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Register a new userTransport signatures of surface states in a Weyl semimetal: evidence of field driven Fermi arc interferometry
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A signature property of Weyl semimetals is the existence of topologically protected surface states - arcs in momentum space that connect Weyl points in the bulk. However, the presence of bulks states makes detection of surface contributions to the transport challenging. Here we present a magnetoresistance study of high-quality samples of the prototypical Weyl semimetal, TaAs. By measuring the Shubnikov de Haas effect, we reveal the presence of a two-dimensional cyclotron orbit. This orbit is quantitatively consistent with the interference of coherent quasiparticles traversing two distinct Fermi arcs on the [001] crystallographic surface. The observation of this effect suggests that high magnetic fields can be used to study not only the transport properties of Fermi arcs, but also the interference of their quantum mechanical wavefunctions.
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We experimentally investigate charge transport through the interface between a niobium superconductor and a three-dimensional WTe$_2$ Weyl semimetal. In addition to classical Andreev reflection, we observe sharp non-periodic subgap resistance resonances. From an analysis of their positions, magnetic field and temperature dependencies, we can interpret them as an analog of Tomasch oscillations for transport along the topological surface state across the region of proximity-induced superconductivity at the Nb-WTe$_2$ interface. Observation of distinct geometrical resonances implies a specific transmission direction for carriers, which is a hallmark of the Fermi arc surface states.
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.
Bulk-surface correspondence in Weyl semimetals assures the formation of topological Fermi-arc surface bands whose existence is guaranteed by bulk Weyl nodes. By investigating three distinct surface terminations of the ferromagnetic semimetal Co3Sn2S2 we verify spectroscopically its classification as a time reversal symmetry broken Weyl semimetal. We show that the distinct surface potentials imposed by three different terminations modify the Fermi-arc contour and Weyl node connectivity. On the Sn surface we identify intra-Brillouin zone Weyl node connectivity of Fermi-arcs, while on Co termination the connectivity is across adjacent Brillouin zones. On the S surface Fermi-arcs overlap with non-topological bulk and surface states that ambiguate their connectivity and obscure their exact identification. By these we resolve the topologically protected electronic properties of a Weyl semimetal and its unprotected ones that can be manipulated and engineered.
The Fermi surface of a conventional two-dimensional electron gas is equivalent to a circle, up to smooth deformations that preserve the orientation of the equi-energy contour. Here we show that a Weyl semimetal confined to a thin film with an in-plane magnetization and broken spatial inversion symmetry can have a topologically distinct Fermi surface that is twisted into a $mbox{figure-8}$ $-$ opposite orientations are coupled at a crossing which is protected up to an exponentially small gap. The twisted spectral response to a perpendicular magnetic field $B$ is distinct from that of a deformed Fermi circle, because the two lobes of a mbox{figure-8} cyclotron orbit give opposite contributions to the Aharonov-Bohm phase. The magnetic edge channels come in two counterpropagating types, a wide channel of width $beta l_m^2propto 1/B$ and a narrow channel of width $l_mpropto 1/sqrt B$ (with $l_m=sqrt{hbar/eB}$ the magnetic length and $beta$ the momentum separation of the Weyl points). Only one of the two is transmitted into a metallic contact, providing unique magnetotransport signatures.
We investigate electron transport along the surface of WTe$_2$ three-dimensional single crystals, which are characterized by coexistence of Weyl semimetal conductivity and ferroelectricity at room temperature. We find that non-linear behavior of $dV/dI(I)$ WTe$_2$ differential resistance is accompanied by slow relaxation process, which appears as the $dV/dI(I)$ dependence on the sign of the current change. This observation is confirmed by direct investigation of time-dependent relaxation curves. While strongly non-linear differential resistance should be expected for the zero-gap WTe$_2$, the slow relaxation in transport is very unusual for well-conducting semimetals at room temperature. We establish, that non-monotonous dependence of the amplitude of the effect on driving current $Delta dV/dI(I)$ well corresponds to the known Sawyer-Towers ferroelectric hysteresis loop. This conclusion is also confirmed by gate voltage dependencies, so our results can be understood as a direct demonstration of WTe$_2$ ferroelectric polarization in charge transport experiment.
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