544 research outputs found
Dirac dispersion and non-trivial Berry's phase in three-dimensional semimetal RhSb3
We report observations of magnetoresistance, quantum oscillations and
angle-resolved photoemission in RhSb, a unfilled skutterudite semimetal
with low carrier density. The calculated electronic band structure of RhSb
entails a quantum number in analogy to
strong topological insulators, and inverted linear valence/conduction bands
that touch at discrete points close to the Fermi level, in agreement with
angle-resolved photoemission results. Transport experiments reveal an
unsaturated linear magnetoresistance that approaches a factor of 200 at 60 T
magnetic fields, and quantum oscillations observable up to 150~K that are
consistent with a large Fermi velocity ( ms), high
carrier mobility ( /Vs), and small three dimensional hole pockets
with nontrivial Berry phase. A very small, sample-dependent effective mass that
falls as low as bare masses scales with Fermi velocity, suggesting
RhSb is a new class of zero-gap three-dimensional Dirac semimetal.Comment: 9 pages, 4 figure
Influence of the shaping effect on hardness homogeneity by Vickers indentation analysis
In this study, indentation technique (Vickers indentation) has been unconventionally used to evaluate the homogeneity of barium zirconate ceramic samples which have been shaped through different routes. Statistical tools have been used to estimate the con-elation which can be established between heterogeneities within the samples and their shaping ways. (c) 2005 Elsevier Ltd. All rights reserved
Emergence of pseudogap from short-range spin-correlations in electron doped cuprates
Electron interactions are pivotal for defining the electronic structure of
quantum materials. In particular, the strong electron Coulomb repulsion is
considered the keystone for describing the emergence of exotic and/or ordered
phases of quantum matter as disparate as high-temperature superconductivity and
charge- or magnetic-order. However, a comprehensive understanding of
fundamental electronic properties of quantum materials is often complicated by
the appearance of an enigmatic partial suppression of low-energy electronic
states, known as the pseudogap. Here we take advantage of ultrafast
angle-resolved photoemission spectroscopy to unveil the temperature evolution
of the low-energy density of states in the electron-doped cuprate
NdCeCuO, an emblematic system where
the pseudogap intertwines with magnetic degrees of freedom. By photoexciting
the electronic system across the pseudogap onset temperature T*, we report the
direct relation between the momentum-resolved pseudogap spectral features and
the spin-correlation length with an unprecedented sensitivity. This transient
approach, corroborated by mean field model calculations, allows us to establish
the pseudogap in electron-doped cuprates as a precursor to the incipient
antiferromagnetic order even when long-range antiferromagnetic correlations are
not established, as in the case of optimal doping.Comment: 17 pages, 3 figure
Deciphering the local Interstellar spectra of primary cosmic ray species with HelMod
Local interstellar spectra (LIS) of primary cosmic ray (CR) nuclei, such as
helium, oxygen, and mostly primary carbon are derived for the rigidity range
from 10 MV to ~200 TV using the most recent experimental results combined with
the state-of-the-art models for CR propagation in the Galaxy and in the
heliosphere. Two propagation packages, GALPROP and HelMod, are combined into a
single framework that is used to reproduce direct measurements of CR species at
different modulation levels, and at both polarities of the solar magnetic
field. The developed iterative maximum-likelihood method uses GALPROP-predicted
LIS as input to HelMod, which provides the modulated spectra for specific time
periods of the selected experiments for model-data comparison. The interstellar
and heliospheric propagation parameters derived in this study are consistent
with our prior analyses using the same methodology for propagation of CR
protons, helium, antiprotons, and electrons. The resulting LIS accommodate a
variety of measurements made in the local interstellar space (Voyager 1) and
deep inside the heliosphere at low (ACE/CRIS, HEAO-3) and high energies
(PAMELA, AMS-02).Comment: 13 pages, 13 figures, 6 tables, ApJ in press. arXiv admin note: text
overlap with arXiv:1704.0633
Influence of Spin Orbit Coupling in the Iron-Based Superconductors
We report on the influence of spin-orbit coupling (SOC) in the Fe-based
superconductors (FeSCs) via application of circularly-polarized spin and
angle-resolved photoemission spectroscopy. We combine this technique in
representative members of both the Fe-pnictides and Fe-chalcogenides with ab
initio density functional theory and tight-binding calculations to establish an
ubiquitous modification of the electronic structure in these materials imbued
by SOC. The influence of SOC is found to be concentrated on the hole pockets
where the superconducting gap is generally found to be largest. This result
contests descriptions of superconductivity in these materials in terms of pure
spin-singlet eigenstates, raising questions regarding the possible pairing
mechanisms and role of SOC therein.Comment: For supplementary information, see
http://qmlab.ubc.ca/ARPES/PUBLICATIONS/articles.htm
HelMod in the works: from direct observations to the local interstellar spectrum of cosmic-ray electrons
The local interstellar spectrum (LIS) of cosmic-ray (CR) electrons for the
energy range 1 MeV to 1 TeV is derived using the most recent experimental
results combined with the state-of-the-art models for CR propagation in the
Galaxy and in the heliosphere. Two propagation packages, GALPROP and HelMod,
are combined to provide a single framework that is run to reproduce direct
measurements of CR species at different modulation levels, and at both
polarities of the solar magnetic field. An iterative maximum-likelihood method
is developed that uses GALPROP-predicted LIS as input to HelMod, which provides
the modulated spectra for specific time periods of the selected experiments for
model-data comparison. The optimized HelMod parameters are then used to adjust
GALPROP parameters to predict a refined LIS with the procedure repeated subject
to a convergence criterion. The parameter optimization uses an extensive data
set of proton spectra from 1997-2015. The proposed CR electron LIS accommodates
both the low-energy interstellar spectra measured by Voyager 1 as well as the
high-energy observations by PAMELA and AMS-02 that are made deep in the
heliosphere; it also accounts for Ulysses counting rate features measured out
of the ecliptic plane. The interstellar and heliospheric propagation parameters
derived in this study agree well with our earlier results for CR protons,
helium nuclei, and anti-protons propagation and LIS obtained in the same
framework.Comment: 11 pages, 14 figures, 4 tables; ApJ, in pres
Doping dependent charge order correlations in electron-doped cuprates
Understanding the interplay between charge order (CO) and other phenomena
(e.g. pseudogap, antiferromagnetism, and superconductivity) is one of the
central questions in the cuprate high-temperature superconductors. The
discovery that similar forms of CO exist in both hole- and electron-doped
cuprates opened a path to determine what subset of the CO phenomenology is
universal to all the cuprates. Here, we use resonant x-ray scattering to
measure the charge order correlations in electron-doped cuprates (La2-xCexCuO4
and Nd2-xCexCuO4) and their relationship to antiferromagnetism, pseudogap, and
superconductivity. Detailed measurements of Nd2-xCexCuO4 show that CO is
present in the x = 0.059 to 0.166 range, and that its doping dependent
wavevector is consistent with the separation between straight segments of the
Fermi surface. The CO onset temperature is highest between x = 0.106 and 0.166,
but decreases at lower doping levels, indicating that it is not tied to the
appearance of antiferromagnetic correlations or the pseudogap. Near optimal
doping, where the CO wavevector is also consistent with a previously observed
phonon anomaly, measurements of the CO below and above the superconducting
transition temperature, or in a magnetic field, show that the CO is insensitive
to superconductivity. Overall these findings indicate that, while verified in
the electron-doped cuprates, material-dependent details determine whether the
CO correlations acquire sufficient strength to compete for the ground state of
the cuprates.Comment: Supplementary information available upon reques
Collapse of superconductivity in cuprates via ultrafast quenching of phase coherence
The possibility of driving phase transitions in low-density condensates
through the loss of phase coherence alone has far-reaching implications for the
study of quantum phases of matter. This has inspired the development of tools
to control and explore the collective properties of condensate phases via phase
fluctuations. Electrically-gated oxide interfaces, ultracold Fermi atoms, and
cuprate superconductors, which are characterized by an intrinsically small
phase-stiffness, are paradigmatic examples where these tools are having a
dramatic impact. Here we use light pulses shorter than the internal
thermalization time to drive and probe the phase fragility of the
BiSrCaCuO cuprate superconductor, completely melting
the superconducting condensate without affecting the pairing strength. The
resulting ultrafast dynamics of phase fluctuations and charge excitations are
captured and disentangled by time-resolved photoemission spectroscopy. This
work demonstrates the dominant role of phase coherence in the
superconductor-to-normal state phase transition and offers a benchmark for
non-equilibrium spectroscopic investigations of the cuprate phase diagram.Comment: 24 pages, 9 figures, Main Text and Supplementary Informatio
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