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Takayuki Kubo

Publications and source records attributed to Takayuki Kubo.

At least 37 records · Page 2Linked to original sources
arXiv2019

Field-dependent nonlinear surface resistance and its optimization by surface nano-structuring in superconductors

We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong radio-frequency (RF) field, taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. The Usadel equations were solved to obtain the quasiparticle density of states (DOS) and the low-frequency surface resistance $R_s$ as functions of the RF field amplitude $H_0$. It is shown that the interplay of the broadening of the DOS peaks and a decrease of a quasiparticle gap caused by the RF currents produces a minimum in $R_s(H_0)$ and an extended rise of the quality factor $Q(H_0)$ with the RF field. Paramagnetic impurities shift the minimum in $R_s(H_0)$ to lower fields and can reduce $R_s(H_0)$ in a wide range of $H_0$. Subgap states in the DOS can give rise to a residual surface resistance while reducing $R_s$ at higher temperatures. A proximity-coupled normal layer at the surface can shift the minimum in $R_s(H_0)$ to either low and high fields and can reduce $R_s$ below that of an ideal surface. The theory shows that the behavior of $R_s(H_0)$ changes as the temperature and the RF frequency are increased, and the field dependence of $Q(H_0)$ can be very sensitive to the materials processing. Our results suggest that the nonlinear RF losses can be minimized by tuning pairbreaking effects at the surface using impurity management or surface nanostructuring.

cond-mat.supr-con↗
arXiv2019

Lower Critical Field Measurement System based on Third-Harmonic Method for Superconducting RF Materials

We develop a lower critical field (Hc1) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. Parameter Hc1 is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally applied to one side of the sample mimics the magnetic field within superconducting radio-frequency (SRF) cavities and prevents edge effects of the superconducting sample. With this approach, our measurement system can potentially characterize surface-engineered SRF materials such as Superconductor-Insulator-Superconductor multilayer structure (S-I-S structure). As a validation test, we measure the temperature dependence of Hc1 of two high-RRR bulk Nb samples and obtain results consistent with the literature. We also confirm that our system can apply magnetic fields of at least 120 mT at 4-5 K without any problem of heat generation of the coil. This field value is higher than those reported in previous works and makes it possible to more accurately estimate Hc1 at lower temperatures.

physics.acc-ph↗
arXiv2017

Surface impedance and optimum surface resistance of a superconductor with imperfect surface

We calculate a low-frequency surface impedance of a dirty, s-wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer. Such structures model realistic surfaces of superconducting materials which can contain oxide layers, absorbed impurities or nonstoichiometric composition. We solved the Usadel equations self-consistently and obtained spatial distributions of the order parameter and the quasiparticle density of states which then were used to calculate a low-frequency surface resistance $R_s(T)$ and the magnetic penetration depth $λ(T)$ as functions of temperature in the limit of local London electrodynamics. It is shown that the imperfect surface in a single-band s-wave superconductor results in a non-exponential temperature dependence of $Z(T)$ at $T\ll T_c$ which can mimic the behavior of multiband or d-wave superconductors. The imperfect surface and the broadening of the gap peaks in the quasiparticle density of states $N(ε)$ in the bulk give rise to a weakly temperature-dependent residual surface resistance. We show that the surface resistance can be optimized and even reduced below its value for an ideal surface by engineering $N(ε)$ at the surface using pairbreaking mechanisms, particularly, by incorporating a small density of magnetic impurities or by tuning the thickness and conductivity of the normal layer and its contact resistance. The results of this work address the limit of $R_s$ in superconductors at $T\ll T_c$, and the ways of engineering the optimal density of states by surface nano-structuring and impurities to reduce losses in superconducting micro-resonators, thin film strip lines, and radio frequency cavities for particle accelerators.

cond-mat.supr-con↗
arXiv2017

Large-gap magnetic topological heterostructure formed by subsurface incorporation of a ferromagnetic layer

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4 /Bi2Se3 heterostructure which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the co-deposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4 /Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac-cone gap opening of ~100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future " topotronics" devices.

cond-mat.mtrl-sci↗
arXiv2017

Accurate method of verified computing for solutions of semilinear heat equations

We provide an accurate verification method for solutions of heat equations with a superlinear nonlinearity. The verification method numerically proves the existence and local uniqueness of the exact solution in a neighborhood of a numerically computed approximate solution. Our method is based on a fixed-point formulation using the evolution operator, an iterative numerical verification scheme to extend a time interval in which the validity of the solution can be verified, and rearranged error estimates for avoiding the propagation of an overestimate. As a result, compared with the previous verification method using the analytic semigroup, our method can enclose the solution for a longer time. Some numerical examples are presented to illustrate the efficiency of our verification method.

math.NA↗
arXiv2017

The Next SRF Technologies

In this talk, I introduce the proposed next superconducting radio-frequency (SRF) technologies that will make it possible to achieve much higher accelerating electric field than the present SRF technologies. Audiences are assumed to be non-experts. We start from a brief review of basics of SRF, history of the high gradient technologies and the layered structure behind it. The multiple benefit of the layered structure is introduced. We then move to the next SRF technologies: superconductor-superconductor (SS) structure and superconductor-insulator-superconductor (SIS) structure. We discuss the SS structure in detail. Experimental results are also introduced and compared with theoretical considerations.

physics.acc-ph↗
arXiv2016

Flux trapping in superconducting accelerating cavities during cooling down with a spatial temperature gradient

During the cool-down of a superconducting accelerating cavity, a magnetic flux is trapped as quantized vortices, which yield additional dissipation and contribute to the residual resistance. Recently, cooling down with a large spatial temperature gradient attracts much attention for successful reductions of trapped vortices. The purpose of the present paper is to propose a model to explain the observed efficient flux expulsions and the role of spatial temperature gradient during the cool-down of cavity. In the vicinity of a region with a temperature close to the critical temperature Tc,the critical fields are strongly suppressed and can be smaller than the ambient magnetic field. A region with a lower critical field smaller than the ambient field is in the vortex state. As a material is cooled down, a region with a temperature close Tc associating the vortex state domain sweeps and passes through the material. In this process, vortices contained in the vortex state domain are trapped by pinning centers that randomly distribute in the material. A number of trapped vortices can be naively estimated by using the analogy with the beam-target collision event. Based on this result, the residual resistance is evaluated. We find that a number of trapped vortices and the residual resistance are proportional to the strength of the ambient magnetic field and the inverse of the temperature gradient. The obtained residual resistance agrees well with experimental results. A material property dependence of a number of trapped vortices is also discussed.

physics.acc-ph↗
arXiv2015

Model of Flux Trapping in Cooling Down Process

The flux trapping that occurs in the process of cooling down of the superconducting cavity is studied. The critical fields $B_{c2}$ and $B_{c1}$ depend on a position when a material temperature is not uniform. In a region with $T\simeq T_c$, $B_{c2}$ and $B_{c1}$ are strongly suppressed and can be smaller than the ambient magnetic field, $B_a$. A region with $B_{c2}\le B_a$ is normal conducting, that with $B_{c1}\le B_a < B_{c2}$ is in the vortex state, and that with $B_{c1}> B_a$ is in the Meissner state. As a material is cooled down, these three domains including the vortex state domain sweep and pass through the material. In this process, vortices contained in the vortex state domain are trapped by pinning centers distributing in the material. A number of trapped fluxes can be evaluated by using the analogy with the beam-target collision event, where beams and a target correspond to pinning centers and the vortex state domain, respectively. We find a number of trapped fluxes and thus the residual resistance are proportional to the ambient magnetic field and the inverse of the temperature gradient. The obtained formula for the residual resistance is consistent with experimental results. The present model focuses on what happens at the phase transition fronts during a cooling down, reveals why and how the residual resistance depends on the temperature gradient, and naturally explains how the fast cooling works.

physics.acc-ph↗
arXiv2015

Field limit and nano-scale surface topography of superconducting radio-frequency cavity made of extreme type II superconductor

The field limit of superconducting radio-frequency cavity made of type II superconductor with a large Ginzburg-Landau parameter is studied with taking effects of nano-scale surface topography into account. If the surface is ideally flat, the field limit is imposed by the superheating field. On the surface of cavity, however, nano-defects almost continuously distribute and suppress the superheating field everywhere. The field limit is imposed by an effective superheating field given by the product of the superheating field for ideal flat surface and a suppression factor that contains effects of nano-defects. A nano-defect is modeled by a triangular groove with a depth smaller than the penetration depth. An analytical formula for the suppression factor of bulk and multilayer superconductors are derived in the framework of the London theory. As an immediate application, the suppression factor of the dirty Nb processed by the electropolishing is evaluated by using results of surface topographic study. The estimated field limit is consistent with the present record field of nitrogen-doped Nb cavities. Suppression factors of surfaces of other bulk and multilayer superconductors, and those after various surface processing technologies can also be evaluated by using the formula.

physics.acc-ph↗
arXiv2014

Superconducting nano-layer coating without insulator

The superconducting nano-layer coating without insulator layer is studied. The magnetic-field distribution and the forces acting on a vortex are derived. Using the derived forces, the vortex-penetration field and the lower critical magnetic field can be discussed. The vortex-penetration field is identical with the multilayer coating, but the lower critical magnetic field is not. Forces acting on a vortex from the boundary of two superconductors play an important role in evaluations of the free energy.

physics.acc-ph↗
arXiv2014

Two-dimensional models of the magnetic-field enhancement at pit and bump

Formulae of magnetic field enhancement at a two-dimensional semi-elliptical bump and a two-dimensional pit with chamfered edges are derived by using the method of conformal mapping. The latter can be regarded as an approximated model of the two-dimensional pit with round edges.

physics.acc-ph↗
arXiv2014

Review of the multilayer coating model

The recent theoretical study on the multilayer-coating model published in Applied Physics Letters [1] is reviewed. Magnetic-field attenuation behavior in a multilayer coating model is different from a semi-infinite superconductor and a superconducting thin film. This difference causes that of the vortex-penetration field at which the Bean-Livingston surface barrier disappears. A material with smaller penetration depth, such as a pure Nb, is preferable as the substrate for pushing up the vortex-penetration field of the superconductor layer. The field limit of the whole structure of the multilayer coating model is limited not only by the vortex-penetration field of the superconductor layer, but also by that of the substrate. Appropriate thicknesses of superconductor and insulator layers can be extracted from contour plots of the field limit of the multilayer coating model given in Ref.[1].

physics.acc-ph↗
arXiv2014

Radio-frequency electromagnetic field and vortex penetration in multilayered superconductors

A multilayered structure with a single superconductor layer and a single insulator layer formed on a bulk superconductor is studied. General formulae for the vortex-penetration field of the superconductor layer and the magnetic field on the bulk superconductor, which is shielded by the superconductor and insulator layers, are derived with a rigorous calculation of the magnetic field attenuation in the multilayered structure. The achievable peak surface field depends on the thickness and its material of the superconductor layer, the thickness of the insulator layer and material of the bulk superconductor. The calculation shows a good agreement with an experimental result. A combination of the thicknesses of superconductor and insulator layers to enhance the field limit can be given by the formulae for any given materials.

physics.acc-ph↗
arXiv2013

Models of the magnetic field enhancement at pits

Models of the magnetic field enhancement at pits are discussed. In order to build a model of pit, parameters that characterize a geometry of edges of pit, such as a curvature radius and a slope angle of edge, should be included, because the magnetic field is enhanced at an edge of a pit. A shape of the bottom of the pit is not important, because the magnetic field attenuates at the bottom. The simplest model of the pit is given by the two-dimensional pit with a triangular section. The well-like pit, which is known to many researchers, is a special case of this model. In this paper, idea and methods to analytically evaluate the magnetic field enhancement factor of such models are mentioned in detail. The well-like pit is considered as an instructive exercise, where the famous results by Shemelin and Padamsee are reproduced analytically. The triangular pit model, which is practically important for studies of the quench of SRF cavity, are discussed in detail. Comparisons between the prediction of the triangular-pit model and the vertical test results are also shown.

physics.acc-ph↗
arXiv2013

RF field-attenuation formulae for the multilayer coating model

Formulae that describe the RF electromagnetic field attenuation for the multilayer coating model with a single superconductor layer and a single insulator layer deposited on a bulk superconductor are derived from a rigorous calculation with the Maxwell equations and the London equation.

physics.acc-ph↗
arXiv2013

Analytical model of the magnetic field enhancement at pits on the surface of superconducting accelerating cavity

A simple model of the magnetic field enhancement at pits on the surface of superconducting accelerating cavity is proposed. The model consists of a two-dimensional pit with a slope angle, depth, width, and radius of round edge. An analytical formula that describes the magnetic field enhancement factor of the model is derived. The formula is given as a function of a slope angle and a ratio of half a width to a round-edge radius. Agreements between the formula and numerical calculations are also demonstrated. Using the formula, the field vortices start to penetrate can be evaluated for a given geometry of pit.

physics.acc-ph↗