Atomic thermal fluctuation reduction method for robust local lattice structure identification in finite-temperature molecular dynamics

Plastic deformation of crystalline materials is mostly mediated by the motion of dislocations. It is known that the dislocation motion encounters drag related to the crystal lattice and various defects such as vacancies, impurity atoms, and grain boundaries. Under the conditions of quasi-static straining, the dislocations overcome this resistance due to the joint action of the applied stress and thermal fluctuations of atoms. The action of the thermal fluctuations of atoms on any region of a dislocation has a discrete character, since the time intervals during which the thermal fluctuations of atoms favor the motion alternate with time intervals where this action is absent. In accordance with the laws of statistical physics, this alternation takes place at a very high frequency. For this reason, the discrete character is not manifested under the usual experimental conditions. However, in connection with the investigations of material properties under impact loading of ultrashort duration, which have been conducted extensively in recent years [1], a question naturally arises: how short should the external loading duration be in order for thermal fluctuations of atoms to have, with a large probability, no time to set dislocations in motion during this loading? In this study, it is shown on the basis of the Einstein‐ Debye fluctuation theory and the results of our previous investigations [2] that, during the impact loading of a crystal with a duration of 10 –5 to 10 –6 s and a stress level on the order of the dynamic yield strength, the thermal fluctuations of atoms required for the dislocation motion do not occur (with a probability of nearly unity). As a result, the crystal remains plastically undeformed, though at the same stress level but for more prolonged loading it would experience plastic deformation. This effect can be compensated, that is, the plastic deformation of a sample subjected to a short loading pulse can be achieved, by increasing the stress level during the pulse. This is indicative of an increase in the yield strength of a material with decreasing duration of impact loading. Let us consider a crystal as an elastic isotropic body containing dislocations of one slip system. It is known that the following condition must be fulfilled in order for a dislocation at rest to be set in motion: in the vicinity of the dislocation, the tangential component of the stress tensor acting in the dislocation slip plane in the direction of the Burgers vector must exceed in absolute value a certain threshold σ 0 which characterizes the resistance impeding the dislocation motion. We will analyze a situation in which the external stress component σ B does not reach the threshold level; for certainty, let σ B be positive, so that we have 0 < σ B < σ 0 . In this case, favorable action of the thermal motion of atoms is needed for the onset of dislocation motion.

EPR theoretical study of local lattice structure in ZnS:Cr 2+ system

Theoretical study of spin singlets contributions to zero-field splitting and local lattice structure of Cr 2+ in CdGa 2S 4

Theoretical study of EPR spectra and local structure for (NiO 6) 10− cluster in LiNbO 3:Ni 2+ and Al 2O 3:Ni 2+ systems

In the last decades, applications of lithium ion batteries have expanded, and thus it is required to improve energy density of batteries. In this regard, the solid solutions of Li2MnO3 and LiMO2(M= Mn, Ni, Co, etc.) have attracted attention over the years because they deliver high discharge capacity of over 200 mAh/g. In our laboratory, we have focused on xLi2MnO3-(1-x)LiMO2(M= Mn, Ni, Co) with =0.4 and 0.5 because of the good electrochemical performance, and then performed average and local crystal structure analysis on the materials1). However, the influence of operating temperature has not been studied. In this work, we reported the temperature dependencies of average, local and electronic structures of 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2during first discharge process by using neutron and synchrotron X-ray total scatterings. In addition, we observed the electrode by TEM and STEM/EDS, and compared the result with those of the crystal structure analysis, and examined the structural changes of the particle surface and inside. 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 sample was synthesized by co-precipitation method. From the powder X-ray diffractions, it was found that a product can be attributed to a single phase of the layered structure with space group of C2/m. In the cycle tests at room and high temperatures at 60 oC, 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 electrode was able to deliver high capacity of over 280 mAh/g in the first discharge process within the voltages of 2.5 V and 4.8 V vs. Li/Li+ at the high temperature, and the capacity was higher than the value at room temperature. In order to clarify an origin of the electrode characteristics change, some electrodes were prepared with the different discharge depths in first discharge process, and examined the average structure by Rietveld technique using neutron diffraction measurements at BL20, J-PARC and synchrotron X-ray diffraction measurements at BL02B2, BL19B2, SPring-8. As a result, it was found that the ordering of the transition metal might be changed by the operating temperature and Ni cation mixing might be increased at high temperature. We also investigated the cation-distribution uniformity within the particle by STEM/EDS, and confirmed that there was not significant variation in the cation distribution. In addition, we constructed local structure models by extending the refined unit cell, and then analyzed the local structure by PDF technique using neutron total scattering measurements at BL21, J-PARC and synchrotron X-ray total scattering measurements at BL04B2, SPring-8(Fig.1).Although the cathode kept the layered structure regardless of the operating temperature, the difference of the distortion parameters, σ2, of Ni-O6 octahedral of transition metal layer and Li layer was small at high temperature. It may be one of the reasons for high discharge capacity at high temperature. However, a different trend was seen in the electron beam diffraction image: that is, there was a possibility that a part of the structure might be transformed to the spinel. In addition, we could observe a modulated structure of the layered structure with space group of C2/min the electron diffraction image, which corresponded to the results of PDF fitting. 1) Yasushi Idemoto, et al., Electrochimica Acta 153, 20 (2015). 2) Yasushi Idemoto, et al., J. Mater. Sci., to be published. Figure 1

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

X-ray absorption fine structure (XAFS) is a spectroscopic technique which can investigate the physical and chemical structure of materials at the atomic scale. X-rays are applied in this technique to be near and above the binding energy of a particular core electronic level of a particular atomic species. Over the last decades, XAFS has emerged as a highly informative probe of the local structure around selected atomic species in solids, liquids, and molecular gases. It offers both element specificity and local structure sensitivity. Foremost among its strengths are its ability to probe the local atomic environments of different elements in the sample by selecting the corresponding incident X-ray energy. In the first part of this thesis, FeGaB alloys, which are of value as soft magnetic materials having relatively large magnetostriction coefficient, were fabricated in which varying amounts of boron were added to the host FeGa alloy to investigate its impact upon local atomic structure and magnetic and microwave properties. The impact of B upon the local atomic structure in FeGaB films were investigated by extended X-ray absorption fine structure (EXAFS) analysis. EXAFS fitting results revealed a contraction of lattice parameters with the introduction of B. The Debye-Waller factor determined from EXAFS fitting gradually increases as a function of boron addition and abruptly increases during the structural evolution from crystalline to amorphous. Upon the onset of this transition the static and microwave magnetic properties became exceptionally softer, with values of coercivity and ferromagnetic linewidth reducing dramatically. In the second part of this thesis, metastable alloys of the composition FeCuZr were synthesized by high energy ball milling and measured by EXAFS . The fitting results demonstrate that nanocrystalline or amorphous alloys have been obtained depending on the Zr content.

The local lattice structures in Mn-doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ are investigated by Cu $K$-edge and Mn $K$-edge x-ray-absorption near-edge structure and extended x-ray-absorption fine structure from $10\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The results confirm that the doped Mn ions are located at the Cu site of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ phase and the impurity phases, such as ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$, are not presented in the samples. It is found that the local lattice structures of Cu are not affected by Mn impurities and the local lattice environments of Mn are similar to those of Cu. We consider that the Mn impurities only break the Cu-O $p\text{\ensuremath{-}}d$ hybridization locally, while the local lattice environments and the superconductivity in the regions away from the impurities remain untouched. This result implicates that local lattice inhomogeneity plays an important role in the mechanism of high-temperature superconductivity.

It is a long debated question whether catalytic activities of enzymes, which lie on the millisecond timescale, are possibly already reflected in variations in atomic thermal fluctuations on the pico- to nanosecond timescale. To shed light on this puzzle, the enzyme human acetylcholinesterase in its wild-type form and complexed with the inhibitor huperzine A were investigated by various neutron scattering techniques and molecular dynamics simulations. Previous results on elastic neutron scattering at various timescales and simulations suggest that dynamical processes are not affected on average by the presence of the ligand within the considered time ranges between 10 ps and 1 ns. In the work presented here, the focus was laid on quasi-elastic (QENS) and inelastic neutron scattering (INS). These techniques give access to different kinds of individual diffusive motions and to the density of states of collective motions at the sub-picoseconds timescale. Hence, they permit going beyond the first approach of looking at mean square displacements. For both samples, the autocorrelation function was well described by a stretched-exponential function indicating a linkage between the timescales of fast and slow functional relaxation dynamics. The findings of the QENS and INS investigation are discussed in relation to the results of our earlier elastic incoherent neutron scattering and molecular dynamics simulations.

A tunable plasmonic vortex lens (PVL) that can adjust the size of vortex by changing optical polarization is proposed. The tunable PVL provides a novel degree of freedom in managing surface plasmon polariton fields.

Electronic transport in semiconductors that possess high internal spontaneous and piezoelectric polarization opens up a new field of pyroelectronics and pyrosensors. The pyroelectric character of group-III-nitrides with wurtzite crystal structure yields a novel degree of freedom in designing and tailoring devices for modern micro- and nanoelectronic applications. Furthermore, spontaneous and piezoelectric polarization induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of ions, gases and polar liquids. We present a review of both theoretical and experimental studies of spontaneous and piezoelectric polarization present in AlGaN/GaN heterostructures as well as the electronic transport properties of polarization induced two-dimensional electron gases which are formed at the AlGaN/GaN interface due to the difference in the total polarization of two adjacent III-nitride layers. We demonstrate that the two-dimensional electron gases (2DEGs) achieved without modulation doping are very suitable as channel of high electron mobility transistors optimally suited for high power and high frequency applications (PART A) as well as for various kinds of sensors which can be operated in harsh environments (PART B). (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Particle and nuclear physics are moving toward a new generation of experiments to stress-test the Standard Model (SM), search for novel degrees of freedom, and comprehensively map the internal structure of hadrons. Due to the complex nature of QCD and wide array of past, present, and possible future experiments, measurements taken at these next-generation facilities will inhabit an expansive space of high-energy data. Maximizing the impact of each future collider program will depend on identifying its place within this sprawling landscape. As an initial exploration, we use the recently-developed PDFSense framework to assess the PDF sensitivity of two future high-energy facilities --- the high-luminosity upgrade to the LHC (HL-LHC) and the Large Hadron-electron Collider (LHeC) proposal --- as well as the electron-ion collider (EIC) proposed to map the few-GeV quark-hadron transition region. We report that each of these experimental facilities occupies a unique place in the kinematical parameter space with specialized pulls on particular collinear quantities. As such, there is a clear complementarity among these programs, with an opportunity for each to mutually reinforce and inform the others.

Nanophotonics and metamaterials have revolutionized the way we think about optical space (ɛ,μ), enabling us to engineer the refractive index almost at will, to confine light to the smallest of the volumes, and to manipulate optical signals with extremely small footprints and energy requirements. Significant efforts are now devoted to finding suitable materials and strategies for the dynamic control of the optical properties. Transparent conductive oxides exhibit large ultrafast nonlinearities under both interband and intraband excitations. Here we show that combining these two effects in aluminium-doped zinc oxide via a two-colour laser field discloses new material functionalities. Owing to the independence of the two nonlinearities, the ultrafast temporal dynamics of the material permittivity can be designed by acting on the amplitude and delay of the two fields. We demonstrate the potential applications of this novel degree of freedom by dynamically addressing the modulation bandwidth and optical spectral tuning of a probe optical pulse.

In this paper, we theoretically propose and experimentally demonstrate the manipulation of a novel degree of freedom in ring resonators, which is the coupling from the clockwise input to the counterclockwise propagating mode (and vice versa). We name this mechanism backcoupling, in contrast with the normal forward-coupling of a directional coupler. It is well known that internal reflections will cause peak splitting in a ring resonator. Our previous research demonstrated that the peak asymmetry will be strongly influenced by the backcoupling. Thus, it is worth manipulating the backcoupling in order to gain full control of a split resonance for the benefit of various resonance-splitting-based applications. While it is difficult to directly manipulate the backcoupling of a conventional directional coupler, here we design a circuit explicitly for manipulating the backcoupling. It can be potentially developed for applications such as single sideband filter, resonance splitting elimination, Fano resonance, and ultrahigh-Q and finesse.

The pair density function (PDF) used in the analysis of pulsed neutron diffraction data of La{sub 1{minus}x}Sr{sub x}CoO{sub 3} revealed new structural effects which are correlated to the susceptibility and transport transitions. The transition in the spin configuration of the Co ions from the low-spin (LS) to the high-spin (HS) state in the Co perovskite oxides can potentially induce structural distortions due to the coupling of the spin to the lattice and charge. The ground state of the pure compound, LaCoO{sub 3}, is in the LS state and is non-magnetic. A transition occurs to the HS state at {approximately} 50 K as indicated from the susceptibility measurements due to the thermal excitation of electrons to the e{sub g} level. The Co{sub LS}O{sub 6} octahedra associated with the Co ions in the LS configuration are distinguished from the Co{sub HS}O{sub 6} octahedra with the Co in the HS configuration because the Co{sub LS}-O bond length is shorter than the Co{sub HS}-O distance due to the different size of the corresponding Co ions. Such bond lengths are clearly identified in the local structure between 15--300 K. This finding is in contrast to the average structure which shows only one type of bond length in this temperature range but two types of bond lengths are suggested at considerably higher temperatures. This suggests that whereas the LS and HS CoO{sub 6}-octahedra coexist, they are randomly distributed in the crystal lattice at lower temperatures and become ordered at higher temperatures.