We report the observation of a Skyrmion lattice in the chiral multiferroic insulator Cu2OSeO3 using Cu L3-edge resonant soft x-ray diffraction. We observe the unexpected existence of two distinct Skyrmion sublattices that arise from inequivalent Cu sites with chemically identical coordination numbers but different magnetically active orbitals. The Skyrmion sublattices are rotated with respect to each other, implying a long wavelength modulation of the lattice. The modulation vector is controlled with an applied magnetic field, associating this moirélike phase with a continuous phase transition. Our findings will open up a new class of science involving manipulation of quantum topological states.

The dynamics of an order parameter's amplitude and phase determines the collective behaviour of novel states emerging in complex materials. Time-and momentum-resolved pump-probe spectroscopy, by virtue of measuring material properties at atomic and electronic time scales out of equilibrium, can decouple entangled degrees of freedom by visualizing their corresponding dynamics in the time domain. Here we combine time-resolved femotosecond optical and resonant X-ray diffraction measurements on charge ordered La 1.75 sr 0.25 nio 4 to reveal unforeseen photoinduced phase fluctuations of the charge order parameter. such fluctuations preserve long-range order without creating topological defects, distinct from thermal phase fluctuations near the critical temperature in equilibrium. Importantly, relaxation of the phase fluctuations is found to be an order of magnitude slower than that of the order parameter's amplitude fluctuations, and thus limits charge order recovery. This new aspect of phase fluctuations provides a more holistic view of the phase's importance in ordering phenomena of quantum matter.

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We report the observation of ferromagnetic resonance (FMR) in SrRuO3 using the time-resolved magneto-optical Kerr effect. The FMR oscillations in the time-domain appear in response to a sudden, optically induced change in the direction of easy-axis anisotropy. The high FMR frequency, 250 GHz, and large Gilbert damping parameter, alpha approximately 1, are consistent with strong spin-orbit coupling. We find that the parameters associated with the magnetization dynamics, including alpha, have a nonmonotonic temperature dependence, suggestive of a link to the anomalous Hall effect.

We report time-resolved measurements of the photoinduced change in reflectivity, R, in the Bi 2 Sr 2 Ca 1ÿy Dy y Cu 2 O 8 (BSCCO) system of cuprate superconductors as a function of hole concentration. We find that the kinetics of quasiparticle decay and the sign of R both change abruptly where the superconducting transition temperature T c is maximal. These coincident changes suggest that a sharp transition in quasiparticle dynamics takes place precisely at optimal doping in the BSCCO system. DOI: 10.1103/PhysRevLett.95.117005 PACS numbers: 74.25.Gz, 78.47.+p Pump and probe methods in optical spectroscopy have opened a new window on the properties of quasiparticles in cuprate superconductors and other highly correlated electron systems [1]. In experiments based on these methods, ultrashort pump pulses inject quasiparticles at densities that are continuously variable from well above to well below the thermal equilibrium level. Time-delayed probe pulses measure changes in the reflectivity or transmissivity that result from the presence of nonequilibrium quasiparticles, providing information about their recombination rates, transport, and optical properties. These studies have been carried out extensively in the cuprate superconductors, yielding a rich, complex, yet poorly understood array of experimental observations. One of the central observations, and possibly the most puzzling, has been the behavior of the quasiparticle recombination rate, , as a function of temperature, T, and photoinjected density, n ph . Two classes of behavior are found: in class (1) appears to vanish as T [2 -6] and n ph [2,5,7,8] tend to zero, while in class (2) remains essentially constant with decreasing T [9-11] and n ph [12]. Another, seemingly distinct, puzzle concerns the sign of the photoinduced change in sample reflectivity, R, which can be either positive or negative [2,9,13-15].Here we report measurements of R and in the Bi 2 Sr 2 Ca 1ÿy Dy y Cu 2 O 8 (BSCCO) system of cuprate superconductors as a function of hole concentration, x, that considerably clarify the conditions under which these behaviors appear. As discussed below, the key to successfully exploring the BSCCO system was to eliminate the effects of laser-induced heating. Once this is accomplished, we find that the dynamics change from class (1) to (2) at exactly x m , the value for which the superconducting transition temperature T c is maximal. Moreover, we find that the sign of R reverses at x m as well. These coincident changes suggest that an abrupt transition in quasiparticle dynamics takes place precisely at optimal doping in the BSCCO system.Time-resolved optical spectroscopy was performed using pump and probe pulses of photon energy 1.5 eV and duration 80 fs from a mode-locked Ti:Sapphire oscillator. Because the BSCCO crystals are optically thick at the laser wavelength of 820 nm, the changes in optical response were probed by measuring the reflected probe power. Figure 1 is a plot of the initial reflectivity change, R, normalized to the reflectivity R, as...

We investigate the order parameter dynamics of the stripe-ordered nickelate, La1.75Sr0.25NiO4, using time-resolved resonant X-ray diffraction. In spite of distinct spin and charge energy scales, the two order parameters' amplitude dynamics are found to be linked together due to strong coupling. Additionally, the vector nature of the spin sector introduces a longer re-orientation time scale which is absent in the charge sector. These findings demonstrate that the correlation linking the symmetrybroken states does not unbind during the non-equilibrium process, and the time scales are not necessarily associated with the characteristic energy scales of individual degrees of freedom. PACS numbers: Valid PACS appear hereIn solids, the dynamics of electronic states are often represented in the energy domain. For example, spin dynamics in ferromagnets are often characterized by the spin wave (i.e. magnon) whose bandwidth is proportional to the energy of spin exchange coupling. Charge dynamics are represented by characteristic energy scales in the single-particle excitations or the charge-charge correlation functions, such as energy gap, bandwidth, and collective mode energy. However, in complex materials, the strongly intertwined degrees of freedom can self-organize a large number of charges and spins into one or more collective broken symmetry states[1, 2], forming a rich phase diagram that is one of the hallmarks of strongly correlated materials [3]. In these cases, the dynamics of coexisting broken-symmetry states inevitably couple, and the energy representation associated with a given degree of freedom may not provide a complete description of the dynamics of these collective states.Stripe-ordered nickelate[4] is a good example of materials, in which the dynamics of its broken-symmetry states is difficult to comprehend only using energy representation. In these striped nickelates, as shown in Fig. 1 (a), two broken-symmetry states of distinct degrees of freedom coexist [5][6][7]: (1) charge order (CO), where doped charge carriers form one-dimensional charge density waves and break translational symmetry, and (2) spin order (SO), where antiferromagnetically ordered spin stripes are separated by the charge stripes and break both translational and rotational symmetry. The known energy scales for the spin and charge degrees of freedom differ by at least an order of magnitude (20 meV for spin [8] and >200 meV [9, 10] for charge); however, the significant energetic differences do not seem to be reflected in the emergent thermodynamic properties of CO and SO. More specifically, for a wide range of doping, the periodicity of the SO is always twice that of the CO, and their transition temperatures exhibit similar doping dependences [11], where the formation of the SO requires pre-existing CO. These observations are argued to provide supporting evidence that CO and SO are coupled strongly [12], which may be elusive in the energy scale description. Furthermore, such a coupling effect has not been explicitly revealed by experim...

We present resonant soft X-ray scattering (RSXS) results from small band width manganites (Pr,Ca)MnO3, which show that the CE-type spin ordering (SO) at the phase boundary is stabilized only below the canted antiferromagnetic transition temperature and enhanced by ferromagnetism in the macroscopically insulating state (FM-I). Our results reveal the fragility of the CE-type ordering that underpins the colossal magnetoresistance (CMR) effect in this system, as well as an unexpected cooperative interplay between FM-I and CE-type SO which is in contrast to the competitive interplay between the ferromagnetic metallic (FM-M) state and CE-type ordering.

Electronic orderings of charges, orbitals and spins are observed in many strongly correlated electron materials, and revealing their dynamics is a critical step toward undertsanding the underlying physics of important emergent phenomena. Here we use time-resolved resonant soft x-ray scattering spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the manganite Pr0.7Ca0.3MnO3 following ultrafast photo-exitation. Our studies reveal a glass-like recovery of the spin ordering and a crossover in the dimensionality of the restoring interaction from quasi-1D at low pump fluence to 3D at high pump fluence. This behavior arises from the metastable state created by photo-excitation, a state characterized by spin disordered metallic droplets within the larger charge- and spin-ordered insulating domains. Comparison with time-resolved resistivity measurements suggests that the collapse of spin ordering is correlated with the insulator-to-metal transition, but the recovery of the insulating phase does not depend on the re-establishment of the spin ordering.