1,087 research outputs found
Tenfold way and many-body zero modes in the Sachdev-Ye-Kitaev model
The Sachdev-Ye-Kitaev (SYK) model, in its simplest form, describes k Majorana fermions with random all-to-all four-body interactions. We consider the SYK model in the framework of a many-body Altland-Zirnbauer classification that sees the system as belonging to one of eight (real) symmetry classes depending on the value of k mod 8. We show that, depending on the symmetry class, the system may support exact many-body zero modes with the symmetries also dictating whether these may have a nonzero contribution to Majorana fermions, i.e., single-particle weight. These zero modes appear in all but two of the symmetry classes. When present, they leave clear signatures in physical observables that go beyond the threefold (Wigner-Dyson) possibilities for level spacing statistics studied earlier. Signatures we discover include a zero-energy peak or hole in the single-particle spectral function, depending on whether symmetries allow or forbid zero modes to have single-particle weight. The zero modes are also shown to influence the many-body dynamics, where signatures include a nonzero long-time limit for the out-of-time-order correlation function. Furthermore, we show that the extension of the four-body SYK model by quadratic terms can be interpreted as realizing the remaining two complex symmetry classes; we thus demonstrate how the entire tenfold Altland-Zirnbauer classification may emerge in the SYK model
Symmetry classes, many-body zero modes, and supersymmetry in the complex Sachdev-Ye-Kitaev model
The complex Sachdev-Ye-Kitaev (cSYK) model is a charge-conserving model of
randomly interacting fermions. The interaction term can be chosen such that the
model exhibits chiral symmetry. Then, depending on the charge sector and the
number of interacting fermions, level spacing statistics suggests a fourfold
categorization of the model into the three Wigner-Dyson symmetry classes. In
this work, inspired by previous findings for the Majorana Sachdev-Ye-Kitaev
model, we embed the symmetry classes of the cSYK model in the Altland-Zirnbauer
framework and identify consequences of chiral symmetry originating from
correlations across different charge sectors. In particular, we show that for
an odd number of fermions, the model hosts exact many-body zero modes that can
be combined into a generalized fermion that does not affect the system's
energy. This fermion directly leads to quantum-mechanical supersymmetry that,
unlike explicitly supersymmetric cSYK constructions, does not require
fine-tuned couplings, but only chiral symmetry. Signatures of the generalized
fermion, and thus supersymmetry, include the long-time plateau in
time-dependent correlation functions of fermion-parity-odd observables: The
plateau may take nonzero value only for certain combinations of the fermion
structure of the observable and the system's symmetry class. We illustrate our
findings through exact diagonalization simulations of the system's dynamics.ERC Starting Grant No. 678795 TopInS
Тренинговые услуги в украинском туризме: состояние, тенденции, проблемы
Целью данной статьи является анализ места и роли тренингов в системе туризма Украины на фоне общемировых тенденций
Functional changes induced by extrusion during cocoa alkalization
[EN] Polyphenols, a group of secondary metabolites, have well-known relevant effects on human health. During traditional alkalization, this content dramatically lowers. We aimed to evaluate an alternative alkalization method based on extrusion on cocoa functional characteristics. The results showed that the antioxidant capacity and total phenolic values increased as alkali concentration and temperature did, and these values doubled under less extreme conditions. Comparing the functional properties between extruded and traditionally produced powders revealed that catechin, epicatechin and dimers B1 and B2 contents were 43%, 33%, 54% and 34% lower in the extruded samples, respectively. However, this reduction was partially balanced by increased clovamide content up to 50%. Thus the total phenol content and antioxidant capacity of the extruded samples were statistically above those of the commercial one. Hence extrusion alkalization should be considered a new processing alternative to avoid markedly reducing functional properties.This work was funded by the Spanish Government and European Regional Development Fund (Project RTC-2016-5241-2).Valverde-Garcia, D.; Behrends, B.; Pérez-Esteve, É.; Kuhnert, N.; Barat Baviera, JM. (2020). Functional changes induced by extrusion during cocoa alkalization. Food Research International. 136:1-10. https://doi.org/10.1016/j.foodres.2020.10946911013
Provider reported implementation of nutrition-related practices in childcare centers and family childcare homes in rural and urban Nebraska
Approximately 15 million children under age 6 are in childcare settings, offering childcare providers an opportunity to influence children’s dietary intake. Childcare settings vary in organizational structure – childcare centers (CCCs) vs. family childcare homes (FCCHs) – and in geographical location – urban vs. rural. Research on the nutrition-related best practices across these childcare settings is scarce. The objective of this study is to compare nutrition-related best practices of CCCs and FCCHs that participate in the Child and Adult Care Food Program (CACFP) in rural and urban Nebraska. Nebraska providers (urban n = 591; rural n = 579) reported implementation level, implementation difficulty and barriers to implementing evidence-informed food served and mealtime practices. Chi-square tests comparing CCCs and FCCHs in urban Nebraska and CCCs and FCCHs in rural Nebraska showed sub-optimal implementation for some practices across all groups, including limiting fried meats and high sugar/ high fat foods, using healthier foods or non-food treats for celebrations and serving meals family style. Significant differences (p \u3c .05) between CCCs and FCCHs also emerged, especially with regard to perceived barriers to implementing best practices. For example, CCCs reported not having enough money to cover the cost of meals for providers, lack of control over foods served and storage problems, whereas FCCHs reported lack of time to prepare healthier foods and sit with children during mealtimes. Findings suggest that policy and public health interventions may need to be targeted to address the unique challenges of implementing evidence-informed practices within different organizational structures and geographic locations
Tenfold way and many-body zero modes in the Sachdev-Ye-Kitaev model
The Sachdev-Ye-Kitaev (SYK) model, in its simplest form, describes
Majorana fermions with random all-to-all four-body interactions. We consider
the SYK model in the framework of a many-body Altland-Zirnbauer classification
that sees the system as belonging to one of eight (real) symmetry classes
depending on the value of . We show that, depending on the symmetry
class, the system may support exact many-body zero modes with the symmetries
also dictating whether these may have a nonzero contribution to Majorana
fermions, i.e., single-particle weight. These zero modes appear in all but two
of the symmetry classes. When present, they leave clear signatures in physical
observables that go beyond the threefold (Wigner-Dyson) possibilities for level
spacing statistics studied earlier. Signatures we discover include a
zero-energy peak or hole in the single-particle spectral function, depending on
whether symmetries allow or forbid zero modes to have single-particle weight.
The zero modes are also shown to influence the many-body dynamics, where
signatures include a nonzero long-time limit for the out-of-time-order
correlation function. Furthermore, we show that the extension of the four-body
SYK model by quadratic terms can be interpreted as realizing the remaining two
complex symmetry classes; we thus demonstrate how the entire tenfold
Altland-Zirnbauer classification may emerge in the SYK model
Probing the wave function and dynamics of the quintet multiexciton state with coherent control in a singlet fission material
High-spin states play a key role in chemical reactions found in nature. In artificial molecular systems, singlet fission produces a correlated triplet-pair state, a spin-bearing excited state that can be harnessed for more efficient solar-energy conversion and photocatalysis. In particular, triplet-pair states with overall quintet character (total spin S=2) have been discovered, but many of the fundamental properties of these biexciton states remain unexplored. The net spin of these pair states makes spin-sensitive probes attractive for their characterization. Combined with their surprisingly long spin coherence (of order microseconds), this opens up techniques relying on coherent spin control. Here we apply coherent manipulation of triplet-pair states to (i) isolate their spectral signatures from coexisting free triplets and (ii) selectively couple quintet and triplet states to specific nuclear spins. Using this approach, we separate quintet and triplet transitions and extract the relaxation dynamics and hyperfine couplings of the fission-borne spin states. Our results highlight the distinct properties of correlated and free triplet excitons and demonstrate optically induced nuclear spin polarization by singlet fission
Rare processes and coherent phenomena in crystals
We study coherent enhancement of Coulomb excitation of high energy particles
in crystals. We develop multiple scattering theory description of coherent
excitation which consistently incorporates both the specific resonant
properties of particle-crystal interactions and the final/initial state
interaction effects typical of the diffractive scattering. Possible
applications to observation of induced radiative neutrino transitions are
discussed.Comment: 8 pages, LaTe
Model-based comparison of organ at risk protection between VMAT and robustly optimised IMPT plans
The comparison between intensity-modulated proton therapy (IMPT) and volume-modulated arc therapy (VMAT) plans, based on models of normal tissue complication probabilities (NTCP), can support the choice of radiation modality. IMPT irradiation plans for 50 patients with head and neck tumours originally treated with photon therapy have been robustly optimised against density and setup uncertainties. The dose distribution has been calculated with a Monte Carlo (MC) algorithm. The comparison of the plans was based on dose-volume parameters in organs at risk (OARs) and NTCP-calculations for xerostomia, sticky saliva, dysphagia and tube feeding using Langendijk's model-based approach. While the dose distribution in the target volumes is similar, the IMPT plans show better protection of OARs. Therefore, it is not the high dose confirmation that constitutes the advantage of protons, but it is the reduction of the mid-to-low dose levels compared to photons. This work investigates to what extent the advantages of proton radiation are beneficial for the patient's post-therapeutic quality of life (QoL). As a result, approximately one third of the patients examined benefit significantly from proton therapy with regard to possible late side effects. Clinical data is needed to confirm the model-based calculations
New Approach for Measuring at Future -Factories
It is suggested that the measurements of hadronic invariant mass ()
distributons in the inclusive decays can be
useful in extracting the CKM matrix element . We investigated
hadronic invariant mass distributions within the various theoretical models of
HQET, FAC and chiral lagrangian as well as ACCMM model. It is also emphasized
that the distribution even at the region in the inclusive
are effetive in selecting the events, experimentally viable at
the future asymmetric factories, with better theoretical understandings.Comment: 11 pages not including 1 figur
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