Nonthermal Bonding Origin of a Novel Photoexcited Lattice Instability in SnSe.

Lattice dynamics measurements are often crucial tools for understanding how materials transform between different structures. We report time-resolved x-ray scattering-based measurements of the nonequilibrium lattice dynamics in SnSe, a monochalcogenide reported to host a novel photoinduced lattice instability. By fitting interatomic force models to the fluence dependent excited-state dispersion, we determine the nonthermal origin of the lattice instability to be dominated by changes of interatomic interactions along a bilayer-connecting bond, rather than of an intralayer bonding network that is of primary importance to the lattice instability in thermal equilibrium.

Ultrafast X-Ray Scattering Reveals Composite Amplitude Collective Mode in the Weyl Charge Density Wave Material (TaSe_{4})_{2}I.

We report ultrafast x-ray scattering experiments of the quasi-1D charge density wave (CDW) material (TaSe_{4})_{2}I following ultrafast infrared photoexcitation. From the time-dependent diffraction signal at the CDW sidebands we identify a 0.11 THz amplitude mode derived primarily from a transverse acoustic mode of the high-symmetry structure. From our measurements we determine that this mode interacts with the valence charge indirectly through another collective mode, and that the CDW system in (TaSe_{4})_{2}I has a composite nature supporting multiple dynamically active structural degrees of freedom.

Generic character of charge and spin density waves in superconducting cuprates.

Charge density waves (CDWs) have been observed in nearly all families of copper-oxide superconductors. But the behavior of these phases across different families has been perplexing. In La-based cuprates, the CDW wavevector is an increasing function of doping, exhibiting the so-called Yamada behavior, while in Y- and Bi-based materials the behavior is the opposite. Here, we report a combined resonant soft X-ray scattering (RSXS) and neutron scattering study of charge and spin density waves in isotopically enriched La1.8−xEu0.2SrxCuO4 over a range of doping 0.07≤x≤0.20. We find that the CDW amplitude is temperature independent and develops well above experimentally accessible temperatures. Further, the CDW wavevector shows a nonmonotonic temperature dependence, exhibiting Yamada behavior at low temperature with a sudden change occurring near the spin ordering temperature. We describe these observations using a Landau­Ginzburg theory for an incommensurate CDW in a metallic system with a finite charge compressibility and spin-CDW coupling. Extrapolating to high temperature, where the CDW amplitude is small and spin order is absent, our analysis predicts a decreasing wavevector with doping, similar to Y and Bi cuprates. Our study suggests that CDW order in all families of cuprates forms by a common mechanism.

Visualization of dynamic polaronic strain fields in hybrid lead halide perovskites.

Excitation localization involving dynamic nanoscale distortions is a central aspect of photocatalysis1, quantum materials2 and molecular optoelectronics3. Experimental characterization of such distortions requires techniques sensitive to the formation of point-defect-like local structural rearrangements in real time. Here, we visualize excitation-induced strain fields in a prototypical member of the lead halide perovskites4 via femtosecond resolution diffuse X-ray scattering measurements. This enables momentum-resolved phonon spectroscopy of the locally distorted structure and reveals radially expanding nanometre-scale strain fields associated with the formation and relaxation of polarons in photoexcited perovskites. Quantitative estimates of the magnitude and shape of this polaronic distortion are obtained, providing direct insights into the dynamic structural distortions that occur in these materials5-9. Optical pump-probe reflection spectroscopy corroborates these results and shows how these large polaronic distortions transiently modify the carrier effective mass, providing a unified picture of the coupled structural and electronic dynamics that underlie the optoelectronic functionality of the hybrid perovskites.

Unconventional Charge Density Wave Order in the Pnictide Superconductor Ba(Ni_{1-x}Co_{x})_{2}As_{2}.

Ba(Ni_{1-x}Co_{x})_{2}As_{2} is a structural homologue of the pnictide high temperature superconductor, Ba(Fe_{1-x}Co_{x})_{2}As_{2}, in which the Fe atoms are replaced by Ni. Superconductivity is highly suppressed in this system, reaching a maximum T_{c}=2.3 K, compared to 24 K in its iron-based cousin, and the origin of this T_{c} suppression is not known. Using x-ray scattering, we show that Ba(Ni_{1-x}Co_{x})_{2}As_{2} exhibits a unidirectional charge density wave (CDW) at its triclinic phase transition. The CDW is incommensurate, exhibits a sizable lattice distortion, and is accompanied by the appearance of α Fermi surface pockets in photoemission [B. Zhou et al., Phys. Rev. B 83, 035110 (2011)PRBMDO1098-012110.1103/PhysRevB.83.035110], suggesting it forms by an unconventional mechanism. Co doping suppresses the CDW, paralleling the behavior of antiferromagnetism in iron-based superconductors. Our study demonstrates that pnictide superconductors can exhibit competing CDW order, which may be the origin of T_{c} suppression in this system.

Flujo espiratorio pico (FEP) en niños de Juliaca, a 3824 msnm / Peak expiratory flow rate (PEFR) in children of Juliaca (3824 mosl)

Se estudia a 857 niños, 413 arones y 444 mujeres de 6 a 15 años de edad, de la ciudad de Juliaca, a 3824 msnm, para determinar los valores normales del flujo esp;iratorio p;ico (FEP), en relación con la edad, peso y talla. El promedio del pico espiratorio forzado (PEF) de todo el grupo fue de 255.4 L/minuto, para los niños de 273.6 L/min y para las niñas 238.4 L/min, existiendo diferencia estadística altamente significativa p > 0.01, lo que demuestra que los varones presentan valores mayores de PEF en relación con las mujeres. Los modelos que se adecuaron al estudio fueron los de regresión lineal simple (dos variables) y múltiple (tres variables), así como el análisis de correlación (r), encontrándose que existge alta corfrelación positiva entre el PEF y las variables edad, peso y talla, a medida que aumenta la variable independiente, como se expresa en las gráficas respectivas

Tiempo de trânsito intestinal en nativos del Altiplano / Intestinal transit time of the Peruvian natives from high lands

En treinta sujetos jóvenes (media=18 años), nativos de Puno (3,850 metros de altura) y que consumen dieta mixta, se estudió el tiempo de tránsito intestinal (TTI) y el peso promedio de sus heces evacuadas en 24 horas (PDH). Para este propósito se utilizó el método de León Barúa y Col. y la fórmula de correlación con el método de Hinton. La media del TTI encontrada (media ñ d.s. = 39.50 ñ 10.04 horas) es menor a las reportadas en otras poblaciones que consumen dieta mixta (p < 0.05). Cuando se compara con poblaciones vegetarianas de medio urbano nuestra cifra es menor (39.50 vs 42.4 Hs); con respecto a grupos rurales con dieta no refinada es mayor (39.17 vs 35.7 Hs). El peso de las heces (PDH) en promedio fué: media ñ d.s. = 327.23 ñ 208.47 gramos en 24 horas, mayor a las encontradas en grupos similares (165 Grs.) o en vegetarianos (225 Grs.); y menor a la de los grupos rurales del Africa (470 Grs.). La correlación entre los TTI y PDH fué negativa (r = 0.441), similar a lo reportado por otros autores. Los resultados obtenidos podrían explicarse por la posibilidad de que algunos componentes de la dieta mixta consumida puedan contribuir con más fibra que la asumida cualitativamente; otro factor a definir es la influencia de la altura sobre el tránsito intestinal