Session K44: Dirac and Weyl Semimetals: Transport II

Thermomagnetic Transport in the Weyl Semimetal NbP

Weyl semimetals (WSM) combine both topological and semi-metallic effects in the transport of fermions through both bulk and surface states.~ The thermal transport properties, magneto-thermal conductivity, magneto-thermopower, and Anomalous Nernst Effect (ANE) of Weyl fermions are predicted$^{1}$~to contain interesting topological signatures.~~So far, one publication$^{2}$~reports experiments on the magneto-thermopower and ANE on Cd$_{3}$As$_{2}$, a Dirac semimetal.~ An inversion symmetry-breaking group of transition metal monopnictides, including NbP, TaP, NbAs, and TaAs, has been discovered and proven to be WSM.~ Here, we report on thermal transport in NbP on samples that are proven$^{3}$~to have an unsaturated, large magnetoresistance and ultrahigh mobilities.~ Specifically, we map the magneto-thermopower and ANE thermomagnetic tensor elements of single-crystal NbP in order to determine the effect of Fermi arcs on this transport, and we compare our results to computational models. 1. G. Sharma, et al.~ \textit{Phys. Rev. B}~\textbf{93}, 035116 (2016). 2. T. Liang, et al.~~arXiv:1610.02459 (2016). 3. C. Shekhar, et al.~~\textit{Nat. Phys.}~\textbf{11}~(2015).   

The theory of thermomagnetic transport in topological Weyl semimetals

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. Although Weyl semimetals have been demonstrated to exhibit a variety novel signatures in electronic transport, thermal transport remains less understood. We report on calculations of thermo-magneto transport in Weyl semimetals. In particular, we calculate the dependence of the Seebeck effect, the Nernst effect and the thermal conductivity on magnetic field and temperature. We identify signatures of Weyl nodes in our bulk transport calculations and predict how topological Fermi arcs can be definitively identified in thermomagnetic transport. Our work is applicable to several classes of material realizations of Weyl semimetals.   

Dynamical piezoelectric and magnetopiezoelectric effects in polar metals from Berry phases and orbital moments

The polarization of a material and its response to applied electric and magnetic fields are key solid-state properties with a long history in insulators, although a satisfactory theory required new concepts such as Berry-phase gauge fields. In metals, quantities such as static polarization and magnetoelectric $\theta$-term cease to be well-defined. In polar metals there can be analogous dynamical current responses, which we study in a common theoretical framework. We find that current responses to dynamical strain in polar metals depend on both the first and second Chern forms, related to polarization and magnetoelectricity in insulators, as well as the orbital magnetization on the Fermi surface. We provide realistic estimates that predict that the latter contribution will dominate and investigate the feasibility of experimental detection of this effect.   

Magnetic field induced Weyl node annihilation in TaP

The discovery of the Weyl semimetal TaAs family has motivated the interest for investigating the topological properties of the Weyl quasiparticles. Particularly interesting is the unique transport features associated with the chirality of the Weyl quasiparticles, for example, negative longitudinal magnetoresistance induced by an ABJ anomaly. I will show our electrical transport measurement results for TaAs and TaP in a magnetic field. Our comprehensive data allow us to exclude other possible origins of the observed negative longitudinal magnetoresistance. Moreover, we observe that the Hall resistivity of TaP exhibits a sharp sign reversal and a step-like feature in an intense magnetic field. This phenomenon occurs at a specific magnetic field value (33T) that corresponds to the momentum space separation of the Weyl node. These findings demonstrate a magnetic field induced Weyl node annihilation and a gap opening in a Weyl semimetal.   

Predicted Unusual Magnetoresponse in Type-II Weyl Semimetals

We show several distinct signatures in the magnetoresponse of type-II Weyl semimetals. The energy tilt tends to squeeze the Landau levels (LLs), and, for a type-II Weyl node, there always exists a critical angle between the B-field and the tilt, at which the LL spectrum collapses, regardless of the field strength. Before the collapse, signatures also appear in the magneto-optical spectrum, including the invariable presence of intraband peaks, the absence of absorption tails, and the special anisotropic field dependence.   

High-field magnetoconductivity study of a candidate Weyl semimetal

The monopnictides have attracted considerable attention recently due to their conjectured linear band crossings in momentum space. Upon breaking time-reversal or inversion symmetry, the spin-degeneracy at a Dirac node can be lifted to reveal two Weyl points that are characterized by a right- and left-handedness (chirality). These `Weyl semimetals' are expected to host exotic phenomena, including topological Fermi arc surface states and unusual magnetotransport properties. The search for such materials, however, is complicated by the additional presence of trivial (non-Weyl) sections of Fermi surface. Thus, we use high magnetic fields to isolate the the zeroth Landau levels at the Weyl nodes, and measure microstructured transport devices to avoid ``current-jetting'' effects common in such high mobility systems. We measure the longitudinal and transverse magnetoconductivity in the quantum limit of NbP, a candidate Weyl semimetal, to look for experimental signatures that characterize a Weyl state.   

Quantum oscillations in the type-II Weyl fermion metal PtSe$_2$

Relativistic massless fermions described by the Weyl equation have not been observed as elementary particles so far, yet analogous quasiparticles have been identified in solids. While the band structure topology protects the existence of un-gapped linear band crossings, the presence of the crystal lattice as a distinguishable rest frame breaks the Lorentz-invariance. Therefore in solids an additional class of type-II Weyl fermions is allowed that is described by a tilted dispersion cone which is forbidden in elementary particles obeying Lorentz-invariance. It physically corresponds to a sign change of the Fermi velocity along the tilting direction, and their electronic structures are characterized by a topologically protected touching point of an electron-like and a hole-like Fermi surface. PtSe$_2$ has been proposed as a candidate material hosting type-II Weyl fermions. We have performed quantum oscillation measurements in pulsed fields up to 65T on Focused Ion Beam microstructured crystallites that agree well with the Fermi surfaces of current band structure calculations. The present results support a band structure that hosts type-II Weyl fermions in the valence band.   

Magnetotransport study of Dirac fermions in YbMnBi2 and CaMnBi2

It is well known that AMnBi$_{2}$ (A $=$ alkaline earth) with two dimensional (2D) bismuth layer host quasi-2D Dirac states similar to graphene and topological insulators. The Dirac state is significantly affected by the alkaline earth in the block layer. Angle-resolved photoemission spectroscopy (ARPES) indicates that YbMnBi$_{2}$ could be the first Weyl semimetal with time-reversal symmetry breaking, whereas the anisotropic Dirac state in SrMnBi$_{2}$ can host a valley-polarized interlayer current through magnetic valley control. Here, we study in-plane magnetotransport in YbMnBi$_{2}$, and interlayer magnetotransport in CaMnBi$_{2}$. The angular-dependent magnetoresistance, nonzero Berry phase, and small cyclotron mass confirm the presence of Dirac fermion and quasi-2D fermi surface in YbMnBi$_{2}$. The interlayer electronic transport in CaMnBi$_{2}$ suggest valley polarized conduction and a Dirac state on the side wall of the warped cylindrical Fermi surface of CaMnBi$_{2}$.   

Observation of two-dimensional Fermi surface and Dirac dispersion in the new material YbMnSb$_2$

We present the synthesis, crystal structure, electronic structure, and transport properties of the new material YbMnSb$_2$. Our measurements reveal that this system is a low-carrier-density semimetal with a 2D Fermi surface arising from a 3D Dirac dispersion. This Fermi surface is consistent with the predictions of antiferromagnetic density functional theory calculations and the Fermi surface observed via angle-resolved photoemission spectroscopy. The quantitative agreement between these measurements and calculations indicates that YbMnSb$_2$ may be a new topological semimetal in the presence of magnetic order.   

Observation of quantized Hall plateaus in a bulk antiferromagnet EuMnBi$_2$ with magnetically confined 2D Dirac fermions

Dirac fermions in solids have been of current interest for their unique transport properties under magnetic field. To further explore distinct magneto-transport properties, interplay between Dirac fermions and magnetic order is desirable as shown in magnetic topological insulators. In this study, we have focused on a layered bulk antiferromagnet EuMnBi$_2$, where the Bi square nets hosting quasi-2D Dirac fermion and the magnetic insulating layers stack alternatively, and demonstrated that the quantum transport of Dirac fermions is highly enriched by Eu antiferromagnetic(AFM) order. Below the AFM transition temperature, external magnetic field induces spin-flop transition of Eu moments, which suppresses the interlayer coupling between the conducting Bi layers and dramatically enhances interlayer resistivity $\rho_{zz}$. Furthermore, this high-$\rho_{zz}$ state is accompanied by plateau-like structures in the Hall resistivity and giant SdH oscillations, which signify the quantum Hall effect in a bulk magnet. [H. Masuda \textit{et al.}, Sci. Adv. \textbf{2}, e1501117 (2016)]   

Anomalous Hall Effect in topological Heusler semimetal ZrCo2Sn.

Besides non-magnetic Weyl semimetals without inversion centers, Weyl Fermions are also predicted to exist in certain topological materials with time-reverse symmetry breaking. Due to the topological non-trivial state, the system exhibits novel properties compared to the normal metal. In this talk, we will present the systematic magneto-transport study on the magnetic Heusler ZrCo{\$}\textunderscore 2{\$}Sn single crystal, a proposed candidate of ferromagnetic Weyl semimetal. Ruderman--Kittel--Kasuya--Yosida (RKKY) picture will be discussed regarding the isotropic negative magnetoresistance observed.   

Interplay between magnetism and relativistic fermions in Eu doped (Sr/Ba)MnSb$_{\mathbf{2}}$

Layered compounds AMnBi$_{2}$ (A$=$Ca, Sr, Ba, Eu, and Yb) have been established as Dirac materials with fascinating properties. In our previous work, we have demonstrated that Sr$_{1-}_{y}$Mn$_{1-}_{z}$Sb$_{2}$ (y, z \textless 0.1), isostructural to AMnBi$_{2}$, not only host relativistic fermions, but also exhibit ferromagnetic properties, with its ferromagnetism being coupled to the relativistic fermions' transport. To gain further insight into the relativistic fermion-magnetism coupling, we have synthesized a series of Eu doped (Sr/Ba)MnSb$_{2}$ single crystals and found Eu moments order antiferromagnetically. Through neutron scattering experiments, we determined the magnetic structures for Sr$_{1-x}$Eu$_{x}$MnSb$_{2}$ with x $=$ 0.2, 0.5, and 0.8. From magnetotransport measurements, we find the Eu antiferromagnetism is also coupled to relativistic fermion transport. More importantly, we observed a novel quantum phase with saturated magnetoresistivity near the quantum limit for the 10{\%} Eu doped BaMnSb$_{2}$ sample. We will discuss possible mechanisms for this novel phase.   

Magnetic and Electrical Transport Properties of Dirac Compound BaMnSb$_{\mathrm{2}}$*

BaMnSb$_{\mathrm{2}}$ is a layered compound containing Sb square nets that is theoretically predicted to host Dirac fermions. We have carried out experimental investigations on electrical transport and magnetic properties of BaMnSb$_{\mathrm{2}}$ single crystals. Both in-plane ($\rho _{\mathrm{ab}})$ and c-axis ($\rho_{\mathrm{c}})$ resistivities show metallic behavior with a small bump in $\rho_{\mathrm{c}}$ located near 40 K, while there is large anisotropy $\rho_{\mathrm{c}}$ / $\rho _{\mathrm{ab}}$ (\textasciitilde 100 at 300 K) that increases with decreasing temperature to 1500 at 2 K. Interestingly, Shubnikov-de Hass (SdH) oscillations are observed for both $\rho_{\mathrm{ab}}$ and $\rho _{\mathrm{c}}$ over a wide temperature and magnetic field range. Quantitative analysis indicates that large amplitude SdH oscillations result from nearly massless Dirac Fermions. Furthermore, our magnetic measurements indicate an A-type antiferromagnetic magnetic ordering below 286 K where ferromagnetic ordering is observed in the ab plane with antiferromagnetic coupling along the c direction. These results indicate that BaMnSb$_{\mathrm{2}}$ is a 2D magnetic Dirac material.