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 ...

Engineering and enhancing inversion symmetry breaking in solids is a major goal in condensed matter physics and materials science, as a route to advancing new physics and applications ranging from improved ferroelectrics for memory devices to materials hosting Majorana zero modes for quantum computing. Here, we uncover a new mechanism for realising a much larger energy scale of inversion symmetry breaking at surfaces and interfaces than is typically achieved. The key ingredient is a pronounced asymmetry of surface hopping energies, i.e. a kinetic energy-driven inversion symmetry breaking, whose energy scale is pinned at a significant fraction of the bandwidth. We show, from spin-and angle-resolved photoemission, how this enables surface states of 3d and 4d-based transition-metal oxides to surprisingly develop some of the largest Rashba-like spin splittings that are known. Our findings open new possibilities to produce spin textured states in oxides which exploit the full potential of the bare atomic spin-orbit coupling, raising exciting prospects for oxide spintronics. More generally, the core structural building blocks which enable this are common to numerous materials, providing the prospect of enhanced inversion symmetry breaking at judiciously-chosen surfaces of a plethora of compounds, and suggesting routes to interfacial control of inversion symmetry breaking in designer heterostructures.The lifting of inversion symmetry is a key prerequisite for stabilising a wide range of striking physical properties such as chiral magnetism, ferroelectricity, odd-parity multipolar orders, and the creation of Weyl fermions and other spin-split electronic states without magnetism [1][2][3][4][5][6][7] . Inversion symmetry is naturally broken at surfaces and interfaces of materials, opening exciting routes to stabilise electronic structures distinct from those of the bulk [8][9][10][11][12][13][14][15] . A striking example is found in materials which also host significant spin-orbit interactions, where inversion symmetry breaking (ISB) underpins the formation of topologically-protected surface states 11,16 and Rashba 10,17 spin splitting of surface or interfacelocalised two-dimensional electron gases 14,15,[18][19][20][21][22] , generically characterised by a locking of the quasiparticle spin perpendicular to its momentum. Such effects lie at the heart of a variety of proposed applications in spin-based electronics 10,[23][24][25][26][27] , and provide new routes to stabilise novel physical regimes such as spiral RKKY interactions, enhanced electron-phonon coupling, localisation by a weak potential, large spin-transfer torques, and mixed singlet-triplet superconductivity [28][29][30][31][32][33][34][35] . Conventional wisdom about how to maximize the Rashba effect has been to work with heavy elements whose atomic spin-orbit coupling is large. However, the energetic spin splittings obtained are usually only a small fraction of the atomic spin-orbit energy scale. This is because the key physics is not exclusively that of spi...

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

A detailed investigation of the first-order ferrimagnetic (FRI) to antiferromagnetic (AFM) transition in Mn1.85Co0.15Sb is carried out. These measurements demonstrate anomalous thermomagnetic irreversibility and a glass-like frozen FRI phase at low temperatures. The irreversibility arising between the supercooling and superheating spinodals is distinguished in an ingenious way from the irreversibility arising due to kinetic arrest. Field annealing measurements show a re-entrant FRI–AFM–FRI transition with increasing temperature. In this system the kinetic arrest band and supercooling band are also shown to be anticorrelated (i.e. the regions which are kinetically arrested at higher temperature have lower supercooling temperature and vice versa), which has been a universal feature of the AFM/ferromagnetic transition so far.

The hybridization between localized 4f electrons and itinerant electrons in rare-earth-based materials gives rise to their exotic properties like valence fluctuations, Kondo behaviour, heavy-fermions, or unconventional superconductivity. Here we present an angle-resolved photoemission spectroscopy (ARPES) study of the Kondo lattice antiferromagnet CeRh2Si2, where the surface and bulk Ce-4f spectral responses were clearly resolved. The pronounced 4f 0 peak seen for the Ce terminated surface gets strongly suppressed in the bulk Ce-4f spectra taken from a Si-terminated crystal due to much larger f-d hybridization. Most interestingly, the bulk Ce-4f spectra reveal a fine structure near the Fermi edge reflecting the crystal electric field splitting of the bulk magnetic 4f 15/2 state. This structure presents a clear dispersion upon crossing valence states, providing direct evidence of f-d hybridization. Our findings give precise insight into f-d hybridization penomena and highlight their importance in the antiferromagnetic phases of Kondo lattices.