Niobium arsenide
| Names | |
|---|---|
| Other names
Niobium monoarsenide | |
| Identifiers | |
3D model (JSmol) |
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| ECHA InfoCard | 100.032.263 |
| EC Number |
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PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |
| NbAs | |
| Molar mass | 167.83 g/mol |
| Appearance | Dark grey crystalline solid |
| Density | 7.93 g/cm3 |
| Insoluble in water | |
| Structure | |
| Tetragonal | |
| I41md, No. 109 | |
a = 0.3452 nm, c = 1.168 nm | |
Formula units (Z) |
4 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Niobium arsenide is an inorganic compound of niobium and arsenic with the chemical formula NbAs. It is a dark grey crystalline solid and a member of the non-centrosymmetric transition-metal monopnictides.
NbAs is a Weyl semimetal. In 2015, angle-resolved photoemission spectroscopy measurements directly observed bulk Weyl cones and topological Fermi arc surface states in single crystals of NbAs.[1]
Preparation
[edit]Niobium arsenide can be prepared by direct reaction of stoichiometric amounts of elemental niobium and arsenic at high temperature:
- Nb + As → NbAs
Polycrystalline NbAs can be prepared by sealing niobium and arsenic under vacuum, initially heating the mixture to about 600 °C and subsequently to approximately 1050 °C.[2]
Single crystals can be grown from the polycrystalline material by chemical vapor transport using iodine as the transport agent. In one method, NbAs and iodine are sealed under vacuum in a quartz ampoule and maintained in a temperature gradient of approximately 1150–1000 °C for several weeks.[2]
Structure
[edit]Niobium arsenide crystallizes in the tetragonal crystal system, in the non-centrosymmetric space group I41md (No. 109).[3]
Room-temperature lattice parameters are approximately a = 3.45 Å and c = 11.68 Å, with four formula units per unit cell.[4]
The structure is closely related to those of tantalum arsenide, niobium phosphide and tantalum phosphide. It lacks inversion symmetry, an important requirement for the Weyl-semimetal electronic structure of NbAs.[1]
Electronic and physical properties
[edit]NbAs is a three-dimensional Weyl semimetal. First-principles calculations predict 24 Weyl nodes in the first Brillouin zone, occurring in pairs of opposite chirality.[4] In 2015, angle-resolved photoemission spectroscopy directly observed the bulk Weyl cones and associated surface Fermi arc states.[1]
Its Fermi surface contains both electron and hole pockets. Shubnikov–de Haas oscillation measurements have identified both topologically non-trivial and conventional carrier pockets, with one reported carrier pocket having a Berry phase close to π and an effective mass of about 0.033 times the free-electron mass.[5] Electron–hole compensation contributes to the large magnetoresistance of NbAs.[5]
Under sufficiently strong magnetic fields, NbAs enters the quantum limit. Magneto-optical and transport measurements have observed chiral Landau levels associated with its Weyl bands,[4] while magnetic torque measurements show a pronounced anomaly near the quantum limit.[6]
Infrared and magneto-optical spectroscopy have also been used to probe the interband transitions and Landau-level structure of NbAs.[7]
The surface electronic structure can be modified without eliminating the bulk Weyl nodes. Alkali-metal surface dosing, for example, has been used to induce topological Lifshitz transitions and change the connectivity of the Fermi arcs.[2]
Thin films
[edit]Epitaxial and textured thin films of NbAs have been grown on gallium arsenide substrates by molecular-beam epitaxy. Depending on substrate orientation and surface termination, films oriented predominantly along either the [001] or [100] crystallographic directions can be obtained.[8]
Low-temperature resistivities of approximately 420–450 μΩ cm and carrier densities of about 1021–1022 cm−3 have been measured in such films.[8]
References
[edit]- 1 2 3 Xu, Su-Yang; Alidoust, Nasser; Belopolski, Ilya (2015). "Discovery of a Weyl fermion state with Fermi arcs in niobium arsenide". Nature Physics. 11: 748–754. arXiv:1504.01350. doi:10.1038/nphys3437.
- 1 2 3 "Topological Lifshitz transitions and Fermi arc manipulation in Weyl semimetal NbAs". Nature Communications. 10 3478. 2019. doi:10.1038/s41467-019-11491-4. PMC 6677823.
- ↑ Boller, H.; Parthé, E. (1963). "The transposition structure of NbAs and of similar monophosphides and arsenides of niobium and tantalum". Acta Crystallographica. 16 (11): 1095–1101. doi:10.1107/S0365110X63002930.
- 1 2 3 "Chiral Landau levels in Weyl semimetal NbAs with multiple topological carriers". Nature Communications. 9 1936. 2018. doi:10.1038/s41467-018-04080-4. PMC 5945645.
- 1 2 Luo, Yongkang; Ghimire, N. J.; Wartenbe, M. (2015). "Electron-hole compensation effect between topologically trivial electrons and nontrivial holes in NbAs". Physical Review B. 92 205134. arXiv:1506.01751. doi:10.1103/PhysRevB.92.205134.
- ↑ "Magnetic torque anomaly in the quantum limit of Weyl semimetals". Nature Communications. 7 12492. 2016. doi:10.1038/ncomms12492. PMC 4996949.
- ↑ Polatkan, S.; Uykur, E.; Wyzula, J. (2023). "Magneto-optical response of the Weyl semimetal NbAs: Experimental results and hyperbolic-band computations". Physical Review B. 108 L241201. arXiv:2312.03094. doi:10.1103/PhysRevB.108.L241201.
- 1 2 "Thin film growth of the Weyl semimetal NbAs". Physical Review Materials. 8 034204. 2024. doi:10.1103/PhysRevMaterials.8.034204. hdl:11336/265394.