Depression of Quasiparticle Density of States at Zero Energy inLa1.9Sr0.1Cu1−

C. F. Chang

¹

,

Jiunn-Yuan Lin

²

,

H. D. Yang

³

Abstract

We have measured low-temperature specific heat C(T, H) of La 1.9 Sr 0.1 Cu 1-x Zn x O 4 (x=0, 0.01, and 0.02) both in zero and applied magnetic fields. A pronounced dip of C/T below 2 K was first observed in Zn-doped samples, which is absent in the nominally clean one. If the origin of the dip in C/T is electronic, the quasiparticle density of states N(E) in Zn-doped samples may be depressed below a small energy scale E 0 . The present data can be well described by the model N(E)=N(0)+aE 1/2 , with a non-zero N(0) and positive a. N(E) is argued to vanish as E or E 2 depending on whether the time reversal is a good symmetry or not.In principle, the low-temperature specific heat (LTSH) C(T) is a powerful probe of N(E) of the quasiparticle low energy excitation. Nevertheless, previous LTSH experiments in impurity-doped (especially Zn-doped) [14,15,20] cuprates usually suffered a upturn in C/T at low temperatures. This upturn hinders the investigation of the low-temperature electronic contribution in C, and is presumably due to either a hyperfine contribution or the local magnetic moment both of which probably are associated with defects in samples.To shed light on the issue of the low-energy quasiparticle N(E), we have carefully prepared Zn-and Ni-doped La 1.9 Sr 0.1 CuO 4 . These samples show no upturn in C/T down to the lowest experiment temperature 0.6 K. Therefore, LTSH can be readily used to probe N(E) and provide

Low-energy quasiparticles in high- T c cuprates

“…This implies that in contrast to the optimally doped samples, ® … 0 † in overdoped La2¡xSrxCuO4 might also contain an additional contribution due to a small fraction of normal phase. Finally, the experimental situation in La2¡xSrxCuO4 has been complicated further by the latest speci®c heat data on polycrystalline Zn-doped La 1:9 Sr 0:1 CuO 4 [81]. For the undoped (yˆ0) sample, no T 2 term was observed, whilst in the Znand Ni-doped samples, there is a clear dip in C=T , in contrast to previous studies in both La2¡xSrxCuO4 [75] and YBa2Cu3O7¡¯ [70].…”

Defects in correlated metals and superconductors

“…tematic studies with Zn doping in single crystals were performed by several groups (Chang et al, 2000;Loram et al, 1990b;Momono and Ido, 1996;Sisson et al, 2000), verifying the expected dependence of the residual density of states on impurity concentration. (Sisson et al, 2000) were the first group to do a systematic comparison of the magnetic field dependence of Zn-doped samples with the Schottky anomaly expected on the basis of the assumption of local moments of order 0.3µ B , the typical order of magnitude found for these moments above T c in optimally doped samples in early direct magnetization experiments (Mendels et al, 1994b;Zagoulaev et al, 1995) on polycrystalline samples.…”

Details of Disorder Matter in 2Dd-Wave Superconductors

“…4(d)]. We speculate that this may be the case with Zn, which is clearly a unitary scatterer in BSCCO [14], but appears to produce a depression in the DOS at small energies in LSCO, as seen in recent specific heat experiments [21].…”