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A quantum pumping protocol through which the quasiparticles of Weyl/multi‐Weyl and nodal‐line semimetals are subjected to a time‐periodic rectangular potential well is considered. The presence of an oscillating potential of frequency ω creates equispaced Floquet side‐bands with spacing ℏω$\hbar \omega$. As a result, a Fano resonance is observed when the difference in the Fermi energy (i.e., the energy of the incident quasiparticle), and the energy of one of the (quasi)bound state levels of the well, coincides with the energy of an integer number of photons (each carrying energy quantum ℏω$\hbar \omega$). Using the Floquet theory and the scattering matrix approach in the zero‐temperature non‐adiabatic pumping limit, characteristic Fano resonance patterns are found in the transmission coefficients. The inflection points in the pumped shot noise spectra also serve as a proxy for the corresponding Fano resonances. Therefore, the pumped shot noise is also numerically evaluated. Finally, the existence of the Fano resonance points is correlated to the (quasi)bound states of the well, by explicitly calculating the bound states of the static well (which are a subset of the bound states of the driven system). Since semimetals with anisotropic dispersions are considered, all the features observed depend on the orientation of the potential well. [ABSTRACT FROM AUTHOR]

The realization of Dirac and Weyl physics in solids has made topological materials one of the main focuses of condensed matter physics. Recently, the topic of topological nodal line semimetals, materials in which Dirac or Weyl-like crossings along special lines in momentum space create either a closed ring or line of degeneracies, rather than discrete points, has become a hot topic in topological quantum matter. Here, we review the experimentally confirmed and theoretically predicted topological nodal line semimetals, focusing in particular on the symmetry protection mechanisms of the nodal lines in various materials. Three different mechanisms: a combination of inversion and time-reversal symmetry, mirror reflection symmetry, and non-symmorphic symmetry and their robustness under the effect of spin orbit coupling are discussed. We also present a new Weyl nodal line material, the Te-square net compound KCu $ _2 $ EuTe $ _4 $ , which has several Weyl nodal lines including one extremely close to the Fermi level ( $ < $ 30 meV below E $ _F $ ). Finally, we discuss potential experimental signatures for observing exotic properties of nodal line physics.

With the use of simple models, we investigated the optical conductivity of a nodal-line semimetal (NLSM) whose crossing of the conduction and valence bands near the origin ( O point) in the (kx,ky) plane of a small cubic region can be adjusted by a parameter α . The Hamiltonian of the NLSM is based on thek⋅pmodel for the low-lying energy bands. When α  = 0, these bands touch each other along a continuous closed loop but the opening of a band gap corresponding to finite values of α and the varying of the carrier concentration can be adjusted. This provides a tunable semiconductor gap, around the O point and the valence and conduction bands can meet at a pair of points within the small cubic region in k space. The optical conductivity of such a NLSM is calculated using the Kubo formula with emphasis on the optical spectral weight redistribution, deduced from appropriate Green's functions, brought about by changes in gap and chemical potential due to modifying α . We derived closed-form semi-analytic expressions for the longitudinal components of the optical conductivity for these model systems of NLSM and compare results for chosen α and chemical potential. We also present results for the heat capacity when the system is in thermal equilibrium for various chosen α and chemical potential., (© 2023 IOP Publishing Ltd.)

Nodal-line superconductor NaAlSi with a transition temperature (Tc) of 7 K has attracted considerable attention in recent years, whereas its Ge counterpart, NaAlGe, does not superconduct down to the lowest temperature regardless of their similar atomic and electrical structures. To tackle this enigma, we resort to the growth of NaAlGe single crystal and characterize its ground state. Interestingly, when hole doped by oxidation or extracting Na, single-crystalline NaAlGe transforms from a semimetal/semiconductor to a superconductor (Tc=1.8∼3.3 K) with zero resistivity and a diamagnetic shielding fraction over 100%, but without a thermodynamic response in heat capacity. Continuous x-ray diffraction reveals a transient new structure with a larger c axis, which is suggested to have arisen from the minor loss of Na and to be responsible for the emergence of the delicate superconductivity. Our findings place NaAlGe on an equal footing with NaAlSi and provide an alternative for studying the intriguing relationship between superconductivity and nodal-line topology., (© 2022 IOP Publishing Ltd.)