*3.1. Magnetic Measurements*

Figure 1a,b compare the temperature dependence of magnetic moment *M* with *H*//*c* for two FST films (film-A and film-B) before and after irradiation with 1 <sup>×</sup> <sup>10</sup><sup>15</sup> and <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> dose, respectively. Both the zero-field-cooled (ZFC) and field-cooled

(FC) magnetizations in 2 Oe magnetic field parallel to the *c*-axis indicate the appearance of superconductivity (obtained by the bifurcation of ZFC and FC) in pristine FST films at 16.8 K for film-A and 16.6 K for film-B. After the irradiation, the superconducting transitions occurred at 16.8 K for film-A and 16.8 K for film-B, indicating that 1.5 MeV proton irradiations with 1 <sup>×</sup> <sup>10</sup><sup>15</sup> and 1 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> dose have little impact on *<sup>T</sup>*<sup>c</sup> mag . However, the diamagnetic signal was enhanced with a sharper superconducting transition in the FST film-B irradiated with 1 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> dose. A degradation of *<sup>T</sup>*<sup>c</sup> after the ion irradiation is commonly reported in iron-based superconductors [19], although there have been a few reports on an increased *T*<sup>c</sup> in iron-based superconductors irradiated with proton and electron [16,20,21]. In previous work, the Fe(Se,Te) films were covered by Al foil with 80 <sup>µ</sup>m thickness and irradiated with 3.5 MeV protons at doses of 2.68 <sup>×</sup> <sup>10</sup><sup>16</sup> and 5.35 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> , corresponding to 2.30 <sup>×</sup> <sup>10</sup>–3 and 4.59 <sup>×</sup> <sup>10</sup>–3 dpa, respectively [22–24]. The average bombarding energy of the protons on the Fe(Se,Te) film was calculated to be 1.43 <sup>±</sup> 0.07 MeV. As a result, the irradiations to doses of 2.68 <sup>×</sup> <sup>10</sup><sup>16</sup> and 5.35 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> slightly suppressed *T*<sup>c</sup> from 17.7 K for pristine film to 17.3 K and 17.1 K, respectively. Given these results, the primary reason of the almost same *T*cs before and after the irradiation in our study would be a lower fluence than that in the previous works. zations in 2 Oe magnetic field parallel to the *c*-axis indicate the appearance of superconductivity (obtained by the bifurcation of ZFC and FC) in pristine FST films at 16.8 K for film-A and 16.6 K for film-B. After the irradiation, the superconducting transitions occurred at 16.8 K for film-A and 16.8 K for film-B, indicating that 1.5 MeV proton irradiations with 1 × 1015 and 1 × 1016 p/cm2 dose have little impact on *T*cmag. However, the diamagnetic signal was enhanced with a sharper superconducting transition in the FST film-B irradiated with 1 × 1016 p/cm2 dose. A degradation of *T*c after the ion irradiation is commonly reported in iron-based superconductors [19], although there have been a few reports on an increased *T*c in iron-based superconductors irradiated with proton and electron [16,20,21]. In previous work, the Fe(Se,Te) films were covered by Al foil with 80 μm thickness and irradiated with 3.5 MeV protons at doses of 2.68 × 1016 and 5.35 × 1016 p/cm2, corresponding to 2.30 × 10–3 and 4.59 × 10–3 dpa, respectively [22–24]. The average bombarding energy of the protons on the Fe(Se,Te) film was calculated to be 1.43 ± 0.07 MeV. As a result, the irradiations to doses of 2.68 × 1016 and 5.35 × 1016 p/cm2 slightly suppressed *T*c from 17.7 K for pristine film to 17.3 K and 17.1 K, respectively. Given these results, the primary reason of the almost same *T*cs before and after the irradiation in our study would be a lower fluence than that in the previous works.

Figure 1a,b compare the temperature dependence of magnetic moment *M* with *H*//*c* for two FST films (film-A and film-B) before and after irradiation with 1 × 1015 and 1 × 1016 p/cm2 dose, respectively. Both the zero-field-cooled (ZFC) and field-cooled (FC) magneti-

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 3 of 8

**3. Results and Discussion**  *3.1. Magnetic Measurements* 

**Figure 1.** Temperature dependences of magnetic moment *M* for both zero-field-cooled (ZFC) and field-cooled (FC) process at a magnetic field of *H* = 2 Oe applied along the *c*-axis for FST films before and after 1.5 MeV proton irradiation with (**a**) 1 × 1015 and (**b**) 1 × 1016 p/cm2 dose, respectively. **Figure 1.** Temperature dependences of magnetic moment *M* for both zero-field-cooled (ZFC) and field-cooled (FC) process at a magnetic field of *H* = 2 Oe applied along the *c*-axis for FST films before and after 1.5 MeV proton irradiation with (**a**) 1 <sup>×</sup> <sup>10</sup><sup>15</sup> and (**b**) 1 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> dose, respectively.

Figure 2 shows the magnetic field dependence of *J*c for the FST film-B at 5, 8, 10 K before and after 1.5 MeV proton irradiation at a dose of 1 × 1016 p/cm2. The *J*c was estimated from the magnetization hysteresis (*M*–*H*) loops using the critical-state Bean model [25,26]. For a rectangular prism-shaped crystal of dimensions *a* < *b*, we obtained the in-plane critical current density *J*c*ab* in the magnetic field parallel to the *c*-axis as *J*c*ab* = 20*ΔM*/(*a*(1 − *a*/3*b*)), where Δ*M* is the difference in magnetization *M*(emu/cm3) between the top and bottom branches of the *M*-*H* loop. In the inset of Figure 2, the *M*–*H* loop in FST film-B at 5 K before and after the irradiation of a dose of 1 × 1016 p/cm2 is plotted. A large irreversibility is noticeable up to around 4 T at 5 K. We attained a 30% increase in *J*c in the magnetic field below 1 T, which indicates that the irradiation defects contribute to vortex pinning. In contrast, we observed almost no change in the in-field *J*c above 1 T. Irradiation with MeV protons could produce mostly random point defects and nanocluster [27] due to ion–nucleus collisions. Sylva et al. reported that 3.5 MeV proton irradiation with 6.40 × 1016 p/cm2 dose (corresponding to 2.27 × 10–3 dpa) yields *J*c improvement of about 40% at 4.2 K and 7 Figure 2 shows the magnetic field dependence of *J*<sup>c</sup> for the FST film-B at 5, 8, 10 K before and after 1.5 MeV proton irradiation at a dose of 1 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> . The *J*<sup>c</sup> was estimated from the magnetization hysteresis (*M*–*H*) loops using the critical-state Bean model [25,26]. For a rectangular prism-shaped crystal of dimensions *a* < *b*, we obtained the in-plane critical current density *J*<sup>c</sup> *ab* in the magnetic field parallel to the *c*-axis as *J*<sup>c</sup> *ab* = 20∆*M*/(*a*(1 <sup>−</sup> *<sup>a</sup>*/3*b*)), where ∆*M* is the difference in magnetization *M*(emu/cm<sup>3</sup> ) between the top and bottom branches of the *M*-*H* loop. In the inset of Figure 2, the *M*–*H* loop in FST film-B at 5 K before and after the irradiation of a dose of 1 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> is plotted. A large irreversibility is noticeable up to around 4 T at 5 K. We attained a 30% increase in *J*<sup>c</sup> in the magnetic field below 1 T, which indicates that the irradiation defects contribute to vortex pinning. In contrast, we observed almost no change in the in-field *J*<sup>c</sup> above 1 T. Irradiation with MeV protons could produce mostly random point defects and nanocluster [27] due to ion–nucleus collisions. Sylva et al. reported that 3.5 MeV proton irradiation with 6.40 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> dose (corresponding to 2.27 <sup>×</sup> <sup>10</sup>–3 dpa) yields *<sup>J</sup>*<sup>c</sup> improvement of about 40% at 4.2 K and 7 T with respect to the pristine film almost without a decrease in *T*<sup>c</sup> [22]. On the contrary, *J*<sup>c</sup> of 3.5 MeV proton irradiated Fe(Se,Te) films covered with 80 µm thick Al foil decreased by up to 80% after irradiation at 4.2 K. The in-field *J*<sup>c</sup> performance in the irradiated FST films in our study could be attributed to the small number of vortex pinning defects created by the irradiation at low fluence.

ated by the irradiation at low fluence.

**Figure 2.** Magnetic field dependence of critical current density *J*<sup>c</sup> *ab*(*H*) at 5, 8 and 10 K calculated using the critical-state Bean model for FST film-B pre- and post- 1.5 MeV proton irradiation with 1 × 1016 p/cm2 dose. The inset shows magnetic hysteresis loop under *H*//*c* at 5 K. **Figure 2.** Magnetic field dependence of critical current density *J*<sup>c</sup> *ab*(*H*) at 5, 8 and 10 K calculated using the critical-state Bean model for FST film-B pre- and post- 1.5 MeV proton irradiation with <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> dose. The inset shows magnetic hysteresis loop under *<sup>H</sup>*//*<sup>c</sup>* at 5 K. dose of 1.5 MeV proton beam under 1 and 3 T at 4.2K in Figure 5. The pristine film has a less-anisotropic *J*c angular dependence at 1 and 3 T without a prominent *J*c peak at *H*//*c*,

), of

T with respect to the pristine film almost without a decrease in *T*c [22]. On the contrary, *J*<sup>c</sup> of 3.5 MeV proton irradiated Fe(Se,Te) films covered with 80 μm thick Al foil decreased by up to 80% after irradiation at 4.2 K. The in-field *J*c performance in the irradiated FST films in our study could be attributed to the small number of vortex pinning defects cre-

#### *3.2. Transport Measurement 3.2. Transport Measurement* which is often observed in YBa2Cu3O*y* films [28]. A small *J*c-anisotropy, *γJ*c (*J*c*H*//*ab*/*J*c*H*//*<sup>c</sup>*

In transport measurements, the current is forced to flow through the sample in a particular direction, enabling the direct characterization of superconductivity as a function of temperature, applied magnetic field and field angle. However, we observed an obvious degradation of superconducting properties in the transport measurement of the FST film-B. This could be due to sample degradation, sample handling during mounting and unmounting in a measurement system and possible damage by the laser cutting for patterning the bridge on FST films. In this section, we refer to the FST film-A. Figure 3 presents the temperature dependence of the electrical resistivity before and after irradiation for FST film-A with 1 × 1015 p/cm2 dose of 1.5 MeV proton. The FST films before and after the irradiation showed metallic behavior below 200 K. Additionally, 1.5 MeV proton irradiation with 1 × 1015 p/cm2 dose has little effect on normal-state resistivity due to the low dpa. On the contrary, the normal-state resistivity shows nearly upwards parallel-shift upon 6 MeV Au-ion irradiation with a dose of 1 × 1012 Au/cm2, corresponding to 6.42 × 10–3 dpa [11]. We observed no change in *T*c,10 (=17.5 K) before and after the 1.5 MeV protons irradiation with 1 × 1015 p/cm2 dose. This could be due to the low fluence, i.e., low dpa. In transport measurements, the current is forced to flow through the sample in a particular direction, enabling the direct characterization of superconductivity as a function of temperature, applied magnetic field and field angle. However, we observed an obvious degradation of superconducting properties in the transport measurement of the FST film-B. This could be due to sample degradation, sample handling during mounting and unmounting in a measurement system and possible damage by the laser cutting for patterning the bridge on FST films. In this section, we refer to the FST film-A. Figure 3 presents the temperature dependence of the electrical resistivity before and after irradiation for FST film-A with 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> dose of 1.5 MeV proton. The FST films before and after the irradiation showed metallic behavior below 200 K. Additionally, 1.5 MeV proton irradiation with 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> dose has little effect on normal-state resistivity due to the low dpa. On the contrary, the normal-state resistivity shows nearly upwards parallel-shift upon 6 MeV Au-ion irradiation with a dose of 1 <sup>×</sup> <sup>10</sup><sup>12</sup> Au/cm<sup>2</sup> , corresponding to 6.42 <sup>×</sup> <sup>10</sup>–3 dpa [11]. We observed no change in *T*c,10 (=17.5 K) before and after the 1.5 MeV protons irradiation with 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> dose. This could be due to the low fluence, i.e., low dpa. 1.7 is observed at 1 T. This value is smaller than the value of Fe(Se,Te) films grown on Febuffered MgO substrates (*γJ*c = 2.6) [29] while it is larger than the value of Fe(Se,Te) films grown on CaF2 substrates [30,31]. These differences might arise from the difference of the substrate and buffer layer. Upon irradiation with 1.5 MeV proton, the *J*c increases for most of the field orientations, retaining a small *γJ*c of 1.7 at 1 T, indicating that the vortex pinning defects would be less anisotropic and randomly distributed. At 3 T, there is a significant decrease in *J*c in the angular range ±30° from *H*//*ab*. Iron-based and cuprate high-temperature superconductors commonly possess inherent layered structures, consisting of alternating conducting and insulating atomic planes. In general, the strong *J*c peak for *H*//*ab* could be ascribed to the vortex pinning by the intrinsic pinning and planar defects such as intergrowths and stacking faults, parallel to the *ab* plane [32–35]. In the iron–chalcogenide Fe(Se,Te) compound, which is composed of only the Fe–Se(Te) layer, *J*c(*θ*) has a maximum at *H*//*ab* due to intrinsic pinning from the Fe–Se(Te) intralayer and Van der Waals interlayer couplings [29,34,35]. Hence, the *J*c suppression at around *H*//*ab* would occur because of the reduction in the density of intrinsic pinning upon the irradiation.

**Figure 3.** Temperature dependences of electrical resistivity at 0 T for the FST film-A before and after 1.5 MeV proton irradiation with 1 × 1015 p/cm2 dose. Inset shows a magnified temperature region near *T*c. **Figure 3.** Temperature dependences of electrical resistivity at 0 T for the FST film-A before and after 1.5 MeV proton irradiation with 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> dose. Inset shows a magnified temperature region near *T*c.

Figure 4 presents the magnetic field dependence of transport critical current density *J*<sup>c</sup> with *H*//*c* for the FST film-A before and after irradiation with 1.5 MeV protons to a dose of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> at 4.2 K. Comparing *<sup>J</sup>*cs obtained from magnetization and transport measurements, the values of *J*<sup>c</sup> obtained from transport measurement are larger than those of *J*<sup>c</sup> calculated from magnetization measurement. This would come from the difference of criterion to determine the *J*<sup>c</sup> values. The positive effect of the proton irradiation on *J*<sup>c</sup> at 4.2 K is unambiguous in the magnetic field below 1 T. As the magnetic field increased, the difference between pristine and the irradiated FST film became smaller. Similar behavior was observed in *J*c(*H*) (calculated from magnetization measurement in Figure 2) for FST film-B irradiated with 1 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> dose. **Figure 3.** Temperature dependences of electrical resistivity at 0 T for the FST film-A before and after 1.5 MeV proton irradiation with 1 × 1015 p/cm2 dose. Inset shows a magnified temperature region near *T*c.

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 5 of 8

dose of 1.5 MeV proton beam under 1 and 3 T at 4.2K in Figure 5. The pristine film has a less-anisotropic *J*c angular dependence at 1 and 3 T without a prominent *J*c peak at *H*//*c*, which is often observed in YBa2Cu3O*y* films [28]. A small *J*c-anisotropy, *γJ*c (*J*c*H*//*ab*/*J*c*H*//*<sup>c</sup>*

1.7 is observed at 1 T. This value is smaller than the value of Fe(Se,Te) films grown on Febuffered MgO substrates (*γJ*c = 2.6) [29] while it is larger than the value of Fe(Se,Te) films grown on CaF2 substrates [30,31]. These differences might arise from the difference of the substrate and buffer layer. Upon irradiation with 1.5 MeV proton, the *J*c increases for most of the field orientations, retaining a small *γJ*c of 1.7 at 1 T, indicating that the vortex pinning defects would be less anisotropic and randomly distributed. At 3 T, there is a significant decrease in *J*c in the angular range ±30° from *H*//*ab*. Iron-based and cuprate high-temperature superconductors commonly possess inherent layered structures, consisting of alternating conducting and insulating atomic planes. In general, the strong *J*c peak for *H*//*ab* could be ascribed to the vortex pinning by the intrinsic pinning and planar defects such as intergrowths and stacking faults, parallel to the *ab* plane [32–35]. In the iron–chalcogenide Fe(Se,Te) compound, which is composed of only the Fe–Se(Te) layer, *J*c(*θ*) has a maximum at *H*//*ab* due to intrinsic pinning from the Fe–Se(Te) intralayer and Van der Waals interlayer couplings [29,34,35]. Hence, the *J*c suppression at around *H*//*ab* would occur because of the reduction in the density of intrinsic pinning upon the irradiation.

), of

**Figure 4.** Magnetic field dependence of critical current density *J*<sup>c</sup> obtained from transport measurement at 4.2 K for FST film-A pre- and post-1.5 MeV proton irradiation with 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> dose.

A more detailed representation of the pinning efficiency can be obtained from the angular dependence of *<sup>J</sup>*c. We show *<sup>J</sup>*c(*θ*) for the FST film-A irradiated with <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> dose of 1.5 MeV proton beam under 1 and 3 T at 4.2K in Figure 5. The pristine film has a less-anisotropic *J*<sup>c</sup> angular dependence at 1 and 3 T without a prominent *J*<sup>c</sup> peak at *H*//*c*, which is often observed in YBa2Cu3O*<sup>y</sup>* films [28]. A small *J*c-anisotropy, *γJ*<sup>c</sup> (*J*c *<sup>H</sup>*//*ab*/*J*<sup>c</sup> *H*//*c* ), of 1.7 is observed at 1 T. This value is smaller than the value of Fe(Se,Te) films grown on Fe-buffered MgO substrates (*γJ*<sup>c</sup> = 2.6) [29] while it is larger than the value of Fe(Se,Te) films grown on CaF<sup>2</sup> substrates [30,31]. These differences might arise from the difference of the substrate and buffer layer. Upon irradiation with 1.5 MeV proton, the *J*<sup>c</sup> increases for most of the field orientations, retaining a small *γJ*<sup>c</sup> of 1.7 at 1 T, indicating that the vortex pinning defects would be less anisotropic and randomly distributed. At 3 T, there is a significant decrease in *J*<sup>c</sup> in the angular range ±30◦ from *H*//*ab*. Iron-based and cuprate high-temperature superconductors commonly possess inherent layered structures, consisting of alternating conducting and insulating atomic planes. In general, the strong *J*<sup>c</sup> peak for *H*//*ab* could be ascribed to the vortex pinning by the intrinsic pinning and planar defects such as intergrowths and stacking faults, parallel to the *ab* plane [32–35]. In the iron–chalcogenide Fe(Se,Te) compound, which is composed of only the Fe–Se(Te) layer, *J*c(*θ*) has a maximum at *H*//*ab* due to intrinsic pinning from the Fe–Se(Te) intralayer and Van der Waals interlayer couplings [29,34,35]. Hence, the *J*<sup>c</sup> suppression at around *H*//*ab* would occur because of the reduction in the density of intrinsic pinning upon the irradiation.

**Figure 5.** Angular field dependence of the critical current density *J*c obtained from transport measurement for FST film-A before and after 1.5 MeV proton irradiation with 1 × 1015 p/cm2 dose measured at 4.2 K under 1 and 3 T. **Figure 5.** Angular field dependence of the critical current density *J*c obtained from transport measurement for FST film-A before and after 1.5 MeV proton irradiation with 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> dose measured at 4.2 K under 1 and 3 T.

**Figure 4.** Magnetic field dependence of critical current density *J*c obtained from transport measurement at 4.2 K for FST film-A pre- and post-1.5 MeV proton irradiation with 1 × 1015 p/cm2 dose.

#### **4. Conclusions 4. Conclusions**

We conclude a study on the effect of 1.5 MeV proton irradiation on superconducting properties of FST films. Upon the irradiation up to 1 × 1016 p/cm2 dose, *T*c remains virtually unchanged in magnetization as well as in transport measurement. An approximately 30% enhancement of *J*c in the magnetic field below 1 T is observed using 1.5 MeV proton irradiation with 1 × 1016 p/cm2. Transport properties of a pristine film and an irradiated film with a fluence of 1 × 1015 p/cm2 show a small anisotropy of *J*c in the applied magnetic field range at 4.2 K. The enhancement of *J*c for almost all the field orientations was accomplished by the irradiation at a dose of 1 × 1015 p/cm2 at 4.2 K and 1 T. These results indicate that 1.5 MeV proton irradiation is effective in providing less anisotropic pinning defects in the magnetic field below 1 T in iron–chalcogenide superconducting films. Additionally, by fine tuning an irradiation fluence of proton, superconducting properties can be further improved. We conclude a study on the effect of 1.5 MeV proton irradiation on superconducting properties of FST films. Upon the irradiation up to 1 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> dose, *<sup>T</sup>*<sup>c</sup> remains virtually unchanged in magnetization as well as in transport measurement. An approximately 30% enhancement of *J*<sup>c</sup> in the magnetic field below 1 T is observed using 1.5 MeV proton irradiation with 1 <sup>×</sup> <sup>10</sup><sup>16</sup> p/cm<sup>2</sup> . Transport properties of a pristine film and an irradiated film with a fluence of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> show a small anisotropy of *J*<sup>c</sup> in the applied magnetic field range at 4.2 K. The enhancement of *J*<sup>c</sup> for almost all the field orientations was accomplished by the irradiation at a dose of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> p/cm<sup>2</sup> at 4.2 K and 1 T. These results indicate that 1.5 MeV proton irradiation is effective in providing less anisotropic pinning defects in the magnetic field below 1 T in iron–chalcogenide superconducting films. Additionally, by fine tuning an irradiation fluence of proton, superconducting properties can be further improved.

**Author Contributions:** Conceptualization, T.O.; sample preparation, T.K. and T.O.; ion irradiation, R.I., T.K. and T.O.; transport measurement, T.K., I.K. and T.O.; magnetization measurement, T.O. and T.K.; data curation, T.K. and T.O.; writing—original draft preparation, T.O.; writing—review and editing, I.K. and R.I. All authors have read and agreed to the published version of the manu-**Author Contributions:** Conceptualization, T.O.; sample preparation, T.K. and T.O.; ion irradiation, R.I., T.K. and T.O.; transport measurement, T.K., I.K. and T.O.; magnetization measurement, T.O. and T.K.; data curation, T.K. and T.O.; writing—original draft preparation, T.O.; writing—review and editing, I.K. and R.I. All authors have read and agreed to the published version of the manuscript.

script. **Funding:** This work was partly supported by Foundation of Kinoshita Memorial Enterprise.

**Funding:** This work was partly supported by Foundation of Kinoshita Memorial Enterprise. **Acknowledgments:** This research has been performed under the collaboration program between **Acknowledgments:** This research has been performed under the collaboration program between Kwansei Gakuin University, Kyoto University and Wakasa Wan Energy Research Center.

Kwansei Gakuin University, Kyoto University and Wakasa Wan Energy Research Center. **Conflicts of Interest:** The authors declare no conflict of interest. **Conflicts of Interest:** The authors declare no conflict of interest.
