Some of the most intriguing problems in solid-state physics arise when the motion of one electron dramatically affects the motion of surrounding electrons. Traditionally, such highly correlated ...electron systems have been studied mainly in materials with complex transition metal chemistry. Over the past decade, researchers have learned to confine one or a few electrons within a nanometre-scale semiconductor 'artificial atom', and to understand and control this simple system in detail3. Here we combine artificial atoms to create a highly correlated electron system within a nano-engineered semiconductor structure. We tune the system in situ through a quantum phase transition between two distinct states, each a version of the Kondo state, in which a bound electron interacts with surrounding mobile electrons. The boundary between these competing Kondo states is a quantum critical point-namely, the exotic and previously elusive two-channel Kondo state, in which electrons in two reservoirs are entangled through their interaction with a single localized spin.
Urban greening can potentially help mitigate heat-related mortality and flooding facing the >4 billion urban population worldwide. However, the geographical variation of the relative combined ...hydrological and thermal performance benefits of such interventions are unknown. Here we quantify globally, using a hydrological model, how climate-driven trade-offs exist between hydrological retention and cooling potential of urban greening such as green roofs and parks. Using a Budyko framework, we show that water retention generally increases with aridity in water-limited environments, while cooling potential favors energy-limited climates. Our models suggest that common urban greening strategies cannot yield high performance simultaneously for addressing both urban heat-island and urban flooding problems in most cities globally. Irrigation, if sustainable, may enhance cooling while maintaining retention performance in more arid locations. Increased precipitation variability with climate change may reduce performance of thinner green-infrastructure more quickly compared to greened areas with thicker soils and root systems. Our results provide a conceptual framework and first-order quantitative guide for urban development, renewal and policymaking.
We present a new classical spin liquid on the pyrochlore lattice by extending spin ice with further neighbour interactions. We find that this disorder-free spin model exhibits a form of dynamical ...heterogeneity with extremely slow relaxation for some spins, while others fluctuate quickly down to zero temperature. We thus call this state spin slush, in analogy to the heterogeneous mixture of solid and liquid water. This behaviour is driven by the structure of the ground-state manifold which extends the celebrated two-in/two-out ice states to include branching structures built from three-in/one-out, three-out/one-in and all-in/all-out tetrahedra defects. Distinctive liquid-like patterns in the magnetic correlations serve as a signature of this intermediate range order. Possible applications to materials as well the effects of quantum tunnelling are discussed.
Red supergiants are the most common final evolutionary stage of stars that have initial masses between 8 and 35 times that of the Sun
. During this stage, which lasts roughly 100,000 years
, red ...supergiants experience substantial mass loss. However, the mechanism for this mass loss is unknown
. Mass loss may affect the evolutionary path, collapse and future supernova light curve
of a red supergiant, and its ultimate fate as either a neutron star or a black hole
. From November 2019 to March 2020, Betelgeuse-the second-closest red supergiant to Earth (roughly 220 parsecs, or 724 light years, away)
-experienced a historic dimming of its visible brightness. Usually having an apparent magnitude between 0.1 and 1.0, its visual brightness decreased to 1.614 ± 0.008 magnitudes around 7-13 February 2020
-an event referred to as Betelgeuse's Great Dimming. Here we report high-angular-resolution observations showing that the southern hemisphere of Betelgeuse was ten times darker than usual in the visible spectrum during its Great Dimming. Observations and modelling support a scenario in which a dust clump formed recently in the vicinity of the star, owing to a local temperature decrease in a cool patch that appeared on the photosphere. The directly imaged brightness variations of Betelgeuse evolved on a timescale of weeks. Our findings suggest that a component of mass loss from red supergiants
is inhomogeneous, linked to a very contrasted and rapidly changing photosphere.
Understanding and managing groundwater resources in drylands is a challenging task, but one that is globally important. The dominant process for dryland groundwater recharge is thought to be as ...focused, indirect recharge from ephemeral stream losses. However, there is a global paucity of data for understanding and quantifying this process and transferable techniques for quantifying groundwater recharge in such contexts are lacking. Here we develop a generalized conceptual model for understanding water table and groundwater head fluctuations due to recharge from episodic events within ephemeral streams. By accounting for the recession characteristics of a groundwater hydrograph, we present a simple but powerful new water table fluctuation approach to quantify focused, indirect recharge over both long term and event time scales. The technique is demonstrated using a new, and globally unparalleled, set of groundwater observations from an ephemeral stream catchment located in NSW, Australia. We find that, following episodic streamflow events down a predominantly dry channel system, groundwater head fluctuations are controlled by pressure redistribution operating at three time scales from vertical flow (days to weeks), transverse flow perpendicular to the stream (weeks to months), and longitudinal flow parallel to the stream (years to decades). In relative terms, indirect recharge decreases almost linearly away from the mountain front, both in discrete monitored events as well as in the long‐term average. In absolute terms, the estimated indirect recharge varies from 80 to 30 mm/a with the main uncertainty in these values stemming from uncertainty in the catchment‐scale hydraulic properties.
Key Points:
A water table fluctuation method to quantify indirect recharge is presented
Indirect recharge decreases almost linearly away from a semiarid mountain front
This spatial pattern is persistent both in the long term and on an event basis
The interaction between groundwater and surface water along the Tambo and Nicholson rivers, southeast Australia, was investigated using super(222)Rn, Cl, differential flow gauging, head gradients, ...electrical conductivity (EC) and temperature profiles. Head gradients, temperature profiles, Cl concentrations and super(222)Rn activities all indicate higher groundwater fluxes to the Tambo River in areas of increased topographic variation where the potential to form large groundwater-surface water gradients is greater. Groundwater discharge to the Tambo River calculated by Cl mass balance was significantly lower (1.48 x 10 super(4) to 1.41 x 10 super(3) m super(3) day super(-1)) than discharge estimated by super(222)Rn mass balance (5.35 x 10 super(5) to 9.56 x 10 super(3) m super(3) day super(-1)) and differential flow gauging (5.41 x 10 super(5) to 6.30 x 10 super(3) m super(3) day super(-1)) due to bank return waters. While groundwater sampling from the bank of the Tambo River was intended to account for changes in groundwater chemistry associated with bank infiltration, variations in bank infiltration between sample sites remain unaccounted for, limiting the use of Cl as an effective tracer. Groundwater discharge to both the Tambo and Nicholson rivers was the highest under high-flow conditions in the days to weeks following significant rainfall, indicating that the rivers are well connected to a groundwater system that is responsive to rainfall. Groundwater constituted the lowest proportion of river discharge during times of increased rainfall that followed dry periods, while groundwater constituted the highest proportion of river discharge under baseflow conditions (21.4% of the Tambo in April 2010 and 18.9% of the Nicholson in September 2010).
The adventure of carbon stars Rau, G; Hron, J; Paladini, C ...
Astronomy and astrophysics (Berlin),
04/2017, Letnik:
600
Journal Article
Recenzirano
Context. Modeling stellar atmospheres is a complex and intriguing task in modern astronomy. A systematic comparison of models with multi-technique observations is the only efficient way to constrain ...the models. Aims. We intend to perform self-consistent modeling of the atmospheres of six carbon-rich AGB stars (R Lep, R Vol, Y Pav, AQ Sgr, U Hya, and X TrA) with the aim of enlarging the knowledge of the dynamic processes occurring in their atmospheres. Methods. We used VLTI/MIDI interferometric observations, in combination with spectro-photometric data, and compared them with self-consistent, dynamic model atmospheres. Results. We found that the models can reproduce spectral energy distribution (SED) data well at wavelengths longer than 1 mu m, and the interferometric observations between 8 mu m and 10 mu m. Discrepancies observed at wavelengths shorter than 1 mu m in the SED, and longer than 10 mu m in the visibilities, could be due to a combination of data- and model-related effects. The models best fitting the Miras are significantly extended, and have a prominent shell-like structure. On the contrary, the models best fitting the non-Miras are more compact, showing lower average mass loss. The mass loss is of episodic or multi-periodic nature but causes the visual amplitudes to be notably larger than the observed ones. A number of stellar parameters were derived from the model fitting: T sub(Ross), L sub(Ross), M, C/O, and M. Our findings agree well with literature values within the uncertainties. T sub(Ross), and L sub(Ross) are also in good agreement with the temperature derived from the angular diameter T(theta sub(()V-K)) and the bolometric luminosity from the SED fitting L sub(bol), except for AQ Sgr. The possible reasons are discussed in the text. Finally, theta sub(Ross) and theta sub(()V-K) agree with one another better for the Miras than for the non-Miras targets, which is probably connected to the episodic nature of the latter models. We also located the stars in the H-R diagram, comparing them with evolutionary tracks. We found that the main derived properties (L, T sub(eff), C/O ratios and stellar masses) from the model fitting are in good agreement with TP-AGB evolutionary calculations for carbon stars carried out with the COLIBRI code.
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