arXiv subjects

F. Giubileo

Publications and source records attributed to F. Giubileo.

21 records · Page 2Linked to original sources

arXiv2004

Two regimes in the magnetic field response of superconducting MgB$_2$

Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the $π$-band of the two-band superconductor MgB$_2$. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value : The gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the $π$-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the $σ$-band. Our findings allow a deeper understanding of the unique phase diagram of MgB$_2$.

cond-mat.supr-con↗

arXiv2001

Two Gap State Density in MgB$_{2}$: A True Bulk Property or A Proximity Effect?

We report on the temperature dependence of the quasiparticle density of states (DOS) in the simple binary compound MgB2 directly measured using scanning tunneling microscope (STM). To achieve high quality tunneling conditions, a small crystal of MgB2 is used as a tip in the STM experiment. The ``sample'' is chosen to be a 2H-NbSe2 single crystal presenting an atomically flat surface. At low temperature the tunneling conductance spectra show a gap at the Fermi energy followed by two well-pronounced conductance peaks on each side. They appear at voltages V$_{S}\simeq \pm 3.8$ mV and V$_{L}\simeq \pm 7.8$ mV. With rising temperature both peaks disappear at the Tc of the bulk MgB2, a behavior consistent with the model of two-gap superconductivity. The explanation of the double-peak structure in terms of a particular proximity effect is also discussed.

cond-mat.supr-con↗

arXiv2001

Strong Coupling and Double Gap Density of States in Superconducting MgB2

Using scanning tunneling spectroscopy at T = 4.2 K, we perform simultaneously the topographic imaging and the quasiparticle density of states (DOS) mapping in granular MgB2. We observe a new type of spectrum, showing a pronounced double gap, with the magnitudes of Delta_S = 3.9 meV and Delta_L = 7.5 meV, i.e. well below and well above the BCS limit. The largest gap value gives the ratio 2*Delta_L/k_B T_c = 4.5, which implies strong electron-phonon coupling. Other superconducting regions are found to have a characteristic BCS-shaped DOS. However, the variation of the spectral shape and lower gap widths, from 2.0 meV to 6.5 meV, indicate the importance of surface inhomogeneity and proximity effects in previously published tunneling data. Our finding gives no evidence for any important gap anisotropy. Instead, it strongly supports the multiple gap scenario in MgB2 in the clean limit, and the single gap scenario in the dirty limit.

cond-mat.supr-con↗