Characterization of topological band structures away from the Fermi level by the anomalous Nernst effect

Abstract

Resolving the structure of energy bands in transport experiments is a major challenge in condensed matter physics and material science. Sometimes, however, traditional electrical conductance or resistance measurements only provide very small signals, and thus limit the ability to obtain direct band structure information. In this case, it has been proven beneficial to employ thermoelectric measurements which are sensitive to the first derivative of the density of states with respect to energy, rather than to its value itself. Due to the large interest in topological effects these days, it is important to identify a similar concept for detecting the Berry curvature in a band structure. Nowadays, the common way to access the Berry curvature directly via measurements is the anomalous Hall effect, but the corresponding signal can be too small to be detected when the topological features of the band structure lie too far off the Fermi level. In this work we propose to investigate topological band structure features utilizing the anomalous Nernst effect which is tied to the derivative of the anomalous Hall effect with respect to energy. Thereby, also signatures become resolvable, which are elusive in anomalous Hall measurements. We demonstrate the underlying mechanisms for a minimal effective four-band model and exemplary for the real Heusler compounds Co2FeX (X=Ge,Sn), which host topological nodal lines about 100 meV above the Fermi level. This work demonstrates that anomalous Nernst measurements can be an effective tool for the characterization of topological band structures, both at room temperature and in the quantum transport regime at cryogenic temperatures.Electrical conductance or resistance measurements have been widely used as a powerful tool to probe the energetic band structure of materials. This is because these electrical transport coefficients are directly proportional to the density of states.However, detecting fine structures can be a challenge when the electrical signals become very small.Due to such limitations, thermoelectric measurements have recently been established as a complementary tool to resolve energy bands and various related quantum phenomena. At low temperatures, the thermopower provides a measure of the first derivative of the electrical characteristics with respect to the energy 1 and is thereby sensitive to the change of the density of states, rather than to its value itself.Therefore, although not containing additional information, the thermoelectric transport coefficients can provide large signals when the electrical transport can hardly be resolved 2 . Hence, as long as the density of states is not constant, the thermopower can give deep insights into the underlying energy structure that would be elusive with other methods.In addition to the interest in their energetic structure, there has been a focus on the Berry curvature (BC) and the related topological properties of electronic bands over the last few years. The BC is intimately linked to the anomalous Hall conductivity (AHC), w...

Giant anomalous Nernst effect in noncollinear antiferromagnetic Mn-based antiperovskite nitrides

“…The ANC in Mn 3 NiN is nearly one order of magnitude larger than that in noncollinear antiferromagnetic Mn 3 Sn (∼0.2 AK −1 m −1 ). The pronounced ANC that we predict for Mn 3 NiN is substantially larger than for most of the typical ferromagnets (0.01∼1 AK −1 m −1 ), and the calculated value is only slightly smaller than for the two ferromagnetic Weyl semimetals Co 2 FeGe [25] and Co 2 MnGa, [9,11] , as summarized in Figure 4a. Here, we stress that Co 2 FeGe and Co 2 MnGa as intrinsic ferromagnets do not play any role for antiferromagnetic spin caloritronics as they are not free of parasitic stray fields.…”

“…a) The ANC recorded for various ferromagnets and two antiferromagnets (Mn3Sn and Mn3NiN). Part of the data is taken from the previous works [9,10,25,26]. b)-c) The AHC and ANC of Mn3NiN as a function of energy.…”

Spin-polarized Weyl cones and giant anomalous Nernst effect in ferromagnetic Heusler films

“…34 ). the k i = 0 planes (i = x, y, z) 9,15,28,38,39 . For example, the nodal points of bands C and D in Fig.…”

All Electrical Access to Topological Transport Features in Mn1.8PtSn Films

“…Additionally, when decreasing the temperature to below 50 K, the Nernst signal decreases by roughly one order of magnitude. This behaviour is expected for the anomalous thermal signals due to the Mott relation: Since the weighting function of the anomalous Nernst approaches a delta distribution, the anomalous Nernst signals will be suppressed towards T = 0 K. [3][4][5][6] Please note, that the Nernst and Hall signals have opposite signs with respect to their anomalous counterparts below T SR which was not observed in the only other report of the topological Hall and Nernst in the same sample. 7…”

“…Non-coplanar spin configurations, such as Skyrmions or other topological configurations are very exciting. [1][2][3][4][5][6] In magnetic materials, such topological spin structures can give rise to a finite scalar spin chirality S i · (S j × S k ) between neighbouring spins S i,j,k on the sites i, j, k. 7 In a semi-classical picture, itinerant electrons moving through a spin texture with finite scalar spin chirality or a Skyrmion lattice 5 accumulate a Berry phase, with the latter acting like an additional magnetic field on those electrons. [7][8][9][10][11] This (fictitious) magnetic field in turn leads to an additional contribution to the Hall and Nernst effects.…”