The dielectric behavior of a linear cluster of two or more living cells connected by tight juncti... more 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.
Linear clusters made by tightly connecting two or more metallic nanoparticles have new types of s... more 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.
The effect of smooth shape changes of metallic nanoparticles on localized surface plasmon resonan... more 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 greater than 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.
Beyond the rotating-wave approximation, the dynamics of a quantum oscillator interacting strongly... more 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.
We have calculated electronic and optical properties of Si/BeSe 0.41 Te 0.59 heterostructures by ... more We have calculated electronic and optical properties of Si/BeSe 0.41 Te 0.59 heterostructures by a semiempirical sp 3 s * tight-binding method. Tight-binding parameters and band bowing of BeSe 0.41 Te 0.59 are considered through a recent model for highly mismatched semiconductor alloys. The band bowing and the measurements of conduction band offset lead to a type II heterostucture for Si/BeSe 0.41 Te 0.59 with conduction band minimum in the Si layer and valence band maximum in the BeSe 0.41 Te 0.59 layer. The electronic structure and optical properties of various (Si 2 ) n /(BeSe 0.41 Te 0.59 ) m [001] superlattices have been considered. Two bands of interface states were found within the bandgap of bulk Si. Our calculations indicate that the optical edges are below the fundamental bandgap of bulk Si and the transitions are optically allowed.
CAS 2010 Proceedings (International Semiconductor Conference), 2010
We analyse the adequacy of the single-band model in Si based resonant tunneling devices (RTD's) a... more We analyse the adequacy of the single-band model in Si based resonant tunneling devices (RTD's) as opposed to a multi-band model which is closer to real systems due to the fact that Si is an indirect bandgap semiconductor. Our calculations based on nonequilibrium Green function formalism show that a single band model with tunneling light electrons in Si is quantitatively sound in simulating Si based RTD's. It is found that light electrons contribute not only through 2-dimensional (2D) transverse density of states but also through the transparency of the barriers. Thus a single-band model can be safely used in applications.
The capacitance of an arbitrarily shaped object is calculated with the same second-kind integral ... more 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.
Beryllium selenide telluride BeSe_0.41Te_0.59 is a II-VI semiconductor lattice-matched to silicon... more Beryllium selenide telluride BeSe_0.41Te_0.59 is a II-VI semiconductor lattice-matched to silicon. For Si-based optoelectronic heterostructures, BeSeTe has planar epitaxial growth, covalent fault-resistant bonding, and thermal and chemical stability. BeSeTe has an indirect bandgap of 3.0eV, from a tight-binding model alloy of the indirect-gap constituents BeTe (2.7eV) and BeSe (4.5eV). Growth, fabrication, and vertical transport measurements were done for several thin
The epitaxial growth of multiple layers of BeSeTe/Si films on arsenic passivated vicinal Si (100)... more The epitaxial growth of multiple layers of BeSeTe/Si films on arsenic passivated vicinal Si (100) substrates is reported. These are interesting wide bandgap heterostructures that are lattice matched to silicon. Reflection high-energy electron diffraction (RHEED) was used to investigate the entire growth regime and to optimize the initial growth conditions, which is important for reducing interface defects. Atomic layer epitaxy
BeTe and BeSe are wide-band gap and highly mismatched semiconductors (HMS). The band bowing of Be... more BeTe and BeSe are wide-band gap and highly mismatched semiconductors (HMS). The band bowing of BeSexTe1-x originates in the mismatch of size and orbital energies between Te and Se and is under-predicted by virtual crystal approximation. The recent band anti-crossing (BAC) model explains the band bowing in highly mismatched semiconductors. We extend the BAC model to empirical tight-binding (ETB) theory. We use the sp3s* model and calculate the band bowing of the on-site (self-) energies for an arbitrary concentration x. The results are applied to calculation of electronic and optical properties of BeSe0.41Te0.59 lattice matched to Si in a superlattice configuration.
CAS 2011 Proceedings (2011 International Semiconductor Conference), 2011
ABSTRACT Localized surface plasmon resonances (LSPRs) in metallic nanoparticles (NPs) are calcula... more ABSTRACT Localized surface plasmon resonances (LSPRs) in metallic nanoparticles (NPs) are calculated by a boundary integral equation (BIE) method. The response to the incident electromagnetic field and the NP polarizability are shown to depend on eigenvalues and eigenfunctions of the integral operator associated with BIE. The NP polarizability is conveniently expressed as an eigenmode sum of analytic terms when the Drude model of metals is used. The proposed polarizability decomposition is proven to offer an analytical tool for the design of plasmonic nanostructures.
ABSTRACT The capacitance of an arbitrarily shaped object is calculated with the same second-kind ... more ABSTRACT 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-Poincar\'{e} 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.
2014 International Semiconductor Conference (CAS), 2014
The effect of dielectric embedding on the capacitance of back-gated nanowires can be accurately c... more The effect of dielectric embedding on the capacitance of back-gated nanowires can be accurately captured as an effective dielectric constant that depends solely on the difference between the nanowire-gate distance and the dielectric thickness. When used for sensing purposes this property provides the maximum sensitivity within a range of two diameters around the center of the nanowire.
CAS 2012 (International Semiconductor Conference), 2012
Extinction spectra and near-field enhancement of metallic nanoparticles are calculated with a bou... more Extinction spectra and near-field enhancement of metallic nanoparticles are calculated with a boundary integral equation (BIE) method. With the BIE method the far-field response and the near-field evanescent coupling are expressed as an eigenmode sum of resonant terms. In particular, the near-field enhancement around nanoparticles is obtained as a sum of resonant terms which acquire the spatial dependence of the eigenfunctions of the BIE operators. Moreover, the presented method permits a direct link between near-field and far field spectral properties. Finally, a numerical example is given.
CAS 2013 (International Semiconductor Conference), 2013
The integral equations of the first-and secondkind can be used to calculate the capacitance and c... more The integral equations of the first-and secondkind can be used to calculate the capacitance and charging of an arbitrary conductor. In this communication we discuss the relationship between these two formulations and their numerical implementations in terms of a spectral method. Also a relationship between the second-kind integral equation formulation of capacitance and a boundary integral equation method for calculation of surface plasmon resonances in metallic nanoparticles is established and a numerical example is analyzed.
2014 International Semiconductor Conference (CAS), 2014
Usually, fluorescence of a two-level system increases with the coupling strength to an electromag... more Usually, fluorescence of a two-level system increases with the coupling strength to an electromagnetic field. However, we show numerically that the fluorescence is quenched in the ultra-strong coupling limit of a two-level system interacting with an electromagnetic field. This behavior is attributed to the onset of multi-photon processes which occur for the ultra-strong coupling between the two-level system and the electromagnetic field.
CAS 2012 (International Semiconductor Conference), 2012
The near-field enhancement of rod-like nanoantenna is studied in two variants: in the electrostat... more The near-field enhancement of rod-like nanoantenna is studied in two variants: in the electrostatic approximation and in a fully retarded approach. A boundary integral equation (BIE) method has been used in the electrostatic approximation. The BIE method allows a transparent eigenmode decomposition of both the far-field and near-field, which are valid as long as the size of the nanoantenna is much smaller than every representative light wavelength. Our near-field calculations show that for a rod-like nanoantenna with a length below 100 nm the electrostatic and the fully retarded results are in good agreement with each other.
In a recent paper [Phys. Rev. B 72, 153314 (2005)], the k 3 -Dresselhaus term in the contacts and... more 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.
Based on first principles electronic structure calculations using the Coherent Potential Approxim... more Based on first principles electronic structure calculations using the Coherent Potential Approximation (CPA) in the Blackman-Esterling-Berk (BEB) multiscattering formalism and the variable range hopping (VRH) model proposed by Mott, we evaluate the low temperature dc conductivity and its temperature dependence for n-doped wurtzite-type M:ZnO, with M = Al, Ti, Mn, at concentrations of 2, 5 and 10 at.% respectively. We theoretically determine the phenomenologic quantities in the expression of the hopping conductivity, as well as the temperature range in which the VRH model is applicable to the investigated compounds. We show that self-consistent CPA-BEB and LSDA+U calculations yield reasonable band gaps, dopant state localization and also spin magnetic moments for the Ti and Mn systems. These results are discussed in comparison with reported data obtained by supercell LSDA+U calculations for similar systems. The results in this study point to 2-5 at.% Ti and approximately 2 at.% Al codoping in wurtzite-type ZnO as an interesting option to obtain a material with an increased low temperature dc conductivity and ferromagnetic background.
The dielectric behavior of a linear cluster of two or more living cells connected by tight juncti... more 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.
Linear clusters made by tightly connecting two or more metallic nanoparticles have new types of s... more 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.
The effect of smooth shape changes of metallic nanoparticles on localized surface plasmon resonan... more 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 greater than 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.
Beyond the rotating-wave approximation, the dynamics of a quantum oscillator interacting strongly... more 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.
We have calculated electronic and optical properties of Si/BeSe 0.41 Te 0.59 heterostructures by ... more We have calculated electronic and optical properties of Si/BeSe 0.41 Te 0.59 heterostructures by a semiempirical sp 3 s * tight-binding method. Tight-binding parameters and band bowing of BeSe 0.41 Te 0.59 are considered through a recent model for highly mismatched semiconductor alloys. The band bowing and the measurements of conduction band offset lead to a type II heterostucture for Si/BeSe 0.41 Te 0.59 with conduction band minimum in the Si layer and valence band maximum in the BeSe 0.41 Te 0.59 layer. The electronic structure and optical properties of various (Si 2 ) n /(BeSe 0.41 Te 0.59 ) m [001] superlattices have been considered. Two bands of interface states were found within the bandgap of bulk Si. Our calculations indicate that the optical edges are below the fundamental bandgap of bulk Si and the transitions are optically allowed.
CAS 2010 Proceedings (International Semiconductor Conference), 2010
We analyse the adequacy of the single-band model in Si based resonant tunneling devices (RTD's) a... more We analyse the adequacy of the single-band model in Si based resonant tunneling devices (RTD's) as opposed to a multi-band model which is closer to real systems due to the fact that Si is an indirect bandgap semiconductor. Our calculations based on nonequilibrium Green function formalism show that a single band model with tunneling light electrons in Si is quantitatively sound in simulating Si based RTD's. It is found that light electrons contribute not only through 2-dimensional (2D) transverse density of states but also through the transparency of the barriers. Thus a single-band model can be safely used in applications.
The capacitance of an arbitrarily shaped object is calculated with the same second-kind integral ... more 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.
Beryllium selenide telluride BeSe_0.41Te_0.59 is a II-VI semiconductor lattice-matched to silicon... more Beryllium selenide telluride BeSe_0.41Te_0.59 is a II-VI semiconductor lattice-matched to silicon. For Si-based optoelectronic heterostructures, BeSeTe has planar epitaxial growth, covalent fault-resistant bonding, and thermal and chemical stability. BeSeTe has an indirect bandgap of 3.0eV, from a tight-binding model alloy of the indirect-gap constituents BeTe (2.7eV) and BeSe (4.5eV). Growth, fabrication, and vertical transport measurements were done for several thin
The epitaxial growth of multiple layers of BeSeTe/Si films on arsenic passivated vicinal Si (100)... more The epitaxial growth of multiple layers of BeSeTe/Si films on arsenic passivated vicinal Si (100) substrates is reported. These are interesting wide bandgap heterostructures that are lattice matched to silicon. Reflection high-energy electron diffraction (RHEED) was used to investigate the entire growth regime and to optimize the initial growth conditions, which is important for reducing interface defects. Atomic layer epitaxy
BeTe and BeSe are wide-band gap and highly mismatched semiconductors (HMS). The band bowing of Be... more BeTe and BeSe are wide-band gap and highly mismatched semiconductors (HMS). The band bowing of BeSexTe1-x originates in the mismatch of size and orbital energies between Te and Se and is under-predicted by virtual crystal approximation. The recent band anti-crossing (BAC) model explains the band bowing in highly mismatched semiconductors. We extend the BAC model to empirical tight-binding (ETB) theory. We use the sp3s* model and calculate the band bowing of the on-site (self-) energies for an arbitrary concentration x. The results are applied to calculation of electronic and optical properties of BeSe0.41Te0.59 lattice matched to Si in a superlattice configuration.
CAS 2011 Proceedings (2011 International Semiconductor Conference), 2011
ABSTRACT Localized surface plasmon resonances (LSPRs) in metallic nanoparticles (NPs) are calcula... more ABSTRACT Localized surface plasmon resonances (LSPRs) in metallic nanoparticles (NPs) are calculated by a boundary integral equation (BIE) method. The response to the incident electromagnetic field and the NP polarizability are shown to depend on eigenvalues and eigenfunctions of the integral operator associated with BIE. The NP polarizability is conveniently expressed as an eigenmode sum of analytic terms when the Drude model of metals is used. The proposed polarizability decomposition is proven to offer an analytical tool for the design of plasmonic nanostructures.
ABSTRACT The capacitance of an arbitrarily shaped object is calculated with the same second-kind ... more ABSTRACT 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-Poincar\'{e} 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.
2014 International Semiconductor Conference (CAS), 2014
The effect of dielectric embedding on the capacitance of back-gated nanowires can be accurately c... more The effect of dielectric embedding on the capacitance of back-gated nanowires can be accurately captured as an effective dielectric constant that depends solely on the difference between the nanowire-gate distance and the dielectric thickness. When used for sensing purposes this property provides the maximum sensitivity within a range of two diameters around the center of the nanowire.
CAS 2012 (International Semiconductor Conference), 2012
Extinction spectra and near-field enhancement of metallic nanoparticles are calculated with a bou... more Extinction spectra and near-field enhancement of metallic nanoparticles are calculated with a boundary integral equation (BIE) method. With the BIE method the far-field response and the near-field evanescent coupling are expressed as an eigenmode sum of resonant terms. In particular, the near-field enhancement around nanoparticles is obtained as a sum of resonant terms which acquire the spatial dependence of the eigenfunctions of the BIE operators. Moreover, the presented method permits a direct link between near-field and far field spectral properties. Finally, a numerical example is given.
CAS 2013 (International Semiconductor Conference), 2013
The integral equations of the first-and secondkind can be used to calculate the capacitance and c... more The integral equations of the first-and secondkind can be used to calculate the capacitance and charging of an arbitrary conductor. In this communication we discuss the relationship between these two formulations and their numerical implementations in terms of a spectral method. Also a relationship between the second-kind integral equation formulation of capacitance and a boundary integral equation method for calculation of surface plasmon resonances in metallic nanoparticles is established and a numerical example is analyzed.
2014 International Semiconductor Conference (CAS), 2014
Usually, fluorescence of a two-level system increases with the coupling strength to an electromag... more Usually, fluorescence of a two-level system increases with the coupling strength to an electromagnetic field. However, we show numerically that the fluorescence is quenched in the ultra-strong coupling limit of a two-level system interacting with an electromagnetic field. This behavior is attributed to the onset of multi-photon processes which occur for the ultra-strong coupling between the two-level system and the electromagnetic field.
CAS 2012 (International Semiconductor Conference), 2012
The near-field enhancement of rod-like nanoantenna is studied in two variants: in the electrostat... more The near-field enhancement of rod-like nanoantenna is studied in two variants: in the electrostatic approximation and in a fully retarded approach. A boundary integral equation (BIE) method has been used in the electrostatic approximation. The BIE method allows a transparent eigenmode decomposition of both the far-field and near-field, which are valid as long as the size of the nanoantenna is much smaller than every representative light wavelength. Our near-field calculations show that for a rod-like nanoantenna with a length below 100 nm the electrostatic and the fully retarded results are in good agreement with each other.
In a recent paper [Phys. Rev. B 72, 153314 (2005)], the k 3 -Dresselhaus term in the contacts and... more 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.
Based on first principles electronic structure calculations using the Coherent Potential Approxim... more Based on first principles electronic structure calculations using the Coherent Potential Approximation (CPA) in the Blackman-Esterling-Berk (BEB) multiscattering formalism and the variable range hopping (VRH) model proposed by Mott, we evaluate the low temperature dc conductivity and its temperature dependence for n-doped wurtzite-type M:ZnO, with M = Al, Ti, Mn, at concentrations of 2, 5 and 10 at.% respectively. We theoretically determine the phenomenologic quantities in the expression of the hopping conductivity, as well as the temperature range in which the VRH model is applicable to the investigated compounds. We show that self-consistent CPA-BEB and LSDA+U calculations yield reasonable band gaps, dopant state localization and also spin magnetic moments for the Ti and Mn systems. These results are discussed in comparison with reported data obtained by supercell LSDA+U calculations for similar systems. The results in this study point to 2-5 at.% Ti and approximately 2 at.% Al codoping in wurtzite-type ZnO as an interesting option to obtain a material with an increased low temperature dc conductivity and ferromagnetic background.
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Papers by Titus Sandu