Gapped excitation in dense Kondo lattice CePtZn

We report on neutron scattering and muon spin relaxation measurements of dense Kondo lattice CePtZn. The system develops long-range incommensurate magnetic order as the temperature is reduced below $T_N=1.75$ K. Interestingly, a $Q$-independent gap at $E=0.65$ meV in the energy spectrum is found to co-exist with the long-range magnetic order. The gap persists to a very high temperature of $T\simeq 100$ K. The $Q$-independent characteristic and its persistence to high temperature hint that the gapped excitation may be manifesting the excited state of the ground-state doublet of the crystal-field energy levels. However, the observed broadness in the linewidth with distinct temperature and field dependencies makes it a nontrivial phenomenon. Qualitative analysis of the experimental data suggests the possible co-existence of a local critical behavior, which is onset near the critical field of $H\simeq 3$ T, with the crystal-field excitation in the dynamic properties.

Figure 1

Crystal structure and elastic neutron scattering measurement of CePtZn. (a) Crystal structure of CePtZn. (b) Powder neutron diffraction confirms the high quality of the powder. (c) The resolution limited elastic peaks are found to develop at low temperature in the elastic scan, indicating long-range magnetic order in the system. (d) Magnetic order parameter as a function of temperature is plotted in this figure. Fitting of the order parameter using a power law yields a Neel temperature of $T_N=1.75$ K. (e) Plot of an elastic peak intensity, at $Q=1.45$ ${\text{Å}}^{-1}$, as a function of field shows that the magnetic order disappears at $H_c=3$ T. The inset shows a week enhancement in the nuclear peak position as a function of field. It arises due to the alignment of the uncorrelated Ce ions to the applied field direction, as the field strength increases.

Figure 2

$\mu \mathrm{SR}$ measurements of CePtZn. (a) Time-dependent asymmetry for selected temperatures between 0.1 and 3 K. The solid lines are fits of the data using Eq. (1). (b) Temperature dependence of $B_{\max }$ extracted from fit. Inset shows the external field dependence of the initial asymmetry. (c) Fraction of the sample in the magnetically ordered state as a function of temperature. (d) Dynamic or longitudinal depolarization as a function of temperature.

Figure 3

Inelastic neutron scattering data as a function of temperature. (a) and (b) Energy scans at fixed $Q$ and at different temperatures are shown in this figure. A peak-type structure, centered at $E=0.65$ meV, tends to develop as temperature is reduced. Inelastic spectra is fitted using damped harmonic oscillator model (see text). The inset illustrates the development of the gapped excitation below $T\simeq 30$ K. (c) and (d) Momentum-energy ($Q\text{−}E$) color map, at two temperatures of $T=80$ mK and 2 K, respectively, depicting the $Q$-independent nature of the inelastic excitation across the magnetic order transition temperature. The gap seems to be independent of the development of long-range magnetic order. The dashed line highlights the scattering from helium roton excitation. (e) and (f) $Q\text{−}E$ maps at $T=80$ mK and 2 K at $H=6$ T, respectively. Clearly, the application of magnetic field significantly suppresses inelastic excitation.

Figure 4

Inelastic measurements in applied magnetic field. (a)–(d) $Q\text{−}E$ maps at different magnetic field application of $H=0$, 2, 4, and 6 T at $T=80$ mK, respectively. Remnants of gapped excitation can still be seen at $H=4$ T and 6 T, which is larger than the critical field to suppress the long-range order. (e) and (f) Plots of dynamic susceptibility as a function of energy at different fields and at $T=1.5$ K and 80 mK, respectively. $\chi {}^{\prime \prime }$ was deduced from the high-resolution fixed-$Q$ scans on SPINS at final neutron energy of $E_f=2.5\mathrm{meV}$. Inelastic data are well described by the damped harmonic oscillator model (see text). The excitation persists to a high field of $H=8$ T. At $H=9$ T, the excitation is completely suppressed. Inset [in (f)] shows the plot of estimated linewidth, FWHM, of the gapped excitation as a function of field. The linewidth of the excitation remains field-independent. (g) and (h) Integrated intensity vs applied magnetic field at $T=1.5$ K and 80 mK, respectively, are plotted. (i) and (j) In this plot, we show the field dependence of the center of the excitation as a function of field at $T=1.5$ K and 80 mK, respectively. A dramatic change in the peak position of the excitation around the critical field value, $H_c=3$ T, is observed at low temperature.