arXiv subjects

S. Jiang

Publications and source records attributed to S. Jiang.

12 recordsLinked to original sources

arXiv2019

Magnetodynamics in orthogonal nanocontact spin-torque nano-oscillators based on magnetic tunnel junctions

We demonstrate field and current controlled magnetodynamics in nanocontact spin-torque nano-oscillators (STNOs) based on orthogonal magnetic tunnel junctions (MTJs). We systematically analyze the microwave properties (frequency $f$, linewidth $Δf$, power $P$, and frequency tunability $df/dI$) with their physical origins---perpendicular magnetic anisotropy (PMA), damping-like and field-like spin transfer torque (STT), and voltage-controlled magnetic anisotropy (VCMA). These devices present several advantageous characteristics: high emission frequencies ($f> 20$ GHz), high frequency tunability ($df/dI=0.25$~GHz/mA), and zero-field operation ($f\sim 4$ GHz). Furthermore, a detailed investigation of $f(H, I)$ reveals that $df/dI$ is mostly governed by the large VCMA (287~fJ/(V$\cdot$m)), while STT plays a negligible role.

physics.app-ph↗

arXiv2018

CMOS compatible W/CoFeB/MgO spin Hall nano-oscillators with wide frequency tunability

We demonstrate low-operational-current W/Co$_{20}$Fe$_{60}$B$_{20}$/MgO spin Hall nano-oscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates. Thanks to a record high spin Hall angle of the $β$-phase W ($θ_{SH}$ = -0.53), a very low threshold current density of 3.3 $\times$ 10$^{7}$ A/cm$^2$ can be achieved. Together with their very wide frequency tunability (7-28 GHz), promoted by a moderate perpendicular magnetic anisotropy, this makes HiR-Si/W/CoFeB based SHNOs potential candidates for wide-band microwave signal generation. Their CMOS compatibility offers a promising route towards the integration of spintronic microwave devices with other on-chip semiconductor microwave components.

physics.app-ph↗

arXiv2018

Tuning the magnetodynamic properties of all-perpendicular spin valves using He+ irradiation

Using He+ ion irradiation, we demonstrate how the magnetodynamic properties of both ferromagnetic layers in all-perpendicular [Co/Pd]/Cu/[Co/Ni] spin valves can be tuned by varying the He+ ion fluence. As the perpendicular magnetic anisotropy of both layers is gradually reduced by the irradiation, different magnetic configurations can be achieved from all-perpendicular, through orthogonal, to all in-plane. In addition, both the magnetic damping and the inhomogeneous broadening of the Co/Ni layer improve substantially with increasing fluence. GMR of the spin valve is negatively affected and decreases linearly from an original value of 1.14% to 0.4% at the maximum fluence of 50*10^14 He+/cm^2.

cond-mat.mtrl-sci↗

arXiv2016

Nonlinear and time-resolved optical study of the 112-type iron-based superconductor parent Ca$_{1-x}$La$_{x}$FeAs$_{2}$ across its structural phase transition

The newly discovered 112-type ferropnictide superconductors contain chains of As atoms that break the tetragonal symmetry between the $a$ and $b$ axes. This feature eliminates the need for uniaxial strain that is usually required to stabilize large single domains in the electronic nematic state that exists in the vicinity of magnetic order in the iron-based superconductors. We report detailed structural symmetry measurements of 112-type Ca$_{0.73}$La$_{0.27}$FeAs$_{2}$ using rotational anisotropy optical second harmonic generation. This technique is complementary to diffraction experiments and enables a precise determination of the point group symmetry of a crystal. By combining our measurements with density functional theory calculations, we uncover a strong optical second harmonic response of bulk electric dipole origin from the Fe and Ca $3d$-derived states that enables us to assign $C_2$ as the crystallographic point group. This makes the 112-type materials high-temperature superconductors without a center of inversion, allowing for the possible mixing of singlet and triplet Cooper pairs in the superconducting state. We also perform pump-probe transient reflectivity experiments that reveal a 4.6 THz phonon mode associated with the out-of-plane motion of As atoms in the FeAs layers. We do not observe any suppression of the optical second harmonic response or shift in the phonon frequency upon cooling through the reported monoclinic-to-triclinic transition at 58 K. This allows us to identify $C_1$ as the low-temperature crystallographic point group but suggests that structural changes induced by long-range magnetic order are subtle and do not significantly affect electronic states near the Fermi level.

cond-mat.str-el↗

arXiv2015

Arbitrary tunable spaser based on double-Fano resonance of two sets of disk-ring nanostructure

This paper demonstrates an arbitrary tunable spaser based on double-Fano resonance of a plasmonic nanostructure consisting of two sets of disk-ring (TSDR) nanostructure. TSDR nanostructure supports two Fano resonances, which can be served as the lasing mode and the pumping mode of a spaser. These two mode can be tuned in a very wide wavelength range because of the good tunability of the two nanorings. The tuning range of the lasing mode and the pumping mode can be reached to 710 nm and 620 nm. These results represent a significant step in the pursuit of ultimate spasers and propose a approach to manipulate lasing mode and pumping mode over a broad spectral range.

physics.optics↗

arXiv2015

Microengineering laser plasma interactions at relativistic intensities

We report on the first successful proof-of-principle experiment to manipulate laser-matter interactions on the microscale using highly ordered Si microwire arrays. The interaction of a high contrast short pulse laser with a flat target via periodic Si microwires yields a substantial enhancement in both total and cut-off energies of the produced electron beam. The self generated electric and magnetic fields behave as an electromagnetic lens that confines and guides electrons between the microwires as they acquire relativistic energies via direct laser acceleration (DLA).

physics.plasm-ph↗

arXiv2014

Persistent detwinning of iron pnictides by small magnetic fields

Our comprehensive study on EuFe$_2$As$_2$ reveals a dramatic reduction of magnetic detwinning fields compared to other AFe$_2$As$_2$ (A = Ba, Sr, Ca) iron pnictides by indirect magneto-elastic coupling of the Eu$^{2+}$ ions. We find that only 0.1T are sufficient for persistent detwinning below the local Eu$^{2+}$ ordering; above $T_\text{Eu}$ = 19K, higher fields are necessary. Even after the field is switched off, a significant imbalance of twin domains remains constant up to the structural and electronic phase transition (190K). This persistent detwinning provides the unique possibility to study the low temperature electronic in-plane anisotropy of iron pnictides without applying any symmetrybreaking external force.

cond-mat.supr-con↗

arXiv2014

Enhancing Bremsstrahlung Production From Ultraintense Laser-Solid Interactions With Front Surface Structures

We report the results of a combined study of particle-in-cell and Monte Carlo modeling that investigates the production of Bremsstrahlung radiation produced when an ultraintense laser interacts with a tower-structured target. These targets are found to significantly narrow the electron angular distribution as well as produce significantly higher energies. These features combine to create a significant enhancement in directionality and energy of the Bremstrahlung radiation produced by a high-Z converter target. These studies employ short-pulse, high intensity laser pulses, and indicate that novel target design has potential to greatly enhance the yield and narrow the directionality of high energy electrons and $γ$-rays. We find that the peak $γ$-ray brightness for this source is 6.0$\times$10$^{19}$ ${\rm s^{-1}mm^{-2}mrad^{-2}}$ at 10MeV and 1.4$\times$10$^{19}$ ${\rm s^{-1}mm^{-2}mrad^{-2}}$ at 100MeV (0.1$\%$ bandwidth).

physics.plasm-ph↗

arXiv2013

The Effects of Front-Surface Target Structures on Properties of Relativistic Laser-Plasma Electrons

We report the results of a study of the role of prescribed geometrical structures on the front of a target in determining the energy and spatial distribution of relativistic laser-plasma electrons. Our 3D PIC simulation studies apply to short-pulse, high intensity laser pulses, and indicate that a judicious choice of target front-surface geometry provides the realistic possibility of greatly enhancing the yield of high energy electrons, while simultaneously confining the emission to narrow (< 5 degree) angular cones.

physics.plasm-ph↗

arXiv2013

EuFe2(As1-xPx)2: reentrant spin glass and superconductivity

By systematic investigations of the magnetic, transport and thermodynamic properties of single crystals of EuFe2(As1-xPx)2 we explore the complex interplay of superconductivity and Eu2+ magnetism. Below 30K, two magnetic transitions are observed for all P substituted crystals suggesting a revision of the phase diagram. In addition to the canted A-type antiferromagnetic order of Eu2+ at approximately 20K, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of EuFe2(As1-xPx)2 coexists with superconductivity around x = 0.2.

cond-mat.supr-con↗

arXiv2009

Effects of the magnetic orderings on the dynamical conductivity: optical investigations of EuFe$_2$As$_2$ single crystals

The magnetic, transport and optical properties of EuFe$_2$As$_2$ single crystals have been investigated parallel and perpendicular to the $ab$-plane. The anisotropy $ρ_c/ρ_{ab}\approx 8$ depends only slightly on temperature. In both orientations, the spin-density wave transition at $T_{\rm SDW}=189$ K shows up as a considerable increase in the dc resistivity. Susceptibility measurements evidence the magnetic order of the Eu$^{2+}$ moments at $T_N=19$ K with little influence on the electronic transport taking place in the FeAs layers. Polarization-dependent infrared spectroscopy reveals strongly anisotropic optical properties and yields a carrier density of only $4.2\times 10^{21} {\rm cm}^{-3}$ and a bandmass of $m_b=2m_0$. A sizeable Drude contribution is present at all temperatures and narrows upon cooling. Below $T_{\rm SDW}$, the spin-density-wave gap develops in the in-plane optical conductivity; no appreciable change is detected for the perpendicular polarization. Modifications in the phonon features are associated with changes of the electronic properties at $T_{\rm SDW}$. The extended Drude analysis yields a linear behavior of the frequency-dependent scattering rate below $T_{\rm SDW}$, indicating an interaction between the charge carriers and spin fluctuations in the spin-density-wave state.

cond-mat.supr-con↗

arXiv2001

Experimental Limit to Interstellar 244Pu Abundance

Short-lived nuclides, now extinct in the solar system, are expected to be present in the interstellar medium (ISM). Grains of ISM origin were recently discovered in the inner solar system and at Earth orbit and may accrete onto Earth after ablation in the atmosphere. A favorable matrix for detection of such extraterrestrial material is presented by deep open-sea sediments with very low sedimentation rates (0.8-3 mm/kyr). We report here on the measurement of Pu isotopic abundances in a 1-kg deep-sea dry sediment collected in 1992 in the North Pacific. Our measured value of (3+-3)x10^5 244Pu atoms in the Pu-separated fraction of the sample shows no excess over the expected stratospheric nuclear fallout content and under reasonable assumptions we derive a limit of 2x10^-11 g-244Pu/g-ISM for the abundance of 244Pu in ISM.

astro-ph↗