In a quantum fluid without an associated lattice, such as ! He, the momentum of the fluid is conserved except where it interacts with the walls of a channel through which it is flowing. As the temperature decreases and the quasiparticle--quasiparticle mean free path within the fluid increases because of the decrease of its quasiparticle scattering rate, interactions with the walls become more probable, and the viscosity and flow resistance increase. This is intuitively at odds with the behavior seen for electrons moving in a crystalline lattice, whose flow resistance decreases as increases. The resolution of this apparent paradox is that coupling to the lattice and its excitations means that the large majority of collisions in the electron fluid (electron--impurity, normal electron--phonon, Umklapp electron--electron and Umklapp electron--phonon) relax momentum, taking the fluid far from the hydrodynamic limit. At least some of these momentum--relaxing collisions are inevitable in any real material. Strictly speaking, momentum of the electron fluid can never be conserved, even in a bulk sample for which boundary scattering is insignificant. This does not, however, mean that the electronic viscosity needs to play no role in determining electrical resistance. A pragmatic benchmark is whether momentum--conserving processes are faster or slower than momentum--relaxing ones. If the electron fluid's momentum is relaxed slowly, it can be thought of as being quasi--conserved, and hydrodynamic signatures might be observable (1--9).The search for hydrodynamic effects in electrons in solids has been given extra impetus by the introduction of the "holographic correspondence" to condensed matter physics (10).This technique introduced the concept of a minimum viscosity, argued to be applicable to strongly interacting fluids as diverse as the quark--gluon plasma and cold atomic gases (11). 3Hydrodynamic effects have also been postulated to be at the root of the T--linear resistivity of the high temperature superconductors (6, 7), but because momentum--relaxing scattering is strong in those materials, it is difficult to perform an analysis of the experimental data that unambiguously separates the two effects. In a pioneering experiment, unusual current--voltage relationships in a semiconductor wire were convincingly ascribed to hydrodynamic effects (3), but that avenue of research has not been widely pursued, even though the large difference between transport and electron--electron scattering rates in semiconductors was subsequently demonstrated by direct non--equilibrium measurements (12).Here we sought to identify a material in which momentum--relaxing scattering is anomalously suppressed in order to investigate whether a hydrodynamic contribution to electrical transport could be clearly separated from the more standard contributions from momentum--relaxing processes. The material that we chose was PdCoO ! , a layered metal with a series of unusual properties (13--21). Its electronic structure is deceptively simple, with one ...

In metallic samples of small enough size and sufficiently strong momentum-conserving scattering, the viscosity of the electron gas can become the dominant process governing transport. In this regime, momentum is a long-lived quantity whose evolution is described by an emergent hydrodynamical theory. Furthermore, breaking time-reversal symmetry leads to the appearance of an odd component to the viscosity called the Hall viscosity, which has attracted considerable attention recently due to its quantized nature in gapped systems but still eludes experimental confirmation. Based on microscopic calculations, we discuss how to measure the effects of both the even and odd components of the viscosity using hydrodynamic electronic transport in mesoscopic samples under applied magnetic fields.The semiclassical theory of electronic conduction, based on relaxation of total momentum by impurities, phonons and umklapp scattering, occupies a central place in condensed matter physics. It is therefore of particular interest to study the cases for which it fails. One case that has attracted much interest is the possibility of a hydrodynamic regime, where transport is dominated by viscous effects . One needs a large separation of scales between momentum-relaxing and momentum-conserving scattering in order to see these effects. This was recently achieved in graphene [23,24] and PdCoO 2 [25,26].Interest in such a hydrodynamic regime also emanated from a conjectured bound on diffusion constants for the hydrodynamics of strongly interacting quantum systems [27,28]. Even though the physics described in this work is semiclassical and probably still quite far from these quantum-mechanical bounds, the observations that we hope to stimulate would constitute an important first step towards the understanding of emergent hydrodynamical regimes in electronic systems.A further motivation for the work is that reaching a viscous regime for a charged fluid enables one to break time-reversal symmetry by adding a magnetic field and hence to study a non-dissipative component to the viscosity tensor called the Hall viscosity. The recent interest in this Hall viscosity emanates from the fact that it is topologically quantized in gapped systems [29]. In order to study this effect experimentally, in analogy with the Hall conductivity, the first step would obviously be to measure the classical Hall viscosity. We show in this letter how this measurement could be done by describing specific size effects from Hall viscosity in transport in restricted 2D channels under transverse magnetic fields. This paper is organized as follows. We start by assuming a perfect hydrodynamic regime and calculate ρ xx and ρ xy . We show that the 1/W 2 component of ρ xy is proportional to the Hall viscosity, thereby providing a way of measuring it. In order to have realistic predictions to compare with experiments, one should also take into account other, non-viscous effects that can lead to a sizedependent resistivity. We thus perform a kinetic Boltzmann calculation in which t...

Cuprates exhibit antiferromagnetic, charge density wave (CDW), and high-temperature superconducting ground states that can be tuned by means of doping and external magnetic fields. However, disorder generated by these tuning methods complicates the interpretation of such experiments. Here, we report a high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa2Cu3O6.67under uniaxial stress, and we show that a three-dimensional long-range-ordered CDW state can be induced through pressure along theaaxis, in the absence of magnetic fields. A pronounced softening of an optical phonon mode is associated with the CDW transition. The amplitude of the CDW is suppressed below the superconducting transition temperature, indicating competition with superconductivity. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials.

Transport and ARPES reveal extremely good metallicity arising from almost free-electron behavior in single-crystal PtCoO2.

2. -A long-standing question in the physics of solids has been whether the viscosity of the electron fluid plays an observable role in determining the resistance. Experimental evidence is given in this paper that the resistance of restricted channels of ultrapure two-dimensional PdCoO 2 has a large viscous contribution. The electronic viscosity is estimated to be in the range from 6 x 10 -3 to 3 x 10 -4 kg/(m s). -(MOLL, P. J. W.; KUSHWAHA, P.; NANDI, N.; SCHMIDT, B.; MACKENZIE*, A. P.; Science (Washington, DC, U. S.) 351 (2016) 6277, 1061-1064, http://dx.doi.org/10.1126/science.aac8385 ; MPI Chem. Phys. fester Stoffe,