Exotic Vortex Research Articles

Exotic vortex effect on the alternating order around impurities in two-dimensional antiferromagnets

It is well known that a magnetic field applied onto a Heisenberg antiferromagnet acts as an effective easy-plane anisotropy leading, in the case of two dimensions, to a Berezinskii–Kosterlitz–Thouless transition at a field-dependent temperature. If a nonmagnetic impurity is introduced in the lattice, an island of finite staggered magnetization develops, for which a recently proposed effective model predicts a universal shape that is almost independent of temperature and whose amplitude decays exponentially with the distance from the impurity. This picture was confirmed for the two-dimensional spin-12 case by quantum Monte Carlo simulations, whose outcomes, however, display also a counterintuitive effect: as the temperature approaches the critical value, the field-induced staggered magnetization surprisingly increases its range, which reaches a maximum before definitely dropping to zero in the disordered phase. In this paper an explanation of such a strange behavior is given as the result of vortex excitation.

Vortex jamming in superconductors and granular rheology

We demonstrate that a highly frustrated anisotropic Josephson junction array (JJA) on a square lattice exhibits a zero-temperature jamming transition, which shares much in common with those in granular systems. Anisotropy of the Josephson couplings along the horizontal and vertical directions plays roles similar to normal load or density in granular systems. We studied numerically static and dynamic response of the system against shear, i.e. injection of external electric current at zero temperature. Current–voltage curves at various strength of the anisotropy exhibit universal scaling features around the jamming point much as do the flow curves in granular rheology, shear-stress versus shear-rate. It turns out that at zero temperature the jamming transition occurs right at the isotropic coupling and anisotropic JJA behaves as exotic fragile vortex matter: it behaves as a superconductor (vortex glass) in one direction, whereas it is a normal conductor (vortex liquid) in the other direction even at zero temperature. Furthermore, we find a variant of the theoretical model for the anisotropic JJA quantitatively reproduces universal master flow-curves of the granular systems. Our results suggest an unexpected common paradigm stretching over seemingly unrelated fields—the rheology of soft materials and superconductivity.

Neutron scattering investigations of the Abrikosov state of high-temperature superconductors

The magnetic phase diagram of high-temperature superconductors can contain many exotic vortex phases not observed in conventional superconducting materials. For example, the familiar vortex lattice may melt at high temperatures into a vortex liquid. The influence of defects, which pin the vortices, is of particular interest from both a theoretical and an experimental point of view. We have used small angle neutron scattering (SANS) to probe the order of the vortex system on a microscopic scale and have succeeded, for the first time, to measure a well-ordered vortex lattice (VL) structure at all doping regimes of La 2− x Sr x CuO 4. In the optimally to overdoped regime a field-induced transition from hexagonal to square coordination is reported. The possible connections of our SANS results to photoemission data, as well as the implications for various competing theoretical models will be discussed.

Double transition and vortex-sheet structure in multi-component superconductors

A simulation study using the time-dependent Ginzburg–Landau theory is performed for the vortex state in two-component superconductors, in order to investigate the nature of the double transition and exotic vortex state, such as the vortex structure at the domain wall. Half flux-quantum vortices are aligned along the domain wall, forming vortex-sheet structure. The flux flow and the pinning of the vortex sheet are also investigated.

Coreless and singular vortex lattices in rotating spinor Bose-Einstein condensates

We theoretically investigate vortex-lattice phases of rotating spinor Bose-Einstein condensates (BEC) with the ferromagnetic spin-interaction by numerically solving the Gross-Pitaevskii equation. The spinor BEC under slow rotation can sustain a rich variety of exotic vortices due to the multi-component order parameters, such as the Mermin-Ho and Anderson-Toulouse coreless vortices (the 2-dimensional skyrmion and meron) and the non-axisymmetric vortices with the sifting vortex cores. Here, we present the spin texture of various vortex-lattice states at higher rotation rates and in the presence of the external magnetic field. In addition, the vortex phase diagram is constructed in the plane by the total magnetization $M$ and the external rotation frequency $\Omega$ by comparing the free energies of possible vortices. It is shown that the vortex phase diagram in a $M$-$\Omega$ plane may be divided into two categories; (i) the coreless vortex lattice formed by the several types of Mermin-Ho vortices and (ii) the vortex lattice filling in the cores with the pure polar (antiferromagnetic) state. In particular, it is found that the type-(ii) state forms the composite lattices of coreless and polar-core vortices. The difference between the type-(i) and type-(ii) results from the existence of the singularity of the spin textures, which may be experimentally confirmed by the spin imaging within polarized light recently proposed by Carusotto and Mueller. We also discussed on the stability of triangular and square lattice states for rapidly rotating condensates.

Exotic vortex states in layered superconductors created by tilted magnetic field: Josephson vortices, solitons, vortex chains…

In very anisotropic layered superconductors tilted magnetic field generates two interpenetrating vortex sublattices. This set of crossing lattices contains a sublattice of Josephson vortices (JVs) and a sublattice of pancake vortex (PV) stacks. The PV sublattice modifies structure of an isolated JV. The JV phase field is composed of the regular and vortex phases. The contribution from the vortex phase smoothly takes over with increase of the magnetic field. The core structure experiences qualitative evolution with anisotropy decrease. At large anisotropies JV weakly distorts PV crystal and the JV core contains many PV rows. At smaller anisotropies the JV core shrinks to one PV row and PV stacks in this central row form a soliton-like structure. At very small c-axis field (≲1 G) the PV stacks form chains located at JVs. At certain field the crossing-lattices chains transform into the tilted-vortices chains. We present a simple criterion for this transition.

Unconventional vortices and phase transitions in rapidly rotating superfluid3He

This paper studies vortex-lattice phases of rapidly rotating superfluid ^3He based on the Ginzburg-Landau free-energy functional. To identify stable phases in the p-Omega plane (p: pressure; Omega: angular velocity), the functional is minimized with the Landau-level expansion method using up to 3000 Landau levels. This system can sustain various exotic vortices by either (i) shifting vortex cores among different components or (ii) filling in cores with components not used in the bulk. In addition, the phase near the upper critical angular velocity Omega_{c2} is neither the A nor B phases, but the polar state with the smallest superfluid density as already shown by Schopohl. Thus, multiple phases are anticipated to exist in the p-Omega plane. Six different phases are found in the present calculation performed over 0.0001 Omega_{c2} <= Omega <= Omega_{c2}, where Omega_{c2} is of order (1- T/T_c) times 10^{7} rad/s. It is shown that the double-core vortex experimentally found in the B phase originates from the conventional hexagonal lattice of the polar state near Omega_{c2} via (i) a phase composed of interpenetrating polar and Scharnberg-Klemm sublattices; (ii) the A-phase mixed-twist lattice with polar cores; (iii) the normal-core lattice found in the isolated-vortex calculation by Ohmi, Tsuneto, and Fujita; and (iv) the A-phase-core vortex discovered in another isolated-vortex calculation by Salomaa and Volovik. It is predicted that the double-core vortex will disappear completely in the experimental p-T phase diagram to be replaced by the A-phase-core vortex for Omega >~ 10^{3} ~ 10^{4} rad/s. C programs to minimize a single-component Ginzburg-Landau functional are available at {http://phys.sci.hokudai.ac.jp/~kita/index-e.html}.

μSR studies of the vortex state in type-II superconductors

The authors present a review of recent muon spin rotation (\ensuremath{\mu}SR) studies of the vortex state in type-II superconductors. There are significant gaps in our understanding of this unusual phase of matter, especially in unconventional superconductors, for which the description of the vortex structure is a subject of great controversy. The \ensuremath{\mu}SR technique provides a sensitive local probe of the spatially inhomogeneous magnetic field associated with the vortex state. For the case of a regular vortex lattice, the magnetic penetration depth \ensuremath{\lambda} and the coherence length \ensuremath{\xi} can be simultaneously extracted from the measured internal field distribution. The penetration depth is directly related to the density of superconducting carriers in the material, and measurements of its variation with temperature, magnetic field, and impurities can provide essential information on the symmetry of the order parameter. The coherence length measured with \ensuremath{\mu}SR is the length scale for spatial variations of the order parameter within a vortex core. A primary goal of this review article is to show that measurements of these fundamental length scales are fairly robust with respect to the details of how the field distribution is modeled. The reliability of the results is demonstrated by a comparison of the \ensuremath{\mu}SR experiments with relevant theories and with other experimental techniques. The authors also review \ensuremath{\mu}SR measurements that have focused on the study of pinning-induced spatial disorder and vortex fluctuation phenomena. The \ensuremath{\mu}SR technique has proven to be a powerful tool for investigating exotic vortex phases, where vortex transitions are directly observable from changes in the \ensuremath{\mu}SR line shape. Particular emphasis is given to \ensuremath{\mu}SR experiments performed on high-temperature superconductors since high-quality single crystals have become available.

Investigation of Vortex Behavior in the Organic Superconductorκ−(BEDT−TTF)2Cu(SCN)2Using Muon Spin Rotation

Muon spin rotation ({mu}SR) measurements have been performed on the organic superconductor {kappa}-(BEDT-TTF){sub 2}Cu(SCN){sub 2} in order to investigate its exotic vortex behavior. Previously unobserved features of the {mu}SR line shapes have been measured at low fields and temperatures. In the mixed state the existence of a lattice composed of linear vortices is demonstrated at low fields. The breakup of this well-ordered lattice has furthermore been observed as a function of both field and temperature. {copyright} {ital 1997} {ital The American Physical Society}

Neutron diffraction from the vortex lattice in the heavy fermion superconductor UPt3 (invited) (abstract)

The heavy fermion superconductor UPt3 is thought to have a d-wave pairing ground state. The principal experimental evidence for this consists of the anisotropy of the power-law behavior observed in transverse ultrasound and μ+ SR measurements. The observation of a complex phase diagram in the superconducting state in ultrasound, torsional oscillator, and specific heat measurements may be a further indication of an unconventional pairing state. Theoretical investigations suggest the possibility of vortex lattices that are unconventional in their symmetry, their quantization, or the structure of their composite vortex cores. Transitions between such exotic vortex lattices are in principle allowed and could explain the observed features at H≊0.6 Hc2 (for H∥ĉ) and H≊0.3Hc2 (for H⊥ĉ). Neutron diffraction is an ideal bulk probe of the microscopic properties of the vortex lattice. We have studied the vortex lattice with H⊥ĉ and T≊50 mK in the field range 0.75&amp;lt;H&amp;lt;10 kG. The structure of the vortex lattice and the quantization of the vortices, in addition to the London penetration depth, λL, the coherence length, ξ, and the effective mass anisotropy are all well determined by our measurements. The lattice is oblique hexagonal with conventional quantization. Its anisotropy can be explained by considering a combination of Fermi surface and gap anisotropy. However, the lattice does not appear to change near the transition between superconducting phases identified by other techniques.

Monopoles in the rotating superfluid helium-3 A–B interface

The 3He A–B interface provides an unprecedented type of super-fluid boundary between two degenerate macroscopically coherent quantum systems which display different broken symmetries. As a consequence, the rotating superfluid A–B phase boundary is here predicted to exhibit novel physical features: vortices in superfluid He cannot perforate as such across the A–B interface. Instead, at the rotating phase boundary, the macroscopic rotation condition is fulfilled by a lattice of monopoles—or by exotic vortices with half-integer circulation quanta. Here we suggest that the resulting areal density of monopoles which persist in the phase boundary possibly enables, for the first time, their experimental detection in superfluid 3He.

Superfluid 3He in rotation

Experiments on vortices in rotating superfluid 3He have revealed phenomena which have not been observed in any other quantum liquid and have implications in fields beyond low-temperature physics. In 3He-A, vorticity may be supported by a continuous winding of the order parameter which gives rise to the observed continuous coreless vortices with two flow quanta. Exotic vortices with half-integer circulation quanta may also exist in this phase. In 3He-B, an abrupt change in the vortex-core structure has been explained theoretically as a topological phase transition involving half-quantum vortices. NMR experiments on rotating 3He-B have revealed that the cores of vortices observed possess spontaneously broken parity, manifested by their ferromagnetic superfluid cores.