Inverse pressure-induced Mott transition in TiPO4_4

TiPO$_4$ shows interesting structural and magnetic properties as temperature and pressure are varied, such as a spin-Peierls phase transition and the development of incommensurate modulations of the lattice. Recently, high pressure experiments for TiPO$_4$ reported two new structural phases appearing at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem. Int. Ed. 55, 15053]. The latter was shown to include the first example of 5-fold O-coordinated P-atoms in an inorganic phosphate compound. In this work we characterize the electronic structure and other physical properties of these new phases by means of ab-initio calculations, and investigate the structural transition. We find that the appearance of phases IV and V coincides with a collapse of the Mott insulating gap and quenching of magnetism in phase III as pressure is applied. Remarkably, our calculations show that in the high pressure phase V, these features reappear, leading to an antiferromagnetic Mott insulating phase, with robust local moments

Synthesis of FeN₄ at 180 GPa and its crystal structure from a submicron-sized grain

Iron tetranitride, FeN4, was synthesized from the elements in a laser-heated diamond anvil cell at 180 (5) GPa and 2700 (200) K. Its crystal structure was determined based on single-crystal X-ray diffraction data collected from a submicron-sized grain at the synchrotron beamline ID11 of ESRF. The compound crystallizes in the triclinic space group P\overline{1}. In the asymmetric unit, the Fe atom occupies an inversion centre (Wyckoff position 1d), while two N atoms occupy general positions (2i). The structure is made up from edge-sharing [FeN6] octahedra forming chains along [100] and being interconnected through N—N bridges. N atoms form catena-poly[tetraz-1-ene-1,4-diyl] anions [–N=N—N—N–]∞2− running along [001]. In comparison with the previously reported structure of FeN4 at 135 GPa [Bykov et al. (2018). Nat. Commun. 9, 2756], the crystal structure of FeN4 at 180 GPa is similar but the structural model is significantly improved in terms of the precision of the bond lengths and angles

Second harmonic generation in multilayer graphene induced by direct electric current

Optical second harmonic generation (SHG) is studied from multilayer graphene films in the presence of DC electric current flowing in the sample plane. Graphene layers are manufactured by chemical vapour deposition (CVD) technique and deposited on an oxidised Si(001) substrate. SHG intensity from graphene layer is found to be negligible in the absence of the DC current, while it increases dramatically with the application of the electric current. The current-induced change of the SHG intensity rises linearly with the current amplitude and changes its sign under the reversal of the current direction to the opposite. The observed effect is explained in terms of the interference of second harmonic radiation reflected from the Si surface and that induced by the DC current in multilayer graphene

Photovoltaic Current Response of Mesoscopic Conductors to Quantized Cavity Modes

We extend the analysis of the effects of electromagnetic (EM) fields on mesoscopic conductors to include the effects of field quantization, motivated by recent experiments on circuit QED. We show that in general there is a photovoltaic (PV) current induced by quantized cavity modes at zero bias across the conductor. This current depends on the average photon occupation number and vanishes identically when it is equal to the average number of thermal electron-hole pairs. We analyze in detail the case of a chaotic quantum dot at temperature T_e in contact with a thermal EM field at temperature T_f, calculating the RMS size of the PV current as a function of the temperature difference, finding an effect ~pA.Comment: 4 pages, 2 figure

Crystal structures of cristobalite-type and coesite-type PON redetermined on the basis of single-crystal X-ray diffraction data

Hitherto, phosphorus oxonitride (PON) could not be obtained in the form of single crystals and only powder diffraction experiments were feasible for structure studies. In the present work we have synthesized two polymorphs of phosphorus oxonitride, cristobalite-type (cri-PON) and coesite-type (coe-PON), in the form of single crystals and reinvestigated their crystal structures by means of in house and synchrotron single-crystal X-ray diffraction. The crystal structures of cri-PON and coe-PON are built from PO2N2 tetrahedral units, each with a statistical distribution of oxygen and nitrogen atoms. The crystal structure of the coe-PON phase has the space group C2/c with seven atomic sites in the asymmetric unit [two P and three (N,O) sites on general positions, one (N,O) site on an inversion centre and one (N,O) site on a twofold rotation axis], while the cri-PON phase possesses tetragonal I-42d symmetry with two independent atoms in the asymmetric unit [the P atom on a fourfold inversion axis and the (N,O) site on a twofold rotation axis]. In comparison with previous structure determinations from powder data, all atoms were refined with anisotropic displacement parameters, leading to higher precision in terms of bond lengths and angles

Luminescent upconversion nanoparticles evaluating temperature-induced stress experienced by aquatic organisms due to environmental variations

Growing anthropogenic activities are significantly influencing the environment and especially aquatic ecosystems. Therefore, there is an increasing demand to develop techniques for monitoring and assessing freshwater habitats changes so that interventions can prevent irrevocable damage. We explore an approach for screening the temperature-induced stress experienced by aquatic organisms due to environmental variations. Luminescent spectra of upconversion [Y2O3: Yb, Er] particles embedded within Caridina multidentata shrimps are measured, while ambient temperature gradient is inducing stress conditions. The inverse linear dependence of logarithmic ratio of the luminescence intensity provides an effective means for temperature evaluation inside aquatic species in vivo. The measured luminescence shows high photostability on the background of complete absence of biotissues autofluorescence, as well as no obscuration of the luminescence signal from upconversion particles. Current approach of hybrid sensing has a great potential for monitoring of variations in aquatic ecosystems driven by climate changes and pollution

Nitride Spinel: An Ultraincompressible High‐Pressure Form of BeP2N4

Owing to its outstanding elastic properties, the nitride spinel γ‐Si3N4 is of considered interest for materials scientists and chemists. DFT calculations suggest that Si3N4‐analog beryllium phosphorus nitride BeP2N4 adopts the spinel structure at elevated pressures as well and shows outstanding elastic properties. Herein, we investigate phenakite‐type BeP2N4 by single‐crystal synchrotron X‐ray diffraction and report the phase transition into the spinel‐type phase at 47 GPa and 1800 K in a laser‐heated diamond anvil cell. The structure of spinel‐type BeP2N4 was refined from pressure‐dependent in situ synchrotron powder X‐ray diffraction measurements down to ambient pressure, which proves spinel‐type BeP2N4 a quenchable and metastable phase at ambient conditions. Its isothermal bulk modulus was determined to 325(8) GPa from equation of state, which indicates that spinel‐type BeP2N4 is an ultraincompressible material