Simple two-dimensional model for the elastic origin of cooperativity among spin states of spin-crossover complexes

We study the origin of the cooperative nature of spin crossover (SC) between low-spin and high-spin (HS) states from the viewpoint of elastic interactions among molecules. As the size of each molecule changes depending on its spin state, the elastic interaction among the lattice distortions provides the cooperative interaction of the spin states. We develop a simple model of SC with intra and intermolecular potentials which accounts for the elastic interaction including the effect of the inhomogeneity of the spin states and apply constant temperature molecular dynamics based on the Nosé-Hoover formalism. We demonstrate that, with increase of the strength of the intermolecular interactions, the temperature dependence of the HS component changes from a gradual crossover to a first-order transition.     

Magnetic Properties and Spin Crossover in Transition Metal Oxides with d 5 Ions at High Pressures

We analyze the influence of cooperative effects on the magnetic properties and spin crossover between the high-spin (HS) term S = 5/2 and low-spin (LS) term S = 1/2 in Mott–Hubbard dielectrics with 3d5 ions under high pressures. Two cooperation mechanisms (superexchange interaction and effective interaction via the elastic system) are considered. The sign of the exchange interaction changes because of the crossover from the antiferromagnetic in the HS state to the ferromagnetic in the LS state. In view of the large difference between the ionic radii of the HS and LS states, the systems with spin crossover acquire an additional strong coupling via the elastic system. Using the Hubbard operator representation and considering the electronic states of the two terms simultaneously, we obtain the effective Hamiltonian with allowance for the cooperative effects. The magnetic phase diagram and the spin crossover are investigated in the mean field approximation. It is shown that the inclusion of cooperative effects at low temperatures leads to a first-order phase transition between the antiferromagnetic HS state and the ferromagnetic LS state. At higher temperatures, more complicated sequences of phase transitions are possible upon an increase in pressure, including the HS paramagnet–HS antiferromagnet–LS paramagnet and HS antiferromagnet–LS paramagnet–LS ferromagnet transitions.     

Quench dynamics in spin crossover induced by high pressure

In this paper we have analytically and numerically studied the dynamics of spin crossover induced by time-dependent pressure. We show that quasi static pressure, with a slow dependence on time, yields a spin crossover leading to transition from the high spin (HS) quantum system state to the low spin (LS) state. However, quench dynamics under shockwave load are more complicated. The final state of the system depends on the amplitude and pulse velocity, resulting in the mixture of the HS and LS states.     

Study on the spin crossover transition and glass transition for Fe(II) complex film, [Fe(II)(H-triazole) 3 ]@Nafion, by means of M?ssbauer spectroscopy

[Fe(II)(H-trz)₃]@Nafion (trz = triazole) is a transparent spin crossover complex film, where the spin crossover transition between the low-spin (S = 0) and the high-spin (S = 2) states takes place between 225?K and 300?K. In this film, two doublets corresponding to the low-spin and high-spin states were observed in the ⁵⁷Fe M?ssbauer spectra, reflecting the spin crossover transition. From the analysis of ⁵⁷Fe M?ssbauer spectra, the Debye temperatures of the low-spin and high-spin sites were estimated at 185?K and 176?K, respectively, in the temperature range between 10?K and 150?K. In this film, the total intensity of the M?ssbauer spectra corresponding to the low-spin and high-spin sites drastically decreases above 200?K, reflecting the glass transition of Nafion, where the lattice vibration of [Fe(H-trz)₃] $_{\rm n}^{\,\,\rm 2n+}$ is softened just as in solution due to micro-Brown motion of the segment of Nafion polymer membrane.     

Calculation of the electronic energy differences of spin crossover complexes

Density functional calculations using different functionals and basis sets have been carried out to calculate the electronic energy difference between the high- and low-spin isomers of spin crossover complexes with a transition metal center. The reparameterized B3LYP* method is confirmed to be most suitable for the calculation of electronic energy differences between isomers with different spin states. If only changes of the electronic energy difference upon modifications of the complex are considered all employed density functional methods show a similar performance. Basis sets with effective core potentials seem to perform as well as large all electron basis sets. Calculations using the polarizable continuum model have been performed to investigate the effect of solvents. In addition the effect of hydrogen bonding between the spin crossover complex and a water molecule is studied.     

Modeling the effect of spin polarization of magnetic moments of electrons on crossover formation

We present the results from investigations of the effect of spin polarization on crossover formation in electron microscopes. Spatial distributions of magnetic states of electrons for the electron beams in the crossover of the electron gun are analyzed by the example of a self-consistent spin-polarization field. The factors able to affect the spatial resolving ability and sensitivity of scanning microscopy are evaluated.     

Ab initio study on phase transition and magnetism of BiFeO3 under pressure

In this paper the first-principles calculations within local spin density approximation (LSDA)+U show that BiFeO3 experiences a mixed phase state with P4mm structure being the intermediate phase before the pressure of phase transition is reached. The critical pressure for the insulator-metal transition (IMT) is found to be about 50 GPa. A pressure induced crossover of high-spin states and low-spin states is observed close to the IMT pressure in R3c structure. The LSDA+U calculations account well for the mechanism of the IMT and crossover of spin states predicted in recent experiment (Ref.[1]).     

Nin(n≤8)团簇的结构

用高斯程序的B3LYP交换相关函数和全电子基组DGDZVP、TZVP优化Ni_n、Ni_n⁺和Ni_n^-(n=2~8),得到这些体系的基态几何构型、离化能、电子亲和能及键能.结果表明:Ni₂,Ni₃,Ni₄,Ni₅和Ni₆团簇的基态分别是5,7,9,11和13重态.表明:Ni₂,Ni₃,Ni₄,Ni₅和Ni₆团簇中分别有4,6,8,10和12个平行自旋的电子,在每一个Ni_n团簇中非耦合的电子最多,这些电子占据不同的轨道,导致每一个Ni_n团簇的能量最小,说明在这些Ni_n团簇中电子平行的效应大于电子耦合的效应,这显然与d电子的离域效应有关,称为自旋极化效应,符合宏特规则.Ni_n、Ni_n⁺和Ni_n^-(n=2~8)体系的磁矩分别是Ni_n具有8μ_B(n=6~8),Ni_n⁺具有9μ_B(n=6~8),Ni_n具有7μ_B(n=4~8).     

Non-perturbative effects in two-dimensional lattice O(N) models

Non-abelian analogues of Kosterlitz-Thouless vortices may have important effects in two-dimensional lattice spin systems with O(N) symmetries. Renormalization group equations which include these effects are developed in two ways. The first set of equations extends the renormalization group equations of Kosterlitz to O(N) spin systems, in a form suggested by Cardy and Hamber. The second is derived from a Villain-type O(N) model using Migdal's recursion relations. Using these equations, the part played by topological excitations in the crossover from weak to strong coupling behavior is studied. Another effect which influences crossover behavior is also discussed: irrelevant operators which occur naturally in lattice theories can make important contributions to the renormalization group flow in the crossover region. When combined with conventional perturbative results, these two effects may explain the observed crossover behavior of these models.     

High spin band structure in 139Nd

High-spin states in 139Nd nucleus have been reinvestigated with the reaction 128Te (16O, 5n) at a beam energy of 90 MeV. The level scheme has been expanded with spin up to 47/2 h. At the low spin states,the yrast collective structure built on the vh(-1)(11/2) multiplet shows a transitional shape with γ≈32° according to calculations of the triaxial rotor-plus-particle model. Three collective oblate bands with γ～-60° at the high spin states were identified for the first time. A band crossing is observed around hw ～0.4 MeV in one oblate band based on the 25/2- level.     

Specific features of spin ordering in an Ising antiferromagnet with single-ion easy-plane anisotropy

A thermodynamic analysis of spin ordering in an Ising antiferromagnet with single-ion easy-plane anisotropy is carried out. The solutions of the equations of state are analyzed, and the phase diagram is constructed. It is shown that an increase in the single-ion anisotropy constant as a result of increasing competition between single-ion anisotropy and exchange interaction brings about a crossover from the continuous phase transition to the abrupt phase transition. This is initiated by a change in the ion state populations due to single-ion anisotropy and by the freezing of ionic states with a nonzero spin projection. An increase in the single-ion anisotropy also leads to a crossover of the phase transformation between the paramagnetic and antiferromagnetic states from the order-disorder to order-order type.     

First-principles calculation of the structural,electronic,and magnetic properties of cubic perovskite RbXF_3(X = Mn,V,Co,Fe)

First-principles calculations by means of the full-potential linearized augmented plane wave method using the generalized gradient approximation with correlation effect correction(GGA+U) within the framework of spin polarized density functional theory(DFT+U) are used to study the structural,electronic,and magnetic properties of cubic perovskite compounds RbXF_3(X = Mn,V,Co,and Fe).It is found that the calculated structural parameters,i.e.,lattice constant,bulk modulus,and its pressure derivative are in good agreement with the previous results.Our results reveal that the strong spin polarization of the 3d states of the X atoms is the origin of ferromagnetism in RbXF_3.Cohesive energies and the magnetic moments of RbXF_3 have also been calculated.The calculated electronic properties show the half-metallic nature of RbCoF_3 and RbFeF_3,making these materials suitable for spintronic applications.     

Lamb–Mössbauer factor of electronically excited molecular states measured by time‐differential Mössbauer emission spectroscopy

The Lamb-Mössbauer Factor (LMF) of molecular crystals is expected to depend on the electronic molecular states by their different intramolecular vibrational frequencies. Revisiting Mössbauer spectra obtained by time differential Mössbauer emission spectroscopy of the low spin compound [⁵⁷Co/Mn(bipy)₃](PF₆)₂ (bipy= 2,2'-bipyridine) a ratio of 1.25 for the LMFs of the low spin ground state and of an excited high spin state decaying in the Mössbauer time window could be evaluated. The difference found is in line with the change of LMF observed for spin crossover compounds where the excited high spin state is populated by the so‐called LIESST effect. The initial population of the high spin state is close to 100%.     

含stubs量子波导系统的电子自旋极化输运性质

理论上研究了含stubs的Rashba自旋轨道耦合（spin-orbit coupling, SOC）量子波导系统的自旋极化输运性质. 利用晶格格林函数方法,发现由于stubs和SOC产生的势阱使系统中出现束缚态,这些束缚态与传播态之间相互干涉导致电导中出现Fano共振结构,同时在对应的自旋极化率中也出现Fano共振或反共振结构. 此外,由于系统结构的突变使电子被反向散射和量子干涉效应,电导中出现一系列的共振峰. 但是,当系统加上外磁场后,所有这些效应都被抑制, 系统重新出现量子化电导, 同时自旋电导也出 关键词： 量子波导 自旋极化输运 自旋轨道耦合     

New Class of Photo-induced Cooperative Phenomena in Organic and Inorganic Hybrid Complexes

We review experimental results which show the occurrence of a new class of cooperative photo-effect so called as photo-induced phase transition (PIPT) in metal-organic hybrid system. We focus on the photo-induced spin state phase transition in spin crossover complex, and report the dynamic behavior and magnetic field effect on it. Obtained results demonstrate that cooperative spin-lattice interaction in crystal plays a key role for driving PIPT phenomena in spin crossover complexes.     

二维声子晶体中简单旋转操作导致的拓扑相变

构建了一种简单的二维声子晶体:由两个横截面为三角形的钢柱所组成的复式元胞按三角点阵的形式排列在空气中,等效地形成了一个蜂巢点阵结构.当三角形钢柱的取向与三角点阵的高对称方向一致时,整个体系具有C_(6v)对称性.研究发现:在保持钢柱填充率不变的条件下,只需要将所有三角柱绕着自己的中心旋转180°,就可实现二重简并的p态和d态在布里渊区中心Γ点处的频率反转,且该能带反转过程实质上是一个拓扑相变过程.通过利用Γ点的P态和d态的空间旋转对称性,构造了一个赝时反演对称性,并在声学系统中实现了类似于电子系统中量子自旋霍尔效应的赝自旋态.随后通过k·p微扰法导出了Γ点附近的有效哈密顿量,并分别计算了拓扑平庸和非平庸系统的自旋陈数,揭示了能带反转和拓扑相变的内在联系.最后通过数值模拟演示了受到拓扑不变量保护的声波边界态的单向传输行为和对缺陷的背向散射抑制.文中所研究的声波体系,尽管材料普通常见,但其拓扑带隙的相对宽度超过21%,比已报道的类似体系的带隙都要宽,且工作原理涵盖从次声波到超声波的很大频率范围,从而在实际应用上具有较大的优势和潜力.     

Spin transition in iron(II) complexes induced by heat,pressure, light,and nuclear decay

The phenomenon of temperature-dependent spin transition will be introduced and the numerous chemical and physical influences affecting the spin transition characteristics will be discussed. We shall mainly concentrate on the spin crossover system [Fe(2-pic)₃]X₂·Sol (2-pic=2-aminomethylpyridine; X=Cl, Br; Sol=C₂H₅OH, CH₃OH) and demonstrate how the behaviour of the spin transition⁵T_2g(O_h)?¹A_1g(O_h) is influenced by substituting the metalion, the non-coordinating anions X, the crystal solvent molecules Sol and by isotopic exchange with H/D and¹⁴N/¹⁵N. It will also be shown that the spin transition is very susceptible to pressure. A quantitative spin state conversion from low spin to high spin can also be achieved by illuminating the crystals of a spin crossover system at sufficiently low temperatures. The metastable quintet state can be trapped with practically infinite lifetimes. Several examples for this “Light-Induced Excited Spin State Trapping” (LIESST) will be given. Finally, the occurrence of short-lived anomalous spin quintet states following the⁵⁷Co(EC)⁵⁷Fe nuclear decay, which have been observed in the Mössbauer emission spectra of⁵⁷Co-doped complex compounds, will be discussed with particular references to the LIESST effect.     

Stripe fluctuations, carriers, spectroscopies, transport, and BCS-BEC crossover in the high-Tc cuprates

The quasiparticles of the high-T_c cuprates are found to consist of: polaron-like ‘stripons’ carrying charge, and associated primarily with large-U orbitals in stripe-like inhomogeneities; ‘quasi-electrons’ (QE) carrying charge and spin, and associated with hybridized small-U and large-U orbitals; and ‘svivons’ carrying spin and lattice distortion. It is shown that this electronic structure leads to the systematic behavior of spectroscopic and transport properties of the cuprates. High-T_c pairing results from transitions between pair states of stripons and QEs through the exchange of svivons. The cuprates fall in the regime of crossover between BCS and preformed-pairs Bose-Einstein condensation behaviors.     

Pressure-induced electron spin transition in the paramagnetic phase of the GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> Heisenberg magnet

The HS → LS spin crossover effect (high-spin → low-spin transition) induced by high pressure in the range 45–53 GPa is observed in trivalent Fe³⁺ ions in the paramagnetic phase of a Gd⁵⁷Fe₃(BO₃)₄ gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K _β high-resolution x-ray emission spectroscopy (XES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.     

An investigation of the properties of single-particle states in the second minimum of 237Pu

In connection with the recent measurement of the magnetic moments of the fission isomeric states in ²³⁷Pu, we investigate the following subjects: (i) The spin alignment of the lowest-lying states in a potential minimum which are populated by (α, 2nγ) reactions, and the fragment angular distributions in fission from isomeric states. (ii) The single-particle level scheme in a very deformed Woods-Saxon potential. (iii) The spin polarizability as a function of the quadrupole deformation.