Unconventional superconductivity

Stewart, G. R.

doi:10.1080/00018732.2017.1331615

Cited by 221 publications

(177 citation statements)

References 700 publications

(811 reference statements)

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“…The relative influence of λ and ∆ can be inferred from the dispersion of the magnetic excitations that are displayed as a function of momentum and energy. 54 . In particular, multiple lines of evidence akin to those in the iron-based superconductors have demonstrated a key role of collective electronic modes-especially spin fluctuations-in driving these transitions 55 .…”

Section: Excitonic Insulatorsmentioning

confidence: 99%

The physics of quantum materials

2017

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The physical description of all materials is rooted in quantum mechanics, which describes how atoms bond and electrons interact at a fundamental level. Although these quantum e ects can in many cases be approximated by a classical description at the macroscopic level, in recent years there has been growing interest in material systems where quantum e ects remain manifest over a wider range of energy and length scales. Such quantum materials include superconductors, graphene, topological insulators, Weyl semimetals, quantum spin liquids, and spin ices. Many of them derive their properties from reduced dimensionality, in particular from confinement of electrons to two-dimensional sheets. Moreover, they tend to be materials in which electrons cannot be considered as independent particles but interact strongly and give rise to collective excitations known as quasiparticles. In all cases, however, quantum-mechanical e ects fundamentally alter properties of the material. This Review surveys the electronic properties of quantum materials through the prism of the electron wavefunction, and examines how its entanglement and topology give rise to a rich variety of quantum states and phases; these are less classically describable than conventional ordered states also driven by quantum mechanics, such as ferromagnetism.T he way we think about manifestations of quantum physics in materials has recently undergone a profound change of perspective. Although materials scientists and engineers have long exploited quantum effects in a range of electronic deviceswell-known examples are the quantized electronic energy levels and optical selection rules at the heart of optoelectronics, and the tunnel effect that underlies the upcoming generation of harddisk drives 1,2 -the past decade has seen a dramatic increase in our understanding of how subtle quantum effects control the macroscopic behaviour of a whole range of different materials.Two strange and beautiful aspects of quantum mechanics have come to the fore. One is the topological nature of quantum wavefunctions. A familiar example is the existence of quantized vortices in superconductors. These vortices exist because of the requirement that the superconducting condensate have a well-defined phase, and gauge invariance fixes how this phase couples to magnetic flux. The phase can wind only by an integer multiple of 2π around a vortex, and this integer winding number is a simple example of a topological invariant: a quantity that remains fixed under smooth changes of a system. Similar topological quantities turn out to govern many other kinds of materials, not just superconductors, and these support phenomena ranging from dissipationless transport to novel quasiparticle excitations.Another deep feature of quantum mechanics is the non-local entanglement of some quantum states that is spectacularly highlighted in teleportation experiments with two photons separated over macroscopic distances 3 . Even the wavefunction of two spins in a singlet is entangled, in that the wavefunction of...

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Section: Excitonic Insulatorsmentioning

confidence: 99%

The physics of quantum materials

2017

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“…In this theory, the existence of the superconducting phase is explained following the fundamental observation by Cooper [2] that the ground-state energy of an attractively interacting system is significantly decreased by the collective formation of Cooper pairsnon-trivially correlated states of two fermions with exactly opposite momenta. Based on this idea of collective pairing, a plethora of other pairing mechanisms have been proposed and investigated [3][4][5]. One of the most influential extensions of the Cooper's idea comes from the observation that in the case of imbalanced systems, due to the mismatch of Fermi spheres of different components, the formation of correlated pairs forced by attractive mutual interactions is inseparably connected with resulting non-zero net momentum of the pair [6,7].…”

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confidence: 99%

Signatures of unconventional pairing in spin-imbalanced one-dimensional few-fermion systems

Pęcak

Sowiński

2020

Phys. Rev. Research

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A system of a few attractively interacting fermionic 6 Li atoms in one-dimensional harmonic confinement is investigated. Non-trivial inter-particle correlations induced by interactions in a particleimbalanced system are studied in the framework of the noise correlation. In this way, it is shown that evident signatures of strongly correlated fermionic pairs in the Fulde-Ferrell-Larkin-Ovchinnikov state are present in the system and they can be detected by measurements directly accessible within state-of-the-art techniques. The results convincingly show that the exotic pairing mechanism is a very universal phenomenon and can be captured in systems being essentially non-uniform and far from the many-body limit.One of the cornerstones of our understanding of strongly correlated states of quantum matter is based on the theory of superconductivity by Bardeen, Cooper, and Schrieffer [1]. In this theory, the existence of the superconducting phase is explained following the fundamental observation by Cooper [2] that the ground-state energy of an attractively interacting system is significantly decreased by the collective formation of Cooper pairsnon-trivially correlated states of two fermions with exactly opposite momenta. Based on this idea of collective pairing, a plethora of other pairing mechanisms have been proposed and investigated [3][4][5]. One of the most influential extensions of the Cooper's idea comes from the observation that in the case of imbalanced systems, due to the mismatch of Fermi spheres of different components, the formation of correlated pairs forced by attractive mutual interactions is inseparably connected with resulting non-zero net momentum of the pair [6,7]. This unconventional pairing mechanism named after Fulde, Ferrell, Larkin, and Ovchinnikov (FFLO) has been extensively examined theoretically, mostly in the case of various solid-state systems like iron-based superconductors [8][9][10][11], heavy-fermion compounds [11][12][13][14][15], or organic conductors [16][17][18]. However, it is also viewed as one of the possible ways to understand fundamental properties of neutron stars [19][20][21], specific quantum chromodynamics models [22], or fermionic ultra-cold gases [23]. The latter example is of high importance since ultra-cold atomic systems, due to their tremendous tunability, are believed to be the best candidates for the first experimental observation of the FFLO state. Unfortunately, up to this day, the FFLO state is ephemeral and there are only indirect signs of this state of matter (see [24] for a recent review).In this Letter, we show that the many-body groundstate of a few 6 Li atoms confined in a harmonic trap (in the presence of mutual attractions) possesses many characteristic properties of the FFLO state which can be experimentally captured. For example, if one would com-0 1 2 3 4 0 1 2 3 4 q 0 [ℏ/µm] Δp F [ℏ/µm] FIG. 1. The most probable FFLO momentum q0 as a function of the Fermi momenta mismatch ∆pF. Different points correspond to different number of particles and differen...

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“…Model.-In many theories e-p and e-e interactions have been considered to be mutually exclusive in mediating SC. This belief is partly due to the failures of the phononbased pairing mechanism of the BCS model in unconventional superconductors [24], but is also reinforced by the study of Hamiltonians that combine e-e and e-p interactions. The simplest is the Hubbard-Holstein model (HHM), where at each site a dispersionless phonon is coupled to the charge density:…”

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confidence: 99%

Superconductivity due to cooperation of electron-electron and electron-phonon interactions at quarter filling

Clay

Roy

2020

Phys. Rev. Research

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We present the results of Quantum Monte Carlo calculations for a two dimensional frustrated Hubbard model coupled to bond phonons. The model is known to have a d-wave superconducting ground state in the limit of large phonon frequency for sufficiently strong electron-phonon coupling. In the absence of electron-phonon coupling the Hubbard interaction U enhances superconducting pairing in the quarter-filled (density ρ = 0.5) band. We show here that at ρ = 0.5 electron-electron and electron-phonon interactions cooperatively reinforce d-wave pairing, while competing with each other at all other densities. Cooperative degrees of freedom are found in many phase transitions and are essential to understanding superconductivity in strongly correlated materials.

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Unconventional superconductivity

Cited by 221 publications

References 700 publications

The physics of quantum materials

The physics of quantum materials

Signatures of unconventional pairing in spin-imbalanced one-dimensional few-fermion systems

Superconductivity due to cooperation of electron-electron and electron-phonon interactions at quarter filling

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