Disorder-induced multifractal superconductivity in monolayer niobium dichalcogenides

Nature Physics 15, 904 (2019)

The interplay between disorder and superconductivity is a subtle and fascinating phenomenon in quantum many-body physics. Conventional superconductors are insensitive to dilute non-magnetic impurities, known as Anderson’s theorem. Destruction of superconductivity and even superconductor–insulator transitions occur in the regime of strong disorder. Hence, disorder-enhanced superconductivity is rare and has been observed only in some alloys or granular states. Owing to the entanglement of various effects, the mechanism of enhancement is still under debate. Here, we report a well-controlled disorder effect in the recently discovered monolayer NbSe₂ superconductor. The superconducting transition temperatures of NbSe₂ monolayers are substantially increased by disorder. Realistic theoretical modelling shows that the unusual enhancement possibly arises from the multifractality of electron wavefunctions. This work provides experimental evidence of the multifractal superconducting state.

Stripes developed at the strong limit of nematicity in FeSe film

Nature Physics 13, 957 (2017)

A single monolayer of iron selenide grown on strontium titanate shows an impressive enhancement of superconductivity compared with the bulk, as well as a novel Fermi surface topology, extreme two-dimensionality, and the possibility of phonon-enhanced electron pairing. For films thicker than one unit cell, however, the electronic structure is markedly different, with a drastically suppressed superconductivity and strong nematicity appearing. The physics driving this extraordinary dichotomy of superconducting behaviour is far from clear. Here, we use low-temperature scanning tunnelling microscopy to study multilayers of iron selenide grown by molecular beam epitaxy, and find a stripe-type charge ordering instability that develops beneath the nematic state. The charge ordering is visible and pinned in the vicinity of impurities. And as it emerges in the strong limit of nematicity, it suggests that a magnetic fluctuation with a rather small wavevector may be competing with the ordinary collinear antiferromagnetic ordering in multilayer films. The existence of stripes in iron-based superconductors, which resemble the stripe order in cuprates, not only suggests that electronic anisotropy and correlation are playing an important role, but also provides a platform for probing the complex interactions between nematicity, charge ordering, magnetism and superconductivity in high-temperature superconductors

Charge ordering in stoichiometric FeTe: Scanning tunneling microscopy and spectroscopy 

Physical Review B 93, 041101(R) (2016)

We use scanning tunneling microscopy and spectroscopy to reveal a unique stripy charge order in a parent phase of iron-based superconductors in stoichiometric FeTe epitaxy films. The charge order has unusually the same period as the spin order. We also found highly anisotropic electron band dispersions being large and little along the ferromagnetic and antiferromagnetic directions, respectively. Our data suggest that the microscopic mechanism is likely of the Stoner type driven by interatomic Coulomb repulsion. The Coulomb repulsion and charge fluctuations, so far much neglected, are important to the understanding of iron-based superconductors.  

Superconductivity in a single-layer alkali-doped FeSe: A weakly coupled two-leg ladder system 

Physical Review B 88, 140506(R) (2013)

We prepare single-layer potassium-doped iron selenide (110) film by molecular-beam expitaxy. Such a single layer film can be viewed as a two-dimensional system composed of weakly coupled two-leg iron ladders. Scanning tunneling spectroscopy reveals that superconductivity is developed in this two-leg ladder system. The superconducting gap is similar to that of the multilayer films. However, the Fermi-surface topology in this quasi-one-dimensional system is remarkably different from that of the bulk materials. Our results suggest that superconducting pairing is very short ranged or takes place rather locally in iron chalcogenides. The superconductivity is most likely driven by electron-electron correlation effect and is insensitive to the change of Fermi surfaces.

KFe₂Se₂ is the parent compound of K-doped iron selenide superconductors 

Physical Review Letters 109, 057003 (2012)

Phase separation and magnetic order in K-doped iron selenide superconductor 

Nature Physics 8, 126 (2012)

The stoichiometric KFe₂Se₂ with √2x√2 charge ordering was identified as the parent compound of K_xFe_2-ySe_2-z superconductor using scanning tunneling microscopy and spectroscopy. The superconductivity is induced in KFe₂Se₂ by either Se vacancies or interacting with the antiferromagnetic K₂Fe₄Se₅ compound. In total, four phases were found to exist in K_xFe_2-ySe_2-z: parent compound KFe₂Se₂, superconducting KFe₂Se₂ with √2x√5 charge ordering, superconducting KFe₂Se_2-z with Se vacancies, and insulating K₂Fe₄Se₅ with √5x√5 Fe vacancy order. Interfacing with K₂Fe₄Se₅ induces superconductivity in the parent compound KFe₂Se₂. Measurement of the magnetic field dependence of the Fe-vacancy induced bound states reveals a magnetically related bipartite order in the tetragonal iron lattice of KFe₂Se₂. These findings elucidate the existing controversies  in K-doped iron selenide superconductors and provide atomistic information on the interplay between magnetism and superconductivity in iron-based superconductors.

Direct observation of nodes and twofold symmetry in FeSe superconductor 

Science 332, 1410 (2011)

We investigated the electron-pairing mechanism in an iron-based superconductor, iron selenide (FeSe), using scanning tunneling microscopy and spectroscopy. Tunneling conductance spectra of stoichiometric FeSe crystalline films in their superconducting state revealed evidence for a gap function with nodal lines. Electron pairing with twofold symmetry was demonstrated by direct imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms, and Se vacancies. The twofold pairing symmetry was further supported by the observation of striped electronic nanostructures in the slightly Se-doped samples. The anisotropy can be explained in terms of the orbital-dependent reconstruction of electronic structure in FeSe.

Superconductivity in one-atomic-layer metal films grown on Si(111) 

Nature Physics 6, 104 (2010)

The two-dimensional (2D) superconducting state is a fragile state of matter susceptible to fluctuations. Although superconductivity has been observed in ultrathin metal films down to a few layers, it is still not known whether a single layer of ordered metal atoms, which represents the ultimate 2D limit of a crystalline film, could be superconducting. Here we report scanning tunneling microscopy measurements on single atomic layers of Pb and In grown epitaxially on Si(111) substrate, and demonstrate unambiguously that superconductivity does exist at such a 2D extreme. The film shows a superconducting transition temperature of 1.83 K for SIC-Pb, 1.52 K for √7x√3-Pb and 3.18 K for √7x√3-In, respectively. We confirm the occurrence of superconductivity by the presence of superconducting vortices under magnetic field.

High-resolution tunneling spectroscopy of magnetic impurity induced bound states in the superconducting gap of Pb thin films 

Physical Review Letters 100, 226801 (2008)

Tunneling spectra for individual atoms and dimers of Mn and Cr adsorbed on superconducting Pb thin films were measured by a low temperature scanning tunneling microscope. Multiple-resonance structures within the superconducting gap on the adsorbates were resolved and interpreted as the magnetic impurity induced bound states associated with different scattering channels. The experiment demonstrates a spectroscopic approach to characterizing the spin states of magnetic structures and exploring the competition between superconductivity and magnetism at the nanometer scale. Recently, this work has been given much attention in searching Majorana states.