Magnetic quantum criticality in quasi‐one‐dimensional Heisenberg antiferromagnet

We analyze exciting recent measurements [Phys. Rev. Lett. 114 (2015) 037202] of the magnetization, differential susceptibility and specific heat on one dimensional Heisenberg antiferromagnet Cu(C₄H₄N₂)(NO₃)₂ (CuPzN) subjected to strong magnetic fields. Using the mapping between magnons (bosons) in CuPzN and fermions, we demonstrate that magnetic field tunes the insulator towards quantum critical point related to so‐called fermion condensation quantum phase transition (FCQPT) at which the resulting fermion effective mass diverges kinematically. We show that the FCQPT concept permits to reveal the scaling behavior of thermodynamic characteristics, describe the experimental results quantitatively, and derive for the first time the (temperature—magnetic field) phase diagram, that contains Landau‐Fermi‐liquid, crossover and non‐Fermi liquid parts, thus resembling that of heavy‐fermion compounds.     

Fast two‐dimensional detection for X‐ray photon correlation spectroscopy using the PILATUS detector

The first X‐ray photon correlation spectroscopy experiments using the fast single‐photon‐counting detector PILATUS (Paul Scherrer Institut, Switzerland) have been performed. The short readout time of this detector permits access to intensity autocorrelation functions describing dynamics in the millisecond range that are difficult to access with charge‐coupled device detectors with typical readout times of several seconds. Showing no readout noise the PILATUS detector enables measurements of samples that either display fast dynamics or possess only low scattering power with an unprecedented signal‐to‐noise ratio.     

Electron‐counting rules,three‐dimensional aromaticity,and the boundaries of the Periodic Table

To describe aromaticity of planar and three‐dimensional molecules, different electron counting rules are employed. Here, the relationship between the Hückel 4n + 2 rule and the Hirsch 2(n + 1)² rule is established based on formal approach considering the electrons as objects in an arbitrary n‐dimensional space of states. Two types of three‐dimensional aromaticity, referred to as ‘spherical’ (following the 4n + 2 electron counting rule) and ‘spatial’ (following the 2(n + 1)² electron counting rule) are distinguished. A conclusion concerning the boundaries of the Periodic Table of Elements is made based on the same formal approach that no g‐ (or higher) elements can exist and that possible extension of the Periodic Table beyond the seventh period must be followed by filling of the 8p or inner 6f or 7d levels. Copyright © 2011 John Wiley & Sons, Ltd.     

Phase‐specific Raman analysis of n‐alkane melting by moving‐window two‐dimensional correlation spectroscopy

We use moving‐window two‐dimensional correlation spectroscopy (MW‐2DCOS) for phase‐specific Raman analysis of the n‐alkane (C₂₁H₄₄) during melting from the crystalline solid phase to the intermediate rotator phase and to the amorphous molten phase. In MW‐2DCOS, individual peak‐to‐peak correlation analysis within a small subset of spectra provides both temperature‐resolved and spectrally disentangled Raman assignments conducive to understanding phase‐specific molecular interactions and chain configurations. We demonstrate that autocorrelation MW‐2DCOS can determine the phase transition temperatures with a higher resolving power than commonly used analysis methods including individual peak intensity analysis or principal component analysis. Besides the enhanced temperature resolving power, we demonstrate that asynchronous 2DCOS near the orthorhombic‐to‐rotator transition temperature can spectrally resolve the two overlapping peaks embedded in the Raman CH₂ twisting band in the orthorhombic phase, which had been only predicted but not observed because of thermal broadening near the melting temperature. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.     

Ab initio prediction of a new allotrope of two‐dimensional silicon

Silicene, a promising candidate for future electronic devices, has been fabricated only on supporting substrates as silicon atom prefers to form the sp³ hybridization structure. Therefore, it's important to search more stable two‐dimensional (2D) silicon allotropes and several 2D silicon allotropes have been proposed recently. In this work, we predict a new type of 2D silicon allotrope (called OTDS) based on ab initio structure, phonon‐mode and molecular dynamics calculations. OTDS has the in‐plane octagonal tiling (OT) pattern with dumbbell‐like structures and silicon atoms in OTDS are four‐ and three‐coordinated. OTDS is a semiconductor with a large band gap (about 1.5 eV by HSE calculation) and the band gap can be tuned effectively by the in‐plane strain.

Perspective and side views of the atomic structure of OTDS.     

Transmission of an X‐ray beam through a two‐dimensional photonic crystal and the Talbot effect

Results of computer simulations of the transmission of an X‐ray beam through a two‐dimensional photonic crystal as well as the propagation of an X‐ray beam in free space behind the photonic crystal are reported. The photonic crystal consists of a square lattice of silicon cylinders of diameter 0.5 µm. The amount of matter in the path of the X‐ray beam rapidly decreases at the sides of the cylinder projections. Therefore the transmission is localized near the boundaries, and appears like a channeling effect. The iterative method of computer simulations is applied. This method is similar to the multi‐slice method that is widely used in electron microscopy. It allows a solution to be obtained with acceptable accuracy. A peculiarity in the intensity distribution inside the Talbot period z_T in free space was found when the intensity is approximately equal to the initial value at a distance 0.46z_T, and it is shifted by half a period at distance 0.5z_T. The reason for this effect is the existence of a periodic phase of the wavefunction of radiation inside the intensity peaks. Simulations with zero phase do not show this effect. Symmetry rules for the Talbot effect are discussed.     

Linear differential equations for the one‐dimensional scattering problem

The transmission and reflection amplitudes of an electron moving in a one dimensional potential of arbitrary form are obtained using the transfer matrix method. It is shown that the one‐dimensional scattering problem, in its most general form, can be reduced to Cauchy problem for a set of two linear differential equations.     

Development of a two‐dimensional imaging system of X‐ray absorption fine structure

A two‐dimensional imaging system of X‐ray absorption fine structure (XAFS) has been developed at beamline BL‐4 of the Synchrotron Radiation Center of Ritsumeikan University. The system mainly consists of an ionization chamber for I₀ measurement, a sample stage, and a two‐dimensional complementary metal oxide semiconductor (CMOS) image sensor for measuring the transmitted X‐ray intensity. The X‐ray energy shift in the vertical direction, which originates from the vertical divergence of the X‐ray beam on the monochromator surface, is corrected by considering the geometrical configuration of the monochromator. This energy correction improves the energy resolution of the XAFS spectrum because each pixel in the CMOS detector has a very small vertical acceptance of ～0.5 µrad. A data analysis system has also been developed to automatically determine the energy of the absorption edge. This allows the chemical species to be mapped based on the XANES feature over a wide area of 4.8 mm (H) × 3.6 mm (V) with a resolution of 10 µm × 10 µm. The system has been applied to the chemical state mapping of the Mn species in a LiMn₂O₄ cathode. The heterogeneous distribution of the Mn oxidation state is demonstrated and is considered to relate to the slow delocalization of Li⁺‐defect sites in the spinel crystal structure. The two‐dimensional‐imaging XAFS system is expected to be a powerful tool for analyzing the spatial distributions of chemical species in many heterogeneous materials such as battery electrodes.     

Piezoelectricity in two‐dimensional group III–V buckled honeycomb monolayers

We studied the elastic and piezoelectric properties of buckled honeycomb group III–V monolayers (GaP, GaAs, GaSb, InP, InAs and InSb) by DFT calculations. Those buckled monolayers are ferroelectric and have nonzero e₁₁, e₃₁, d₁₁ and d₃₁ piezoelectric coefficients. Our calculations show that those monolayers are good piezoelectric materials and a pronounced periodic trend of the piezoelectric coefficients e₁₁, e₃₁, d₁₁ and d₃₁ was found. Group III–V monolayers are promising candidates for future atomically thin piezoelectric applications such as transducers, sensors, and energy harvesting devices.     

Ultra‐short and ultra‐intense X‐ray free‐electron laser single pulse in one‐dimensional photonic crystals

The propagation within a one‐dimensional photonic crystal of a single ultra‐short and ultra‐intense pulse delivered by an X‐ray free‐electron laser is analysed with the framework of the time‐dependent coupled‐wave theory in non‐linear media. It is shown that the reflection and the transmission of an ultra‐short pulse present a transient period conditioned by the extinction length and also the thickness of the structure for transmission. For ultra‐intense pulses, non‐linear effects are expected: they could give rise to numerous phenomena, bi‐stability, self‐induced transparency, gap solitons, switching, etc., which have been previously shown in the optical domain.     

Spin–orbit stiffness of the spin‐polarized electron gas

In a spin‐polarized electron gas, Coulomb interaction couples the spin and motion degrees of freedom to build propagating spin waves. The spin wave stiffness S_sw quantifies the energy cost to trigger such excitation by perturbing the kinetic energy of the electron gas (i.e. putting it in motion). Here we introduce the concept of spin–orbit stiffness, S_so, as the energy necessary to excite a spin wave with a spin polarization induced by spin–orbit coupling. This quantity governs the Coulombic enhancement of the spin–orbit field acting of the spin wave. First‐principles calculations and electronic Raman scattering experiments carried out on a model spin‐polarized electron gas, embedded in a CdMnTe quantum well, demonstrate that S_so = S_sw. Through optical gating of the structure, we demonstrate the reproducible tuning of S_so by a factor of 3, highlighting the great potential of spin–orbit control of spin waves in view of spintronics applications. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Tails of the density of states of two‐dimensional Dirac fermions

The density of states of Dirac fermions with a random mass on a two‐dimensional lattice is considered. We give the explicit asymptotic form of the single‐electron density of states as a function of both energy and (average) Dirac mass, in the regime where all states are localized. We make use of a weak‐disorder expansion in the parameter g/m², where g is the strength of disorder and m the average Dirac mass for the case in which the evaluation of the (supersymmetric) integrals corresponds to non‐uniform solutions of the saddle point equation. The resulting density of states has tails which deviate from the typical pure Gaussian form by an analytic prefactor.     

Non‐magnetic defects in the bulk of two‐dimensional topological insulators

We found that non‐magnetic defects in two‐dimensional topological insulators induce bound states of two kinds for each spin orientation: electron‐ and hole‐like states. Depending on the sign of the defect potential these states can be also of two kinds with different distribution of the electron density. The density has a maximum or minimum in the center. A surprising effect caused by the topological order is a singular dependence of the bound‐state energy on the defect potential.

     

Fabrication of three‐dimensional photonic crystals in quantum‐dot‐based materials

Controlling spontaneous emission (SE) is of fundamental importance to a diverse range of photonic applications including but not limited to quantum optics, low power displays, solar energy harvesting and optical communications. Characterized by photonic bandgap (PBG) property, three‐dimensional (3D) photonic crystals (PCs) have emerged as a promising synthetic material, which can manipulate photons in much the same way as a semiconductor does to electrons. Emission tunable nanocrystal quantum dots (QDs) are ideal point sources to be embedded into 3D PCs towards active devices. The challenge however lies in the combination of QDs with 3D PCs without degradation of their emission properties. Polymer materials stand out for this purpose due to their flexibility of incorporating active materials. Combining the versatile multi‐photon 3D micro‐fabrication techniques, active 3D PCs have been fabricated in polymer‐QD composites with demonstrated control of SE from QDs. With this milestone novel miniaturized photonic devices can thus be envisaged.     

Radiation damage to protein specimens from electron beam imaging and diffraction: a mini‐review of anti‐damage approaches,with special reference to synchrotron X‐ray crystallography

Recent research progress using X‐ray cryo‐crystallography with the photon beams from third‐generation synchrotron sources has resulted in recognition that this intense radiation commonly damages protein samples even when they are held at 100 K. Other structural biologists examining thin protein crystals or single particle specimens encounter similar radiation damage problems during electron diffraction and imaging, but have developed some effective countermeasures. The aim of this concise review is to examine whether analogous approaches can be utilized to alleviate the X‐ray radiation damage problem in synchrotron macromolecular crystallography. The critical discussion of this question is preceded by presentation of background material on modern technical procedures with electron beam instruments using 300–400 kV accelerating voltage, low‐dose exposures for data recording, and protection of protein specimens by cryogenic cooling; these practical approaches to dealing with electron radiation damage currently permit best resolution levels of 6 Å (0.6 nm) for single particle specimens, and of 1.9 Å for two‐dimensional membrane protein crystals. Final determination of the potential effectiveness and practical value of using such new or unconventional ideas will necessitate showing, by experimental testing, that these produce significantly improved protection of three‐dimensional protein crystals during synchrotron X‐ray diffraction.     

Two‐dimensional stimulated ultraviolet resonance Raman spectra of tyrosine and tryptophan: a simulation study

We report an ab initio simulation study of the ultrafast broad bandwidth ultraviolet stimulated resonance Raman spectra (SRRS) of l ‐tyrosine, l ‐tryptophan, and trans‐l ‐tryptophan‐l ‐tyrosine (WY) dipeptide. Two‐pulse one‐dimensional SRRS and three‐pulse two‐dimensional SRRS that reveal inter‐residue and intra‐residue vibrational correlations are simulated using electronically resonant or pre‐resonant pulse configurations that select the Raman signal and discriminate against excited state pathways. Multimode effects are incorporated via the cumulant expansion. The two‐dimensional SRRS technique is more sensitive to residue couplings than spontaneous Raman. Copyright © 2013 John Wiley & Sons, Ltd.     

Interaction of a relativistic dense electron beam with a laser wiggler in a vacuum: self‐field effects on the electron orbits and free‐electron laser gain

Employing laser wigglers and accelerators provides the potential to dramatically cut the size and cost of X‐ray light sources. Owing to recent technological developments in the production of high‐brilliance electron beams and high‐power laser pulses, it is now conceivable to make steps toward the practical realisation of laser‐pumped X‐ray free‐electron lasers (FELs). In this regard, here the head‐on collision of a relativistic dense electron beam with a linearly polarized laser pulse as a wiggler is studied, in which the laser wiggler can be realised using a conventional quantum laser. In addition, an external guide magnetic field is employed to confine the electron beam against self‐fields, therefore improving the FEL operation. Conditions allowing such an operating regime are presented and its relevant validity checked using a set of general scaling formulae. Rigorous analytical solutions of the dynamic equations are provided. These solutions are verified by performing calculations using the derived solutions and well known Runge–Kutta procedure to simulate the electron trajectories. The effects of self‐fields on the FEL gain in this configuration are estimated. Numerical calculations indicate that in the presence of self‐fields the sensitivity of the gain increases in the vicinity of resonance regions. Besides, diamagnetic and paramagnetic effects of the wiggler‐induced self‐magnetic field cause gain decrement and enhancement for different electron orbits, while these diamagnetic and paramagnetic effects increase with increasing beam density. The results are compared with findings of planar magnetostatic wiggler FELs.     

Electron cooling in three‐dimensional Dirac fermion systems at low temperature: Effect of screening

Hot electron cooling rate P, due to acoustic phonons, is investigated in three‐dimensional Dirac fermion systems at low temperature taking account of the screening of electron–acoustic phonon interaction. P is studied as a function of electron temperature T_e and electron concentration n_e. Screening is found to suppress P very significantly for about T_e < 0.5 K and its effect reduces considerably for about T_e > 1 K in Cd₃As₂. In Bloch–Grüneisen (BG) regime, for screened (unscreened) case the T_e dependence is P ～ T_e⁹(T_e⁵) and the n_e dependence gives P ～ n_e^–5/3 (n_e^–1/3). The T_e dependence is characteristic of 3D phonons and n_e dependence is characteristics of 3D Dirac fermions. The plot of P /T_e⁴ vs. T_e shows a maximum at temperature T_em which shifts to higher values for larger n_e. Interestingly, the maximum is nearly same for different n_e and T_em/n_e^1/3 being nearly constant. More importantly, we propose, the n_e dependent measurements of P would provide a clearer signature to identify 3D Dirac semimetal phase. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)