Large copper isotope effect on the pseudogap in the high-temperature superconductor HoBa 2 Cu 4 O 8
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The copper isotope effect (63Cu vs.65Cu) on the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature superconductor HoBa2Cu4O8 has been investigated by inelastic neutron scattering. For the 63Cu compound there is clear evidence for the opening of an electronic gap in the normal state at T * ≈ 160 K far above T c = 79.0 K. Upon substitution of 63Cu by 65Cu, T c decreases marginally to 78.6 K, whereas T * is increased to about 185 K. This large copper isotope shift ΔT * (Cu) = T * ( 65 Cu) - T * ( 63 Cu) ≈ 25 K - together with the corresponding oxygen isotope shift ΔT * (O) = T * ( 18 O) - T * ( 16 O) ≈ 50 K found in an earlier investigation - suggests that phonons or lattice fluctuations involving both the copper and the oxygen ions are important for the pairing mechanism in high-T c materials.
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