The separation of the Weyl nodes in a broken time-reversal symmetric Weyl semimetal leads to helical quasi-particle excitations at the Weyl nodes, which, when coupled with overall spin conservation allows only inter-nodal transport at the junction of the Weyl semimetal with a superconductor. This leads to an unusual periodic oscillation in the Josephson current as a function of k0L, where L is the length of the Weyl semimetal and 2k0 is the inter-nodal distance. This oscillation is robust and should be experimentally measurable, providing a direct path to confirming the existence of chiral nodes in the Weyl semimetal. PACS numbers: 74.45.+c, 74.50.+r, Introduction.-Weyl semimetals (WSM), which have received much interest recently due to their non-trivial transport characteristics, are 3D topological systems where conduction and valence bands touch at two or more 'Weyl' points 1-5 . According to a no-go theorem 6 , gapless Weyl nodes in a WSM appear as pairs in momentum space with each of the nodes having a definite 'chirality', a quantum number that depends on the Berry flux enclosed by a closed surface around the node. Gauss law prevents the annihilation of the nodes unless two of them with opposite chirality are brought together, which provides the 'topological' protection of the Weyl nodes 7 . A WSM phase requires broken time-reversal and/or inversion symmetry and a growing number of systems has been put forward which realize the WSM phase 8-10 .The separation of the chiral nodes, allows charge pumping between the nodes in the presence of parallel electric and magnetic fields, as a consequence of the chiral anomaly 11 , and this has led to detailed studies of transport in Weyl semi-metals in several recent papers .In this paper we study the current in a simple Josephson junction setup, depicted in Fig. 1(a). The helical quasi-particle excitations at the Weyl nodes, due to the overall spin conserving processes at a WSMsuperconductor (SC) junction, allow only inter-nodal transport 33 . Further, we show, unlike in a normal metal-SC interface, the inter-nodal 'normal' (electron to electron) reflection process in a WSM-SC interface is not suppressed even for energies close to Fermi-energy, due to the broken time-reversal symmetry separating the Weyl nodes. The Josephson current, flowing through the bound levels formed by multiple inter-nodal 'normal' and Andreev (electron to hole) processes in a SC-WSM-SC system, consequently, acquires a specific periodicity as a function of the length of the WSM which depends only on the separation of the Weyl nodes in the momentum space (see Fig. 1(b)). We argue that both of these features are robust because they are not only bulk effects, but they are also protected by the robustness of the Weyl nodes. We also discuss the feasibility of experimental observations of this transition in our system, which can confirm the presence of chiral nodes in WSM.
We study transport through a Weyl semimetal quantum dot sandwiched between an s-wave superconductor and a normal lead. The conductance peaks at regular intervals and exhibits double periodicity with respect to two characteristic frequencies of the system, one that originates from Klein tunneling in the system and the other coming from the chiral nature of the excitations. Using a scattering matrix approach as well as a lattice simulation, we demonstrate the universal features of the conductance through the system and discuss the feasibility of observing them in experiments.
Transition metal dichalcogenide (TMD) monolayers are interesting materials in part because of their strong spin-orbit coupling. This leads to intrinsic spin-splitting of opposite signs in opposite valleys, so the valleys are intrinsically spin-polarized when hole-doped. We study spin response in a simple model of these materials, with an eye to identifying sharp collective modes (i.e, spin-waves) that are more commonly characteristic of ferromagnets. We demonstrate that such modes exist for arbitrarily weak repulsive interactions, even when they are too weak to induce spontaneous ferromagnetism. The behavior of the spin response is explored for a range of hole dopings and interaction strengths.
Smooth interfaces of topological systems are known to host massive surface states in addition to the topologically protected chiral one. We show that in Weyl semimetals these massive states, along with the chiral Fermi arc, strongly alter the form of the Fermi-arc plasmon. Most saliently, they yield further collective plasmonic modes that are absent in a conventional interface. The plasmon modes are completely anisotropic as a consequence of the underlying anisotropy in the surface model and expected to have a clear-cut experimental signature, e.g., in electron-energy loss spectroscopy.
Moiré superlattices engineer band properties and enable observation of fractal energy spectra of Hofstadter butterfly. Recently, correlated-electron physics hosted by flat bands in small-angle moiré systems has been at the foreground. However, the implications of moiré band topology within the single-particle framework are little explored experimentally. An outstanding problem is understanding the effect of band topology on Hofstadter physics, which does not require electron correlations. Our work experimentally studies Chern state switching in the Hofstadter regime using twisted double bilayer graphene (TDBG), which offers electric field tunable topological bands, unlike twisted bilayer graphene. Here we show that the nontrivial topology reflects in the Hofstadter spectra, in particular, by displaying a cascade of Hofstadter gaps that switch their Chern numbers sequentially while varying the perpendicular electric field. Our experiments together with theoretical calculations suggest a crucial role of charge polarization changing concomitantly with topological transitions in this system. Layer polarization is likely to play an important role in the topological states in few-layer twisted systems. Moreover, our work establishes TDBG as a novel Hofstadter platform with nontrivial magnetoelectric coupling.
Optical response of crystalline solids is to a large extent driven by excitations that promote electrons among individual bands. This allows one to apply optical and magneto‐optical methods to determine experimentally the energy band gap —a fundamental property crucial to our understanding of any solid—with a great precision. Here it is shown that such conventional methods, applied with great success to many materials in the past, do not work in topological Dirac semimetals with a dispersive nodal line. There, the optically deduced band gap depends on how the magnetic field is oriented with respect to the crystal axes. Such highly unusual behavior is explained in terms of band‐gap renormalization driven by Lorentz boosts which results from the Lorentz‐covariant form of the Dirac Hamiltonian relevant for the nodal line at low energies.
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