Interface between heavy fermions and normal electrons investigated by spatially resolved nuclear magnetic resonance

We have studied the superlattices with alternating block layers (BLs) of heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$ and conventional-metal ${\mathrm{YbCoIn}}_5$ by site-selective nuclear magnetic resonance spectroscopy, which uniquely offers spatially resolved dynamical magnetic information. We find that the presence of antiferromagnetic fluctuations is confined to the Ce BLs, indicating that magnetic degrees of freedom of $f\mathrm{electrons}$ are quenched inside the Yb BLs. Contrary to simple expectations that the two dimensionalization enhances fluctuations, we observe that antiferromagnetic fluctuations are rapidly suppressed with decreasing Ce BL thickness. Moreover, the suppression is more prominent near the interfaces between the BLs. These results imply significant effects of local inversion symmetry breaking at the interfaces.

Figure 1

(a) Crystal structure of $\mathrm{Ce}\left(\mathrm{Yb}\right){\mathrm{CoIn}}_5$. The largest principal axes of the electric field gradient at the Co, In(1), and In(2) sites are shown by the arrows. (b) Schematic representation of the ${\mathrm{CeCoIn}}_5\left(5\right)/{\mathrm{YbCoIn}}_5$(5) superlattice. (c), (d) Field-sweep NMR spectra at 4.2 K on thin-film ${\mathrm{CeCoIn}}_5$ with the thickness of 500 nm, ${\mathrm{CeCoIn}}_5\left(9\right)/{\mathrm{YbCoIn}}_5\left(5\right)$ and ${\mathrm{CeCoIn}}_5\left(5\right)/{\mathrm{YbCoIn}}_5\left(5\right)$ superlattices, and thin-film ${\mathrm{YbCoIn}}_5$ with the thickness of 350 nm. Panels of (c) show the field range between 10.4 and 13.5 T. Panels of (d) are the expanded views near the central transitions of the In(1) site.

Figure 2

The nuclear spin-lattice relaxation rate divided by temperature ${\left(T_{1}T\right)}^{-1}$ at the In(1) site in thin-film ${\mathrm{CeCoIn}}_5$ (magenta squares), thin-film ${\mathrm{YbCoIn}}_5$ (khaki squares), and ${\mathrm{CeCoIn}}_5\left(n\right)/{\mathrm{YbCoIn}}_5\left(5\right)$ superlattices with $n=9$ (triangles) and $n=5$ (circles). In the superlattices, ${\left(T_{1}T\right)}^{-1}\mathrm{s}$ in the Ce BLs (orange and red) and Yb BLs (dark and light green) are shown separately. ${\left(T_{1}T\right)}^{-1}$ in bulk ${\mathrm{CeCoIn}}_5$ measured at ${\mu }_{0}H\sim 12.1$ T is also shown with dashed line.

Figure 3

(a), (b) ${}^{115}\mathrm{In}$(1)-NMR spectra of the $n=9$ (a) and 5 (b) superlattices at 3.2 K and with fixed NMR frequency ($f=114.5\mathrm{MHz})$. In (a), the red (12.115 T), yellow (12.180 T), and green (12.262 T) arrows show the magnetic field at which we observed relaxation curves. (c) Field dependence of ${\left(T_{1}T\right)}^{-1}$ in the $n=9$ and $n=5$ superlattices. (d) Relaxation curves vs $t$ plot at 12.262, 12.115, and 12.180 T with $f=114.5\mathrm{MHz}$ in ${\mathrm{CeCoIn}}_5\left(9\right)/{\mathrm{YbCoIn}}_5\left(5\right)$. The red (green) dashed line represents the fitted relaxation curve of ${\mathrm{CeCoIn}}_5\left({\mathrm{YbCoIn}}_5\right)$ thin film. (e) The schematic Ce and Yb arrangements of these superlattices. From the comparison of two ${}^{115}\mathrm{In}$(1)-NMR spectra, the field-dependent NMR signals are approximately assigned to arise from the colored layers in (e).

Figure 4

Plot of $T_{1}T$ at the In(1) site of Ce BLs against $T$ in the $n=9$ and $n=5$ superlattices compared with the result of pure ${\mathrm{CeCoIn}}_5$ films. The data can be fitted by the Curie-Weiss law $[T_{1}T=(T+\theta )/C]$.