Browsing by Subject "Crystal lattices"
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Item Open Access Anharmonicity in GaTe layered crystals(Wiley-VCH Verlag GmbH & Co. KGaA, 2002) Aydınlı, Atilla; Gasanly, N. M.; Uka, A.; Efeoglu, H.The temperature dependencies (10-300 K) of seven Raman-active mode frequencies in layered semiconductor gallium telluride have been measured in the frequency range from 25 to 300 cm -1. Softening and broadening of the optical phonon lines are observed with increasing temperature. Comparison between the experimental data and theories of the shift of the phonon lines during heating of the crystal showed that the experimental dependencies can be explained by contributions from thermal expansion and lattice anharmonicity. Lattice anharmonicity is determined to be due to three-phonon processes.Item Open Access Effect of disorder on the interacting fermi gases in a one-dimensional optical lattice(World Scientific Publishing Co., 2008) Xianlong, G.; Polini, M.; Tosi, M. P.; Tanatar, BilalInteracting two-component Fermi gases loaded in a one-dimensional (1D) lattice and subjected to a harmonic trapping potential exhibit interesting compound phases in which fluid regions coexist with local Mott-insulator and/or band-insulator regions. Motivated by experiments on cold atoms inside disordered optical lattices, we present a theoretical study of the effects of a correlated random potential on these ground-state phases. We employ a lattice version of density-functional theory within the local-density approximation to determine the density distribution of fermions in these phases. The exchange-correlation potential is obtained from the Lieb-Wu exact solution of Fermi-Hubbard model. On-site disorder (with and without Gaussian correlations) and harmonic trap are treated as external potentials. We find that disorder has two main effects: (i) it destroys the local insulating regions if it is suffciently strong compared with the on-site atom-atom repulsion, and (ii) it induces an anomaly in the inverse compressibility at low density from quenching of percolation. For suffciently large disorder correlation length the enhancement in the inverse compressibility diminishes.Item Open Access Evolution of the Hofstadter butterfly in a tunable optical lattice(American Physical Society, 2015) Yllmaz, F.; Ünal, F. N.; Oktel, M. O.Recent advances in realizing artificial gauge fields on optical lattices promise experimental detection of topologically nontrivial energy spectra. Self-similar fractal energy structures generally known as Hofstadter butterflies depend sensitively on the geometry of the underlying lattice, as well as the applied magnetic field. The recent demonstration of an adjustable lattice geometry [L. Tarruell, D. Greif, T. Uehlinger, G. Jotzu, and T. Esslinger, Nature (London) 483, 302 (2012)NATUAS0028-083610.1038/nature10871] presents a unique opportunity to study this dependence. In this paper, we calculate the Hofstadter butterflies that can be obtained in such an adjustable lattice and find three qualitatively different regimes. We show that the existence of Dirac points at zero magnetic field does not imply the topological equivalence of spectra at finite field. As the real-space structure evolves from the checkerboard lattice to the honeycomb lattice, two square-lattice Hofstadter butterflies merge to form a honeycomb lattice butterfly. This merging is topologically nontrivial, as it is accomplished by sequential closings of gaps. Ensuing Chern number transfer between the bands can be probed with the adjustable lattice experiments. We also calculate the Chern numbers of the gaps for qualitatively different spectra and discuss the evolution of topological properties with underlying lattice geometry.Item Open Access Fabry-Perot-type resonances in metallic photonic crystals(IEEE, 1998) Serpengüzel, Ali; Savran, KerimA metallic photonic crystals (MPC) with a face-centered cubic (FCC) Bravais lattice is investigated in the microwave regime. It was found that the transmission below and above the stop band is unity, which is quite remarkable since the metallic surface would have reflected 99.97% of the incident radiation in the given microwave frequency range. The effect of the incidence angle on the transmission of the MPC was also studied.Item Open Access Hofstadter butterfly evolution in the space of two-dimensional bravais lattices(American Physical Society, 2017) Yllmaz, F.; Oktel, M. Ö.The self-similar energy spectrum of a particle in a periodic potential under a magnetic field, known as the Hofstadter butterfly, is determined by the lattice geometry as well as the external field. Recent realizations of artificial gauge fields and adjustable optical lattices in cold-atom experiments necessitate the consideration of these self-similar spectra for the most general two-dimensional lattice. In a previous work [F. Yllmaz, Phys. Rev. A 91, 063628 (2015)PLRAAN1050-294710.1103/PhysRevA.91.063628], we investigated the evolution of the spectrum for an experimentally realized lattice which was tuned by changing the unit-cell structure but keeping the square Bravais lattice fixed. We now consider all possible Bravais lattices in two dimensions and investigate the structure of the Hofstadter butterfly as the lattice is deformed between lattices with different point-symmetry groups. We model the optical lattice with a sinusoidal real-space potential and obtain the tight-binding model for any lattice geometry by calculating the Wannier functions. We introduce the magnetic field via Peierls substitution and numerically calculate the energy spectrum. The transition between the two most symmetric lattices, i.e., the triangular and the square lattices, displays the importance of bipartite symmetry featuring deformation as well as closing of some of the major energy gaps. The transitions from the square to rectangular lattice and from the triangular to centered rectangular lattices are analyzed in terms of coupling of one-dimensional chains. We calculate the Chern numbers of the major gaps and Chern number transfer between bands during the transitions. We use gap Chern numbers to identify distinct topological regions in the space of Bravais lattices.Item Open Access Intimate relationship between structural deformation and properties of single-walled carbon nanotubes(Cambridge, 2002) Yıldırım, Taner; Gülseren, Oğuz; Çıracı, SalimCarbon nanotubes continue to surprise scientists with their novel properties. Recently we have discovered many intimate relationships between structural deformation and the properties of single-walled nanotubes (SWNT), that could be important in technological applications. From first-principles we show that by using pressure, carbon nanotubes can be covalently joined to form one and two-dimensional networks of interlinked nanotubes. We also find that the band gap of an insulating nanotube can be engineered by elliptical distortion, which is found to be in the elastic range. This could allow the fine-tuning of the properties of SWNTs via reversible deformation and ultimately lead to variable quantum devices. Finally, we have very recently shown that the chemical reactivity of nanotubes can be tuned by elliptical deformation, which may provide a way to attach various atoms such as H and metals to a specific location on a nanotube.Item Open Access Lattice dynamics and elastic properties of lanthanum monopnictides(2008) Gökoǧlu G.; Erkişi, A.In this study, first principles calculation results of the second order elastic constants and lattice dynamics of two lanthanum monopnictides, LaN and LaBi, which crystallize in rock-salt structure (B1 phase), are presented. Calculations were based on plane wave basis sets and pseudopotential methods in the framework of Density Functional Theory (DFT) with generalized gradient approximation. Elastic constants are calculated by tetragonal and orthorhombic distortions on cubic structure. Phonon dispersion spectra was constructed in the linear response approach of the Density Functional Perturbation Theory (DFPT). The complete phonon softening with negative frequencies and large elastic anisotropy were observed for LaN single crystal as a sign of the structural instability. The phonon dispersion curve for LaBi is typical for lanthanum monopnictides and does not show any anomalous physical property. The calculated structural quantities for both LaN and LaBi systems agree well with the available experimental and theoretical data. © 2008 Elsevier Ltd. All rights reserved.Item Open Access Pairing and vortex lattices for interacting fermions in optical lattices with a large magnetic field(The American Physical Society, 2010) Zhai, H.; Umucalılar, R. O.; Oktel, M. Ö.We study the structure of a pairing order parameter for spin-1/2 fermions with attractive interactions in a square lattice under a uniform magnetic field. Because the magnetic translation symmetry gives a unique degeneracy in the single-particle spectrum, the pair wave function has both zero and finite-momentum components coexisting, and their relative phases are determined by a self-consistent mean-field theory. We present a microscopic calculation that can determine the vortex lattice structure in the superfluid phase for different flux densities. Phase transition from a Hofstadter insulator to a superfluid phase is also discussed.Item Open Access Phase boundary of the boson Mott insulator in a rotating optical lattice(The American Physical Society, 2007) Umucalilar, R. O.; Oktel, M. Ö.We consider the Bose-Hubbard model in a two-dimensional rotating optical lattice and investigate the consequences of the effective magnetic field created by rotation. Using a Gutzwiller-type variational wave function, we find an analytical expression for the Mott insulator (MI)-superfluid (SF) transition boundary in terms of the maximum eigenvalue of the Hofstadter butterfly. The dependence of phase boundary on the effective magnetic field is complex, reflecting the self-similar properties of the single particle energy spectrum. Finally, we argue that fractional quantum Hall phases exist close to the MI-SF transition boundaries, including MI states with particle densities greater than one.Item Open Access Photonic band gap via quantum coherence in vortex lattices of Bose-Einstein condensates(The American Physical Society, 2005) Müstecaplioǧlu, O. E.; Oktel, M. Ö.We investigate the optical response of an atomic Bose-Einstein condensate with a vortex lattice. We find that it is possible for the vortex lattice to act as a photonic crystal and create photonic band gaps, by enhancing the refractive index of the condensate via a quantum coherent scheme. If high enough index contrast between the vortex core and the atomic sample is achieved, a photonic band gap arises depending on the healing length and the lattice spacing. A wide range of experimentally accessible parameters are examined and band gaps in the visible region of the electromagnetic spectrum are found. We also show how directional band gaps can be used to directly measure the rotation frequency of the condensate.Item Open Access Quantum correlations of spin-1 atoms in an optical lattice(Institute of Physics Publishing, 2009) Öztop, B.; Oktel, M. Ö.; Müstecaplioǧlu, Ö. E.In this work, we investigate the system of cold spin-1 atoms in a one dimensional optical lattice in relation with squeezing and entanglement. By using the corresponding Bose-Hubbard Hamiltonian, both superfluid and Mott-insulator phases are studied by using numerical methods in the mean-field approximation. To observe the presence of entanglement, we used a squeezing measure as a criterion for quantum correlations. We further investigate the two interaction regimes, namely ferromagnetic and antiferromagnetic in the case of zero and nonzero but very small angle between the counterpropagating laser beams that form the optical lattice. States in the superfluid phase are calculated analytically by using the perturbation theory.Item Open Access Quantum entanglement of spin-1 bosons with coupled ground states in optical lattices(IOP Institute of Physics Publishing, 2009) Öztop, B.; Oktel, M. Ö.; Müstecapliolu, Ö. E.; You, L.We examine particle entanglement, characterized by pseudo-spin squeezing, of spin-1 bosonic atoms with coupled ground states in a one-dimensional optical lattice. Both the superfluid and Mott-insulator phases are investigated separately for ferromagnetic and antiferromagnetic interactions. Mode entanglement is also discussed in the Mott-insulating phase. The role of a small but nonzero angle between the polarization vectors of counter-propagating lasers forming the optical lattice on quantum correlations is investigated as well.Item Open Access Strong coupling characterisation of quasi-1D polarons in cylindrical QW-wires(Pergamon Press, 1996) Erçelebi, A.; Senger, R. T.We retrieve, within the strong-coupling theory, the quasi-one dimensional analog of the standard optical polaron relevant to a cylindrical quantum well wire. Under the assumption of perfect confinement the ground state binding energy, effective polaronic mass and the phonon-coupling-induced potential well profiles are given as a function of the wire radius and the electron-phonon interaction strength.Item Open Access Temperature dependence of the first-order Raman scattering in GaS layered crystals(Pergamon Press, 2000) Gasanly, N. M.; Aydınlı, A.; Özkan, H.; Kocabaş, C.The temperature dependence (15-293 K) of the six Raman-active mode frequencies and linewidths in gallium sulfide has been measured in the frequency range from 15 to 380 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison between the experimental data and theories of the shift and broadening of the interlayer and intralayer phonon lines during the heating of the crystal showed that the experimental dependencies can be explained by the contributions from thermal expansion and lattice anharmonicity. The pure-temperature contribution (phonon-phonon coupling) is due to three- and four-phonon processes.Item Open Access Temperature-dependent Raman scattering spectra of ε-GaSe layered crystal(Elsevier Science, 2002) Gasanly, N. M.; Aydnl, A.; Özkan, H.; Kocabaş, C.The temperature dependencies (15-300 K) of seven Raman-active mode frequencies and linewidths in layered gallium selenide have been measured in the frequency range from 10 to 320 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison between the experimental data and theories of the shift and broadening of the intralayer phonon lines during heating of the crystal showed that the experimental dependencies can be explained by the contributions from thermal expansion, lattice anharmonicity and crystal disorder. The pure-temperature contribution (phonon-phonon coupling) is due to three-phonon processes. Moreover, it was established that the effect of crystal disorder on the linewidth broadening of TO mode is stronger than that of LO mode.Item Open Access Time dependent study of quantum bistabiliity(1995) Ecemiş, Mustafa IhsanThe analysis of quantum transport phenomena in small systems is a prominent topic of condensed matter physics due to its numerous technological applications. The current analytical theories are not adequate for studying realistic problems. Computational methods provide the most convenient approaches. Numerical integration of the time-dependent Schrödinger equation is one of the most powerful tools albeit the implementation of the blackbody boundary conditions is problematic. In this work, a novel method which render possible this implementation is described. A number of sample calculations are presented. The method is applied to several one- and two-dimensional systems. A description of the time-dependent behavior of quantum bistable switching is given.Item Open Access Trapped fermi gases in rotating optical lattices: realization and detection of the topological hofstadter insulator(The American Physical Society, 2008) Umucalilar, R. O.; Zhai, H.; Oktel, M. Ö.We consider a gas of noninteracting spinless fermions in a rotating optical lattice and calculate the density profile of the gas in an external confinement potential. The density profile exhibits distinct plateaus, which correspond to gaps in the single particle spectrum known as the Hofstadter butterfly. The plateaus result from insulating behavior whenever the Fermi energy lies within a gap. We discuss the necessary conditions to realize the Hofstadter insulator in a cold atom setup and show how the quantized Hall conductance can be measured from density profiles using the StÅ™eda formula.Item Open Access Vortex lattice of a Bose-Einstein condensate as a photonic band gap material(IOP Institute of Physics Publishing, 2009) Taşgin, M. E.; Müstecaplioǧlu, Ö. E.; Oktel, M. Ö.Photonic crystal behavior of a rotating Bose-Einstein condensate with a triangular vortex lattice is reviewed and a scheme for getting much wider band gaps is proposed. It is shown that photonic band gaps can be widened an order of magnitude more by using a Raman scheme of index enhancement, in comparison to previously considered upper level microwave scheme.Item Open Access Vortex lattices in dipolar two-compenent Bose-Einstein condensates(American Physical Society, 2014-02-21) Ghazanfari, N.; Keles, A.; Oktel, M. O.We consider a rapidly rotating two-component Bose-Einstein condensate with short-range s-wave interactions as well as dipolar coupling. We calculate the phase diagram of vortex lattice structures as a function of the intercomponent s-wave interaction and the strength of the dipolar interaction. We find that the long-range interactions cause new vortex lattice structures to be stable and lead to a richer phase diagram. Our results reduce to the previously found lattice structures for short-range interactions and single-component dipolar gases in the corresponding limits.Item Open Access Widely tunable resonant cavity enhanced detectors built around photonic crystals(Society of Photo-Optical Instrumentation Engineers, Bellingham, WA, United States, 1999) Temelkuran, B.; Özbay, EkmelWe report a resonant cavity enhanced (RCE) detector built around a three-dimensional photonic band gap crystal. We have demonstrated the resonant cavity enhanced (RCE) effect by placing microwave detectors in defect structures built around dielectric and metallic based photonic crystals. We measured a power enhancement factor of 3450 for planar cavity structures built around dielectric based photonic crystals. The tuning bandwidth of the RCE detector extends from 10.5 to 12.8 GHz. We also demonstrated the RCE effect in cavities built around metallic structures. The power enhancement for the EM wave within these defect structures were measured to be around 190. These measurements show that detectors embedded inside photonic crystals can be used as frequency selective RCE detectors with increased sensitivity and efficiency when compared to conventional detectors.