We investigate the charge excitations of a Weyl semimetal in the axionic charge density wave (axionic CDW) state. While it has been shown that the topological response (anomalous Hall conductivity) is protected against the CDW state, we find that the long wavelength plasmon excitation is radically influenced by the dynamics of the CDW order parameter. In the normal state, we show that an undamped collective mode should exist at q ≈ QCDW if there is an attractive interaction favoring the formation of the CDW state. The undamped nature of this collective mode is attributed to a gap-like feature in the particle-hole continuum at q ≈ QCDW due to the chirality of the Weyl nodes, which is not seen in other materials with CDW instability. In the CDW state, the long wavelength plasmon excitations become more dispersive due to the additional interband scattering not allowed in the normal state. Moreover, because the translational symmetry is spontaneously broken, Umklapp scattering, the process conserving the total momentum only up to n QCDW with n an integer and QCDW the ordering wave vector, emerges in the CDW state. We find that the plasmon excitation couples to the phonon mode of the CDW order via the Umklapp scattering, leading to two branches of resonant collective modes observable in the density-density correlation function at q ≈ 0 and q ≈ QCDW . Based on our analysis, we propose that measuring these resonant plasmon-axion excitations around q ≈ 0 and q ≈ QCDW by the momentum-resolved electron energy loss spectroscopy (M-EELS) could serve as a reliable way to detect the axionic CDW state in Weyl semimetals.PACS numbers: 71.45. Gm,71.45.Lr,75.70.Tj Introduction-The Weyl semimetal[1, 2] is a new gapless state of matter attracting a lot of attention in the past few years due to its rich topological properties. [3][4][5][6][7][8][9][10][11][12][13][14] In the wake of the study on topological insulators [15,16], the search for semimetals exhibiting linear band dispersion near the Fermi energy has been one of the most active fields in condensed matter physics. One typical example is the Dirac semimetal [17][18][19][20] in which the conduction and valence bands become degenerate at certain points in the Brillouin zone, namely the Dirac nodes, and the low energy single particle states near the Dirac nodes can therefore be described by Dirac equations with a fourcomponent spinor. If the Dirac semimetal has a zero mass, the Dirac equation can be further reduced to two independent sets of Weyl equations with two-component spinors. As a result, the Weyl semimetal is a special case of the Dirac semimetal with zero mass, which could be obtained by breaking either the inversion or the timereversal symmetries in a Dirac semimetal. In addition, it can be proved [21][22][23] that Weyl nodes must appear in pairs with opposite chirality if the Weyl equations are imposed on a lattice. Therefore, if a crystalline material is a Weyl semimetal, the low energy single particle excitations can be described by a minimal model of the Weyl ...

Oxygen vacancy is known to play an important role for the physical properties in SrTiO3(STO)-based systems. On the surface, rich structural reconstructions had been reported owing to the oxygen vacancies, giving rise to metallic surface states and unusual surface phonon modes. More recently, an intriguing phenomenon of a huge superconducting transition temperature enhancement was discovered in a monolayer FeSe on STO substrate, where the surface reconstructed STO (SR-STO) may play a role. In this work, SR-STO substrates were prepared via thermal annealing in ultra-high vacuum followed by low energy electron diffraction analyses on surface structures. Thin Nb films with different thicknesses (d) were then deposited on the SR-STO. The detailed studies of the magnetotransport and superconducting property in the Al(1 nm)/Nb(d)/SR-STO samples revealed a large positive magnetoresistance and a pronounced resistance peak near the onset of the resistive superconducting transition in the presence of an in-plane field. Remarkably, the amplitude of the resistance peak increases with increasing fields, reaching a value of nearly 57% of the normal state resistance at 9 T. Such resistance peaks were absent in the control samples of Al(1 nm)/Nb(d)/STO and Al(1 nm)/Nb(d)/SiO2. Combining with DFT calculations for SR-STO, we attribute the resistance peak to the interface resistance from the proximity coupling of the superconducting niobium to the field-enhanced long-range magnetic order in SR-STO that arises from the spin-polarized in-gap states due to oxygen vacancies.