Browsing by Author "Bulutay, Ceyhun"
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Item Open Access Atomistic structure simulation of silicon nanocrystals driven with suboxide penalty energies(American Scientific Publishers, 2008) Yılmaz, Dündar E.; Bulutay, Ceyhun; Çağın, T.The structural control of silicon nanocrystals embedded in amorphous oxide is currently an important technological problem. In this work, an approach is presented to simulate the structural behavior of silicon nanocrystals embedded in amorphous oxide matrix based on simple valence force fields as described by Keating-type potentials. After generating an amorphous silicon-rich-oxide, its evolution towards an embedded nanocrystal is driven by the oxygen diffusion process implemented in the form of a Metropolis algorithm based on the suboxide penalty energies. However, it is observed that such an approach cannot satisfactorily reproduce the shape of annealed nanocrystals. As a remedy, the asphericity and surface-to-volume minimization constraints are imposed. With the aid of such a multilevel approach, realistic-sized silicon nanocrystals can be simulated. Prediction for the nanocrystal size at a chosen oxygen molar fraction matches reasonably well with the experimental data when the interface region is also accounted. The necessity for additional shape constraints suggests the use of more involved force fields including long-range forces as well as accommodating different chemical environments such as the double bonds.Item Open Access Carrier-induced refractive index change in InN(Wiley, 2008) Bulutay, Ceyhun; Zakhleniuk, N. A.Rapid development of InN technology demands comprehensive assessment of the electronic and optoelectronic potential of this material. In this theoretical work the effect of free electrons on the optical properties of the wurtzite phase of InN is investigated. The blue shift of the optical absorption edge by the free-carrier band filling is known as the Burstein-Moss effect for which InN offers to be a very suitable candidate as has been recently demonstrated experimentally. Due to well known Kramers-Kronig relations, a change in absorption is accompanied by a change in the index of refraction. Considering n-type InN samples with free electron concentrations ranging from 5x10 17 to 5x1020 cm-3, and employing a nonlocal empirical pseudopotential band structure, it is shown that this leads to a few percent change of the index of refraction. These carrier-induced refractive index changes can be utilized in optical switches, futhermore it needs to be taken into account in the design of InN-based optical devices such as lasers and optical modulators.Item Open Access Cat-state generation and stabilization for a nuclear spin through electric quadrupole interaction(American Physical Society, 2017) Bulutay, CeyhunSpin cat states are superpositions of two or more coherent spin states (CSSs) that are distinctly separated over the Bloch sphere. Additionally, the nuclei with angular momenta greater than 1/2 possess a quadrupolar charge distribution. At the intersection of these two phenomena, we devise a simple scheme for generating various types of nuclear-spin cat states. The native biaxial electric quadrupole interaction that is readily available in strained solid-state systems plays a key role here. However, the fact that built-in strain cannot be switched off poses a challenge for the stabilization of target cat states once they are prepared. We remedy this by abruptly diverting via a single rotation pulse the state evolution to the neighborhood of the fixed points of the underlying classical Hamiltonian flow. Optimal process parameters are obtained as a function of electric field gradient biaxiality and nuclear-spin angular momentum. The overall procedure is seen to be robust under 5% deviations from optimal values. We show that higher-level cat states with four superposed CSS can also be formed using three rotation pulses. Finally, for open systems subject to decoherence we extract the scaling of cat-state fidelity damping with respect to the spin quantum number. This reveals rates greater than the dephasing of individual CSSs. Yet, our results affirm that these cat states can preserve their fidelities for practically useful durations under the currently attainable decoherence levels.Item Open Access Defect states in monolayer hexagonal BN: A comparative DFT and DFT-1/2 study(Elsevier, 2020) Aksu-Korkmaz, Yağmur; Bulutay, Ceyhun; Sevik, C.Hexagonal boron nitride (h-BN) acts like a semiconductor vacuum to point defects enabling stable and controllable spin states at room temperature which qualifies them for quantum technological applications. To characterize their properties first-principles techniques constitute indispensable tools. The currently established paradigm for such solid-state electronic structure calculations is the density functional theory (DFT). Recently its variant, so-called DFT-1/2 method was introduced with the promise of accurate band gaps without a computational overhead with respect to ordinary DFT. Here, for the monolayer h-BN we contrast DFT and DFT-1/2 results for carbon substitutional impurities (CB, CN), boron and nitrogen single vacancies (VB, VN), divacancy, and Stone-Wales defects. Comparisons with more sophisticated, yet computationally costly techniques namely, hybrid functional DFT and the GW are also made, where available. From the standpoint of defect states embedded in the band gap region we demonstrate a clear advantage of DFT-1/2 in revealing the localized states otherwise buried within either the valence or conduction band continuum due to well-known gap underestimation syndrome of the standard DFT implementations. Thus, DFT-1/2 can serve for the rapid screening of candidate defect systems before more demanding considerations.Item Open Access Electron ground state g factor in embedded InGaAs quantum dots: an atomistic study(American Physical Society, 2021-03-03) Kahraman, Mustafa; Bulutay, CeyhunWe present atomistic computations within an empirical pseudopotential framework for the electron s-shell ground state g tensor of InGaAs quantum dots (QDs) embedded to host matrices that grant electronic confinement. A large structural set consisting of geometry, size, and molar fraction variations is worked out which also includes a few representative uniform strain cases. The tensor components are observed to display insignificant discrepancies even for the highly anisotropic shapes. The family of g-factor curves associated with these parameter combinations coalesces to a single universal one when plotted as a function of the gap energy, thus confirming a recent assertion reached under much restrictive conditions. Our work extends its validity to alloy QDs with various shapes and finite confinement that allows for penetration to the host matrix, placing it on a more realistic basis. Accordingly, the electrons in InGaAs QDs having s-shell transition energies close to 1.13 eV will be least susceptible to magnetic field. We also show that low indium concentration offers limited g-factor tunability under shape or confinement variations. These findings can be taken into consideration in the fabrication and the use of InGaAs QDs with g-near-zero or other targeted g values for spintronic or electron spin resonance-based direct quantum logic applications.Item Open Access Elements of nanocrystal high-field carrier transport modeling(Wiley, 2007) Sevik, Cem; Bulutay, CeyhunEmbedded semiconductor nanocrystals (NCs) within wide bandgap oxide materials are being considered for light emission and solar cell applications. One of the fundamental issues is the high-field transport in NCs. This requires the combination of a number of tools: ensemble Monte Carlo carrier transport simulation, ab initio band structure of the bulk oxide, Fermi's golden rule modeling of impact ionization and Auger processes and the pseudopotential-based atomistic description of the confined NC states. These elements are outlined in this brief report.Item Open Access Experimental and theoretical investigations of electronic and atomic structure of Si-nanocrystals formed in sapphire by ion implantation(IOP, 2008) Wainstein, D.; Kovalev, A.; Tetelbaum, D.; Mikhailov, A.; Bulutay, Ceyhun; Aydınlı, AtillaThe semiconductor nanocomposites based on Si nanocrystals in dielectric matrices attract a great amount of attention due to their ability for luminescence in visible and near-IR part of the electromagnetic spectrum. Si nanocrystals in sapphire matrix were formed by Si+ ion implantation with doses from 5×1016 to 3×1017 cm -2 at an accelerating voltage 100 kV and post-implantation annealing at 500-1100 d̀C for 2 hours. Depth distribution of lattice defects, impurities and Si nanocrystals, the peculiarities of interband electronic transitions were investigated by XPS and HREELS. The molecular orbitals and local electronic structure of the Al2O3 matrix with Si nanocrystals was calculated using an atomistic pseudopotential technique. The electronic structure of Si nanocrystals as determined from HREELS measurements is in good agreement with the theoretically calculated electronic structure for Si nanocrystals.Item Open Access Geometric band properties in strained monolayer transition metal dichalcogenides using simple band structures(American Institute of Physics, 2019) Aas, Shahnaz; Bulutay, CeyhunMonolayer transition metal dichalcogenides (TMDs) bare large Berry curvature hotspots readily exploitable for geometric band effects. Tailoring and enhancement of these features via strain is an active research direction. Here, we consider spinless two- and three-band and spinful four-band models capable to quantify the Berry curvature and the orbital magnetic moment of strained TMDs. First, we provide a k⋅p parameter set for MoS2, MoSe2, WS2, and WSe2 in the light of the recently released ab initio and experimental band properties. Its validity range extends from the K valley edge to about one hundred millielectron volts into valence and conduction bands for these TMDs. To expand this over a larger part of the Brillouin zone, we incorporate strain to an available three-band tight-binding Hamiltonian. With these techniques, we demonstrate that both the Berry curvature and the orbital magnetic moment can be doubled compared to their intrinsic values by applying typically a 2.5% biaxial tensile strain. These simple band structure tools can find application in the quantitative device modeling of the geometric band effects in strained monolayer TMDs.Item Open Access Gunn oscillations in GaN channels(IOP, 2004) Sevik, Cem; Bulutay, CeyhunGallium nitride with its high negative differential mobility threshold is an appealing material for high power millimetre-wave oscillators as a Gunn diode. By means of extensive ensemble Monte Carlo simulations, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz to 650 GHz is studied in detail. Their operations are checked under both impressed single-tone sinusoidal bias and external tank circuit conditions. The width of the doping notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency up to a critical value, beyond which sustained Gunn oscillations cease. The degeneracy effects due to the Pauli exclusion principle are also considered, but their effects are seen to be negligible within the realistic bounds of the Gunn diode operation.Item Open Access Harmonic enhancement of Gunn oscillations in GaN(American Institute of Physics, 2005) Sevik, Cem; Yılmaz, Dündar E.; Bulutay, CeyhunHigh field transport in wide bandgap semiconductors like GaN is of great technological importance. The negative differential mobility regime at high fields, under suitable conditions, can lead to millimeter-wave Gunn oscillations. Using extensive simulation based an ensemble Monte Carlo technique, the prospects of GaN Gunn diodes are theoretically investigated. The possibility of operating these Gunn diodes at their higher harmonic modes are explored. Main finding of this research is that the carrier dynamics in GaN can be tailored by an optimum choice of doping profile, temperature and bias conditions so that the efficiency of higher harmonic Gunn oscillations can be boosted.Item Open Access High-energy electron relaxation and full-band electron dynamics in aluminium nitride(Elsevier, 2002) Bulutay, Ceyhun; Ridley, B. K.; Zakhleniuk, N. A.Material properties of AlN, particularly its wide band gap around 6 eV, warrant its operation in the high-field transport regimes reaching MV/cm fields. In this theoretical work, we examine the full-band scattering of conduction band electrons in AlN due to polar optical phonon (POP) emission, which is the main scattering channel at high fields. First, we obtain the band structure for the wurtzite phase of AlN using the empirical pseudopotential method. Scattering rates along the full length of several high-symmetry directions are computed efficiently through the Lehmann-Taut Brillouin zone integration technique. In order to shed light on the behaviour of the velocity-field characteristics at extremely high electric fields, in the order of a few MV/cm, we resort to an Esaki-Tsu estimation. Comparison of these results for AlN is made with our similar work on GaN. With typically more than 50% higher POP scattering rate compared to GaN, AlN has poorer high-field prospects. Availability of these data for AlN and GaN paves the way for practical assessment of the high-energy electron dynamics for the ternary alloy, AlGaN.Item Open Access k · p Parametrization and linear and circular dichroism in strained monolayer (janus) transition metal dichalcogenides from first-principles(American Chemical Society, 2021-04-08) Aksu Korkmaz, Yağmur; Bulutay, Ceyhun; Sevik, C.Semiconductor monolayer transition metal dichalcogenides (TMDs) have brought a new paradigm by introducing optically addressable valley degree of freedom. Concomitantly, their high flexibility constitutes a unique platform that links optics to mechanics via valleytronics. With the intention to expedite the research in this direction, we investigated ten TMDs, namely MoS2, MoSe2, MoTe2, WS2, WSe2, WTe2, MoSSe, MoSeTe, WSSe, and WSeTe, which particularly includes their so-called janus types (JTMDs). First, we obtained their electronic band structures using regular and hybrid density functional theory (DFT) calculations in the presence of the spin–orbit coupling and biaxial or uniaxial strain. Our DFT results indicated that against the expectations based on their reported piezoelectric behavior, JTMDs typically interpolated between the standard band properties of the constituent TMDs without producing a novel feature. Next, by fitting to our DFT data we generated both spinless and spinful k · p parameter sets which are quite accurate over the K valley where the optical activity occurs. As an important application of this parametrization, we considered the circular and linear dichroism under strain. Among the studied (J)TMDs, WTe2 stood out with its largest linear dichroism under uniaxial strain because of its narrower band gap and large K valley uniaxial deformation potential. This led us to suggest WTe2 monolayer membranes for optical polarization-based strain measurements, or conversely, as strain tunable optical polarizers.Item Open Access Loschmidt echo driven by hyperfine and electric-quadrupole interactions in nanoscale nuclear spin baths(American Physical Society, 2018) Güldeste, E. T.; Bulutay, CeyhunThe nuclear spin bath (NSB) dynamics and its quantum control are of importance for the storage and processing of quantum information within a semiconductor environment. In the presence of a carrier spin, primarily it is the hyperfine interaction that rules the high frequency NSB characteristics. Here, we first study the overall coherence decay and rephasings in a hyperfine-driven NSB through the temporal and spectral behaviors of the so-called Loschmidt echo (LE). Its dependence on the NSB size, initial polarization, and coupling inhomogeneity are separately investigated, which leads to a simple phenomenological expression that can accommodate all of these attributes. Unlike the prevailing emphasis on spin 1/2, the NSBs with larger spin quantum numbers are equally considered. For this case, additionally the effect of nuclear electric quadrupole interaction is taken into account where its biaxiality term is influential on the decoherence. The insights gained from model systems are then put to use for two generic realistic semiconductor systems, namely, a donor center and a quantum dot that represent small and large nanoscale NSB examples, respectively. The spectrum of LE for large quantum dots can reach the 100 MHz range, whereas, for donor centers, it reduces to a few MHz, making them readily amenable for dynamical decoupling techniques. The effect of quadrupole interaction on LE is seen to be negligible for large quantum dots, while it becomes significant for donor centers, most notably in the form of depolarizing a polarized NSB.Item Open Access Physica Status Solidi (C) Current Topics in Solid State Physics: Preface(2008) Rorison, Judy; Balkan, Naci; Bulutay, Ceyhun; Saarinen, Mika[No abstract available]Item Open Access Pseudopotential-based full zone k · p technique for indirect bandgap semiconductors: Si, Ge, diamond and SiC(TÜBİTAK, 2006) Bulutay, CeyhunThe k · p is a versatile technique that describes the semiconductor band structure in the vicinity of the bandgap. The technique can be extended to full Brillouin zone by including more coupled bands into consideration. For completeness, a detailed formulation is provided where the associated k · p parameters are extracted from the local empirical pseudopotential method in the form of band edge energies and generalized momentum matrix elements. We demonstrate the systematic improvement of the technique with the proper choice of the band edge states for the group-IV indirect bandgap semiconductors: Si, Ge, diamond and SiC of the 30 cubic phase. The full zone agreement is observed to span an energy window of more than 20 eV for Si, and 40 eV for the diamond with the 15-band pseudopotential-based k · p approach. © TÜBİTAK.Item Open Access Static synthetic gauge field control of double optomechanically induced transparency in a closed-contour interaction scheme(American Physical Society, 2021-09-07) Sütlüoğlu, Beyza; Bulutay, CeyhunWe study theoretically an optical cavity and a parity-time (PT)-symmetric pair of mechanical resonators, where all oscillators are pairwise coupled, forming an optomechanical system with a closed-contour interaction. Due to the presence of both gain and feedback, we explore its stability and the root loci over a wide coupling range. Under the red-sideband pumping and for the so-called PT-unbroken phase, it displays a double optomechanically induced transparency (OMIT) for an experimentally realizable parameter set. We show that both the transmission amplitude and the group delay can be continuously steered from the lower transmission window to the upper one by the loop coupling phase which breaks the time-reversal symmetry and introduces a static synthetic gauge field. In the PT -unbroken phase both the gain-bandwidth and delay-bandwidth products remain constant over the full range of the controlling phase. Tunability in transmission and bandwidth still prevails in the PT -broken phase, albeit over a reduced range. In essence, we suggest a simple scheme that grants coupling phase-dependent control of the single and double OMIT phenomena within an effective PT-symmetric optomechanical system.Item Open Access Strain dependence of photoluminescence and circular dichroism in transition metal dichalcogenides: a k. p analysis(OSA - The Optical Society, 2018) Aas, Shahnaz; Bulutay, CeyhunWithin a two-band k p method we analyze different types of strain for the k valley optical characteristics of a freestanding monolayer mos2 Mose2 Ws2 andwse2. we predict that circular polarization selectivity for energies above the direct transition onset deteriorates/improves by tensile/compressive strain. wide range of available strained-sample photoluminescence data can be reasonably reproduced by this simple bandstructure combined with accounting for excitons at a variational level. according to this model strain impacts optoelectronic properties through its hydrostatic component.Whereas the shear strain only causes a rigid wavevector shift of the valley. furthermore Under the stress loading of flexible substrates the presence of poisson's effect or the lack of it are examined individually for the reported measurements.Item Open Access Wavelet resolved coherence beating in the Overhauser field of a thermal nuclear spin ensemble(American Physical Society, 2022-02-14) Güldeste, Ekrem Taha; Bulutay, CeyhunThis work introduces the so-called synchrosqueezed wavelet transform, to shed light on the dipolar fluctuations of a thermal ensemble of nuclear spins in a diamond crystal structure, hyperfine-coupled to a central spin. The raw time series of the nuclear spin bath coherent dynamics is acquired through the two-point correlation function computed using the cluster correlation expansion method. The dynamics can be conveniently analyzed according to zero-, single-, and double-quantum transitions derived from the dipolar pairwise spin flips. We show that in the early-time behavior when the coherence is preserved in the spin ensemble, the Overhauser field fluctuations are modulated by dipole-dipole-induced small inhomogeneous detunings of nearly resonant transitions within the bath. The resulting beating extending over relatively longer time intervals is featured on the scalograms where both temporal and spectral behaviors of nuclear spin noise are unveiled simultaneously. Moreover, a second kind of beating that affects faster dynamics is readily discernible, originating from the inhomogeneous spread of the hyperfine coupling of each nucleus with the central spin. Additionally, any quadrupolar nuclei within the bath imprint as beating residing in the zero-quantum channel. The nuclear spin environment can be directionally probed by orienting the hyperfine axis. Thereby, crucial spatial information about the closely separated spin clusters surrounding the central spin are accessible. Thus, a wavelet-based postprocessing can facilitate the identification of proximal nuclear spins as revealed by their unique beating patterns on the scalograms. Finally, when these features are overwhelmed by either weakly or strongly coupled classical noise sources, we demonstrate the efficacy of thresholding techniques in the wavelet domain in denoising contaminated scalograms.