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Register a new userAmbipolar suppression of superconductivity by ionic gating in optimally-doped BaFe2(As,P)2 ultrathin films
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Superconductivity (SC) in the Ba-122 family of iron-based compounds can be controlled by aliovalent or isovalent substitutions, applied external pressure, and strain, the combined effects of which are sometimes studied within the same sample. Most often, the result is limited to a shift of the SC dome to different doping values. In a few cases, the maximum SC transition at optimal doping can also be enhanced. In this work, we study the combination of charge doping together with isovalent P substitution and strain by performing ionic gating experiments on BaFe$_2$(As$_{0.8}$P$_{0.2}$)$_2$ ultrathin films. We show that the polarization of the ionic gate induces modulations to the normal-state transport properties that can be mainly ascribed to surface charge doping. We demonstrate that ionic gating can only shift the system away from the optimal conditions, as the SC transition temperature is suppressed by both electron and hole doping. We also observe a broadening of the resistive transition, which suggests that the SC order parameter is modulated nonhomogeneously across the film thickness, in contrast with earlier reports on charge-doped standard BCS superconductors and cuprates.
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We use inelastic neutron scattering to study temperature and energy dependence of spin excitations in optimally P-doped BaFe2(As0.7P0.3)2 superconductor (Tc = 30 K) throughout the Brillouin zone. In the undoped state, spin waves and paramagnetic spin excitations of BaFe2As2 stem from antiferromagnetic (AF) ordering wave vector QAF= (1/-1,0) and peaks near zone boundary at (1/-1,1/-1) around 180 meV. Replacing 30% As by smaller P to induce superconductivity, low-energy spin excitations of BaFe2(As0.7P0.3)2form a resonance in the superconducting state and high-energy spin excitations now peaks around 220 meV near (1/-1,1/-1). These results are consistent with calculations from a combined density functional theory and dynamical mean field theory, and suggest that the decreased average pnictogen height in BaFe2(As0.7P0.3)2 reduces the strength of electron correlations and increases the effective bandwidth of magnetic excitations.
The Weyl semimetal MoTe$_2$ offers a rare opportunity to study the interplay between Weyl physics and superconductivity. Recent studies have found that Se substitution can boost the superconductivity up to 1.5K, but suppress the Td structure phase that is essential for the emergence of Weyl state. A microscopic understanding of possible coexistence of enhanced superconductivity and the Td phase has not been established so far. Here, we use scanning tunneling microscopy (STM) to study a optimally doped new superconductor MoTe$_{1.85}$Se$_{0.15}$ with bulk Tc ~ 1.5K. By means of quasiparticle interference imaging, we identify the existence of low temperature Td phase with broken inversion symmetry where superconductivity globally coexists. Consistently, we find that the superconducting coherence length, extracted from both the upper critical field and the decay of density of states near a vortex, is much larger than the characteristic length scale of existing dopant derived chemical disorder. Our findings of robust superconductivity arising from a Weyl semimetal normal phase in MoTe$_{1.85}$Se$_{0.15}$, makes it a promising candidate for realizing topological superconductivity.
We establish quasi-two-dimensional thin films of iron-based superconductors (FeSCs) as a new high-temperature platform for hosting intrinsic time-reversal-invariant helical topological superconductivity (TSC). Based on the combination of Dirac surface state and bulk extended $s$-wave pairing, our theory should be directly applicable to a large class of experimentally established FeSCs, opening a new TSC paradigm. In particular, an applied electric field serves as a topological switch for helical Majorana edge modes in FeSC thin films, allowing for an experimentally feasible design of gate-controlled helical Majorana circuits. Applying an in-plane magnetic field drives the helical TSC phase into a higher-order TSC carrying corner-localized Majorana zero modes. Our proposal should enable the experimental realization of helical Majorana fermions.
We perform single- and multi-band Migdal-Eliashberg (ME) calculations with parameters exctracted from density functional theory (DFT) simulations to study superconductivity in the electric-field-induced 2-dimensional hole gas at the hydrogenated (111) diamond surface. We show that according to the Eliashberg theory it is possible to induce a high-T$_{text{c}}$ superconducting phase when the system is field-effect doped to a surface hole concentration of $6times10^{14},$cm$^{-2}$, where the Fermi level crosses three valence bands. Starting from the band-resolved electron-phonon spectral functions $alpha^2F_{jj}(omega)$ computed ab initio, we iteratively solve the self-consistent isotropic Migdal-Eliashberg equations, in both the single-band and the multi-band formulations, in the approximation of a constant density of states at the Fermi level. In the single-band formulation, we find T$_{text{c}}approx40,$K, which is enhanced between $4%$ and $8%$ when the multi-band nature of the system is taken into account. We also compute the multi-band-sensistive quasiparticle density of states to act as a guideline for future experimental works.
We present an in-depth classification of the topological phases and Majorana fermion (MF) excitations that arise from the bulk interplay between unconventional multiband spin-singlet superconductivity and various magnetic textures. We focus on magnetic texture crystals with a periodically-repeating primitive cell of the helix, whirl, and skyrmion types. Our analysis is relevant for a wide range of layered materials and hybrid devices, and accounts for both strong and weak, as well as crystalline topological phases. We identify a multitude of accessible topological phases which harbor flat, uni- or bi-directional, (quasi-)helical, or chiral MF edge modes. This rich variety of MFs originates from the interplay between topological phases with gapped and nodal bulk energy spectra, with the resulting types of spectra and MFs controlled by the size of the pairing and magnetic gaps.
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