Browsing by Subject "Quantum theory"

Now showing 1 - 20 of 54

Open Access
Atomic and electronic structure of carbon strings
(IOP Publishing Ltd., 2005) Tongay, S.; Dag, S.; Durgun, Engin; Senger, R. T.; Çıracı, Salim
This paper presents an extensive study of various string and tubular structures formed by carbon atomic chains. Our study is based on first-principles pseudopotential plane wave and finite-temperature ab initio molecular dynamics calculations. Infinite- and finite-length carbon chains exhibit unusual mechanical and electronic properties such as large cohesive energy, axial strength, high conductance, and overall structural stability even at high temperatures. They are suitable for structural and chemical functionalizations. Owing to their flexibility and reactivity they can form linear chain, ring, helix, two-dimensional rectangular and honeycomb grids, three-dimensional cubic networks, and tubular structures. Metal-semiconductor heterostructures and various quantum structures, such as multiple quantum wells and double-barrier resonant tunnelling structures, can be formed from the junctions of metallic carbon and semiconducting BN linear chains. Analysis of atomic and electronic structures of these periodic, finite, and doped structures reveals fundamentally and technologically interesting features, such as structural instabilities and chiral currents. The double covalent bonding of carbon atoms depicted through self-consistent charge density analysis underlies the chemical, mechanical, and electronic properties.
Open Access
Bounding the Set of Finite Dimensional Quantum Correlations
(American Physical Society, 2015) Navascués, M.; Vértesi, T.
We describe a simple method to derive high performance semidefinite programing relaxations for optimizations over complex and real operator algebras in finite dimensional Hilbert spaces. The method is very flexible, easy to program, and allows the user to assess the behavior of finite dimensional quantum systems in a number of interesting setups. We use this method to bound the strength of quantum nonlocality in Bell scenarios where the dimension of the parties is bounded from above. We derive new results in quantum communication complexity and prove the soundness of the prepare-and-measure dimension witnesses introduced in Gallego et al., Phys. Rev. Lett. 105, 230501 (2010). Finally, we propose a new dimension witness that can distinguish between classical, real, and complex two-level systems. © 2015 American Physical Society. © 2015 American Physical Society.
Open Access
Chiral single-wall gold nanotubes
(American Physical Society, 2004) Senger, R. T.; Dag, S.; Çıracı, Salim
The formation of freestanding and tip-suspended chiral-wall (n,m) nanotubes, which were composed of helical atomic strands, from gold atoms was investigated using first-principles calculations, where (n,m) notation defines the structure of the tube. The tubes with 3≤n≤5 were found to be stable and exhibited electronic and transport properties investigated. The (5,3) gold tube was energetically the most favourable. It was observed from the quantum ballistic conductance, band structure and charge density analysis that the current on these wires was less chiral, and no direct correlation between the numbers of conduction channels and helical strands was found.
Open Access
Collective oscillations in a two-dimensional Bose-Einstein condensate with a quantized vortex state
(The American Physical Society, 2005) Banerjee, A.; Tanatar, Bilal
We study the effect of lower dimensional geometry on the frequency splitting of the quadrupole oscillations of a harmonically trapped Bose-Einstein condensate due to the presence of a quantized vortex. To study the effect of two-dimensional geometry we consider a pancake-shaped condensate and employ various models for the coupling parameter depending on the thickness of the condensate relative to the value of the scattering length. Using these models and the sum-rule approach we obtain analytical expressions for the frequency splitting. These expressions are valid for positive scattering length and large N. We show that the frequency splitting of the quadrupole oscillations are significantly altered by the reduced dimensionality and also study the evolution of the splitting as the system makes transition from one scattering regime to the other.
Open Access
A critical look at quantum diffusion and some of its interesting aspects
(Springer, 2007) Mukhopadhyay, S.; Saglam, M.; Chatterjee, A.
A fresh and critical look has been given to the long-time behaviour of the quantum diffusion problem and a marginally more accurate solution has been obtained as compared to the one reported in the recent literature. Attempt has also been made to bring out a few interesting generic features of this problem which may have important bearing on real systems in the quantum domain.
Open Access
Cubic-phase zirconia nano-island growth using atomic layer deposition and application in low-power charge-trapping nonvolatile-memory devices
(Institute of Physics Publishing Ltd., 2017) El-Atab, N.; Ulusoy, T. G.; Ghobadi, A.; Suh, J.; Islam, R.; Okyay, Ali Kemal; Saraswat, K.; Nayfeh, A.
The manipulation of matter at the nanoscale enables the generation of properties in a material that would otherwise be challenging or impossible to realize in the bulk state. Here, we demonstrate growth of zirconia nano-islands using atomic layer deposition on different substrate terminations. Transmission electron microscopy and Raman measurements indicate that the nano-islands consist of nano-crystallites of the cubic-crystalline phase, which results in a higher dielectric constant (κ ∼ 35) than the amorphous phase case (κ ∼ 20). X-ray photoelectron spectroscopy measurements show that a deep quantum well is formed in the Al2O3/ZrO2/Al2O3 system, which is substantially different to that in the bulk state of zirconia and is more favorable for memory application. Finally, a memory device with a ZrO2 nano-island charge-trapping layer is fabricated, and a wide memory window of 4.5 V is obtained at a low programming voltage of 5 V due to the large dielectric constant of the islands in addition to excellent endurance and retention characteristics.
Open Access
Direct imaging of localized surface plasmon polaritons
(Optical Society of America, 2011-08) Balcı, Sinan; Karademir, Ertuğrul; Kocabaş, Coşkun; Aydınlı, Atilla
In this Letter, we report on dark field imaging of localized surface plasmon polaritons (SPPs) in plasmonic waveguiding bands formed by plasmonic coupled cavities. We image the light scattered from SPPs in the plasmonic cavities excited by a tunable light source. Tuning the excitation wavelength, we measure the localization and dispersion of the plasmonic cavity mode. Dark field imaging has been achieved in the Kretschmann configuration using a supercontinuum white-light laser equipped with an acoustooptic tunable filter. Polarization dependent spectroscopic reflection and dark field imaging measurements are correlated and found to be in agreement with finite-difference time-domain calculations.
Open Access
Disorder and localization in the lowest Landau level in the presence of dilute point scatterers
(Pergamon Press, 1999) Gedik, Z.; Bayındır, Mehmet
We study the localization properties of a two-dimensional noninteracting electron gas in the presence of randomly distributed short-range scatterers in very high magnetic fields. We evaluate the participation number of the eigenstates obtained by exact diagonalization technique. At low impurity concentrations we obtain self-averaged values showing that all states, except those exactly at the Landau level, are localized with finite localization length. We conclude that in this dilute regime the localization length does not diverge. We also find that the maximum localization length increases exponentially with impurity concentration. Our calculations suggest that scaling behavior may be absent even for higher concentrations of scatterers.
Open Access
Does the donor-acceptor concept work for designing synthetic metals? 2. theoretical investigation of copolymers of 4-(dicyanomethylene)-4H-cyclopenta[2, 1-b: 3, 4-b′]dithiophene and 3, 4-(ethylenedioxy)thiophene
(American Chemical Society, 2002) Salzner, U.; Köse, M. E.
Density functional theory (DFT) calculations were performed on oligomers of 3,4-(ethylenedioxy)thiophene (EDOT), 4-(dicyanomethylene)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CDM), and co-oligomers (CDM/ EDOT). Oligomer data were extrapolated to polymer values. Theoretical band gaps reproduce λmax from UV spectroscopy for PEDOT and are about 1 eV larger than electrochemical band gaps. λmax of PCDM/EDOT is predicted to be 0.42 eV smaller than that of PEDOT and 0.15 eV smaller than that of PCDM. PCDM/EDOT has a wide valence and an extremely narrow conduction "band". It is probably better not to refer to these localized states as a band at all. This rationalizes the mobility ratio of 500 between p-type and n-type charge carriers and the low n-type conductivity of PCDM/EDOT. The lack of dispersion of the conduction band is due to the very different EAs of EDOT and CDM.
Open Access
Effect of cross-sectional geometry on the RPA plasmons of quantum wires
(Pergamon Press, 1994) Bennett, C. R.; Tanatar, Bilal; Constantinou, N. C.; Babiker, M.
The effect of cross-sectional geometry on both the intrasubband plasmon and intersubband plasmon of a quantum wire is investigated within a two-subband RPA scheme. Exact analytical electronic wavefunctions for circular, elliptical and rectangular wires are employed within the infinite barrier approximation. It is found that for fixed cross-sectional area and linear electron concentration, the intrasubband plasmon energy is only marginally dependent on the wire geometry whereas the intersubband plasmon energy may change considerably due to its dependence on the electronic subband energy difference. © 1994.
Open Access
Effective electron-electron interactions and magnetic phase transition in a two-dimensional electron liquid
(Elsevier B.V., 2007) Asgari, R.; Esmailian, A.; Tanatar, Bilal
We investigate the spin-dependent effective electron-electron interactions in a uniform system of two-dimensional electrons to understand the spontaneous magnetization expected to occur at very low density. For this purpose, we adopt the Kukkonen-Overhauser form for the effective interactions which are built by accurately determined local-field factors describing the charge and spin fluctuations. The critical behavior of the effective interaction for parallel spin electrons allows us to quantitatively locate the transition to the ferromagnetic state at rs ≈ 27. When the finite width effects are approximately taken into account the transition occurs at rs ≈ 30 in agreement with recent quantum Monte Carlo calculations.
Open Access
Electromagnetically induced left-handedness in a dense gas of three-level atoms
(The American Physical Society, 2004) Oktel, M. Ö.; Müstecaphoǧlu, Ö. E.
Ways in which a three-level system can be used to change the frequency-dependent magnetic permeability of an atomic gas were discussed. The resulting macroscopic electrodynamics was also discussed. The two levels were separated at optical frequencies while having a nonvanishing magnetic dipole matrix element. It was found that such level splittings require large external magnetic fields.
Open Access
Electronic transport through a kink in an electron waveguide
(Institute of Electrical and Electronics Engineers, 1994) Yalabik, M. C.
The current-voltage denendence correspondinp to electronic transport through a kink in an electronic waveguide is analyzed. No phase breaking dissipation mechanisms are considered, but the effects of the Coulomb interaction are included through a self consistent approximation. The results indicate very nonlinear transport properties, including negative differential resistance and bistability. © 1994 IEEE
Open Access
Experimental investigation of critical Casimir forces in binary liquid mixtures by blinking optical tweezers
(OSA, 2017) Magazzu, Alessandro; Schmidt, F.; Callegari, Agnese; Gambassi, A.; Dietrich, S.; Volpe, Giovanni
We investigate, for the first time and by blinking optical tweezers, the effects of critical Casimir forces (CCFs) on the free dynamics of a pair of spherical colloidal particles, immersed in binary liquid mixtures upon approaching their critical points.
Open Access
Fast multipole methods in service of various scientific disciplines
(IEEE, 2014) Gürel, Levent
For more than two decades, several forms of fast multipole methods have been extremely successful in various scientific disciplines. Reduced complexity solutions are obtained for solving different forms of equations that are derived from Maxwell's equations, such as Helmholtz's equation for electrodynamics and Laplace's equation for electrostatics. Fast multipole solvers are developed for and applied to the integral equations derived from Helmholtz's and Laplace's equations. Fast multipole solvers are kernel-dependent techniques, i.e., they rely on certain analytical properties of the integral-equation kernels, such as diagonalizability. Electromagnetics is not the only discipline benefiting from the fast multipole methods; a plethora of computations in various disciplines, such as the solution of Schroedinger's equation in quantum mechanics and the calculation of gravitational force in astrophysics, to name a few, exploit the reduced-complexity nature of the fast multipole methods. Acoustics, molecular dynamics, structural mechanics, and fluid dynamics can be mentioned as other disciplines served by the fast multipole methods. © 2014 IEEE.
Open Access
Fractional quantum Hall states in the vicinity of Mott plateaus
(The American Physical Society, 2010) UmucalIlar, R. O.; Mueller, E. J.
We perform variational Monte Carlo calculations to show that bosons in a rotating optical lattice will form analogs of fractional quantum Hall states when the tunneling is sufficiently weak compared to the interactions, and the deviation of density from an integer is commensurate with the effective magnetic field. We compare the energies of superfluid and correlated states to one another and to the energies found in full configuration-interaction calculations on small systems. We look at overlaps between our variational states and the exact ground state, characterizing the ways in which fractional quantum Hall effect correlations manifest themselves near the Mott insulating state. We explore the experimental signatures of these states.
Open Access
Generation of long-living entanglement between two separate three-level atoms
(The American Physical Society, 2005) Çakir, Ö.; Dung, H. T.; Knöll, L.; Welsch, Dirk- Gunnar
A scheme for nonconditional generation of long-living maximally entangled states between two spatially well separated atoms is proposed. In the scheme, A-type atoms pass a resonatorlike equipment of dispersing and absorbing macroscopic bodies giving rise to body-assisted electromagnetic field resonances of well-defined heights and widths. Strong atom-field coupling is combined with weak atom-field coupling to realize entanglement transfer from the dipole-allowed transitions to the dipole-forbidden transitions, whereby the entanglement is preserved when the atoms depart from the bodies and from each other. The theory is applied to the case of atoms passing by a microsphere.
Open Access
High-efficiency optical gain in type-II semiconductor nanocrystals of alloyed colloidal quantum wells
(American Chemical Society, 2017) Guzelturk, B.; Kelestemur Y.; Olutas M.; Li, Q.; Lian, T.; Demir, Hilmi Volkan
Colloidal nanocrystals having controlled size, tailored shape, and tuned composition have been explored for optical gain and lasing. Among these, nanocrystals having Type-II electronic structure have been introduced toward low-threshold gain. However, to date, their performance has remained severely limited due to diminishing oscillator strength and modest absorption cross-section. Overcoming these problems, here we realize highly efficient optical gain in Type-II nanocrystals by using alloyed colloidal quantum wells. With composition-tuned core/alloyed-crown CdSe/CdSexTe1-x quantum wells, we achieved amplified spontaneous emission thresholds as low as 26 μJ/cm2, long optical gain lifetimes (τgain ≈ 400 ps), and high modal gain coefficients (gmodal ≈ 930 cm-1). We uncover that the optical gain in these Type-II quantum wells arises from the excitations localized to the alloyed-crown region that are electronically coupled to the charge-transfer state. These alloyed heteronanostructures exhibiting remarkable optical gain performance are expected to be highly appealing for future display and lighting technologies.
Open Access
High-stability, high-efficiency organic monoliths made of oligomer nanoparticles wrapped in organic matrix
(American Chemical Society, 2016) Soran-Erdem Z.; Erdem, T.; Gungor K.; Pennakalathil, J.; Tuncel, D.; Demir, Hilmi Volkan
Oligomer nanoparticles (OL NPs) have been considered unsuitable for solid-state lighting due to their low quantum yields and low temperature stability of their emission. Here, we address these problems by forming highly emissive and stable OL NPs solids to make them applicable in lighting. For this purpose, we incorporated OL NPs into sucrose matrix and then prepared their all-organic monoliths. We show that wrapping the OL NPs in sucrose significantly increases their quantum yield up to 44%, while the efficiency of their dispersion and direct solid-film remain only at ∼6%. We further showed ∼3-fold improved temperature stability of OL NP emission within these monoliths. Our experiments revealed that a physical passivation mechanism is responsible from these improvements. As a proof-of-concept demonstration, we successfully employed these high-stability, high-efficiency monoliths as color converters on a blue LED chip. Considering the improved optical features, low cost, and simplicity of the presented methodology, we believe that this study holds great promise for a ubiquitous use of organic OL NPs in lighting and possibly in other photonic applications.
Open Access
Hybrid plasmon-phonon polariton bands in graphene-hexagonal boron nitride metamaterials [Invited]
(Optical Society of America, 2017) Hajian, H.; Ghobadi, A.; Dereshgi, S. A.; Butun, B.; Özbay, Ekmel
We theoretically investigate mid-infrared electromagnetic wave propagation in multilayered graphene-hexagonal boron nitride (hBN) metamaterials. Hexagonal boron nitride is a natural hyperbolic material that supports highly dispersive phonon polariton modes in two Reststrahlen bands with different types of hyperbolicity. Due to the hybridization of surface plasmon polaritons of graphene and hyperbolic phonon polaritons of hBN, each isolated unit cell of the graphene-hBN metamaterial supports hybrid plasmon-phonon polaritons (HPPs). Through the investigation of band structure of the metamaterial we find that, due to the coupling between the HPPs supported by each unit cell, the graphene-hBN metamaterial can support HPP bands. The dispersion of these bands can be noticeably modified for different thicknesses of hBN layers, leading to the appearance of bands with considerably flat dispersions. Moreover, analysis of light transmission through the metamaterial reveals that this system is capable of supporting high-k propagating HPPs. This characteristic makes graphene-hBN metamaterials very promising candidates for the modification of the spontaneous emission of a quantum emitter, hyperlensing, negative refraction, and waveguiding. © 2017 Optical Society of America.