The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.
The ultrafast dynamics of excited carriers in graphene is closely linked to the Dirac spectrum and plays a central role for many electronic and optoelectronic applications. Harvesting energy from excited electron-hole pairs, for instance, is only possible if these pairs can be separated before they lose energy to vibrations, merely heating the lattice. Until now, the hot carrier dynamics in graphene could only be accessed indirectly. Here, we present a dynamical view on the Dirac cone by time- and angle-resolved photoemission spectroscopy. This allows us to show the quasi-instant thermalization of the electron gas to a temperature of ≈2000 K, to determine the time-resolved carrier density, and to disentangle the subsequent decay into excitations of optical phonons and acoustic phonons (directly and via supercollisions).
Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S2 and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.
Topological semimetals materialize a new state of quantum matter where massless fermions protected by a specific crystal symmetry host exotic quantum phenomena. Distinct from well-known Dirac and Weyl fermions, structurally-chiral topological semimetals are predicted to host new types of massless fermions characterized by a large topological charge, whereas such exotic fermions are yet to be experimentally established. Here, by using angle-resolved photoemission spectroscopy, we experimentally demonstrate that a transition-metal silicide CoSi hosts two types of chiral topological fermions, spin-1 chiral fermion and double Weyl fermion, in the center and corner of the bulk Brillouin zone, respectively. Intriguingly, we found that the bulk Fermi surfaces are purely composed of the energy bands related to these fermions. We also find the surface states connecting the Fermi surfaces associated with these fermions, suggesting the existence of the predicted Fermi-arc surface states. Our result provides the first experimental evidence for the chiral topological fermions beyond Dirac and Weyl fermions in condensed-matter systems, and paves the pathway toward realizing exotic electronic properties associated with unconventional chiral fermions.
Topological semimetals (TSs) in structurally chiral crystals (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, such as new fermionic excitations with large topological charge 1 , long Fermi-arc surface states 2,3 , unusual magnetotransport 4 and lattice dynamics 5 , as well as a quantized response to circularly polarized light 6 . To date, however, all experimentally confirmed TSs crystallize in space groups that contain mirror operations, which forces the aforementioned properties to vanish. Here, by employing angle-resolved photoelectron spectroscopy and ab-initio calculations, we show that AlPt is a structurally chiral TS that hosts new fourfold and sixfold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. Remarkably, these multifold fermions are located at high symmetry points with Chern numbers larger than those in Weyl-semimetals, thus resulting in multiple Fermi-arcs that thread through the full diagonal of the surface Brillouin zone (BZ), spanning the largest portion of the BZ of any material. By imaging these long Fermi-arcs, we can experimentally determine the magnitude and sign of their Chern number, which allows us to relate their dispersion to the handedness of their host crystal.Revised manuscript 2 An object that cannot be superimposed with its mirror image is said to be chiral, a concept first proposed by Lord Kelvin 7 that has found widespread applications across the modern sciences, from high energy physics to biology 8,9 . In condensed matter physics (where the properties of crystalline materials are tightly constrained by spatial lattice symmetries), chiral crystals can only be found in the 65 Sohncke space groups, which contain only orientation-preserving operations, and can therefore be assigned a handedness. The structural chirality of these systems can endow them with fascinating properties, such as natural optical activity 10 , negative refraction in metamaterials 11 , non-reciprocal effects such as magnetochiral birefringence of light or electronic magnetochiral anisotropy 12,13 , chiral magnetic textures such as helices and skyrmions 14 , or unusual superconductivity 15 . In the recently discovered Weyl-semimetals, not just the crystal structure itself, but also the electronic wavefunctions can exhibit a chirality at point-like twoband crossings of the quasiparticle dispersion, which are known as Weyl fermions [16][17][18][19] . These crossings are topologically protected because they carry a topological charge, a quantized Berry flux through any surface enclosing them in momentum space. This charge has integer magnitude C= ±1, which is known as the Chern number and gives the electronic wavefunction a handedness. This topological property results in a plethora of exotic phenomena, such as Fermiarc surface states 20 , unconventional magnetoresistance 16,21,22 , nonlocal transport 23 , and many
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