The properties of cuprate high-temperature superconductors are largely shaped by competing phases whose nature is often a mystery. Chiefly among them is the pseudogap phase, which sets in at a doping p* that is material-dependent. What determines p* is currently an open question. Here we show that the pseudogap cannot open on an electron-like Fermi surface, and can only exist below the doping p FS at which the large Fermi surface goes from hole-like to electron-like, so that p* ≤ p FS. We derive this result from high-magnetic-field transport measurements in La1.6−xNd0.4SrxCuO4 under pressure, which reveal a large and unexpected shift of p* with pressure, driven by a corresponding shift in p FS. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.
The nature of the pseudogap phase of cuprates remains a major puzzle 1,2 . One ofnew signatures is a large negative thermal Hall conductivity κ xy , which appears for dopings p below the pseudogap critical doping p*, but whose origin is as yet unknown 3 . Because this large κ xy is observed even in the undoped Mott insulator La 2 CuO 4 , it cannot come from charge carriers, these being localized at p = 0. Here we show that the thermal Hall conductivity of La 2 CuO 4 is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO 2 planes, i.e. κ zy (T) ≈ κ xy (T). This shows that the Hall response must come from phonons, these being the only heat carriers able to move as easily normal and parallel to the planes 4 . At p > p*, in both La 1.6-x Nd 0.4 Sr x CuO 4 and La 1.8-x Eu 0.2 Sr x CuO 4 with
BackgroundOptogenetic tools enable cell selective and temporally precise control of neuronal activity; yet, difficulties in delivering sufficient light to the spinal cord of freely behaving animals have hampered the use of spinal optogenetic approaches to produce analgesia. We describe an epidural optic fiber designed for chronic spinal optogenetics that enables the precise delivery of light at multiple wavelengths to the spinal cord dorsal horn and sensory afferents.ResultsThe epidural delivery of light enabled the optogenetic modulation of nociceptive processes at the spinal level. The acute and repeated activation of channelrhodopsin-2 expressing nociceptive afferents produced robust nocifensive behavior and mechanical sensitization in freely behaving mice, respectively. The optogenetic inhibition of GABAergic interneurons in the spinal cord dorsal horn through the activation of archaerhodopsin also produced a transient, but selective induction of mechanical hypersensitivity. Finally, we demonstrate the capacity of optogenetics to produce analgesia in freely behaving mice through the inhibition of nociceptive afferents via archaerhodopsin.ConclusionEpidural optogenetics provides a robust and powerful solution for activation of both excitatory and inhibitory opsins in sensory processing pathways. Our results demonstrate the potential of spinal optogenetics to modulate sensory behavior and produce analgesia in freely behaving animals.
The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La2CuO4 were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures and magnetic orders – Nd2CuO4 and Sr2CuO2Cl2 – and show that two potential mechanisms can be excluded – the scattering of phonons by rare-earth impurities and by structural domains. Our comparative study further reveals that orthorhombicity, apical oxygens, the tilting of oxygen octahedra and the canting of spins out of the CuO2 planes are not essential to the mechanism of chirality. Our findings point to a chiral mechanism coming from a coupling of acoustic phonons to the intrinsic excitations of the CuO2 planes.
The thermal conductivity κ of the Kondo insulator SmB6 was measured at low temperature, down to 70 mK, in magnetic fields up to 15 T, on single crystals grown using both the floating-zone and the flux methods. The residual linear term κ0/T at T → 0 is found to be zero in all samples, for all magnetic fields, in agreement with previous studies. There is therefore no clear evidence of fermionic heat carriers. In contrast to some prior data, we observe a large enhancement of κ(T ) with increasing field. The effect of field is anisotropic, depending on the relative orientation of field and heat current (parallel or perpendicular), and with respect to the cubic crystal structure. We interpret our data in terms of heat transport predominantly by phonons, which are scattered by magnetic impurities.
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