Yong Seung Kwon

Evidence of a pseudogap for superconducting iron-pnictide Ba0.6+δK0.4−δFe2As2 single crystals from optical conductivity measurements

We report the observation of pseudogap (PG) behavior in optical conductivity in an underdoped Ba0.6+δK0.4−δFe2As2 (Tonsetcxa0≈xa036xa0K) single crystal far above Tc, up to 100xa0K (≈3Tc). Wide separation of the energy scales of the magnetic and superconducting (SC) correlations in the Ba0.6+δK0.4−δFe2As2 sample enabled us to establish that the PG structures observed in optical conductivity in the range of 50–150xa0cm−1 are uniquely driven by the SC correlation, and the magnetic correlation coexists in the far separated range of about 700xa0cm−1. Theoretical calculations, based on the preformed Cooper pair model, provided an excellent description of the temperature evolution of the SC correlation in optical conductivity data from below to above Tc.

Optical properties of the iron-based superconductor LiFeAs single crystal

We have measured the reflectivity spectra of the LiFeAs (Tc = 17.6 K) single crystal in the temperature range from 4 to 300 K. In the superconducting (SC) state (T Tc), the optical conductivity spectra display typical metallic behavior with the Drude-type spectra at low frequencies, but we found that the introduction of the two Drude components best fits the data, indicating the multiband nature of this compound. A theoretical analysis of the low-temperature data (T = 4 K 200 cm−1), we have also performed the local-density approximation band calculation and calculated the optical spectra of the interband transitions. This theoretical result provided qualitative agreement with the experimental data below 4000 cm−1 .

Electron-boson spectral density of LiFeAs obtained from optical data

We analyze existing optical data in the superconducting state of LiFeAs at T = 4 K, to recover its electron-boson spectral density. A maximum entropy technique is employed to extract the spectral density I(2)χ(ω) from the optical scattering rate. Care is taken to properly account for elastic impurity scattering which can importantly affect the optics in an s-wave superconductor, but does not eliminate the boson structure. We find a robust peak in I(2)χ(ω) centered about Ω(R) ≅ 8.0 meV or 5.3 k(B)Tc (with Tc = 17.6 K). Its position in energy agrees well with a similar structure seen in scanning tunneling spectroscopy (STS). There is also a peak in the inelastic neutron scattering (INS) data at this same energy. This peak is found to persist in the normal state at T = 23 K. There is evidence that the superconducting gap is anisotropic as was also found in low temperature angular resolved photoemission (ARPES) data.

Vortex fluctuation effect evaluated using reversible magnetization in optimally doped single crystal NaFe0.97Co0.03As superconductor

The reversible magnetization curves for various magnetic fields are investigated for a single crystal of the optimally doped NaFe0.97Co0.03As superconductor with the superconducting transition temperature Tc?=?20.9 K (?(T)?=?0) in the case where H is parallel to c-axis. A well-formed rounding effect is observed in the temperature dependence of the magnetization, and it widens on increasing the strength of the applied magnetic fields. At 20.2 K, we observe a crossing of the M(T) curves for different applied magnetic fields, indicating important fluctuation effects. The irreversibility line Hirr(T) obeys a universal scaling relation Hirr(T)??=??H0(1???T/Tc)n characterized by n?????1.537, which confirms the moderately anisotropic nature of the sample. The scaling analysis of the reversible magnetization in the region near Tc(H) supports the three-dimensional nature of the vortex fluctuations in NaFe0.97Co0.03As.

Fluctuation-induced magnetoconductivity in pristine and proton-irradiated Ca8.5La1.5(Pt3As8)(Fe2As2)5 single crystals

The influence of the proton irradiation on the thermally fluctuation-induced conductivity in Ca8.5La1.5(pt3As8)(Fe2As2)5 single crystal was investigatyed. The in-plane magnetoconductivity was measured up to H=13T. It is observed that the Tc was suppressed up to 30.3 from 32.5 K when the proton is irradiated whereas the amplitude of the fluctuation effect is the almost same in both samples. The results were analyzed by the Ullah and Dorsey scaling functions based on the Hartree approximation to the Ginzburg-Landau free energy, including the critical-fluctuation regime. In the pristine sample, the 3D-2D crossover in the fluctuation effect was observed near the Tc. When the 3D-2D occurs, it is newly found that there is a regime simultaneously described by 2D and 3D fluctuation behaviors. Meanwhile, the proton-irradiated sample showed the 3D fluctuation.

Thermal activation energy of 3D vortex matter in NaFe 1−x Co x As ( x = 0.01, 0.03 and 0.07) single crystals

We report on the thermally activated flux flow dependency on the doping dependent mixed state in NaFe1−xCoxAs (xu2009=u20090.01, 0.03, and 0.07) crystals using the magnetoresistivity in the case of B//c-axis and B//ab-plane. It was found clearly that irrespective of the doping ratio, magnetoresistivity showed a distinct tail just above the Tc,offset associated with the thermally activated flux flow (TAFF) in our crystals. Furthermore, in TAFF region the temperature dependence of the activation energy follows the relation

Fermi Surface Variation of Ce 4f-electrons in Hybridization Controlled Heavy-Fermion Systems

U(T,B)={U}_{0}(B){(1-T/{T}_{c})}^{q}

Polarization-dependent three-dimensional angle-resolved photoemission study on LiFeAs

U(T,B)=U0(B)(1−T/Tc)q with qu2009=u20091.5 in all studied crystals. The magnetic field dependence of the activation energy follows a power law of