Thesis advisor: Kevin S. Bedell / The Landau Fermi liquid theory is a very successful theory in condensed matter physics. It provides a phenomenological framework for describing thermodynamics, transport and collective modes of itinerant fermionic systems. In 1957, Silin described the spin waves in polarized Fermi liquids based on Landau Fermi liquid theory, which are related to series of components of the spherical harmonic expansion of the fermi surface. It has been proved by Pomeranchuck that for the Fermi surface to be stable, the Landau parameters should satisfy the relation: $F_l^{s,a}>-(2l+1)$. Whenever the relation is violated, there will exist an instability of the Fermi surface known as a Pomeranchuck instability, such as the Stoner ferromagnetism when $F_0^a→ -1^+$, or phase separation when $F_0^s→ -1^+$. In 1959, Abrikosov and Dzyaloshinskii developed a ferromagnetic Fermi liquid theory(FFLT) of itinerant ferromagnetism based on Landau Fermi liquid theory, whose microscopic foundations were established later by Dzyaloshiskii and Kondratenko. Further studies had been made of this state using a generalized Pomeranchuck instability based on the FFLT of Blagoev, Engelbrecht and Bedell and Bedell and Blagoev. In this thesis, I study a magnetically ordered system with spin orbit magnetism, where the order parameter has a net spin current and no net magnetization in both two dimension and three dimension. Starting from a Fermi liquid theory, similar to that for a weak ferromagnet, I have shown that this excitation emerges from an exotic magnetic Fermi liquid state that is protected by a generalized Pomeranchuck condition. I derive the propagating mode using the Landau kinetic equation, and find that the dispersion of the mode has a $sqrt q$ behavior in leading order in 2D. I also find an instability toward superconductivity induced by this exotic mode, and a further analysis based on the forward scattering sum rule strongly suggests that this superconductivity has triplet pairing symmetry. I perform similar studies in the 3D case, with a slightly different magnetic system and find that the mode leads to a Lifshitz-like instability most likely toward an inhomogeneous magnetic state in one of the phases. I also study the collective modes in itinerant ferromagnetic system, which is related to the $F_0^a$ pomeranchuck instability. Using FFLT, I obtained the well-known magnon (Nambu-Goldstone) mode and a gapped mode that was first found by Bedell and Blagoev. I have identified this mode as the Higgs boson (amplitude mode) of a ferromagnetic metal. This is identified as the Higgs since it can be shown that it corresponds to a fluctuation of the amplitude of the order parameter. I use this model to describe the itinerant-electron ferromagnetic material MnSi. By fitting the model with the existing experimental results, I calculate the dynamical structure function and see well-defined peaks contributed from the magnon and the Higgs. From my estimates of the relative intensity of the Higgs amplitude mode I expect that it can be seen in neutron scattering experiments on MnSi. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

No description available.

This thesis describes experiments carried out on two novel strongly correlated electron systems. The first, FeCrAs, is a new material that has not been studied before, while the second, Sr3Ru2O7, has been previously shown to have a very novel so-called ‘nematic’ phase around the metamagnetic quantum critical end point (QCEP). For these studies, a new variation on an established method for measuring the field dependence of susceptibility in a BeCu clamp cell has been developed, and is described, as is a relaxation heat capacity cell that works from 4 K down to 300 mK. A method of growing stoichiometric crystals of the hexagonal iron-pnictide FeCrAs has been developed, and transport and thermodynamic measurements carried out. The in-plane resistivity shows an unusual “non-metallic” dependence on temperature T, rising continuously with decreasing T from ∼800 K to below 100 mK. The c-axis resistivity is similar, except for a sharp drop upon entry into an antiferromagnetic state at T_N ∼ 125 K. Below 10 K the resistivity follows a non-Fermi-liquid power law, ρ(T) = ρ_0 − AT^x with x < 1, while the specific heat shows Fermi liquid behaviour with a large Sommerfeld coefficient, γ ∼ 30 mJ/molK^2. The high temperature properties are reminiscent of those of the parent compounds of the new layered iron-pnictide superconductors, however the T → 0 K properties suggest a new class of non-Fermi liquid. The metamagnetic critical end point temperature T^∗ in Sr3Ru2O7 as a function of hydrostatic pressure with H//ab has been studied using the ac susceptibility. It is found that T^∗ falls monotonically with increasing pressure, going to zero at Pc = 14±0.3 kbar. One sign of the nematic phase observed in the field-angle tuning, i.e. T^∗ rises as the novel phase emerges, has not been seen in our study. However, we see a slope change in T^∗ vs P at ∼12.8 kbar, and a shoulder at the upper field side of the peak in χ′ from ∼12.8 kbar to ∼16.7 kbar. These new features indicate that some new physics sets in near the pressure-tuned QCEP.

Thesis advisor: Kevin S. Bedell / In 1956, Lev Davidovich Landau put forth his theory on systems of interacting fermions, or fermi liquids. A year later, Viktor Pavlovich Silin described spin waves that such a system of fermions would support. The treatment of the contribution of the molecular field to the spin wave dispersion was a novel aspect of these spin waves. Silin predicted that there would exist a hierarchy of spin waves in a fermi liquid, one for each component of the spherical harmonic expansion of the fermi surface. In 1968, Anthony J. Leggett and Michael J. Rice derived from fermi liquid theory how the behavior of the spin diffusion coefficient of a fermi liquid could be directly experimentally observable via the spin echo effect. Their prediction, that the diffusion coefficient of a fermi liquid would not decay exponentially with temperature, but rather would have a maximum at some non-zero temperature, was a direct consequence of the fermi liquid molecular field and spin wave phenomena, and this was corroborated by experiment in 1971 by Corruccini, et al. A parallel advancement in the theory of fermi liquid spin waves came with the extension of the theory to describe weak ferromagnetic metals. In 1959, Alexei Abrikosov and I. E. Dzyaloshiski put forth a theoretical description of a ferromagnetic fermi liquid. In 2001, Kevin Bedell and Krastan Blagoev showed that a non-trivial contribution to the dispersion of the ferromagnetic current spin wave arises from the necessary consideration of higher harmonic moments in the distortion of the fermi surface from its ground state. In the chapters to follow, the author presents new results for transverse spin waves in a fermi liquid, which arise from a novel ground state of a fermi liquid--one in which an l=1 harmonic distortion exists in the ground state polarization. It is shown that such an instability can lead to spin waves with dispersions that are characterized by a linear dependence on the wave number at long wavelengths, or can lead to spin waves that are characterized by a square root dependence on the wave number at long wavelength. The author also presents new results for spin waves in a fermi liquid that has a spin density wave in its ground state. A spin density wave is characterized by a spiral magnetization in the ground state, and is observed to occur in materials such as MnSi. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

This thesis describes experiments carried out on two novel strongly correlated electron systems. The first, FeCrAs, is a new material that has not been studied before, while the second, Sr3Ru2O7, has been previously shown to have a very novel so-called ‘nematic’ phase around the metamagnetic quantum critical end point (QCEP). For these studies, a new variation on an established method for measuring the field dependence of susceptibility in a BeCu clamp cell has been developed, and is described, as is a relaxation heat capacity cell that works from 4 K down to 300 mK. A method of growing stoichiometric crystals of the hexagonal iron-pnictide FeCrAs has been developed, and transport and thermodynamic measurements carried out. The in-plane resistivity shows an unusual “non-metallic” dependence on temperature T, rising continuously with decreasing T from ∼800 K to below 100 mK. The c-axis resistivity is similar, except for a sharp drop upon entry into an antiferromagnetic state at T_N ∼ 125 K. Below 10 K the resistivity follows a non-Fermi-liquid power law, ρ(T) = ρ_0 − AT^x with x < 1, while the specific heat shows Fermi liquid behaviour with a large Sommerfeld coefficient, γ ∼ 30 mJ/molK^2. The high temperature properties are reminiscent of those of the parent compounds of the new layered iron-pnictide superconductors, however the T → 0 K properties suggest a new class of non-Fermi liquid. The metamagnetic critical end point temperature T^∗ in Sr3Ru2O7 as a function of hydrostatic pressure with H//ab has been studied using the ac susceptibility. It is found that T^∗ falls monotonically with increasing pressure, going to zero at Pc = 14±0.3 kbar. One sign of the nematic phase observed in the field-angle tuning, i.e. T^∗ rises as the novel phase emerges, has not been seen in our study. However, we see a slope change in T^∗ vs P at ∼12.8 kbar, and a shoulder at the upper field side of the peak in χ′ from ∼12.8 kbar to ∼16.7 kbar. These new features indicate that some new physics sets in near the pressure-tuned QCEP.

No description available.

No description available.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2015. / Recently there has been signi cant interest in new types of metals called non-Fermi liquids, which cannot be described by Landau Fermi liquid theory. Landau Fermi liquid theory is a theoretical model used to describe low energy interacting fermions or quasiparticles. There is a growing interest in constructing an e ective eld theory for these types of metals. One of the paradigms to understand these metals is by the use of Wilsonian renormalization group (RG) to study a theoretical toy model consisting of fermions coupled to a gapless order parameter eld. Here we will study fermions coupled to gapless bosons (order parameter) below the upper critical dimension (d = 3). We will treat both fermions and bosons on equal footing and construct an e ective eld theory which only integrates out high momentum modes. Then we compute the one-loop RG ows for the Yukawa coupling and four-Fermi interaction. We will discuss log2 and log3 subleties associated with the one loop RG ows for the four-Fermi interaction and how they can be circumvented.

No description available.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 43-45). Also available in electronic version.