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8 5 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

Signature of quantum criticality in cuprates by charge density fluctuations, R. Arpaia (a), L. Martinelli (b), M. Moretti Sala (b), S. Caprara (c), A. Nag (d), N.B. Brookes (e), P. Camisa (b), Q. Li (f), Q. Gao (f), X. Zhou (f), M. Garcia-Fernandez (d), K.-J. Zhou (d), E. Schierle (g), T. Bauch (a), Y.Y. Peng (f), C. Di Castro (c), M. Grilli (c), F. Lombardi (a), L. Braicovich (b,e), G. Ghiringhelli (b,h), Nat. Commun. 14, 7198 (2023); https:/doi.org/10.1038/s41467-023-42961-5 (a) Chalmers University, Göteborg (Sweden) (b) Politecnico di Milano, Milan (Italy) (c) Università La Sapienza, Rome (Italy) (d) Diamond Light Source, Didcot (UK) (e) ESRF (f) Peking University, Beijing (China) (g) BESSY II, Berlin (Germany) (h) CNR-SPIN, Milan (Italy)

REFERENCES

[1] P. W. Phillips et al., Science 377, eabh4273 (2022). [2] S. Sachdev, Phys. Status Solidi (b) 247, 537 (2010). [3] R. Arpaia et al., Science 365, 906 (2019). [4] R. Arpaia, G. Ghiringhelli, Phys. Soc. Jpn. 90, 111005 (2021). [5] G. Seibold et al., Commun. Phys. 4, 7 (2021).

and pervading the whole strange metal region above and below the pseudogap temperature T* [4]. Due to their characteristics presence in a broad range of doping in the phase diagram, finite energy and short correlation length that implies a broad, almost isotropic q-space distribution they were immediately connected by various theories to the strange metal phase of the cuprates [5].

In this work, CDF were investigated using resonant inelastic X-ray scattering (RIXS) measurements (Figure 64), at beamline ID32 and at Diamond Light Source (UK). To nail down exactly where the QCP is located in the phase diagram, CDF were extensively studied in two families of cuprate superconductors across a wide range of doping levels and temperatures.

The results of the experiments pinpoint a significant finding: by approaching the absolute zero temperature, at a specific critical doping level, p* ≈ 0.19, the putative QCP, the CDF intensity reaches its maximum, while the characteristic energy of these fluctuations is minimal (Figure 65). This phenomenon carves out a distinctive wedge-shaped region in the phase diagram as a function of doping a strong indication of a quantum critical behaviour.

Fig. 65: The CDF energy Δ, directly measured at q = qCDF in the high-resolution RIXS spectra and at low temperature, is plotted as a function of the doping level p. It is lowest at p = p* = 0.19, while it increases at all dopings. The values of Δ, converted into kelvin, define a characteristic wedge with a minimum at p*, lining up with the border of the strange metal phase as determined by transport.

These results uncover the connection of the strange metal phase to quantum criticality and support the leading role of charge order in driving this unconventional phenomenology. Ultimately, these interconnections add new elements to our understanding of high critical- temperature superconductivity.