Changhyuk Choi

Lithium phosphorous oxynitride (Lipon) films were deposited in gas atmosphere with different radio-frequency magnetron sputtering power from 80 to 160 W with 20 W step increase. Lipon films deposited at lower sputtering power showed higher ionic conductivities than the films deposited at higher sputtering power. The results of impedance measurements showed that nitrogen incorporation into the glass structure increased the ionic conductivity and this nitrogen content in the Lipon films increased

Very-low-energy (\ensuremath{\approxeq}28 eV), high-flux (\ensuremath{\approxeq}0.4 mA/${\mathrm{cm}}^{2}$) Ar-ion irradiation during molecular-beam epitaxy changed the nucleation of GaAs on Si(100) from Stranski-Krastanov to a mechanism approaching layer-by-layer growth. While three-dimensional island nucleation was eliminated, the growth surface exhibited low-amplitude undulations. The results are explained by ion-induced removal of atoms from stable 3D islands, which suppressed 3D island nucl

An impurity in an unconventional superconductor perturbs the order parameter out to distances of order \ensuremath{\xi}(T). When the order parameter is ``ferromagnetic'' in character a localized flow of supercurrent is also induced; this supercurrent creates a magnetic field at the site of the impurity. We discuss these supercurrents, and estimate the size of the resulting magnetic field.

We observed the growth of epitaxial Al(111) films on Si(111) at room temperature by the partially ionized beam deposition technique. The films were deposited in a conventional vacuum condition without in situ cleaning. The beam contained 0.3% of Al self-ions and a bias potential of 1 kV was applied to the substrate during deposition. X-ray diffraction (pole figure) revealed that one of the two possible twin structures, with the Al〈1̄10〉∥Si〈1̄10〉 orientation, was preferentially grown on the Si su

Nucleation and growth of InAs on Si(100) substrates during ion-assisted deposition has been investigated. Reflection high-energy electron diffraction studies showed that InAs films nucleated by the Stranski–Krastanov (SK) mechanism. Ar ion irradiation with energies E near 30 eV and a flux of 4 ions per deposited atom prolonged the layer-by-layer stage of SK nucleation. The transition from layer-by-layer to island growth was at 2–3 monolayers (ML) for growth temperature Ts=380 °C and E=13 eV. Wit

We consider the superfluid density tensor ${\ensuremath{\rho}}_{s}$ in both singlet and triplet unconventional superconductors. We develop theoretical expressions for ${\ensuremath{\rho}}_{s}$ which incorporate both band-structure effects and impurity scattering; these expressions simplify considerably in the Ginzburg-Landau region close to ${T}_{c}$. We also present the results of numerical calculations showing ${\ensuremath{\rho}}_{s}$ as a function of temperature for several different unconve

We study the upper critical field for various possible order parameters of a heavy-fermion superconductor. We pay particular attention to the question of how the anisotropy of the upper critical field changes by effects of paramagnetism, impurity scattering, and Fermi-surface anisotropy in the presence of strong spin-orbit coupling. The paramagnetic limit can have a dramatic effect on the anisotropy in odd-parity superconductors, and provide important information on the symmetry of underlying or

We consider the superfluid density tensor ${\ensuremath{\rho}}_{s}$ for two unconventional spin-triplet gaps: an A phase and a polar phase. We pay particular attention to the effect of impurity scattering and how such scattering influences the anisotropy of ${\ensuremath{\rho}}_{s}$.

The nucleation mechanism and crystalline perfection of InAs on Si(100) were found to depend strongly on the substrate bias voltage Vs used to accelerate Ar ions to the growing film during triode ion-assisted deposition. Increasing the Vs value used during the first ∼10 monolayers (ML) of growth from 15 to 30 V decreased the x-ray diffraction rocking curve full width at half maximum (FWHM) by a factor of ∼2. Further increases in Vs yielded an increase in the FWHM. Reflection high-energy electron

We discuss the Ginzburg-Landau gradient-free-energy density for unconventional superconductors. In particular, we consider a certain term related to the intrinsic angular momentum of the Cooper pairs. The coefficient of this term vanishes in weak-coupling theory for a pure metal; as pointed out by Gorkov, it can acquire a nonzero value when impurity scattering is present. We display calculations of this coefficient for both even- and odd-parity gaps using the quasiclassical theory of superconduc

An impurity in an unconventional superconductor can perturb the order parameter out to a distance of order the coherence length. This can lead to a large change in the density of states near an impurity. We compute the density of states as a function of position and frequency for a two-dimensional d-wave order parameter, while taking into account the effect of impurity scattering both in the Born and unitarity limits. We pay particular attention to the effect of anisotropy of the order parameter

An impurity in an unconventional superconductor strongly perturbs the order parameter out to distances of order \ensuremath{\xi}(T); when the order parameter breaks time-reversal symmetry, a pattern of supercurrents J(R) is set up in the vicinity of the impurity. In this paper we address the question of whether J(R) integrates up to produce a net magnetic moment. In a wide variety of situations the moment vanishes, if the perturbation in the order parameter goes to zero rapidly enough at large d

Suppression of three-dimensional (3D) island nucleation during growth of InAs on Si (100), achieved by using very low energy, high-flux Ar ion irradiation, reduced planar defect densities. For 13 eV ion irradiation, 3D islands nucleated after ∼2 monolayers (ML) of deposition, similar to conventional molecular beam epitaxy. High-resolution transmission electron microscopy studies of nominally 18-ML-thick films showed 3D InAs islands with {111} facets. A high density of {111} twins and stacking fa

The nucleation and defect structure of GaAs films grown on Ge(001) by molecular-beam epitaxy (MBE) and ion-assisted MBE (IAMBE) were studied using reflection high-energy electron diffraction and transmission electron microscopy. Three-dimensional (3D) islands nucleated during the initial stages of MBE growth at 580 °C, and continued growth led to nonplanar surfaces and a high density of stacking faults and twins in the films. Irradiation by 33 eV Ar ions during IAMBE growth at 580 °C suppressed