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This paper is devoted to the investigation of electron sound -- oscillations of the electron distribution function coupled with elastic deformation and propagating with the Fermi velocity. The amplitude-phase relations characterizing the behavior of the electron sound in Ga single crystals are determined experimentally. A model problem of excitation of electron sound in a compensated metal with equivalent bands is solved for a finite sample with diffusive scattering of electrons at the interfaces. It was found that the displacement amplitude of the receiving interface is two orders of magnitude larger than the elastic amplitude of the wave due to electron pressure. It was established that the changes occurring in the amplitude and phase of the electron sound waves at a superconducting transition do not depend on the path traversed by the wave, i.e. they refer only to the behavior of the transformation coefficient.
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In this document, I present a personal view on the heavy-fermion problem, within a phenomenological approach guided by experiments. This review presents a set of historical works which established the ground bases of the thematic during the last decades. An exhaustive and systematic approach is privileged. After a general presentation in Chapter 2, the properties of heavy-fermion paramagnets, antiferromagnets, and ferromagnets are considered in Chapters 3, 4, and 5, respectively. Chapters 6 and 7 are dedicated to two specific compounds, URu$_2$Si$_2$ for which a hidden-order phase constitutes a more-than-thirty-years-old unsolved mystery, and UTe$_2$, where multiple superconducting phases have been discovered in the last two years. Experiments performed using a panel of techniques ranging from microscopic (neutron scattering, NMR, etc.) to thermodynamic (specific heat, magnetization, etc.) and transport (electrical resistivity, etc.) probes, under extreme conditions of low temperatures, intense magnetic fields and high pressures, are reviewed. They show that magnetism plays a central role in the quantum critical properties of heavy-fermion systems. An emphasis is given to the intersite magnetic fluctuations, presented as the driving force for a heavy Fermi liquid, precursor of quantum magnetic criticality ending in magnetically-ordered phases. They are also suspected to drive an unconventional mechanism for superconductivity, which develops in the vicinity of quantum magnetic phase transitions induced under pressure or magnetic field. The appearance of magnetic fluctuations and ultimately magnetic order in heavy-fermion compounds occurs in a nearly-integer-valence regime, in which $f$ electrons have a dual itinerant-localized character. Fermi-surface and valence studies, which give complementary information about this duality, are also considered.
We study two techniques to create electrons in a liquid helium environment. One is thermionic emission of tungsten filaments in a low temperature cell in the vapor phase with a superfluid helium film covering all surfaces; the other is operating a glowing filament immersed in bulk liquid helium. We present both the steady state and rapid sweep I-V curves and the electron current yield. These curves, having a negative dynamic resistance region, differ remarkably from those of a vacuum tube filament. A novel low temperature vapor-phase electron collector for which the insulating helium film on the collector surface can be removed is used to measure emission current. We also discuss our achievement of producing multi-electron bubbles (MEBs) in liquid helium by a new method.
To clarify the nature of correlations in Hund metals and its relationship with Mott physics we analyze the electronic correlations in multiorbital systems as a function of intraorbital interaction U, Hunds coupling JH and electronic filling n. We show that the main process behind the enhancement of correlations in Hund metals is the suppression of the double-occupancy of a given orbital, as it also happens in the Mott-insulator at half-filling. However, contrary to what happens in Mott correlated states the reduction of the quasiparticle weight Z with JH can happen on spite of increasing charge fluctuations. Therefore, in Hund metals the quasiparticle weight and the mass enhancement are not good measurements of the charge localization. Using simple energetic arguments we explain why the spin polarization induced by Hunds coupling produces orbital decoupling. We also discuss how the behavior at moderate interactions, with correlations controlled by the atomic spin polarization, changes at large $U$ and $J_H$ due to the proximity to a Mott insulating state.
Theoretical descriptions of Yu-Shiba-Rusinov (YSR) states induced by magnetic impurities inside the gap of a superconductor typically rely on a classical spin model or are restricted to spin-1/2 quantum spins. These models fail to account for important aspects of YSR states induced by transition-metal impurities, including the effects of higher quantum spins coupled to several conduction-electron channels, crystal or ligand-field effects, and magnetic anisotropy. We introduce and explore a zero-bandwidth model, which incorporates these aspects, is readily solved numerically, and analytically tractable in several limiting cases. The principal simplification of the model is to neglect Kondo renormalizations of the exchange couplings between impurity spin and conduction electrons. Nevertheless, we find excellent correspondence in those cases, in which we can compare our results to existing numerical-renormalization-group calculations. We apply the model to obtain and understand phase diagrams as a function of pairing strength and magnetic anisotropy as well as subgap excitation spectra. The single-channel case is most relevant for transition-metal impurities embedded into metallic coordination complexes on superconducting substrates, while the multi-channel case models transition-metal adatoms.
Theoretically, it is commonly held that in metals near a nematic quantum critical point the electronic excitations become incoherent on the entire `hot Fermi surface, triggering non Fermi liquid behavior. However, such conclusions are based on electron-only theories, ignoring a symmetry-allowed coupling between the electronic nematic variable and a suitable crystalline lattice strain. Here we show that including this coupling leads to entirely different conclusions because the critical fluctuations are mostly cutoff by the non-critical lattice shear modes. At sufficiently low temperatures the thermodynamics remain Fermi liquid type, while, depending on the Fermi surface geometry, either the entire Fermi surface stays cold, or at most there are hot spots. In particular, our predictions are relevant for the iron-based superconductors.
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