Axionic charge-density wave in the Weyl semimetal (TaSe4)2I

Abstract

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...

Long‐Range Ordered Amorphous Atomic Chains as Building Blocks of a Superconducting Quasi‐One‐Dimensional Crystal

“…[1,19,20] Very recently, an exotic axionic CDW state has also been reported for this material. [1,6] Due to the formation of fluctuating CDW, [16,21,22] (TaSe 4 ) 2 I even showed a steep decrease in the optical absorption at room temperature. [18] These experimental results testified the strong intrachain confinement of electrons and thus the quasi-1D nature of (TaSe 4 ) 2 I.…”

“…Synthesis of the Starting Material: The (TaSe 4 ) 2 I single crystals were synthesized with chemical vapor transport method. [1] Stoichiometric Ta, Se powders, and I 2 pieces were ground together and loaded into a quartz tube. The tube was sealed in vacuum and placed in a two-zone furnace.…”

“…The linear chain compound (TaSe 4 ) 2 I has drawn considerable attention due to its intriguing electronic properties correlated to the quasi-1D structure. [1][2][3][4][5][6][7][8][9][10] TaSe 4 chains, parallelly aligned in the caxis and separated by strands of iodine atoms, serve as the building blocks of this paradigmatic quasi-1D material (Figure 1a). [11][12][13][14] Under ambient conditions, (TaSe 4 ) 2 I has a body-centered tetragonal unit cell (space group I422, no.…”

“…[2,7,8,17] Due to the instability of the atomic lattice along the c-axis, the TaSe 4 chains demonstrate periodic modulations of the electron density, giving rise to various CDW-correlated phenomena in the electrical transport, magnetic, and optical properties of (TaSe 4 ) 2 I. [1,2,7,8,[17][18][19][20] For example, around the CDW transition temperature T p ≈ 260 K, a kink was observed in both its electrical resistivity and magnetic susceptibility. [1,19,20] Very recently, an exotic axionic CDW state has also been reported for this material.…”

“…[1,2,7,8,[17][18][19][20] For example, around the CDW transition temperature T p ≈ 260 K, a kink was observed in both its electrical resistivity and magnetic susceptibility. [1,19,20] Very recently, an exotic axionic CDW state has also been reported for this material. [1,6] Due to the formation of fluctuating CDW, [16,21,22] (TaSe 4 ) 2 I even showed a steep decrease in the optical absorption at room temperature.…”