Browsing by Subject "Fermions"

Now showing 1 - 14 of 14

Open Access
Bell solitons in ultra-cold atomic Fermi gas
(2013) Khan, A.; Panigrahi P.K.
We demonstrate the existence of supersonic bell solitons in the Bardeen-Cooper-Schrieffer-Bose-Einstein condensate crossover regime. Starting from the extended Thomas-Fermi density functional theory of superfluid order parameter, a density transformation is used to map the hydrodynamic mean field equation to a Lienard-type equation. As a result, bell solitons are obtained as exact solutions, which is further verified by the numerical solution of the dynamical equation. The stability of the soliton is established and its behaviour in the entire crossover domain is obtained. It is found that, akin to the case of vortices, bell solitons yield highest contrast in the BEC regime. © 2013 IOP Publishing Ltd.
Open Access
Collective modes in a bilayer dipolar fermi gas and the dissipationless drag effect
(Springer, 2013) Tanatar, Bilal
We consider the collective modes of a bilayer dipolar Fermi system in which the particles interact via long range (∼1/r 3) interaction. Assuming that each layer has a background flow which varies little and that the dynamics of the superfluid near T=0 is the same as that of a normal fluid, we obtain the dispersion relations for the collective modes in the presence of background flow. Decomposing the background flow into two parts, the center-of-mass flow and counterflow, we focus on the properties of the counterflow. We first find an estimate of the change in the zero-point energy ΔE ZP due to counterflow for a unit area of bilayer. Combining this with the free energy F of the system and taking the partial derivatives with respect to background velocities in the layers, we determine the current densities which reveal the fact that current in one layer does not only depend on the velocity in the same layer but also on the velocity of the other layer. This is the drag effect and we calculate the drag coefficient.
Open Access
Dimensional crossover in two-dimensional Bose-Fermi mixtures
(Institute of Physics Publishing, 2010) Subaşi, A. L.; Sevinçli, S.; Vignolo, P.; Tanatar, Bilal
We investigate the equilibrium properties of boson-fermion mixtures consisting of a Bose condensate and spin-polarized Fermi gas confined in a harmonic two-dimensional (2D) trap using mean-field theory. Boson-boson and boson-fermion coupling constants have a logarithmic dependence on the density because of the two-dimensional scattering events when the s-wave scattering lengths are on the order of mixture thickness. We show that this modifies the density profiles significantly. It is also shown that the dimensional crossover stabilizes the mixture against collapse and spatial demixing is observed for the case of a negative boson-fermion scattering length.
Open Access
Drag effect in double-layer dipolar fermi gases
(IOP, 2014) Tanatar, Bilal; Renklioğlu, Başak; Öktel, M. Özgür
We consider two parallel layers of two-dimensional spin-polarized dipolar Fermi gas without any tunneling between the layers. The effective interactions describing screening and correlation effects between the dipoles in a single layer (intra-layer) and across the layers (interlayer) are modeled within the Hubbard approximation. We calculate the rate of momentum transfer between the layers when the gas in one layer has a steady flow. The momentum transfer induces a steady flow in the second layer which is assumed initially at rest. This is the drag effect familiar from double-layer semiconductor and graphene structures. Our calculations show that the momentum relaxation time has temperature dependence similar to that in layers with charged particles which we think is related to the contributions from the collective modes of the system.
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, Bilal
Interacting 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.
Open Access
Excitonic condensation under spin-orbit coupling and BEC-BCS crossover
(The American Physical Society, 2007) Hakioǧlu T.; Şahin, M.
The condensation of electron-hole pairs is studied at zero temperature and in the presence of a weak spin-orbit coupling (SOC) in coupled quantum wells. Under realistic conditions, a perturbative SOC can have observable effects in the order parameter of the condensate. First, the fermion exchange symmetry is absent. As a result, the condensate spin has no definite parity. Additionally, the excitonic SOC breaks the rotational symmetry yielding a complex order parameter in an unconventional way; i.e., the phase pattern of the order parameter is a function of the condensate density. This is manifested through finite off-diagonal components of the static spin susceptibility, suggesting a new experimental method to confirm an excitonic condensate.
Open Access
Heat transfer through dipolar coupling: Sympathetic cooling without contact
(American Physical Society, 2016) Renklioglu, B.; Tanatar, Bilal; Oktel, M. Ö.
We consider two parallel layers of dipolar ultracold Fermi gases at different temperatures and calculate the heat transfer between them. The effective interactions describing screening and correlation effects between the dipoles in a single layer are modeled within the Euler-Lagrange Fermi-hypernetted-chain approximation. The random-phase approximation is used for the interactions across the layers. We investigate the amount of transferred power between the layers as a function of the temperature difference. Energy transfer arises due to the long-range dipole-dipole interactions. A simple thermal model is established to investigate the feasibility of using the contactless sympathetic cooling of the ultracold polar atoms and molecules. Our calculations indicate that dipolar heat transfer is effective for typical polar molecule experiments and may be utilized as a cooling process.
Open Access
Impurity coupled to an artificial magnetic field in a Fermi gas in a ring trap
(American Physical Society, 2015) Ünal, F. N.; Hetényi, B.; Oktel, M. Ö.
The dynamics of a single impurity interacting with a many-particle background is one of the central problems of condensed-matter physics. Recent progress in ultracold-atom experiments makes it possible to control this dynamics by coupling an artificial gauge field specifically to the impurity. In this paper, we consider a narrow toroidal trap in which a Fermi gas is interacting with a single atom. We show that an external magnetic field coupled to the impurity is a versatile tool to probe the impurity dynamics. Using a Bethe ansatz, we calculate the eigenstates and corresponding energies exactly as a function of the flux through the trap. Adiabatic change of flux connects the ground state to excited states due to flux quantization. For repulsive interactions, the impurity disturbs the Fermi sea by dragging the fermions whose momentum matches the flux. This drag transfers momentum from the impurity to the background and increases the effective mass. The effective mass saturates to the total mass of the system for infinitely repulsive interactions. For attractive interactions, the drag again increases the effective mass which quickly saturates to twice the mass of a single particle as a dimer of the impurity and one fermion is formed. For excited states with momentum comparable to number of particles, effective mass shows a resonant behavior. We argue that standard tools in cold-atom experiments can be used to test these predictions.
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.
Open Access
Pairing of Fermions with Unequal Effective Charges in an Artificial Magnetic Field
(American Physical Society, 2016) Ünal, F. N.; Oktel, M. Ö.
Artificial magnetic fields (AMFs) created for ultracold systems depend sensitively on the internal structure of the atoms. In a mixture, each component experiences a different AMF depending on its internal state. This enables the study of Bardeen-Cooper-Schrieffer pairing of fermions with unequal effective charges. In this Letter, we investigate the superconducting (SC) transition of a system formed by such pairs as a function of field strength. We consider a homogeneous two-component Fermi gas of unequal effective charges but equal densities with attractive interactions. We find that the phase diagram is altered drastically compared to the usual balanced charge case. First, for some AMFs there is no SC transition and isolated SC phases are formed, reflecting the discrete Landau level (LL) structure. SC phases become reentrant both in AMF and temperature. For extremely high fields where both components are confined to their lowest LLs, the effect of the charge imbalance is suppressed. Charge asymmetry reduces the critical temperature even in the low-field semiclassical regime. We discuss a pair breaking mechanism due to the unequal Lorentz forces acting on the components of the Cooper pairs to identify the underlying physics.
Open Access
Two-and one-dimensional honeycomb structures of silicon and germanium
(American Physical Society, 2009) Cahangirov, S.; Topsakal, M.; Aktürk, E.; Şahin, H.; Çıracı, Salim
First-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, these puckered structures are ambipolar and their charge carriers can behave like a massless Dirac fermion due to their π and π* bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.
Open Access
Two-dimensional boson-fermion mixtures
(2009) Subaşi, A.L.; Sevinçli, S.; Vignolo P.; Tanatar, Bilal
Using mean-field theory, we study the equilibrium properties of boson-fermion mixtures confined in a harmonic pancake-shaped trap at zero temperature. When the modulus of the s -wave scattering lengths are comparable to the mixture thickness, two-dimensional scattering events introduce a logarithmic dependence on density in the coupling constants, greatly modifying the density profiles themselves. We show that, for the case of a negative boson-fermion three-dimensional s -wave scattering length, the dimensional crossover stabilizes the mixture against collapse and drives it toward spatial demixing. © 2009 The American Physical Society.
Open Access
Two-dimensional boson-fermion mixtures in harmonic traps
(Elsevier Science B.V., 2003) Tanatar, Bilal; Erdemir, E.
The density profiles of bosonic and fermionic components in a system of trapped two-dimensional (2D) boson-fermion (BF) mixture are studied. We employ the variational approach to minimize the total energy functional of the BF mixture subject to the conservation of particle numbers of the species. We consider repulsive interactions between bosons and investigate the repulsive and attractive interactions between bosons and fermions. Our results are qualitatively similar to those in 3D, despite the fact that the structure of equations in 2D are different.
Open Access
Vortices in trapped boson-fermion mixtures
(Springer, 2005) Taşgin, M. E.; Subaşi, A. L.; Oktel, M. Ö.; Tanatar, Bilal
We consider a trapped system of atomic boson-fermion mixture with a quantized vortex. We investigate the density profiles of bosonic and fermionic components as functions of the boson-boson and boson-fermion short-range interaction strengths within the mean-filed approach. Stability of a vortex and conditions for the phase segregation are studied. We compare and contrast our results with the related system of droplets of 3He-4He mixtures.