arXiv subjects

Takahiro Shimojima

Publications and source records attributed to Takahiro Shimojima.

6 recordsLinked to original sources

arXiv2026

Momentum-dependent charge-density-wave gap formation in ZrTe_{2.98}Se_{0.02}

We investigated the energy gap formation across the charge density wave (CDW) transition inof ZrTe_{2.98}Se_{0.02}. By employing a laser photoemission microscopy, we clearly resolved one elliptical Fermi surface (FS) around the Brillouin zone (BZ) center, and two quasi-one-dimensional FSs along the BZ boundary. We further mapped the intensity difference between the FSs below and above the CDW transition temperature. We found that the energy CDW gap formation is limited to the momentum region 0.25 Å^{-1} < ky < 0.8 Å^{-1} along \bar{B}-\bar{D} line, which coincides with the location of one of the quasi-one-dimensional FSs. Characteristic momentum dependence in the energy CDW gap suggests the importance of both FS nesting and band-dependent electron-phonon coupling for understanding the CDW state in ZrTe_{3} system.

cond-mat.other↗

arXiv2026

Indication of Stochastic Photothermal Dynamics around a Topological Defect in a Chiral Magnet

Chiral magnets host topologically protected spin textures whose nonequilibrium dynamics are crucial in phase transitions and domain evolution, yet ultrafast defect-mediated processes remain poorly understood. Here, we investigate photothermally induced helical-to-paramagnetic phase transition in Co$_9$Zn$_9$Mn$_2$ using pump-probe Lorentz transmission electron microscopy (LTEM). Following the suppression of the magnetic stripe contrast induced by femtosecond pulsed laser, we observe a directional recovery process of magnetic order driven by the anisotropic thermal diffusion, toward the thick region that effectively acts as a heat sink. Remarkably, around a magnetic edge dislocation, the magnetic contrast recovery exhibits a pronounced delay accompanied by a transient blurring of LTEM contrast. These findings suggest that the recovery dynamics around the magnetic edge dislocation proceed through multiple relaxation paths that are selected stochastically. Our results indicate a possible enhancement of stochasticity around topological defects during the recovery dynamics of magnetic phase transitions.

cond-mat.mes-hall↗

arXiv2026

Fermi surface of Kagome metal CsCr$_3$Sb$_5$ observed by laser photoemission microscopy

We investigated the Fermi surface (FS) in the paramagnetic state of Kagome metal CsCr$_3$Sb$_5$ by employing a laser photoemission microscopy. We found a circular FS and two hexagonal FSs around the Brillouin zone (BZ) center. Polarization-dependent measurements further enable us to detect small FS pockets at the BZ boundary. According to the density functional theory calculations, the orbital characters of the FSs were determined from their shape and orientations. We found that the size of the FS is strongly modified for the d$_{xz}$ orbital, suggesting the orbital-dependent correlation effect. These results provide an electronic basis for exploring the interplay of antiferromagnetic/charge density wave order and possible unconventional superconductivity in this compound.

cond-mat.str-el↗

arXiv2024

Development of ultrafast four-dimensional precession electron diffraction

Ultrafast electron diffraction/microscopy technique enables us to investigate the nonequilibrium dynamics of crystal structures in the femtosecond-nanosecond time domain. However, the electron diffraction intensities are in general extremely sensitive to the excitation errors (i.e., deviation from the Bragg condition) and the dynamical effects, which had prevented us from quantitatively discussing the crystal structure dynamics. Here, we develop a four-dimensional precession electron diffraction (4D-PED) system by which time ($t$) and electron-incident-angle ($ϕ$) dependences of electron diffraction patterns ($q_x,q_y$) are recorded. Nonequilibrium crystal structure refinement on VTe$_{2}$ demonstrates that the ultrafast change in the crystal structure can be quantitatively determined from 4D-PED. We further perform the analysis of the $ϕ$ dependence, from which we can qualitatively estimate the change in the reciprocal lattice vector parallel to the optical axis. These results show the capability of the 4D-PED method for the quantitative investigation of ultrafast crystal structural dynamics.

cond-mat.mtrl-sci↗

arXiv2016

Low-Temperature and High-Energy-Resolution Laser Photoemission Spectroscopy

We present a review on the developments in the photoemission spectrometer with a vacuum ultraviolet laser at Institute for Solid State Physics at the University of Tokyo. The advantages of high energy resolution, high cooling ability, and bulk sensitivity enable applications with a wide range of materials. We introduce some examples of fine electronic structures detected by laser photoemission spectroscopy and discuss the prospects of research on low-transition-temperature superconductors exhibiting unconventional superconductivity.

cond-mat.supr-con↗

arXiv2014

Superconductivity in an electron band just above the Fermi level: possible route to BCS-BEC superconductivity

Conventional superconductivity follows Bardeen-Cooper-Schrieffer(BCS) theory of electrons-pairing in momentum-space, while superfluidity is the Bose-Einstein condensation(BEC) of atoms paired in real-space. These properties of solid metals and ultra-cold gases, respectively, are connected by the BCS-BEC crossover. Here we investigate the band dispersions in FeTe$_{0.6}$Se$_{0.4}$($T_c$ = 14.5 K $\sim$ 1.2 meV) in an accessible range below and above the Fermi level($E_F$) using ultra-high resolution laser angle-resolved photoemission spectroscopy. We uncover an electron band lying just 0.7 meV ($\sim$ 8 K) above $E_F$ at the $Γ$-point, which shows a sharp superconducting coherence peak with gap formation below $T_c$. The estimated superconducting gap $Δ$ and Fermi energy $ε_F$ indicate composite superconductivity in an iron-based superconductor, consisting of strong-coupling BEC in the electron band and weak-coupling BCS-like superconductivity in the hole band. The study identifies the possible route to BCS-BEC superconductivity.

cond-mat.supr-con↗