Scaling of the Quantum Anomalous Hall Effect as an Indicator of Axion Electrodynamics

Grauer, S.; Fijalkowski, Kajetan M.; Schreyeck, S.; Winnerlein, Martin; Βrunner, Karl; Thomale, Ronny; Gould, C.; Molenkamp, Laurens W.

doi:10.1103/physrevlett.118.246801

Cited by 84 publications

(74 citation statements)

References 34 publications

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“…The signatures of the topological magnetoelectric effect, a.k.a. static axion electrodynamics, were recently reported as quantized magneto-optical effects in TIs [41][42][43], and quantized magneto and electrical resistance changes in artificial antiferromagnetic heterostructures of magnetically doped TIs [44][45][46]. Finally, dynamical axion quasiparticles (AQ) in the form of magnetic fluctuations were predicted in magnetically doped TIs (MTI) [35], spin-orbit coupled Mott insulators [47], in MTI superlattices [40], and also an inverse chiral magnetoelectric effect was suggested [48].…”

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confidence: 99%

Proposal to Detect Dark Matter using Axionic Topological Antiferromagnets

et al. 2019

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Antiferromagnetically doped topological insulators (A-TI) are among the candidates to host dynamical axion fields and axion-polaritons; weakly interacting quasiparticles that are analogous to the dark axion, a long sought after candidate dark matter particle. Here we demonstrate that using the axion quasiparticle and antiferromagnetic fluctuations in A-TI's in conjunction with low-noise methods of detecting THz photons presents a viable route to detect axion dark matter with mass 0.7 to 3.5 meV, a range currently inaccessible to other dark matter detection experiments and proposals.

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confidence: 99%

Proposal to Detect Dark Matter using Axionic Topological Antiferromagnets

et al. 2019

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“…The anomalous Hall effect (AHE), despite being first reported over a hundred years ago, [1] remains of significant modern research interest as its investigation in novel magnetic materials continues to yield rich physics. Recent examples of this are the quantum anomalous Hall effect [2][3][4][5][6][7][8][9][10][11][12], which offers both potential for metrological applications [13,14] and for the academic study of axion electrodynamics [5,12,15,16], as well as reports on alleged contribution to the Hall effect [17][18][19][20][21][22][23][24] associated with skyrmion magnetic textures.…”

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“…All investigations are performed on samples with a standard Hall bar geometry prepared by optical lithography, and are equipped with an electrostatic gate that allows tuning of the Fermi level in the layers. The topological insulator heterostructures are (Bi,Sb) 2 Te 3 based, with the magnetic layers doped by V. They are grown using molecular beam epitaxy (MBE) on insulating, hydrogen passivated Si (111) substrates, and are covered in-situ with 10 nm of Te to protect the surface from aging [12,31]. The Bi/Sb ratio is 0.79/0.21 for all samples.…”

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Coexistence of Surface and Bulk Ferromagnetism Mimics Skyrmion Hall Effect in a Topological Insulator

Fijalkowski¹,

Hartl²,

Winnerlein³

et al. 2020

Phys. Rev. X

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Here we report the investigation of the anomalous Hall effect in the magnetically doped topological insulator (V,Bi,Sb)2Te3. We find it contains two contributions of opposite sign. Both components are found to depend differently on carrier density, leading to a sign inversion of the total anomalous Hall effect as a function of applied gate voltage. The two contributions are found to have different magnetization reversal fields, which in combination with a temperature dependent study points towards the coexistence of two ferromagnetic orders in the system. Moreover, we find that the sign of total anomalous Hall response of the system depends on the thickness and magnetic doping density of the magnetic layer. The thickness dependence suggests that the two ferromagnetic components originate from the surface and bulk of the magnetic topological insulator film. We believe that our observations provide insight on the magnetic behavior, and thus will contribute to an eventual understanding of the origin of magnetism in this material class. In addition, our data bears a striking resemblance to anomalous Hall signals often associated with skyrmion contributions. Our analysis provides a straightforward explanation for both the magnetic field dependence of the Hall signal and the observed change in sign without needing to invoke skyrmions, and thus suggest that caution is needed when making claims of effects from skyrmion phases. arXiv:1912.02766v1 [cond-mat.mes-hall]

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“…[23]. Grauer et al [25] also claimed the observation of an 'axion insulator' in a uniformly doped QAH sample in the 3D regime. However, the presence of the 1D chiral edge mode of the 3D QAH state implies that side surfaces are not gapped, thus violating the condition (ii) noted above.…”

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“…Furthermore, the observed large  xx also exclude the existence of helical side surface states in the sandwich heterostructure, reflecting the effect of quantum confinement. Since there is no conduction channel in the 3-5-3 sandwich heterostructure with antiparallel magnetization alignment, the axion insulator state is much more insulating than that induced by scattering from multi-magnetic domains in uniformly magnetically doped QAH samples [10][11][12][13][14][15]25,31,32]. Thus, the external  0 H can turn OFF/ON the 1D chiral edge state in antiparallel/parallel magnetization alignment regions, leading to an 'OFF/ON' ratio of ~for  xx .…”

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Realization of the Axion Insulator State in Quantum Anomalous Hall Sandwich Heterostructures

et al. 2018

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Three-dimensional (3D) topological insulators (TIs) are unusual quantum materials that host conducting helical Dirac states on their surfaces, which are protected by time reversal symmetry (TRS), but are electrically insulating in the bulk [1,2]. TI is distinct from a trivial insulator by its unique electromagnetic response, described by the so-called  term shown below in addition to the ordinary Maxwell terms [3][4][5][6]. Here E and B are the conventional electric and magnetic fields inside an insulator, e is electron charge, and θ is the dimensionless pseudo-scalar parameter describing the insulator. For a trivial insulator, θ=0, while for a TI, θ=. When TRS is preserved, θ is either 0 or , reflecting its topological nature. This  term is related to the axion electrodynamics in particle physics [7]. Since the EB  term can be rewritten as a total derivative, its effect manifests on the surface states. A half-integer quantum Hall effect on the TI surface occurs once the surface Dirac Fermions acquire a mass, i.e. the surface state is gapped by magnetism. Such half-integer quantum Hall effect on TI surface can lead to a variety of exotic phenomena such as the quantum anomalous Hall (QAH) effect [3,[8][9][10][11][12][13][14][15], the quantized magneto-optical effect [3,16,17], the topological magnetoelectric (TME) effect [3][4][5][6]18], and the image magnetic monopole [19]. The QAH and quantized magneto-optical effects have been experimentally demonstrated in pure or magnetic TI films [10][11][12][13][20][21][22]. The TME effect refers to the quantized response of electric 3 polarization to applied magnetic fields and vice versa. The realization of the TME effect requires the following three conditions: (i) the TI film should be in the 3D regime; (ii) all the surfaces are gapped with the chemical potential lying within the gaps; (iii) the interior of the TI maintains TRS or inversion symmetry to maintain  in the bulk. A material system allowing for the realization of TME effect is known as an axion insulator [3][4][5].Recently, two papers reported the possible realization of the axion insulator. (Figs. 3k and 3m). We note that the zero  yx plateau is absent in either uniformly doped or the Cr modulation doped QAH samples [23,26,31,32]. The 3-5-3 SH2 was magnetically trained first by an upward sweep up to  0 H=1.5T before being swept downward. When  0 H=-0.01T, the MFM contrast is uniform (red), indicating that both top V-and bottom Cr-doped TI layers have upward magnetization (Fig. 3c). At  0 H=-0.05T, some reversed magnetic domains (green regions in Fig. 3d) appear, presumably in the 'softer' Cr-doped TI layer. As  0 H is swept further, the green regions expand and fill up the whole scan area at  0 H=-0.09T, indicating the uniform antiparallel magnetization alignment over the entire 3-5-3 SH2. When  0 H is further swept toward H c1 , new reversed magnetic domains (blue regions in Fig. 3g) nucleate at different locations, presumably in the 'harder' V-doped TI layer. Downward parallel magnetiza...

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Scaling of the Quantum Anomalous Hall Effect as an Indicator of Axion Electrodynamics

Cited by 84 publications

References 34 publications

Proposal to Detect Dark Matter using Axionic Topological Antiferromagnets

Proposal to Detect Dark Matter using Axionic Topological Antiferromagnets

Coexistence of Surface and Bulk Ferromagnetism Mimics Skyrmion Hall Effect in a Topological Insulator

Realization of the Axion Insulator State in Quantum Anomalous Hall Sandwich Heterostructures

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