arXiv subjects

J. Yuan

Publications and source records attributed to J. Yuan.

At least 37 records · Page 2Linked to original sources

arXiv2019

Emergence of superconductivity in strongly correlated hole-dominated Fe1-xSe

Here we establish a more complete phase diagram for FeSe system, based on experimental results of nonstoichiometric Fe1-xSe single crystals that we have developed recently, as well as nearly stoichiometric FeSe single crystals. The electronic correlation is found to be strongly enhanced in hole-dominated Fe1-xSe, as compared with electron-dominated FeSe, from the magnetic susceptibility and electrical transport measurements in the normal state. A superconducting dome is found to emerge starting from the strongly correlated hole-dominated regime with electron doping, while the tetragonal-orthorhombic phase transition at ~90 K is observed only at higher electron-doping levels in the electron-dominated regime.

cond-mat.supr-con↗

arXiv2019

Recent advances in high-throughput superconductivity research

Superconducting materials find applications in a rapidly growing number of technological areas, and searching for novel superconductors continues to be a major scientific task. However, the steady increase in the complexity of candidate materials presents a big challenge to the researchers in the field. In particular, conventional experimental methods are not well suited to efficiently search for candidates in compositional space exponentially growing with the number of elements; neither do they permit quick extraction of reliable multidimensional phase diagrams delineating the physical parameters that control superconductivity. New research paradigms that can boost the speed and the efficiency of superconducting materials research are urgently needed. High-throughput methods for rapid screening and optimization of materials have demonstrated their utility for accelerating research in bioinformatics and pharmaceutical industry, yet remain rare in quantum materials research. In this paper, we will briefly review the history of high-throughput research paradigm and then focus on some recent applications of this paradigm in superconductivity research. We consider the role these methods can play in all stages of materials development, including high-throughput computation, synthesis, characterization, and the emerging field of machine learning for materials. The high-throughput paradigm will undoubtedly become an indispensable tool of superconductivity research in the near future.

cond-mat.supr-con↗

arXiv2019

Final results for the neutron $β$-asymmetry parameter $A_0$ from the UCNA experiment

The UCNA experiment was designed to measure the neutron $β$-asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for $A_0$ was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008--2009, 2010, and 2011--2013, which ultimately culminated in a 0.67\% precision result for $A_0$.

nucl-ex↗

arXiv2019

SCOPE: SCUBA-2 Continuum Observations of Pre-protostellar Evolution - Survey Description and Compact Source Catalogue

We present the first release of the data and compact-source catalogue for the JCMT Large Program SCUBA-2 Continuum Observations of Pre-protostellar Evolution (SCOPE). SCOPE consists of 850-um continuum observations of 1235 Planck Galactic Cold Clumps (PGCCs) made with the Submillimetre Common-User Bolometer Array 2 on the James Clerk Maxwell Telescope. These data are at an angular resolution of 14.4 arcsec, significantly improving upon the 353-GHz resolution of Planck at 5 arcmin, and allowing for a catalogue of 3528 compact sources in 558 PGCCs. We find that the detected PGCCs have significant sub-structure, with 61 per cent of detected PGCCs having 3 or more compact sources, with filamentary structure also prevalent within the sample. A detection rate of 45 per cent is found across the survey, which is 95 per cent complete to Planck column densities of $N_{H_{2}}$ $>$ 5 $\times$ 10$^{21}$ cm$^{-2}$. By positionally associating the SCOPE compact sources with YSOs, the star formation efficiency, as measured by the ratio of luminosity to mass, in nearby clouds is found to be similar to that in the more distant Galactic Plane, with the column density distributions also indistinguishable from each other.

astro-ph.GA↗

arXiv2019

Even-odd effect in higher-order holographic production of electron vortex beams with nontrivial radial structures

Structured electron beams carrying orbital angular momentum are currently of considerable interest, both from a fundamental point of view and for application in electron microscopy and spectroscopy. Until recently, most studies have focused on the azimuthal structure of electron vortex beams with well-defined orbital angular momentum. To unambiguously define real electron-beam states and realise them in the laboratory, the radial structure must also be specified. Here we use a specific set of orthonormal modes of electron (vortex) beams to describe both the radial and azimuthal structures of arbitrary electron wavefronts. The specific beam states are based on truncated Bessel beams localised within the lens aperture plane of an electron microscope. We show that their Fourier transform set of beams can be realised at the focal planes of the probe-forming lens using a binary computer generated electron hologram. Using astigmatic transformation optics, we demonstrate that the azimuthal indices of the diffracted beams scale with the order of the diffraction through phase amplification. However, their radial indices remain the same as those of the encoding beams for all the odd diffraction orders or are reduced to the zeroth order for the even-order diffracted beams. This simple even-odd rule can also be explained in terms of the phase amplification of the radial profiles. We envisage that the orthonormal cylindrical basis set of states could lead to new possibilities in phase contrast electron microscopy and spectroscopy using structured electron beams.

quant-ph↗

arXiv2017

Matrix-assisted fabrication and exotic charge mobility of (Li,Fe)OHFeSe superconductor films

Superconducting (Li1-xFex)OHFe1-ySe films are attractive for both the basic research and practical application. However, the conventional vapor deposition techniques are not applicable in synthesizing the films of such a complex system. So no intrinsic charge transport measurements on the films are available so far to reveal the nature of charge carriers, which is fundamental to understanding the iron-based superconductivity mechanism. Herein we report a soft chemical film technique (matrix-assisted hydrothermal epitaxial growth), by which we have succeeded in growing a series of (Li1-xFex)OHFe1-ySe films covering the whole superconducting regime, with the superconducting transition temperature (Tc) from 4 K up to 42 K. This film technique opens up a new way for fabricating other complex functional materials as well. Furthermore, our systematic transport investigation on the film samples indicates that both the electron and hole carriers contribute to the charge transport, with the scattering rates deviating from the Fermi liquid. We find that the superconductivity occurs upon the electron and hole mobility becoming divergent. And in the high Tc samples, the electron carriers are found much more mobile than the holes, a feature distinct from the low Tc samples. Hence, our transport results provide key insights into the underlying physics for iron-based high-Tc superconductivity.

cond-mat.supr-con↗

arXiv2016

Robust and adjustable C-shaped vortex beams

Wavefront engineering is an important quantum technology. Here, we demonstrate the design and production of a robust C-shaped and orbital angular momentum (OAM) carrying beam in which the doughnut shaped structure contains an adjustable gap. We find that the presence of the vortex line in the core of the beam is crucial for the robustness of the C-shape against beam propagation. The topological charge of the vortex core controls mainly the size of the C, while its opening angle is controlled by the presence of vortex-anti-vortex loops. We demonstrate the generation and characterisation of C-shaped electron vortex beams, although the result is equally applicable to other quantum waves. Applications of C-shaped vortex beams include lithography, dynamical atom sorting and atomtronics.

physics.optics↗

arXiv2016

CO observations and investigation of triggered star formation towards N10 infrared bubble and surroundings

We studied the environment of the dust bubble N10 in molecular emission. Infrared bubbles, first detected by the GLIMPSE survey at 8.0 $μ$m, are ideal regions to investigate the effect of the expansion of the HII region on its surroundings eventual triggered star formation at its borders. In this work, we present a multi-wavelength study of N10. This bubble is especially interesting as infrared studies of the young stellar content suggest a scenario of ongoing star formation, possibly triggered, on the edge of the HII region. We carried out observations of $^{12}$CO(1-0) and $^{13}$CO(1-0) emission at PMO 13.7-m towards N10. We also analyzed the IR and sub-mm emission on this region and compare those different tracers to obtain a detailed view of the interaction between the expanding HII region and the molecular gas. We also estimated the parameters of the denser cold dust condensation and of the ionized gas inside the shell. Bright CO emission was detected and two molecular clumps were identified, from which we have derived physical parameters. We also estimate the parameters for the densest cold dust condensation and for the ionized gas inside the shell. The comparison between the dynamical age of this region and the fragmentation time scale favors the "Radiation-Driven Implosion" mechanism of star formation. N10 reveals to be specially interesting case with gas structures in a narrow frontier between HII region and surrounding molecular material, and with a range of ages of YSOs situated in region indicating triggered star formation.

astro-ph.GA↗

arXiv2015

Thermal and electromagnetic properties of Bi$_2$Sr$_2$CaCu$_2$O$_8$ intrinsic Josephson junction stacks studied via one-dimensional coupled sine-Gordon equations

We used one-dimensional coupled sine-Gordon equations combined with heat diffusion equations to numerically investigate the thermal and electromagnetic properties of a $300\,μ\mathrm{m}$ long intrinsic Josephson junction stack consisting of $N = 700$ junctions. The junctions in the stack are combined to $M$ segments where we assume that inside a segment all junctions behave identically. Most simulations are for $M = 20$. For not too high bath temperatures there is the appearence of a hot spot at high bias currents. In terms of electromagnetic properties, robust standing wave patterns appear in the current density and electric field distributions. These patterns come together with vortex/antivortex lines across the stack that correspond to $π$ kink states, discussed before in the literature for a homogeneous temperature distribution in the stack. We also discuss scaling of the thermal and electromagnetic properties with $M$, on the basis of simulations with $M$ between 10 and 350.

cond-mat.supr-con↗

arXiv2013

Chiral specific electron vortex beam spectroscopy

Electron vortex beams carry well-defined orbital angular momentum (OAM) about the propagation axis. Such beams are thus characterised by chirality features which make them potentially useful as probes of magnetic and other chiral materials. An analysis of the inelastic processes in which electron vortex beams interact with atoms and which involve OAM exchange is outlined, leading to the multipolar selection rules governing this chiral specific electron vortex beam spectroscopy. Our results show clearly that the selection rules are dependent on the dynamical state and location of the atoms involved. In the most favorable scenario, this form of electron spectroscopy can induce magnetic sublevel transitions which are commonly probed using circularly polarized photon beams.

quant-ph↗

arXiv2013

Modeling the linewidth dependence of coherent terahertz emission from intrinsic Josephson junction stacks in the hot-spot regime

Recently it has been found that, when operated at large input power, the linewidth of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime a hot spot coexists with regions which are still superconducting. Surprisingly, the linewidth was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature T_b. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of T_b. In the presence of a spatial gradient of the junction parameters critical current and resistance, the linewidth deceases with increasing T_b, similar to the experimental observation.

cond-mat.supr-con↗

arXiv2013

Hot-spot formation in stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8

We have studied experimentally and numerically temperature profiles and the formation of hot spots in intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8 (BSCCO). The superconducting stacks are biased in a state where all junctions are resistive. The formation of hot spots in this system is shown to arise mainly from the strongly negative temperature coefficient of the c-axis resistivity of BSCCO at low temperatures. This leads to situations where the maximum temperature in the hot spot can be below or above the superconducting transition temperature Tc. The numerical simulations are in good agreement with the experimental observations.

cond-mat.supr-con↗

arXiv2012

Measurement of the neutron $β$-asymmetry parameter $A_0$ with ultracold neutrons

We present a detailed report of a measurement of the neutron $β$-asymmetry parameter $A_0$, the parity-violating angular correlation between the neutron spin and the decay electron momentum, performed with polarized ultracold neutrons (UCN). UCN were extracted from a pulsed spallation solid deuterium source and polarized via transport through a 7-T magnetic field. The polarized UCN were then transported through an adiabatic-fast-passage spin-flipper field region, prior to storage in a cylindrical decay volume situated within a 1-T $2 \times 2π$ solenoidal spectrometer. The asymmetry was extracted from measurements of the decay electrons in multiwire proportional chamber and plastic scintillator detector packages located on both ends of the spectrometer. From an analysis of data acquired during runs in 2008 and 2009, we report $A_0 = -0.11966 \pm 0.00089_{-0.00140} ^{+0.00123}$, from which we extract a value for the ratio of the weak axial-vector and vector coupling constants of the nucleon, $λ= g_A/g_V = -1.27590 \pm 0.00239_{-0.00377}^{+0.00331}$. Complete details of the analysis are presented.

nucl-ex↗

arXiv2012

Interaction of electron vortices and optical vortices with matter and processes of orbital angular momentum exchange

The quantum processes involved in the interaction of matter with, separately, an electron vortex(EV) and an optical vortex (OV) are described, with matter modelled in terms of a neutral two particle atomic system, allowing for both the internal (electronic-type) motion and the gross (center of mass-type) motion of matter to be taken into account. The coupling of the atomic system to the EV is dominated by Coulomb forces, while that of the OV is taken in the $\mathbf{p}\cdot\mathbf{A}$ canonical form which couples $\mathbf{A}$, the transverse vector potential of the optical vortex, to the linear momenta of the two-particle system. An analysis of the dipole active transition matrix element is carried out in each case. The electron vortex is found to be capable of exchanging its orbital angular momentum (OAM) with both the electronic and the center of mass motions of the atomic system in an electric dipole transition. In contrast, for electric dipole transitions the optical vortex is found to be capable of exchanging OAM only with the center of mass. The predictions are discussed with reference to recent experiments on electron energy loss spectroscopy (EELS) using EVs traversing magnetised iron thin film samples and those involving OVs interacting with chiral molecules.

quant-ph↗

arXiv2012

Unusual linewidth dependence of coherent THz emission measured from intrinsic Josephson junction stacks in the hot-spot regime

We report on measurements of the linewidth Δf of THz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution limited measurements indicated that Δf may be below 1 GHz - much smaller than expected from a purely cavity-induced synchronization. While at low bias we found Δf to be not smaller than ? 500 MHz, at high bias, where a hotspot coexists with regions which are still superconducting, Δf turned out to be as narrow as 23 MHz. We attribute this to the hotspot acting as a synchronizing element. Δf decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or ESR, but hard to explain in standard electrodynamic models of Josephson junctions.

cond-mat.supr-con↗

arXiv2011

A Device to Measure the Propulsive Power of Nematodes

In the fluid dynamics video, we present a microfluidic device to measure the propulsive power of nematodes. The device consists of a tapered conduit filled with aqueous solution. The conduit is subjected to a DC electric field with the negative pole at the narrow end and to pressure-driven flow directed from the narrow end. The nematode is inserted at the conduit's wide end. Directed by the electric field (through electrotaxis), the nematode swims deliberately upstream toward the negative pole of the DC field. As the conduit narrows, the average fluid velocity and the drag force on the nematode increase. Eventually, the nematode arrives at an equilibrium position, at which its propulsive force balances the viscous drag force induced by the adverse flow. The equilibrium position of different animals, with similar body lengths, was measured as a function of the flow rate. The flow field around the nematode was obtained by direct numerical simulations with the experimentally imaged gait and the tapered geometry of the conduit as boundary conditions. The flow field generated by a swimming worm is similar to the one induced by two pairs of counter rotating rotors. Equilibrium positions under different flow rates were identified by finding the positions at which the horizontal component of the total force exerted on the worm body vanishes. The theoretically predicted equilibrium positions were compared and favorably agreed with the experimental data. The nematode's propulsive power was calculated by integrating the product of velocity and total stress over the worm's body surface. The device is useful to retain the nematodes at a nearly fixed position for prolonged observations of active animals under a microscope, to keep the nematode exercising, and to estimate the nematode's power consumption based on the conduit's width at the equilibrium position.

physics.flu-dyn↗

arXiv2010

Interaction of hot spots and THz waves in Bi_2Sr_2CaCu_2O_8 intrinsic Josephson junction stacks of various geometry

At high enough input power in stacks of Bi_2Sr_2CaCu_2O8 intrinsic Josephson junctions a hot spot (a region heated to above the superconducting transition temperature) coexists with regions still in the superconducting state. In the ``cold'' regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong coherent THz emission. We investigate the interplay of hot spots and standing electromagnetic waves by low temperature scanning laser microscopy and THz emission measurements, using stacks of various geometries. For a rectangular and a arrow-shaped structure we show that the standing wave can be turned on and off in various regions of the stack structure, depending on the hot spot position. We also report on standing wave and hot spot formation in a disk shaped mesa structure.

cond-mat.supr-con↗

arXiv2010

Coherent THz emission of Bi_2Sr_2CaCu_2O_8 intrinsic Josephson junction stacks in the hot spot regime

We report on THz emission measurements and low temperature scanning laser imaging of Bi_2Sr_2CaCu_2O_8 intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the "cold" part of the stack, coexist. By varying the hot spot size the cavity resonance frequency and the emitted radiation can be tuned. The linewidth of radiation is much smaller than expected from the quality factor of the cavity mode excited. Thus, an additional mechanism of synchronization seems to play a role, possibly arising from nonequilibrium processes at the hot spot edge.

cond-mat.supr-con↗