arXiv subjects

Takeharu Kato

Publications and source records attributed to Takeharu Kato.

10 recordsLinked to original sources

arXiv2026

Higher critical currents yet faster vortex creep in EuBa$_2$Cu$_3$O$_y$ films containing coherent artificial pinning centers

The electromagnetic properties of type-II superconductors depend on vortices -- magnetic flux lines whose motion introduces dissipation that can be mitigated by pinning from material defects. The material disorder landscape is tuned by the choice of materials growth technique and incorporation of impurities that serve as vortex pinning centers. For example, metal organic deposition (MOD) and pulsed laser deposition (PLD) produce high-quality superconducting films with uncorrelated versus correlated disorder, respectively. Here, we study vortex dynamics in PLD-grown EuBa$_2$Cu$_3$O$_y$ films containing varying concentrations of BaHfO$_3$ inclusions and compare our results with those of MOD-grown (Y,Gd)Ba$_2$Cu$_3$O$_y$ films. Despite both systems exhibiting behavior consistent with strong pinning theory, which predicts the critical current density $J_c$ based on vortex trapping by randomly distributed spherical inclusions, we find striking differences in the vortex dynamics owing to the correlated versus uncorrelated disorder. Specifically, we find that the EuBa$_2$Cu$_3$O$_y$ films grown without inclusions exhibit surprisingly slow vortex creep, comparable to the slowest creep rates achieved in (Y,Gd)Ba$_2$Cu$_3$O$_y$ films containing high concentrations of BaHfO$_3$. Whereas adding inclusions to (Y,Gd)Ba$_2$Cu$_3$O$_y$ is effective in slowing creep, BaHfO$_3$ increases creep in EuBa$_2$Cu$_3$O$_y$ even while concomitantly improving $J_c$. Lastly, we find evidence of variable range hopping and that $J_c$ is maximized at the BaHfO$_3$ concentration that hosts creep of large vortex bundles or a Bose glass state.

cond-mat.supr-con↗

arXiv2022

Thermodynamic approach for enhancing superconducting critical current performance

The addition of artificial pinning centers has led to an impressive increase in critical current density ($J_{\rm c}$) in a superconductor, enabling record-breaking all-superconducting magnets and other applications. $J_{\rm c}$ has reached $\sim 0.2$-$0.3$ $J_{\rm d}$, where $J_{\rm d}$ is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying $λ$ and/or $ξ$, the penetration depth and coherence length, respectively, we can increase $J_{\rm d}$. For (Y$_{0.77}$Gd$_{0.23}$)Ba$_2$Cu$_3$O$_y$ ((Y,Gd)123) we achieve this by controlling the carrier density, which is related to $λ$ and $ξ$. We also tune $λ$ and $ξ$ by controlling the chemical pressure in the Fe-based superconductors, BaFe$_2$(As$_{1-x}$P$_x$)$_2$ films. The variation of $λ$ and $ξ$ leads to an intrinsic improvement of $J_{\rm c}$, via $J_{\rm d}$, obtaining extremely high values of $J_{\rm c}$ of $130$ MA/cm$^2$ and $8.0$ MA/cm$^2$ at $4.2$ K, consistent with an enhancement of $J_{\rm d}$ of a factor of $2$ for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe$_2$(As$_{1-x}$P$_x$)$_2$ films, showing that this new material design is useful to achieving high critical current densities for a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached $\sim 3.17$ TN/m$^3$ at $4.2$ K and 18 T (${\bf H}\parallel c$), the highest values ever reported in any superconductor.

cond-mat.supr-con↗

arXiv2021

Designing high-performance superconductors with nanoparticle inclusions: comparisons to strong pinning theory

One of the most promising routes for achieving unprecedentedly high critical currents in superconductors is to incorporate dispersed, non-superconducting nanoparticles to control the dissipative motion of vortices. However, these inclusions reduce the overall superconducting volume and can strain the interlaying superconducting matrix, which can detrimentally reduce $T_c$. Consequently, an optimal balance must be achieved between the nanoparticle density $n_p$ and size $d$. Determining this balance requires garnering a better understanding of vortex-nanoparticle interactions, described by strong pinning theory. Here, we map the dependence of the critical current on nanoparticle size and density in (Y$_{0.77}$,Gd$_{0.23}$)Ba$_2$Cu$_3$O$_{7-δ}$ films in magnetic fields up to 35 T, and compare the trends to recent results from time-dependent Ginzburg-Landau simulations. We identify consistencies between the field-dependent critical current $J_c(B)$ and expectations from strong pinning theory. Specifically, we find that that $J_c \propto B^{-α}$, where $α$ decreases from $0.66$ to $0.2$ with increasing density of nanoparticles and increases roughly linearly with nanoparticle size $d/ξ$ (normalized to the coherence length). At high fields, the critical current decays faster ($\sim B^{-1}$), suggestive that each nanoparticle has captured a vortex. When nanoparticles capture more than one vortex, a small, high-field peak is expected in $J_c(B)$. Due to a spread in defect sizes, this novel peak effect remains unresolved here. Lastly, we reveal that the dependence of the vortex creep rate $S$ on nanoparticle size and density roughly mirrors that of $α$, and compare our results to low-$T$ nonlinearities in $S(T)$ that are predicted by strong pinning theory.

cond-mat.supr-con↗

arXiv2011

Unusually large enhancement of thermopower in an electric field induced two-dimensional electron gas

Two-dimensionally confined electrons showing unusually large thermopower (S) have attracted attention as a potential approach for developing high performance thermoelectric materials. However, enhanced S has never been observed in electric field induced two-dimensional electron gas (2DEG). Here we demonstrate electric field modulation of S for a field effect transistor (FET) fabricated on a SrTiO3 crystal using a water-infiltrated nanoporous glass as the gate insulator. An electric field application confined carrier electrons up to ~2E15 /cm^2 in an extremely thin (~2 nm) 2DEG. Unusually large enhancement of |S| was observed when the sheet carrier concentration exceeded 2.5E14 /cm^2, and it modulated from ~600 (~2E15 /cm^2) to ~950 μV/K (~8E14 /cm^2), which were approximately five times larger than those of the bulk, clearly demonstrating that an electric field induced 2DEG provides unusually large enhancement of |S|.

cond-mat.mtrl-sci↗

arXiv2011

Electric field thermopower modulation analysis of an interfacial conducting layer formed between Y2O3 and rutile TiO2

Electric field modulation analysis of thermopower (S) - carrier concentration (n) relation of a bilayer laminate structure composed of a 1.5-nm thick conducting layer, probably TinO2n-1 (n=2, 3,...) Magnéli phase, and rutile TiO2 was performed. The results clearly showed that both the rutile TiO2 and the thin interfacial layer contribute to carrier transport: the rutile TiO2 bulk region (mobility mu~0.03 cm2V-1s-1) and the 1.5-nm thick interfacial layer (mu~0.3 cm2V-1s-1). The effective thickness of the interfacial layer, which was obtained from the S-n relation, was below ~ 3 nm, which agrees well with that of the TEM observation (~1.5 nm), clearly showing that electric field modulation measurement of S-n relation can effectively clarify the carrier transport properties of a bilayer laminate structure.

cond-mat.mtrl-sci↗

arXiv2010

Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal

Here we demonstrate that water-infiltrated nanoporous glass electrically switches an oxide semiconductor from an insulator to metal. We fabricated the field effect transistor structure on an oxide semiconductor, SrTiO3, using 100%-water-infiltrated nanoporous glass - amorphous 12CaO*7Al2O3 - as the gate insulator. For positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO3 surface at room temperature, leading to the formation of a thin (~3 nm) metal layer with an extremely high electron concentration of 10^15-10^16 cm^-2, which exhibits exotic thermoelectric behaviour.

cond-mat.mtrl-sci↗

arXiv2010

Electric field modulation of thermopower for transparent amorphous oxide thin film transistors

To clarify the electronic density of states (DOS) around the conduction band bottom for state of the art transparent amorphous oxide semiconductors (TAOSs), InGaZnO4 and In2MgO4, we fabricated TAOS-based transparent thin film transistors (TTFTs) and measured their gate voltage dependence of thermopower (S). TAOS-based TTFTs exhibit an unusual S behavior. The |S|-value abruptly increases, but then gradually decreases as Vg increases, clearly suggesting the anti-parabolic shaped DOS is hybridized with the original parabolic shaped DOS around the conduction band bottom.

cond-mat.mtrl-sci↗

arXiv2010

Experimental characterization of the electronic structure of anatase TiO2: Thermopower modulation

Thermopower (S) for anatase TiO2 epitaxial films (n3D: 1E17-1E21 /cm3) and the gate voltage (Vg) dependence of S for thin film transistors (TFTs) based on TiO2 films were investigated to clarify the electronic density of states (DOS) around the conduction band bottom. The slope of the |S|-log n3D plots was -20 μV/K, which is an order magnitude smaller than that of semiconductors (-198 μV/K), and the |S| values for the TFTs increased with Vg in the low Vg region, suggesting that the extra tail states are hybridized with the original conduction band bottom.

cond-mat.mtrl-sci↗

arXiv2009

A Single-Crystalline, Epitaxial SrTiO3 Thin-Film Transistor

We report herein fabrication and characterization of a thin-film transistor (TFT) using single-crystalline, epitaxial SrTiO3 film, which was grown by a pulsed laser deposition technique followed by the thermal annealing treatment in an oxygen atmosphere. Although TFTs on the polycrystalline epitaxial SrTiO3 films (as-deposited) exhibited poor transistor characteristics, the annealed single-crystalline SrTiO3 TFT exhibits transistor characteristics comparable with those of bulk single-crystal SrTiO3 FET: an on/off current ratio >10^5, sub-threshold swing ~2.1 V/decade, and field-effect mobility ~0.8 cm^2/Vs. This demonstrates the effectiveness of the appropriate thermal annealing treatment of epitaxial SrTiO3 films.

cond-mat.mtrl-sci↗

arXiv2009

Electric-Field Modulation of Thermopower for the KTaO3 Field Effect Transistors

We show herein fabrication and field-modulated thermopower for KTaO3 single-crystal based field-effect transistors (FETs). The KTaO3 FET exhibits field effect mobility of ~8 cm2/Vs, which is ~4 times larger than that of SrTiO3 FETs. The thermopower of the KTaO3 FET decreased from 600 to 220 microV/K by the application of gate electric field up to 1.5 MV/cm, ~400 microV/K below that of an SrTiO3 FET, clearly reflecting the smaller carrier effective mass of KTaO3.

cond-mat.str-el↗