The Anomalous Photo‐Nernst Effect of Massive Dirac Fermions In HfTe₅

Abstract

The quantum geometric Berry curvature results in an anomalous correction to the band velocity of crystal electrons with a corresponding transverse (thermo)electric conductivity. However, time‐reversal symmetry typically constrains the direct observation and exploitation of anomalous transport to magnetic compounds. Here, it is demonstrated the anomalous Hall and Nernst conductivities are essential for describing the optoelectronic transport in thin films of the non‐magnetic, weakly gapped semimetal HfTe5 subject to an external magnetic field. A focused photoexcitation adresses the symmetries of the local Nernst conductivity, which unveils a hitherto hidden, anomalous photo‐Nernst effect of three‐dimensional (3D) massive Dirac fermions. The experimental temperature and density dependencies are compared with a semiclassical Boltzmann transport model. For HfTe5 thin films with the Fermi level close to the gap, the model suggests that the anomalous photo‐Nernst currents originate from an intrinsic Berry curvature mechanism, where the Zeeman interaction effectively breaks time‐reversal symmetry of the massive Dirac fermions already at moderate external magnetic fields.

Probing the Spatial Homogeneity of Exfoliated HfTe5 Films

“…Several studies provided evidence, including photoemission spectroscopy and electron transport, that the fundamental gaps of HfTe 5 , and similarly ZrTe 5 , can be strain-tuned across a weak to a strong topological insulator phase. ,,, Moreover, the location of additional, trivial carrier pockets depends on strain as well. , Therefore, our results may explain partly the considerable variation of results seen in the literature. While some studies reported a quasi-quantized Hall resistance of gapped Dirac Fermions with well-developed Shubnikov-de-Haas oscillations, , other studies reported signatures of anomalous Hall effects with largely suppressed Shubnikov-de-Haas oscillations, ,, sometimes even in nominally identical materials, and further studies described the transport features taking into account additional carrier pockets. − Based on the substantial inhomogeneity of exfoliated films observed here, it may be advantageous to carefully and systematically differentiate between measurements on bulk crystal (mm-scale) and exfoliated films (μm-scale). We note that, in previous work on HfTe 5 , we already observed such systematic differences in the magnetotransport properties of bulk reference crystals and exfoliated films .…”

“…While some studies reported a quasi-quantized Hall resistance of gapped Dirac Fermions with well-developed Shubnikov-de-Haas oscillations, , other studies reported signatures of anomalous Hall effects with largely suppressed Shubnikov-de-Haas oscillations, ,, sometimes even in nominally identical materials, and further studies described the transport features taking into account additional carrier pockets. − Based on the substantial inhomogeneity of exfoliated films observed here, it may be advantageous to carefully and systematically differentiate between measurements on bulk crystal (mm-scale) and exfoliated films (μm-scale). We note that, in previous work on HfTe 5 , we already observed such systematic differences in the magnetotransport properties of bulk reference crystals and exfoliated films . Depending on the used contacts within a multiterminal, exfoliated device, one may probe homogeneous areas with little to no built-in strain or one may probe inhomogeneous areas, which might consist of patches with varying density, varying gap size, or even varying topological nature of the bands.…”

“…Strikingly, the Raman intensity reveals a strongly inhomogeneous distribution, with distinct hot spots close to the center of the crystal. The gray-shaded regions mark gold contacts, which were evaporated after the exfoliation process (see Methods and ref ( 28 )). Comparison of the full Raman spectra at three distinct positions (marked by the stars in Figure 4 a) reveals indeed that the hot spots coincide with an enhanced intensity of the additional peak (Figure 4 b, the spectra are offset vertically for clarity).…”

“… 20 , 31 Therefore, our results may explain partly the considerable variation of results seen in the literature. While some studies reported a quasi-quantized Hall resistance of gapped Dirac Fermions with well-developed Shubnikov-de-Haas oscillations, 10 , 15 other studies reported signatures of anomalous Hall effects with largely suppressed Shubnikov-de-Haas oscillations, 28 , 32 , 33 sometimes even in nominally identical materials, and further studies described the transport features taking into account additional carrier pockets. 34 − 36 Based on the substantial inhomogeneity of exfoliated films observed here, it may be advantageous to carefully and systematically differentiate between measurements on bulk crystal (mm-scale) and exfoliated films (μm-scale).…”

“…We note that, in previous work on HfTe 5 , we already observed such systematic differences in the magnetotransport properties of bulk reference crystals and exfoliated films. 28 Depending on the used contacts within a multiterminal, exfoliated device, one may probe homogeneous areas with little to no built-in strain or one may probe inhomogeneous areas, which might consist of patches with varying density, varying gap size, or even varying topological nature of the bands. For example, recent studies on ZrTe 5 found evidence for a nodal-line semimetal phase with a strong magnetochiral anisotropy and nonlinear transport, which were argued to be signatures of spatial inhomogeneity and charge puddling.…”