16 Search Results

Thin film synthesis allows for the potential to orient crystals in different orientations, permitting measurement of orientation-dependent material aspects such as band structures and transport anisotropy. Here, Dirac semimetal Cd₃As₂ films are epitaxially grown on GaAs$[110]$ substrates, which has a $[001]$ orientation in-plane. Films contain domains of two different c-axis orientations resulting from an aligned a-axis in-plane. Magnetoresistance measurements performed along both $$[1\bar{1}0]$$ and $[001]$ substrate directions reveal similar mobility and carrier concentration, but much larger magnetoresistance along the $[001]$ direction, which can be explained by the guiding center diffusion model as arising from anisotropic disorder and different atomic spacings.

Ternary transition metal (TM) nitrides have gained significant attention in thin film research due to their promising properties for a broad range of applications. Particularly, some of the ternary TM nitrides have been predicted to adopt layered structures that make them interesting for thermoelectric conversion and quantum materials applications. Unfortunately, synthesis of TM ternary nitride films by physical vapor deposition often favors disordered 3D structures rather than the predicted 2D-like layered structure. In this study, we investigate the structural interplay in the Sc-Ta-N ternary system using a combinatorial approach. Combinatorial libraries S⁢c_𝑥⁢T⁢a_1−𝑥⁢N are synthesized following a two-step method: First, depositmore » film precursors by cosputtering and then process the resulting 3D-structured samples with rapid thermal annealing. Synchrotron grazing-incidence wide-angle x-ray scattering on films annealed at 1200 ⁢°⁢C for 20 min leads to the nucleation of ScTaN₂ layered structure (𝑃⁢6₃/𝑚⁢𝑚⁢𝑐) near stoichiometry. We find that the layered structure can accommodate large off-stoichiometry in the Ta-rich region (𝑥 < 0.5), facilitated by the alloying with quasi-isostructural Ta₅⁢N₆ compound that exists on a composition tie line at 𝑥 = 0. While focusing on ScTaN₂, we estimate the long-range order parameter in near-stoichiometric films to be 0.86, corresponding to a fraction of Sc/Ta antisites of 7%. Transport measurements on ScTaN₂ reveal a nearly temperature-independent high carrier density (10²¹ c⁢m⁻³), suggesting a heavily doped semiconductor or semimetallic character, consistent with a small positive Seebeck coefficient of +19 µV/K. The carrier mobility at 2 K is relatively small (9.5c⁢m² V⁻¹ s⁻¹) and the residual-resistivity ratio is minor, suggesting that electrical conduction is dominated by defects or disorder. Measured magnetoresistance suggests possible weak antilocalization at 2 K. This paper highlights the interplay between ScTaN₂ and Ta₅⁢N₆ crystal structures in stabilizing layered materials, emphasizes the importance of cation order/disorder for potential tunable alloys, and suggests that ScTaN₂ is a promising platform for exploring electronic properties.« less

The prospect of transforming a Dirac topological semimetal (TSM) into a Weyl TSM phase, following doping by magnetic impurities, is central to TSM applications. The magnetic field from polarized 𝑑 levels of magnetic impurities produces a field with a sharp local structure. To what extent magnetic impurities act in the same manner as an applied field and what are the effects of such a field on the electronic structure of a Dirac TSM are the subject of this paper. We present electronic structure calculations of bulk Cd₃⁢As₂ with substitutional doping of Mn impurities in the dilute alloy range. Quasiparticle 𝐺⁡𝑊more » (QS⁢𝐺⁡𝑊) ab initio electronic structure calculations are used in conjunction with 𝑘 · 𝑝 model Hamiltonian calculations. As expected, we observe the splitting of the Dirac points into pairs of Weyl points following the doping with Mn. We also show that the electronic structure of Mn-doped Cd_3⁢As₂ can be emulated by the electronic structure of pristine Cd_3⁢As₂ with an appropriate external magnetic field. Some properties of the conductivity of bulk Cd₃⁢As₂ for different magnetic field orientations are also investigated. Our results inform future opportunities for unique device functionality based on band structure tuning not found in conventional magnetic Weyl TSM.« less

In studying compensated semimetals, the two-band model has proven extremely useful in capturing electrical conductivity under magnetic field, as a function of density and mobility of electronlike and holelike carriers. However, it rarely offers practical insight into magnetothermoelectric properties. Here, in this work, we report the field dependence of thermoelectric (TE) coefficients in a highly compensated semimetal NbSb₂, where we find the Seebeck (𝑆_𝑥⁢𝑥) and Nernst (𝑆_𝑥⁢𝑦) coefficients increase quadratically and linearly with applied magnetic field, respectively. Such field dependence was predicted in previous work that studied a system of two parabolic bands, within semiclassical Boltzmann transport theory when themore » following two conditions are simultaneously met: 𝜔_𝑐⁢𝜏 ≫ 1 and tan ⁡𝜃_𝐻 ≪ 1, where 𝜔_𝑐 and 𝜏 refer to the cyclotron frequency and relaxation time, respectively, and 𝜃_𝐻 is the Hall angle. Under these conditions, we find the field dependence of the TE coefficients directly provides a relation between the electronlike (𝑛_𝑒) and holelike (𝑛_ℎ) carrier densities, which in turn can be used to refine two-band model fitting. With this, we find the compensation factor ($$\frac{|Δ⁢𝑛|}{𝑛_{𝑒}}$$, where Δ⁢𝑛 = 𝑛_𝑒−𝑛_ℎ) of NbSb₂ is two orders of magnitude smaller than what was found in unrestricted fitting, resulting in a larger saturation-field scale for magnetoresistance. Within the same framework of the semiclassical theory, we also deduce that the thermoelectric Hall angle tan⁡ 𝜃_𝛾 = $$\frac{𝑆_{𝑥⁢𝑦}}{𝑆_{𝑥⁢𝑥}}$$ can be expressed as ($$\frac{Δ⁢𝑛}{𝑛_{𝑒}}$$ ×𝜔_𝑐⁢𝜏)⁻¹, which serves as a parameter to predict the degree of compensation. Our findings offer crucial insights into identifying empirical conditions for field-induced enhancement of TE performance and into engineering-efficient thermoelectric devices based on semimetallic materials.« less

Eu(II)-containing chalcogenides are an emerging class of materials that are of great interest due to their high optical activity and intriguing magnetism. Here, we synthesized Eu₂SiSe₄ as red-colored single crystals and characterized its structure with single-crystal X-ray diffraction, confirming the reported chiral monoclinic P2₁ symmetry at room temperature. The crystal structure of Eu₂SiSe₄ comprises distorted SiSe₄ tetrahedral units and charge-balancing Eu(II) cations. Here, we develop a two-step solid-state synthesis method for Eu₂SiSe₄ and compare it to the known boron chalcogenide method. We find the second-harmonic generation (SHG) activity of polycrystalline Eu₂SiSe₄ to be ∼7 × AgGaS₂, placing it among themore » highest-known SHG-active chalcogenides. No symmetry lowering is observed down to 100 K in single-crystal X-ray diffraction, although an anomalous expansion in the b-axis lattice parameter occurs and may be correlated to lattice modes of the SiSe₄ tetrahedra. We investigate the physical properties of Eu₂SiSe₄ using magnetometry and heat capacity measurements and find a transition to an antiferromagnetic ground state at T_N ≈ 5.5 K. The low-temperature transition releases less entropy than expected, which may be due to the complex crystal electric field effects of Eu(II).« less

The recent demonstration of the superconducting diode effect (SDE) has generated renewed interest in superconducting electronics, in which devices such as compact superconducting diodes that can perform signal rectification with low-energy operations are needed. In this article, we present our results of robust and symmetric-in-magnetic-field SDE in asymmetric superconducting quantum interference devices (SQUIDs) realized in high-quality Dirac semimetal Cd₃As₂ thin film grown by the molecular beam epitaxy technique. Consistent with previous work, a zero magnetic field SDE is observed. Furthermore, the difference in switching current is independent of the strength and polarity of an out-plane magnetic field in the rangemore » of −10 and 10 mT. We speculate that this robust symmetric-in-field SDE in our Dirac semimetal SQUIDs is due to the formation of helical spin texture, theoretically predicted in Dirac semimetals.« less

Experimental synthesis and characterization of theoretically predicted compounds are important steps in the materials discovery pipeline. Here, we report on the synthesis of Co₃PdN, which was recently predicted to be a stable magnetic antiperovskite. The Co₃PdN thin films were grown by reactive sputtering and were confirmed to form in an antiperovskite crystal structure. The thermal stability of the compound is demonstrated up to 600 K by in situ X-ray diffraction, though the phase persists at slightly higher temperatures (700 K) in an air-free magnetometer. Both ab initio calculations and magnetization measurements find Co₃PdN to be ferromagnetic with an experimentally determinedmore » Curie temperature of T_C = 560 ± 5 K. The saturation magnetization of 1.2 μ_B/Co found in the experiment is slightly lower than the 1.7 μ_B/Co value expected by theory. A narrow magnetic hysteresis loop with a coercive field of 100 Oe at low temperature suggests that Co₃PdN might be useful in electronic applications requiring fast switching of the magnetization vector. While prior prediction of Co₃PdN showed a gapped electronic band structure for each spin channel, we show that this was due to incomplete sampling of Brillouin zone paths and that band crossings exist along R-X|M and X|M-R paths. The metallic nature of Co₃PdN is further confirmed by temperature-dependent transport measurements, which also show a considerable anomalous Hall effect. Altogether, this work represents an appreciable step toward understanding the synthesis, structure, stability, and properties of a new magnetic material.« less

Abstract Transition metal oxides with insulator-metal transitions (IMTs) are uniquely suited for volatile memristor devices that mimic the spiking of biological neurons. Unlike most non-volatile memristors, which often operate via ion migration into filaments, volatile devices utilize a reversible phase change that returns to a ground state in the absence of applied stimulus. In these devices, Joule heating triggers the IMT and changes the bulk resistivity rather than influencing conduction through defects, as in previous studies. This volatile resistive switching behavior has previous been leveraged in niobium and vanadium oxides, but not in rare-earth nickelates, despite their tunable transition temperatures.more » This study demonstrates an electrically driven IMT in the prototypical rare-earth nickelate, NdNiO ₃ , in large area devices. While previous work examining the electrically-driven IMT in NdNiO ₃ suggests defect-dominated conduction, this study shows clear s-type negative differential resistance (NDR) consistent with temperature-dependent resistivity measurements. The NDR peak-to-valley voltage scales linearly with temperature as expected for conductivity pathways dominated by bulk IMT behavior. Unlike other transition metal oxides, which are modeled using the insulator-metal phase fraction as the internal state variable, a thermoelectric model with temperature as the internal state variable is found to more accurately describe the current–voltage characteristic of NdNiO ₃ volatile memristors. Overall, we report the synthesis, fabrication, and characterization of NdNiO ₃ volatile memristors with resistivity dominated by bulk-like IMT behavior which is scalable and not dependent upon oxygen vacancy migration or defect mediated conduction pathways.« less

Device applications of topological semimetals await the development of epitaxial films in the ultrathin limit. Weak antilocalization (WAL) has been extensively utilized in the understanding of surface states in topological insulators and shows promise for use in elucidating the properties of thin film topological semimetals. Here, we report insights from WAL in the surface state and interface transport properties of our recently synthesized single-crystal-like thin films of the Weyl semimetal TaAs(001) grown on GaAs(001). We observe robust, anisotropic WAL in the magnetoconductance from 2 to 20 K in films from 10 to 200 nm thick. We link the anisotropic WAL magnetoconductance tomore » anisotropic mobility stemming from film topography. We conclude that WAL in the films likely originates from the antilocalization of topological surface states. The WAL magnetoconductance is impacted by the film thickness and topography, solidifying the useful role of WAL in the study of topological semimetal/semiconductor heterointerfaces. Published by the American Physical Society 2024« less