Heat transport in the cuprate superconductors YBa2Cu3Oy and La2−xSrxCuO4 was measured at low temperatures as a function of doping. A residual linear term κ0/T is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from κ0/T is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component (ix) appearing at a putative quantum phase transition and argues against a non-superconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.

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Single crystals of the perovskite-type 3d 1 metallic alloy system Ca 1Ϫx Sr x VO 3 were synthesized in order to investigate metallic properties near the Mott transition. The substitution of a Ca 2ϩ ion for a Sr 2ϩ ion reduces the bandwidth W due to a buckling of the V-O-V bond angle from ϳ180°for SrVO 3 to ϳ160°for CaVO 3 . Thus, the value of W can be systematically controlled without changing the number of electrons making Ca 1Ϫx Sr x VO 3 : one of the most ideal systems for studying bandwidth effects. The Sommerfeld-Wilson ratio (Ӎ2), the Kadowaki-Woods ratio ͑in the same region as heavy fermion systems͒, and a large T 2 term in the electric resistivity, even at 300 K, substantiate a large electron correlation in this system, though the effective mass, obtained by thermodynamic and magnetic measurements, shows only a systematic but moderate increase in going from SrVO 3 to CaVO 3 , in contrast to the critical enhancement expected from the Brinkmann-Rice picture. It is proposed that the metallic properties observed in this system near the Mott transition can be explained by considering the effect of a nonlocal electron correlation. ͓S0163-1829͑98͒03232-9͔ PHYSICAL REVIEW B 15 AUGUST 1998-II VOLUME 58, NUMBER 8 PRB 58 0163-1829/98/58͑8͒/4372͑12͒/$15.00 4372

Many recent studies show that superconductivity not only exists in atomically thin monolayers but can exhibit enhanced properties such as a higher transition temperature and a stronger critical field. Nevertheless, besides being unstable in air, the weak tunability in these intrinsically metallic monolayers has limited the exploration of monolayer superconductivity, hindering their potential in electronic applications (e.g., superconductor-semiconductor hybrid devices). Here we show that using field effect gating, we can induce superconductivity in monolayer WS grown by chemical vapor deposition, a typical ambient-stable semiconducting transition metal dichalcogenide (TMD), and we are able to access a complete set of competing electronic phases over an unprecedented doping range from band insulator, superconductor, to a reentrant insulator at high doping. Throughout the superconducting dome, the Cooper pair spin is pinned by a strong internal spin-orbit interaction, making this material arguably the most resilient superconductor in the external magnetic field. The reentrant insulating state at positive high gating voltages is attributed to localization induced by the characteristically weak screening of the monolayer, providing insight into many dome-like superconducting phases observed in field-induced quasi-2D superconductors.