Universal TT-linear resistivity and Planckian dissipation in overdoped cuprates

International audienceThe perfectly linear temperature dependence of the electrical resistivity observed as T$\rightarrow$ 0 in a variety of metals close to a quantum critical point is a major puzzle of condensed-matter physics . Here we show that T-linear resistivity as T$\rightarrow$0 is a generic property of cuprates, associated with a universal scattering rate. We measured the low-temperature resistivity of the bilayer cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+\lambda}$ and found that it exhibits a T-linear dependence with the same slope as in the single-layer cuprates Bi$_2$Sr$_2$CuO$_{6+\delta}$ , La$_{1.6−x}$Nd$_{0.4}$Sr$_x$CuO$_4$ and La$_{2−x}$Sr$_x$CuO$_4$ , despite their very different Fermi surfaces and structural, superconducting and magnetic properties. We then show that the T-linear coefficient (per CuO$_2$ plane), A1$^□$, is given by the universal relation A1$^□$T$_F$=$h/2e^2$ , where $e$ is the electron charge, $h$ is the Planck constant and $T_F$ is the Fermi temperature. This relation, obtained by assuming that the scattering rate 1/$\tau$ of charge carriers reaches the Planckian limit, whereby $\hbar$/$\tau$=$k_BT$, works not only for holedoped cuprates but also for electron-doped cuprates, despite the different nature of their quantum critical point and strength of their electron correlations

The variability of multisensory processes of natural stimuli in human and non-human primates in a detection task

BACKGROUND:Behavioral studies in both human and animals generally converge to the dogma that multisensory integration improves reaction times (RTs) in comparison to unimodal stimulation. These multisensory effects depend on diverse conditions among which the most studied were the spatial and temporal congruences. Further, most of the studies are using relatively simple stimuli while in everyday life, we are confronted to a large variety of complex stimulations constantly changing our attentional focus over time, a modality switch that can impact on stimuli detection. In the present study, we examined the potential sources of the variability in reaction times and multisensory gains with respect to the intrinsic features of a large set of natural stimuli. METHODOLOGY/PRINCIPLE FINDINGS:Rhesus macaque monkeys and human subjects performed a simple audio-visual stimulus detection task in which a large collection of unimodal and bimodal natural stimuli with semantic specificities was presented at different saliencies. Although we were able to reproduce the well-established redundant signal effect, we failed to reveal a systematic violation of the race model which is considered to demonstrate multisensory integration. In both monkeys and human species, our study revealed a large range of multisensory gains, with negative and positive values. While modality switch has clear effects on reaction times, one of the main causes of the variability of multisensory gains appeared to be linked to the intrinsic physical parameters of the stimuli. CONCLUSION/SIGNIFICANCE:Based on the variability of multisensory benefits, our results suggest that the neuronal mechanisms responsible of the redundant effect (interactions vs. integration) are highly dependent on the stimulus complexity suggesting different implications of uni- and multisensory brain regions. Further, in a simple detection task, the semantic values of individual stimuli tend to have no significant impact on task performances, an effect which is probably present in more cognitive tasks