The interface between transition metal compounds provides a rich playground for emergent phenomena. Recently, significantly enhanced superconductivity has been reported for single-layer FeSe on Nb-doped SrTiO 3 substrate. Yet it remains mysterious how the interface affects the superconductivity. Here we use in situ angle-resolved photoemission spectroscopy to investigate various FeSe-based heterostructures grown by molecular beam epitaxy, and uncover that electronic correlations and superconducting gap-closing temperature (T g ) are tuned by interfacial effects. T g up to 75 K is observed in extremely tensile-strained single-layer FeSe on Nb-doped BaTiO 3 , which sets a record high pairing temperature for both Fe-based superconductor and monolayer-thick films, providing a promising prospect on realizing more cost-effective superconducting device. Moreover, our results exclude the direct correlation between superconductivity and tensile strain or the energy of an interfacial phonon mode, and highlight the critical and non-trivial role of FeSe/oxide interface on the high T g , which provides new clues for understanding its origin.

Single-layer FeSe films with extremely expanded in-plane lattice constant of 3.99±0.02 Å are fabricated by epitaxially growing FeSe/Nb:SrTiO 3 /KTaO 3 heterostructures, and studied by in situ angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillion zone corner are resolved with negligible hybridization between them, indicating the symmetry of the low energy electronic structure remains intact as a free-standing single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70 K for the iron based superconductors. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results put strong constraints on the current theories, and support the coexistence of both even and odd parity spin-singlet pairing channels as classified by the lattice symmetry. [3][4][5]. For these systems, weak coupling theories based on spin-fluctuations predict a dwave pairing symmetry [6,7]. However, it is inconsistent with the isotropic superconducting gap observed by angle resolved photoemission spectroscopy (ARPES) [2,3,8,9], together with evidences for nodeless superconducting gap from specific heat [10], nuclear magnetic resonance [11], etc. On the other hand, the sign preserving s-wave pairing symmetry [12][13][14][15] could not account for the spin-resonance mode found in Rb x Fe 2−y Se 2 by inelastic neutron scattering [16], which suggests the sign change of the superconducting order parameter on different Fermi surface sections [17].To explain the sign changing isotropic gap in e-FeHTSs, several novel pairing scenarios were proposed. For example, it is argued in the bonding-antibonding s ± pairing scenario that with strong hybridization between electron pockets, the two reconstructed electron pockets can have different signs [18]. A further study suggested that this pairing likely coexists with the d-wave to form an s + id-wave pairing symmetry [19]. More recently, the importance of the parity of the 2-Fe unit cell has been emphasized [20], and it has been proposed that there are even and odd parity s-wave spin singlet pairing states, and the coexistence of both states gives a fully gapped state with varied signs in different Fermi surface sections [21,22]. The hybridization between the two electron pockets is not necessary in this scenario. So far, these scenarios could not be convincingly tested, since the detailed structure of the two electron pockets could not be resolved in all known e-FeHTSs.Two recent ARPES studies have found a gap in singlelayer FeSe/STO, which closes at 65 K and suggests a possible record high superconducting transition temperature (T c ) of 65 K for FeHTSs [4,5]; or at least, it is the pair-formation temperature record, if the superconducting transition there is a two dimensional Berezinskii-Kosterlitz-Thouless (BKT) type. Particularly, our previous ARPES study has found that the high T c in single-layer FeSe/STO is in...

Monolayer FeSe thin film grown on SrTiO 3 (001) (STO) shows the sign of T c > 77 K, which is higher than the T c -record of 56 K for the bulk FeAs-based superconductors. However, little is known about the magnetic ground state of FeSe, which should be closely related to its unusual superconductivity. Previous studies presume the collinear stripe antiferromagnetic (AFM) state as the ground state of FeSe, same to that in FeAs superconductors. Here we find a novel magnetic order named "pair-checkboard AFM" as the magnetic ground state of tetragonal FeSe. The novel pair-checkboard order results from the interplay between the nearest, the next-nearest and the unnegligible next-next-nearest neighbor magnetic exchange couplings of Fe atoms. The monolayer FeSe in pair-checkbord order shows an unexpected insulating behavior with a Dirac-cone-like band structure related to the specific orbital order of d xz and d yz characters of Fe atoms, which could explain recently observed insulatorsuperconductor transition. The present results cast new insights on the magnetic ordering in FeSe monolayer and its derived superconductors. The high temperature (high-T c ) superconductivity discovered in the iron-based superconductors [1-3] breaks the conventional knowledge that the magnetic atoms like Fe should not contribute to the superconductivity. This inspires that the magnetism plays an important role in the mechanism of the high-T c superconductivity in iron-based superconductors [4]. Although the electronic properties for different families of iron-based superconductors can be somehow different [5], they all are believed to share the common feature of AFM ordered parent compound [6]. While the magnetism contributing to the high-Tc superconductivity has attracted wide attention [7], the magnetic ground states for the parent compounds of iron-based superconductors remain unclear. Recently, the sign of over 77 K unconventional high T c superconductivity [8-11] has been observed in monolayer FeSe grown on STO substrate [12-17],which is much higher than the highest T c record in the intensively studied FeAs systems [18,19]. For FeAs-based materials, the collinear AFM (or the stripe AFM) has been verified as the ground state for the parent compounds by neutron scattering [7]. However, the ground state for the compound based on FeSe is still waiting to be clarified. Previous theoretical Supplemental Material Figure S4 [38]). The properties of the predicted gapped insulating ground state are in good agreement with the recent experimental observations [17,39,40].The pair-checkboard AFM order in FeSe shed new lights on the understanding of high-T c superconductivity in FeSe monolayer on the oxides substrates and FeSe-layer derived superconductors. The novel pair-checkboard AFM order we predicted would call more direct experiments for investigating the magnetic properties in high-quality FeSe samples.

Recently, the signs of both superconducting transition temperature (T c ) beyond 60 K and spin density wave (SDW) have been observed in FeSe thin film on SrTiO 3 (STO) substrate, which suggests a strong interplay between superconductivity and magnetism. With the first-principles calculations, we find that the substrate-induced tensile strain tends to stabilize the SDW state in FeSe thin film by enhancing of the next-nearest-neighbor superexchange antiferromagnetic interaction bridged through Se atoms. On the other hand, we find that when there are oxygen vacancies in the substrate, the significant charge transfer from the substrate to the first FeSe layer would suppress the magnetic order there, and thus the high-temperature superconductivity could occur.In addition, the stability of the SDW is lowered when FeSe is on a defect-free STO substrate due to the redistribution of charges among the Fe 3d-orbitals. Our results provide a comprehensive microscopic explanation for the recent experimental findings, and build a foundation for the further exploration of the superconductivity and magnetism in this novel superconducting interface.

Using non-equilibrium molecular dynamics method(NEMD), we have found that the thermal conductivity of multilayer graphene nanoribbons monotonously decreases with the increase of the number of layers, such behavior can be attributed to the phonon resonance effect of out-of-plane phonon modes. The reduction of thermal conductivity is found to be proportional to the layer size, which is caused by the increase of phonon resonance. Our results are in agreement with recent experiment on dimensional evolution of thermal conductivity in few layer graphene.