Normal state specific heat in the cuprate superconductors La2−xSrxCuO4 and Bi2

Abstract

The specific heat C of the cuprate superconductors La2−xSrxCuO4 and Bi2+ySr2−x−yLaxCuO6+ was measured at low temperatures (down to 0.5 K) for dopings p close to p, the critical doping for the onset of the pseudogap phase. A magnetic field up to 35 T was applied to suppress superconductivity, giving direct access to the normal state at low temperatures, and enabling a determination of Ce, the electronic contribution to the normal-state specific heat at T→0. In La2−xSrxCuO4 at x=p=0.22, 0.24 and 0.25, Ce/T=15to16mJmol−1K−2 at T=2K, values that are twice as large as those measured at higher doping (p>0.3) and lower doping (p<0.15). This confirms the presence of a broad peak in the doping dependence of Ce at p0.19 as previously reported for samples in which superconductivity was destroyed by Zn impurities. Moreover, at those three dopings, we find a logarithmic growth as T→0 such that Ce/TBln(T0/T). The peak versus p and the logarithmic dependence versus T are the two typical thermodynamic signatures of quantum criticality. In the very different cuprate Bi2+ySr2−x−yLaxCuO6+, we again find that Ce/TBln(T0/T) at pp, strong evidence that this ln(1/T) dependence of the electronic specific heat-first discovered in the cuprates La1.8−xEu0.2SrxCuO4 and La1.6−xNd0.4SrxCuO4-is a universal property of the pseudogap critical point.

Transport signatures of the pseudogap critical point in the cuprate superconductor Bi2Sr2−xLaxCuO6Lizaire1, Legros2, Gourgout3 et al. 2021Phys. Rev. BSelf Cite253160View full textAdd to dashboardCite

“…Data at H = 10 and 15 T are practically superimposed, which indicates that the normal state is very nearly reached in the bulk. (Measurements of the specific heat on the same two samples show that the critical field for OD10K is indeed 15 T, while it is ∼25 T for OD18K [52].) This is reminiscent of what was found in Nd-LSCO, in which the normal state in thermal conductivity measurements was also reached at a smaller field than in electric transport measurements [20].…”

“…In Eu-LSCO, Nd-LSCO and Bi2201 a log(1/T ) divergence is also observed in the electronic specific heat, (C el /T ), at p [17,52]. This thermodynamic evidence of quantum criticality, combined with the T -linear resistivity, suggests that the log(1/T ) dependence of the Seebeck coefficient is also a signature of quantum criticality, as found in some theoretical models [65].…”

“…(Note that in the paper by Legros et al [13], the value of the specific heat in Bi2201 had to be extrapolated to higher magnetic fields than measured in Ref. [51] in order to extract the normal-state electronic specific heat at H c2 , yielding γ = 10 ± 2 mJ/K −2 mol, but recent specificheat data in Bi2201 obtained in high magnetic fields on our sample OD10K show a good agreement with the extrapolated value: γ = 8 ± 1 mJ/K −2 mol at 3 K [52]. With this last value, we get a theoretical slope, A/d = 6.3 ± 1.1 /K.)…”

Spin fluctuations associated with the collapse of the pseudogap in a cuprate superconductor

“…The single-layer cuprate superconductor La 2−x Sr x CuO 4 (LSCO) can be doped across the superconducting phase diagram, where the hole doping p = x. Normal-state heat-capacity measurements have been made at T ≈ T c [16,17] and also at lower temperatures with superconductivity suppressed by Zn doping [18], a high magnetic field [19], or a high magnetic field and Nd/Eu doping [14]. It is found that the specific heat γ(p) shows a peak at p = p c ≈ 0.22 (see Fig.…”

“…Thus, it is natural to ask whether spin fluctuations contribute to the large γ values near p c in LSCO. The situation in LSCO is subtle because the Fermi energy E F passes through a van-Hove singularity (VHS) [6,7] near p c as the doping is increased and the pseudogap [11] terminates at a critical doping p ≈ 0.19 [13,19] which is close to p c .…”