arXiv subjects

H. Li

Publications and source records attributed to H. Li.

At least 217 records · Page 12Linked to original sources

arXiv2010

The Paired-Electron Crystal in the Two-Dimensional Frustrated Quarter-Filled Band

The competition between antiferromagnetic and spin-singlet ground states within quantum spin models and the 1/2-filled band Hubbard model has received intense scrutiny. Here we demonstrate a frustration-induced transition from Néel antiferromagnetism to spin-singlet in the interacting 1/4-filled band on an anisotropic triangular lattice. While the antiferromagnetic state has equal charge densities 0.5 on all sites, the spin-singlet state is a paired-electron crystal, with pairs of charge-rich sites separated by pairs of charge-poor sites. The paired-electron crystal provides a natural description of the spin-gapped state proximate to superconductivity in many organic charge-transfer solids. Pressure-induced superconductivity in these correlated-electron systems is likely a transition from the 1/4-filled band valence bond solid to a valence bond liquid.

cond-mat.str-el↗

arXiv2009

Experimental Probing of Photonic Density of States in Hyperbolic Metamaterial

In the metamaterial with hyperbolic dispersion (an array of silver nanowires in alumina membrane) we have observed six-fold reduction of the emission life-time of dye deposited onto the metamaterials surface. This serves as the evidence of the earlier predicted high density of photonic states in hyperbolic metamaterials.

physics.optics↗

arXiv2009

Bipolaron Density-Wave Driven By Antiferromagnetic Correlations and Frustration in Organic Superconductors

We describe the Paired Electron Crystal (PEC) which occurs in the interacting frustrated two-dimensional 1/4-filled band. The PEC is a charge-ordered state with nearest-neighbor spin singlets separated by pairs of vacant sites, and can be thought of as a bipolaron density wave. The PEC has been experimentally observed in the insulating state proximate to superconductivity in the organic charge-transfer solids. Increased frustration drives a PEC-to-superconductor transition in these systems.

cond-mat.str-el↗

arXiv2009

Non-commutative Combinatorial Inverse Systems

We introduce the notion of a combinatorial inverse system in non-commutative variables. We present two important examples, some conjectures and results. These conjectures and results were suggested and supported by computer investigations.

math.RA↗

arXiv2009

Polarimetry of DG Tau at 350 microns

We present the first 350 micron polarization measurement for the disk of the T Tauri star (TTS) DG Tau. The data were obtained using the SHARP polarimeter at the Caltech Submillimeter Observatory. We did not detect polarization but we provide constraints on the normalized Stokes parameters q and u. We derive information about the polarization spectrum by comparing our 350 micron result with an 850 micron polarization detection previously published for this source. This empirical information concerning the polarization spectrum disagrees with the predictions of a recent model for TTS disk polarization. We conclude, however, that adding more mass to the model disk would probably result in model polarization spectra that agree much better with the 350 and 850 micron polarimetric observations. We suggest that multiwavelength polarimetry of TTS disk emission may provide a promising method for probing the opacity of TTS disks.

astro-ph.SR↗

arXiv2009

The Need for Plasma Astrophysics in Understanding Life Cycles of Active Galaxies

In this White Paper, we emphasize the need for and the important role of plasma astrophysics in the studies of formation, evolution of, and feedback by Active Galaxies. We make three specific recommendations: 1) We need to significantly increase the resolution of VLA, perhaps by building an EVLA-II at a modest cost. This will provide the angular resolution to study jets at kpc scales, where, for example, detailed Faraday rotation diagnosis can be done at 1GHz transverse to jets; 2) We need to build coordinated programs among NSF, NASA, and DOE to support laboratory plasma experiments (including liquid metal) that are designed to study key astrophysical processes, such as magneto-rotational instability (origin of angular momentum transport), dynamo (origin of magnetic fields), jet launching and stability. Experiments allowing access to relativistic plasma regime (perhaps by intense lasers and magnetic fields) will be very helpful for understanding the stability and dissipation physics of jets from Supermassive Black Holes; 3) Again through the coordinated support among the three Agencies, we need to invest in developing comprehensive theory and advanced simulation tools to study the accretion disks and jets in relativistic plasma physics regime, especially in connecting large scale fluid scale phenomena with relativistic kinetic dissipation physics through which multi-wavelength radiation is produced.

astro-ph.HE↗

arXiv2009

Dispersion of Observed Position Angles of Submillimeter Polarization in Molecular Clouds

One can estimate the characteristic magnetic field strength in GMCs by comparing submillimeter polarimetric observations of these sources with simulated polarization maps developed using a range of different values for the assumed field strength. The point of comparison is the degree of order in the distribution of polarization position angles. In a recent paper by H. Li and collaborators, such a comparison was carried out using SPARO observations of two GMCs, and employing simulations by E. Ostriker and collaborators. Here we reexamine this same question, using the same data set and the same simulations, but using an approach that differs in several respects. The most important difference is that we incorporate new, higher angular resolution observations for one of the clouds, obtained using the Hertz polarimeter. We conclude that the agreement between observations and simulations is best when the total magnetic energy (including both uniform and fluctuating field components) is at least as large as the turbulent kinetic energy.

astro-ph↗

arXiv2008

Type I Planet Migration in Nearly Laminar Disks

We describe 2D hydrodynamic simulations of the migration of low-mass planets ($\leq 30 M_{\oplus}$) in nearly laminar disks (viscosity parameter $α< 10^{-3}$) over timescales of several thousand orbit periods. We consider disk masses of 1, 2, and 5 times the minimum mass solar nebula, disk thickness parameters of $H/r = 0.035$ and 0.05, and a variety of $α$ values and planet masses. Disk self-gravity is fully included. Previous analytic work has suggested that Type I planet migration can be halted in disks of sufficiently low turbulent viscosity, for $α\sim 10^{-4}$. The halting is due to a feedback effect of breaking density waves that results in a slight mass redistribution and consequently an increased outward torque contribution. The simulations confirm the existence of a critical mass ($M_{cr} \sim 10 M_{\oplus}$) beyond which migration halts in nearly laminar disks. For $α\ga 10^{-3}$, density feedback effects are washed out and Type I migration persists. The critical masses are in good agreement with the analytic model of Rafikov (2002). In addition, for $α\la 10^{-4}$ steep density gradients produce a vortex instability, resulting in a small time-varying eccentricity in the planet's orbit and a slight outward migration. Migration in nearly laminar disks may be sufficiently slow to reconcile the timescales of migration theory with those of giant planet formation in the core accretion model.

astro-ph↗

arXiv2008

A Fast Potential and Self-Gravity Solver for Non-Axisymmetric Disks

Disk self-gravity could play an important role in the dynamic evolution of interaction between disks and embedded protoplanets. We have developed a fast and accurate solver to calculate the disk potential and disk self-gravity forces for disk systems on a uniform polar grid. Our method follows closely the method given by Chan et al. (2006), in which an FFT in the azimuthal direction is performed and a direct integral approach in the frequency domain in the radial direction is implemented on a uniform polar grid. This method can be very effective for disks with vertical structures that depend only on the disk radius, achieving the same computational efficiency as for zero-thickness disks. We describe how to parallelize the solver efficiently on distributed parallel computers. We propose a mode-cutoff procedure to reduce the parallel communication cost and achieve nearly linear scalability for a large number of processors. For comparison, we have also developed a particle-based fast tree-code to calculate the self-gravity of the disk system with vertical structure. The numerical results show that our direct integral method is at least two order of magnitudes faster than our optimized tree-code approach.

astro-ph↗

arXiv2008

Local singlets, frustration, and unconventional superconductivity in the organic charge-transfer solids

We suggest that superconductivity (SC) in the organic charge transfer solids (CTS) is reached from a Bond-Charge Density Wave (BCDW). We discuss an effective model for the BCDW to SC transition, an attractive U extended Hubbard Hamiltonian with repulsive nearest neighbor interaction V. We discuss experimental consequences of the theory for different classes of CTS superconductors as well as related inorganic materials.

cond-mat.supr-con↗

arXiv2008

Bulk photonic metamaterial with hyperbolic dispersion

In this work, we demonstrate a self-standing bulk three-dimensional metamaterial based on the network of silver nanowires in an alumina membrane. This constitutes an anisotropic effective medium with hyperbolic dispersion, which can be used in sub-diffraction imaging or optical cloaks. Highly anisotropic dielectric constants of the material range from positive to negative, and the transmitted laser beam shifts both toward the normal to the surface, as in regular dielectrics, and off the normal, as in anisotropic dielectrics with the refraction index smaller than one. The designed photonic metamaterial is the thickest reported in the literature, both in terms of its physical size 1cm x 1cm x 51 mm, and the number of vacuum wavelengths, N=61 at l=0.84 mm.

cond-mat.mtrl-sci↗

arXiv2008

Statistical Global Modeling of Beta-Decay Halflives Systematics Using Multilayer Feedforward Neural Networks and Support Vector Machines

In this work, the beta-decay halflives problem is dealt as a nonlinear optimization problem, which is resolved in the statistical framework of Machine Learning (LM). Continuing past similar approaches, we have constructed sophisticated Artificial Neural Networks (ANNs) and Support Vector Regression Machines (SVMs) for each class with even-odd character in Z and N to global model the systematics of nuclei that decay 100% by the beta-minus-mode in their ground states. The arising large-scale lifetime calculations generated by both types of machines are discussed and compared with each other, with the available experimental data, with previous results obtained with neural networks, as well as with estimates coming from traditional global nuclear models. Particular attention is paid on the estimates for exotic and halo nuclei and we focus to those nuclides that are involved in the r-process nucleosynthesis. It is found that statistical models based on LM can at least match or even surpass the predictive performance of the best conventional models of beta-decay systematics and can complement the latter.

nucl-th↗

arXiv2008

Entanglement dynamics of two-qubit system in different types of noisy channels

In this paper, we study entanglement dynamics of a two-qubit extended Werner-like state locally interacting with independent noisy channels, i.e., amplitude damping, phase damping and depolarizing channels. We show that the purity of initial entangled state has direct impacts on the entanglement robustness in each noisy channel. That is, if the initial entangled state is prepared in mixed instead of pure form, the state may exhibit entanglement sudden death (ESD) and/or be decreased for the critical probability at which the entanglement disappear.

quant-ph↗

arXiv2008

The entanglement in one-dimensional random XY spin chain with Dzyaloshinskii-Moriya interaction

The impurities of exchange couplings, external magnetic fields and Dzyaloshinskii--Moriya (DM) interaction considered as Gaussian distribution, the entanglement in one-dimensional random $XY$ spin systems is investigated by the method of solving the different spin-spin correlation functions and the average magnetization per spin. The entanglement dynamics at central locations of ferromagnetic and antiferromagnetic chains have been studied by varying the three impurities and the strength of DM interaction. (i) For ferromagnetic spin chain, the weak DM interaction can improve the amount of entanglement to a large value, and the impurities have the opposite effect on the entanglement below and above critical DM interaction. (ii) For antiferromagnetic spin chain, DM interaction can enhance the entanglement to a steady value. Our results imply that DM interaction strength, the impurity and exchange couplings (or magnetic field) play competing roles in enhancing quantum entanglement.

quant-ph↗

arXiv2008

Entanglement and quantum phase transition in alternating XY spin chain with next-nearest neighbour interactions

By using the method of density-matrix renormalization-group to solve the different spin-spin correlation functions, the nearest-neighbouring entanglement(NNE) and next-nearest-neighbouring entanglement(NNNE) of one-dimensional alternating Heisenberg XY spin chain is investigated in the presence of alternating nearest neighbour interactions of exchange couplings, external magnetic fields and next-nearest neighbouring interactions. For dimerized ferromagnetic spin chain, NNNE appears only above the critical dimerized interaction, meanwhile, the dimerized interaction effects quantum phase transition point and improves NNNE to a large value. We also study the effect of ferromagnetic or antiferromagnetic next-nearest neighboring (NNN) interactions on the dynamics of NNE and NNNE. The ferromagnetic NNN interaction increases and shrinks NNE below and above critical frustrated interaction respectively, while the antiferromagnetic NNN interaction always decreases NNE. The antiferromagnetic NNN interaction results to a larger value of NNNE in comparison to the case when the NNN interaction is ferromagnetic.

quant-ph↗

arXiv2008

Entanglement control in one-dimensional $s=1/2$ random XY spin chain

The entanglement in one-dimensional random XY spin systems where the impurities of exchange couplings and the external magnetic fields are considered as random variables is investigated by solving the different spin-spin correlation functions and the average magnetization per spin. The entanglement dynamics near particular locations of the system is also studied when the exchange couplings (or the external magnetic fields) satisfy three different distributions(the Gaussian distribution, double-Gaussian distribution, and bimodal distribution). We find that the entanglement can be controlled by varying the strength of external magnetic field and the different distributions of impurities. Moreover, the entanglement of some nearest-neighboring qubits can be increased for certain parameter values of the three different distributions.

quant-ph↗

arXiv2008

Design and Electronics Commissioning of the Physics Prototype of a Si-W Electromagnetic Calorimeter for the International Linear Collider

The CALICE collaboration is studying the design of high performance electromagnetic and hadronic calorimeters for future International Linear Collider detectors. For the electromagnetic calorimeter, the current baseline choice is a high granularity sampling calorimeter with tungsten as absorber and silicon detectors as sensitive material. A ``physics prototype'' has been constructed, consisting of thirty sensitive layers. Each layer has an active area of 18x18 cm2 and a pad size of 1x1 cm2. The absorber thickness totals 24 radiation lengths. It has been exposed in 2006 and 2007 to electron and hadron beams at the DESY and CERN beam test facilities, using a wide range of beam energies and incidence angles. In this paper, the prototype and the data acquisition chain are described and a summary of the data taken in the 2006 beam tests is presented. The methods used to subtract the pedestals and calibrate the detector are detailed. The signal-over-noise ratio has been measured at 7.63 +/- 0.01. Some electronics features have been observed; these lead to coherent noise and crosstalk between pads, and also crosstalk between sensitive and passive areas. The performance achieved in terms of uniformity and stability is presented.

physics.ins-det↗

arXiv2008

From valence bond solid to unconventional superconductivity in the organic charge-transfer solids

We show that superconductivity is absent within the 1/2-filled band triangular lattice repulsive Hubbard model that has been proposed for organic charge-transfer solids. We posit that organic superconductivity is rather reached from a Bond-Charge Density Wave that either constitutes the insulating state proximate to superconductivity, or is extremely close in energy to the antiferromagnetic state, and replaces the latter under pressure. The Bond-Charge Density Wave can be described within an effective attractive $U$ extended Hubbard Hamiltonian with repulsive nearest neighbor interaction $V$. A first-order transition from the insulating to the superconducting state occurs within the model with increasing frustration.

cond-mat.supr-con↗