Disorder and superconducting-state conductivity of single crystals ofYBa2Cu3O6.95
Physical Review B (Condensed Matter)
The ab-plane optical conductivity of YBa&Cu306.95 has been studied by infrared re8ectance spectroscopy in the presence of disorder introduced by ion irradiation. The frequency dependence of the conductivity in the superconducting state suggests that ion irradiation has caused carrier localization. Localization explains both the suppression of T, and the reduction of the super6uid fraction. The temperature dependence of the penetration depth in pure and disordered YBaqCu306. 95 supports the
... 5 supports the hypothesis of non-8-wave superconductivity. High-T, materials can undergo a metal-insulator transition, either through reduced carrier concentration by chemical doping or by the addition of scattering From defects introduced by ion irradiation. In this paper we show evidence that low energy ions have an advantage, since they influence the normal-state transport and suppress the critical temperature without modiFying the chemical composition of the sample. This opens up new possibilities for studying the relationship between normaland superconducting-state properties in materials with identical chemical compositions. Moreover, following Anderson's theorem, 2 nonmagnetic impurities are expected to have negligible influence on superconductivity in the case of 8-wave pairing whereas a superconductor with an unconventional pairing state (p or d wave) is strongly afFected by impurities. s Thus, a study of disorder may provide an experimental test of the nature of the superconducting state in high-T, materials. We focused on changes in the ab-plane optical conductivity of YBa2Cu306 95 single crystals resulting &om disorder introduced by ion irradiation. The conductivity of the damaged crystal is consistent with carrier localization. This allows us to explain a variety of effects resulting from the ion irradiation of YBa2CusOs ss (YBCO). The temperature dependence of the superfluid density changes &om a linear law at low temperature in the pure crystal to 1 -(T/T, )" with n between 2 and 4 in the strongly damaged sample suggesting non-8-wave superconductivity in the YBCO system. High quality single crystals of YBa2Cu306 95 with mirror smooth ab faces were grown using a flux technique. Inductively coupled plasma (ICP) mass spectroscopy shows impurities at the 0.1 level. The crystals have a 0.5 K wide transition at 93.5 K and demonstrate a sharp peak in heat capacity and an abrupt drop in microwave losses at T,. 5 These crystals are almost defect &ee since the scattering rate at T = 4 K is as small as 1 cm The ab face of the crystal was irradiated with 160 keV He+ ions which provide homogeneous damage to a depth of 400 nm. The penetration depth along the c axis in the superconducting state is enhanced in the irradiated samples, but remains less than 400 nm so that the re-flectivity spectra measured &om the ab face of the crystal are solely determined by the optical constants of the damaged layer. While the thickness of the damaged layer is sufficient for the in&ared spectroscopy, measuring the transport properties is problematic. We irradiated c axis YBa2Cu30~thin films to evaluate the correlation between the ion dose and the critical temperature of the irradiated YBCO system. Figure 1 shows that the ternperature dependence of the resistivity of 40 nm thick YBa2Cu307 films remains linear for dose levels of less than 10~H e/cm2, but with suppressed T,. In agreement with the results of Valles et at. , the absolute resistivity is increased by a constant amount, proportional to the dose level. We observed similar effects induced by ion irradiation in 200 nm thick films. Based on the thin film data, we expect a reduction in the crystals' critical temperature &om 93.5 K before irradiation down to 85, 80, and 70 K with dose levels of 2, 4, and 8 x10 He/cm, respectively.