arXiv subjects

H. Zhang

Publications and source records attributed to H. Zhang.

At least 235 records · Page 13Linked to original sources

arXiv2009

Trapping of dark vector solitons in a fiber laser

We report on the experimental observation of incoherently coupled dark vector soliton trapping in a fiber laser. Dark vector solitons along the two orthogonal polarization directions though with large difference in central frequency, energy and darkness could be still synchronously trapped as a group velocity locked dark vector soliton in virtue of the incoherent interactions. Our numerical simulation could well rebirth the experimental observations and confirm the existence of incoherently coupled dark vector soliton under certain conditions.

physics.optics↗

arXiv2009

Multiwavelength Passively Mode Locked Fiber Laser: all normal dispersion

We report on the generation of multiwavelength gain-guided soliton (GGS) in an all normal dispersion birefringent fiber laser passively mode-locked with a semiconductor saturable absorber mirror (SESAM). Determined by the cavity birefringence, we observe the stable single-, dual- and triple- wavelength mode locked GGSs, which is a natural consequence of the mutual nonlinear interaction among the normal cavity dispersion, cavity fiber nonlinear Kerr effect, laser gain saturation and bandwidth limitation resulting from the intra-cavity birefringent filter.

physics.optics↗

arXiv2009

Thermodynamic Fluctuations in Magnetic Phase Transition: Invar as a Prototype

We propose a first-principles formulism for system with spin fluctuations and apply it to the ordered Fe3Pt to uncover the Invar anomalies, including negative thermal expansion and spontaneous magnetization. The theory has coherently predicted the finite temperature intermixing between the fully ferromagnetic configuration and the spin-flipping configurations. We also discover a tri-critical point at which a high-temperature second-order phase transition, between the fully ferromagnetic configuration and the spin-flipping configurations, becomes first-order at low temperatures.

cond-mat.mtrl-sci↗

arXiv2009

Observation of high-order polarization-locked vector solitons in a fiber laser

We report on the experimental observation of a novel type of polarization locked vector soliton in a passively mode-locked fiber laser. The vector soliton is characterized by that not only the two orthogonally polarized soliton components are phase locked, but also one of the components has a double-humped intensity profile. Multiple such phase-locked high order vector solitons with identical soliton parameters and harmonic mode-locking of the vector solitons were also obtained in the laser. Numerical simulations confirmed the existence of stable high-order vector solitons in fiber lasers.

physics.optics↗

arXiv2009

Coherent energy exchange between vector soliton components in fiber lasers

We report on the experimental evidence of four wave mixing (FWM) between the two polarization components of a vector soliton formed in a passively mode-locked fiber laser. Extra spectral sidebands with out-of-phase intensity variation between the polarization resolved soliton spectra was firstly observed, which was identified to be caused by the energy exchange between the two soliton polarization components. Other features of the FWM spectral sidebands and the soliton internal FWM were also experimentally investigated and numerically confirmed.

physics.optics↗

arXiv2008

Comparison of Resonant Inelastic X-Ray Scattering Spectra and Dielectric Loss Functions in Copper Oxides

We report empirical comparisons of Cu K-edge indirect resonant inelastic x-ray scattering (RIXS) spectra, taken at the Brillouin zone center, with optical dielectric loss functions measured in a number of copper oxides. The RIXS data are obtained for Bi$_2$CuO$_4$, CuGeO$_3$, Sr$_2$Cu$_3$O$_4$Cl$_2$, La$_2$CuO$_4$, and Sr$_2$CuO$_2$Cl$_2$, and analyzed by considering both incident and scattered photon resonances. An incident-energy-independent response function is then extracted. The dielectric loss functions, measured with spectroscopic ellipsometry, agree well with this RIXS response, especially in Bi$_2$CuO$_4$ and CuGeO$_3$.

cond-mat.str-el↗

arXiv2008

First Study of Neutron Tagging with a Water Cherenkov Detector

A first study of neutron tagging is conducted in Super--Kamiokande, a 50,000-ton water Cherenkov detector. The tagging efficiencies of thermal neutrons are evaluated in a 0.2 % GdCl$_{3}$-water solution and pure water. They are determined to be, respectively, 66.7 % for events above 3 MeV and 20 % with corresponding background probabilities of 2 $\times$ 10$^{-4}$ and 3 $\times$ 10$^{-2}$. This newly developed technique may enable water Cherenkov detectors to identify $\bar ν_{e}$'s geological or astrophysical sources as well as those produced by commercial reactors via the delayed coincidence scheme.

hep-ex↗

arXiv2008

Pressure Effect on Superconductivity of Iron-based Arsenic-oxide ReFeAsO0.85 (Re=Sm and Nd)

Here we report pressure effect on superconducting transition temperature (Tc) of ReFeAsO0.85 (Re= Sm and Nd) system without fluorine doping. In-situ measurements under high pressure showed that Tc of the two compounds decrease monotonously over the pressure range investigated. The pressure coefficients dTc/dP in SmFeAsO0.85 and Nd FeAsO0.85 were different, revealing the important influence of the deformation in layers on Tc. Theoretical calculations suggested that the electron density of states decrease with increasing pressure, following the same trend of experimental data.

cond-mat.supr-con↗

arXiv2008

Current Helicity and Twist as Two Indicators of The Mirror Asymmetry of solar Magnetic Fields

A comparison between the two tracers of magnetic field mirror asymmetry in solar active regions, twist and current helicity, is presented. It is shown that for individual active regions these tracers do not possess visible similarity while averaging by time over the solar cycle, or by latitude, reveals similarities in their behaviour. The main property of the dataset is anti-symmetry over the solar equator. Considering the evolution of helical properties over the solar cycle we find signatures of a possible sign change at the beginning of the cycle, though more systematic observational data are required for a definite confirmation. We discuss the role of both tracers in the context of the solar dynamo theory.

astro-ph↗

arXiv2007

Disappearance of antiferromagnetic spin excitations in over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$

Magnetic excitations in the energy range up to 100 meV are studied for over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.25$ and 0.30, using time-of-flight neutron spectroscopy. Comparison of spectra integrated over the width of an antiferromagnetic Brillouin zone demonstrates that the magnetic scattering at intermediate energies, $20 \lesssim ω\lesssim 100$ meV, progressively decreases with over-doping. This strongly suggests that the magnetism is not related to Fermi surface nesting, but rather is associated with a decreasing volume fraction of (probably fluctuating) antiferromagnetic bubbles.

cond-mat.supr-con↗

arXiv2007

On a synthetical method to constrain 3-D structure of emission regions of pulsars

The main idea and procedure are presented for a new synthetical method on constraining 3-D structure of emission regions of pulsars. With this method the emission regions can be synthetically constrained by fitting multi-wavelength observations features, e.g. pulse widths, phase offsets between radio pulse profiles and high energy light curves, and radio polarization properties. The main technique is based on numerically calculating the emission directions along any open field line via including aberration and retardation effects under a certain magnetic field configuration, e.g. vacuum dipole field. It can be easily extended by involving some further effects or magnetic field line configuration. The role of observed linear polarization properties in the method is discussed. The future application to known X/gamma-ray and radio pulsars will be helpful to test and improve current theoretical models on pulsar emission.

astro-ph↗

arXiv2006

Indication for $π^+ π^-$ scattering in $p+p$ collisions at $\sqrt{s_{_{NN}}} = $200 GeV

A $ρ(770)^0$ mass shift of about -40 MeV/$c^2$ was measured in $p+p$ collisions at $\sqrt{s_{_{NN}}} = $ 200 GeV at RHIC. Previous mass shifts have been observed at CERN-LEBC-EHS and CERN-LEP. We will show that phase space does not account for the $ρ(770)^0$ mass shift measured at RHIC, CERN-LEBC-EHS and CERN-LEP and conclude that there are significant scattering interactions in $p+p$ collisions.

hep-ex↗

arXiv2006

The distribution of current helicity at the solar surface at the beginning of the solar cycle

A fraction of solar active regions are observed to have current helicity of a sign that contradicts the polarity law for magnetic helicity; this law corresponds to the well-known Hale polarity law for sunspots. A significant excess of active regions with the "wrong" sign of helicity is seen to occur just at the beginning of the cycle. We compare these observations with predictions from a dynamo model based on principles of helicity conservation, discussed by Zhang et al. (2006). This model seems capable of explaining only a fraction of the regions with the wrong sign of the helicity. We attribute the remaining excess to additional current helicity production from the twisting of rising magnetic flux tubes, as suggested by Choudhuri et al. (2004). We estimate the relative contributions of this effect and that connected with the model based on magnetic helicity conservation.

astro-ph↗

arXiv2005

Resonant inelastic X-ray scattering study of overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$

Resonant inelastic x-ray scattering (RIXS) at the copper K absorption edge has been performed for heavily overdoped samples of La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x= 0.25$ and 0.30. We have observed the charge transfer and molecular-orbital excitations which exhibit resonances at incident energies of $E_i= 8.992$ and 8.998 keV, respectively. From a comparison with previous results on undoped and optimally-doped samples, we determine that the charge-transfer excitation energy increases monotonically as doping increases. In addition, the $E_i$-dependences of the RIXS spectral weight and absorption spectrum exhibit no clear peak at $E_i = 8.998$ keV in contrast to results in the underdoped samples. The low-energy ($\leq 3$ eV) continuum excitation intensity has been studied utilizing the high energy resolution of 0.13 eV (FWHM). A comparison of the RIXS profiles at $(π~0)$ and $(π~π)$ indicates that the continuum intensity exists even at $(π~π)$ in the overdoped samples, whereas it has been reported only at $(0 ~0)$ and $(π~0)$ for the $x=0.17$ sample. Furthermore, we also found an additional excitation on top of the continuum intensity at the $(π~π)$ and $(π~0)$ positions.

cond-mat.supr-con↗

arXiv2005

Magnetic properties of the overdoped superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ with and without Zn impurities

The magnetic properties of the Zn-substituted overdoped high-$T_c$ superconductor La$_{2-x}$Sr$_{x}$Cu$_{1-y}$Zn$_{y}$O$_{4}$ have been studied by magnetization measurements and neutron scattering. Magnetization measurements reveal that for Zn-free samples with $x \geq 0.22$ a Curie term is induced in the temperature dependence of the magnetic susceptibility implying the existence of local paramagnetic moments. The induced Curie constant corresponds to a moment of 0.5 $μ_B$ per additional Sr$^{2+}$ ion that exceeds $x=0.22$. Zn-substitution in the overdoped \lsco also induces a Curie term that corresponds to 1.2 $μ_B$ per Zn$^{2+}$ ion, simultaneously suppressing $T_c$. The relationship between $T_c$ and the magnitude of the Curie term for Zn-free \lsco with $x \geq 0.22$ and for Zn-substituted \lsco with $x = 0.22$ are closely similar. This signifies a general competitive relationship between the superconductivity and the induced paramagnetic moment. Neutron scattering measurements show that Zn-substitution in overdoped \lsco anomalously enhances the inelastic magnetic scattering spectra around the $(π, π)$ position, peaking at $ω\sim 7$ meV. These facts are discussed on the basis of a "swiss-cheese" model of Zn-substituted systems as well as a microscopic phase separation scenario in the overdoped region indicated by muon-spin-relaxation measurements.

cond-mat.supr-con↗

arXiv2005

Some Global Aspects of Gauge Anomalies of Semisimple Gauge Groups and Fermion Generations in GUT and Superstring Theories

We study more extensively and completely for global gauge anomalies with some semisimple gauge groups as initiated in ref.1. A detailed and complete proof or derivation is provided for the Z_2 global gauge anomaly given in ref.1 for a gauge theory with the semisimple gauge group SU(2)\times SU(2)\times SU(2) in D=4 dimensions and Weyl fermions in the irreducible representation (IR) ω=(2,2,2) with 2 denoting the corresponding dimensions. This Z_2 anomaly was used in the discussions related to generic SO(10) and supersymmetric SO(10) unification theories^1 for the total generation numbers of fermions and mirror fermions. Our result^1 that the global anomaly coefficient formula is given by A(ω)=exp[iπQ_2(\Box)]=-1 in this case with Q_2(\Box) being the Dynkin index for SU(8) in the fundamental IR (\Box)=(8) is also discussed, and as shown by our results^1 that the semisimple gauge transformations collectively may have physical consequences which do not correspond to successive simple gauge transformations. The similar result given in ref.1 for the Z_2 global gauge anomaly of gauge group SU(2)\times SU(2) is also discussed. We also give a complete proof for some relevent topological results. Gauge anomalies for the relevant semisimple gauge groups are also briefly discussed in higher dimensions, especially for self-contragredient representations, with discussions involving trace identities relating to ref.14. We also remark the connection of our results and discussions to the total generation numbers in relevant theories.

hep-ph↗

arXiv2005

Delocalized fermions in underdoped cuprates

Low temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly-doped cuprate YBa$_{2}$Cu$_{3}$O$_{6.33}$. Samples were measured on either side of the superconducting phase boundary, in both zero and applied magnetic field. We report the observation of delocalized fermionic excitations at zero energy in the non-superconducting state, which shows that the ground state of underdoped cuprates is metallic. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e. nodal. The insulating ground state observed in underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ is attributed to the competing spin-density-wave order present in that system.

cond-mat.supr-con↗

arXiv2005

The Radial Distribution of Magnetic Helicity in the Solar Convective Zone: Observations and Dynamo Theory

We continue our attempt to connect observational data on current helicity in solar active regions with solar dynamo models. In addition to our previous results about temporal and latitudinal distributions of current helicity (Kleeorin et al. 2003), we argue that some information concerning the radial profile of the current helicity averaged over time and latitude can be extracted from the available observations. The main feature of this distribution can be presented as follows. Both shallow and deep active regions demonstrate a clear dominance of one sign of current helicity in a given hemisphere during the whole cycle. Broadly speaking, current helicity has opposite polarities in the Northern and Southern hemispheres, although there are some active regions that violate this polarity rule. The relative number of active regions violating the polarity rule is significantly higher for deeper active regions. A separation of active regions into `shallow', `middle' and `deep' is made by comparing their rotation rate and the helioseismic rotation law. We use a version of Parker's dynamo model in two spatial dimensions, that employs a nonlinearity based on magnetic helicity conservation arguments. The predictions of this model about the radial distribution of solar current helicity appear to be in remarkable agreement with the available observational data; in particular the relative volume occupied by the current helicity of "wrong" sign grows significantly with the depth.

astro-ph↗