Session A29: Dirac Semimetal

Ultrafast reflectance of photoexcited Weyl and Dirac semimetals TaAs and ZrSiS

We report ultrafast pump-probe and transient-grating (TG) measurements of the Weyl semimetal TaAs and the Dirac line-node semimetal ZrSiS, and contrast these results with prior measurements on the Dirac semimetal Cd$_3$As$_2$. After absorption of photons from the pump pulse, we monitor the samples' recovery to equilibrium by measuring the change in reflectance of a time-delayed probe pulse. For TaAs, the reflectance recovers in just 1.2 ps, significantly faster than the 3.1 ps measured in Cd$_3$As$_2$. This rapid recovery appears not to change when temperature is varied from 300 K to 8 K, when a magnetic field of order 0.3 T is applied, or when the excitation fluence is increased by a factor of 20. TG measurements allow us to assign the changes in reflectance to changes in either the dispersive (real) or absorptive (imaginary) parts of the index of refraction. Intriguingly, and in contrast to Cd$_3$As$_2$, the initial change in reflectance is caused by an abrupt reduction in the dispersive part, followed by a slower reduction in the absorptive part. For ZrSiS, the recovery after photoexcitation is even faster, at 0.3 ps. We will discuss the implications of these findings for carrier dynamics in topological semimetals.   

Ultrafast Photo-Carrier Dynamics and Coherent Phonon Excitations in Topological Dirac Semimetal Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$

Three dimensional (3D) topological Dirac semimetal has attracted growing research interest owing to its intriguing quantum properties such as high bulk carrier mobility and quantum spin Hall effects. However, so far, the ultrafast dynamics of a typical 3D topological Dirac semimetal, Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$, as well as its coherent phonon has not been thoroughly investigated. Here we report the ultrafast dynamics of Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ by using femtosecond pump-probe spectroscopy. Two distinct relaxation processes was observed, with the lifetimes (at 5 K) of 2.4 ps and 18.6 ps, respectively. Variable temperature experiment from 5 K to 295 K also reveals signatures of phase transitions. Furthermore, coherent optical (8.1 meV) and acoustic (0.036 THz) phonon modes were generated and detected, respectively, with signatures of hybrid-excitation of the two modes.   

Gate-tunable quantum oscillations in ambipolar Cd3As2 thin films

Cd3As2, a three-dimensional (3D) analog of graphene with extraordinary carrier mobility, was predicted to be a 3D Dirac semimetal, a feature confirmed by recent experiments. Here we report on the first observation of a gate-induced transition from band conduction to hopping conduction in single-crystalline Cd3As2 thin films via electrostatic doping by solid electrolyte gating. The extreme charge doping enables the unexpected observation of p-type conductivity in a 50-nm-thick Cd3As2 thin film grown by molecular beam epitaxy. More importantly, the gate-tunable Shubnikov--de Haas oscillations and the temperature-dependent resistance reveal a unique band structure and bandgap opening when the dimensionality of Cd3As2 is reduced. This is also confirmed by our first-principle calculations. The present results offer new insights toward nanoelectronic and optoelectronic applications of Dirac semimetals and provide new routes in the search for the intriguing quantum spin Hall effect in low-dimension Dirac semimetals. Reference: Y. Liu. et.al. NPG Asia Mater. 7\textbf{,} e221 (2015)   

Quantum Oscillations in Weyl and Dirac Semimetal Ultra-Thin Films

We show that a thin film of Weyl or Dirac semimetal with a strong in-plane magnetic field becomes a novel two-dimensional Fermi liquid with interesting properties. The Fermi surface in this system is strongly anisotropic, consisting of a combination of chiral bulk channels and the Fermi arcs. The area enclosed by the Fermi surface is proportional to the magnetic field component parallel to the Weyl/Dirac node splitting, which leads to unusual behavior in quantum oscillations when the magnetic field is tilted out of the plane. We estimate the oscillation frequencies and the regimes where such effects could be seen in Cd$_3$As$_2$ and TaAs.   

Dirac Cone Protected by Non-Symmorphic Symmetry and highly dispersive 3D Dirac crossings in ZrSiS

Materials harboring exotic quasiparticles, such as Dirac and Weyl fermions have garnered much attention from the physics and material science communities. Here, we show with angle resolved photoemission studies supported by ab initio calculations that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones which form a Fermi surface with a diamond-shaped line of Dirac nodes. We also experimentally show, for the first time, that the square Si lattice in ZrSiS is an excellent template for realizing the new types of 2D Dirac cones protected by non-symmophic symmetry and image an unforseen surface state that arises close to the 2D Dirac cone. Finally, we find that the energy range of the linearly dispersed bands is as high as 2 eV above and below the Fermi level; much larger than of any known Dirac material so far. We will discuss why these characteristics make ZrSiS very promising for future applications.   

Helical Spin Order from Topological Dirac and Weyl Semimetals

We study dynamical mass generation and the resultant helical spin orders in topological Dirac and Weyl semimetals, including the edge states of quantum spin Hall insulators, the surface states of weak topological insulators, and the bulk materials of Weyl semimetals. In particular, the helical spin textures of Weyl semimetals manifest the spin-momentum locking of Weyl fermions in a visible manner. The spin-wave fluctuations of the helical order carry electric charge density; therefore, the spin textures can be electrically controlled in a simple and predictable manner.   

Magnetotransport in Dirac semimetals: Chiral magnetic effect and quantum oscillations

Dirac semimetals are characterized by the linear dispersion of fermionic quasiparticles, with the Dirac point hidden inside a Fermi surface. We study the magnetotransport in these materials using chiral kinetic theory to describe within the same framework both the negative magnetoresistance caused by the chiral magnetic effect and quantum oscillations in the magnetoresistance due to the existence of the Fermi surface [1]. We also consider the role of Fermi Arcs and their contribution for the SdH modes. We discuss the relevance of obtained results to recent measurements on $Cd_3As_2$. \newline [1] G. Monteiro, A. Abanov and D. Kharzeev, Phys. Rev. \textbf{B} 92, 165109 (2015).   

Are the surface Fermi arcs in Dirac semimetals topologically protected?

Motivated by recent experiments probing double Fermi arcs on the surface of Dirac semimetals (DSMs) Na3Bi and Cd3As2, we raise the question posed in the title. We find that, in marked contrast to Weyl semimetals, the Fermi arcs of DSMs are not topologically protected in general, except at certain time-reversal invariant momenta. For a simple 4-band model with a pair of Dirac nodes at k = (0, 0, ±Q) gapless surface states are protected only at kz = 0. We identify symmetry allowed bulk perturbations that destroy Fermi arcs, but show that they are necessarily “small”, i.e., higher order than terms kept in usual k · p theory. We validate our conclusions about the absence of a topological invariant protecting the surface states in DSMs using a K-theory analysis for the space groups of Na3Bi and Cd3As2   

Angular Magnetoresistance and Hall Measurements in New Dirac Material, ZrSiS

Dirac and Weyl materials have shot to the forefront of condensed matter research in the last few years. Recently, the square-net material, ZrSiS, was theorized and experimentally shown (via ARPES) to host several highly dispersive Dirac cones, including the first Dirac cone demanded by non-symmorphic symmetry in a Si square net. Here we report the magnetoresistance and Hall Effect measurements in this compound. ZrSiS samples with RRR $=$ 40 were found to have MR values up to 6000{\%} at 2 K, be predominantly p-type with a carrier concentration of \textasciitilde 8 x 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$ and mobility \textasciitilde 8500 cm$^{\mathrm{2}}$/Vs. Angular magnetoresistance measurements reveal a peculiar behavior with multiple local maxima, depending on field strength, indicating of a sensitive and sensitive Fermi surface. SdH oscillations analysis confirms Hall and angular magnetoresistance measurements. These results, in the context of the theoretical and ARPES results, will be discussed.   

Observation of quasi-two-dimensional Dirac fermions in ZrTe5

Since the discovery of graphene, layered materials have attracted extensive interests owing to their unique electronic and optical characteristics. Among them, Dirac semimetal, one of the most appealing categories, has been a long-sought objective in layered systems beyond graphene. Recently, layered pentatelluride ZrTe5 was found to host signatures of Dirac semimetal. However, the low Fermi level in ZrTe5 strongly hinders a comprehensive understanding of the whole picture of electronic states through photoemission measurements, especially in the conduction band. Here, we report the observation of Dirac fermions in ZrTe5 through magneto-optics and magneto-transport. By applying magnetic field, we observe a square-root-B-dependence of inter-Landau-level resonance and Shubnikov-de Haas oscillations with non-trivial Berry phase, both of which are hallmarks of Dirac fermions. The angular-dependent SdH oscillations show a clear quasi-two-dimensional feature with highly anisotropic effective mass and Fermi velocity, in stark contrast to the 3D Dirac semimetal such as Cd3As2. With the confined interlayer dispersion and reducible dimensionality, our work establishes ZrTe5 as an ideal platform for exploring exotic physical phenomena of Dirac fermions. Another work about the optics on Cd3As2 thin film will also be discussed.   

Angle-resolved photoemission study on potential topological insulator ZrTe$_5$

ZrTe$_5$ is a layered-structure material which is predicted to exhibit the quantum spin hall effect in its monolayer limit. Bulk ZrTe$_5$ material is of scientific interest as well, as it might lie within the transition boundary between weak and strong topological insulator. We are using angle-resolved photoemission spectroscopy (ARPES) to investigate the band structure of bulk ZrTe$_5$. Synchrotron data with varied photon energies shows little k$_z$ dependence, which indicates a quasi-two-dimensional band structure; in addition, we observe circular dichroism, which suggests possible spin polarization. We are also working on time-resolved ARPES measurements, hoping to reveal the band structure above the Fermi level, which might give information about the material’s topological properties.   

Magneto-infrared spectroscopy of Landau levels and Zeeman splitting of three-dimensional massless Dirac Fermions in ZrTe$_5$

We present a magneto-infrared spectroscopy study on a newly identified three-dimensional (3D) Dirac semimetal ZrTe$_5$. We observe clear transitions between Landau levels and their further splitting under magnetic field. Both the sequence of transitions and their field dependence follow quantitatively the relation expected for 3D \emph{massless} Dirac fermions. The measurement also reveals an exceptionally low magnetic field needed to drive the compound into its quantum limit, demonstrating that ZrTe$_5$ is an extremely clean system and ideal platform for studying 3D Dirac fermions. The splitting of the Landau levels provides a direct and bulk spectroscopic evidence that a relatively weak magnetic field can produce a sizeable Zeeman effect on the 3D Dirac fermions, which lifts the spin degeneracy of Landau levels. Our analysis indicates that the compound evolves from a Dirac semimetal into a topological line-node semimetal under current magnetic field configuration. Refs: R. Y. Chen et al., Phys. Rev. B 92, 075107 (2015); R. Y. Chen et al., Phys. Rev. Lett. 115, 176404 (2015).   

Detection of chiral anomaly and valley transport in Dirac semimetals

Chiral anomaly is a non-conservation of chiral charge pumped by the topological nontrivial gauge field, which has been predicted to exist in the emergent quasiparticle excitations in Dirac and Weyl semimetals. However, so far, such pumping process hasn't been clearly demonstrated and lacks a convincing experimental identification. Here, we report the detection of the charge pumping effect and the related valley transport in Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ driven by external electric and magnetic fields (E\textbullet B). We find that the chiral imbalance leads to a non-zero gyrotropic coefficient, which can be confirmed by the E\textbullet B-generated Kerr effect. By applying B along the current direction, we observe a negative magnetoresistance despite the giant positive one at other directions, a clear indication of the chiral anomaly. Remarkably, a robust nonlocal response in valley diffusion originated from the chiral anomaly is persistent up to room temperature when B is parallel to E. The ability to manipulate the valley polarization in Dirac semimetal opens up a brand-new route to understand its fundamental properties through external fields and utilize the chiral fermions in valleytronic applications.   

A coupled wire model of topological Weyl and Dirac semimetal I: topological insulating texture and gapping interaction

Weyl and Dirac semimetals in three dimensions have semi-robust massless electronic structures. We mimic these gapless systems using an array of coupled Dirac wires, and analytically study the gapping effect of many-body interactions. The Dirac wires are arranged in a way so that the charge conserving model exhibits an antiferromagnetic time reversal symmetry as well as a p2mg wallpaper group symmetry, which contains twofold rotations, reflections and glide planes. The gapless electrons can aquire a mass upon symmetry breaking dimerizations, or more interestingly, symmetry preserving many-body interactions. This involves the introduction of a topological insulating texture in the bulk supported by layers of gapped symmetric interacting surfaces of topological insulators. The resulting massive system is a three dimensional {\em geometric topological state}.   

A coupled wire model of topological Weyl and Dirac fermion II: three-dimensional geometric topological phase

We mimic Weyl and Dirac semimetals in three dimensions by a coupled Dirac wire model, and introduce many-body gapping interactions that preserve symmetries. The construction relies on additional layers of gapped symmetric interacting surfaces of topological insulators, each carrying fractional charge excitations and containing Ising-like surface topological order. The three dimensional stack supports mutually non-local fractional point charges and flux tubes. Moreover the flux tubes, when directed in an appropriate direction, can carry Majorana zero modes and give rise to non-Abelian "3-loop braiding". Due to the highly anisotropic nature of the coupled wire model, the topological phase also exhibits geometric properties beyond a topological field theory description.