arXiv subjects

Mun-Seog Kim

Publications and source records attributed to Mun-Seog Kim.

At least 19 recordsLinked to original sources

arXiv2020

Quantum mechanical current-to-voltage conversion with quantum Hall resistance array

Accurate measurement of the electric current requires a stable and calculable resistor for an ideal current to voltage conversion. However, the temporal resistance drift of a physical resistor is unavoidable, unlike the quantum Hall resistance directly linked to the Planck constant h and the elementary charge e. Lack of an invariant high resistance leads to a challenge in making small current measurements below 1 muA with an uncertainty better than one part in 106. In this work, we demonstrate a current to voltage conversion in the range from a few nano amps to one microamp with an invariant quantized Hall array resistance. The converted voltage is directly compared with the Josephson voltage reference in the framework of Ohm's law. Markedly distinct from the classical conversion, which relies on an artifact resistance reference, this current-to-voltage conversion does not demand timely resistance calibrations. It improves the precision of current measurement down to 8 10 -8 at 1 muA.

physics.ins-det↗

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↗

arXiv2004

The Role of Thermal Phase Fluctuations in Underdoped YBCO Films

The effect of thermal phase fluctuations (TPF's) on the ab-plane penetration depth, lambda(T), of thin YBa2Cu3O(7-delta) (YBCO) films is found to be much smaller than expected from the paradigm of cuprates as weakly-coupled 2D superconducting layers. A 2D vortex-pair-unbinding transition is observed, but the effective thickness for fluctuations is the film thickness, not a CuO bilayer thickness. In a strongly underdoped YBCO film, Tc= 34 K, TPF's suppress Tc by only about 3 K. They cannot be a significant factor in the suppression of Tc and emergence of the pseudogap with underdoping.

cond-mat.supr-con↗

arXiv2003

Magnetic Penetration Depth Measurements of Pr$_{2-x}$Ce$_x$CuO$_{4-δ}$ Films on Buffered Substrates: Evidence for a Nodeless Gap

We report measurements of the inverse squared magnetic penetration depth, $λ^{-2}(T)$, in Pr$_{2-x}$Ce$_{x}$CuO$_{4-δ}$ ($0.115 \leq x \leq 0.152$) superconducting films grown on SrTiO$_3$ (001) substrates coated with a buffer layer of insulating Pr$_{2}$CuO$_{4}$. $λ^{-2}(0)$, $T_c$ and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, $0.124\leq x \leq 0.144$, $λ^{-2}(T)$ at low $T$ is flat: it changes by less than 0.15% over a factor of 3 change in $T$, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in $λ^{-2}(T)$ produce values of the minimum superconducting gap in the range of $0.29\leqΔ_{\rm min}/k_BT_c\leq1.01$.

cond-mat.supr-con↗

arXiv2003

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y}

From measurements of the magnetic penetration depth, $λ(T)$, from 1.6 K to $T_c$ in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, $N_s(E)$ at T=0 by using a simple model. In this framework, the flat behavior of $λ^{-2}(T)$ at low $T$ implies $N_s(E)$ is small, possibly gapped, at low energies. The upward curvature in $λ^{-2}(T)$ near $T_c$ seen in overdoped films implies that superfluid comes from an anomalously small energy band within about $3k_BT_c$ of the Fermi surface.

cond-mat.supr-con↗

arXiv2002

Effects of unreacted Mg impurities on the transport properties of MgB2

We synthesized polycrystalline MgB2 from a stoichiometric mixture of Mg and the 11 B isotope under different conditions. All the samples showed bulk superconductivity with Tc = 38~39 K. The samples containing the least amount of unreacted Mg showed the highest Tc and the sharpest transition width (Delta_Tc). A residual resistivity ratio (RRR) of ~ 5.8, and a magnetoresistance (MR), at 40 K, of 12% were obtained for these samples. Moreover, there was no upturn of resistivity in a low temperature region at 10 Tesla. The samples containing appreciable amounts of unreacted Mg showed quite different behaviors; the values of Delta_Tc, RRR, and MR were much larger. An upturn appeared in resistivity of the samples below about 50 K at 10 T and is thought to be due to the unreacted Mg.

cond-mat.supr-con↗

arXiv2002

Reflection of two-gap nature in penetration depth measurements of MgB$_2$ film

The magnetic penetration depth, $λ^{-2}(T)$, in the basal plane of a magnesium diboride (MgB$_2$) film was measured using a two-coil mutual inductance technique at 50 kHz. This film has $T_c\simeq 38$ K, $ΔT_c \leq 1$ K, and $λ(0)\sim 1500$ Å. At low temperatures, $λ^{-2}(T)$ shows a clear exponential temperature dependence, indicating s-wave superconducting order parameter symmetry. However, the data are not quantitatively well described by theory assuming a single gap. From the data fit by the full BCS calculation assuming a double gap, the values of the two distinct gaps were obtained: $Δ_S(0)=2.61\pm0.41$ meV and $Δ_L(0)=6.50\pm0.33$ meV. The contributions of the small and the large gaps to the total superfluid density at T=0 were estimated to be 21% and 79%, respectively. Finally, we consider the effect of gap anisotropy on the penetration depth measurements, and find that the anisotropy does not play a significant role in determining the temperature dependence of the penetration depth.

cond-mat.supr-con↗

arXiv2002

Evidence for a Transition in the Pairing Symmetry of the Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y}

We present measurements of the magnetic penetration depth, λ^{-2}(T), in Pr_{2-x}Ce_{x}CuO_{4-y} and La_{2-x}Ce_{x}CuO_{4-y} films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, λ^{-2}(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, λ^{-2}(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominally optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.

cond-mat.supr-con↗

arXiv2001

Synthesis and pinning properties of the infinite-layer superconductor Sr0.9La0.1CuO

We report the high-pressure synthesis of the electron-doped infinite-layer superconductor Sr0.9La0.1CuO2 and its superconducting properties. A Rietveld analysis of X-ray powder diffraction data showed that, within the resolution of the measurement, the sample had purely an infinite-layer structure without any discernible impurities. The superconducting volume fraction and the transition width were greatly improved compared to those in previous reports. The irreversibility field line and the intragranular critical current density were much higher than those of La1.85Sr0.15CuO4 and Nd1.85Ce0.15CuO4. The stronger pinning behaviors are consistent with the strong interlayer coupling due to the short distance between CuO2 planes.

cond-mat.supr-con↗

arXiv2001

High current-carrying capability in c-axis-oriented superconducting MgB2 thin films

In high-quality c-axis-oriented MgB2 thin films, we observed high critical current densities (Jc) of 16 MA/cm^2 at 15 K under self fields comparable to those of cuprate high-temperature superconductors. The extrapolated value of Jc at 5 K was estimated to be 40 MA/cm^2. For a magnetic field of 5 T, a Jc of 0.1 MA/cm^2 was detected at 15 K, suggesting that this compound would be a very promising candidate for practical applications at high temperature and lower power consumption. The vortex-glass phase is considered to be a possible explanation for the observed high current-carrying capability.

cond-mat.supr-con↗

arXiv2001

Substitution for Cu in the electron-doped infinite-layer superconductor Sr0.9La0.1CuO2, Ni reduces Tc much faster than Zn

We report on the effect of substitution for Cu on Tc of electron-doped infinite-layer superconductors Sr0.9La0.1Cu1-xRxO2, R = Zn and Ni. We found that Tc was nearly constant until x = 0.03 for R = Zn, while superconductivity was nearly suppressed for x = 0.02 with dTc/dx = 20 K/% for R = Ni. This behavior is very similar to that of conventional superconductors. These findings are discussed in terms of the superconducting gap symmetry in the cuprate superconductors including another electron-doped superconductor, (Nd,Ce)2CuO4-y.

cond-mat.supr-con↗

arXiv2001

Superconducting properties of well-shaped MgB2 single crystal

We report measurements of the transport and the magnetic properties of high-quality, sub-millimeter-sized MgB2 single crystals with clear hexagonal-plate shapes. The low-field magnetization and the magnetic hysteresis curves show the vortex pinning of these crystals to be very weak. The Debye temperature of $Θ_{D}\sim 1100$ K, obtained from the zero-field resistance curve, suggests that the normal-state transport properties are dominated by electron-phonon interactions. The resistivity ratio between 40 K and 300 K was about 5, and the upper critical field anisotropy ratio was 3 $\pm$ 0.2 at temperatures around 32 K.

cond-mat.supr-con↗

arXiv2001

Three-Dimensional Superconductivity in the Infinite-Layer Compound Sr_{0.9}La_{0.1}CuO_2 in Entire Region below $T_c$

The infinite-layer compound ACuO$_{2}$ (A $=$ alkaline-earth ions) is regarded as the most suitable material for exploring the fundamental nature of the CuO$_2$ plane because it does not contain a charge-reservoir block, such as a rock-salt or a fluorite like block. We report that superconductivity in the infinite-layer compound Sr$_{0.9}$La$_{0.1}$CuO$_2$ is of a three-dimensional nature, in contrast to the quasi two-dimensional superconducting behavior of all other cuprates. The key observation is that the $c$-axis coherence length is longer than the $c$-axis lattice constant even at zero temperature. This means that the superconducting order parameter of one CuO$_{2}$ plane overlaps with those of neighboring CuO$_{2}$ planes all the temperatures below the $T_c$. Among all cuprates, only the infinite-layer superconductor shows such a feature.

cond-mat.supr-con↗

arXiv2001

Prominent bulk pinning effect in the MgB_2 superconductor

We report the magnetic-field dependence of the irreversible magnetization of the recently discovered binary superconductor MgB$_{2}$. For the temperature region of $T< 0.9T_c$, the contribution of the bulk pinning to the magnetization overwhelms that of the surface pinning. This was evident from the fact that the magnetization curves, $M(H)$, were well described by the critical-state model without considering the surface pinning effect. It was also found that the $M(H)$ curves at various temperatures scaled when the field and the magnetization were normalized by the characteristic scaling factors $H^\ast(T)$ and $M^\ast(T)$, respectively. This feature suggests that the pinning mechanism determining the hysteresis in $M(H)$ is unique below $T=T_c$.

cond-mat.supr-con↗

arXiv2001

Temperature- and magnetic-field-dependent resistivity of MgB2 sintered at high temperature and high pressure condition

We report the temperature- and magnetic-field-dependent resistivity of MgB2 sintered at high temperature and high pressure condition. The superconducting transition width for the resistivity measurement was about 0.4 K, and the low-field magnetization showed a sharp superconducting transition with a transition width of about 1 K. The resistivity in the normal state roughly followed T^2 behavior with smaller residual resistivity ratio (RRR) of 3 over broad temperature region above 100 K rather than reported T^3 behavior with larger RRR value of ~ 20 in the samples made at lower pressures. Also, the resistivity did not change appreciably with the applied magnetic field, which was different from previous report. These differences were discussed with the microscopic and structural change due to the high-pressure sintering.

cond-mat.supr-con↗

arXiv2001

Origin of the high DC transport critical current density for the MgB2 superconductor

If the critical current density Jc is very high for a superconductor, then estimating its value from transport measurements is not very easy. In such cases, the value of Jc, called Jcm for magnetic Jc, is usually obtained from the measured magnetic hysteresis loop measurements by using a proper critical state model such as Bean's model (ref. 1). However, for bulk polycrystalline high temperature superconductors, the values of Jcm are much higher than the values, Jct, obtained from the transport measurements. This is due to the fact that the Jct is interrupted by weakly linked grain boundaries. However, for the recently discovered superconductor MgB2 (ref. 2), the grain boundary effect is negligible and these two values seem to coincide. Moreover, Jc increases drastically with decreasing the temperature. Consequently, the critical current densities for bulk wires can be very high, suggesting that numerous applications for the power transport. In this letter, we report the origin of the large current carrying capability of MgB2 based on direct measurements of the current-voltage relation in high magnetic fields. A strong coupling between the grains may be one reason for the absence of the weak link effect. Another reason may be the fact that, instead of a weak pinning mechanism such as thermally activated flux hopping, strong pinning due to a vortex glass phase is found in this material. The vortex phase diagram obtained from transport measurements shows that, in H-T space, a wide region below the Hc2 line is covered by a vortex glass phase.

cond-mat.supr-con↗

arXiv2001

Fluctuation Magnetoconductance in MgB2

We report fluctuation magnetoconductance in a MgB2 superconductor prepared under a high pressure of 3 GPa. This excess magnetoconductance, $Δσ(H)$, follows the three-dimensional scaling function for the critical fluctuations proposed by Ullah and Dorsey. This feature is inferred to originate from the isotropic nature of the MgB2 compound.

cond-mat.supr-con↗

arXiv2001

Effect of sintering temperature under high pressure in the uperconductivity for MgB2

We report the effect of the sintering temperature on the superconductivity of MgB2 pellets prepared under a high pressure of 3 GPa. The superconducting properties of the non-heated MgB2 in this high pressure were poor. However, as the sintering temperature increased, the superconducting properties were vastly enhanced, which was shown by the narrow transition width for the resistivity and the low-field magnetizations. This shows that heat treatment under high pressure is essential to improve superconducting properties. These changes were found to be closely related to changes in the surface morphology observed using scanning electron microscopy.

cond-mat.supr-con↗