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We study the dynamics of skyrmions in a metallic chiral magnet. First, we show that skyrmions can be created dynamically by destabilizing the ferromagnetic background state through a spin polarized current. We then treat skyrmions as rigid particles and derive the corresponding equation of motion. The dynamics of skyrmions is dominated by the Magnus force, which accounts for the weak pinning of skyrmions observed in experiments. Finally, we discuss the quantum motion of skyrmions. [ABSTRACT FROM AUTHOR]

Lars Onsager and Richard Feynman envisaged that the three-dimensional (3D) superfluid-to-normal λ transition in 4He occurs through the proliferation of vortices. This process should hold for every phase transition in the same universality class. The role of topological defects in symmetry-breaking phase transitions has become a prime topic in cosmology and high-temperature superconductivity, even though direct imaging of these defects is challenging. Here we show that the U(1) continuous symmetry that emerges at the ferroelectric critical point of multiferroic hexagonal manganites leads to a similar proliferation of vortices. Moreover, the disorder field (vortices) is coupled to an emergent U(1) gauge field, which becomes massive by means of the Higgs mechanism when vortices condense (span the whole system) on heating above the ferroelectric transition temperature. Direct imaging of the vortex network in hexagonal manganites offers unique experimental access to this dual description of the ferroelectric transition, while enabling tests of the Kibble-Zurek mechanism. [ABSTRACT FROM AUTHOR]

We study the triangular lattice Ising model with a finite number of vertically stacked layers and demonstrate a low temperature reentrance of two Berezinskii-Kosterlitz-Thouless transitions, which results in an extended disordered regime down to T = 0. Numerical results are complemented with the derivation of an effective low-temperature dimer theory. Contrary to order by disorder, we present a new scenario for fluctuation-induced ordering in frustrated spin systems. While short-range spin-spin correlations are enhanced by fluctuations, quasi-long-range ordering is precluded at low enough temperatures by proliferation of topological defects. [ABSTRACT FROM AUTHOR]

In borocarbide ErNi2B2C, the phase transition to the commensurate spin density wave at 2.3 K leaves 1 /20 part of Ising-like Er spins practically free. Vortices polarize these spins nonuniformly and repolarize them when moving. At a low spin relaxation rate and at low bias currents, vortices carrying magnetic polarization clouds become polaronlike and their velocities are determined by the effective drag coefficient, which is significantly bigger than the Bardeen-Stephen (BS) one. As current increases, at a critical current Jc vortices release polarization clouds and the velocity as well as the voltage in the I-V characteristics jump to values corresponding to the BS drag coefficient. The nonuniform components of the magnetic field and magnetization drop as velocity increases, resulting in weaker polarization and discontinuous dynamic dissociation depinning transition. As current decreases, on the way back, vortices are retrapped by polarization clouds at the current Jr < Jc. [ABSTRACT FROM AUTHOR]

In this work, we investigate the Josephson effect between a two-band superconductor either with the s++ (two energy gaps have the same sign and are fully gapped) pairing symmetry or j± (two energy gaps have n phase difference and are fully gapped) pairing symmetry and a conventional s-wave superconductor. The ground state, critical current, plasma modes, flux-flow dynamics, response to external ac electric field, and possible soliton solutions are investigated. For junctions with the charge neutrality breaking, we find a new plasma mode for junctions, which gives rise to new resonance peaks in the Josephson flux-flow region. Because of the frustrated interaction injunctions with s± pairing symmetry, time-reversal symmetry (TRS) can be broken if the frustration is optimized. In the TRS broken (TRSB) state, there is a nontrivial phase difference between the two Josephson tunneling channels, which results in a nontrivial interference. Furthermore, we find a massless plasma mode at the TRSB transition for junctions with the charge neutrality breaking. In the TRSB state, a spontaneous magnetic flux appears where there is a spatial inhomogeneity in the Josephson coupling, thus providing a possible smoking-gun evidence for the underlying pairing symmetry. [ABSTRACT FROM AUTHOR]

The Leggett mode associated with out-of-phase oscillations of the superconducting phase in multiband superconductors usually is heavy due to interband coupling, which makes its excitation and detection difficult. We report on the existence of a massless Leggett mode in three-band superconductors with time-reversal-symmetry breaking. The mass of this Leggett mode is small close to the time-reversal-symmetry-breaking transition and vanishes at the transition point, and thus locates within the smallest superconducting energy gap, which makes it stable and detectable, e.g., by means of the Raman spectroscopy. The thermodynamic consequences of this massless mode and possible realization in iron-based superconductors are also discussed. [ABSTRACT FROM AUTHOR]

Beside the spin density wave (SDW) order inside the superconducting phase in CeCoIn5 at high magnetic fields, recent neutron scattering measurements have found Bragg peaks in 5% Nd-doped CeCoIn5 at low fields. The intensity of Bragg peaks in low fields is suppressed by increasing field. Based on the phenomenological and microscopic modeling, we show that for the Pauli limited d-wave superconductors in the vicinity of SDW instability relevant for CeCoIn5, magnetic impurities locally induce droplets of SDW order. Because of the strong anisotropy in the momentum space in the spin fluctuations guaranteed by the d-wave pairing symmetry, sharp peaks in spin structure factor at Qs are produced by the impurities, even when the droplets of SDW do not order. At zero field, the Nd impurity spins are along the c axis due to the coupling to the conduction electrons with an easy c axis, besides their own crystal field effect. The in-plane magnetic field cants the impurity moments toward the ab plane, which suppress the droplets of SDW order. At high fields, the long-range SDW inside the superconducting phase is stabilized as a consequence of magnon condensation. Our results are consistent with the recent neutron scattering and thermal conductivity measurements. [ABSTRACT FROM AUTHOR]