Babaev, Egor

Job Title

Assistant Professor, Physics Department, College of Natural Sciences

Last Name

Babaev

First Name

Egor

Discipline

Physics

Expertise

Condensed Matter Theory

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Now showing 1 - 10 of 12

Hidden vortex lattices in a thermally paired superfluid
(2008-01) Dahl, E. K.; Babaev, E.; Sudbø, A.
We study the evolution of rotational response of a statistical mechanical model of two-component superfluid with a nondissipative drag interaction as the system undergoes a transition into a paired superfluid phase at finite temperature. The transition manifests itself in a change of (i) vortex-lattice symmetry and (ii) nature of the vortex state. Instead of a vortex lattice, the system forms a highly disordered tangle which constantly undergoes merger and reconnecting processes involving different types of vortices with a “hidden” breakdown of translation symmetry.
Effects of boundaries and density inhomogeneity on states of vortex matter in Bose-Einstein condensates at finite temperature
(2008-01) Kragset, S.; Babaev, E.; Sudbø, A. Sudbø
Most of the literature on quantum vortices predicting various states of vortex matter in three dimensions at finite temperatures in quantum fluids is based on the assumption of an extended and homogeneous system. This is well known not to be the case in actual Bose-Einstein condensates in traps, which are finite systems with nonuniform density. This raises the question to what extent one can speak of different aggregate states of vortex matter (vortex lattices, liquids, and tensionless vortex tangles) in these systems. To address this point, in the present work we focus on the finite-size, boundaries and density inhomogeneity effects on thermal vortex matter in a Bose-Einstein condensate. To this end we perform Monte Carlo simulations on a model system describing trapped Bose-Einstein condensates. Throughout the paper, we draw on analogies with results for vortex matter obtained for extended systems. We also consider, for comparison, the cylindrical confinement geometry with uniform density profile from the center out to the edge of the trap. The trapping potential is taken to be generically of an anharmonic form in such a way as to interpolate between a harmonic trap and a cylindrical confinement geometry. It also allows for a dip in the density profile at the center. We find distinct thermal equilibrium properties of the vortex system as the qualitative characteristics of the trapping potential are varied. For a uniform cylindrical confinement geometry, we find a vortex lattice at the center of the trap as well as ringlike thermal fluctuations of the vortex system as the trap edge is approached. For a harmonic trap, we find a distinct region at the edge of the trap where the vortex lines appear to have lost their line tension. Due to the steep density gradient, this crosses directly over to a vortex-line lattice at the center of the trap at low temperatures. At higher temperatures, an intermediate tensionful vortex liquid may exist. For an anharmonic trap where the ground state condensate density features a local minimum at the center of the trap, we find a corresponding region which appears to feature a tensionless vortex-line liquid phase. This work suggests that, finiteness and intrinsic inhomogeneity of the system notwithstanding, one nonetheless can approximately invoke the notion of distinct aggregate states of vortex matter realized at certain length scales. This might be helpful, in particular, in the search for possible new states of vortex matter in Bose-Einstein condensates with multiple components and different symmetries.
Non-pairwise intervortex interaction forces
(2011-01) Carlstrom, Johan; Garaud, Julien; Babaev, Egor
We demonstrate the existence of a new kind of non-pairwise multivortex interaction forces, which are present between superconducting vortices along with pairwise vortex interactions. We show that the multibody forces are especially important in compact vortex clusters in two-component type-1.5 superconductors and result in extremely rich physics of multivortex bound states.
Type-1.5 superconductivity in multiband systems: the effects of interband couplings
(2010-01) Carlstrom, Johan; Babaev, Egor; Speight, Martin
In contrast to single-component superconductors, which are described at the level of Ginzburg- Landau theory by a single parameter and are divided in type-I < 1= p 2 and type-II > 1= p 2 classes, two-component systems in general possesses three fundamental length scales and have been shown to possess a separate \type-1.5" superconducting state1,2. In that state, as a consequence of the extra fundamental length scale, vortices attract one another at long range but repel at shorter ranges, and therefore should form clusters in low magnetic fields. In such clusters one can dene a negative interface energy inside a cluster and at the same there one can dene a positive interface energy associated with the cluster's boundary. In this work we present a detailed study of the appearance of type-1.5 superconductivity and the interpretation of the fundamental length scales in the case of two active bands with substantial interband couplings such as intrinsic Josephson coupling, mixed gradient coupling and density-density interactions. We show that in the presence of substantial intercomponent interactions of the above types the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by mixed combinations of the density fields.
Unusual States of Vortex Matter in Mixtures of Bose-Einstein Condensates on Rotating Optical Lattices
(2008-01) Dahl, E. K.; Babaev, E.; Sudbø, A.
In a single-component superfluid under rotation a broken symmetry in the order parameter space results in a broken translational symmetry in real space: a vortex lattice. If translational symmetry is restored, the phase of the order parameter disorders and thus the broken symmetry in the order parameter space is also restored. We show that for Bose-Einstein condensate mixtures in optical lattices with negative dissipationless drag, a new situation arises. This state is a modulated vortex liquid which breaks translational symmetry in the direction transverse to the rotation vector.
Unconventional Rotational Responses of Hadronic Superfluids in a Neutron Star Caused by Strong Entrainment and a Σ- Hyperon Gap
(2009-01) Babaev, Egor
I show that the usual model of the rotational response of a neutron star, which predicts rotation-induced neutronic vortices and no rotation-induced protonic vortices, does not hold (i) beyond a certain threshold of entrainment interaction strength nor (ii) in the case of nonzero Σ- hyperon gap. I show that in both of these cases the rotational response involves the creation of phase windings in an electrically charged condensate. Lattices of bound states of vortices which result from these phase windings can (for a range of parameters) strongly reduce the interaction between rotation-induced vortices with magnetic-field carrying superconducting components.
Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors
(2010-01) Babaev, Egor; Carlstrom, Johan; Speight, Martin
We show that in multiband superconductors, even an extremely small interband proximity effect can lead to a qualitative change in the interaction potential between superconducting vortices by producing long-range intervortex attraction. This type of vortex interaction results in an unusual response to low magnetic fields leading to phase separation into domains of two-component Meissner states and vortex droplets.
Magnetic Field Delocalization and Flux Inversion in Fractional Vortices in Two-Component Superconductors
(2009-01) Babaev, Egor; Jaykka, Juha; Speight, Martain
We demonstrate that, in contrast with the single-component Abrikosov vortex, in two-component superconductors vortex solutions with an exponentially screened magnetic field exist only in exceptional cases: in the case of vortices carrying an integer number of flux quanta and in a special parameter limit for half-quantum vortices. For all other parameters, the vortex solutions have a delocalized magnetic field with a slowly decaying tail. Furthermore, we demonstrate a new effect which is generic in two-component systems but has no counterpart in single-component systems: on exactly half of the parameter space of the U(1)×U(1) Ginzburg-Landau model, the magnetic field of a generic fractional vortex inverts its direction at a certain distance from the vortex core.
Preemptive vortex-loop proliferation in multicomponent interacting Bose-Einstein condensates
(2008-01) Dahl, E. K.; Babaev, E.; Kragset, S.; Sudbo, A.
We use analytical arguments and large-scale Monte Carlo calculations to investigate the nature of the phase transitions between distinct complex superfluid phases in a two-component Bose-Einstein condensate when a nondissipative drag between the two components is being varied. We focus on understanding the role of topological defects in various phase transitions and develop vortex-matter arguments, allowing an analytical description of the phase diagram. We find the behavior of fluctuation induced vortex matter to be much more complex and substantially different from that of single-component superfluids. We propose and numerically investigate a drag-induced “preemptive vortex loop proliferation” scenario. Such a transition may be a quite generic feature in many multicomponent systems where symmetry is restored by a gas of several kinds of competing vortex loops.
Phase transitions in a three dimensional U(1)×U(1) lattice London superconductor: Metallic superfluid and charge-4e superconducting states
(2010-01) Herland, Egil V; Babaev, Egor; Sudbø, Asle
We consider a three dimensional lattice U(1)×U(1) and [U(1)]N superconductors in the London limit with individually conserved condensates. The U(1)×U(1) problem, generically, has two types of intercomponent interactions of different characters. First, the condensates are interacting via a minimal coupling to the same fluctuating gauge field. A second type of coupling is the direct dissipationless drag represented by a local intercomponent current-current coupling term in the free-energy functional. In this work, we present a study of the phase diagram of a U(1)×U(1) superconductor which includes both of these interactions. We study phase transitions and two types of competing paired phases which occur in this general model: (i) a metallic superfluid phase (where there is order only in the gauge-invariant phase difference of the order parameters), (ii) a composite superconducting phase where there is order in the phase sum of the order parameters which has many properties of a single-component superconductor but with a doubled value of electric charge. We investigate the phase diagram with particular focus on what we call “preemptive phase transitions.” These are phase transitions unique to multicomponent condensates with competing topological objects. A sudden proliferation of one kind of topological defects may come about due to a fluctuating background of topological defects in other sectors of the theory. For U(1)×U(1) theory with unequal bare stiffnesses where components are coupled by a noncompact gauge field only, we study how this scenario leads to a merger of two U(1) transitions into a single U(1)×U(1) discontinuous phase transition. We also report a general form of vortex-vortex bare interaction potential and possible phase transitions in an N-component London superconductor with individually conserved condensates.