Realization of Type-II Dirac Semi-metal in NiTe2 using Structural, Electrical and Magnetic Investigation
Abstract
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.
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- Physics (PHY) [453]