained when propagating. We derive a universal analytical expression for the energy of arbitrary shape excitations, investigate their influence on the dynamics of a vortex line, and discuss conditions where such excitations can be created. Finally, we show that such excitations play the role of a clock for a relativistically-moving Josephson vortex and suggest an experiment to measure a time dilation effect analogous to that in special relativity.

cond-mat.supr-con↗