Polarization-dependent surface-bulk scattering in the Weyl semimetal NbAs
Yaomin DaiBing ShenLi‐Juan ZhaoBing XuYongkang LuoA. P. ChenRun YangXianggang QiuGenfu ChenNi NiS. A. TrugmanJian‐Xin ZhuAntoinette J. TaylorDmitry YarotskiRohit P. Prasankumar
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Ultrafast optical spectroscopy reveals surface-bulk scattering in the Weyl semimetal NbAs within 50 femtoseconds. The direction of this scattering can be controlled by the pump and probe polarizations, suggesting potential ultrafast device applications.Keywords:
Weyl semimetal
Surface States
Weyl semimetals have been classified into type-I and type-II with respect to the geometry of their Fermi surfaces at the Weyl points. Here, we propose a new class of Weyl semimetal, whose unique Fermi surface contains two electron or two hole pockets touching at a multi-Weyl point, dubbed as type-III Weyl semimetal. Based on first-principles calculations, we first show that quasi-one-dimensional compound (TaSe4)2I is a type-III Weyl semimetal with larger chiral charges. (TaSe4)2I can support four-fold helicoidal surface states with remarkably long Fermi arcs on the (001) surface. Angle-resolved photoemission spectroscopy measurements are in agreement with the gapless nature of (TaSe4)2I at room temperature and reveal its characteristic dispersion. In addition, our calculations show that external strain could induce topological phase transitions in (TaSe4)2I among the type-III, type-II, and type-I Weyl semimetals, accompanied with the Lifshitz transitions of the Fermi surfaces. Therefore, our work first experimentally indicates (TaSe4)2I as a type-III Weyl semimetal and provides a promising platform to further investigate the novel physics of type-III Weyl fermions.
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The development of Ti:sapphire femtosecond laser is reviewed. The basic principl es of its mode-locking, ultrashort pulse generation and its amplifiers are intro duced. The ultrafast time-resolved laser spectroscopy, including femtosecond flu orescence up-conversion and optical Kerr effect as well as its applications are presented.
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We report angle-resolved photoemission experiments identifying an arc-like surface state connecting the bulk electron and hole pockets of the candidate type-II Weyl semimetal WTe2. This surface state can be isolated clearly on one of two distinct surface terminations observed experimentally, which we associate with the in-equivalent top and bottom surfaces of the non-centrosymmetric crystal structure. We further use the identification of the two different surfaces to clarify the number of bulk Fermi surface sheets in WTe2.
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With electron and hole pockets touching at the Weyl node, type-II Weyl semimetal is a newly proposed topological state distinct from its type-I cousin. We numerically study the localization effect for tilted type-I as well as type-II Weyl semimetals and give the global phase diagram. For dis- ordered type-I Weyl semimetal, an intermediate three-dimensional quantum anomalous Hall phase is confirmed between Weyl semimetal phase and diffusive metal phase. However, this intermediate phase is absent for disordered type-II Weyl semimetal. Besides, near the Weyl nodes, comparing to its type-I cousin, type-II Weyl semimetal possesses even larger ratio between the transport lifetime along the direction of tilt and the quantum lifetime. Near the phase boundary between the type-I and the type-II Weyl semimetals, infinitesimal disorder will induce an insulating phase so that in this region, the concept of Weyl semimetal is meaningless for real materials.
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Highly circularly polarized (CP) infrared thermal radiation is greatly in demand because of its significant potential in mid-infrared (mid-IR) applications. To exploit the magnitude and quality factor of circular dichroism (CD) simultaneously, a lithography-free platform consisting of a Weyl semimetal (WSM)/dielectric (Ge)/WSM stack sitting on a metallic substrate (Mo) is proposed. A chiral response and varying CD values from -1 to 0.957 have been demonstrated. The numerical results from a generalized 4 × 4 transfer matrix algorithm verify that the chiral structure manifests a remarkably high quality factor (
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The recent explosion of research interest in Weyl semimetals has led to many proposed Weyl semimetal candidates and a few experimental observations of a Weyl semimetal in real materials. Through this experience, we have come to appreciate that typical Weyl semimetals host many Weyl points. For instance, the first Weyl semimetal observed in experiment, TaAs, hosts 24 Weyl points. Similarly, the Mo$_x$W$_{1-x}$Te$_2$ series, recently under study as the first Type II Weyl semimetal, has eight Weyl points. However, it is well-understood that for a Weyl semimetal without inversion symmetry but with time-reversal symmetry, the minimum number of Weyl points is four. Realizing such a minimal Weyl semimetal is fundamentally relevant because it would offer the simplest "hydrogen atom" example of an inversion-breaking Weyl semimetal. At the same time, transport experiments and device applications may be simpler in a system with as few Weyl points as possible. Recently, TaIrTe$_4$ has been predicted to be a minimal, inversion-breaking Weyl semimetal. However, crucially, the Weyl points and Fermi arcs live entirely above the Fermi level, making them inaccessible to conventional angle-resolved photoemission spectroscopy (ARPES). Here we use pump-probe ARPES to directly access the band structure above the Fermi level in TaIrTe$_4$. We directly observe Weyl points and topological Fermi arcs, showing that TaIrTe$_4$ is a Weyl semimetal. We find that, in total, TaIrTe$_4$ has four Weyl points, providing the first example of a minimal inversion-breaking Weyl semimetal. Our results hold promise for accessing exotic transport phenomena arising in Weyl semimetals in a real material.
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Weyl semimetals are recently predicted class of materials that can be regarded as three-dimensional analogs of graphene breaking time reversal or inversion symmetry. Electrons in a Weyl semimetal behave as Weyl fermions, which have many exotic properties, such as chiral anomaly and magnetic monopoles in the crystal momentum space. The surface state of a Weyl semimetal displays pairs of entangled Fermi arcs at two opposite surfaces. However, the existence of Weyl semimetals has not yet been proved experimentally. Here we report the experimental realization of a Weyl semimetal in TaAs by observing Fermi arcs formed by its surface states using angle-resolved photoemission spectroscopy. Our first-principles calculations, matching remarkably well with the experimental results, further confirm that TaAs is a Weyl semimetal.
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In this paper, we theoretically study the topological phase transition from nodal-line semimetal to Weyl semimetal. The nodal-line structure is protected by mirror symmetry and located on the ${k}_{x}\text{\ensuremath{-}}{k}_{y}$ mirror reflection plane, and the Hamiltonian of nodal-line semimetal has an emergent chiral symmetry on this plane. When the mirror symmetry is broken, the topological nodal line opens the gap and the nodal-line semimetal transition to Weyl semimetal with Weyl points on the ${k}_{x}$ axis or the ${k}_{y}$ axis. In addition, we break the chiral symmetry and realize the Weyl semimetal with the Weyl points on the ${k}_{z}$ axis. Destruction of the chiral symmetry leads to the gradual bending of the energy bands. With the evolution of the energy bands, the type-II nodal-line semimetal, the type-II Weyl semimetal and the type-I Weyl semimetal are successively realized. Furthermore, we also study the surface states of the nodal-line semimetal and the corresponding Weyl semimetals after the phase transition. Our work provides more ways to study the phase transition between nodal-line semimetal and Weyl semimetal and helps realize possible applications in topological electronic devices in the future.
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Considerable progress has taken place in the generation and application of ultrashort optical pulses. The methods and techniques for extending time-resolved measurements into the femtosecond (10(-15) second) time domain are described, and recent applications and fertile areas for investigation with femtosecond pulses are discussed.
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