Browsing by Author "Lee, Sangyun"

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Enhanced critical current density in the pressure-induced magnetic state of the high-temperature superconductor FeSe
(2015-11-09)
Jung, Soon-Gil
;
Kang, Ji-Hoon
;
Park, Eunsung
;
Lee, Sangyun
;
Lin, Jiunn-Yuan
We investigate the relation of the critical current density (J(c)) and the remarkably increased superconducting transition temperature (T-c) for the FeSe single crystals under pressures up to 2.43 GPa, where the T-c is increased by similar to 8 K/GPa. The critical current density corresponding to the free flux flow is monotonically enhanced by pressure which is due to the increase in T-c, whereas the depinning critical current density at which the vortex starts to move is more influenced by the pressure-induced magnetic state compared to the increase of T-c. Unlike other high-T-c superconductors, FeSe is not magnetic, but superconducting at ambient pressure. Above a critical pressure where magnetic state is induced and coexists with superconductivity, the depinning Jc abruptly increases even though the increase of the zero-resistivity T-c is negligible, directly indicating that the flux pinning property compared to the T-c enhancement is a more crucial factor for an achievement of a large J(c). In addition, the sharp increase in J(c) in the coexisting superconducting phase of FeSe demonstrates that vortices can be effectively trapped by the competing antiferromagnetic order, even though its antagonistic nature against superconductivity is well documented. These results provide new guidance toward technological applications of high-temperature superconductors.
Pressure dependence of upper critical fields in FeSe single crystals
(2016-03)
Kang, Ji-Hoon
;
Jung, Soon-Gil
;
Lee, Sangyun
;
Park, Eunsung
;
Lin, Jiunn-Yuan
We investigate the pressure dependence of the upper critical fields (mu H-0(c2)) for FeSe single crystals with pressure up to 2.57 GPa. The superconducting (SC) properties show a disparate behavior across a critical pressure where the pressure-induced antiferromagnetic phase coexists with superconductivity. The magnetoresistance for H//ab and H//c is very different: for H//c, magnetic field induces and enhances a hump in the resistivity close to the T-c for pressures higher than 1.2 GPa, while it is absent for H//ab. Since the measured mu H-0(c2) for FeSe samples is smaller than the orbital limited upper critical field (H-c2(orb)) estimated by the Werthamer, Helfand and Hohenberg model, the Maki parameter (alpha) related to Pauli spin-paramagnetic effects is additionally considered to describe the temperature dependence of mu H-0(c2)(T). Interestingly, the alpha value is hardly affected by pressure for H//ab, while it strongly increases with pressure for H//c. The pressure evolution of the mu H-0(c2)(0) for the FeSe single crystals is found to be almost similar to that of T-c(P), suggesting that the pressure-induced magnetic order adversely affects the upper critical fields as well as the SC transition temperature.