876 research outputs found
Observation of Weyl nodes in robust type-II Weyl semimetal WP2
Distinct to type-I Weyl semimetals (WSMs) that host quasiparticles described
by the Weyl equation, the energy dispersion of quasiparticles in type-II WSMs
violates Lorentz invariance and the Weyl cones in the momentum space are
tilted. Since it was proposed that type-II Weyl fermions could emerge from
(W,Mo)Te2 and (W,Mo)P2 families of materials, a large numbers of experiments
have been dedicated to unveil the possible manifestation of type-II WSM, e.g.
the surface-state Fermi arcs. However, the interpretations of the experimental
results are very controversial. Here, using angle-resolved photoemission
spectroscopy supported by the first-principles calculations, we probe the
tilted Weyl cone bands in the bulk electronic structure of WP2 directly, which
are at the origin of Fermi arcs at the surfaces and transport properties
related to the chiral anomaly in type-II WSMs. Our results ascertain that due
to the spin-orbit coupling the Weyl nodes originate from the splitting of
4-fold degenerate band-crossing points with Chern numbers C = 2 induced by
the crystal symmetries of WP2, which is unique among all the discovered WSMs.
Our finding also provides a guiding line to observe the chiral anomaly which
could manifest in novel transport properties.Comment: 13 pages, 3 figure
Evolution from a nodeless gap to d(x2-y2) form in underdoped La(2-x)SrxCuO4
Using angle-resolved photoemission (ARPES), it is revealed that the
low-energy electronic excitation spectra of highly underdoped superconducting
and non-superconducting La(2-x)SrxCuO4 cuprates are gapped along the entire
underlying Fermi surface at low temperatures. We show how the gap function
evolves to a d(x2-y2) form as increasing temperature or doping, consistent with
the vast majority of ARPES studies of cuprates. Our results provide essential
information for uncovering the symmetry of the order parameter(s) in strongly
underdoped cuprates, which is a prerequisite for understanding the pairing
mechanism and how superconductivity emerges from a Mott insulator.Comment: 5 pages, 4 figure
Spin-Orbit Coupling in Iridium-Based 5d Compounds Probed by X-ray Absorption Spectroscopy
We have performed x-ray absorption spectroscopy (XAS) measurements on a
series of Ir-based 5d transition metal compounds, including Ir, IrCl3, IrO2,
Na2IrO3, Sr2IrO4, and Y2Ir2O7. By comparing the intensity of the "white-line"
features observed at the Ir L2 and L3 absorption edges, it is possible to
extract valuable information about the strength of the spin-orbit coupling in
these systems. We observe remarkably large, non-statistical branching ratios in
all Ir compounds studied, with little or no dependence on chemical composition,
crystal structure, or electronic state. This result confirms the presence of
strong spin-orbit coupling effects in novel iridates such as Sr2IrO4, Na2IrO3,
and Y2Ir2O7, and suggests that even simple Ir-based compounds such as IrO2 and
IrCl3 may warrant further study. In contrast, XAS measurements on Re-based 5d
compounds, such as Re, ReO2, ReO3, and Ba2FeReO6, reveal statistical branching
ratios and negligible spin-orbit coupling effects.Comment: 9 pages, 4 figure
Momentum-Resolved Electronic Structure of the High- Superconductor Parent Compound BaBiO
We investigate the band structure of BaBiO, an insulating parent
compound of doped high- superconductors, using \emph{in situ}
angle-resolved photoemission spectroscopy on thin films. The data compare
favorably overall with density functional theory calculations within the local
density approximation, demonstrating that electron correlations are weak. The
bands exhibit Brillouin zone folding consistent with known BiO breathing
distortions. Though the distortions are often thought to coincide with
Bi/Bi charge ordering, core level spectra show that bismuth is
monovalent. We further demonstrate that the bands closest to the Fermi level
are primarily oxygen derived, while the bismuth states mostly contribute
to dispersive bands at deeper binding energy. The results support a model of
Bi-O charge transfer in which hole pairs are localized on combinations of the O
orbitals.Comment: minor changes to text and other figures; includes link to online
Supplemental Material; accepted to Phys. Rev. Let
Exotic Kondo crossover in a wide temperature region in the topological Kondo insulator SmB6 revealed by high-resolution ARPES
Temperature dependence of the electronic structure of SmB6 is studied by
high-resolution ARPES down to 1 K. We demonstrate that there is no essential
difference for the dispersions of the surface states below and above the
resistivity saturating anomaly (~ 3.5 K). Quantitative analyses of the surface
states indicate that the quasi-particle scattering rate increases linearly as a
function of temperature and binding energy, which differs from Fermi-Liquid
behavior. Most intriguingly, we observe that the hybridization between the d
and f states builds gradually over a wide temperature region (30 K < T < 110
K). The surface states appear when the hybridization starts to develop. Our
detailed temperature-dependence results give a complete interpretation of the
exotic resistivity result of SmB6, as well as the discrepancies among
experimental results concerning the temperature regions in which the
topological surface states emerge and the Kondo gap opens, and give new
insights into the exotic Kondo crossover and its relationship with the
topological surface states in the topological Kondo insulator SmB6.Comment: 8 pages, 5 figure
Bulk electronic structure of superconducting LaRu2P2 single crystals measured by soft x-ray angle-resolved photoemission spectroscopy
We present a soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES)
study of the stoichiometric pnictide superconductor LaRu2P2. The observed
electronic structure is in good agreement with density functional theory (DFT)
calculations. However, it is significantly different from its counterpart in
high-temperature superconducting Fe-pnictides. In particular the bandwidth
renormalization present in the Fe-pnictides (~2 - 3) is negligible in LaRu2P2
even though the mass enhancement is similar in both systems. Our results
suggest that the superconductivity in LaRu2P2 has a different origin with
respect to the iron pnictides. Finally we demonstrate that the increased
probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is
essential in determining the bulk electronic structure in the experiment.Comment: 4 pages, 4 figures, 1 supplemental material. Accepted for publication
in Physical Review Letter
Trivial topological phase of CaAgP and the topological nodal-line transition in CaAg(P1-xAsx)
By performing angle-resolved photoemission spectroscopy and first-principles
calculations, we address the topological phase of CaAgP and investigate the
topological phase transition in CaAg(P1-xAsx). We reveal that in CaAgP, the
bulk band gap and surface states with a large bandwidth are topologically
trivial, in agreement with hybrid density functional theory calculations. The
calculations also indicate that application of "negative" hydrostatic pressure
can transform trivial semiconducting CaAgP into an ideal topological nodal-line
semimetal phase. The topological transition can be realized by partial
isovalent P/As substitution at x = 0.38.Comment: 20 pages, 4 figure
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