Realization of Epitaxial NbP and TaP Weyl Semimetal Thin Films

Abstract

Weyl Semimetals (WSMs), a recently discovered topological state of matter, exhibit an electronic structure governed by linear band dispersions and degeneracy (Weyl) points leading to rich physical phenomena, which are yet to be exploited in thin film devices. While WSMs were established in the monopnictide compound family several years ago, the growth of thin films has remained a challenge. Here, we report the growth of epitaxial thin films of NbP and TaP by means of molecular beam epitaxy. Single crystalline films are grown on MgO (001) substrates using thin Nb (Ta) buffer layers, and are found to be tensile strained (1%) and with slightly P-rich stoichiometry with respect to the bulk crystals. The resulting electronic structure exhibits topological surface states characteristic of a P-terminated surface, linear dispersion bands in agreement with the calculated band structure, and a Fermi-level shift of -0.2 eV with respect to the Weyl points. Consequently, the electronic transport is dominated by both holes and electrons with carrier mobilities close to 10 3 cm 2 /Vs at room-temperature. The growth of epitaxial thin films opens up the use of strain and controlled doping to access and tune the electronic structure of Weyl Semimetals on demand, paving the way for the rational design and fabrication of electronic devices ruled by topology.

Large Fermi‐Energy Shift and Suppression of Trivial Surface States in NbP Weyl Semimetal Thin Films

“…This challenge needs to be overcome to achieve Fermi-level engineered Weyl semimetal heterostructures, leading to a plethora of novel platforms to explore both fundamental phenomena and device applications based on topology.In this work, we show two striking modifications of the electronic structure of the type-I Weyl semimetal NbP, that become accessible due to a successful epitaxial thin film growth synthesis route. [25] First, a full suppression of the bowtie-like (trivial) surface states of NbP is obtained due to the saturation of surface dangling bonds by an ordered phosphorous termination, that manifests itself in a (√2 × √2) surface reconstruction. Second, by chemically doping the surface with Se-atoms, the Fermi-energy undergoes a substantial shift of around +0.3 eV (electron doping) while preserving the pristine NbP bandstructure features, thereby enabling the first experimental visualization of the topological band dispersion well above the Weyl points, and highlighting the large Fermi-level tunability that can be achieved by surface chemical doping in a molecular beam epitaxy process.…”

Crystalline materials for quantum computing: Semiconductor heterostructures and topological insulators exemplars

“…Materials grown by conventional epitaxy include Cd 3 As 2 , NbP, TaP, Na 3 Bi, Sr 3 PbO, LaAlGe, Co 3 Sn 2 S 2 , and TaIrTe 4 . [83][84][85][86][87][88][89][90][91] In most of these cases, the more volatile element (i.e., As, P, S, Te) is held in excess, while the less volatile element controls the growth rate. However, in the cases of materials like LaAlGe or Na 3 Bi, more careful flux matching is needed.…”

Van Der Waals Epitaxy of Soft Twisted Bilayers: Lattice Relaxation and Static Distortion Waves

“…Epitaxial engineering of thin film devices has been a critically important feature of many modern optoelectronic devices, where the interfacial registration forces between two different materials can be used to modify structural, electrical and optical properties. [1][2][3] Rational tuning of material properties via epitaxy is being increasingly applied to socalled 2-dimensional van der Waals or soft materials for applications such as switchable ferromagnets, [4][5][6][7] tunable field effect and logic devices, 2,8-13 volatile organics sensors, 14 high-efficiency photodetectors, 15,16 quantum communication devices 17 and superlubricity. 18 Since the discovery of superconductivity in twisted magic-angle graphene bilayers by Jarillo-Herrero and co-workers in 2018, 19 there has been an explosion of research into understanding the properties of twisted bilayer van der Waals heterostructures .…”

“…18 Since the discovery of superconductivity in twisted magic-angle graphene bilayers by Jarillo-Herrero and co-workers in 2018, 19 there has been an explosion of research into understanding the properties of twisted bilayer van der Waals heterostructures . 1,[20][21][22][23][24][25][26] Critical to the functional properties of these 2-dimensional van der Waals bilayer heterostructures is the local atomic structure and orientation that they adopt upon epitaxial registration, which can significantly influence electronic and magnetic properties. 20,[27][28][29] The epitaxial structure and orientation of 2-dimensional van der Waals bilayer heterostructures, including twisted magic-angle graphene bilayers, has been explained by the conceptually and mathematically simple 20-year-old model of superposition of plane waves, resulting in well-defined superstructure patterns.…”