Emergent symmetry in a low-dimensional superconductor on the edge of Mottness.

Upon cooling, condensed-matter systems typically transition into states of lower symmetry. The converse-i.e., the emergence of higher symmetry at lower temperatures-is extremely rare. In this work, we show how an unusually isotropic magnetoresistance in the highly anisotropic, one-dimensional conductor Li0.9Mo6O17 and its temperature dependence can be interpreted as a renormalization group (RG) flow toward 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 and an experimentally measurable quantity, we uncover a path through which emergent symmetry might be identified in other candidate materials.

Incoherent transport across the strange-metal regime of overdoped cuprates.

Strange metals possess highly unconventional electrical properties, such as a linear-in-temperature resistivity1-6, an inverse Hall angle that varies as temperature squared7-9 and a linear-in-field magnetoresistance10-13. Identifying the origin of these collective anomalies has proved fundamentally challenging, even in materials such as the hole-doped cuprates that possess a simple bandstructure. The prevailing consensus is that strange metallicity in the cuprates is tied to a quantum critical point at a doping p* inside the superconducting dome14,15. Here we study the high-field in-plane magnetoresistance of two superconducting cuprate families at doping levels beyond p*. At all dopings, the magnetoresistance exhibits quadrature scaling and becomes linear at high values of the ratio of the field and the temperature, indicating that the strange-metal regime extends well beyond p*. Moreover, the magnitude of the magnetoresistance is found to be much larger than predicted by conventional theory and is insensitive to both impurity scattering and magnetic field orientation. These observations, coupled with analysis of the zero-field and Hall resistivities, suggest that despite having a single band, the cuprate strange-metal region hosts two charge sectors, one containing coherent quasiparticles, the other scale-invariant 'Planckian' dissipators.

Thermodynamic signatures of the field-induced states of graphite.

When a magnetic field confines the carriers of a Fermi sea to their lowest Landau level, electron-electron interactions are expected to play a significant role in determining the electronic ground state. Graphite is known to host a sequence of magnetic field-induced states driven by such interactions. Three decades after their discovery, thermodynamic signatures of these instabilities are still elusive. Here we report the detection of these transitions with sound velocity measurements. The evolution of elastic constant anomalies with temperature and magnetic field allows to draw a detailed phase diagram which shows that the ground state evolves in a sequence of thermodynamic phase transitions. Our analysis indicates that the electron-electron interaction is not the sole driving force of these transitions and that lattice degrees of freedom play an important role.

The world's smallest capacitive dilatometer, for high-resolution thermal expansion and magnetostriction in high magnetic fields.

For the characterization of novel quantum phases of matter, it is often required to study materials under multi-extreme conditions, in particular down to very low temperatures and in very high magnetic fields. We developed the world's smallest high-resolution capacitive dilatometer suitable for temperatures down to 10 mK and usage in high magnetic fields up to 37.5 T. Despite the extreme miniaturization, the capacitive dilatometer can resolve length changes down to 0.01 Å. This is an unprecedented resolution in a capacitive dilatometer of this compact size. Many cryogenic devices have limited space. Due to the extremely reduced cell size (3 cm3, 12 g), implementation or new applications in many of these sample space lacking devices are now possible. As an important example, the minute device can now be rotated in any standard cryostat, including dilution refrigerators or the commercial physical property measurement system. The present super compact design provides also for high resolution thermal expansion and magnetostriction measurements in a 15.2 mm diameter tube, enabling its use in the 32 mm bore, 37.5 T Bitter magnet at the High Field Magnet Laboratory in Nijmegen down to a temperature of 300 mK.