Leggett's plasma resonances and two-gap structures in the CVCs of MgB${}_2$ break junctions $-$ a direct evidence for a two-gap superconductivity in MgB${}_2$

In the present investigation (2002) the current-voltage characteristics (CVCs) of break junctions in polycrystalline $Mg{B}_2$ samples have been studied in the temperature range $4.2K<T<T_c$. An inelastic Cooper pair tunneling accompanied by the emission of Leggett plasmons has been found for the first time in $Mg{B}_2$ Josephson contacts. A fine structure in the CVCs of $Mg{B}_2$ Andreev point contacts caused by a resonant emission of Leggett plasmons has been observed for the first time. An energy of the Leggett plasmon has been estimated: $E_0$ ~ 4 meV. Distinct two-gap structures have been found in the CVCs of $Mg{B}_2$ tunneling contacts and $Mg{B}_2$ Andreev point contacts with 7.5 meV $<{\mathrm{\Delta }}_L<$ 11 meV and 1.5 meV $<{\mathrm{\Delta }}_S<$ 2.5 meV. An intrinsic tunneling effect (ITE) and intrinsic multiple Andreev reflections effect (IMARE) have been observed due to the layered structure of $Mg{B}_2$. The obtained experimental results give a direct evidence for a two-gap superconductivity in $Mg{B}_2$. (“V. Tarasov’s Readings”, November 2002, Russia, Moscow, conference proceedings.)