No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Magnetic and superconducting properties of doped (Sr,Ca)10Cu17O29-type single crystals
Abstract
Magnetization, resistivity and electron spin resonance (ESR) measurements have been performed on single crystals of A10Cu17O29 (A=Ca5.9, Sr3.5, Bi0.3, Pb0.1, Y0.1, Al0.1) of the S=1/2 quasi-one-dimensional system, which has both simple chains and two-leg ladders of copper ions. Substantial hole doping has been achieved in the studied crystals, which led to superconductivity with a high critical temperature (Tc≈80 K). The values of the penetration depth have been estimated for temperatures in the range 30–60 K using the reversible magnetization data. A rough estimation of the Ginzburg–Landau parameter, κ, indicates that the superconductivity in the investigated ladder material should be described as an extreme type-II limit. It has been suggested that the superconductivity in the studied system should be related to the two-leg ladders rather than to the chains.
... The highest superconducting transition temperature of about 80 K was observed in A Cu O (m/n" ) [12]. In A Cu O (m/n" ) the superconductivity with ¹ "10 K appeared only under hydrostatic pressure [13]. ...
... Cu O , doped with Y, Bi, Pb and Al, focusing on the temperature dependence of the intensity of the absorption line, as well as on the temperature dependencies of the resonance "eld and the line width. Superconducting properties of this system have been presented elsewhere [12]. ...
... They were grown from a melt using a self-#ux method [8]. (Details concerning the decanting technique and the crystal quality have been described elsewhere [8,12].) ...
We report on electron paramagnetic resonance (EPR) measurements on the quasi-one-dimensional system (Sr,Ca)10Cu17O29, doped with Y, Bi, Pb and Al, which has simple chains and two-leg ladder chains of copper ions. A very broad absorption line, coming from the ladder chains, dominates near room temperature. The EPR signals observed below room temperature mainly come from the simple (dimerized) chains. A detailed analysis of the temperature dependencies of the resonance field, line width and intensity of EPR signal is presented. EPR spectra of Cu2+ ions located in chains are described in terms of Wannier excitons. It is also found that at low temperatures the EPR line experiences a sharp broadening and shifts to the lower field region. These findings are discussed by taking into account two different mechanisms: the magnetic dipole interactions among short-range ordered Cu2+ spins in the chains and the exchange interaction between Cu2+ ions located in chains and holes (“bottleneck effect”).
... It is well known that the Bi2Sr2CaCu2O8 + δ phase has also T, close to 80 K and consists of the same elements. Nevertheless, the magnetization measurements have shown that at least 10% of crystal volume exhibits the transition to superconducting state at the temperature of about 80 K [10]. It is distinctly larger than a possible volume of Bi2Sr2CaCu2O8+δ phase, which follows from the chemical composition. ...
Transmission electron microscopy and X-ray diffraction proved chain/ ladder incommensurate single crystal structure of investigated samples. The incommensurate ratio was determined from the X-ray and electron diffraction being equal to 0.704. Diffuse scattering intensities localised on the planes perpendicular to the c*-axis and passing through the spots originating from the periodicity of chain sublattice were detected. High-angle grain boundary or twinning formed by rotation of 33.3° around [100] direction was observed. High-resolution electron microscopy images revealed the stacking faults in ac planes.
... The anisotropy of the susceptibility observed in these samples (especially in a higher temperature range for Τ > 100 K) is typical of all compounds of the studied family in which only Cu is characterized by the magnetic moment. The character of anisotropy does not depend on nonmagnetic substitutions in A-sites, even for such which can lead to magnetic order at low temperature [12] or to a significant change of transport properties (even to superconductivity [13][14][15]). ...
... The compound A10Cu17O29 is a member of larger family (A 2 Cu2 O3 )m (CuO 2 )n with m/n = 5/7, 7/10, and 9/13 [4]. Recently, the superconductivity observations in this compound have been reported with the critical temperature of about 80 K [5,6]. Superconductivity under pressure was also found in 7/10 compound [7]. ...
Polarized Raman spectra of high temperature superconducting single crystals of A10Cu17O29 (A10 = Ca4.7Sr4.7Sr4.1Bi0.3) were studied in various scattering configurations in the range of 40-700 cm-1. In very distinctive spectra there were found over 20 peaks. It was observed that the flat continuum of electronic excitations in the normal state was redistributed below the critical temperature. The frequency dependent redistribution is consistent with the value of energy gap estimated using tunnelling spectroscopy techniques.
... The superconducting and nonsuperconducting single crys- tals with ladder-type structure were obtained by the flux method using Bi 2 CuO 4 and BaCuO 2 as fluxes. The flux method was chosen because it allows one to adjust easily the crystal growth parameters. ...
We performed tunneling experiments on single crystals of hole-doped
two-leg ladders (SrCa)10Cu17O29 with
Tc≅75 K and, for reference purposes, on textured bulk
samples of Bi2Sr2CaCu2O8
(Bi2212). Point-contact and break-junction (Bi2212 only) junction
configurations were used. The point-contact junctions, both
superconductor-insulator-normal-metal and superconductor-normal-metal,
were using Ag counterelectrodes. In all experiments the tunneling
direction was along Cu2O3 planes (for the ladders)
and CuO2 planes (for Bi2212). The recorded tunneling
conductance spectra were reproducible with sharp, well-defined
energy-gap structures. All junctions exhibited a pronounced zero-bias
conductance peak disappearing at the critical temperature. Experimental
results are consistent with the formation of zero-energy Andreev bound
states at the surface of the superconductors and
dx2-y2-wave symmetry of the order
parameter Δ in both compounds.
... Kato et al. [5], Szymczak et al. [6], Shvanskaya et al. [7] grew single crystals of doped (Sr,Ca) 10 Cu 17 O 29 and determined their crystal structures in three-dimensional space (space group F 222). In the present work, the crystal structure of Bi-doped (Sr,Ca) 10 Cu 17 O 29 spin-ladder compound, representative of the [M 2 Cu 2 O 3 ] m [CuO 2 ] n series with m = 5 and n = 7, is refined in three-and four-dimensional space and discussed. ...
The crystal structure of the (Bi(0.02)Sr(0.39)Ca(0.59))(10)Cu(17)O(29) spin-ladder compound was determined from single-crystal X-ray diffraction data. The precursor powder was prepared from stoichiometric amounts of SrCO(3), CaCO(3), CuO, and Bi(2)O(3) by calcination at 820 degrees C for 20 h and pressed to form a rod used for the crystal growth. The travelling-solvent-floating-zone method was applied in an image furnace to grow large single crystals. The composite commensurate structure was solved and refined in space group F222, Pearson symbol oF224, a = 11.340(2), b = 12.829(2), c = 19.441(3) angstrom, Z = 4. The composite incommensurate structure of the (Bi(0.02)Sr(0.39)Ca(0.59))(10)Cu(17)O(29) compound was solved and refined in the (3 + 1)-space group F222(00 gamma), a = 11.340(2), b = 12.829(2), c(1) = 2.7773(4), c(2) = 3.8882(6) angstrom, q = 0.7143.
Magnetization M(T) and M(H) data of single crystals of the 5 = 1/2 quasi-one-dimensional system (A2Cu2O3)m(CuO2) n with various A-site constituents are compared. The studied orthorhombic, incommensurate crystals are characterized by m/n = 5/7, 7/10, and 9/13. The role of rare-earth ions located in A-sites is discussed.
The structural, optical, and magnetic properties are studied in the (M2Cu2O3)m(CuO2) n-type single crystals with the surface on which the Bi-2212-type phase was indicated. The physical properties of such samples were interpreted to some extent by the contributions of two phases. When the secondary phases on the surface of the samples were removed by mechanical polishing or etching by Ar+-ion plasma, the physical properties were typical of the (M2Cu2O3)m(CuO2) n-type crystals. The optical method was shown to be effective for indication of secondary Bi-rich phases on the surface of (M2Cu2O3)m(CuO2) n samples.
A review is given of the structural, magnetic and transport studies on
two-leg spin-ladder ({M}_2{Cu}_2{O}_3)_m({CuO}_2)_n systems (where M are
divalent or{M}_2{Cu}_2{O}_3] is composed of ({Cu}_2{O}_3) two-leg ladder
planes and M ions coordinated to them. The second Subsystem consists of
{CuO}_2 1D-chains. In these materials, the superconductivity was
discovered for m/n = 1/1, 5/7, 7/10. The intrinsic and extrinsic
superconducting properties of spin-ladder Systems are presented and
discussed in detail. The important role played by the hole transfer from
{CuO}_2 planes to the spin-ladder planes is stressed. It is shown that
the superconductivity in this new family of high-temperature
superconductors should be described as an extreme type II limit.
Electron paramagnetic resonance (EPR) measurements were performed on the S=1/2 spin ladder system A10Cu17O29 (where /A=Sr,Ca,Y,Bi,Pb). The EPR signal intensity shows a broad peak around 70 K which indicates the presence of an energy gap in the spin excitation spectrum originating from simple (dimerized) chains. EPR spectra of Cu2+ ions in chains are described in terms of Wannier excitons. It is also shown that these ions are in a strong bottleneck regime due to the coupling to current carriers (holes) by the exchange interaction.
The bond-valence-sum method is used to calculate the hole distribution in the spin ladders of [M2Cu2O3]m[CuO2]n-type cuprates with m = n = 1 (where M are divalent or trivalent cations). It is shown that the appearance of superconductivity in this system is related to hole transfer from chain planes to the spin-ladder plane. The role of the valency and size of the M atoms is discussed in detail.
A review is given of the structural, magnetic and transport studies on two-leg spin-ladder (M
2Cu2O3)m(CuO2)n systems (where M are divalent or/and trivalent cations). The crystals belonging to these systems consist of two interpenetrated subsystems.
The first subsystem [M
2CU2O3] is composed of (CU2O3) two-leg ladder planes and M ions coordinated to them. The second subsystem consists of CuO2 ID-chains. In these materials, the superconductivity was discovered for m/n = 1/1, 5/7, 7/10. The intrinsic and extrinsic superconducting properties of spin-ladder systems are presented and discussed
in detail. The important role played by the hole transfer from CuO2 planes to the spin-ladder planes is stressed. It is shown that the superconductivity in this new family of high-temperature
superconductors should be described as an extreme type II limit.
Spin ladders are low-dimensional magnetic quantum systems that were recently discovered. A comprehensive overview of their
structural and magnetic characteristics is summarised here and the possibilities for supramolecular chemistry in the development
of such fascinating materials are discussed. By using molecules as spin carrier units the interacting spins are located in
molecular orbitals with a dominant role of π electrons. On the basis of a review of the few existing examples of molecular spin ladders it is clear that typical supramolecular
and crystal engineering criteria, such as π-π overlap, S···S and C—H· · ·S interactions and the complementary nature of size and shape, are useful in the construction
of these magnetic quantum systems with intermediate dimensionalities between ID chains and 2D square lattices.
Band structure calculations for Cu03 and Cu02 chain compounds as well as for zigzag Cu204 double chain compounds are reported.
These cuprate chains form the basic elements of a rich and beautiful variety of cuprate structures. From the dispersion of
the antibonding band crossing the Fermi energy the exchange interaction is estimated and qualitative trends in the antiferromagnetic
ordering at low temperatures are well explained. From the comparison of exact diagonalization studies of corresponding periodic
chains with frequently used approximations a restricted validity of the latter with respect to the hole doping amount is found.
The possibility of CDW-BOW states is discussed on the basis of self-consistent adiabatic studies. Chain aspects for superconductivity
and the possible relationship to the stripe scenario are briefly discussed.
The influence of hydrostatic pressure up to 10 kbar on the superconducting transition temperature in doped single crystal of (Sr,Ca)10Cu17O29 has been studied. A strong decrease of the normal state electrical resistivity and the pressure effect dTc/dP as high as 0.4 K/kbar were obtained. It is suggested that the superconductivity in the doped (Sr,Ca)10Cu17O29 system is related to the inhomogeneous charge carrier distribution. The existence of quasi-one-dimensional (quasi-1D) metallic and spin-gapped regions in close electrical contact can explain the observed magnetic properties of (Sr,Ca)10Cu17O29 and the effect of pressure on its transport properties.
A comprehensive theoretical and experimental study is presented of the magnetic susceptibility versus temperature \chi(T) of spin S = 1/2 two- and three-leg Heisenberg ladders and ladder oxide compounds. Extensive quantum Monte Carlo simulations of \chi(T) were carried out for both isolated and coupled two-leg ladders with spatially anisotropic intraladder exchange. Accurate fits to these and related literature QMC data were obtained. We have also calculated the one- and two-magnon dispersion relations and the dynamical spin structure factor for anisotropic isolated 2 x 12 ladders. The exchange constants in the two-leg ladder compound SrCu2O3 are estimated from LDA+U calculations. We report the detailed crystal structure of SrCu2O3 and of the three-leg ladder compound Sr2Cu3O5. New experimental \chi(T) data are reported for the two-leg ladder cuprates SrCu2O3 and LaCuO_{2.5}, and for the (nominally) two-leg ladder vanadates CaV2O5 and MgV2O5. The new and literature \chi(T) data for these compounds and for Sr2Cu3O5 are modeled using our QMC \chi(T) simulation fits, and the exchange coupling constants between the spins-1/2 are thereby estimated for each material. The surpisingly strong spatial anisotropy of the bilinear intraladder exchange constants in the cuprate compounds is discussed together with the results of other experiments sensitive to this anisotropy. Recent theoretical predictions are discussed including those which indicate that a four-spin cyclic exchange interaction within a Cu4 plaquette is important to determining the magnetic properties and which can significantly influence the exchange interactions estimated from \chi(T) data assuming the presence of only bilinear exchange.
A new A14Cu24O41-type phase of the composition (Nd3.66Y1.34Ca6.5Sr2.5)(Cu 23.15Ca0.425Sr0.425)O41 has been obtained by spontaneous crystallization from flux. The structure consisting of two sublattices incommensurate in one direction is determined and refined by single-crystal X-ray analysis (692 crystallographically nonequivalent reflections) in the commensurate approximation up to R = 0.0512. The parameters of the averaged unit cell are a = 12.515(2) Å, b = 11.294(6) Å, and c = 27.544(8) Å = 7c′ = 10c″, where c′ = 3.936(1) Å and c″ = 2.7537(4) Å are the parameters of two incommensurate sublattices, sp. gr. Abmm. The incommensurability factor q = 0.6996(6) provided the consideration of the structure as a commensurate one within the accuracy of determining the translation parameters. The structure determined differs from the known A14Cu24O41-type structures by the mutual arrangement of the structural units ([CuO2]-ribbons) in one of the sublattices, which is explained by the presence of Nd-atoms in the other sublattice. The partial substitution of Cu by Ca and Sr atoms never observed for such structures earlier is established. The almost complete incommensurability observed is interpreted in terms of the interaction between the two sublattices forming cation-oxygen bonds.
We have extended Bean’s critical state model to explicitly include anisotropic critical currents. Measurements at 30 K of the critical currents parallel to the Cu‐O planes but with vortex motion either parallel or across twin boundaries show twin boundaries are probably not an important cause of vortex pinning. In the critical state, current flow perpendicular to the Cu‐O planes is about 30 times smaller than flow parallel to these planes.
The crystal structure of Y2BaNiO5 is characterized by the existence of isolated linear magnetic chains. Nickel is located within compressed oxygen octahedra sharing two opposite corners to form (NiO5)n chains along the a-axis separated by Y and Ba atoms. Within a large temperature range, the magnetic susceptibility can be fitted using a one dimensional S = 1 Heisenberg model. The best fit is obtained for J/k ≈ −285 K (Tmax ≈ 410 K). The three dimensional long range magnetic ordering has not been observed by neutron diffraction down to 1.8 K, implying an inter-intrachain coupling ratio J'/J ⪡ 10−2. Inelastic neutron scattering experiments have given evidences for a singlet ground state and two gaps in the excitation spectrum at energies Δxy ≈ 8.5 meV and Δz ≈ 16 meV. Our experimental data are interpreted quantitatively within the framework of the Haldane conjecture for the S = 1 antiferromagnetic chain.
The interplay between structure and magnetic properties of Sr14Cu24O41+delta with CuO2 chain and Cu2O3 ladder building blocks is studied as a function of oxygen nonstoichiometry. The characteristic decrease in the magnetic susceptibility below 80 K for delta~=0 disappears both with increasing and decreasing delta, and, correspondingly, the periodicity of the superstructure, which arises from the lattice mismatch along the chain direction between the two structure blocks, shows a significant change. The microscopic origin of the singlet ground state is suggested to be the localization of electrons at low temperature in dimers on structurally modulated CuO2 chains.
We discuss an electronic model consisting of two chains or planes, each described by a t-J model, coupled by t'-J' interactions between them. For J'~J or larger we show the presence of a spin-gap and hole-pair formation upon doping. The model exhibits superconducting pairing correlations away from half filling. We support our claims by numerical studies of the spin gap, the binding energy of holes, and the pairing correlations on finite clusters. The hole-pairing operator is a spin singlet with one member of the pair in each chain or plane. Our model belongs to the universality class of the U>t. We argue that this model may be physically realized by doping the orthorhombic compound (VO)2P2O7.
Some fundamentals of the magnetic structure in anisotropic superconductors are presented. The approach focuses on the vortex structure in layered superconductors, the most important example being the high-Tc cuprates. The GinzburgLandau theory is discussed, as well as its extensions to include anisotropy via an anisotropic effective mass tensor. The Lawrence-Doniach theory is introduced, and it is used to show that a vortex parallel to the layers has a Josephson core confined to the interlayer junction. Two-dimensional (2D) pancake vortices are discussed, and the fields and currents generated by them are presented. The Josephson coupling energy between 2D pancake vortices in adjacent layers is estimated. Some basics of flux pinning in the layered superconductors are examined. Finally, an explanation is given of why the activation energy is much larger than the single-pancake pinning energy in strongly coupled superconductors but is approximately equal to the single-pancake pinning energy in weakly coupled superconductors.
We have investigated spin dynamics of both the ladder and the chain sites in Sr 14 Cu 24 O 41 by Cu-NMR and Cu-NQR. From the NMR shift, K spin , and nuclear spin-lattice relaxation time, T 1 , the energy gaps in the spin excitation spectrum are confirmed to be Δ=140 K for the chain site and Δ=470 K for the ladder site for Sr 14 Cu 24 O 41 . Doping effects on the spin gaps and electric field gradient at the Cu sites were also investigated by the substituting of Ca, Y and La for Sr. The spin gap of the ladder site decreases with increasing x for Sr 14- x Ca x Cu 24 O 41 and seems to collapse at around x =13 for Ca substitution.
The single crystal habit and structure of new incommensurate-type phases
in the system [A 2Cu 2O 3]
m[CuO 2] n (where A is alkaline earth
and trivalent metals) were analyzed. Needle- and platelet-like crystals
were grown from the melt by slow-cooling or decantation. Three
modifications of the phases with m/n values equal to {5}/{7}, {7}/{10}
and {9}/{13} were distinguished. The distortion of the second structural
fragment, the plane of Cu-O ribbons, was determined to be significantly
larger than for the first fragment composed of a ladder between two
A-cation layers. Superconducting crystals ( Tc=80-85 K) were
attributed to the modification with m=5 and n=7.
Anisotropic electrical resistivity and magnetic susceptibility are measured on single crystals of Sr14-xAxCu24O41 (A = Ca, Y). This material contains two-leg antiferromagnetic spin ladders that are progressively doped with hole carriers upon Ca substitution and thus determine the charge-transport properties. Application of hydrostatic pressure to this system has an effect similar to Ca substitution, increasing the hole density in the ladders. The resistivity shows remarkable anisotropy, indicative of a one-dimensional electronic state.
Magnetic measurements have been performed on the S = 1/2 quasi-one dimensional system of(Sr,Ca)(14)Cu24O41, which has both simple chains and two-legged ladders of copper ions. The total susceptibility of studied compounds has been shown to be anisotropic with anisotropy coming mainly from the Curie-Weiss term. The chains have a spin gap in the excitation spectrum, which originates from a dimerized state. Substantial doping of the system leads to super conductivity with a high critical temperature.
Single crystals of structural type A14Cu24O41 were grown and their structural, optical and magnetic properties were investigated. On the base of the data obtained it is proposed that superconductivity in the crystals of incommensurate phases is to be correlated with the Cu-deficiency and the corresponding changes in characteristic CuO structural units.
We have investigated the out-of-plane resistivity (varrhoc) of a number of copper oxides with various carrier concentration systematically, using well-characterized single crystals. The out-of-plane conduction is strongly dependent on crystal structure, and in contrast to the metallic T-linear in-plane resistivity, varrhoc is non-metallic in most superconducting compounds, suggesting an unconventional conduction mechanism. The most remarkable observation is that varrhoc decreases with increasing carrier concentrations much more rapidly than varrhoab.
The compound M10 Cu17 O29 with M = Bi0.06 Sr0.39 Ca0.55 has been shown to have a composite structure consisting of two interpenetrating, modulated sublattices. One substructure is a sandwich. Adjacent layers are Cu2O3 sheets extending in the ac-plane, and Bi, Sr and Ca atoms, which are statistically distributed over one atomic site. The other substructure is interleaved between the layers of metal atoms and consists of CuO2 ribbons extending along the c-axis. Both substructures are orthorhombic, with the same a- and b-axis values, but different and mutually incommensurate c-axis values, which accounts for the modulation. First-order satellite reflections are observed. The structure is described and refined in a four-dimensional formalism taking into account measured first-order satellite reflections. All atoms exhibit significant modulation amplitudes in particular along the b- and c-axes. Atoms in the CuO2 ribbons have relatively large mean-square amplitudes along the ribbon axis, which supports a model of modulated ribbons with some phase disorder of the longitudinal modulation waves. Interatomic distances are calculated considering the effect of modulation.
We report on measurements of the out-of-plane resistivity and magnetoresistance in the normal state region above the critical temperature Tc in optimally doped and overdoped Bi2Sr2CaCu2O8+x single crystals. The temperature dependence of the out-of-plane resistivity indicates thermally activated hopping of carriers over an interplane pseudogap. In the overdoped samples this behavior graduall changes to a metallic-like resistivity at higher temperatures. The out-of-plane magnetoresistance (measured with the magnetic field parallel to the current) is negative, indicating a shrinking of the pseudogap in magnetic fields. Our experimental results are in good accordance with a recent model for the reduction of the quasiparticle density of states due to condensation of quasiparticles into superconducting fluctuations. Such a fluctuation-induced pseudogap is expected to be suppressed in high magnetic fields according to a reduction of the in-plane fluctuation amplitude.
The London equations with a phenomenlogical mass tensor are used to analyze the vortex structure near the lower critical field. The anisotropy results in a transverse magnetic field in the Abrikosov vortex. This field attenuates exponentially at large distances from the vortex axis. The strongly anisotropic attenuation length is evaluated. The line energy is found in the London approximation.
The magnetization of YBa2Cu3O6+x antiferromagnetic films was experimentally studied as a function of magnetic field, temperature, oxygen index, and photoillumination dose. The results are evidence of the formation of ferromagnetic clusters, promoted by oxygen holes, in a general accordance with the phase separation approach. However, field and concentration dependences of magnetization display some essential features which can be explained only with allowance for the motion of holes in long-distance potential wells, created in the CuO2 plane (where the clusters are formed) by charged copper-oxygen chains (allocated in the CuOx plane where excess oxygen ions are incorporated). A significant straggling of ferromagnetic clusters over sizes (from 100 to 104 or more spins per cluster) was discovered, the properties of clusters being essentially dependent on their size.
We have performed magnetic susceptibility and electron spin resonance measurements on the quasi-one-dimensional magnet Sr14Cu24O41, which has simple chains and two-leg ladder chains of copper ions. The magnetic signals which we observed below room temperature mainly come from the simple chains. The experimental results show that the simple chain unexpectedly has a spin gap in the excitation spectrum, which originates from a dimerized state. © 1996 The American Physical Society.
We report comprehensive Cu NMR studies on single crystals of
Sr14-xCaxCu24O41, which
contain simple CuO2 chains and two-leg
Cu2O3 ladders. From measurements of the
63Cu NMR shift, it is clear that the spin gap in the ladders
decreases with isovalent Ca substitution from Δ=550+/-30 K for
Sr14Cu24O41 (Sr14) to 350+/-30 K,
280+/-30 K, and 270+/-30 K for x=6 (Ca6), x=9 (Ca9), and x=11.5
(Ca11.5), respectively. The exponential decrease of the nuclear
spin-lattice relaxation rate 1/T1 below ~130 K is consistent
with the presence of the spin gap in the spin excitation spectrum. In
the T range higher than ~200 K, we observed the following dependences:
1/T1=const and the square of Gaussian spin-echo decay time,
T22G~T which are consistent with the scaling
theory for the S=1/2 one-dimensional (1D) Heisenberg model. The value of
T2G/T1T is compatible with the theoretical
prediction of an exchange constant along the leg J∥~1800
K for Ca6 and J∥~1500 K for Ca9 and Ca11.5. A notable
finding is that the magnitude of the spin gap remains nearly constant
and characteristics of novel 1D-like spin dynamics are maintained in the
content varying from Ca9 to Ca11.5. On the other hand, the charge
transport changes with increasing Ca content so that the more conductive
Ca11.5 exhibits pressure-induced superconductivity exceeding 3.5 GPa. We
have found that T22G, which is proportional to the
inverse spin correlation length ξ-1, deviates from a
linear T dependence upon cooling and is described by A+BT
exp(-Δ/T), regardless of the Ca substitution. We point out that
the value of T22G(T=0)=A is proportional to the
finite value of ξ-10=Δ/c1D,
where c1D=(π/2)J∥ is the spin-wave
velocity. From the result that the values of
A-1~ξeff for Ca6, Ca9, and Ca11.5 are
significantly reduced compared to that for Sr14, it is suggested that
ξeff is dominated at low T by an average distance d among
mobile holes obeying the relation ξeff~d=ξh.
From an estimate of ξ0/a~ 5.2 for Sr14, where a is the
Cu-Cu distance along the leg, ξh/a is obtained as ~3.5,
2.3, and 2.0, and hole content x as ~0.14, 0.22, and 0.25 per
Cu2O3 ladder for Ca6, Ca9, and Ca11.5,
respectively. These values were consistent with x= 0.14, 0.2, and 0.22
for Ca6, Ca9, and Ca11 estimated from the optical conductivity
experiment by Osafune et al. [Phys. Rev. Lett. 78, 1980 (1997)]. The
Sr14-xCaxCu24O41 compounds
are thus hole-doped two-leg spin-ladder systems which reveal the
metallic behavior dominated by the 1D-like spin dynamics at high T and
accompanied by the spin gap formation at low T. For Ca11.5, as the spin
gap is formed upon cooling below ~180 K, the resistivity increases in
the direction perpendicular to the ladder, whereas the conductivity
along the ladder remains metallic, followed by the localization of
mobile holes in both directions below TL~60 K. We point out
that preformed pairs are confined in each ladder and localized below ~60
K at an ambient pressure.
We have observed superconductivity in the ladder material
Sr0.4Ca13.6Cu24O41.84 under
pressures of 3 GPa and 4.5 GPa by means of electrical measurements. The
superconducting transition temperatures T c (onset) are 12 K
and 9 K at 3 and 4.5 GPa, respectively. The superconducting volume
fraction was obtained to be about 5% from magnetization measurement
under 3.5 GPa at 4.2 K, indicating the bulk nature of the
superconductivity in this system.
Experimental investigations of phase separation in high-Tc cuprates are reviewed with special emphasis on recent results obtained on doped La2CuO4+x single crystals. Particular attention is paid to the magnetic properties. The experiments give very strong evidence that magnetic polarons are responsible for macroscopic phase separation observed in La2CuO4+x for x>0.
We have measured the absolute value and temperature dependence of the magnetic penetration depth λ(T) of magnetically aligned cobalt and zinc doped YBa2Cu3O7-δ (YBCO) using a commercial AC susceptometer. For pure YBCO, our values of λab(0)≅140 nm and λc(0)≅1040 nm agree with other published values. We find increasing the Co doping increase both the fluctuation effects and the anisotropy ratio λc/λab, probably due to the reduction of coupling in the c-direction, while doping with Zn substantially increases both λab and λc. Our data imply a large, highly anisotropic gap whose maximum value is 2Δ(0)≅10kBTc. All samples show a T2 term in both λab(T) and λc(T) at low temperatures (T<0.6Tc), in apparent disagreement with conventional s-wave BCS pairing. The possible explanations of this result are discussed.
We have investigated spin dynamics of both the ladder and the chain sites in Sr14Cu24O41 by Cu-NMR and Cu-NQR. From the NMR shift, Kspin, and nuclear spin-lattice relaxation time, T1, the energy gaps in the spin excitation spectrum are confirmed to be Δ = 140 K for the chain site and Δ = 470 K for the ladder site for Sr14Cu24O41 . Doping effects on the spin gaps and electric field gradient at the Cu sites were also investigated by the substituting of Ca, Y and La for Sr. The spin gap of the ladder site decreases with increasing x for Sr14-xCaxCu24O41 and seems to collapse at around x = 13 for Ca substitution.
New 80 K superconductor with structural type A14Cu24O41 was grown and the physical properties of single crystals grown were studied.
We have investigated the transport and magnetic properties of the ladder system with two legs Sr14−xAxCu24O41 (A Ba and Ca). For x = 0, both electrical resistivity and thermoelectric power exhibit semiconductive behavior. It becomes more insulating with the increase in x(Ba), while it becomes more conductive with the increase in x(Ca). A metal-insulator transition is found at x(Ca) = 6.0–8.4 from the thermoelectric power measurements. The magnetic susceptibility exhibits a broad peak around 80 K for x = 0. Subtracting the Curie component at low temperatures, the remainder of the susceptibility χs(T) decreases toward zero with decreasing temperature below ∼80 K, which implies the presence of a spin gap. The temperature at which the broad peak shows the maximum changes little through the partial substitution of Ba and Ca for Sr. However, the value of χs(T) decreases with the increase in x(Ba), while it tends to increase with the increase in x(Ca). These properties are discussed in terms of the redistribution of holes between the two different sites of Cu. It appears that the observed spin gap behavior is due to Cu2+ spins in the CuO2 chain rather than in the Cu2O3 plane.
The influence of oxygen ordering on the temperature dependence of the magnetic field penetration depth λ is investigated in submicron YBa2Cu3O6+x particles obtained by low-temperature (Ts = 800 − 850°C) synthesis from a sol-gel precursor. Additional OO bonds created at low temperature synthesis hinder the oxygen ordering in Cu(1)O plane despite of its high content (x > 0.82). Oxygen exchange as well as additional annealing stimulate tetra-ortho transition and oxygen ordering. A wide linear range is observed in λab−2 (T) for samples with highly disordered oxygen. This is attributed to strong phase fluctuations caused by local strains. A gauge-glass model with random distribution of order parameter phases describes the behavior of λab−2(T), revealed at initial stage of the hindered tetra-ortho transition. Transformation of the linear λab−2(T) fuction to a quadratic one with oxygen ordering is related to decreasing of local deformations and to transformation from a glass state to the usual superconducting orthophase. Two essentially different ways of oxygen ordering, reoxidation and additional annealing, result in the same final state with a similar power law of λab−2(T).
Three kinds of cupric spin ladder compounds, SrCu2O3(2-leg), Sr2Cu3O5(3-leg), and LaCuO2.5(2-leg), have been discovered in the course of studies of high-pressure chemistry carried out at 6 GPa and 1200 K, typically. Structures, electrical and magnetic properties of these intriguing compounds, which exemplify the initial stage of the transition from one- to two-dimensionality, will be summarized.
Phases crystallizing in a new structure-type with general formula (A1−xA′x)14Cu24O41 (A = alkaline earth metal, A′ = trivalent metal) and symmetry Cccm with an extended stoichiometry range have been found in studies of the SrYCuO, BaSrBiCuO and CaLaCuO systems. Single crystal X-ray studies on several crystals grown from different alkaline earth/metal oxide-cuprate melts reveal a common orthorhombic F-centered subcell of . Superstructure is observed in crystals, leading to a 7-fold increase of the c-axis and a change in symmetry to space group Cccm. As in Ba2YCu3O7, the Cu atoms are found as CuO planes and linear CuO chains. Due to shear in the planes, half of the CuO squares share edges, producing CuCu zigzag chains, similar to the planes observed in CaCu2O3. In the linear CuO chains, the CuO squares share edges as well, leading to a short CuCu contact of 2.75Å. Experiments on ceramic samples indicate that the oxygen content is fixed and that the samples are semiconducting.
The structures of two compositions of the title compound, Sr8Ca6Cu24O41 and Sr14Cu24O41, have been determined by x-ray, single-crystal diffraction techniques. The structure of the first is best described as two interpenetrating structures: one with (Sr,Ca)(Cu2O3 sheets)(Sr,Ca) layers in an orthorhombic cell with dimensions of a = 11.375(2), b = 12.9027(2), and a second structure with layers of (CuO2 chains) in a cell having identical a and b values but with . The title formulation arises because these two subcells are nearly commensurate at 7 × c (sheet) = 27.372(2) and 10 × c (chain) = 27.534. The structure of the pure strontium phase is very similar but the adjacent CuO2 chains in a given layer shift to a less constrained conformation: a = 11.459(2), b = 13.368(5), c (sheet) = 3.931(1), . X-ray powder diffraction studies have shown that x may vary from 0 to ∼ 8, with the layer axis, b, showing the most sensitivity to composition. Single crystals of these phases were found to be semiconductors.
We propose a theoretical model which allows to explain high-Tc superconductivity in perovskites. It starts from a Hubbard-type Hamiltonian and from a description of the antiferromagnetic state. We show that when doping with holes immobile ferromagnetic clusters are formed which at higher hole concentrations percolate to metallic lines. The superconductivity occurring along these one-dimensional lines is caused by deformation coupling to Jahn-Teller active optical vibrations. Only a small isotope effect is predicted. The model also explains the spin-glass behaviour for intermediate concentrations and the destruction of superconductivity at high concentrations.
The problem of an anisotropic uniaxial superconducting particle of a general ellipsoidal form in the magnetic field ${H}_{0}$ such that ${H}_{c1}$\ll${}{H}_{0}$\ll${}{H}_{c2}$, is considered. It is shown\char22{}in agreement with experimental data\char22{}that particles of high-${T}_{c}$ materials at $T<{T}_{c}$ must orient with their $c$ axis normal to the applied field. The torque acting on a particle is evaluated and shown to be independent of the ellipsoid shape. The torque is associated with the magnetization component perpendicular to the field direction.
In high-Tc superconductors there exists a broad field domain,
in which the reversible magnetization M is linear in the logarithm of
the applied field Ha. The dependence M(lnHa) is
obtained taking the strong uniaxial anisotropy into account. It is
shown, that for an arbitrary orientation of the single crystal with
respect to Ha the magnetization vector has a component normal
to the applied field comparable to the usually measured component
parallel to Ha. A procedure is suggested for extracting the
value of the penetration depth from the linear part of
M(lnHa). Being applied to the data available for
Y1Ba2Cu3O7, the method
yields λ~(Tc-T)-1/2 near Tc up
to temperatures (Tc-T)/Tc~10-3. This
puts an upper bound upon the value of the temperature domain, in which
the underlying mean-field theory is valid.
The structure of the vortex lattice in anisotropic, unaxial superconductors in the domain Hc1HHc2 is considered within the London approach. To first order in the small parameter (L)2, where L is the average intervortex spacing and is the average penetration depth, there exists a continuum of different lattices with the same free energy for any direction of the magnetic induction B with respect to the crystal. It is shown that the degeneracy is removed if terms in the free energy of order (L)4 are taken into account, yielding both a unique structure and a preferred orientation of the vortex lattice with respect to the direction of B within the crystal. Parameters of the primitive cell for this structure are obtained and evaluated for known values of the anisotropy of YBa2Cu3O7. For the particular case of vortices parallel to the Cu-O planes the degeneracy remains exact (within the London approach), which should make this lattice more susceptible to disorder. The magnetization is shown to be almost parallel to the c^ crystal direction for all orientations of the external field H0, except in a narrow domain where H0 is nearly normal to c
The c-axis resistivity rhoc in Bi2Sr2CaCu2O8+delta and oxygen-reduced YBa2Cu3O6+x is quite sensitive to a magnetic field H. The magnetoresistance (MR) is large (1% at 14 T and 100 K), weakly anisotropic, and negative over a wide range of temperatures T above the critical temperature Tc. The magnitude of the MR is activated in temperature with a characteristic energy UL that varies with the oxygen content delta. We interpret the activated form of rhoc and the negative MR in terms of a pseudogap Delta that is slightly reduced by H.
Crystals of orthorhombic Sr14Cu24O41 are semiconducting, with an activation energy of 0.18(2) eV for conduction along c, the direction lying parallel to both the (Cu2O3) planes and the (CuO2) chains. The magnetic susceptibility above 80 K follows a Curie-Weiss law with theta=-95 K and peff=0.65muB/Cu in the ac plane and theta=-190 and peff=0.94muB/Cu along b, the direction perpendicular to the (Cu2O3) planes and the (CuO2) chains. There is an indication of antiferromagnetic order below TN=60 K, and a Curie contribution at low temperature due to localized free spins [0.4(1) with S=(1/2 per formula]. The specific heat measured in the range 2-12 K is of the form AT-2+gammaT+betaT3, where A=1080 mJ K/mole, gamma=100 mJ/mole K2, and beta=5.93 mJ/mole K3. A 5-T magnetic field greatly enhances the low-temperature specific heat, an effect which is attributed to the localized states with S=(1/2. The linear term is attributed to a localized density of states with no net spin, possibly associated with peroxide ions. The relevance of these results to the low-temperature specific heat of copper-oxide superconductors is discussed.
The magnetic susceptibility of single-crystal CuGeO3, a linear Cu2+ (spin-1/2) chain compound, was measured. The susceptibilities in all the directions rapidly drop to small constant values with decreasing temperature below a phase transition temperature near 14 K. The magnetic-field dependence of the transition temperature quantitatively agrees with both theoretical predictions and experimental results for organic spin-Peierls systems. This Letter is the first report of an unambiguous determination of the existence of the spin-Peierls transition in an inorganic compound.
Substantial hole doping has been achieved in (La,Sr,Ca)14Cu24O41 compounds with CuO2 chain and Cu2O3 ladder building blocks. Holes enter the structural units with larger O/Cu ratios which are the CuO2 chains. The ladders remain undoped and magnetically inert due to a large spin gap. The moments in the undoped chains interact weakly, and every hole added creates a nonmagnetic CuO2 unit and decreases the resistivity. At the maximum hole concentration \(~0.6/CuO2\) the residual moments form a singlet ground state possibly due to dimerization with an excitation gap of ~140 K to the triplet state.
- Nagaev