Topological and superconducting properties of monolayered CoN and CoP:A first-principles comparative study

Two-dimensional systems that simultaneously harbor superconductivity and nontrivial band topology may serve as appealing platforms for realizing topological superconductivity with promising applications in fault-tolerant quantum computing.Here,based on first-principles calculations,we show that monolayered Co N and Co P with the isovalent Fe Se-like structure are stable in freestanding form,even though their known bulk phases have no resemblance to layering.The two systems are further revealed to display intrinsic band inversions due to crystal field splitting,and such orderings are preserved with the inclusion of spin-orbit coupling(SOC),which otherwise is able to open a curved band gap,yielding a non-zero Z2 topological invariant in each case.Such a mechanism of topologicalization is distinctly contrasted with that identified recently for the closely related monolayers of CoX(X=As,Sb,Bi),where the SOC plays an indispensable role in causing a nontrivial band inversion.Next,we demonstrate that,by applying equi-biaxial tensile strain,the electron-phonon coupling strength in monolayered CoN can be significantly enhanced,yielding a superconducting transition temperature(Tc)up to 7-12 K for the Coulomb pseudopotential ofμ*=0.2-0.1,while the CoP monolayer shows very low Tc even under pronounced strain.Their different superconducting behaviors can be attributed to different variations in lattice softening and electronic density of states around the Fermi level upon pressuring.Our central findings enrich the understanding of different mechanisms of band inversions and topologicalization and offer platforms for achieving the coexistence of superconductivity and nontrivial band topology based on two-dimensional systems.

Two-dimensional superconductors with intrinsic p-wave pairing or nontrivial band topology

Over the past fifteen years,tremendous efforts have been devoted to realizing topological superconductivity in realistic materials and systems,predominately propelled by their promising application potentials in fault-tolerant quantum information processing.In this article,we attempt to give an overview on some of the main developments in this field,focusing in particular on two-dimensional crystalline superconductors that possess either intrinsic p-wave pairing or nontrivial band topology.We first classify the three different conceptual schemes to achieve topological superconductor(TSC),enabled by real-space superconducting proximity effect,reciprocal-space superconducting proximity effect,and intrinsic TSC.Whereas the first scheme has so far been most extensively explored,the subtle difference between the other two remains to be fully substantiated.We then move on to candidate intrinsic or p-wave superconductors,including Sr2Ru O4,UTe2,Pb3Bi,and graphene-based systems.For TSC systems that rely on proximity effects,the emphases are mainly on the coexistence of superconductivity and nontrivial band topology,as exemplified by transition metal dichalcogenides,cobalt pnictides,and stanene,all in monolayer or few-layer regime.The review completes with discussions on the three dominant tuning schemes of strain,gating,and ferroelectricity in acquiring one or both essential ingredients of the TSC,and optimizations of such tuning capabilities may prove to be decisive in our drive towards braiding of Majorana zero modes and demonstration of topological qubits.

Correlation-enhanced electron-phonon coupling for accurate evaluation of the superconducting transition temperature in bulk FeSe

It has been widely recognized that,based on standard density functional theory calculations of the electron-phonon coupling,the superconducting transition temperature(T_(c))in bulk FeSe is exceptionally low(almost 0 K)within the Bardeen-Cooper-Schrieffer formalism.Yet the experimentally observed T_(c)is much higher(∼10 K),and the underlying physical origin remains to be fully explored,especially at the quantitative level.Here we present the first accurate determination of T_(c)in FeSe where the correlation-enhanced electron-phonon coupling is treated within first-principles dynamical mean-field theory.Our studies treat both the multiple electronic bands across the Fermi level and phononic bands,and reveal that all the optical phonon modes are effectively coupled with the conduction electrons,including the important contributions of a single breathing mode as established by previ-ous experiments.Accordingly,each of those phonon modes contributes pronouncedly to the electron pairing,and the resultant T_(c)is drastically enhanced to the experimentally observed range.The approach developed here should be broadly applicable to other superconducting systems where correlation-enhanced electron-phonon coupling plays an important role.