arXiv subjects

Yasunori Mawatari

Publications and source records attributed to Yasunori Mawatari.

At least 19 recordsLinked to original sources

arXiv2024

Doping Dependence of Upper Critical Field of High-Tc Cuprate Bi2+xSr2-xCaCu2O8+d Estimated from Irreversibility Field at Zero Temperature

We investigated the temperature (T) dependence of the irreversibility field Hirr(T) in high-critical-temperature cuprate Bi2+xSr2-xCa1-yYyCu2O8+d (Bi-2212) single crystals over a wide range of hole doping level (p). Hirr(T) was evaluated by measuring the magnetization hysteresis loop. The value of Hirr(T) extrapolated to T = 0 K [Hirr(0)], is either equal to or sets the lower boundary for the upper critical field at T = 0 K [Hc2(0)]. Tc shows a parabolic p-dependence (peak at p = 0.16), whereas Hirr(0) increases monotonically with p by approximately one order of magnitude, from 19 T for the most underdoped sample (p = 0.065, Tc = 24 K) to 209 T for the most overdoped sample (p = 0.200, Tc = 75 K). The present results qualitatively agree with Hc2(0) values evaluated from the specific heat measurements. The observed p-dependence of Hirr(0) in Bi-2212 is distinct from those in YBa2Cu3O7-d and HgBa2CuO6+d, in which a pronounced dip structure appears in the underdoped region. Considering that the dip structures observed in these two systems are likely associated with the formation of competing orders (most likely field-induced charge orders), the present results indicate that the influence of the competing order in Bi-2212 is less prominent than that in the other two systems.

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arXiv2024

Evaluation of Doppler shifts in $d$-wave superconductor tunneling junctions

The energy levels of quasiparticles in superconductors experience Doppler shifts due to the influence of a finite current flow. The influence of the Doppler shift is particularly pronounced in unconventional superconductors with gap nodes. In the case of a normal/insulator/superconductor junction of $d$-wave superconductors, a zero-bias conductance peak exhibits a peak shift in an applied magnetic field. Although the shifts have been detected in experiments, the shifts showed different field responses depending on each experiment. In this paper, we evaluate the magnitude of the shift based on realistic junction shapes to elucidate the origin of the variability in experimental Doppler shifts. We find that the peak shift depends on the junction shape; the splitting is significantly suppressed when the junction is formed at a protruding position. Also, the shift is suppressed by the penetration of magnetic flux quanta, resulting in a nonlinear response to the magnetic field. The present results explain the origin of the variability in experimental Doppler shifts.

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arXiv2024

Ginzburg-Landau simulations of three-terminal operation of a superconducting nanowire cryotron

Superconducting nanowire cryotrons (nTrons) are expected to be used as interfaces for super-high-performance hybrid devices in which superconductor and semiconductor circuits are combined. However, nTrons are still under development, and diverse analyses of these devices are needed. Accordingly, we have developed a numerical technique to simulate the three-terminal operation of an nTron by using the finite element method to solve the time-dependent Ginzburg-Landau (TDGL) equation and the heat-diffusion equation. Simulations using this technique offer understanding of the dynamics of the order parameter, the thermal behavior, and the characteristics of three-terminal operation, and the TDGL model reproduces qualitatively the results of nTron experiments. In addition, we investigated how some geometric and physical parameters (the design elements) affect the operation characteristics. The TDGL model has far fewer free parameters compared with the lumped-element electrothermal model commonly used for simulating superconducting devices. Furthermore, the TDGL model provides time-dependent visual information about the superconducting state and the normal state, thereby offering insights into the relationship between nTron geometry and three-terminal operation. These simulation results offer a route to nTron optimization and the development of nTron applications.

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arXiv2024

Magnetic field dependence of critical currents of cross-type Josephson junctions with inhomogeneous critical current density under oblique magnetic fields

Studies of the magnetic interference of sandwich-type Josephson junctions in which perpendicular or oblique magnetic fields are applied to the junction plane have received less attention than those where the applied magnetic fields are parallel. Recently, it has been theoretically demonstrated that a variety of magnetic interferences of the critical currents appear when oblique magnetic fields are applied to a cross-type junction with homogeneous critical current density. We theoretically investigated the effect of the inhomogeneous critical current density in the junction plane, and found that more complicated magnetic interferences appeared. We considered the distribution of the current density flowing through the junction plane to explore the cause of these complex magnetic interferences.

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arXiv2024

Synthesis of $c$-axis textured CaKFe$_4$As$_4$ superconducting bulk via spark plasma texturing technique

Grain alignment is a key factor that determines the performance of a superconducting bulk. In this study, the spark plasma texturing (SPT) technique was used to fabricate a CaKFe$_4$As$_4$ superconducting bulk. X-ray diffraction and electron backscatter diffraction revealed that the $c$-axes of the CaKFe$_4$As$_4$ grains in the SPT bulk are aligned, demonstrating that the SPT technique is effective in achieving $c$-axis texture. In addition, chemical composition analysis showed that oxide impurities, which affect the grain boundary characteristics that determine the inter-grain critical current density ($J_c$), are randomly distributed in the SPT bulk. Magnetization measurements showed high $J_c$ values of the SPT bulk, reaching 127 kA cm$^{-2}$ and 26 kA cm$^{-2}$ at 4.2 K under a self-field and magnetic field of 5 T, respectively. These results suggest that the SPT technique is a promising approach for obtaining a high-performance superconducting bulk material for high-field applications.

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arXiv2024

Synthesis of CaKFe$_4$As$_4$ bulk samples with high critical current density using a spark plasma sintering technique

A high density CaKFe$_4$As$_4$ bulk sample was successfully synthesized using a spark plasma sintering (SPS) technique. The density of the synthesized sample was 5.02 g cm$^{-3}$, corresponding to 96.2% of the theoretical density of CaKFe$_4$As$_4$. Moreover, a reasonably high Vickers hardness of 1 GPa was measured. The electrical resistivity of the SPS bulk sample was as low as approximately 600 $μΩ$ cm at 300 K, which is smaller than that of the ordinary sintered polycrystalline sample by nearly one order of magnitude, and exhibited a sharp superconducting transition, with the transition width $Δ\textit{T}_c$ less than 2 K, indicating an improved grain connectivity. The critical current density of the SPS bulk sample, as calculated from the magnetization hysteresis loops (magnetic $\textit{J}_c$), reached 18 kA cm$^{-2}$ at 4.2 K under 5 T, which is the highest among the iron-based superconductor polycrystalline samples reported thus far.

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arXiv2023

Field resilient superconductivity in atomic layer crystalline materials

A recent study [S. Yoshizawa {\it et al}., Nature Communications {\bf 12}, 1462 (2021)] reported the occurrence of field-resilient superconductivity, that is, enhancement of the in-plane critical magnetic field $H^{||}_{\rm c2}$ beyond the paramagnetic limiting field, in atomic-layer crystalline ($\sqrt{7}\times\sqrt{3}$)-In on a Si(111) substrate. The present article elucidates the origin of the observed field-resilient noncentrosymmetric superconductivity in this highly crystalline two-dimensional material. We develop the quasiclassical theory of superconductivity by incorporating the Fermi surface anisotropy together with an anisotropic spin splitting and texture specific to atomic-layer crystalline systems. In Si(111)-($\sqrt{7}\times\sqrt{3}$)-In, a typical material with a large antisymmetric spin-orbit coupling (ASOC), we show an example where the combination of the ASOC and disorder effect suppresses the paramagnetic depairing and can lead to an enhancement of $H^{||}_{\rm c2}$ compared to an isotropic system only when a magnetic field is applied in a particular direction due to an anisotropic spin texture. We also study the parity-mixing effect to demonstrate that the enhancement of $H^{||}_{\rm c2}$ is limited in the moderately clean regime because of the fragile $s$+$p$-wave pairing against nonmagnetic scattering in the case of the dominant odd-parity component of a pair wavefunction. Furthermore, from analysis of the transition line, we identify the field-resilience factor taking account of the scattering and suppression of paramagnetic effects and discuss the origin of the field-resilient superconductivity. Through fitting of the $H^{||}_{\rm c2}$ data, the normal-state electron scattering is discussed with a prime focus on the role of atomic steps on a Si(111) surface.

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arXiv2023

Kinetic inductance of superconducting nanostrips with turns

Kinetic inductances of superconducting nanostrips with a meander pattern are theoretically investigated based on the London model, and the effect of the current crowding at the turns of the nanostrips is considered. The complex current approach is developed for analytical investigation of the kinetic inductance of nanostrips with turns for thin $d<λ$ and narrow $w\ll λ^2/d$ superconducting strips, where $d$ is the strip thickness, $w$ is the strip width, and $λ$ is the London penetration depth. We show that the current distribution in superconducting nanostrips of $wd\llλ^2$ is identical to that in normal conducting nanostrips of $wd\llδ^2/2$, where $δ$ is the skin depth, and the dependence of the kinetic inductance on the nanostrip geometry is identical to that of the normal resistance. Effects of the edge defects of superconducting strips upon the kinetic inductance are also considered.

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arXiv2022

Anomalous magnetic interference of cross-type Josephson junctions exposed to oblique magnetic fields

Gauge-invariant phase difference and critical currents of cross-type Josephson junctions with thin and narrow superconducting strips exposed to three-dimensional magnetic fields are theoretically investigated. When a sandwich-type Josephson junction in the xy plane is exposed to parallel magnetic fields Hx and Hy, the phase difference linearly depends on the spatial coordinates, x and y, and the critical currents exhibit the standard Fraunhofer-type magnetic interference. The perpendicular field Hz, on the other hand, nonlinearly modulates the distribution of the phase difference and the critical currents as the functions of the oblique field exhibit anomalous magnetic interference. We obtain simple analytical expressions for critical currents of small cross-type junctions by neglecting the effects of self-field and trapped vortices. The resulting dc critical currents show anomalous and diverse interference patterns depending on the parallel and perpendicular magnetic fields

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arXiv2021

Round-Core-Radius-Dependent Electromagnetic Coupling of Multifilament Helical Superconducting Tapes in a Swept Magnetic Field

With the excitation and demagnetization of a magnet for magnetic resonance imaging in mind, we theoretically and numerically investigated electromagnetic coupling--especially its dependence on the round core radius $R$--of multifilament helically wound superconducting tapes under steady-state conditions in a constantly ramped magnetic field. We found that even in a rapidly ramped magnetic field, the electromagnetic coupling can be suppressed by reducing $R$ to close to the tape width. We also clarified that the coupling sweep rate at which the electromagnetic coupling starts scales as $R^{-2}$, showing that the dependence on $R$ reflects the penetration of magnetic flux from the edges of the tape. Even when the round core is as narrow as the tape width, the behavior is considered to be similar to the electromagnetic response of a flat tape rather than that of a tubular wire.

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arXiv2019

Efficient Numerical Modeling of the Magnetization Loss on a Helically Wound Superconducting Tape in a Ramped Magnetic Field

We investigate theoretically the dependence of magnetization loss of a helically wound superconducting tape on the round core radius $R$ and the helical conductor pitch in a ramped magnetic field. Using the thin-sheet approximation, we identify the two-dimensional equation that describes Faraday's law of induction on a helical tape surface in the steady state. Based on the obtained basic equation, we simulate numerically the current streamlines and the power loss $P$ per unit tape length on a helical tape. For $R \gtrsim w_0$ (where $w_0$ is the tape width), the simulated value of $P$ saturates close to the loss power $\sim(2/π)P_{\rm flat}$ (where $P_{\rm flat}$ is the loss power of a flat tape) for a loosely twisted tape. This is verified quantitatively by evaluating power loss analytically in the thin-filament limit of $w_0/R\rightarrow 0$. For $R \lesssim w_0$, upon thinning the round core, the helically wound tape behaves more like a cylindrical superconductor as verified by the formula in the cylinder limit of $w_0/R\rightarrow 2π$, and $P$ decreases further from the value for a loosely twisted tape, reaching $\sim (2/π)^2 P_{\rm flat}$.

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arXiv2019

Electromagnetic coupling of twisted multi-filament superconducting tapes in a ramped magnetic field

We investigate theoretically the magnetization loss and electromagnetic coupling of twisted multi-filament superconducting (SC) tapes in a ramped magnetic field. Based on the two-dimensional reduced Faraday--Maxwell equation for a tape surface obtained with a thin-sheet approximation, we simulate numerically the power loss $P$ per unit length on twisted multi-filament tapes in the steady state. The current density profile clearly shows electromagnetic coupling between the SC filaments upon increasing the field sweep rate $β$. Although the $β$ dependence of $P/β$ for twisted multi-filament SC tapes closely resembles that for filaments in an alternating field, we show that the mechanism for electromagnetic coupling in a ramped field differs from that in an alternating field. We also identify the conditions under which electromagnetic coupling is suppressed for the typical sweep rate of a magnet used for magnetic resonance imaging.

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arXiv2018

Theoretical evaluation of the screening-current-induced magnetic field in superconducting coils with tape wires

We theoretically investigate the physical mechanism of the screening-current-induced field (SCIF) in solenoid coils wound with superconducting tape wires. We derive the direct relationship between the SCIF and the magnetization of tape wires, and a scaling law for the SCIF and the coil dimensions is demonstrated. A simple analytical expression of the SCIF is obtained as functions of current load factor, tape wire width, and the coil dimensions. We verify that the published data for the precise numerical computation of SCIF are roughly fitted by our theoretical results for flat coils where the height is smaller than the outer diameter.

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arXiv2018

Analysis of Magnetization Loss on a Twisted Superconducting Strip in a Constantly Ramped Magnetic Field

Magnetization loss on a twisted superconducting (SC) tape in a ramped magnetic field is theoretically investigated through the use of a power law for the electric field--current density characteristics and a sheet current approximation. First, the Maxwell equation in a helicoidal coordinate system is derived to model a twisted SC tape, taking account of the response to the perpendicular field component in the steady state. We show that a loosely twisted tape can be viewed as the sum of a portion of tilted flat tapes of infinite length by examining the perpendicular field distribution on a twisted tape. The analytic formulae for both magnetization and loss power in the tilted flat tape approximation are verified based on the analytic solution of the reduced Maxwell equation in the loosely twisted tape limit of $L_{\rm p}\rightarrow \infty$ with the twist pitch length $L_{\rm p}$. These analytic formulae show that both magnetization and loss power decrease by a factor of $B(1+1/2n,1/2)/π$ (where $B$ is the beta function) for an arbitrary power of SC nonlinear resistivity $n$, compared with those in a flat tape of infinite length. Finally, the effect of the field-angle dependence of the critical current density $J_{\rm c}$ on the loss power is investigated, and we demonstrate that it is possible to obtain an approximate estimate of the loss power value via $J_{\rm c}$ in an applied magnetic field perpendicular to the tape surface (i.e., parallel to the $c$ axis).

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arXiv2018

Detection of the phase shift of an alternating-current magnetic field by quantum sensing with multiple-pulse decoupling sequences

Magnetometry utilizing a spin qubit in a solid state possesses high sensitivity. In particular, a magnetic sensor with a high spatial resolution can be achieved with the electron-spin states of a nitrogen vacancy (NV) center in diamond. In this study, we demonstrated that NV quantum sensing based on multiple-pulse decoupling sequences can sensitively measure not only the amplitude but also the phase shift of an alternating-current (AC) magnetic field. In the AC magnetometry based on decoupling sequences, the maximum phase accumulation of the NV spin due to an AC field can be generally obtained when the $π$-pulse period in the sequences matches the half time period of the field and the relative phase difference between the sequences and the field is zero. By contrast, the NV quantum sensor acquires no phase accumulation if the relative phase difference is $π/2$. Thus, this phase-accumulation condition does not have any advantage for the magnetometry. However, we revealed that the non-phase-accumulation condition is available for detecting a very small phase shift of an AC field from its initial phase. This finding is expected to provide a guide for realizing sensitive measurement of a complex AC magnetic field in micrometer and nanometer scales.

quant-ph↗

arXiv2018

Influence of dynamical decoupling sequences with finite-width pulses on quantum sensing for AC magnetometry

Dynamical decoupling sequences with multiple pulses can be considered to exhibit filter functions for the time evolution of a qubit superposition state. They contribute to the improvement of coherence time and qubit-phase accumulation due to a time-varying field and can thus achieve high-frequency-resolution spectroscopy. Such behaviors find useful application in highly sensitive detection based on qubits for various external fields such as a magnetic field. Hence, decoupling sequences are indispensable tools for quantum sensing. In this study, we experimentally and theoretically investigated the effects of finite-width pulses in the sequences on AC magnetometry utilizing nitrogen-vacancy centers in an isotopically-controlled diamond. We revealed that the finite pulse widths cause a deviation of the optimum time to acquire the largest phase accumulation due to the sensing field from that expected by filter functions neglecting the pulse widths, even if the widths are considerably shorter than the time period of the sensing field. Moreover, we experimentally demonstrated that the deviation can be corrected by an appropriate time-frequency conversion. Our results provide a guideline for the detection of an AC field with an accurate frequency and linewidth in quantum sensing with multiple-pulse sequences.

quant-ph↗

arXiv2015

Inversion symmetry of Josephson current as test of chiral domain wall motion in Sr$_{2}$RuO$_{4}$

Clarifying the chiral domains structure of superconducting Sr$_{2}$RuO$_{4}$ has been a long-standing issue in identifying its peculiar topological superconducting state. We evaluated the critical current $I_{c}$ versus the magnetic field $H$ of Nb/Sr$_{2}$RuO$_{4}$ Josephson junctions, changing the junction dimension in expectation of that the number of domains in the junction is controlled. $I_{c}(H)$ exhibits a recovery from inversion symmetry breaking to invariance when the dimension is reduced to several microns. This inversion invariant behavior indicates the disappearance of domain walls; thus, the size of a single domain is estimated at approximately several microns.

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arXiv2013

Two-dimensional arrays of superconducting and soft magnetic strips as dc magnetic metamaterials

We have theoretically investigated the magnetic response of two-dimensional (2D) arrays of superconducting and soft magnetic strips, which are regarded as models of dc magnetic metamaterials. The anisotropy of the macroscopic permeabilities depends on whether the applied magnetic field is parallel to the wide surface of the strips ($μ_{\parallel}$) or perpendicular ($μ_{\perp}$). For the 2D arrays of superconducting strips, $0<μ_{\perp}/μ_0\ll μ_{\parallel}/μ_0\simeq 1$, whereas for the 2D arrays of soft magnetic strips, $μ_{\parallel}/μ_0\ggμ_{\perp}/μ_0\simeq 1$, where $μ_0$ is the vacuum permeability. We also demonstrate that strong anisotropy of the macroscopic permeability can be obtained for hybrid arrays of superconducting and soft magnetic strips, where $μ_{\parallel}/μ_0\gg 1\gg μ_{\perp}/μ_0>0$.

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