820 research outputs found

    Odd q-State Clock Spin-Glass Models in Three Dimensions, Asymmetric Phase Diagrams, and Multiple Algebraically Ordered Phases

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    Distinctive orderings and phase diagram structures are found, from renormalization-group theory, for odd q-state clock spin-glass models in d=3 dimensions. These models exhibit asymmetric phase diagrams, as is also the case for quantum Heisenberg spin-glass models. No finite-temperature spin-glass phase occurs. For all odd q5q\geqslant 5, algebraically ordered antiferromagnetic phases occur. One such phase is dominant and occurs for all q5q\geqslant 5. Other such phases occupy small low-temperature portions of the phase diagrams and occur for 5q155 \leqslant q \leqslant 15. All algebraically ordered phases have the same structure, determined by an attractive finite-temperature sink fixed point where a dominant and a subdominant pair states have the only non-zero Boltzmann weights. The phase transition critical exponents quickly saturate to the high q value.Comment: Published version, 9 pages, 10 phase diagrams, 5 figures, 1 tabl

    Overfrustrated and Underfrustrated Spin-Glasses in d=3 and 2: Evolution of Phase Diagrams and Chaos Including Spin-Glass Order in d=2

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    In spin-glass systems, frustration can be adjusted continuously and considerably, without changing the antiferromagnetic bond probability p, by using locally correlated quenched randomness, as we demonstrate here on hypercubic lattices and hierarchical lattices. Such overfrustrated and underfrustrated Ising systems on hierarchical lattices in d=3 and 2 are studied. With the removal of just 51 % of frustration, a spin-glass phase occurs in d=2. With the addition of just 33 % frustration, the spin-glass phase disappears in d=3. Sequences of 18 different phase diagrams for different levels of frustration are calculated in both dimensions. In general, frustration lowers the spin-glass ordering temperature. At low temperatures, increased frustration favors the spin-glass phase (before it disappears) over the ferromagnetic phase and symmetrically the antiferromagnetic phase. When any amount, including infinitesimal, frustration is introduced, the chaotic rescaling of local interactions occurs in the spin-glass phase. Chaos increases with increasing frustration, as seen from the increased positive value of the calculated Lyapunov exponent λ\lambda, starting from λ=0\lambda =0 when frustration is absent. The calculated runaway exponent yRy_R of the renormalization-group flows decreases with increasing frustration to yR=0y_R=0 when the spin-glass phase disappears. From our calculations of entropy and specific heat curves in d=3, it is seen that frustration lowers in temperature the onset of both long- and short-range order in spin-glass phases, but is more effective on the former. From calculations of the entropy as a function of antiferromagnetic bond concentration p, it is seen that the ground-state and low-temperature entropy already mostly sets in within the ferromagnetic and antiferromagnetic phases, before the spin-glass phase is reached.Comment: Published version, 18 phase diagrams, 12 figures, 10 page

    Implementation of atmospheric proton spectrum in GEANT4 simulations for space applications

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    A major part of cosmic rays consists of the primary protons, and this portion plays a crucial role in the space applications such as shielding of spacecrafts. In this study, the proton flux values measured at the top of the atmosphere through the BESS-TeV spectrometer in 2004 are introduced into the GEANT4 simulations by using a probability grid that generates the corresponding discrete kinetic energies with a certain discrete probability. The introduced scheme is tested over a set of the shielding materials such as aluminum, polypropylene, Kevlar, polyethylene, and water by computing the total absorbed dose, which is the measure of the cumulative energy deposited in the investigated target volumes by protons per unit mass in Gy. It is shown that the present recipe provides the opportunity to use the discrete energy values together with the experimental flux values, thereby demonstrating a beneficial capability in the GEANT4 simulations for diverse space applications.Comment: 8 pages, 3 figures, 4 table

    From task structures to world models: What do LLMs know?

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    In what sense does a large language model have knowledge? The answer to this question extends beyond the capabilities of a particular AI system, and challenges our assumptions about the nature of knowledge and intelligence. We answer by granting LLMs "instrumental knowledge"; knowledge defined by a certain set of abilities. We then ask how such knowledge is related to the more ordinary, "worldly" knowledge exhibited by human agents, and explore this in terms of the degree to which instrumental knowledge can be said to incorporate the structured world models of cognitive science. We discuss ways LLMs could recover degrees of worldly knowledge, and suggest such recovery will be governed by an implicit, resource-rational tradeoff between world models and task demands

    Vertical Convergence of Turbulent and Double-Diffusive Heat Flux Drives Warming and Erosion of Antarctic Winter Water in Summer

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    The seasonal warming of Antarctic Winter Water (WW) is a key process that occurs along the path of deep water transformation to intermediate waters. These intermediate waters then enter the upper branch of the circumpolar overturning circulation. Despite its importance, the driving mechanisms that mediate the warming of Antarctic WW remain unknown, and their quantitative evaluation is lacking. Using 38 days of glider measurements of microstructure shear, we characterize the rate of turbulent dissipation and its drivers over a summer season in the northern Weddell Sea. Observed dissipation rates in the surface layer are mainly forced by winds and explained by the stress scaling (r2 = 0.84). However, mixing to the base of the mixed layer during strong wind events is suppressed by vertical stratification from sea ice melt. Between the WW layer and the warm and saline circumpolar deep water, a subsurface layer of enhanced dissipation is maintained by double-diffusive convection (DDC). We develop a WW layer temperature budget and show that a warming trend (0.2°C over 28 days) is driven by a convergence of heat flux through mechanically driven mixing at the base of the mixed layer and DDC at the base of the WW layer. Notably, excluding the contribution from DDC results in an underestimation of WW warming by 23%, highlighting the importance of adequately representing DDC in ocean models. These results further suggest that an increase in storm intensity and frequency during summer could increase the rate of warming of WW with implications for rates of upper-ocean water mass transformation.publishedVersio

    Ventilator-Associated Pneumonia as a Quality Indicator for Patient Safety?

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    The economic and clinical burden of ventilator-associated pneumonia (VAP) is uncontested. In many hospitals, VAP surveillance is conducted to identify outbreaks and to monitor infection rates. Here, we discuss the concept of benchmarking in health care as modeled on industry, and we contribute personal arguments against considering the VAP rate as a potential candidate for benchmarking or for monitoring the quality of patient care. Accurate benchmarking of VAP rates currently seems to be unfeasible, because the patient case mix is often too diverse and complicated to be adjusted for, and diagnostic criteria and surveillance protocols vary. Thus, the risk of drawing inaccurate comparisons is high. In contrast, some risk factors for VAP are modifiable and can be monitored and used as quality indicators. Process-oriented surveillance permits bypass of case-mix and diagnostic constraints. A well-defined interhospital surveillance system is necessary to prove that interventions on procedures do really lead to a reduction of VAP rate
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