梯形化合物NaV2O4F电子结构的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理赝势平面波方法, 通过自旋极化的广义梯度近似(GGA)电子结构计算对梯形化合物NaV₂O₄F进行了研究. 考虑了四种假想的自旋有序态,计算结果表明该化合物的磁基态具有二维反铁磁(AFM)结构, 即沿梯阶和梯腿方向都表现为AFM作用. 能带结构显示NaV₂O₄F为绝缘体材料, 带隙约为1.0eV. 方锥体中的晶体场劈裂使得VO₄F方锥体中的 V^{4+ 关键词： 2O₄F')" href="#">NaV₂O₄F 梯形化合物 第一性原理计算 电子结构}     

Specific heat of Na1−x V2O5 single crystals

Temperature dependences of the specific heat C and the magnetic susceptibility χ of Na_1?x V₂O₅ single crystals (x=0, 0.01, 0.02, 0.03, and 0.04) are studied. In NaV₂O₅, the transition to the spin-gap state (T _c =34 K) is accompanied by a sharp decrease in χ, while C exhibits a λ-shaped anomaly. At low temperatures, the specific heat of NaV₂O₅ is approximated by the sum of phonon ～T ³ and magnon ～exp(?Δ/T) contributions, which makes it possible to estimate the Debye temperature Θ_D=336 K and the gap in the magnetic excitation spectrum Δ=112 K. With the departure from stoichiometry, the anomalies observed in the behavior of χ and C are spread and shifted to lower temperatures. The low-temperature specific heat of nonstoichiometric samples is determined by the sum of phonon and magnon components and the contribution due to the presence of defects. The values of magnetic entropy characterizing the phase transitions in Na_1?x V₂O₅ are calculated.     

A mean-field theory for strongly disordered non-frustrated antiferromagnets

Motivated by impurity-induced magnetic ordering phenomena in spin-gap materials like TlCuCl₃, we develop a mean-field theory for strongly disordered antiferromagnets, designed to capture the broad distribution of coupling constants in the effective model for the impurity degrees of freedom. Based on our results, we argue that in the presence of random magnetic couplings the conventional first-order spin-flop transition of an anisotropic antiferromagnet is split into two transitions at low temperatures, associated with separate order parameters along and perpendicular to the field axis. We demonstrate the existence of either a bicritcal point or a critical endpoint in the temperature–field phase diagram, with the consequence that signatures of the spin flop are more pronounced at elevated temperature.     

Field-controlled phase separation at the impurity-induced magnetic ordering in the spin-Peierls Magnet CuGeO3

The paramagnetic fraction surviving at the impurity-induced antiferromagnetic phase transition in the spin-Peierls magnet CuGeO3 is found to increase with an external magnetic field. This effect is explained by the competition of the Zeeman interaction and of the exchange interaction of local antiferromagnetic clusters formed on the spin-gap background near impurities.     

Crystal and magnetic properties of the new copper-sodium borate CuNaB3O6 · 0.842H2O

A new compound CuNaB₃O₆ · 0.842H₂O was grown for the first time. Its crystal structure, magnetic susceptibility, and magnetic resonance properties are presented. It was proposed that CuNaB₃O₆ · 0.842H₂O is a spin-Peierls magnet with the transition temperature T _SP ～ 128 K and a ladder spin structure. The possibility of a structural phase transition at T < T _SP is predicted.     

Anomalous thermal conductivity of single crystal Cu2Te2O5Cl2

A strong increase of the thermal conductivity is observed at the phase transition (T _c=18.2 K) in Cu₂Te₂O₅Cl₂ single crystal. This behavior is compared with that of the spin-Peierls system NaV₂O₅, where a similar experimental observation has been made, and the conventional spin-Peierls system CuGeO₃, where a modest kink in the thermal conductivity curve has been observed. The strong increase of the thermal conductivity atT _c in Cu₂Te₂O₅Cl₂ could be partially attributed to the opening of the energy gap in the magnetic excitation spectrum evident from the magnetic susceptibility measurements. However, the main reason for the anomaly of the thermal conductivity could be explained by a strong spin-lattice coupling in this system, which what is in agreement with the preliminary X-band electron spin resonance measurement.     

Electronic structure and magnetic coupling in CaV<Subscript>2</Subscript>O<Subscript>5</Subscript>: spin dimer versus spin ladder

The electronic structure and magnetic exchange interactions of the ladder vanadate CaV₂O₅ have been studied by ab initio electronic structure calculations based on density functional theory (DFT). Geometry optimization and electronic structure calculations are performed using spin-polarized generalized gradient approximation (GGA) exchange-correlation functionals for four possible spin-ordered states. The experimentally observed insulating behavior has been reproduced successfully in the framework of the band theory by considering the magnetic ordering. Calculated results reveal that the true magnetic ground state of CaV₂O₅ is the antiferromagnetic (AFM) state with AFM exchange interactions both inside the rungs and along the ladder legs. Calculated exchange parameters indicate that the ladder structural vanadate CaV₂O₅ should be described as weakly coupled dimer system rather than as spin ladder compound. The AFM interactions inside the dimer are crucial to the insulating ground state and magnetic characteristics of CaV₂O₅.     

Monte Carlo simulations on magnetic behavior of a spin-chain system in triangular lattice doped with antiferromagnetic bonds

A three-dimensional Ising-like model doped with anti-ferromagnetic (AFM) bonds is proposed to investigate the magnetic properties of a doped triangular spin-chain system by using a Monte-Carlo simulation. The simulated results indicate that a steplike magnetization behavior is very sensitive to the concentration of AFM bonds. A low concentration of AFM bonds can suppress the stepwise behavior considerably, in accordance with doping experiments on Ca₃Co₂O₆. The analysis of spin snapshots demonstrates that the AFM bond doping not only breaks the ferromagnetic ordered linear spin chains along the hexagonal c-axis but also has a great influence upon the spin configuration in the ab-plane.      

Magnetic relaxation in rare-earth oxide pyrochlores

A theory of magnetic relaxation is developed for geometrically frustrated three-dimensional magnets that can be described by an antiferromagnetic Ising model. These magnetic materials are exemplified by some of the recently synthesized rare-earth oxide pyrochlores, such as Dy₂Ti₂O₇, Ho₂Ti₂O₇, or Yb₂Ti₂O₇. A model based on an analogy between the spin ordering in Ising magnets and proton ordering in ice is proposed. In this model, magnetic point defects treated as noninteracting quasiparticles characterized by well-defined energies, mobilities, and effective magnetic charges play a fundamental role analogous to that of ion defects in the physics of ice or by electrons and holes in semiconductors. The proposed model is used to derive expressions for magnetic susceptibility as a function of frequency and temperature.     

Magnetic anomalies in nanocrystalline , a geometrically frustrated spin-chain compound

We have investigated the magnetic behavior of nano crystals, synthesized by high-energy ball-milling, for a well-known geometrically frustrated spin-chain system, Ca₃CoRhO₆, and compared its magnetic characteristics with those of the bulk form by measuring ac and dc magnetization. The features attributable to the onset of ‘partially disordered antiferromagnetism’ (characterizing the bulk form) are not seen in the magnetization data of the nano particles; the magnetic moment at high fields in the very low temperature range in the magnetically ordered state gets relatively enhanced in the nano particles. It appears that the ferromagnetic intrachain interaction, judged by the sign of the paramagnetic Curie temperature, is preserved in the nano particles. These trends are opposite to those seen in the nano particles of Ca₃Co₂O₆. However, the complex spin-dynamics as evidenced by large frequency dependence of ac susceptibility is retained in the nano particles as well. Thus, there are some similarities and dissimilarities between the properties of the nano particles and those of the bulk. We believe that these findings will be useful to understand correlation lengths deciding various properties of geometrical frustration and/or spin-chain phenomena.     

Magnetic excitations in charge ordered a' - {N_a}{V_2}{O_5}

An investigation of the spin excitation spectrum of charge ordered (CO) NaV₂O₅ is presented. We discuss several different exchange models which may be relevant for this compound, namely in-line and zig-zag chain models with weak as well as strong inter-chain coupling and also a ladder model and a CO/MV (mixed valent) model. We put special emphasis on the importance of large additional exchange across the diagonals of V-ladders and the presence of exchange anisotropies on the excitation spectrum. It is shown that the observed splitting of transverse dispersion branches may both be interpreted as anisotropy effect as well as acoustic-optic mode splitting in the weakly coupled chain models. In addition we calculate the field dependence of excitation modes in these models. Furthermore we show that for strong inter-chain coupling, as suggested by recent LDA + U results, an additional high energy optical excitation appears and the spin gap is determined by anisotropies. The most promising CO/MV model predicts a spin wave dispersion perpendicular to the chains which agrees very well with recent results obtained by inelastic neutron scattering. Received 30 April 1999 and Received in final form 5 October 1999     

Local density of states of one-dimensional Mott insulators and charge-density wave states with a boundary

We determine the local density of states of one-dimensional incommensurate charge-density wave states in the presence of a strong impurity potential, which is modeled by a boundary. We find that the charge-density wave gets pinned at the impurity, which results in a singularity in the Fourier transform of the local density of states at momentum 2k_{F}. At energies above the spin gap we observe dispersing features associated with the spin and charge degrees of freedom, respectively. In the presence of an impurity magnetic field we observe the formation of a bound state localized at the impurity. All of our results carry over to the case of 1D Mott insulators by exchanging the roles of spin and charge degrees of freedom. We discuss the implications of our result for scanning tunneling microscopy experiments on spin-gap systems such as two-leg ladder cuprates.     

Magnetic and resonance properties of LiCu2O2 single crystals

We study the structural, magnetic, and resonance properties of LiCu₂O₂ single crystals grown by the spontaneous crystallization method. The data are interpreted on the assumption that the crystalline structure of the grown single crystals is orthorhombic. Long-range antiferromagnetic order sets in at temperatures below 22.5 K, while above this temperature the dependence of the magnetic susceptibility has a shape characteristic of interacting antiferromagnetic Heisenberg chains. We hypothesize that long-range magnetic order sets in below 22.5 K through the destruction of the ideal ladder structure of LiCu₂O₂ because of partial redistribution of copper and lithium ions at the crystal lattice sites and because of the presence of other defects in the crystalline structure. Zh. éksp. Teor. Fiz. 113, 1866–1876 (May 1998)     

Two-leg spin ladder model for Ag2VOP2O7 from mapping analysis based on first principles density functional calculations

The magnetic properties of Ag₂V OP₂O₇ were examined by evaluating its spin exchange interactions in terms of spin dimer analysis based on tight binding calculations and mapping analysis based on first principles density functional theory calculations. Both calculations show that a strong spin exchange interaction occurs through the super-superexchange path J₁ with the V…V distance of 5.293 Å. This strong antiferromagnetic interaction forms isolated spin dimer units, which are coupled antiferromagnetically by the spin exchange path J₃ to form a two-leg spin ladder that has no spin frustration. The inter–dimer interaction J₂ is found to be ferromagnetic, and does not lead to spin frustration.     

自旋梯状化合物Sr14Cu24O41+δ的Raman散射谱的研究

利用常规的固相反应法制备了单相多晶样品Sr₁₄Cu₂₄O_41+δ,并在不同的温度下进行退火,改变样品中的氧含量.能量色散谱(EDS)显示样品中的氧含量随退火温度的增加而减少.磁化率温度特性的研究显示,600 ℃下退火的样品中的二聚体数最多.Raman光谱的研究显示,伴随着样品中氧含量的偏离,由无序或低能磁激发诱导的一些Raman振动模出现规律性的变化.进一步的分析证实这种Raman光谱的变化行为与晶体结构中由于氧含量的不同     

Anomalous critical behavior of NaV2O5

The temperature dependences and critical behavior of the dielectric constant were studied in NaV₂O₅ along the c axis in a frequency range of 1 MHz-1GHz and a temperature range of 4.2–300 K. An analysis of the data obtained, along with literature data on the heat capacity, magnetic losses, and the ultrasound velocity, indicates that the various physical quantities demonstrate similar temperature dependences in the vicinity of the transition, which corroborates the conclusion on the universality of the critical behavior in NaV₂O₅. Deviations from the predictions of the standard theory of second-order phase transitions were found, such as the asymmetry of the critical behavior above and below the transition and the presence of an anomalous base line.     

Strongly interacting superspins in Fe3O4 nanoparticles

In this paper we report structural and magnetic properties of Fe₃O₄ nanoparticles synthesized by thermal decomposition of ball milled iron nitrate and citric acid in N₂ and air ambient. The XRD pattern of samples which are prepared in air shows some impurity phases, while the samples synthesized in the N₂ atmosphere are almost pure Fe₃O₄ phase. The result shows that by increasing the particle size, the magnetization of the samples increases. The increase of magnetization by increasing the particle size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel -Brown model for superparamagnetic relaxation, but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass like phase in these samples.     

Flow equations and the Ruderman-Kittel-Kasuya-Yosida interaction

In the past different models for the magnetic salt vanadyl pyrophosphate (VO)₂P₂O₇ were discussed. Neither a spin ladder nor an alternating chain are capable to describe recently measured magnetic excitations. In this paper we propose a 2D model that fits better to experimental observations. Received: 29 May 1998 / Received in final form: 25 September 1998 / Accepted: 30 September 1998     

Superspin glass state in MnFe2O4 nanoparticles

Nanosized MnFe₂O₄ ferrites were synthesized by a simple method, which is based on the solid state ball-milling and calcinations of nitrate precursors and citric acid. The samples were characterized by using different methods. The results indicate that the products mainly consist of MnFe₂O₄ nanoparticles. The effect of different annealing temperatures on particle sizes and crystallinity of the samples was also studied. By increasing the particle size, the coercivity and magnetization of the samples increase. The increase of magnetization by increasing the crystallite size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. Our analysis of ac susceptibility measurements shows that the interparticle magnetic interaction leads to the superspin glass-like behavior in these nanoparticle samples.     

Dynamical properties of low-dimensional and spin-Peierls systems

Properties of low-dimensional spin-Peierls systems are described by using a one-dimensional S =1/2 antiferromagnetic Heisenberg chain linearly coupled to a single phonon mode of wave vector (whose contribution is expected to be dominant). By exact diagonalizations of small rings with up to 24 sites supplemented by a finite size scaling analysis, static and dynamical properties are investigated. Numerical evidences are given for a spontaneous discrete symmetry breaking towards a spin gapped phase with a frozen lattice dimerization. Special emphasis is put on the comparative study of the two inorganic spin-Peierls compounds CuGeO₃ and NaV₂O₅ and the model parameters are determined from a fit of the experimental spin gaps. We predict that the spin-phonon coupling is 2 or 3 times larger in NaV₂O₅ than in CuGeO₃. Inelastic neutron scattering spectra are calculated and similar results are found in the single phonon mode approximation and in the model including a static dimerization. In particular, the magnon S =1 branch is clearly separated from the continuum of triplet excitations by a finite gap. Received: 30 July 1997 / Revised: 16 September 1997 / Accepted: 10 October 1997