Searcharxiv⌕ Search

arXiv subjects

Titus Sandu

Publications and source records attributed to Titus Sandu.

At least 19 recordsLinked to original sources
arXiv2022

Insights into electron transport in a ferroelectric tunnel junction

The success of a ferroelectric tunnel junction (FTJ) depends on the asymmetry of electron tunneling as given by the tunneling electroresistance (TER) effect. This characteristic is mainly assessed considering three transport mechanisms: direct tunneling, thermionic emission, and Fowler-Nordheim tunneling. Here, by analyzing the effect of temperature on TER, we show that taking into account only these mechanisms may not be enough in order to fully characterize the performance of FTJ devices. We approach the electron tunneling in FTJ with the non-equilibrium Green function (NEGF) method, which is able to overcome the limitations affecting the three mechanisms mentioned above. We bring evidence that the performance of FTJs is also affected by temperature, in a non-trivial way, via resonance (Gamow-Siegert) states, which are present in the electron transmission probability and are usually situated above the barrier. Although the NEGF technique does not provide direct access to the wavefunctions, we show that, for single-band transport, one can find the wavefunction at any given energy and in particular at resonant energies in the system.

cond-mat.mes-hall↗
arXiv2019

Modal Approach to the Theory of Energy Transfer Mediated by a Metallic Nanosphere

Theoretically, the presence of a metallic nanoparticle enhances the intermolecular energy transfer. We calculate this enhancement factor with a modal approach pertaining analytical results in the case of a nanosphere. We calculate the Green's function of the system relaying on the spectral properties of the electrostatic operator, fully known for spherical geometry. In contrast to other treatments, the present calculations are straightforward for any molecular orientation giving modal information about the response of the system. Numerical calculations and further discussions are also provided.

cond-mat.mes-hall↗
arXiv2019

A perspective on effective medium models of thermal conductivity in (ultra)nanocrystalline diamond films

Thermal conductivity of nanocrystalline and ultra-nanocrystalline films is analyzed with effective medium theory (EMT) models. The existing EMT models use the spherical inclusion approximation. Although this approximation works quite well it is inconsistent, mostly with respect to the maximal packing of 74{\%}, which may be unrealistic for polycrystalline films. To check the consistency of these models we devise an EMT model with arbitrarily shaped inclusions. We pick the EMT model with cubic inclusions and we compare its results with the results of the EMT model with spherical inclusions. It is found a very good agreement between both calculations. This agreement is explained by general geometrical arguments. We further employ these models to analyze thermal conductivity of nanocrystalline and ultra-nanocrystalline diamond films. It is noticed that the effective conductivity is strongly affected not only by the boundary Kapitza resistance but also by intra-grain scattering for grain sizes below 100 nm. Generally, both intra-grain conductivity and Kapitza resistance increase with grain size. However, the effect of Kapitza resistance increase is negligible due to the geometrical factor accompanying Kapitza resistance contribution to the effective conductivity.

cond-mat.mtrl-sci↗
arXiv2015

Dielectric Sensing with Back-Gated Nanowires

Extensive numerical calculations show that the capacitance of back-gated nanowires with various degrees of dielectric embeddings is accurately described with an effective dielectric constant as long as the difference between the dielectric thickness and the gate-nanowire distance is held constant. This is valid for dielectrics with permittivities ranging from simple air to water. However, due to screening the scaling is not valid if the dielectric lies down well below the nanowire. Moreover, when only the dielectric thickness varies the capacitance characteristics are S-shaped with three distinct regions, of which only the first two can be used for dielectric sensing. The first region is almost linear while the middle region, with a span of two diameters around the center of the nanowire, is the most sensitive.

cond-mat.mes-hall↗
arXiv2015

The Rabi Hamiltonian in the dispersive regime

The Rabi Hamiltonian is studied in the dispersive regime and ultra-strong coupling. We employ a recent unitary transformation to obtain not only the approximate Hamiltonian and its energy levels but also its eigenfunctions. The relationship of the approximation with other regimes and their approximations are also discussed.

cond-mat.mes-hall↗
arXiv2014

Applications of electrostatic capacitance and charging

The capacitance of an arbitrarily shaped object is calculated with the same second-kind integral equation method used for computing static and dynamic polarizabilities. The capacitance is simply the dielectric permittivity multiplied by the area of the object and divided by the squared norm of the Neumann-Poincaré operator eigenfunction corresponding to the largest eigenvalue. The norm of this eigenfunction varies slowly with shape thus enabling the definition of two scale-invariant shape factors and perturbative calculations of capacitance. The result is extended to a special class of capacitors in which the electrodes are the equipotential surfaces generated by the equilibrium charge on the object. This extention allows analytical expressions of capacitance for confocal spheroidal capacitors and finite cylinders. Moreover, a second order formula for thin constant-thickness capacitors is given with direct applications for capacitance of membranes in living cells and of supercapacitors. For axisymmetric geometries a fast and accurate numerical method is provided.

cond-mat.mes-hall↗
arXiv2013

Eigenmode decomposition of the near-field enhancement in localized surface plasmon resonances of metallic nanoparticles

I present a direct and intuitive eigenmode method that evaluates the near-field enhancement around the surface of metallic nanoparticles of arbitrary shape. The method is based on the boundary integral equation in the electrostatic limit. Besides the nanoparticle polarizability and the far-field response, the near-field enhancement around nanoparticles can be also conveniently expressed as an eigenmode sum of resonant terms. Moreover, the spatial configuration of the near-field enhancement depends explicitly on the eigenfunctions of both the BIE integral operator and of its adjoint. It is also established a direct physical meaning of the two types of eigenfunctions. While it is well known that the eigenfunctions of the BIE operator are electric charge modes, it is less known and used that the eigenfunctions of the adjoint represent the electric potential generated by the charge modes. For the enhanced spectroscopies the present method allows an easy identification of hot spots which are located in the regions with maximum charge densities and/or regions with fast variations of the electric potential generated by the charge modes on the surface. This study also clarifies the similarities and the differences between the far-field and the near-field behavior of plasmonic systems. Finally, the analysis of concrete examples like the nearly touching dimer, the prolate spheroid, and the nanorod illustrate some modalities to improve the near-field enhancement.

cond-mat.mes-hall↗
arXiv2013

Capacitance and charging of metallic objects

The capacitance of arbitrarily shaped objects is reformulated in terms of the Neumann-Poincaré operator. Capacitance is simply the dielectric permittivity of the surrounding medium multiplied by the area of the object and divided by the squared norm of the Neumann-Poincaré eigenfunction that corresponds to its largest eigenvalue. The norm of this eigenfunction varies slowly with shape changes and allows perturbative calculations. This result is also extended to capacitors. For axisymmetric geometries a numerical method provides excellent results against finite element method results. Two scale-invariant shape factors and the capacitance of nanowires and of membrane in biological cells are discussed.

cond-mat.mes-hall↗
arXiv2013

Shape effects on localized surface plasmon resonances in metallic nanoparticles

The effect of smooth shape changes of metallic nanoparticles on localized surface plasmon resonances is assessed with a boundary integral equation method. The boundary integral equation method allows compact expressions of nanoparticle polarizability which is expressed as an eigenmode sum of terms that depends on the eigenvalues and eigenfunctions of the integral operator associated to the boundary integral equation method. Shape variations change not only the eigenvalues but also their coupling weights to the electromagnetic field. Thus, rather small changes in the shape may induce large variations of the coupling weights. It has been found that shape changes that bring volume variations >12% induce structural changes in the extinction spectrum of metallic nanoparticles. Also, the largest variations in eigenvalues and their coupling weights are encountered by shape changes along the smallest cross-sections of nanoparticles. These results are useful as guiding rules in the process of designing plasmonic nanostrucrures.

cond-mat.mes-hall↗
arXiv2011

Surface plasmon resonances of clustered nanoparticles

Linear clusters made by tightly connecting two or more metallic nanoparticles have new types of surface plasmon resonances as compared to isolated nanoparticles. These new resonances are related to the size of the junction and to the number of interconnected particles and have direct interpretation as eigenmodes of a Boundary Integral Equation (BIE). This formulation allows effective separation of geometric and shape contribution from electric properties of the constituents. Results for particles covered by a thin shell are also provided. In addition, the present analysis sheds a new light on the interpretation of recent experiments from literature.

cond-mat.mes-hall↗
arXiv2010

Comments on "Spin-dependent tunneling through a symmetric semiconductor barrier: The Dresselhaus effect"

In a recent paper [Phys. Rev. B 72, 153314 (2005)], the $k^{3}$-Dresselhaus term in the contacts and the full form of the current operator are considered for spin-dependent tunneling through a symmetric barrier. The authors found that the full form of the current operator has a much larger influence on the spin polarization than it was initially thought. In this Comment we will show that their treatment of the other problem, the $k^{3}$-Dresselhaus term in the contacts, is incorrect. Their proposed solution in the contacts simply does not obey the Schrödinger equation. In this context we also comment on the definition and the suitability of spin polarization in contacts with spin-orbit coupling.

cond-mat.mes-hall↗
arXiv2010

Bandgaps and band bowing in semiconductor alloys

The bandgap and band bowing parameter of semiconductor alloys are calculated with a fast and realistic approach. The method is a dielectric scaling approximation that is based on a scissor approximation. It adds an energy shift to the bandgap provided by the local density approximation (LDA) of the density functional theory (DFT). The energy shift consists of a material-independent constant weighted by the inverse of the high-frequency dielectric constant. The salient feature of the approach is the fast calculation of the dielectric constant of alloys via the Green function (GF) of the TB-LMTO (tight-binding linear muffin-tin orbitals) in the atomic sphere approximation (ASA). When it is applied to highly mismatched semiconductor alloys (HMAs) like Zn Te$_x$ Se$_{1-x}$, this method provides a band bowing parameter that is different from the band bowing parameter calculated with the LDA due to the bowing exhibited also by the high-frequency dielectric constant.

cond-mat.mtrl-sci↗
arXiv2010

Linear dielectric response of clustered living cells

The dielectric behavior of a linear cluster of two or more living cells connected by tight junctions is analyzed using a spectral method. The polarizability of this system is obtained as an expansion over the eigenmodes of the linear response operator, showing a clear separation of geometry from electric parameters. The eigenmode with the second largest eigenvalue dominates the expansion as the junction between particles tightens, but only when the applied field is aligned with the cluster axis. This effect explains a distinct low-frequency relaxation observed in the impedance spectrum of a suspension of linear clusters.

physics.bio-ph↗
arXiv2009

Generalized band anti-crossing model for highly mismatched semiconductors applied to BeSe$_{x}$Te$_{1 - x}$

We report a new model for highly mismatched semiconductor (HMS) alloys. Based on the Anderson impurity Hamiltonian, the model generalizes the recent band anti-crossing (BAC) model, which successfully explains the band bowing in highly mismatched semiconductors. Our model is formulated in empirical tight-binding (ETB) theory and uses the so called sp$^{3}$s* parameterization. It does not need extra parameters other than bulk ones. The model has been applied to BeSe$_{x}$Te$_{1 - x}$ alloy. BeTe and BeSe are wide-band gap and highly mismatched semiconductors. Calculations show large band bowing, larger on the Se rich side than on the Te rich side. Linear interpolation is used for an arbitrary concentration $x$. The results are applied to calculation of electronic and optical properties of BeSe$_{0.41}$Te$_{0.59}$ lattice matched to Si in a superlattice configuration.

cond-mat.other↗
arXiv2009

Dynamics of a quantum oscillator strongly and off-resonantly coupled with a two-level system

Beyond the rotating-wave approximation, the dynamics of a quantum oscillator interacting strongly and off-resonantly with a two-level system exhibit beatings, whose period equals the revival time of the two-level system. On a longer time scale, the quantum oscillator shows collapses, revivals and fractional revivals, which are encountered in oscillator observables like the mean number of oscillator quanta and in the two-level inversion population. Also the scattered oscillator field shows doublets with symmetrically displaced peaks.

cond-mat.mes-hall↗
arXiv2009

Ab-initio calculations of spin tunneling through an indirect barrier

We use a fully relativistic layer Green's functions approach to investigate spin-dependent tunneling through a symmetric indirect band gap barrier like GaAs/AlAs/GaAs heterostructure along [100] direction. The method is based on Linear Muffin Tin Orbitals and it is within the Density Functional Theory (DFT) in the Local Density Approximation (LDA). We find that the results of our {\it ab-initio} calculations are in good agreement with the predictions of our previous empirical tight binding model [Phys. Rev. {\bf B}, 075313 (2006)]. In addition we show the $k_{||}$-dependence of the spin polarization which we did not previously include in the model. The {\it ab-initio} calculations indicate a strong $k_{||}$-dependence of the transmission and the spin polarization due to band non-parabolicity. A large window of 25-50 % spin polarization was found for a barrier of 8 AlAs monolayers at $k_{||}$ = 0.03 $2π/a$. Our calculations show clearly that the appearance of energy windows with significant spin polarization depends mostly on the location of transmission resonances and their corresponding zeros and not on the magnitude of the spin splitting in the barrier.

cond-mat.mtrl-sci↗
arXiv2006

Dynamics of a two-level system coupled with a quantum oscillator in the very strong coupling limit

The time-dependent behavior of a two-level system interacting with a quantum oscillator system is analyzed in the case of a coupling larger than both the energy separation between the two levels and the energy of quantum oscillator ($Ω< ω< λ$, where $Ω$ is the frequency of the transition between the two levels, $ω$ is the frequency of the oscillator, and $λ$ is the coupling between the two-level system and the oscillator). Our calculations show that the amplitude of the expectation value of the oscillator coordinate decreases as the two-level system undergoes the transition from one level to the other, while the transfer probability between the levels is staircase-like. This behavior is explained by the interplay between the adiabatic and the non-adiabatic regimes encountered during the dynamics with the system acting as a quantum counterpart of the Landau-Zener model. The transition between the two levels occurs as long as the expectation value of the oscillator coordinate is driven close to zero. On the contrary, if the initial conditions are set such that the expectation values of the oscillator coordinate are far from zero, the system will remain locked on one level.

cond-mat.mes-hall↗