The smallest observable elements of magnetic flux

We have followed disappearing elements of magnetic flux to determine the smallest elements detectable with the Big Bear videomagnetograph. All the elements followed were disappearing through interaction with elements of opposite polarity. The last remaining visible segment of magnectic field of such features can be used to infer the total magnetic flux of these and other small flux elements visible on the magnetograms. We used both photographic and digital videomagnetograms combining 4096 Zeeman frames made at Big Bear. Fifteen elements were measured near the vanishing point, in a 2–8 hr period. The minimum observable fluxes fall in the range of 1.0 × 10¹⁶ to 1.4 × 10¹⁷ Mx, and the apparent size of these elements is in the range of 1 to 9 square arc sec. The process of disappearance appears to be a smooth one. The smallest detectable elements of network field and ephemeral regions (ER) appear to be the same as the small intra-network (IN) field elements. The present limit is still instrumental; elements smaller than 1 × 10¹⁶ would not have been detected

Phase diagrams on composition-spread Fey_yTe1−x_{1-x}Sex_x films

Fe$_y$Te$_{1-x}$Se$_x$, an archetypical iron-based high-temperature superconductor with a simple structure but rich physical properties, has attracted lots of attention because the two end compositions, Se content $x = 0$ and 1, exhibit antiferromagnetism and nematicity, respectively, making it an ideal candidate for studying their interactions with superconductivity. However, what is clearly lacking to date is a complete phase diagram of Fe$_y$Te$_{1-x}$Se$_x$ as functions of its chemical compositions since phase separation usually occurs from $x\sim 0.6$ to 0.9 in bulk crystals. Moreover, fine control of its composition is experimentally challenging because both Te and Se are volatile elements. Here we establish a complete phase diagram of Fe$_y$Te$_{1-x}$Se$_x$, achieved by high-throughput film synthesis and characterization techniques. An advanced combinatorial synthesis process enables us to fabricate an epitaxial composition-spread Fe$_y$Te$_{1-x}$Se$_x$ film encompassing the entire Se content $x$ from 0 to 1 on a single piece of CaF$_2$ substrate. The micro-region composition analysis and X-ray diffraction show a successful continuous tuning of chemical compositions and lattice parameters, respectively. The micro-scale pattern technique allows the mapping of electrical transport properties as a function of relative Se content with an unprecedented resolution of 0.0074. Combining with the spin patterns in literature, we build a detailed phase diagram that can unify the electronic and magnetic properties of Fe$_y$Te$_{1-x}$Se$_x$. Our composition-spread Fe$_y$Te$_{1-x}$Se$_x$ films, overcoming the challenges of phase separation and precise control of chemical compositions, provide an ideal platform for studying the relationship between superconductivity and magnetism.Comment: 13 pages,5 figures and Supplementary Material 3 pages,3 figure

Rifting/spreading propagation interacts with preexisting transform faults: 3D geodynamic modeling

The divergent rifting/spreading centers and the strike-slip transform faults are the essential tectonic units on Earth, the dynamic evolution of which regulates the development of rifting/spreading basins. The propagation of rifting/spreading centers may interact with pre-existing transform faults, but how they interact with each other remains enigmatic. Here we use three-dimensional geodynamical numeric models to systematically simulate the interaction between rifting/spreading propagation and the pre-existing transform faults. Our model results provide the following findings. 1) The pre-existing transform faults affect rifting/spreading propagation promoting the formation of ridge segments with an offset distance, facilitating the process of spreading of the western sea basin and restraining the propagation of the east sea basin. Yet, the evolution of the transform faults is regulated by rifting/spreading propagation, featured by the increase of its length, the change in its width along strike and the presence of lineated magmatism. 2) The initial length and orientation of the pre-existing transform faults largely affect rifting/spreading propagation, i.e., large transform fault length favors the formation of large offset between ridge segments, and oblique transform faults facilitate the formation of overlapped spreading centers. 3) Model results shed new light on the evolution of the South China Sea basin, implying that the observed ridge segments in the east and southwest sub-basins, the difference of the Zhongnan Fault Zone width along strike and the lineated volcanos along the Zhongnan Fault Zone are the results of the interaction between the rifting/spreading propagation and the pre-existing transform fault

Breakdown of Three-dimensional Dirac Semimetal State in pressurized Cd3As2

We report the first observation of a pressure-induced breakdown of the 3D-DSM state in Cd3As2, evidenced by a series of in-situ high-pressure synchrotron X-ray diffraction (XRD) and single crystal transport measurements. We find that Cd3As2 undergoes a structural phase transition from a metallic tetragonal (T) phase in space group I41/acd to a semiconducting monoclinic (M) phase in space group P21/c at critical pressure 2.57 GPa, above this pressure, an activation energy gap appears, accompanied by distinct switches in Hall resistivity slope and electron mobility. These changes of crystal symmetry and corresponding transport properties manifest the breakdown of the 3D-DSM state in pressurized Cd3As2.Comment: 17 pages, 4 figure

The Distribution of the Mid-to-Lower Crustal Low-Velocity Zone Beneath the Northeastern Tibetan Plateau Revealed from Ambient Noise Tomography

We collected continuous seismic data recorded between 2007 and 2010 by 208 broadband stations from the Chinese Provincial Digital Seismic Networks, A Seismic Collaborative Experiment of Northern Tibet, and the Northeastern Tibet Seismic experiment. Cross correlations of vertical component records are computed to extract the Rayleigh wave empirical Green\u27s functions. Group and phase velocities are then constructed from the empirical Green\u27s functions in 8 to 50 s period. At periods ≤25 s, more than 10% lower velocities are imaged beneath the Qaidam Basin, and high velocities are observed beneath the nonbasin regions. At periods ≥30 s, up to 10% lower velocities are imaged in the Qiangtang and Songpan-Ganze Terranes. From these group and phase velocity maps, a three-dimensional (3-D) Vsv model of the crust is derived. The model shows that the Qiangtang and Songpan-Ganze Terranes have a very thick crust with a prominent low-velocity zone (LVZ) in the middle crust. The LVZ thins out in the vicinity of the eastern Kunlun Mountains, providing a new constraint on the mode of deformation across the Tibetan Plateau. The northwestern Qilian Orogen, where receiver functions reveal a Moho deeper than the surrounding areas, also features a relatively weak midcrustal LVZ, which we interpret as an intracrustal response associated with the shortening between the North China Craton and the Tibetan Plateau