Takao Watanabe

Elucidating the origin of the pseudogap remains a central challenge in copper oxide high\leavevmode\hbox{-}${T}_{c}$ superconductors. By investigating the magnetotransport properties of Bi\leavevmode\hbox{-}2212 and Bi\leavevmode\hbox{-}2223 single crystals, the authors demonstrate here that the pseudogap and superconducting fluctuations are distinct phenomena. Their results strongly suggest that the pseudogap arises from preformed Cooper pairing in the BCS--BEC crossover regime, imposing stringent constraints on theoretical descriptions of high\leavevmode\hbox{-}${T}_{c}$ cuprates beyond the conventional BCS paradigm.

We investigated the pressure dependence of the superconducting transition temperature (Tc) in optimally doped Bi2Sr2Ca2Cu3O10+δ (Bi-2223) single crystals using different pressure-transmitting media. Previous high-pressure studies have reported conflicting behaviors, ranging from a resurgence of Tc of optimally doped Bi-2223 in fluid media to an insulating-like transition in solid media. However, a direct comparison of the effects of different pressure-transmitting media is lacking. Here, we employed wide-field quantum microscopy based on nitrogen-vacancy centers to probe the magnetic response under high pressure, utilizing cBN and KBr as media. We observed that a diamagnetic response near 70 K, indicative of the superconducting transition, persisted up to 23 GPa in KBr, whereas it disappeared above 11 GPa and 70 K in cBN. These results demonstrate the high sensitivity of Bi-2223 to the pressure environment and highlight the critical role of hydrostatic pressure in cuprate superconductors.

We performed angle-resolved photoemission spectroscopy studies on the triple-layer Bi₂Sr₂Ca₂Cu₃O_10+δ over a wide doping range. Although the doping level of the inner CuO₂ plane is extremely low in underdoped samples, the d-wave SC gap is enhanced to the unprecedentedly large value of Δ₀ ~ 80–100 meV at the antinode. This gap persists well above T_c without a Fermi arc, indicating a “nodal metal”. We attribute the nodal metallic behavior to the unique local environment of the inner clean CuO₂ plane, sandwiched by nearly optimally-doped two outer planes and hence subject to strong proximity effect from both sides. In the nodal metal, quasiparticle peaks show electron-hole symmetry, suggesting d-wave pairing fluctuations. Thus the proximity effect on the innermost CuO₂ plane is the strongest in the triple-layer cuprates, which explains why the T_c reaches the maximum at the layer number of three in every multi-layer cuprate family.

Time-reversal symmetry breaking (TRSB) in magnetic topological insulators induces a Dirac gap in the topological surface state (TSS), leading to exotic phenomena such as the quantum anomalous Hall effect. Yet, the interplay between TRSB and topology in superconductors remains underexplored due to limited suitable materials. Here we employ zero-field muon spin relaxation (μSR) as a sensitive probe of TRSB to map out the electronic phase diagrams of iron-chalcogenide superconductors FeSe_1-xTe_x. For the Te composition x = 0.64 with the highest superconducting transition temperature T_c = 14.5 K, which is known to host a TSS and Majorana zero modes within vortices, we detect spontaneous magnetic fields below T_c distinct from a magnetic order. This signifies a TRSB superconducting state in the bulk, revealing the convergence of unconventional TRSB superconductivity with topologically nontrivial electronic structures in FeSe_1-xTe_x. Given the relatively high T_c and the tunability of the Fermi level through chemical substitution, iron-chalcogenide superconductors offer an intriguing platform for investigating the synergy between topological superconductivity and TRSB.

This study presents the first observation of a continuous electromagnetic terahertz wave emission from mesoscopic mesas of the trilayer high- T c superconductor, Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi-2223), a putative Josephson plasma emitter utilizing intrinsic Josephson junctions. Two types of Bi-2223 crystals, optimally doped and underdoped, were prepared via thermal annealing under controlled partial oxygen pressures, and their electrical and emission characteristics were compared. A weak, but detectable terahertz emission was observed from the underdoped Bi-2223 mesas, whereas no radiation was observed from the optimally doped ones. We associate the low terahertz emission output with the weak capacitive coupling between the intrinsic junctions in the trilayer system.

ARPES studies have established that the high - T _c cuprates with single and double CuO ₂ layers evolve from the Mott insulator to the pseudogap state with a Fermi arc, on which the superconducting (SC) gap opens. In four- to six-layer cuprates, on the other hand, small hole Fermi pockets are formed in the innermost CuO ₂ planes, indicating antiferromagnetism. Here, we performed ARPES studies on the triple-layer Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _10+δ over a wide doping range, and found that, although the doping level of the inner CuO ₂ plane was extremely low in underdoped samples, the d -wave SC gap was enhanced to the unprecedentedly large value of Δ ₀ ～100 meV at the antinode and persisted well above T _c without the appearance of a Fermi arc, indicating a robust ″nodal metal″. We attribute the nodal metallic behavior to the unique local environment of the inner clean CuO ₂ plane in the triple-layer cuprates, sandwiched by nearly optimally-doped two outer CuO ₂ planes and hence subject to strong proximity effect from both sides. In the nodal metal, quasiparticle peaks showed electron-hole symmetry, suggesting d -wave pairing fluctuations. Thus the proximity effect on the innermost CuO ₂ plane is the strongest in the triple-layer cuprates, which explains why the T _c reaches the maximum at the layer number of three in every multi-layer cuprate family.