arXiv subjects

P. Zhang

Publications and source records attributed to P. Zhang.

At least 145 records · Page 8Linked to original sources

arXiv2013

Hot-carrier transport and spin relaxation on the surface of topological insulator

We study the charge and spin transport under high electric field (up to several kV/cm) on the surface of topological insulator Bi$_2$Se$_3$, where the electron-surface optical phonon scattering dominates except at very low temperature. Due to the spin mixing of conduction and valence bands, the electric field not only accelerates electrons in each band, but also leads to inter-band precession. In the presence of the electric field, electrons can transfer from the valence band to the conduction one via the inter-band precession and inter-band electron-phonon scattering. The electron density in each band varies with the electric field linearly when the electric field is strong. Due to the spin-momentum locking, a transverse spin polarization, with the magnitude proportional to the momentum scattering time, is induced by the electric field. The induced spin polarization depends on the electric field linearly when the latter is small. Moreover, its magnitude is inversely proportional to the temperature and is insensitive to the electron density at high temperature. Our investigation also reveals that due to the large relative static dielectric constant, the Coulomb scattering is too weak to establish a drifted Fermi distribution with a unified hot-electron temperature in the steady state under the electric field. After turning off the electric field in the steady state, the hot carriers cool down in a time scale of energy relaxation which is very long (of the order of 100-1000 ps) while the spin polarization relaxes in a time scale of momentum scattering which is quite short (of the order of 0.01-0.1 ps).

cond-mat.mes-hall↗

arXiv2012

Electronic band structure of BaCo$_{2}$As$_2$: a fully-doped ferropnictide with reduced electronic correlations

We report an angle-resolved photoemission spectroscopy investigation of the Fermi surface and electronic band structure of BaCo$_{2}$As$_2$. Although its quasi-nesting-free Fermi surface differs drastically from that of its Fe-pnictide cousins, we show that the BaCo$_{2}$As$_2$ system can be used as an approximation to the bare unoccupied band structure of the related BaFe$_{2-x}$Co$_x$As$_2$ and Ba$_{1-x}$K$_x$Fe$_2$As$_2$ compounds. However, our experimental results, in agreement with dynamical mean field theory calculations, indicate that electronic correlations are much less important in BaCo$_{2}$As$_2$ than in the ferropnictides. Our findings suggest that this effect is due to the increased filling of the electronic 3$d$ shell in the presence of significant Hund's exchange coupling.

cond-mat.str-el↗

arXiv2012

Performance of a Downconverter Test-electronics with MTCA-based Digitizers for Beam Position Monitoring in 3.9 GHz Accelerating Cavities

Beam-excited higher order modes (HOM) in 3.9GHz accelerating cavities are planned to be used for beam position monitoring at the European XFEL. The selected HOMs are located around 5440MHz, with a bandwidth of 100MHz and 9060MHz, with a bandwidth of 50MHz. A downconverter electronics, built for tests at FLASH, converts the HOMs to an intermediate frequency of 70MHz. The MTCA (Micro Telecommunications Computing Architecture) standard will be used for the XFEL. Thus it is important to have a performance study of the downconverter test-electronics using the MTCA digitizer card SIS8300. In the digitizer the IF frequency of 70MHz is undersampled with a clock frequency of 108MS/s. The paper presents the performance of the digitizer together with the test-electronics. A comparison with a 216MS/s VME (Versa Module Eurocard) digitizer is made.

physics.acc-ph↗

arXiv2012

Long-term Monitoring on Mrk 501 for Its VHE gamma Emission and a Flare in October 2011

As one of the brightest active blazars in both X-ray and very high energy $γ$-ray bands, Mrk 501 is very useful for physics associated with jets from AGNs. The ARGO-YBJ experiment is monitoring it for $γ$-rays above 0.3 TeV since November 2007. Starting from October 2011 the largest flare since 2005 is observed, which lasts to about April 2012. In this paper, a detailed analysis is reported. During the brightest $γ$-rays flaring episodes from October 17 to November 22, 2011, an excess of the event rate over 6 $σ$ is detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase of the $γ$-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steady emission. In particular, the $γ$-ray flux above 8 TeV is detected with a significance better than 4 $σ$. Based on time-dependent synchrotron self-Compton (SSC) processes, the broad-band energy spectrum is interpreted as the emission from an electron energy distribution parameterized with a single power-law function with an exponential cutoff at its high energy end. The average spectral energy distribution for the steady emission is well described by this simple one-zone SSC model. However, the detection of $γ$-rays above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and $γ$-rays are also investigated.

astro-ph.HE↗

arXiv2012

A Study of Beam Position Diagnostics with Beam-excited Dipole Higher Order Modes using a Downconverter Test Electronics in Third Harmonic 3.9 GHz Superconducting Accelerating Cavities at FLASH

Beam-excited higher order modes (HOM) in accelerating cavities contain transverse beam position information. Previous studies have narrowed down three modal options for beam position diagnostics in the third harmonic 3.9 GHz cavities at FLASH. Localized modes in the beam pipes at approximately 4.1 GHz and in the fifth cavity dipole band at approximately 9 GHz were found, that can provide a local measurement of the beam position. In contrast, propagating modes in the first and second dipole bands between 4.2 and 5.5 GHz can reach a better resolution. All the options were assessed with a specially designed test electronics built by Fermilab. The aim is to define a mode or spectral region suitable for the HOM electronics. Two data analysis techniques are used and compared in extracting beam position information from the dipole HOMs: direct linear regression and singular value decomposition. Current experiments suggest a resolution of 50 μm accuracy in predicting local beam position using modes in the fifth dipole band, and a global resolution of 20 μm over the complete module. Based on these results we decided to build a HOM electronics for the second dipole band and the fifth dipole band, so that we will have both high resolution measurements for the whole module, and localized measurements for individual cavity. The prototype electronics is being built by Fermilab and planned to be tested in FLASH by the end of 2012.

physics.acc-ph↗

arXiv2012

Higher Order Modes for Beam Diagnostics in Third Harmonic 3.9 GHz Accelerating Module

An international team is currently investigating the best way to use Higher Order Modes (HOM) for beam diagnostics in 3.9 GHz cavities. HOMs are excited by charged particles when passing through an accelerating structure. Third harmonic cavities working at 3.9 GHz have been installed in FLASH to linearize the bunch energy profile. A proof-of-principle of using HOMs for beam monitoring has been made at FLASH in the TESLA 1.3 GHz cavities. Since the wakefields generated in the 3.9 GHz cavities are significantly larger, their impact on the beam should be carefully minimized. Therefore our target is to monitor HOMs and minimize them by aligning the beam on the cavity axis. The difficulty is that, in comparison to the 1.3 GHz cavities, the HOM-spectrum is dense, making it difficult to identify individual modes. Also, most modes propagate through the whole cryo-module containing several cavities, making it difficult to measure local beam properties. In this paper the options for the HOM-based beam position monitors are discussed.

physics.acc-ph↗

arXiv2012

Simulations of higher order modes in the ACC39 module of FLASH

This study is focused on the development of a HOM-based BPM system for the ACC39 module currently installed and in operation at FLASH. A similar system is anticipated to be installed at XFEL. Coupled inter-cavity modes are simulated together with a limited band of trapped modes. A suite of finite element computer codes (including HFSS and ACE3P) and globalised scattering matrix calculations (GSM) are used to investigate the modes in these cavities with a view to providing guidance on their use as a cavity beam diagnostic.

physics.acc-ph↗

arXiv2012

Effects of Ru Substitution on Dimensionality and Electron Correlations in Ba(Fe_{1-x}Ru_x)_2As_2

We report a systematic angle-resolved photoemission spectroscopy study on Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ for a wide range of Ru concentrations (0.15 $\leq$ \emph{x} $\leq$ 0.74). We observed a crossover from two-dimension to three-dimension for some of the hole-like Fermi surfaces with Ru substitution and a large reduction in the mass renormalization close to optimal doping. These results suggest that isovalent Ru substitution has remarkable effects on the low-energy electron excitations, which are important for the evolution of superconductivity and antiferromagnetism in this system.

cond-mat.supr-con↗

arXiv2012

Electron spin relaxation in graphene with random Rashba field: Comparison of D'yakonov-Perel' and Elliott-Yafet--like mechanisms

Aiming to understand the main spin relaxation mechanism in graphene, we investigate the spin relaxation with random Rashba field induced by both adatoms and substrate, by means of the kinetic spin Bloch equation approach. The charged adatoms on one hand enhance the Rashba spin-orbit coupling locally and on the other hand serve as Coulomb potential scatterers. Both effects contribute to spin relaxation limited by the D'yakonov-Perel' mechanism. In addition, the random Rashba field also causes spin relaxation by spin-flip scattering, manifesting itself as an Elliott-Yafet--like mechanism. Both mechanisms are sensitive to the correlation length of the random Rashba field, which may be affected by the environmental parameters such as electron density and temperature. By fitting and comparing the experiments from the Groningen group [Józsa {\it et al.}, Phys. Rev. B {\bf 80}, 241403(R) (2009)] and Riverside group [Pi {\it et al.}, Phys. Rev. Lett. {\bf 104}, 187201 (2010); Han and Kawakami, {\it ibid.} {\bf 107}, 047207 (2011)] which show either D'yakonov-Perel'-- (with the spin relaxation rate being inversely proportional to the momentum scattering rate) or Elliott-Yafet--like (with the spin relaxation rate being proportional to the momentum scattering rate) properties, we suggest that the D'yakonov-Perel' mechanism dominates the spin relaxation in graphene. The latest experimental finding of a nonmonotonic dependence of spin relaxation time on diffusion coefficient by Jo {\it et al.} [Phys. Rev. B {\bf 84}, 075453 (2011)] is also well reproduced by our model.

cond-mat.mes-hall↗

arXiv2012

Electron spin relaxation in rippled graphene with low mobilities

We investigate spin relaxation in rippled graphene where curvature induces a Zeeman-like spin-orbit coupling with opposite effective magnetic fields along the graphene plane in ${\bf K}$ and ${\bf K}^\prime$ valleys. The joint effect of this Zeeman-like spin-orbit coupling and the intervalley electron-optical phonon scattering opens a spin relaxation channel, which manifests itself in low-mobility samples with the electron mean free path being smaller than the ripple size. Due to this spin relaxation channel, with the increase of temperature, the relaxation time for spins perpendicular to the effective magnetic field first decreases and then increases, with a minimum of several hundred picoseconds around room temperature. However, the spin relaxation along the effective magnetic field is determined by the curvature-induced Rashba-type spin-orbit coupling, leading to a temperature-insensitive spin relaxation time of the order of microseconds. Therefore, the in-plane spin relaxation in low-mobility rippled graphene is anisotropic. Nevertheless, in the presence of a small perpendicular magnetic field, as usually applied in the Hanle spin precession measurement, the anisotropy of spin relaxation is strongly suppressed.

cond-mat.mes-hall↗

arXiv2012

Observation of TeV gamma rays from the Cygnus region with the ARGO-YBJ experiment

We report the observation of TeV gamma-rays from the Cygnus region using the ARGO-YBJ data collected from 2007 November to 2011 August. Several TeV sources are located in this region including the two bright extended MGRO J2019+37 and MGRO J2031+41. According to the Milagro data set, at 20 TeV MGRO J2019+37 is the most significant source apart from the Crab Nebula. No signal from MGRO J2019+37 is detected by the ARGO-YBJ experiment, and the derived flux upper limits at 90% confidence level for all the events above 600 GeV with medium energy of 3 TeV are lower than the Milagro flux, implying that the source might be variable and hard to be identified as a pulsar wind nebula. The only statistically significant (6.4 standard deviations) gamma-ray signal is found from MGRO J2031+41, with a flux consistent with the measurement by Milagro.

astro-ph.HE↗

arXiv2011

A Study of Beam Alignment Based on Coupling Modes in Third Harmonic Superconducting Cavities at FLASH

This paper presents a study of beam alignment in accelerating cavities based on beam-excited higher order modes (HOM). Among the transverse HOMs, dipole modes have the dominant contribution, and may have an adverse effect on the beam quality. Since they depend linearly on the transverse beam offset, their effect can be reduced by aligning the beam in the cavity. The study has been made on the four third harmonic superconducting cavities installed in the ACC39 module at FLASH. Modes with strong beam coupling were monitored while moving the beam and searching for the minimum excited HOM power.

physics.acc-ph↗

arXiv2011

Higher Order Modes in Coupled Cavities of the Flash Module ACC39

We analyse the higher order modes (HOM's) in the 3.9GHz bunch shaping cavities installed in the FLASH facility at DESY. A suite of finite element computer codes (including HFSS and ACE3P) and globalised scattering matrix calculations (GSM) are used to investigate the modes in these cavities. This study is primarily focused on the dipole component of the multiband expansion of the wakefield, with the emphasis being on the development of a HOM-based BPM system for ACC39. Coupled inter-cavity modes are simulated together with a limited band of trapped modes.

physics.acc-ph↗

arXiv2011

Study of Beam Diagnostics with Trapped Modes in Third Harmonic Superconducting Cavities at FLASH

Off-axis beams passing through an accelerating cavity excite dipole modes among other higher order modes (HOMs). These modes have linear dependence on the transverse beam offset from the cavity axis. Therefore they can be used to monitor the beam position within the cavity. The fifth dipole passband of the third harmonic superconducting cavities at FLASH has modes trapped within each cavity and do not propagate through the adjacent beam pipes, while most other cavity modes do. This could enable the beam position measurement in individual cavities. This paper investigates the possibility to use the fifth dipole band for beam alignment in the third harmonic cavity module. Simulations and measurements both with and without beam-excitations are presented. Various analysis methods are used and compared. A good correlation of HOM signals to the beam position is observed.

physics.acc-ph↗

arXiv2011

Isotropic superconducting gaps with enhanced pairing on electron Fermi surfaces in FeTe0.55Se0.45

The momentum distribution of the energy gap opening at the Fermi level of superconductors is a direct fingerprint of the pairing mechanism. While the phase diagram of the iron-based superconductors promotes antiferromagnetic fluctuations as a natural candidate for electron pairing, the precise origin of the interaction is highly debated. We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe1-xSex, which has the simplest structure of all iron-based superconductors. We found isotropic superconducting gaps on all Fermi surfaces whose sizes can be fitted by a single gap function derived from a strong coupling approach, strongly suggesting local antiferromagnetic exchange interactions as the pairing origin.

cond-mat.supr-con↗

arXiv2011

Long-term monitoring of the TeV emission from Mrk 421 with the ARGO-YBJ experiment

ARGO-YBJ is an air shower detector array with a fully covered layer of resistive plate chambers. It is operated with a high duty cycle and a large field of view. It continuously monitors the northern sky at energies above 0.3 TeV. In this paper, we report a long-term monitoring of Mrk 421 over the period from 2007 November to 2010 February. This source was observed by the satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray band. Mrk 421 was especially active in the first half of 2008. Many flares are observed in both X-ray and gamma-ray bands simultaneously. The gamma-ray flux observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag between the X-ray and gamma-ray photons longer than 1 day is found. The evolution of the spectral energy distribution is investigated by measuring spectral indices at four different flux levels. Hardening of the spectra is observed in both X-ray and gamma-ray bands. The gamma-ray flux increases quadratically with the simultaneously measured X-ray flux. All these observational results strongly favor the synchrotron self-Compton process as the underlying radiative mechanism.

astro-ph.HE↗

arXiv2011

Beam-based HOM Study in Third Harmonic SC Cavities for Beam Alignment at FLASH

An electron beam entering an accelerator cavity excites higher order modes (HOM). These are radiated to HOM couplers and subsequently damped. They can also be used to facilitate beam position monitoring. The modes excited by off-axis beams are the focus of this study. Results are presented on the first analysis of HOM dependency on the beam position for the third harmonic cavities at FLASH. The analysis is based on the beampipe and cavity dipole modes. Various analysis methods are used and compared to satisfy the needs emerging from the characteristics of those modes. The electric center of each mode is observed by looking for the minima of the HOM signals of different beam positions. A single electron bunch per RF pulse is used.

physics.acc-ph↗

arXiv2011

Electron spin diffusion and transport in graphene

We investigate the spin diffusion and transport in a graphene monolayer on SiO$_2$ substrate by means of the microscopic kinetic spin Bloch equation approach. The substrate causes a strong Rashba spin-orbit coupling field $\sim 0.15$ meV, which might be accounted for by the impurities initially present in the substrate or even the substrate-induced structure distortion. By surface chemical doping with Au atoms, this Rashba spin-orbit coupling is further strengthened as the adatoms can distort the graphene lattice from $sp^2$ to $sp^3$ bonding structure. By fitting the Au doping dependence of spin relaxation from Pi {\sl et al.} [Phys. Rev. Lett. {\bf 104}, 187201 (2010)], the Rashba spin-orbit coupling coefficient is found to increase approximately linearly from 0.15 to 0.23 meV with the increase of Au density. With this strong spin-orbit coupling, the spin diffusion or transport length is comparable with the experimental values. In the strong scattering limit (dominated by the electron-impurity scattering in our study), the spin diffusion is uniquely determined by the Rashba spin-orbit coupling strength and insensitive to the temperature, electron density as well as scattering. With the presence of an electric field along the spin injection direction, the spin transport length can be modulated by either the electric field or the electron density. (The remaining is omitted due to the limit of space)

cond-mat.mes-hall↗