Exotic magnetic states in Pauli-limited superconductors

Abstract

Magnetism and superconductivity compete or interact in complex and intricate ways. Here we review the special case where novel magnetic phenomena appear due to superconductivity, but do not exist without it. Such states have recently been identified in unconventional superconductors. They are different from the mere coexistence of magnetic order and superconductivity in conventional superconductors, or from competing magnetic and superconducting phases in many materials. We describe the recent progress in the study of such exotic magnetic phases, and articulate the many open questions in this field.

Field-induced magnetic instability within a superconducting condensate

“…Because the Kondo breakdown in Nd 1− x Ce x CoIn 5 occurs around x = 0.5 ( 28 ), we exclude the possibility that the observed magnetic instability for x = 0.95 is driven by charge valence fluctuations. On the other hand, it is well known that Pauli-limited superconductors feature instabilities toward complex superconducting phases at high fields, including modulated superconductivity and phases of coexisting magnetic and superconducting order ( 20 , 31 , 32 ). Our results may be explained by a modification of the superconducting condensate that induces a change in the magnetic properties.…”

Spin fluctuation anisotropy as a probe of orbital-selective hole-electron quasiparticle excitations in detwinned Ba(Fe1−xCox)2As2Tian1, Li2, Xu3 et al. 2019Phys. Rev. B145110View full textAdd to dashboardCite

“…Unconventional superconductors such as copper oxides, iron pnictides, and heavy Fermions are interesting because superconductivity in these materials is derived from their long-range antiferromagnetic (AF) ordered parent compounds 1 . Although there is no consensus on the microscopic origin of superconductivity, there is increasing evidence that electron pairing in these superconductors is mediated by spin fluctuations (excitations) [1][2][3][4][5] . In particular, superconductivity is intertwined with magnetic degrees of freedom, and forms a state coexisting with the static AF order in the underdoped regime [1][2][3][4][5] .…”

“…Although there is no consensus on the microscopic origin of superconductivity, there is increasing evidence that electron pairing in these superconductors is mediated by spin fluctuations (excitations) [1][2][3][4][5] . In particular, superconductivity is intertwined with magnetic degrees of freedom, and forms a state coexisting with the static AF order in the underdoped regime [1][2][3][4][5] . Therefore, to understand the fundamental interactions that lead to unconventional superconductivity, it is important to investigate how magnetism interacts with superconductivity in the coexisting regime of unconventional superconductors.…”

Spin-isotropic continuum of spin excitations in antiferromagnetically ordered Fe1.07Te

“…Although the isotropic spin excitations cannot be ascribed to spin waves from a long-range ordered local moment antiferromagnet, the continuum can result from the bicollinear magnetic order ground state of Fe1.07Te being quasi-degenerate with plaquette magnetic order. PACS numbers: 74.25.Ha, 78.70.Nx Unconventional superconductivity in cuprate and heavy fermion superconductors emerge in the vicinity of multiple exotic orders that are quasi-degenerate in energy [1][2][3][4], providing a plethora of fluctuations that may enhance or even generate superconductivity. Iron-based superconductors are found close to several different magnetic instabilities [5][6][7][8][9][10][11][12][13][14], suggesting an important role for magnetism in their superconductivity [15,16].…”