Single to Multiquasiparticle Excitations in the Itinerant Helical Magnet ${\mathrm{CeRhIn}}_5$

${\mathrm{CeRhIn}}_5$ is an itinerant magnet where the ${\mathrm{Ce}}^{3+}$ spins order in a simple helical phase. We investigate the spin excitations and observe sharp spin waves parameterized by a nearest-neighbor exchange, $J_{\mathrm{RKKY}}=0.88\pm 0.05\mathrm{meV}$. At higher energies, the spin fluctuations are heavily damped, where single-quasiparticle excitations are replaced by a momentum- and energy-broadened continuum constrained by kinematics of energy and momentum conservation. The delicate energy balance between localized and itinerant characters results in the breakdown of the single-quasiparticle picture in ${\mathrm{CeRhIn}}_5$.

Figure 1

The magnetic structure of ${\mathrm{CeRhIn}}_5$ investigated using spherical polarimetry. (a) An illustration of the magnetic structure. (b) A plot of the polarization matrix elements $P_{\text{measured}}$ vs $P_{\text{calculated}}$ based upon the isotropic helical model shown in (a).

Figure 2

Constant energy scans taken on IN12 and MACS in the antiferromagnetic phase. (a)–(b) illustrate fluctuations polarized along $c$ with the horizontal bar being the spectrometer resolution. (c)–(e) show constant energy slices showing the energy dependence of the spin fluctuations. Fluctuations at large $L$ characteristic of predominately $a-b$-plane polarized fluctuations are present to high-energy transfers.

Figure 3

Constant energy [(a)–(c)] and $Q$ [(d)–(f)] cuts integrating over $L=[-4,-1.25]$ sensitive to predominately in-plane fluctuations. The solid lines in (a)–(c) are to Gaussians displaced from the commensurate ($\frac{1}{2},\frac{1}{2}$) position. The solid lines in (d)–(f) are fits to damped harmonic oscillators. The shaded region is the broad heavily damped component. (d)–(f) are integrated over $\pm (0.025,0.025)$. The solid bars represent the experimental resolution.

Figure 4

Constant-$Q$ scans taken on IN12 and MACS. (a) Illustrates the energy dependence of the $c$-axis-polarized spin fluctuations. (b) The momentum integrated spectral weight as a function of energy. (c) A constant-$Q$ slice taken on MACS (integrating $L=[-4,-1.25]$), where the solid points are fits to constant-$Q$ scans and the open circles fits to constant energy. A continuum of scattering is present above the top of the 1-magnon band. (d) A calculation considering the parameterization in single (“1”) and multiparticle (“2”) states with the $\overrightarrow{Q}$ integrated intensities plotted in (b).