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In recent years, there has been an intense search for materials realizing the Kitaev quantum spin liquid model. A number of edge-shared compounds with strong spin-orbit coupling, such as RuCl3 and iridates, have been proposed to realize this model. Nevertheless, an effective spin Hamiltonian derived from the microscopic model relevant to these compounds generally contains terms that are antagonistic toward the quantum spin liquid. This is consistent with the fact that the zero magnetic field ground state of these materials is generally magnetically ordered. It is a pressing issue to identify protocols to drive the system to the limit of the Kitaev quantum spin model. In this work, we propose Floquet engineering of these Kitaev quantum magnets by coupling materials to a circularly polarized laser. We demonstrate that all the magnetic interactions can be tuned in situ by the amplitude and frequency of the laser, hence providing a route to stabilize the Kitaev quantum spin liquid phase. Kitaev materials are prime candidates to study quantum spin liquids but despite many known examples it is still a challenge to realize systems that have a total absence of magnetic ordering. Here, the authors propose Floquet engineering and the use of light-matter interactions to achieve a Kitaev quantum spin liquid phase. [ABSTRACT FROM AUTHOR]

Exploring and controlling topological textures such as merons and skyrmions has attracted enormous interests from the perspective of fundamental research and spintronic applications. It has been predicted theoretically and proved experimentally that the lattice form of topological meron-skyrmion transformation can be realized with the requirement of external magnetic fields in chiral ferromagnets. However, such topological transition behavior has yet to be verified in other materials. Here, we report real-space observation of magnetic topology transformation between meron pairs and skyrmions in the localized domain wall of ferrimagnetic GdFeCo films without the need of magnetic fields. The topological transformation in the domain wall of ferrimagnet is introduced by temperature-induced spin reorientation transition (SRT) and the underlying mechanism is revealed by micromagnetic simulations. The convenient electric-controlling topology transformation and driving motion along the confined domain wall is further anticipated, which will enable advanced application in magnetic devices. Merons and Skyrmions, two topological spin-textures, have attracted a lot of interests due to their potential use in information storage. Here, the authors demonstrate the transformation of Meron pairs into Skyrmions without an applied magnetic field within domain walls of GdFeCo films. [ABSTRACT FROM AUTHOR]

Unusual magnetic textures can be stabilized in f-electron materials due to the interplay between competing magnetic interactions, complex Fermi surfaces, and crystalline anisotropy. Here we investigate CeAuSb2, an f-electron incommensurate antiferromagnet hosting both single-Q and double-Q spin textures as a function of magnetic fields (H) applied along the c axis. Experimentally, we map out the field-temperature phase diagram via electrical resistivity and thermal expansion measurements. Supported by calculations of a Kondo lattice model, we attribute the puzzling magnetoresistance enhancement in the double-Q phase to the localization of the electronic wave functions caused by the incommensurate magnetic texture. The magnetic, electronic, and structural properties of a quantum material are intrinsically linked. In f-electron systems, such as cerium-based materials, this interplay can be exceedingly complex. Here, the authors investigate the antiferromagnet CeAuSb2 using a Kondo lattice model in order to resolve the changes in conductance that occur with variations in the magnetic spin texture as function of applied magnetic field. [ABSTRACT FROM AUTHOR]

We study the equilibrium and dynamical properties of skyrmions in thin films of chiral magnets with oblique magnetic field. The shape of an individual skyrmion is noncircular and the skyrmion density decreases with the tilt angle from the normal of films. As a result, the interaction between two skyrmions depends on the relative angle between them in addition to their separation. The triangular lattice of skyrmions under a perpendicular magnetic field is distorted into a centered rectangular lattice for a tilted magnetic field. For a low skyrmion density, skyrmions form a chainlike structure. The dynamical response of the noncircular skyrmions depends on the direction of external currents. [ABSTRACT FROM AUTHOR]

We introduce a minimal spin model for describing the magnetic properties of CuCrCO2. Our Monte Carlo simulations of this model reveal a rich magnetic-field-induced phase diagram, which explains the measured field dependence of the electric polarization. The sequence of phase transitions between different multiferroic states arises from a subtle interplay between spatial and spin anisotropy, magnetic frustration, and thermal fluctuations. Our calculations are compared to new measurements up to 92 T. [ABSTRACT FROM AUTHOR]

A spin texture called skyrmion has been recently observed in certain chiral magnets without inversion symmetry. The observed skyrmions are extended objects with typical linear sizes of 10 to 100 nm that contain 103 to 105 spins and can be deformed in response to external perturbations. Weak deformations are characterized by internal modes, which are localized around the skyrmion center. Knowledge of internal modes is crucial to assess the stability and rigidity of these topological textures. Here, we compute the internal modes of a skyrmion in a ferromagnetic background state by numerical diagonalization of the dynamical matrix. We find several internal modes below the magnon continuum, such as the mode corresponding to the translational motion and different kinds of breathing modes. The number of internal modes is larger for lower magnetic fields. Indeed, several modes become gapless in the low-field region indicating that the single skyrmion solution becomes unstable, although a skyrmion lattice remains thermodynamically stable. On the other hand, only three internal modes exist at high fields and the skyrmion texture remains locally stable even when the ferromagnetic state becomes thermodynamically stable. We also show that the presence of out-of-plane easy-axis anisotropy stabilizes the single skyrmion solution. Finally, we discuss the effects of damping and possible experimental observations of these internal modes. [ABSTRACT FROM AUTHOR]

A topological excitation called skyrmion has been observed experimentally in chiral magnets without spatial inversion symmetry. The dynamics of a skyrmion is equivalent to an electron moving in a strong magnetic field. As a skyrmion involves large number of spins, it is not clear whether there exist observable quantum effects. In this work, we study the quantum dynamics of a skyrmion in a pinning potential. Without a pinning potential, the skyrmion performs cyclotron motion due to the strong emergent magnetic field originating from the Berry phase of spins, and all skyrmions occupy the lowest Landau level. Their wave functions are strongly localized in a region with radius less than 1 Å when no driving force is applied. Thus in most circumstances, the quantum effects of a skyrmion are weak. In the presence of a pinning potential, the lowest Landau level for skyrmions is split into quantized levels, characterized by the orbital moments. The transition between quantized levels can be observed experimentally by microwave absorption measurements in low temperature region. The quantum effects are more prominent for a skyrmion with a small size, which can be realized in magnets with a large Dzyaloshinskii-Moriya interaction. [ABSTRACT FROM AUTHOR]

A skyrmion in a nanosized disk of a chiral magnet can be used as a bit of information. To this end, it is desirable to control the creation and removal of a skyrmion only by currents without using external magnetic fields. Here, we propose to create a skyrmion by applying a current pulse to a nanodisk. The skyrmion can be removed from the disk by applying a dc current. We show that the dynamics of the created skyrmion can lead to a rectification effect, in which a dc voltage is generated by the motion of skyrmion in the presence of an ac current. [ABSTRACT FROM AUTHOR]

The fabrication of high-quality thin superconducting films is essential for single-photon detectors. Their device performance is crucially affected by their material parameters, thus requiring reliable and nondestructive characterization methods after the fabrication and patterning processes. Important material parameters to know are the resistivity, superconducting transition temperature, relaxation time of quasiparticles, and uniformity of patterned wires. In this work, we characterize micropatterned thin NbN films by using transport measurements in magnetic fields. We show that from the instability of vortex motion at high currents in the flux-flow state of the / V characteristic, the inelastic lifetime of quasiparticles can be determined to be about 2 ns. Additionally, from the depinning transition of vortices at low currents, as a function of magnetic field, the size distribution of grains can be extracted. This size distribution is found to be in agreement with the film morphology obtained from scanning electron microscopy and high-resolution transmission electron microscopy images. [ABSTRACT FROM AUTHOR]

We propose to use a superconductor-magnet multilayer structure to achieve high critical current density by invoking the polaronic mechanism of pinning. The magnetic layers should have large magnetic susceptibility to enhance the coupling between the vortices and magnetization in those layers. The relaxation of die magnetization should be slow. When the velocity of vortices is low, they are dressed by nonuniform magnetization and move as polarons. In mis case, the viscosity of vortices, which is proportional to the magnetic relaxation time, is enhanced significantly. As the velocity increases, the polarons dissociate and the viscosity drops to me usual Bardeen-Stephen value, resulting in a jump in the I-V curve. Experimentally the jump shows up as a depinning transition and the corresponding current at the jump is the depinning current. For Nb and an appropriate magnet multilayer structure, we estimate the critical current density Jc ~ 109 A/m² at magnetic field B ≈ 1 T. [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]

The working principle of magnetic force microscopy and scanning SQUID microscopy is introducing a magnetic source near a superconductor and measuring the magnetic field distribution near the superconductor, from which one can obtain the penetration depth. We investigate the magnetic field distribution near the surface of a magnetic superconductor when a magnetic source is placed close to the superconductor, which can be used to extract both the penetration depth &lgr;L and magnetic susceptibility X by magnetic force microscopy or scanning SQUID microscopy. When the magnetic moments are parallel to the surface, one extracts &lgr;L/√l -- 4&pgr;X. When the moments are perpendicular to the surface, one obtains &lgr;L. By changing the orientation of the crystal, one thus is able to extract both X and &lgr;L. [ABSTRACT FROM AUTHOR]

The Josephson-like interband couplings in multi-band superconductivity exhibit degenerate energy minima, which support states with kinks in phase of superconductivity. When the interband couplings in systems of three or more components are frustrated, the time-reversal symmetry (TRS) can be broken, which generates another type of phase kink between the two time-reversal symmetry breaking (TRSB) pair states. In this work, we focus on these novel states of phase kinks, and investigate their stability, similarity, differences and physical consequences. The main results can be summarized as follows: (i) we find a new type of phase slip when the kink becomes unstable. (ii) In the kink region, TRS is broken and spontaneous magnetic fields are induced. (iii) In superconductors with TRSB, composite topological excitations associated with variations of both the superconductivity phase and amplitude can be created by local perturbations or due to proximity effect between normal metals. [ABSTRACT FROM AUTHOR]

A single vortex overcoming the surface barrier in a mesoscopic superconductor with lateral dimensions of several coherence lengths and thickness of several nanometers provides an ideal platform to study thermal activation of a single vortex. In the presence of thermal fluctuations, there is nonzero probability for vortex penetration into or exclusion from the superconductor even when the surface barrier does not vanish. We consider the thermal activation of a single vortex in a mesoscopic superconducting disk of circular shape. To obtain statistics for the penetration and exclusion magnetic fields, slow and periodic magnetic fields are applied to the superconductor. We calculate the distribution of the penetration and exclusion fields from the thermal activation rate. This distribution can also be measured experimentally, which allows for a quantitative comparison. [ABSTRACT FROM AUTHOR]

We investigate the behavior of vortices of multicomponent superconductivity with intercomponent coupling, realized in MgB2 and Fe-based superconductors, within the framework of Ginzburg-Landau (GL) theory in terms of numerical calculations of the time-dependent GL equations and the variational method. It is revealed that close to the critical point of the composite system the intercomponent coupling makes the system behave as a single component superconductivity. However, when the bare mean-field critical points of the two components coincide with each other, and furthermore the interband coupling disappears at the same temperature, interesting phenomena occur as follows. Vortices interact attractively at large separation and repulsively at short distance in certain parameter space. Because of the nonmonotonic interaction profile, phase separations between vortex clusters of triangular order and the Meissner state take place, which indicates a first-order phase transition associated with the penetration of the magnetic field into a superconductor sample. Phase diagrams of vortex states are then constructed with the associated magnetization curve. It is found that the system shares some features of both type-I and -II superconductors. [ABSTRACT FROM AUTHOR]

A new family of dynamic states are found in a stack of inductively coupled intrinsic Josephson junctions in the absence of an external magnetic field. In this state, (2 m l+1) π phase kinks with integers m l’s stack along the c axis and lock neighboring junctions together. A significant part of the dc power is pumped into plasma oscillations via kinks at the cavity resonances. The plasma oscillation is uniform along the c axis with the frequency satisfying the ac Josephson relation. Thus this state supports strong terahertz radiation and seems to be compatible with the recent experimental observations. [ABSTRACT FROM AUTHOR]

The d-wave superconductor CeCoIn5 has been proposed as a strong candidate for supporting the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state near the low-temperature boundary of its upper critical field. Neutron diffraction, however, finds spin-density-wave (SDW) order in this part of the phase diagram for field in the a-b plane, and evidence for the SDW disappears as the applied field is rotated toward the tetragonal c axis. It is important to understand the interplay between the SDW and a possible FFLO state in CeCoIn5, as the mere existence of an SDW does not necessarily exclude an FFLO state. Here, based on a model constructed on the basis of available experiments, we show that an FFLO state competes with an SDW phase. The SDW state in CeCoIn5 is stabilized when the field is directed close to the a-b plane. When the field is rotated toward the c axis, the FFLO state emerges, and the SDW phase disappears. In the FFLO state, the nodal planes with extra quasiparticles (where the superconducting order parameter is zero) are perpendicular to the field, and in the SDW phase, the quasiparticle density of states is reduced. We test this model prediction by measuring heat transported by normal quasiparticles in the superconducting state. As a function of field, we observe a reduction of thermal conductivity for field close to the a-b plane and an enhancement of thermal conductivity when field is close to the c axis, consistent with theoretical expectations. Our modeling and experiments, therefore, indicate the existence of the FFLO state when field is parallel to the c axis. [ABSTRACT FROM AUTHOR]

We study the effect of a nonmagnetic impurity inserted in a two-dimensional frustrated ferromagnet above its saturation magnetic field Hsat for arbitrary spin S. We demonstrate that the ground state includes a magnetic vortex that is nucleated around the impurity over a finite range of magnetic field Hsat≤H≤HsatI. Upon approaching the quantum critical point at H=Hsat, the radius of the magnetic vortex diverges as the magnetic correlation length: ξ∝√1/H-Hsat. These results are derived both for the lattice and in the continuum limit. [ABSTRACT FROM AUTHOR]