Browsing by Subject "Optical lattices"
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Item Open Access Co doping induced structural and optical properties of sol-gel prepared ZnO thin films(Elsevier BV, 2014-11) Gungor, E.; Gungor, T.; Caliskan, D.; Ceylan, A.; Özbay, EkmelThe preparation conditions for Co doping process into the ZnO structure were studied by the ultrasonic spray pyrolysis technique. Structural and optical properties of the Co:ZnO thin films as a function of Co concentrations were examined. It was observed that hexagonal wurtzite structure of ZnO is dominant up to the critical value, and after the value, the cubic structural phase of the cobalt oxide appears in the X-ray diffraction patterns. Every band-edge of Co:ZnO films shifts to the lower energies and all are confirmed with the PL measurements. Co substitution in ZnO lattice has been proved by the optical transmittance measurement which is observed as the loss of transmission appearing in specific region due to Co2+ characteristic transitions. © 2014 Elsevier B.V. All rights reserved.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 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.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 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 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 Raman superradiance and spin lattice of ultracold atoms in optical cavities(IOP Institute of Physics Publishing, 2013) Safaei, S.; Müstecaplioǧlu, Ö. E.; Tanatar, BilalWe investigate the synthesis of a hyperfine spin lattice in an atomic Bose-Einstein condensate, with two hyperfine spin components, inside a one-dimensional high-finesse optical cavity, using off-resonant superradiant Raman scattering. Spatio-temporal evolution of the relative population of the hyperfine spin modes is examined numerically by solving the coupled cavity-condensate mean-field equations in the dispersive regime. We find, analytically and numerically, that beyond a certain threshold of the transverse laser pump, Raman superradiance and self-organization of the hyperfine spin components occur simultaneously and as a result a magnetic lattice is formed. The effects of an extra laser pump parallel to the cavity axis and the time dependence of the pump strength on the synthesis of a sharper lattice are also addressed.