Effect of spin–orbit coupling on spin transport at graphene/transition metal interface

In spite of large spin coherence length in graphene due to small spin–orbit coupling, the created potential barrier and antiferromagnetic coupling at graphene/transition metal (TM) contacts strongly reduce the spin transport behavior in graphene. Keeping these critical issues in mind in the present work, ferromagnetic (Co, Ni) nanosheets are grown on graphene surface to elucidate the nature of interaction at the graphene/ferromagnetic interface to improve the spin transistor characteristics. Temperature dependent magnetoconductance shows unusual behavior exhibiting giant enhancement in magnetoconductance with increasing temperature. A model based on spin–orbit coupling operated at the graphene/TM interface is proposed to explain this anomalous result. We believe that the device performance can be improved remarkably tuning the spin–orbit coupling at the interface of graphene based spin transistor. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Inelastic electron lifetimes in simple metals: Influence of spin–orbit coupling

Inelastic single‐particle lifetimes due to electron–electron Coulomb interaction are computed ab‐initio for aluminium, silver, gold and copper using an all‐electron density‐functional calculation and a parameter‐free evaluation of the dielectric function. The novel feature is the inclusion of spin– orbit coupling in the wave functions. We show that, even in light metals, spin–orbit interaction is important for the calculation of inelastic lifetimes because it influences the scattering matrix elements. The importance of spin mixing on the lifetimes in aluminium is examined. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Interplay of spin–orbit coupling and superconducting correlations in germanium telluride thin films

There is much current interest in combining superconductivity and spin–orbit coupling in order to induce the topological superconductor phase and associated Majorana‐like quasiparticles which hold great promise towards fault‐tolerant quantum computing. Experimentally these effects have been combined by the proximity‐coupling of super‐conducting leads and high spin–orbit materials such as InSb and InAs, or by controlled Cu‐doping of topological insu‐lators such as Bi₂Se₃. However, for practical purposes, a single‐phase material which intrinsically displays both these effects is highly desirable. Here we demonstrate coexisting superconducting correlations and spin–orbit coupling in molecular‐beam‐epitaxy‐grown thin films of GeTe. The former is evidenced by a precipitous low‐temperature drop in the electrical resistivity which is quelled by a magnetic field, and the latter manifests as a weak antilocalisation (WAL) cusp in the magnetotransport. Our studies reveal several other intriguing features such as the presence of two‐dimensional rather than bulk transport channels below 2 K, possible signatures of topological superconductivity, and unexpected hysteresis in the magnetotransport. Our work demonstrates GeTe to be a potential host of topological SC and Majorana‐like excitations, and to be a versatile platform to develop quantum information device architectures. (© 2016 The Authors. Phys. Status Solidi RRL published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First‐principle investigations of the peculiar magnetic behavior of Sr2FeOsO6

Structural, electronic and magnetic properties of Sr₂FeOsO₆ have been revisited by using the first‐principle calculations. Semiconducting behavior is reproduced. The band gap is 0.09 eV from generalized gradient approximation (GGA) and 0.30 eV by considering both SOC and U, a bit larger than the experimental observed 0.125 eV. In the C‐type antiferromagnetic configuration, spin frustration is found by analysing the magnetic exchange parameters, explaining the experimental observed magnetic complexity.

     

Quantum transport phenomena in disordered electron systems with spin–orbit coupling in two dimensions and below

Electron transport phenomena in disordered electron systems with spin–orbit coupling in two dimensions and below are studied numerically. The scaling hypothesis is checked by analyzing the scaling of the quasi-1D localization length. A logarithmic increase of the mean conductance is also confirmed. These support the theoretical prediction that the two-dimensional metal in systems with spin–orbit coupling has a perfect conductivity. Transport through a Sierpinski carpet is also reported.     

Rotational disorder in 2‐piperidyl‐5‐nitro‐6 ‐methylpyridine: structural phase transition and its vibrational characteristics

X‐ray diffraction (XRD) studies have shown that 2‐piperidyl‐5‐nitro‐6‐methylpyridine, C₁₁H₁₅N₃O₂, undergoes a structural phase transition at T = 240 K. The room temperature structure is tetragonal, space group I4₁/a, with the unit‐cell dimensions a = 13.993(2) and c = 23.585(5) Å. The pyridine ring takes trans conformation with respect to the piperidine unit. While pyridine is well ordered, the piperidine moiety shows apparent disorder resulting from a libration about the linking N C bond. The low‐temperature phase is monoclinic, space group I2/a. Contraction of the unit‐cell volume by 2.3% at 170 K enables the C H···O linkage between the molecules of the neighbouring stacks. As result, the asymmetric unit becomes bi‐molecular. The thermal librations of the piperidine and methyl groups become considerably reduced at 170 K and nearly fully reduced at about 100 K. The IR spectra and polarised Raman spectra agree with the X‐ray structure and confirm the disorder effect on the piperidine ring. The assignment of the bands observed was made on the basis of DFT chemical quantum calculations. Copyright © 2008 John Wiley & Sons, Ltd.     

Observation of insulating–insulating monoclinic structural transition in macro‐sized VO2 single crystals

VO₂ single crystals with unprecedented quality, exhibiting a first‐order metal–insulator transition (MIT) at 67.8 °C and an insulator –insulator transition (IIT) at ～49 °C, are grown using a self‐flux evaporation method. Using synchrotron‐based X‐ray microdiffraction analysis, it is shown that the IIT is related to a structural phase transition (SPT) from the monoclinic M2 phase to the M1 phase upon heating while the MIT occurs together with a SPT of M1 to the rutile R phase. All previous reports have shown that VO₂ exists in the M1 phase at room temperature in contrast to the M2 phase observed in this work. We suggest that internal strain inside single crystal VO₂ may generate the previously unobserved IIT and the unusual room temperature structure. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Yb3+ site occupation and host sensitization luminescence of a novel near‐infrared emitting Sr2CaMoO6:Yb3+ phosphor

The host sensitized near‐infrared (NIR) emitting phosphor Sr₂CaMoO₆:Yb³⁺ was fabricated by the solid state reaction method. The structural refinement and Raman spectra elucidate that Yb³⁺ ions preferentially occupy Ca²⁺ sites. The phosphor can harvest ultraviolet (UV)–blue photons and exhibits intense NIR emission at around 1012 nm with full‐width‐at‐half‐maximum of 1635 cm^–1. Moreover, the absolute NIR photoluminesence quantum yield (PLQY) is estimated to be about 9%. The Sr₂CaMoO₆:Yb³⁺ phosphor may be a promising luminescence downshifting material for improving the spectral response of solar cells in the UV region. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural transformations in a single‐crystal Rb2NaYF6: Raman scattering study

This paper reports Raman spectroscopy investigation of phase transitions in Rb₂NaYF₆ crystal. The experimental spectra were compared with the calculated one. The spectra were obtained in temperature range from 8 to 300 K. The Raman spectra shows anomalous temperature‐dependent behavior at T₁ = 154 and T₂ = 122 K. Soft mode restoration has been found, which allows us to attribute first transition at 154 K to displacive type. Detailed analysis temperature dependencies of the line positions and widths have been performed. We found no effects of possible lattice disorder anywhere, except narrow (about 20 K) range above the T₁ temperature. The Raman spectra of Rb₂NaYF₆ crystal have been obtained and analyzed under hydrostatic pressure up to 4.33 GPa (at T = 295 K). The high pressure experiment up to 4.33 GPa did not disclose any effects associated with phase transitions. The lattice vibration spectra were calculated up to 10 GPa. The calculation has been demonstrated that the Rb₂NaYF₆ does not undergo high pressure phase transition. Copyright © 2013 John Wiley & Sons, Ltd.     

Dzyaloshinskii–Moriya interactions and magnetic texture in Fe films deposited on transition‐metal dichalcogenides

The magnetic properties of materials based on two‐dimensional transition‐metal dichalcogenides (TMDC), namely bulk Fe_1/4TaS₂ compound as well as TMDC monolayers with deposited Fe films, have been investigated by means of first‐principles DFT calculations. Changing the structure and the composition of these two‐dimensional systems resulted in considerable variations of their physical properties. For the considered systems the Dzyaloshinskii– Moriya (DM) interaction has been determined and used for the subsequent investigation of their magnetic structure using Monte Carlo simulations. Rather strong DM interactions as well as large | D ₀₁|J₀₁ ratios have been obtained in some of these materials, which can lead to the formation of skyrmionic structures varying with the strength of the applied external magnetic field. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Development of an energy‐domain 57Fe‐Mössbauer spectrometer using synchrotron radiation and its application to ultrahigh‐pressure studies with a diamond anvil cell

An energy‐domain ⁵⁷Fe‐Mössbauer spectrometer using synchrotron radiation (SR) with a diamond anvil cell (DAC) has been developed for ultrahigh‐pressure measurements. The main optical system consists of a single‐line pure nuclear Bragg reflection from an oscillating ⁵⁷FeBO₃ single crystal near the Néel temperature and an X‐ray focusing device. The developed spectrometer can filter the Doppler‐shifted single‐line ⁵⁷Fe‐Mössbauer radiation with a narrow bandwidth of neV order from a broadband SR source. The focused incident X‐rays make it easy to measure a small specimen in the DAC. The present paper introduces the design and performance of the SR ⁵⁷Fe‐Mössbauer spectrometer and its demonstrative applications including the newly discovered result of a pressure‐induced magnetic phase transition of polycrystalline ⁵⁷Fe₃BO₆ and an unknown high‐pressure phase of Gd⁵⁷Fe₂ alloy placed in a DAC under high pressures up to 302 GPa. The achievement of Mössbauer spectroscopy in the multimegabar range is of particular interest to researchers studying the nature of the Earth's core.     

Phonon anomalies and structural transition in spin ice Dy2Ti2O7: a simultaneous pressure‐dependent and temperature‐dependent Raman study

We revisit the assignment of Raman phonons of rare‐earth titanates by performing Raman measurements on single crystals of O¹⁸ isotope‐rich spin ice and nonmagnetic pyrochlores and compare the results with their O¹⁶ counterparts. We show that the low‐wavenumber Raman modes below 250 cm⁻¹ are not due to oxygen vibrations. A mode near 200 cm⁻¹, commonly assigned as F_2g phonon, which shows highly anomalous temperature dependence, is now assigned to a disorder‐induced Raman active mode involving Ti⁴⁺ vibrations. Moreover, we address here the origin of the ‘new’ Raman mode, observed below T_C ~ 110 K in Dy₂Ti₂O₇, through a simultaneous pressure‐dependent and temperature‐dependent Raman study. Our study confirms the ‘new’ mode to be a phonon mode. We find that dT_C/dP = + 5.9 K/GPa. Temperature dependence of other phonons has also been studied at various pressures up to ~8 GPa. We find that pressure suppresses the anomalous temperature dependence. The role of the inherent vacant sites present in the pyrochlore structure in the anomalous temperature dependence is also discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

Recent X-ray diffraction studies on α-quartz (SiO₂) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO₂ and α-AlPO₄ (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO₂.     

Symmetry Reductions,Group‐Invariant Solutions,and Conservation Laws of a (2+1)‐Dimensional Nonlinear Schrödinger Equation in a Heisenberg Ferromagnetic Spin Chain

Nonlinear spin excitations in ferromagnetic spin chains are studied for spintronic and magnetic devices including magnetic‐field sensors and for high‐density data storage. Here, (2+1)‐dimensional nonlinear Schrödinger equation is investigated, which describes the nonlinear spin dynamics for a Heisenberg ferromagnetic spin chain. Lie point symmetry generators and Lie symmetry groups of that equation are derived. Lie symmetry groups are related to the time, space, scale, rotation transformations, and Galilean boosts of that equation. Certain solutions, which are associated with the known solutions, are constructed. Based on the Lie symmetry generators, the reduced systems of such an equation are obtained. Based on the polynomial expansion and through one of the reduced systems, group‐invariant solutions are constructed. Soliton‐type group‐invariant solutions are graphically investigated and effects of the magnetic coupling coefficients, that is, α₁, α₂, α₃, and α₄, on the soliton's amplitude, width, and velocity are discussed. It is seen that α₁, α₂, α₃, and α₄ have no influence on the soliton's amplitude, but can affect the soliton's velocity and width. Lax pair and conservation laws of such an equation are derived.     

4‐Phenyl‐1,2,4‐triazoline‐3,5‐dione in the ene reactions with cyclohexene, 1‐hexene and 2,3‐dimethyl‐2‐butene. The heat of reaction and the influence of temperature and pressure on the reaction rate

The values of the enthalpy (53.3; 51.3; 20.0 kJ mol^?1), entropy (?106; ?122; ?144 J mol^?1K^?1), and volume of activation (?29.1; ?31.0; ?cm³ mol^?1), the reaction volume (?25.0; ?26.6; ?cm³ mol^?1) and reaction enthalpy (?155.9; ?158.2; ?150.2 kJ mol^?1) have been obtained for the first time for the ene reactions of 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione 1 , with cyclohexene 4 , 1‐hexene 6 , and with 2,3‐dimethyl‐2‐butene 8 , respectively. The ratio of the values of the activation volume to the reaction volume (?V^≠_corr/ΔV_{r ? n}) in the ene reactions under study, 1 + 4 → 5 and 1 + 6 → 7 , appeared to be the same, namely 1.16. The large negative values of the entropy and the volume of activation of studied reactions 1 + 4 → 5 and 1 + 6 → 7 better correspond to the cyclic structure of the activated complex at the stage determining the reaction rate. The equilibrium constants of these ene reactions can be estimated as exceeding 10¹⁸ L mol^?1, and these reactions can be considered irreversible. Copyright © 2014 John Wiley & Sons, Ltd.     

Orthorhombic to tetragonal phase transition in high TC superconductor YBa2Cu3O7-y and NdBa2Cu3O7-z under high pressure

Abstract

X-ray powder diffraction measurements for YBa₂Cu₃O_7-y and NdBa₂Cu₃O_7-y were made at the intense synchrotron radiation source under high pressure up to 5 GPa. These samples were wrapped tightly in platinum foil to avoid deoxidizing atmosphere. The orthorhombic to tetragonal transition temperature increases with pressure in both samples. These results are discussed on the basis of the disordering of the oxygen atoms on the chain sites.     

A first principles lattice dynamics and Raman spectra of the ferroelastic rutile to CaCl2 phase transition in SnO2 at high pressure

A first principles calculation of the lattice dynamical properties of rutile SnO₂ has been performed using density functional perturbation theory at ambient and high‐pressure conditions to understand the pressure‐induced phase transition. The calculated zone centre phonon modes at ambient and high pressures have been compared with Raman scattering and infrared measurements. Full phonon dispersion curves and phonon densities of states and Raman intensities at high pressures are calculated and given for the first time in literature. The ferroelastic transition from the rutile to the CaCl₂‐type structure was confirmed. It is clearly illustrated that the first transition is associated with macroscopic shear instability which arises from the strong coupling between elastic constants and softening of Raman active B_1g mode. The observed pressure of phase transition in experimental measurements was reproduced more accurately than in previous calculations, and the difference between observed and calculated transition pressure is only of the order of 2%. The mode Grüneisen parameter is quantitatively as well as qualitatively different from the earlier reported values. Copyright © 2013 John Wiley & Sons, Ltd.     

The excited state dynamics study of di‐2‐pyridylketone in the A‐band and B‐band absorptions by using resonance Raman spectroscopy,IR and UV–visible spectroscopy

Excitation wavelengths of 282.4, 273.9 (A band), 252.7, 239.5 and 228.7 nm (B band) resonance Raman spectra were acquired for di‐2‐pyridylketone, and density functional calculations were carried out to help in the elucidation of the photo relaxation dynamics of A‐band and B‐band electronic transitions. The resonance Raman spectra show that the intensity pattern of the A band presents great difference from that of the B band, which indicate that the short‐time A‐band (S₀→S₄) photo relaxation dynamics have substantial difference from that of B band (S₀→S₁₀) . The overall picture of short‐time dynamics and the vibronic coupling mechanisms are interpreted using Albrecht's theory. Copyright © 2012 John Wiley & Sons, Ltd.