Magnetic Field Splitting of the Spin Resonance inCeCoIn5

Abstract

Neutron scattering in strong magnetic fields is used to show the spin-resonance in superconducting CeCoIn5 (Tc=2.3 K) is a doublet. The underdamped resonance (hΓ=0.069 ± 0.019 meV) Zeeman splits into two modes at E±=hΩ0 ±gµBµ0H with g=0.96 ± 0.05. A linear extrapolation of the lower peak reaches zero energy at 11.2 ± 0.5 T, near the critical field for the incommensurate "Q-phase" (Ref. 1) indicating that the Q-phase is a bose condensate of spin excitons.PACS numbers: 75.40. Gb, 74.70.Tx, 75.50.Cc The presence of an underdamped resonance peak in the neutron scattering response has proven to be a strong indication of unconventional superconductivity where magnetism and electronic properties are strongly coupled. [2,3] Spin resonances have been reported in a series of heavy fermion, cuprate and iron based superconductors and have been associated with the gap function undergoing a change in sign (∆(q + Q 0 ) = −∆(q)). Therefore, neutron scattering can be used to probe the electronic superconducting gap symmetry.It is to be expected that applied magnetic fields, which suppress the superconducting order parameter, should have a strong effect on the spin resonance. Such effects have been difficult to pursue in the cuprates and iron based superconductors where chemical doping is required and resonance energies are high.[4] CeCoIn 5 is, however, particularly well suited owing to the stoichiometric nature of the compound and the accessible field and energy scales.CeCoIn 5 displays an unconventional superconducting phase at ambient pressures and at temperatures below 2.3 K with a gap characterized by d-wave symmetry. [5][6][7][8] The structure is layered tetragonal with magnetic Ce 3+ ions in Ce-In(1) planes stacked along the c-axis and separated by a Co-In(2) network.[9] Despite the two-dimensional magnetic network, the Fermi surface is characterized by three dimensional sheets. [10-12] However, the superconductivity does reflect the underlying lamellar structure with a critical field of ∼12 T for fields applied with the a − b plane and significantly lower ∼5 T for fields applied along c at temperatures below 0.1 K. [13][14][15][16] Neutron scattering [Ref. 17] shows the normal state has overdamped magnetic excitations peaked near Q 0 =(1/2,1/2,1/2) indicative of antiferromagnetic interactions between Ce 3+ ions both within the a − b plane and along c. The commensurate magnetic spin response differs from non-superconducting, though metallic, CeRhIn 5 which displays a magnetic Bragg peak at the incommensurate point Q=(1/2,1/2,0.297) with a spiral magnetic structure.[18] On entering the superconducting phase in CeCoIn 5 , an underdamped resonance peak at hΩ 0 =0.60 meV develops gathering spectral weight from low-energies. These results indicated a strong coupling between f-electron d-wave superconductivity and magnetism. A similar result and analysis has been applied to the heavy fermion superconductor CeCu 2 Si 2 where a spin resonance has also been observed in the superconducting phase. [19] While no magnetic Bragg p...

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…Surprisingly, the application of a magnetic field on heavy Fermion superconductor CeCoIn 5 splits the resonance into two peaks as shown in Figs. 40(b)-40(d) (Stock et al, 2012). This would suggest that the resonance in CeCoIn 5 is a doublet instead of a singlet-totriplet excitation.…”

Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe1.85Ni0.15As

“…Although there have been attempts to split the resonance for copper oxide 30 and iron-based superconductors 31,32 in this way, the results are inconclusive and it has not been possible determine if the mode is indeed a singlet-to-triplet excitation. In a very recent neutron experiment performed on the heavy Fermion superconductor CeCoIn 5 , the resonance was shown to be a doublet excitation 33 , thus casting doubt on its direct connection with superconducting Cooper pairs 34 .…”

“…Assuming that the resonance is a spin-1 singlet-to-triplet excitation of the Cooper pairs 29 , it should be possible to split it into three peaks under the influence of a magnetic field via the Zeeman effect by an amount ∆E = ±gµ B B, where g = 2 is the Lande factor and B is the magnitude of the field [30][31][32][33] . Although there have been attempts to split the resonance for copper oxide 30 and iron-based superconductors 31,32 in this way, the results are inconclusive and it has not been possible determine if the mode is indeed a singlet-to-triplet excitation.…”

Magnétisme de bande et Supraconductivité : l'apport de la diffusion inélastique des neutrons

“…Dans le second cas, on peut par exemple appliquer un champ magnétique dans l'état supraconducteur. Il faut alors prendre en compte l'effet orbital qui est lié à l'action d'un champ magnétique sur les particules chargés et l'effet Zeeman qui est lié au couplage du champ magnétique avec le spin des électrons [51][52][53][54].…”