Yan Qi-Wei

Structure analysis of new superconductor MgB2

The superconductivity of a sintered pellet of the newly discovered MgB2 superconductor has been studied with magnetic measurements and its crystal structure was analysed using the Rietveld method of powder X-ray patterns. It has hexagonal symmetry (S.G. P6/mmm) with unit cell lattice parameters a=0.308136(14)nm and c=0.351782(17)nm.

Crystallographic and magnetic structures of Pr6Fe13Ge studied by powder neutron diffraction

Crystallographic and magnetic structures of Pr6Fe13Ge have been investigated by high-resolution powder neutron diffraction in the temperature range of 10?300 K. The magnetic structure consists of ferromagnetic Pr6Fe13 slabs that alternate antiferromagnetically, along c, with the next Pr6Fe13 slab separated by a non-magnetic Ge layer. The magnetic moments lie within the ab-planes. The propagation vector of this structure is k=(001) with respect to the conventional reciprocal lattice of the I-centred structure. However, the temperature-dependence of neutron-scattering intensity of the (110) Bragg peak, very similar to the temperature-dependent magnetization measured by SQUID magnetometer, indicates that a small c-axis ferromagnetic component should be added to the above antiferromagnetic model.

Spin Dynamics of Supramolecular Dimer [Mn 4 ] 2 Interacting With a Spin-polarized Electron

A new family of the supramolecular antiferromagnetically exchange-coupled dimer of single-molecules magnets, [Mn4]2, really behave as a quantum mechanical coupled system and can be used for quantum computation [Tiron et al (2003) Phys. Rev. Lett. 91 227203]. We study the time evolution of [Mn4]2 dimer interacting with an injected electron by solving the time-dependent Schrodinger equations. We find that the variations of [Mn4]2 dimer magnetization and the electron spin crucially depend on the exchange interaction strength. Time evolution of the entanglement between the injected electron and supramolecular dimer [Mn4]2 is evaluated. It is observed that entanglement oscillates in time and is closely related to the spin variation of injected electron. From these characteristics the technique for the reversing and read-out of supramolecular dimer [Mn4]2 spin states is suggested.

Generation of Bell states in two-component Bose－Einstein condensates

We examine analytically the generation of Bell state in Bose condensates of two interacting species trapped in a double well configuration. The density of probability for finding the entangled Bell state is given. The effect of the tunnelling rate and the interspecies interaction strength on the generation of Bell state is discussed. We find that the oscillation amplitude of the density of probability for finding the entangled Bell state becomes greater as the tunnelling rate Ω increases and the self-interaction strength of the component A(B) has no effect on it.

Larmor Precession and Tunnelling Time of a Non-Relativistic Neutral Spin-1/2 Particle Through an Arbitrary Potential Barrier

We investigate the Larmor precession of a non-relativistic neutral spin-1/2 particle in a uniform constant magnetic field confined to the region of a one-dimensional arbitrary potential barrier. With the help of a general spin coherent state it is explicitly shown that the spin precession time is equal to the dwell time. For the special case of a symmetric rectangular potential barrier, the precession time is also in agreement with the transmission time. We present a numerical estimation of the precession time showing an apparent superluminal tunnelling.

Calculations of radioactivity and afterheat in the components of the CSNS target station

This paper shows the calculations of radioactivity and afterheat in the components of the China Spallation Neutron Source (CSNS) target station, with the Monte Carlo codes LAHET, MCNP4C and the multigroup code CINDER’90. These calculations provide essential data for the detailed design and maintenance of the CSNS target station.

Shape dependence of low-temperature magnetic relaxation of Mn(12)Ac

We report the discovery that the low-temperature magnetic relaxation in Mn(12)Ac single crystals strongly depends on the shape of the samples. The relaxation time exhibits a minimum at the phase transition point between ferromagnetic and antiferromagnetic phases. The shape dependence is attributed to the dipolar interaction between molecular magnets.We report the discovery that the low-temperature magnetic relaxation in Mn(12)Ac single crystals strongly depends on the shape of the samples. The relaxation time exhibits a minimum at the phase transition point between ferromagnetic and antiferromagnetic phases. The shape dependence is attributed to the dipolar interaction between molecular magnets.

Magnetic Quantum Tunnelling in Faster Relaxation Process in Mn12Ac Molecular Magnets

The field-tuned ac susceptibility of Mn12Ac single crystal has been measured as functions of temperature and frequency. Two relaxation processes appear in our measurement. One is related to the magnetic quantum tunnelling of a collective spin S = 12 by a 61 K magnetic barrier and it has been well studied, while the other is faster than the former and its mechanism is unclear. We find that the relaxation time for the faster process shows the minima at H = 0 and H = 3.4 kOe, indicating that the quantum tunnelling also takes place in this process while the resonant fields are different from what we have known before.

Resonant tunnelling of the magnetization in molecular crystal of [(Mn1-xCrx)12O12 (CH3COO)16(H2O)4] ·2CH3COOH·4H2O (x=0,0.03,0.04,0.05)

The quantum tunnelling of magnetization (QTM) in single crystals of the single molecule magnet (Mn1-xCrx)12-Ac (x=0, 0.03, 0.04, 0.05) has been investigated. In comparison with its parent Mn12-Ac, a greater rate of magnetization relaxation and a lower effective potential-energy barrier have been observed in Cr-doping samples. This modulation of QTM due to the Cr-doping could be attributed to the small change of Sz due to the smaller spin of Cr itself and additional intrinsic but distributed transverse and longitudinal anisotropy raised by a subtle change of the local environment in the magnetic Mn12 core.

Bifurcation of a periodic instanton and quantum-classical transition in the biaxial nano-ferromagnet with a magnetic field along hard axis

Crossover from classical to quantum regimes of the barrier transition rate in a biaxial ferromagnetic magnet with a magnetic field applied along hard anisotropy axis is investigated. We show that the type of action-temperature diagrams can be determined by counting the number of bifurcation points. The model possesses not only the known type I and II, but also the interesting type III and IV of transition which do not occur in general.