Sub-Kelvin scanning tunneling spectroscopy in the Chevrel phases SnMo6S8 and PbMo6S8 reveals two distinct superconducting gaps with Δ1=3 meV, Δ2∼1.0 meV and Δ1=3.1 meV, Δ2∼1.4 meV, respectively. The gap distribution is strongly anisotropic, with Δ2 predominantly seen when scanning across unit-cell steps on the (001) sample surface. The spectra are well fitted by an anisotropic two-band BCS s-wave gap function. Our spectroscopic data are confirmed by electronic heat capacity measurements, which also provide evidence for a twin-gap scenario.
Using scanning tunnelling microscopy at 400 mK, we have obtained maps of around 100 vortices in SnMo6S8 from 2 -9 T. The orientational and positional disorder at 5 and 9 T show that these are the first large-scale images of a vortex glass. At higher temperature a magnetisation peak effect is observed, whose upper boundary coincides with a lambda anomaly in the specific heat. Our data favour a kinetic glass description of the vortex melting transition, indicating that vortex topological disorder persists at fields and temperatures far below the peak effect in low-Tc superconductors.PACS numbers: 74.25. Qt, 74.70.Dd Twenty years after the first attempt to produce a generalised vortex phase diagram for type-II superconductors, there is still no general consensus on this subject. In particular, confusion remains over the nature and topology of the "vortex glass" phase [1, 2, 3] and its relation to the peak effect observed in DC magnetisation and AC susceptibility of numerous type-II systems. It has been claimed that the peak effect in low-T c materials is associated with the transition from a Bragg glass to a vortex glass [4]. Other peak effect interpretations include the elastic lattice softening model [5] and a multi-dynamic vortex liquid scenario [6]. However, recent studies of Nb 3 Sn do not give any indication of a phase transition from a Bragg glass to an intermediate disordered state within the peak regime: instead, the peak effect is interpreted as arising from the metastability of an underlying first-order vortex melting transition [7,8]. It may therefore be considered as a zone dominated by strong thermal fluctuations and consequentially enhanced pinning.Extended real-space vortex imaging is the best method of clarifying the extent of disorder in the (H,T ) phase diagram. However, it remains a considerable challenge, with the inherent difficulties varying with the choice of superconductor. In low-T c materials the disordered vortex phase typically spans a narrow window of phase space, thus limiting experimental accessibility [7]. Images of the disordered phase have only been obtained in NbSe 2 , where magnetic decoration reveals static disorder [9] and scanning tunnelling microscopy (STM) in the peak effect regime displays a crossover from collective vortex motion to positional fluctuations [10]. In contrast, disorder occupies a far greater portion of phase space in high-T c s due to their small coherence volumes and hence increased influence of thermal fluctuations. Unfortunately high vortex mobility severely complicates the detection of any stable high-field vortex solid in these compounds [11].The Chevrel phase SnMo 6 S 8 is an attractive system to investigate since its extremely short coherence length ξ ∼ 3 nm lies close to those of the high-T c s, suggesting that disordered zones of its phase diagram may be more extensive and easily experimentally accessible. An additional advantage is its quasi-3D crystal structure which should help to stabilise any disordered solid phase against melting at non-ze...
Low dimensionality, broken symmetry and easily-modulated carrier concentrations provoke novel electronic phase emergence at oxide interfaces. However, the spatial extent of such reconstructions - i.e. the interfacial “depth” - remains unclear. Examining LaAlO3/SrTiO3 heterostructures at previously unexplored carrier densities n2D ≥ 6.9 × 1014 cm−2, we observe a Shubnikov-de Haas effect for small in-plane fields, characteristic of an anisotropic 3D Fermi surface with preferential dxz,yz orbital occupancy extending over at least 100 nm perpendicular to the interface. Quantum oscillations from the 3D Fermi surface of bulk doped SrTiO3 emerge simultaneously at higher n2D. We distinguish three areas in doped perovskite heterostructures: narrow (<20 nm) 2D interfaces housing superconductivity and/or other emergent phases, electronically isotropic regions far (>120 nm) from the interface and new intermediate zones where interfacial proximity renormalises the electronic structure relative to the bulk.
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