arXiv subjects

X. G. Wu

Publications and source records attributed to X. G. Wu.

At least 19 recordsLinked to original sources

arXiv2018

T0 Fan-out for Back-n White Neutron Facility at CSNS

the main physics goal for Back-n white neutron facility at China Spallation Neutron Source (CSNS) is to measure nuclear data. The energy of neutrons is one of the most important parameters for measuring nuclear data. Method of time of flight (TOF) is used to obtain the energy of neutrons. The time when proton bunches hit the thick tungsten target is considered as the start point of TOF. T0 signal, generated from the CSNS accelerator, represents this start time. Besides, the T0 signal is also used as the gate control signal that triggers the readout electronics. Obviously, the timing precision of T0 directly affects the measurement precision of TOF and controls the running or readout electronics. In this paper, the T0 fan-out for Back-n white neutron facility at CSNS is proposed. The T0 signal travelling from the CSNS accelerator is fanned out to the two underground experiment stations respectively over long cables. To guarantee the timing precision, T0 signal is conditioned with good signal edge. Furthermore, techniques of signal pre-emphasizing and equalizing are used to improve signal quality after T0 being transmitted over long cables with about 100 m length. Experiments show that the T0 fan-out works well, the T0 signal transmitted over 100 m remains a good time resolution with a standard deviation of 25 ps. It absolutely meets the required accuracy of the measurement of TOF.

physics.ins-det↗

arXiv2018

Electronics of Time-of-flight Measurement for Back-n at CSNS

Back-n is a white neutron experimental facility at China Spallation Neutron Source (CSNS). The time structure of the primary proton beam make it fully applicable to use TOF (time-of-flight) method for neutron energy measuring. We implement the electronics of TOF measurement on the general-purpose readout electronics designed for all of the seven detectors in Back-n. The electronics is based on PXIe (Peripheral Component Interconnect Express eXtensions for Instrumentation) platform, which is composed of FDM (Field Digitizer Modules), TCM (Trigger and Clock Module), and SCM (Signal Conditioning Module). T0 signal synchronous to the CSNS accelerator represents the neutron emission from the target. It is the start of time stamp. The trigger and clock module (TCM) receives, synchronizes and distributes the T0 signal to each FDM based on the PXIe backplane bus. Meantime, detector signals after being conditioned are fed into FDMs for waveform digitizing. First sample point of the signal is the stop of time stamp. According to the start, stop time stamp and the time of signal over threshold, the total TOF can be obtained. FPGA-based (Field Programmable Gate Array) TDC is implemented on TCM to accurately acquire the time interval between the asynchronous T0 signal and the global synchronous clock phase. There is also an FPGA-based TDC on FDM to accurately acquire the time interval between T0 arriving at FDM and the first sample point of the detector signal, the over threshold time of signal is obtained offline. This method for TOF measurement is efficient and not needed for additional modules. Test result shows the accuracy of TOF is sub-nanosecond and can meet the requirement for Back-n at CSNS.

physics.ins-det↗

arXiv2018

New high-spin structure and possible chirality in $^{109}$In

High-spin structure of $^{109}$In has been investigated with the $^{100}$Mo($^{14}$N, 5$n$)$^{109}$In reaction at a beam energy of 78 MeV using the in-beam $γ$ spectroscopic method. The level scheme of $^{109}$In has been modified considerably and extended by 46 new $γ$-rays to the highest excited state at 8.979 MeV and $J^π$=(45/2$^{+}$). The new level scheme consists of eight bands, six of which are identified as dipole bands. The configurations have been tentatively assigned with the help of the systematics of neighboring odd-$A$ indium isotopes and the experimental aligned angular momenta. The dipole bands are then compared with the titled axis cranking calculation in the framework of covariant density function theory (TAC-CDFT). The results of theoretical calculation based on the configurations, which involve one proton hole at the $g_{9/2}$ orbital and two or four unpaired neutrons at $g_{7/2}$, $d_{5/2}$ and $h_{11/2}$ orbitals, show that the shape of $^{109}$In undergoes an evolution on both $β$ and $γ$ deformations and possible chirality is suggested in $^{109}$In.

nucl-ex↗

arXiv2015

Landau level transitions in InAs/AlSb/GaSb quantum wells

The electronic structure of InAs/AlSb/GaSb quantum wells embedded in AlSb barriers and in the presence of a perpendicular magnetic field is studied theoretically within the $14$-band ${\bf k}\cdot{\bf p}$ approach without making the axial approximation. At zero magnetic field, for a quantum well with a wide InAs layer and a wide GaSb layer, the energy of an electron-like subband can be lower than the energy of hole-like subbands. As the strength of the magnetic field increases, the Landau levels of this electron-like subband grow in energy and intersect the Landau levels of the hole-like subbands. The electron-hole hybridization leads to a series of anti-crossing splittings of the Landau levels. The energies of some Landau level transitions and their corresponding transition strengthes are calculated. The magnetic field dependence of some dominant transitions is shown with their corresponding initial-states and final-states indicated. This information should be useful in analyzing an experimentally measured magneto-optical spectrum. At high magnetic fields, multiple transitions due to the initial-state splitting can be observed. The dominant transitions at high fields can be roughly viewed as two spin-split Landau level transitions with many electron-hole hybridization induced splittings. The energy separations between the dominant transitions may decrease or increase versus the magnetic field locally, or may be almost field independent. The separations can be tuned by changing the width of InAs layer or the width of middle AlSb layer. When the magnetic field is tilted, the electron-like Landau level transitions show additional anti-crossing splittings due to the subband-Landau level coupling.

cond-mat.mtrl-sci↗

arXiv2013

Exact wave functions for the edge state of a disk-shaped two dimensional topological insulator

We report the exact wave functions for the eigen state of a disk-shaped two dimensional topological insulator. The property of the edge state whose energy lies inside the bulk gap is studied. It is found that the edge state energy is affected by the radius of the disk. For a fixed angular momentum index, there is a critical disk radius below which there exists no edge state. The value of this critical radius increases as the angular momentum index increases. In the limit of large disk radius, the energy of the edge state approaches a limiting value determined by the system parameters and independent of the angular momentum index. The derivation from this limiting value is inversely proportional to the radius with a coefficient proportional to the angular momentum index. In the general case, the energy differences between two edge states with adjacent angular momentum indexes are not equal. The exact and analytical wave functions also facilitates the investigation of electronic state in other structures of the two dimensional topological insulator.

cond-mat.mes-hall↗

arXiv2013

Influence of bulk inversion asymmetry on the magneto-optical spectrum of a HgTe topological insulator

The influence of bulk inversion asymmetry in [001] and [013] grown HgTe quantum wells is investigated theoretically. The bulk inversion asymmetry leads to an anti-crossing gap between two zero-mode Landau levels in a HgTe topological insulator, i.e., the quantum well with inverted band structure. It is found that this is the main contribution to the anti-crossing splitting observed in recent experimental magneto spectroscopic measurements. The relevant optical transitions involve different subbands, but the electron-electron interaction induced depolarization shift is found to be negligibly small. It is also found that the splitting of this anti-crossing only depends weakly on the tilting angle when the magnetic field is tilted away from the perpendicular direction to the quantum well. Thus, the strength of bulk inversion asymmetry can be determined via a direct comparison between the theoretical calculated one-electron energy levels and experimentally observed anti-crossing energy gap.

cond-mat.mes-hall↗

arXiv2013

Efficiency of electrical manipulation on two-dimensional topological insulators

We investigate the efficiency of electrical manipulation on two-dimensional topological insulators by considering a lateral potential superlattice on the system. The electronic states under various conditions are examined carefully. It is found that the dispersion of the mini-band and the electron distribution in the potential well region display an oscillatory behavior as the potential strength of the lateral superlattice increases. The probability of finding an electron in the potential well region can be larger or smaller than the average as the potential strength varies. This indicates that the electric manipulation efficiency on two-dimensional topological insulators is not as high as expected, which should be carefully considered in designing a device application that bases on two-dimensional topological insulators. These features can be attributed to the coupled multiple-band nature of the topological insulator model. In addition, it is also found that these behaviors are not sensitive to the gap parameter of the two-dimensional topological insulator model.

cond-mat.mes-hall↗

arXiv2013

Identification of a $9/2^-$[505] isomer in the neutron-rich $^{193}$Os nucleus

The neutron rich nucleus $^{193}$Os was produced in the $^{192}$Os($^{7}$Li,$^{6}$Li)$^{193}$Os reaction. An isomeric state based on the $9/2^-$[505] nilsson orbital was identified in the present work. Half-life of the isomeric state was extracted and discussed in terms of the $K$ quantum number. Level scheme built on the isomeric state was proposed based on the experimental data.

nucl-ex↗

arXiv2007

Spin-Flip Interactions and the Puzzle of psi's Polarization at Tevatron

Nonrelativistic QCD provides a systematic approach for inclusive decays and productions of a quarkonium. By taking color-octet components into account, the approach can explain the $ψ'$-anomaly at Tevatron, where the measured production rate at large transverse momentum $p_\perp$ is in order of magnitude larger than the predicted with color-singlet components only. With the approach one can predict that the produced $J/ψ$ and $ψ'$ at large $p_\perp$ will be transversely polarized. But the prediction fails in confronting with experimental measurements and this generates a puzzle. We examine the role of spin-flip interactions in the spin density matrix of the transition of a color-octet charm quark pair into $J/ψ$ and $ψ'$. These interactions will introduce new nonperturbative parameters in the spin density matrix. Our result shows that the impact of the interactions is always to dilute the polarization and can be very significant. Taking the impact into account, predictions for the polarization are more close to the measured than the previous predicted. The same can also be expected for the polarization of $J/ψ$.

hep-ph↗

arXiv2006

Interplay between s-d exchange interaction and Rashba effect: spin-polarized transport

We investigate the spin-polarized transport properties of a two-dimensional electron gas in a n-type diluted magnetic narrow gap semiconductor quantum well subjected to a perpendicular magnetic and electric field. Interesting beating patterns in the magneto resistance are found which can be tuned significantly by varying the electric field. A resonant enhancement of spin-polarized current is found which is induced by the competition between the s-d exchange interaction and the Rashba effect [Y. A. Bychkov and E. I. Rashba, J. Phys. C 17, 6039 (1984)].

cond-mat.mes-hall↗

arXiv2005

Spin-polarized transport in a lateral two-dimensional diluted magnetic semiconductor electron gas

The transport property of a lateral two-dimensional diluted magnetic semiconductor electron gas under a spatially periodic magnetic field is investigated theoretically. We find that the electron Fermi velocity along the modulation direction is highly spin-dependent even if the spin polarization of the carrier population is negligibly small. It turns out that this spin-polarized Fermi velocity alone can lead to a strong spin polarization of the current, which is still robust against the energy broadening effect induced by the impurity scattering.

cond-mat.mes-hall↗

arXiv2004

Pion Form Factor in the $k_T$ Factorization Formalism

Based on the light-cone (LC) framework and the $k_T$ factorization formalism, the transverse momentum effects and the different helicity components' contributions to the pion form factor $F_π(Q^2)$ are recalculated. In particular, the contribution to the pion form factor from the higher helicity components ($λ_1+λ_2=\pm 1$), which come from the spin-space Wigner rotation, are analyzed in the soft and hard energy regions respectively. Our results show that the right power behavior of the hard contribution from the higher helicity components can only be obtained by fully keeping the $k_T$ dependence in the hard amplitude, and that the $k_T$ dependence in LC wave function affects the hard and soft contributions substantially. As an example, we employ a model LC wave function to calculate the pion form factor and then compare the numerical predictions with the experimental data. It is shown that the soft contribution is less important at the intermediate energy region.

hep-ph↗

arXiv1996

Skyrmions in the Fractional Quantum Hall Effect

It is verified that, at small Zeeman energies, the charged excitations in the vicinity of 1/3 filled Landau level are skyrmions of composite fermions, analogous to the skyrmions of electrons near filling factor unity. These are found to be relevant, however, only at very low magnetic fields.

cond-mat↗

arXiv1996

Excitons of Composite Fermions

The low-energy excitations of filled Landau levels (LL's) of electrons involve promotion of a single electron from the topmost filled LL to the lowest empty LL. These are called excitons or collective modes. The incompressible fractional quantum Hall states are understood as filled LL's of composite fermions, and the low-energy neutral excitations are excitons of composite fermions. New techniques are developed to study large systems, which provide detailed information about the dispersions of the composite fermion excitons. In particular, it is found that the interaction energy of the exciton is well described by the `unprojected' composite fermion theory.

cond-mat↗

arXiv1995

Composite fermion theory of collective excitations in fractional quantum Hall effect

The low energy neutral excitations of incompressible fractional quantum Hall states are called collective modes or magnetic excitons. This work develops techniques for computing their dispersion at general filling fractions for reasonably large systems. New structure is revealed; in particular, the collective mode at 1/3 is found to possess several minima, with the energy of the principal minimum significantly smaller than the earlier estimate. \pacs{73.40.Hm, 73.20.Dx, 73.20.Mf}

cond-mat↗

arXiv1994

Excitation Spectrum and Collective Modes of Composite Fermions

According to the composite fermion theory, the interacting electron system at filling factor $ν$ is equivalent to the non-interacting composite fermion system at $ν^*=ν/(1-2mν)$, which in turn is related to the non-interacting electron system at $ν^*$. We show that several eigenstates of non-interacting electrons at $ν^*$ do not have any partners for interacting electrons at $ν$, but, upon composite fermion transformation, these states are eliminated, and the remaining states provide a good description of the spectrum at $ν$. We also show that the collective mode branches of incompressible states are well described as the collective modes of composite fermions. Our results suggest that, at small wave vectors, there is a single well defined collective mode for all fractional quantum Hall states. Implications for the Chern-Simons treatment of composite fermions will be discussed.

cond-mat↗

arXiv1994

Excitation Spectrum of Composite Fermions

We show that the excitation spectrum of interacting electrons at filling factor $ν=ν^*/(2ν^*+1)$ is well described in terms of non-interacting composite fermions at filling factor $ν^*$, but does not have a one-to-one correspondence with the excitation spectrum of non-interacting electrons at $ν^*$. In particular, the collective modes of the fractional quantum Hall states are not analogous to those of the integer quantum Hall states. We also speculate on the nature of the compressible state at $ν=1/2$.

cond-mat↗

arXiv1993

Hund's Rule for Composite Fermions

We consider the ``fractional quantum Hall atom" in the vanishing Zeeman energy limit, and investigate the validity of Hund's maximum-spin rule for interacting electrons in various Landau levels. While it is not valid for {\em electrons} in the lowest Landau level, there are regions of filling factors where it predicts the ground state spin correctly {\em provided it is applied to composite fermions}. The composite fermion theory also reveals a ``self-similar" structure in the filling factor range $4/3>ν>2/3$.

cond-mat↗