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The bulk electronic structure of T d -MoTe2 features large hole Fermi pockets at the Brillouin zone center (Γ) and two electron Fermi surfaces along the Γ − X direction. However, the large hole pockets, whose existence has important implications for the Weyl physics of T d -MoTe2, had never been conclusively detected in quantum oscillations. This raises doubt on the realizability of Majorana states in T d -MoTe2, because these exotic states rely on the existence of Weyl points, which originated from the same band structure predicted by DFT. Here, we report an unambiguous detection of these elusive hole pockets via Shubnikov-de Haas (SdH) quantum oscillations. At ambient pressure, the quantum oscillation frequencies for these pockets are 988 T and 1513 T, when the magnetic field is applied along the c-axis. The quasiparticle effective masses m * associated with these frequencies are 1.50 me and 2.77 me, respectively, indicating the importance of Coulomb interactions in this system. We further measure the SdH oscillations under pressure. At 13 kbar, we detected a peak at 1798 T with m * = 2.86 me. Relative to the oscillation data at a lower pressure, the amplitude of this peak experienced an enhancement, which can be attributed to the reduced curvature of the hole pockets under pressure. Combining with DFT + U calculations, our data shed light on why these important hole pockets had not been detected until now.

Pressure has been established as a powerful way of tuning material properties and studying various exotic quantum phases. Nonetheless, measurements under pressure are no trivial matter. To ensure a stable pressure environment, several experimental restrictions must be imposed including the limited size of a sample chamber. These have created difficulties in assembling high-pressure devices and conducting measurements. Hence, novel sensing methods that are robust and compatible with high-pressure devices under pressure are highly in demand. In this review, we discuss the nitrogen-vacancy (NV) center in diamond as a versatile quantum sensor under pressure. The excellent sensitivity and superior resolution of the NV center enable exciting developments in recent years. The NV center has great potential in sensing under pressure, especially beneficial to magnetic-related measurements.

The emergence of high transition temperature (T c) superconductivity in bulk FeSe under pressure is associated with the tuning of nematicity and magnetism. However, sorting out the relative contributions from magnetic and nematic fluctuations to the enhancement of T c remains challenging. Here, we design and conduct a series of high-pressure experiments on FeSe thin flakes. We find that as the thickness decreases the nematic phase boundary on temperature–pressure phase diagrams remains robust while the magnetic order is significantly weakened. A local maximum of T c is observed outside the nematic phase region, not far from the extrapolated nematic end point in all samples. However, the maximum T c value is reduced associated with the weakening of magnetism. No high-T c phase is observed in the thinnest sample. Our results strongly suggest that nematic fluctuations alone can only have a limited effect while magnetic fluctuations are pivotal on the enhancement of T c in FeSe.