99 research outputs found
Dirichlet-based Gaussian Processes for Large-scale Calibrated Classification
This paper studies the problem of deriving fast and accurate classification algorithms with uncertainty quantification. Gaussian process classification provides a principled approach, but the corresponding computational burden is hardly sustainable in large-scale problems and devising efficient alternatives is a challenge. In this work, we investigate if and how Gaussian process regression directly applied to classification labels can be used to tackle this question. While in this case training is remarkably faster, predictions need to be calibrated for classification and uncertainty estimation. To this aim, we propose a novel regression approach where the labels are obtained through the interpretation of classification labels as the coefficients of a degenerate Dirichlet distribution. Extensive experimental results show that the proposed approach provides essentially the same accuracy and uncertainty quantification as Gaussian process classification while requiring only a fraction of computational resources
Content Replication in Mobile Networks
Performance and reliability of content access in mobile networks is conditioned by the number and location of content replicas deployed at the network nodes. In this work, we design a practical, distributed solution to content replication that is suitable for dynamic environments and achieves load balancing. Simulation results show that our mechanism, which uses local measurements only, approximates well an optimal solution while being robust against network and demand dynamics. Also, our scheme outperforms alternative approaches in terms of both content access delay and access congestio
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
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
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
Stable Weyl points, trivial surface states and particle-hole compensation in WP2
A possible connection between extremely large magneto-resistance and the
presence of Weyl points has garnered much attention in the study of topological
semimetals. Exploration of these concepts in transition metal phosphide WP2 has
been complicated by conflicting experimental reports. Here we combine
angle-resolved photoemission spectroscopy (ARPES) and density functional theory
(DFT) calculations to disentangle surface and bulk contributions to the ARPES
intensity, the superposition of which has plagued the determination of the
electronic structure in WP2. Our results show that while the hole- and
electron-like Fermi surface sheets originating from surface states have
different areas, the bulk-band structure of WP2 is electron-hole-compensated in
agreement with DFT. Furthermore, the detailed band structure is compatible with
the presence of at least 4 temperature-independent Weyl points, confirming the
topological nature of WP2 and its stability against lattice distortions.Comment: 6 pages, 4 figure
Establishing non-thermal regimes in pump-probe electron-relaxation dynamics
Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the
electronic structure of solids under optical excitation, and is a powerful
technique for studying the coupling between electrons and collective modes. One
approach to infer electron-boson coupling is through the relaxation dynamics of
optically-excited electrons, and the characteristic timescales of energy
redistribution. A common description of electron relaxation dynamics is through
the effective electronic temperature. Such a description requires that
thermodynamic quantities are well-defined, an assumption that is generally
violated at early delays. Additionally, precise estimation of the non-thermal
window -- within which effective temperature models may not be applied -- is
challenging. We perform TR-ARPES on graphite and show that Boltzmann rate
equations can be used to calculate the time-dependent electronic occupation
function, and reproduce experimental features given by non-thermal electron
occupation. Using this model, we define a quantitative measure of non-thermal
electron occupation and use it to define distinct phases of electron relaxation
in the fluence-delay phase space. More generally, this approach can be used to
inform the non-thermal-to-thermal crossover in pump-probe experiments.Comment: 18 pages, 10 figure
Direct determination of mode-projected electron-phonon coupling in the time-domain
Ultrafast spectroscopies have become an important tool for elucidating the
microscopic description and dynamical properties of quantum materials. In
particular, by tracking the dynamics of non-thermal electrons, a material's
dominant scattering processes -- and thus the many-body interactions between
electrons and collective excitations -- can be revealed. Here we present a new
method for extracting the electron-phonon coupling strength in the time domain,
by means of time and angle-resolved photoemission spectroscopy (TR-ARPES). This
method is demonstrated in graphite, where we investigate the dynamics of
photo-injected electrons at the K point, detecting quantized energy-loss
processes that correspond to the emission of strongly-coupled optical phonons.
We show that the observed characteristic timescale for spectral-weight-transfer
mediated by phonon-scattering processes allows for the direct quantitative
extraction of electron-phonon matrix elements, for specific modes, and with
unprecedented sensitivity.Comment: 19 pages, 4 figure
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