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Register a new userEnhanced Thermoelectric Properties of Dirac Semimetal Cd3As2
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We report an investigation of temperature- and doping-dependent thermoelectric behaviors of topological semimetal Cd3As2. The electrical conductivity, thermal conductivity, Seebeck coefficient, and figure of merit (ZT) are calculated by using Boltzmann transport theory. The calculated thermoelectric properties of the pristine Cd3As2 match well the experimental results. The electron or hole doping, especially the latter, is found improving much the thermoelectric behaviors of the material. The optimum merit ZT of Cd3As2 with electron doping is found to be about 0.5 at T=700 K with n=1x1020 cm-3, much larger than the maximum experimental value obtained for the pristine Cd3As2 (~0.15). For the p-type Cd3As2, the maximal value of the Seebeck coefficient as a function of temperature increases apparently with the increase of the hole doping concentration and its position shifts drastically towards the lower temperature region compared to that of the n-type Cd3As2, leading to the optimum merit ZT of about 0.5 obtained at low temperature of 500K (p=1x1020 cm-3) in the p-type Cd3As2.
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Three dimensional (3D) Dirac semimetals are 3D analogue of graphene, which display Dirac points with linear dispersion in k-space, stabilized by crystal symmetry. Cd3As2 and Na3Bi were predicted to be 3D Dirac semimetals and were subsequently demonstrated by photoemission experiments. As unveiled by transport measurements, several exotic phases, such as Weyl semimetals, topological insulators, and topological superconductors, can be deduced by breaking time reversal or inversion symmetry. Here, we reported a facile and scalable chemical vapor deposition method to fabricate high-quality Dirac semimetal Cd3As2 microbelts, they have shown ultrahigh mobility up to 1.15*10^5 cm^2/V s and pronounced Shubnikov-de Haas oscillations. Such extraordinary features are attributed to the suppression of electron backscattering. This research opens a new avenue for the scalable fabrication of Cd3As2 materials towards exciting electronic applications of 3D Dirac semimetals.
Cadmium arsenide (Cd3As2) - a time-honored and widely explored material in solid-state physics - has recently attracted considerable attention. This was triggered by a theoretical prediction concerning the presence of 3D symmetry-protected massless Dirac electrons, which could turn Cd3As2 into a 3D analogue of graphene. Subsequent extended experimental studies have provided us with compelling experimental evidence of conical bands in this system, and revealed a number of interesting properties and phenomena. At the same time, some of the material properties remain the subject of vast discussions despite recent intensive experimental and theoretical efforts, which may hinder the progress in understanding and applications of this appealing material. In this review, we focus on the basic material parameters and properties of Cd3As2, in particular those which are directly related to the conical features in the electronic band structure of this material. The outcome of experimental investigations, performed on Cd3As2 using various spectroscopic and transport techniques within the past sixty years, is compared with theoretical studies. These theoretical works gave us not only simplified effective models, but more recently, also the electronic band structure calculated numerically using ab initio methods.
Rapid progress of quantum transport study in topological Dirac semimetal, including observations of quantum Hall effect in two-dimensional (2D) Cd$_{mathrm{3}}$As$_{mathrm{2}}$ samples, has uncovered even more interesting quantum transport properties in high-quality and three-dimensional (3D) samples. However, such 3D Cd$_{mathrm{3}}$As$_{mathrm{2}}$ films with low carrier density and high electron mobility have been hardly obtained. Here we report the growth and characterization of 3D thick Cd$_{mathrm{3}}$As$_{mathrm{2}}$ films adopting molecular beam epitaxy. The highest electron mobility ($mu$ = 3 $times$ 10$^{4}$ cm$^{2}$/Vs) among the reported film samples has been achieved at a low carrier density ($textit{n} = 5$ $times$ 10$^{16}$ cm$^{-3}$). In the magnetotransport measurement, Hall plateau-like structures are commonly observed in spite of the 3D thick films ($textit{t} = 120$ nm). On the other hand, field angle dependence of the plateau-like structures and corresponding Shubunikov-de Haas oscillations rather shows a 3D feature, suggesting the appearance of unconventional magnetic orbit, also distinct from the one described by the semiclassical Weyl-orbit equation.
The three-dimensional topological semimetals represent a new quantum state of matter. Distinct from the surface state in the topological insulators that exhibits linear dispersion in two-dimensional momentum plane, the three-dimensional semimetals host bulk band dispersions linearly along all directions, forming discrete Dirac cones in three-dimensional momentum space. In addition to the gapless points (Weyl/Dirac nodes) in the bulk, the three-dimensional Weyl/Dirac semimetals are also characterized by topologically protected surface state with Fermi arcs on their specific surface. The Weyl/Dirac semimetals have attracted much attention recently they provide a venue not only to explore unique quantum phenomena but also to show potential applications. While Cd3As2 is proposed to be a viable candidate of a Dirac semimetal, more experimental evidence and theoretical investigation are necessary to pin down its nature. In particular, the topological surface state, the hallmark of the three-dimensional semimetal, has not been observed in Cd3As2. Here we report the electronic structure of Cd3As2 investigated by angle-resolved photoemission measurements on the (112) crystal surface and detailed band structure calculations. The measured Fermi surface and band structure show a good agreement with the band structure calculations with two bulk Dirac-like bands approaching the Fermi level and forming Dirac points near the Brillouin zone center. Moreover, the topological surface state with a linear dispersion approaching the Fermi level is identified for the first time. These results provide strong experimental evidence on the nature of topologically non-trivial three-dimensional Dirac cones in Cd3As2.
Raman scattering in the three-dimensional Dirac semimetal Cd3As2 shows an intricate interplay of electronic and phonon degrees of freedom. We observe resonant phonon scattering due to interband transitions, an anomalous anharmonicity of phonon frequency and intensity, as well as quasielastic (E~0) electronic scattering. The latter two effects are governed by a characteristic temperature scale T* ~ 100 K that is related to mutual fluctuations of lattice and electronic degrees of freedom. A refined analysis shows that this characteristic temperature corresponds to the energy of optical phonons which couple to interband transitions in the Dirac states of Cd3As2. As electron-phonon coupling in a topological semimetal is primarily related to phonons with finite momenta, the back action on the optical phonons in only observed as anharmonicities via multi-phonon processes involving a broad range of momenta. The resulting energy density fluctuations of the coupled system have previously only been observed in low dimensional or frustrated spin systems with suppressed long range ordering.
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