Kyuil Cho

The superconducting properties of YBa2Cu3O7−δ thin-films were investigated by conducting 1.7 MeV proton irradiations with a total fluence of 2.64×1017p/cm2. The superconducting critical temperature (Tc) was reduced from 89.4 K to 10.1 K. The experimental procedure was similar to the previous study (0.6 MeV proton irradiation). We compared the effectiveness of Tc suppression by varying the proton energy from 0.6 to 1.7 MeV and found that in general both protons of 1.7 MeV and 0.6 MeV were effective in suppressing the Tc of YBCO. In particular, both results were consistent with the theoretical expectation (extended d-wave AG theory) when the zero-temperature London penetration depth (λ0) = 215 nm is assumed for thin-film YBCO. For heavily irradiated cases (more than 80% Tc suppression), however, 1.7 MeV protons were more effective in suppressing Tc than 0.6 MeV protons. This can be understood by the fact that in thin-film limit, higher energy protons tend to produce less clustered point-defects while lower energy protons tend to create agglomeration of point-defects.

We investigated the effect of 0.6 MeV proton irradiation on the superconducting and normal state properties of thin-film YBa2Cu3O7−δ superconductors. A thin-film YBCO superconductor (≈ 567 nm thick) was subject to a series of proton irradiations with a total fluence of 7.6×1016p/cm2. Upon irradiation, Tc was drastically decreased from 89.3 K towards zero with a corresponding increase in its normal state resistivity above Tc. This increase in resistivity which indicates the increase of defects inside the thin-film sample can be converted to the dimensionless scattering rate. We found that the relation between Tc and dimensionless scattering rate obtained during proton irradiation approximates the generalized d-wave Abrikosov-Gor’kov theory better than the previous results obtained from electron irradiations. This is an unexpected result since the electron irradiation is known to be most effective to suppress superconductivity over other heavier ion irradiations such as proton irradiation. It suggests that the type of defects created by proton irradiation evolves from cascade defects (in bulk single crystals) to point-like defects (in thin-film single crystals) as the thickness decreases.

Solid state chemistry has produced a plethora of materials with properties not found in nature. For example, high-temperature superconductivity in cuprates is drastically different from the superconductivity of naturally occurring metals and alloys and is frequently referred to as unconventional. Unconventional superconductivity is also found in other synthetic compounds, such as iron-based and heavy-fermion superconductors. Here, we report compelling evidence of unconventional nodal superconductivity in synthetic samples of Rh₁₇S₁₅ (T_c = 5.4 K), which is also found in nature as the mineral miassite. We investigated the temperature-dependent variation of the London penetration depth Δλ(T) and the disorder evolution of the critical superconducting temperature T_c and the upper critical field H_c2(T) in single crystalline Rh₁₇S₁₅. We found a T − linear temperature variation of Δλ(T) below 0.3T_c, which is consistent with the presence of nodal lines in the superconducting gap of Rh₁₇S₁₅. The nodal character of the superconducting state is supported by the observed suppression of T_c and H_c2(T) in samples with a controlled level of non-magnetic disorder introduced by 2.5 MeV electron irradiation. We propose a nodal sign-changing superconducting gap in the A_1g irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the derived superfluid density. To the best of our knowledge, this establishes miassite as the only mineral known so far that reveals unconventional superconductivity in its clean synthetic form, though it is unlikely that it is present in natural crystals because of unavoidable impurities that quickly destroy nodal superconductivity.

Unconventional superconductivity has long been believed to arise from a lab-grown correlated electronic system. Here we report compelling evidence of unconventional nodal superconductivity in a mineral superconductor Rh₁₇S₁₅. We investigated the temperature-dependent London penetration depth Δλ(T) and disorder evolution of the critical temperature T_c and upper critical field H_c2(T) in synthetic miassite Rh₁₇S₁₅. We found a power-law behavior of Tⁿ with n≈1.1 at low temperatures below 0.3T_c (T_c = 5.4 K), which is consistent with the presence of lines of the node in the superconducting gap of Rh₁₇S₁₅. The nodal character of the superconducting state in Rh₁₇S₁₅ was supported by the observed pairbreaking effect in T_c and H_c2(T) in samples with the controlled disorder that was introduced by low-temperature electron irradiation. We propose a nodal sign-changing superconducting gap in the A_1g irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the superfluid density, λ²(0)/λ²(T).

Purpose: The purpose of this study was to examine how preservice teachers (PSTs) developed their adaptive competence in lesson planning through repeated rehearsals and reflections in an online learning environment. Methods: A case study design utilizing descriptive analysis was used to analyze data collected from a series of lesson plan iterations made by nine PSTs in a physical education teacher education program. All participants attended one online introductory methods course, which consisted of a synchronous lecture and laboratory components to learn fundamental instruction and management skills in teaching physical education. Findings: PSTs made positive adaptations in five core practices in their lesson plan iterations. However, the frequency of adaptations in each core practice varied in different lesson components. The findings suggest that the complexity of content being taught and opportunities embedded in each lesson component may influence how adaptations were made in lesson plans. Conclusions: Repeated rehearsals and reflections can facilitate PSTs’ development of adaptations in lesson planning in an online environment. However, learning environments may prevent PSTs from adapting certain core practices.