2026/07/20 by Ilmo Sung
Physics and Astronomy · #Physics of Superconductivity and Magnetism #Surface and Thin Film Phenomena #Topological Materials and Phenomena #cond-mat.str-el #cond-mat.supr-con
paper · pdf · doi:10.1088/1361-648x/ae8d6c
published as J. Phys.: Condens. Matter 38, 305601 (2026) · 17 pages, 2 figures, 1 table; accepted for publication in Journal of Physics: Condensed Matter
arxiv created 2026/07/23 · arxiv updated 2026/07/31
Momentum-resolved electron energy-loss spectroscopy (EELS) on strange-metal Bi2Sr2CaCu2O8+x (Bi-2212) observes a broad charge continuum that is nearly momentum independent over an extended finite-momentum regime, whereas optical spectroscopy determines a metallic local conductivity. We analyze the long-wavelength response function that connects these regimes: the proper homogeneous bulk charge-density polarization. For any homogeneous U(1)-conserving metal whose longitudinal matter conductivity remains finite in the q→0 limit at fixed nonzero frequency, the Ward identity gives χ''ρρ(\bf q,ω)=q2ReσL(\bf q,ω)/ω. We then prove that any nonnegative normalized momentum-resolution kernel with a self-similar q-scaled profile, uniformly bounded second moment, and tight second-moment tails preserves this q2 asymptotic behavior, changing only the prefactor. Consequently, a nearly local finite-q continuum in Bi-2212, if continuously connected to a regular metallic optical limit of the same proper bulk response, is naturally described by a crossover surface q_∗(ω,T). The theorem is a constraint on the proper bulk density response, or on a positive q-scaled convolution of it; screened loss functions and surface EELS observables require the corresponding electrodynamic conversion before the constraint is applied.