# Dept. of Physics - Ph.D. / Sc.D.

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Item Open Access A1GaN UV photodetectors : from micro to nano(Bilkent University, 2011) Bütün, SerkanShow more The absorption edge of AlGaN based alloys can be tuned from deep UV to near UV by changing the composition. This enables the use of the material in various technological applications such as military, environmental monitoring and biological imaging. In this thesis, we proposed and demonstrated various UV photodetectors for different purposes. The multi-band photodetectors have the unique ability to sense the UV spectrum in different portions at the same time. We demonstrated monolithically integrated dual and four-band photodetectors with multi layer structures grown on sapphire. This was achieved through epitaxial growth of multi AlGaN layers with decreasing Al content. We suggested two different device architectures. First one has separate filter and active layers, whereas the second one has all active layers which are used as filter layers as well. The full width at half maximum (FWHM) values for the dual band photodetector was 11 and 22 nm with more than three orders of magnitude inter-band rejection ratio. The self-filtering four band photodetector has FWHMs of 18, 17, 22 and 9 nm from longer to shorter bands. Whereas photodetector with separate filter layers has FWHMs of 8, 12, 11 and 8 nm, from longer to shorter bands. The overall inter-band rejection ration was increased from about one to two of magnitude after incorporating the passive filter layers. The plasmonic enhancement of photonic devices has attracted much attention for the past decade. However, there is not much research that has been conducted in UV region. In the second part of this thesis, we fabricated nanostructures on GaN based photodetectors and improved the responsivity of the device. We have fabricated Al nano-particles on sapphire with e-beam lithography. We characterized their response via spectral extinction measurements. We integrated these particles with GaN photodetectors and had enhancement of %50 at the plasmonic resonance of the nano-particles. Secondly, we have fabricated sub-wavelength photodetectors on GaN coupled with linear gratings. We had 8 fold enhancement in the responsivity at the plasmonic resonance frequency of the grating at normal incidence. Numerical simulations revealed that both surface plasmons and the unbound leaky surface waves played a role in the enhancement. We, finally, conducted basic research on the current transport mechanisms in Schottky barriers of AlGaN based materials. Experiments revealed that the tunneling current plays a major role in current transport. In addition incorporation, of a thin insulator between metalsemiconductor interface reduces the undesired surface states thereby improving the device performance.Show more Item Open Access The adiabatic and non-adiabatic behavior of a particle in optical lattices(Bilkent University, 2018-06) Yılmaz, FıratShow more The cold atom experiments provide a clean and controlled environment for realizing many body systems. Recent realizations of artificial gauge fields and adjustable optical lattices paved the way for the study of effectively charged particles with neutral atoms in various lattice and continuum systems. Moreover, it is possible to precisely control the external system parameters, i.e. the artificial gauge fields much faster or slower than the time scales associated with atomic motion in the lattice. It still needs further analysis to fully understand how the adiabatic and non-adiabatic changes affect the stationary and dynamical behavior of the system. We first investigate the effect of the adiabatic changes in the artificial gauge fields, and focus on the famous problem: A charged particle in a periodic potential under magnetic field. This simple system leads a complicated and involved selfsimilar energy spectrum, the Hofstadter butterfly. The whole structure of this energy spectrum is determined by the lattice geometry as well as the external field. In this regard, we consider all possible Bravais lattices in two dimensions and investigate the structure of the Hofstadter butterfly as the different point symmetry groups of the lattices are adiabatically deformed from one into another. We find that each 2D Bravais lattice is uniquely mapped to a fractal energy spectrum and it is possible to understand the interplay between the point symmetry groups and the energy spectrum. This beautiful spectrum, in addition, consists of infinitely many topologically distinct regions as a function of magnetic flux and gap number. The topological character of energy bands are determined through their Chern numbers. We calculate the Chern numbers of the major gaps and Chern number transfer between bands during the topological transitions. In the second part, we investigate the dramatic effect of the non-adiabatic changes in the artificial gauge fields. In a synthetic lattice, the precise control over the hopping matrix elements makes it possible to change this artificial magnetic field non-adiabatically even in the quench limit. We consider such a magneticflux quench scenario in synthetic dimensions. Sudden changes have not been considered for real magnetic fields as such changes in a conducting system would result in large induced currents. Hence we first study the difference between a time varying real magnetic field and an artificial magnetic field using a minimal six-site model which leads to gauge dependent results. This model proves the relation between the gauge dependant dynamics and the absence of scalar potential terms connecting different gauge potentials. In this context, we secondly search for clear indication of the gauge dependent dynamics through magnetic flux quenches of wave packets in two- and three-leg synthetic ladders. We show that the choice of gauge potentials have tremendous effect on the post-quench dynamics of wave packets. Even trivially distinct two vector potentials by an additive constant can produce observable effects, we investigate the effects on the Landau levels and the Laughlin wave function for a filling factor ν = 1/q. We also show that edge solutions in a wide synthetic ladder are protected under a flux quench only if there is another edge state solution in the quenched Hamiltonian.Show more Item Open Access Analysis of nonequilibrium steady-states(Bilkent University, 2016-11) Yeşil, Ayşe FerhanShow more Non-equilibrium is the state of the almost all systems in the universe. Unlike equilibrium systems, they interfere with their surroundings which results in never ceasing uxes. There is no unified theory to understand these systems, since their complexity have no bounds. However, there is a restricted subset of them, namely a steady state, in which system maintains constant uxes and its macroscopic observables are not changing in time. Majority of the non-equilibrium problems that the scientific community is interested in comprise systems at steady states or the way such systems relax to steady states, due to their relative ease of analysis. Steady states of Totally Asymmetric Simple Exclusion Processes (TASEPs) are the main focus of this dissertation. We analyze them through Monte Carlo (MC) simulations. The technique is basically a computational experiment done by utilizing random numbers. Performing a computational experiment is a natural way to study these systems since most of the time they are still too complex to have analytical solutions. We present MC simulation results of our studies on the response of TASEP steady states to sinusoidal boundary oscillations. Typically over-damped systems, such as TASEPs, give monotonous frequency response to sinusoidal driving. However, there are exceptions to these all which draw significant attention from the community, e.g., stochastic resonance. We report a novel resonance phenomena on over-damped systems. We present our results in two different but related works. In our first work, we study the motion of shock profiles of TASEP with single class of particles under oscillatory boundary conditions using MC analysis. We also model its dynamics as a Fokker-Planck (FP) system, which incorporates a retarded-oscillatory force with a static single well potential. We solve the FP system by numerical integration. We showed that amplitudes of statistical quantities in both of these systems, (e.g., average position), display resonant effects and their results are qualitatively very similar. In our second work, we showed that by periodically manipulating the boundary conditions of TASEP with two classes of particles, we can achieve otherwise unreachable states of the system by the same parameters. We also report the hysteresis behavior in the same system, existence of which leads to the identifi- cation of typical velocity of the system. All these phenomena are the results of resonant response of the particle number density of the system.Show more Item Open Access Ballistic transport and tunneling in small systems(Bilkent University, 1990) Tekman, A ErkanShow more Ballistic transport and tunneling of electrons in mesoscopic systems have become one of the most important subjects of condensed matter physics. The quantum point contacts and scanning tunneling microscope form the basic experimental tools in this area and have been used for understanding many features of small systems. In this work ballistic transport and tunneling in small systems are investigated theoretically. Ballistic transport through narrow constrictions is investigated for a variety of configurations. It is found that for a uniform constriction the conductance is quantized in units of the quantum of conductance (2e^/A) for long channels. The interference of waves in the constriction gives rise to the resonance structure superimposed on the quantized steps. The lack of the resonance structure in the experimental results are attributed to temperature effects and/or adiabatic transport due to tapering of the constriction. It is shown that elastic scattering by an impurity distorts the quantization of conductance. Novel resonant tunneling effects due to formation of bound states are predicted for an attractive impurity or a local widening at the center of the constriction. It is shown that the probing in scanning tunneling microscopy have very much in common with narrow constrictions. The transition from tunneling to point contact regime is explained by the vanishing effective potential barrier as a result of tip-sample interaction. For noble and simple metals it is conjectured that lateral position dependent interaction between the tip and sample leads to corrugation of the potential barrier and in turn to atomic corrugation observed by scanning tunneling microscopy. The focused field emission of electrons from point sources is analyzed in a systematical way. The effective barrier due to the lateral confinement and nonadiabatic transport through the horn-like opening are found to be responsible for focusing. The nonequilibrium nature of transport is investigated by use of Keldysh Green’s function technique. The effects of elastic and inelastic scattering are analyzed in a strictly one-dimensional geometry. The features of voltage and current probes are studied and the Landauer formulae are examined for multiprobe measurements.Show more Item Open Access Beaming and localization of electromagnetic waves in periodic structures(Bilkent University, 2010) Çağlayan, HümeyraShow more We want to manipulate light for several applications: microscopy, data storage, leds, lasers, modulators, sensor and solarcells to make our life healthier, easier or more comfortable. However, especially in small scales manipulating light have many difficulties. We could not focus or localize light into subwavelength dimensions easily, which is the key solution to beat today’s devices both in performance and cost. Achievements in three key research fields may provide the answer to these problems. These emerging research fields are metamaterials, photonic crystals and surface plasmons. In this thesis, we investigated beaming and localization of electromagnetic waves in periodic structures such as: subwavelength metallic gratings, photonic crystals and metamaterials. We studied off-axis beaming from both a metallic subwavelength aperture and photonic crystal waveguide at microwave regime. The output surfaces are designed asymmetrically to change the beaming angle. Furthermore, we studied frequency dependent beam steering with a photonic crystal with a surface defect layer made of dimmers. The dispersion diagram reveals that the dimer-layer supports a surface mode with negative slope. Thus, a photonic crystal based surface wave structure that acts as a frequency dependent leaky wave antenna was presented. Additionally, we investigated metamaterial based cavity systems. Since the unit cells of metamaterials are much smaller than the operation wavelength, we observed subwavelength localization within these metamaterial cavity structures. Moreover, we introduced coupled-cavity structures and presented the transmission spectrum of metamaterial based coupled-cavity structures. Finally, we demonstrated an ultrafast bioassay preparation method that overcomes the today’s bioassay limitations using a combination of low power microwave heating and split ring resonator structures.Show more Item Open Access Boron nitride and graphene 2D nanostructures from first-principles(Bilkent University, 2010) Ovalı, Rasim VolgaShow more In this thesis, the structures as well as mechanical and electronic properties of various boron nitride (BN) and graphene based two dimensional (2D) nano-structures are investigated in detail from rst-principle calculations using planewave pseudopotential method based on density functional theory. At the beginning of the thesis, essentials of the density functional theory (DFT) and a guidance for performing ab-initio calculations in the framework of DFT is presented. In addition, fundamentals about the exchange-correlation potential as well as approaches approximating it like local density approximation (LDA) and generalized gradient approximation (GGA) are discussed. Along with this thesis, rst of all, in order to understand the relation between the hexagonal boron nitride (h-BN) and cubic boron nitride (c-BN) and the growth of three dimensional (3D) BN structures, various defect structures introduce to BN monolayer, including point defects and especially highly curved defects such as n-fold rings, are investigated in detail. The calculated formation energies and structural analysis showed that 4-fold BN rings are the transient phase between h-BN to c-BN during c-BN nucleation. The charge density plots and density of states analysis further provide information about the electronic structure of these defect formations. Second of all, we have studied the formation of boron-nitride-carbon (BNC) ternary thin lms, so we observed the carbon nucleation in BN monolayer. These DFT based calculations show that carbon prefers the nitrogen site at rst step and the calculated defect energy indicates that carbon atoms tends to aggregate in BN hexagonal network, and hence increases the number of C-C bonds. BNC structures have magnetization of =1.0 B for odd number of carbon adsorption. Further substitution of carbon atoms into BN layer showed that carbon atomsform hexagonal rings instead of armchair or zigzag formations. Moreover, we calculated the vibrational modes of BN monolayer and BNC structures, and phonon density of states graphs are presented. The phonon frequencies intrinsic to C-C bond oscillations are observed, which is in good agreement with the experiment. Finally, point defects and ring formations on graphene are investigated in order to understand the Y-junction and kink formation in carbon nanotubes (CNTs). Pentagonal rings are the good candidates to initiate such 2D networks in CNTs. The curvature increases with increasing number of pentagonal rings. Moreover, interaction of sulphur atoms with graphene defects is studied. Final geometries and binding energies suggest that sulphur prefers to adsorb on defected regions, but it is not responsible for the formation of these structures or defects.Show more Item Open Access Carrier dynamics in silicon and Germanium nanocrystals(Bilkent University, 2008) Sevik, CemShow more This is a computational work on the Si and Ge nanocrystals (NCs) embedded in wide band gap host matrices. As the initial task, extensive ab initio work on the structural and electronic properties of various NC host matrices, namely, SiO2, GeO2, Si3N4, and Al2O3 are preformed. The structural parameters, elastic constants, static and optical dielectric constants are obtained in close agreement with the available results. Furthermore, recently reported high density cubic phase of SiO2 together with GeO2 and SnO2 are studied and their stable highdielectric constant alloys are identified. Based on the ab initio study of host matrices, two related high field phenomena, vital especially for the electroluminescence in Si and Ge NCs, are examined. These are the hot carrier transport through the SiO2 matrix and the subsequent quantum-confined impact ionization (QCII) process which is responsible for the creation of electron-hole pairs within the NCs. First, the utility and the validity of the ab initio density of states results are demonstrated by studying the high field carrier transport in bulk SiO2 up to fields of 12 MV/cm using the ensemble Monte Carlo technique. Next, a theoretical modeling of the impact ionization of NCs due to hot carriers of the bulk SiO2 matrix is undertaken. An original expression governing the QCII probability as a function of the energy of the hot carriers is derived. Next, using an atomistic pseudopotential approach the electronic structures for embedded Si and Ge NCs in wide band-gap matrices containing several thousand atoms are employed. Effective band-gap values as a function of NC diameter reproduce very well the available experimental and theoretical data. To further check the validity of the electronic structure on radiative processes, direct photon emission rates are computed. The results for Si and Ge NCs as a function of diameter are in excellent agreement with the available ab initio calculations for small NCs. In the final part, non-radiative channels, the Auger recombination (AR) and carrier multiplication (CM) in Si and Ge NCs are investigated again based on the atomistic pseudopotential Hamiltonian. The excited electron and excited hole type AR and CM and biexciton type AR lifetimes are calculated for different sized and shaped NCs embedded in SiO2 and Al2O3. Asphericity is also observed to increase the AR and CM rates. An almost monotonous size-scaling and satisfactory agreement with experiment for AR lifetime is obtained considering a realistic interface region between the NC core and the host matrix. It is further shown that the size-scaling of AR can simply be described by slightly decreasing the established bulk Auger constant for Si to 1.0×10−30cm6 s −1 . The same value for germanium is extracted as 1.5×10−30cm6 s −1 which is very close to the established bulk value. It is further shown that both Si and Ge NCs are ideal for photovoltaic efficiency improvement via CM due to the fact that under an optical excitation exceeding twice the band gap energy, the electrons gain lion’s share from the total excess energy and can cause a CM. Finally, the electron-initiated CM is predicted to be enhanced by couple orders of magnitude with a 1 eV of excess energy beyond the CM threshold leading to subpicosecond CM lifetimes.Show more Item Open Access Chalcogenide micro and nanostructures and applications(Bilkent University, 2014) Aktaş, OzanShow more Chalcogenides, which are glasses consist of S, Se and Te elements, are promising materials for photonics as silicon for modern electronics, due to their extraordinary material properties such as high nonlinearity and wide mid-IR transparency. However, the biggest barrier before their full extend technological exploitation is the difficulty in utilization of these unique material properties within photonic devices with various forms of desired geometries including nanowires, microspheres, and microdisks as necessitated by unique optical functionalities for specific applications, some of which are optical microresonators, modulators, and photodetection devices. In this study, the author explore new routes for the fabrication of on-chip photonic elements with chalcogenides and consider a low cost high-yield production method with a compatible and extendable integration phase. The study illustrates production of chalcogenide optical cavities embedded in a polymer fiber, on-chip integration of the cavities having spherical, spheroidal, and ellipsoidal boundaries, and results of their optical characterizations. Besides the fabrication of active photonic devices with electro-optical capabilities, tapered chalcogenide fibers are also considered as evanescent couplers for the resonators of high index materials. In addition, a large area chalcogenide nanowire based photodetection device is demonstrated including fabrication of photoconductive pixels, design of an electronic readout circuit, development of a custom software for a pattern detection application. Keywords: Chalcogenides glasses, nanowires, optical microresonators, asymmetric resonant cavities, electro-optical Kerr effect, modulators, whispering gallery mode resonators, photonics, fiber drawing.Show more Item Open Access Characterization and applications of negative-index metamaterials(Bilkent University, 2008) Aydın, KorayShow more Metamaterials offer novel electromagnetic properties and promising applications including negative refraction, flat-lenses, superlenses, cloaking devices. In this thesis, we characterized the negative-index metamaterials that is composed of periodic arrangements of split-ring resonators (providing negative permeability) and thin wire (providing negative permittivity) arrays. The resonances of split-ring resonators (SRR) are investigated experimentally and theoretically. By combining SRR and wire arrays together, we observed a transmission band where both permittivity and permeability are simultaneously negative, indicating a left-handed behavior. Reflection measurements reveal that the impedance is matched to the free space at a certain frequency range. The lefthanded metamaterial is also shown to exhibit negative refractive index by using three different experimental methods namely, refraction from a wedge-shaped negative-index metamaterial (NIM), beam-shift from a slab-shaped NIM and phase shift from NIMs with different lengths. Flat-lens behavior is observed from a slabshaped negative-index metamaterial based microwave lenses. Furthermore, we demonstrated subwavelength imaging and subwavelength resolution by using thin superlenses constructed from SRR-wire arrays with an effective negative index. We have been able to image a point source with a record-level, λ/8 resolution. SRRand wire arrays exhibit negative index provided that the wave propagates parallel to the plane of SRR structure which makes it hard to fabricate at higher frequencies. An alternative structure called fishnet metamaterial however could yield negative index with wave propagation normal to the structure. We observed left-handed transmission and negative phase velocity in fishnet type metamaterials. Finally, we studied enhanced transmission from a single subwavelength aperture by coupling incident electromagnetic wave to a single SRR placed at the near-field of the aperture.Show more Item Open Access Controlling electromagnetic waves with active graphene devices(Bilkent University, 2015) Balcı, OsmanShow more The dynamic control of electromagnetic waves forms the basis of modern communication technologies. Although sources of microwaves can be controlled by electrical means, the active control of microwaves in the free space has been a challenge due to the lack of an active material. Graphene, the 2-dimensional crystal of carbon, provides a unique platform to control light-matter interaction in a broad spectrum. This thesis describes a new approach to control microwaves using large area active graphene devices. Our strategy relies on electrostatic tuning of the density of high mobility charge carriers on an atomically thin graphene electrode which operates as a tunable metal in microwave frequencies. We developed a method to synthesize large area graphene (20x20 cm2) by chemical vapor deposition. Using large area graphene electrodes, we demonstrate a new class of active surfaces capable of real-time electrical control of reflection, transmission, and absorption of microwaves over a broad spectrum. These active devices allow us to fabricate electrically tunable microwave surfaces such as switchable radar absorbing surfaces and tunable metamaterials with modulation depth of 50𝑑𝐵 and operation voltage of 3𝑉. Large modulation depth, simple device architecture, and mechanical flexibility are the key attributes of the graphene-enabled active microwave surfaces that could find a wide range of applications ranging from active signal processing to adaptive camouflage.Show more Item Open Access Decoherence in open quantum systems : a realistic approach(Bilkent University, 2006) Savran, KerimShow more Decoherence mechanisms of opcu quantum systems ın intcraction with an enviromental bath is investigated using the master equation formalism. Widely used two-level approximation is questioned.Show more Item Open Access Deformation and finite size effects in cooperative molecular motors(Bilkent University, 2002-07) Taneri, SencerShow more Motor protein systems have been of considerable interest lately. In these studies muscle contraction is modeled as the sliding of two filaments made of protein particles over one another, that is the sliding of the backbone filament on the track filament. In order to make the analytical analysis easy these filaments are assumed to be of infinite length or mass. This enables the understanding of the sliding of motility assays with constant velocity and generation of constant force. However, finite size in length and mass brings fluctuationsuctuations in velocity around certain values, and changes in direction through intermittent transitions. It is possible to associate time constants to this kind of behavior. It turns out that the magnitude of the time constant being created during the process is proportional to both the length of the filament and the mass of the protein particles. Deformation phenomenon stems from internally generated forces which so far has been examined as axonemal deformations. The elastic coupling of the protein particles to the backbone has been studied separately, which in fact is also related to the generation of internal forces. Instead of focusing on the axonemal deformations, we implemented an Ising-like potential contribution to our computation to study the elastic coupling which makes the computation easier. We found out that for certain range of parameters that measures the deformation strength, one attains a better motor because of more intense force generation at the expanse of getting a lower sliding velocity.Show more Item Open Access Development of force fields for novel 2D materials for temperature dependent vibrational properties(Bilkent University, 2019-09) Mobaraki, ArashShow more A new era of nanodevice engineering has been started after fabricating graphene. This motivated vast number of researches for predicting, fabricating and utilizing 2D materials. Temperature dependent properties are essential for device applications. Although rigorous density functional theory based approaches are able to predict electronic and mechanical properties accurately, but they are mostly limited to zero temperature and ab initio based molecular dynamics are computationally very demanding. Classical molecular dynamics is a very powerful alternative, however its accuracy is basically depend on the interatomic potential used for describing the considered system and therefore constructing accurate force fields is always an open problem, especially for the emerging 2D materials with extra ordinary properties. Single-layer transition metal dichalcogenides (TMDs) are new class of 2D materials which are shown to be good candidates for thermoelectric applications, flexible electronic and optoelectronic devices. In order to investigate thermal properties of TMDs, Stillinger-Weber type potentials are developed using particle swarm optimization method. These potentials are validated by comparing the resulted phonon dispersion curves and thermal conductivities with available first principle and experimental results. In addition, for understanding the anharmonic effects imposed by the generated force fields the trends of the shifts of the optical phonon frequencies at point with variation in the temperature are compared with available experimental data. In all cases, optimized potentials generate results which are in agreement with the target data. In the second step, spectral energy density method together with phonon mode decomposition is used for obtaining temperature dependent phonon frequencies and lifetimes in entire Brillouin zone. The contribution of each phonon branch in thermal conductivity is predicted utilizing the obtained phonon lifetimes and group velocities within the framework of relaxation time approximation. Eventually, with the aim of constructing transferable potentials for describing 2D and bulk structures, a very fast and reliable optimization method is presented. Combining local and global optimization methods and utilizing the energy curves obtained from first principle method, novel Stillinger-Weber type potentials for graphene, silicene and group III nitrides are developed. The proposed approach provides a solid framework for parameter selection and investigating the role of each parameter in the resulted phonon dispersion curves.Show more Item Open Access Development of nano hall sensors for high resolution scanning hall probe microscopy(Bilkent University, 2008-09) Dede, MünirShow more Scanning Hall Probe Microscopy (SHPM) is a quantitative and non invasive method of local magnetic field measurement for magnetic and uperconducting materials with high spatial and field resolution. Since its demonstration in 1992, it is used widely among the scientific community and has already commercialized. In this thesis, fabrication, characterization and SHPM imaging of different nano-Hall sensors produced from heterostructure semiconductors and Bismuth thin films with effective physical probe sizes ranging between 50nm‐1000nm, in a wide temperature range starting from 4.2K up to 425K is presented. Quartz crystal tuning fork AFM feedback is demonstrated for the first time for SHPM over a large temperature range. Its performance has been analyzed in detail and experiments carried with 1×1μm Hall probes has been successfully shown for a hard disk sample in the temperature range of 4.2K to 425K. Other samples, NdFeB demagnetized magnet, Bi substituted iron garnet and, single crystal BSCCO(2212) High Temperature superconductor were also imaged with this method to show the applicability of the method over a wide range of specimens. By this method, complex production steps proposed in the literature to inspect the non‐conductive samples were avoided. A novel Scanning Hall probe gradiometer has also been developed and a new method to image x, y & z components of the magnetic field on the sample surface has been demonstrated for the first time with 1μm resolution. 3D field distribution of a Hard Disk sample is successfully measured at 77K using this novel approach to prove the concept.Show more Item Open Access Electro-magnetic properties and phononic energy dissipation in graphene based structures(Bilkent University, 2008) Sevinçli, HaldunShow more With the synthesis of a single atomic plane of graphite, namely graphene honeycomb structure, active research has been focused on the massless Dirac fermion behavior and related artifacts of the electronic bands crossing the linearly at the Fermi level. This thesis presents a theoretical study on the electronic and magnetic properties of graphene based structures, and phononic energy dissipation. First, functionalization of these structures by 3d-transition metal (TM) atoms is investigated. The binding energies, electronic and magnetic properties have been investigated for the cases where TM-atoms adsorbed to a single side and double sides of graphene. It is found that 3d-TM atoms can be adsorbed on graphene with binding energies ranging between 0.10 to 1.95 eV depending on their species and coverage density. Upon TM-atom adsorption graphene becomes a magnetic metal. TM-atoms can also be adsorbed to graphene nanoribbons with armchair edge shapes (AGNRs). Binding of TM-atoms to the edge hexagons of AGNR yield the minimum energy state for all TM-atom species examined in this work and in all ribbon widths under consideration. Dependingon the ribbon width and adsorbed TM-atom species, AGNR, a non-magnetic semiconductor, can either be a metal or a semiconductor with ferromagnetic or anti-ferromagnetic spin alignment. Interestingly, Fe or Ti adsorption makes certain AGNRs half-metallic with a 100% spin polarization at the Fermi level. These results indicate that the properties of graphene and graphene nanoribbons can be strongly modified through the adsorption of 3d TM atoms. Second, repeated heterostructures of zigzag graphene nanoribbons of different widths are shown to form multiple quantum well structures. Edge states of specific spin directions can be confined in these wells. The electronic and magnetic state of the ribbon can be modulated in real space. In specific geometries, the absence of reflection symmetry causes the magnetic ground state of whole heterostructure to change from antiferromagnetic to ferrimagnetic. These quantum structures of different geometries provide novel features for spintronic applications. Third, as a possible device application, a resonant tunnelling double barrier structure formed from a finite segment of armchair graphene nanoribbon with varying widths has been proposed based on first-principles transport calculations. Highest occupied and lowest unoccupied states are confined in the wider region, whereas the narrow regions act as tunnelling barriers. These confined states are identified through the energy level diagram and isosurface charge density plots which give rise to sharp peaks originating from resonant tunnelling effect. Finally, we studied dynamics of dissipation of local vibrations to the surrounding substrate. A model system consisting of an excited nano-particle which is weakly coupled with a substrate is considered. Using three different methods, the dynamics of energy dissipation for different types of coupling between the nano-particle and the substrate is studied, where different types of dimensionality and phonon densities of states were also considered for the substrate. Results of this theoretical analysis are verified by a realistic study. To this end the phonon modes and interaction parameters involved in the energy dissipation from an excited benzene molecule to the graphene are calculated performing first-principles calculations.Show more Item Open Access Electronic structure and optical properties of monolayer semiconductors: a computational study(Bilkent University, 2021-09) Korkmaz, Yağmur AksuShow more Interest on monolayer semiconductors is rapidly growing in recent years. One of the prominent members is hexagonal boron nitride (h-BN). At room tem-perature, it harbors an environment similar to semiconductor vacuum for point defects which is crucial for stable and controllable spin states. This qualiﬁcation makes h-BN a suitable medium for quantum technological applications. First-principles calculations are essential in order to characterize such systems. Density functional theory (DFT) is one of the most reliable methods used for these type of calculations. Recently, a variant called as DFT-1/2, has been proposed to calculate the band gaps of the materials more accurately without a signiﬁcant additional computational cost. In the ﬁrst part of thesis, we have compared the results of DFT and DFT-1/2 for carbon impurities (CB, CN ), single vacan-cies (VB, VN ), double vacancy (divacancy) and Stone-Wales defect in monolayer h-BN. Subsequently, results from computationally expensive techniques such as hybrid or GW are presented and compared with the obtained DFT-1/2 results. Especially for the defect states seemingly hidden in valence or conduction band, DFT-1/2 technique is instrumental in revealing these states while widening the band gap. Thus, we recommend the DFT-1/2 method for a quick screening of candidate band gap defect states. Another outstanding group of semiconductors is transition metal dichalco-genides (TMDs). They owe their advantages to optically addressable valley and bringing optics and mechanics together as in valleytronics, thanks to their high ﬂexibility. In the second part of this thesis, ten TMDs including their janus coun-terparts (JTMDs), namely, MoS2, MoSe2, MoTe2, WS2, WSe2, WTe2, MoSSe, MoSeTe, WSSe, and WSeTe have been computationally studied. To begin with, the electronic band structure of the speciﬁed materials have been computed using DFT followed by hybrid calculations over these, with the addition of spin-orbit coupling. Biaxial and uniaxial strain calculations are subsequently performed. JTMDs were previously proclaimed to have a good piezoelectric characteristic. According to our DFT results, JTMDs exhibit band structure and electronic properties in between its constituent TMDs, and in this respect they do not dis-play an outstanding behaviour. Based on the acquired DFT data, spinless and spinful k · p parameters are extracted by ﬁtting around optically active K valley. With the help of k·p parametrization, linear and circular dichroic behaviours are studied for unstrained and strained cases. In consideration of all these materials, WTe2 displays the largest linear dichroic responsitivity for uniaxial strain, since it has the smallest band gap and the greatest uniaxial deformation potential at the K valley. Thus, we propose monolayer WTe2 membranes to be considered for optical polarization based strain measurements, as well as, strain adjustable optical polarizers.Show more Item Open Access Electronic structure of graphene under the influence of external fields(Bilkent University, 2012) İslamoğlu, SelcenShow more In this thesis, the electronic structure of graphene under the influence of external fields such as strain or magnetic fields is investigated by using tight-binding method. Firstly, we study graphene for a band gap opening due to uniaxial strain. In contrast to the literature, we find that by considering all the bands (both σ and π bands) in graphene and including the second nearest neighbor interactions, there is no systematic band gap opening as a function of applied strain. Our results correct the previous works on the subject. Secondly, we examine the band structure and Hall conductance of graphene under the influence of perpendicular magnetic field. For graphene, we demonstrate the energy spectrum in the presence of magnetic field (Hofstadter Butterfly) where all orbitals are included. We recover both the usual and the anomalous integer quantum Hall effects depending on the proximity of the Dirac points for pure graphene and the usual integer quantum Hall effect for pure square lattice. Then, we explore the evolution of electronic properties when imperfections are introduced systematically to the system. We also demonstrate the results for a square lattice which has a distinct position in cold atom experiments. For the energy spectrum of imperfect graphene and square lattice under magnetic field (Hofstadter Butterflies), we find that impurity atoms with smaller hopping constants result in highly localized states which are decoupled from the rest of the system. The bands associated with these states form close to E = 0 eV line. On the other hand, impurity atoms with higher hopping constants are strongly coupled with the neighboring atoms. These states modify the Hofstadter Butterfly around the minimum and maximum values of the energy and for the case of graphene they form two self similar bands decoupled from the original butterfly. We also show that the bands and gaps due to the impurity states are robust with respect to the second order hopping. For the Hall conductance, in accordance with energy spectra, the localized states associated to the smaller hopping constant impurities or vacancies do not contribute to Hall conduction. However the higher hopping constant impurities are responsible for new extended states which contribute to Hall conduction. Our results for Hall conduction are also robust with respect to the second order interactions.Show more Item Open Access Electronic structure of low dimensional semiconductor systems(Bilkent University, 1992) Gülseren, OğuzShow more Recent progress made in the growth techniques has led to the fabrication of the artificial semiconductor systems of lower dimension. Electrons and holes in these materials have quantization different from those of the three dimensional systems presenting unusual electronic properties and novel device applications. In this work, the important features of the free carriers in semiconductor superlattices are examined, and the electronic structure of some novel 2D semiconductor systems are investigated theoretically. This thesis studies various systems of lower dimensionality such as: the strained Si/Ge superlattices, i-doping. Si (100) surface and the tip-sample interaction in scanning tunneling microscopy (STM) study of this surface, and Wannier-Stark localization in finite length superlattices. The electronic energy structure of pseudomorphic Ge„i/Si„ superlattices is investigated by using the empirical tight binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. It is found that Ge„i/Si„ superlattices grown on Ge (001) can have a direct band gap, if m + n = 10 and m = 6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the center of the superlattice Brillouin zone. The electronic structure of the Si i-layer in germanium is explored by using the Green’s function formalism with layer orbitals. We found two dimensional parabolic subbands near the band edges. This approach is extended to treat the electronic structure of a single quantum well without invoking the periodically repeating models. Quantum well formation in Ge,„Si„ superlattices is also studied by using different number of ^-layers. Subband structure is observed by changing the height of the Si quantum well. The confinement of acoustical modes within 2DEG due to only the electronphonon interaction is proposed. The confined modes split out from the bulk phonons, if the 2DEG is created by means of modulation doping. This occurs even if the lattice has uniform parameters. The effect is more pronounced when the wave vector q of the modes increases and is maximum a,t q = 2kp {kp is the Fermi wave vector). In the case of several electron sheets the additional features of the confinement effect appear. Green’s function method is also applied to treat the modifications of electronic state density in STM. The tip-sample interaction in STM study of Si (100) surface is explored by calculating the Gieen’s function within the empirical tight binding method. Both of the proposed reconstruction models, buckled and symmetrical dimer model, is investigated. A dip occurs in the change of density of states of surface atoms at the energy of surface states for small tip-sample distances, and it decreases with increasing tip-sample separation. Although, in-plane tip position (above the up- or down-surface atom) affects the surface atoms differently in buckled dimer model, it influences the surface atoms symmetrically in symmetric dimer model. Recent experimental studies revealed the significant information on the Wannier-Stark localization. Following these experimental results, the WannierStark ladder is investigated by carrying out numerical calculations on a multiple quantum well structure under an applied electric field. The variation of the Wannier-Stark ladder energies and localization of the corresponding wave II function are examined for a wide range of applied electric field. Our results show that Wannier-Stark ladder do exist for finite but periodic system which consists of a large number of quantum well having multi-miniband structure. It is found that the miniband states are localized in the well regions with the applied electric field, while the continuum states preserve their extended character. Energies of the well states show a linear shift with the electric field except the small field values in which a nonlinear shift is resulted. Multiband calculations show that there is a mixing between the different band states although they are localized in different well regions.Show more Item Open Access Entanglement : quantification via uncertainties and search among ultracold bosons in optical lattices(Bilkent University, 2009) Öztop, BarışShow more In the first part of the Thesis, the known measures of entanglement for finite dimensional systems are reviewed. Both the simplest case of pure states that belong to bipartite systems and more general case of mixed states are discussed. The multipartite extensions are also mentioned. In addition to the already existing ones, we propose a new measure of entanglement for pure states of bipartite systems. It is based on the dynamical symmetry group approach to quantum systems. The new measure is given in terms of the total uncertainty of basic observables for the corresponding state. Unlike conventional measures concurrence and 3-tangle, which measure the amount of entanglement of different groups of correlated parties, our measure gives the total amount of multipartite entanglement in a specific state. In the second part of the Thesis, the trapping of bosonic atoms in optical lattices is reviewed. The band structure together with Bloch functions and Wannier basis are discussed for this system. In relation with that, the corresponding Bose-Hubbard model and by the use of this model, the resulting superfluid to Mott-insulator quantum phase transition is summarized. In this regard, the Bose-Hubbard Hamiltonian of a specific system, namely ultracold spin-1 atoms with coupled ground states in an optical lattice is considered. For this system we examine particle entanglement, that is characterized by pseudo-spin squeezing both for the superfluid and Mott-insulator phases in the case of ferromagnetic and antiferromagnetic interactions. The role of a small but nonzero angle between the polarization vectors of counterpropagating lasers forming the optical lattice on quantum correlations is investigated as well.Show more Item Open Access Entanglement in atom-photon systems(Bilkent University, 2004) Can, Muhammet AliShow more In this work we propose a new principle from the point of view of quantum fluctuations of observables. This new principle can be considered as an operational definition of ME states. Moreover, we show the existence of perfect entangled states of a single “spin-1” particle. We give physical examples related to the photons, and particle physics. We show that a system of 2n identical two-level atoms interacting with n cavity photons manifests entanglement and that the set of entangled states coincides with the so-called SU(2) phase states. In particular, violation of classical realism in terms of Greenberger-Horne-Zeilinger (GHZ) and Clauser-Horne-Shimoni-Holt (GHSH) conditions is proved. We also show that generation of entangled states in the atom-photon systems under consideration strongly depends on the choice of initial conditions In order to obtain maximally robust entangled states we have combined maximum principle with minimum of energy requirement for stabilization, called Mini-max principle. We discuss the generation and monitoring of durable atomic entangled state via Raman-type process, which can be used in the quantum information processing. It is shown that the system of two three-level atoms in Λ configuration in a cavity can evolve to a long-lived maximum entangled state if the Stokes photons vanish from the cavity by means of either leakage or damping. We presented some results based on the application of spherical wave representation to description of quantum properties of multipole radiation generated by atomic transitions. In particular, the angular momentum of photons including the angular momentum entanglement, the quantum phase of photons, and the spatial properties of polarization are discussed.Show more