Superconductivity and possible overdoping in Lu1−x Ca x Ba2Cu3O7−δ

Structural and superconducting properties of the system Lu_1−x Ca _x Ba₂ Cu₃O_7−δ, both in bulk as well as thin film form, have been investigated. Presence of large Ca-ions at the Lu-sites is responsible for phase stability (in 1–2–3 phase) in bulk. In argon annealed tetragonal samples (δ:1) superconductivity is reinstated as in these samples, presence of Ca-ions lead to the generation of holes in the Cu−O planes. In oxygen annealed bulk samples and thin films, there is a large depression ofT _c due to divalent Ca-ions present in concentrationx<0.2. Our data indicate that this depression ofT _c is likely due to overdoping effect.     

Role of processing in improving the critical current of Ag/BPSCCO tapes

Silver-clad tapes of highT _c Bi-Pb-Sr-Ca-Cu-O superconductors have been fabricated through the powder-in-tube method. The critical current density, J _c , of a thick tape was 534 A.cm⁻² (77 K, 0 T). Subsequent rolling and sintering of the tapes led to a gradual decrease inJ _c, instead of the expected increase. This was caused by the microcracks developed in the core material by a rather drastic reduction during the rolling of the tapes. A modified and well controlled rolling technique, on the other hand, resulted in much improvedJ _c values. Repeated rolling and sintering resulted in a good grain alignment and no microcracks were observed. In the present studies, maximumJ _c of 1846 A.cm⁻² (77 K, 0 T) and 2.43 × 10⁴ A.cm⁻² (4.2 K, 0 T) have been obtained Optimization of the processing and sintering parameters are expected to lead to still higherJ _c values.     

Intrinsic Josephson effects in 60-K phase YBCO thin film mesas

We fabricatedc-axis oriented meas on YBa₂Cu₃O_7−δ thin films. Columnar structures with an area of 20×20 μm² and a depth of 0.1 μm were formed on oxygen deficient films with a critical temperature (T _c ) of 64 K or less. The devices showed hysteretic I–V curves without any branches. The dependence of the critical current density on the temperature nearT _c is explained by the Ambegaokar-Baratoff relation.     

Muon spin relaxation study on magnetism in high quality single crystal of a high transition temperature superconductor La2−xSrxCuO4−σ (0.11≤x≤0.14)

The positive muon spin relaxation method is applied to probe magnetic ordering in the superconducting phase of a high quality single crystal of La_2−xSr_xCuO_4−σ (0.11≤x≤0.14). The well characterized crystal ofx=0.11 (T _c=34.5 K) with nearly complete flux exclusion exhibits spin freezing at 8K(T _f) with significant spin fluctuation up to 20 K. The onset of spin fluctuation andT _f decrease against increasingx towardsx=0.15, suggesting an existence of a magnetic phase boundary aroundx whereT _c becomes maximum.     

Electronic states of boron in superconducting MgB2 studied by11B NMR

Nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation rateT ₁⁻¹ of¹¹B have been measured in superconducting polycrystalline MgB₂ with 7_c^ons = 39.5 K. It is shown that (T ₁T⁻¹ and the Knight shiftK _s are independent of temperature and nearly isotropic aboveT _c. Both of these quantities are decreased gradually in going to the superconducting state. According to NMR data the density of states near the Fermi level is flat at the scale of about 500 K. Some conclusions on the orbital content of the density of states at the Fermi level were drawn and compared with the results of the band structure calculations.     

Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

The spin-spin relaxation rate ⁶³ T ₂ ⁻¹ of ⁶³Cu nuclei in CuO₂ layers is measured in the normal and superconducting states of the compound YBa₂Cu₃O_6.9 (T _c ^onset =94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to T _c ^onset =68 K (Φ=7×10¹⁸ cm⁻²) and T _c ^onset <4 K (Φ=12×10¹⁸ cm⁻²). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction ⁶³ T _2G ⁻¹ , which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χ_s(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x ²−y ² symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-T _c cuprates. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 172–177 (10 February 1998)     

Micro-Raman response and phonon oscillator parameters in solid solutions Pb1 ? xCaxTiO3 (0.40 < x < 0.62)

The temperature (295 K < T < 650 K) investigations of the polarized Raman spectra measured for a single-crystal sample of the Pb_0.54Ca_0.46TiO₃ solid solution in the frequency range 10 < ν < 1200 cm⁻¹ have established the existence of at least one specific temperature (T _c ≈ 392 K) at which the oscillator parameters of phonon modes exhibit noticeable anomalies. Similar phenomena are observed in solutions with 0.4 < x < 0.5. A sharp increase in the intensity of the spectral lines corresponding to polarization waves at temperatures immediately below T _c suggests a transformation of Pb_{1 −x} Ca _x TiO₃ (0.4 < x < 0.5) into a polar state, which does not exist for x > 0.5. Original Russian Text ? A.N. Chabanyuk, V.I. Torgashev, Yu.I. Yuzyuk, V.V. Lemanov, P. Simon, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 155–163.     

The transition width and critical current density measurements on slow-cooled YBa2Cu3O7−x superconductor

It is shown that by adopting a very gradual programmed cooling procedure in oxygen environment one can obtain a sharp transition (ΔT _c⋍1 K) in YBa₂Cu₃O_7−x while retaining the high-T _c value (⋍105 K) of samples prepared by a semi-wet route. This is attributed to a maximum occupancy of oxygen at 0 1/2 0 sites and a near-perfect ordering of vacancies at 1/2 0 0 sites in the orthorhombic unit cell which maximizes the availability of conduction paths in the form of continuous CuO₄ chains. Critical current densities (J _c) of 204 A cm⁻² are obtained for bulk samples at 77 K. It is suggested that the intergrain coupling is weak and thus limits the transportJ _c-values.     

μ+SR study of vacancies in thermal equilibrium in ferromagnets

Muon spin precession frequencies and transverse relaxation rates have been measured on demagnetized iron, cobalt, and FeCo alloys (3 at%–50 at% Co) between room temperature and the Curie temperatureT _c. The increase of the relaxation rate in iron between 930 K and 1010 K could be quantitatively attributed to the trapping of positive muons by vacancies in thermal equilibrium, resulting in an enthalpy of monovacancy formation ofH _1V ^F =(1.7±0.1) eV. the smallest vacancy concentrations detected are = 10⁻⁸.     

Comparison of superconducting parameters of A 15- and σ-phases of Nb−Pt

The superconducting properties of A 15- and σ-phases of the system Nb−Pt were investigated. Alloys of compositions Nb₃Pt, Nb₂Pt, and Nb₆₂Pt₃₈ were prepared by arc melting and subjected to optical microscopic and x-ray analysis for structure determination. The transition temperatures were measured inductively as well as resistively. The temperature dependences of critical fields were measured up to 35 kG. The A 15-phase has aT _c value of 8.97K and a zero temperature upper critical fieldH _c2(0) of 100 kG as compared to aT _c value of 2.14K and anH _c2(0) value of 14kG for the σ-phase. A comparison with theoretical upper critical field values seems to indicate that a spin-orbit coupling mechanism is responsible for the higher value ofH _c2(0) in the case of the A 15-phase alloy. The temperature dependent resistance up to 300 K is also measured and discussed for both the phases.     

Metal-insulator transition in electron-doped Ba1−x La x MnO3 compounds

Electron-doped (Ba_1−x La _x )MnO₃ compounds were prepared for x=0−0.5. Measurements of X-ray diffraction (XRD) at room temperature and temperature variation of dc electrical resistivity down to 20 K were carried out. Samples with x=0.2–0.5 exhibit metal-insulator (M-I) transition. The maximum M-I transition temperature (T _c) of 289 K was observed for 30% of La doping (x=0.3). XRD patterns of these samples (x=0.2−0.5) were analyzed using Rietveld refinement. These samples are found to be mostly in single-phase form with orthorhombic symmetry (space group Pbnm). We have found strong correlation between Mn-O-Mn bond angles and T _c of M-I transition. The resistivity data below T _c could be fitted to the expression ρ=ρ ₁+ρ ₂ T ² and this shows that double exchange interaction plays a major role even in Mn⁴⁺-rich compound. Above T _c the resistivity data were fitted to variable range hopping and small polaron models.     

Zero-field current-voltage characteristics in high-Tc superconductors

Zero-field current-voltage (I–V) characteristics of various high-temperature superconductor samples are analyzed in the context of the current-temperature (I−T) phase diagram. After establishing the validity and relevance of the phase diagram to these materials, the anisotropy factor is extracted from the slope ofI _c ¹ (T) (the current defined by the onset of resistance). It is concluded that studying theI−V characteristics of amples in the context of theI−T phase diagram is a simple, useful tool for comparing samples. Work supported by the Office of Naval Research.     

Phase transitions in solid solutions (1 ? x)BaTiO3-xBaMg1/3B2/3O3 (B = Nb, Ta)

The dielectric properties of ceramic samples of (1 − x)BaTiO₃-xBaMg_1/3Nb_2/3O₃ and (1 − x)BaTio₃-xBaMg_1/3Ta_2/3O₃ solid solutions (x = 0–0.25) are investigated in the frequency range from 10 Hz to 100 kHz at temperatures of 77–450 K. It is shown that the (x−T) phase diagrams of these solid solutions at x = 0.05 have a multiphase point at which the lines of all three phase transitions of BaTiO₃ converge. Original Russian Text ? V.G. Zalesskiĭ, V.V. Lemanov, E.P. Smirnova, A.V. Sotnikov, N. V. Zaĭtseva, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 1, pp. 108–112.     

Influence of plastic deformation on superconductivity of Vanadium and Niobium

AT _c increase up to 0.5 K was produced by plastic deformation at low temperatures of Nb and V in a solid pressure cell. Annealing experiments on these materials show that the increase ofT _c disappears below 100 K while the change of the residual resistance removes only partially up to room temperature. This can be explained by tensile stress producing regions with enhancedT _c .     

Superconductivity of A15-and σ-phase of Nb-Ir

The superconducting properties of A15-, σ-, and tetragonal phases of the system Nb−Ir were investigated. The alloys were prepared by sintering and arc melting. They were subjected to optical and scanning electron microscopy. The lattice parameters were determined by x-ray diffraction technique. Superconducting transition temperatures,T _c , as well as upper critical fields,H _c2, were measured inductively and resistively. TheT _c -values of the σ-phase vary between 2.15 and 2.40 K whereas for the A15-phase they vary between 1.83 and 2.73 K. TheT _c -value of the tetragonal phase is 3.81 K. The upper critical fields of the tetragonal and the A 15-phase are nearly the same (≈13 kG) and lower compared with that of the σ-phase (≈18 kG). Several theoretically predicted values ofH _c2(0) are evaluated and compared with the experimental ones.     

Effect of hydrostatic compression on the indium-impurity-induced superconducting transition in Pb<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>Te

This paper reports on a study of the low-temperature conductivity and parameters of the superconducting state, namely, the critical temperature T _c and the second critical magnetic field H_c2, in the (Pb_0.3Sn_0.7)_0.95In_0.05Te solid solution under hydrostatic pressure P ≤ 9 kbar at T = 4.2 K. The choice of this material has been motivated by the fact that, according to earlier observations, it undergoes a superconducting transition at T _c ∼ 2.3 K, i.e., close to the maximum value T _c ∼ 2.9 K found for the (Pb _z Sn_{1 − z} )_0.95In_0.05Te solid solutions with a lead content z ∼ 0.15–0.25. It has been demonstrated that an increase in the pressure to P ≤ 9 kbar leads to a bell-shaped dependence T _c (P). The observed dependences are assigned to the effect of hydrostatic compression on the band structure of the solid solution and indicate a shift in the position of the Fermi level E _F with increasing pressure within the impurity band of the In quasi-local states. In this case, E _F passes through a maximum in the density of impurity states at P = 3–5 kbar.     

Superconductivity of thin films of lead,indium, and tin prepared in the presence of oxygen

We have studied the influence of oxygen on the superconducting properties of thin films of lead, indium and tin deposited on glass or sapphire substrates. In addition, the morphological microstructure was investigated by scanning electron microscopy. The film thickness was 1.0 μm, and the partial pressure of O₂ during the film deposition was raised up to 1×10⁻⁴ Torr. In all three materials the development of a granular structure and a strong increase in the residual electric resistivity was observed due to the O₂-treatment. Whereas in the Pb films no change of the critical temperature was found, the In films deposited on glass substrates showed a slight increase ofT _c due to the oxygen. The strongest increase ofT _c (up to 8%) was observed in the O₂-treated Sn films. These results are discussed in terms of the McMillan theory. From our measurements of the critical current densityj _c we conclude that edge pinning is dominant in the undoped films. All three materials showed a strong increase ofj _c due to the O₂-treatment which must be interpreted in terms of bulk pinning.     

Pulsed laser deposition of Zr–N codoped <Emphasis Type="Italic">p</Emphasis>-type ZnO thin films

Present p-type ZnO films tend to exhibit high resistivity and low carrier concentration, and they revert to their natural n-type state within days after deposition. One approach to grow higher quality p-type ZnO is by codoping the ZnO during growth. This article describes recent results from the growth and characterization of Zr–N codoped p-type ZnO thin films by pulsed laser deposition (PLD) on (0001) sapphire substrates. For this work, both N-doped and Zr–N codoped p-type ZnO films were grown for comparison purposes at substrate temperatures ranging between 400 to 700 °C and N₂O background pressures between 10⁻⁵ to 10⁻² Torr. The carrier type and conduction were found to be very sensitive to substrate temperature and N₂O deposition pressure. P-type conduction was observed for films grown at pressures between 10⁻³ to 10⁻² Torr. The Zr–N codoped ZnO films grown at 550 °C in 1×10⁻³ Torr of N₂O show p-type conduction behavior with a very low resistivity of 0.89 Ω-cm, a carrier concentration of 5.0×10¹⁸ cm⁻³, and a Hall mobility of 1.4 cm² V⁻¹ s⁻¹. The structure, morphology and optical properties were also evaluated for both N-doped and Zr–N codoped ZnO films.     

Ion transport in the anion-deficient nonstoichiometric phases La0.95(Ba1−x Srx)0.05F2.95 (0⩽x⩽1)

The fluoride-ion conductivity of the nonstoichiometric tysonite phases La_0.95(Ba_1−x Sr_x)_0.05Fe_2.95 (0⩽x⩽1) is investigated by impedance spectroscopy. Electrophysical measurements are performed in the frequency range 5–5×10⁵ Hz and temperature range 300–700 K. A discontinuity is observed in the temperature dependence of the conductivity at T _c=410–430 K. The behavior of the temperature dependence of the electrical conductivity is explained within a transport model taking into account the migration of fluoride ions between different inequivalent structural sites. The maximum value of the conductivity at room temperature (293 K) is 2×10⁻⁴ Ω⁻¹ cm⁻¹ for the solid solution La_0.95Sr_0.05F_2.95. The fluorine-ion conductivity in La_0.95(Ba_1−x Sr_x)_0.05F_2.95 single crystals is almost an order of magnitude larger than the value for the commercial solid electrolyte La_0.992Eu_0.008F_2.992 (a fluorine-selective membrane) having a tysonite structure. Fiz. Tverd. Tela (St. Petersburg) 40, 658–661 (April 1998)     

Effect of Zr,Ti substitutions onT c in superconducting YBa2Cu3O7−δ system

Zr and Ti have been substituted in the system Y_1−x M _x Ba₂Cu₃O_7−δ(M=Zr, Ti,x=0.05,0.1). We find that theT _c value is unchanged, ∼90K for Zr substitutions up to 10% and in the case of Ti substitutions theT _c drops significantly. These changes may arise from their site preference.