arXiv subjects

Y. Zhang

Publications and source records attributed to Y. Zhang.

At least 613 records · Page 34Linked to original sources

arXiv2008

Diversity of rationality affects cooperation in spatial prisoner's dilemma game

In real world, individual rationality varies for the sake of the diversity of people's individuality. In order to investigate how diversity of agent's rationality affects the evolution of cooperation, we introduce the individual rationality proportional to the $β$th power of the each agent's degree. Simulation results on heterogeneous scale-free network show that the dynamic process is greatly affected by the diversity of rationality. Both promotion and inhibition of cooperative behavior can be observed at different region of parameter $β$. We present explanation to these results by quantitative and qualitative analysis. The nodes with middle degree value are found to play a critical role in the evolutionary processes. The inspiration from our work may provide us a deeper comprehension towards some social phenomenon.

physics.soc-ph↗

arXiv2008

First principles perspective on the microscopic model for Cs2CuCl4 and Cs2CuBr4

We investigate the microscopic model for the frustrated layered antiferromagnets Cs2CuCl4 and Cs2CuBr4 by performing ab initio density functional theory (DFT) calculations and with the help of the tight-binding method. The combination of both methods provide the relevant interaction paths in these materials, and we estimate the corresponding exchange constants. We find for Cs2CuCl4 that the calculated ratio of the strongest in-plane exchange constants J'/J between the spin-1/2 Cu ions agrees well with neutron scattering experiments. The microscopic model based on the derived exchange constants is tested by comparing the magnetic susceptibilities obtained from exact diagonalization with experimental data. The electronic structure differences between Cs2CuCl4 and Cs2CuBr4 are also analyzed.

cond-mat.str-el↗

arXiv2008

Analytic Spectra of CMB Anisotropies and Polarization Generated by Relic Gravitational Waves with Modification due to Neutrino Free-Streaming

We present an analytical calculation of the spectra of CMB anisotropies and polarizations generated by relic gravitational waves (RGWs). As a substantial extension to the previous studies, three new ingredients are included in this work. Firstly, the analytic $C_l^{TT}$ and $C_l^{TE}$ are given; especially the latter can be useful to extract signal of RGWs from the observed data in the zero multipole method. Secondly, a fitting formula of the decaying factor on small scales is given, coming from the visibility function around the photon decoupling. Thirdly, the impacts by the neutrino free-streaming (NFS) is examined, a process that occurred in the early universe and leaves observable imprints on CMB via RGWs. It is found that the analytic $C_l^{TT}$ and $C_l^{TE}$ have profiles agreeing with the numeric ones, except that $C^{TT}_l$ in a range $l \le 10$ and the $1^{st}$ trough of $C_l^{TE}$ around $l \sim 75$ have some deviations. With the new damping factor, the analytic $C^{EE}_l$ and $C^{BB}_l$ match with the numeric ones with the maximum errors only $\sim 3%$ up to the first three peaks for $l\le 600$, improving the previous studies substantially. The correspondence of the positions of peaks of $C^{XX}_l$ and those of RGWs are also demonstrated explicitly. We also find that NFS reduces the amplitudes of $C^{XX}_l$ by $(20% \sim 35%)$ for $l\simeq(100\sim 600)$ and shifts slightly their peaks to smaller angles. Detailed analyses show that the zero multipoles $l_0$, where $C_l^{TE}$ crosses 0, are shifted to larger values by NFS. This shifting effect is as important as those causedby different inflation models and different baryon fractions.

astro-ph↗

arXiv2008

Primary role of the barely occupied states in the charge density wave formation of NbSe2

NbSe2 is a prototypical charge-density-wave (CDW) material, whose mechanism remains mysterious so far. With angle resolved photoemission spectroscopy, we mapped out the CDW gap and recovered the long-lost nesting condition over a large broken-honeycomb region in the Brillouin zone, which consists of six saddle band point regions with high density of states (DOS), and large regions away from Fermi surface with negligible DOS at the Fermi energy. We show that the major contributions to the CDW come from these barely occupied states rather than the saddle band points. Our findings not only resolve a long standing puzzle, but also overthrow the conventional wisdom that CDW is dominated by regions with high DOS.

cond-mat.str-el↗

arXiv2008

O- and H- induced surface core level shifts on Ru(0001): Prevalence of the additivity rule

In previous work on adsorbate-induced surface core level shifts (SCLSs), the effects caused by O atom adsorption on Rh(111) and Ru(0001) were found to be additive: the measured shifts for first layer Ru atoms depended linearly on the number of directly coordinated O atoms. Density-functional theory calculations quantitatively reproduced this effect, allowed separation of initial and final state contributions, and provided an explanation in terms of a roughly constant charge transfer per O atom. We have now conducted similar measurements and calculations for three well-defined adsorbate and coadsorbate layers containing O and H atoms: (1 x 1)-H, (2 x 2)-(O+H), and (2 x 2)-(O+3H) on Ru(0001). As H is stabilized in fcc sites in the prior two structures and in hcp sites in the latter, this enables us to not only study coverage and coadsorption effects on the adsorbate-induced SCLSs, but also the sensitivity to similar adsorption sites. Remarkably good agreement is obtained between experiment and calculations for the energies and geometries of the layers, as well as for all aspects of the SCLS values. The additivity of the next-neighbor adsorbate-induced SCLSs is found to prevail even for the coadsorbate structures. While this confirms the suggested use of SCLSs as fingerprints of the adsorbate configuration, their sensitivity is further demonstrated by the slightly different shifts unambiguously determined for H adsorption in either fcc or hcp hollow sites.

cond-mat.mtrl-sci↗

arXiv2008

Correlation effects of exchange splitting and coexistence of spin-density-wave and superconductivity in single crystalline Sr1-xKxFe2As2

The nature of spin-density wave and its relation with superconductivity are crucial issues in the newly discovered Fe-based high temperature superconductors. Particularly it is unclear whether the superconducting phase and spin density wave (SDW) are truly exclusive from each other as suggested by certain experiments. With angle resolved photoemission spectroscopy, we here report exchange splittings of the band structures in Sr1-xKxFe2As2 (x=0,0.1,0.2), and the non-rigid-band behaviors of the splitting. Our data on single crystalline superconducting samples unambiguously prove that SDW and superconductivity could coexist in iron-pnictides.

cond-mat.supr-con↗

arXiv2008

Relic Gravitational Waves And CMB Polarization In The Accelerating Universe

In this paper we briefly present our works on the relic gravitational waves (RGW) and the CMB polarization in the accelerating universe. The spectrum of RGW has been obtained, showing the influence of the dark energy. Compared with those from non-accelerating models, the shape of the spectrum is approximately similar, nevertheless, the amplitude of RGW now acquires a suppressing factor of the ratio of matter over dark energy $\propto Ω_m/Ω_Λ\sim 0.4$ over almost the whole range of frequencies. The RGW spectrum is then used as the source to calculate the spectra of CMB polarization. By a two half Gaussian function as an approximation to the visibility function during the photon decoupling, both the "electric" and "magnetic" spectra have been analytically derived, which are quite close to the numerical ones. Several physical elements that affect the spectra have been examined, such as the decoupling process, the inflation, the dark energy, the baryons, etc.

astro-ph↗

arXiv2008

Electronic structure and exotic exchange splitting in spin-density-wave states of BaFe$_2$As$_2$

The magnetic properties in the parent compounds are often intimately related to the microscopic mechanism of superconductivity. Here we report the first direct measurements on the electronic structure of a parent compound of the newly discovered iron-based superconductor, BaFe$_2$As$_2$, which provides a foundation for further studies. We show that the energy of the spin density wave (SDW) in BaFe$_2$As$_2$ is lowered through exotic exchange splitting of the band structure, rather than Fermi surface nesting of itinerant electrons. This clearly demonstrates that a metallic SDW state could be solely induced by interactions of local magnetic moments, resembling the nature of antiferromagnetic order in cuprate parent compounds.

cond-mat.supr-con↗

arXiv2008

Nonlinear current-voltage behavior and electrically driven phase transition in charge frustrated LuFe2O4

Electric transport measurements of the charge frustrated LuFe2O4, in which the charge ordering (CO) and electronic ferroelectricity are found, reveal strong nonlinear electric conduction upon application of electrical field in both single crystalline and polycrystalline samples. The threshold electric fields (Et) in single crystalline LuFe2O4 are estimated respectively to be about 60V/cm and 10V/cm with E parallel and perpendicular to the c-axis direction. Experimental measurements also demonstrate that the I-V nonlinearity increases quickly with lowering temperature. Furthermore, our in-situ TEM investigations evidently reveal that the nonlinear I-V behavior is intrinsically in correlation with a current driven charge ordering insulator-metal transition, and the applied electrical field triggers a visible CO collapse recognizable as the fading of satellite spots of the CO modulations.

cond-mat.str-el↗

arXiv2008

Novel electronic structure induced by a highly strained oxide interface with incommensurate crystal fields

The misfit oxide, Bi$_{2}$Ba$_{1.3}$K$_{0.6}$Co$_{2.1}$O$_{y}$, made of alternating rocksalt-structured [BiO/BaO] layers and hexagonal CoO$_{2}$ layers, was studied by angle-resolved photoemission spectroscopy. Detailed electronic structure of such a highly strained oxide interfaces is revealed for the first time. We found that under the two incommensurate crystal fields, electrons are confined within individual sides of the interface, and scattered by umklapp scattering of the crystal field from the other side. In addition, the high strain on the rocksalt layer raises its chemical potential and induces large charge transfer to the CoO$_{2}$ layer. Furthermore, a novel interface effects, the interfacial enhancement of electron-phonon interactions, is discovered. Our findings of these electronic properties lay a foundation for designing future functional oxide interfaces.

cond-mat.str-el↗

arXiv2008

Doping-insensitive density-of-states suppression in polycrystalline iron-based superconductor SmO$_{1-x}$F$_{x}$FeAs

We investigated the temperature dependence of the density-of-states in the iron-based superconductor SmO_1-xF_xFeAs (x=0, 0.12, 0.15, 0.2) with high resolution angle-integrated photoemission spectroscopy. The density-of-states suppression is observed with decreasing temperature in all samples, revealing two characteristic energy scales (10meV and 80meV). However, no obvious doping dependence is observed. We argue that the 10meV suppression is due to an anomalously doping-independent normal state pseudogap, which becomes the superconducting gap once in the superconducting state; and alert the possibility that the 80meV-scale suppression might be an artifact of the polycrystalline samples.

cond-mat.supr-con↗

arXiv2008

An Updated Search of Steady TeV $γ-$Ray Point Sources in Northern Hemisphere Using the Tibet Air Shower Array

Using the data taken from Tibet II High Density (HD) Array (1997 February-1999 September) and Tibet-III array (1999 November-2005 November), our previous northern sky survey for TeV $γ-$ray point sources has now been updated by a factor of 2.8 improved statistics. From $0.0^{\circ}$ to $60.0^{\circ}$ in declination (Dec) range, no new TeV $γ-$ray point sources with sufficiently high significance were identified while the well-known Crab Nebula and Mrk421 remain to be the brightest TeV $γ-$ray sources within the field of view of the Tibet air shower array. Based on the currently available data and at the 90% confidence level (C.L.), the flux upper limits for different power law index assumption are re-derived, which are approximately improved by 1.7 times as compared with our previous reported limits.

astro-ph↗

arXiv2008

A spectral line survey in the 2 mm and 1.3 mm windows toward the carbon rich envelope of IRC +10216

We present the results of our spectral line surveys in the 2 mm and 1.3 mm windows toward the carbon rich envelope of IRC +10216. Totally 377 lines are detected, among which 360 lines are assigned to 57 known molecules (including 29 rare isotopomers and 2 cyclic isomers). Only 17 weak lines remain unidentified. Rotational lines of isotopomers 13CCH and HN13C are detected for the first time in IRC +10216. The detection of the formaldehyde lines in this star is also confirmed. Possible abundance difference among the three 13C substituted isotopic isomers of HC3N is reported. Isotopic ratios of C and O are confirmed to be non-solar while those of S and Si to be nearly solar. Column densities have been estimated for 15 molecular species. Modified spectroscopic parameters have been calculated for NaCN, Na13CN, KCN and SiC2. Transition frequencies from the present observations were used to improve the spectroscopic parameters of Si13CC, 29SiC2 and 30SiC2.

astro-ph↗

arXiv2008

Angle integrated photoemission study of SmO$_{0.85}$F$_{0.15}$FeAs

The electronic structure of the new superconductor, SmO$_{1-x}$F$_x$FeAs ($x=0.15$), has been studied by angle-integrated photoemission spectroscopy. Our data show a sharp feature very close to the Fermi energy, and a relative flat distribution of the density of states between 0.5 eV and 3 eV binding energy, which agrees best with band structure calculations considering an antiferromagnetic ground state. No noticeable gap opening was observed at 12 Kelvin below the superconducting transition temperature, indicating the existence of large ungapped regions in the Brillouin zone.

cond-mat.supr-con↗

arXiv2008

Comparison between experiment and calculated band structures for DyN and SmN

We investigate the electronic band structure of two of the rare-earth nitrides, DyN and SmN. Resistivity measurements imply that both materials have a semiconducting ground state, and both show resistivity anomalies coinciding with the magnetic transition, despite the different magnetic states in DyN and SmN. X-ray absorption and emission measurements are in excellent agreement with LSDA+U calculations, although for SmN the calculations predict a zero band gap.

cond-mat.str-el↗

arXiv2008

Multi-ferroelectricity in charge ordered LuFe2O4(LuFeO3)n with n=1

Electronic ferroelectricity from charge ordering (CO) is currently a significant issue that has been extensively investigated in the charge/spin frustrated LuFe2O4 system. Chemical substitution and structural layer intercalation have been considered as potentially effective approaches in our recent study, and we herein report on the structural and multi-ferromagnetic properties in the layered series of LuFe2O4(LuFeO3)n (n=0, 1, and 2). Experimental investigations reveal that the (RFeO3)-layer intercalation results in notable changes in ferroelectricity, magnetic properties and CO features. These results demonstrate that further exploration of this series of layered members might prove fruitful.

cond-mat.str-el↗

arXiv2008

Pion Production by Protons on a Thin Beryllium Target at 6.4, 12.3, and 17.5 GeV/c Incident Proton Momenta

An analysis of inclusive pion production in proton-beryllium collisions at 6.4, 12.3, and 17.5 GeV/c proton beam momentum has been performed. The data were taken by Experiment 910 at the Alternating Gradient Synchrotron at the Brookhaven National Laboratory. The differential $π^+$ and $π^-$ production cross sections ($d^2σ/dpdΩ$) are measured up to 400 mRad in $θ_π$ and up to 6 GeV/c in $p_π$. The measured cross section is fit with a Sanford-Wang parameterization.

nucl-ex↗

arXiv2008

Superconducting coherence peak in the electronic excitations of a single layer cuprate superconductor $Bi_2 Sr_{1.6} La_{0.4} Cu O_{6+δ}$

Angle resolved photoemission spectroscopy study is reported on a high quality optimally doped Bi2Sr1.6La0.4CuO6+delta high Tc superconductor. In the antinodal region with maximal d-wave gap, the symbolic superconducting coherence peak, which has been widely observed in multi-CuO2-layer cuprate superconductors, is unambiguously observed in a single layer system. The associated peak-dip separation is just about 19 meV, which is much smaller than its counterparts in multi-layered compounds, but correlates with the energy scales of spin excitations in single layer cuprates.

cond-mat.supr-con↗