Discovery of topological nodal-line fermionic phase in a magnetic material GdSbTe

Abstract

Topological Dirac semimetals with accidental band touching between conduction and valence bands protected by time reversal and inversion symmetry are at the frontier of modern condensed matter research. A majority of discovered topological semimetals are nonmagnetic and conserve time reversal symmetry. Here we report the experimental discovery of an antiferromagnetic topological nodal-line semimetallic state in GdSbTe using angle-resolved photoemission spectroscopy. Our systematic study reveals the detailed electronic structure of the paramagnetic state of antiferromagnetic GdSbTe. We observe the presence of multiple Fermi surface pockets including a diamond-shape, and small circular pockets around the zone center and high symmetry X points of the Brillouin zone (BZ), respectively. Furthermore, we observe the presence of a Dirac-like state at the X point of the BZ and the effect of magnetism along the nodal-line direction. Interestingly, our experimental data show a robust Dirac-like state both below and above the magnetic transition temperature (T_N = 13 K). Having a relatively high transition temperature, GdSbTe provides an archetypical platform to study the interaction between magnetism and topological states of matter.

Figure 1

Crystal structure and sample characterization of GdSbTe. (a) Tetragonal crystal structure. Layers of Sb atoms form a square net. Sheets of Gd atoms are separated by two Te layers. (b) Core-level spectrum. Here, we clearly observe sharp peaks due to Te 4d (~40 eV), Sb 4d (~33 eV) and Gd 4 f (~8.5 eV) states. The black dashed line represents the Fermi level. (c) Temperature dependence of the reciprocal magnetic susceptibility measured in a magnetic field of 0.5 T applied within the crystallographic a-b plane. Solid line represents the fit of Curie-Weiss law to the experimental data. Upper inset: low-temperature magnetic susceptibility data. Lower inset: magnetic field variation of the magnetization taken at 1.72 K with increasing (full circles) and decreasing (open circles) magnetic field strength. (d) Ab-initio calculated bulk band structure along the high-symmetry directions. Red circle indicates the approximate position of the Dirac point.

Figure 2

Fermi surface and constant energy contour plots. (a) Experimentally measured Fermi surface maps at various photon energies and at different high symmetry directions. The rightmost panel represents the Fermi level in a different orientation with high symmetry points. Photon energies are noted in the plots. (b) Constant energy contour plots at various binding energies. Energies are noted in the plots. High symmetry points are indicated in the leftmost plot. All the measurements were performed at the ALS beamline 4.0.3 at a temperature of 21 K.

Figure 3

Dispersion map along the high symmetry directions. (a) ARPES measured dispersion maps along the M-Γ-M direction at various photon energies. Nodal-line is observed to be in the vicinity of the chemical potential. (b) Dispersion maps along the high symmetry X-Γ-X direction. (c) Band dispersion along the M-X-M direction. Dirac-like state is observed. Measured photon energies are noted in the plots. Inset shows the zoomed view near the Dirac point marked with dashed white rectangular box. All the measurements were performed at the ALS beamline 4.0.3 at a temperature of 21 K. We note that, NL = nodal line, BS = bulk state, SS = surface state, DP = Dirac point.

Figure 4

Temperature dependent measurement of dispersion maps along the M-X-M direction. Measured temperature are noted on the plots. Re_8 K indicates the dispersion map after thermal recycle (8 K → 21 K → 53 K → 8 K). All the measurements were performed at the ALS beamline 4.0.3 using a photon energy of 90 eV.

Figure 5

Low-temperature thermodynamic properties of single-crystalline GdSbTe. (a) Temperature dependence of the magnetic susceptibility measured in a magnetic field of 0.1 T applied along the crystallographic c axis (triangles) and the tetragonal a–b plane (circles). (b) Temperature dependence of the specific heat.