high-Tc Cuprate Superconductors Research Articles

Photo-induced transition to additional symmetry broken state in cuprate superconductors

The high Tc cuprate superconductors are found to exhibit both d( x 2− y 2) and s-wave pairing symmetry. The possibility of radiation-induced development of a small d( xy) component of the superconducting order parameter with the breaking of time reversal and parity symmetry in some cuprate superconductors is suggested.

Zeeman and orbital effects of an in-plane magnetic field in cuprate superconductors

We discuss the effects of a magnetic field applied parallel to the Cu–O (ab) plane of the high Tc cuprate superconductors. After briefly reviewing the Zeeman effect of the field, we study the orbital effects, using the Lawrence–Doniach model for layered superconductors as a guide to the physics. We argue that the orbital effect is qualitatively different for in-plane and interlayer mechanisms for superconductivity. In the case of in-plane mechanisms, interlayer couplings may be modeled as a weak interlayer Josephson coupling, whose effects disappear as H→∞; in this case Zeeman dominates the effect of the field. In contrast, in the interlayer mechanism the Josephson coupling is the driving force of superconductivity, and we argue that the in-plane field suppresses superconductivity and provides an upper bound for Hc2 which we estimate very crudely.

Scaling approach to the magnetic torque on an anisotropic superconductor

We present a new, self-contained and simplified derivation of the magnetic response of an anisotropic type-II superconductor to an applied magnetic field in the regime Hc1 << H << Hc2. The method exploits angular scaling and avoids both a detour through Fourier space and explicit coordinate system changes. We conclude on the magnetic torque and relate to the relevant case of high-Tc cuprate superconductors.

High-pressure synthesis and superconducting properties of the oxychloride superconductor(Sr,Ca)3Cu2O4+δCl2−y

Chlorine-containing (Sr,Ca)(3)CU2O4+deltaCl2-y superconductors have been synthesized at 5.0 Ga and similar to 1000 degrees C. The double-[CuO2] layered compound crystallizes into (La,Sr)(2)CaCu2O6 structure with Cl residing at the apical anion site relative to the [CuO2] plane. The critical transition temperature T-c 80K has been achieved in these superconductors. The Sr2.3Ca0.7Cu2O4+deltaCl1.3 superconductor was studied in terms of irreversibility field Hi, and intragrain critical current density I,. It is found that the irreversibility field of the superconductor can be described by the power law dependence (I-T/T-c)(n), with n similar to 2. The reduced H-irr(T/T-c) line is positioned between those of YBa2Cu3O7 and Bi2Sr2CaCu2O8 in the H-T plane, close to HgBa2CaCu2O6+delta. The critical current density J(c)(T) was extracted from the Bean critical-state model and showed a smooth temperature dependence. No significant difference was found for the general behavior of H-irr(T) and J(c)(T) of (Sr,Ca)(3)Cu2O4+deltaCl2-y from those of oxide high-T-c superconducting cuprates (HTSC). The off-plane space remains the important parameter to evaluate interlayer coupling of the oxyhalide HTSC, which is consistent with oxide HTSC. The results suggest the potential application capability of the oxyhalide superconductors even though the off-plane block is composed of oxyhalide instead of the routine oxides.

A POSSIBLE FIRST PRINCIPLE ARGUMENTS ON THE SYMMETRY OF SUPERCONDUTING GAP PARAMETER

The symmetry of superconducting gap parameter in high-Tc cuprate superconductors has been studied based on general physical considerations. The disagreement between theoretical prediction and experimental data is discussed.

Correlation of low-temperature thermal expansion with oxygen nonstoichiometry in Bi1.7Pb0.3Sr2Ca2Cu3O10+y

Low-temperature thermal expansion and its correlation with oxygen nonstoichiometry have been studied for Bi1.7Pb0.3Sr2Ca2Cu3O10+y samples. An anomalously big jump of the thermal expansion in the c direction of Bi1.7Pb0.3Sr2Ca2Cu3O10+y has been observed at a temperature slightly above the superconductivity transition temperature. The corresponding temperature and the magnitude of the jump are found to be correlated with the oxygen nonstoichiometry, carrier concentration and superconductivity. The results reveal that the anomalously big jump of the thermal expansion is an electron-assisted lattice softening transition. Its correlation with the superconductivity suggests that phonon-electron interaction still plays a certain role in the superconductivity in high-Tc cuprates.

Effect of mixing RE elements (Nd, Sm, Gd, Eu, Y, Yb) on the RE2BaCuO5/Nd4-2xBa2+2xCu2-xO10-2x phases in RE cuprate high-Tc superconductors

The RE2BaCuO5/RE4Ba2Cu2O10 phases (where RE is a mixture of Nd, Sm, Gd, Eu, Y and Yb) have been synthesized in an oxygen atmosphere and subsequently characterized. The mixing of RE elements which inherently form the RE2BaCuO5 phase through the peritectic decomposition of REBa2Cu3O7-x (RE123) (i.e. Sm, Gd, Eu, Y, Yb), was observed to give homogeneous mixing of the elements in the 211 phase. In contrast it was found that on mixing Nd with other RE elements a mixture of the Nd4-2xBa2+2xCu2-xO10-2x (Nd422) and RE2BaCuO5 (RE211) phases resulted. It was also observed that on mixing Nd with other REs a finite amount of the RE is substituted into the Nd422 phase and Nd into the RE211.

Heterostructured Bi-Based Cuprate High-Tc Superconductors with Unusually Coordinated Metal Halides

Attempts have been made to prepare various high-Tc superconducting intercalation compounds, M–X–Bi2Sr2Can−1CunO2n+4+δ (M=metal; X=halogen; n=1, 2, and 3), with the synthetic strategy of hard–soft–acid–base interaction. The chimie douce route such as vapor transport and ionic diffusion is found to be effective in intercalating soft Lewis acidic guest species (I−3, AgxIx−ww, HgIδ−2, HgBrδ−2, and AuIδ−3) into the Lewis basic Bi2Sr2Can−1CunO2n+4+δ lattice. According to the X-ray diffraction and magnetic property measurements on these intercalates, it becomes clear that the basal increment upon intercalation (3.25–7.35 Å) is governed by the interlayer geometry of the intercalant, while the accompanying Tc change depends on the charge transfer between host and guest. The extended X-ray absorption fine structure and Raman studies reveal that mercuric halides are intercalated as linear molecules as in vapor state, while silver iodide forms a two-dimensionally extended ionic-conducting channel. In the case of new gold iodide intercalate, unique trigonal planar AuI3 species is identified as the first example of an isolated gold iodide molecule.

Electronic Raman Scattering in Y1?xCaxBa2Cu3O7?? Single Crystals

Polarized Raman scattering measurements have been performed on single crystals of the high temperature superconductor Y0.8Ca0.2Ba2Cu3O7—δ as a function of carrier concentration. For strongly overdoped crystals we found that positions of the pair-breaking peaks decrease with increasing overdoping and, unlike in optimally doped crystals, almost coincide. In both doping states the Raman response follows a linear power law at low frequencies for the A1g channel and a αω + βω3 dependence for the B1g one. Our results agree with similar data on overdoped Bi-2212 and Tl-2201, and should therefore represent a universal property of the overdoped cuprate high-Tc superconductors. For the strongly underdoped sample, no superconductivity-induced change in the electronic continuum has been observed.

Tunneling spectroscopy study of spin-polarized quasiparticle injection effects in cuprate/manganite heterostructures

Scanning tunneling spectroscopy was performed at 4.2 K on epitaxial thin-film heterostructures comprising YBa2Cu3O7−δ and La0.7Ca0.3MnO3, to study the microscopic effects of spin-polarized quasiparticle injection from the half-metallic ferromagnetic manganite on the high Tc cuprate superconductor. The quasiparticle tunneling characteristics observed were consistent with d-wave pairing symmetry, with a gap-maximum Δ0≈22 meV, up to at least 35 mA (7×103 A/cm2) injection. Spectral smearing observed at higher injections could be fitted to elevated effective quasiparticle temperatures, even though negligible sample heating was detected by in situ thermometry. The overall spectral evolution with the injection current also appears to be nonthermal in character, showing a nonmonotonic change in both the zero-bias tunneling conductance and the area under the conductance spectrum. We discuss general implications of these results for the scenario of dynamic pair breaking by a nonequilibrium distribution of spin-polarized quasiparticles.

A new modelling approach to superconductor layered structures

The analysis of the structural data of high Tc superconducting cuprates (HTSCs) shows that large differences exist in X-ray diffraction (XRD) patterns collected from powders and thin films with respect to the single crystal case, depending in many cases on the synthesis routes and material treatments. These differences are often disregarded and the assessment on the purity of the crystalline phase is made on the basis of XRD pattern indexing only. However, when relative intensities of XRD reflections belonging to the same family (e.g. (001)) differ from those obtained from the single crystal structure, the simple pattern indexing is not sufficient for the structural characterization and important structural features are missed. These might be helpful for discussing magnetic or electrical behaviour of specimens, as the related properties may depend on the structural correlation along the c axis. In this article a structural modelling of cuprates of the Bi–Sr–Ca–Cu–O family is proposed by considering the electronic density in the projection perpendicular to the layers of the structure. In particular, the structures of Bi-2201, Bi-2212 and Bi-2223 HTSCs were reduced to monomers and intergrowth effects are simulated by generating polymers with different percentages of these phases. By using this model, much information in terms of intergrowth and site occupancy can be gained from a careful analysis of intensities and widths of 001 reflections.

Reservoir Layers in High Tc Mercury Cuprates

AbstractWe put forward the hypothesis that cations with 6s electrons (Hg,Tl,Pb,Bi) in the charge reservoir layers of high Tc cuprate superconductors actively participate in the pairing interaction as negative-U centers. We further argue that the Hg-cuprates are outstanding superconductors (Tc &gt; 160 K) because they can exist as two-ion negative-U centers, . Their electrons are less localized than in single-site centers (negative-U or bipolaron) and can have a strong pairing interaction with a smaller increase in effective mass. The centers are oriented in the x and y directions and can have phase differences compatible with the d-wave symmetry of the CuO2 planes.

D-wave condensate and essential phenomenological description of some properties of highTc cuprate superconductors

The discovery of high Tc superconducting cuprates occurred over a decade ago but the cause of the superconducting condensation and electronic structure of such compounds is still a matter of considerable debate. While there is no agreement as to the pairing mechanism, there is, on the other hand, a wide consensus about the main properties which a theoretical description should provide. In this article, a theory is presented which accounts in a straightforward way for many of the essential properties of the high Tc cuprate superconductors. Some further developments of the model are suggested, particularly relating to the normal state spin-gap which our model does not currently describe. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 617–626, 1999

Thermodynamics of the 2D Hubbard model

A theoretical analysis of the thermodynamic response functions of the 2D single-band Hubbard model is realized by means of the composite operator method. It is shown that all the features of these quantities can be explained by looking at the dependence of the thermodynamic variables on their conjugate ones. It is found that the numerical data from quantum Monte Carlo techniques for the internal energy and electronic specific heat are well reproduced. The anomalous normal state properties in hole-doped cuprate high Tc superconductors are also well described. Actually, the results for the linear coefficient of the electronic specific heat are in agreement with those obtained by using a pure fermionic theoretical scheme. Finally, we obtain several characteristic crossing points for the response functions when reported as functions of some thermodynamic variables. These peculiar features indicate the existence of more than one energy scale competing with thermal excitations in the 2D Hubbard model.

Electronic structures and instabilities of ZrNCl and HfNCl: implications for superconductivity in the doped compounds

Amongst the many rules of thumb that prevail in the search for new superconductors is a belief that square nets of metal atoms are crucial to the achievement of high transition temperatures. The recent finding of superconductivity below 26 K in some intercalated β-HfNCl phases suggests that high transition temperatures might be achievable in compounds with triangular or graphite-like nets as well. Here we present key features of high level ab initio band structure calculations on the insulating ZrNCl and HfNCl parent compounds. Electron doping through intercalation has been modeled within a rigid-band framework. Features in the energy isosurfaces of the ‘doped’ compounds are examined for nesting instabilities of the kind that have been implicated in the superconducting properties of high-T c compounds with square nets. Despite very different electron counts, bonding patterns and atom topologies, it would seem that certain important aspects of the Fermi surfaces of the superconducting Zr(Hf)NCl phases are in many respects very similar to the Fermi surfaces of the cuprate high-T c superconductors.

Lattice instability of high-Tc superconductor

Lattice parameters of high-Tc cuprate superconductors La2−xSrxCuO4 (LSCO) and YBa2Cu3O7−δ (YBCO) have been measured at high pressure for a wide variety of x and δ. It is found that the linear and volume compressibilities have peaks in the vicinity of x and δ at which the superconducting state becomes unstable. Some anomalous properties in the linear compressibility κi (i=a, b and c) of YBCO are pointed out and discussed in connection with the oxygen deficient δ.

On the mechanism of critical temperature enhancement in cuprate high-Tc superconductors under pressure

The results of experiments [C. Y. Han et al., Low Temp. Phys. 24, 305 (1998)] on the influence of quasi-hydrostatic pressure P on the superconducting transition temperature Tc in polycrystalline samples of cuprate compounds Tl2Ba2Ca2Cu3Ox (Tl-2223) with the initial value of Tc=129 K and the maximum value Tc=255.4 K at P=4.3 GPa are considered. A hypothesis on the existence of two mechanisms of the effect of pressure on Tc is proposed on the basis of an analysis of the nonmonotonic dependence of Tc on P. The first mechanism operating in the pressure range P 3 GPa is due to hydrostatic compression of the ionic lattice and an increase in the bulk concentration of charge carriers (holes) in view of the electroneutrality condition.

Group theoretical identification of active localized orbital space in high Tc cuprate superconductors

There is a widely shared consensus that the pair condensate wavefunction in cuprate superconductors has a dominant d x 2− y 2 symmetry. In this paper a group theoretical analysis of the pair condensate wavefunction in real space for cuprate superconductors is presented. The analysis indicates that there is a degenerate pair of active bands in cuprate superconductors which are principally derived from localized e-orbitals and two possible singlet pair states formed from two different orbitals with symmetries a 1, b 1 or a 2, b 2 and all three may contribute to the condensate wavefunction. Hund's rule and the experimental observation that singlet pairing with a short coherence length occurs in cuprate superconductors, indicates that the first option is the most likely candidate in these materials and that hole doping mainly occurs from 1 B 1 pairs of e-orbitals giving rise to singlet electron pairing and the phenomenon of high T c cuprate superconductivity.

Muon-spin-rotation study of Zn-induced magnetic moments in cuprate high-Tcsuperconductors

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C-Axis Penetration Depth of Hg-1201 Single Crystals

We have magnetically imaged interlayer Josephson vortices emerging from an ac face of single crystals of the single layer cuprate high-Tc superconductor (Hg,Cu)Ba2CuO4+d. These images provide a direct measurement of the c-axis penetration depth of about 10 microns. This length is a factor of 10 longer than predicted by the interlayer tunneling model for the mechanism of superconductivity in layered compounds, indicating that the condensation energy available through this mechanism is 100 times smaller than is required for superconductivity.