231 research outputs found
Anomalous pumping in the non-Hermitian Rice-Mele model
We study topological charge pumping (TCP) in the Rice-Mele (RM) model with
irreciprocal hopping. The non-Hermiticity gives rise to interesting pumping
physics, owing to the presence of skin effect and exceptional points. In the
static 1D RM model, we observe two independent tuning knobs that drive the
topological transition, viz., non-Hermitian parameter and system size
. To elucidate the system-size dependency, we made use of the finite-size
generalized Brillouin zone (GBZ) scheme. This scheme captures the state pumping
of topological edge modes in the static 1D RM model and provides further
insight into engineering novel gapless exceptional edge modes with the help of
adiabatic drive. Finally, we apply three types of adiabatic protocols to study
TCP in the 1+1D RM model. We further explain the number of pumped charges (in
each period) using a non-Bloch topological invariant. This exactly explains the
presence of different pumping phases in the non-Hermitian RM model as we tune
the non-Hermitian parameter . We observe that in a non-Hermitian
system, even a trivial adiabatic protocol can lead to pumping that has no
Hermitian counterpart.Comment: 16 pages, 18 figure
Differential rates of perinatal maturation of human primary and nonprimary auditory cortex
Abstract Primary and nonprimary cerebral cortex mature along different timescales; however, the differences between the rates of maturation of primary and nonprimary cortex are unclear. Cortical maturation can be measured through changes in tissue microstructure detectable by diffusion magnetic resonance imaging (MRI). In this study, diffusion tensor imaging (DTI) was used to characterize the maturation of Heschl’s gyrus (HG), which contains both primary auditory cortex (pAC) and nonprimary auditory cortex (nAC), in 90 preterm infants between 26 and 42 weeks postmenstrual age (PMA). The preterm infants were in different acoustical environments during their hospitalization: 46 in open ward beds and 44 in single rooms. A control group consisted of 15 term-born infants. Diffusion parameters revealed that (1) changes in cortical microstructure that accompany cortical maturation had largely already occurred in pAC by 28 weeks PMA, and (2) rapid changes were taking place in nAC between 26 and 42 weeks PMA. At term equivalent PMA, diffusion parameters for auditory cortex were different between preterm infants and term control infants, reflecting either delayed maturation or injury. No effect of room type was observed. For the preterm group, disturbed maturation of nonprimary (but not primary) auditory cortex was associated with poorer language performance at age two years
Characterization of Overlap in Observational Studies
Overlap between treatment groups is required for non-parametric estimation of
causal effects. If a subgroup of subjects always receives the same
intervention, we cannot estimate the effect of intervention changes on that
subgroup without further assumptions. When overlap does not hold globally,
characterizing local regions of overlap can inform the relevance of causal
conclusions for new subjects, and can help guide additional data collection. To
have impact, these descriptions must be interpretable for downstream users who
are not machine learning experts, such as policy makers. We formalize overlap
estimation as a problem of finding minimum volume sets subject to coverage
constraints and reduce this problem to binary classification with Boolean rule
classifiers. We then generalize this method to estimate overlap in off-policy
policy evaluation. In several real-world applications, we demonstrate that
these rules have comparable accuracy to black-box estimators and provide
intuitive and informative explanations that can inform policy making.Comment: To appear at AISTATS 202
Solar-assisted heat pump system for cost-effective space heating and cooling
The use of heat pumps for the utilization of solar energy is studied. Two requirements for a cost-effective system are identified: (1) a special heat pump whose coefficient of performance continues to rise with source temperature over the entire range appropriate for solar assist, and (2) a low-cost collection and storage subsystem able to supply solar energy to the heat pump efficiently at low temperatures. Programs leading to the development of these components are discussed. A solar assisted heat pump system using these components is simulated via a computer, and the results of the simulation are used as the basis for a cost comparison of the proposed system with other solar and conventional systems
Processing and evaluation of dissimilar Al-SS friction welding of pipe configuration : nondestructive inspection, properties, and microstructure
In the present investigation, dissimilar friction welding between AA6063-T6 and SS 304L materials of pipe joint configuration with an outer diameter of 88.90 mm and a wall thickness of 5.4 mm was performed. Four different experimental conditions were varied based on visual inspection after each weld. The welded pipe joints were evaluated by vacuum leak detection, thermal shock test, pneumatic pressure test, tensile test, optical and scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractions, X-ray elemental mapping, and hardness measurements. The results revealed that friction welded Al-SS bimetallic pipes sustained ultra-high vacuum pressure and cryogenic working environments without leak detection. Al-SS friction welded pipe resulted in high tensile strength with 72% of joint efficiency as compared to AA6063-T6 base material. Microstructure variations were observed significant towards AA6063-T6 material close to the Al-SS interface. The intermetallic compound of Fe3Al phase was identified with a reaction layer between Al-SS joints with varying thickness of 1.1 µm to 2.0 µm
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