Magnetic enhancement of superconductivity from electron spin domains

Abstract

Since the discovery of superconductivity, there has been a drive to understand the mechanisms by which it occurs. The BCS (Bardeen-Cooper-Schrieffer) model successfully treats the electrons in conventional superconductors as pairs coupled by phonons (vibrational modes of oscillation) moving through the material, but there is as yet no accepted model for high-transition-temperature, organic or 'heavy fermion' superconductivity. Experiments that reveal unusual properties of those superconductors could therefore point the way to a deeper understanding of the underlying physics. In particular, the response of a material to a magnetic field can be revealing, because this usually reduces or quenches superconductivity. Here we report measurements of the heat capacity and magnetization that show that, for particular orientations of an external magnetic field, superconductivity in the heavy-fermion material CeCoIn(5) is enhanced through the magnetic moments (spins) of individual electrons. This enhancement occurs by fundamentally altering how the superconducting state forms, resulting in regions of superconductivity alternating with walls of spin-polarized unpaired electrons; this configuration lowers the free energy and allows superconductivity to remain stable. The large magnetic susceptibility of this material leads to an unusually strong coupling of the field to the electron spins, which dominates over the coupling to the electron orbits.

Emerging evidence for FFLO states in layered organic superconductors (Review Article)

“…These claims, however, later had to be revised or are inconclusive (see [7] for an overview). For the heavyfermion compound CeCoIn 5 , clear thermodynamic evidence for a field-induced phase inside the superconducting state was found [21,22]. This phase, however, does not show the features expected from the FFLO prediction, but rather an antiferromagnetically ordered state [23,24].…”

Superconductivity and Non-Fermi-Liquid Behavior in the Heavy-Fermion Compound CeCo1−xNixIn5

“…[7][8][9] In magnetic fields, the spin degrees of freedom are significantly coupled with the stability of the SC order; a strong Pauli paramagnetic effect gives rise to a first-order transition at the SC upper critical field H c2 below 0.7 K, 7,[10][11][12] and the SC phase coexistent with AFM spin modulation evolves just below H c2 at very low temperatures. [13][14][15][16][17][18] Furthermore, the existence of the AFM-QCP at ∼ H c2 is strongly suggested from the observations of the NFL behavior in the paramagnetic phase above H c2 , including the − ln T divergence in specific heat divided by temperature, the T -linear dependence in magnetization, and electrical resistivity. 10,19,20) In fact, the long-range AFM orders are generated by substituting the ions for the elements in CeCoIn 5 , such as Nd for Ce, 21,22) Rh for Co, [23][24][25][26] and Cd, Hg, and Zn for In.…”

Fulde-Ferrell-Larkin-Ovchinnikov state in the dimensional crossover between one- and three-dimensional lattices

“…Recent experiments realized a weakly coupled 2D array of 1D tubes with ultracold 6 Li gases, observing the density profile characteristics of attractively interacting spin-polarized 1D Fermi gases. 15 This system can be further extended toward the quasi-1D regime by adjusting the optical lattice potentials, and, if needed, discreteness along the tube direction can be realized by 3D lattices.…”

“…[2][3][4][5] The Fulde-Ferrell-LarkinOvchinnikov (FFLO) state would arise with this interplay, 2,3 but it still remains elusive in spite of indirect experimental evidence observed. 6 The FFLO state is characterized by the Cooper pair carrying finite momentum causing a spatially modulated order parameter. One peculiar feature of this exotic phase is that apparently its stability is largely affected by the dimensionality of the system.…”