1,169 research outputs found
Two-dimensional conical dispersion in ZrTe5 evidenced by optical spectroscopy
Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal,
with a single conical band, located at the center of the Brillouin zone. The
cone's lack of protection by the lattice symmetry immediately sparked vast
discussions about the size and topological/trivial nature of a possible gap
opening. Here we report on a combined optical and transport study of ZrTe5,
which reveals an alternative view of electronic bands in this material. We
conclude that the dispersion is approximately linear only in the a-c plane,
while remaining relatively flat and parabolic in the third direction (along the
b axis). Therefore, the electronic states in ZrTe5 cannot be described using
the model of 3D Dirac massless electrons, even when staying at energies well
above the band gap 6 meV found in our experiments at low temperatures.Comment: Physical Review Letters 122, 217402 (2019). Corrected acknowledgment
Revealing the KH2PO4 soft-mode coupling mechanism with infrared spectroscopy under pressure
We measured the far-infrared reflectivity of a KH2PO4 single crystal up to
pressures of 2 GPa in the ferroelectric and paraelectric phases. We find that
the nu4 vibrational mode of the PO4 tetrahedron is strongly affected by the
applied pressure. At ambient pressure this phonon is destabilized by the
presence of the H ions and hence shows a highly damped character, beyond the
phonon propagation threshold. Applying a pressure close to 0.6 GPa makes this
phonon clearly underdamped. Its behavior closely follows the soft-mode behavior
observed in Raman spectroscopy. Our results solve a long standing open problem,
demonstrating that the nu4 mode is the excitation mediating the coupling of the
hydrogen network to the lattice modes that create the ferroelectic polarization
in KH2PO4.Comment: 5 pages, 4 figure
Optical conductivity signatures of open Dirac nodal lines
We investigate the optical conductivity and far-infrared magneto-optical
response of BaNiS, a simple square-lattice semimetal characterized by Dirac
nodal lines that disperse exclusively along the out-of-plane direction. With
the magnetic field aligned along the nodal line the in-plane Landau level
spectra show a nearly behavior, the hallmark of a conical-band
dispersion with a small spin-orbit coupling gap. The optical conductivity
exhibits an unusual temperature-independent isosbestic line, ending at a Van
Hove singularity. First-principles calculations unambiguously assign the
isosbestic line to transitions across Dirac nodal states. Our work suggests a
universal topology of the electronic structure of Dirac nodal lines
Linear behavior of the optical conductivity and incoherent charge transport in
Optical conductivity measurements on a BaCoS2 single crystal unveil an unusual linear behavior over a broad spectral range. In the paramagnetic phase above 300 K, the spectrum shows no gap, which contradicts the previously proposed scenario of a charge-transfer Mott insulator. Ab initio dynamical mean field theory calculations including a retarded Hubbard interaction explain the data in terms of an incipient opening of a Co(3d)−S(3p) charge-transfer gap concomitant to incoherent charge transport driven by electronic correlations. These results point to a non-Fermi liquid scenario with Hund's metal properties in the paramagnetic state, which arises from an incipient Mott phase destabilized by low-energy charge fluctuations across the vanishing 3d−3p charge-transfer gap
Two-dimensional conical dispersion in evidenced by optical spectroscopy
Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. The cone’s lack of protection by the lattice symmetry immediately sparked vast discussions about the size and topological or trivial nature of a possible gap opening. Here, we report on a combined optical and transport study of ZrTe5, which reveals an alternative view of electronic bands in this material. We conclude that the dispersion is approximately linear only in the a-c plane, while remaining relatively flat and parabolic in the third direction (along the b axis). Therefore, the electronic states in ZrTe5 cannot be described using the model of 3D Dirac massless electrons, even when staying at energies well above the band gap 2Δ ¼ 6 meV found in our experiments at low temperatures
Bulk charge density wave and electron-phonon coupling in superconducting copper oxychlorides
Bulk charge density waves (CDWs) are now reported in nearly all
high-temperature superconducting (HTS) cuprates, with the noticeable exception
of one particular family: the copper oxychlorides. Here, we used resonant
inelastic X-ray scattering (RIXS) to reveal a bulk CDW in these materials.
Combining RIXS with non-resonant IXS, we investigate the interplay between the
lattice excitations and the CDW, and evidence bond-stretching (BS) phonon
anomalies at the CDW wave-vector. We propose that such electron-phonon
anomalies occur in the presence of dispersive charge excitations emanating from
the CDW and interacting with the BS phonon. Our observations in a structurally
simple cuprate promises to better connect bulk and surface properties and
bridge the gap between theory and experiment
Darwin -— an experimental astronomy mission to search for extrasolar planets
As a response to ESA call for mission concepts for its Cosmic Vision 2015–2025 plan, we propose a mission called Darwin. Its primary goal is the study of terrestrial extrasolar planets and the search for life on them. In this paper, we describe different characteristics of the instrument
EuCdAs: a magnetic semiconductor
EuCdAs is now widely accepted as a topological semimetal in which a
Weyl phase is induced by an external magnetic field. We challenge this view
through firm experimental evidence using a combination of electronic transport,
optical spectroscopy and excited-state photoemission spectroscopy. We show that
the EuCdAs is in fact a semiconductor with a gap of 0.77 eV. We show
that the externally applied magnetic field has a profound impact on the
electronic band structure of this system. This is manifested by a huge decrease
of the observed band gap, as large as 125~meV at 2~T, and consequently, by a
giant redshift of the interband absorption edge. However, the semiconductor
nature of the material remains preserved. EuCdAs is therefore a
magnetic semiconductor rather than a Dirac or Weyl semimetal, as suggested by
{\em ab initio} computations carried out within the local spin-density
approximation.Comment: Accepted for publication in Physical Review Letter
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