Spin-resolved spectra of Shiba multiplets from Mn impurities in MgB_2 (original) (raw)
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Shiba multiplets due to Mn impurities in MgB_2
We study the effect of magnetic Mn ions on the two-band superconductor MgB 2 , and compute both the total and spin resolved scanning tunneling spectrum in the vicinity of the magnetic impurity. We show that when the internal structure of the Mn ion's d-shell is taken into account, multiple Shiba states appear in the spectrum. These multiplets were missed by previous calculations based on simplistic models, and their presence could alter significantly the overall interpretation of local tunneling spectra for a wide range of superconducting hosts and magnetic impurities.
Scanning tunneling spectroscopy in MgB2
Physica C: Superconductivity, 2003
We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter Γ. The value of the superconducting gap is 5.2 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair-breaking as seen in the conductance spectra in fields up to 6 T.
Momentum-dependent scanning tunneling spectroscopy in MgB2
Physica C: Superconductivity, 2003
We present study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy. The results reveal two distinct energy gaps at ∆ 1 =2.3 meV and ∆ 2 =7.1 meV. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multi-band system. Our experimental observations are consistent with the existence of two-band superconductivity in the presence of interband superconducting pair interaction and quasiparticle scattering. Temperature evolution of the tunneling spectra follows the BCS scenario with both gaps vanishing at the bulk Tc. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB 2 proposed in the multigap model by Liu et al. [1]
Scanning tunneling spectroscopy on single crystal MgB2
Physica C: Superconductivity, 2003
We report on the results of scanning tunneling spectroscopy measurements on single crystals of MgB 2 . Tunneling was performed both parallel and perpendicular to the crystalline c-axis. In the first case, a single superconducting gap (∆ π = 2.2 meV) associated with the π-band is observed. Tunneling parallel to the ab-plane reveals an additional, larger gap (∆ σ ∼ 7 meV) originating in the highly two-dimensional σband. Vortex imaging in the π-band was performed with the field and tunnel current parallel to the c-axis. The vortices have a large core size compared to estimates based on H c2 , and show an absence of localized states in the core. Furthermore, superconductivity between the vortices is rapidly suppressed by an applied field. A comparison to specific heat measurements is performed.
Two regimes in the magnetic field response of superconducting MgB2
The European Physical Journal B, 2007
Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the π-band of the two-band superconductor MgB2. 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 MgB2.
Physical Review Letters, 2006
We present the first results of directional point-contact measurements in Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3 K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model. Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the junctions T_c^A, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principle band structure calculations.
Low temperature physics, 2019
The systematic study of the nanoscale local electronic states on the MgB 2 surface was performed using the low-temperature scanning tunnel microscopy/spectroscopy (STM/STS). The STM topography shows the atomic image of the hexagonal lattice with the constant parameter a′ = 0.31 nm, which is identified as mainly the Mg site occupancy. The temperature-dependent STS measurements were analyzed assuming the existence of two energy gaps. As a result, the fitting gap amplitudes ∆ fit 10.2 meV and 4.8 meV were found at T = 4.9 K. The scanned conductance (dI / dV) maps in the area of 4×2 nm 2 show homogenous distributions of the gaps associated with the π-band. In addition, the conductance peaks at zero-bias voltage were observed through defined lines with lengths about ∼0.8 nm, which is much smaller than the superconducting coherence length ξ ab ~ 40 nm of MgB 2. The form of the zero-bias peaks looks like that in the case of the Andreev-Saint-James reflection at the tip-sample contact.
Two-Band Superconductivity in MgB2
Physical Review Letters, 2002
The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at ∆1=2.3 meV and ∆2=7.1 meV. Different spectral weights of the partial superconducting density of states (PDOS) are a reflection of different tunneling directions in this multi-band system. Our experimental observations are consistent with the existence of two-band superconductivity in the presence of interband superconducting pair interaction and quasiparticle scattering. Temperature evolution of the tunneling spectra follows the BCS scenario [1] with both gaps vanishing at the bulk Tc. Indeed, the study of tunneling junctions exhibiting only the small gap (c-axis tunneling) clearly and reproducibly show that this gap persists up to the bulk Tc. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. .
Journal of Superconductivity and Novel Magnetism, 2007
We performed point-contact spectroscopy (PCS) measurements in Mg1−x Mnx B2 single crystals, with x≤0.015 and bulk T c down to 13.3 K. The gaps Δσ and Δπ were obtained by fitting the conductance curves of the point contacts with the two-band Blonder–Tinkham–Klapwijk (BTK) model. Both Δσ and Δπ decrease with the critical temperature of the junctions T c A , but remain clearly distinct down to the lowest critical temperature (T c A ≅9 K). Once analyzed within the Eliashberg theory, the gap trends as a function of T c A can be explained by a doping-induced increase in the pair-breaking scattering within the σ band, with smaller contributions from the π-π or the σ-π channels.
Strong coupling and double-gap density of states in superconducting MgB 2
Europhysics Letters (EPL), 2002
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 ∆S = 3.9 meV and ∆L = 7.5 meV, i.e. well below and well above the BCS limit. The largest gap value gives the ratio 2∆L/kBTc = 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.