35 research outputs found
Temperature independent band structure of WTe2 as observed from ARPES
Extremely large magnetoresistance (XMR), observed in transition metal
dichalcogendies, WTe, has attracted recently a great deal of research
interests as it shows no sign of saturation up to the magnetic field as high as
60 T, in addition to the presence of type-II Weyl fermions. Currently, there
has been a lot of discussion on the role of band structure changes on the
temperature dependent XMR in this compound. In this contribution, we study the
band structure of WTe using angle-resolved photoemission spectroscopy
(ARPES) and first-principle calculations to demonstrate that the temperature
dependent band structure has no substantial effect on the temperature dependent
XMR as our measurements do not show band structure changes on increasing the
sample temperature between 20 and 130 K. We further observe an electronlike
surface state, dispersing in such a way that it connects the top of bulk
holelike band to the bottom of bulk electronlike band. Interestingly, similar
to bulk states, the surface state is also mostly intact with the sample
temperature. Our results provide invaluable information in shaping the
mechanism of temperature dependent XMR in WTe.Comment: 7 pages, 3 figures. arXiv admin note: text overlap with
arXiv:1705.0721
Electrical, Thermal and Spectroscopic Characterization of Bulk Bi2Se3 Topological Insulator
We report electrical (angular magneto-resistance, and Hall), thermal (heat
capacity) and spectroscopic (Raman, x-ray photo electron, angle resolved photo
electron) characterization of bulk Bi2Se3 topological insulator, which is being
is grown by self flux method through solid state reaction from high temperature
(950C) melt and slow cooling (2C/hour) of constituent elements. Bi2Se3
exhibited metallic behaviour down to 5K. Magneto transport measurements
revealed linear up to 400% and 30% MR at 5K under 14 Tesla field in
perpendicular and parallel field direction respectively. We noticed that the
magneto-resistance (MR) of Bi2Se3 is very sensitive to the angle of applied
field. MR is maximum when the field is normal to the sample surface, while it
is minimum when the field is parallel. Hall coefficient (RH) is seen nearly
invariant with negative carrier sign down to 5K albeit having near periodic
oscillations above 100K. Heat capacity (Cp) versus temperature plot is seen
without any phase transitions down to 5K and is well fitted (Cp = gammaT +
betaT3) at low temperature with calculated Debye temperature (ThetaD) value of
105.5K. Clear Raman peaks are seen at 72, 131 and 177 cm-1 corresponding to
A1g1, Eg2 and A1g2 respectively. Though, two distinct asymmetric characteristic
peak shapes are seen for Bi 4f7/2 and Bi 4f5/2, the Se 3d region is found to be
broad displaying the overlapping of spin - orbit components of the same.
Angle-resolved photoemission spectroscopy (ARPES) data of Bi2Se3 revealed
distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point
(DP) and bulk valence bands (BVB) and 3D bulk conduction signatures are clearly
seen. Summarily, host of physical properties for as grown Bi2Se3 crystal are
reported here.Comment: 6 Pages Text + Figs; Comments Suggestions welcom
Emergent magnetic state in (111)-oriented quasi-two-dimensional spinel oxides
We report on the emergent magnetic state of (111)-oriented CoCr2O4 ultrathin
films sandwiched by Al2O3 in the quantum confined geometry. At the
two-dimensional crossover, polarized neutron reflectometry reveals an anomalous
enhancement of the total magnetization compared to the bulk value. Synchrotron
x-ray magnetic circular dichroism (XMCD) demonstrates the appearance of
long-range ferromagnetic ordering of spins on both Co and Cr sublattices.
Brillouin function analyses further corroborates that the observed phenomena
are due to the strongly altered magnetic frustration, manifested by the onset
of a Yafet-Kittel type ordering as the new ground state in the ultrathin limit,
which is unattainable in the bulk
Anomalous orbital structure in two-dimensional titanium dichalcogenides
Generally, lattice distortions play a key role in determining the ground
states of materials. Although it is well known that trigonal distortions are
generic to most two-dimensional transition metal dichalcogenides, the impact of
this structural distortion on the electronic structure has not been understood
conclusively. Here, by using a combination of polarization dependent X-ray
absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and
atomic multiplet cluster calculations, we have investigated the electronic
structure of titanium dichalcogenides TiX2 (X=S, Se, Te), where the magnitude
of the trigonal distortion increase monotonically from S to Se and Te. Our
results reveal the presence of an anomalous and large crystal filed splitting.
This unusual kind of crystal field splitting is likely responsible for the
unconventional electronic structure of TiX2 compounds. Our results also
indicate the drawback of the distorted crystal field picture in explaining the
observed electronic ground state of these materials and emphasize the key
importance of metal-ligand hybridization and electronic correlation in defining
the electronic structures near Fermi energy
Chemical exfoliation of MoS2 leads to semiconducting 1T' phase and not the metallic 1T phase
A trigonal phase existing only as small patches on chemically exfoliated few
layer, thermodynamically stable 1H phase of MoS2 is believed to influence
critically properties of MoS2 based devices. This phase has been most often
attributed to the metallic 1T phase. We investigate the electronic structure of
chemically exfoliated MoS2 few layered systems using spatially resolved (lesser
than 120 nm resolution) photoemission spectroscopy and Raman spectroscopy in
conjunction with state-of-the-art electronic structure calculations. On the
basis of these results, we establish that the ground state of this phase is a
small gap (~90 meV) semiconductor in contrast to most claims in the literature;
we also identify the specific trigonal (1T') structure it has among many
suggested ones
High photon energy spectroscopy of NiO: experiment and theory
We have revisited the valence band electronic structure of NiO by means of
hard x-ray photoemission spectroscopy (HAXPES) together with theoretical
calculations using both the GW method and the local density approximation +
dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity
problem in DMFT is solved through the exact diagonalization (ED) method. We
show that the LDA+DMFT method alone cannot explain all the observed structures
in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and
p derived states to properly position their binding energies. Our results
establish that a combination of the GW and DMFT methods is necessary for
correctly describing the electronic structure of NiO in a proper ab-initio
framework. We also demonstrate that the inclusion of photoionization cross
section is crucial to interpret the HAXPES spectra of NiO.We argue that our
conclusions are general and that the here suggested approach is appropriate for
any complex transition metal oxide.Comment: 16 pages, 5 figure
Electrical, thermal and spectroscopic characterization of bulk Bi2Se3 topological insulator
We report the electrical (angular magnetoresistance and Hall), thermal (heat capacity) and spectroscopic (Raman, X-ray photoelectron, angle-resolved photoelectron)
characterization of a bulk Bi2Se3 topological
insulator, which was grown by self-flux method through solid-state reaction from high-temperature (950 ◦C) melt and slow cooling (2 ◦C/h) of constituent elements. Bi2Se3
exhibited metallic behaviour down to 5 K. Magnetotransport measurements revealed linear up to 400 and 30% magneto-resistance (MR) at 5 K under a 14-T field in perpendicular
and parallel field directions, respectively. We
noticed that the MR of Bi2Se3 is very sensitive to the angle of the applied field. The MR is maximum when the field is normal to the sample surface, while it is minimum when
the field is parallel. The Hall coefficient (RH) is seen nearly invariant with a negative carrier sign down to 5 K albeit having near-periodic oscillations above 100 K. The heat capacity (Cp) versus temperature plot is seen without any phase transitions down to 5 K and is well fitted (Cp = γT + βT 3) at low temperature with a calculated Debye temperature (θD) value of 105.5 K. Clear Raman peaks are
seen at 72, 131 and 177 cm−1 corresponding to A1 1g, E2g and A2 1g, respectively. Though two distinct asymmetric characteristic peak shapes are seen for Bi 4f7/2 and Bi 4f5/2, the Se 3d region is found to be broad, displaying the overlapping of spin-orbit components of the same. Angle-resolved photoemission
spectroscopy (ARPES) data of Bi2Se3 revealed
distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point (DP) and bulk valence bands (BVB), and 3D bulk conduction signatures are clearly seen. Summarily, a host of physical properties for the as-grownBi2Se3 crystal are reported here