arXiv subjects

Yumiko Katayama

Publications and source records attributed to Yumiko Katayama.

7 recordsLinked to original sources

arXiv2026

High mobility holes at germanane/Ge(111) allotropic cross-dimensional heterointerface

Germanane (GeH) is essentially a hydrogen-terminated Ge analog of graphene with a direct gap (~1.6 eV). Record hole mobility mu_h~67,000 cm2/Vs is found at 15 K for a single allotropic cross-dimensional(D) heterointerface. This is enabled by making topotactically-transformed 2D GeH layers meet the 3D bulk Ge(111). Temperature dependence of mu_h implies metallic conduction without ionized impurity scattering between 20 K and 250 K. Sheet hole density for a Fermi sphere n_S=2.8x10^11 /cm2 agrees well with 3.0x10^11 /cm2 of Hall measurements. A 6,500% magnetoresistance at 7 T accompanies Shubnikov-de Haas oscillations visible even at 15 K. These imply single-band conduction of holes with small effective mass in the in-plane directions, invoking a 2D hole gas (2DHG) picture that allotropic cross-D heterointerface between 2D GeH and 3D Ge harbors 2D-confined high-mobility holes. Even without elaborate heteroepitaxy and modulation doping, allotropic cross-D heterostructures pave the way toward facile 2DHG creation.

cond-mat.mtrl-sci↗

arXiv2024

Large Rashba spin-orbit coupling in metallic SrTaO$_3$ thin films

Epitaxial thin films of SrTaO$_3$ with thickness ($t$) smaller than 74 nm were successfully fabricated on an insulator (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ substrate. Films with $t$ above 8.6 nm showed metallic conduction. Both conductivity and a mobility showed a decrease with increasing $t$ above 42 nm, suggesting the instability of thick SrTaO$_3$ films. This instability was also supported by TEM image and XRD intensity. For the metallic films with $t$ below 25 nm, energy band splitting due to spin-orbit coupling ($Δ$$_{so}$) and Rashba parameter ($α$$_R$) were deduced from an analysis of a magnetoresistance using two-dimensional weak antilocalization theory. The values of $Δ$$_{so}$ ranged from 26 to 120 meV, which were the largest among other metallic oxide films, such as SrNbO$_3$, SrIrO$_3$, and La$_{2/3}$Sr$_{1/3}$MnO$_3$ thin films, indicating that spin-orbit coupling in SrTaO$_3$ was the largest among the metallic perovskite oxides reported so far. The values of $α$$_R$ for our SrTaO$_3$ films ranged from $8.8 \times$10$^{-13}$ to $1.7 \times$10$^{-12}$ eV m, which were much larger than those reported for other metallic oxide thin films.

cond-mat.str-el↗

arXiv2022

Tuning the hysteresis loop for the anomalous Hall effect in Pt ultrathin films on $\rm{CoFe_2O_4}$ by electrolyte gating

Pt ultrathin films on ferromagnetic insulators have been widely studied for spintronics applications, and magnetic moments of interface Pt atoms were considered to be ferromagnetically ordered due to a magnetic proximity effect (MPE). An anomalous Hall effect (AHE) is usually used to examine an out-of-plane magnetic moments of the Pt layer. To tune ferromagnetic properties of an Pt ultrathin film, we fabricated electric double layer transistors on Pt thin films with thicknesses of 5.9 nm and 7.0 nm on a $\rm{CoFe_2O_4}$ (CFO) ferrimagnetic insulator. For the Pt (7.0 nm)/CFO sample, a hysteresis loop was observed in the anomalous Hall resistivity without the gate bias, and the coercive field was tuned by applying the gate bias. For the Pt (5.9 nm)/CFO sample, a hysteresis loop was not observed without a gate bias, but was opened by applying a gate bias ($V\rm{_G} =$ $\pm$3 V). This indicated that the long-range ferromagnetic ordering of magnetic moments in the Pt film was switched on and off by the electric field effect. The hysteresis loop was observed up to 19.5 K for a $V\rm{_G}$ of +3 V, while the AHE was observed up to approximately room temperature.

cond-mat.mtrl-sci↗

arXiv2022

Large Rashba parameter for 4d strongly correlated perovskite oxide SrNbO3 ultrathin films

To elucidate the spin relaxation mechanism of SrNbO3 (SNO) ultrathin films, the transport properties of a series of SNO films with various thicknesses were measured on both sides of the metal insulator transition. The spin orbit scattering time was deduced from the analysis of the magnetoresistance with weak antilocalization theory, and it was found that the spin orbit scattering time was inversely proportional to the momentum scattering time. This result was explained in terms of the D`Yakonov Perel` mechanism, indicative of the dominant Rashba effect. The values of the Rashba parameter were largest in the values reported for other ultrathin films of metallic oxides.

cond-mat.str-el↗

arXiv2021

Transport properties around the metal-insulator transition for SrVO3 ultrathin films fabricated by electrochemical etching

By using electrochemical etching, we fabricated conductive ultrathin SrVO3 (SVO) films that exhibited metallic behavior down to 3 monolayers (ML). From an observed systematic change in transport properties with decreasing film thickness, it was found that the disorder in the films remained nearly unchanged during etching, and only the thickness was reduced. This is in contrast to the insulating behavior found for as-deposited SVO ultrathin films. For the etched films, the electron mobility at 200 K decreased with decreasing film thickness below 10 ML, originating from an increased scattering rate and electron effective mass near the metal-insulator transition. A slight upturn in the resistivity and a positive magnetoresistance at low temperatures were typically observed for the etched films down to 3 ML, which was explained by weak anti-localization of electrons in a weakly disordered metal.

cond-mat.str-el↗

arXiv2020

Enhancement of superconducting transition temperature in electrochemically etched FeSe/LaAlO$_3$ films

In this study, we investigated the gate voltage dependence of $T_{\mathrm c}$ in electrochemically etched FeSe films with an electric-double layer transistor structure. The $T_{\mathrm c}^{\mathrm {zero}}$ value of the etched FeSe films with a lower gate voltage ($V_{\mathrm g}$ = 2.5 and 3.3 V) reaches 46 K, which is the highest value among almost all reported values from the resistivity measurements except for the data by Ge et al. This enhanced $T_{\mathrm c}$ remains unchanged even after the discharge process, unlike the results for electrostatic doping without an etching process. Our results suggest that the origin of the increase in $T_{\mathrm c}$ is not electrostatic doping but rather the electrochemical reaction at the surface of an etched films.

cond-mat.supr-con↗

arXiv2018

Superconductivity at 38 K in an electrochemical interface between ionic liquid and Fe(Se0.8Te0.2) on various substrates

Superconducting FeSe0.8Te0.2 thin films on SrTiO3, LaAlO3 and CaF2 substrates were electrochemically etched in an ionic liquid DEME-TFSI electrolyte with a gate bias of 5 V. Superconductivity at 38 K was commonly observed on all substrates after etching the films with a thickness above 30 nm, in spite of different Tc of 8 K, 12 K and 19 K before the etching on SrTiO3, LaAlO3 and CaF2 substrates, respectively. Tc returned to the original value by removing the gate bias. The Tc enhancement on the thick film indicates no relationship between the Tc enhancement and any interface effects between the film and the substrate. The sheet resistance and the Hall coefficient of the surface conducting layer were estimated from the gate bias dependence of the transport properties. The sheet resistance of the surface conducting layer of the films on LaAlO3 and CaF2 showed an identical temperature dependence, and the Hall coefficient is almost temperature independent and -0.05 to -0.2 m2/C, corresponding to 4-17 electrons per one FeSe0.8Te0.2 unit cell area in two dimension. These common transport properties on various substrates suggest that the superconductivity at 38 K appeared in the surface conducting layer produced by electrochemical reaction between the surface of the FeSe0.8Te0.2 thin film and the ionic liquid electrolyte.

cond-mat.supr-con↗