arXiv subjects

H. Li

Publications and source records attributed to H. Li.

At least 199 records · Page 11Linked to original sources

arXiv2015

Observation of Raman active phonon with Fano lineshape in quasi-one-dimensional superconductor K$_2$Cr$_3$As$_3$

We performed a polarized Raman scattering study of quasi-one-dimensional superconductor K$_2$Cr$_3$As$_3$. We detect two A$_1^\prime$ phonons and three E$^\prime$ phonons. One of the A$_1^\prime$ modes exhibits a nearly temperature-independent Fano lineshape. Based on our first-principles calculations, we ascribe this mode to the in-phase vibrations of Cr atoms within one layer. This observation strongly suggests that the magnetic fluctuations in K$_2$Cr$_3$As$_3$ are coupled to the electronic structure \emph{via} the lattice.

cond-mat.supr-con↗

arXiv2015

Energy dynamics and current sheet structure in fluid and kinetic simulations of decaying magnetohydrodynamic turbulence

Simulations of decaying magnetohydrodynamic (MHD) turbulence are performed with a fluid and a kinetic code. The initial condition is an ensemble of long-wavelength, counter-propagating, shear-Alfvén waves, which interact and rapidly generate strong MHD turbulence. The total energy is conserved and the rate of turbulent energy decay is very similar in both codes, although the fluid code has numerical dissipation whereas the kinetic code has kinetic dissipation. The inertial range power spectrum index is similar in both the codes. The fluid code shows a perpendicular wavenumber spectral slope of $k_{\perp}^{-1.3}$. The kinetic code shows a spectral slope of $k_{\perp}^{-1.5}$ for smaller simulation domain, and $k_{\perp}^{-1.3}$ for larger domain. We estimate that collisionless damping mechanisms in the kinetic code can account for the dissipation of the observed nonlinear energy cascade. Current sheets are geometrically characterized. Their lengths and widths are in good agreement between the two codes. The length scales linearly with the driving scale of the turbulence. In the fluid code, their thickness is determined by the grid resolution as there is no explicit diffusivity. In the kinetic code, their thickness is very close to the skin-depth, irrespective of the grid resolution. This work shows that kinetic codes can reproduce the MHD inertial range dynamics at large scales, while at the same time capturing important kinetic physics at small scales.

physics.plasm-ph↗

arXiv2015

Time-resolved measurement of single pulse femtosecond laser-induced periodic surface structure formation

Time-resolved diffraction microscopy technique has been used to observe the formation of laser-induced periodic surface structures (LIPSS) from the interaction of a single femtosecond laser pulse (pump) with a nano-scale groove mechanically formed on a single-crystal Cu substrate. The interaction dynamics (0-1200 ps) was captured by diffracting a time-delayed, frequency-doubled pulse from nascent LIPSS formation induced by the pump with an infinity-conjugate microscopy setup. The LIPSS ripples are observed to form sequentially outward from the groove edge, with the first one forming after 50 ps. A 1-D analytical model of electron heating and surface plasmon polariton (SPP) excitation induced by the interaction of incoming laser pulse with the groove edge qualitatively explains the time-evloution of LIPSS formation.

physics.optics↗

arXiv2015

Cluster magnetic fields through the study of polarized radio halos in the SKA era

Galaxy clusters are unique laboratories to investigate turbulent fluid motions and large scale magnetic fields. Synchrotron radio halos at the center of merging galaxy clusters provide the most spectacular and direct evidence of the presence of relativistic particles and magnetic fields associated with the intracluster medium. The study of polarized emission from radio halos is extremely important to constrain the properties of intracluster magnetic fields and the physics of the acceleration and transport of the relativistic particles. However, detecting this polarized signal is a very hard task with the current radio facilities.We use cosmological magneto-hydrodynamical simulations to predict the expected polarized surface brightness of radio halos at 1.4 GHz. We compare these expectations with the sensitivity and the resolution reachable with the SKA1. This allows us to evaluate the potential for studying intracluster magnetic fields in the surveys planned for SKA1.

astro-ph.CO↗

arXiv2014

Hot Core, Outflows and Magnetic Fields in W43-MM1 (G30.79 FIR 10)

We present submillimeter spectral line and dust continuum polarization observations of a remarkable hot core and multiple outflows in the high-mass star-forming region W43-MM1 (G30.79 FIR 10), obtained using the Submillimeter Array (SMA). A temperature of $\sim$ 400 K is estimated for the hot-core using CH$_3$CN (J=19-18) lines, with detections of 11 K-ladder components. The high temperature and the mass estimates for the outflows indicate high-mass star-formation. The continuum polarization pattern shows an ordered distribution, and its orientation over the main outflow appears aligned to the outflow. The derived magnetic field indicates slightly super-critical conditions. While the magnetic and outflow energies are comparable, the B-field orientation appears to have changed from parsec scales to $\sim$ 0.1 pc scales during the core/star-formation process.

astro-ph.GA↗

arXiv2013

Observations of a Quasi-Periodic Fast Propagating Magnetosonic Wave in Multi-Wavelength and Its Interaction with Other Magnetic Structures

We present an observational study of a quasi-periodic fast propagating (QFP) magnetosonic wave on 2012, April 23. The multiple wave trains were observed along an active region open loop system which has a divergence geometry. The wave trains were first observed in 171 A observations at a distance of 150 Mm from the footpoint of the guiding loop system and with a speed of 689 km/s, then they appeared in 193 A observations after their interaction with a perpendicular underlaying loop system on the path, in the meantime, the wave speed decelerated to 343 km/s quickly within a short timescale. The sudden deceleration of the wave trains and their appearance in 193 A observations caused by the interaction are interpreted through geometric effect and the density increase of the guiding loop system, respectively. On the other hand, with Wavelet and Fourier analysis methods we find that the wave trains has a common period of 80 s with the associated flare. In addition, a few low frequencies are also identified in the QFP wave. We propose that the generation of the period of 80 s was caused by the periodic releasing of energy busts through some nonlinear processes in magnetic reconnection or the so-called oscillatory reconnection mechanism, while the low frequencies detected in the QFP wave were possibly the manifestations of the leakage of pressure-driven oscillations from the photosphere or chromosphere, which could be an important source for driving QFP waves in the low corona. Our observational results also indicate that the properties of the guiding magnetic structure such as the distributions of magnetic field and density as well as geometry are crucial for modulating the propagation behaviors of QFP waves. Therefore, QFP waves could be used for remote diagnostics of the local physical properties of the solar corona.

astro-ph.SR↗

arXiv2013

Exciton-Trion Coherent Interactions in a CdTe/CdMgTe Quantum Well

We present a collection of zero-, one- and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the exciton and trion nonlinear optical response. Exciton-exciton and exciton-trion coherent coupling is manifest as distinct peaks in the spectra, whereas signatures of trion-trion interactions are absent. Excellent agreement using density matrix calculations is obtained, which highlights the essential role of many-body effects on coherent interactions in the quantum well.

cond-mat.mes-hall↗

arXiv2013

Polarization of cluster radio halos with upcoming radio interferometers

Synchrotron radio halos at the center of merging galaxy clusters provide the most spectacular and direct evidence of the presence of relativistic particles and magnetic fields associated with the intracluster medium. The study of polarized emission from radio halos has been shown to be extremely important to constrain the properties of intracluster magnetic fields. However, detecting this polarized signal is a very hard task with the current radio facilities.We investigate whether future radio observatories, such as the Square Kilometer Array (SKA) and its precursors and pathfinders, will be able to detect the polarized emission of radio halos in galaxy clusters.On the basis of cosmological magnetohydrodynamical simulations with initial magnetic fields injected by active galactic nuclei, we predict the expected radio halo polarized signal at 1.4 GHz. We compare these expectations with the limits of current radio facilities and explore the potential of the forthcoming radio interferometers to investigate intracluster magnetic fields through the detection of polarized emission from radio halos.The resolution and sensitivity values that are expected to be obtained in future sky surveys performed at 1.4 GHz using the SKA precursors and pathfinders (like APERTIF and ASKAP) are very promising for the detection of the polarized emission of the most powerful (L1.4GHz>10e25 Watt/Hz) radio halos. Furthermore, the JVLA have the potential to already detect polarized emission from strong radio halos, at a relatively low resolution.However, the possibility of detecting the polarized signal in fainter radio halos (L1.4GHz~10e24 Watt/Hz) at high resolution requires a sensitivity reachable only with SKA.

astro-ph.CO↗

arXiv2013

Carrier density dependence of the magnetic properties in iron-doped Bi2Se3 topological insulator

The electronic and magnetic properties of iron-doped topological insulator Bi1.84-xFe0.16CaxSe3 single crystals were studied. By co-doping Fe and Ca atoms, ferromagnetic bulk states with different carrier density (from n-type to p-type) were obtained. Effective magnetic moments for each Fe atom was estimated as small as about 0.07$μ$B. Magnetic and non-magnetic phases separation was observed in all samples. Our results suggest that the bulk ferromagnetism in Fe-doped Bi2Se3 is not intrinsic and regardless of carrier density.

cond-mat.mtrl-sci↗

arXiv2012

Selective surface functionalization at regions of high local curvature in graphene

Monolayer graphene was deposited on a Si wafer substrate decorated with SiO2 nanoparticles (NPs) and then exposed to aryl radicals that were generated in situ from their diazonium precursors. Using micro-Raman mapping, the aryl radicals were found to selectively react with the regions of graphene that covered the NPs. The enhanced chemical reactivity was attributed to the increased strain energy induced by the local mechanical deformation of the graphene.

cond-mat.mtrl-sci↗

arXiv2012

Fifth-order nonlinear optical response of excitonic states in an InAs quantum dot ensemble measured with 2D spectroscopy

Exciton, trion and biexciton dephasing rates are measured within the inhomogeneous distribution of an InAs quantum dot (QD) ensemble using two-dimensional Fourier-transform spectroscopy. The dephasing rate of each excitonic state is similar for all QDs in the ensemble and the rates are independent of excitation density. An additional spectral feature -- too weak to be observed in the time-integrated four-wave mixing signal -- appears at high excitation density and is attributed to the $χ^{\textrm{(5)}}$ biexcitonic nonlinear response.

cond-mat.mes-hall↗

arXiv2012

HZ Recoil Mass and Cross Section Analysis in ILD

This note describes the details of a simulation study of the Higgs boson production for processes in which the Higgs is produced together with a well measurable di-lepton system using the proposal of the ILD detector for its Letter of Intent. The analysis is optimised for the measurement of the Higgs-strahlung process, i.e. e+e- -> HZ. The cross section can be determined with a precision of 2-3% and by combining the decay channels a precision of ~30 MeV is obtained for the mass of the Higgs boson. The background can be largely reduced and the analysis exhibits a sensitivity to the configuration of the accelerator.

hep-ex↗

arXiv2011

Beyond the quantum spin liquid concept in frustrated two dimensional organic superconductors

The occurrence of antiferromagnetism in kappa-(ET)_2X can be understood within an effective 1/2-filled band with dimers of ET molecules containing one hole each. We argue that while this effective model can describe the presence of antiferromagnetism, a complete description for these materials requires the correct carrier density of one-half per molecule. For dimerized and strongly frustrated 1/4-filled lattices we show that a singlet-paired state coexisting with charge ordering occurs that we have termed the Paired Electron Crystal (PEC). Here we investigate the 1/4-filled model on a dimerized lattice, showing regions where AFM, PEC, and the Wigner-crystal occur. We point out the need to go beyond quantum spin liquid concepts for highly frustrated materials such as kappa-(ET)_2Cu_2(CN)_3 and beta'-EtMe_3Sb[Pd(dmit)_2]_2 which we believe are PECs at low temperatures.

cond-mat.str-el↗

arXiv2011

Evolution of Hard X-ray Sources and Ultraviolet Solar Flare Ribbons for a Confined Eruption of a Magnetic Flux Rope

We study the magnetic field structures of hard X-ray sources and flare ribbons of the M1.1 flare in active region NOAA 10767 on 2005 May 27. We have found in a nonlinear force-free field extrapolation, over the same polarity inversion line, a small pre-eruptive magnetic flux rope located next to sheared magnetic arcades. Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Transition Region and Coronal Explorer (TRACE) observed this confined flare in the X-ray bands and ultraviolet (UV) 1600 Å\ bands, respectively. In this event magnetic reconnection occurred at several locations. It first started at the location of the pre-eruptive flux rope. Then, the observations indicate that magnetic reconnection occurred between the pre-eruptive magnetic flux rope and the sheared magnetic arcades more than 10 minutes before the flare peak. It implied the formation of the larger flux rope, as observed with TRACE. Next, hard X-ray (HXR) sources appeared at the footpoints of this larger flux rope at the peak of the flare. The associated high-energy particles may have been accelerated below the flux rope, in or around a reconnection region. Still, the close spatial association between the HXR sources and the flux rope footpoints favors an acceleration within the flux rope. Finally, a topological analysis of a large solar region including the active regions NOAA 10766 and 10767 shows the existence of large-scale Quasi-Separatrix Layers (QSLs) before the eruption of the flux rope. No enhanced emission was found at these QSLs during the flare, but the UV flare ribbons stopped at the border of the closest large-scale QSL.

astro-ph.SR↗

arXiv2011

Darker than black: radiation-absorbing metamaterial

We show that corrugated surfaces of hyperbolic metamaterials scatter light preferentially inside the media, resulting in a very low reflectance and ultimate dark appearance in the spectral range of hyperbolic dispersion. This phenomenon of fundamental importance, demonstrated experimentally in arrays of silver nanowires grown in alumina membranes, originates from a broad-band singularity in the density of photonic states. It paves the road to a variety of applications ranging from the stealth technology to high-efficiency solar cells and photodetectors.

physics.optics↗

arXiv2011

Theory of carrier concentration-dependent electronic behavior in layered cobaltates

A natural explanation for the carrier concentration-dependent electronic behavior in the layered cobaltates emerges within correlated-electron Hamiltonians with finite on-site and significant nearest neighbor hole-hole Coulomb repulsions. The nearest neighbor repulsion decreases hole double-occupancy below hole density 1/3, but increases the same at higher hole densities. Our conclusion is valid for both single-band and three-band extended Hubbard Hamiltonians, and sheds light on concentration-dependent $e_g^\prime$ hole occupancy within the latter.

cond-mat.str-el↗

arXiv2011

The Paired Electron Crystal: order from frustration in the quarter-filled band

We present a study of the effects of simultaneous charge- and spin-frustration on the two-dimensional strongly correlated quarter-filled band on an anisotropic triangular lattice. The broken-symmetry states that dominate in the weakly frustrated region near the rectangular lattice limit are the well known antiferromagnetic state with in-phase lattice dimerization along one direction, and the Wigner crystal state with the checkerboard charge order. For moderate to strong frustration, however, the dominant phase is a novel spin-singlet paired-electron crystal (PEC), consisting of pairs of charge-rich sites separated by pairs of charge-poor sites. The PEC, with coexisting charge-order and spin-gap in two dimension, is the quarter-filled band equivalent of the valence bond solid (VBS) that can appear in the frustrated half-filled band within antiferromagnetic spin Hamiltonians. We discuss the phase diagram as a function of on-site and intersite Coulomb interactions as well as electron-phonon coupling strength. We speculate that the spin-bonded pairs of the PEC can become mobile for even stronger frustration, giving rise to a paired-electron liquid. We discuss the implications of the PEC concept for understanding several classes of quarter-filled band materials that display unconventional superconductivity, focusing in particular on organic charge transfer solids. Our work points out the need to go beyond quantum spin liquid (QSL) concepts for highly frustrated organic charge-transfer solids such as kappa-(BEDT-TTF)_2Cu_2(CN)_3 and EtMe_3Sb[Pd(dmit)_2]_2, which we believe show frustration-induced charge disproportionation at low temperatures. We discuss possible application to layered cobaltates and 1/4-filled band spinels.

cond-mat.str-el↗

arXiv2010

Cooperative orbital ordering and Peierls instability in the checkerboard lattice with doubly degenerate orbitals

It has been suggested that the metal-insulator transitions in a number of spinel materials with partially-filled t_2g d-orbitals can be explained as orbitally-driven Peierls instabilities. Motivated by these suggestions, we examine theoretically the possibility of formation of such orbitally-driven states within a simplified theoretical model, a two-dimensional checkerboard lattice with two directional metal orbitals per atomic site. We include orbital ordering and inter-atom electron-phonon interactions self-consistently within a semi-classical approximation, and onsite intra- and inter-orbital electron-electron interactions at the Hartree-Fock level. We find a stable, orbitally-induced Peierls bond-dimerized state for carrier concentration of one electron per atom. The Peierls bond distortion pattern continues to be period 2 bond-dimerization even when the charge density in the orbitals forming the one-dimensional band is significantly smaller than 1. In contrast, for carrier density of half an electron per atom the Peierls instability is absent within one-electron theory as well as mean-field theory of electron-electron interactions, even for nearly complete orbital ordering. We discuss the implications of our results in relation to complex charge, bond, and orbital-ordering found in spinels.

cond-mat.str-el↗