A MHz X-ray diffraction set-up for dynamic compression experiments in the diamond anvil cell.

An experimental platform for dynamic diamond anvil cell (dDAC) research has been developed at the High Energy Density (HED) Instrument at the European X-ray Free Electron Laser (European XFEL). Advantage was taken of the high repetition rate of the European XFEL (up to 4.5 MHz) to collect pulse-resolved MHz X-ray diffraction data from samples as they are dynamically compressed at intermediate strain rates (≤103 s-1), where up to 352 diffraction images can be collected from a single pulse train. The set-up employs piezo-driven dDACs capable of compressing samples in ≥340 µs, compatible with the maximum length of the pulse train (550 µs). Results from rapid compression experiments on a wide range of sample systems with different X-ray scattering powers are presented. A maximum compression rate of 87 TPa s-1 was observed during the fast compression of Au, while a strain rate of ∼1100 s-1 was achieved during the rapid compression of N2 at 23 TPa s-1.

Is Trait Mindfulness in College Students Related to Yoga and Meditation Experience?

Background: Although meditation and yoga have been associated with higher trait mindfulness in general, they may help practitioners cultivate different qualities of mindfulness that manifest in different trait mindfulness profile scores. Primary Study Objective: This study tested the hypothesis that college students who practice both meditation and yoga report the highest overall trait mindfulness scores and also explored the possibility that scores on specific facets of mindfulness may differ as a function of the meditation or yoga experience. Methods/Design: The study used a cross-sectional online survey (n = 529). Analysis of covariance (ANCOVA) controlling for gender was used to compare students with neither meditation nor yoga experience, only meditation experience, only yoga experience, and both meditation and yoga experience, in terms of overall trait mindfulness (CAMS-R), five specific facets of trait mindfulness (FFMQ), decentering, and self-compassion. An exploratory analysis was also conducted to assess the construct validity of the FFMQ as a function of the meditation/yoga experience. Setting: Public university in Mid-Atlantic U.S. Participants: An undergraduate student convenience sample (age: 18-56 years; gender: 66% F; ethnicity: 32% racial minority). Results: There were no significant effects of the meditation/yoga experience on any outcome measure. Pairwise comparisons show that non-reactivity was significantly higher in the practitioners of both meditation and yoga compared to non-practitioners (P = .046, d = .026). Overall trait mindfulness was significantly higher in practitioners of both, compared to meditators only (P = .045, d = .064). Reliability and convergent validity of the FFMQ-SF were generally worse in non-practitioners. Conclusion: Testing for differences in trait mindfulness using the FFMQ-SF may not be reliable or valid in college students, particularly when students have little meditation or yoga experience. Future research with this population should utilize the long form of the FFMQ, collect additional data about participants' meditation and yoga practice, and strongly consider using multiple methods to assess mindfulness.

Mindfulness, cognitive functioning, and academic achievement in college students:the mediating role of stress.

Higher trait mindfulness may be associated with better cognitive functioning and academic achievement in college students. Although mediating mechanisms are unclear, lower stress levels could explain this relationship. Participants: Cross-sectional online survey (n = 534; 33% non-white; Apr 2018 - Sep 2019). Path analysis tested Perceived Stress as a mediator between specific facets of trait mindfulness and three measures of self-reported cognitive functioning and academic achievement: Cognitive Abilities, Cognitive Concerns, and GPA. Perceived Stress fully or partially mediated the relationship between all facets of trait mindfulness and perceived cognitive functioning. Only Decentering, however, was associated with higher GPA as a function of lower stress. Lower stress can explain the link between higher trait mindfulness and better cognitive functioning, but not necessarily academic achievement. Future research is needed to address causality, examine objective measures of cognitive functioning, and extend this explanatory model to mindfulness training.

The High Energy Density Scientific Instrument at the European XFEL.

The European XFEL delivers up to 27000 intense (>1012 photons) pulses per second, of ultrashort (≤50 fs) and transversely coherent X-ray radiation, at a maximum repetition rate of 4.5 MHz. Its unique X-ray beam parameters enable groundbreaking experiments in matter at extreme conditions at the High Energy Density (HED) scientific instrument. The performance of the HED instrument during its first two years of operation, its scientific remit, as well as ongoing installations towards full operation are presented. Scientific goals of HED include the investigation of extreme states of matter created by intense laser pulses, diamond anvil cells, or pulsed magnets, and ultrafast X-ray methods that allow their diagnosis using self-amplified spontaneous emission between 5 and 25 keV, coupled with X-ray monochromators and optional seeded beam operation. The HED instrument provides two target chambers, X-ray spectrometers for emission and scattering, X-ray detectors, and a timing tool to correct for residual timing jitter between laser and X-ray pulses.

High-Pressure Melt Curve and Phase Diagram of Lithium.

We investigate the phase diagram of lithium at temperatures of 200 to 400 K, to pressures over 100 GPa using x-ray diffraction in diamond anvil cells, covering the region in which the melting curve is disputed. To overcome degradation of the diamond anvils by dense lithium we utilize a rapid compression scheme taking advantage of the high flux available at modern synchrotrons. Our results show the hR1 and cI16 phases to be stable to higher temperature than previously reported. The melting minima of lithium is found to be close to room temperature between 40 and 60 GPa, below which the solid is crystalline. Analysis of the stability fields of the cI16 and oC88 phases suggest the existence of a triple point between these and an undetermined solid phase at 60 GPa between 220 and 255 K.

Evidence for phonon hardening in laser-excited gold using x-ray diffraction at a hard x-ray free electron laser.

Studies of laser-heated materials on femtosecond timescales have shown that the interatomic potential can be perturbed at sufficiently high laser intensities. For gold, it has been postulated to undergo a strong stiffening leading to an increase of the phonon energies, known as phonon hardening. Despite efforts to investigate this behavior, only measurements at low absorbed energy density have been performed, for which the interpretation of the experimental data remains ambiguous. By using in situ single-shot x-ray diffraction at a hard x-ray free-electron laser, the evolution of diffraction line intensities of laser-excited Au to a higher energy density provides evidence for phonon hardening.

Release dynamics of nanodiamonds created by laser-driven shock-compression of polyethylene terephthalate.

Laser-driven dynamic compression experiments of plastic materials have found surprisingly fast formation of nanodiamonds (ND) via X-ray probing. This mechanism is relevant for planetary models, but could also open efficient synthesis routes for tailored NDs. We investigate the release mechanics of compressed NDs by molecular dynamics simulation of the isotropic expansion of finite size diamond from different P-T states. Analysing the structural integrity along different release paths via molecular dynamic simulations, we found substantial disintegration rates upon shock release, increasing with the on-Hugnoiot shock temperature. We also find that recrystallization can occur after the expansion and hence during the release, depending on subsequent cooling mechanisms. Our study suggests higher ND recovery rates from off-Hugoniot states, e.g., via double-shocks, due to faster cooling. Laser-driven shock compression experiments of polyethylene terephthalate (PET) samples with in situ X-ray probing at the simulated conditions found diamond signal that persists up to 11 ns after breakout. In the diffraction pattern, we observed peak shifts, which we attribute to thermal expansion of the NDs and thus a total release of pressure, which indicates the stability of the released NDs.

The high-pressure lithium-palladium and lithium-palladium-hydrogen systems.

The lithium-palladium and lithium-palladium-hydrogen systems are investigated at high pressures at and above room temperature. Two novel lithium-palladium compounds are found below [Formula: see text]. An ambient temperature phase is tentatively assigned as [Formula: see text], with [Formula: see text] Å at 8.64 GPa, isostructural with [Formula: see text]. The other phase occurs at high-temperature and is [Formula: see text], [Formula: see text] Å at 3.88 GPa and 200 [Formula: see text], similar to [Formula: see text], which is also known at high pressure. The presence of hydrogen in the system results in an [Formula: see text] structure with [Formula: see text] Å at 9.74 GPa. This persists up to [Formula: see text], the highest pressure studied. Below [Formula: see text] an fcc phase with a large unit cell, [Formula: see text] Å at 0.39 GPa, is also observed in the presence of hydrogen. On heating the hydrogen containing system at 4 GPa the [Formula: see text] phases persists to the melting point of lithium. In both systems melting the lithium results in the loss of crystalline diffraction from palladium containing phases. This is attributed to dissolution of the palladium in the molten lithium, and on cooling the palladium remains dispersed.

Perceived Stress Mediates the Relationship Between Trait Mindfulness and Physical Symptoms of Stress: a Replication Study Using Structural Equation Modeling.

Objectives: The mindfulness stress buffering account posits mindfulness may benefit physical health by reducing stress. Previous research supports this account and suggests the non-judging facet of mindfulness may be most strongly associated with physical symptoms of stress, via lower perceived stress. The current replication study used structural equation modeling to analyze relationships between multiple facets of mindfulness, perceived stress, and physical symptoms of stress. Methods: Undergraduate students (n = 534, 68% White, 65% female) completed surveys measuring trait mindfulness (Five Facet Mindfulness Questionnaire-Short Form), perceived stress (Perceived Stress Scale), and physical symptoms of stress (Cohen-Hoberman Inventory of Physical Symptoms). Results: As hypothesized, results showed the negative relationship between four facets of mindfulness (describing, non-judging, non-reactivity, and acting with awareness) and physical symptoms of stress was partially mediated by lower perceived stress. Observing, however, was associated with more physical symptoms of stress. Conclusions: The current findings successfully replicated the results of two previous studies in an independent sample, using a more parsimonious analytic strategy that included all variables in a single path model. Results confirm the stress-buffering effect of trait mindfulness, particularly non-judging. Future research may test whether changes in trait mindfulness, particularly non-judging, explain individual differences in objective measures of stress and physical health.

Trait mindfulness and mind-body health in students: The role of gender, race, and ethnicity.

Objective: Trait mindfulness is associated with well-being in college students, yet it is unclear whether these associations are consistent across demographics. Participants: Undergraduate students (n = 534; 33% nonwhite; Apr2018-Sep2019). Methods: A cross-sectional online survey was performed. Pearson correlations tested the relationship between specific facets of trait mindfulness and four domains of mind-body health: stress, well-being, cognitive functioning, and health behaviors. Gender, race, and ethnicity were tested as moderators. Results: In general, higher trait mindfulness is consistently associated with better mind-body health across demographics. However, in men, some health behavior variables correlated more strongly with mindfulness. Among Black students, the relationship between Non-Reactivity and some outcome variables was null or counterintuitive. In Asian students, several predicted associations were significantly stronger. Conclusion: Trait mindfulness corresponds to mind-body health in college students, but relationships may not be universal. Future research is needed to replicate these findings and to examine possible demographic differences in response to mindfulness training.

Diamond formation kinetics in shock-compressed C─H─O samples recorded by small-angle x-ray scattering and x-ray diffraction.

Extreme conditions inside ice giants such as Uranus and Neptune can result in peculiar chemistry and structural transitions, e.g., the precipitation of diamonds or superionic water, as so far experimentally observed only for pure C─H and H2O systems, respectively. Here, we investigate a stoichiometric mixture of C and H2O by shock-compressing polyethylene terephthalate (PET) plastics and performing in situ x-ray probing. We observe diamond formation at pressures between 72 ± 7 and 125 ± 13 GPa at temperatures ranging from ~3500 to ~6000 K. Combining x-ray diffraction and small-angle x-ray scattering, we access the kinetics of this exotic reaction. The observed demixing of C and H2O suggests that diamond precipitation inside the ice giants is enhanced by oxygen, which can lead to isolated water and thus the formation of superionic structures relevant to the planets' magnetic fields. Moreover, our measurements indicate a way of producing nanodiamonds by simple laser-driven shock compression of cheap PET plastics.

Compression-rate dependence of pressure-induced phase transitions in Bi.

It is qualitatively well known that kinetics related to nucleation and growth can shift apparent phase boundaries from their equilibrium value. In this work, we have measured this effect in Bi using time-resolved X-ray diffraction with unprecedented 0.25 ms time resolution, accurately determining phase transition pressures at compression rates spanning five orders of magnitude (10-2-103 GPa/s) using the dynamic diamond anvil cell. An over-pressurization of the Bi-III/Bi-V phase boundary is observed at fast compression rates for different sample types and stress states, and the largest over-pressurization that is observed is ΔP = 2.5 GPa. The work presented here paves the way for future studies of transition kinetics at previously inaccessible compression rates.

X-ray Free Electron Laser-Induced Synthesis of ε-Iron Nitride at High Pressures.

The ultrafast synthesis of ε-Fe3N1+x in a diamond-anvil cell (DAC) from Fe and N2 under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transformation of α- to γ-iron. Ultimately, the Fe and N2 reacted uniformly throughout the beam path to form Fe3N1.33, as deduced from its established equation of state (EOS). We thus demonstrate that the activation energy provided by intense X-ray exposures in an XFEL can be coupled with the source time structure to enable exploration of the time-dependence of reactions under high-pressure conditions.