Quantum Critical Scaling in the Single-Particle Spectrum of a Novel Anisotropic Metal

We report an angle resolved photoemission spectroscopy study of quantum critical scaling in the single-particle spectral function of a novel anisotropic metal ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$. We find a temperature ($T$) scaling exponent value and also low-$T$ angle resolved photoemission spectroscopy line shapes that are very challenging for current one-dimensional theory frameworks. These results add a new spectroscopic component to a growing collection of puzzling low-$T$ transport behaviors of this material.

Figure 1

ARPES spectra of ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$. (a) Intensity map of 300 K spectra of $T$-dependent data set analyzed quantitatively in this Letter. Band D is shown as a dashed line because [12, 13, 14] it is too weak to observe in this particular experimental geometry. (b) Overplot of spectra in range of box in (a). (c) 30 K spectra show sharpening. Different colors in (b,c) represent different $k$ values with increments 3.6% of $\Gamma \mathrm{\text{−}}Y$. (d) Comparison of $k_F$ spectra of (b,c) to TL model spectra [16], showing lack of full sharpening predicted for decreasing $T$.

Figure 2

$T$ scaling of ARPES spectra. (a) Spectra for $k=k_F$ normalized as described in text. (b) Log-log plot of $T$ dependence of inverse of scaling factors obtained from data of (a) and used to obtain Figs. 3b, 3d. Lines show that power-law fitting data are essentially $T^{\alpha }$.

Figure 3

$\omega /T$ scaling of the spectra. (a) $k=k_F$ spectra; (b) $k=k_F$ spectra after $T$ scaling, and plotted vs $(E-E_F)/k_{B}T$. The edges fall together onto a $T$-independent line shape (solid line). (c,d) Same as (a,d) but for $k=cT$. Low-temperature deviations from scaling above $E_F$ in (b) and (d) can be accounted for by the experimental resolutions, as shown in the insets and described in the text.