Photoinduced phase transitions are of special interest in condensed matter physics because they can be used to change complex macroscopic material properties on the ultrafast timescale. Cooperative interactions between microscopic degrees of freedom greatly enhance the number and nature of accessible states, making it possible to switch electronic, magnetic or structural properties in new ways. Photons with high energies, of the order of electron volts, in particular are able to access electronic states that may differ greatly from states produced with stimuli close to equilibrium. In this study we report the photoinduced change in the lattice structure of a charge and orbitally ordered Nd(0.5)Sr(0.5)MnO(3) thin film using picosecond time-resolved X-ray diffraction. The photoinduced state is structurally ordered, homogeneous, metastable and has crystallographic parameters different from any thermodynamically accessible state. A femtosecond time-resolved spectroscopic study shows the formation of an electronic gap in this state. In addition, the threshold-like behaviour and high efficiency in photo-generation yield of this gapped state highlight the important role of cooperative interactions in the formation process. These combined observations point towards a 'hidden insulating phase' distinct from that found in the hitherto known phase diagram.

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Proteins harbor a number of cavities of relatively small volume. Although these packing defects are associated with the thermodynamic instability of the proteins, the cavities also play specific roles in controlling protein functions, e.g., ligand migration and binding. This issue has been extensively studied in a well-known protein, myoglobin (Mb). Mb reversibly binds gas ligands at the heme site buried in the protein matrix and possesses several internal cavities in which ligand molecules can reside. It is still an open question as to how a ligand finds its migration pathways between the internal cavities. Here, we report on the dynamic and sequential structural deformation of internal cavities during the ligand migration process in Mb. Our method, the continuous illumination of native carbonmonoxy Mb crystals with pulsed laser at cryogenic temperatures, has revealed that the migration of the CO molecule into each cavity induces structural changes of the amino acid residues around the cavity, which results in the expansion of the cavity with a breathing motion. The sequential motion of the ligand and the cavity suggests a self-opening mechanism of the ligand migration channel arising by induced fit, which is further supported by computational geometry analysis by the Delaunay tessellation method. This result suggests a crucial role of the breathing motion of internal cavities as a general mechanism of ligand migration in a protein matrix.hydrophobic cavity ͉ molecular movie ͉ protein dynamics ͉ time-resolved crystallography L ocalized electronic excitation by photons often induces largescale structural modulations and novel physical properties in condensed matter (1, 2). Myoglobin (Mb), often referred to as the hydrogen atom of biology and a paradigm of complexity (3), has played a central role in research on the photo-induced response of proteins and migration of gases, solvents, and ligands in the protein matrix (3, 4). Despite the large number of details known about Mb dynamics, it remains unclear how a ligand molecule escapes from the protein matrix to the solvent and how the protein matrix responds to the ligand migration at the atomic level. A number of time-resolved spectroscopic measurements of Mb photoproducts have revealed a complex ligand-binding reaction with multiple kinetic intermediates (4-8). After dissociation from the heme iron atom, ligand gas molecules either rebind internally from the distal pocket (DP) (Fig. 1) or escape into the solvent. It has been deduced that the escape of the ligand is assisted by the thermal f luctuations that transiently open exit channels. Lowering the temperature slows down the thermal f luctuations, and the internal binding process becomes dominant (4, 5).The multiple kinetic intermediates scheme of Mb has motivated researchers to characterize the structural features of the the intermediates by using both time-resolved (9-14) and cryogenic crystallographic measurements (15)(16)(17)(18)(19)(20)(21)(22). A general picture emerging from these experiments is that Mb...

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The magnetic field induced antiferromagnetic moment M(AF) at low magnetic fields in the antiferroquadrupolar (AFQ) ordered phase of CeB6 was investigated by elastic neutron diffraction experiments for H parallel [110]. The peak intensity at the AF magnetic reciprocal point (1 / 2,1 / 2,1 / 2) corresponding to M(2)(AF) increases with decreasing temperature below the AFQ ordering temperature T(Q), and exhibits a broad maximum at T approximately 3 K and decreases with a further decrease of temperature. This unusual behavior of M(AF) at low fields is explained as a result of the competition between the AF-octupolar and AF-exchange interactions in the O(xy) type AFQ ordered state.