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

Bedoya-Pinto, Amílcar; Liu, Defa; Tan, Hengxin; Pandeya, Avanindra K.; Chang, Kai; Zhang, Jibo; Parkin, S. S. P.

doi:10.1002/adma.202008634

Cited by 9 publications

(7 citation statements)

References 32 publications

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“…The periodicity of the contour matches the one expected for the (110) surface termination, in agreement with previous studies [13,24,26,42]. Our data cannot resolve the presence of surface states and Fermi arcs predicted by theory [13], whose signal could be enhanced or suppressed by proper surface preparation, as similarly reported in other Weyl semimetals [43,44]. Heat-treating (TaSe 4 ) 2 I has already proved to be a viable tool to modify the band structure by partial loss of the iodine atoms at the surface [42].…”

Section: Resultssupporting

confidence: 90%

Optically induced changes in the band structure of the Weyl charge-density-wave compound (TaSe4)2I

et al. 2022

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Collective modes are responsible for the emergence of novel quantum phases in topological materials. In the quasi-one dimensional Weyl semimetal $\mathrm{(TaSe_4)_2I}$, a charge density wave (CDW) opens band gaps at the Weyl points, thus turning the system into an axionic insulator. Melting the CDW would restore the Weyl phase, but 1D fluctuations extend the gapped regime far above the 3D transition temperature (T$_{CDW}$ = 263 K), thus preventing the investigation of this topological phase transition with conventional spectroscopic methods. Here we use a non-equilibrium approach: we perturb the CDW phase by photoexcitation, and we monitor the dynamical evolution of the band structure by time- and angle-resolved photoelectron spectroscopy (trARPES). We find that, upon optical excitation, electrons populate the linearly dispersing states at the Fermi level (E$_\mathrm{F}$), and fill the CDW gap. The dynamics of both the charge carrier population and the band gap renormalization (BGR) show a fast component with a characteristic time scale of a few hundreds femtoseconds. However, the BGR also exhibits a second slow component on the $\mathrm{\mu}$s time scale. The combination of an ultrafast response and of persistent changes in the spectral weight at E$_\mathrm{F}$, and the resulting sensitivity of the linearly dispersing states to optical excitations, may explain the high performances of $\mathrm{(TaSe_4)_2I}$ as a material for broadband infrared photodetectors.

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Section: Resultssupporting

confidence: 90%

Optically induced changes in the band structure of the Weyl charge-density-wave compound (TaSe4)2I

et al. 2022

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“…A fitting of the nonzero tensor elements are shown by solid lines in Figure 2c,d, giving a good agreement with the experimental data (more details can be found in the Supporting Information). As it was recently shown that the surface states in identically fabricated NbP thin films are almost exclusively topological, due to a suppression of the trivial surfaces’ states via atomic engineering, [ 30 ] we attribute the high observed efficiency to the topological surface states of NbP, in particular as they were shown the generate extremely high signal in other topological materials. [ 42 ] Figure 2e shows the relative THG efficiency over the excitation wavelength range from 1400–1950 nm with a clear peak ≈1800 nm.…”

Section: Resultsmentioning

confidence: 78%

“…NbP (001) thin films were grown by molecular‐beam epitaxy along the [100]‐direction of atomically flat MgO substrates. [ 30 ] To compensate for the MgO–NbP lattice mismatch, a Nb seed layer is grown with an in‐plane rotation of 45° to the MgO substrate and phosphorized prior to the epitaxial growth of NbP (001). The layer growth is monitored in situ via reflection high‐energy electron diffraction (RHEED) at all stages.…”

Section: Resultsmentioning

confidence: 99%

“…[ 42 ] Figure 2e shows the relative THG efficiency over the excitation wavelength range from 1400–1950 nm with a clear peak ≈1800 nm. A comparison with calculations of the bandstructure of NbP [ 24,30 ] indicates that the small plateau in the conduction band, located at the Γ‐point of the Brillouin zone is responsible for this resonance. Here the energy difference of valence and conduction bands equals the energy of the third‐harmonic photon, enhancing the third‐order NL process with two virtual electron states (for more details refer to Supporting Information).…”

Section: Resultsmentioning

confidence: 99%

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Ultrafast Sub‐100 fs All‐Optical Modulation and Efficient Third‐Harmonic Generation in Weyl Semimetal Niobium Phosphide Thin Films

et al. 2022

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Since their experimental discovery in 2015, Weyl semimetals have generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular, in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, the first steps toward Weyl semimetal nanophotonics are performed here by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to the near‐infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third‐harmonic generation studies demonstrate high conversion efficiencies up to 10−4% that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump–probe measurements with sub‐10 fs pulses reveal a maximum modulation depth of ≈1%, completely decaying within 100 fs and therefore suggesting the possibility of developing all‐optical switching devices based on NbP. Altogether, this work reveals the promising NL optical properties of Weyl semimetal thin films, which outperform bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top‐down nanostructuring, such as light‐harvesting, on‐chip frequency conversion, and all‐optical processing.

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“…5a) the observed open arc-like contours evolve from the Fermi-arcs and have thus topological character. The fact that only these topological surface states are observed in our films merits further investigation and will be discussed elsewhere [45]. The Fermi-level shift (ΔE) of the epitaxial films is further visualized in the second derivative plot of the AA' (panel (ii) in Figure 5c), considering the extrapolation to the band crossing point and the comparison to the shape of the bulk crystal dispersion data (inset of Figure 5c (ii)), taken from Ref.…”

Section: Resultsmentioning

confidence: 96%

Realization of Epitaxial NbP and TaP Weyl Semimetal Thin Films

et al. 2020

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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.

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

Cited by 9 publications

References 32 publications

Optically induced changes in the band structure of the Weyl charge-density-wave compound (TaSe4)2I

Optically induced changes in the band structure of the Weyl charge-density-wave compound (TaSe4)2I

Ultrafast Sub‐100 fs All‐Optical Modulation and Efficient Third‐Harmonic Generation in Weyl Semimetal Niobium Phosphide Thin Films

Realization of Epitaxial NbP and TaP Weyl Semimetal Thin Films

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