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.

Quantum materials exhibit intriguing properties with important scientific values and huge technological potential. Electrical transport measurements under hydrostatic pressure have been influential in unraveling the underlying physics of many quantum materials in bulk form. However, such measurements have not been applied widely to samples in the form of thin flakes, in which new phenomena can emerge, due to the difficulty in attaching fine wires to a thin sample suitable for high-pressure devices. Here, we utilize a home-built direct laser writing system to functionalize a diamond anvil to directly integrate the capability of conducting electrical transport measurements of thin flakes with a pressure cell. With our methodology, the culet of a diamond anvil is equipped with a set of custom-designed conducting tracks. We demonstrate the superiority of these tracks as electrodes for the studies of thin flakes by presenting the measurement of pressure-enhanced superconductivity and quantum oscillations in a flake of MoTe2.

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Charge density wave (CDW) instability is often found in phase diagrams of superconductors such as cuprates and certain transition-metal dichalcogenides. This proximity to superconductivity triggers the question on whether CDW instability is responsible for the pairing of electrons in these superconductors. However, this issue remains unclear and new systems are desired to provide a better picture. Here, we report the temperature–pressure phase diagram of a recently discovered BiS2 superconductor La2O2Bi3AgS6, which shows a possible CDW transition at T* ∼ 155 K and a superconducting transition at T c ∼ 1.0 K at ambient pressure, via electrical resistivity measurements. Upon applying pressure, T* decreases linearly and extrapolates to 0 K at 3.9 GPa. Meanwhile, T c is enhanced and reaches maximum value of 4.1 K at 3.1 GPa, forming a superconducting dome in the temperature–pressure phase diagram.

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