Abstract
Weyl semimetals (WSMs) exhibit an electronic structure governed by linear band dispersions and degenerate (Weyl) points that lead to exotic physical phenomena. While WSMs were established in bulk monopnictide compounds several years ago, the growth of thin films remains a challenge. Here, we report the bottom-up synthesis of single-crystalline NbP and TaP thin films, 9 to 70 nm thick, by means of molecular beam epitaxy. The as-grown epitaxial films feature a phosphorus-rich stoichiometry, a tensile-strained unit cell, and a homogeneous surface termination, unlike their bulk crystal counterparts. These properties result in an electronic structure governed by topological surface states as directly observed using in situ momentum photoemission microscopy, along with a Fermi-level shift of −0.2 eV with respect to the intrinsic chemical potential. Although the Fermi energy of the as-grown samples is still far from the Weyl points, carrier mobilities close to 103 cm2/(V s) have been measured at room temperature in patterned Hall-bar devices. The ability to grow thin films of Weyl semimetals that can be tailored by doping or strain, is an important step toward the fabrication of functional WSM-based devices and heterostructures.
Highlights
Weyl semimetals (WSMs) exhibit an electronic structure governed by linear band dispersions and degenerate (Weyl) points that lead to exotic physical phenomena
Both phosphide compounds have been conclusively shown to be type-I Weyl semimetals in the bulk crystal form by angle-resolved photoemission experiments.[8−11] Our epitaxial layers present clear differences with respect to the bulk crystals: (i) both in-plane and out-of plane lattice parameters are larger by more than 1%, (ii) the composition of the films is slightly in the P-rich regime, and (iii) the film surfaces exhibit sub-unit cell steps (2.8 Å) that point to a single surface termination
For a reasonable lattice matching on typical cubic oxide insulators, the NbP (TaP) growth has to be stabilized with an in-plane rotation of 45° in the basal plane with respect to the substrate
Summary
Weyl semimetals (WSMs) exhibit an electronic structure governed by linear band dispersions and degenerate (Weyl) points that lead to exotic physical phenomena. The as-grown epitaxial films feature a phosphorus-rich stoichiometry, a tensile-strained unit cell, and a homogeneous surface termination, unlike their bulk crystal counterparts These properties result in an electronic structure governed by topological surface states as directly observed using in situ momentum photoemission microscopy, along with a Fermi-level shift of −0.2 eV with respect to the intrinsic chemical potential. ACS Nano www.acsnano.org an unusual twisting of the Fermi surface,[30] the emergence of Floquet topological insulator phases,[31] a metal−insulator transition upon thickness confinement,[32] or even a special interplay of long- and short-range surface plasmon-polariton modes.[33] On the other hand, promising application areas of WSMs have been addressed, pointing to them as efficient hydrogen catalysts,[34] colossal photovoltaic materials,[25] midinfrared detectors,[35] and most recently, topological magnets.[36−39] But the most important implication of WSM thin films is the possibility to fabricate atomically engineered heterostructures and functional interfaces It allows the interplay of WSMs with other materials, such as superconductors, (anti)ferromagnetic, or ferroelectric materials, to be explored. The successful realization of epitaxial thin films will allow for the use of strain and controlled doping to tailor the electronic structure of Weyl semimetals, paving the way towards the fabrication of functional WSM heterostructures
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