Optical conductivity of cuprates in a new light

Abstract

Understanding the physical properties of unconventional superconductors as well as of other correlated materials presents a formidable challenge. Their unusual evolution with doping, frequency, and temperature, has frequently led to non-Fermi-liquid (non-FL) interpretations. Optical conductivity is a major challenge in this context. Here, the optical spectra of two archetypal cuprates, underdoped HgBa 2 CuO 4+δ and optimally-doped Bi 2 Sr 2 CaCu 2 O 8+δ , are interpreted based on the standard Fermi liquid (FL) paradigm. At both dopings, perfect frequency-temperature FL scaling is found to be modified by the presence of a second, gapped electronic subsystem. This non-FL component emerges as a well-defined mid-infrared spectral feature after the FL contribution-determined independently by transport-is subtracted. Temperature, frequency and doping evolution of the MIR feature identifies a gapped rather than dissipative response.In contrast, the dissipative response is found to be relevant for pnictides and ruthenates. Such an unbiased FL/non-FL separation is extended across the cuprate phase diagram, providing a natural explanation why the superfluid density is attenuated on the overdoped side. Thus, we obtain a unified interpretation of optical responses and transport measurements in all analyzed physical regimes and all analyzed compounds.

Fermi liquid-like behaviour of cuprates in the pseudogap phase simulated via T-dependent electron-boson spectral density

“…This result is very similar to the experimentally observed results of underdoped cuprate systems 4 . We could not find the reason why our data show a p-value of 2.0 rather than values less than 2 as in the published papers 4,6 yet. It is worth noting that the optical scattering rate in this paper ( 1/τ op (ω) = −2� op 2 (ω) ) is different from the optical scattering rate ( /τ (ω) ) in the literature by Kumar et al 6 , i.e., /τ (ω) = 1/τ op (ω)(m b /m * (ω)) , where m b is the band mass and m * (ω) is the optical effective mass and m * (ω)/m b ≡ 1 + [−2� op 1 (ω)]/ω .…”

“…We could not find the reason why our data show a p-value of 2.0 rather than values less than 2 as in the published papers 4,6 yet. It is worth noting that the optical scattering rate in this paper ( 1/τ op (ω) = −2� op 2 (ω) ) is different from the optical scattering rate ( /τ (ω) ) in the literature by Kumar et al 6 , i.e., /τ (ω) = 1/τ op (ω)(m b /m * (ω)) , where m b is the band mass and m * (ω) is the optical effective mass and m * (ω)/m b ≡ 1 + [−2� op 1 (ω)]/ω . However, even though the optical scattering rates in the absence of the pseudogap fall into a single curve at low values of ξ 2 for p = 1.5 and 1.7 (Fig.…”

“…7 and is getting worse as the temperature increases. Therefore, the results are not consistent with those of underdoped cuprates in the literature 6 .…”

“…As we can see in the figure, the conductivity consists of two components: a Drude component and a Lorentz one. The Lorentz one is known as the mid-infrared absorption (or the conformal tail) 5,6,35 . The results are consistent with those of underdoped cuprates 5,6 ; the quality of the fit is getting better as the temperature increases.…”

“…These findings can provide important information to better understand the metallic state and high-temperature superconductivity in cuprates. There are some recent intriguing optical studies on Fermi-liquid-like behavior and the strange (or bad) metal states of cuprates 5,6 . The authors showed that the optical conductivity in the low-energy region consisted of two components, which are the Drude component and the mid-infrared absorption one.…”