Shiyan Li

Gate-tunable phase transitions in thin flakes of 1T-TaS2

The ability to tune material properties using gating by electric fields is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as the observation of gate electric-field-induced superconductivity and metal-insulator transitions. Here, we describe an ionic field-effect transistor (termed an iFET), in which gate-controlled Li ion intercalation modulates the material properties of layered crystals of 1T-TaS2. The strong charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS2 and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density wave states in 1T-TaS2 collapse in the two-dimensional limit at critical thicknesses. Meanwhile, at low temperatures, the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS2 thin flakes, with five orders of magnitude modulation in resistance, and superconductivity emerges in a textured charge-density wave state induced by ionic gating. Our method of gate-controlled intercalation opens up possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

Nodal superconductivity in FeS: Evidence from quasiparticle heat transport

We report low-temperature heat transport measurements on superconducting iron sulfide FeS with

Doping dependence of the superconducting gap in Tl 2 Ba 2 CuO 6 + δ from heat transport

T_c approx

Nodeless superconductivity in the SnAs-based van der Waals-type superconductor NaSn2As2

5 K, which has the same crystal structure and similar electronic band structure to the superconducting iron selenide FeSe. In zero magnetic field, a significant residual linear term

The emergence of topologically protected surface states in epitaxial Bi(111) thin films

kappa_0/T

Ultralow-Temperature Thermal Conductivity of the Kitaev Honeycomb Magnet α-RuCl 3 across the Field-Induced Phase Transition

is observed. At low field,

Quantum transport in the three-dimensional Dirac semimetal Cd3As2

kappa_0/T

Unveiling the quantum critical point of an Ising chain

increases rapidly with the increase of field. These results provide strong evidence for nodal superconducting gap in FeS. We compare it with the sister compound FeSe, and other iron-based superconductors with nodal gap.

Ultralow-temperature thermal conductivity of Pr2Ir2O7: a metallic spin-liquid candidate with quantum criticality

We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_{6+delta} throughout the overdoped regime. In the T -> 0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap, Delta. We find Delta to decrease with increasing doping, with a magnitude consistent with spectroscopic measurements (photoemission and tunneling). This argues for a pure and simple d-wave superconducting state in the overdoped region of the phase diagram, which appears to extend into the underdoped regime down to a hole concentration of 0.1 hole/Cu. As hole concentration is decreased, the gap-to-Tc ratio increases, showing that the suppression of the superconducting transition temperature Tc (relative to the gap) begins in the overdoped regime.

Wiedemann-Franz Law and Abrupt Change in Conductivity across the Pseudogap Critical Point of a Cuprate Superconductor

We grew the single crystals of the SnAs-based van der Waals (vdW)-type superconductor NaSn2As2 and systematically measured its resistivity, specific heat, and ultralow-temperature thermal conductivity. The superconducting transition temperature of our single crystal is 0.3 K higher than that previously reported. A weak but intrinsic anomaly situated at 193 K is observed in both resistivity and specific heat, which likely arises from a charge-density-wave (CDW) instability. Ultralow-temperature thermal conductivity measurements reveal a fully gapped superconducting state with a negligible residual linear term in zero magnetic field, and the field dependence of further suggests NaSn2As2 is an s-wave superconductor.