We present results from experiments at the Linac Coherent Light Source (LCLS) demonstrating that serial femtosecond crystallography (SFX) can be performed to high resolution (~2.5 Å) using protein microcrystals deposited on an ultra-thin silicon nitride membrane and embedded in a preservation medium at room temperature. Data can be acquired at a high acquisition rate using x-ray free electron laser sources to overcome radiation damage, while sample consumption is dramatically reduced compared to flowing jet methods. We achieved a peak data acquisition rate of 10 Hz with a hit rate of ~38%, indicating that a complete data set could be acquired in about one 12-hour LCLS shift using the setup described here, or in even less time using hardware optimized for fixed target SFX. This demonstration opens the door to ultra low sample consumption SFX using the technique of diffraction-before-destruction on proteins that exist in only small quantities and/or do not produce the copious quantities of microcrystals required for flowing jet methods.

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.

Sr 2 CuTeO 6 presents an opportunity for exploring low-dimensional magnetism on a square lattice of S ¼ 1=2 Cu 2þ ions. We employ ab initio multireference configuration interaction calculations to unravel the Cu 2þ electronic structure and to evaluate exchange interactions in Sr 2 CuTeO 6 . The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr 2 CuTeO 6 is an almost ideal realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV. DOI: 10.1103/PhysRevLett.117.237203 Mott insulators are a subject of intense interest due to the observation of many different quantum phenomena [1,2]. In low-dimensional systems, frustration and quantum fluctuations can destroy long-range magnetic order giving rise to quantum paramagnetic phases such as valence-bond solids with broken lattice symmetry or spin liquids, where symmetry is conserved but with possible new collective behaviors involving emergent gauge fields and fractional excitations [3][4][5]. The spin-1=2 frustrated square lattice with nearest-neighbor (NN) J 1 and nextnearest neighbor J 2 exchange interactions is one of the simplest models for valence-bond solids and spin liquids [4,6]. Yet, despite the many theoretical efforts, experimental realizations of the J 1 -J 2 model have been rather scarce. The double perovskite oxides are particularly interesting as magnetic interactions can be tuned by changing structure, stoichiometry, and cation order [7,8].In the search for a quantum magnet with weak exchange energies, Sr 2 CuTeO 6 has been proposed [9,10].The tetragonal crystal structure of the double perovskite Sr 2 CuTeO 6 [11] consists of corner sharing CuO 6 and TeO 6 octahedra that are rotated in a staggered fashion about the c axis; see Figs. 1(a) and 1(b). The CuO 6 octahedra are elongated along the c axis, effectively resulting in the ground state of a Cu 2þ (3d 9 ) ion having a hole in the inplane d x 2 −y 2 orbital, where z is along the c axis. This could eventually result in quasi-2D magnetism in Sr 2 CuTeO 6 with dominant intraplane exchange interactions. In the basal ab plane, the exchange that couples the Cu 2þ ions is the super-superexchange interaction mediated through the bridging TeO 6 octahedra as shown in Fig. 1(b), which is expected to reduce the coupling strength in Sr 2 CuTeO 6 .Magnetic susceptibility and heat capacity measurements on Sr 2 CuTeO 6 indicate a quasi-2D magnetic behavior, suggesting that it is a realization of the square-lattice J 1 -J 2 model [10]. More recently, neutron diffraction measurements on Sr 2 CuTeO 6 have shown it to order in a Néel antiferromagnetic (AFM) structure below T N ≃ 29 K with moments in the ab plane [12]; see Fig. 1(a). The ordered moment at 1.5 K was found to be reduced to 0.69ð6Þμ B , from the classical value of 1 μ B [12], indicating a renormalization by q...

Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity.

We have studied the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)_{2}(ClO4)_{2} (where pz denotes pyrazine), using neutron inelastic scattering and series expansion calculations. We show that the presence of antiferromagnetic next-nearest-neighbor interactions enhances quantum fluctuations associated with resonating valence bonds. Intermediate magnetic fields lead to a selective tuning of resonating valence bonds and a spectacular inversion of the zone-boundary dispersion, providing novel insight into 2D antiferromagnetism in the quantum limit.