Quantum fluid dynamics based current-density functional study of a helium atom in a strong time-dependent magnetic field

Evolution of the helium atom in a strong time-dependent (TD) magnetic field (B) of strength up to 10¹¹ G is investigated through a quantum fluid dynamics (QFD) based current-density functional theory (CDFT). The TD-QFD-CDFT computations are performed through numerical solution of a single generalized nonlinear Schr?dinger equation employing vector exchange-correlation potentials and scalar exchange-correlation density functionals that depend both on the electronic charge-density and the current-density. The results are compared with that obtained from a B-TD-QFD-DFT approach (based on conventional TD-DFT) under similar numerical constraints but employing only scalar exchange-correlation potential dependent on electronic charge-density only. The B-TD-QFD-DFT approach, at a particular TD magnetic field-strength, yields electronic charge- and current-densities as well as exchange-correlation potential resembling with that obtained from the time-independent studies involving static (time-independent) magnetic fields. However, TD-QFD-CDFT electronic charge- and current-densities along with the exchange-correlation potential and energy differ significantly from that obtained using B-TD-QFD-DFT approach, particularly at field-strengths >10⁹ G, representing dynamical effects of a TD field. The work concludes that when a helium atom is subjected to a strong TD magnetic field of order >10⁹ G, the conventional TD-DFT based approach differs “dynamically” from the CDFT based approach under similar computational constraints.     

Current-density functional theory study of the H2 molecule evolving under a strong ultrashort magnetic field

Hydrogen molecule in a strong ultrashort magnetic field is investigated through a current-density functional theory (CDFT) and quantum fluid dynamics (QFD) based approach employing current-density dependent vector exchange-correlation potential and energy density functional derived with a vorticity variable. The numerical computations through the CDFT based approach are performed for the H₂ molecule, starting initially from its field-free ground state, in a parallel internuclear axis and magnetic field-axis configuration with the internuclear separation R ranging from 0.1 a.u. to 14.0 a.u., and the strength of the time-dependent (TD) magnetic field varying between 0−10¹¹ G over a few femtoseconds. The numerical results are compared with that obtained using an approach based on the current-density independent approximation under similar computational constraints but employing only scalar exchange-correlation potential dependent on the electronic charge-density alone. The current-density based approach yields exchange- and correlation energy as well as electronic charge-density of the H₂ molecule drastically different from that obtained using current-independent approach, in particular, at TD magnetic field-strengths >10⁹ G during a typical time-period of the field when the magnetic-field had attained maximum applied field-strength and is switched to a decreasing ramp function. This nonadiabatic behavior of the TD electronic charge-density is traced to the TD vorticity-dependent vector exchange-correlation potential of the CDFT based approach. The interesting electron dynamics of the H₂ molecule in strong TD magnetic field is further elucidated by treating electronic charge-density as an ‘electron-fluid’. The present work also reveals interesting real-time dynamics on the attosecond time-scale in the electronic charge-density distribution of the hydrogen molecule.     

Delayed formation of muon-containing species in organic liquids observed by spin resonance technique

The resonance technique has been applied to observe diamagnetic muons and, for the first time by the resonance, Mu-substituted radicals in organic liquids under strong decoupling magnetic fields. In benzoquinone solutions in benzene the relaxation of Mu-cyclohexadienyl radicals through reaction with quinone was directly observed by the radical resonance technique. The product of this reaction was then observed by the diamagnetic muon resonance as a slow formation. Similar slow formation was observed for diamagnetic muons in neat CS₂ and for Mu-radicals in benzene and styrene. Such slow formation can never be observed by the rotation technique due to dephasing problem, and thus the previous method is expected to provide new source of information on slow reaction dynamics of muon containing species.     

Quasiparticle density of states of 2H-NbSe2 single crystals revealed by low-temperature specific heat measurements according to a two-component model

Low-temperature specific heat in a dichalcogenide superconductor 2H-NbSe2 is measured in various magnetic fields. It is found that the specific heat can be described very well by a simple model concerning two components corresponding to vortex normal core and ambient superconducting region, separately. For calculating the specific heat outside the vortex core region, we use the Bardeen-Cooper Schrieffer （BCS） formalism under the assumption of a narrow distribution of the superconducting gaps. The field-dependent vortex core size in the mixed state of 2H-NbSe2, determined by using this model, can explain the nonlinear field dependence of specific heat coefficient γ（H）, which is in good agreement with the previous experimental results and more formal calculations. With the high-temperature specific heat data, we can find that, in the multi-band superconductor 2H-NbSe2, the recovered density of states （or Fermi surface） below Tc under a magnetic field seems not to be gapped again by the charge density wave （CDW） gap, which suggests that the superconducting gap and the CDW gap may open on different Fermi surface sheets.     

Effect of magnetic annealing on magnetization of BiFeO3 ceramics

The effect of magnetic annealing treatment on the magnetization of multiferroic BiFeO₃ was studied systematically. A series of pelletized nano-sized BiFeO₃ powders were annealed at high temperature under different magnetic fields. Typical ferromagnetic hysteresis loops were obtained at room temperature of the ceramics which were derived from ferromagnetic BiFeO₃ precursors. On the other hand, antiferromagnetic behaviors were observed in other samples synthesized from nonmagnetic precursors. The enhanced magnetic properties were ascribed to the magnetic anisotropy which was induced by the strong magnetic fields. This work indicates that the strong magnetic annealing method is an alternative approach to tuning the magnetic properties of high performance multiferroic materials with canted antiferromagnetic ordering.     

Nonlinear dynamics of filamentation instability and current filament merging in a high density current-driven plasma

The magnetic field generation due to the filamentation instability (FI) of a high density current-driven plasma is studied through a new nonlinear diffusion equation. This equation is obtained on the basis of quantum hydrodynamic model and numerically solved by applying the Crank–Nicolson method. The spatiotemporal evolution of the magnetic field and the electron density distribution exhibits the current filament merging as a nonlinear phase of the FI which is responsible for the strong magnetic fields in the current-driven plasmas. It is found that the general behaviour of the FI is the same as that of the classical case but the instability growth rate, its magnitude, and the saturation time are affected by the quantum effects. It is eventually concluded that the quantum effects can play a stabilizing role in such situation.     

Measurement of silver vapor density by an optical method

The density of silver saturated vapor has been measured in the temperature range 710 °C–820 °C. The density values were obtained by measuring the absorption coefficient of the resonance lineλ=3 383 Å by magnetic scanning. The experimental density points have been fitted with the equation lgN=A?B/T? lgT and the best values ofA andB were found to beA=24.2525,B=10416.2.     

Non-equilibrium effects in the magnetic behavior of Co3O4 nanoparticles

We report detailed studies of the non-equilibrium magnetic behavior of antiferromagnetic Co₃O₄ nanoparticles. The temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects, and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the system shows some features that are characteristic of nanoparticle magnetism such as bifurcation of field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at which the ZFC magnetization peaks coincides with the bifurcation temperature and does not shift on application of magnetic fields up to 1 kOe, unlike most other nanoparticle systems. Aging effects in these particles are negligible in both FC and ZFC protocols, and memory effects are present only in the FC protocol. We show that Co₃O₄ nanoparticles constitute a unique antiferromagnetic system which enters into a blocked state above the average Néel temperature.     

High-Pressure Mössbauer Spectroscopy of Perovskite Iron Oxides

Using a diamond anvil cell and a low-temperature cryostat with a superconducting solenoid, high-pressure ⁵⁷Fe Mössbauer spectroscopy with a radioactive source at 4.5 K under external magnetic fields up to 7 T has been developed. Pressure-temperature magnetic phase-diagrams for perovskite iron oxides, SrFeO₃, CaFeO₃ and Sr₂LaFe₃O₉ are presented up to about 70 GPa. High-pressure Mössbauer measurements under external magnetic fields proved that the electronic ground states of these oxides switch to uniform-charge and ferromagnetic (and most probably metallic) states under extremely high pressure.

     

Mechanism of magnetic field effect on OH density distribution in a methane–air premixed jet flame

The mechanism of magnetic field effect on OH density distribution in a methane-air premixed jet flame was investigated by means of PLIF measurement and numerical simulation. In the experiment, magnetically induced change in the OH density profile in the flame in a N₂ atmosphere was much smaller than that in air (mixture of 80% N₂ and 20% O₂), and such a phenomenon was qualitatively reproduced by solving the equations for reactive gas dynamics and magnetism in the numerical simulation. Here, N₂ is diamagnetic and little affected by the magnetic field, while O₂ is paramagnetic and influenced due to the magnetic field. This provided the experimental and numerical verification for the mechanism of the magnetic field effect suggested in our previous study. That is, the magnetic force does not directly and selectively induce the change in the diffusion velocity of OH itself. Alternatively, the magnetic force acting on O₂ in the surrounding air, whose mass density and magnetic susceptibility are much larger than that of other chemical species in the flame, causes the change in the convection velocity of the gas mixture and displaces the OH density distribution indirectly and passively. In other words, the most important cause of the OH density change is not the diffusion of OH, but the convection of air containing a large amount of O₂. Furthermore, by careful examination of the magnetic field effect on the flame in the N₂ atmosphere, it was found out that the magnetic force does not only act on O₂ in the surrounding air, but also on O₂ in the premixed gas injected from the burner.     

A polarised neutron study of magnetic moment density in Cu2MnAl

A polarised neutron study of the ferromagnetic Heusler alloy Cu₂Mn_0.863Al_1.057 has been made. It has been concluded that the magnetic moment density is primarily situated on the Mn ions. On assigning the Mn-moment value, the observed magnetic form factor is found to be in good agreement with the Mn²⁺ free ion form factor calculated by Watson and Freeman. A slight asphericity has been observed in the moment density. It is estimated that there are about 3% excess 3d-electrons in the Eg states compared to spherical distribution. There is evidence of a very small positive polarisation of the Cu atoms. No appreciable conduction electron polarisation is found.     

Quantum wells under an in-plane magnetic field: Effect of the composition parameters on excited electron energy splitting

The dependence of the electronic spin-splitting energy on the composition parameters (x,y) in In_xGa_1?xAs–In_yAl_1?yAs-based quantum wells, has been calculated. InGaAs narrow gap structures, subjected to in-plane magnetic fields, have been selected because these structures have a big Landè factor. The dependence of the Landé factor both on the applied fields and composition parameters has been included for fixed well width and external electric field. Contributions from the interfaces and strain, which also depend on the composition, are included. Spin-splitting energy and density of states show a strong dependence on the above parameters.     

Anisotropic pinning in macroscopic electrodynamics of superconductors

Anisotropy of critical currents and electric fields in superconductors with strong pinning has been ascribed in the macroscopic model to features of the material equation system relating the electric field to the current density in a superconductor. The anisotropy of the pinning proper is described by an operator relating the pinning force density to the vectors of magnetic induction and Lorentz force. In the approximation of an extended critical state model, a feasible expression of this operator is given in the form of an algorithm based on the concept of a collective anisotropic potential well containing fluxoids. The current-carrying capacity of a strongly anisotropic niobium-titanium foil as a function of the orientation of the current density and applied field with respect to the principal axes of the material has been investigated in detail. Given measurements of the transverse electric fields in the foil under magnetic fields normal to the foil plane, we can plot cross sections of surfaces describing the pinning force density in the space of magnetic induction and Lorentz force. Zh. éksp. Teor. Fiz. 112, 1055–1081 (September 1997)     

Density matrix for an electron confined in quantum dots under uniform magnetic field and static electrical field

Using unitary transformations, this paper obtains the eigenvalues and the common eigenvector of Hamiltonian and a new-defined generalized angular momentum (L_z) for an electron confined in quantum dots under a uniform magnetic field (UMF) and a static electric field (SEF). It finds that the eigenvalue of L_z just stands for the expectation value of a usual angular momentum l_z in the eigen-state. It first obtains the matrix density for this system via directly calculating a transfer matrix element of operator \exp( -\beta H) in some representations with the technique of integral within an ordered products (IWOP) of operators, rather than via solving a Bloch equation. Because the quadratic homogeneity of potential energy is broken due to the existence of SEF, the virial theorem in statistical physics is not satisfactory for this system, which is confirmed through the calculation of thermal averages of physical quantities.     

Spontaneous and field-induced magnetic transitions in YBaCo2O5.5

A detailed study of magnetic properties of cobaltite YBaCo₂O_5.5 has been performed in high (up to 35 T) magnetic fields and under hydrostatic pressure up to 0.8 GPa. The temperatures of paramagnet-ferromagnet (PM-FM) and ferromagnet-antiferromagnet (FM-AF) phase transitions and their pressure derivatives have been determined. It has been revealed that in the compound with yttrium, in contrast to those with magnetic rare earth atoms, the AF-FM field-induced magnetic phase transition is accompanied by a considerable field hysteresis below 240 K, and the magnetic field of 35 T is not sufficient to complete this transition at low temperatures. The hysteresis value depends on the magnetic field sweep rate, which considered as an evidence of magnetic viscosity that is especially strong in the region of coexistence of the FM and AF phases. High values of susceptibility for the field-induced FM phase show that Co spin state in these compounds changes in strong magnetic field.     

Energy gap of a charge density wave in NbSe3 induced by a high magnetic field above the Peierls transition temperature

The effect of a magnetic field on the energy gap of the charge density wave (CDW) in NbSe₃ near the temperature T _p2 of the lower Peierls transition has been investigated using interlayer tunneling spectroscopy. It has been shown that the magnetic field increases the energy gap and can even induce it at temperatures higher than T _p2 by 15–20 K. As the field strength increases, the peak amplitude of the gap singularity of the tunneling spectrum first increases, reaches its maximum at 20–30 T, and then decreases. The increase in the gap peak amplitude is attributed to the field-induced improvement of the condition of the CDW nesting, while the decrease in the amplitude in high fields, to the breakdown of the ground state caused by its Zeeman splitting.     

Application of the dissipative particle dynamics method to magnetic colloidal dispersions

The validity of the application of the dissipative particle dynamics (DPD) method to ferromagnetic colloidal dispersions has been investigated by conducting DPD simulations for a two–dimensional system. First, the interaction between dissipative and magnetic particles has been idealized as some model potentials, and DPD simulations have been carried out using such model potentials for a two magnetic particle system. In these simulations, attention has been focused on the collision time for the two particles approaching each other and touching from an initially separated position, and such collision time has been evaluated for various cases of mass and diameter of dissipative particles and model parameters, which are included in defining the equation of motion of dissipative particles. Next, a multi–particle system of magnetic particles has been treated, and particle aggregates have been evaluated, together with the pair correlation function along an applied magnetic field direction. Such characteristics of aggregate structures have been compared with the results of Monte Carlo and Brownian dynamics simulations in order to clarify the validity of the application of the DPD method to particle dispersion systems. The present simulation results have clearly shown that DPD simulations with the model interaction potential presented here give rise to physically reasonable aggregate structures under circumstances of strong magnetic particle–particle interactions as well as a strong external magnetic field, since these aggregate structures are in good agreement with those of Monte Carlo and Brownian dynamics simulations.     

Si2O2 molecule: structure of ground state and the excited properties under different external electric fields

Geometry and vibrational frequencies of the ground state of Si 2 O 2 molecule are studied using density function theory (DFT) at the level of cc-pvtz and 6-311++G.It is found that the optimizing value by B3lyp/cc-pvtz is closer to the experimental data.The excited properties under different external electric fields are also investigated by the time-dependent-DFT method.Transitions from the ground state of Si 2 O 2 molecule to the first singlet state under different external electric fields can take place more easily.The corresponding absorption spectral line is about 360 nm in wavelength and the excitation energy is about 3.4 eV.     

Effect of heat treatment on electronic phase in underdoped La2-xSrxCuO4 single crystal

Superconducting La1.937Sr0.063CuO4 crystals grown by the travelling-solvent floating-zone technique were thermally treated under various temperatures and oxygen pressures for moderately adjusting the oxygen content. The response of intrinsic electronic property of the crystals to the change of hole density in La2-xSrxCuO4 in the vicinity of the magic doping of x= 1/16 （= 0.0625） is studied in detail by magnetic measurements under various fields up to 1 T. It is found that when the superconducting critical temperature （Tc） increases with the oxygen content, there appears also a new subtle electronic state that can be detected from the differential curves of diamagnetic susceptibility dx/dT of the crystal sample. In contrast with the intrinsic state, the new subtle electronic state is very fragile under the magnetic fields. Our results indicate that a moderate change in oxygen doping does not significantly modify the intrinsic electronic state originally existing at the magic doping level.     

Magnetoelectric and magnetoelastic interactions in NdFe3(BO3)4 multiferroics

Complex experimental and theoretical investigations of the magnetic, magnetoelectric, and magnetoelastic properties of neodymium iron borate NdFe₃(BO₃)₄ along various crystallographic directions have been carried out in strong pulsed magnetic fields up to 230 kOe in a temperature range of 4.2–50 K. It has been found that neodymium iron borate, as well as gadolinium iron borate, is a multiferroic. It has a much larger (above 300 μC/m²) electric polarization controlled by the magnetic field and giant quadratic magnetoelectric effect. The exchange field between the rare-earth and iron subsystems (～50 kOe) has been determined for the first time from experimental data. The theoretical analysis based on the magnetic symmetry and quantum properties of the Nd ion in the crystal provides an explanation of the unusual behavior of the magnetoelectric and magnetoelastic properties of neodymium iron borate in strong magnetic fields and correlation observed between them.