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 500 µV K-1 and a corresponding thermoelectric power factor over 30 mW K-2 m-1 are observed at 5 K 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 cryogenic temperatures.

Zeeman Field-Induced Two-Dimensional Weyl Semimetal Phase in Cadmium Arsenide.

We report a topological phase transition in quantum-confined cadmium arsenide (Cd_{3}As_{2}) thin films under an in-plane Zeeman field when the Fermi level is tuned into the topological gap via an electric field. Symmetry considerations in this case predict the appearance of a two-dimensional Weyl semimetal (2D WSM), with a pair of Weyl nodes of opposite chirality at charge neutrality that are protected by space-time inversion (C_{2}T) symmetry. We show that the 2D WSM phase displays unique transport signatures, including saturated resistivities on the order of h/e^{2} that persist over a range of in-plane magnetic fields. Moreover, applying a small out-of-plane magnetic field, while keeping the in-plane field within the stability range of the 2D WSM phase, gives rise to a well-developed odd integer quantum Hall effect, characteristic of degenerate, massive Weyl fermions. A minimal four-band k·p model of Cd_{3}As_{2}, which incorporates first-principles effective g factors, qualitatively explains our findings.

Edge Channel Transmission through a Quantum Point Contact in the Two-Dimensional Topological Insulator Cadmium Arsenide.

Cadmium arsenide (Cd3As2) thin films feature a two-dimensional topological insulator (2D TI) phase for certain thicknesses, which theoretically hosts a set of counterpropagating helical edge states that are characteristic of a quantum spin Hall (QSH) insulator. In devices containing electrostatically defined junctions and for magnetic fields below a critical value, chiral edge modes of the quantum Hall effect can coexist with QSH-like edge modes. In this work, we use a quantum point contact (QPC) device to characterize edge modes in the 2D TI phase of Cd3As2 and to understand how they can be controllably transmitted, which is important for use in future quantum interference devices. We investigate equilibration among both types of modes and find non-spin-selective equilibration. We also demonstrate the effect of the magnetic field on suppressing equilibration. We discuss the potential role of QSH-like modes in a transmission pathway that precludes full pinch-off.

Two-Dimensional Topological Insulator State in Cadmium Arsenide Thin Films.

Two-dimensional topological insulators (2D TIs) are a highly desired quantum phase but few materials have demonstrated clear signatures of a 2D TI state. It has been predicted that 2D TIs can be created from thin films of three-dimensional TIs by reducing the film thickness until the surface states hybridize. Here, we employ this technique to report the first observation of a 2D TI state in epitaxial thin films of cadmium arsenide, a prototype Dirac semimetal in bulk form. Using magnetotransport measurements with electrostatic gating, we observe a Landau level spectrum and quantum Hall effect that are in excellent agreement with those of an ideal 2D TI. Specifically, we observe a crossing of the zeroth Landau levels at a critical magnetic field. We show that the film thickness can be used to tune the critical magnetic field. Moreover, a larger change in film thickness causes a transition from a 2D TI to a 2D trivial insulator, just as predicted by theory. The high degree of tunability available in epitaxial cadmium arsenide heterostructures can thus be used to fine-tune the 2D TI, which is essential for future topological devices.

Ultralow thermal conductivity of turbostratically disordered MoSe2 ultra-thin films and implications for heterostructures.

Films containing 8, 16, 24, 32 and 64 MoSe2 layers were synthesized using the modulated elemental reactants method. X-ray reflectivity patterns showed that the annealed films were the targeted number of MoSe2 layers thick with atomically smooth interfaces. In-plane x-ray diffraction (XRD) scans contained only hk0 reflections for crystalline MoSe2 monolayers. Specular XRD patterns contained only 00l reflections, also indicating that the hk0 plane of the MoSe2 layers are parallel to the substrate. Both XRD and electron microscopy techniques indicated that the hk0 planes are rotationally disordered with respect to one another, with all orientations equally probable for large areas. The rotational disorder between MoSe2 layers is present even when analyzed spots are within 10 nm of one another. Cross-plane thermal conductivities of 0.07-0.09 W m-1 K-1 were measured by time domain thermoreflectance, with the thinnest films exhibiting the lowest conductivity. The structural analysis suggests that the ultralow thermal conductivity is a consequence of rotational disorder, which increases the separation between MoSe2 layers. The bonding environment of the Se atoms also becomes significantly distorted from C 3v symmetry due to the rotational disorder between layers. This structural disorder efficiently reduces the group velocity of the transverse phonon modes but not that of longitudinal modes. Since rotational disorder between adjacent layers in heterostructures is expected if the constituents have incommensurate lattices, this study indicates that these heterostructures will have very low cross-plane thermal conductivity.

Structural Changes as a Function of Thickness in [(SnSe)1+δ]mTiSe2 Heterostructures.

Single- and few-layer metal chalcogenide compounds are of significant interest due to structural changes and emergent electronic properties on reducing dimensionality from three to two dimensions. To explore dimensionality effects in SnSe, a series of [(SnSe)1+δ]mTiSe2 intergrowth structures with increasing SnSe layer thickness (m = 1-4) were prepared from designed thin-film precursors. In-plane diffraction patterns indicated that significant structural changes occurred in the basal plane of the SnSe constituent as m is increased. Scanning transmission electron microscopy cross-sectional images of the m = 1 compound indicate long-range coherence between layers, whereas the m ≥ 2 compounds show extensive rotational disorder between the constituent layers. For m ≥ 2, the images of the SnSe constituent contain a variety of stacking sequences of SnSe bilayers. Density functional theory calculations suggest that the formation energy is similar for several different SnSe stacking sequences. The compounds show unexpected transport properties as m is increased, including the first p-type behavior observed in (MSe)m(TiSe2)n compounds. The resistivity of the m ≥ 2 compounds is larger than for m = 1, with m = 2 being the largest. At room temperature, the Hall coefficient is positive for m = 1 and negative for m = 2-4. The Hall coefficient of the m = 2 compound changes sign as temperature is decreased. The room-temperature Seebeck coefficient, however, switches from negative to positive at m = 3. These properties are incompatible with single band transport indicating that the compounds are not simple composites.

Kinetically Controlled Formation and Decomposition of Metastable [(BiSe)1+δ] m[TiSe2] m Compounds.

Preparing homologous series of compounds allows chemists to rapidly discover new compounds with predictable structure and properties. Synthesizing compounds within such a series involves navigating a free energy landscape defined by the interactions within and between constituent atoms. Historically, synthesis approaches are typically limited to forming only the most thermodynamically stable compound under the reaction conditions. Presented here is the synthesis, via self-assembly of designed precursors, of isocompositional incommensurate layered compounds [(BiSe)1+δ] m[TiSe2] m with m = 1, 2, and 3. The structure of the BiSe bilayer in the m = 1 compound is not that of the binary compound, and this is the first example of compounds where a BiSe layer thicker than a bilayer in heterostructures has been prepared. Specular and in-plane X-ray diffraction combined with high-resolution electron microscopy data was used to follow the formation of the compounds during low-temperature annealing and the subsequent decomposition of the m = 2 and 3 compounds into [(BiSe)1+δ]1[TiSe2]1 at elevated temperatures. These results show that the structure of the precursor can be used to control reaction kinetics, enabling the synthesis of kinetically stable compounds that are not accessible via traditional techniques. The data collected as a function of temperature and time enabled us to schematically construct the topology of the free energy landscape about the local free energy minima for each of the products.

High-κ Lanthanum Zirconium Oxide Thin Film Dielectrics from Aqueous Solution Precursors.

Metal oxide thin films are critical components in modern electronic applications. In particular, high-κ dielectrics are of interest for reducing power consumption in metal-insulator-semiconductor (MIS) field-effect transistors. Although thin-film materials are typically produced via vacuum-based methods, solution deposition offers a scalable and cost-efficient alternative. We report an all-inorganic aqueous solution route to amorphous lanthanum zirconium oxide (La2Zr2O7, LZO) dielectric thin films. LZO films were spin-cast from aqueous solutions of metal nitrates and annealed at temperatures between 300 and 600 °C to produce dense, defect-free, and smooth films with subnanometer roughness. Dielectric constants of 12.2-16.4 and loss tangents <0.6% were obtained for MIS devices utilizing LZO as the dielectric layer (1 kHz). Leakage currents <10-7 A cm-2 at 4 MV cm-1 were measured for samples annealed at 600 °C. The excellent surface morphology, high dielectric constants, and low leakage current densities makes these LZO dielectrics promising candidates for thin-film transistor devices.