Quantized surface transport in topological Dirac semimetal films

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.

Evidences of Topological Surface States in the Nodal-Line Semimetal SnTaS2 Nanoflakes

Exploring the topological surface state of a topological semimetal by the transport technique has always been a big challenge because of the overwhelming contribution of the bulk state. In this work, we perform systematic angular-dependent magnetotransport measurements and electronic band calculations on SnTaS₂ crystals, a layered topological nodal-line semimetal. Distinct Shubnikov-de Haas quantum oscillations were observed only in SnTaS₂ nanoflakes when the thickness was below about 110 nm, and the oscillation amplitudes increased significantly with decreasing thickness. By analysis of the oscillation spectra, together with the theoretical calculation, a two-dimensional and topological nontrivial nature of the surface band is unambiguously identified, providing direct transport evidence of drumhead surface state for SnTaS₂. Our comprehensive understanding of the Fermi surface topology of the centrosymmetric superconductor SnTaS₂ is crucial for further research on the interplay of superconductivity and nontrivial topology.

Elemental Topological Dirac Semimetal α-Sn with High Quantum Mobility

α-Sn provides an ideal avenue to investigate novel topological properties owing to its rich diagram of topological phases and simple elemental material structure. Thus far, however, the realization of high-quality α-Sn remains a challenge, which limits the understanding of its quantum transport properties and device applications. Here, epitaxial growth of α-Sn on InSb (001) with the highest quality thus far is presented. The studied samples exhibit unprecedentedly high quantum mobilities of both the surface state (30 000 cm² V^-1 s^-1 ), which is ten times higher than the previously reported values, and the bulk heavy-hole state (1800 cm² V^-1 s^-1 ), which is never obtained experimentally. These excellent features allow quantitative characterization of the nontrivial interfacial and bulk band structure of α-Sn via a thorough investigation of Shubnikov-de Haas oscillations combined with first-principles calculations. The results firmly identify that α-Sn grown on InSb (001) is a topological Dirac semimetal (TDS). Furthermore, a crossover from the TDS to a 2D topological insulator and a subsequent phase transition to a trivial insulator when varying the thickness of α-Sn are demonstrated. This work indicates that α-Sn is an excellent model system to study novel topological phases and a prominent material candidate for topological devices.

Field-Tunable One-Sided Higher-Order Topological Hinge States in Dirac Semimetals

Recently, higher-order topological matter and 3D quantum Hall effects have attracted a great amount of attention. The Fermi-arc mechanism of the 3D quantum Hall effect proposed to exist in Weyl semimetals is characterized by the one-sided hinge states, which do not exist in all the previous quantum Hall systems, and more importantly, pose a realistic example of the higher-order topological matter. The experimental effort so far is in the Dirac semimetal Cd_{3}As_{2}, where, however, time-reversal symmetry leads to hinge states on both sides of the top and bottom surfaces, instead of the aspired one-sided hinge states. We propose that under a tilted magnetic field, the hinge states in Cd_{3}As_{2}-like Dirac semimetals can be one sided, highly tunable by field direction and Fermi energy, and robust against weak disorder. Furthermore, we propose a scanning tunneling Hall measurement to detect the one-sided hinge states. Our results will be insightful for exploring not only the quantum Hall effects beyond two dimensions, but also other higher-order topological insulators in the future.

Intrinsic coupling between spatially-separated surface Fermi-arcs in Weyl orbit quantum Hall states

Topological semimetals hosting bulk Weyl points and surface Fermi-arc states are expected to realize unconventional Weyl orbits, which interconnect two surface Fermi-arc states on opposite sample surfaces under magnetic fields. While the presence of Weyl orbits has been proposed to play a vital role in recent observations of the quantum Hall effect even in three-dimensional topological semimetals, actual spatial distribution of the quantized surface transport has been experimentally elusive. Here, we demonstrate intrinsic coupling between two spatially-separated surface states in the Weyl orbits by measuring a dual-gate device of a Dirac semimetal film. Independent scans of top- and back-gate voltages reveal concomitant modulation of doubly-degenerate quantum Hall states, which is not possible in conventional surface orbits as in topological insulators. Our results evidencing the unique spatial distribution of Weyl orbits provide new opportunities for controlling the novel quantized transport by various means such as external fields and interface engineering.

The spatial distribution of the quantized transport due to the presence of Weyl orbits in topological semimetals remains elusive. Here, the authors report concomitant modulation of doubly-degenerate quantum Hall states, evidencing intrinsic coupling between two spatially separated surface states in the Weyl orbits of a Dirac semimetal film.

Strain engineering of the magnetic multipole moments and anomalous Hall effect in pyrochlore iridate thin films

The recent observation of the anomalous Hall effect (AHE) without notable magnetization in antiferromagnets has suggested that ferromagnetic ordering is not a necessary condition. Thus, recent theoretical studies have proposed that higher-rank magnetic multipoles formed by clusters of spins (cluster multipoles) can generate the AHE without magnetization. Despite such an intriguing proposal, controlling the unconventional AHE by inducing these cluster multipoles has not been investigated. Here, we demonstrate that strain can manipulate the hidden Berry curvature effect by inducing the higher-rank cluster multipoles in spin-orbit-coupled antiferromagnets. Observing the large AHE on fully strained antiferromagnetic Nd₂Ir₂O₇ thin films, we prove that strain-induced cluster T ₁-octupoles are the only source of observed AHE. Our results provide a previously unidentified pathway for generating the unconventional AHE via strain-induced magnetic structures and establish a platform for exploring undiscovered topological phenomena via strain in correlated materials.

Controllable p-n junctions in three-dimensional Dirac semimetal Cd3As2 nanowires

We demonstrate a controllable p-n junction in a three-dimensional Dirac semimetal (DSM) Cd₃As₂ nanowire with two recessed bottom gates. The device exhibits four different conductance regimes with gate voltages, the unipolar (n-n and p-p) and bipolar (n-p and n-p) regimes, where p-n junctions are formed. The conductance in the p-n junction regimes decreases drastically when a magnetic field is applied perpendicular to the nanowire. In these regimes, the device shows quantum dot behavior, whereas the device exhibits conductance plateaus in the n-n regime at high magnetic fields. Our experiment shows that the ambipolar tunability of DSM nanowires can enable the realization of quantum devices based on quantum dots and electron optics.