Bongjoo Kang

We report on specific-heat, high-magnetic-field transport, and ac-susceptibility measurements on ${\mathrm{MgB}}_{2}$ single crystals. The upper critical field for magnetic fields perpendicular and parallel to the basal planes is presented in the entire temperature range. A very different temperature dependence has been observed in the two directions, which yields a temperature-dependent anisotropy with $\ensuremath{\Gamma}\ensuremath{\sim}5$ at low temperatures and $\ensuremath{\sim}2$ near ${T

The temperature dependence of the upper (H(c2)) and lower (H(c1)) critical fields has been deduced from Hall probe magnetization measurements of high quality MgB2 single crystals along the two main crystallographic directions. We show that Gamma(H(c2))=H(c2 axially ab)/H(c2 axially c) and Gamma(H(c1))=H(c1 axially c)/H(c1 axially ab) differ significantly at low temperature (being approximately 5 and approximately 1, respectively) and have opposite temperature dependencies. We suggest that MgB2 c

The superconducting phase diagram of ${\mathrm{MgB}}_{2}$ was determined from magnetization, magnetotransport, and single-crystal specific-heat measurements. A zero-temperature in-plane coherence length of 9.4 nm is determined. The superconducting anisotropy \ensuremath{\gamma} increases from a value around 2 near ${T}_{c}$ to above 4.5 at 22 K. For $H\ensuremath{\parallel}c$ a pronounced peak effect in the critical current occurs at the upper critical field. Evidence for a surface superconducti

The angular dependence of the upper critical field ${H}_{c2}$ of ${\mathrm{MgB}}_{2}$ single crystals is studied at various temperatures by means of specific-heat and transport measurements in magnetic fields up to $17\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Clear deviations from Ginzburg--Landau behavior are observed at all temperatures and are explained by two-band effects. The angular dependence and temperature dependence of the deviations are in qualitative agreement with theoretical predicti

We demonstrate direct evidence of possible surface superconductivity on small, well-shaped ${\mathrm{MgB}}_{2}$ single crystals. Transport measurements in the range $H<{1.6H}_{c2}^{c},$ where ${H}_{c2}^{c}$ is the bulk upper critical field for the c axis, show non-Ohmic and strongly angular-dependent resistivity. Studies of the alignment of $\mathbf{H}$ with selected crystal surfaces, transport and specific heat measurements on the same crystal, and a physical sculpturing of the crystal surfa

The lower $({H}_{c1})$ and upper $({H}_{c2})$ critical fields of ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2}$ single crystals (for $x=0$, $0.1$, and $\ensuremath{\gtrsim}0.2$) have been deduced from local magnetization and specific heat measurements, respectively. We show that ${H}_{c1}$ and ${H}_{c2}$ are both decreasing with increasing doping content. The corresponding anisotropy parameter ${\ensuremath{\Gamma}}_{{H}_{c2}}(0)={H}_{c2}^{ab}(0)∕{H}_{c2}^{c}(0)$ value also

The first penetration field $({H}_{p})$ has been deduced from local magnetization and specific heat measurements in magnesium diboride single crystals. For ${H}_{a}^{\ensuremath{\Vert}c}$, the geometrical barriers (GB) play a dominant role in the irreversibility mechanism. In thin samples, neglecting the GB in this direction would then lead to a large overestimation of ${H}_{c1}$ deduced from ${H}_{p}$ through the standard elliptical formula. The lower critical field is found to be isotropic at

The upper critical field Hc2 for the fields perpendicular and parallel to Mg and B planes of the magnesium diboride is presented in the temperature range from 5.4 K up to Tc as obtained by comparison of high-field magnetotransport, ac-susceptibility and specific heat measurements. The onset of the finite resistivity yields Hc2, while the end of the resistive transition to the normal state is due to surface effects. Hc2 perpendicular to the planes reveals a conventional temperature dependence wit

The anisotropic field dependence of the Sommerfeld coefficient gamma has been measured down to B-->0 by combining specific heat and Hall probe magnetization measurements in MgB2 single crystals. We find that gamma(B,theta) is the sum of two contributions arising from the sigma and pi band, respectively. We show that gammasigma(B,theta)=B/Bc2(theta) where Bc2(theta)=Bc2ab/sqrt[sin2theta+Gamma2cos2theta] with Gamma approximately 5.4 (theta being the angle between the applied field and the c axis)

The angular dependence of the upper critical field Hc2 of MgB2 single crystals is studied at various temperatures by means of specific heat and transport measurements in magnetic fields up to 17 T. Clear deviations from Ginzburg-Landau behavior are observed at all temperatures and are explained by two-band effects. The angular- and temperature dependence of the deviations are in qualitative agreement with theoretical predictions based on band-structure calculations. Quantitative agreement is obt