90 research outputs found
Strong Electron-Phonon Interaction and Colossal Magnetoresistance in EuTiO
At low temperatures, EuTiO system has very large resistivities and
exhibits colossal magnetoresistance. Based on a first principle calculation and
the dynamical mean-field theory for small polaron we have calculated the
transport properties of EuTiO. It is found that due to electron-phonon
interaction the conduction band may form a tiny subband which is close to the
Fermi level. The tiny subband is responsible for the large resistivity.
Besides, EuTiO is a weak antiferromagnetic material and its magnetization
would slightly shift the subband via exchange interaction between conduction
electrons and magnetic atoms. Since the subband is close to the Fermi level, a
slight shift of its position gives colossal magnetoresistance.Comment: 6 pages, 5 figure
Grassmann Time-Evolving Matrix Product Operators for Quantum Impurity Models
The time-evolving matrix product operators (TEMPO) method, which makes full
use of the Feynman-Vernon influence functional, is the state-of-the-art tensor
network method for bosonic impurity problems. However, for fermionic impurity
problems the Grassmann path integral prohibits application of this method. We
develop Grassmann time-evolving matrix product operators, a full fermionic
analog of TEMPO, that can directly manipulates Grassmann path integrals with
similar numerical cost as the bosonic counterpart. We further propose a zipup
algorithm to compute expectation values on the fly without explicitly building
a single large augmented density tensor, which boosts the efficiency of our
method on top of the vanilla TEMPO. We demonstrate our method on the
non-equilibrium dynamics of the single impurity Anderson models, and find a
favorable performance against existing tensor network influence functional
methods. Our method could significantly change the application landscape of
tensor network based impurity solvers.Comment: 4 pages, 3 figure
Large adiabatic temperature and magnetic entropy changes in EuTiO3
We have investigated the magnetocaloric effect in single and polycrystalline
samples of quantum paraelectric EuTiO3 by magnetization and heat capacity
measurements. Single crystalline EuTiO3 shows antiferromagnetic ordering due to
Eu2+ magnetic moments below TN = 5.6 K. This compound shows a giant
magnetocaloric effect around its Neel temperature. The isothermal magnetic
entropy change is 49 Jkg-1K-1, the adiabatic temperature change is 21 K and the
refrigeration capacity is 500 JKg-1 for a field change of 7 T at TN. The single
crystal and polycrystalline samples show similar values of the magnetic entropy
change and adiabatic temperature changes. The large magnetocaloric effect is
due to suppression of the spin entropy associated with localized 4f moment of
Eu2+ ions. The giant magnetocaloric effect together with negligible hysteresis,
suggest that EuTiO3 could be a potential material for magnetic refrigeration
below 20 K.Comment: 12 pages, 4 figure
Crafting Training Degradation Distribution for the Accuracy-Generalization Trade-off in Real-World Super-Resolution
Super-resolution (SR) techniques designed for real-world applications
commonly encounter two primary challenges: generalization performance and
restoration accuracy. We demonstrate that when methods are trained using
complex, large-range degradations to enhance generalization, a decline in
accuracy is inevitable. However, since the degradation in a certain real-world
applications typically exhibits a limited variation range, it becomes feasible
to strike a trade-off between generalization performance and testing accuracy
within this scope. In this work, we introduce a novel approach to craft
training degradation distributions using a small set of reference images. Our
strategy is founded upon the binned representation of the degradation space and
the Fr\'echet distance between degradation distributions. Our results indicate
that the proposed technique significantly improves the performance of test
images while preserving generalization capabilities in real-world applications.Comment: This paper has been accepted to ICML 202
Scaling Up, Scaling Deep: Blockwise Graph Contrastive Learning
Oversmoothing is a common phenomenon in graph neural networks (GNNs), in
which an increase in the network depth leads to a deterioration in their
performance. Graph contrastive learning (GCL) is emerging as a promising way of
leveraging vast unlabeled graph data. As a marriage between GNNs and
contrastive learning, it remains unclear whether GCL inherits the same
oversmoothing defect from GNNs. This work undertakes a fundamental analysis of
GCL from the perspective of oversmoothing on the first hand. We demonstrate
empirically that increasing network depth in GCL also leads to oversmoothing in
their deep representations, and surprisingly, the shallow ones. We refer to
this phenomenon in GCL as long-range starvation', wherein lower layers in deep
networks suffer from degradation due to the lack of sufficient guidance from
supervision (e.g., loss computing). Based on our findings, we present BlockGCL,
a remarkably simple yet effective blockwise training framework to prevent GCL
from notorious oversmoothing. Without bells and whistles, BlockGCL consistently
improves robustness and stability for well-established GCL methods with
increasing numbers of layers on real-world graph benchmarks. We believe our
work will provide insights for future improvements of scalable and deep GCL
frameworks.Comment: Preprint; Code is available at
https://github.com/EdisonLeeeee/BlockGC
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