arXiv subjects

J. Jiang

Publications and source records attributed to J. Jiang.

At least 109 records · Page 6Linked to original sources

arXiv2012

Surface flux evolution constraints for flux transport dynamos

The surface flux transport (SFT) model of solar magnetic fields involves empirically well-constrained velocity and magnetic fields. The basic evolution of the Sun's large-scale surface magnetic field is well described by this model. The azimuthally averaged evolution of the SFT model can be compared to the surface evolution of the flux transport dynamo (FTD), and the evolution of the SFT model can be used to constrain several near-surface properties of the FTD model. We compared the results of the FTD model with different upper boundary conditions and diffusivity profiles against the results of the SFT model. Among the ingredients of the FTD model, downward pumping of magnetic flux, related to a positive diffusivity gradient, has a significant effect in slowing down the diffusive radial transport of magnetic flux through the solar surface. Provided the pumping was strong enough to give rise to a downflow of a magnetic Reynolds number of 5 in the near-surface boundary layer, the FTD using a vertical boundary condition matches the SFT model based on the average velocities above the boundary layer. The FTD model with a potential field were unable to match the SFT results.

astro-ph.SR↗

arXiv2011

Symmetry breaking via orbital-dependent reconstruction of electronic structure in uniaxially strained NaFeAs

The superconductivity discovered in iron-pnictides is intimately related to a nematic ground state, where the C4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with the polarization dependent angle-resolved photoemission spectroscopy. A uniaxial strain was applied on the sample to overcome the twinning effect in the low temperature C2-symmetric state, and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the dxy- and dyz-dominated bands. These bands strongly hybridize with each other, inducing a band splitting, while the dxz-dominated bands only exhibit an energy shift without any reconstruction. These findings suggest that the development of orbital-dependent spin polarization is likely the dominant force to drive the nematicity, while the ferro-orbital ordering between dxz and dyz orbitals can only play a minor role here.

cond-mat.supr-con↗

arXiv2011

Coherent heavy quasiparticles in CePt5 surface alloy

We report on the results of a high-resolution angle-resolved photoemission (ARPES) study on the ordered surface alloy CePt5. The temperature dependence of the spectra show the formation of the coherent low-energy heavy-fermion band near the Fermi level. This experimental data is supported by a multi-band model calculation in the framework of the dynamical mean field theory (DMFT).

cond-mat.str-el↗

arXiv2011

Superfluid density measurements of Ba(CoxFe1-x)2As2 films near optimal doping

We report the first direct measurements of superfluid density, ns(T) \propto λ-2(T), in films of Fe-pnictide superconductors. The magnetic penetration depth, λ(0), in our epitaxial, single-crystal Ba(CoxFe1-x)2As2 films near optimal doping (x=0.08) is 350 nm to 430 nm, comparable to bulk single crystals. The T-dependence of λ-2 indicates a small s-wave gap, 2Δ(0)/kBTc = 2.2 \pm 0.1. In detail, λ has power-law behavior at low T: λ(T)/λ(0) - 1 \approx 0.60*(T/Tc)2.5\pm0.1. The small gap, together with power-law behavior at low T, suggests strong intraband scattering on the larger-gap Fermi surface and significant interband scattering between large-gap and small-gap Fermi surfaces.

cond-mat.supr-con↗

arXiv2010

Heavily electron-doped electronic structure and isotropic superconducting gap in AxFe2Se2 (A=K,Cs)

The low energy band structure and Fermi surface of the newly discovered superconductor, AxFe2Se2 (A=K,Cs), have been studied by angle-resolved photoemission spectroscopy. Compared with iron pnictide superconductors, AxFe2Se2 (A=K,Cs) is the most heavily electron-doped with Tc~30 K. Only electron pockets are observed with an almost isotropic superconducting gap of ~10.3 meV, while there is no hole Fermi surface near the zone center, which indicates the inter-pocket hopping or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign changed s$_\pm$ pairing symmetry, a leading candidate proposed for iron-based superconductors, becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is a better description.

cond-mat.supr-con↗

arXiv2010

The orbital characters of bands in iron-based superconductor BaFe1.85Co0.15As2

The unconventional superconductivity in the newly discovered iron-based superconductors is intimately related to its multi-band/multi-orbital nature. Here we report the comprehensive orbital characters of the low-energy three-dimensional electronic structure in BaFe$_{1.85}$Co$_{0.15}$As$_2$ by studying the polarization and photon energy dependence of angle-resolved photoemission data. While the distributions of the $d_{xz}$, $d_{yz}$, and $d_{3z^2-r^2}$ orbitals agree with the prediction of density functional theory, those of the $d_{xy}$ and $d_{x^2-y^2}$ orbitals show remarkable disagreement with theory. Our results point out the inadequacy of the existing band structure calculations, and more importantly, provide a foundation for constructing the correct microscopic model of iron pnictides.

cond-mat.supr-con↗

arXiv2010

Evidence for electromagnetic granularity in polycrystalline Sm1111 iron-pnictides with enhanced phase purity

We prepared polycrystalline SmFeAsO1-xFx (Sm1111) bulk samples by sintering and hot isostatic pressing (HIP) in order to study the effects of phase purity and relative density on the intergranular current density. Sintered and HIPped Sm1111 samples are denser with fewer impurity phases, such as SmOF and the grain boundary wetting phase, FeAs. We found quite complex magnetization behavior due to variations of both the inter and intragranular current densities. Removing porosity and reducing second phase content enhanced the intergranular current density, but HIPping reduced Tc and the intragranular current density, due to loss of fluorine and reduction of Tc. We believe that the HIPped samples are amongst the purest polycrystalline 1111 samples yet made. However, their intergranular current densities are still small, providing further evidence that polycrystalline pnictides, like polycrystalline cuprates, are intrinsically granular.

cond-mat.supr-con↗

arXiv2010

Multigap superconductivity and strong electron-boson coupling in Fe-based superconductors: A point-contact Andreev-reflection study of Ba(Fe1-xCox)2As2 single crystals

Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1-xCox)2As2 single crystals (Tc=24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The PCAR spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Omega_b(T) is obtained, very similar to the spin-resonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band s+- Eliashberg model by using an electron-boson spectral function peaked at Omega_0 = 12 meV ~ Omega_b(0).

cond-mat.supr-con↗

arXiv2010

Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies

We measured the THz reflectance properties of a high quality epitaxial thin film of the Fe-based superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ with T$_c$=22.5 K. The film was grown by pulsed laser deposition on a DyScO$_3$ substrate with an epitaxial SrTiO$_3$ intermediate layer. The measured $R_S/R_N$ spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, provides clear evidence of a superconducting gap $Δ_A$ close to 15 cm$^{-1}$. A detailed data analysis shows that a two-band, two-gap model is absolutely necessary to obtain a good description of the measured $R_S/R_N$ spectrum. The low-energy $Δ_A$ gap results to be well determined ($Δ_A$=15.5$\pm$0.5 cm$^{-1}$), while the value of the high-energy gap $Δ_B$ is more uncertain ($Δ_B$=55$\pm$7 cm$^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$Δ/kT_c$ values close to 2 and 7.

cond-mat.supr-con↗

arXiv2010

Observation of a coherence peak and pair-breaking effects in THz conductivity of BaFe$_{2-2x}$Co$_{2x}$As$_2$

We report a study of high quality pnictide superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ thin films using time-domain THz spectroscopy. Near T$_c$ we find evidence for a coherence peak and qualitative agreement with the weak-coupling Mattis-Bardeen form of the conductivity. At low temperature, we find that the real part of the THz conductivity is not fully suppressed and $σ_2$ is significantly smaller than the Matthis-Bardeen expectation. The temperature dependence of the penetration depth $λ$ follows a power law with an unusually high exponent of 3.1. We interpret these results as consistent with impurity scattering induced pair-breaking. Taken together our results are strong evidence for an extended s$\pm$ symmetry order parameter.

cond-mat.supr-con↗

arXiv2010

Surface flux transport modeling for solar cycles 15--21: effects of cycle-dependent tilt angles of sunspot groups

We model the surface magnetic field and open flux of the Sun from 1913 to 1986 using a surface flux transport model, which includes the observed cycle-to-cycle variation of sunspot group tilts. The model reproduces the empirically derived time evolution of the solar open magnetic flux, and the reversal times of the polar fields. We find that both the polar field and the axial dipole moment resulting from this model around cycle minimum correlate with the strength of the following cycle.

astro-ph.SR↗

arXiv2010

Conductance asymmetry in point-contacts on epitaxial thin films of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$

Point-contact spectroscopy is a powerful tool for probing superconductors. One of the most common observations in the point-contact spectra on the recently discovered ferropnictide superconductors is a large conductance asymmetry with respect to voltage across the point-contact. In this paper we show that the antisymmetric part of the point-contact spectrum between a silver tip and an epitaxial thin film of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ shows certain unique features. These features have an interesting evolution with increasing temperature up to a temperature that is 30% larger than the critical temperature $T_c$ of the superconductor. We argue that this evolution can be associated with the rich normal state properties of these materials.

cond-mat.supr-con↗

arXiv2010

Surface and bulk electronic structures of LaOFeAs studied by angle resolved photoemission spectroscopy

The electronic structure of LaOFeAs, a parent compound of iron-arsenic superconductors, is studied by angleresolved photoemission spectroscopy. By examining its dependence on photon energy, polarization, sodium dosing and the counting of Fermi surface volume, both the bulk and the surface contributions are identified. We find that a bulk band moves toward high binding energies below structural transition, and shifts smoothly across the spin density wave transition by about 25 meV. Our data suggest the band reconstruction may play a crucial role in the spin density wave transition, and the structural transition is driven by the short range magnetic order. For the surface states, both the LaO-terminated and FeAs-terminated components are revealed. Certain small band shifts are verified for the FeAs-terminated surface states in the spin density wave state, which is a reflection of the bulk electronic structure reconstruction. Moreover, sharp quasiparticle peaks quickly rise at low temperatures, indicating of drastic reduction of the scattering rate. A kink structure in one of the surface band is shown to be possibly related to the electron-phonon interactions.

cond-mat.supr-con↗

arXiv2010

An efficient method for computing the eigenfunctions of the dynamo equation

We present an elegant method of determining the eigensolutions of the induction and the dynamo equation in a fluid embedded in a vacuum. The magnetic field is expanded in a complete set of functions. The new method is based on the biorthogonality of the adjoint electric current and the vector potential with an inner product defined by a volume integral over the fluid domain. The advantage of this method is that the velocity and the dynamo coefficients of the induction and the dynamo equation do not have to be differentiated and thus even numerically determined tabulated values of the coefficients produce reasonable results. We provide test calculations and compare with published results obtained by the classical treatment based on the biorthogonality of the magnetic field and its adjoint. We especially consider dynamos with mean-field coefficients determined from direct numerical simulations of the geodynamo and compare with initial value calculations and the full MHD simulations.

astro-ph.IM↗

arXiv2010

The effect of activity-related meridional flow modulation on the strength of the solar polar magnetic field

We studied the effect of the perturbation of the meridional flow in the activity belts detected by local helioseismology on the development and strength of the surface magnetic field at the polar caps. We carried out simulations of synthetic solar cycles with a flux transport model, which follows the cyclic evolution of the surface field determined by flux emergence and advective transport by near-surface flows. In each hemisphere, an axisymmetric band of latitudinal flows converging towards the central latitude of the activity belt was superposed onto the background poleward meridional flow. The overall effect of the flow perturbation is to reduce the latitude separation of the magnetic polarities of a bipolar magnetic region and thus diminish its contribution to the polar field. As a result, the polar field maximum reached around cycle activity minimum is weakened by the presence of the meridional flow perturbation. For a flow perturbation consistent with helioseismic observations, the polar field is reduced by about 18% compared to the case without inflows. If the amplitude of the flow perturbation depends on the cycle strength, its effect on the polar field provides a nonlinearity that could contribute to limiting the amplitude of a Babcock-Leighton type dynamo.

astro-ph.SR↗

arXiv2010

Phase-incoherent superconducting pairs in the normal state of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

The normal state properties of the recently discovered ferropnictide superconductors might hold the key to understanding their exotic superconductivity. Using point-contact spectroscopy we show that Andreev reflection between an epitaxial thin film of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ and a silver tip can be seen in the normal state of the film up to temperature $T\sim1.3T_c$, where $T_c$ is the critical temperature of the superconductor. Andreev reflection far above $T_c$ can be understood only when superconducting pairs arising from strong fluctuation of the phase of the complex superconducting order parameter exist in the normal state. Our results provide spectroscopic evidence of phase-incoherent superconducting pairs in the normal state of the ferropnictide superconductors.

cond-mat.supr-con↗

arXiv2010

Strong vortex pinning in Co-doped BaFe2As2 single crystal thin films

We report measurements of the field and angular dependences of Jc of truly epitaxial Co-doped BaFe2As2 thin films grown on SrTiO3/(La,Sr)(Al,Ta)O3 with different SrTiO3 template thicknesses. The films show Jc comparable to Jc of single crystals and a maximum pinning force Fp(0.6Tc) > 5 GN/m3 at H/Hirr ~ 0.5 indicative of strong vortex pinning effective up to high fields. Due to the strong correlated c-axis pinning, Jc for field along the c-axis exceeds Jc for H//ab plane, inverting the expectation of the Hc2 anisotropy. HRTEM reveals that the strong vortex pinning is due to a high density of nanosize columnar defects.

cond-mat.supr-con↗

arXiv2010

Study of the Urban Road Networks of Le Mans

An urban road network of Le Mans in France is analyzed. Some topological properties of network are investigated, such as degree distribution, clustering coefficient, diameter, and characteristic path length. These results suggest that our network is a "small- world" network with short average shortest path and large clustering coefficient. Furthermore, double power-law distribution is found in degree distribution which is distinct from the single power-law and a novel degree distribution function is also given. Some analysis on this function extend the comprehension of the origination of the double power-law distribution widely dispersed in nature.

physics.soc-ph↗