arXiv subjects

Min-Seok Park

Publications and source records attributed to Min-Seok Park.

At least 19 recordsLinked to original sources

arXiv2004

Reversible magnetization measurement of the anisotropy of the London penetration depth in MgB2 single crystals

We have studied the anisotropy of the London penetration depth, which was obtained from reversible magnetization measurements with the magnetic field both parallel and perpendicular to the c-axis. The anisotropy of the London penetration depth has a smaller magnitude than the anisotropy of the upper critical field and increases with temperature while that of the upper critical field decreases as reported earlier. This behavior is in sharp contrast with the behaviors of superconductors with one superconducting energy gap. The temperature dependence of the anisotropies of the London penetration depth and of the upper critical field can be well explained within the theory of two-gap superconductivity in MgB2.

cond-mat.supr-con↗

arXiv2003

Temperature and magnetic field dependent tunneling spectroscopy of PtIr/Sr$_{0.9}$La$_{0.1}$CuO$_2$ point contact

Low barrier quasiparticle tunneling spectroscopy on Sr$_{0.9}$La$_{0.1}$CuO$_2$ has been studied with PtIr/Sr$_{0.9}$La$_{0.1}$CuO$_2$ point contacts at various temperatures and magnetic fields. No zero-bias conductance peaks are observed. By fitting tunneling conductance to Blonder-Tinkham-Klapwijk theory, the temperature dependent s-wave superconducting gaps are obtained. The present results exclude the possibility of d-wave symmetry for the pair gap in Sr$_{0.9}$La$_{0.1}$CuO$_2$, and do not support the conventional phonon mediated pairing in this system either.

cond-mat.supr-con↗

arXiv2003

X-ray absorption and optical spectroscopy studies of (Mg$_{1-x}$Al$_x$)B$_2$

X-ray absorption spectroscopy and optical reflectance measurements have been carried out to elucidate the evolution of the electronic structure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$ for $\emph{x}$ = 0.0,0.1, 0.2, 0.3, and 0.4. The important role of B 2$\emph{p}$ $σ$ hole states to superconductivity has been identified, and the decrease in the hole carrier number is $\emph{quantitatively}$ determined. The rate of the decrease in the hole concentration agree well with the theoretical calculations. On the other hand,while the evolution of the electronic structure is gradual through the doping range, $T_c$ suppression is most significant at $\emph{x}$ = 0.4. These results suggest that the superstructure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$, in addition to the $σ$ holes, can affect the lattice dynamics and contributes to the $T_c$ suppression effect. Other possible explanations like the topological change of the $σ$ band Fermi surface are also discussed.

cond-mat.supr-con↗

arXiv2003

Bulk Evidence for s-Wave Pairing Symmetry in the n-Type Infinite-Layer Cuprate $Sr_{0.9}La_{0.1}CuO_{2}$

Low temperature specific heat of the electron-doped (n-type) infinite-layer cuprate $Sr_{0.9}La_{0.1}CuO_{2}$ has been measured. The quasiparticle density of states (DOS) in the mixed state is found to be consistent with the feature of the s-wave pairing symmetry, agreeing very well with the earlier tunnelling measurement, but being contrary to the d-wave symmetry well confirmed for the hole-doped (p-type) cuprates. Our results indicate that the electronic DOS are mainly contributed by the vortex cores in the present sample being contrast to the p-type cuprates in which the vortex cores are abnormal and contribute very limited low energy DOS as evidenced by many means.

cond-mat.supr-con↗

arXiv2002

Fluctuation Study of the Specific Heat of MgB2

The specific heat of polycrystalline Mg$^{11}$B$_{2}$ has been measured with high resolution ac calorimetry from 5 to 45 K at constant magnetic fields. The excess specific heat above T$_{c}$ is discussed in terms of Gaussian fluctuations and suggests that Mg$^{11}$B$_{2}$ is a bulk superconductor with Ginzburg-Landau coherence length $ξ_{0}=26$ Å. The transition-width broadening in field is treated in terms of lowest-Landau-level (LLL) fluctuations. That analysis requires that $ξ_{0}=20$ Å. The underestimate of the coherence length in field, along with deviations from 3D LLL predictions, suggest that there is an influence from the anisotropy of B$_{c2}$ between the c-axis and the a-b plane.

cond-mat.supr-con↗

arXiv2002

Photoemission and x-ray absorption study of MgC_(1-x)Ni_3

We investigated electronic structure of MgC_(1-x)Ni_3 with photoemission and x-ray absorption spectroscopy. Both results show that overall band structure is in reasonable agreement with band structure calculations including the existence of von Hove singularity (vHs)near E_F. However, we find that the sharp vHs peak theoretically predicted near the E_F is substantially suppressed. As for the Ni core level and absorption spectrum, there exist the satellites of Ni 2p which have a little larger energy separation and reduced intensity compared to the case of Ni-metal. These facts indicate that correlation effects among Ni 3d electrons may be important to understand various physical properties.

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

Strongly correlated s-wave pairing in the n-type infinite-layer cuprate

Quasiparticle tunneling spectra of the electron-doped (n-type) infinite-layer cuprate Sr_{0.9}La_{0.1}CuO_2 reveal characteristics that counter a number of common phenomena in the hole-doped (p-type) cuprates. The optimally doped Sr_{0.9}La_{0.1}CuO_2 with T_c = 43 K exhibits a momentum-independent superconducting gap Δ= 13.0 +- 1.0 meV that substantially exceeds the BCS value, and the spectral characteristics indicate insignificant quasiparticle damping by spin fluctuations and the absence of pseudogap. The response to quantum impurities in the Cu-sites also differs fundamentally from that of the p-type cuprates with d_{x^2-y^2}-wave pairing symmetry.

cond-mat.supr-con↗

arXiv2001

Gap Anisotropy, Spin Fluctuations and Normal-State Properties of the Electron Doped Superconductor Sr0.9La0.1CuO2

We report the results from a thermopower and a Cu nuclear magnetic resonance (NMR) study of the infinite CuO2 layer electron doped high temperature superconducting cuprate (HTSC), Sr0.9La0.1CuO2. We find that the temperature dependence of the thermopower, S(T), is different from that observed in the hole doped HTSC. In particular, we find that dS(T)/dT is positive above ~120 K. However, we show that S(T) can still be described by the same model developed for the hole doped HTSC and hence S(T) is not anomalous and does not imply phonon mediated pairing as has previously been suggested. The Cu NMR data reveal a Knight shift and spin lattice relaxation rate below Tc that are inconsistent with isotropic s-wave pairing. The Cu spin lattice relaxation rate in the normal state, however, is Curie-Weiss like and is comparable to that of the optimally and overdoped hole doped HTSC, La2-xSrxCuO4. The magnitude of the Knight shift indicates that the density of states at the Fermi level is anomalously small when compared with the hole doped HTSC with the same Tc, indicating that the size of the density of states at the Fermi level is of little importance in the HTSC. We find no evidence of the normal state pseudogap that is observed in the hole doped HTSC and which was recently reported to exist in the electron doped HTSC, Nd1.85Ce0.15CuO4, from infrared reflectance measurements.

cond-mat.supr-con↗

arXiv2001

Spectroscopic Evidence for Anisotropic S-Wave Pairing Symmetry in MgB2

Scanning tunneling spectroscopy of superconducting MgB$_2$ ($T_c = 39$ K) were studied on high-density pellets and c-axis oriented films. The sample surfaces were chemically etched to remove surface carbonates and hydroxides, and the data were compared with calculated spectra for all symmetry-allowed pairing channels. The pairing potential ($Δ_k$) is best described by an anisotropic s-wave pairing model, with $Δ_k = Δ_{xy} \sin ^2 θ_k + Δ_z \cos ^2 θ_k$, where $θ_k$ is the angle relative to the crystalline c-axis, $Δ_z \sim 8.0$ meV, and $Δ_{xy} \sim 5.0$ meV.

cond-mat.supr-con↗

arXiv2001

A Magnetization and Microwave Study of Superconducting MgB2

We report magnetic field dependent magnetization and microwave impedance measurements on a MgB2 superconductor prepared by high pressure synthesis. We find that the upper critical field is linearly dependent on temperature near Tc and the dc irreversibility field exponent is ~1.4. The microwave data display an excess surface resistance below Tc which is neither observed in low Tc nor in high temperature superconductors (HTSC). The real part of the complex conductivity, sigma1, shows a huge maximum below Tc and the imaginary part, sigma2, is linear for temperatures less than 20 K, which can not be simply accounted for by the weak coupling BCS model with an s-wave superconducting order parameter. We speculate that this may be due to the two gaps reported by other studies. Unlike measurements on the high temperature superconducting cuprates, we find no evidence of weak-links in the superconducting state. By inverting the magnetic field dependent impedance data, we find a vortex depinning frequency that decreases with increasing magnetic field and evidence for an anisotropic upper critical magnetic field.

cond-mat.supr-con↗

arXiv2001

X-ray Photoemission Study of the Infinite-Layer Cuprate Superconductor Sr0.9La0.1CuO2

The electron-doped infinite-layer superconductor Sr0.9La0.1CuO2 is studied with x-ray photoemission spectroscopy (XPS). A nonaqueous chemical etchant is shown to effectively remove contaminants and to yield surfaces from which signals intrinsic to the superconductor dominate. These data are compared to measurements from hole-doped La1.85Sr0.15CuO4, from undoped La2CuO4, and from electron-doped Nd1.85Ce0.15CuO4-d. The Cu 2p core level is consistent with a lower value of the O 2p to Cu 3d charge transfer energy D than in hole-doped cuprates. A clear Fermi edge is observed in the valence band region.

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

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

Pressure dependent thermoelectric power of MgB$_2$ superconductor

We have measured temperature dependence of thermoelectric power (TEP) on MgB$_2$ superconductor under hydrostatic pressure. The sign and temperature dependence of TEP shows metallic hole carriers are dominant with activation type contribution at higher temperature. TEP increases with pressure while $T_c$ decrease with ratio -0.136 K/kbar. The data are discussed in consideration of carriers from different bands and anisotropy of compressibility.

cond-mat.str-el↗

arXiv2001

X-ray Photoemission Study of MgB2

A c-axis oriented thin film and a high density sintered pellet of MgB2 have been studied by x-ray photoemission spectroscopy, and compared to measurements from MgO and MgF2 single crystals. The as-grown surface has a layer which is Mg-rich and oxidized, which is effectively removed by a nonaqueous etchant. The subsurface region of the pellet is Mg-deficient. This nonideal near-surface region may explain varied scanning tunneling spectroscopy results. The MgB2 core level and Auger signals are similar to measurements from metallic Mg and transition metal diborides, and the measured valence band is consistent with the calculated density of states.

cond-mat.supr-con↗

arXiv2001

Optical phonons of superconducting infinite-layer compounds Sr_{0.9}Ln_{0.1}CuO_2 (Ln = La, Gd, Sm)

We have identifies the otical phonon modes of the superconducting infinite-layer compounds Sr$_{0.9}Ln_{0.1}$CuO$_2$ ($Ln$ = La, Gd, Sm; $T_c$ = 43 K for all) from their infrared reflectivity spectra obtained with a Fourier-transform infrared spectrometer on high-quality high-purity samples synthesized under high pressure. The La- and the Gd-doped compounds exhibited only four (2$A_{\rm 2u}$+2$E_{\rm u}$) out of the five (2$A_{\rm 2u}$+3$E_{\rm u}$) infrared-active phonon modes predicted by a group theoretical analysis whereas the Sm-doped compound exhibited all five modes. We propose the atomic displacement pattern for each phonon mode based on reported lattice dynamics calculations and through comparison with the phonon modes of other single-layer high-$T_{\rm c}$ cuprate superconductors.

cond-mat.supr-con↗