Realization of Type-II Dirac Semi-metal in NiTe2 using Structural, Electrical and Magnetic Investigation

Includes bibliographical references

PhD;2023;Physics

Dirac semimetal (DSM) is a topological semi-metal with a fourfold degenerate linear band crossing in the bulk, in the presence of time-reversal symmetry and inversion symme- try, exhibiting nontrivial spin-momentum locking mediated by spin-orbit coupling (SOC). NiTe2, a potential candidate of type-II DSM, has topologically nontrivial surface states below the Fermi level due to the nontrivial Z2 topological gap. DFT calculations on NiTe2 have implied weak topological superconducting properties with two Dirac points located at k = (0, 0, ±0.35)(2π/c) on both sides of the Γ point along the [001] direction. The topological nature of bulk NiTe2 was addressed through Angle-resolved Photoemission spectroscopy (ARPES), revealing the existence of Dirac nodes. Crystals of NiTe2 were grown by the Vertical gradient freeze technique and Physical vapor deposition method. X-ray diffraction, Raman spectroscopy at room temperature, and elemental analysis tech- niques are utilized to validate the synthesis. Fermi surface (FS) morphology of NiTe2 by de Haas–van Alphen (dHvA) quantum oscillations was undertaken. Quantum oscillations analysis for out-of-plane and in-plane magnetizations for crystals revealed the presence of Dirac fermions. The temperature-dependent structural characterization of NiTe2 in the form of a bulk single crystal and a nano-flake (200 nm thick) is studied. Temperature- dependent x-ray diffraction study along with Rietveld refinement analysis reveals linear thermal expansion coefficient (αT) of 5.56×10−6 K−1 and 22.5×10−6 K−1 along a / b and c lattice directions, respectively. Temperature evolution of Raman spectra shows non-linear variations in the phonon frequency and full-width half maxima (FWHM) of the out-of-plane A1g and in-plane Eg modes. Raman mode E1 2g disappears on decreas- ing the thickness from bulk to nano-flake. Quantitative analysis with anharmonic model yields dominating electron-phonon interaction over phonon-phonon interaction mediated by three and four-phonon processes. No signature of Dirac fermions has been obtained till now in electrical transport measurements in bulk NiTe2. Chiral magnetic effect (CME), weak anti-localization (WAL), and nonlinear Hall Effect have been identified in the de- vices made out of exfoliated nano-flakes via electrical transport measurement. The CME obtained due to chiral symmetry breaking is observed as negative magneto-resistance for the electric field direction parallel to the magnetic field. The WAL analysis using the Hikami-Larkin-Nagoka theory yields a single-phase coherent conducting channel, a possible indication of chiral Dirac surface states. Thickness-dependent resistivity with temperature depicts the shifting of chemical potential away from the Dirac point with an increase in thickness. The presence of ambipolar transport and compensation of charge carriers was revealed in Hall measurement. NiTe2 is a promising candidate for exploring topological superconductivity, topological Dirac fermions, and other emergent phenomena for the development of spintronics or topological devices.