Dirac Semimetal Cd3As2 Research Articles

Enhancement of the thermoelectric figure of merit in the dirac semimetal Cd3As2 by band-structure and -filling control

ABSTRACT Topological materials attract a considerable research interest because of their characteristic band structure giving rise to various new phenomena in quantum physics. Beside this, they are tempting from a functional materials point of view: Topological materials bear potential for an enhanced thermoelectric efficiency because they possess the required ingredients, such as intermediate carrier concentrations, large mobilities, heavy elements etc. Against this background, this work reports an enhanced thermoelectric performance of the topological Dirac semimetal Cd3As2 upon alloying the trivial semiconductor Zn3As2. This allows to gain fine-tuned control over both the band filling and the band topology in Cd3-x Zn x As2. As a result, the thermoelectric figure of merit exceeds 0.5 around x = 0.6 and x = 1.2 at elevated temperatures. The former is due to an enhancement of the power factor, while the latter is a consequence of a strong suppression of the thermal conductivity. In addition, in terms of first-principle band structure calculations, the thermopower in this system is theoretically evaluated, which suggests that the topological aspects of the band structure change when traversing x = 1.2 .

Extraordinary Thermoelectric Properties of Topological Surface States in Quantum-Confined Cd3As2 Thin Films.

Topological insulators and semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport associated with the Fermi arc topological surface states on topological Dirac semimetals remains less explored. This work systematically examines thermoelectric transport in a series of topological Dirac semimetal Cd3As2 thin films grown by molecular beam epitaxy. Surprisingly, significantly enhanced Seebeck effect and anomalous Nernst effect are found at cryogenic temperatures when the Cd3As2 layer is thin. In particular, a peak Seebeck coefficient of nearly 500µVK-1 and a corresponding thermoelectric power factor over 30mWK-2m-1 are observed at 5K in a 25-nm-thick sample. Combining angle-dependent quantum oscillation analysis, magnetothermoelectric measurement, transport modeling, and first-principles simulation, the contributions from bulk and surface conducting channels are isolated and the unusual thermoelectric properties are attributed to the topological surface states. The analysis showcases the rich thermoelectric transport physics in quantum-confined topological Dirac semimetal thin films and suggests new routes to achieving high thermoelectric performance at cryogenictemperatures.

Leggett modes in a Dirac semimetal

Experiments have shown that several materials, including MgB2, iron-based superconductors and monolayer NbSe2, are multiband superconductors. Superconducting pairing in multiple bands can give rise to phenomena not available in a single band, including Leggett modes. A Leggett mode is the collective periodic oscillation of the relative phase between the phases of the superconducting condensates formed in the different bands. The experimental observation of Leggett modes is challenging because multiband superconductors are rare and because these modes describe charge fluctuations between bands and therefore are hard to probe directly. Also, the excitation energy of a Leggett mode is often larger than the superconducting gaps, and therefore they are strongly overdamped via relaxation processes into the quasiparticle continuum. Here, we show that Leggett modes and their frequency can be detected in a.c. driven superconducting quantum interference devices. We then use the results to analyse the measurements of such a quantum device, one based on a Dirac semimetal Cd3As2, in which superconductivity is induced by proximity to superconducting Al. These results show the theoretically predicted signatures of Leggett modes, and therefore we conclude that a Leggett mode is present in the two-band superconducting state of Cd3As2.

MAGNETO-OPTICAL KERR SPECTROSCOPY OF (СD 3 AS 2)100-X (MNAS) X COMPOSITES

Composite Films (Сd3As2)100-X(MnAs)X on silicon and sitall substrates with a concentration of Mn 5.8-16.4 at.% were obtained by vacuum-thermal evaporation. The structural properties of the films were investigated by X-ray phase analysis and scanning electron microscopy. Magneto-optical properties were studied by the method of the equatorial Kerr e ect (EEC) in the energy range of 0.5-4.0 eV in magnetic elds with a strength of up to 3 kE at a temperature of 20-300 K. In the geometry of the equatorial Kerr effect, spectral, field and temperature dependences of the EEC are obtained. The analysis of experimental data showed that with a Mn content of more than 12.9 at.% of the films contain the α′′-phase of the topological Dirac semimetal Cd3As2 in the form of large granules, as well as ferromagnetic MNAs granules. The Curie temperature of lms depends on the Mn content in them and is less than the temperature of bulk samples ofMPas. When the Mp content in the lm is 5.8 at.% and 6.4 at.%, the magneto-optical response is not detected, which indicates the formation of a superparamagnetic state or a spin glass state at low concentrations of Mn. With a film content of Mn 9.9 at%. A signi cant change in magneto-optical spectra was detected, which indicates the formation of MNAs nanoclusters and partial dissolution of Mn in the Cd3As2 matrix.

Study of the electrophysical and magnetic properties of a Dirac 3D semimetal Cd3As2 with nanogranules of MnAs

We report on the main results of studying the electrical and magnetoresistance (MR) of a composite material consisting of 70 % mol. Dirac semi-metal Cd3As2 and 30 % mol. ferromagnet MnAs at pressures up to 50 GPa in a diamond anvil cell with a «rounded cone-flat» type anvils, as well as magnetization at hydrostatic pressures up to 6 GPa in a toroid-shaped high-pressure cell, both at room temperature and in the temperature range of 180 – 350 K at atmospheric pressure. A mixture of methanol and ethanol in a ratio of 4:1 was used as a pressure transmitting medium. Elemental analysis of Cd3As2 + 30 % mol MnAs composites showed that much of the volume is occupied by the Cd3As2 phase. The proportion of MnAs phase inclusions is less than 5 %. The feature of Cd3As2 + MnAs is the presence of a significant region of non-mixing of the Cd3As2 and MnAs phase melts. A negative MR was revealed with increasing pressure in the entire studied baric zone. The maximum negative MR is observed in the baric zone of 22 – 26 GPa. Further increase in the pressure up to the maximum level result in the appearance of several extrema on the ΔR/R0(P) curve, with negative MR not exceeding 4 %. Upon pressure release from 50 GPa, the baric dependence of ΔR/R0(P) is characterized by an inversion of the MR sign: at pressures around 40 GPa, a negative MR is replaced by a positive MR, and at around 20 GPa, the maximum value of positive MR of ~5.3 % is observed. Signs of the instability of the monoclinic structure of Cd3As2 resulted from its partial decomposition upon decompression were revealed. The results obtained can be used in spintronics when using appropriate composite materials.

Electric modulation of the Fermi arc spin transport via three-terminal configuration in topological semimetal nanowires

Spin–momentum locking is a key feature of the topological surface state, which plays an important role in spintronics. The electrical detection of current-induced spin polarization protected by the spin–momentum locking in nonmagnetic systems provides a new platform for developing spintronics, while previous studies were mostly based on magnetic materials. In this study, the spin transport measurement of Dirac semimetal Cd3As2 was studied by three-terminal geometry, and a hysteresis loop signal with high resistance and low resistance state was observed. The hysteresis was reversed by reversing the current direction, which illustrates the spin–momentum locking feature of Cd3As2. Furthermore, we realized the on–off states of the spin signals through electric modulation of the Fermi arc via the three-terminal configuration, which enables the great potential of Cd3As2 in spin field-effect transistors.

Influence of Magnetic and Electric Fields on Universal Conductance Fluctuations in Thin Films of the Dirac Semimetal Cd3As2.

Time-reversal invariance (TRS) and inversion symmetry (IS) are responsible for the topological band structure in Dirac semimetals (DSMs). 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 using universal conductance fluctuations (UCF) in the prototypical DSM, Cd3As2. With increasing magnetic field, the magnitude of the UCF decreases by a factor of , in agreement with numerical calculations of the effect of broken TRS. In contrast, the magnitude of the UCF increases monotonically when the chemical potential is gated away from the charge neutrality point. We attribute this to Fermi surface anisotropy rather than broken IS. The concurrence between experimental data and theory provides unequivocal evidence that UCF are the dominant source of fluctuations and offers a general methodology for probing broken-symmetry effects in topological quantum materials.

Nonlinear current response and electric quantum oscillations in the Dirac semimetal Cd3As2

Chiral anomaly is a distinct quantum anomaly associated with chiral fermions in Dirac or Weyl semimetals. The use of negative magnetoresistance (negative MR) as a signature for this anomaly remains contentious, as trivial mechanisms such as current jetting and weak localization can also induce negative MR. In this study, we report a novel nonlinear behavior of the chiral anomaly in the longitudinal direction, which we observed by applying parallel current and magnetic field to the Dirac semimetal Cd3As2. This nonlinear characteristic peaks at an intermediate magnetic field of approximately 5 T, displaying a resistance-increasing property concomitant with strengthening of the current source. Through angle-dependence experiments, we were able to rule out trivial factors, such as thermal effects, geometric artifacts, and anisotropy. Furthermore, additional electric quantum oscillations were observed when the direct current (DC) was applied as high as 300 μA. Such an unusual phenomenon is ascribed to the formation of quantized levels due to Bloch oscillation in the high DC regime, suggesting that an oscillatory density distribution may arise as the electric field increases. The non-Ohmic electric quantum oscillations open a new avenue for exploring chiral anomaly and other nontrivial topological properties, which is also one of the salient features of nonequilibrium steady states in condensed matter physics.

Narrowband, Angle-Tunable, Helicity-Dependent Terahertz Emission from Nanowires of the Topological Dirac Semimetal Cd3As2.

All-optical control of terahertz pulses is essential for the development of optoelectronic devices for next-generation quantum technologies. Despite substantial research in THz generation methods, polarization control remains difficult. Here, we demonstrate that by exploiting band structure topology, both helicity-dependent and helicity-independent THz emission can be generated from nanowires of the topological Dirac semimetal Cd3As2. We show that narrowband THz pulses can be generated at oblique incidence by driving the system with optical (1.55 eV) pulses with circular polarization. Varying the incident angle also provides control of the peak emission frequency, with peak frequencies spanning 0.21-1.40 THz as the angle is tuned from 15 to 45°. We therefore present Cd3As2 nanowires as a promising novel material platform for controllable terahertz emission.

Epitaxial Integration of Dirac Semimetals with Si(001)

Topological semimetals contain novel combinations of properties that make them useful in a variety of applications, including optoelectronics, spintronics and low energy computing, and catalysis. Although they have been grown with high quality as bulk single crystals, incorporation with semiconductor substrates will ultimately be required to maximize their technological reach. Here, epitaxial growth of the Dirac semimetal Cd3As2 on Si(001) is demonstrated through two routes. First, Cd3As2(112) epilayers are grown on Si(001) via an intermediate CdTe(111) buffer layer. Second, Cd3As2(112) is grown directly on Si(001). This work sets the foundation for integration of novel semimetal materials with existing CMOS technology.

Anisotropy of the Paramagnetic Susceptibility in the 3D Dirac Semimetal Cd-=SUB=-3-=/SUB=-As-=SUB=-2-=/SUB=- Caused by Chromium Impurity: the EPR on Cr-=SUP=-3+-=/SUP=- Ions

In the literature devoted to magnetic impurities in Dirac semimetals, the appearance of anisotropic and long-range non-oscillating contributions to the Ruderman--Kittel--Kasuya--Yosida interaction is predicted. These contributions are due to the hybridization of the wave functions of magnetic impurities with the wave functions of Dirac electrons, which have a linear dispersion law. We have studied electron paramagnetic resonance (EPR) in powder samples of topological 3D Dirac semimetal Cd3As2 with an admixture of chromium ions. Anisotropy of the magnetic susceptibility in the paramagnetic state was found from the behavior of the resonance field. The appearance of magnetic anisotropy in the paramagnetic state is associated with an increase in the contribution of the orbital moments of chromium ions Cr2+ and Cr3+ due to their interaction with the orbital moments of donor Dirac electrons, which have anomalously large values of g factors. Keywords: magnetic resonance, topological materials, magnetic impurities.

Анизотропия парамагнитной восприимчивости дираковского полуметалла Cd-=SUB=-3-=/SUB=-As-=SUB=-2-=/SUB=-, обусловленная примесью хрома: ЭПР на ионах Cr-=SUP=-3+-=/SUP=-

In the literature devoted to magnetic impurities in Dirac semimetals, the appearance of anisotropic and long-range non-oscillating contributions to the Ruderman–Kittel–Kasuya–Yosida interaction is predicted. These contributions are due to the hybridization of the wave functions of magnetic impurities with the wave functions of Dirac electrons, which have a linear dispersion law. We have studied electron paramagnetic resonance (EPR) in powder samples of topological 3D Dirac semimetal Cd3As2 with an admixture of chromium ions. Anisotropy of the magnetic susceptibility in the paramagnetic state was found from the behavior of the resonance field. The appearance of magnetic anisotropy in the paramagnetic state is associated with an increase in the contribution of the orbital moments of chromium ions Cr2+ and Cr3+ due to their interaction with the orbital moments of donor Dirac electrons, which have anomalously large values of g factors.

Recent advances in photonics of three-dimensional Dirac semimetal Cd3As2

Due to their unusual features in condensed matter physics and their applicability in optical and optoelectronic applications, three-dimensional Dirac semimetals (3D DSMs) have garnered substantial interest in recent years. In contrast to monolayer graphene, 3D DSM exhibits linear band dispersion despite its macroscopic thickness. Therefore, being a bulk material, it is easy to make nanostructures with 3D DSM, just as one normally does with metals such as gold and silver. Among 3D DSMs, cadmium arsenide (Cd3As2) is quite famous and considered an excellent 3D DSM due to its chemical stability in air and extraordinary optical response. In this review, advances in 3D DSM Cd3As2 fabrication techniques and recent progress in the photonics of 3D DSM Cd3As2 are given and briefly reviewed. Various photonic features, including linear and nonlinear plasmonics, optical absorption, optical harmonic generation, and ultrafast dynamics, have been explored in detail. It is expected that Cd3As2 would share an excellent tunable photonic response like graphene. We envision that this article may serve as a concise overview of the recent progress of photonics in 3D DSM Cd3As2 and provides a compact reference for young researchers.

Ultrafast Optical Probe of Coherent Acoustic Phonons in Dirac Semimetal Cd3As2 Film Epitaxied on GaAs(111)B Substrate.

Coherent longitudinal acoustic phonon (CAP) generation in epitaxial Dirac semimetal Cd3As2 films with different thicknesses was investigated by a time-resolved reflectance technique. The short-lived weak CAP oscillations can be observed only in the thicker Cd3As2 films, and their central frequency of 0.039 THz has no dependence on sample thickness, but is nearly inversely proportional to the probe wavelength. For the 20 nm thin film, the observed long-lived CAP with a central frequency of 0.049 THz is generated in the GaAs(111)B substrate underneath. A sound velocity of 3800 m/s for the Cd3As2 film and 5360 m/s for the GaAs(111)B substrate is thus deduced. In addition, the opposite CAP amplitude and lifetime dependence on temperature further confirms the electronic and thermal stress origination of CAP generated in GaAs(111)B and Cd3As2 film, respectively, based on the propagating strain pulse model. The central frequency of CAP is found to be stable with increasing pumping fluence and temperature, which makes Cd3As2 a potential material for thermoelectric device applications.

Unusual thermodynamics of low-energy phonons in the Dirac semimetal Cd3As2

By studying the thermal conductivity, specific heat, elastic modulus, and thermal expansion as a function of temperature for Cd3As2, we have unveiled a couple of important thermodynamic features of the low-energy phonons strongly interacting with Dirac electrons. The existence of soft optical phonons, as inferred from the extremely low thermal conductivity, is unambiguously confirmed by low-temperature specific heat revealing significant deviation from Debye’s description. The estimated Debye temperature is small in the range of 100–200 K and varies significantly depending upon the measurement used in its experimental determination. The thermodynamic Grüneisen ratio γ reveals a remarkable reduction below about 100 K, an energy scale that is highly relevant to the Dirac states, towards negative values below about 10 K that are indicative of lattice instability.

Photodetection and Infrared Imaging Based on Cd3As2 Epitaxial Vertical Heterostructures

Due to the nontrivial electronic structure, Cd3As2 is predicted to possess various transport properties and outstanding photoresponses. Photodetectors based on topological materials are mostly made up of nanoplates, yet monolithic in situ heteroepitaxial Cd3As2 photodetectors are rarely reported to date owing to the crystal mismatch between Cd3As2 and semiconductors. Here, we demonstrate Cd3As2/ZnxCd1-xTe/GaSb vertical heteroepitaxial photodetectors via molecule beam epitaxy. By constructing dual-Schottky junctions, these photodetectors show high responsivity and external quantum efficiency in a broadband spectrum. Based on the strong and fast photoresponse, we achieved visible light to near-infrared imaging using a one-pixel imaging system with a galvo. Our results illustrate that the integration of three-dimensional Dirac semimetal Cd3As2 with semiconductors has potential applications in broadband photodetection and infrared cameras.

Evidence of decoupling of surface and bulk states in Dirac semimetal Cd3As2

Dirac semimetals have attracted a great deal of current interests due to their potential applications in topological quantum computing, low-energy electronic devices, and single photon detection in the microwave frequency range. Herein are results from analyzing the low magnetic (B) field weak-antilocalization behaviors in a Dirac semimetal Cd3As2 thin flake device. At high temperatures, the phase coherence length l ϕ first increases with decreasing temperature (T) and follows a power law dependence of l ϕ ∝ T −0.4. Below ∼3 K, l ϕ tends to saturate to a value of ∼180 nm. Another fitting parameter α, which is associated with independent transport channels, displays a logarithmic temperature dependence for T > 3 K, but also tends to saturate below ∼3 K. The saturation value, ∼1.45, is very close to 1.5, indicating three independent electron transport channels, which we interpret as due to decoupling of both the top and bottom surfaces as well as the bulk. This result, to our knowledge, provides first evidence that the surfaces and bulk states can become decoupled in electronic transport in Dirac semimetal Cd3As2.

Tracking Ultrafast Change of Multiterahertz Broadband Response Functions in a Photoexcited Dirac Semimetal Cd3As2 Thin Film.

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd3As2 by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 μm), which covers the crossover between the inter- and intraband response. We resolve dynamics of the photoexcited nonthermal electrons, which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics.

Gate modulation of anisotropic superconductivity in Al–Dirac semimetal Cd3As2 nanoplate–Al Josephson junctions

Three-dimensional Dirac semimetal Cd3As2, hosting a pair of Dirac cones and Fermi arc-like surface states, displays numerous exotic properties in transport experiments. In particular, when proximitized with a superconductor, Cd3As2 is expected to realize topological superconductivity and Majorana zero modes, which are essential for fault-tolerant quantum computing. Here, using electronic transport measurements on superconductor Al–Cd3As2 nanoplate–Al heterostructures, we investigate the effect of gate modulation and magnetic field on the superconducting properties of Cd3As2. A proximity-induced superconducting state is well achieved in the junction, which can be effectively tuned by the gate voltage. The critical current oscillations under out-of-plane magnetic fields are well fitted with the Fraunhofer function. The critical supercurrent shows a slower decay as the gate voltage is tuned to negative under in-plane magnetic fields, which may arise from the enhanced contribution of surface states. Anisotropic superconductivity is also observed with in-plane rotating magnetic fields. Our results report the gate modulation of supercurrents in different magnetic field directions, which should be valuable for further exploring the topological superconductivity in Dirac semimetals.

Challenges to magnetic doping of thin films of the Dirac semimetal Cd3As2

Magnetic doping of topological quantum materials provides an attractive route for studying the effects of time-reversal symmetry breaking. Thus motivated, we explore the introduction of the transition metal Mn into thin films of the Dirac semimetal Cd3As2 during growth by molecular beam epitaxy. Scanning transmission electron microscopy measurements show the formation of a Mn-rich phase at the top surface of Mn-doped Cd3As2 thin films grown using both uniform doping and delta doping. This suggests that Mn acts as a surfactant during epitaxial growth of Cd3As2, resulting in phase separation. Magnetometry measurements of such samples indicate a ferromagnetic phase with out-of-plane magnetic anisotropy. Electrical magneto-transport measurements of these films as a function of temperature, magnetic field, and chemical potential reveal a lower carrier density and higher electron mobility compared to pristine Cd3As2 films grown under similar conditions. This suggests that the surfactant effect might also serve to getter impurities. We observe robust quantum transport (Shubnikov-de Haas oscillations and an incipient integer quantum Hall effect) in very thin (7 nm) Cd3As2 films despite being in direct contact with a structurally disordered surface ferromagnetic overlayer.