Upon cooling, condensed matter systems typically transition into states of lower symmetry. While the converse, i.e. the emergence of higher symmetry at lower temperatures, has been hypothesized, it is extremely rare. Here, we show how an unusually isotropic 25 magnetoresistance in the highly anisotropic, one-dimensional conductor Li0.9Mo6O17 (LMO), and its temperature dependence, can be interpreted as a renormalization group (RG) flow towards a so-called separatrix. This approach is equivalent to an emergent symmetry in the system. The existence of two distinct ground states – Mott insulator and superconductor – can then be traced back to two opposing RG trajectories. By establishing a direct link between quantum field theory 30 and an experimentally measurable quantity, we uncover a path through which emergent symmetry might be identified in other candidate materials.

Hall effect and magnetoresistance (MR) measurements were carried out using a standard ac lockin detection technique in three different magnet cryostat systems: an 8 T Cryogen Free Measurement System from CryogenicTM, a 16 T superconducting magnet from Oxford InstrumentsTM and a 35 T Bitter magnet from the High Field Magnet Laboratory at Radboud University, Nijmegen. For all measurements, field sweeps were performed in both polarities. For the thermal conductivity measurements, we used a zero-field set-up housed in a He-4 flow cryostat that covers the temperature range 10K < T < 300K. We employed a modified steady-state method in which a temperature gradient, measured using a differential thermocouple, is set up across the sample through a pair of calibrated heat-links attached to each end.

In the main manuscript, only Figures 3 and 4 contain data. Panels A/C/E of Fig. 3 contain zero-field resistivity data of LMO with the current applied along the chain. Panels B/D/F of Fig. 3 contain the corresponding MR results, namely the (inverse square root of the) coefficients of the low-field H^2 MR at each temperature (obtained from the raw data files shown in the Supplementary Material). Panel G of Fig. 3 is the theoretical prediction, while panel H shows the MR coefficients, similarly plotted, for field and current along different crystallographic axes. Panel A of Fig. 4 shows Hall effect R_H(T) data in LMO at two different field strengths, whle panel B compares the T-dependence of R_H(T) with the Lorenz ratio whose data were reported in a different publication. 

In the Supplementary Material, there are 8 figures containing experimental data (Figs. S1-S8). Figures S1 to S7 all show essentially the same thing, namely a zero-field resistivity curve of a particular sample (panel A) and MR field sweeps of the corresponding sampe at different temperatures (plotted in various ways in panels B-D). Figure S8 shows the zero-field resistivity (panel A) and Hall effect data (panel B) for two LMO crystals, one superconducting, the other non-superconducting. Finally, Figures S9-S11 are results from theoretical modelling, as described in the text.