arXiv subjects

Hiroshi Eisaki

Publications and source records attributed to Hiroshi Eisaki.

At least 19 recordsLinked to original sources

arXiv2026

Remote magnon sensing detects the universal phase stiffness jump in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Phase stiffness governs macroscopic quantum coherence in superfluids and superconductors, providing a sensitive probe of condensate depletion, fluctuations, and phase transitions. However quantitative and spatially resolved measurements remain challenging. Here we report a contactless, local, all-optical measurement of superconducting phase stiffness using propagating antiferromagnetic magnons as remote sensors. In CrSBr, strong exciton-magnon coupling enables optical excitation and detection of magnon wavepackets and precise determination of their group velocity. When CrSBr is interfaced with underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, we find that the magnon group velocity is substantially enhanced below the superconducting transition and quantitatively encodes the phase stiffness through Meissner screening currents. Using this capability, we identify a parallel-field-induced Berezinskii-Kosterlitz-Thouless transition through direct observation of the universal phase-stiffness jump, revealing a field-controlled route to expose intrinsic two-dimensional phase-ordering physics in a bulk cuprate. Our work establishes propagating magnons as local probes of superconducting electrodynamics and phase-coherent phenomena in low-dimensional quantum materials.

cond-mat.supr-con↗

arXiv2026

Josephson transport in YBa${}_{2}$Cu${}_{3}$O${}_{7}$ weak links created by focused-helium-ion-beam irradiation: Analysis based on diffusive-SNS-junction model

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 $I_{\mathrm{c}}$ and the $I_{\mathrm{c}}R_{\mathrm{n}}$ product are well described by diffusive SNS junction models over a wide temperature range. Analyses show that the observed dose dependences of $I_{\mathrm{c}}$ and $I_{\mathrm{c}}R_{\mathrm{n}}$ cannot be explained solely by variations in the effective Thouless energy $E_{\mathrm{T}}$. 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 $R_{\mathrm{n}}$ 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.

cond-mat.supr-con↗

arXiv2026

Simplified Silicon Nitride Nanomembrane Circuits for van der Waals Integration

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$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ (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.

cond-mat.supr-con↗

arXiv2026

Room-Temperature Calcium Intercalation into Graphite Catalyzed by Sodium

Calcium (Ca) insertion into graphite (C) has been considered to require elevated temperatures, and its occurrence at room temperature (RT) has been regarded as highly unlikely. Here, we demonstrate that sodium (Na) catalysis enables the formation of superconducting CaC$_6$ 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 CaC$_6$ during storage at RT. The superconducting transition temperature increases with storage time, and the amount of CaC$_6$ scales proportionally to the square root of storage time. These findings provide new insights into the mechanism of superconductivity and Na-catalyzed formation of CaC$_6$, and highlight the potential of this RT intercalation process for practical applications such as electrode materials in Ca-ion batteries.

cond-mat.supr-con↗

arXiv2026

Direct laser micromachining of superconducting terahertz Josephson plasma emitters

We demonstrate a rapid, maskless fabrication method for superconducting terahertz Josephson plasma emitters (JPEs) based on direct ultraviolet laser micromachining of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (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 to 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.

cond-mat.supr-con↗

arXiv2026

Ultrafast Two-Dimensional Spectroscopy Uncovers Ubiquitous Electron-Paramagnon Coupling in Cuprate Superconductors

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$_2$Sr$_2$Ca$_{0.92}$Y$_{0.08}$Cu$_2$O$_{8+δ}$, 2DES reveals a pronounced off-diagonal resonance arising from the ultrafast generation of non-thermal bosons with energy $\hbarΩ_\mathbf{q}\simeq200$ 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 $\lesssim 10$ fs, placing a lower bound $λ\gtrsim 0.7$ 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.

cond-mat.supr-con↗

arXiv2026

Energy-Resolved Real-Space Imaging of Orbital Nematicity in an Fe-Based Superconductor

Electronic nematicity in Fe-based superconductors is manifested by spontaneous rotational symmetry breaking and the formation of nematic domains with mutually orthogonal directions of $d_{xz}$/$d_{yz}$ orbital anisotropy. However, its energy dependence has remained largely unexplored in real space. Using 5.82-eV laser-excited photoemission electron microscopy (laser-PEEM) with an energy-selective slit, we visualize the evolution of linear dichroic (LD) contrast within individual nematic domains of Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ ($x\approx0.08$). We discover a sign reversal of the LD contrast at an energy $\sim0.4$ eV below the Fermi level, directly revealing an inversion of orbital anisotropy inside each domain. This behavior reflects a different energy-dependent redistribution of spectral weight between the $d_{xz}$ and $d_{yz}$ states, highlighting the crucial role of orbital-selective coherence in the nematic phase of Fe-based superconductors.

cond-mat.supr-con↗

arXiv2025

Rapid and Scalable Synthesis of Alkali Metal-Intercalated C$_{60}$ Superconductors

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$$C_{60}$ ($A$ = K, Rb, and Cs$_{1/3}$Rb$_{2/3}$) via direct mixing of $A$ and C$_{60}$, realizing the fabrication of high-quality sintered $A_3$$C_{60}$ pellets within just 1 hour of heating at 200-300°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.

cond-mat.supr-con↗

arXiv2024

Direct observation of the Higgs particle in a superconductor by non-equilibrium Raman scattering

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 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 the direct observation of the Higgs particle in the high-temperature superconductor Bi-2212 by developing an innovative technique to study its symmetries and energies after a "soft quench" of the Mexican-Hat potential. Population inversion of the metastable Higgs particle induced by an initial laser pulse allows identifying the polarization-dependent Higgs modes as an additional anti-Stokes Raman-scattering signal. Within Ginzburg-Landau theory, the Higgs-mode energy is connected to the Cooper-pair coherence length. Within a BCS 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.

cond-mat.supr-con↗

arXiv2024

Doping Dependence of Upper Critical Field of High-Tc Cuprate Bi2+xSr2-xCaCu2O8+d Estimated from Irreversibility Field at Zero Temperature

We investigated the temperature (T) dependence of the irreversibility field Hirr(T) in high-critical-temperature cuprate Bi2+xSr2-xCa1-yYyCu2O8+d (Bi-2212) single crystals over a wide range of hole doping level (p). Hirr(T) was evaluated by measuring the magnetization hysteresis loop. The value of Hirr(T) extrapolated to T = 0 K [Hirr(0)], is either equal to or sets the lower boundary for the upper critical field at T = 0 K [Hc2(0)]. Tc shows a parabolic p-dependence (peak at p = 0.16), whereas Hirr(0) increases monotonically with p by approximately one order of magnitude, from 19 T for the most underdoped sample (p = 0.065, Tc = 24 K) to 209 T for the most overdoped sample (p = 0.200, Tc = 75 K). The present results qualitatively agree with Hc2(0) values evaluated from the specific heat measurements. The observed p-dependence of Hirr(0) in Bi-2212 is distinct from those in YBa2Cu3O7-d and HgBa2CuO6+d, in which a pronounced dip structure appears in the underdoped region. Considering that the dip structures observed in these two systems are likely associated with the formation of competing orders (most likely field-induced charge orders), the present results indicate that the influence of the competing order in Bi-2212 is less prominent than that in the other two systems.

cond-mat.supr-con↗

arXiv2024

Accelerated Lanthanide Intercalation into Graphite Catalyzed by Na

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 $Ln$C$_6$ ($Ln$ = Sm, Eu, and Yb) significantly rapidly in high yields. The synthesis process involves the formation of the reaction intermediate NaC$_x$, through the mixing of Na and C, which subsequently reacts with $Ln$ upon heating to form $Ln$C$_6$. Well-sintered $Ln$C$_6$ pellets with low residual Na concentrations ($Ln$:Na = 98:2) were fabricated by the two-step method. The pellets enabled the evaluation of $Ln$C$_6$ 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 $Ln$C$_6$, with potential applications in superconducting and rechargeable battery materials.

cond-mat.mtrl-sci↗

arXiv2024

Doping Dependence of Spin-Momentum Locking in Bismuth-Based High-Temperature Cuprate Superconductors

Non-zero spin orbit coupling has been reported in several unconventional superconductors due to the absence of inversion symmetry breaking. This contrasts with cuprate superconductors, where such interaction has been neglected for a long time. The recent report of a non-trivial spin orbit coupling in overdoped Bi2212 cuprate superconductor, has re-opened an old debate on both the source and role of such interaction and its evolution throughout the superconducting dome. Using high-resolution spin- and angle-resolved photoemission spectroscopy, we reveal a momentum-dependent spin texture throughout the hole-doped side of the superconducting phase diagram for single- and double-layer bismuth-based cuprates. The universality of the reported effect among different dopings and the disappearance of spin polarization upon lead substitution, suggest a common source. We argue that local structural fluctuations of the CuO planes and the resulting charge imbalance may cause local inversion symmetry breaking and spin polarization, which might be crucial for understanding cuprates physics.

cond-mat.str-el↗

arXiv2024

Coherence Length of Electronic Nematicity in Iron-Based Superconductors

Recent developments in laser-excited photoemission electron microscopy (laser-PEEM) advance the visualization of electronic nematicity and nematic domain structures in iron-based superconductors. In FeSe and BaFe$_2$(As$_{0.87}$P$_{0.13}$)$_2$ superconductors, it has been reported that the thickness of the electronic nematic domain walls is unexpectedly long, leading to the formation of mesoscopic nematicity wave [T. Shimojima $\textit{et al.}$, Science $\textbf{373}$ (2021) 1122]. This finding demonstrates that the nematic coherence length $ξ_{\rm nem}$ can be decoupled from the lattice domain wall. Here, we report that the electronic domain wall thickness shows a distinct variation in related materials: it is similarly long in FeSe$_{0.9}$S$_{0.1}$ whereas it is much shorter in undoped BaFe$_2$As$_2$. We find a correlation between the thick domain walls and the non-Fermi liquid properties of normal-state resistivity above the nematic transition temperature. This suggests that the nematic coherence length can be enhanced by underlying spin-orbital fluctuations responsible for the anomalous transport properties.

cond-mat.supr-con↗

arXiv2024

Synthesis of $c$-axis textured CaKFe$_4$As$_4$ superconducting bulk via spark plasma texturing technique

Grain alignment is a key factor that determines the performance of a superconducting bulk. In this study, the spark plasma texturing (SPT) technique was used to fabricate a CaKFe$_4$As$_4$ superconducting bulk. X-ray diffraction and electron backscatter diffraction revealed that the $c$-axes of the CaKFe$_4$As$_4$ grains in the SPT bulk are aligned, demonstrating that the SPT technique is effective in achieving $c$-axis texture. In addition, chemical composition analysis showed that oxide impurities, which affect the grain boundary characteristics that determine the inter-grain critical current density ($J_c$), are randomly distributed in the SPT bulk. Magnetization measurements showed high $J_c$ values of the SPT bulk, reaching 127 kA cm$^{-2}$ and 26 kA cm$^{-2}$ at 4.2 K under a self-field and magnetic field of 5 T, respectively. These results suggest that the SPT technique is a promising approach for obtaining a high-performance superconducting bulk material for high-field applications.

cond-mat.supr-con↗

arXiv2024

Synthesis of CaKFe$_4$As$_4$ bulk samples with high critical current density using a spark plasma sintering technique

A high density CaKFe$_4$As$_4$ bulk sample was successfully synthesized using a spark plasma sintering (SPS) technique. The density of the synthesized sample was 5.02 g cm$^{-3}$, corresponding to 96.2% of the theoretical density of CaKFe$_4$As$_4$. Moreover, a reasonably high Vickers hardness of 1 GPa was measured. The electrical resistivity of the SPS bulk sample was as low as approximately 600 $μΩ$ cm at 300 K, which is smaller than that of the ordinary sintered polycrystalline sample by nearly one order of magnitude, and exhibited a sharp superconducting transition, with the transition width $Δ\textit{T}_c$ less than 2 K, indicating an improved grain connectivity. The critical current density of the SPS bulk sample, as calculated from the magnetization hysteresis loops (magnetic $\textit{J}_c$), reached 18 kA cm$^{-2}$ at 4.2 K under 5 T, which is the highest among the iron-based superconductor polycrystalline samples reported thus far.

cond-mat.supr-con↗

arXiv2024

Single-Crystal Growth and Characterization of Cuprate Superconductor (Hg,Re)Ba$_2$Ca$_2$Cu$_3$O$_{8+δ}$

We grew (Hg,Re)Ba$_2$Ca$_2$Cu$_3$O$_{8+δ}$ ((Hg,Re)1223) single crystals with good reproducibility via the single-step flux method using monoxides as raw materials. A double-sealing method using a thick-walled quartz tube and a stainless-steel container was adopted for explosion protection. The maximum crystal size was approximately 1 mm x 1 mm in the ab plane and 0.04 mm in thickness. The crystal was square-shaped, reflecting the tetragonal crystal structure of (Hg,Re)1223. Magnetic susceptibility measurements indicated a critical temperature of 130 K. The in-plane resistivity exhibited a linear temperature dependence, indicating that the sample was close to optimal doping level. The out-of-plane resistivity was also measured, and the anisotropy parameter was 250-650 at 300 K.

cond-mat.supr-con↗

arXiv2024

Optically Probing Unconventional Superconductivity in Atomically Thin Bi$_2$Sr$_2$Ca$_{0.92}$Y$_{0.08}$Cu$_2$O$_{8+δ}$

Atomically thin cuprates exhibiting a superconducting phase transition temperature similar to bulk have recently been realized, although the device fabrication remains a challenge and limits the potential for many novel studies and applications. Here we use an optical pump-probe approach to noninvasively study the unconventional superconductivity in atomically thin Bi$_2$Sr$_2$Ca$_{0.92}$Y$_{0.08}$Cu$_2$O$_{8+δ}$ (Y-Bi2212). Apart from finding an optical response due to the superconducting phase transition that is similar to bulk Y-Bi2212, we observe that the sign and amplitude of the pump-probe signal in the atomically thin flake vary significantly in different dielectric environments depending on the nature of the optical excitation. By exploiting the spatial resolution of the optical probe, we uncover the exceptional sensitivity of monolayer Y-Bi2212 to the environment. Our results provide the first optical evidence for the intralayer nature of the superconducting condensate in Bi2212, and highlight the role of double-sided encapsulation in preserving superconductivity in atomically thin cuprates.

cond-mat.mes-hall↗

arXiv2023

Discovery of Orbital Ordering in Bi2Sr2CaCu2O8+x

The primordial ingredient of cuprate superconductivity is the CuO2 unit cell. Here, theoretical attention usually concentrates on the intra-atom Coulombic interactions dominating the 3d^9 and 3d^10 configurations of each copper ion. However, if Coulombic interactions also occur between electrons of the 2p^6 orbitals of each planar oxygen atom, spontaneous orbital ordering may split their energy levels. This long predicted intra-unit cell symmetry breaking should then generate an orbital ordered phase, for which the charge-transfer energy E separating the 2p^6 and 3d^10orbitals is distinct for the two oxygen atoms. Here we introduce sublattice resolved E(r) imaging techniques to CuO2 studies and discover intra-unit-cell rotational symmetry breaking of E(r), with energy-level splitting between the two oxygen atoms on the 50 meV scale. Spatially, this state is arranged in disordered Ising domains of orthogonally oriented orbital order that appear bounded by dopant ions, and within whose domain walls low energy electronic quadrupolar two-level systems occur. Overall, these data reveal a Q=0 orbitally ordered state that splits the energy levels of the oxygen orbitals by ~50 meV, in underdoped CuO2.

cond-mat.supr-con↗