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Register a new userDoping-induced topological transition and enhancement of thermopower in the Dirac-semimetal system Cd$_{3-x}$Zn$_x$As$_2$
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Cd$_3$As$_2$ is one of the prototypical topological Dirac semimetals. Here, we manipulate the band inversion responsible for the emergence of Dirac nodes by alloying Cd$_3$As$_2$ with topologically trivial Zn$_3$As$_2$. We observe the expected topological phase transition around a Zn concentration of $xsim 1$ while the carrier density monotonically decreases as $x$ is increased. For larger $x$, the thermoelectric figure of merit exhibits comparably large values exceeding 0.3 at room temperature, due to the combined effects of a strong enhancement of the thermopower, an only moderate increase of the resistivity, and a suppression of the thermal conductivity. Complementary quantum-oscillation data and optical-conductivity measurements allow to infer that the enhanced thermoelectric performance is due to a flattening of the band structure in the higher-$x$ region in Cd$_{3-x}$Zn$_x$As$_2$.
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We present a study on magnetotransport in films of the topological Dirac semimetal Cd$_{3}$As$_{2}$ doped with Sb grown by molecular beam epitaxy. In our weak antilocalization analysis, we find a significant enhancement of the spin-orbit scattering rate, indicating that Sb doping leads to a strong increase of the pristine band-inversion energy. We discuss possible origins of this large enhancement by comparing Sb-doped Cd$_{3}$As$_{2}$ with other compound semiconductors. Sb-doped Cd$_{3}$As$_{2}$ will be a suitable system for further investigations and functionalization of topological Dirac semimetals.
We study the low-energy electronic structure of three-dimensional Dirac semimetal, Cd$_3$(As$_{1-x}$P$_x$)$_2$ [$x$ = 0 and 0.34(3)], by employing the angle-resolved photoemission spectroscopy (ARPES). We observe that the bulk Dirac states in Cd$_3$(As$_{0.66}$P$_{0.34}$)$_2$ are gapped out with an energy of 0.23 eV, contrary to the parent Cd$_3$As$_2$ in which the gapless Dirac states have been observed. Thus, our results confirm the earlier predicted topological phase transition in Cd$_3$As$_2$ with perturbation. We further notice that the critical P substitution concentration, at which the two Dirac points that are spread along the $c$-axis in Cd$_3$As$_2$ form a single Dirac point at $Gamma$, is much lower [x$_c$(P)$<$ 0.34(3)] than the predicted value of x$_c$(P)=0.9. Therefore, our results suggest that the nontrivial band topology of Cd$_3$As$_2$ is remarkably sensitive to the P substitution and can only survive over a narrow substitution range, i.e., 0 $leq$ x (P) $<$ 0.34(3).
To probe the charge scattering mechanism in Cd$_{3}$As$_{2}$ single crystal, we have analyzed the temperature and magnetic field dependence of the Seebeck coefficient ($S$). The large saturation value of $S$ at high field clearly demonstrates the linear energy dispersion of three-dimensional Dirac fermion. A wide tunability of the charge scattering mechanism has been realized by varying the strength of the magnetic field and carrier density via In doping. With the increase in magnetic field, the scattering time crosses over from being nearly energy independent to a regime of linear dependence. On the other hand, the scattering time enters into the inverse energy-dependent regime and the Fermi surface strongly modifies with 2% In doping at Cd site. With further increase in In content from 2 to 4%, we did not observe any Shubnikov-de Haas oscillation up to 9 T field, but the magnetoresistance is found to be quite large as in the case of undoped sample.
Using first-principles density functional theory calculations, combined with a topological analysis, we have investigated the electronic properties of $Cd_3As_2$ and $Na_3Bi$ Dirac topological semimetals doped with non-magnetic and magnetic impurities. Our systematic analysis shows that the selective breaking of the inversion, rotational and time-reversal symmetry, controlled by specific choices of the impurity doping, induces phase transitions from the original Dirac semimetal to a variety of topological phases such as, topological insulator, trivial semimetal, non-magnetic and magnetic Weyl semimetal, and Chern insulator. The Dirac semimetal phase can exist only if the rotational symmetry $C_n$ with $n > 2$ is maintained. One particularly interesting phase emerging in doped $Cd_3As_2$ is a coexisting Dirac-Weyl phase, which occurs when only inversion symmetry is broken while time-reversal symmetry and rotational symmetry are both preserved. To further characterize the low-energy excitations of this phase, we have complemented our density functional results with a continuum four-band $kcdot p$ model, which indeed displays nodal points of both Dirac and Weyl type. The coexisting phase appears as a transition point between two topologically distinct Dirac phases, but may also survive in a small region of parameter space controlled by external strain.
The antiferromagnet and semimetal EuCd$_2$As$_2$ has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the $c$-axis in EuCd$_2$As$_2$. In the search for such phases, we investigate here the effects of hydrostatic pressure in EuCd$_2$As$_2$. For that, we present specific heat, transport and $mu$SR measurements under hydrostatic pressure up to $sim,2.5,$GPa, combined with {it ab initio} density functional theory (DFT) calculations. Experimentally, we establish that the ground state of EuCd$_2$As$_2$ changes from in-plane antiferromagnetic (AFM$_{ab}$) to ferromagnetic at a critical pressure of $,approx,$2,GPa, which is likely characterized by the moments dominantly lying within the $ab$ plane (FM$_{ab}$). The AFM$_{ab}$-FM$_{ab}$ transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our experimental and theoretical results indicate that EuCd$_2$As$_2$ moves closer to the sought-for FM$_c$ state (moments $parallel$ $c$) with increasing pressure further. We predict that a pressure of $approx$,23,GPa will stabilize the FM$_c$ state, if Eu remains in a 2+ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of EuCd$_2$As$_2$ and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.
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