We report an experimental study of the magnetic order and electronic structure and transport of the layered pnictide ${\mathrm{EuMnSb}}_{2}$, performed using neutron diffraction, angle-resolved photoemission spectroscopy (ARPES), and magnetotransport measurements. We find that the Eu and Mn sublattices display antiferromagnetic (AFM) order below ${T}_{\mathrm{N}}^{\mathrm{Eu}}=21(1)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}}^{\mathrm{Mn}}=350(2)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively. The former can be described by an A-type AFM structure with the Eu spins aligned along the $c$ axis (an in-plane direction), whereas the latter has a C-type AFM structure with Mn moments along the $a$ -axis (perpendicular to the layers). The ARPES spectra reveal Dirac-like linearly dispersing bands near the Fermi energy. Furthermore, our magnetotransport measurements show strongly anisotropic magnetoresistance and indicate that the Eu sublattice is intimately coupled to conduction electron states near the Dirac point.

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Magnetic and electronic structure of Dirac semimetal candidate EuMnSb 2

Unconventional features of relativistic Dirac/Weyl quasiparticles in topological materials are most evidently manifested in the two-dimensional quantum Hall effect (QHE), whose variety is further enriched by their spin and/or valley polarization. Although its extension to three dimensions has been long sought and inspired theoretical proposals, material candidates have been lacking. Here, we have discovered valley-contrasting spin-polarized Dirac fermions in a multilayer form in the bulk antiferromagnet ${\mathrm{BaMnSb}}_{2}$, where out-of-plane Zeeman-type spin splitting is induced by in-plane inversion symmetry breaking and spin-orbit coupling in the distorted Sb square net. Furthermore, we have observed well-defined quantized Hall plateaus together with vanishing interlayer conductivity at low temperatures as a hallmark of the half-integer QHE in a bulk form. The Hall conductance of each layer is found to be nearly quantized to $2(N+1/2){e}^{2}/h$, with $N$ being the Landau index, which is consistent with two spin-polarized Dirac valleys protected by the strong spin-valley coupling.

Bulk quantum Hall effect of spin-valley coupled Dirac fermions in the polar antiferromagnet BaMnSb 2

We report on the crystal and magnetic structures and magnetic and transport properties of ${\mathrm{SrMnSb}}_{2}$ single crystals grown by the self-flux method. Magnetic susceptibility measurements reveal an antiferromagnetic (AFM) transition at ${T}_{\mathrm{N}}=295(3)$ K. Above ${T}_{\mathrm{N}}$, the susceptibility slightly increases and forms a broad peak at $T\ensuremath{\sim}420$ K, which is a typical feature of two-dimensional magnetic systems. Neutron diffraction measurements on single crystals confirm the previously reported C-type AFM structure below ${T}_{\mathrm{N}}$. Both de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects are observed in ${\mathrm{SrMnSb}}_{2}$ single crystals. Analysis of the oscillatory component by a Fourier transform shows that the prominent frequencies obtained by the two different techniques are practically the same within error regardless of sample size or saturated magnetic moment. Transmission electron microscopy (TEM) reveals the existence of stacking faults in the crystals, which result from a horizontal shift of Sb atomic layers suggesting possible ordering of Sb vacancies in the crystals. Increase of temperature in susceptibility measurements leads to the formation of a strong peak at $T\ensuremath{\sim}570$ K that upon cooling under magnetic field the susceptibility shows a ferromagnetic transition at ${T}_{\mathrm{C}}\ensuremath{\sim}580$ K. Neutron powder diffraction on crushed single crystals does not support a ferromagnetic phase above ${T}_{\mathrm{N}}$. Furthermore, x-ray magnetic circular dichroism (XMCD) measurements of a single crystal at the ${L}_{2,3}$ edge of Mn shows a signal due to induced canting of AFM moments by the applied magnetic field. All evidence strongly suggests that a chemical transformation at the surface of single crystals occurs above 500 K concurrently producing a minute amount of ferromagnetic impurity phase.

Crystal growth, microstructure, and physical properties of SrMnSb 2

${\mathrm{SrMnSb}}_{2}$ is a candidate Dirac semimetal whose electrons near the $Y$ point have the linear dispersion and low mass of a Dirac cone. Here we demonstrate that ultrafast, 800-nm optical pulses can launch coherent phonon oscillations in ${\mathrm{Sr}}_{0.94}{\mathrm{Mn}}_{0.92}{\mathrm{Sb}}_{2}$, particularly an ${A}_{g}$ mode at 4.4 THz. Through first-principles calculations of the electronic and phononic structure of ${\mathrm{SrMnSb}}_{2}$, we show that high-amplitude oscillations of this mode would displace the atoms in a way that transiently opens and closes a gap at the node of the Dirac cone. The ability to control the nodal gap on a subpicosecond timescale could create opportunities for the design and manipulation of Dirac fermions.

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Using coherent phonons for ultrafast control of the Dirac node of SrMnSb 2

Shubnikov-de Haas oscillations of massive Dirac fermions in a Dirac antiferromagnet SrMnSb2

SrMnSb22 is suggested to be a magnetic topological semimetal. It contains square, 2D Sb planes with non-symmorphic crystal symmetries that could protect band crossings, offering the possibility of a quasi-2D, robust Dirac semi-metal in the form of a stable, bulk (3D) crystal. Here, we report a combined and comprehensive experimental and theoretical investigation of the electronic structure of SrMnSb22, including the first ARPES data on this compound. SrMnSb22 possesses a small Fermi surface originating from highly 2D, sharp and linearly dispersing bands (the Y-states) around the (0,π/a)-point in k-space. The ARPES Fermi surface agrees perfectly with that from bulk-sensitive Shubnikov de Haas data from the same crystals, proving the Y−states to be responsible for electrical conductivity in SrMnSb22. DFT and tight binding (TB) methods are used to model the electronic states, and both show good agreement with the ARPES data. Despite the great promise of the latter, both theory approaches show the Y-states to be gapped above EF, suggesting trivial topology. Subsequent analysis within both theory approaches shows the Berry phase to be zero, indicating the non-topological character of the transport in SrMnSb22, a conclusion backed up by the analysis of the quantum oscillation data from our crystals.

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Adriaan Schiphorst

Papers

Electronic structure of the candidate 2D Dirac semimetal SrMnSb2: a combined experimental and theoretical study