arXiv subjects

Hajime Yamamoto

Publications and source records attributed to Hajime Yamamoto.

8 recordsLinked to original sources

arXiv2026

Raman scattering in cuprate oxychlorides high-temperature superconductors single-crystals

We investigate the Raman response of sodium-doped cuprate oxychloride Na$_x$Ca$_{2-x}$CuO$_2$Cl$_2$ high-temperature superconductors across their entire phase diagram, from the antiferromagnetic to the superconducting region. In addition to the expected Raman-active phonon modes, we detect several additional modes that can be interpreted as being excited via a resonance process enabled by strong electron--phonon coupling. To verify the resonance effect, the Raman response at different incident photon energies was measured. At high energies, there is a well-defined $B_{1g}$ mode in the antiferromagnetic phase, which can be interpreted as a bimagnon We follow its temperature and doping dependence, providing information on the multimagnon excitation in these cuprates, which can be theoretically linked to the exchange interaction.

cond-mat.supr-con↗

arXiv2025

Unveiling two-dimensional electron systems on ultra-wide bandgap semiconductor $\mathrmβ$-Ga$_2$O$_3$

Ultra-wide bandgap (UWBG) semiconductors promise to revolutionize power electronics, yet a fundamental understanding of their interfacial electronic structure has been hindered by the absence of direct experimental observation. Here, we report the first momentum-resolved observation of two-dimensional electron systems on a UWBG material, enabled by angle resolved photoemission spectroscopy (ARPES) on high-purity $β$-Ga$_2$O$_3$ single crystals. Alkaline-metal-induced electron doping forms an isotropic circular Fermi surface, achieving a sheet carrier density of up to $1.0\times10^{14}$ $\mathrm{cm}^{-2}$. Self-consistent Poisson-Schrödinger calculations show that the electrons are confined within 1.2 nm of the surface and reveal an internal electric field of $18$ MV cm$^{-1}$. Crucially, our measurements reveal a pronounced renormalization of the electronic band structure: a series of carrier-density-dependent ARPES measurements shows that as the carrier density increases from $2\times10^{13}$ to $1.0\times10^{14}$ $\mathrm{cm}^{-2}$, the effective mass anomalously increases, nearly doubling to a final value of 0.48 $\textit{m}_{\mathrm{e}}$. This trend is notably opposite to that reported for other oxide semiconductors, pointing towards a unique renormalization mechanism in $β$-Ga$_2$O$_3$. Our findings establish the interfacial electronic structure of $β$-Ga$_2$O$_3$ and demonstrate that UWBG materials provide fertile ground for exploring carrier-density-driven electronic phenomena, opening new avenues for future quantum and power devices.

cond-mat.mtrl-sci↗

arXiv2025

On the cuprates' universal waterfall feature: evidence of a momentum-driven crossover

We study two related universal anomalies of the spectral function of cuprates, so called waterfall and high-energy kink features, by a combined cellular dynamical mean-field theory and angle-resolved photoemission study for the oxychloride Na$_x$Ca$_{2-x}$CuO$_2$Cl$_2$ (Na-CCOC). Tracing their origin back to an interplay of spin-polaron and local correlation effects both in undoped and hole-doped (Na-)CCOC, we establish them as a universal crossover between regions differing in the momentum dependence of the coupling and not necessarily in the related quasiparticles' energies. The proposed scenario extends to doping levels coinciding with the cuprate's superconducting dome and motivates further investigations of the fate of spin-polarons in the superconducting phase.

cond-mat.str-el↗

arXiv2024

Toward the theoretically observable limit of electron density distribution by single-crystal synchrotron X-ray diffraction: The case of orbitally ordered Ti-3d^1 in YTiO_3

The theoretically observable limit of electron density distribution by single-crystal X-ray diffraction is discussed. When F_{orb} and δF are defined as, respectively, the partial structure factor for an orbital and the deviation of the observed F from the true F, the accuracy of electron density attributable to F_{orb} is chiefly determined by the number of reflections satisfying the condition F_{orb}/F > δF/F. Since F_{orb}/F, which is generally small for crystals with large F(0,0,0), is constant under a given set of experimental conditions, δF/F must be reduced to increase the number of reflections satisfying F_{orb}/F > δF/F. The present paper demonstrates how to reduce δF mathematically and experimentally, and the following topics are covered: the Poisson statistics, accumulation of errors in the data collection and reduction procedure, multiple diffraction, conversion error from F^2 to F in refinement programs, which is unavoidable when the input quantities have different dimension from F, weighting of reflections, and tips. For demonstration, observation of the electron density of the Ti-3d^1 orbital in YTiO_3 by synchrotron single-crystal X-ray diffraction is presented.

cond-mat.mtrl-sci↗

arXiv2024

Crystal structures and superconducting properties of metallic double-chain based cuprate Pr2Ba4Cu7O15-delta

We demonstrated the lattice structures and the superconducting phases of metallic double-chain based cuprate Pr2Ba4Cu7O15-delta exhibiting higher Tc. After the oxygen heat treatment on citrate pyrolysis precursors, their reduction treatment followed by a quench procedure caused higher Tc samples with 26.5-30 K. The crystal structural parameters for the superconducting sample (delta = 0.81) were analyzed from the powder synchrotron X-ray diffraction data using RIETAN-FP program. The effect of magnetic field on the superconducting phase of these samples with different oxygen defects (delta =0.73, 0.81 and 0.87) was examined, for our understanding of the superconducting magnetic field-temperature phase diagram. For delta = 0.87 sample with Tc = 30 K, the resistive critical field Hc* was estimated to be 13 T at 4.2 K. The oxygen deficiency dependence on Tc,on for our samples was compared with the data of several other groups.

cond-mat.supr-con↗

arXiv2023

Paramagnon dispersion and damping in doped Na$_{x}$Ca$_{2-x}$CuO$_2$Cl$_2$

Using Resonant Inelastic X-ray Scattering, we measure the paramagnon dispersion and damping of undoped, antiferromagnetic Ca$_2$CuO$_2$Cl$_2$ as well as doped, superconducting Na$_{x}$Ca$_{2-x}$CuO$_2$Cl$_2$. Our estimation of the spin-exchange parameter and width of the paramagnon peak at the zone boundary $X=(0.5,0)$ confirms that no simple relation can be drawn between these parameters and the critical temperature $T_\mathrm{c}$. Consistently with other cuprate compounds, we show that upon doping there is a slight softening at $(0.25,0)$, but not at the zone boundary $X$. In combination with these measurements we perform calculations of the dynamical spin structure factor of the one-band Hubbard model using cluster dynamical mean-field theory. The calculations are in excellent agreement with the experiment in the undoped case, both in terms of energy position and width. While the increase in width is also captured upon doping, the dynamical spin structure factor shows a sizable softening at $X$, which provides insightful information on the length-scale of the spin fluctuations in doped cuprates.

cond-mat.supr-con↗

arXiv2019

Phonon anomalies with doping in superconducting oxychlorides Ca2-xCuO2Cl2

We measure the dispersion of the Cu-O bond-stretching phonon mode in the high-temperature superconducting parent compound Ca$_2$CuO$_2$Cl$_2$. Our density functional theory calculations predict a cosine-shaped bending of the dispersion along both the ($ξ$00) and ($ξξ$0) directions, while comparison with previous results on Ca$_{1.84}$CuO$_2$Cl$_2$ show it only along ($ξ$00), suggesting an anisotropic effect which is not reproduced in calculation at optimal doping. Comparison with isostructural La$_{2-x}$Sr$_x$CuO$_4$ suggests that these calculations reproduce well the overdoped regime, however they overestimate the doping effect on the Cu-O bond-stretching mode at optimal doping.

cond-mat.str-el↗

arXiv2019

Giant Negative Thermal Expansion Induced by the Synergistic Effects of Ferroelectrostriction and Spin-Crossover in PbTiO3-Based Perovskites

The discovery of unusual negative thermal expansion (NTE) provides the opportunity to control the common but much desired property of thermal expansion, which is valuable not only in scientific interests but also in practical applications. However, most of the available NTE materials are limited to a narrow temperature range, and the NTE effect is generally weakened by means of various modifications. Here, we report an enhanced NTE effect that occurs over a wide temperature range (αV = -5.24 * 10-5 °C^-1, 25-575 ° C), and this NTE effect is accompanied by an abnormal enhanced tetragonality, a large spontaneous polarization, and a G-type antiferromagnetic ordering in the present perovskite-type ferroelectric of (1-x)PbTiO3-xBiCoO3. Specifically, for the composition of 0.5PbTiO3-0.5BiCoO3, an extensive volumetric contraction of ~4.8 % has been observed near the Curie temperature of 700 °C, which represents the highest level in PbTiO3-based ferroelectrics. According to our experimental and theoretical results, the giant NTE originates from a synergistic effect of the ferroelectrostriction and spin-crossover of cobalt on the crystal lattice. The actual NTE mechanism is contrasted with previous functional NTE materials, in which the NTE is simply coupled with one ordering such as electronic, magnetic, or ferroelectric ordering. The present study sheds light on the understanding of NTE mechanisms and it attests that NTE could be simultaneouslycoupled with different orderings, which will pave a new way toward the design of large NTE materials.

cond-mat.mtrl-sci↗