Jahn-Teller effect and the orbital ground state of Cu2+ ions in Eu2CuO4 and La2CuO4 crystals

Local ion displacements and their anharmonicity in Eu₂CuO₄ and La₂CuO₄ crystals are studied by high-precision x-ray diffractometry. A symmetry analysis and the temperature behavior of the probability density function of Cu²⁺ ions reveal a degenerate ground orbital state in the Eu₂CuO₄ crystal (tetragonal doublet d _xz, d _yz). The pattern and anharmonicity of the local displacements are found to be determined not only by lattice vibrations but by 2D antiferromagnetic spin fluctuations in the CuO₂ sheet as well. By contrast, in the La₂CuO₄ crystal the orbital ground state of the Cu²⁺ ion is a singlet (d _z 2). The pattern of Eu³⁺ local displacements and their effect on the properties of the Cu subsystem in the Eu₂CuO₄ crystal are also studied. Fiz. Tverd. Tela (St. Petersburg) 39, 1600–1608 (September 1997)     

The contribution to the specific heat from the spin-phonon interaction for (3d)n ions

The contribution, ΔC, to the specific heat arising from the spin-phonon interaction is calculated for general spin values. For a given set of spin-phonon coupling constants and cubic symmetry the ratio ΔC_max/ C(Schottky)_max for ions having S = 2 or $\text{5}\text{2}$ is one or two orders of magnitude greater than that for ions having S = 1 or $\text{3}\text{2}$. The relative low values of ΔC in the latter cases is traced to two distinct reasons, and the differences that the inclusion of a lower symmetry crystals field makes to the ratio is discussed.     

Magnetocrystalline symmetry, spin statics and dynamics in high-temperature superconductors

The symmetry analysis of the magnetic structures of La₂CuO₄ and YBa₂Cu₃O_6+δ (δ 0.4) antiferromagnetic oxide has been carried out. The spin configurations and spin-reorientation transition fields have been found. The frequencies of uniform spin oscillations and the external field dependence of their polarization properties have been calculated. It has been shown that the spin excitation branches may include exchange modes among which some dipole-active modes can be present. The spectrum of the inelastic light scattering accompanied with the excitation of spin and libration (tilting) freedom degrees has been discussed.     

Effect of asymmetry of CuO2 layers in copper oxides on the anomalous neutron scattering near T c

Reasons for critical magnetic scattering of neutrons near T _c in copper oxides with CuO₂ layers whose nearest environment has no “up-down” symmetry are discussed. The intracrystalline electric field, which threads the CuO₂ planes on account of the asymmetry, induces coupling between the spin and momentum of the current carriers. This coupling is shown to result in a manifestation of virtual Cooper pairs in the imaginary part of the spin susceptibility. Thus spin density fluctuations as well as current fluctuations should participate in the scattering. A way of experimentally distinguishing between the two mechanisms is pointed out. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 363–368 (10 March 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Magnetic excitations in antiferromagnetic Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

Magnetic excitations in the antiferromagnetic Bi₂CuO₄ (T _N =42K) are investigated on the basis of anisotropic exchange interaction between spins of Cu²⁺ ions. We calculate the dispersion curves and evaluate the intensity of the inelastic neutron scattering by spin wave excitations. Spin contraction at OK and the effect of spin wave interaction are studied.     

Magnetic resonance of Cu and of Gd in insulating GdSr<Subscript>2</Subscript>Cu<Subscript>2</Subscript>NbO<Subscript>8</Subscript> and in superconducting GdSr<Subscript>2</Subscript>Cu<Subscript>2</Subscript>RuO<Subscript>8</Subscript>

Weak-ferromagnetic (or antiferromagnetic) resonance of Cu and electron spin resonance (ESR) of Gd are observed both in insulating GdSr₂Cu₂NbO₈ and in superconducting GdSr₂Cu₂RuO₈. The Cu resonance implies that the CuO₂ planes are magnetic and indicates that the superconducting layer of GdSr₂Cu₂RuO₈ is SrO (not CuO₂), as in its related superconducting compound without cuprate planes, doped Sr₂YRuO₆.     

Jahn-Teller calculations for CuO4 and FeO4 clusters in the Nd1.85Ce0.15Cu0.99Fe0.01O4−δ

The Jahn-Teller distortion mechanism in the Nd_2?x Ce _x CuO₄ is analyzed. This analysis is based on a quasi-molecular model for the CuO₄ cluster havingD _4h symmetry. In order to compare with experimental Mössbauer measurements in the doped Nd_1.85Ce_0.15Cu_0.99Fe_0.01O_4?δ superconductor, the FeO₄ cluster is also analyzed for the case of Fe²⁺ in the high spin state. The results show not evidence of a Jahn-Teller effect in these superconductors.     

Nonlinear magnetic susceptibility and microwave spin dynamics of R2CuO4 quasi-2D antiferromagnets (R=Eu, Pr, Gd)

An experimental study of nonlinear ac magnetic susceptibility and microwave spin dynamics of R₂CuO₄ quasi-2D Heisenberg antiferromagnets (R=Eu, Pr, Gd) has been carried out. The data obtained can be accounted for if one assumes the existence of a random-field (RF) state in the Eu₂CuO₄ and Pr₂CuO₄ tetragonal crystals within the 77⩽T⩽350 K range covered. If this is so, 3D antiferromagnetic order persists only within limited regions, while in CuO₂ layers there are 2D Heisenberg antiferromagnetic spin fluctuations with large correlation lengths. In the Gd₂CuO₄ crystal there exists, besides uniform 3D antiferromagnetic long-range order with weak ferromagnetism, an admixture of an RF-type state. Fiz. Tverd. Tela (St. Petersburg) 41, 1437–1444 (August 1999)     

Interplay between the magnetic fluctuations and superconductivity in lanthanum cuprates

We report an analysis of the magnetic fluctuations in superconducting La_2?xSr_xCuO₄ and related lanthanum cuprates that have different symmetry of the low-temperature structure. NMR and ESR investigations revealed a dynamical coexistence of superconductivity and the antiferromagnetic correlations in most of the superconductivity region of the phase diagram. We show that, for all compounds, regardless of their low-temperature symmetry and their superconducting properties, the enhancement of the spin stiffness near 1/8 doping takes place.     

On the interpretation of the phase diagram of La<Subscript>2−x</Subscript>M<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

The difference between the phase diagrams of La_2?x(Ba,Sr)_xCuO₄ and Nd_2?xCe_xCuO₄ is discussed. It is proposed that the discrepancy of x-values corresponding to the transition from antiferromagnetic dielectric state to conducting one (respectively, x≈0.06 in La_2?x(Ba,Sr)_xCuO₄ and x≈0.125 in Nd_2?xCe_xCuO₄) results from non-homogeneous doping of La_2?x(Ba,Sr)_xCuO₄ over the range 0.06<x<0.125, when localized holes are added to each second CuO₂ layer. Therefore the actual phase diagram of the “holedoped” superconductor La_2?x(Ba,Sr)_xCuO₄ coincides with one for the “electron-doped” superconductor Nd_2?xCe_xCuO₄. It is shown that all features observed in La_2?x(Ba,Sr)_xCuO₄ around x=0.125 are clarified on this basis.     

Structural phase transition and structural properties in oxygenated La2CuO4+y; y ∼0.02

The crystal structure of the oxygenated La₂CuO_4+y; y ～0.02 has been studied by X-ray diffraction. It is found that the structural phase transition from tetragonal to orthorhombic symmetry occurring at about 450 K (Tc) is of second order and shows diffuse scattering above Tc. The diffuse scattering is related to static and/or dynamic short-range order of cooperative tilts of rigid-like elongated CuO₆ octahedra, which make two-dimensional networks stacked along the [001] axis. The structures at room temperature and 210K are determined using a 4-circle X-ray diffractometer and have the orthorhombic space group ¹⁰D_2h-Pccn. The structure is mainly characterized by anisotropic cooperative tilts of the CuO₄ octahedra around the [100]_p and [010]_p primitive axes in two-dimensional octahedron-networks, where suffix “p” means pseudo K₂NiF₄-type structure. The anisotropy of the tilting-angles results from pushing by excess oxygen atoms to apical oxygen atoms of the CuO₆ octahedra. It is also found by refinement of the structure that the excess oxygen atoms are randomly sited at the atomic coordinates (1/4,1/4,0.244) and/or (1/4, 1/4,0.279).     

The influence of the matrix element on angle-resolved photoemission spectra of insulating cuprates

Microscopic model calculations of the matrix element of a dipole moment are carried out in terms of the cluster model. This matrix element determines the transition probability of electron photoemission from a one-electron orbital with symmetry γμ to a free state. The effect of the matrix element on the angular and polarization dependences in the angle-resolved photoemission spectra of insulating cuprates, such as Sr₂CuO₂Cl₂ and Ca₂CuO₂Cl₂, is analyzed under the assumption of a well-isolated ground state of the two-hole CuO ₄ ^5? cluster, namely, the Zhang-Rice singlet. The angular k dependence of the matrix element gives rise to effects, such as the residual Fermi surface, which are typical of metallic systems. An analysis of the experimental data reveals the presence of another electronic state (with different symmetry) in the vicinity of the Zhang-Rice singlet.     

Electronic structure of pure and doped orthorhombic La2CuO4

The electronic structure of orthorhombic La₂CuO₄ has been investigated by first principles pseudofunction band calculations and group theoretical analysis. We find that pure as well as doped compounds remain metallic at all finite temperatures as a conseqence of the Cmca (D¹⁸_2h) space group symmetry. The experimentally observed rapid rise in resistivity below 30°K suggests a structural transition to a lower symmetry space group that could be driven electronically or magnetically. One possible candidate is monoclinic C2/m (C³_2h) which is a subgroup of Cmca and can be obtained by distorting the CuO bonds or rotating the CuO₆ octahedra. Implications for superconductivity are discussed.     

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd_2−x Ce _x CuO_4−y and a large single crystal of Nd₂CuO_4−y . We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La_2−x Sr _x CuO_4−y , although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd₂CuO_4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.     

Light scattering by spin excitations in copper-oxide-based materials

In this paper we present a study of spin dynamics in CuO, Bi₂CuO₄and CuGeO₃single crystals using Raman spectroscopy. The measurements of polarized Raman scattering spectra are performed in the temperature region from 10 to 300 K in various frequency ranges. We found and assigned the lines in the spectra that belong to magnetically ordered phases. The origin of these excitations, based on the temperature dependence of their energies, linewidths and symmetry arguments, is given. Also, we present calculations of two-magnon intensities, based on the densities of states, and compare them with experimental data.     

μSR on the novel heavy‐fermion system Nd2-yCeyCuO4

The magnetic correlations in Nd_2-yCe_yCuO₄ have been studied by zero and longitudinal field μSR. Nd_1.8Ce_0.2CuO₄ reveals a saturation of the muon relaxation rate below 0.5 K. Even down to 70 mK LF spectra evidence spin fluctuations in the dynamic regime with a rate of \sim 10⁹ s^-1 excluding long range magnetic order or spin glass freezing. The average Nd moment is estimated to be \approx 0.2\mu_B, i.e., strongly reduced from the value determined for the ground state Kramers doublet of Nd₂CuO₄. Extending former μSR measurements on Nd₂CuO₄, a gradual enhancement of the internal field has been detected below 10 K which is accompanied by an increase of the relaxation rate. These results are attributed to the development of ordered Nd moments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Theory for the Gaussian Component of the 63Cu Nuclear Spin-Echo Decay Rate 1/T 2G in La2−x Sr x CuO4

We report calculations of the Gaussian component of the ⁶³Cu nuclear spin-echo decay rate 1/T _2G employing the theory for spin susceptibility as derived within the t-J model starting from carrier-free La₂CuO₄ and right up to optimally doped superconducting layered copper oxide La_2?x Sr _x CuO₄. The theory reproduces the temperature and doping behavior of the experimental data for 1/T _2G. A quantitative agreement with experimental 1/T _2G data in doped La_2?x Sr _x CuO₄ compounds is obtained with account of both the spin–spin and “fermion”–“fermion” correlations.     

Magnetotransport of lanthanum doped RuSr<Subscript>2</Subscript>GdCu<Subscript>2</Subscript>O<Subscript>8</Subscript> – the role of gadolinium

Strongly underdoped RuSr_1.9La_0.1GdCu₂O₈ has been comprehensively studied by dc magnetization, microwave measurements, magnetoresistivity and Hall resistivity in fields up to 9 T and temperatures down to 1.75 K. Electron doping by La reduces the hole concentration in the CuO₂ planes and completely suppresses superconductivity. Microwave absorption, dc resistivity and ordinary Hall effect data indicate that the carrier concentration is reduced and a semiconductor-like temperature dependence is observed. Two magnetic ordering transitions are observed. The ruthenium sublattice orders antiferromagnetically at 155 K in low applied magnetic fields, and the gadolinium sublattice orders antiferromagnetically at 2.8 K. The magnetoresistivity in this compound exhibits a complicated temperature dependence due to the occurence of the two magnetic orders and spin fluctuations. It is shown that the ruthenium magnetism influences the conductivity in the RuO₂ layers while the gadolinium magnetism influences the conductivity in the CuO₂ layers. The magnetoresistivity is isotropic above 4 K, but it becomes anisotropic close to the gadolinium antiferromagnetic order temperature.