Above-T c anomalies of the infrared-active phonons in RBa2Cu4O8 (R=Dy,Ho) and Y2Ba4Cu7O15−δ superconductors

We report a far-infrared reflectivity study of DyBa₂Cu₄O₈, HoBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– superconductors in the temperature range 10–300 K. The assignment of thez-polarized infrared-acitve vibrations of the rare-earth-ion (R) was verified by the substitution ofR. Since the structure of the Y₂Ba₄Cu₇O_15– compound inz-direction is an ordered sequence of YBa₂Cu₃O_7– and YBa₂Cu₄O₈ blocks, the phonon spectrum of Y₂Ba₄Cu₇O_15– is a superposition of their respective phonon modes. The temperature dependence of the infrared-active phonons in RBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– was analyzed. A narrowing of the plane-oxygen phonon upon sample cooling was found to occur atT _c , as it was reported for single-chain YBa₂Cu₃O₇ materials; however a pronounced frequency softening of this mode for both RBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– was found to start well aboveT _c , suggesting the interaction of infrared-active phonons with an excitation gap similar to that observed in spin-dependent measurements.     

μSR Study of Zn-Substituted YBa2Cu3O x

We report zero field and transverse fieldSR. measurements in magnetic YBa₂(Cu_1–y Zn _y )₃O _x . Mild disorder of the antiferromagnetic state (x<6.1), a new disordered magnetic phase abovex=6.4 where superconductivity is destroyed by Zn substitution and, finally, local moments in the paramagnetic state are the most salient features related to Zn substitution described here.     

Temperature effects on the phonon spectrum in YBa2Cu3O7 single crystals and thin films

We have performed detailed investigations on the temperature dependence of the 335 cm^–1 phonon in single crystals and thin films of the YBa₂Cu₃O₇ superconductor. The frequency of this phonon exhibits a downshift of about 5 cm^–1 on passing the superconducting transition from above. The shift of the phonon in thin epitaxial films on MgO or SrTiO₃ substrates is only about 2.5 cm^–1. The width of the asymmetric phonon line displays a slight increase belowT _c due to the electron-phonon interaction in these systems.     

Synthesis and superconductivity of a new 1222-type T1-based layered cuprate (T1, Nb) Sr2(Nd,Ce)2Cu2O z

Synthesis and superconductivity of a new 1222-type layered cuprate (Tl_1–x Nb _x ) Sr₂(Nd_1–y Ce _y )₂Cu₂O _z have been studied. The structure of this cuprate is directly related to that of Nb-1222 NbSr₂(Nd, Ce)₂Cu₂O _z with tetragonal body-center lattice. Partial substitution of Tl for Nb in Nb-1222 phase improves its superconductivity. (Tl_1–x Nb _x ) Sr₂(Nd_0.75Ce_0.25)₂Cu₂O _z samples prepared by the typical procedure exhibit superconductivity withT _c of 30–40 K. Effects of Tl and Nb on superconductivity of this cuprate are briefly discussed.     

Phonons in YBa2Cu3O7−x crystals with site-selective oxygen isotope substitution: Frequencies,normal vectors,non-linear properties

Lattice vibrations of YBa₂Cu₃O_7–x crystals with a site-selective isotope substitution¹⁸O¹⁶O are investigated theoretically. It is shown that shifts of the frequencies of A_1g oxygen vibrations due to this substitution are smaller than under complete one. However, phonon bands and normal vectors are changed much stronger, especially in the case when apical oxygens are substituted. On the basis of the presented analysis anharmonic properties and transition temperatures of the selectively substituted crystals are also briefly discussed.     

Anomalous Zn- and Ni-substitution effects on superconductivity in the superconducting weak ferromagnets RuSr2RCu2O8 (R = Gd,Eu)

The effect of magnetic Ni and non-magnetic Zn impurities on superconducting transition temperature T_c in RuSr₂R(Cu_1?x(Ni, Zn)_x)₂O₈ with R = Gd or Eu (Ni- and Zn-substituted Ru1212Gd(Eu)) was extensively studied. It is found that the suppression rate dT_c/dx of RuSr₂R(Cu_1?x(Ni, Zn)_x)₂O₈ is comparable to that of underdoped YBa₂(Cu_1?x(Ni, Zn)_x)₃O_7?δ. The suppression of superconductivity in Ni-substituted Ru1212Eu samples is more significant than that in Zn-substituted ones, indicative of Ni being a more effective pair-breaker than Zn. In strong contrast, the magnetic Ni impurity atoms have a weaker effect on superconductivity than non-magnetic Zn atoms in Ru1212Gd, similar to what was observed in the high-T_c cuprates. These intriguing findings strongly suggest that the impurity-induced local disturbance of the 3d-spin correlation at Cu sites around Ni/Zn is distinctly different between Ru1212Gd and Ru1212Eu.     

Infrared phonon spectra of 3d-metal doped YBa2(Cu1-x M x )3O7-δ

Infrared (IR) absorption spectra of doped YBa₂(Cu_1–x M _x )₃O_7–, with M representing Co, Fe, Ni, and Zn were observed as a function of concentrationx. TheB _1u IR mode at 580 cm^–1 was observed for all samples which were in the orthorhombic phase, which includes all the Ni and Zn doped samples and the lower concentrations (x<2.5%) for Fe and Co samples. The tetragonal phase of Co and Fe systems (x%>2.5%) shows three new IR modes at 655 cm^–1, 340 cm^–1, and 175 cm^–1. The frequency of the B_1u mode at 580 cm^–1 remains unchanged for all the Ni and Zn concentrations. In contrast, this mode shows a strong monotonic frequency softening with increase in Co substitution, while in the Fe-system it shows a comparable monotonic mode softening for 3%x%0%, and then remains unchanged for 12%x%3%. These observations indicate that Fe and Co substitute on the Cu(1) sites, whereas Ni and Zn most likely substitute on Cu(2) sites. We attribute the difference in the mode softening for Co and Fe to different Cu(1) local environments for these two elements. Indication for a chain-plane charge transfer based on our results is also discussed.     

Preparation of high J c YBa2Cu3O7−δ superconducting thin films by ion beam sputtering deposition

Preparation of high T _c and high J _c YBa₂Cu₃O_7– superconducting thin films by ion beam sputtering deposition is reported. The main factors affecting the composition of the films and the orientation of the crystal grains have been examined. Experimental results show that the Y, Ba and Cu composition of as-deposited films can be conveniently and accurately adjusted by a combined sputtering target which consists of a large sintered target of YBa₂Cu₃O_7– and a small one that is Ba and Cu rich (YBa_2.5Cu_3.3O_x). Fabrication conditions of highly oriented superconducting thin films are described. YBa₂Cu₃O_7– superconducting films with zero resistance at 88–90.5K and critical current density J _c (at 77K) of 1.5×10⁵ A/cm² are obtained.     

Optical study of RE 1 + x Ba 2 - x Cu 3 O 6 (RE = Nd, Sm) aaand YBa 2 Cu 3 O 6 in the mid infrared range

Infrared reflectance, and transmission measurements as well as Raman scattering have been used to study the RE_1+xBa_2-xCu₃O₆ (RE = Nd, Sm) and YBa₂Cu₃O₆ absorption bands in the 1100-1500 cm^-1 infrared range as a function of temperature and beam polarization. In addition to two-phonon absorption between 1100 and 1170 cm^-1, we observe excitations around 1400 cm^-1, occurring in oxygen rich enclosures within the samples, and assign them to an excitation involving two-phonons plus the 270 cm^-1 local mode related to Cu-O broken chains. Thus, the previously reported possible magnetic origin of the 1436 cm^-1 sharp absorption band in YBa₂Cu₃O₆ is contested. Received 14 February 2001 and Received in final form 12 April 2001     

Mössbauer study of tin-doped 1-2-3 and 2-1-4 highT c superconductors

Samples of YBa₂(Cu_0.85Sn_0.15)₃O_7–y (1-2-3), La₂(Cu_0.95Sn_0.05)O4_4–x, and La_1.85Ba_0.15(Cu_0.95Sn_0.05)O_4–x (2-1-4), with different oxygen stoichiometry, have been studied by Mössbauer spectroscopy. These measurements reveal the existence of two inequivalent sites for Sn in the (1-2-3) compounds. The (1-2-3) spectra display two quadrupole doublets which we associate with Sn in Cu(1) and Cu(2) sites, respectively. La compounds show a single quadrupole doublet.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

X-ray photoelectron spectroscopy of the high-temperature superconductor YBa2Cu3O7−x and the copper oxides

The X-ray photoelectron (XPS) core-level spectra of Cu-metal, Cu₂O, CuO, NaCuO₂ and YBa₂Cu₃O_7–x have been determined. It has been shown, that the Cu³⁺ ions in NaCuO₂ are reduced to lower valence states by X-rays in ultrahigh vacuum (UHV). The determination of the electron binding energy of the Cu 2 _{p 3/2} level in NaCuO₂ yields a value of 934.1 eV. The shape of the Cu 2 _{p 3/2} peak as well as the appearence of a shake-up satellite in YBa₂Cu₃O_7–x which is smaller than in CuO show that YBa₂Cu₃O_7–x contains Cu³⁺ ions. The behaviour of YBa₂Cu₃O_7–x also was studied during X-ray irradiation and a reduction was noticeable.     

Superconductivity coexisting with ferromagnetism in mixed-valence Mn doped La1.85−(4/3)xSr0.15+(4/3)xCu1−xMnxO4

The effect of Mn substitution for Cu in mixed-valence Mn doped La_{1.85−(4/3)x}Sr_0.15+(4/3)xCu_1−xMn_xO₄ (x=0.06) has been investigated by electric resistivity, magnetization and electron spin resonance experiments. Coexistence of superconductivity and ferromagnetism was observed.     

Spin triplet pairing in high temperature superconductors

Various superconductivity mechanisms show that the electron pair is in spin singlet (S=0) belowTc. In contrast with these, our EPR measurements show that the electron pair is in spin triplet (S=1) rather than the spin singlet. Since an intense zero field absorption was observed in the high temperature superconductors of YBa₂Cu₃O_7–y , and YBa₂Cu₃O_7–y F₁, which violated various spin-pairing singlet superconductivity mechanisms.     

CNDO studies of Y-Ba-Cu-O superconductors II. Simple models of YBa2Cu3Ox

The electronic structure of YBa₂Cu₃O_x (x=6 and 7) is investigated using the CNDO molecular orbital method. Electronic structures of model clusters [Cu₃O₁₀]^–15, [Ba₈Cu₃O₁₀]⁺¹, [Y₈Cu₃O₁₀]⁺⁹ of the non-superconducting (x=6) and [Cu₃O₁₂]^–17, [Ba₈Cu₃O₁₂]^–1, [Y₈Cu₃O₁₂]⁺⁷ of the superconducting (x=7) phases are compared. Y and Ba layers cause a considerable electron density transfer from the central Cu(1) region.     

Mössbauer studies of YBa2(Cu1−x Fe x )3O7−d annealed in an inert atmosphere

YBa₂(Cu_1–x Fe _x )₃O_7–d annealed in an Ar atmosphere, then reoxygenated at various temperatures, has been studied by Mössbauer spectroscopy at 300 K and low temperature with and without an applied field of 5.5 T. The results are interpreted as being due to Fe clustering remaining in the chains rather than a significant transfer to the planes.     

Superconductivity in CoSr2(Y1−xCax)Cu2O7+δ

The roles of aliovalent Ca^II-for-Y^III substitution and high-pressure-oxygen annealing in the process of ‘superconducterizing’ the Co-based layered copper oxide, CoSr₂(Y_1−xCa_x)Cu₂O_7+δ (Co-1212), were investigated. The as-air-synthesized samples up to x=0.4 were found essentially oxygen stoichiometric (−0.03≤δ≤0.00). These samples, however, were not superconducting, suggesting that the holes created by the divalent-for-trivalent cation substitution are trapped on Co in the charge reservoir. Ultra-high-pressure heat treatment carried out at 5 GPa and 500 °C for 30 min in the presence of Ag₂O₂ as an excess oxygen source induced bulk superconductivity in these samples. The highest T_c was obtained for the high-oxygen-pressure treated x=0.3 sample at ∼40 K.     

The superconductive properties and crystal structure of Ba2(Y1−x Pr x )Cu3O9−y solid solutions

A series of Ba₂(Y_1–x Pr _x )Cu₃O_9–y solid solutions were prepared with the orthorhombic distorted perovskite structure. The compound Ba₂PrCu₃O_9–y belonging to tetragonal system is a good semiconductor. The superconductivity is observed for samples withx<0.6. With the increase of Y content, the distortion of crystal structure is greater and the superconductive transition temperature of zero resistanceTc(0) raises.     

Effect of hole doping and antiferromagnetic coupling on the itinerant ferromagnetism of through Cu substitution at Ru site

Systematic studies on the structural, transport and magnetic properties of SrRu_1−xCu_xO₃ (x=0.0–0.2) compounds have been performed. SrRu_0.8Cu_0.2O₃ shows a tetragonal structure unlike the other compositions which exhibit a pseudo-cubic structure. Low temperature powder X-ray diffraction data of SrRu_0.8Cu_0.2O₃ collected at a synchrotron beam line reveals that the tetragonal structure is stable down to 8 K. Ferromagnetic transition temperatures (T_c) are significantly reduced from 160 to 34 K with Cu doping. All the compositions exhibit irreversibilities in M_ZFC(T) and M_FC(T) curves ascribable to the presence of domain structures. Magnetic susceptibility measurements show that the copper ions are anti-ferromagnetically coupled for concentrations higher than x=0.16. The antiparallel arrangement of Ru⁵⁺ ions with its neighboring cations also contributes to the large reduction in the observed magnetic moment. X-ray photoemission spectroscopy measurements show evidence for both tetravalent and pentavalent Ru ions while copper is in a divalent state. We conclude from our resistivity data that Cu²⁺ substitution promotes a polaronic type of conductivity.     

c-Axis correlated extended defects and critical current in YBa2Cu3Ox films grown on Au and Ag-nano dot decorated substrates

We report measurements of critical current in YBa₂Cu₃O_x films deposited on SrTiO₃ substrates decorated with silver and gold nanodots. An increase in critical current in these films, in comparison with the films deposited on non-decorated substrates, has been achieved. We argue that this increase comes from the c-axis correlated extended defects formed in the films and originated from the nanodots. Additionally to creating extended defects, the nanodots pin them and prevent their exit from the sample during the film growth, thus keeping a high density of defects and providing a lower rate of decrease of the critical current with the thickness of the films. The best pinning is achieved in the samples with silver nanodots by optimising their deposition temperature. The nanodots grown at a temperature of a few hundred °C have a small diameter of a few nanometres and a high surface density of 10¹¹–10¹² particles/cm². We give evidence of c-axis correlated extended defects in YBa₂Cu₃O_x films by planar and cross-sectional atomic force microscopy, transmission electron microscopy and angle-dependent transport measurements of critical current.