• 한국초전도학회:학술대회논문집
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• 2009.07a
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• pp.48-48
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• 2009

• Progress in Superconductivity and Cryogenics
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• v.19 no.1
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• pp.1-4
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• 2017

Forefront ultrafast experimental techniques have recently proven their potential as new approaches to understand materials based on non-equilibrium dynamics in the time domain. The time domain approach is useful especially in disentangling complicated coupling among charge, spin and lattice degrees of freedom. Various ultrafast experiments on Fe-based superconductors have observed strong coherent oscillations of an $A_{1g}$ phonon mode of arsenic ions, which shows strong coupling to the electronic and magnetic states. This paper reviews the recent reports of ultrafast studies on Fe-based superconductor with a focus on the coherent oscillations. Experimental results with ultrashort light sources from the terahertz-infrared pulses to the hard X-rays from a free electron laser will be presented.

• Progress in Superconductivity
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• v.11 no.2
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• pp.112-117
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• 2010

We examine phase transition of the spin density wave and $\pi$ phase shifted superconductivity in the Fe pnictide superconductors. The phase diagram is described in the plane of the temperature T and the doping x with the combination of Ginzburg-Landau expansion of the free energy near the multi-critical temperature $T_c$ and the self-consistent numerical iterations of the gap equations. The phase separation or coexistence is determined by computing the 4-th order terms of the free energy which is confirmed by the numerical calculations. We can show the phase coexistence when the spin density wave is incommensurate. And the first order phase transition is observed near the boundary between commensurate and incommensurate spin density wave.

• Progress in Superconductivity and Cryogenics
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• v.15 no.2
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• pp.16-19
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• 2013

The magneto-resistivity and electric field-current density (E-J) curves were investigated up to a magnetic field 9 T in the optimally doped $BaFe_{1.8}Co_{0.2}As_2$ single crystal with a superconducting temperature ($T_c$) of 24.6 K. The E-J Scaling behaviors below and above vortex glass transition temperature ($T_g$) were found, confirming the existence of the vortex glass phase transition. The critical exponents for the diverging spatial and time correlations at $T_g$, were obtained as v = $1.1{\pm}0.1$ and z = $4.5{\pm}0.3$, respectively. The obtained critical exponents are in good agreement with the predicted values of v ~ 1 - 2 and z > 4 within the 3D vortex glass theory.

• 한국초전도학회:학술대회논문집
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• 2009.07a
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• pp.33-33
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• 2009

• Progress in Superconductivity and Cryogenics
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• v.16 no.4
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• pp.31-35
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• 2014

The upper critical field ($H_{c2}$) was determined by applying a magnetic field along the ab plane and c axis for two single crystals of $BaFe_{2-x}Ru_xAs_2$ (x=0.48 and 0.75). The anisotropy of the $H_{c2}(0)$, ${\gamma}(0)=H_{c2}{^{ab}}(0)/H_{c2}{^c}(0)$, was ~1.6 for x=0.48 and ~2.3 for x=0.75. The angle-dependent resistance measured below $T_c$ allowed perfect scaling features based on anisotropic Ginzburg-Landau theory, leading to consistent anisotropy values. Because only one fitting parameter ${\gamma}$ is used in the scaling for each temperature, the validity of the ${\gamma}$ value was compared with that determined from ${\gamma}=H_{c2}{^{ab}}/H_{c2}{^c}$. The ${\gamma}$ obtained at a temperature close to $T_c$ was 3.0 and decreased to 2.0 at low temperatures. Comparing to the anisotropy determined for electron- or hole-doped $BaFe_2As_2$ using the same method, the present results point to consistent anisotropy in Ru-doped $BaFe_2As_2$ with other electron- or hole-doped $BaFe_2As_2$.

• Journal of the Korean Magnetics Society
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• v.1 no.2
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• pp.15-21
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• 1991

The magnetic properties of the $Y_{1}Ba_{2}Cu_{3-x}Sn_{x}O_{7-y}$ superconductor were studied as a function of Sn concentration by utilizing both the vibrating sample magnetometer (VSM) and torque magnetometer. Unlike the cases where Fe and Co were substituted for Cu, the superconducting transition temperature was maintained above 90 K until x reached the value of 0.36. The lower critical field $H_{c1}(T)$ and upper critical field $H_{c2}(T)$ are measured as a function of temperature and external magnetic field, respectively. By aid of these results, $H_{c1}(0)$.($H_{c2}(0)$), the coherence length ${\varepsilon}_{0}$, the penetration depth ${\lambda}_{0}$, and the Ginzburg-Landau parameter k were oqtained. Flux pinning was also observed in the sample.

• Progress in Superconductivity and Cryogenics
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• v.17 no.4
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• pp.30-33
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• 2015

We report the synthesis of Ni-doped $BaFe_2Se_3$ single crystals by using a flux method. X-ray diffraction (XRD) of $Ba(Fe_{1-x}Ni_x)_2Se_3$ shows a gradual peak shift with an increase in the nominal Ni-doping rate, x = 0, 0.05, and 0.10, due to a decrease in unit-cell volume. All samples show a spin glass transition, and temperature dependence of magnetic susceptibility shows a negligible change in the spin-glass transition temperature ($T_g$) with Ni concentration x. The temperature dependence of electrical resistivity for $BaFe_2Se_3$ shows an insulating behavior, and the resistivity value at 295 K and the activation energy ($E_a$) obtained from the Arrhenius plot decrease with increasing x. These results suggest that the Ni doping can be effectively worked as a dopant for electron charge carriers, but is less efficient in controlling the magnetic property, such as spin glass transition, in the $BaFe_2Se_3$ compound.

• Progress in Superconductivity
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• v.12 no.2
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• pp.110-113
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• 2011

We have grown a Nb/Gd bilayer on a$SiO_2$/Si substrate by using a DC magnetron sputtering system, which was fabricated in situ with silicon stencil masks. In order to investigate proximity effect of the Nb/Gd bilayer, we used a planar tunnel junction with an AlOx tunnel barrier by oxidizing the Al ground electrode at the bottom. A $Co_{60}Fe_{40}$ backing of Al was deposited so as to reduce the superconductivity of the Al, ensuring a normal counterelectrode. With a 50-nm-thick Nb layer, we have measured dI/dV (dynamic conductance) by varying the thickness of Gd, which can reveal the density of states (DOS) of the Nb/Gd bilayer as a function of the Gd thickness resulting from the proximity effect of a superconductor/ferromagnet bilayer (S/F). The SF proximity effect in Nb/Gd will be discussed in comparison to our previous results of the CoFe/Nb, Ni/Nb and CuNi/Nb proximity effect; Gd is expected to show different effects since Gd has f-electrons, while CoFe, Ni, and CuNi have only d-electrons. Our studies will focus on the triplet correlation in a superconducting pair.

• Progress in Superconductivity
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• v.10 no.2
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• pp.87-91
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• 2009

Fluorine-doped $SmFeAsO_{1-x}F_x$ single crystals with the nominal value of x=0.2 were grown at $1350-1450^{\circ}C$ under the pressure of 3.3 GPa by using the self-flux method. Plate-shaped single crystals in the range of a few-150 ${\mu}m$ in their lateral size were obtained. The detailed crystal structure was analyzed by using the x-ray diffractometry. Superconducting transition temperature, determined by the resistive transition, of a single crystal was about 49 K with a narrow resistive transition width of ${\sim}1$ K. A relatively sharp transition, a low residual resistivity, and a large residual resistivity ratio compared with those reported for $REFeAsO_{1-x}F_x$(RE=Sm, Nd) single crystals indicate the high quality of our single crystals.