The stability of the surface of in situ cleaved black phosphorus crystals upon exposure to atmosphere is investigated with synchrotron-based photoelectron spectroscopy. After 2 days atmosphere ...exposure a stable subnanometer layer of primarily P2O5 forms at the surface. The work function increases by 0.1 eV from 3.9 eV for as-cleaved black phosphorus to 4.0 eV after formation of the 0.4 nm thick oxide, with phosphorus core levels shifting by <0.1 eV. The results indicate minimal charge transfer, suggesting that the oxide layer is suitable for passivation or as an interface layer for further dielectric deposition.
Inducing long‐range magnetic order in 3D topological insulators can gap the Dirac‐like metallic surface states, leading to exotic new phases such as the quantum anomalous Hall effect or the axion ...insulator state. These magnetic topological phases can host robust, dissipationless charge and spin currents or unique magnetoelectric behavior, which can be exploited in low‐energy electronics and spintronics applications. Although several different strategies have been successfully implemented to realize these states, to date these phenomena have been confined to temperatures below a few Kelvin. This review focuses on one strategy: inducing magnetic order in topological insulators by proximity of magnetic materials, which has the capability for room temperature operation, unlocking the potential of magnetic topological phases for applications. The unique advantages of this strategy, the important physical mechanisms facilitating magnetic proximity effect, and the recent progress to achieve, understand, and harness proximity‐coupled magnetic order in topological insulators are discussed. Some emerging new phenomena and applications enabled by proximity coupling of magnetism and topological materials, such as skyrmions and the topological Hall effect, are also highlighted, and the authors conclude with an outlook on remaining challenges and opportunities in the field.
The magnetic proximity effect in topological insulator–magnetic materials heterostructures is a promising strategy to optimize new topological phases with potential applications in low‐energy electronics. Progress in this field is summarized with an introduction to the underlying physical mechanisms, a review of emerging material systems for magnetic proximity in topological insulators, and the status and prospects for obtaining new topological phases.
The electric-field-induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor1-4. In this scheme, 'on' is the ...ballistic flow of charge and spin along dissipationless edges of a two-dimensional quantum spin Hall insulator5-9, and 'off' is produced by applying an electric field that converts the exotic insulator to a conventional insulator with no conductive channels. Such a topological transistor is promising for low-energy logic circuits4, which would necessitate electric-field-switched materials with conventional and topological bandgaps much greater than the thermal energy at room temperature, substantially greater than proposed so far6-8. Topological Dirac semimetals are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases3,10-16. Here we use scanning tunnelling microscopy and spectroscopy and angle-resolved photoelectron spectroscopy to show that mono- and bilayer films of the topological Dirac semimetal3,17 Na3Bi are two-dimensional topological insulators with bulk bandgaps greater than 300 millielectronvolts owing to quantum confinement in the absence of electric field. On application of electric field by doping with potassium or by close approach of the scanning tunnelling microscope tip, the Stark effect completely closes the bandgap and re-opens it as a conventional gap of 90 millielectronvolts. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy at room temperature (25 millielectronvolts), suggest that ultrathin Na3Bi is suitable for room-temperature topological transistor operation.
Two key plant adaptations for phosphorus (P) acquisition are carboxylate exudation into the rhizosphere and mycorrhizal symbioses. These target different soil P resources, presumably with different ...plant carbon costs. We examined the effect of inoculation with arbuscular mycorrhizal fungi (AMF) on amount of rhizosphere carboxylates and plant P uptake for 10 species of low‐P adapted Kennedia grown for 23 weeks in low‐P sand. Inoculation decreased carboxylates in some species (up to 50%), decreased plant dry weight (21%) and increased plant P content (23%). There was a positive logarithmic relationship between plant P content and the amount of rhizosphere citric acid for inoculated and uninoculated plants. Causality was indicated by experiments using sand where little citric acid was lost from the soil solution over 2 h and citric acid at low concentrations desorbed P into the soil solution. Senesced leaf P concentration was often low and P‐resorption efficiencies reached >90%. In conclusion, we propose that mycorrhizally mediated resource partitioning occurred because inoculation reduced rhizosphere carboxylates, but increased plant P uptake. Hence, presumably, the proportion of plant P acquired from strongly sorbed sources decreased with inoculation, while the proportion from labile inorganic P increased. Implications for plant fitness under field conditions now require investigation.
Layered transition metal dichalcogenides are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. ...In bulk NbSe2 a CDW sets in at TCDW =33 K and superconductivity sets in at Tc =7.2 K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial.
We report the discovery of room temperature in-plane ferroelectricity in van der Waals In
2
Se
3
with β′ phase.
Van der Waals (vdW) assembly of layered materials is a promising paradigm for creating ...electronic and optoelectronic devices with novel properties. Ferroelectricity in vdW layered materials could enable nonvolatile memory and low-power electronic and optoelectronic switches, but to date, few vdW ferroelectrics have been reported, and few in-plane vdW ferroelectrics are known. We report the discovery of in-plane ferroelectricity in a widely investigated vdW layered material, β′-In
2
Se
3
. The in-plane ferroelectricity is strongly tied to the formation of one-dimensional superstructures aligning along one of the threefold rotational symmetric directions of the hexagonal lattice in the
c
plane. Surprisingly, the superstructures and ferroelectricity are stable to 200°C in both bulk and thin exfoliated layers of In
2
Se
3
. Because of the in-plane nature of ferroelectricity, the domains exhibit a strong linear dichroism, enabling novel polarization-dependent optical properties.
Intrinsic magnetic topological insulators offer low disorder and large magnetic band gaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, ...emergent phenomena such as the quantum anomalous Hall (QAH) effect and axion insulator phases have been realized. These observations occur at temperatures significantly lower than the Néel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gap at the Dirac point in ultrathin MnBi2Te4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verification of a temperature-dependent topological phase transition from a large band gap QAH insulator to a gapless TI paramagnetic phase. Here we utilize temperature-dependent angle-resolved photoemission spectroscopy to study epitaxial ultrathin MnBi2Te4. We directly observe a layer-dependent crossover from a 2D ferromagnetic insulator with a band gap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>70 meV) at 8 K in 3 and 5 SL MnBi2Te4. The QAH gap is confirmed to be magnetic in origin, as it becomes gapless with increasing temperature above 8 K.
Fast and reliable DNA sequencing is a long-standing target in biomedical research. Recent advances in graphene-based electrical sensors have demonstrated their unprecedented sensitivity to adsorbed ...molecules, which holds great promise for label-free DNA sequencing technology. To date, the proposed sequencing approaches rely on the ability of graphene electric devices to probe molecular-specific interactions with a graphene surface. Here we experimentally demonstrate the use of graphene field-effect transistors (GFETs) as probes of the presence of a layer of individual DNA nucleobases adsorbed on the graphene surface. We show that GFETs are able to measure distinct coverage-dependent conductance signatures upon adsorption of the four different DNA nucleobases; a result that can be attributed to the formation of an interface dipole field. Comparison between experimental GFET results and synchrotron-based material analysis allowed prediction of the ultimate device sensitivity, and assessment of the feasibility of single nucleobase sensing with graphene.
Trisodium bismuthide (Na3Bi) is the first experimentally verified topological Dirac semimetal, and is a 3D analogue of graphene hosting relativistic Dirac fermions. Its unconventional momentum–energy ...relationship is interesting from a fundamental perspective, yielding exciting physical properties such as chiral charge carriers, the chiral anomaly, and weak anti‐localization. It also shows promise for realizing topological electronic devices such as topological transistors. Herein, an overview of the substantial progress achieved in the last few years on Na3Bi is presented, with a focus on technologically relevant large‐area thin films synthesized via molecular beam epitaxy. Key theoretical aspects underpinning the unique electronic properties of Na3Bi are introduced. Next, the growth process on different substrates is reviewed. Spectroscopic and microscopic features are illustrated, and an analysis of semiclassical and quantum transport phenomena in different doping regimes is provided. The emergent properties arising from confinement in two dimensions, including thickness‐dependent and electric‐field‐driven topological phase transitions, are addressed, with an outlook toward current challenges and expected future progress.
Na3Bi displays remarkable thickness‐dependent properties, from bulk topological Dirac semimetal to wide‐bandgap 2D topological insulator in ultrathin films with electric‐field‐driven topological switching. An overview of the recent progress on Na3Bi is presented, with a focus on technologically relevant large‐area epitaxial films, and an outlook toward challenges and opportunities for applications to the next generation of topological electronic and spintronic devices.
Topological Dirac semimetals (TDS) are three-dimensional analogues of graphene, with linear electronic dispersions in three dimensions. Nanoscale confinement of TDSs in thin films is a necessary step ...toward observing the conventional-to-topological quantum phase transition (QPT) with increasing film thickness, gated devices for electric-field control of topological states, and devices with surface-state-dominated transport phenomena. Thin films can also be interfaced with superconductors (realizing a host for Majorana Fermions) or ferromagnets (realizing Weyl Fermions or T-broken topological states). Here we report structural and electrical characterization of large-area epitaxial thin films of TDS Na3Bi on single crystal Al2O30001 substrates. Charge carrier mobilities exceeding 6,000 cm2/(V s) and carrier densities below 1 × 1018 cm–3 are comparable to the best single crystal values. Perpendicular magnetoresistance at low field shows the perfect weak antilocalization behavior expected for Dirac Fermions in the absence of intervalley scattering. At higher fields up to 0.5 T anomalously large quadratic magnetoresistance is observed, indicating that some aspects of the low field magnetotransport (μB < 1) in this TDS are yet to be explained.
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