Makoto Hashimoto

The discovery of high-temperature superconductivity in bilayer and trilayer nickelates has spurred intense interest in the Ruddlesden-Popper nickelates, yet the fundamental properties of the NiO 2 layer remain obscured by interlayer coupling. It is therefore imperative to investigate the electronic properties of their single-layer counterpart—though not superconducting—to isolate the intrinsic physics of the Ni-O layer. In this work, we present a systematic study of the single-layer nickelate La 1.4 Sr 0.6 NiO 4 using high-resolution angle-resolved photoemission spectroscopy and theoretical calculations. We reveal strong electron correlation effects manifested by high-energy kinks in band dispersions and a pronounced orbital-dependent band renormalization. Interestingly, we observe a quasi-one-dimensional electronic structure characterized by straight Fermi surface sheets along the diagonal momentum directions. Such a squarelike Fermi surface topology facilitates non-Fermi-liquid behavior consistent with the Luttinger liquid model, as evidenced by the power-law spectral function, robust temperature scaling, and spectroscopic features reminiscent of spin-charge separation. Our results therefore not only point toward an exotic Luttinger liquid phase emerging from the unexpected dimensional reduction in an intrinsically quasi-two-dimensional nickelate but also provide a new perspective for understanding the intriguing physics in multilayer nickelates.

Trilayer nickelates provide a tunable platform for investigating the interplay between structural geometry, electron correlation, and unconventional superconductivity. By directly comparing Pr${}_{4}$Ni${}_{3}$O${}_{10}$ and La${}_{4}$Ni${}_{3}$O${}_{10}$, this work uncovers an orbital differentiation enhanced by structural modification in Pr${}_{4}$Ni${}_{3}$O${}_{10}$, where orbitals become selectively incoherent and significantly renormalized, while orbitals remain coherent. This dichotomy leads to a marked suppression of interorbital hybridization in Pr${}_{4}$Ni${}_{3}$O${}_{10}$. This work suggests that the interlayer bonding angle serves as an active tuning parameter for the electronic properties, bridging multiorbital correlated physics and superconductivity in nickelates.

In multilayered high- T _c cuprates, the inner CuO ₂ planes (IPs) are spatially separated from dopant layers and thus remain cleaner than the outer planes (OPs). Here, we investigate five-layer (Cu,C)Ba ₂ Ca ₄ Cu ₅ O _y (Cu1245) and three-layer Ba ₂ Ca ₂ Cu ₃ O ₆ (F,O) ₂ (F0223) using angle-resolved photoemission spectroscopy (ARPES) and uncover an unprecedented situation, in which only the IPs become superconducting while the OPs remain metallic at low temperatures. Our model calculations indicate that over 95% of the OP wavefunction remains confined to OP itself, rendering superconducting proximity negligible. This interlayer decoupling realizes an ideal configuration—a single superconducting CuO ₂ layer screened from the dopant layer-induced disorder by heavily overdoped metallic OPs. The clean CuO ₂ layer exhibits a large superconducting gap with coherent Bogoliubov peaks extending beyond the antiferromagnetic zone boundary, and a widely extended coherent flat band at the Brillouin zone edge. These findings introduce the “degree of screening” as a physical parameter for probing competition between pseudogap and superconductivity, and for bridging typically disordered real materials with the idealized theoretical models.

Fermi arcs observed in underdoped cuprates have sparked debate over whether they represent segments of a large Fermi surface or small Fermi pockets. This ambiguity has long hindered their classification as either the conventional Bardeen-Cooper-Schrieffer (BCS) regime or the strongly coupled Bose-Einstein condensation (BEC) crossover limit. Here, using angle-resolved photoemission spectroscopy and quantum oscillations, we demonstrate the coexistence of a small Fermi pocket and a large superconducting gap in the clean inner CuO ₂ layers of the four-layer cuprate Ba ₂ Ca ₃ Cu ₄ O ₈ (F,O) ₂ . This coexistence constitutes a hallmark of the BCS-BEC crossover and has remained elusive for decades. Despite the presence of antiferromagnetic (AF) order, the superconducting gap in the small pocket is remarkably large, yielding a gap-to-Fermi energy ratio (Δ _pocket / ε _F ~ 0.6) and a critical-to-Fermi temperature ratio ( T _c / T _F ~ 0.13) that reach the theoretical upper bound for two-dimensional superconductivity. Unexpectedly, this BCS-BEC crossover emerges not as the carrier density decreases but as it increases, abruptly within a narrow doping range of less than 1%. These results provide a long-sought microscopic foundation for the d -wave pairing mechanism in doped AF-Mott insulators.

Among the family of intercalated transition-metal dichalcogenides (TMDs), Fe_{x}NbS_{2} is found to possess unique current-induced resistive switching behaviors, tunable antiferromagnetic states, and a commensurate charge order, all of which are tied to a critical Fe doping of x_{c}=1/3. However, the electronic origin of such extreme stoichiometry sensitivities remains unclear. Combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations, we identify and characterize a dramatic eV-scale electronic restructuring that occurs across the x_{c}. Moment-carrying Fe 3d_{z^{2}} electrons manifest as narrow bands within 200 meV of the Fermi level, distinct from other transition metal intercalated TMD magnets. These states strongly hybridize with itinerant electrons in the TMD layer and rapidly lose coherence above x_{c} due to correlation-driven effects. This sudden quasiparticle decoherence collapses the Fe-Nb hybridization, which explicitly suppresses the out-of-plane effective Fe-Fe exchange interaction, driving the transformation of the magnetic ground state from an antiferromagnetic stripe phase to a zigzag phase. These observations resemble the exceptional electronic and magnetic sensitivity of strongly correlated systems, and demonstrate that quantifying orbital-specific hybridization via ARPES offers an alternative pathway to evaluate effective magnetic exchange in metallic magnets, complementing inelastic neutron and resonant x-ray scattering probes.

As strong topological insulators, Bi 1 − x Sb _x alloys with ∼ 0.07 < x < 0.22 are predicted to host topologically protected conduction pathways along extended defects, such as dislocations. Due to the presence of bulk conduction, these fascinating topological features have been obscured to date. We examine carrier transport and electronic states in high-purity single-crystals of Bi 1 − x Sb _x alloys with 11 at% Sb. Using high magnetic fields, we suppress bulk conduction, enabling the identification of the residual conductivity associated with extended defects. A two-band analysis of magnetotransport measurements in combination with angle-resolved photoemission spectroscopy, reveals a bandgap ⩾ 40 meV, larger than Arrhenius analyses of temperature-dependent resistivities. With mobility as high as 750 000 cm ² V ⁻¹ s ⁻¹ , Bi–Sb alloys are promising candidates for topological electronics and related applications.

Twisted bilayer MoTe 2 ( tMoTe 2 ) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemission spectroscopy measurements of tMoTe 2 , revealing pronounced twist-angle-dependent band reconstruction shaped by orbital character, interlayer coupling, and moiré potential modulation. Density functional theory captures the qualitative evolution, yet underestimates key energy scales across twist angles, highlighting the importance of electronic correlations. Notably, the hole effective mass at the K point exhibits a nonmonotonic dependence on twist angle, peaking near 2°, consistent with band flattening at the magic angle predicted by continuum models. Via electrostatic gating and surface dosing, we further visualize the evolution of electronic structure versus doping, enabling direct observation of the conduction band minimum and confirm tMoTe 2 as a direct band gap semiconductor. These results establish a spectroscopic foundation for modeling and engineering emergent quantum phases in this moiré platform.