28 research outputs found
Correlations and confinement of excitations in an asymmetric Hubbard ladder
Correlation functions and low-energy excitations are investigated in the
asymmetric two-leg ladder consisting of a Hubbard chain and a noninteracting
tight-binding (Fermi) chain using the density matrix renormalization group
method. The behavior of charge, spin and pairing correlations is discussed for
the four phases found at half filling, namely, Luttinger liquid, Kondo-Mott
insulator, spin-gapped Mott insulator and correlated band insulator.
Quasi-long-range antiferromagnetic spin correlations are found in the Hubbard
leg in the Luttinger liquid phase only. Pair-density-wave correlations are
studied to understand the structure of bound pairs found in the Fermi leg of
the spin-gapped Mott phase at half filling and at light doping but we find no
enhanced pairing correlations. Low-energy excitations cause variations of spin
and charge densities on both legs that demonstrate the confinement of the
lowest charge excitations on the Fermi leg while the lowest spin excitations
are localized on the Hubbard leg in the three insulating phases. The velocities
of charge, spin, and single-particle excitations are investigated to clarify
the confinement of elementary excitations in the Luttinger liquid phase. The
observed spatial separation of elementary spin and charge excitations could
facilitate the coexistence of different (quasi-)long-range orders in
higher-dimensional extensions of the asymmetric Hubbard ladder
Ground-state and spectral properties of an asymmetric Hubbard ladder
We investigate a ladder system with two inequivalent legs, namely a Hubbard
chain and a one-dimensional electron gas. Analytical approximations, the
density matrix renormalization group method, and continuous-time quantum Monte
Carlo simulations are used to determine ground-state properties, gaps, and
spectral functions of this system at half-filling. Evidence for the existence
of four different phases as a function of the Hubbard interaction and the rung
hopping is presented. First, a Luttinger liquid exists at very weak interchain
hopping. Second, a Kondo-Mott insulator with spin and charge gaps induced by an
effective rung exchange coupling is found at moderate interchain hopping or
strong Hubbard interaction. Third, a spin-gapped paramagnetic Mott insulator
with incommensurate excitations and pairing of doped charges is observed at
intermediate values of the rung hopping and the interaction. Fourth, the usual
correlated band insulator is recovered for large rung hopping. We show that the
wavenumbers of the lowest single-particle excitations are different in each
insulating phase. In particular, the three gapped phases exhibit markedly
different spectral functions. We discuss the relevance of asymmetric two-leg
ladder systems as models for atomic wires deposited on a substrate.Comment: published versio
Correlated atomic wires on substrates. I. Mapping to quasi-one-dimensional models
We present a theoretical study of correlated atomic wires deposited on
substrates in two parts. In this first part, we propose lattice models for a
one-dimensional quantum wire on a three-dimensional substrate and map them onto
effective two-dimensional lattices using the Lanczos algorithm. We then discuss
the approximation of these two-dimensional lattices by narrow ladder models
that can be investigated with well-established methods for one-dimensional
correlated quantum systems, such as the density-matrix renormalization group or
bosonization. The validity of this approach is studied first for noninteracting
electrons and then for a correlated wire with a Hubbard electron-electron
repulsion using quantum Monte Carlo simulations. While narrow ladders cannot be
used to represent wires on metallic substrates, they capture the physics of
wires on insulating substrates if at least three legs are used. In the second
part [arXiv:1704.07359], we use this approach for a detailed numerical
investigation of a wire with a Hubbard-type interaction on an insulating
substrate
Correlated atomic wires on substrates. II. Application to Hubbard wires
In the first part of our theoretical study of correlated atomic wires on
substrates, we introduced lattice models for a one-dimensional quantum wire on
a three-dimensional substrate and their approximation by quasi-one-dimensional
effective ladder models [arXiv:1704.07350]. In this second part, we apply this
approach to the case of a correlated wire with a Hubbard-type electron-electron
repulsion deposited on an insulating substrate. The ground-state and spectral
properties are investigated numerically using the density-matrix
renormalization group method and quantum Monte Carlo simulations. As a function
of the model parameters, we observe various phases with quasi-one-dimensional
low-energy excitations localized in the wire, namely paramagnetic Mott
insulators, Luttinger liquids, and spin- Heisenberg chains. The validity
of the effective ladder models is assessed by studying the convergence with the
number of legs and comparing to the full three-dimensional model. We find that
narrow ladder models accurately reproduce the quasi-one-dimensional excitations
of the full three-dimensional model but predict only qualitatively whether
excitations are localized around the wire or delocalized in the
three-dimensional substrate
Pair binding and enhancement of pairing correlations in asymmetric Hubbard ladders
Asymmetric two-leg Hubbard ladders with different on-site interactions
and hoppings on each leg are investigated using the density matrix
renormalization group method and exact diagonalizations. The pairing found in
symmetric ladders is robust against the introduction of the leg asymmetry. When
studying pairing, one-band Hubbard ladder models are better described as
one-dimensional correlated two-band models than as sublattices of higher
dimensional systems. The asymmetric Hubbard ladder provides us with a simple
model for studying pairing in the crossover regime between charge transfer and
Mott insulators
Impact of opioid-free analgesia on pain severity and patient satisfaction after discharge from surgery: multispecialty, prospective cohort study in 25 countries
Background: Balancing opioid stewardship and the need for adequate analgesia following discharge after surgery is challenging. This study aimed to compare the outcomes for patients discharged with opioid versus opioid-free analgesia after common surgical procedures.Methods: This international, multicentre, prospective cohort study collected data from patients undergoing common acute and elective general surgical, urological, gynaecological, and orthopaedic procedures. The primary outcomes were patient-reported time in severe pain measured on a numerical analogue scale from 0 to 100% and patient-reported satisfaction with pain relief during the first week following discharge. Data were collected by in-hospital chart review and patient telephone interview 1 week after discharge.Results: The study recruited 4273 patients from 144 centres in 25 countries; 1311 patients (30.7%) were prescribed opioid analgesia at discharge. Patients reported being in severe pain for 10 (i.q.r. 1-30)% of the first week after discharge and rated satisfaction with analgesia as 90 (i.q.r. 80-100) of 100. After adjustment for confounders, opioid analgesia on discharge was independently associated with increased pain severity (risk ratio 1.52, 95% c.i. 1.31 to 1.76; P < 0.001) and re-presentation to healthcare providers owing to side-effects of medication (OR 2.38, 95% c.i. 1.36 to 4.17; P = 0.004), but not with satisfaction with analgesia (beta coefficient 0.92, 95% c.i. -1.52 to 3.36; P = 0.468) compared with opioid-free analgesia. Although opioid prescribing varied greatly between high-income and low- and middle-income countries, patient-reported outcomes did not.Conclusion: Opioid analgesia prescription on surgical discharge is associated with a higher risk of re-presentation owing to side-effects of medication and increased patient-reported pain, but not with changes in patient-reported satisfaction. Opioid-free discharge analgesia should be adopted routinely
Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries
Abstract
Background
Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres.
Methods
This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries.
Results
In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia.
Conclusion
This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries
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Global burden of 288 causes of death and life expectancy decomposition in 204 countries and territories and 811 subnational locations, 1990–2021: a systematic analysis for the Global Burden of Disease Study 2021
BACKGROUND Regular, detailed reporting on population health by underlying cause of death is fundamental for public health decision making. Cause-specific estimates of mortality and the subsequent effects on life expectancy worldwide are valuable metrics to gauge progress in reducing mortality rates. These estimates are particularly important following large-scale mortality spikes, such as the COVID-19 pandemic. When systematically analysed, mortality rates and life expectancy allow comparisons of the consequences of causes of death globally and over time, providing a nuanced understanding of the effect of these causes on global populations. METHODS The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 cause-of-death analysis estimated mortality and years of life lost (YLLs) from 288 causes of death by age-sex-location-year in 204 countries and territories and 811 subnational locations for each year from 1990 until 2021. The analysis used 56 604 data sources, including data from vital registration and verbal autopsy as well as surveys, censuses, surveillance systems, and cancer registries, among others. As with previous GBD rounds, cause-specific death rates for most causes were estimated using the Cause of Death Ensemble model-a modelling tool developed for GBD to assess the out-of-sample predictive validity of different statistical models and covariate permutations and combine those results to produce cause-specific mortality estimates-with alternative strategies adapted to model causes with insufficient data, substantial changes in reporting over the study period, or unusual epidemiology. YLLs were computed as the product of the number of deaths for each cause-age-sex-location-year and the standard life expectancy at each age. As part of the modelling process, uncertainty intervals (UIs) were generated using the 2·5th and 97·5th percentiles from a 1000-draw distribution for each metric. We decomposed life expectancy by cause of death, location, and year to show cause-specific effects on life expectancy from 1990 to 2021. We also used the coefficient of variation and the fraction of population affected by 90% of deaths to highlight concentrations of mortality. Findings are reported in counts and age-standardised rates. Methodological improvements for cause-of-death estimates in GBD 2021 include the expansion of under-5-years age group to include four new age groups, enhanced methods to account for stochastic variation of sparse data, and the inclusion of COVID-19 and other pandemic-related mortality-which includes excess mortality associated with the pandemic, excluding COVID-19, lower respiratory infections, measles, malaria, and pertussis. For this analysis, 199 new country-years of vital registration cause-of-death data, 5 country-years of surveillance data, 21 country-years of verbal autopsy data, and 94 country-years of other data types were added to those used in previous GBD rounds. FINDINGS The leading causes of age-standardised deaths globally were the same in 2019 as they were in 1990; in descending order, these were, ischaemic heart disease, stroke, chronic obstructive pulmonary disease, and lower respiratory infections. In 2021, however, COVID-19 replaced stroke as the second-leading age-standardised cause of death, with 94·0 deaths (95% UI 89·2-100·0) per 100 000 population. The COVID-19 pandemic shifted the rankings of the leading five causes, lowering stroke to the third-leading and chronic obstructive pulmonary disease to the fourth-leading position. In 2021, the highest age-standardised death rates from COVID-19 occurred in sub-Saharan Africa (271·0 deaths [250·1-290·7] per 100 000 population) and Latin America and the Caribbean (195·4 deaths [182·1-211·4] per 100 000 population). The lowest age-standardised death rates from COVID-19 were in the high-income super-region (48·1 deaths [47·4-48·8] per 100 000 population) and southeast Asia, east Asia, and Oceania (23·2 deaths [16·3-37·2] per 100 000 population). Globally, life expectancy steadily improved between 1990 and 2019 for 18 of the 22 investigated causes. Decomposition of global and regional life expectancy showed the positive effect that reductions in deaths from enteric infections, lower respiratory infections, stroke, and neonatal deaths, among others have contributed to improved survival over the study period. However, a net reduction of 1·6 years occurred in global life expectancy between 2019 and 2021, primarily due to increased death rates from COVID-19 and other pandemic-related mortality. Life expectancy was highly variable between super-regions over the study period, with southeast Asia, east Asia, and Oceania gaining 8·3 years (6·7-9·9) overall, while having the smallest reduction in life expectancy due to COVID-19 (0·4 years). The largest reduction in life expectancy due to COVID-19 occurred in Latin America and the Caribbean (3·6 years). Additionally, 53 of the 288 causes of death were highly concentrated in locations with less than 50% of the global population as of 2021, and these causes of death became progressively more concentrated since 1990, when only 44 causes showed this pattern. The concentration phenomenon is discussed heuristically with respect to enteric and lower respiratory infections, malaria, HIV/AIDS, neonatal disorders, tuberculosis, and measles. INTERPRETATION Long-standing gains in life expectancy and reductions in many of the leading causes of death have been disrupted by the COVID-19 pandemic, the adverse effects of which were spread unevenly among populations. Despite the pandemic, there has been continued progress in combatting several notable causes of death, leading to improved global life expectancy over the study period. Each of the seven GBD super-regions showed an overall improvement from 1990 and 2021, obscuring the negative effect in the years of the pandemic. Additionally, our findings regarding regional variation in causes of death driving increases in life expectancy hold clear policy utility. Analyses of shifting mortality trends reveal that several causes, once widespread globally, are now increasingly concentrated geographically. These changes in mortality concentration, alongside further investigation of changing risks, interventions, and relevant policy, present an important opportunity to deepen our understanding of mortality-reduction strategies. Examining patterns in mortality concentration might reveal areas where successful public health interventions have been implemented. Translating these successes to locations where certain causes of death remain entrenched can inform policies that work to improve life expectancy for people everywhere. FUNDING Bill & Melinda Gates Foundation