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$_2$

TL;DR

This study provides direct experimental evidence of two-gap superconductivity in MgB₂ through detailed analysis of current-voltage characteristics, revealing Leggett plasmons and two distinct superconducting gaps.

Contribution

First observation of Leggett plasmons and two-gap structures in MgB₂ junctions, confirming the two-gap superconductivity model with detailed spectroscopic evidence.

Findings

Detection of Leggett plasmons at ~4 meV

Identification of two superconducting gaps: 7.5-11 meV and 1.5-2.5 meV

Observation of intrinsic tunneling and multiple Andreev reflections

Abstract

In the present investigation (2002) the current-voltage characteristics (CVCs) of break junctions in polycrystalline $MgB_2$ samples have been studied in the temperature range $4.2 K < T < T_c$. An inelastic Cooper pair tunneling accompanied by the emission of Leggett plasmons has been found for the first time in $MgB_2$ Josephson contacts. A fine structure in the CVCs of $MgB_2$ 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 $MgB_2$ tunneling contacts and $MgB_2$ Andreev point contacts with 7.5 meV $< \Delta_L <$ 11 meV and 1.5 meV $< \Delta_S <$ 2.5 meV. An intrinsic tunneling effect (ITE) and intrinsic multiple Andreev reflections effect (IMARE) have been observed due to the layered structure of $MgB_2$. The obtained experimental results give a direct evidence for a two-gap superconductivity in $MgB_2$. ("V. Tarasov's Readings", November 2002, Russia, Moscow, conference proceedings.)