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Register a new userBulk-Fermi-Arc Transition Induced Large Photogalvanic Effect in Weyl Semimetals
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The surface Fermi arc, as a hallmark of Weyl semimetals (WSMs), has been well known in current research, but it remains a challenge to unveil novel phenomena associated with the Fermi arc. Here, we predict a heretofore unrecognized process in WSMs, namely, the photoinduced transition between the bulk states and the Fermi arc. We find this process is significant and can lead to a large effective three-dimensional shift current on the boundaries with the Fermi arc in wide terahertz range. Moreover, due to the low symmetry of the boundaries, the surface photogalvanic effect predicted here can appear in a large class of WSMs that do not have bulk shift current. Hence, our work not only unveils a hidden photogalvanic effect in WSMs but also suggests that all the WSMs are promising material candidates for developing efficient terahertz photodetectors.
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Fermi arc surface states are the hallmark of Weyl semimetals, whose identification is usually challenged by their coexistence with gapless bulk states. Surface transport measurements by fabricating setups on the sample boundary provide a natural solution to this problem. Here, we study the Andreev reflection (AR) in a planar normal metal-superconductor junction on the Weyl semimetal surface with a pair of Fermi arcs. For a conserved transverse momentum, the occurrence of normal reflection depends on the relative orientation between the Fermi arcs and the normal of the junction, which is a direct result of the disconnected Fermi arcs. Consequently, a crossover from the suppressed to perfect AR occurs with varying the orientation of the planar junction, giving rise to a change from double-peak to plateau structure in conductance spectra. Moreover, such a crossover can be facilitated by imposing a magnetic field, making electrons slide along the Fermi arcs so as to switch between two regimes of the AR. Our results provide a decisive signature for the detection of Fermi arcs and open the possibilities of exploring novel phenomenology through their interplay with superconductivity.
Weyl semimetals expand research on topologically protected transport by adding bulk Berry monopoles with linearly dispersing electronic states and topologically robust, gapless surface Fermi arcs terminating on bulk node projections. Here, we show how the Nernst effect, combining entropy with charge transport, gives a unique signature for the presence of Dirac bands. The Nernst thermopower of NbP (maximum of 800 microV K-1 at 9 T, 109 K) exceeds its conventional thermopower by a hundredfold and is significantly larger than the thermopower of traditional thermoelectric materials. The Nernst effect has a pronounced maximum near T_M=90 +/- 20 K=mu_0/kB (mu_0 is chemical potential at T=0 K). A self-consistent theory without adjustable parameters shows that this results from electrochemical potential pinning to the Weyl point energy at T>=TM, driven by charge neutrality and Dirac band symmetry. Temperature and field dependences of the Nernst effect, an even function of the charge polarity, result from the intrinsically bipolar nature of the Weyl fermions. Through this study, we offer an understanding of the temperature dependence of the position of the electrochemical potential vis-a-vis the Weyl point, and we show a direct connection between topology and the Nernst effect, a potentially robust experimental tool for investigating topological states and the chiral anomaly.
In topological Weyl semimetals, the low energy excitations are comprised of linearly dispersing Weyl fermions, which act as monopoles of Berry curvature in momentum space and result in topologically protected Fermi arcs on the surfaces. We propose that these Fermi arcs in Weyl semimetals lead to an anisotropic magnetothermal conductivity, strongly dependent on externally applied magnetic field and resulting from entropy transport driven by circulating electronic currents. The circulating currents result in no net charge transport, but they do result in a net entropy transport. This translates into a magnetothermal conductivity that should be a unique experimental signature for the existence of the arcs. We analytically calculate the Fermi arc-mediated magnetothermal conductivity in the low-field semiclassical limit as well as in the high-field ultra-quantum limit, where only the chiral Landau levels are involved. By numerically including the effects of higher Landau levels, we show how the two limits are linked at intermediate magnetic fields. This work provides the first proposed signature of Fermi arc-mediated thermal transport and sets the stage for utilizing and manipulating the topological Fermi arcs in experimental thermal applications.
The Weyl semimetal phase is a recently discovered topological quantum state of matter characterized by the presence of topologically protected degeneracies near the Fermi level. These degeneracies are the source of exotic phenomena, including the realization of chiral Weyl fermions as quasiparticles in the bulk and the formation of Fermi arc states on the surfaces. Here, we demonstrate that these two key signatures show distinct evolutions with the bulk band topology by performing angle-resolved photoemission spectroscopy, supported by first-principle calculations, on transition-metal monophosphides. While Weyl fermion quasiparticles exist only when the chemical potential is located between two saddle points of the Weyl cone features, the Fermi arc states extend in a larger energy scale and are robust across the bulk Lifshitz transitions associated with the recombination of two non-trivial Fermi surfaces enclosing one Weyl point into a single trivial Fermi surface enclosing two Weyl points of opposite chirality. Therefore, in some systems (e.g. NbP), topological Fermi arc states are preserved even if Weyl fermion quasiparticles are absent in the bulk. Our findings not only provide insight into the relationship between the exotic physical phenomena and the intrinsic bulk band topology in Weyl semimetals, but also resolve the apparent puzzle of the different magneto-transport properties observed in TaAs, TaP and NbP, where the Fermi arc states are similar.
After the experimental realization, the Berry curvature dipole (BCD) induced nonlinear Hall effect (NLHE) has attracted tremendous interest to the condensed matter community. Here, we investigate another family of Hall effect, namely, chiral anomaly induced nonlinear Hall effect (CNHE) in multi-Weyl semimetal (mWSM). In contrast to the BCD induced NLHE, CNHE appears because of the combination of both chiral anomaly and anomalous velocity due to non-trivial Berry curvature. Using the semiclassical Boltzmann theory within the relaxation time approximation, we show that, in contrast to the chiral anomaly induced linear Hall effect, the magnitude of CNHE decreases with the topological charge n. Interestingly, we find that unlike the case of n=1, the CNHE has different behaviors in different planes. Our prediction on the behavior of CNHE in mWSM can directly be checked in experiments.
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