arXiv subjects

Tetsuro Nakamura

Publications and source records attributed to Tetsuro Nakamura.

6 recordsLinked to original sources

arXiv2026

Development of ultra-high efficiency soft X-ray angle-resolved photoemission spectroscopy equipped with deep prior-based denoising method

Soft X-ray angle resolved photoemission spectroscopy (SX-ARPES) is one of the most powerful spectroscopic techniques to visualize the three-dimensional bulk electronic structure in reciprocal lattice space. Compared with ARPES employing low-energy photon sources, the time burden imposed by a lower photoelectron yield, stemming from the photoionization cross-section, has been a persistent technical challenge. To address this challenge, we have developed a noise removal system by using the deep prior-based method and integrated it into the micro focused SX-ARPES (μSX-ARPES) system at BL25SU in SPring-8. Our implemented system effectively eliminates the grid and spike noise typically present in ARPES data acquired using the voltage Fixed-mode, within about 30 seconds. We demonstrate, through the μSX-ARPES measurements on a single crystal of CeRu2Si2, that data with sufficient statistical accuracy can be obtained in approximately 40 seconds. In addition, we present the potential of high signal-to-noise ratio ARPES measurement, achieving an energy resolution of 51.6 meV at an excitation energy of 708 eV in μSX-ARPES measurements on polycrystalline gold. Our developed system successfully reduces the time burden in SX-ARPES and paves the way for advancements in lower photoelectron yield measurements, such as those requiring higher energy resolution and three-dimensional nonequilibrium measurements.

cond-mat.str-el↗

arXiv2018

Superconducting transition temperatures in the electronic and magnetic phase diagrams of Sr2VFeAsO3-delta, a superconductor

We elucidate the magnetic phases and superconducting transition temperatures (Tc) in Sr2VFeAsO3-delta (21113V), an iron-based superconductor with a thick-blocking layer fabricated from a perovskite-related transition metal oxide. At low temperatures (T < 37.1 K), 21113V exhibited a superconducting phase in the range 0.031 =< delta =< 0.145 and an antiferromagnetic (AFM) iron sublattice in the range 0.267 =< delta =< 0.664. Mixed-valent vanadium exhibited a dominant AFM phase in 0.031 =< delta =< 0.088, and a partial ferrimagnetic (Ferri.) phase in the range 0.124 =< delta =< 0.664. The Ferri. phase was the most dominant at a delta value of 0.267, showing an AFM phase of Fe at T < 20 K. Increasing the spontaneous magnetic moments reduced the magnetic shielding volume fraction due to the superconducting phase. This result was attributed to the magnetic phase of vanadium, which dominates the superconductivity of Fe in 21113V. The Tc-delta curve showed two maxima. The smaller and larger of Tc maxima occurred at delta = 0.073 and delta = 0.145, respectively; the latter resides on the phase boundary between AFM and the partial Ferri. phases of vanadium. 21113V is a useful platform for verifing new mechanisms of Tc enhancement in iron-based superconductors.

cond-mat.supr-con↗

arXiv2013

Ferromagnetic Quantum Critical Point Induced by Tuning the Magnetic Dimensionality of the Heavy-Fermion Iron Oxypnictide Ce(Ru$_{1-x}$Fe$_x$)PO

We have performed 31P-NMR measurements of the c-axis-aligned poly-crystal Ce(Ru1-xFex)PO with a two-dimensional layered structure in order to understand the origin of TCurie suppression by Fe substitution. The variation in the magnetic-fluctuation character with respect to x is investigated from the in-plane and out-of plane fluctuations and the relationship between the static spin susceptibility and the in-plane fluctuation. It was found that three-dimensional ferromagnetic (FM) correlations are dominant and give rise to the FM ordering in CeRuPO. The out-of-plane fluctuations are significantly suppressed with increasing x, and it was revealed that the FM fluctuations become two dimensional near a FM quantum critical point (QCP). Our NMR results strongly suggest that a unique FM QCP in Ce(Ru1-xFex)PO is induced by the suppression of the magnetic correlations along the c-axis, which is a different mechanism from that presumed in heavy-fermion compounds.

cond-mat.str-el↗

arXiv2012

Ferromagnetic Quantum Critical Point in Heavy-Fermion Iron Oxypnictide Ce(Ru$_{1-x}$Fe$_x$)PO

We have performed ^{31}P-NMR measurements on Ce(Ru_{1-x}Fe_{x})PO in order to investigate ferromagnetic (FM) quantum criticality, since heavy-fermion (HF) ferromagnet CeRuPO with a two-dimensional structure turns to a HF paramagnet by an isovalent Fe-substitution for Ru. We found that Ce(Ru_{0.15}Fe_{0.85})PO shows critical fluctuations down to \sim 0.3 K, as well as the continuous suppression of Curie temperature and the ordered moments by the Fe-substitution. These experimental results suggest the presence of a FM quantum critical point (QCP) at x \sim 0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru_{0.15}Fe_{0.85})PO share the similarity with those in YbRh_{2}Si_{2}, where the local criticality of $f$ electrons has been discussed. We reveal that Ce(Ru_{1-x}Fe_{x})PO is a new system to study FM quantum criticality in HF compounds.

cond-mat.str-el↗

arXiv2012

Paramagnetic Phase of a Heavy-Fermion Compound, CeFePO, Probed by 57Fe Mössbauer Spectroscopy

57Fe Mössbauer spectroscopy was applied to an iron-based layered compound CeFePO. At temperatures from 9.4 to 293 K, no magnetic splitting was observed in the Mössbauer spectra of CeFePO indicating a paramagnetic phase of the Fe magnetic sublattice. All the spectra were fitted with a small quadrupole splitting, and the Debye temperature of CeFePO was found to be \sim448 K. The isomer shift at room temperature, 0.32 mm/s, was almost equal to those of LnFeAsO (Ln = La, Ce, Sm). Comparing s-electron density using the isomer shifts and unit cell volumes, it was found that the Fe of CeFePO has a similar valence state to other layered iron-based quaternary oxypnictides except LaFePO.

cond-mat.str-el↗

arXiv2009

Full Self-Consistent Projection Operator Approach to Nonlocal Excitations in Solids

A self-consistent projection operator method for single-particle excitations is developed. It describes the nonlocal correlations on the basis of a projection technique to the retarded Green function and the off-diagonal effective medium. The theory takes into account long-range intersite correlations making use of an incremental cluster expansion in the medium. A generalized self-consistent coherent potential is derived. It yields the momentum-dependent excitation spectra with high resolution. Numerical studies for the Hubbard model on a simple cubic lattice at half filling show that the theory is applicable in a wide range of Coulomb interaction strength. In particular, it is found that the long-range antiferromagnetic correlations in the strong interaction regime cause shadow bands in the low-energy region and sub-peaks of the Mott-Hubbard bands.

cond-mat.str-el↗