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Işkın, Menderes

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Menderes

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Işkın

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Now showing 1 - 10 of 52
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    Quantum-geometric contribution to the bogoliubov modes in a two-band Bose-Einstein condensate
    (American Physical Society (APS), 2023) N/A; Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    We consider a weakly interacting Bose-Einstein condensate that is loaded into an optical lattice with a two -point basis and described by a two-band Bose-Hubbard model with generic one-body and two-body terms. By first projecting the system onto the lower Bloch band and then applying the Bogoliubov approximation to the resultant Hamiltonian, we show that the inverse effective-mass tensor of the superfluid carriers in the Bogoliubov spectrum has two physically distinct contributions. In addition to the usual inverse band-mass tensor that is originating from the intraband processes within the lower Bloch band, there is also a quantum-geometric contribution that is induced by the two-body interactions through the interband processes. We also discuss the conditions under which the latter contribution is expressed in terms of the quantum-metric tensor of the Bloch states, i.e., the natural Fubini-Study metric on the Bloch sphere.
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    Transverse spin polarization of a Rashba-Zeeman-coupled Fermi superfluid
    (Elsevier, 2021) N/A; Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    We apply the Bogoliubov-de Gennes theory to a superfluid Fermi gas that is confined to a disc, and analyze its transverse spin polarization that is produced by the simultaneous presence of an out-of-plane Zeeman field and an in-plane Rashba spin-orbit coupling. This is a finite-size effect whose physical origin is directly associated with the equilibrium mass-current density. Our numerical findings for the ground state suggest a bulk-boundary correspondence between characteristic features of the edge-bound transverse polarization and the changes in the momentum-space topology of the bulk.
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    Dimers, trimers, tetramers, and other multimers in a multiband Bose-Hubbard model
    (American Physical Society (APS), 2022) Keleş, Ahmet; Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    We study the bound states of N identical bosons that are described by a multiband Bose-Hubbard model with generic hoppings and an attractive on-site interaction. Using a variational approach, we first derive exact integral equations for the dimers, trimers, tetramers, and other multimers, and then apply them to a one-dimensional sawtooth model that features two bands. In particular we reveal the presence of not only the off-site dimer states which consist of two monÖmers on different sites even in the strong-coupling limit but also the off-site trimer states which consist of either a dimer on one site and a monÖmer on another site or three monÖmers on three different sites. Our variational calculations for the ground states of on-site dimers, on-site trimers and off-site trimers benchmark perfectly well with the density matrix renormalization group (DMRG) simulations. We also present DMRG results for the ground states of on-site tetramers, off-site tetramers, on-site pentamers, off-site pentamers, and other multimers.
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    PublicationOpen Access
    BCS theory of time-reversal-symmetric Hofstadter-Hubbard model
    (American Physical Society (APS), 2017) Umucalılar, Rıfat Onur; Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    The competition between the length scales associated with the periodicity of a lattice potential and the cyclotron radius of a uniform magnetic field is known to have dramatic effects on the single-particle properties of a quantum particle, e.g., the fractal spectrum is known as the Hofstadter butterfly. Having this intricate competition in mind, we consider a two-component Fermi gas on a square optical lattice with opposite synthetic magnetic fields for the components, and study its effects on the many-body BCS-pairing phenomenon. By a careful addressing of the distinct superfluid transitions from the semimetal, quantum spin-Hall insulator, or normal phases, we explore the low-temperature phase diagrams of the model, displaying lobe structures that are reminiscent of the well-known Mott-insulator transitions of the Bose-Hubbard model.
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    PublicationOpen Access
    Topological superfluid phases of an atomic Fermi gas with in- and out-of-plane Zeeman fields and equal Rashba-Dresselhaus spin-orbit coupling
    (American Physical Society (APS), 2013) Subaşı, Ahmet Levent; Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    We analyze the effects of in-and out-of-plane Zeeman fields on the BCS-Bose-Einstein condensation (BEC) evolution of a Fermi gas with equal Rashba-Dresselhaus (ERD) spin-orbit coupling (SOC). We show that the ground state of the system involves gapless superfluid phases that can be distinguished with respect to the topology of the momentum-space regions with zero excitation energy. For the BCS-like uniform superfluid phases with zero center-of-mass momentum, the zeros may correspond to one or two doubly degenerate spheres, two or four spheres, two or four concave spheroids, or one or two doubly degenerate circles, depending on the combination of Zeeman fields and SOC. Such changes in the topology signal a quantum phase transition between distinct superfluid phases and leave their signatures on some thermodynamic quantities. We also analyze the possibility of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like nonuniform superfluid phases with finite center-of-mass momentum and obtain an even richer phase diagram.
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    PublicationOpen Access
    Two-band superfluidity and intrinsic Josephson effect in alkaline-earth-metal Fermi gases across an orbital Feshbach resonance
    (American Physical Society (APS), 2016) Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    We first show that the many-body Hamiltonian governing the physical properties of an alkaline-earth Yb-173 Fermi gas across the recently realized orbital Feshbach resonance is exactly analogous to that of two-band s-wave superconductors with contact interactions; i.e., even though the free-particle bands have a tunable energy offset in between and are coupled by a Josephson-type attractive interband pair scattering, the intraband interactions have exactly the same strength. We then introduce two intraband order parameters within the BCS mean-field approximation and investigate the competition between their in-phase and out-of-phase (i.e., the so-called pi-phase) solutions in the entire BCS-BEC evolution at zero temperature.
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    Atom-dimer and dimer-dimer scatterings in a spin-orbit-coupled Fermi gas
    (American Physical Society (APS), 2021) Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    Using the diagrammatic approach, here we study how spin-orbit coupling (SOC) affects the fermion-dimer and dimer-dimer scattering lengths in the Born approximation, and we benchmark their accuracy with the higher-order approximations. We consider both isotropic and Rashba couplings in three dimensions and show that the Born approximation gives accurate results in the 1/(mαas)≪-1 limit, where m is the mass of the fermions, α is the strength of the SOC, and as is the s-wave scattering length between fermions. This is because the higher-loop contributions form a perturbative series in the 1/(mαas)<0 region that is controlled by the smallness of the residue Z of the dimer propagator. In sharp contrast, since Z grows with the square root of the binding energy of the dimer in the 1/(mαas)>0 region, all of the higher-loop contributions are of similar order.
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    PublicationOpen Access
    Mean-field theory for the Mott-insulator–paired-superfluid phase transition in the two-species Bose-Hubbard model
    (American Physical Society (APS), 2010) Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    The standard mean-field theory for the Mott-insulator–superfluid phase transition is not sufficient to describe the Mott-insulator–paired-superfluid phase transition. Therefore, by restricting the two-species Bose-Hubbard Hamiltonian to the subspace of paired particles, and using perturbation theory, here we derive an analytic mean-field expression for the Mott-insulator–paired-superfluid transition boundary.
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    PublicationOpen Access
    Counterflow of spontaneous mass currents in trapped spin-orbit-coupled Fermi gases
    (American Physical Society (APS), 2012) Subaşı, Ahmet Levent; Department of Physics; Işkın, Menderes; Doko, Enis; Faculty Member; Department of Physics; College of Sciences; 29659; N/A
    We use the Bogoliubov-de Gennes formalism and study the ground-state phases of trapped spin-orbit-coupled Fermi gases in two dimensions. Our main finding is that the presence of a symmetric (Rashba-type) spin-orbit coupling spontaneously induces counterflowing mass currents in the vicinity of the trap edge, i.e., up arrow and down arrow particles circulate in opposite directions with equal speed. These currents flow even in noninteracting systems, but their strength decreases toward the molecular Bose-Einstein-condensate limit, which can be achieved by increasing either the spin-orbit coupling or the interaction strength. These currents are also quite robust against the effects of asymmetric spin-orbit couplings in the x and y directions, gradually reducing to zero as the spin-orbit coupling becomes one dimensional. We compare our results with those of chiral p-wave superfluids and superconductors.
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    PublicationOpen Access
    Spin susceptibility of spin-orbit-coupled Fermi superfluids
    (American Physical Society (APS), 2018) Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659
    Under the self-consistent mean-field approach for the BCS-BEC crossover problem, we derive a closed-form analytical expression for the general spin response of noncentrosymmetric Fermi superfluids with arbitrary spin-orbit coupling and Zeeman fields. In addition to the paramagnetic, i.e., the Pauli intrahelicity and Van Vleck type interhelicity, contributions to the spin-susceptibility tensor that have normal-state counterparts, we identify a diamagnetic interhelicity contribution that is unique to the superfluid state. Our extensive numerical calculations for the Weyl, Rashba, and equal Rashba-Dresselhaus spin-orbit couplings illustrate that it is this diamagnetic contribution that grows gradually with pairing and cancels precisely the Van Vleck contribution away from the BCS regime in general.