arXiv subjects

Sung-Ik Lee

Publications and source records attributed to Sung-Ik Lee.

At least 19 recordsLinked to original sources

arXiv2010

Fabrication of FeSe1-x superconducting films with bulk properties

We have fabricated high-quality FeSe1-x superconducting films with a bulk Tc of 11-12 K on different substrates, Al2O3(0001), SrTiO3(100), MgO(100), and LaAlO3(100), by using a pulsed laser deposition technique. All the films were grown at a high substrate temperature of 610 oC, and were preferentially oriented along the (101) direction, the latter being to be a key to fabricating of FeSe1-x superconducting thin films with high Tc. According to the energy dispersive spectroscopy data, the Fe:Se composition ratio was 1:0.90+-0.02. The FeSe1-x film grown on a SrTiO3 substrate showed the best quality with a high upper critical magnetic field [Hc2(0)] of 56 T.

cond-mat.supr-con↗

arXiv2010

Very strong intrinsic supercurrent carrying ability and vortex avalanches in (Ba,K)Fe2As2 superconducting single crystals

We report that single crystals of (Ba,K)Fe2As2 with Tc = 32 K have a pinning potential, U0, as high as 10^4 K, with U0 showing very little field depend-ence. In addition, the (Ba,K)Fe2As2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to Hc2. The rigid vortices in the two dimensional (Ba,K)Fe2As2 distinguish this compound from 2D high Tc cuprate superconductors with 2D vortices, and make it being capable of cearrying very high critical current.Flux jumping due to high Jc was also observed in large samples at low temperatures.

cond-mat.supr-con↗

arXiv2009

In-situ fabrication of cobalt-doped SrFe2As2 thin films by using pulsed laser deposition with excimer laser

The remarkably high superconducting transition temperature and upper critical field of iron(Fe)-based layered superconductors, despite ferromagnetic material base, open the prospect for superconducting electronics. However, success in superconducting electronics has been limited because of difficulties in fabricating high-quality thin films. We report the growth of high-quality c-axis-oriented cobalt(Co)-doped SrFe2As2 thin films with bulk superconductivity by using an in-situ pulsed laser deposition technique with a 248-nm-wavelength KrF excimer laser and an arsenic(As)-rich phase target. The temperature and field dependences of the magnetization showing strong diamagnetism and transport critical current density with superior Jc-H performance are reported. These results provide necessary information for practical applications of Fe-based superconductors.

cond-mat.supr-con↗

arXiv2009

Concurrent Magnetic and Metal-Insulator Transitions in (Eu,Sm)B_6 Single Crystals

The effects of magnetic doping on a EuB_6 single crystal were investigated based on magnetic and transport measurements. A modest 5% Sm substitution for Eu changes the magnetic and transport properties dramatically and gives rise to concurrent antiferromagnetic and metal-insulator transitions (MIT) from ferromagnetic MIT for EuB6. Magnetic doping simultaneously changes the itinerant carrier density and the magnetic interactions. We discuss the origin of the concurrent magnetic MIT in (Eu,Sm)B_6.

cond-mat.dis-nn↗

arXiv2007

Avalanches Injecting Flux into the Central Hole of a Superconducting MgB2 Ring

Magneto-optical imaging was used to observe dendritic flux avalanches connecting the outer and inner edges of a ring-shaped superconducting MgB2 film. Such avalanches create heated channels across the entire width of the ring, and inject large amounts of flux into the central hole. By measuring the injected flux and the corresponding reduction of current, which is typically 15%, we estimate the maximum temperature in the channel to be 100 K, and the duration of the process to be on the order of a microsecond. Flux creep simulations reproduce all the observed features in the current density before and after injection events.

cond-mat.supr-con↗

arXiv2006

Dynamic critical behaviors in two-dimensional Josephson junction arrays with positional disorder

We numerically investigate dynamic critical behaviors of two-dimensional (2D) Josephson-junction arrays with positional disorder in the scheme of the resistively shunted junction dynamics. Large-scale computation of the current voltage characteristics reveals an evidence supporting that a phase transition occurs at a nonzero critical temperature in the strong disorder regime, as well as in the weak disorder regime. The phase transition at weak disorder appears to belong to the Berezinskii-Kosterlitz-Thouless (BKT) type. In contrast, evidence for a non-BKT transition is found in the strong disorder regime. These results are consistent with the recent experiment %by Yun {\it et al.} in cond-mat/0509151 on positionally disordered Josephson-junction arrays; in particular, the critical temperature of the non-BKT transition (ranging from 0.265 down to the minimum 0.22 in units of $E_J/k_B$ with the Josephson coupling strength $E_J$), the correlation length critical exponent $ν=1.2$, and the dynamic critical exponent $z=2.0$ in the strong disorder regime agree with the existing studies of the 2D gauge-glass model.

cond-mat.supr-con↗

arXiv2006

Flux Dendrites of Opposite Polarity in Superconducting MgB$_2$ rings observed with magneto-optical imaging

Magneto-optical imaging was used to observe flux dendrites with opposite polarities simultaneously penetrate superconducting, ring-shaped MgB$_2$ films. By applying a perpendicular magnetic field, branching dendritic structures nucleate at the outer edge and abruptly propagate deep into the rings. When these structures reach close to the inner edge, where flux with opposite polarity has penetrated the superconductor, they occasionally trigger anti-flux dendrites. These anti-dendrites do not branch, but instead trace the triggering dendrite in the backward direction. Two trigger mechanisms, a non-local magnetic and a local thermal, are considered as possible explanations for this unexpected behaviour. Increasing the applied field further, the rings are perforated by dendrites which carry flux to the center hole. Repeated perforations lead to a reversed field profile and new features of dendrite activity when the applied field is subsequently reduced.

cond-mat.supr-con↗

arXiv2006

Onset of dendritic flux avalanches in superconducting films

We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermo-magnetic model published very recently [Phys. Rev. B 73, 014512 (2006)] gives an excellent quantitative description of key features like the instability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of MgB2. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

cond-mat.supr-con↗

arXiv2005

Comparison of $H_{c2}(T,θ)$ in Mg$_{1-x}$Al$_x$B$_2$ single crystals with the dirty-limit two-gap theory

We studied the temperature and the angular dependences of the upper critical field ($H_{c2}(T,θ$)) of Mg$_{1-x}$Al$_x$B$_2$ single crystals ($x = 0.12$ and 0.21) and compared with the dirty-limit two-gap theory. We found that $H_{c2}(T,θ)$'s were well described in a unified way by this theory. The obtained values of the parameters indicated that as the Al concentration was increased, anisotropic impurity scattering increased, making the $σ$ bands less anisotropic. Accordingly, the temperature dependence of the anisotropy ratio of $H_{c2}$ ($γ_H$) systematically decreased, and for $x=0.21$, $γ_H$ was nearly constant. Our results imply that Mg$_{1-x}$Al$_x$B$_2$ single crystals are in dirty-limit and that two-gap nature survives until $x = 0.21$.

cond-mat.supr-con↗

arXiv2005

Instability of defensive alliances in the predator-prey model on complex networks

A model of six-species food web is studied in the viewpoint of spatial interaction structures. Each species has two predators and two preys, and it was previously known that the defensive alliances of three cyclically predating species self-organize in two-dimensions. The alliance-breaking transition occurs as either the mutation rate is increased or interaction topology is randomized in the scheme of the Watts-Strogatz model. In the former case of temporal disorder, via the finite-size scaling analysis the transition is clearly shown to belong to the two-dimensional Ising universality class. In contrast, the geometric or spatial randomness for the latter case yields a discontinuous phase transition. The mean-field limit of the model is analytically solved and then compared with numerical results. The dynamic universality and the temporally periodic behaviors are also discussed.

cond-mat.stat-mech↗

arXiv2005

Effect of magnetic order on the superfluid response of single-crystal ErNi$_{2}$B$_{2}$C: A penetration depth study

We report measurements of the in-plane magnetic penetration depth $Δλ$(T) in single crystals of ErNi$_{2}$B$_{2}$C down to $\sim$0.1 K using a tunnel-diode based, self-inductive technique at 21 MHz. We observe four features: (1) a slight dip in $Δλ$(T) at the N$\acute{e}$el temperature $T_{N}$ = 6.0 K, (2) a peak at $T_{WFM}$ = 2.3 K, where a weak ferromagnetic component sets in, (3) another maximum at 0.45 K, and (4) a final broad drop down to 0.1 K. Converting to superfluid density $ρ_{s}$, we see that the antiferromagnetic order at 6 K only slightly depresses superconductivity. We seek to explain some of the above features in the context of antiferromagnetic superconductors, where competition between the antiferromagnetic molecular field and spin fluctuation scattering determines increased or decreased pairbreaking. Superfluid density data show only a slight decrease in pair density in the vicinity of the 2.3 K feature, thus supporting other evidences against bulk ferromagnetism in this temperature range.

cond-mat.supr-con↗

arXiv2005

Observation of the spontaneous vortex phase in the weakly ferromagnetic superconductor ErNi$_{2}$B$_{2}$C: A penetration depth study

The coexistence of weak ferromagnetism and superconductivity in ErNi$_{2}$B$% _{2}$C suggests the possibility of a spontaneous vortex phase (SVP) in which vortices appear in the absence of an external field. We report evidence for the long-sought SVP from the in-plane magnetic penetration depth $Δλ(T)$ of high-quality single crystals of ErNi$_{2}$B$_{2}$C. In addition to expected features at the Néel temperature $T_{N}$ = 6.0 K and weak ferromagnetic onset at $T_{WFM}=2.3 $K, $Δλ(T)$ rises to a maximum at $T_{m}=0.45$ K before dropping sharply down to $\sim $0.1 K. We assign the 0.45 K-maximum to the proliferation and freezing of spontaneous vortices. A model proposed by Koshelev and Vinokur explains the increasing $Δλ(T)$ as a consequence of increasing vortex density, and its subsequent decrease below $T_{m}$ as defect pinning suppresses vortex hopping.

cond-mat.supr-con↗

arXiv2005

The Dendritic magnetic avalanches in carbon-free MgB$_2$ thin films with and without a deposited Au layer

From the magneto optics images (MOI), the dendritic magnetic avalanche is known to appear dominantly for thin films of the newly discovered MgB$_2$. To clarify the origin of this phenomenon, we studied in detail the MOI of carbon-free MgB$_2$ thin films with and without a deposited gold layer. The MOI indicated carbon contamination was not the main source of the avalanche. The MOI clearly showed that the deposition of metallic gold deposition on top of a MgB$_2$ thin film improved its thermal stability and suppressed the sudden appearance of the dendritic flux avalanche. This is consistent with the previous observation of flux noise in the magnetization.

cond-mat.supr-con↗

arXiv2005

Evidence for a smooth superconductor to normal state transition for nonzero applied magnetic field in Sr0.9La0.1CuO2

We present and analyze field cooled magnetization data, revealing for 0<H<5 T remarkable consistency with a magnetic field induced finite size effect. It is traced back to the fact that the correlation length gsi cannot grow beyond the limiting length scale LH set by the magnetic field, where at temperature Tp(H), gsi(T) = LH. Thus, in sufficiently homogeneous samples and nonzero H the transition from the superconducting to the normal state turns out to be smooth and the appropriately scaled magnetization data fall near Tp(H) on a universal curve. Consistent with the generic behavior of optimally doped cuprates we also show that the pressure effect on Tc is negligibly small, while the negative value of the relative volume change mirrors that of the ansisotropy.

cond-mat.supr-con↗

arXiv2005

Muon-Spin-Rotation Measurements of the Penetration Depth in the Infinite-Layer Electron-Doped Cuprate Superconductor Sr0.9La0.1CuO2

Muon spin rotation (mSR) measurements of the in-plane penetration depth lambda_ab have been performed in the electron-doped infinite layer high-Tc superconductor (HTS) Sr0.9La0.1CuO2. Absence of the magnetic rare-earth ions in this compound allowed to measure for the first time the absolute value of lambda_ab(0) in electron-doped HTS using mSR. We found lambda_ab(0)=116(2) nm. The zero-temperature depolarization rate sigma(0)?1/lambda_ab(0)^2=4.6(1) MHz is more than four times higher than expected from the Uemura line. Therefore this electron-doped HTS does not follow the Uemura relation found for hole-doped HTS.

cond-mat.supr-con↗

arXiv2005

Anomalous Paramagnetic Effects in the Mixed State of LuNi2B2C

Anomalous paramagnetic effects in dc magnetization were observed in the mixed state of LuNi2B2C, unlike any reported previously. It appears as a kink-like feature for H > 30 kOe and becomes more prominent with increasing field. A specific heat jump at the corresponding temperature suggests that the anomaly is due to a true bulk transition. A magnetic flux transition from a square to an hexagonal lattice is consistent with the anomaly.

cond-mat.supr-con↗

arXiv2004

Specific Heat Study of the Magnetic Superconductor HoNi2B2C

The complex magnetic transitions and superconductivity of HoNi2B2C were studied via the dependence of the heat capacity on temperature and in-plane field angle. We provide an extended, comprehensive magnetic phase diagram for B // [100] and B // [110] based on the thermodynamic measurements. Three magnetic transitions and the superconducting transition were clearly observed. The 5.2 K transition (T_{N}) shows a hysteresis with temperature, indicating the first order nature of the transition at B=0 T. The 6 K transition (T_{M}), namely the onset of the long-range ordering, displays a dramatic in-plane anisotropy: T_{M} increases with increasing magnetic field for B // [100] while it decreases with increasing field for B // [110]. The anomalous anisotropy in T_{M} indicates that the transition is related to the a-axis spiral structure. The 5.5 K transition (T^{*}) shows similar behavior to the 5.2 K transition, i.e., a small in-plane anisotropy and scaling with Ising model. This last transition is ascribed to the change from a^{*} dominant phase to c^{*} dominant phase.

cond-mat.supr-con↗

arXiv2004

Evidence for the Coexistence of Anisotropic Superconducting Gap and Nonlocal Effects in the Non-magnetic Superconductor LuNi2B2C

A study of the dependence of the heat capacity Cp(alpha) on field angle in LuNi2B2C reveals an anomalous disorder effect. For pure samples, Cp(alpha) exhibits a fourfold variation as the field H < Hc2 is rotated in the [001] plane, with minima along <100> (alpha = 0). A slightly disordered sample, however, develops anomalous secondary minima along <110> for H > 1 T, leading to an 8-fold pattern. The anomalous pattern is discussed in terms of coexisting superconducting gap anisotropy and non-local effects.

cond-mat.supr-con↗