Unconventional Hund Metal in a Weak Itinerant Ferromagnet

The physics of weak itinerant ferromagnets is challenging due to their small magnetic moments and the ambiguous role of local interactions governing their electronic properties, many of which violate Fermi-liquid theory. While magnetic fluctuations play an important role in the materials’ unusual electronic states, the nature of these fluctuations and the paradigms through which they arise remain debated. Here we use inelastic neutron scattering to study magnetic fluctuations in the canonical weak itinerant ferromagnet MnSi. Data reveal that short-wavelength magnons continue to propagate until a mode crossing predicted for strongly interacting quasiparticles is reached, and the local susceptibility peaks at a coherence energy predicted for a correlated Hund metal by first-principles many-body theory. Scattering between electrons and orbital and spin fluctuations in MnSi can be understood at the local level to generate its non-Fermi liquid character. These results provide crucial insight into the role of interorbital Hund’s exchange within the broader class of enigmatic multiband itinerant, weak ferromagnets

Numerical simulations of disordered superconductors

This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The authors carried out Monte Carlo studies of the critical behavior of superfluid {sup 4}He in aerogel. They found the superfluid density exponent increases in the presence of fractal disorder with a value roughly consistent with experimental results. They also addressed the localization of flux lines caused by splayed columnar pins. Using a Sine-Gordon-type of renormalization group study they obtained an analytic form for the critical temperature. They also determined the critical temperature from I-V characteristics obtained from a molecular dynamics simulation. The combined studies enabled one to construct the phase diagram as a function of interaction strength, temperature, and disorder. They also employed the recently developed mapping between boson world-lines and the flux motion to use quantum Monte Carlo simulations to analyze localization in the presence of disorder. From measurements of the transverse flux line wandering, they determined the critical ratio of columnar to point disorder strength needed to localize the bosons

Impurity Effects on the A_1-A_2 Splitting of Superfluid 3He in Aerogel

When liquid 3He is impregnated into silica aerogel a solid-like layer of 3He atoms coats the silica structure. The surface 3He is in fast exchange with the liquid on NMR timescales. The exchange coupling of liquid 3He quasiparticles with the localized 3He spins modifies the scattering of 3He quasiparticles by the aerogel structure. In a magnetic field the polarization of the solid spins gives rise to a splitting of the scattering cross-section of for `up' vs. `down' spin quasiparticles, relative to the polarization of the solid 3He. We discuss this effect, as well as the effects of non-magnetic scattering, in the context of a possible splitting of the superfluid transition for $\uparrow\uparrow$ vs. $\downarrow\downarrow$ Cooper pairs for superfluid 3He in aerogel, analogous to the A_1-A_2 splitting in bulk 3He. Comparison with the existing measurements of T_c for B< 5 kG, which show no evidence of an A_1-A_2 splitting, suggests a liquid-solid exchange coupling of order J = 0.1 mK. Measurements at higher fields, B > 20 kG, should saturate the polarization of the solid 3He and reveal the A_1-A_2 splitting.Comment: 7 pages, 3 figure

Lectures of Fermi liquid theory

The Fermi liquid theory was first introduced by Landau in 1956 to provide a theoretical basis for the properties of strongly correlated Fermi systems. This theory has proven to be crucial for our understanding of a broad range of materials. These include liquid [sup 3]He, [sup 3]He-[sup 4]He mixtures, simple metals, heavy-fermions, and nuclear matter to name a few. In the high temperature superconductors questions have been raised regarding the applicability of Fermi liquid theory to the normal state behavior of these materials. I will not address this issue in these lectures. My focus will be to summarize the foundations of this theory and to explore the consequences. These lectures are in part a summary of the excellent review article by Baym and Pethick and the books by Pines and Nozieres and Baym and Pethick. They include as well a summary of some articles that I have authored and co-authored. In the main body of the lectures I will not make any additional references to the books or articles. In the absence of reading the original materials, my lectures should provide the essentials of a mini-course in Fermi liquid theory

Similarity of the leading contributions to the self-energy and the thermodynamics in two- and three-dimensional Fermi Liquids

We compare the self-energy and entropy of a two- and three-dimensional Fermi Liquids (FLs) using a model with a contact interaction between fermions. For a two-dimensional (2D) FL we find that there are T[sup 2] contributions to the entropy from interactions separate from those due to the collective modes. These T[sup 2] contributions arise from nonanalytic corrections to the real part of the self-energy and areanalogous to T[sup 3]lnT contributions present in the entropy of a three-dimensional (3D) FL. The difference between the 2D and 3D results arises solely from the different phase space factors

Conservation Of Spin Current As A Consequence Of A 2d Fermi Surface

The possible existence of a previously unseen conservation in the theory of Fermi liquids is proposed for a two-dimensional geometry. If the ground state can be described by a smooth curve in momentum space then one expects spin current to be conserved, as it is not the case in three dimensions. Some immediate consequences that can be checked experimentally are pointed out.28001/04/159394Farinas, P.F., Bedell, K.S., Studart, N., (1999) Phys. Rev. Lett., 82, p. 385

Towards The Detection Of A New Ferromagnetic Spin-wave Mode

We carry a theoretical study of an electron-spin-resonance setup aimed at finding a massive magnon in ferromagnets. We present the transmitted, patterns expected for small, moments. © 2001 Elsevier Science B.V. AU rights reserved.226-230PART I490491Halperin, B.I., Hohenberg, P.C., (1969) Phys. Rev., 188, p. 898Moser, M., Prets, A., Spitzer, W.L., (1999) Phys. Rev. Lett., 83, p. 3542Bedell, K.S., Blagoev, K.B., Phylos. Mag. Lett., , cond-mat0003187 v2 submitted for publicationAbrikosov, A.A., Dzyaloshinskii, I.E., (1959) Sov. Phys. JETP, 35, p. 535Abrikosov, A.A., (1986) Theory of Metals, , Plenum Press, New YorkSchultz, S., Dunifer, G., (1967) Phys. Rev. Lett., 18, p. 283Platzman, P.M., Wolff, P.A., (1967) Phys. Rev. Lett., 18, p. 28