Two relaxation peaks were found in the complex susceptibility of ferrofluids. Both can be described by the Vogel-Fulcher law t t 0 f͑T͒ exp͓E͞k͑T 2 T 0 ͔͒. Nevertheless, the physical origins for these two relaxations are quite different. We found that Néel relaxation strongly depends on the dipoledipole interaction. The dramatic dependence can be described by a surprisingly simple scaling relation: t t 0 exp͓E͞k͑T 2 af 0.8 ͔͒, where f is the volume fraction of the dipoles. In contrast, Brownian relaxation is much less sensitive to the concentration of magnetic moments because the interparticle force is mainly hydrodynamic in nature. [S0031-9007(96)00634-5] PACS numbers: 75.50. Mm, 61.20.Lc, 75.40.Gb, 82.70.Dd 0031-9007͞96͞77(2)͞390(4)$10.00

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We show that the recent magnetoresistance data on thin-film La2−xSrxCuO4 (LSCO) in strong magnetic fields (B) 1 obeys a single-parameter scaling of the form MR(B,, at which point the single-parameter scaling breaks down. The functional form of the MR is distinct from the simple quadratic-tolinear quadrature combination of temperature and magnetic field found in the optimally doped iron superconductor BaFe2(As1−xPx)2 2 . Further, low-temperature departure of the MR in LSCO from its high-temperature scaling law leads us to conclude that the MR curve collapse is not the result of quantum critical scaling. We examine the classical effective medium theory (EMT) previously 3 used to obtain the quadrature resistivity dependence on field and temperature for metals with a T -linear zero-field resistivity. It appears that this scaling form results only for a binary, random distribution of metallic components. More generally, we find a low-temperature, high-field region where the resistivity is simultaneously T and B linear when multiple metallic components are present. Our findings indicate that if mesoscopic disorder is relevant to the magnetoresistance in strange metal materials, the binary-distribution model which seems to be relevant to the iron pnictides is distinct from the more broad-continuous distributions relevant to the cuprates. Using the latter, we examine the applicability of classical effective medium theory to the MR in LSCO and compare calculated MR curves with the experimental data. arXiv:1905.02737v1 [cond-mat.str-el]

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A periodic arrangement of 2D conducting planes is known to host a (bulk) plasmon dispersion that interpolates between the typical, gapped behavior of 3D metals and a gapless, acoustic regime as a function of the out-of-plane wavevector. The semi-infinite system -the configuration relevant to Electron Energy Loss Spectroscopy (EELS) in a reflection geometry, as in High Resolution EELS (HREELS) -is known to host a surface plasmon that ceases to propagate below a cutoff wavevector. As the f-sum rule requires a finite response whether or not there exist sharp excitations, we demonstrate that what remains in the surface loss function -the material response probed by HREELS -is the contribution from the (bulk) plasmon of the infinite system. In particular, we provide a one-to-one mapping between the plasmon continuum and the spectral weight in the surface loss function. In light of this result, we suggest that HREELS be considered a long wavelength probe of the plasmon continuum in layered materials.

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