On the Presence of 1D-Like Stripes in Superconducting Epitaxial La2 -x Sr x CuO4 thin Films

A large positive magnetoresistance (up to tens of percents) is observed in both underdoped (x?<?0.15) and overdoped superconducting La_{2? x}Sr_xCuO₄ (LSCO) epitaxial thin films, at temperatures far above the superconducting critical temperature T _c . In the underdoped regime, this magnetoresistance cannot be described by the Kohler rule and is due to the influence of superconducting fluctuations. On the other hand, in the overdoped regime, the Kohler rule does not seem to be violated. ?The strong magnetoresistance above T _c can be related to the preformed superconducting pairs existing well above T _c but forming a phase coherent superconducting state below T _c . The observations support the idea of a close relation between the pseudogap and the superconducting gap and provide evidence for the presence of pre-pairs above T _c . Both the observed fluctuations and the observed magnetoresistance are in accordance with the existence of 1D-like stripes. These results are further supported by recent high magnetic field measurements (up to 55 T) of the transverse magnetoconductivity σ _xy, which goes to zero for T→0 K.     

The Underdoped Region of the Phase Diagram of YBa2Cu3O6+x

Here we present a reviewed phase diagram of the high-T _c superconducting YBa₂Cu₃O_{6+ x} compound, finely mapped in the strongly underdoped region (0 < x < 0.5), from the pure antiferromagnetic state to the superconducting regime. The Neèl and spin freezing temperatures have been measured by μSR experiments while the hole density per Cu atom in the CuO₂ planes has been determined from the resistive T _c and from Seebeck coefficients at 290 K. The phase diagram is discussed in comparison to those of La_{2− x} Sr _x CuO₄ and Y_{1− x} Ca _x Ba₂Cu₃O₆ cuprate systems.     

High-Temperature Thermoelectric Power of The Metal Oxides: La2?xSrxCuO4 and Bi1?xSrxMnO3

We have investigated the high-temperature thermoelectric power (TEP) of La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35) and Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8) up to 700 K. Based on the TEP results we have discussed the phase transitions on each case. In the case of high-T _C cuprates, La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35), the TEP shows different temperature dependences in three temperature regions. At low temperature, the positive TEP rises showing a broad peak at temperature T _P, which shifts to lower temperature upon Sr doping. Right above T _P, the TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, T _H, showing a saturation behavior. The systematic change of the TEP behavior is discussed in terms of the two-fluids model, which is an intrinsically inhomogeneous state, consisted of bound pairs and independent carriers in the normal state of the high-T _C superconductors. For Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8), the negative TEP is almost temperature-independent in the high temperature regime (T _CO < T < 700 K). Near the charge ordering temperature (T _CO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, T _CO shifts to lower temperature from T _CO ∼ 520 K for x = 0.5 to T _CO ∼ 435 K for x = 0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s² character of Bi³⁺ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion.     

NMR Study Under Pressure on the La1111 Pnictides,LaFeAsO<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>F<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The relationship between antiferromagnetic (AF) fluctuation and superconductivity was investigated in the La1111 series, LaFeAsO_1−x F _x (x=0.05, 0.08, and 0.14) by examining nuclear relaxation rates (1/T ₁) at both ambient pressure and 3.0 GPa. Although the AF fluctuation is enhanced by applying pressure in the underdoped regime (x=0.05, and 0.08), the increase in critical transition temperature (T _c) is small, whereas T _c increases remarkably in the overdoped regime (x=0.14) where the AF fluctuation is absent, suggesting that high T _c above 40 K originates not from the AF fluctuation but from the density of states at the electron pocket.     

Normal and Anomalous Pseudogap of Superconducting Y0.9−x Pr x Ca0.1Ba2[Cu1−y Zn y ]3O7−δ

Electrical resistivity measurements on the superconducting oxides of the compositions Y_0.9−x Pr _x Ca_0.1Ba₂[Cu_1−y Zn _y ]₃O_7−δ (0≤x≤0.20 and 0.0≤y≤0.10) sintered in oxygen atmosphere were carried out to obtain the normal and anomalous pseudogaps in underdoped and overdoped samples. It is observed that pseudogap temperature T ^* decreases with increasing doping level p in the underdoped case. For the overdoped sample with y=0.06, T ^* shows no p dependence.      

Scaling of the Temperature Dependent Resistivity and Hall Effect in Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)As<Subscript>2</Subscript>

We show that the temperature-dependent resistivity ρ(T), Hall number n _H(T) and the cotangent of the Hall angle cot θ _H(T) of Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2) can be scaled using a recently proposed model-independent scaling method (Luo et al. in Phys. Rev. B 77:014529, 2008). The zero field normal-state resistivity above T _c can be reproduced by the expresion r(T) = r₀ +cTexp(- \frac2\varDelta T )\rho(T) = \rho_{0} +cT\exp(- \frac{2\varDelta }{T} ) and scaled using the energy scale Δ, c and the residual resistivity ρ ₀ as scaling parameters. The scaling parameters have been calculated and the compositional variation of 2Δ and ρ ₀ has been determined. The 2Δ(x) dependence show almost linear decreasing in underdoped regime, minimum corresponding to the T _c maximum and increasing in overdoped regime. The latter is different from that reported for cuprates. The existence of a universal metallic ρ(T) curve which, however, is restricted for the underdoped compounds to temperatures above a structural and antiferromagnetic transition is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2).     

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.     

Superconducting Transition Temperatures in the Ru-1232 System, RuSr2(Gd1?xLnxCe1.8Sr0.2)Cu2Oz (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperature T _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 temperature T _co are almost the same for x = 0.00–0.15, and each of the lattice parameters a and c is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample with x = 0.05 shows the maximum values for both the superconducting onset temperature T _co and the zero resistivity temperature T _cz. Especially for the sample with Ln = Dy, the values of T _co and T _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 of T _co, lattice parameters of a and c in the RuSr₂(Gd_{1− x} Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy content x, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Superconductivity at 103 K in CdBa2(Ca0.7Y0.3)Cu2Oy

A cadmium analogue of the mercury system with nominal composition CdBa₂(Ca_1–xY_x)Cu₂O_y has been synthesized. Thex=0 samples contain about 12 vol.% of the 1212 phase but are not superconducting. Thex=0.3 samples are superconducting atT _on = 103 K. The EDX analysis of 18 microcrystals shows a broad cationic distribution of the different components. The observed broad superconducting transition is attributed to the variousT _c of the different microcrystals.     

In-Plane Anisotropy of the Upper Critical Field of La2−x Sr x CuO4 in the Underdoped and Overdoped Regimes

For La_2–x Sr _x CuO₄ single-crystals in the underdoped and overdoped regimes, we have measured the electrical resistivity along the c-axis, _c , under constant magnetic fields, changing the field direction in the ab-plane. At x=0.08, 0.12 and 0.16 in the underdoped and almost optimally-doped regimes, clear anisotropy of _c with the fourfold symmetry in the ab-plane has been observed. This symmetry is regarded as the fourfold symmetry of the upper critical field, H _c2, and explained as being mainly due to the anisotropy of the superconducting energy gap owing to the d_x ^{2-y 2} pairing. The magnitude of the fourfold component of H _c2, H _c2/H _c2, is largest in x=0.08 among the three crystals. At x=0.20 in the overdoped regime, no clear fourfold symmetry has been observed. It appears that the fourfold symmetry is easy to observe in the underdoped regime rather than in the overdoped regime.     

Energy Scales in the High-Tc Superconductor YBa2Cu3O6+x

The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa₂Cu₃O_6+x along the a axis. Differences in N() above and below T _c allow the strength of the superconducting condensate _s to be determined. In the optimally-doped material, _s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties.     

The Transport Properties of Bi-Riched Bi2Sr2CuO6+δ Single Crystal

As-grown superconducting Bi-riched Bi₂Sr₂CuO₆₊ single crystals have been grown by the traveling solvent floating zone technique. The superconducting transition temperature T _c was about 6 K and the room temperature resistivity was about 2×10^–3 Ohm-cm. Transport properties, such as resistivity, magnetoresistance and Hall effect were measured from overdoped to underdoped samples annealed in inert atmosphere at 650°C. The transition temperature can be raised to 12 K after post annealing. The Hall measurement shows that the hole carrier density decrease after annealing. The temperature dependence of Hall angle is T ^1.5, not quadratic as observed for most high-T _c superconducting oxides such as YBa₂Cu₃O₇. The variation of onset T _c with different external magnetic field is very different from high-T _c superconductors. The in-plane conductivity shows the dependence of ln T and can be explained by weak localization theory.     

Effect of pressure onTc in La2-xBaxCuO4 withx=0.125

The superconducting transition temperature,T _c , of La_2–x Ba _x CuO₄ has been measured under high pressure up to 8 GPa.T _c is found to change drastically at the pressure where the structural phase transition takes place. This finding clearly indicates that there exists an intimate relation between the crystal structure and superconductivity.     

Superconductivity of barium-doped Bi2Sr2CaCu2Oy

Sintered ceramic samples of Bi₂Sr_2–x Ba _x CaCu₂O_y with nominal barium fraction 0x0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

T c amplication and pseudogap at a shape resonance in a superlattice of quantum stripes

The shape resonance of the superconducting gap for a 2D electron gas in a superlattice of quantum stripes with a finite 1 D periodic potential barrier is studied when the Fermi level is tuned near the bottom of the third superlattice subband. The maximumT _c . appears at a critical charge densityT _c . Forp > p _c (in the underdoped regime) the chemical potential is tuned in a pseudogap region characterized by a decrease of the density of states and an opening of the partial gap at the Fermi surface. The pseudogap is closed forp< p _c (in the overdoped regime).     

Magnetic penetration depth of Tl2Ba2CuO6+δ :T c ∼σ0 in the overdoped region

We report transverse-field muon-spin-rotation experiments carried out on Tl₂Ba₂CuO_6+δ . This system spans the whole overdoped regime, andT _c is reduced by excess oxygen doping, which increases the normal-state carrier concentration. In the heavily overdoped regimeσ(0) is found to scale linearly with the superconducting critical temperatureT _c , similar to the behavior previously observed for other cuprates in the underdoped regime. However, for the overdoped region one has to explain the reduction ofσ ₀, thus the increase of the magnetic penetration depthλ, in spite of an increasing normal-state carrier concentration. We discuss some possible explanations for this behavior.     

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.     

Scanning Tunneling Spectroscopy on Overdoped Bi2Sr2?xLaxCuO6+δ

We have performed low-temperature scanning tunneling spectroscopy on overdoped Bi₂Sr_2−x La _x CuO_6+δ (x=0.1 and 0.2) at 4.2 K. The tunneling spectra in both samples obviously exhibit two energy gaps: one has clear gap structure with sharp gap edge peaks, and another is observed as kinks inside the larger gap. Both the energy gaps inhomogeneously vary on the surface at nanometer scale, and their values positively correlate each other. The distributions of both gaps normalized to the mean values are identical independent of the doping level. These results suggest that the two gaps are commonly affected by the source of the electronic disorder.     

Magnetic-Field Induced Superconductor–Antiferromagnet Transition in Lightly Doped RBa2Cu3O6+x (R = Lu, Y) Crystals

The remarkable sensitivity of the c-axis resistivity and magnetoresistance in cuprates to the spin ordering is used to clarify the doping-induced transformation from an antiferromagnetic (AF) insulator to a superconducting (SC) metal in RBa₂Cu₃O_6+x (R = Lu, Y) single crystals. The established phase diagram demonstrates that the AF and SC regions apparently overlap: The superconductivity in RBa₂Cu₃O_6+x , in contrast to La_2−x Sr _x CuO₄, sets in before the long-range AF order is completely destroyed by hole doping. Magnetoresistance measurements of superconducting crystals with low T _c ≤15–20 K give a clear view of the magnetic-field induced superconductivity suppression and recovery of the long-range AF state. What still remains to be understood is whether the AF order actually persists in the SC state or just revives when the superconductivity is suppressed, and in the former case, whether the antiferromagnetism and superconductivity reside in nanoscopically separated phases or coexist on an atomic scale.     

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.