${}^{63}\mathrm{C}\mathrm{u}$ Nuclear Relaxation in the Quantum Critical Point System ${\mathrm{C}\mathrm{e}\mathrm{C}\mathrm{u}}_{5.9}{\mathrm{A}\mathrm{u}}_{0.1}$

${}^{63}\mathrm{C}\mathrm{u}$ NQR measurements of the ${}^{63}\mathrm{C}\mathrm{u}$ $T_1$ are reported for the quantum critical point system $\mathrm{C}\mathrm{e}\mathrm{C}{\mathrm{u}}_{\mathrm{5.9}}\mathrm{A}{\mathrm{u}}_{\mathrm{0.1}}$ over temperatures ranging from 0.1 up to 4.2 K. Below $\sim 1\mathrm{K}$ the magnetization recovery exhibits a stable, nonexponential decay function which we believe signals the onset of 2D quantum critical fluctuations, as has been noted in the literature. We find $T_1^{-1}\propto T^{0.75}$ for the region $T<1\mathrm{K}$. The observed temperature dependence is in agreement with a phenomenological model of non-Fermi liquid behavior based on the uniform susceptibility but is inconsistent with calculations based on susceptibility peaks identified via neutron scattering experiments.

Figure 1

${}^{63}\mathrm{C}\mathrm{u}$ $T_1$ magnetization recovery curves from the 6.25 MHz NQR line for temperatures ranging from 0.1 to 1.03 K. Solid lines show fits of the stretched exponential function [Eq. (1)], and the inset shows fitted values of the exponent $\alpha$. The data have been scaled to approach unity as $\Delta t\to \infty$ and are displaced vertically by 0.1 from one another for clarity.

Figure 2

The ${}^{63}\mathrm{C}\mathrm{u}$ $T_1$ data from Fig. 1 for $T<1\mathrm{K}$ (closed circles) are scaled to fit a single stretched exponential function with $\alpha =0.73$, showing that the distribution of magnetic fluctuations is stable in this temperature region. At the top is shown (filled squares) the response curve to an rf pulse sample heating test described in the text. All data are normalized to unity as $\Delta t\to \infty$. A simple exponential decay (dotted line) is shown for comparison.

Figure 3

A log-log plot of ${}^{63}\mathrm{C}\mathrm{u}$ relaxation data $T_{1s}^{-1}$ from Fig. 1 for $T<1\mathrm{K}$ (filled circles), plus exponential $T_1$ data between $T=1.03$ and 4.2 K (open circles). The dotted line shows a simple power law $T^{0.75}$ and the solid line shows the behavior of the NFL model $\chi (\overrightarrow{q},\omega )$ function [Eq. (2)], which has been fitted to the uniform susceptibility data of Ref. 4 as described in the text.