Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.

Novel interplay of spin-orbit coupling and electron correlations in complexOne of the appealing examples of exotic electronic phases produced by the novel interplay of spin-orbit coupling (SOC) and the other relevant parameters in complex Ir oxides is a novel spin-orbital Mott insulator associated with the half-filled J eff = 1/2 band, lately identified in layered perovskite Sr 2 IrO 4 1,2 . The three dimensional counterpart of Sr 2 IrO 4 , SrIrO 3 , is revealed to be a semimetal close to a band insulator 3,4 , where a line of Dirac nodes generated by combination of SOC and lattice symmetry plays a key role 5 . The large SOC may give rise to intriguing topological phases as well in their sister iridium oxides. It was 4 theoretically predicted that iridium oxides with unique geometry of lattice, such as honeycomb or pyrochlore, may have a topological character [6][7][8] . A family of complex iridium oxides is thus a promising playground to explore rich electronic phases, ranging from semimetal, magnetic insulator and even topological insulator, by the subtle control of lattice.The control of dimensionality and the introduction of interface using superlattice structure have been demonstrated to be a useful technique to control the electronic phase of 3d transition metal oxides including titanium 9 and nickel 10 . The approach using superlattice could be even more powerful in complex 5d Ir oxides than their 3d analogues to explore exotic electronic phases, because of the presence of the large SOC. The emergence of a variety of phases in a narrow materials space, as a consequence of the interplay between SOC and other electronic energy scales, means that only a minute change of the control parameters including dimensionality may totally alter the ground states. To realize topological phases, the modification of local symmetry of lattice, for example breaking inversion symmetry, is often essential, which could be done by introducing interfaces. Indeed, a design of topological insulator was theoretically proposed by utilizing superlattice structure of perovskite oxides 5,11 .In this work, we have successfully grown artificial superlattices [(SrIrO 3 ) m , (T) and M(T)indicate that the magnetism, rather than disorder, is a key ingredient for the (semi-)metal to insulator transition. We also note here that for m = 1 and 2, more significantly for m = 1, (T) shows weak insulating behavior even above magnetic ordering, which might suggest the increased Mott character in the limit of reduced dimensionality. 7The tailored magnetism in the superlattice turns out to be a canted in-plane antiferromagnetism. The weak ferromagnetic moments were observed only for magnetic field parallel to IrO 2 plane as indicated in Fig. 3(a). Using those weak ferromagnetic moment as a marker, we are able to investigate the local lattice distortion in thin films, which in general is very difficult to probe. Since the unit-cell volume of SrIrO 3 is larger than that of SrTiO 3 13,14 , the IrO 6 octahedra should rotate within the plane to match t...

Transition-metal substitution in Fe pnictides leading to superconductivity is usually interpreted in terms of carrier doping to the system. We report on a density functional calculation of the local substitute electron density and demonstrate that substitutions like Co and Ni for Fe do not carrier dope but rather are isovalent to Fe. We find that the extra d electrons for Co and Ni are almost totally located within the muffin-tin sphere of the substituted site. We suggest that Co and Ni act more like random scatterers scrambling momentum space and washing out parts of the Fermi surface.

We demonstrate a method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy-sensitive photon detector to measure the partial fluorescence yield (PFY). The x-ray emission from any element or core-hole excitation that is not resonant with the absorption edge under investigation is selected from the PFY. It is found that the inverse of this PFY spectrum, which we term inverse partial fluorescence yield (IPFY), is linearly proportional to the x-ray absorption cross-section without any corrections due to saturation or self-absorption effects. We demonstrate this technique on the Cu L 2,3 and Nd M 4,5 absorption edges of the high-T c cuprate La 1.475 Nd 0.4 Sr 0.125 CuO 4 by measuring the O K α PFY and comparing the total electron yield, total fluorescence yield, and IPFY spectra.

No abstract