M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

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Charge density fluctuations shed light on the enigmatic strange metal phase of cuprates

Charge density fluctuations have been studied with resonant inelastic X-ray scattering as a function of temperature and doping in several cuprate superconductors. The results indicate a quantum critical behaviour, confirming the quantum critical point scenario for the strange metal and strengthening the role of charge order in explaining this unconventional phenomenology.

The normal state of cuprate superconductors is a strange metal phase characterised by highly unconventional properties, including a linear dependence of the resistivity with temperature, and anomalous features in the optical conductivity, magnetoresistance and spin relaxation rate [1]. This phase has intrigued physicists for decades. It has been observed in a variety of superconducting quantum materials, from pnictides to heavy fermions and, more recently, in magic-angle, double-layer graphene. This ubiquitous behaviour raises the question of what underlies the universality of the strange metal phase. The fact that superconductivity is strongest when the strange metal phase is more robust tantalises scientists, with the prospect of uncovering the microscopic mechanisms underpinning superconductivity within cuprate superconductors.

This enigmatic phase of matter is believed to be connected to the existence of a quantum critical point (QCP) at zero temperature [2]. A QCP is a singularity at a temperature of 0 K (−273.15°C), signalling a quantum phase transition between different states of matter. Reaching the absolute zero to directly observe the QCP is practically impossible. Nevertheless, when one gets closer to a quantum critical point, quantum fluctuations emerge. The detection of these fluctuations, which have remained elusive until now, is crucial to confirm if the physics of a strange metal is ruled by a QCP scenario and, in the affirmative case, to determine its true nature.

Charge density fluctuations (CDF) [3] could be the hypothetical quantum fluctuations ruling the physics close to a QCP. CDF are ripples of electric charge generated by patterns of electrons in the material lattice, recently observed in several cuprate families

Fig. 64: High-resolution (ΔE=38 meV) RIXS map along the (H,0) direction taken at a temperature T = 20 K for a strongly

underdoped (p = 0.06) YBa2Cu3O7-δ sample. The map is presented after subtracting the fit of the pure elastic peak from the raw spectra. In addition to lattice (phonons) and magnetic

(paramagnons) excitations, charge density fluctuations emerge at a finite energy, with a broad intensity peak at a

momentum q = qCDF = (0.35,0).