Materials Science (cond-mat.mtrl-sci)

Abstract: Static recrystallization is an important aspect of metal processing. The initial stage of recrystallization - nucleation of new grains - determines its later stages. The accepted mechanisms of recrystallization nucleation are based on the assumption that embryos with orientations of the nuclei preexist in the deformed matrix. However, this standard picture seems to be incomplete. There are indications that, in some cases, the deformed matrix has no orientations observed in recrystallized material. Therefore a mechanism of early stage recrystallization without preexisting embryos is considered. Since the recrystallization growth shows strong orientation selection, and the recrystallization front is believed to migrate through collective shuffling of atoms, it is postulated that the shuffling mechanism responsible for oriented growth of a recrystallizing grain extends to the very beginning of the grain's existence, i.e., that a new orientation can be created via rearrangement of atoms in a strained region. The postulate explains the formation of new orientations, and it has the potential to significantly change the understanding of the early phase of recrystallization.

Abstract: Many ultra-dense lutetium or gadolinium based compounds doped with Eu 3+ have been prepared. This paper reports on the major scintillation performances of these compounds. One of them (Lu 2 O 3 :Eu) is particularly promising and have been deposited on a screen. Performances of such a screen are presented.

Abstract: Ultrasonic liquid phase exfoliation is a promising method for the production of 2D layered materials. A large number of studies have been made in investigating the underlying ultrasound exfoliation mechanisms. Due to the experimental challenge in capturing in real-time the highly transient and dynamic phenomena in sub-microsecond time scale and micrometer length scale at the same time, most theories reported to date are still under intensive debate. Here, we report the exciting new findings from the first scheduled user experiment using the free electron laser MHz X-ray Microscopy at the SPB/SFX instrument of the European X-ray Free-Electron Laser. The ultra-short X-ray pulse (~25 fs) and the unique pulse train time structure allow us to reveal fully the ultrasound cavitation dynamics, including the nucleation, growth, implosion dynamics of cavitation bubbles at different wave amplitudes. Using ma-chine-learning image processing, the instance of bubble cloud shock wave emission, their periodic impact onto the graphite materials and the cyclic fatigue exfoliation mechanism in multi-time scale from sub-microsecond to tens of minutes were all quantified and elucidated in this research.

Abstract: We present an innovative contactless method suitable to study in-plane thermal transport based on beam-offset frequency-domain thermoreflectance using a one-dimensional heat source with uniform power distribution. Using a one-dimensional heat source provides a number of advantages as compared to point-like heat sources, as typically used in time- and frequency-domain thermoreflectance experiments, just to name a few: (i) it leads to a slower spatial decay of the temperature field in the direction perpendicular to the line-shaped heat source, allowing to probe the temperature field at larger distances from the heater, hence, enhancing the sensitivity to in-plane thermal transport; (ii) the frequency range of interest is typically < 100 kHz. This rather low frequency range is convenient regarding the cost of the required excitation laser system but, most importantly, it allows the study of materials without the presence of a metallic transducer with almost no influence of the finite optical penetration depth of the pump and probe beams on the thermal phase lag, which arises from the large thermal penetration depth imposed by the used frequency range. We also show that for the case of a harmonic thermal excitation source, the phase lag between the thermal excitation and thermal response of the sample exhibits a linear dependence with their spatial offset, where the slope is proportional to the inverse of the thermal diffusivity of the material. We demonstrate the applicability of this method to the cases of: (i) suspended thin films of Si and PDPP4T, (ii) Bi bulk samples, and (iii) Si, glass, and highly-oriented pyrollitic graphite (HOPG) bulk samples with a thin metallic transducer. Finally, we also show that it is possible to study in-plane heat transport on substrates with rather low thermal diffusivity, e.g., glass, even using a metallic transducer.

Abstract: The measurement of the momentum distribution of positron annihilation radiation is a powerful method to detect and identify open-volume defects in crystalline solids. The Doppler broadening of the 511 keV line of the $2\gamma$ electron-positron annihilation event reflects the momentum density of annihilating pairs and local electron momenta at positron annihilation sites. It can provide information on the chemical surroundings of vacancies, such as the impurity atoms around them. Accurate methods based on first-principles calculations are crucial for interpreting measured Doppler spectra. In this work we will validate such a method based on variational quantum Monte Carlo by benchmarking results in aluminium nitride and silicon against experimental data measured from defect-free reference samples. The method directly models electron-positron correlations using variational wave functions. We achieve better agreement with experiments for our test set than conventional state-of-the-art methods. We show that normalized Doppler broadening spectra calculated with quantum Monte Carlo converge rapidly as a function of simulation cell size, and backflow transformations have only a minor effect. This makes the method robust and practical to support positron-based spectroscopies.

Abstract: This study focuses on the self-assembly and subsequent diamond growth on SiO$_2$ buffered lithium niobate (LiNbO$_3$) and lithium tantalate (LiTaO$_3$) single crystals. The zeta-potential of LNO and LTO single crystal were measured as a function of pH. They were found to be negative in the pH range 3.5-9.5. The isoelectric point for LNO was found to be at pH $\sim$ 2.91 and that of LTO to be at pH $\sim$ 3.20. X-ray photoelectron spectroscopy performed on the surfaces show presence of oxygen groups which may be responsible for the negative zeta potential of the crystals. Self-assembly of nanodiamond particles on LTO and LNO, using nanodiamond colloid, were studied. As expected, high nanodiamond density was seen when self-assembly was done using a positively charged nanodiamond particles. Diamond growth was attempted on the nanodiamond coated substrates but they were found to be unsuitable for direct growth due to disintegration of substrates in diamond growth conditions.. A $\sim$100nm thick silicon dioxide layer was deposited on the crystals, followed by nanodiamond self assembly and diamond growth. Thin diamond films were successfully grown on both coated crystals. The diamond quality was analysed by Raman spectroscopy and atomic force microscopy.

Abstract: We numerically investigate and develop analytic models for both the DC and pulsed spin-orbit-torque (SOT)-driven response of order parameter in single-domain Mn$_3$Sn, which is a metallic antiferromagnet with an anti-chiral 120$^\circ$ spin structure. We show that DC currents above a critical threshold can excite oscillatory dynamics of the order parameter in the gigahertz to terahertz frequency spectrum. Detailed models of the oscillation frequency versus input current are developed and found to be in excellent agreement with the numerical simulations of the dynamics. In the case of pulsed excitation, the magnetization can be switched from one stable state to any of the other five stable states in the Kagome plane by tuning the duration or the amplitude of the current pulse. Precise functional forms of the final switched state versus the input current are derived, offering crucial insights into the switching dynamics of Mn$_3$Sn. The readout of the magnetic state can be carried out via either the anomalous Hall effect, or the recently demonstrated tunneling magnetoresistance in an all-Mn$_3$Sn junction. We also discuss possible disturbance of the magnetic order due to heating that may occur if the sample is subject to large currents. Operating the device in pulsed mode or using low DC currents reduces the peak temperature rise in the sample due to Joule heating. Our predictive modeling and simulation results can be used by both theorists and experimentalists to explore the interplay of SOT and the order dynamics in Mn$_3$Sn, and to further benchmark the device performance.

Abstract: We present a first-principles study of the effect of 3$d$ transition metal intercalation on the magnetic properties of the 2H-NbS$_2$ system, using spin-resolved density functional theory calculations to investigate the electronic structure of $N_{1/3}$NbS$_2$ ($N$ = Ti, V, Cr, Mn, Fe, Co, Ni). We are able to accurately determine the magnetic moments and crystal field splitting, and find that the magnetic properties of the materials are determined by a mechanism based on filling rigid bands with electrons from the intercalant. We predict the dominant magnetic interaction of these materials by considering Fermi surface nesting, finding agreement with experiment where data are available.

Abstract: The crystal structure of CH3NH3PbBr3 perovskite has been investigated under high-pressure by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm-3m->Im-3->Pmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21, but the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments and visual observations and by the fact that changes induced by pressure up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy which is discussed using the structural information obtained from X-ray diffraction.

Abstract: Magnetic materials with noncollinear spin textures are promising for spintronic applications. To realize practical devices, control over the length and energy scales of such spin textures is imperative. The chiral helimagnets Cr1/3NbS2 and Cr1/3TaS2 exhibit analogous magnetic phase diagrams with different real-space periodicities and field dependence, positioning them as model systems for studying the relative strengths of the microscopic mechanisms giving rise to exotic spin textures. Here, we carry out a comparative study of the electronic structures of Cr1/3NbS2 and Cr1/3TaS2 using angle-resolved photoemission spectroscopy and density functional theory calculations. We show that bands in Cr1/3TaS2 are more dispersive than their counterparts in Cr1/3NbS2 and connect this result to bonding and orbital overlap in these materials. We also unambiguously distinguish exchange splitting from surface termination effects by studying the dependence of their photoemission spectra on polarization, temperature, and beam size. We find strong evidence that hybridization between intercalant and host lattice electronic states mediates the magnetic exchange interactions in these materials, suggesting that band engineering is a route toward tuning their spin textures. Overall, these results underscore how the modular nature of intercalated transition metal dichalcogenides translates variation in composition and electronic structure to complex magnetism.

Abstract: Doped lithium lanthanum zirconium oxide (LLZO) is a promising class of solid electrolytes for lithium-ion batteries due to their good electrochemical stability and compatibility with Li metal anodes. Ionic diffusivity in these ceramics is known to occur via correlated, vacancy mediated, jumps of Li+ between alternating tetrahedral and octahedral sites. Aliovalent doping at the Zr-site increases the concentration of vacancies in the Li+ sublattice and cation diffusivity, but such an increase is universally followed by a decrease for Li+ concentration lower than 6.3 - 6.5 Li molar content. Molecular dynamics simulations based on density functional theory show that the maximum in diffusivity originates from competing effects between the increased vacancy concentration and the increasing occupancy of the low-energy tetrahedral sites by Li+, which increases the overall activation energy associated with diffusion. For the relatively high temperatures of our simulations, Li+ concentration plays a dominant role in transport as compared to dopant chemistry.