Stripes and superconductivity in cuprates

Holes doped into the CuO₂ planes of cuprate parent compounds frustrate the antiferromagnetic order. The development of spin and charge stripes provides a compromise between the competing magnetic and kinetic energies. Static stripe order has been observed only in certain particular compounds, but there are signatures which suggest that dynamic stripe correlations are common in the cuprates. Though stripe order is bad for superconducting phase coherence, stripes are compatible with strong pairing. Ironically, magnetic-field-induced stripe order appears to enhance the stability of superconducting order within the planes.     

A two-band electron-phonon model for superconductivity in graphite intercalation compounds

A two-band electron-phonon model for superconductivity in graphite intercalation compounds is developed. A new mechanism for the relaxation times due to interband scattering of intraband pairs is proposed for a superconductor with a two-component order parameter. Two distinct relaxation times τ₁ and τ₂ of the order parameters are predicted for C₆K and C₈K.     

Analysis of the conditions favoring the onset of long-range order in disordered relaxors of different types

The possibility of realizing a stable relaxor behavior in PbSc_1/2Nb_1/2O₃ (PSN) compounds and the conditions favoring the formation of a phase with long-range order in PbZn_1/3Nb_2/3O₃ (PZN) are studied using optical methods. It is found that the replacement of only 6% of Pb ions with Ba ions (PBSN-6) freezes the relaxor state in PSN compounds, while adding even 7% of PbTiO₃ to the PZN relaxor (PZN-7PT) leads to the formation of a phase with long-range order and a phase transition occurring at T _maxε through the percolation mechanism. Common features in the formation and destruction of the long-range order phase in relaxors of different types are revealed. The differences associated with the presence of spontaneously polarized ferroelectric regions in the cubic nonpolar matrix of PSN-type compounds are determined.     

Surface composition of graphite intercalation compounds

We present Auger spectroscopy measurements on graphite intercalation compounds, for donor (cesium) and for acceptor (MnCl₂) intercalants. The surface composition of both compounds is steeply different from their bulk composition. In the case of high stage cesium compounds, a segregation of the intercalant towards the surface layers is observed, leading to an increased concentration between the first two graphite layers (CsC₈ or CsC₆). On the opposite, an intercalant depletion between these first layers is observed for the first stage (MnCl₂)C₆ compounds.Both effects are explained by the screening of surface states, which involves an electrostatic energy of the same order of magnitude as the intercalant chemical potential. This is thus sufficient to allow the local intercalant concentration to change.     

Probabilities of octahedral clusters depending on long-range order parameters and composition in nonstoichiometric titanium monoxide TiO y

A method for calculating the probabilities of cluster configurations in ordered superstructures of strongly nonstoichiometric compounds depending on the composition and the order parameter is described using the Ti₅O₅ superstructure of nonstoichiometric titanium monoxide TiO _y . Analytic expressions are derived for the dependences of the probabilities of the main cluster configurations in the Ti₅O₅ superstructure on the fraction of atomic positions in the titanium and oxygen sublattices and on the long-range order parameter. The probabilities of configurations are calculated for various long-range order parameters taking into account experimental data on the concentration of structural vacancies in the titanium and oxygen sublattices. The dependences of the probabilities of cluster configurations on the short-range order parameter are established from the relations between the superstructure long-range and short-range order parameters.     

Magnetization processes caused by iron substitution in cubic laves phases

The magnetic properties of cubic Laves phases RE(Fe, M)₂, M = Al, Co, Rh are discussed. Pauli-paramagnetism is obtained for all three YM₂ compounds. The appearance of a percolation limit for the onset of long-range ferromagnetic order which depends strongly on the matrix susceptibility is characteristic for all investigated series Y (Fe, M)₂. Mictomagnetic behaviour can be deduced from magnetic, Mössbauer- and neutron depolarization measurements below the percolation limit. A Co moment of approximately 1 μ_B is induced in RECo₂ compounds, RE = Gd, Dy, Ho. On the contrary Rh has no magnetic moment in these compounds. Within the RE(Fe, Co)₂ series a Co moment is induced by both the RE and Fe atoms, whereas within RE(Fe, Rh)₂ the Rh atoms will only be influenced by the substituted Fe. In all compounds the spin of the heavy RE atoms couples antiparallel to the spin of the transition metal atoms. Local environment effects are important for all Fe containing series whereas the properties of RECo₂ compounds evidence itinerant electron metamagnetism.     

Magnetization and AC susceptibility of TbxY1-xCo2 compounds

Magnetization and ac susceptibility measurements have been performed on Tb_xY_1-xCo₂ compounds. Samples with x > 0.1 order ferromagnetically below room temperature. The cobalt magnetic moments in these compounds are induced by the internal magnetic field exerted by the terbium moments on the cobalt atoms. The ac susceptibility measurements indicate a change from second order to first order in the ferromagnetic transition for samples with x-values equal or smaller than 0.5.     

Gigantic stimulated Raman scattering in one-dimensional Mott-Hubbard insulators: A possible THz source

One-dimensional Mott-Hubbard insulators like Sr₂CuO₃, halogen-bridged Nickel chain compounds have orders-of-magnitude nonlinear optical properties compared to other one-dimensional organic or inorganic compounds. We show theoretically, that the stimulated Raman scattering susceptibility for such insulators could be order(s)-of-magnitude larger even compared to other nonlinear optical susceptibilities. The lowest two-photon state is at lower energy than the lowest one-photon state in some of these insulators. This leads to a potential for strong Stokes generation in the THz regime from these compounds. Our results and conclusions are based on exact numerical solution of finite size two-band extended Hubbard model.     

An EXAFS study of interionic distances in complex lanthanide oxides

When the smaller vanadium ion is substituted for niobium in NdNbO₄, the volume of the unit cell is almost unchanged; in the lanthanum and cerium counterparts the volume actually increases. In order to understand these phenomena we have employed the technique of extended x-ray absorption fine structure (EXAFS) to determine all of the near neighbor anion-cation distances in the series of mixed-B-ion oxides, NdNb_1?xV_xO₄, including the end-member compounds, NdNbO₄ and NdVO₄. We find that the average Nb-O, V-O and Nd-O interionic distances in the mixed compounds closely resemble the distances exhibited in the corresponding end-member compounds, but there are large variances associated with these averages. We take this to mean that, while crystal symmetry is not broken by the substitution of the smaller V⁵⁺ ion, there are substantial local distortions in bond lengths and angles in order to meet the ion-size requirements of immediate neighbors.     

Third order elastic constants of V3Si and V3Ge

General expressions have been derived for the second order elastic constants and third order elastic constants of the A-15 structure compounds with the nearest neighbour central interaction. The second order elastic constants, the third order elastic constants and the pressure derivatives of the second order elastic constants of V₃Si and V₃Ge are reported and compared with the available experimental measurements.     

YTi(Fe1-xNix)11的晶体结构和内禀磁性

当x<0.7时,YTi(Fe_1-xNi_x)₁₁可形成单相的ThMn₁₂型四方结构,空间群14/mmm。本文研究了以Ni原子代换Fe原子时,对饱和磁矩、磁晶各向异性和居里温度的影响。通过代换研究,并讨论了Fe原子和Ni原子在稀土化合物中所表现的两种不同的特性。稀土-铁(R-Fe)金属间化合物的磁性依赖于Fe-Fe近邻原子的间距和数目;而R-Ni金属间化合物的磁性取决于稀土金属传导电子对Ni的3d能带的影响。为进一步确证这 关键词：     

Internal energy and parameters of the order-disorder phase transition in titanium monoxide TiO y

Quantum-mechanical ab initio calculations are used to simulate the free energy functions for titanium monoxide TiO _y . The effect of the long-range order of the Ti₅O₅ type superstructure on the internal energy of the compound is studied by the supercell method. The dependences of the configuration entropy and free energy on the long-range order parameter are determined. It is found that the order-disorder phase transition in titanium monoxide must occur in accordance with the mechanism of the first-order phase transition with a critical value of the long-range order parameter of 0.971. The calculated parameters of the phase transition are compared with the experimental data and the results obtained using the model of point charges and by calculating the Madelung energy. It is concluded that the short-range order and the phonon entropy must be taken into account in calculating the equilibrium phase diagrams for strongly nonstoichiometric compounds.     

First-principles study on the crystal, electronic structure and mechanical properties of hexagonal Al3RE (RE = La, Ce, Pr, Nd, Sm, Gd) intermetallic compounds

The structural properties, elastic properties and electronic structures of hexagonal Al₃RE intermetallic compounds are calculated by using first-principles calculations based on density functional theory. Since there exists strong on-site Coulomb repulsion between the highly localized 4f electrons of RE atoms, we present a combination of the GGA and the LSDA+U approaches in order to obtain the appropriate results. The GGA calculated lattice constants for the hexagonal Al₃RE intermetallic compounds are in good agreement with available experimental values. The results of cohesive energy indicate that these compounds can be stable under absolute zero Kelvin and the stability of Al₃Gd is the strongest in all of the hexagonal Al₃RE compounds. The densities of states for GGA and LSDA+U approaches are also obtained for the Al₃RE intermetallic compounds. The mechanical properties are calculated from the GGA method in this paper. According to the computed single crystal elastic constants, Al₃La, Al₃Sm and Al₃Gd are mechanically unstable, while Al₃Ce, Al₃Pr and Al₃Nd are stable. The polycrystalline elastic modulus and Poisson’s ratio have been deduced by using Voigt-Reuss-Hill (VRH) approximations, and the calculated ratio of bulk modulus to shear modulus indicates that Al₃La compound is ductile material, but Al₃Ce, Al₃Pr, Al₃Nd, Al₃Sm and Al₃Gd are brittle materials.     

LiBX2 (B=Ga, In; X= S, Se, Te)光学性质与力学性质的第一性原理计算

采用基于第一性原理的赝势平面波方法系统地计算了LiBX₂ (B= Ga, In; X= S,Se,Te) 晶体的光学性质与力学性质. 由禁带宽度推断出晶体抗激光损伤阈值的大小顺序为LiGaS₂ > LiInS₂ > LiGaSe₂ > LiInSe₂ > LiGaTe₂ > LiInTe₂. 六种晶体在常压下均满足机械力学稳定性要求, 且铟化合物可塑性及延展性强于镓化合物. 这些晶体的静态电介电常数 ε₁(0)、静态折射率n(0)和双折射率Δn 理论计算值与实验值相符. LiGaS₂, LiInS₂, LiGaSe₂, LiInSe₂和LiGaTe₂五种化合物双折射率较高, 并且它们的吸收谱与反射谱在中远红外区是透过的, 因此可推断出这五种化合物可以成为优异的中远红外非线性光学材料.     

Generalized Fulde-Ferrell-Larkin-Ovchinnikov state in heavy-fermion and intermediate-valence systems

Anomalous properties of the magnetization, the crystal dilatation, and other physical quantities observed in the mixed state of compounds such as UPd₂Al₃ and CeRu₂ suggest the occurrence of a new state near H _c2. In such compounds with large spin susceptibility and high H _c2, a new inhomogeneous superconducting state is theoretically predicted to occur in high magnetic fields. In this inhomogeneous state which is shown to be a generalized Fulde-Ferrell-Larkin-Ovchinnikov state, the order parameter is spatially modulated, and planar nodes of the order parameter are periodically aligned perpendicular to the vortices. Various theoretical predictions are compared with experimental results on UPd₂Al₃ and CeRu₂ related to this new superconducting high-field state.     

Magnetism and hyperfine interactions in EuM2Ge2 and GdM2Ge2(M = Mn,Fe, Co,Ni, Cu)

X-ray, magnetic susceptibility and ¹⁵¹Eu, ¹⁵⁵Gd Mössbauer effect studies of EuM₂Ge₂ and GdM₂Ge₂ were performed. All compounds crystallize in the ThCr₂Si₂ body centered tetragonal structure. In all compounds, except those with M = Mn and in EuM₂Ge₂, the M component carries no magnetic moment. All compounds except those with Mn are antiferromagnetic at low temperatures. In EuMn₂Ge₂ the Mn moments order ferromagnetically at 330 K and change to antiferromagnetic order when the Eu moments order ferromagnetically (9 K). This behaviour is different from that in GdMn₂Ge₂, where the Mn sublattice orders antiferromagnetically at 365 K and becomes ferromagnetic and antiparallel to the ferromagnetic Gd sublattice at 96 K. The Mössbauer studies of ¹⁵¹Eu and ¹⁵¹Gd provide values for the magnetic hyperfine fields, the quadrupole interactions and the orientation of the magnetic moments relative to the local fourfold axis (c-axis). It turns out that in the Eu compounds the easy axis of magnetization is close to the c-axis, while in the Gd compounds it is in the basal plane. In all systems, excluding those with Mn, the interatomic rare earth-rare earth distances have the dominant effect on the conduction electron charge density and polarization at the rare earth site and on the Curie point.     

Temperature induced valence instabilities in ternary Eu-pnictides: a comprehensive view

We report on measurements of the lattice constants, magnetic susceptibility, L_III X-ray absorption and Mössbauer-effect on EuNiP and EuPdP, which crystallize in the hexagonal layered Ni₂In structure. In both compounds the Eu valence above 510 K is 2.33. With decreasing temperature they show one (EuPdP) or two (EuNiP) first order phase transitions with a valence increase of about 0.16. At the same time thec-axis shrinks while the a-axis even increases. From Mössbauer measurements we show that the nature of the valence mixing is static. In contrast, the valence mixing in isostructural EuPtP is static at low temperatures, too, but it becomes homogeneous valent above a first order phase transition at 235 K. The behaviour of these compounds (as well as that of EuPdAs) is explained in a new model of electrostatically charged layers. In this model we can explain the temperature dependence of the lattice constants, the static valence mixing and the occurrence of preferred valences of the order 2 6/n. Together with the compression shift model of Hirst we can also understand the mechanism of the phase transitions. A comparison with EuT₂Si₂ compounds in ThCr₂Si₂ structure shows that in EuTX compounds only the electronic structure of the transition elements is relevant for the occurrence of mixed valency.     

Magnetic structure and physical properties of the multiferroic compound PrMn2O5

RMn₂O₅ (R=lanthanide, Bi, Y) multiferroic compounds are intensively studied for their potential application in the spintronic field. In these systems, the key issue is to understand the origin of the strong coupling between the ferroelectric and magnetic orders and to investigate the influence of the nature of the R ions in this coupling. While the phase diagram of RMn₂O₅ compounds with small R size is well established, this of large R size compounds is missing due to the lack of samples originating with difficulties of synthesis. We present in this paper the first investigation of the thermodynamic, structural and magnetic properties of high quality polycrystalline PrMn₂O₅ samples. Our work shows that PrMn₂O₅ presents two magnetic transitions corresponding to commensurate magnetic orderings. We also evidence a weak lattice effect coupled to the magnetic order. Our results point out that the physical properties of PrMn₂O₅ differ from those of the parent compounds with magnetic R ions.     

Magnetic neutron diffraction study of UGA3 and UGA2

The magnetic ordering of the intermetallic compounds UGa₃ and UGa₂ has been examined by time-of-flight neutron diffraction, and the nature of the magnetic order of these compounds is confirmed.