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Register a new userSpin and charge interconversion in Dirac semimetal thin films
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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.
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The close approach of the Fermi energy EF of a Dirac semimetal to the Dirac point ED uncovers new physics such as velocity renormalization,1,2,3 and the Dirac plasma 4,5 at |EF -ED| < kBT, where kBT is the thermal energy. In graphene, substrate disorder drives fluctuations in EF. Three-dimensional topological Dirac semimetals (TDS)6,7 obviate the substrate, and should show reduced EF fluctuations due to better metallic screening and higher dielectric constants. Here we map the potential fluctuations in TDS Na3Bi using a scanning tunneling microscope. The rms potential fluctuations are significantly smaller than room temperature ({Delta}EF,rms = 4-6 meV = 40-70 K) and comparable to the highest quality graphene on h-BN;8 far smaller than graphene on SiO2,9,10 or the Dirac surface state of a topological insulator.11 Surface Na vacancies produce a novel resonance close to the Dirac point with surprisingly large spatial extent and provides a unique way to tune the surface density of states in a TDS thin-film material.
Three-dimensional topological Dirac semimetals have hitherto stimulated unprecedented research interests as a new class of quantum materials. Breaking certain types of symmetries has been proposed to enable the manipulation of Dirac fermions; and that was soon realized by external modulations such as magnetic fields. However, an intrinsic manipulation of Dirac states, which is more efficient and desirable, remains a significant challenge. Here, we report a systematic study of quasi-particle dynamics and band evolution in Cd3As2 thin films with controlled Chromium (Cr) doping by both magneto-infrared spectroscopy and electrical transport. For the first time, we observe square-root-B relation of inter-Landau-level resonance in undoped Cd3As2 Dirac semimetal, an important signature of ultra-relativistic Dirac state inaccessible in previous optical experiments. A crossover from quantum to quasi-classical behavior makes it possible to directly probe the mass of Dirac fermions. Importantly, Cr doping allows for a Dirac mass acquisition and topological phase transition enabling a desired dynamic control of Dirac fermions. Corroborating with the density-functional theory calculations, we show that the mass generation is essentially driven by explicit C4 rotation symmetry breaking and the resultant Dirac gap engineering through Cr substitution for Cd atoms. The manipulation of the system symmetry and Dirac mass in Cd3As2 thin films provides a tuning knob to explore the exotic states stemming from the parent phase of Dirac semimetals.
Three-dimensional (3D) topological Dirac semimetal, when thinned down to 2D few layers, is expected to possess gapped Dirac nodes via quantum confinement effect and concomitantly display the intriguing quantum spin Hall (QSH) insulator phase. However, the 3D-to-2D crossover and the associated topological phase transition, which is valuable for understanding the topological quantum phases, remain unexplored. Here, we synthesize high-quality Na3Bi thin films with R3*R3 reconstruction on graphene, and systematically characterize their thickness-dependent electronic and topological properties by scanning tunneling microscopy/spectroscopy in combination with first-principles calculations. We demonstrate that Dirac gaps emerge in Na3Bi films, providing spectroscopic evidences of dimensional crossover from a 3D semimetal to a 2D topological insulator. Importantly, the Dirac gaps are revealed to be of sizable magnitudes on 3 and 4 monolayers (72 and 65 meV, respectively) with topologically nontrivial edge states. Moreover, the Fermi energy of a Na3Bi film can be tuned via certain growth process, thus offering a viable way for achieving charge neutrality in transport. The feasibility of controlling Dirac gap opening and charge neutrality enables realizing intrinsic high-temperature QSH effect in Na3Bi films and achieving potential applications in topological devices.
The linear band crossings of 3D Dirac and Weyl semimetals are characterized by a charge chirality, the parallel or anti-parallel locking of electron spin to its momentum. Such materials are believed to exhibit a ${bf E} cdot {bf B}$ chiral magnetic effect that is associated with the near conservation of chiral charge. Here, we use magneto-terahertz spectroscopy to study epitaxial Cd$_3$As$_2$ films and extract their conductivities $sigma(omega)$ as a function of ${bf E} cdot {bf B}$. As field is applied, we observe a remarkably sharp Drude response that rises out of the broader background. Its appearance is a definitive signature of a new transport channel and consistent with the chiral response, with its spectral weight a measure of the net chiral charge and width a measure of the scattering rate between chiral species. The field independence of the chiral relaxation establishes that it is set by the approximate conservation of the isospin that labels the crystalline point-group representations.
The analogues of elementary particles have been extensively searched for in condensed matter systems because of both scientific interests and technological applications. Recently massless Dirac fermions were found to emerge as low energy excitations in the materials named Dirac semimetals. All the currently known Dirac semimetals are nonmagnetic with both time-reversal symmetry $mathcal{T}$ and inversion symmetry $mathcal{P}$. Here we show that Dirac fermions can exist in one type of antiferromagnetic systems, where $mathcal{T}$ and $mathcal{P}$ are broken but their combination $mathcal{PT}$ is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyze the robustness of the Dirac points with symmetry protections, and demonstrate its distinctive bulk dispersions as well as the corresponding surface states by emph{ab initio} calculations. Our results give a new route towards the realization of Dirac materials, and provide a possible platform to study the interplay of Dirac fermion physics and magnetism.
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