Applying a temperature gradient in a magnetic material generates a voltage that is perpendicular to both the heat flow and the magnetization. 1,2 This is the anomalous Nernst effect (ANE), 3,4 which was thought to be proportional to the value of the magnetization for a long time. However, more generally, the ANE has been predicted to originate from a net Berry curvature of all bands near the Fermi level (EF. 5,6 Subsequently, a large anomalous Nernst thermopower ( ) has recently been observed in topological materials with no net magnetization but large net Berry curvature [n(k)] around EF. 7-9 These experiments clearly fall outside the scope of the conventional magnetization-model of the ANE, but a significant question remains: Can the value of the ANE in topological ferromagnets exceed the highest values observed in conventional ferromagnets? Here, we report a remarkably high -value of ~6.0 µV K −1 at 1 T in the ferromagnetic topological Heusler compound Co2MnGa at room temperature, which is around 7-times larger than any anomalous Nernst thermopower value ever reported for a conventional ferromagnet. Combined electrical, thermoelectric and first-principles calculations reveal that this high value of the ANE arises from a large net Berry curvature near the Fermi level associated with nodal lines and Weyl points.

Atomically thin materials such as graphene or MoS are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region.

An axion insulator is a correlated topological phase, predicted to arise from the formation of a charge density wave in a Weyl semimetal. The accompanying sliding mode in the charge density wave phase, the phason, is an axion. It is expected to cause anomalous magneto-electric transport effects. However, this axionic charge density wave has so far eluded experimental detection. In this paper, we report the observation of a large, positive contribution to the magneto-conductance in the sliding mode of the charge density wave Weyl semimetal (TaSe4)2I for collinear electric and magnetic fields (E||B).The positive contribution to the magneto-conductance originates from the anomalous axionic contribution of the chiral anomaly to the phason current, and is locked to the parallel alignment of E and B. By rotating B, we show that the angular dependence of the magneto-conductance is consistent with the anomalous transport of an axionic charge density wave. 3 Axions refer to elementary particles that have long been known in quantum field theory, 1,2 but have yet to be observed in nature. However, it has been recently understood that axions can emerge as collective electronic excitations in certain crystals, so-called axion insulators. 3 Despite being fully gapped to single-particle excitations in the bulk and at the surface, an axion insulator is characterized by an effective action, which includes a topological EB-term, where E and B are the electromagnetic fields inside the insulator, and  plays the role of the dynamical axion field. Physically, the average value of  is determined by the microscopic details of the band structure of the system, and gives rise to unusual magnetoelectric response properties such as quantum anomalous Hall conductivities 4-9 , the quantized circular photo-galvanic 4,10,11 effect, and the chiral magnetic effect. 4,[12][13][14] The prospect of realizing an axion insulator has inspired much theoretical and experimental work. Only very recently, signatures of a dynamic axion field have been found on the surface of magnetically doped topological insulator thin films. [15][16][17] However, the axionic quasi-particle in these systems-the axionic polariton 3 -has so far eluded experimental detection. Alternatively, axion insulators have been predicted to arise in Weyl semimetals that are unstable towards the formation of a charge density wave (CDW). [18][19][20][21][22][23] In their parent state, Weyl semimetals are materials in which the low-energy electronic quasiparticles behave as chiral relativistic fermions without rest mass, known as Weyl fermions. [24][25][26][27] The Weyl fermions exist at isolated crossing points of conductance and valence bands-so called Weyl nodes-and their energy can be approximated with a linear dispersion relation ( Fig. 1 (a)). The Weyl nodes always occur in pairs of opposite "handedness" or chirality. At low energies and in the absence of interactions the chirality is a conserved quantum number, and the two chiral populations do not mix. Parallel electric and mag...

The interplay of magnetism and topology opens up the possibility for exotic linear response effects, such as the anomalous Hall effect and the anomalous Nernst effect, which can be strongly enhanced by designing a large Berry curvature in the electronic structure. Magnetic Heusler compounds are a promising class of materials for this purpose because they are versatile, show magnetism, and their electronic structure hosts strong topological features. Here, we provide a comprehensive study of the intrinsic anomalous transport for magnetic cubic full Heusler compounds and we illustrate that several Heusler compounds outperform the best so far reported materials. The results reveal the importance of symmetries, especially mirror planes, in combination with magnetism for giant anomalous Hall and Nernst effects, which should be valid in general for linear responses (spin Hall effect, spin orbital torque, etc.) dominated by intrinsic contributions.

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...