Suchergebnisse

The tungsten ditelluride WTe2 was suggested to belong to the Weyl semimetal family. We studied 125Te spin-lattice relaxation and NMR spectra in a WTe2 single crystal within a large range from 28 K up to room temperature. Measurements were carried out on a Bruker Avance 500 NMR pulse spectrometer for the crystalline c axis directed in parallel and perpendicular to the magnetic field. Relaxation proved to be single-exponential. The relaxation time varied depending on the sample position in magnetic field and frequency offset. The relaxation rate increased about linearly with temperature below 70 K, however the dependence became nearly quadratic at higher temperatures. The relaxation rate within the total temperature range was fitted using a theoretical model developed in Ref. [41] for Weyl semimetals and assuming the decrease of the chemical potential with increasing temperature. The results obtained for 125Te spin-lattice relaxation evidence in favor of the topological nontriviality of the WTe2 semimetal. The 125Te NMR spectra agreed with the occurrence of nonequivalent tellurium sites and varied insignificantly with temperature. © 2020 The Author(s) ; The authors acknowledge the financial support from RFBR , grants 19-57-52001 , 19-29-03004 , and 19-07-00028 . The work was partly supported by Minobrnauki of Russia ("Spin" No. AAAA-A18-118020290104-2 ) and Russian Government (contract No. 02.A03.21.0006 ). Measurements were partly carried out using the equipment of the Research park of St. Petersburg State University.

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 668119 (project "IDentIFY"). The role of COST Action CA15209 (European Network on NMR Relaxometry) is also acknowledged. ; Peer reviewed ; Publisher PDF

The first-principles calculation of many material properties, in particular related to defects and disorder, starts with the relaxation of the atomic positions of the system under investigation. This procedure is routine for nonmagnetic and magnetically ordered materials. However, when it comes to magnetically disordered systems, in particular the paramagnetic phase of magnetic materials, it is not clear how the relaxation procedure should be performed or which geometry should be used. Here we propose a method for the structural relaxation of magnetic materials in the paramagnetic regime, in an adiabatic fast-magnetism approximation within the disordered local moment (DLM) picture in the framework of density functional theory. The method is straightforward to implement using any ab initio code that allows for structural relaxations. We illustrate the importance of considering the disordered magnetic state during lattice relaxations by calculating formation energies and geometries for an Fe vacancy and C insterstitial atom in body-centered cubic (bcc) Fe as well as bcc Fe1-xCrx random alloys in the paramagnetic state. In the vacancy case, the nearest neighbors to the vacancy relax toward the vacancy of 0.14 angstrom (-5% of the ideal bcc nearest-neighbor distance), which is twice as large as the relaxation in the ferromagnetic case. The vacancy formation energy calculated in the DLM state on these positions is 1.60 eV, which corresponds to a reduction of about 0.1 eV compared to the formation energy calculated using DLM but on ferromagnetic-relaxed positions. The carbon interstitial formation energy is found to be 0.41 eV when the DLM relaxed positions are used, as compared to 0.59 eV when the FM-relaxed positions are employed. For bcc Fe0.5Cr0.5 alloys, the mixing enthalpy is reduced by 5 meV/atom, or about 10%, when the DLM state relaxation is considered, as compared to positions relaxed in the ferromagnetic state. ; Funding Agencies|Swedish Research Council (VR) [2014-6336]; Marie Sklodowska Curie Actions, Cofund [INCA 600398]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFOMatLiU) [2009 00971]; Swedish Foundation for Strategic Research

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; We investigate the dynamic susceptibility of Fe4 single-molecule magnets with integer spin (S=5) in the form of pure crystals as well as diluted in crystals of isostructural, but nonmagnetic, Ga4 clusters. Below approximately 1 K, the spin-lattice relaxation becomes dominated by a temperature-independent process. The spin-lattice relaxation time τ measured in this >quantum regime> is 12 orders of magnitude shorter than the characteristic time scale of direct phonon-induced processes but agrees with the relaxation times of pure (i.e., not assisted by phonons) spin tunneling events. The present results show that the latter phenomenon, despite conserving the energy of the ensemble of electronic and nuclear spins, drives the thermalization of electronic spins at very low temperatures. The spin-lattice relaxation time scales with the concentration of Fe4, thus suggesting that the main effect of dipolar interactions is to block tunneling. The data show therefore no evidence for the contribution of collective phonon emission processes, such as phonon superradiance, to the spin-lattice relaxation. © 2014 American Physical Society. ; This paper has been partly funded by the Spanish MINECO (Grant No. MAT2009-13977-C03), the Gobierno de Aragón (project MOLCHIP), the European Union (ERANET project NanoSci-ERA: Nanoscience in European Research Area SMMTRANS), the Italian MIUR (PRIN2008 project) and the Université Joseph Fourier (Grenoble, France) through a visiting professorship to A.C. ; Peer Reviewed

In iron-based superconductors the interactions driving the nematic order that breaks the lattice four-fold rotational symmetry in the iron plane may also facilitate the Cooper pairing, but experimental determination of these interactions is challenging because the temperatures of the nematic order and the order of other electronic phases appear to match each other or to be close to each other. Here we performed field-dependent 77Se-nuclear magnetic resonance (NMR) measurements on single crystals of iron-based superconductor FeSe, with magnetic field B 0 up to 16 T. The 77Se-NMR spectra and Knight shift split when the direction of B 0 is away from the direction perpendicular to the iron planes (i.e. B 0 ∥ c) upon cooling in temperature, with a significant change in the distribution and magnitude of the internal magnetic field at the 77Se nucleus, but these do not happen when B 0 is perpendicular to the iron planes, thus demonstrating that there is an orbital ordering. Moreover, stripe-type antiferromagnetism is absent, while giant antiferromagnetic spin fluctuations measured by the NMR spin-lattice relaxation gradually developed starting at ∼40 K, which is far below the nematic order temperature T nem = 89 K. These results provide direct evidence of orbital-driven nematic order in FeSe. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft. ; Chinese Academy of Sciences, CAS: 51477167, 41527802 ; Ministério da Educação e Ciência, MEC: 2-2017-084 ; National Natural Science Foundation of China, NSFC: 61474096, 1804291 ; BK20180890, 20180889 ; Russian Foundation for Basic Research, RFBR: 17-29-10007 ; Government Council on Grants, Russian Federation ; Work at YZU was supported by National Science Foundation of China (NSFC) (Grants #61474096 and 1804291) and NSF of Jiangsu (Grants #BK20180889 and BK20180890), and at CAS by NSFC (Grants# 51477167 and 41527802).DACthanks supports by the program 211 of the Russian Federation Government (RFG), agreement 02.A03.21.0006 and by the RFG Program of Competitive Growth of KFU.ANVthanks supports by Russian Foundation for Basic Research Grant#17-29-10007, by the Ministry of Education and Science of the RFG in the framework of ICP of NUST MISiS (Grant#K2-2017-084), and by Act 211 of RFG, agreements 02.A03.21.0004, 02.A03.21.0006, and 02.A03.21.0011.

Intro -- Preface -- Acknowledgments -- Contents -- Nomenclature, Symbols, and Abbreviations -- Greek Symbols -- Superscript -- Subscript -- Acronyms, Abbreviations -- Symbols, Operators -- Part I Fundamental Theory and Implementation of the Lattice Boltzmann Method -- 1 Introduction -- 1.1 Introduction -- 1.2 Review of Navier-Stokes Equations (NSE)-Based Computational Fluid Dynamics (CFD) in the Built Environment -- 1.2.1 Continuity Assumption of Fluid -- 1.2.2 Governing Equations -- 1.2.3 Important Physical Quantities in CFD -- 1.2.4 Dimensional Analysis and Dimensionless Form of the NSE -- 1.2.5 Similarity -- 1.2.6 NSE-Based CFD Simulation Methods -- 1.3 Development of the Lattice Boltzmann Method (LBM) -- 1.3.1 Lattice Gas Automata (LGA) -- 1.3.2 From LGA to LBM -- 1.3.3 LBM Development -- 1.3.4 LBM Application in the Built Environment -- 1.4 Purpose and Outline of This Book -- References -- 2 Fundamental Theory of the Lattice Boltzmann Method -- 2.1 Introduction -- 2.2 Fluid From a Mesoscopic Perspective -- 2.2.1 Distribution Functions -- 2.2.2 Equilibrium Distributions and Maxwell-Boltzmann Distributions -- 2.3 Lattice Boltzmann Equation (LBE) -- 2.3.1 Stream and Collision -- 2.3.2 LBE -- 2.4 Discrete Velocity Scheme -- 2.4.1 Discrete Velocity and DdQq Schemes -- 2.4.2 D2Q9 Scheme for 2D Problems -- 2.4.3 D3Q19 and D3Q27 Schemes for 3D Problems -- 2.4.4 Equilibrium Distribution Function in the DdQq Scheme -- 2.5 Collision Function: Relaxation Time Scheme -- 2.5.1 Moment -- 2.5.2 Single-Relaxation Time (SRT) Scheme and the BGK Model -- 2.5.3 Multi-Relaxation Time (MRT) Scheme -- 2.5.4 Two-Relaxation Time (TRT) Scheme -- 2.5.5 Other Advanced Collision Function Schemes -- 2.6 Summary -- References -- 3 Implementation of the Boundary Conditions -- 3.1 Introduction -- 3.2 First-Type BC: The Dirichlet BC -- 3.3 Second-Type BC: The Neumann BC.

The evolution of n-systems of spins in interaction via the lattice is considered. Equations for the evolution of the n spin temperatures [formula] are established and solved. Generally speaking, the inverse of each spin temperatures varies in time as the sum of n exponentials. The relaxation times of these exponentials can be expressed in terms of transition probabilities corresponding to spin-spin interaction via the lattice. The case of two spin systems is considered in more detail.

We have performed a numerical study of the process of electron localization in reduced ceria. Our results show that different localized charge distributions can be attained in a bulk system by varying the lattice parameter. We demonstrate that the effect of electron localization is mainly determined by lattice relaxation and an accurate account for the effects of electronic correlation is necessary to achieve localized charge distribution. ; Funding Agencies|Swedish Research Council [2014-4750]; Swedish Energy Agency (STEM); Carl Tryggers Foundation; LiLi-NFM; Swedish Government Strategic Research Area Grant in Materials Science

Carbonated hydroxyapatite (CHA) is the basic mineral component of animal and human bone. Therefore, it is widely used in medicine to repair bone defects. In orthopedic surgeries, ceramic implants are usually used as a biologically active defect filler. In the lattice of CHA carbonate ions can occupy two non-equivalent positions - A and B. A position corresponds to the position of OH- anions in the lattice of hydroxyapatite (HA), and B - PO43-. It is well known that substitution of B-positions with carbonate groups leads to significant distortions of HA lattice, which causes microstresses and crystalline defects in it. Therefore, CHA ceramics as a result of sintering is characterized by significant internal stresses whose relaxation at room temperature can lead to a change in both its phase composition and biological activity. By methods of chemical and X-ray structural analysis, infrared spectroscopy and electron scanning microscopy the ageing process of pressed CHA at room temperature, sintered in an atmosphere of dry carbon dioxide at temperatures 800÷1200 °C was studied. The phase composition and structure of freshly prepared and aged for two years ceramic samples were compared. It is shown that relaxation of internal stresses arising during sintering of presses causes plastic deformation of crystallites accompanied by redistribution of carbonate ions from B to A-position. As a result, displacement of OH- ions from channel (A) positions and decomposition of B-type CHA on CaO and A-type CHA becomes energetically advantageous.

We propose an intrinsic spin scattering mechanism in graphene originated by the interplay of atomic spin-orbit interaction and the local curvature induced by flexural distortions of the atomic lattice. Starting from a multiorbital tight-binding Hamiltonian with spin-orbit coupling considered nonperturbatively, we derive an effective Hamiltonian for the spin scattering of the Dirac electrons due to flexural distortions. We compute the spin lifetime due to both flexural phonons and ripples and we find values in the microsecond range at room temperature. Interestingly, this mechanism is anisotropic on two counts. First, the relaxation rate is different for off-plane and in-plane spin quantization axis. Second, the spin relaxation rate depends on the angle formed by the crystal momentum with the carbon-carbon bond. In addition, the spin lifetime is also valley dependent. The proposed mechanism sets an upper limit for spin lifetimes in graphene and will be relevant when samples of high quality can be fabricated free of extrinsic sources of spin relaxation. ; This work has been financially supported by MEC-Spain (GrantsNo. FIS2010-21883-C02-01, No. FIS2009-08744, and No. CONSOLIDER CSD2007-0010) and European Union, as well as Generalitat Valenciana, grant Prometeo 2012-11.

The authors would like to thank the clinical teams of the Royal Aberdeen Infirmary for their support, in particular Dr German Guzman-Guttierez, Mr Paddy Ashcroft, Dr Tanja Gagliardi, Prof Steven Heys, Prof Alison Murray and Prof Graeme Murray. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 668119 (project "IDentIFY"). ; Peer reviewed ; Publisher PDF

Fe-doped Bi12MnO20–BiMn2O5 ceramics was sintered at 1130 K for 6 h in ambient air. Two centro-symmetric phases formed thermodynamically stable self-composite material that was deduced from X-ray pattern analysis. The lattice parameters were a = 10.147(8) Å—for the cubic I23 Bi12MnO20 phase; and a = 7.545(4) Å, b = 8.538(1) Å, c = 5.758(3) Å—for the orthorhombic Pbam BiMn2O5 phase. The 57Fe Mössbauer spectrum, recorded at room temperature, has shown pure electronic quadrupolar split. The major doublets reflected the occurrence of Fe3+ ions distributed in two sites, i.e., octahedral Fe4+O6 and square pyramidal Fe3+O5, with preferential occupation of the pyramidal sites, that was consistent with the Pbam phase symmetry. The third doublet resulted from the presence of iron Fe3+ in tetrahedral Fe3+O4 coordination and corresponded to a small admixture of the I23 phase. The DC resistivity ρDC(T) dependence on temperature has shown thermally activated features, and the value of Ea,DC varied in the range of 0.22–0.37 eV. The electric impedance was measured in the f = 20 Hz–1 MHz and 100–690 K range. Two electrical relaxations were determined using the electric modulus formalism M″(T). Low-temperature relaxation has shown the temperature-dependent activation energy EA,1 = 0.14–0.20 eV and characteristic time values of τ01 = 10−10–10−12 s in 100–200 K range. It was attributed to the charge transfer between Mn4+/Mn3+ sites. The other relaxation occurred in the 170–220 K range, and it exhibited the following values: τ02 = 10−11 s, and EA,2 = 0.27 eV. A disorder-related VRH polaron model was proposed for ρDC(T) and for electric relaxation processes. ; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²