Calcium (Ca) intercalation into graphite (C) has been considered to require elevated temperatures, and its occurrence at room temperature (RT) has long been regarded as highly unlikely. Here, we demonstrate that sodium (Na) catalysis enables the formation of superconducting CaC6 even at RT. In mixtures of Ca, Na, and graphite, the gradual development of superconducting diamagnetism and the emergence of x-ray diffraction peaks confirm the formation of CaC6 during storage at RT. The superconducting transition temperature increases with storage time, and the volume fraction of the formed CaC6 scales proportionally to the square root of time, indicating that CaC6 formation proceeds via a diffusion-controlled reaction. These findings provide new insights into both the superconducting mechanism of CaC6 and the intercalation chemistry of graphite. Finally, we propose a possible scenario explaining the Na-catalyzed Ca intercalation process.

The maximum superconducting transition temperature Tc,max in cuprate superconductors exhibits a strong dependence on the number of CuO2 planes n per structural cell, exceeding 100 K in trilayer compounds (n = 3). Elucidating the mechanism responsible for this enhancement of superconductivity in trilayer systems is therefore a central issue. Here we investigate the quasiparticle dynamics of optimally doped Bi-based cuprates Bi2Sr2Can-1CunO2n+4+delta(n = 1-3), using time-resolved pump-probe reflectivity measurements, with particular focus on the trilayer Bi2223 (n = 3). In all compounds, the transient reflectivity reveals two characteristic components: a slow superconducting (SC) response and a fast pseudogap (PG) response with opposite signs under our probe conditions. These responses exhibit a distinct and systematic temperature evolution depending on n. The photoinduced SC phase depletion energy densities UdSC for all three compounds follow a universal c , whereas the energy density required for PG suppression in Bi2223 is found to be approximately three times larger than the values for La-Bi2201 and Bi2212. These results provide bulk-sensitive evidence that the enhancement of Tc,max in trilayer cuprates is accompanied by a robust pseudogap.

The coupling between electronic excitations and collective bosonic modes is fundamental to the emergence of high-temperature superconductivity in cuprates. Despite extensive effort, conventional equilibrium and pump-probe optical spectroscopies still struggle to disentangle couplings to different bosonic modes when their energy scales overlap. Here we overcome this limitation using ultrafast two-dimensional electronic spectroscopy (2DES), which correlates coherent excitation and detection photon energies with femtosecond time resolution. Applied to optimally doped Bi_2Sr_2Ca_0.92Y_0.08Cu_2O_8+δ, 2DES reveals a pronounced off-diagonal resonance arising from the ultrafast generation of non-thermal bosons with energy _𝐪≃200 meV. By comparing the measured spectra with a theoretical framework that explicitly includes the interaction between charge-transfer and magnetic excitations, we identify these bosons as paramagnons with momenta centered near (π/2,π/2) and extending toward (0,π) and (π,0). The resonance persists across a large range of temperatures and doping concentrations, demonstrating that high-energy paramagnons are ubiquitously and strongly coupled to electronic excitations throughout the cuprate phase diagram. Time-domain analysis constrains the build-up of the paramagnon population to ≲ 10 fs, placing a lower bound λ≳ 0.7 on the coupling strength. More broadly, our results establish 2DES as a powerful approach for disentangling mode-selective electron-boson interactions and addressing decoherence dynamics, thereby establishing a new avenue for investigating strongly correlated quantum materials. These findings also provide a direct framework for future time-resolved resonant inelastic X-ray scattering experiments aimed at tracking the ultrafast dynamics of magnetic excitations.

Electronic nematicity in Fe-based superconductors is manifested by spontaneous rotational symmetry breaking and the formation of nematic domains with mutually orthogonal directions of dxZ)dyZ orbital anisotropy. However, their energy dependence remains largely unexplored in real space. Using a 5.82-eV laser-excited photoemission electron microscope (laser-PEEM) with an energy-selective slit, we visualized the evolution of linear dichroic (LD) contrast within individual nematic domains of Ba1-xNaxFe2As2 (x approximate to 0.08). We discovered a sign reversal of the LD contrast at an energy similar to 0.4 eV below the Fermi level, directly revealing an inversion of the orbital anisotropy inside each domain. This behavior reflects a different energy-dependent redistribution of the spectral weight between the dxZ and dyZ states, highlighting the crucial role of orbital-selective coherence in the nematic phase of Fe-based superconductors.

Two-dimensional (2D) materials and van der Waals (vdW) heterostructures provide an exceptional platform for engineering quantum devices, yet realizing their potential requires electrical integration without compromising the pristine properties of atomically thin crystals through conventional nanofabrication. Transferable circuitry addresses this challenge by decoupling circuit fabrication from device assembly, enabling electrical contacting without directly processing the active material. Here, we introduce SiN_x nanomembrane (NMB) circuits realized through a simplified top-down strategy that reduces fabrication complexity, processing steps and specialized tools required by our previous bottom-up approach. As a stringent benchmark of material preservation, we electrically integrate a four-unit-cell-thick, optimally doped Bi_2Sr_2-xLa_xCuO_6+δ (Bi2201) flake and observe a superconducting transition at T_c^inf 32K, close to the T_c^onset 34K measured by susceptibility in the parent crystals. The preservation of superconductivity demonstrates electrical integration of fragile layered materials without direct exposure to conventional cleanroom procedures, providing a versatile platform for integrating increasingly complicated vdW heterostructures, moiré materials, and hybrid quantum architectures.

Fabrication of YBCO weak links by focused helium ion beam irradiation is a promising approach for realizing high-temperature superconducting Josephson junction devices. Although empirical dose-characteristic relationships have been established, the underlying transport mechanisms remain unclear. In this study, we perform a detailed investigation of the transport properties of YBCO weak links fabricated using a helium ion microscope (HIM) and provide a unified phenomenological description of the observed behavior based on the theory of SNS junctions with a diffusive metallic interlayer. We demonstrate that the temperature dependence of the critical current I_c and the I_cR_n product are well described by diffusive SNS junction models over a wide temperature range. Analyses show that the observed dose dependences of I_c and I_cR_n cannot be explained solely by variations in the effective Thouless energy E_T. The discrepancy suggests reduced interface transparency and a reduction in the density of states, leading to a decrease in the effective number of conducting channels contributing to transport. This interpretation is also consistent with the observed exponential increase in R_n with irradiation dose. These results provide a diffusion-based framework for understanding Josephson transport and guiding junction design in helium-ion-irradiated YBCO weak links.

The solid solution Rh1-xPtxSb, bridging the structurally distinct MnP-type RhSb and NiAs-type PtSb, was systematically investigated. A temperature-induced structural phase transition between the MnP- and NiAs-type phases occurs at x ≈ 0.1 near room temperature, accompanied by pronounced resistivity hysteresis. Superconductivity emerges for x ≥ 0.2, and the critical temperature (Tc) reaches a maximum value of 4.25 K at x = 0.4─the highest Tc among transition-metal monoantimonides. The electron-phonon coupling constant (λep ≈ 0.6) and normalized specific-heat jump (ΔCel/γTc ≈ 1.6) classify Rh1-xPtxSb as a weak-coupling superconductor. The compositional dependence of both the Debye temperature (ΘD) and the electronic density of states at the Fermi level (N(EF)) correlates closely with Tc, suggesting that they are responsible for the Tc enhancement. A comparison with the high-entropy analogue M1-xPtxSb (M = equimolar Ru, Rh, Pd, and Ir) revealed distinct differences in ΘD and N(EF), which can account for the observed difference in Tc between the two systems.

We demonstrate a rapid, maskless fabrication method for superconducting terahertz Josephson plasma emitters (JPEs) based on direct ultraviolet laser micromachining of Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals. Although machining debris is formed near the processed regions, uniform stacks of intrinsic Josephson junctions are preserved inside the crystal, enabling stable terahertz emission. Devices fabricated with Ag, Cu, and Cr electrodes all exhibited terahertz radiation, with Cu electrodes showing performance comparable with Ag while offering a low-cost alternative. Spectroscopic and polarization analyses indicate that the emitted radiation is elliptically polarized and dominated by the geometrical cavity resonance mode. Structural and electrical characterizations reveal that the machining width and depth are not limited by the optical spot size but are governed by the anisotropic thermal conductivity of Bi-2212, consistent with a thermally dominated laser ablation process. This direct laser micromachining approach provides a fast and versatile fabrication technique for JPEs and is broadly applicable to superconducting electronics and terahertz devices.

We report the physical properties of the novel Ba-Pt-B ternary compound Ba6Pt19-xB16-y (x = 1.6, y = 4), which crystallizes in the new prototype noncentrosymmetric cubic structure [I43m, a = 11.1093(1) angstrom]. The crystal structure is characterized by an open-cage composed of Pt and B atoms, encapsulating a Ba6Pt octahedron. The electrical resistivity, magnetization, specific heat measurements revealed that Ba6Pt19-xB16-y exhibits superconductivity at approximately 1.0 K and is classified as an intermediate coupling superconductor with an electron-phonon coupling constant of 0.47. Electronic structure calculations indicate that the density of states at the Fermi level is predominantly contributed by the d states of Pt atoms forming the cage.

The doping dependence of the critical current density ( J c ) in a Bi 2 Sr 2 CaCu 2 O 8+ δ single crystal was studied via magnetization measurements following repeated post-annealing on the same crystal. At high temperatures, where vortex lines are pinned, J c increased monotonically with hole concentration (p). In contrast, at low temperatures, where pancake vortices are pinned, J c exhibited two distinct peaks at p ~ 0.12 and p ~ 0.17. These results indicate that different pinning mechanisms dominate in each regime. The low-temperature two peaks suggest inhomogeneous superconductivity, likely induced by electronic and/or structural modifications near these doping levels.

We successfully synthesized the new compound Ba5Ir7Ge4 at ambient pressure by adding Ge to BaIr2, which is a high-pressure stable MgCu2-type Laves phase. Rietveld analysis revealed that Ba5Ir7Ge4 crystallizes by replacing part of the Ir4 tetrahedra in BaIr2 with Ge4Ir tetrahedra, forming in a network of corner-sharing Ge4Ir and Ir3Ge tetrahedra. While BaIr2 has a cubic structure, Ba5Ir7Ge4 adopts a new prototype structure with a tetragonal space group I41/a (no.88) (a = 12.9726(2) & Aring;, c = 8.2703(2) & Aring;). Our experiments demonstrated that Ba5Ir7Ge4 is a typeII superconductor with a transition temperature (Tc) of 3.2 K, which is slightly higher than that of BaIr2. Electronic structure calculations show that the density of states (DOS) at the Fermi energy is primarily contributed by the d states in the Ir atoms forming the Ir3Ge tetrahedra. Despite having smaller DOS at the Fermi level, Ba5Ir7Ge4 exhibits a slightly higher Tc than that of BaIr2, which can be attributed to its higher Debye temperature.

Antiferromagnetic spin fluctuations are the most promising candidate as the pairing glue of high critical temperature (Tc) superconductivity in cuprates. However, many-body states and intertwined orders have made it difficult to determine how electrons couple with fluctuating spins to form Cooper pairs. Recent experimental and theoretical studies have suggested spin fluctuation-driven quasiparticle band folding, but the relationship between the resultant Fermi pockets and superconductivity remains unclear. Here, using angle-resolved photoemission spectroscopy and numerical simulations, we show a proportional relationship between Tc and the quasiparticle weight of the incipient hole pocket near the nodal point in electron-doped Pr1−xLaCexCuO4±δ. Through complementary muon spin spectroscopy measurements, we uncover that the hole pocket forms only in the regime of the fluctuating antiferromagnetic ground state around a presumed quantum critical point. Our observations highlight the significance of the electron-spin fluctuation interaction in enhancing the hole pocket and consequently driving superconductivity. The authors study electron-doped cuprate superconductor Pr1−xLaCexCuO4-δ using ARPES and muon spin spectroscopy. They find that Tc is proportional to the quasiparticle weight of the hole pocket near the nodal points, which arises from Fermi-surface reconstruction associated with antiferromagnetic order.

Even before its role in electroweak symmetry breaking, the Anderson-Higgs mechanism was introduced to explain the Meissner effect in superconductors. Spontaneous symmetry-breaking yields massless phase modes representing the low-energy excitations of the Mexican-Hat potential. Only in superconductors the phase mode is shifted towards higher energies owing to the gauge field of the charged condensate. This results in a low-energy excitation spectrum governed by the Higgs mode. Consequently, the Bardeen-Cooper-Schrieffer-like Meissner effect signifies a macroscopic quantum condensate in which a photon acquires mass, representing a one-to-one analogy to high-energy physics. We report on an innovative spectroscopic technique to study symmetries and energies of the Higgs modes in the high-temperature superconductor Bi2Sr2CaCu2O8 after a soft quench of the Mexican-Hat potential. Population inversion induced by an initial laser pulse leads to an additional anti-Stokes Raman-scattering signal, which is consistent with polarization-dependent Higgs modes. Within Ginzburg-Landau theory, the Higgs-mode energy is connected to the Cooper-pair coherence length. Within a Bardeen-Cooper-Schrieffer weak-coupling model we develop a quantitative and coherent description of single-particle and two-particle channels. This opens the avenue for Higgs Spectroscopy in quantum condensates and provides a unique pathway to control and explore Higgs physics.

The development of a technology for the microfabrication of Bi2Sr2CaCu2O8+δ (Bi2212) crystals is essential for realizing high-performance terahertz emitting devices based on Bi2212 single crystals. We developed an anisotropic wet-etching method using potassium hydroxide solution to improve the etching accuracy of Bi2212 crystal chips. Etching solutions with potassium hydroxide concentration of 10–13 wt. % and temperatures of approximately 40–45 °C are suitable for sample etching. The developed etching method enabled us to obtain crystal chips with sidewall angles of approximately 90°. In the case of a crystal chip with a thickness of ∼6 μm, the undercuts from the edges of the photomask were ∼1.5 μm, which were significantly shorter than those obtained in previous studies using acidic solutions (∼5–10 μm). The etching rate of the developed solution (0.1 μm/min) was lower than that of the acidic solutions (∼20 μm/min), which provided suitable etching conditions for the samples. Devices using Bi2212 crystal chips, fabricated using the developed technique, exhibited clear terahertz emissions, similar to those reported in previous studies. The enhanced accuracy of the proposed etching process is expected to improve the device characteristics of Bi2212 terahertz emitters, particularly in terms of the emission power and frequency.

Single crystals of layered oxychalcogenide Sr2MCu2Se2O2 (M = Co, Ni) have been successfully grown by the melt-solidification method. Thermal analysis and X-ray diffraction measurement results revealed that these compounds are congruent melting under a sealed environment similar to Sr2ZnCu2Se2O2, while melting points are slightly different. The plate-like single crystals on the order of mm2 were obtained by melt solidification. The anisotropy of resistivity in Sr2CoCu2Se2O2 showed that the resistivity in the c-axis direction was about 200 times higher than in the a(b)-axis direction, reflecting the two-dimensional crystal structure. These results indicate that there are several congruent melting layered mixed-anion compounds, even though they have complex crystal structures with multiple components. Thus, the single crystal growth method by melt-solidification under a sealed environment has the potential to be applied to other relative compounds.

Alkali metal-intercalated C-60, A(3)C(60) (A = K, Rb, Cs, and their combinations), holds significant potential for practical applications due to its high superconducting transition temperature (33 K), high upper critical field (900 kOe), and isotropic superconductivity. However, application-oriented research has been limited by the lack of an efficient A(3)C(60) synthesis process. In this study, we demonstrate a rapid and scalable synthesis of A(3)C(60) (A = K, Rb, and Cs1/3Rb2/3) via direct mixing of A and C-60, realizing the fabrication of high-quality sintered A(3)C(60) pellets within just 1 h of heating at 200 degrees C-300 degrees C. The pellets exhibited large superconducting shielding volume fractions with sharp transitions, and the relationship between the lattice constant and transition temperature was in good agreement with previous reports. This direct mixing method enables simple and rapid production of large quantities of A(3)C(60), which is expected to accelerate research into applications such as superconducting wires and bulk magnets.

Electronic nematic states, characterized by broken rotational symmetry, are prevalent in correlated materials. In most iron-based superconductors, the nematic anisotropy aligns with the Fe–Fe direction of the iron square lattice. However, recent investigations propose a unique form of nematicity oriented along the diagonal Fe–As direction in heavily hole-doped AFe2As2 (A=Rb or Cs). Yet, the transport studies focusing on the fluctuations of such nematicity yield conflicting outcomes regarding the presence and orientation of the nematic fluctuations. Here, we report high-resolution heat capacity measurements conducted under in-plane field rotation in RbFe2As2. While no discernible anomaly associated with the nematic transition is found in the temperature dependence of specific heat, the field-angle dependence near the superconducting transition (at ∼2.8 K) reveals clear 2-fold oscillations within the plane, providing thermodynamic evidence for the presence of diagonal nematicity. Moreover, we find that Mössbauer spectroscopy sensitively probes the nematic transition at ∼50 K with no evidence of static magnetism. These findings imply that the diagonal nematicity in RbFe2As2 has a distinct mechanism involving charge degrees of freedom, exhibiting unusual thermodynamic properties of the transition.

We performed a comprehensive study of light-induced superconducting-like responses in La-based cuprate superconductors, La2-x-yNdySrxCuO4, which exhibit charge stripe orders or a short-range charge density wave (CDW) order depending on the concentration of Nd and Sr, by using near-infrared optical pump and terahertz probe spectroscopy. The light-induced plasma edge was observed in the c-axis terahertz reflectivity for all the samples far above Tc, regardless of long-range stripe or short-range CDW order. Remarkably, the frequency of light-induced plasma edge above Tc coincides with that of Josephson plasma resonance of La2-xSrxCuO4 at the similar doping in the low-temperature superconducting phase, imposing a strong constraint on the interpretation of the phenomenon. We elucidated a close correlation between the emergence of the light-induced c-axis plasma edge and the CDW order or fluctuations in the temperature-doping phase diagram, unveiling the intimate interplay between the equilibrium CDW and the light-induced coherent charge carriers along the c axis in nonequilibrium. The possibility of the existence of incoherent Cooper pairs above Tc that show up upon the photoexcitation is discussed from the optical conductivity analysis.

Lanthanides (Ln) are notoriously difficult to intercalate into graphite. We investigated the possibility of using Na to catalyze the formation of Ln-intercalated graphite and successfully synthesized LnC6 (Ln = Sm, Eu, and Yb) significantly rapidly in high yields. The synthesis process involves the formation of the reaction intermediate NaCx, through the mixing of Na and C, which subsequently reacts with Ln upon heating to form LnC6. Well-sintered LnC6 pellets with low residual Na concentrations (Ln:Na ≈ 98:2) were fabricated by the two-step method. The pellets enabled the evaluation of LnC6 by powder X-ray diffraction and electrical resistivity measurements. This study highlights the versatility of the Na-catalyzed method and lays the foundation for the rapid mass production of LnC6, with potential applications in superconducting and rechargeable battery materials.