Ferromagnetic superconductivity driven by changing Fermi surface topology.
TL;DR: A simple but powerful zero temperature Stoner model is introduced to explain the unusual phase dia-gram of the ferromagnetic superconductor, UGe2, and may be derived from a tight-binding, quasi-one-dimensional band structure inspired by previous band-structure calculations.
Abstract: We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase diagram of the ferromagnetic superconductor, ${\mathrm{U}\mathrm{G}\mathrm{e}}_{2}$. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetization. The twin-peak DOS may be derived from a tight-binding, quasi-one-dimensional band structure, inspired by previous band-structure calculations.
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TL;DR: In this article, the authors discuss the instabilities of the Fermi-liquid state of conduction electrons in metals with particular emphasis on magnetic quantum critical points, with the aim of assessing the validity of presently available theory.
Abstract: This review discusses instabilities of the Fermi-liquid state of conduction electrons in metals with particular emphasis on magnetic quantum critical points. Both the existing theoretical concepts and experimental data on selected materials are presented; with the aim of assessing the validity of presently available theory. After briefly recalling the fundamentals of Fermi-liquid theory, the local Fermi-liquid state in quantum impurity models and their lattice versions is described. Next, the scaling concepts applicable to quantum phase transitions are presented. The Hertz-Millis-Moriya theory of quantum phase transitions is described in detail. The breakdown of the latter is analyzed in several examples. In the final part experimental data on heavy-fermion materials and transition-metal alloys are reviewed and confronted with existing theory.
1,420 citations
TL;DR: The experimental status of the study of the superconducting phases of $f$-electron compounds is reviewed in this paper, where superconductivity has been found at the border of magnetic order as well as deep within ferromagnetic and antiferromagnetically ordered states.
Abstract: Intermetallic compounds containing $f$-electron elements display a wealth of superconducting phases, which are prime candidates for unconventional pairing with complex order parameter symmetries. For instance, superconductivity has been found at the border of magnetic order as well as deep within ferromagnetically and antiferromagnetically ordered states, suggesting that magnetism may promote rather than destroy superconductivity. Superconducting phases near valence transitions or in the vicinity of magnetopolar order are candidates for new superconductive pairing interactions such as fluctuations of the conduction electron density or the crystal electric field, respectively. The experimental status of the study of the superconducting phases of $f$-electron compounds is reviewed.
529 citations
TL;DR: The proximity to a ferromagnetic instability, the defect sensitivity of T(s), and the absence of Pauli limiting, suggest triplet superconductivity mediated by criticalferromagnetic fluctuations.
Abstract: We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T(C)=3 K and a small ordered moment m(0)=0.03 micro(B). Superconductivity is observed with a resistive transition temperature T(s)=0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T(s), and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.
449 citations
TL;DR: In this article, a large set of experimental data acquired over the past decade by several groups, and demonstrate how it can be used to construct a detailed picture of the low-temperature metallic state of the unconventional superconductor Sr2RuO4.
Abstract: In this paper, we review a large set of experimental data acquired over the past decade by several groups, and demonstrate how it can be used to construct a detailed picture of the low-temperature metallic state of the unconventional superconductor Sr2RuO4. We show how the normal state properties can be consistently and quantitatively explained in terms of Landau quasi-particles moving on a quasi-two-dimensional Fermi surface. Besides presenting our full and extensive data sets, we explain the details of some novel data analysis tools that can be used within the general context of quasi-two-dimensional metals. We then use the experimental Fermi surface and band dispersion to reassess several issues relevant to the unconventional superconductivity in Sr2RuO4, such as the spin-fluctuation spectrum, quasi-particle renormalization, interlayer dispersion and pressure dependence.
241 citations
TL;DR: In this article, the authors discuss a paradigm that has become of increasing importance in the theory of quantum phase transitions, namely, the coupling of the order-parameter fluctuations to other soft modes and the resulting impossibility of constructing a simple Landau-Ginzburg-Wilson theory in terms of order parameter only.
Abstract: This review discusses a paradigm that has become of increasing importance in the theory of quantum phase transitions, namely, the coupling of the order-parameter fluctuations to other soft modes and the resulting impossibility of constructing a simple Landau-Ginzburg-Wilson theory in terms of the order parameter only. The soft modes in question are manifestations of generic scale invariance, i.e., the appearance of long-range order in whole regions in the phase diagram. The concept of generic scale invariance and its influence on critical behavior is explained using various examples, both classical and quantum mechanical. The peculiarities of quantum phase transitions are discussed, with emphasis on the fact that they are more susceptible to the effects of generic scale invariance than their classical counterparts. Explicit examples include the quantum ferromagnetic transition in metals, with or without quenched disorder; the metal-superconductor transition at zero temperature; and the quantum antiferromagnetic transition. Analogies with classical phase transitions in liquid crystals and classical fluids are pointed out, and a unifying conceptual framework is developed for all transitions that are influenced by generic scale invariance.
198 citations
References
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Book•
01 Jan 1994
TL;DR: In this article, the theoretical and experimental state of affairs of two novel types of broken symmetry ground states of metals, charge, and spin density waves are discussed, as the consequence of electron-phonon and electron-electron interactions in low-dimensional metals.
Abstract: ?Density Waves in Solids is written for graduate students and scientists interested in solid-state sciences. It discusses the theoretical and experimental state of affairs of two novel types of broken symmetry ground states of metals, charge, and spin density waves. These states arise as the consequence of electron-phonon and electron-electron interactions in low-dimensional metals.Some fundamental aspects of the one-dimensional electron gas, and of the materials with anisotropic properties, are discussed first. This is followed by the mean field theory of the phases transitions?discussed using second quantized formalism?together with the various experimental observations on the transition and on the ground states. Fluctuation effects and the collective excitations are reviewed next, using the Ginzburg-Landau formalism, followed by the review of the interaction of these states with the underlying lattice and with impurities. The final chapters are devoted to the response of the ground states to external perturbations.
1,179 citations
Book•
25 Jan 1999
TL;DR: Atoms, ions, and molecules crystal field theory Mott transition and Hubbard model Mott insulators Heisenberg magnets itinerant electron magnetism ferromagnetism in Hubbard models the Gutzwiller variational method.
Abstract: Atoms, ions, and molecules crystal field theory Mott transition and Hubbard model Mott insulators Heisenberg magnets itinerant electron magnetism ferromagnetism in Hubbard models the Gutzwiller variational method the correlated metallic state mixed valence and heavy fermions quantum hall effect hydrogen atom single-spin-flip ansatz Gutzwiller approximation Schrieffer-Wolff transformation.
537 citations