Two-Dimensional Massless Dirac Fermions in Antiferromagnetic AFe2As2

Abstract

We report infrared studies of AFe 2 As 2 (A¼Ba, Sr), two representative parent compounds of iron-arsenide superconductors, at magnetic fields (B) up to 17.5 T. Optical transitions between Landau levels (LLs) were observed in the antiferromagnetic states of these two parent compounds. Our observation of a ffiffiffi ffi B p dependence of the LL transition energies, the zero-energy intercepts at B ¼ 0 T under the linear extrapolations of the transition energies and the energy ratio (∼2.4) between the observed LL transitions, combined with the linear band dispersions in two-dimensional (2D) momentum space obtained by theoretical calculations, demonstrates the existence of massless Dirac fermions in the antiferromagnet BaFe 2 As 2 . More importantly, the observed dominance of the zeroth-LL-related absorption features and the calculated bands with extremely weak dispersions along the momentum direction k z indicate that massless Dirac fermions in BaFe 2 As 2 are 2D. Furthermore, we find that the total substitution of the barium atoms in BaFe 2 As 2 by strontium atoms not only maintains 2D massless Dirac fermions in this system, but also enhances their Fermi velocity, which supports that the Dirac points in iron-arsenide parent compounds are topologically protected. DOI: 10.1103/PhysRevLett.119.096401 In condensed matter, massless Dirac fermions (MDF), which represent a type of quasiparticles with linear energymomentum dispersions, provide a cornerstone for various quantum phenomena, such as the novel quantum Hall effect [1-3], Klein tunneling [4], and giant linear magnetoresistance [5]. Because of their crucial role in diverse quantum phenomena, searching for MDF in new systems is one of the most active research areas in condensed matter science. Generally, massless Dirac fermions with their valence and conduction band touching at a degenerate point (i.e., Dirac point) are protected by symmetries in solids [6][7][8][9][10]. However, the formation of magnetic order is always accompanied with time-reversal symmetry breaking and sometimes followed by crystalline symmetry lowering. Therefore, it is a challenge to achieve MDF in magnetic ground states of solid-state electronic systems [11,12]. Moreover, two-dimensional (2D) MDF, which were realized in 2D materials and on the surfaces of threedimensional (3D) topological insulators [1][2][3][13][14][15][16][17], have rarely been observed in the bulk of 3D crystals [11]. The discovery of iron-arsenide superconductors not only brings people a new class of unconventional superconductivity [18], but also sheds light on seeking 2D MDF in magnetic compounds [19,20].The parent compounds of iron-arsenide superconductors (PCIS) exhibit collinear antiferromagnetic (AFM) order at low temperature (T) [21,22]. After the AFM phase transition, the paramagnetic Brillouin zone of PCIS would be folded along the AFM wave vector connecting the hole-type Fermi surfaces (FSs) surrounding the Γ point and the electron-type FSs at the M point, which leads to the intersection between the electron...

Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

“…However, a real challenge for this interpretation has been provided by the optical spectra of a detwinned BaFe 2 As 2 single crystal for which the absorption peak in the response along the b axis acquires a very singular shape [29]. More recently, the magnetic-field dependence of this absorption feature has been interpreted in terms of an interband transition of two-dimensional Dirac fermions [13] that appear in the nodal regions of the SDW gap due to band backfolding effects. Here, we follow up on this interpretation for which the Dirac nodes are protected by the combination of the physical symmetry and the topology of the band structure [7,9,10].…”

“…Subsequent, theoretical and experimental studies revealed that the Dirac cones exist in pairs [11,12]. More recently, magnetooptical experiments have demonstrated that the Dirac fermion state in BaFe 2 As 2 and SrFe 2 As 2 is two-dimensional [13]. The observation of such a Dirac fermion state with protected Dirac cones in the IBSs' parent compounds provides us with a new kind of topological material, after cuprates [14], graphene [15], topological insulators [16,17], and Weyl semimetals [18,19].…”

“…Although optical investigations into the parent compounds of IBSs have been conducted by many groups [25][26][27][28][29][30][31][32][33], the optical signatures of the Dirac fermions have so far been only discussed for the 122 compound [13]. The evolution of these Dirac fermions with electron and hole doping has also not yet been reported.…”

Thermoelectric signature of the nematic phase in hole-doped iron-based superconductors

“…2a. While ν(T) in CaCo0 and CaCo3 exhibit a clear step-like change at the structural/magnetic transition (which might be related to the presence of the Dirac fermions in the SDW phase of the 122 iron based superconductors[24,25]) such an anomaly is absent in CaCo7.Figure 2b, presenting the temperature and doping evolution of the Nernst anisotropy calculated as Δν = νaνb, indicates that Δν in CaCo0 and CaCo3 changes sign below Ttr, whereas for all three samples Δν is positive in the tetragonal state. This is different from Ba(Fe1-xCox)2As2, where (νa -νb) is negative for all samples being studied except of the most doped Ba(Fe0.94Co0.06)2As2, where Δν is small but positive.…”

Symmetry and Topology in Antiferromagnetic Spintronics

“…(c) Single Dirac cone at M point in FeSe monolayer AF with a stripe order. Panel (a) adapted from[54], panel (b) from[70], and panel (c) from[82].…”