Evidence for Three Fluctuation Channels in the Spin Resonance of the Unconventional Superconductor ${\mathrm{CeCoIn}}_5$

Polarized inelastic neutron scattering under a magnetic field is used to get a microscopic insight into the spin resonance of the heavy fermion superconductor ${\mathrm{CeCoIn}}_5$. The resonance line shape is found to depend on the neutron polarization: Some of the spectral weight is common to the two polarization channels while the remaining part is distributed equally between them. This is evidence for the spin resonance being a degenerate mode with three fluctuation channels: A Zeeman split contribution and an additional longitudinal mode.

Figure 1

Magnetic excitation spectra measured for initial polarization $P_0=1$ (open circles) and $P_0=-1$ (full circles). The left and central panels show constant $\mathbf{Q}$ scans performed at $\mathbf{Q}=(1/2,1/2,1/2)$ for (a) $T=50\mathrm{mK}$ and $H=1\mathrm{T}$ (b) $T=50\mathrm{mK}$ and $H=2\mathrm{T}$ and (c) $T=3\mathrm{K}$, and $H=2\mathrm{T}$. The background (squares) is measured at $\mathbf{Q}=(0.38,0.38,0.9)$ and averaged over the two polarizations. The right panels show constant energy scan performed at $T=50\mathrm{mK}$ and for $H=2\mathrm{T}$ for (d) 0.5 meV and (e) 0.7 meV. Those scans are obtained by rocking the sample, the label $Q_L$ is given for indication. All lines are fits to the data as explained in the text.

Figure 2

Excitation spectra measured at $T=50\mathrm{mK}$ and $H=2\mathrm{T}$ (a) and (b) show the data obtained with the background fit subtracted. Panel (c) shows $I^{-0}-I^{+0}$ and panel (d) shows the sum $I^{+0}+I^{-0}$. The lines are fits as indicated in the text.

Figure 3

Magnetic field dependence of the resonance energy. The full (empty) squares are obtained by unpolarized INS data with magnetic field along [1,1,0] [10] ([1,1,1] [11]). The full circles are the modes with chiral character and the empty circles are the nonchiral contribution. The cross at $H=0\mathrm{T}$ is obtained by Stock et al. [7]. Lines are guides for the eyes.