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Multiferroics have attracted strong interest for potential applications where electric fields control magnetic order. The ultimate speed of control via magnetoelectric coupling, however, remains largely unexplored. Here, we report an experiment in which we drove spin dynamics in multiferroic TbMnO3 with an intense few-cycle terahertz (THz) light pulse tuned to resonance with an electromagnon, an electric-dipole active spin excitation. We observed the resulting spin motion using time-resolved resonant soft x-ray diffraction. Our results show that it is possible to directly manipulate atomic-scale magnetic structures with the electric field of light on a sub-picosecond time scale.

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown1, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator–metal, or Verwey, transition has long remained inaccessible2, 3, 4, 5, 6, 7, 8. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase9. Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics10

We present tunneling data from optimally-doped, superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor, clear coherence-like peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, ∆. The average magnitude of 2∆ is ∼7.4 kBTc, which exceeds the BCS weak coupling value for either s-or d-wave superconductivity. The characteristic length scales of the deviations from the average gap value, and of an anti-correlation discovered between the gap magnitude and the zero bias conductance, match well with the average separation between the Co dopant ions in the superconducting FeAs planes. The tunneling spectra themselves possess a peak-dip-hump lineshape, suggestive of a coupling of the superconducting electronic system to a well-defined bosonic mode of energy 4.7 kBTC , such as the spin resonance observed recently in inelastic neutron scattering.

We present detailed temperature-dependent optical data on BaFe2−xCoxAs2 (BCFA), with x = 0.14, between 4 meV and 6.5 eV. We analyze our spectra to determine the main optical parameters and show that in this material the interband conductivity already starts at energies as low as 10 meV. We determine the superfluid density ρs/(2πc) 2 = 2.2 ± 0.5 • 10 7 cm −2 , which places optimally doped BFCA close to the Uemura line. Our experimental data shows clear signs of a superconducting gap with 2∆1 = 6.2 ± 0.8 meV. In addition, from comparing the experimental spectra to model calculations we obtain indications for an additional band of strongly scattered carriers with a larger gap, 2∆2 = 14 ± 2 meV.

We elucidate the termination surface of cleaved single crystals of the BaFe 2−x Co x As 2 and Fe y Se 1−x Te x families of the high-temperature iron-based superconductors. By combining scanning tunneling microscopic data with low-energy electron diffraction we prove that the termination layer of the BaFe 2 As 2 systems is a remnant of the Ba layer, which exhibits a complex diversity of ordered and disordered structures. The observed surface topographies and their accompanying superstructure reflections in electron diffraction depend on the cleavage temperature. In stark contrast, Fe y Se 1−x Te x possesses only a single termination structure-that of the tetragonally ordered Se 1−x Te x layer.

From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we show that BaFe2As2 possesses essentially two-dimensional electronic states, with a strong change of orbital character of two of the Γ-centered Fermi surfaces as a function of kz. Upon Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases. 74.25.Jb, Since the discovery of high T c superconductivity in Fepnictides [1], many experiments have been carried out to reveal the physical and electronic properties of these materials [2,3,4,5]. The parent compounds of Fepnictide superconductors are antiferromagnetic (AFM) metals. Both electron and hole doping suppresses the AFM order and leads to a superconducting phase. The AFM ordering is supposed to occur by nesting of hole pockets at the center of the Brillouin zone (BZ) and electron pockets at the zone corner. Nesting may be also important for the pairing mechanism in these compounds [6] although there are alternative scenarios based on the high polarizability of the As ions [7]. The nesting scenario could explain why in the SmFeAsO-based superconductors [8], predicted to have an almost twodimensional electronic structure [9, 10], higher superconducting transition temperatures T c are observed than in BaFe 2 As 2 -based systems [2] which are predicted to have a more three-dimensional electronic structure [11]. In general, reduction of the dimensionality increases the number of states that could be considered to be well nested. Furthermore, we point out that the orbital character of the states at the Fermi level E F is very important for the nesting conditions as the interband transitions which determine the electronic susceptibility, as calculated by the Lindhard function, are (in weak coupling scenarios) by far strongest when the two Fermi surfaces have the same orbital character [12]. The admixture of threedimensionality, arising from interlayer coupling, makes the materials potentially more useful in devices and other applications. Thus the dimensionality of the electronic structure, i.e., the k z dispersion of the electronic states is of great importance for the understanding and application of these new superconductors.Although angle-resolved photoemission spectroscopy (ARPES) is an ideal tool to study the dispersion of bands parallel and perpendicular to the FeAs layers there exist only a few experimental studies of these issues [13,14,15]. In this letter, we report a systematic study of the dimensionality of the electronic structure of BaFe 2−x Co x As 2 (x= 0 to 0.4) using polarization dependent ARPES, uncovering two new factors which are of great signi cance for the nesting of the Fermi surfaces of these systems. Firstly we show that the Co d...

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