Observation of giant and tunable thermal diffusivity of a Dirac fluid at room temperature

Conducting materials typically exhibit either diffusive or ballistic charge transport. When electron–electron interactions dominate, a hydrodynamic regime with viscous charge flow emerges1–13. More stringent conditions eventually yield a quantum-critical Dirac-fluid regime, where electronic heat can flow more efficiently than charge14–22. However, observing and controlling the flow of electronic heat in the hydrodynamic regime at room temperature has so far remained elusive. Here we observe heat transport in graphene in the diffusive and hydrodynamic regimes, and report a controllable transition to the Dirac-fluid regime at room temperature, using carrier temperature and carrier density as control knobs. We introduce the technique of spatiotemporal thermoelectric microscopy with femtosecond temporal and nanometre spatial resolution, which allows for tracking electronic heat spreading. In the diffusive regime, we find a thermal diffusivity of roughly 2,000 cm2 s−1, consistent with charge transport. Moreover, within the hydrodynamic time window before momentum relaxation, we observe heat spreading corresponding to a giant diffusivity up to 70,000 cm2 s−1, indicative of a Dirac fluid. Our results offer the possibility of further exploration of these interesting physical phenomena and their potential applications in nanoscale thermal management.

Generation of highly silicic magmas at ultra-high temperature conditions : evidence from melt inclusions in peritectic garnet

<p>Metamorphism at ultra-high temperature (UHT) conditions (i.e., T >900°C and pressures from 7 to 13 kbar) is now recognized as a fundamental process of Earth’s crust, and although progress has been achieved on its understanding, constraining melt generation and fluid regime at such extreme conditions is still poorly explored.</p><p>In this study we use former melt inclusions found in peritectic garnet to investigate anatexis and fluid regime of metapelitic granulites in samples from the Rundvågshetta area, the thermal axis of the Lützow-Holm Complex (East Antarctica). Peak P-T estimates are 925-1039°C at 11.5-15 kbar. The studied rock is a coarse-grained heterogeneous metapelitic granulite with a predominant mafic residual domain and a relatively more felsic, melt-rich domain. The mineral association in the mafic domain typically contains orthopyroxene (Al<sub>2</sub>O<sub>3</sub>6-8.1 wt.%) + sillimanite + quartz + garnet (Prp<sub>42-55</sub>Alm<sub>40-52</sub>Grs<sub>3-4</sub>Sps<sub>0.2-1</sub>; X<sub>Mg</sub>0.5) + K-feldspar (Kfs) + cordierite (X<sub>Mg</sub>0.86) + rutile ± sapphirine ±biotite (X<sub>Mg</sub>0.75; TiO<sub>2</sub>3.7-5.8 wt.%) ±plagioclase (An<sub>35-46</sub>). Interstitial Kfs and quartz with low dihedral angles are often present, in particular as thin films between sillimanite and quartz; these features are interpreted as evidence for the presence of former melt along the grain boundaries. In contrast, the more felsic, melt-rich domain is composed of mesoperthite + quartz + garnet + sillimanite + brown biotite (X<sub>Mg</sub>0.7; TiO<sub>2</sub>3.7-5.4 wt.%) + rutile, but is free of orthopyroxene. Cores of garnet porphyroblasts (0.2-0.8 cm, Prp<sub>54-57</sub>Alm<sub>39-42</sub>Grs<sub>3-4</sub>Sps<sub>0.2-0.6</sub>, X<sub>Mg</sub>0.57) in the melt-rich domains contain clusters of primary glassy inclusions (GI) and crystallized melt inclusions (nanogranitoids; NI) together with multiphase fluid inclusions (MFI) and accessory phases (mainly rutile and apatite).</p><p>The GI (5-20 µm) have negative crystal shapes and contain shrinkage bubbles with or without CO<sub>2</sub>and N<sub>2</sub>. In some cases, GI may have trapped apatite and rutile. Micro-Raman investigation suggest that the H<sub>2</sub>O contents of these glasses range from 0 to 3.4 wt.%. Glasses are weakly peraluminous (ASI=1-1.1), have high SiO<sub>2</sub>(76-78 wt.%), very high K<sub>2</sub>O (6.5-10 wt.%) and extremely low CaO and FeO+MgO contents.</p><p>The NI have variable sizes (10-150 µm) and often contains intergrowth of plagioclase + quartz, K-feldspar (Kfs) and biotite (Bt). Less frequently NI may have euhedral to subhedral grains of Kfs and Bt. Trapped phases are apatite and rutile, except for one inclusion that contains the sapphirine + quartz pair indicating that melt inclusions were trapped at UHT conditions.</p><p>The MFI are composed of CO<sub>2</sub>(with densities from 0.23 to 0.93 g/cm<sup>3</sup>) and step-daughter magnesite, pyrophyllite. Methane, N<sub>2</sub>or H<sub>2</sub>O were not detected.</p><p>Our results show that anatexis of metapelites at extremely hot conditions occurred in the presence of COHfluids and generated highly silicic, weakly peraluminous, mildly to strongly potassic magmas with low H<sub>2</sub>O contents. Additional trace element data will be acquired to shed light on further geochemical fingerprints of these peculiar magmas.</p>

Thermodynamic Curvature of the Binary van der Waals Fluid

The thermodynamic Ricci curvature scalar R has been applied in a number of contexts, mostly for systems characterized by 2D thermodynamic geometries. Calculations of R in thermodynamic geometries of dimension three or greater have been very few, especially in the fluid regime. In this paper, we calculate R for two examples involving binary fluid mixtures: a binary mixture of a van der Waals (vdW) fluid with only repulsive interactions, and a binary vdW mixture with attractive interactions added. In both of these examples, we evaluate R for full 3D thermodynamic geometries. Our finding is that basic physical patterns found for R in the pure fluid are reproduced to a large extent for the binary fluid.

Mineralogy and Fluid Regime of Formation of the REE-Late-Stage Hydrothermal Mineralization of Petyayan-Vara Carbonatites (Vuoriyarvi, Kola Region, NW Russia)

The Vuoriyarvi Devonian alkaline–ultramafic complex (northwest Russia) contains magnesiocarbonatites with rare earth mineralization localized in the Petyayan-Vara area. High concentrations of rare earth elements are found in two types of these rocks: (a) ancylite-dominant magnesiocarbonatites with ancylite–baryte–strontianite–calcite–quartz (±late Ca–Fe–Mg carbonates) ore assemblage, i.e., “ancylite ores”; (b) breccias of magnesiocarbonatites with a quartz–bastnäsite matrix (±late Ca–Fe–Mg carbonates), i.e., “bastnäsite ores.” We studied fluid inclusions in quartz and late-stage Ca–Fe–Mg carbonates from these ore assemblages. Fluid inclusion data show that ore-related mineralization was formed in several stages. We propose the following TX evolution scheme for ore-related processes: (1) the formation of ancylite ores began under the influence of highly concentrated (>50 wt.%) sulphate fluids (with thenardite and anhydrite predominant in the daughter phases of inclusions) at a temperature above300–350 °C; (2) the completion of the formation of ancylite ores and their auto-metasomatic alteration occurred under the influence of concentrated (40–45 wt.%) carbonate fluids (shortite and synchysite–Ce in fluid inclusions) at a temperature above 250–275 °C; (3) bastnäsite ores deposited from low-concentrated (20–30 wt.%) hydrocarbonate–chloride fluids (halite, nahcolite, and/or gaylussite in fluid inclusions) at a temperature of 190–250 °C or higher. Later hydrothermal mineralization was related to the low-concentration hydrocarbonate–chloride fluids (<15 wt.% NaCl-equ.) at 150–200 °C. The presented data show the specific features of the mineral and fluid evolution of ore-related late-stage hydrothermal rare earth element (REE) mineralization of the Vuoriyarvi alkaline–ultramafic complex.