Structures and localized vibrational states of defects in graphite by tight-binding calculations.

The structural and vibrational properties of pristine graphite and point defects in graphite are studied by tight-binding (TB) calculations using a three-center TB potential model. We showed that the three-center TB potential without "ad hoc" van der Waals interaction corrections can accurately describe the inter-layer distance of graphite and the lowest-energy structures and stabilities of typical point defects in graphite. The results from our TB calculations are in good agreement with those from density-functional theory calculations with van der Waals interaction corrections. We also investigated the vibrational properties to gain better understanding on the localization of vibrational states induced by the point defects. Our calculation results show that although localized or quasi-localized vibrational modes can be found in all defected graphite, the localization induced by Frenkel pair, dual-vacancy, and dual-interstitial defects is much stronger. Atomic displacements associated with the localized vibrational modes induced by these three point defects are also analyzed.

Localized electronic and vibrational states in amorphous diamond.

Amorphous diamond structures are generated by quenching high-density high-temperature liquid carbon using tight-binding molecular-dynamics simulations. We show that the generated amorphous diamond structures are predominated by strong tetrahedral bonds with the sp3 bonding fraction as high as 97%, thus exhibit an ultra-high incompressibility and a wide band gap close to those of crystalline diamond. A small amount of sp2 bonding defects in the amorphous sample contributes to localized electronic states in the band gap while large local strain gives rise to localization of vibrational modes at both high and low frequency regimes.

Molecular dynamics simulation of metallic Al-Ce liquids using a neural network machine learning interatomic potential.

Al-rich Al-Ce alloys have the possibility of replacing heavier steel and cast irons for use in high-temperature applications. Knowledge about the structures and properties of Al-Ce alloys at the liquid state is vital for optimizing the manufacture process to produce desired alloys. However, reliable molecular dynamics simulation of Al-Ce alloy systems remains a great challenge due to the lack of accurate Al-Ce interatomic potential. Here, an artificial neural network (ANN) deep machine learning (ML) method is used to develop a reliable interatomic potential for Al-Ce alloys. Ab initio molecular dynamics simulation data on the Al-Ce liquid with a small unit cell (∼200 atoms) and on the known Al-Ce crystalline compounds are collected to train the interatomic potential using ANN-ML. The obtained ANN-ML model reproduces well the energies, forces, and atomic structure of the Al90Ce10 liquid and crystalline phases of Al-Ce compounds in comparison with the ab initio results. The developed ANN-ML potential is applied in molecular dynamics simulations to study the structures and properties of the metallic Al90Ce10 liquid, which would provide useful insight into the guiding experimental process to produce desired Al-Ce alloys.

Ultrafast Control of Excitonic Rashba Fine Structure by Phonon Coherence in the Metal Halide Perovskite CH_{3}NH_{3}PbI_{3}.

We discover hidden Rashba fine structure in CH_{3}NH_{3}PbI_{3} and demonstrate its quantum control by vibrational coherence through symmetry-selective vibronic (electron-phonon) coupling. Above a critical threshold of a single-cycle terahertz pump field, a Raman phonon mode distinctly modulates the middle excitonic states with persistent coherence for more than ten times longer than the ones on two sides that predominately couple to infrared phonons. These vibronic quantum beats, together with first-principles modeling of phonon periodically modulated Rashba parameters, identify a threefold excitonic fine structure splitting, i.e., optically forbidden, degenerate dark states in between two bright ones with a narrow, ∼3 nm splitting. Harnessing of vibronic quantum coherence and symmetry inspires light-perovskite quantum control and sub-THz-cycle "Rashba engineering" of spin-split bands for ultimate multifunction device.

Characterization of three phases of liquid carbon by tight-binding molecular dynamics simulations.

We have performed systematic molecular dynamics simulations to study the structures of liquid carbon at 5000 K with the weight density ranging from 1.4 to 3.5 g cm-3, using a three-center tight-binding potential of carbon. The simulation results show that the bonding characteristics of the liquid changes predominately from twofold to threefold, and then to fourfold coordination as the density increases. Signals of two structural changes at the densities of about 1.9 and 3.0 g cm-3 respectively are revealed by the slope changes in the density dependence of structural, electronic and dynamical properties. Our simulation results suggest that there are three distinct liquid carbon phases in this density range. However, further thermodynamics calculations, e.g., free energy calculations, would be required to clarify the possible liquid-liquid transitions.

Development of interatomic potential for Al-Tb alloys using a deep neural network learning method.

An interatomic potential for the Al-Tb alloy around the composition of Al90Tb10 is developed using the deep neural network (DNN) learning method. The atomic configurations and the corresponding total potential energies and forces on each atom obtained from ab initio molecular dynamics (AIMD) simulations are collected to train a DNN model to construct the interatomic potential for the Al-Tb alloy. We show that the obtained DNN model can well reproduce the energies and forces calculated by AIMD simulations. Molecular dynamics (MD) simulations using the DNN interatomic potential also accurately describe the structural properties of the Al90Tb10 liquid, such as partial pair correlation functions (PPCFs) and bond angle distributions, in comparison with the results from AIMD simulations. Furthermore, the developed DNN interatomic potential predicts the formation energies of the crystalline phases of the Al-Tb system with an accuracy comparable to ab initio calculations. The structure factors of the Al90Tb10 metallic liquid and glass obtained by MD simulations using the developed DNN interatomic potential are also in good agreement with the experimental X-ray diffraction data. The development of short-range order (SRO) in the Al90Tb10 liquid and the undercooled liquid is also analyzed and three dominant SROs, i.e., Al-centered distorted icosahedron (DISICO) and Tb-centered '3661' and '15551' clusters, respectively, are identified.

Tailored Plasmons in Pentacene/Graphene Heterostructures with Interlayer Electron Transfer.

van der Waals (vdW) heterostructures, which are produced by the precise assemblies of varieties of two-dimensional (2D) materials, have demonstrated many novel properties and functionalities. Here we report a nanoplasmonic study of vdW heterostructures that were produced by depositing ordered molecular layers of pentacene on top of graphene. We find through nanoinfrared (IR) imaging that surface plasmons formed due to the collective oscillations of Dirac Fermions in graphene are highly sensitive to the adjacent pentacene layers. In particular, the plasmon wavelength declines systematically but nonlinearly with increasing pentacene thickness. Further analysis and density functional theory (DFT) calculations indicate that the observed peculiar thickness dependence is mainly due to the tunneling-type electron transfer from pentacene to graphene. Our work unveils a new method for tailoring graphene plasmons and deepens our understanding of the intriguing nano-optical phenomena due to interlayer couplings in novel vdW heterostructures.

Novel superconducting structures of BH2 under high pressure.

The crystal structures of boron hydrides in a pressure range of 50-400 GPa were studied using the genetic algorithm (GA) method combined with first-principles density functional theory calculations. BH4 and BH5 are predicted to be thermodynamically unstable. Two new BH2 structures with Cmcm and C2/c space group symmetries, respectively, were predicted, in which the B atoms tend to form two-dimensional sheets. The calculated band structures showed that in the pressure range of 50-150 GPa, the Cmcm-BH2 phase has very small gaps, while the C2/c-BH2 phase at 200-400 GPa is metallic. The superconductivity of the C2/c-BH2 structure was also investigated, and electron-phonon coupling calculations revealed that the estimated Tc values of C2/c-BH2 are about 28.18-37.31 K at 250 GPa.

Development of a semi-empirical potential suitable for molecular dynamics simulation of vitrification in Cu-Zr alloys.

The fast increase in available computation power allowed us to decrease the cooling rate in molecular dynamics (MD) simulation of vitrification by several orders of magnitude. While the reliability of the MD simulation should obviously benefit from this increase in the computational power, in some cases, it led to unexpected results. In particular, Ryltsev et al. [J. Chem. Phys. 149, 164502 (2018)] found that the most popular potentials for the Cu-Zr and Cu-Zr-Al alloys from Mendelev et al. [Philos. Mag. 89, 967 (2009)] and Cheng et al. [Phys. Rev. Lett. 102, 245501 (2009)] do not actually describe good glass forming systems but in contradiction with experiment predict rather fast crystallization of the Cu64.5Zr35.5 alloy which is the well-known example of bulk metallic glasses. In this paper, we present a new Cu-Zr semiempirical potential suitable to simulate vitrification. No crystal nucleation was observed in MD simulation using this potential in the concentration range from 75% to 5% of Zr. Since the new potential leads to about the same liquid structure and viscosity as the Cu-Zr potential from Mendelev et al. [Philos. Mag. 89, 967 (2009)] which failed to describe the good glass formability, our study clearly shows that no reliable conclusions about the glass formability can be deduced based solely on the analysis of the liquid properties and a nucleation/crystal growth study should be performed to address this question.

Helicity-dependent terahertz photocurrent and phonon dynamics in hybrid metal halide perovskites.

We report the discovery of helicity-dependent ultrafast photocurrent generation in organic-inorganic perovskite by measuring terahertz (THz) electric field emissions in the time-domain. We find signatures of the circular photogalvanic effect (CPGE) where right circularly polarized light and left circularly polarized light lead to different photocurrent generation. The direction of photocurrent is also resolved by measuring the polarization of the emitted THz pulses. Furthermore, we observe distinct wavelength-dependent, coherent phonon dynamics using THz pump-induced differential reflectivity, indicative of multiple exciton resonances. Both the CPGE and exciton fine structure, together with theoretical simulations, provide compelling and complementary evidence for the existence of Rashba-type bands in perovskite.

Structural signature and transition dynamics of Sb2Te3 melt upon fast cooling.

Crystalline Sb2Te3 is widely studied due to its important applications in memory materials and topological insulators. The liquid and amorphous structures of this compound as well as the dynamics upon quenching, however, are yet to be fully understood. In this work, we have systematically studied the dynamical properties and local structure of Sb2Te3 at different temperatures using ab initio molecular dynamics simulations. The calculated structure factors agree well with the experimental results. The atomic number density and mean-squared displacement as a function of temperature clearly indicate three states as the temperature decreases, namely, melt, undercooled liquid and glass state, respectively. By analyzing the chemical environments and bond-angle distribution functions, we demonstrate that the most probable short-range motifs in the Sb2Te3 system are defective octahedrons, and they are connected with each other via four-fold rings. This interesting structural feature may be responsible for the high fragility and easy phase transition upon glass forming that is applied in memory devices.

Adverse outcomes after planned surgery with anticipated intensive care admission in out-of-office-hours time periods: a multicentre cohort study.

BACKGROUND: Increasing mortality for patients admitted to hospitals during the weekend is a contentious but well described phenomenon. However, it remains uncertain whether adverse outcomes, including prolonged hospital length-of-stay (LOS), may also occur after patients undergoing major planned surgery are admitted to an intensive care unit (ICU) out-of-office-hours, either during weeknights (after 18:00) or on weekends. METHODS: All planned surgical admissions requiring admission to one of 183 ICUs across Australia and New Zealand between 2006 and 2016 were included in this retrospective population-based cohort study. Primary outcomes were hospital LOS and hospital mortality. RESULTS: Of the total 504 713 planned postoperative ICU admissions, 33.6% occurred during out-of-office-hours. After adjusting for available risk factors, out-of-office-hours ICU admissions were associated with a significant increase in hospital LOS [+2.6 days, 95% confidence interval (CI) 2.5-2.6], mortality [odd ratio (OR) 1.5, 95%CI 1.4-1.6], and a reduced chance of being directly discharged home (OR 0.8, 95%CI 0.8-0.8). The strongest association for adverse outcomes occurred with weekend ICU admissions (hospital LOS: +3.0 days, 95%CI 3.2-3.6; hospital mortality: OR 1.7, 95%CI 1.6-1.8). Clustering of adverse outcomes by hospitals was not observed in the generalised estimating equation analyses. CONCLUSIONS: Despite a greater clinical staff availability and higher monitoring levels, planned surgery requiring anticipated out-of-office-hours ICU admission was associated with a prolonged hospital LOS, reduced discharge directly home, and increased mortality compared with in-office-hours admissions. Our findings have potential clinical, economic and health policy implications on how complex planned surgery should be planned and managed.

Molecular dynamics simulation of the solid-liquid interface migration in terbium.

We developed a Tb embedded atom method potential which properly reproduces the liquid structure obtained from the ab initio molecular dynamics simulation, the hexagonal close packed (hcp)-body-centered cubic (bcc) phase transformation, and melting temperatures. At least three crystal phases [hcp, face-centered cubic (fcc), and bcc] described by this potential can coexist with the liquid phase. Thus, the developed potential provides an excellent test bed for studies of the completive phase nucleation and growth in a single component system. The molecular dynamics simulation showed that all crystal phases can grow from the liquid phase close to their melting temperatures. However, in the cases of the hcp and fcc growth from the liquid phase at very large supercoolings, the bcc phase forms at the solid-liquid interface in the close packed orientations in spite of the fact that both hcp and fcc phases are more stable than the bcc phase at these temperatures. This bcc phase closes the hcp and fcc phase from the liquid such that the remaining liquid solidifies into the bcc phase. The initial hcp phase then slowly continues growing in expense of the bcc phase.

Odd-Integer Quantum Hall States and Giant Spin Susceptibility in p-Type Few-Layer WSe_{2}.

We fabricate high-mobility p-type few-layer WSe_{2} field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level crossing effects at ultralow coincident angles, revealing that the Zeeman energy is about 3 times as large as the cyclotron energy near the valence band top at the Γ valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe_{2}, in which itinerant or QH ferromagnetism likely occurs. Evidently, the Γ valley of few-layer WSe_{2} offers a unique platform with unusually heavy hole carriers and a substantially enhanced g factor for exploring strongly correlated phenomena.

Crystal genes in a marginal glass-forming system of Ni50Zr50.

The marginal glass-forming ability (GFA) of a binary Ni-Zr system is an issue to be explained considering numerous bulk metallic glasses (BMGs) found in a Cu-Zr system. Using molecular dynamics, the structures and dynamics of Ni50Zr50 metallic liquid and glass are investigated at the atomistic level. To achieve a well-relaxed glassy sample, a sub-Tg annealing method is applied and the final sample is closer to the experiments than the models prepared by continuous cooling. With the state-of-the-art structural analysis tools such as cluster alignment and pair-wise alignment methods, two glass-forming motifs with some mixed traits of a metastable B2 crystalline phase and a crystalline Ni-centered B33 motif are found to be dominant in the undercooled liquid and glass samples. A new chemical order characterization on each short-range order (SRO) structure is accomplished based on the cluster alignment method. The significant amount of the crystalline motif and the few icosahedra in the glassy sample deteriorate the GFA.

Intraoperative dexamethasone alters immune cell populations in patients undergoing elective laparoscopic gynaecological surgery.

BACKGROUND: Anaesthetists use dexamethasone principally for its anti-emetic effect. The purpose of this study was to characterize the effects of a single intraoperative dose of dexamethasone on cellular and metabolic components of the immune system in patients undergoing laparoscopic surgical procedures. METHODS: In this prospective double-blind trial, female patients undergoing elective major laparoscopic surgery were randomized to receive saline (Control group, n =16) or dexamethasone 4 mg (Dexamethasone group, n =16) i.v. after the induction of anaesthesia. Inflammatory markers and immune cell counts were examined at 24 and 48 h and 6 weeks after surgery. The changes from baseline preoperative values were compared between groups using a Mann-Whitney U -test, and linear mixed models were used to validate the findings. RESULTS: No differences in concentrations of serum glucose and interleukin-6 were observed between groups after surgery. The increase in C-reactive protein concentration at 24 h after surgery was greater in the control group [median (interquartile range), 33 (25-65) vs 17 (7-26) mg dl -1 ; P =0.018]. Extensive changes in the counts of white cells, including most lymphocyte subsets, were observed 24 h after surgery, and dexamethasone appeared to attenuate most of these changes. Changes at 48 h and 6 weeks did not differ between groups. CONCLUSIONS: In female patients undergoing elective laparoscopic gynaecological surgery, dexamethasone administration appears to attenuate inflammation and to alter immune cell counts at 24 h, with no effects identified after this time. The importance of these changes for postoperative immune function is unknown. TRIAL REGISTRATION: Australia and New Zealand Clinical Trials Registry (ACTRN12608000340336).

Theoretical Prediction of Si2-Si33 Absorption Spectra.

The optical absorption spectra of Si2-Si33 clusters were systematically studied by a time-dependent density functional theory approach. The calculations revealed that the absorption spectrum becomes significantly broad with increasing cluster size, stretching from ultraviolet to the infrared region. The absorption spectra are closely related to the structural motifs. With increasing cluster size, the absorption intensity of cage structures gradually increases, but the absorption curves of the prolate and the Y-shaped structures are very sensitive to cluster size. If the transition energy reaches ∼12 eV, it is noted that all the clusters have remarkable absorption in deep ultraviolet region of 100-200 nm, and the maximum absorption intensity is ∼100 times that in the visible region. Further, the optical responses to doping in the Si clusters were studied.

Fe-Si networks in Na2FeSiO4 cathode materials.

Using a combination of adaptive genetic algorithm search, motif-network search scheme and first-principles calculations, we have systematically studied the low-energy crystal structures of Na2FeSiO4. We show that the low-energy crystal structures with different space group symmetries can be classified into several families based on the topologies of their Fe-Si networks. In addition to the diamond-like network which is shared by most of the low-energy structures, another three robust Fe-Si networks are also found to be stable during the charge/discharge process. The electrochemical properties of representative structures from these four different Fe-Si networks in Na2FeSiO4 and Li2FeSiO4 are investigated and found to be strongly correlated with the Fe-Si network topologies. Our studies provide a new route to characterize the crystal structures of Na2FeSiO4 and Li2FeSiO4 and offer useful guidance for the design of promising cathodes for Na/Li ion batteries.

Concentration-dependent effect of hypocalcaemia on in vitro clot strength in patients at risk of bleeding: a retrospective cohort study.

AIM: It is uncertain whether hypocalcaemia is associated with an increased risk of bleeding. This study assessed the dose-related relationship between ionised calcium concentrations and in vitro clot strength measured by maximum amplitude (MA) on the thromboelastograph (TEG). METHODS: A total of 610 patients who were at risk of bleeding or had active bleeding between 2010 and 2014 were considered in this retrospective cohort study. A scatter plot with Pearson correlation coefficient (r) and multiple linear regression was used to assess the dose-related relationship between ionised calcium concentrations and MA on the TEG. RESULTS: The mean ionised calcium of the patients was 1·10 mmol L(-1) (interquartile range: 1·04-1·17) and 235 (38·5%) of them had hypocalcaemia (<1·1 mmol L(-1) ). Hypocalcaemia was more common in patients with significant coexisting coagulopathy. Ionised calcium concentrations (r = 0·285, 95% confidence interval (CI) 0·211-0·356, P = 0·001), as well as fibrinogen concentrations, platelet counts, international normalised ratio (INR) and activated Partial Thromboplastin Time (aPTT), had a significant linear correlation with the MA on the TEG. Ionised calcium concentrations and its interaction term with platelet count were both significantly associated with the MA on the TEG (slope of the regression line 1·1 per 0·1 mmol L(-1) increment, 95%CI 0·3 to 1·9, P = 0·011), after adjusting for fibrinogen concentrations, platelet counts, INR and aPTT. CONCLUSIONS: Ionised calcium concentrations had a concentration-dependent association with in vitro clot strength after adjusting for other coagulation abnormalities in patients with coexisting coagulopathy. Maintaining a normal ionised calcium concentration, >1 mmol L(-1) , during critical bleeding is recommended.

van der Waals epitaxial growth of atomically thin Bi2Se3 and thickness-dependent topological phase transition.

Two-dimensional (2D) atomic-layered heterostructures stacked by van der Waals interactions recently introduced new research fields, which revealed novel phenomena and provided promising applications for electronic, optical, and optoelectronic devices. In this study, we report the van der Waals epitaxial growth of high-quality atomically thin Bi2Se3 on single crystalline hexagonal boron nitride (h-BN) by chemical vapor deposition. Although the in-plane lattice mismatch between Bi2Se3 and h-BN is approximately 65%, our transmission electron microscopy analysis revealed that Bi2Se3 single crystals epitaxially grew on h-BN with two commensurate states; that is, the (1̅21̅0) plane of Bi2Se3 was preferably parallel to the (1̅100) or (1̅21̅0) plane of h-BN. In the case of the Bi2Se3 (2̅110) ∥ h-BN (11̅00) state, the Moiré pattern wavelength in the Bi2Se3/h-BN superlattice can reach 5.47 nm. These naturally formed thin crystals facilitated the direct assembly of h-BN/Bi2Se3/h-BN sandwiched heterostructures without introducing any impurity at the interfaces for electronic property characterization. Our quantum capacitance (QC) measurements showed a compelling phenomenon of thickness-dependent topological phase transition, which was attributed to the coupling effects of two surface states from Dirac Fermions at/or above six quintuple layers (QLs) to gapped Dirac Fermions below six QLs. Moreover, in ultrathin Bi2Se3 (e.g., 3 QLs), we observed the midgap states induced by intrinsic defects at cryogenic temperatures. Our results demonstrated that QC measurements based on h-BN/Bi2Se3/h-BN sandwiched structures provided rich information regarding the density of states of Bi2Se3, such as quantum well states and Landau quantization. Our approach in fabricating h-BN/Bi2Se3/h-BN sandwiched device structures through the combination of bottom-up growth and top-down dry transferring techniques can be extended to other two-dimensional layered heterostructures.