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Register a new userOxygen isotope effect on superconductivity and magnetism in Y_1-xPr_xBa_2Cu_3O_7-delta
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Oxygen isotope (^{16}O/^{18}O) effects (OIEs) on the superconducting transition (T_c), the spin-glass ordering (T_g), and the antiferromagnetic ordering (T_N) temperatures were studied for Y_1-xPr_xBa_2Cu_3O_7-delta as a function of Pr content (0.0leq x leq 1.0). The OIE on T_c increases with increasing x up to xapprox0.55, where superconductivity disappears. For decreasing x the OIEs on T_N and T_g increase down to xapprox 0.7 where antiferromagnetic order and down to xapprox0.3 where spin-glass behavior vanish, respectively. The OIEs on T_g and T_N are found to have {it opposite signs} as compared to the OIE on T_c. All OIEs are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers.
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In this paper, pressure effect on superconductivity and magnetism has been investigated in FeSex (x = 0.80, 0.88). The magnetization curves display anomaly at Ts1 106 K and Ts2 78 K except for the superconducting diamagnetic transition around Tc 8 K. The magnetic anomaly at Ts1 and Ts2 can be related to a ferromagnetic and an antiferromagnetic phase transition, respectively, as revealed by specific heat measurements. The application of pressure not only raises Tc, but also increases both Ts1 and Ts2. This system shows clear evidence that superconductivity arises in a phase with strong magnetic character and the superconductivity coexists with magnetism. In addition, the specific heat anomaly associated with the superconducting transition seems to be absent.
Muon-spin rotation (muSR) studies of the oxygen isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth lambda_{ab} in cuprate high-temperature superconductors (HTS) are presented. First, the doping dependence of the OIE on the transition temperature T_c in various HTS is briefly discussed. It is observed that different cuprate families show a similar doping dependence of the OIE on T_c. Then, bulk muSR, low-energy muSR, and magnetization studies of the total and site-selective OIE on lambda_{ab} are described in some detail. A substantial OIE on lambda_{ab} was observed in various cuprate families at all doping levels, suggesting that cuprate HTS are non-adiabatic superconductors. The experiments clearly demonstrate that the total OIE on T_c and lambda_{ab} arise from the oxygen sites within the superconducting CuO_2 planes, demonstrating that the phonon modes involving the movement of planar oxygen are dominantly coupled to the supercarriers. Finally, it is shown that the OIE on T_c and lambda_{ab} exhibit a relation that appears to be generic for different families of cuprate HTS. The observation of these unusual isotope effects implies that lattice effects play an essential role in cuprate HTS and have to be considered in any realistic model of high-temperature superconductivity.
Oxygen-isotope effect has been investigated in a recently discovered superconductor Sr_{0.4}K_{0.6}BiO_{3}. This compound has a distorted perovskite structure and becomes superconducting at about 12 K. Upon replacing ^{16}O with ^{18}O by 60-80%, the T_c of the sample is shifted down by 0.32-0.50 K, corresponding to an isotope exponent of alpha_{O} = 0.40(5). This isotope exponent is very close to that for a similar bismuthate superconductor Ba_{1-x}K_{x}BiO_{3} with T_c = 30 K. The very distinctive doping and T_c dependencies of alpha_{O} observed in bismuthates and cuprates suggest that bismuthates should belong to conventional phonon-mediated superconductors while cuprates might be unconventional supercondutors.
We report the first direct observation of the oxygen-isotope ($^{16}$O/$^{18}$O) effect on the in-plane penetration depth $lambda_{ab}$ in a nearly optimally doped YBa$_2$Cu$_3$O$_{7-delta}$ film using the novel low-energy muon-spin rotation technique. Spin polarized low energy muons are implanted in the film at a known depth $z$ beneath the surface and precess in the local magnetic field $B(z)$. This feature allows us to measure directly the profile $B(z)$ of the magnetic field inside the superconducting film in the Meissner state and to make a model independent determination of $lambda_{ab}$. A substantial isotope shift $Deltalambda_{ab}/lambda_{ab}=2.8(7)$% at 4 K is observed, implying that the in-plane effective supercarrier mass $m_{ab}^ast$ is oxygen-isotope dependent with $Delta m_{ab}^ast/m_{ab}^ast = 5.5(1.4)%$.
We have prepared oxygen isotope exchanged crystals of impurity-free YBCO with various oxygen concentents, and examined pure doping ($p$) dependance of isotope effect on superconducting transition temperature. With decreasing oxygen contents, the isotope exponent $alpha$ monotonously increases without any anomaly around $p = 1/8$. The monotonous increase in $alpha$ indicates that phonons are involved in the mechanism which causes the monotonous $T_c$ suppression with underdoping.
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