Evidence for superconducting fluctuations at 180 K: influence of Zn, Fe and Co doping on the phonon renormalization of YBa₂Cu₃O_{7- delta}

Abstract

Detailed studies on the mid-infrared-active phonon spectra 01 YBa&u-,M,)30i-b; M = CO, Fe and Zn for 0 Q 1: Q IO%, have been carried out from 30 K to mom temperature. The phonon mode near 580 cm-' softens at the supermnducting transition temperature (Tc), as reponed earlier. The new observation is a marked difference in the teniperature dependence of the phonon in Co. and Fe-doped compounds as opposed to Zndoped and undoped material. We observe thal the phonon begins to soften at temperatures well obove 7;-in some cases N ZT, (= 180 K)-in the CO-and Fe-doped YBazCuaOi-8, whilst no such behaviour is detected in Zndoped and undoped samples We a s u e that this precursor phonon softening above T, in COand Fewbslituted samples is due to superconducting Huctuations. Our proposal is that the transition-metal-ion substitution affect3 the diniom'onalily of the system and the ramifications of this a# crucial for explaining the observed differences between Ztilundoped YBa2Cu301-6 and ColFedoped compounds. Our observations cast doubt on the idea that m superconductors like YBazCuaOi-6 show a Kosterlitz-i%ouless behaviour. Instead, it appears tllat T, -0 K and fluctuations are reduced for the fully decoupled layea.

Optical Study on Phonon Anomalies and Spin Gap inYBa2(Cu1−xHauff1, Tajima2, Jang3 et al. 1996Phys. Rev. Lett.374211View full textAdd to dashboardCite

“…Concerning the phonon anomalies above T c there has been an attempt to explain them in the framework of a renormalization of the phonon self-energy due to a coupling to superconducting fluctuations [23]. However, normal state phonon anomalies are always observed at T s 150 K in YBCO compounds (T c 45 60 K) [7] and at T s 120 150 K in Y124 (T c 78 82 K) [10,11,13], which means T s does not scale with T c .…”

Superconducting fluctuations and anomalous phonon renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

“…We note that phonon anomalies have been reported for cuprate superconductors at temperature as high as ~ 2T c attributed to the superconducting order-parameter fluctuations. 26 The fact that precursor effects seen in this work have not been observed in other ironbased superconductors is possibly related to the reduced dimensionality of the Cooper pair wavefunction due to large distance d Fe between the two Fe-As layers in the unit cell. The large anomalous softening of the frequency at T o and significant broadening of the observed mode suggests that the phonon frequency is very close to the superconducting gap (i.e 2∆ ~ 230 cm -1 ) in superconducting clusters existing above T c .…”

Effects of Zn substitution for Cu on Raman phonon anomalies in double-chainYBa2Cu4Kall1, Litvinchuk2, Börjesson3 et al. 1996Phys. Rev. B10010View full textAdd to dashboardCite

“…[11][12][13][14] The onset temperatures of the above-T c phonon anomalies are similar to the temperature at which several experiments have indicated an opening of a spin gap 15,16 accompanied by changes in in-plane resistivity 17,18 and structure. 19 At present it is not clear what the mechanism would be for a coupling of spin fluctuations to certain phonons.…”