Session D68: Transport measurements of Dirac semimetals

Strain tuning of unconventional Zeeman effect in ZrTe5

In condensed matter systems, while Zeeman effect has been extensively studied over the past decades, its connection to band  topology has emerged only recently. Besides splitting the spin-degenerate Fermi surface,  Zeeman effect also induces non-trivial Berry phase which is dependent on the band structure. Here we demonstrate the strain tuning of unconventional Zeeman effect in ZrTe5 and its impact on the amplitude of the Shubnikov de Haas (SdH) oscillations in 3D Dirac semimetal ZrTe5. Varying the uniaxial strain, a strong TI to weak TI transition is identified. The dependence of the SdH oscillation amplitude on Fermi energy and Dirac mass gap, both tunable through the strain, is analyzed and compared with the quantum oscillation theory.  It is found that the Berry curvature induced by Zeeman effect is a critical factor in modeling the SdH oscillations in such topological materials.   

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Extensive studies of electron transport in Dirac materials have shown positive magneto-resistance (MR) and positive magneto-thermopower (MTP) in a magnetic field perpendicular to the excitation current or thermal gradient. In contrast, measurements of electron transport often show a negative longitudinal MR and negative MTP for a magnetic field oriented along with the excitation current or thermal gradient; this is attributed to the chiral anomaly in Dirac materials. Here, we report a very different magneto-thermoelectric transport behavior in the massive Dirac material ZrTe₅. Although thin flakes show a positive MR in a perpendicular magnetic field, akin to other Dirac materials, we observe a sharp negative MTP. In a parallel magnetic field, we still observe a negative longitudinal MR but a remarkable positive MTP is observed for the fields parallel to the thermal gradients. Our theoretical calculations suggest that this anomalous magneto-thermoelectric behavior can be attributed to screened Coulomb scattering. This work demonstrates the significance of impurity scattering in understanding novel transport phenomena in topological quantum materials and provides deep insight into the magneto-transport properties of Dirac materials.   

Exact Results on the Anomalous Hall Effect in a Dirac Semimetal

We study in detail the Berry curvature in Dirac semimetals across different regimes of its magnetic field dependence in terms of a "k.p model'' which is valid close to the Dirac point. Using analytical methods, we extract from this linear model exact predictions about the anomalous Hall effect that follow from the topology of the band structure and calculate how these change with the inclusion of Landau levels. These features can be discerned in the temperature and magnetic field dependence of the anomalous Hall effect, which we compare with recent experimental data in ZrTe₅ and with band structure calculations.   

Anomalous Hall effect at the Lifshitz transition in massive Dirac semimetal ZrTe5

The intrinsic anomalous Hall effect is a direct manifestation of the Berry curvature in the band structure of a material. In the massive Dirac semimetal ZrTe₅, it has been shown that the strong Zeeman splitting gives rise to a sizeable Berry curvature which can be directly observed by transport measurements. In this work, we study the anomalous Hall effect in ZrTe₅ in the presence of a temperature induced Lifshitz transition, in which the Fermi level goes from the conduction band to the valence band with increasing temperature. The relative influence of the anomalous contribution increases as the Fermi level approaches the Dirac node, as expected for a Dirac semimetal and in agreement with first principles calculations. Our results provide an insight of the interplay between topology and electronic states in the system and show that ZrTe₅ is a highly tunable platform for studying topological effects in crystals.   

In-situ uniaxial strain tuning of topological pentatellurides

In recent years, the subject of topological order has been at the center of attention in condensed matter physics research. Topological materials, representing a new quantum state of matter, are a family of quantum materials with boundary states whose physical properties are robust against certain types of transformations or disorders. Therefore, there have been few examples for a topological phase transition realized experimentally, even fewer cases for an in-situ tuning of a topological phase. We have applied uniaxial strain along different crystallographic directions for a pentatelluride topological material and performed electrical transport measurements at low temperatures and high magnetic fields. We have observed a dramatic change of the resistivity, as a function of uniaxial strain, temperature, magnetic field and crystallographic direction. Our results point to a topological phase transition of the system tuned by uniaxial strain in situ.   

Photo-induced Thermoelectric chiral anomaly in ZrTe5

ZrTe5, as a canonical Dirac semimetal, possesses three-dimensional linear dispersion and has unique properties such as Fermi arc surface states, negative magnetoresistance (NMR), quantum anomalous Hall effect, etc. Here, we report light-induced photocurrent of ZrTe5 in the presence of magnetic field parallel and perpendicular to the current direction, to reveal characteristic thermopower and magnetoresistance as a signature of chiral anomaly at below 10 Kelvin.   

A Detailed Investigation of the Short- and Long-Range Structure of ZrTe5 via Temperature-dependent Rietveld Refinement and PDF Studies

Zirconium pentatelluride (ZrTe₅) has been investigated heavily for its anomalous transport properties, with transition temperatures of ~60 K and ~150 K. The origin of the anomalies observed in its resistivity curve has been hotly debated in the literature. To provide further insight into this complex system, a thorough temperature-dependent structural study was conducted with high resolution datasets collected from the National Synchrotron Light Source II at Brookhaven National Laboratory (NSLS-II, BNL). This exhaustive structural investigation at various temperatures shows subtle structural changes consistent to the anomaly temperatures reported in literature. Both Rietveld refinements of in-situ powder X-ray diffraction (PXRD) and pair distribution function (PDF) fittings shed light on intricate changes in the short- and long-range of the crystal structure and the interesting physics associated with ZrTe₅. The results of these refinements will be discussed in detail to relate effects of temperature on the crystal structure, electronic structure, and physical properties.   

Band Energy Dependence of Defect Formation in Cd3As2

Cadmium Arsenide (Cd­₃As₂) is a prototypical Dirac semimetal, manifesting topological properties in a 3D bulk material. In defect-free Cd₃As₂, the Fermi level E_F lies at a minimum in the density of states at the Dirac point, but Cd₃As₂ forms as intrinsically n-type with an elevated E_F. Therefore, Fermi level control is desirable to fully access and utilize the topological features. To elucidate the interplay between electronic structure and doping, we performed band structure and defect calculations using the SCAN meta-GGA functional with inclusion of spin-orbit coupling and the 80-atom primitive cell of the Cd₃As₂ ground state (space group # 142).

The SCAN functional maintains the band structure topology of DFT functionals but induces significant quantitative changes in the band energies near the Dirac point, raising the energies of bands with s-like character relative to E_F. At the same time, the formation energies for electron donating and accepting defects in Cd₃As₂ have an innate concentration dependence based upon the occupation and depopulation of band states above and below the Dirac point, respectively. We show that the band energies of defect free Cd₃As₂ can be used to extrapolate the defect formation energies to specific electron concentrations.   

Unusual quantum Hall effect in ultrathin films of the Dirac semimetal Cd3As2

The synthesis of thin films of the Dirac semimetal (DSM) Cd₃As₂ by molecular beam epitaxy (MBE) provides an attractive avenue for studying novel phenomena in a DSM as a function of quantum confinement and chemical potential. Here, we use MBE to grow 10 nm Cd₃As₂ thin films on miscut GaAs (111)B substrates with a GaSb buffer layer. We measure electronic transport in lithographically-patterned top-gated Hall bar devices as a function of gate voltage and magnetic field at T = 20 mK. Apparent Quantum Hall plateaus with the filling factor v = 1, 2, 4, 6, 8, 10… are observed by varying the gate voltage at µ₀H = 9 T. At chemical potentials close to the Dirac point, the magnetic field dependence of the longitudinal and transverse resistivity show transitions directly into the v = 1 or v = 2 quantum Hall states without any sign of quantum oscillations or plateaus at higher filling factors. We will interpret these quantum Hall data using in vacuo angle-resolved photoemission spectroscopy investigation and theoretical calculations of the band structure of ultrathin Cd₃As₂ films.   

Growth and Electronic Properties of (ZnxCd1-x)3As2 Thin Films

The three-dimensional Dirac semimetal Cd₃As₂ has been utilized as a prototypical system for studying the physics of Dirac states and the interesting transport and optical properties they support. Additionally, the ability to epitaxially grow thin films on zinc blende semiconductors has opened the door to utilizing Cd₃As₂ in devices. Successfully exploiting the distinct properties of Cd₃As₂ for technological applications will require advanced understanding of their dependence on disorder, doping, and defects. The ability to further tune its electronic properties will offer pathways for ultimately designing this material for next-generation technologies.

Here, the addition of Zn is explored as a method to alter electronic properties as well change the crystal structure in thin films grow by molecular beam epitaxy. Large percentages of Zn substitution on the Cd sublattice (>25%) are found to shift the as-grown Cd₃As₂ epilayers from n-type to p-type, while small percentages lower the carrier concentration preserve the high mobility of electrons. Zn alloying is further found to result in smoother surfaces, and X-ray diffraction and Raman reveal that it additionally alters the structure and symmetry of the thin films. In particular, Raman modes are broadened, and polarization dependence is reduced with increasing Zn content.   

Controlling defect formation and transport properties in Cd3As2 through epitaxial growth

Three-dimensional topological semimetals have been demonstrated to host extraordinary properties such as high mobilities and magnetoresistances. The Dirac semimetal Cd₃As₂ in particular has been the focus of intense study and has been synthesized in thin film form. This is an essential step towards implementing it in next generation electronic devices. However, it is crucial to now understand the predominant types of defects induced by film growth and their role in the magneto-transport properties. This includes both minimizing defects to optimize properties and harnessing them to tailor the electronic structure for specific applications. We study Cd₃As₂ using a combination of molecular beam epitaxy and magnetoresistance measurements and demonstrate control over the Fermi level and scattering potentials of Cd₃As₂ thin films using tuning parameters uniquely accessible via epitaxial growth. In particular we study the formation of As vacancies, how they impact the electron concentration, and develop strategies to control them.   

Topological phase transition and Dirac fermion in SrCd2(As1-xSbx)2

SrCd₂As₂ is the layered analogue of the prototypical Dirac semimetal Cd₃As₂, yet it is a trivial insulator. Recent theoretical work suggested that the negative pressure induces a band inversion in SrCd₂As₂ and stabilizes the topological Dirac semimetal phase. In this work, we successfully grew SrCd₂(As_1-xSb_x)₂ single crystals, where Sb substitution generates a negative chemical pressure. As the Sb doping level increasing, the resistivity changes from insulating to metallic behavior. For high Sb concentration, we observed pronounced single-frequency Shubnikov-de Haas oscillations. These oscillations correspond to a small hole pocket with an extremely light effective mass 0.05 m_*, consistent with the existence of Dirac dispersion. Therefore, we argued that SrCd₂(As_1-xSb_x)₂ exhibits a topological phase transition from a trivial insulator SrCd₂As₂ to a topological Dirac semimetal, and the material offers a platform to understand the evolution of non-trivial electronic band topology.   

Realization of elemental topological Dirac semimetal α-Sn with high quantum mobility

α-Sn, the only elemental material that shows multiple topological phases, is promising for topological physics and devices [1]. Here, we have successfully grown α-Sn thin films with the highest quality thus far on InSb(001) substrates by molecular beam epitaxy. Transmission electron microscopy lattice images confirm a diamond-type crystal structure of a-Sn and a perfectly flat interface with an InSb buffer layer. The conductance exhibits strong Shubnikov-de Haas (SdH) oscillations, which are observed at a magnetic field as small as 0.3 T at 2 K and persist up to 20 K, manifesting the high sample quality. From the SdH oscillations, we estimate unprecedentedly high quantum mobilities of both the topological surface states (TSS, 30000 cm²/Vs), which is ten times higher than the previously reported values [2], and the bulk heavy-hole (HH) state (1800 cm²/Vs). Both the TSS and HH bands have nontrivial phase shift, indicating that our α-Sn samples are in a topological Dirac semimetal (TDS) phase. Furthermore, we demonstrate a crossover from TDS to a two-dimensional topological insulator and a subsequent phase transition to a trivial insulator when varying the thickness of α-Sn [3].

[1] D. Zhang, et al., Phys. Rev. B 97, 195139 (2018).[2] Q. Barbedienne et al., Phys. Rev. B 98, 195445 (2018). [3] L. D. Anh et al., Adv. Mater, 2104645 (2021).   

Experimental probe of Berry phase in Dirac nodal line systems

In topological semimetals, a non-zero Berry phase originates from topologically non-trivial band crossings, such as Dirac nodes. A non-trivial Berry phase in the fundamental oscillations of the quantum oscillations (QOs) has been treated as a smoking gun for Dirac or Weyl nodal lines. A recent study suggested that this criterion did not uniquely identify non-trivial Berry curvature, and rather analysis of higher order harmonics in magnetization (de Haas can Alphen [dHvA]) was needed [1]. This motivates the search for high quality topological semimetals where higher harmonic QOs are observed, which, in turn, would allow the unambiguous determination of non-trivial Berry curvatures. We take this approach in a series of isostructural AlB2-type compounds, where strong dHvA oscillations with higher harmonics (up to 4^th order) are observed. We utilize the LK formula [1] including the spin damping term and higher harmonic contributions to fit the temperature-dependent QOs and determine the Berry phase. Together with the angle-dependent quantum oscillations, density functional theory (DFT) calculations and angle resolved photoemission spectroscopy (ARPES), we discuss on the fermiology and band topology of different Fermi pockets in these materials.

[1] Phys. Rev. X 8, 011027 (2018)   

Non-trivial Berry Phase and strong Zeeman splitting in Topological Dirac semimetal BaAl4 probed by thermoelectric quantum oscillations

Three-dimensional topological Dirac semimetals hosting Dirac/Weyl fermions are a new kind of Dirac materials in which conduction and valence bands cross each other. Quantum oscillation measurement is an important technique to extract the relativistic natures of Dirac fermions in topological quantum materials. Topological materials exhibit nontrivial Berry phase which also can be determined by the analysis of quantum oscillations. Here we report systematic study of thermoelectric power quantum oscillation of recently discovered topological Dirac nodal-line semimetal BaAl₄ single-crystal. We have shown that the thermoelectric power is a sensitive probe to study multiple quantum oscillations where two of these oscillations are shown to originate from three-dimensional Dirac band. The detected Berry phase develops an evidence of angular dependence and non-trivial state under high magnetic fields. We also have probed the signatures of Zeeman splitting, from which we have extracted the Landé g factor.