Magnetic Fluctuations at a Field-Induced Quantum Phase Transition

We report an inelastic neutron-scattering study at the field-induced magnetic quantum phase transition of ${\mathrm{CeCu}}_{5.8}{\mathrm{Au}}_{0.2}$. The data can be described better by the spin-density-wave scenario than by a local quantum critical point, while the latter scenario was shown to be applicable to the zero-field concentration-tuned quantum phase transition in ${\mathrm{CeCu}}_{6-x}{\mathrm{Au}}_x$ for $x=0.1$. This constitutes direct microscopic evidence for a difference in the quantum fluctuation spectra at a magnetic quantum critical point driven by different tuning parameters.

Figure 1

Elastic neutron-scattering scans (rocking scans) across the position of a magnetic superstructure reflection at $\mathbf{Q}\approx (1.38,0,1.74)$ in ${\mathrm{CeCu}}_{5.8}{\mathrm{Au}}_{0.2}$ for different temperatures at zero magnetic field. Inset: Néel temperature $T_N$ of ${\mathrm{CeCu}}_{6-x}{\mathrm{Au}}_x$ as a function of Au concentration $x$ for $x\le 0.5$ determined by specific heat (triangles) and magnetic susceptibility (circles) [31]. In the inset, results on single crystals are marked by open symbols while closed symbols represent polycrystal measurements.

Figure 2

Integrated intensity and linewidth (rocking width) of the (1.38 0 1.74) magnetic Bragg reflection in ${\mathrm{CeCu}}_{5.8}{\mathrm{Au}}_{0.2}$ as a function of temperature $T$ in zero magnetic field (a),(b) and as a function of magnetic field applied along the $c$ direction at $T=60\mathrm{mK}$ (c),(d).

Figure 3

Neutron-scattering intensity in ${\mathrm{CeCu}}_{5.8}{\mathrm{Au}}_{0.2}$ as a function of energy transfer $\hslash \omega$ at $\mathbf{Q}=(1.38,0,1.74)$ and $B=0.35\mathrm{T}$, $B||c$ for $T=60\mathrm{mK}$ and 5 K. Solid lines represent fits to the data comprised of the incoherent elastic signal (dotted lines) and the quasielastic magnetic response with Lorentzian line shape convoluted with the resolution (dashed lines). The inset shows the tails of the energy resolution of the spectrometer as measured on vanadium at room temperature (RT), where the data have been normalized to 100 for the elastic peak value. The resolution, especially the tails, cannot satisfactorily be described by a function with a single Gaussian line (dashed line) but by a sum of two Gaussians (solid line).

Figure 4

Scaling plots of the imaginary part of the dynamic susceptibility ${\chi }^{\prime \prime }(\omega ,T)$ at $\mathbf{Q}=(1.38,0,1.74)$, $B=0.35\mathrm{T}$, ($B\parallel c$) in ${\mathrm{CeCu}}_{5.8}{\mathrm{Au}}_{0.2}$ plotted as ${\chi }^{\prime \prime }{T}^{\alpha }=f(\omega /T^{\beta })$ with (a) $\alpha =0.8$, $\beta =1.0$ and (b) $\alpha =1.5$, $\beta =1.5$. Solid lines represent fits to the data. For details see text.