Workshop on fast structural changes

Copper-oxide (cuprate) high-temperature superconductors are doped Mott insulators. The undoped parent compounds are antiferromagnetic insulators, and superconductivity occurs only when an appropriate number of charge carriers (electrons or holes) are introduced by doping. All cuprate materials contain CuO₂ planes (Figure 1a) in their crystal structure; the doped carriers are believed to go into these CuO₂ planes, which are responsible for high-temperature superconductivity. High-temperature superconductors are characterized by their unusual physical properties, both in the superconducting state (below the superconducting transition temperature T_c) and in the normal state (above T_c). Since the discovery of high-temperature superconductivity in 1986 [1], these unusual physical properties and the mechanism of superconductivity have been prominent issues in condensed matter physics [2].     

磁性离子Fe和Ni替代YBCO体系的结构特征和载流子局域化

为阐明磁性离子在不同替代位置对YBCO体系超导电性的影响机制，利用正电子湮没及相关实验手段结合数值模拟，系统研究了Fe和Ni掺杂的YBa₂Cu₃O_7-δ体系. 结果表明，Fe和Ni离子在替代过程中均以离子团簇的形式进入晶格. 当离子进入CuO₂面时，由于团簇改变了周围的电子结构，造成电子的局域化，并直接影响了电子对的配对和输运，因而强烈抑制了体系的超导电性.而当掺杂离子进入Cu-O链区时，它们同样通过团簇的形式改变周围 关键词： YBCO超导体 磁性离子替代 正电子湮没 数值模拟     

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.     

The role of doping and pressure in Hg based high Tc cuprates: A theoretical study

We present a series of first-principles calculations for Hg based high T_c cuprates investigating the effect of pressure, doping, and composition on the electronic and crystalline structure. In particular, the total and site-projected hole concentration in the CuO₂ planes and the density of states are studied in detail. We discuss effects of inhomogeneity introduced by doping and the limitations on creating holes by either doping, pressure, or the number of CuO₂ layers per unit cell. From an analysis and comparison of our results to available experimental data on the pressure dependence of T_c, we conclude that the effective coupling constant to the boson mediating the Cooper pairing is of the order of 1 ruling out the weak coupling approaches.     

An X-ray absorption study of the electric field effect mechanism in “123” cuprates

X-ray absorption spectroscopy in the presence of an electric field has been used to study the doping mechanism of the CuO₂ planes in Nd_1+xBa_2-xCu₃O₇ compounds. The electric field effect doping is well-known as a method to modify the electrical properties of a thin film using an external gate voltage and in the copper based high critical temperature superconductors (HTS), it has been used to shift the critical temperature and even to induce phase transitions. Field effect experiments in ultra-thin HTS are usually interpreted by supposing that the induced charges develop into carriers in the CuO₂ conducting planes, thus changing the filling of the Zhang-Rice (ZR) band. Here we show that the polarization charges in both insulating and superconducting films, are mainly confined in the charge reservoir, and in particular in the CuO chains. The characteristics of the charge reservoir layer determine the doping of the CuO₂ planes, achieved by transfer of a fraction of the total injected charges. Moreover we found that holes injection in the CuO₂ planes is reduced in oxygen deficient NdBCO films.     

In-plane polarized collective modes in detwinned YBa2Cu3O6.95 observed by spectral ellipsometry

The in-plane dielectric response of detwinned YBa₂Cu₃O_6.95 has been studied by far-infared ellipsometry. A surprisingly large number of in-plane polarized modes are observed. Some of them correspond to pure phonon modes. Others possess a large electronic contribution which strongly increases in the superconducting state. The free carrier response and the collective modes exhibit a pronounced a–b anisotropy. We discuss our results in terms of a charge density wave state in the 1-D CuO chains and induced charge density fluctuations within the 2-D CuO₂ planes.     

Low-energy excitations and stripes in superconducting cuprate La1.87Sr0.13CuO4

The conductivity and dielectric permittivity spectra of single-crystalline La_1.87Sr_0.13CuO₄ are directly measured with the electric field polarized perpendicular to the CuO planes (E∥c) covering the frequency range 10-40 cm⁻¹ and temperatures 5-300 K. We observe in the superconducting state a well pronounced excitation with strongly temperature dependent parameters. We suggest that the excitation is caused by the transverse Josephson plasma mode that appears due to the different strengths of Josephson coupling between the superconducting charge stripes in the neighboring and next-nearest neighboring copper-oxygen planes of La_1.87Sr_0.13CuO₄. A strongly enhanced low-frequency (below 15 cm⁻¹) absorption is seen in the superconducting state that is assigned to delocalized quasiparticles of as yet unknown origin.     

The anomalous anisotropy in the ac susceptibility of La1.45Nd0.4Sr0.15CuO4 single crystal

The dependencies of ac susceptibility on the superimposed dc magnetic fields for the stripe-ordered La_1.45Nd_0.4Sr_0.15CuO₄ single crystal with two superconducting transitions have been studied in the 3D and 2D superconducting ranges for the fields along different orientations of the crystal. The results show that with increasing fields the interlayer Josephson coupling and the in-plane superconductivity are suppressed orderly for the fields perpendicular to CuO₂ planes. The influences of the field parallel to CuO₂ planes on the 3D and 2D superconductivity are much weaker than those of the field parallel to the c-axis. The irreversibility lines for the 3D and 2D superconducting states are also studied.     

Polarons in thet-J model

A convenient form of the Peierls-Hubbard Hamiltonian is obtained for the case when the Hubbard repulsion is the largest energy parameter. It allows to consider in the spin-wave approximation the properties of the one-hole low-lying excitations of a 2d lattice. For the parameters approximately corresponding to La₂CuO₄ it is shown that the hole polarons in the CuO₂ planes of lightly doped samples are of large size with a solitonlike-shaped highly asymmetric wave function oriented along the diagonals of the planes or of small size depending on the value of the electron-phonon coupling. In both cases the cooperative effect of the electron-phonon and electron-magnon interactions leads to a large effective mass and to hopping transport of the excitations, with preferential jumps along the diagonals in the former case and rotationally symmetric in the latter. For hoping matrix elements which are small in comparison with a phonon quantum the competition between the interactions leads to the decrease of the total spin in the ground state with increasing electron-phonon coupling.     

Anisotropic transport and optical properties of superconducting (La1-xSrx)2CuO4 single crystal thin films

An experimental study on the resistivity, Hall coefficient, upper critical field, and optical reflection and transmission characteristics is described for single-crystalline superconducting (La_1-xSr_x)₂CuO₄ thin films epitaxially grown on SrTiO₃ single crystal substrates by rf magnetron sputtering. These properties exhibit pronounced anisotropy between the c-axis direction and the Cu-O plane. The characteristics are consistently explained from the point of view of the two-diemensional (2D) electronic structure. The most noteworthy implication of the experimental results is that the transport along the c-axis is far from an itinerant picture. This leads to the idea that the superconductivity in the series of cuprate materials is caused by the 2D electronic system arising from the Cu-O planes, and some interactions between them, which is not possibly due to itinerant carriers.     

How Tc can go above 100 K in the YBCO family

We report the results of the electronic structure calculation of a newly discovered member of the YBCO high-T_c family, i.e., Y₃Ba₅Cu₈O₁₈ (Y358) with T_c>100, based on the full-potential linearized augmented plane waves method (FP-LAPW) of density functional theory in the generalized gradient approximation (GGA). The evolution of the number of hole carriers in different sites of the CuO₂ planes and CuO chains has been investigated in comparison with the other YBCO family members, i.e., Y123, Pr123, Y124, and Y247. The results suggest that pumping hole carriers out of the chains toward the planes enhances the transition temperature. The band structure calculations have been performed for Y358, and the results show similar features with the other family members. Most notably, a van Hove singularity forms near the X point of the Brillouin zone below the Fermi level and within the energy of the buckling phonon mode, for which the interplay is discussed.     

A model for layered high-temperature superconductors with two CuO2 layers per unit cell

We extend a model for layered high-temperature superconductors to systems with two CuO₂ layers per unit cell and two interlayer spacings with different physical properties. The carriers are assumed to occupy Fermi liquid states, forming narrow tight-binding bands. The layers are coupled by weak interlayer-hopping matrix elements between adjacent sheets, as well as by an attractive interaction between carriers in neighboring layers in addition to an on-site intralayer coupling. We solve the Gorkov equations for this model to obtain the critical temperature and the density of states of the oneparticle excitations from the superconducting condensate, and discuss various parameter regimes concerning the coupling between the two layers. We compare our results with current experimental findings for high-temperature superconductors. The presence of two CuO₂ layers leads to multi-peak features in the superconducting density of states, as has been observed in recent tunneling measurements.     

原子簇La8-xBaxCuO6的原子磁矩和自旋极化的电子结构研究

利用MS-Xα方法研究了化合物La_2-yBa_yCuO₄的原子磁矩和自旋极化的电子结构.理论计算得到母相氧化物La₂CuO₄的Cu原子磁矩为0.37μ_B，与实验值0.48±0.15μ_B基本一致. 研究结果显示, 由于Ba原子对部分La的替代，使构成化合物的基本原子簇La_8-xBa_xCuO₆的 关键词： 电子结构 自旋极化 磁矩 态密度 超导电性     

Low energy electronic spectroscopy of an infinite-layer cuprate: A resonant inelastic X-ray scattering study of CaCuO2

We report the results of Oxygen K-edge soft X-ray absorption and emission spectroscopy that was performed on an infinite-layer insulating cuprate thin film CaCuO₂. Experimentally obtained spectra are consistent with local density approximation calculations. X-ray absorption spectra show a close resemblance to spectra obtained from homologous single crystal cuprates. In addition to d–d excitations, X-ray emission spectra reveal the presence of Zhang-Rice singlet states in the infinite-layer CuO₂ planes. The question of whether the Zhang-Rice singlet features are masked by the O 2p main-band is addressed: it is possible to quantify the position of the Zhang-Rice singlet using emission intensity profiles. X-ray emission is demonstrated as a tool for understanding CuO₂ planar electronic correlation in the prototypical infinite-layer. The energy difference, 2.0 eV, between the oxygen main-band and the Zhang-Rice singlet band is found to match values obtained theoretically using established planar electronic correlation parameters.     

The electronic structure of grain-boundary of YBa2Cu3O7 doped with 3d transition-metal atoms

We systemically study the electronic structures of Σ5 grain-boundary of YBa₂Cu₃O₇ with and without dopants of 3d transition-metal atoms based on the density functional theory. The partial density of states (PDOS) shows that the electronic structure of grain boundary is very different from that of crystal and the interesting orbital-reconstruction of interface is found. Generally super-current will significantly decrease when transmitting across grain boundary. The main reasons for suppressed super-current are that (1) the carriers are not uniformly distributed near grain-boundary regions and (2) the number of CuO₄-squares in CuO₂ layer, which are essentially important to transport properties, sharply decreases near grain-boundary region. The preferentially substituting sites of 3d transition-metal atoms in YBa₂Cu₃O₇ are predicted and some of them such as Co, Ni and Zn are consistent with the reported experimental analysis.     

Valence fluctuations in La2−xSrxCuO4

Ab-initio electronic band structure calculations are presented for the perovskite La₂CuO₄ and for this material doped with Sr for a supercell of composition La₃SrCu₂O₈. This material is close to the high Tc superconductor La_2−xSr_xCuO₄ discovered recently. The Sr doping gives rise to strong valence fluctuations. We discuss the effect of the valence fluctuations on the stability of the lattice, indicating a small value of U and enhancing the electron-phonon coupling λ, mainly by a mechanism of incipient peroxide formation.     

Synthesis and characterization of the infinite-layer superconductor Sr0.9La0.1CuO2

We report the high-pressure synthesis of the electron-doped infinite-layer superconductor Sr_0.9La_0.1CuO₂. A Rietveld analysis using X-ray powder diffraction data showed that, within the resolution of the measurement, the sample was purely an infinite-layer structure without any discernible impurities. The superconducting volume fraction and the transition width were greatly improved compared to those in the previous reports. Also the irreversibility field line was much higher than that of (La,Sr)₂CuO₄. The higher value seems to originate from the strong interlayer coupling due to the reduced average distance between the CuO₂ planes.     

Effect of ordering of mobile oxygen on spectra of two-magnon and electronic Raman scattering of light in YBa2Cu3O6+x crystals with different doping levels

The effect of ordering of mobile chain-site oxygen in YBa₂Cu₃O_6+x crystals at different doping levels x on the kinetics of the intensity change of the two-magnon line and the extended structureless electronic continuum in optical Raman spectra and on the superconducting transition temperature T _c , has been studied in detail. An increase in the chain-site oxygen content x leads to a higher contribution of free carriers to the electronic continuum in Raman spectra. The kinetics of the electronic continuum becomes slower with x, whereas the relaxation rate of the two-magnon scattering is a nonmonotonic function of the stoichiometric index. Computer simulations of the relaxation of nonequilibrium states using the Monte Carlo technique qualitatively describe the kinetics observed in experiments. Our results lead us to a conclusion about local inhomogeneities in the electronic and spin systems in CuO₂ planes with scales of several lattice constants. Zh. éksp. Teor. Fiz. 116, 684–703 (August 1999)     

Centers of charge nonuniformity and reduction of copper oxide CuO during irradiation with nitrogen ions

Long-range reduction of CuO to Cu₂O and Cu was observed by irradiating polycrystals and different planes of CuO single crystals ((110) and (020)) with 16-MeV nitrogen ions with fluence 10¹⁷ cm⁻². The infrared absorption spectra show an increase in the number of hole [CuO₄]⁵⁻ and electronic [CuO₄]⁷⁻ centers. The highest density of electronic centers and reduction occur near the surfaces of the samples. Fiz. Tverd. Tela (St. Petersburg) 41, 1564–1567 (September 1999)     

Location of superconductivity in La2-βSrβCuO4

Evidence is presented that the superconductivity of La_{2 ?β} Sr_β CuO₄ and La₂CuO4_+δ occurs via holes in the LaO planes, rather than in the CuO₂ planes.