Kevin S. Bedell

Strongly correlated electronic materials

This is the final report of a 3-year project. Novel electronic materials characterized by strong electronic correlations display a number of unexpected, often extraordinary, properties. These are likely to play a major role in purpose-specific high-technology electronic materials of the future developed for electronic, magnetic, and optical applications. This project sought to develop predictive control of the novel properties by formulating, solving and applying many-body models for the underlying microscopic physics. This predictive control required the development of new analytical and numerical many-body techniques and strategies for materials of varying strengths of interactions, dimensionality and geometry. Results are compared with experiment on classes of novel materials, and the robust techniques are used to predict additional properties and motivate key additional experiments.

Effect of Ferromagnetic Spin Correlations on Superconductivity in Ferromagnetic Metals

We study the renormalization of quasiparticle properties in weak ferromagnetic metals due to spin fluctuations, away from the quantum critical point for small magnetic moment. We explain the origin of the s -wave superconducting instability in the ferromagnetic phase and find that the vertex corrections are small and that Migdal{close_quote}s theorem is satisfied away from the quantum critical point. {copyright} {ital 1998} {ital The American Physical Society}

Superconductivity near Itinerant Ferromagnetic Quantum Criticality

Superconductivity mediated by spin fluctuations in weak and nearly ferromagnetic metals is studied close to the zero-temperature magnetic transition. We solve analytically the Eliashberg equations for p-wave pairing and obtain the quasiparticle self-energy and the superconducting transition temperature T(c) as a function of the distance to the quantum critical point (QCP). We show that the reduction of quasiparticle coherence and lifetime due to scattering by quasistatic spin fluctuations is the dominant pair-breaking process, which leads to a rapid suppression of T(c) to a nonzero value near the QCP. We point out the differences and similarities of the problem to that of paramagnetic impurities in superconductors.

Absence of Wigner crystallization in graphene

Graphene, a single sheet of graphite, has attracted tremendous attention due to recent experiments which demonstrate that carriers in it are described by massless fermions with linear dispersion. In this Brief Report, we consider the possibility of Wigner crystallization in graphene in the absence of an external magnetic field. We show that the ratio of potential and kinetic energy is independent of the carrier density, the tuning parameter that usually drives Wigner crystallization, and find that for given material parameters (dielectric constant and Fermi velocity), Wigner crystallization is not possible. We comment on how these results change in the presence of a strong external magnetic field.

Nonanalytic contributions to the self-energy and the thermodynamics of two-dimensional Fermi liquids

We calculate the entropy of a two-dimensional Fermi liquid using a model with a contact interaction between fermions. We find that there are [ital T][sup 2] contributions to the entropy from interactions separate from those due to the collective modes. These [ital T][sup 2] contributions arise fron nonanalytic corrections to the real part of the self-energy which may be calculated from the leading-log dependence of the imaginary part of the self-energy through the Kramers-Kronig relation. We find no evidence of a breakdown in Fermi liquid theory in 2D and conclude that Fermi liquids in 2D are similar to 3D Fermi liquids.

Enhancedp-wave pairing in polarized3He-4He mixtures

The potential model of Bardeen, Baym, and Pines is generalized to describe polarized3He-4He-mixtures. The zero-field strength and range of the potential as a function of3He-density and pressure are determined by transport data and, for the first time, the magnetostriction. It is found that at higher pressures the magnetostriction leads to a3He-density-dependence in the strength of the potential, an effect which only negligibly appears when fitting the transport data. The model predicts ap-wave superfluid transition with a transition temperatureTcpwhich is highest at the maximum solubility. Including the correction for the polarization-dependence of the maximum solubility, a large increase inTcpwith polarization is found. The size of the enhancement depends on the zero fieldTcpwhich in turn depends on which set of normal state data we fit.

LUTTINGER THEOREM IN ONE DIMENSIONAL METALS

One dimensional metals are described by Luttinger liquid theory. Recent experiments have addressed the relation between this non-Fermi liquid behavior and the existence of a Fermi surface. We show that Luttingers theorem, with few modifications, holds for the one dimensional Tomonaga-Luttinger model. The implications for the high temperature superconductors are discussed.

Spin-Polarizing Concentrated 3He-4He Mixtures by the Rapid-Melting Method

We have achieved high-spin polarizations in concentrated mixtures of 3He in 4He using the rapid-melting method originally suggested by Castaing and Nozieres for pure 3He. Polarizations well above the equilibrium value (greater than 10%) were obtained at T ≈ 350 mK. The observed relaxation time T1 = 4000 s was of the same order as the theoretical predictions of the bulk relaxation time, and was long enough to suggest that it should be possible to study the properties of these strongly polarized mixtures.

S -wave superconductivity in weak ferromagnetic metals

We investigate the behaviour of weak ferromagnetic metals close to the ferromagnetic critical point. We show that in the limit of small magnetic moment the low temperature metallic phase is rigorously described by a local ferromagnetic Fermi liquid that has a momentum-independent self-energy. Whereas, non-Fermi liquid features develop at higher temperatures. Furthermore, we find that an instability towards s -wave superconductivity is possible when the exchange splitting is comparable to the superconducting gap.

Criteria for the occurence of ferromagnetism and weak magnetic order in narrow-band metals

Abstract Phenomenological correlations between the occurence of different types of magnetic order, the value of the electronic specific heat divided by temperature γ∗, and the shortest distance d between Ce and U atoms in a number of metallic compounds are presented. Ferromagnetism is found to occur over restricted ranges of d and γ∗, and the magnetic ordering temperature Tm and the ordered moment μor are maximized for d≈4.0−4.2 A. “Weak magnetic order”, for which μor⪡0.5μB, is found to occur in two distinct regimes: Large values of Tm occur for d less than the “Hill limits” dH≈3.4 A (Ce) and 3.5 A (U), with γ∗≈3.5×104erg cm-3K-2, while very low Tm are found in several heavy fermion compounds near d≈4.15 A and γ∗≈1×105erg cm-3K-2. A plot of Tm vs. volume-scaled μor implies that metallic 4f, 5f and 3d transition metal materials can be systematically described within a unified picture. The small exchange enhancement of the magnetic susceptibility of heavy fermion compounds and the surprising trend that ferromagnetism occurs in relatively strongly hybridized systems may be explained by the wavevector dependence of the electronic self-energy that is responsible for large effective mass.