The Electronic Structural Gene Of Unconventional Superconductivity And Topological Critical Dirac Semimetal Materials

Since the Cu-based and iron-based high T,superconductors were discovered,the superconducting mechanism behind unconventional high-temperature superconduc-tors has become a great challenge in condensed-matter physics.Researchers in this field are sharply divided and disagree with each other on the basic physical properties that are relevant to the cause of superconductivity.Many reasons can be attributed to the failure of answering the cause of superconductivity.For example,material com-plexity makes theoretical modeling difficult,rich physical phenomena blind us from distinguishing main causes from side ones,and insufficient theoretical methods leave theoretical calculation doubtable.However,beyond all these difficulties,the lack of successfully realistic guiding principles to search for new high Tc superconductors from theoretical studies is the major reason.Can valuable guiding principles be pro-vided from the theoretical side ahead of the potential discovery of the third family of high Tc superconductors?We believe that we can answer the question based on the following two reasons.First,the intensive research on cuprates and iron-based su-perconductors has brought much invaluable information,and we believe that cuprates and iron-based superconductors should share a common high Tc mechanism.On one side,iron-based superconductors and cuprates share many common features,but on the other side they are not clones of each other,and the similarities and differences can thus speak promising clues.Second,from the past massive searching efforts,it has become increasingly clear that unconventional high Tc superconductors are rare materials.Moreover,for the two known families,their superconductivities are carried robustly on Cu02 layers in cupates and on FeAs/Se layers in iron-based superconduc-tors,respectively.The simultaneous existence of the rareness and robustness suggests that the unconventional high Tc superconductivity is tied to special ingredients in the electronic world,which define the electronic structural gene of unconventional high Tc superconductivity.Thus,the electronic structural gene can open a new window to search for high Tc superconductors.Comparing with cuprates and iron-based superconductors,Hu proposed two ba-sic principles to unify the understanding for both high-Tc superconductors:The corre-spondence principle notes that the short-range magnetic exchange interactions and the Fermi surfaces act collaboratively to achieve high-Tc superconductivity and determine pairing symmetries;The selective magnetic pairing rule notes that the superconduc-tivity is only induced by the magnetic exchange couplings from the superexchange mechanism through cation-anion-cation chemical bonds but not those from direct ex-change couplings resulted from the direct cation' s d-d chemical bonds.These two principles provide an unified explanation why the d-wave pairing symmetry and the s-wave pairing symmetry are robust in cuprates and iron-based superconductors,re-spectively.Following above guides,we predict that the electronic structural gene of unconventional high T,superconductivity can be realized in two special crystal struc-tures.The first one is a two dimensional hexagonal lattice formed by edge-shared trigonal biprymidal complexes.The conditions are satisfied when the electron filling configuration in cation atoms is near d7.Hence,the layered Co2+/Ni3+ based materi-als can be promising high T,families and have d±id pairing symmetry.The second one is a two dimensional square lattice formed by vertex-shared tetrahedra complexes,which has one Fe sublattice in iron-based superconductors.When the d7 configuration in cation atoms is satisfied,the electronic structure near fermi level is full attributed to the degenerated t2g orbitals.What's more,we predict that they host superconduct-ing states with a d-wave pairing symmetry with Tc potentially higher than those of iron-based superconductors.According to our proposed two families of the electronic structural gene of un-conventional high Tc superconductivity,we propose possible new high temperature superconductors by combining CALYPSO structure prediction method with first prin-ciples calculation.The first family is three-dimensional(3D)and two-dimensional(2D)Mg2CoO3.By comparing with ground state energy,the most stable crystalline forms are 3D a-Mg2Co03 and 2D e-Mg2Co03.To further confirm the stability of a(e)-Mg2CoO3,we calculate phonon dispersions,and no imaginary frequencies are ob-served throughout the whole Brillouin zone in phonon dispersions.The electronic structures of a(e)-Mg2CoO3 are calculated,consistent with that of YNiO3,and satisfy the above principles.The second family is A2O2CoSe2(A=La,Ce,Tm)and R2F2CoSe2(R=Ca,Sr,Ba).By calculating ground state energy and phonon dispersions,the most stable crystalline forms are A2O2CoSe2-I and R2F2CoSe2-I,and The electronic struc-tures are consistent with that of YNi03.Besides,in current material database we found a family of material BaCoAO which are electronically close to our second proposed structures.Their parental compounds possess antiferromagnetic(AFM)Mott insulat-ing states through pure superexchange interactions and the low energy physics near Fermi surfaces upon doping is attributed mainly to the t2g orbitals.We predict that a d-wave-like superconducting state with reasonable high transition temperature can emerge by suppressing the AFM ordering.In the process of searching material,The KMgBi with the crytal structure of the 111 family of iron-based superconductors is discovered.By using first principles cal-culation,that AMgBi(A=K,Rb,Cs)are symmetry-protected topological semimetals near the boundary of type-? and type-? Dirac semimetal phases,dubbed topological critical Dirac semimetals.Doping Rb or Cs into KMgBi can drive the transition be-tween the two phases.An effective theory is developed to describe the bands near the Fermi energy,by which we calculate the surface Fermi arcs and the Landau levels throughout the transition.We predict the key features of critical Dirac semimetals that can be observed in photoemission,quantum oscillation and transport measurements.