The bulk photovoltaic effect (BPVE) rectifies light into the dc current in a single-phase material and attracts the interest to design high-efficiency solar cells beyond the pn junction paradigm. Because it is a hot electron effect, the BPVE surpasses the thermodynamic Shockley–Queisser limit to generate above-band-gap photovoltage. While the guiding principle for BPVE materials is to break the crystal centrosymmetry, here we propose a magnetic photogalvanic effect (MPGE) that introduces the magnetism as a key ingredient and induces a giant BPVE. The MPGE emerges from the magnetism-induced asymmetry of the carrier velocity in the band structure. We demonstrate the MPGE in a layered magnetic insulator CrI 3 , with much larger photoconductivity than any previously reported results. The photocurrent can be reversed and switched by controllable magnetic transitions. Our work paves a pathway to search for magnetic photovoltaic materials and to design switchable devices combining magnetic, electronic, and optical functionalities.
The metal-insulator transition (MIT) is a hallmark of strong correlation in solids [1][2][3] . Quantum MITs at zero temperature have been observed in various systems tuned by either carrier doping or bandwidth 1 . However, such transitions have rarely been induced by application of magnetic field, as normally the field scale is too small in comparison with the charge gap, whose size is a fraction of the Coulomb repulsion energy (∼1 eV). Here we report the discovery of a quantum MIT tuned by a field of ∼10 T, whose magnetoresistance exceeds 60,000%. In particular, our anisotropic magnetotransport measurements on the cubic insulator Comprehensive surveys of the R 2 Ir 2 O 7 compounds show a systematic decrease in the thermal MIT temperature with increasing ionic radius of R, reaching zero 'between' R= Nd and Pr, so that Nd 2 Ir 2 O 7 is the closest insulator to the T = 0 quantum MIT. Indeed, Nd 2 Ir 2 O 7 shows an apparently continuous MIT at T MI ∼ 32 K, which is exceptionally low, especially relative to the gap E g ∼ 45 meV observed experimentally 5 , leading to the unusually large ratio E g /T MI ∼ 16. Recent neutron diffraction experiments suggest that in the low-temperature phase both the Nd 3+ and the Ir 4+ moments have an AIAO magnetic structure, an Ising type order with all the moments pointing inward or outward from the centre of each tetrahedron (Fig. 1a) 15 . What made this compound even more interesting is the recent proposal that this AIAO state may stabilize a Weyl semimetallic state [10][11][12][13][14]16 , raising the question of the nature of the MIT proximate to such topological phenomena.First we describe the key experimental observations to verify the MIT in our single crystals of Nd 2 Ir 2 O 7 . The temperature dependence of the resistivity exhibits a MIT at T MI ∼ 27 K under zero field (Fig. 1b). This transition temperature is slightly lower than the value ∼32 K observed in polycrystalline samples, most likely as a result of carrier doping by slight off-stoichiometry within 1% (see Methods). Below T MI , ρ(T ) shows insulating (negative dρ/dT ) behaviour (Fig. 1b inset). It is known that the AIAO magnetic order sets in concomitantly with the MIT (ref. 4). Indeed, exactly below T MI ∼ 27 K, we found that the zero-field-cooled (ZFC) and field-cooled (FC) magnetization bifurcate owing to the magnetic transition (Fig. 1c). Now, we discuss our main discovery of the field-induced MIT and strongly anisotropic magnetoresistance. Figure 2a presents the angle dependence of the transverse magnetoresistance measured using pulsed high magnetic fields up to 50 T and a Nd 2 Ir 2 O 7 single crystal with zero-field T MI ∼ 20 K. Here we note that to reveal the field evolution of the continuous MIT peculiar to the Ir 5d bands, a single crystal with T MI > 15 K is indispensable, as on cooling Nd moments freeze below 15 K (ref. 15). The field B was rotated within the (001) plane, perpendicularly to the current direction [110], with the angle (θ) between the field and the [001] direction, as schematically shown in...
A new mechanism of skew scattering and anomalous Hall effect due to the spin chirality fluctuation is proposed theoretically.
We report a nonmonotonic magnetic field dependence of the anomalous Hall effect due to the change of the Zeeman splitting.
Using first-principles calculations, we investigate the photogalvanic effect in the Weyl semimetal material TaAs. We find colossal photocurrents caused by the Weyl points in the band structure in a wide range of laser frequency. Our calculations reveal that the photocurrent is predominantly contributed by the three-band transition from the occupied Weyl band to the empty Weyl band via an intermediate band away from the Weyl cone, for excitations both by linearly and circularly polarized lights. Therefore, it is essential to sum over all three-band transitions by considering a full set of Bloch bands (both Weyl bands and trivial bands) in the first-principles band structure while it does not suffice to only consider the two-band direct transition within a Weyl cone. The calculated photoconductivities are well consistent with recent experiment measurements. Our work provides the first first-principles calculation on nonlinear optical phenomena of Weyl semimetals and serves as a deep understanding of the photogalvanic effects in complexed materials. Introduction. Weyl fermions correspond to the massless solutions of Dirac equation [1]and have been observed in solids as quasiparticles recently [2][3][4][5][6]. Related materials are called Weyl semimetals (WSM) [7][8][9][10][11][12][13]. A WSM gives rise to linearly band-crossing points called Weyl points (WPs) in the momentum space. WPs are monopoles of the Berry curvature [14,15] with finite chirality and are related to the chiral anomaly in the context of high-energy physics [16][17][18][19] and unique surface Fermi arcs [2].
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