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Register a new userAbsence of the 1/8 -anomaly in oxygen isotope effect on YBa$_{2}$Cu$_{3}$O$_{y}$
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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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Inelastic neutron scattering data from a twinned single-crystal of YBa2Cu3O6.95 are presented that show a distinct a-b plane anisotropy in the oxygen vibrations. The Cu-O bond-stretching type phonons are simultaneously observed along the a and b directions due to a 4 meV splitting arising from the orthorhombicity. The present results show the bond-stretching branch along b (parallel to the chain) has a continuous dispersion, while the branch along a is discontinuous, suggesting a possibility of short-range cell-doubling along a. Furthermore, the LO mode along a is split in en-ergy from its TO partner at non-zero q-vectors, while the b mode is not. These results imply strong anisotropy in the electronic screening and a one-dimensional character in underlying charge fluctuations.
Systematic measurements of the $^{63}$Cu(2) NQR line width were performed in underdoped YBa$_2$Cu$_3$O$_{7-y}$ samples over the temperature range 4.2 K $<T<300$ K. It was shown that the copper NQR line width monotonically increases upon lowering temperature in the below-critical region, resembling temperature behavior of the superconducting gap. The observed dependence is explained by the fact that the energy of a condensate of sliding charge-current states of the charge-density-wave type depends on the phase of order parameter. Calculations show that this dependence appears only at $T<T_c$. Quantitative estimates of the line broadening at $T<T_c$ agree with the measurement results.
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 report the infrared (IR) response of Cu-O chains in the high-$T_{c}$ superconductor YBa$_{2}$Cu$_{3}$O$_{y}$ over the doping range spanning $% y=6.28-6.75$. We find evidence for a power law scaling at mid-IR frequencies consistent with predictions for Tomonaga-Luttinger liquid, thus supporting the notion of one-dimensional transport in the chains. We analyze the role of coupling to the CuO$_{2}$ planes in establishing metallicity and superconductivity in disordered chain fragments.
Superconducting critical currents $j_{c} > 10^{5}$ A/cm$^{2}$ at temperatures $T sim 50$ K and magnetic fields $B sim 6$ T are reported for the YBa$_{2}$Cu$_{3-x}$Mo$_{x}$O$_{7+d}$ compound with $x = 0.02$. Clear evidence for the increased pinning force $F_p$ was found from a peak effect present for $j_{c}(B)$. The pinning force was analyzed by a scaling procedure using Kramers approach. For a wide range of fields and temperatures, we were able to express all $F_p$ data as a single function of a reduced field $b = B/B_k$, where the scaling field $B_{k} << H_{c2}$ was related to the irreversibility filed $B_{irr}$. Analyses of the field dependence of $j_{c}(B,T)$ and $F_{p}(b)$ show that the effective pinning centers act as weakly interacting extended point-like defects. We propose that the pinning centers are randomly distributed small defects, most likely the dimers of MoO$_6$ octahedra in the CuO chains.
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