Abnormal Structural Behavior of Y0.8Ca0.2Ba1.8Nd0.2Cu3Oy at Low Temperature

The variation of structure and transport property for the nominal composition of Y_0.8Ca_0.2Ba_1.8Nd_0.2Cu₃O _y (YCBNC) at temperature range of 78 K < T < 284 K were studied. YCBNC shows a superconducting transition temperature (zero resistivity) of T _c0 ～ 83 K. There is no phase transition detected in the accuracy of our experiments in the whole temperature range. The dependences of lattice constants upon temperature can be discussed in two temperature regions. The other atomic parameters, or the deduced structural parameters, such as the orthorhombic strain, the lattice strain, some selected bond lengths and bond angles all depend on temperature. The dependences of the atomic structural parameters on temperature indicate that these atomic structural parameters may relate to the superconductivity to some extent.     

Stability of the low-temperature tetragonal phase in La2?xBaxCuO4 and related compounds

In La_2–x Ba _x CuO₄ (LBCO), the structural transition to a low-temperature tetragonal phase below 60 K and suppression of superconductivity are observed when the carrier density isp 1 /8 per copper. The replacement by divalent ions smaller than Ba²⁺ suppresses the static deformation of the lattice. We have found that the variationsT _d2 and superconducting transition temperatureT _c are quantitatively characterized by the averaged ionic radius at the La site or lattice parameters. This aspect of substitution could be regarded as the effect of chemical pressure, since similar variations have been reported on applying hydrostatic pressure. In La_2–x–y Nd _y (Ba, Sr) _x CuO₄,T _d2 increases with increasingy in a wide range ofp whileT _c is suppressed only at p l /8. The structural transition atT _d2 here should be ascribed mainly to the crystallochemical origin.     

Superconducting transition temperatures in the Ru-1232 system, RuSr2(Gd1? x Ln x Ce1.8Sr0.2)Cu2O z (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperatureT _c in the Ru-1232 system, RuSr₂(Gd_1−x Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperatureT _co are almost the same forx=0.00−0.15, and each of the lattice parametersa andc is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample withx=0.05 shows the maximum values for both the superconducting onset temperatureT _co and the zero resistivity temperatureT _cz. Especially for the sample with Ln = Dy, the values ofT _co andT _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations ofT _co, lattice parameters ofa andc in the RuSr₂(Gd_1−x Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy contentx, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Microscopic Charge Inhomogeneities in Underdoped La2?xSrxCuO4: Local Structural Evidence

We present local structural evidence for the existence of charge inhomogeneities at low temperature in underdoped and optimally doped La_2–x Sr _x CuO₄. The inhomogeneities disappear for x 0.2. The evidence for the charge inhomogeneities comes from an anomalous increase in the inplane Cu–O bond length distribution in the underdoped samples as well as evidence for CuO₆ octahedral tilt inhomogeneities in the intermediate range structure. Preliminary analysis of the temperature dependence of this phenomenon indicates that the inhomogeneities set in at temperatures in the range 60 K < T _co < 130 K, which depends on doping.     

Comparison of Superconductivity and Structure for YBa2Cu3Oy with Potassium and Sodium Doping

The nominal composition of YBa_2–x M _x Cu₃O _y (M = K, Na) cuprates with x 0.30 were synthesized by the standard solid-state reaction technique. X-ray diffraction (XRD) and the resistivity measurements were used to characterize the structure and the superconductivity of the doped cuprates. There was no impurity phase detected within the whole doping range. The structure of the main phase (123) has the orthorhombic with P _mmm symmetry. With increasing the content of dopants, the lattice constants and some other structural parameters are almost unchanged for M = K, whereas they changed for M = Na. The refined contents of dopants are consistent with that of the nominal ones. The zero resistance temperature T _c0 decreases sharply with the increase of the content of potassium in potassium-doped samples as x 0.20. For sodium-doped YBa_2–z Na _x Cu₃O _y cuprates, T _c0 varies very little. The difference in superconductivity depression may result from the shift oxygen, which transfers conducting carriers from Cu-O chains to Cu-O₂ sheets or the structural stress effect.     

Temperature and spectral dependences of persistent photoconductivity in YBa2Cu3Ox thin films

Temperature and spectral dependences of photoinduced changes of resistance were measured in YBa₂Cu₃O _x thin films with oxygen content ranging as 6.35 <x < 6.75. The absolute value of efficiency of initiation of photoinduced changes decreases with increase in oxygen content, but the position of peaks in the spectral dependence does not change with a change ofx. Temperature dependences of efficiency have an anomaly atT220 K, which is present in all the samples studied, and correlates with anomalies observed by other experimental techniques. Qualitatively similar temperature and spectral dependences of efficiency for the samples in both the insulating and metallic phases may be considered as an indication that the persistent photoconductivity effect in YBCO on both sides of the metal-insulator transition has a common origin.     

How “Musical” Are High-Tc Superconductors and What Can Empirical Rules Tell About Their Origin?

It is shown that parameters such as optimal T _c for cuprate superconductors or details of their doping curves can be organized on phenomenological rules. Accordingly, T _c in a range between a kink and the optimum scale linearly with the number of effective holes h _e, according to T _e = h _e T _c ^e, with T _c ^e = 600 K. Effective holes are composed of the difference between holes in the Cu—O bonds in the CuO₂ planes, h _p, and holes in the Cu—O bonds with the c axis or apical O, h _e, according to h _e = h _p– h_c = fh _p. The deleterious effect of the apical O manifests itself in three levels, depending on the basic modes of its coordination of the CuO₂ planes in zero, one, or two sheets (according to factor f = 1, 2/3, ¹/₂). The values of h _p at T _c optimum tend to rational fractions, ranging from 1/6 to 1/3, and are determined by lattice pressure. This musical or harmonic T _c matrix, originating from two structurally determined factors, groups optimal T _c into families. Knight shift data, establishing h _p, bear out the general assumptions. Some flexibility in the range within families is observed. This flexibility indicates the operation of more complex influences from structural detail, such as the varying distance of Cu to the apical O. The existence of ranges within optimal T _c families indicate a somewhat tunable rather than a strict musical T _c-level scheme with measured intervals. The details of the doping curves are similarly organized. These phenomenological rules suggest the operation of bond ordering effects. Arguments for the actual nature of the bond orderings are presented in terms of local pairs of doped bonds in trijugate positions. These quantitative concepts can be expanded to other characteristic features in the doping curves of cuprates and other high-T _c materials such as C or B containing systems, providing a universal frame for explaining high-T _c superconductivity in bond ordering terms.     

Structural instabilities and superconductivity in La-214 compounds

In La_2–x Ba _x CuO₄ (LBCO) the transition to a low-temperature tetragonal phase and the suppression of superconductivity occur at the carrier concentration p 1/8 per copper. We will discuss the roles of various material parameters that control this instability. An unusual lattice softening has been found by ultrasonic measurement on La_2–x Sr _x CuO₄ (LSCO). This softening is present only in an in-plane shearing mode and is ascribed to the growth of structural fluctuations in the normal state.These phenomena are closely related because both the structural change in LBCO and the applied strain in LSCO lift the degeneracy of in-plane oxygen sites. They clarify the importance of strong coupling between the normal-state electronic system and the lattice by a Peierls-type mechanism.     

Spin Gap Effects on Bulk R1+xba2?x Cu3O7?δ (R=Eu or Nd and 0≤x≤0.4)

Spin gap effects on the underdoping states of the bulk system of R_1+x Ba_2–x Cu₃O_7– (R = Eu or Nd and 0 x 0.4) were investigated through transport property measurements. The underdoping states were achieved by, alternatively substituting R³⁺ for Ba²⁺ ions in the system rather than adjusting the oxygen deficiency. The excess R³⁺ ions were to occupy the Ba sites of the crystalline lattice as revealed from Rietveld analysis for powder X-ray diffraction. The underdoped materials were observed to first undergo spin pairing transition in the temperature range well above T _c, and come across with superconducting transition at T _c. The increasing feature observed for spin gap temperature and the decreasing one for T _c, as the concentration of holes decreases, are in qualitatively good agreement with theoretical predictions from the mean-field RVB model.     

Musical, Bond Ordered Superconductors—Does a Common Phenomenological Model Hold for All High Tc Materials [Cuprates, C60, MgB2, C–S Composites etc.]?

The presence of high T _c (e.g., 117 K) in doped C₆₀ poses the question for the ultimate condition of superconductivity in this and related systems, which are devoid of complex ingredients such as magnetic transition metals. To answer this question, essential aspects of cuprate phenomenology are generalized. A quantitative and universal T _c formalism is developed from empirical rules for the cuprates. Accordingly, T _c increases linearly on doping up to a sharp peak, corresponding to an optimal charge order filling, and decreases thereafter. The hole filling degree of an optimal bond order depends on lattice pressure. Shape of the T _c-hole dependence and magnitude of T _c on the bond order model are born out by transistor doping of CaCuO₂, which is here used as a calibration. Inferences are drawn concerning the optimal bond order. Accordingly, linear local pair kernels are based on a newly postulated trijugate bond system that is mobile, while electronically frozen orders are based on tetrajugate bonding. In the superconductor, linear pair patterns are correlated into a two-dimensional bond order pattern by energies such as elasticities. This concept is here extended to materials based on other electronegative materials. Accordingly, arguments are presented that a common basis for their superconductivity lies in a novel chemical bonding state (e.g., trijugate) in a doped covalent lattice. The hallmarks of the bond order model are also born out by the curve shape of transistor doping of C₆₀. A disproportionation of pairs per molecule is proposed for optimal doping. Complex redox equilibria within laminated layers in the structure can lead to self-doping and can relieve lattice pressure in complex cuprates, MgB₂, C₆₀, or C–S composites.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1?xNbx)Sr2(GdCe1.8Sr0.2)Cu2Oz System

The Ru-1232 compounds have been synthesized in the (Ru_1–xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇- superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The 0.2m thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇- powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×10⁵ A/cm² at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT _c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 m, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.     

Superconductivity of Ba1 ? xKxBiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba_{1 – x} K _x BiO₃ (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T _c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T _c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T _c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T _c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T _c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.     

Observation of X-band SQUID signals in a High-Tc superconducting film device

A microwave superconducting magnetometer is described in which a microstrip resonator is coupled to a two-hole high-T _c thin-film SQUID device. Both the microstrip circuit and the thin-film SQUID were fabricated by photolithography techniques. The YBCO thin film was deposited on single-crystal substrate of yttria-stabilized zirconia [YSZ(100)] by an ion beam sputtering technique producing a superconducting transition measured at a critical temperature ofT _c =92 K to within T 3 K. Non linear oscillatory behavior was observed in the microstrip resonator when inductively coupled to the SQUID. This nonlinear behavior yielded a microwave device in which the reflected microwave power varied with applied DC magnetic flux.     

Investigation on the Structure and Superconducting Properties of Mg1 ? xMxB2 (M = deficiency or Ca)

We report the preparation of Mg_{1 – x} M _x B₂ (M = deficiency or Ca) compounds and their structure and superconducting properties. For Mg_{1 – x} B₂, although nearly single-phase samples can be obtained for x = 0, MgB₄ coexists with the MgB₂ phase and some minor impurity phases, and the amount of MgB₄ increases with x for 0 < x 0.5. The lattice parameters a and c of MgB₂ decreases and increases, respectively, with the increase of x, and T _c also decreases. While for Mg_{1 – x} Ca _x B₂, the superconducting transition temperature remains unchanged for x 0.3 and loss of superconductivity occurs for x > 0.3. X-ray diffraction patterns for x 0.3 samples show that MgB₂ phase coexists with CaB₆, Mg, and MgO. With increasing x, the amount of CaB₆, Mg, and MgO increases, while the amount of MgB₂ decreases. The lattice parameters of MgB₂ phase do not show any obvious change in contrast to Mg_{1 – x} B₂. The results were discussed by considering some possible contributions.     

On structure and stability of (Hg, Mo)-1212 superconductors based on SAED and EDS analysis

A superconductor with nominal composition Hg_0.8Mo_0.2Sr₂Y_0.5Ca_0.5Cu₆₊ withT _c up to 96 K has been isolated and carefully examined by electron diffraction and X-ray energy dispersive spectrum analyses. Its structure can be attributed to space groupP4/mmm and lattice constantsa=3.82 å andc=11.91 å. Inhomogeneous distribution of the cations was observed, which is considered to be the key to the broadening ofT _c in theR-T curve. The effect of humid atmosphere on the stability of the sample has also been investigated.     

Radiative and Optical Properties of La1−xSrxMnO3 (0≤x≤0.4) in the Vicinity of Metal–Insulator Transition Temperatures from 173 to 413K

Radiative and optical properties of polycrystalline La_1–x Sr _x MnO₃ (0x0.4) in the vicinity of the metal–insulator transition are presented. The temperature dependence of the total hemispherical emittance _H of La_1–x Sr _x MnO₃ was measured by the calorimetric method in the temperature range from 173 to 413K. It was confirmed that _H showed unexpected variation as a result of changes in the hole concentration (x). Especially in the case of La_0.825Sr_0.175MnO₃, _H remains high above the transition temperature T _C due to insulator-like behavior; on the other hand, it decreases sharply below T _C because of metallic behavior. The spectral reflectance was measured by FT-IR in the wavelength range of 0.25 to 100 m at room temperature. The optical constants were calculated by Kramers–Kronig analysis of the spectral reflectance data. An insulator-like character of the optical properties appears at lower Sr²⁺ doping levels while a metallic one exists at higher Sr²⁺ doping levels.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7?y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1–x Zn _x )₃O_7–y withy0.1 has been measured forx0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7–y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx 0.10. YBa₂(Cu_1–x Zn _x )₃O_7–y retains the YBa₂Cu₃O_7-y orthorhombic structure forx0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,(T), is metallic-like (d/dT>0) and increases gradually with increasing Zn content. However, forx 0.10,(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.     

Magnetic order and evolution of the electronic state aroundx=0.12 in La2?xBaxCuO4 and La2?xSrxCuO4

Muon spin rotation ( ⁺SR) measurement provides clear evidence of the antiferromagnetic order of Cu moments below 35 K for La_2–x Ba _x CuO₄ and below 15 K for La_2–x Sr _x CuO₄ in the narrow range ofx where the high-T _c superconductivity (SC) is suppressed remarkably. The results suggest that the change of the electronic state coupled with the lattice instability is relevant to the local suppression of SC and freezing of spin fluctuations of the Cu moment.     

The Material-Dependent Parameter Controlling the Universal Phase Diagram of Cuprates

The critical temperature T _c in the universal phase diagram of cuprate superconductors is a function of two variables: the hole-doping δ and a material dependent parameter. Here we focus on the behavior of T _c,max as a function of the material dependent parameter (MDP) at the optimum hole doping. We have discussed the correlation between (1) the average Cu—O (planar) distance, or the strain of the Cu—O bond, (2) the nearest-neighbor hopping t′ and (3) the Lifshitz parameter z. These Lifshitz parameter z = μ_{δ = 0.16}−E _vHs which are all material dependent parameters, where μ_{δ = 0.16} is the chemical potential at optimum doping and E_vHs is the energy of the Van Hove singularity, defines the proximity to the Fermi surface topological transition from electron-like to hole-like. The results show that the striped phases occur for z < 0, the highest T _c,max for and the drop of T _c,max for z > 75 meV.