Breaking time-reversal symmetry in a Dirac semimetal ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ through doping with magnetic ions or by the magnetic proximity effect is expected to cause a transition to other topological phases (such as a Weyl semimetal). To this end, we investigate the possibility of proximity-induced ferromagnetic ordering in epitaxial Dirac semimetal (${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$)/ferromagnetic semiconductor (${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Sb}$) heterostructures grown by molecular beam epitaxy. We report the comprehensive characterization of these heterostructures using structural probes (atomic force microscopy, x-ray diffraction, scanning transmission electron microscopy), angle-resolved photoemission spectroscopy, electrical magnetotransport, magnetometry, and polarized neutron reflectometry. Measurements of the magnetoresistance and Hall effect in the temperature range 2--20 K show signatures that could be consistent with either a proximity effect or spin-dependent scattering of charge carriers in the ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ channel. Polarized neutron reflectometry sets constraints on the interpretation of the magnetotransport studies by showing that (at least for temperatures above 6 K) any induced magnetization in the ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ itself must be relatively small $(<14\phantom{\rule{0.16em}{0ex}}\mathrm{emu}/{\mathrm{cm}}^{3})$.
The quantum anomalous Hall (QAH) state is a two-dimensional topological insulating state that has quantized Hall resistance of h/Ce2 and vanishing longitudinal resistance under zero magnetic field, where C is called the Chern number. The QAH effect has been realized in magnetic topological insulators (TIs) and magic-angle twisted bilayer graphene. Despite considerable experimental efforts, the zero magnetic field QAH effect has so far been realized only for C = 1. Here we used molecular beam epitaxy to fabricate magnetic TI multilayers and realized the QAH effect with tunable Chern number C up to 5. The Chern number of these QAH insulators is tuned by varying the magnetic doping concentration or the thickness of the interior magnetic TI layers in the multilayer samples. A theoretical model is developed to understand our experimental observations and establish phase diagrams for QAH insulators with tunable Chern numbers. The realization of QAH insulators with high tunable Chern numbers facilitates the potential applications of dissipationless chiral edge currents in energy-efficient electronic devices and opens opportunities for developing multi-channel quantum computing and higher-capacity chiral circuit interconnects.
We report spin-to-charge and charge-to-spin conversion at room temperature in heterostructure devices that interface an archetypal Dirac semimetal, Cd3As2, with a metallic ferromagnet, Ni0.80Fe0.20 (permalloy). The spin-charge interconversion is detected by both spin torque ferromagnetic resonance and ferromagnetic resonance driven spin pumping. Analysis of the symmetric and anti-symmetric components of the mixing voltage in spin torque ferromagnetic resonance and the frequency and power dependence of the spin pumping signal show that the behavior of these processes is consistent with previously reported spin-charge interconversion mechanisms in heavy metals, topological insulators, and Weyl semimetals. We find that the efficiency of spin-charge interconversion in Cd3As2/permalloy bilayers can be comparable to that in heavy metals. We discuss the underlying mechanisms by comparing our results with first principles calculations.
There is growing interest in using multiterminal Josephson junctions (MTJJs) as a platform to artificially emulate topological phases and to investigate superconducting mechanisms such as multiplet Cooper pairings. Current experimental signatures in MTJJs have led to conflicting interpretations of the salient features. In this work, we report a collaborative experimental and theoretical investigation of graphene-based four-terminal Josephson junctions. We observe resonant features in the differential resistance maps that resemble those ascribed to multiplet Cooper pairings. To understand these features, we model our junctions using a circuit network of resistively and capacitively shunted junctions (RCSJs). We find that the RCSJ model successfully reproduces the observed multiplet features. Therefore, our study suggests that differential resistance measurements alone are insufficient to conclusively distinguish resonant Andreev reflection processes from semiclassical circuit-network effects.
Magnetic topological insulators (MTIs) host topologically protected edge states, but the role that these edge states play in electronic transport remains unclear. Using scanning superconducting quantum interference device (SQUID) microscopy, we performed local measurements of the current distribution in a quantum anomalous Hall (QAH) insulator at large bias currents, where the quantization of the conductivity tensor breaks down. We find that bulk currents in the channel interior coexist with edge currents at the sample boundary. While the position of the edge current changes with the reversal of the magnetic field, it does not depend on the current direction. To understand our observations, we introduce a model which includes contributions from both the sample magnetization and currents driven by chemical potential gradients. To parameterize our model, we use local measurements of the chemical potential induced changes in the sample magnetization. Our model reveals that the observed edge currents can be understood as changes in the magnetization generated by the electrochemical potential distribution in the sample under bias. Our work underscores the complexity of electronic transport in MTIs and highlights both the value and challenges of using magnetic imaging to disentangle various contributions to the electronic transport signatures.
Time-reversal invariance and inversion symmetry are responsible for the topological band structure in Dirac semimetals. These symmetries can be broken by applying an external magnetic or electric field, resulting in fundamental changes to the ground state Hamiltonian and a topological phase transition. We probe these changes via the magnetic-field dependence and gate voltage-dependence of universal conductance fluctuations in top-gated nanowires of the prototypical Dirac semimetal Cd3As2. As the magnetic field is increased beyond the phase-breaking field, we find a factor of sqrt(2) reduction in the magnitude of the universal conductance fluctuations, in agreement with numerical calculations that study the effect of broken time reversal symmetry in a 3D Dirac semimetal. In contrast, the magnitude of the fluctuations increases monotonically as the chemical potential is gated away from the charge neutrality point. This effect cannot be attributed to broken inversion symmetry, but can be explained by Fermi surface anisotropy. The concurrence between experimental data and theory in our study provides unequivocal evidence that universal conductance fluctuations are the dominant source of intrinsic transport fluctuations in mesoscopic Cd3As2 devices and offers a promising general methodology for probing the effects of broken symmetry in topological quantum materials.
We have demonstrated a robust magnetic tunnel junction (MTJ) with a resistance-area product RA=8 Ω—μm 2 that simultaneously satisfies the statistical requirements of high tunneling magnetoresistance TMR ≫ 15σ(R p ), write threshold spread σ(Vw)/≪Vw≫ ≪7.1%, breakdown-to-write voltage margin over 0.5V, read-induced disturbance rate below 10 −9 , and sufficient write endurance, and is free of unwanted write-induced magnetic reversal. The statistics suggest that a 64Mb chip at the 90-nm node is feasible.
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