Weak antilocalization effect of topological crystalline insulator Pb(1-x)Sn(x)Te nanowires with tunable composition and distinct {100} facets

Controlled Vapor-Liquid-Solid Growth of Long and Remarkably Thin Pb1-xSnxTe Nanowires with Strain-Tunable Ferroelectric Phase Transition

The Pb1-xSnxTe family of compounds possess a wide range of intriguing and useful physical properties, including topologically protected surface states, robust ferroelectricity, remarkable thermoelectric properties, and potential topological superconductivity. Compared to bulk crystals, one-dimensional (1D) nanowires (NWs) offer a unique platform to enhance the functional properties and enable new capabilities, e.g., to realize 1D Majorana zero modes for quantum computations. However, it has been challenging to achieve controlled synthesis of ultrathin Pb1-xSnxTe (0 ≤ x ≤ 1) nanowires in the truly 1D region. In this work, we report on a Au-catalyzed vapor-liquid-solid (VLS) growth of remarkably thin (20-30 nm) and sufficiently long (several to tens of micrometers) Pb1-xSnxTe nanowires of high single-crystalline quality in a controlled fashion. This controlled growth was achieved by enhancing the incorporation of Te into the Au catalyst particle to facilitate the precipitation of the Sn/Pb species and suppress the enlargement of the particle, which we identified as a major challenge for the growth of ultrathin nanowires. Our growth strategy can be easily extended to other compound and alloy nanowires, where the constituent elements have different incorporation rates into the catalyst particle. Furthermore, the growth of thin Pb1-xSnxTe nanowires enabled strain-dependent electrical transport measurements, which shows an enhancement of electrical resistance and ferroelectric transition temperature induced by uniaxial tensile strain along the nanowire axial direction, consistent with density functional theory calculations of the structural phase stability.

Pentagonal nanowires from topological crystalline insulators: a platform for intrinsic core-shell nanowires and higher-order topology

We report on the experimental realization of Pb1-xSnx Te pentagonal nanowires (NWs) with [110] orientation using molecular beam epitaxy techniques. Using first-principles calculations, we investigate the structural stability of NWs of SnTe and PbTe in three different structural phases: cubic, pentagonal with [001] orientation and pentagonal with [110] orientation. Within a semiclassical approach, we show that the interplay between ionic and covalent bonds favors the formation of pentagonal NWs. Additionally, we find that this pentagonal structure is more likely to occur in tellurides than in selenides. The disclination and twin boundary cause the electronic states originating from the NW core region to generate a conducting band connecting the valence and conduction bands, creating a symmetry-enforced metallic phase. The metallic core band has opposite slopes in the cases of Sn and Te twin boundaries, while the bands from the shell are insulating. We finally study the electronic and topological properties of pentagonal NWs unveiling their potential as a new platform for higher-order topology and fractional charge. These pentagonal NWs represent a unique case of intrinsic core-shell one-dimensional nanostructures with distinct structural, electronic and topological properties between the core and the shell region.

Density Functional Theory Study of the Spin-Orbit Insulating Phase in SnTe Cubic Nanowires: Implications for Topological Electronics

We investigate the electronic, structural, and topological properties of the SnTe and PbTe cubic nanowires using ab initio calculations. Using standard and linear-scale density functional theory, we go from the ultrathin limit up to the nanowire thicknesses observed experimentally. Finite-size effects in the ultrathin limit produce an electric quadrupole and associated structural distortions; these distortions increase the band gap, but they get reduced with the size of the nanowires and become less and less relevant. Ultrathin SnTe cubic nanowires are trivial band gap insulators; we demonstrate that by increasing the thickness, there is an electronic transition to a spin-orbit insulating phase due to trivial surface states in the regime of thin nanowires. These trivial surface states with a spin-orbit gap of a few meV appear at the same k-point of the topological surface states. Going to the limit of thick nanowires, we should observe the transition to the topological crystalline insulator phase with the presence of two massive surface Dirac fermions hybridized with the persistent trivial surface states. Therefore, we have the copresence of massive Dirac surface states and trivial surface states close to the Fermi level in the same region of the k-space. According to our estimation, the cubic SnTe nanowires are trivial insulators below the critical thickness tc1 = 10 nm, and they become spin-orbit insulators between tc1 = 10 nm and tc2 = 17 nm, while they transit to the topological phase above the critical thickness of tc2 = 17 nm. These critical thickness values are in the range of typical experimental thicknesses, making the thickness a relevant parameter for the synthesis of topological cubic nanowires. Pb1-xSnxTe nanowires would have both these critical thicknesses tc1 and tc2 at larger values depending on the doping concentration. We discuss the limitations of density functional theory in the context of topological nanowires and the consequences of our results on topological electronics.

Large magnetoresistance and quantum oscillations in Sn0.05Pb0.95Te

We have synthesized high-quality single crystals of Sn_xPb_1-xTe and carried out detailed studies of the magnetotransport properties of one of the samples, Sn_0.05Pb_0.95Te. Longitudinal magnetoresistance increases almost linearly with increasing applied field (H) and reaches ∼310% atH= 13 T. At higher fields, both longitudinal and Hall resistance show clear Shubnikov de Haas oscillations. The oscillations are smooth and periodic, and there exists only one frequency,f_α∼ 57 T. However, an additional frequency,f_β∼ 69 T, appears as the angle between the field direction and the normal to the sample surface (θ) is increased. Bothf_αandf_βexhibitθ-dependence;f_αdecreases whereasf_βincreases gradually with increasingθ. The presence of two frequencies in Sn_0.05Pb_0.95Te indicates that there exist two Fermi surface pockets (αandβpockets). We have constructed the Landau-level fan plot and determined the Berry phase (δ) for theαpocket to beδ∼ 0.1. Thisδvalue is very close to the expected value of 0 for a topologically trivial system.

Development of topological insulator and topological crystalline insulator nanostructures

Topological insulators (TIs), a class of quantum materials with time reversal symmetry protected gapless Dirac-surface states, have attracted intensive research interests due to their exotic electronic properties. Topological crystalline insulators (TCIs), whose gapless surface states are protected by the crystal symmetry, have recently been proposed and experimentally verified as a new class of TIs. With high surface-to-volume ratio, nanoscale TI and TCI materials such as nanowires and nanoribbons can have significantly enhanced contribution from surface states in carrier transport and are thus ideally suited for the fundamental studies of topologically protected surface state transport and nanodevice fabrication. This article will review the synthesis and transport device measurements of TIs and TCIs nanostructures.

Topological phase transition and highly tunable topological transport in topological crystalline insulator Pb1-x Sn x Te (111) thin films

We report the magneotransport studies on the topological crystalline insulator (TCI) Pb_1-x Sn _x Te (111) single crystal thin films grown by molecular beam epitaxy. By decreasing Sn content, an enhanced sheet resistance and decreased hole density are observed in Pb_1-x Sn _x Te (111) thin films. A weak antilocalization likely related to the topological surface states is observed in transport of Pb_1-x Sn _x Te (x > 0.4) thin films, whereas a weak localization is displayed in Pb_1-x Sn _x Te (x < 0.4) thin films. This tunable weak antilocalization to weak localization transition is attributed to the open of Dirac gap because of the topological phase transition in TCI Pb_1-x Sn _x Te. Our research has a potential application in the tunable electronic and spintronic devices and is very significant to the fundamental research based on TCI Pb_1-x Sn _x Te thin film.

Prediction of topological crystalline insulators and topological phase transitions in two-dimensional PbTe films

Topological phases, especially topological crystalline insulators (TCIs), have been intensively explored and observed experimentally in three-dimensional (3D) materials. However, two-dimensional (2D) films are explored much less than 3D TCIs, and even 2D topological insulators. Based on ab initio calculations, here we investigate the electronic and topological properties of 2D PbTe(001) few-layer films. The monolayer and trilayer PbTe are both intrinsic 2D TCIs with a large band gap reaching 0.27 eV, indicating a high possibility for room-temperature observation of quantized conductance. The origin of the TCI phase can be attributed to the p_x,y-p_z band inversion, which is determined by the competition of orbital hybridization and the quantum confinement effect. We also observe a semimetal-TCI-normal insulator transition under biaxial strains, whereas a uniaxial strain leads to Z₂ nontrivial states. In particular, the TCI phase of a PbTe monolayer remains when epitaxially grown on a NaI semiconductor substrate. Our findings on the controllable quantum states with sizable band gaps present an ideal platform for realizing future topological quantum devices with ultralow dissipation.

Topological, Valleytronic, and Optical Properties of Monolayer PbS

A PbS monolayer is demonstrated to be a novel platform for topological, valleytronic, and optical phenomena. Compressive strain can turn the trivial monolayer into a topological insulator. Optical pumping can facilitate charge, spin, and valley Hall effects tunable by external strain and light ellipticity. Similar results apply to other IV-VI semiconductors.

Large-Scale Surfactant-Free Synthesis of p-Type SnTe Nanoparticles for Thermoelectric Applications

A facile one-pot aqueous solution method has been developed for the fast and straightforward synthesis of SnTe nanoparticles in more than ten gram quantities per batch. The synthesis involves boiling an alkaline Na₂SnO₂ solution and a NaHTe solution for short time scales, in which the NaOH concentration and reaction duration play vital roles in controlling the phase purity and particle size, respectively. Spark plasma sintering of the SnTe nanoparticles produces nanostructured compacts that have a comparable thermoelectric performance to bulk counterparts synthesised by more time- and energy-intensive methods. This approach, combining an energy-efficient, surfactant-free solution synthesis with spark plasma sintering, provides a simple, rapid, and inexpensive route to p-type SnTe nanostructured materials.

Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires

Locally induced, magnetic order on the surface of a topological insulator nanowire could enable room-temperature topological quantum devices. Here we report on the realization of selective magnetic control over topological surface states on a single facet of a rectangular Bi2Te3 nanowire via a magnetic insulating Fe3O4 substrate. Low-temperature magnetotransport studies provide evidence for local time-reversal symmetry breaking and for enhanced gapping of the interfacial 1D energy spectrum by perpendicular magnetic-field components, leaving the remaining nanowire facets unaffected. Our results open up great opportunities for development of dissipation-less electronics and spintronics.

Plasmons of topological crystalline insulator SnTe with nanostructured patterns

Using the finite-difference time-domain method and density functional theory, we theoretically investigate the plasmons of topological crystalline insulator (TCI) SnTe with nanostructured patterns.

Low-Dimensional Topological Crystalline Insulators

Topological crystalline insulators (TCIs) are recently discovered topological phase with robust surface states residing on high-symmetry crystal surfaces. Different from conventional topological insulators (TIs), protection of surface states on TCIs comes from point-group symmetry instead of time-reversal symmetry in TIs. The distinct properties of TCIs make them promising candidates for the use in novel spintronics, low-dissipation quantum computation, tunable pressure sensor, mid-infrared detector, and thermoelectric conversion. However, similar to the situation in TIs, the surface states are always suppressed by bulk carriers, impeding the exploitation of topology-induced quantum phenomenon. One effective way to solve this problem is to grow low-dimensional TCIs which possess large surface-to-volume ratio, and thus profoundly increase the carrier contribution from topological surface states. Indeed, through persistent effort, researchers have obtained unique quantum transport phenomenon, originating from topological surface states, based on controllable growth of low-dimensional TCIs. This article gives a comprehensive review on the recent progress of controllable synthesis and topological surface transport of low-dimensional TCIs. The possible future direction about low-dimensional TCIs is also briefly discussed at the end of this paper.