arXiv subjects

Takao Sasagawa

Publications and source records attributed to Takao Sasagawa.

At least 19 recordsLinked to original sources

arXiv2026

Selective coupling of high-order phonons in La2-xSrxCuO4

In crystals and molecules with strong coupling between the charge carriers and atomic vibrations, high order harmonics of the vibrational excitations can be observed with intensity following the Franck-Condon envelope. Here, we uncover a new regime of electron-phonon coupling in La2-xSrxCuO4 using resonant inelastic X-ray scattering. In the undoped compound, we find sharp peaks at approximately 85$\pm$5 meV, 180$\pm$7 meV, and 330$\pm$5 meV with no observable dispersion, and an absence of appreciable intensity from other modes observed in Raman spectroscopy in this energy range. The 180 meV and 330 meV excitations have energies approximately consistent with two-phonon and four-phonon in-plane Cu-O bond stretching mode, suggesting that the resonantly excited valence electrons strongly couple to this mode. These excitations exhibit significant doping dependence, eventually becoming unresolvable at 10% hole doping, where the low energy charge excitations are dominated by dispersive plasmons. The observation of selective coupling to even-ordered phonons and stark contrast with the Raman spectra indicate anomalous electron-phonon coupling beyond the Franck-Condon picture. One intriguing possibility is the existence of locally paired quasiparticles in insulating cuprates.

cond-mat.str-el↗

arXiv2026

Universal Magnetoresistance Scaling in Layered Pd-based Multiband Metals Beyond Compensated Semimetal Regime

We systematically investigated the magnetoresistance ratio (MR) of single-crystalline, nonmagnetic layered Pd-based metals, including centrosymmetric PdTe2, PdPb2, and beta-PdBi2 and noncentrosymmetric alpha-PdBi. Our study identifies a distinct class of large MR in multiband, high-carrier-density systems. Unlike well-studied extremely large MR materials, such as Dirac and Weyl semimetals described by simple compensated-carrier models, these compounds possess complex Fermi surfaces, as validated by our first-principles calculations. Nevertheless, they exhibit a remarkably simple MR scaling governed by carrier mobility, manifested in systematic dependencies on magnetic field, temperature, and the residual resistivity ratio (RRR). The validity of Kohler's rule in high-RRR crystals indicates that MR is governed by a single effective scattering time, even in these multiband systems. The field and RRR dependences of MR follow an intermediate power-law behavior between linear and quadratic, attributable to imperfect carrier compensation and a distribution of carrier mobilities. Among the studied compounds, alpha-PdBi exhibits the largest MR, reaching 1500% (2 K, 7 T), owing to its exceptionally high RRR (approximately 660). However, when compared on an equal-RRR basis, its MR is smaller than that of its centrosymmetric counterparts. This trend suggests that additional scattering channels arising from spin-orbit-induced band splitting in noncentrosymmetric systems reduce the effective carrier mobility. Our results establish a new class of large MR in clean multiband metals where complex electronic structures give rise to emergent single-parameter scaling, highlighting the interplay between disorder, mobility, and symmetry.

cond-mat.mtrl-sci↗

arXiv2026

Thermal Hall Resistivity as a Unifying Description of Phonon Thermal Hall Effect in Various Insulators

A considerable phonon thermal Hall effect was recently discovered across a diverse collection of materials. To clarify this enigmatic thermal Hall response in various insulators, we investigate the doped Mott insulator La$_{5/3}$Sr$_{1/3}$NiO$_4$ as an example material system and reveal a characteristic phonon-dominated thermal Hall effect. As a sensitive probe of the transverse thermal response, the thermal Hall resistivity $w_{xy}$ exhibits an insulating-like temperature dependence $w_{xy}(T)$, a linear magnetic-field dependence $w_{xy}(H)$ near $H=0$, and a $T$-linear thermal Hall angle at low temperatures. The presence of similar phenomena across a series of insulators suggests that $w_{xy}$ serves as a unifying description of phonon thermal Hall effect, corroborated by an apparent correlation between the insulating-like $w_{xy}(T)$ and material's localized electronic state.

cond-mat.str-el↗

arXiv2026

Systematic study of superconductivity in few-layer $T_d$-MoTe$_2$

We present a systematic investigation of superconductivity in a topological superconductor candidate $T_{\rm d}$-MoTe$_2$ in the few-layer limit. By examining multiple mechanically exfoliated samples with different thicknesses, substrates and crystal qualities, we quantitatively correlate superconducting temperature ($T_c$) with disorder, carrier density, carrier type and mobility. By integrating these experimental findings with first-principles calculations, we reveal the relationship between the band structure and superconductivity in this material. Notably, in 2 L samples we access a highly hole-doped regime that has not been systematically explored in previous experiments, providing a complementary perspective to earlier studies. In this regime, we demonstrate that superconductivity can be realized in a manner consistent with a conventional phonon-mediated $s_{(++)}$-wave pairing.

cond-mat.mes-hall↗

arXiv2025

Direct observation of the surface superconducting gap in the topological superconductor candidate β-PdBi2

β-PdBi2 is one of the candidates for topological superconductors with a superconducting (SC) transition temperature (Tc) of 5.3 K, in which parity mixing of spin singlet and spin triplet has been anticipated, being crucial for the further understanding of relationship with inversion symmetry and parity mixing in the superconductivity. In this work, we measured the SC gap in high-quality single crystal of β-PdBi2 by using high-resolution laser angle-resolved photoemission spectroscopy below Tc. We found the isotropic SC gaps in momentum space for multiple bands, and observed that the difference between the SC gap of the topological surface bands and the bulk bands is about 0.1 meV, consistent with other experimental results. These direct and quantitative experimental results support the possibility of β-PdBi2 as a topological superconductor, characterized by unique crystal and electronic band structures.

cond-mat.supr-con↗

arXiv2025

Valley polarization of Landau levels driven by residual strain in the ZrSiS surface band

In a multi-valley electronic band structure, lifting of the valley degeneracy is associated with rotational symmetry breaking in the electronic fluid, and may emerge through spontaneous symmetry breaking order, or through a large response to a small external perturbation such as strain. In this work we use scanning tunneling microscopy to investigate an unexpected rotational symmetry breaking in Landau levels formed in the unusual floating surface band of ZrSiS. We visualize a ubiquitous splitting of Landau levels into valley-polarized sub-levels. We demonstrate methods to measure valley-selective Landau level spectroscopy, to infer unknown Landau level indices, and to precisely measure each valley's Berry phase in a way that is agnostic to the band structure and topology of the system. These techniques allow us to obtain each valley's dispersion curve and infer a rigid valley-dependent contribution to the band energies. Ruling out spontaneous symmetry breaking by establishing the sample-dependence of this valley splitting, we explain the effect in terms of residual strain. A quantitative estimate indicates that uniaxial strain can be measured to a precision of $ \lt 0.025 \% $. The extreme valley-polarization of the Landau levels results from as little as $ \sim 0.1 \% $ strain, and this suggests avenues for manipulation using deliberate strain engineering.

cond-mat.mes-hall↗

arXiv2025

Cu NMR study of lightly doped La$_{2-x}$Sr$_{x}$CuO$_4$

We present a single crystal Cu NMR study of the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_4$ with hole doping levels between $x=2$\% and $8$\%. Measurements with short spin echo times enable us to systematically study the local properties of the electronic spin system in the region of the phase diagram where the material evolves from the insulating antiferromagnetic parent phase to the superconducting state. We find evidence for qualitative changes as the Sr concentration increases through $x=5$\% in both NMR spectra and relaxation times, which we interpret as signatures of a low-temperature transition from a state with disconnected metallic islands to a granular metal with tunneling between the grains. These results provide microscopic insight into the physics of a doped charge-transfer insulator in the presence of quenched substitutional disorder, indicate that a connectivity transition occurs, and demonstrate that complex nanoscale electronic phase separation is ubiquitous in LSCO around the doping level where superconductivity first appears in the phase diagram.

cond-mat.supr-con↗

arXiv2024

Closing of the Mott gap near step edges in NiS2

A prototypical charge-transfer type Mott insulator NiS2 pyrite exhibits a metal-insulator transition with bandwidth control. Recent discoveries on surface-specific electronic states on other 3d transition-metal disulfide pyrites motivate us to further investigate the surface of NiS2, where metallic surface conduction is discussed. Here, the spectroscopic-imaging scanning-tunneling-microscopy observations revealed that the surface is not metallic, contrary to the expectation. Instead, the Mott gap is closed near step edges, suggesting possible electrical conduction from one-dimensional channels. The edge anomaly was observed irrespective of its magnetic order and is limited to the insulator phases.

cond-mat.str-el↗

arXiv2024

Photoemission angular distribution beyond the single wavevector description of photoelectron final states

We develop a simulation procedure for angle-resolved photoemission spectroscopy (ARPES), where a photoelectron wave function is set to be an outgoing plane wave in a vacuum associated with the emitted photoelectron wave packet. ARPES measurements on the transition metal dichalcogenide $1T$-$\mathrm{Ti}\mathrm{S}_2$ are performed, and our simulations exhibit good agreement with experiments. Analysis of our calculated final state wave functions quantitatively visualizes that they include various waves due to the boundary condition and the uneven crystal potential. These results show that a more detailed investigation of the photoelectron final states is necessary to fully explain the photon-energy- and light-polarization-dependent ARPES spectra.

cond-mat.mtrl-sci↗

arXiv2024

Broken Screw Rotational Symmetry in the Near-Surface Electronic Structure of $AB$-Stacked Crystals

We investigate the electronic structure of $2H$-$\mathrm{Nb}\mathrm{S}_2$ and $h$-$\mathrm{BN}$ by angle-resolved photoemission spectroscopy (ARPES) and photoemission intensity calculations. Although in bulk form, these materials are expected to exhibit band degeneracy in the $k_z=π/c$ plane due to screw rotation and time-reversal symmetries, we observe gapped band dispersion near the surface. We extract from first-principles calculations the near-surface electronic structure probed by ARPES and find that the calculated photoemission spectra from the near-surface region reproduce the gapped ARPES spectra. Our results show that the near-surface electronic structure can be qualitatively different from the bulk one due to partially broken nonsymmorphic symmetries.

cond-mat.mtrl-sci↗

arXiv2023

Gate-defined Josephson weak-links in monolayer $\mathrm{WTe_2}$

Systems combining superconductors with topological insulators offer a platform for the study of Majorana bound states and a possible route to realize fault tolerant topological quantum computation. Among the systems being considered in this field, monolayers of tungsten ditelluride ($\mathrm{WTe_2}$) have a rare combination of properties. Notably, it has been demonstrated to be a Quantum Spin Hall Insulator (QSHI) and can easily be gated into a superconducting state. We report measurements on gate-defined Josephson weak-link devices fabricated using monolayer $\mathrm{WTe_2}$. It is found that consideration of the two dimensional superconducting leads are critical in the interpretation of magnetic interference in the resulting junctions. The reported fabrication procedures suggest a facile way to produce further devices from this technically challenging material and the results mark the first step toward realizing versatile all-in-one topological Josephson weak-links using monolayer $\mathrm{WTe_2}$.

cond-mat.mes-hall↗

arXiv2023

A robust weak topological insulator in a bismuth halide Bi4Br2I2

We apply a topological material design concept for selecting a bulk topology of 3D crystals by different van-der-Waals stacking of 2D topological insulator layers, and find a bismuth halide Bi4Br2I2 to be an ideal weak topological insulator (WTI) with the largest band gap (~230 meV) among all the WTI candidates, by means of angle-resolved photoemission spectroscopy (ARPES), density functional theory (DFT) calculations, and resistivity measurements. Our results vastly expand future opportunities for fundamental research and device applications with a robust WTI.

cond-mat.mtrl-sci↗

arXiv2022

Large anomalous Hall effect induced by weak ferromagnetism in the noncentrosymmetric antiferromagnet $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$

We study the mechanism of the exceptionally large anomalous Hall effect (AHE) in the noncentrosymmetric antiferromagnet $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ by angle-resolved photoemission spectroscopy (ARPES) and magnetotransport measurements. From ARPES measurements of $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ and its family compounds ($\mathrm{Fe}\mathrm{Nb}_3\mathrm{S}_6$ and $\mathrm{Ni}\mathrm{Nb}_3\mathrm{S}_6$), we find a band dispersion unique to the Co intercalation existing near the Fermi level. We further demonstrate that a slight deficiency of sulfur in $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ eliminates the ferromagnetism and the AHE simultaneously while hardly changing the band structure, indicating that the weak ferromagnetism is responsible for the emergence of the large AHE. Based on our results, we propose Weyl points near the Fermi level to cause the large AHE.

cond-mat.mtrl-sci↗

arXiv2021

Visualizing superconductivity in an inversion-symmetry-broken doped Weyl semimetal

The Weyl semimetal MoTe$_2$ offers a rare opportunity to study the interplay between Weyl physics and superconductivity. Recent studies have found that Se substitution can boost the superconductivity up to 1.5K, but suppress the Td structure phase that is essential for the emergence of Weyl state. A microscopic understanding of possible coexistence of enhanced superconductivity and the Td phase has not been established so far. Here, we use scanning tunneling microscopy (STM) to study a optimally doped new superconductor MoTe$_{1.85}$Se$_{0.15}$ with bulk Tc ~ 1.5K. By means of quasiparticle interference imaging, we identify the existence of low temperature Td phase with broken inversion symmetry where superconductivity globally coexists. Consistently, we find that the superconducting coherence length, extracted from both the upper critical field and the decay of density of states near a vortex, is much larger than the characteristic length scale of existing dopant derived chemical disorder. Our findings of robust superconductivity arising from a Weyl semimetal normal phase in MoTe$_{1.85}$Se$_{0.15}$, makes it a promising candidate for realizing topological superconductivity.

cond-mat.supr-con↗

arXiv2021

Observation of near EF Fermi-arc van Hove singularity with prominent coupling to phonon in a van der Waals coupled Weyl semimetal

A van der Waals coupled Weyl semimetal material NbIrTe4 is investigated by combining scanning tunneling microscopy/spectroscopy and first principles calculations. We observe a sharp peak in the tunneling conductance near the zero bias energy, and its origin is ascribed to a van Hove singularity associated with a Lifshitz transition of the topologically none trivial Fermi arc states. Furthermore, tunneling spectroscopy measurements show a surprisingly large signature of electron boson coupling, which presumably represents anomalously enhanced electron phonon coupling through the enhanced charge susceptibility. Our finding in van der Waals coupled material is particularly invaluable due to applicable exfoliation technology for searching exotic topological states by further manipulating near Fermi energy van Hove singularity in nanometer scale flakes and their devices.

cond-mat.mes-hall↗

arXiv2021

Local Observations of Orbital Diamagnetism and Excitation in Three-Dimensional Dirac Fermion Systems Bi$_{1-x}$Sb$_x$

Dirac fermions display a singular response against magnetic and electric fields. A distinct manifestation is large diamagnetism originating in the interband effect of Bloch bands, as observed in bismuth alloys. Through $^{209}$Bi NMR spectroscopy, we extract diamagnetic orbital susceptibility inherent to Dirac fermions in the semiconducting bismuth alloys Bi$_{1-x}$Sb$_x$ ($x = 0.08 - 0.16$). The $^{209}$Bi hyperfine coupling constant provides an estimate of the effective orbital radius. In addition to the interband diamagnetism, Knight shift includes an anomalous temperature-independent term originating in the enhanced intraband diamagnetism under strong spin-orbit coupling. The nuclear spin-lattice relaxation rate $1/T_1$ is dominated by orbital excitation and follows cubic temperature dependence in the extensive temperature range. The result demonstrates the robust diamagnetism and low-lying orbital excitation against the small gap opening, whereas $x$-dependent spin excitation appears at low temperatures.

cond-mat.str-el↗

arXiv2021

Material design with the van der Waals stacking of bismuth-halide chains realizing a higher-order topological insulator

The van der Waals (vdW) materials with low dimensions have been extensively studied as a platform to generate exotic quantum properties. Advancing this view, a great deal of attention is currently paid to topological quantum materials with vdW structures. Here, we provide a new concept of designing topological materials by the vdW stacking of quantum spin Hall insulators (QSHIs). Most interestingly, a slight shift of inversion center in the unit cell caused by a modification of stacking is found to induce the topological variation from a trivial insulator to a higher-order topological insulator (HOTI). Based on that, we present the first experimental realization of a HOTI by investigating a bismuth bromide Bi4Br4 with angle-resolved photoemission spectroscopy (ARPES). The unique feature in bismuth halides capable of selecting various topology only by differently stacking chains, combined with the great advantage of the vdW structure, offers a fascinating playground for engineering topologically non-trivial edge-states toward future spintronics applications.

cond-mat.mtrl-sci↗

arXiv2020

High-Mobility Carriers Induced by Chemical Doping in the Candidate Nodal-Line Semimetal CaAgP

We report the electronic properties of single crystals of candidate nodal-line semimetal CaAgP. The transport properties of CaAgP are understood within the framework of a hole-doped nodal-line semimetal. In contrast, Pd-doped CaAgP shows a drastic increase of magnetoresistance at low magnetic fields and a strong decrease of electrical resistivity at low temperatures probably due to weak antilocalization. Hall conductivity data indicated that the Pd-doped CaAgP has not only hole carriers induced by the Pd doping, but also high-mobility electron carriers in proximity of the Dirac point. Electrical resistivity of Pd-doped CaAgP also showed a superconducting transition with onset temperature of 1.7-1.8 K.

cond-mat.mtrl-sci↗