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1.
We report evidence for phase coexistence of orbital orderings of different symmetry in SmVO3 by high resolution x-ray powder diffraction. The phase coexistence is triggered by an antiferromagnetic ordering of the vanadium spins near 130 K, below an initial orbital ordering near 200 K. The phase coexistence is the result of the intermediate ionic size of samarium coupled to exchange striction at the vanadium spin ordering.  相似文献   

2.
Resonant transitions predicted earlier between low-lying electron-nuclear sublevels of the Tm3+ ground state were observed at frequencies up to 700 MHz in a dielectric Van Vleck paramagnet—thulium ethylsulfate single crystal. It is shown that, due to the distortion of the 4f-electron shell of a rare-earth ion in an applied magnetic field, the parameters of electron-nuclear interaction become field-dependent.  相似文献   

3.
4.
Ni(II) with bis(acetylacetone)ethylenediamine ligand forms complexes which crystallizes as semi-hydrate with C2/c space group in monoclinic system and anhydrous form with Pna2(1) space group in orthorhombic system. 13C and 15N CP/MAS experiments were employed for structural characterization of both forms and searching of process of reversible water exchange in the crystal lattice. Comparative analysis of 13C and 15N principal elements of chemical shift tensors for ligand and complexes showed the sensitivity of δii elements both for complexation and phase reaction processes. Theoretical 13C and 15N NMR shielding parameters σii were computed employing ONIOM approach and correlated with experimental δii data. The applicability of ONIOM in structural analysis of coordination compounds (CC) is discussed.  相似文献   

5.
Carbazole derivatives have drawn increasing attention recently in organic electronic device applications because of their particular optoelectronic properties. An in‐depth theoretical investigation was elaborated in this paper to reveal the molecular structures, optoelectronic properties, and the structure‐property relationships of different carbazole‐linked functional groups. The geometric and electronic structures in ground and the mobility for the hole and electron are both calculated by density functional theory method. The excited‐state geometries of these compounds were obtained through Single‐excitation Configuration Interaction method, and time‐dependent density functional theory calculation results described the absorption and emission spectra properties, respectively. Some conclusions are as follows: (1) enlarging the π‐conjugated area, the corresponding spectra red shifted markedly; (2) by introducing the electron‐donor such as carbazole, the spectra blue shifted slightly; (3) compared with compound 1, the spectra for these compounds are hardly influenced by introducing an electron‐acceptor or heterocyclic substitution. On all accounts, these compounds are interesting optoelectronic functional materials. On the basis of their structural modifiability, the arylamine derivatives substituted carbazole compounds have great potential in the applications of organic light‐emitting diodes, organic solar cells, and sensors. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

6.
With an original modulation technique, the Gd3+ electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa2Cu3O6+x (123) and YBa2Cu4O8 (124) compounds doped with 1% Gd. In the 123 sample withx = 0.9T c = 90 K), theT 1 behavior within 50 <T< 200 K reveals the [1 ? tanh2(Δ/2kT)]/T dependence typical of a spin gap opening with Δ ≈ 240 K. Below 50 K, the exponential slowing down ofT 1 is limited by the Korringa-like behaviorT 1 T = const); the same Korringa-like law is found in the 123 sample withx = 0.59 (T c = 56 K) within the total 4.2–200 K temperature range. This is interpreted in terms of microscopic separation of the normal and superconducting phases allowing for the electron spin cross-relaxation between them. In the 124 sample (T c = 82 K), the Gd3+ relaxation rate below 60 K is found to obey a power lawT n with an exponentn ≈ 3. Such a behavior (previously reported for nuclear spin relaxation) is indicative of the d-wave superconducting pairing. Additional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in theT 1 temperature dependence atTT* ≈ 180–200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd3+ ions, suggesting microscopic phase separation and a change in the relaxation mechanism due to electronic crossover related with the opening of the spin gap. This hypothesis is supported by some “180 K anomalies” previously reported by other authors.  相似文献   

7.
王文彬  朱银燕  殷立峰  沈健 《物理学报》2018,67(22):227502-227502
复杂氧化物可以呈现出高温超导、庞磁阻以及多铁效应等诸多新奇的物理现象.这类材料中的电荷/自旋/轨道和晶格自由度之间的强耦合相互作用,可以导致多种相互竞争且能量非常接近的电子态的空间共存,这就是电子相分离现象.如果可以将材料的空间尺寸缩小到电子相分离的特征长度,其物理性质甚至电子关联作用本身都会发生根本的变化,从而有可能实现复杂氧化物中的量子调控.本文综述了我们课题组在过去几年中针对复杂氧化物中电子相分离的量子调控取得的进展,内容包括:发现了锰氧化物边缘电子态,通过氧化物微纳加工技术,实现了量子态空间分布的调控,提高了庞磁阻锰氧化物的临界温度;研究了当材料空间尺度小于其电子相分离特征尺度时电子相分离的表现,确定了在电子相分离消失以后体系的磁结构;通过超晶格生长技术调控了材料中的掺杂有序度,对锰氧化物中大尺度的电子相分离的物理机理从实验上给出了解释.  相似文献   

8.
2D NMR studies in single crystals of CoSiF6.6D2O, CoTiF6.6D2O and ZnTiF6.6D2O reveal that all of them transform from rhombohedral to monoclinic symmetry through an intermediate phase as the temperature is lowered from 300 to 100K. The results throw light on the hydrogen bond network in different phases.  相似文献   

9.
10.
We present new evaluations of phase boundaries in the NaxTiS2 system from electrochemical intercalation and from X-ray and NMR measurements in samples intercalated using the liquid ammonia technique. After a critical discussion of the influence of the method of intercalation on the phase limits we present an extensive NMR study of the system. 23Na quadrupolar coupling determinations support the fact that the low concentration (x < 0.25) phase II is a stage 2 phase. 23Na Knight shift results show that the transition from the Ib trigonal prismatic phase to the Ia trigonal antiprismatic phase with increasing Na concentration takes place with a change in the electronic band structure. The stability of this phase is discussed in terms of the balance between elastic and electronic energies.  相似文献   

11.
An orbital ordering transition and electronic phase coexistence have been discovered in SrCrO3. This cubic, orbitally-degenerate perovskite transforms to a tetragonal phase with partial orbital order. The tetragonal phase is antiferromagnetic below 35-40 K, whereas the cubic phase remains paramagnetic at low temperatures. The orbital ordering temperature (35-70 K) and coexistence of the two electronic phases are very sensitive to lattice strain. X-ray measurements show a preferential conversion of the most strained regions in the cubic phase. This reveals that small fluctuations in microstrain are sufficient to drive long range separation of competing electronic phases even in undoped cubic oxides.  相似文献   

12.
13.
A comparative analysis of nuclear chemical shift predictions of proteins in the solid state by rapid algorithms trained on and verified with solution-state NMR assignments is presented. The precision of predictions by four dedicated computer programs (SHIFTS, PROSHIFTS, SHIFTX and SPARTA) was found to be close to values obtained for proteins in solution. Correlation coefficients depend on the NMR nucleus (N, C′, Cα and Cβ) and on secondary structure (β-strand, random coil and α-helix), but also on the molecular environment (membrane-integral or not). The findings establish a quantitative basis for using chemical shift prediction programs for solid-state NMR applications. On the other hand, prediction inaccuracies identified for certain resonance kind, residue type, and molecular environment point to possible areas of methodological improvement.  相似文献   

14.
We discuss the influence of the Coulomb interaction between localized and conduction electrons on the properties of magnetic impurities in metals and on electronic phase transitions such as the γ—α—α' transitions in Ce and the insulator—metal transition in SmS. Due to excitonic pairing between ?-holes and s-electrons, similar to that in excitonic insulators, the virtual ?-levels in metals may acquire an extra width, which, in contrast to the width in the Anderson model, depends upon the position of the ?-level, the width being the largest when the ?-level crosses the Fermi-level. This effect stabilizes the intermediate valence phase. As a result, in the Falicov model we get either a gradual phase transition (like that found in SmTe), or a first order one, followed by the intermediate valence phase (SmS), or, which is most interesting, two successive jump-like transitions with a mixed valence in between, similar to the γ—α—α' transitions in Ce. The mixed valence phase is described here as a kind of an “excitonic insulator”. The theory also predicts the correct slopes of the phase equilibrium lines for both Ce and SmS.  相似文献   

15.
The13C NMR of camphene in the solid plastic crystalline phase was studied at room temperature. Chemical shifts, spin-lattice relaxation times (T1), nuclear Overhauser enhancement factors and 13C-1H coupling constants were measured and compared with the corresponding values obtained in solution.  相似文献   

16.
Nuclear magnetic resonance spectroscopy has had extensive applications for the characterization of numerous metal-hydrogen systems. Although the greatest emphasis of proton NMR has been to evaluate diffusion behavior, increasing attention has been addressed upon the correlation of proton Knight shifts and the conduction electron contributions to proton spin-lattice relaxation times to the electronic structure properties of the hydride. The general principles of NMR, that pertain to the usual situations for most transition metal hydrides, will be briefly reviewed. Several specific examples from some recent research will be discussed in greater detail. In particular, the roles of host crystal structure and hydrogen site occupancy to hydrogen diffusion behavior are examined for the Ti1-y Cu y H x and Zr1-y Pd y H x systems. The proton hyperfine parameters in TiH x and ZrH x , as well as several related ternary hydrides, are used to qualitatively assess the character of the Fermi level electronic states. The relationship between the tetragonal distortions of the Ti and Zr dihydrides and a solid-state Jahn-Teller mechanism will also be examined. Mound is operated by the Monsanto Research Corporation for the U.S. Department of Energy under Contract No. DE-AC04-76-DP00053.  相似文献   

17.
A superconductive sign found in electric resistivity behavior of mixed ceramic Pr124/Pr123 is searched and discussed, to advance preceding discussion about the superconductive region induced by intergrain charge transfer effect in ceramic Pr247 containing the related impurity phases. Observed reduction effects for largely impure ceramic of Pr247 with Pr124 and Pr123 and for simply dual-phase sample of Pr124/Pr123 inform us that the oxygen-reduction brings not only superconductive electron doping but also normal resistivity enhancement. Milder heat treatment condition is expected to assist possible attainment of zero-resistivity for Pr124/Pr123.  相似文献   

18.
19.
We have investigated the structural and electronic properties of monophospides of thorium, uranium and neptunium. The total energy as a function of volume is obtained by means of the self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From the present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl-type structure at ambient pressure. The structural stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B1-phase) structure to CsCl-type (B2-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP-NpP). We also calculate lattice parameter (a0), bulk modulus (B0), band structure and density of states. From energy band diagram it is observed that ThP, UP and NpP exhibit metallic behavior. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.  相似文献   

20.
Construction has begun on the National Synchrotron Light Source II (NSLS-II), a state-of-the-art, third-generation medium energy storage ring to be built at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory (BNL).  相似文献   

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