Abstract Body

Progress toward a complete understanding of cuprate superconductors has been hindered by their intricate phase diagram, potentially linked to a quantum critical point (QCP). However, conclusive evidence for the QCP is lacking, as the presumed QCP is buried under the superconducting dome, disguising its presence. Here, we use high-resolution resonant inelastic X-ray scattering to examine the dynamical charge-charge correlation in La2−xSrxCuO4 and uncover the quantum critical scaling, a key feature required for a QCP. Specifically, we observed that the inverse correlation lengths for various doping levels and temperatures collapsed onto a universal scaling curve, yielding a critical exponent ν of 0.074 0.08. The non-negativity of this exponent confirms the presence of a QCP. Remarkably, the value of ν suggests that while the QCP is manifested through the charge-density wave, other orders also participate, such that the QCP appears to belong to the universality class characterized by the O(4) symmetry, reminiscent of the microscopic SO(4) symmetry in the Hubbard model at half-filling. Further analysis indicates that the QCP is highly dissipative with a short quasi-particle lifetime, reflecting the intertwined quantum fluctuations due to its being buried inside the superconducting state.

Abstract Body

The superconductivity with a high critical temperature (T_c) is generated by carrier doping to a Mott insulator in cuprates. Therefore, the understanding of the electronic properties of CuO₂ planes with a low carrier concentration close to the half-filed Mott state is essential to elucidate the mechanism of the high-T_c superconductivity. The electronic phase diagram established in cuprates so far has been based on studies for single and double-layered cuprates. However, the conductive CuO₂ sheets in these compounds are adjacent to dopant layers which causes a strong disorder effect as revealed by scanning tunneling spectroscopy. Hence, the intrinsic nature of the electronic state, especially of a lightly doped region that is sensitive to disorder due to a weak screening effect, might not have yet been understood well. This circumstance might have even been a major cause hindering one from unveiling the superconducting mechanism in cuprates. This difficult situation, however, could be overcome by focusing as a research target on the inner CuO₂ plane in multilayer cuprates, which avoids direct contact with dopant layers and thus can realize a clean, nearly ideal electronic state [1]. Especially note that the formation of small Fermi pockets predicted in a dropped Mott state has not been observed in the prototype single- and double-layered cuprates; instead, an arc-like Fermi surface has been observed in them. The origin of the Fermi arc and its relationship with a small Fermi pocket and large Femi surface is still under debate, and it has been one of the central topics not only in cuprate research but also in condensed matter physics.

In my presentation, I will introduce our recent ARPES studies of multi-layered cuprates Ba₂Ca_n−1Cu_nO_2n(F,O)₂ [2,3]. These compounds have clean,  thus ideal CuO₂ planes in the inner layers, which, thus, may reveal the phase diagram inherent for cuprates that has not been unveiled so far. Most importantly, we found small Fermi surface pockets around (π/2,π/2) consistently by ARPES and quantum oscillation measurements [2]. We also find that the d-wave superconducting gap opens along the pocket, thus the superconductivity and antiferromagnetic order coexist in the same CuO₂ sheet. By increasing the number of CuO₂ planes per unit cell up to six, we could reduce the carrier concentration of the innermost CuO₂ planes down to less than 1%. Surprisingly, we found a tiny Fermi pocket to exhibit well-defined quasiparticle peaks, lacking the polaronic feature that was expected in the prototype single- and double-layered cuprates [3].  This indicates that the slightest amount of carriers is enough to turn a Mott-insulating state into a metallic state with long-lived quasiparticles in CuO₂ with the disorder removed. By tuning hole carriers, furthermore, we found an unexpected phase transition from the superconducting to metallic states. These results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates, possibly indicating more intrinsic electronic states in the lightly doped cuprates.

References

[1] H. Mukuda et al., Journal of the Physical Society of Japan 81, 011008 (2012).

[2] S. Kunisada et al., Science 369, 833 (2020).

[3] K. Kurokawa et al., Nature communications 14, 4064 (2023).

Abstract Body

Study of the Cu-spin correlation is indispensable in elucidating the mechanism of superconductivity in high-T_c cuprates. In the hole-doped cuprates, neutron-scattering [1] and muon-spin-relaxation (μSR) [2] measurements revealed that the antiferromagnetic (AF) spin correlation weakened with overdoping of holes and disappeared in the non-superconducting (SC) heavily overdoped regime, suggesting the intimate relation between the AF spin correlation and superconductivity. In the electron-doped cuprate Pr_1-xLaCe_xCuO₄, on the other hand, neutron-scattering experiments suggested the robust AF correlation in non-SC heavily overdoped regime [3], whereas μSR measurements suggested the disappearance of the development of Cu-spin correlation together with superconductivity in the heavily overdoped regime [4]. However, details remain unclear because of predominant effects of the Pr³⁺ moment on the μSR spectra.

In this study, to clarify the relationship between the Cu-spin correlation and superconductivity in the electron-doped cuprates, we performed transport and μSR measurements of La_2-xCe_xCuO₄ (LCCO) without rare-earth moments.

Overdoped LCCO (x = 0.13, 0.17) target samples were prepared by solid-state reaction [5]. Thin films were fabricated on the SrTiO₃ substrate by the pulsed-laser deposition method using a third harmonic of Nd:YAG laser (wavelength : 355 nm) as a light source, which is different from the previous report [6]. By varying the oxygen partial pressure and reduction annealing conditions, LCCO thin films with x = 0.13 and 0.17 having T_c = 20 K and 3 K were fabricated, respectively. μSR measurements using low-energy muons were performed at the MuE4 beamline at the Paul Scherrer Institut in Switzerland.

Figures show zero-field μSR spectra of optimally reduced LCCO with x = 0.13 and 0.17. Fast relaxation of muon spins is observed at low temperatures in both samples, indicating the development of the Cu-spin correlation. As the temperature dependence of the spectra is almost identical between x = 0.13 and 0.17, this suggests that the development of the Cu-spin correlation is less doping-dependent, which is different from the results of Pr_1-xLaCe_xCuO₄ [4]. The development of the Cu-spin correlation at x = 0.17, where superconductivity is almost suppressed, may be related to a recently proposed ferromagnetic order [7].

References

[1] S. Wakimoto et al., Phys. Rev. Lett. 72, 064521 (2005).

[2] Risdiana et al., Phys. Rev. B 77, 054516 (2008).

[3] M. Fujita et al., Phys. Rev. Lett. 101, 107003 (2008).

[4] M. A. Baqiya et al., Phys. Rev. B 100, 064514 (2019).

[5] T. Yamada et al., Jpn. J. Appl. Phys. 33, L168 (1994).

[6] A. Sawa et al., Phys. Rev. B 66, 014531 (2002).

[7] T. Sarkar et al., Science 368, 532 (2020).

Acknowledgment

Helpful advises on the preparation of LCCO thin films by A. Maeda, F. Nabeshima, I. Tsukada are gratefully acknowledged. A part of the resistivity and Hall measurements were performed by using the commercial apparatus (PPMS) at the CROSS-user laboratory.

Figure 1. Zero-field μSR spectra of La_2-xCe_xCuO₄ with (a) x = 0.13 and (b) x = 0.17.

Keywords: High-T_c superconducting cuprate, Thin film, Spin correlation, Muon spin relaxation