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Register a new userObservation of Negative THz Photoconductivity in Large Area Type-II Dirac Semimetal PtTe2
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As a newly emergent type-II Dirac semimetal, Platinum Telluride (PtTe2) stands out from other 2D noble-transition-metal dichalcogenides for the unique structure and novel physical properties, such as high carrier mobility, strong electron-phonon coupling and tunable bandgap, which make the PtTe2 a good candidate for applications in optoelectronics, valleytronics and far infrared detectors. Although the transport properties of PtTe2 have been studied extensively, the dynamics of the nonequilibrium carriers remain nearly uninvestigated. Herein we employ optical pump-terahertz (THz) probe spectroscopy (OPTP) to systematically study the photocarrier dynamics of PtTe2 thin films with varying pump fluence, temperature, and film thickness. Upon photoexcitation the THz photoconductivity (PC) of 5 nm PtTe2 film shows abrupt increase initially, while the THz PC changes into negative value in a subpicosecond time scale, followed by a prolonged recovery process that lasted hundreds of picoseconds (ps). This unusual THz PC response observed in the 5 nm PtTe2 film was found to be absent in a 2 nm PtTe2 film. We assign the unexpected negative THz PC as the small polaron formation due to the strong electron-Eg-mode phonon coupling, which is further substantiated by pump fluence- and temperature-dependent measurements as well as the Raman spectroscopy. Moreover, our investigations give a subpicosecond time scale of sequential carrier cooling and polaron formation. The present study provides deep insights into the underlying dynamics evolution mechanisms of photocarrier in type-II Dirac semimetal upon photoexcitation, which is fundamental importance for designing PtTe2-based optoelectronic devices.
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We report experimental observation of the Planar Hall effect (PHE) in a type-II Dirac semimetal PtTe$_2$. This unusual Hall effect is not expected in nonmagnetc materials such as PtTe$_2$, and has been observed previously mostly in magnetic semiconductors or metals. Remarkably, the PHE in PtTe$_2$ can be observed up to temperatures near room temperature which indicates the robustness of the effect. This is in contrast to the chiral anomaly induced negative longitudnal magnetoresistance (LMR), which can be observed only in the low temperature regime and is sensitive to extrinsic effects, such as current jetting and chemical inhomogeneities in crystals of high mobility. Planar Hall effect on the other hand is a purely intrinsic effect generated by the Berry curvature in Weyl semimetals. Additionally, the PHE is observed for PtTe$_2$ even though the Dirac node is $approx 0.8$~eV away from the Fermi level. Thus our results strongly indicate that PHE can be used as a crucial transport diagnostic for topological character even for band structures with Dirac nodes slightly away from the Fermi energy.
We report angle-resolved photoemission experiments resolving the distinct electronic structure of the inequivalent top and bottom (001) surfaces of WTe2. On both surfaces, we identify a surface state that forms a large Fermi-arc emerging out of the bulk electron pocket. Using surface electronic structure calculations, we show that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. This implies that the observation of surface Fermi arcs alone does not allow the identification of WTe2 as a topological Weyl semimetal. We further use the identification of the two different surfaces to clarify the number of Fermi surface sheets in WTe2.
The study of electronic properties in topological systems is one of the most fascinating topics in condensed matter physics, which has generated enormous interests in recent times. New materials are frequently being proposed and investigated to identify their non-trivial band structure. While sophisticated techniques such as angle-resolved photoemission spectroscopy have become popular to map the energy-momentum relation, the transport experiments lack any direct confirmation of Dirac and Weyl fermions in a system. From band structure calculations, VAl$_{3}$ has been proposed to be a type II topological Dirac semimetal. This material represents a large family of isostructural compounds, all having similar electronic band structure and is an ideal system to explore the rich physics of Lorentz symmetry violating Dirac fermions. In this work, we present a detailed analysis on the magnetotransport properties of VAl$_{3}$. A large, non-saturating magnetoresistance has been observed. Hall resistivity reveals the presence of two types of charge carriers with high mobility. Our measurements show a large planar Hall effect in this material, which is robust and can be easily detectable up to high temperature. This phenomenon originates from the relativistic chiral anomaly and non-trivial Berry curvature, which validates the theoretical prediction of the Dirac semimetal phase in VAl$_{3}$.
We report on a magneto-transport and quantum oscillations study on high quality single crystals of the transition metal di-tellurides PtTe$_2$ and PdTe$_2$. The de Haas-van Alphen (dHvA) oscillations in the magnetization measurements on PtTe$_2$ reveal a complicated, anisotropic band structure characterized by low effective masses and high mobilities for the carriers. Extracted transport parameters for PtTe$_2$ reveal a strong anisotropy which could be related to the tilted nature of Dirac cone. Using a Landau level fan diagram analysis we find at least one Fermi surface orbit with a Berry phase of $pi$ consistent with Dirac electrons for both PtTe$_2$ and PdTe$_2$. The light effective mass and high mobility are also consistent with Dirac electrons in PtTe$_2$. Our results suggest that similar to PdTe$_2$, PtTe$_2$ might also be a three dimensional Dirac semimetal.
Extremely large positive magnetoresistance (XMR) was found in a nonmagnetic semimetal InBi. Using several single crystals with different residual resistivity ratios (RRRs), we revealed that the XMR strongly depended on the RRR (sample quality). Assuming that there were no changes in effective mass m* and carrier concentrations in these single crystals, this dependence was explained by a semiclassical two-carrier model. First-principle calculations including the spin-orbit interactions (SOI) unveiled that InBi had a compensated carrier balance and SOI-induced hidden three-dimensional (3D) Dirac bands at the M and R points. Because the small m* and the large carrier mobilities will be realized, these hidden 3D Dirac bands should play an important role for the XMR in InBi. We suggest that this feature can be employed as a novel strategy for the creation of XMR semimetals.
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