Ferromagnetic phases in spin-Fermion systems

Spin-Fermion systems which obtain their magnetic properties from a system of localized magnetic moments being coupled to conducting electrons are considered. The dynamical degrees of freedom are spin-s operators of localized spins and spin-1/2 Fermi operators of itinerant electrons. Renormalized spin-wave theory, which accounts for the magnon-magnon interaction, and its extension are developed to describe the two ferromagnetic phases in the system: low temperature phase 0 < T < T*, where all electrons contribute the ordered ferromagnetic moment, and high temperature phase T* < T < T _C , where only localized spins form magnetic moment. The magnetization as a function of temperature is calculated. The theoretical predictions are utilize to interpret the experimentally measured magnetization-temperature curves of UGe₂.     

Liouville and Fokker–Planck dynamics for classical plasmas and radiation

We consider a nonequilibrium statistical system formed by many classical non‐relativistic particles of opposite electric charges (plasma) and by the classical dynamical electromagnetic (EM) field. The charges interact with one another directly through instantaneous Coulomb potentials and with the dynamical degrees of freedom of the transverse EM field. The system may also be subject to external influences of: i) either static, but spatially inhomogeneous, electric and magnetic fields (case 1)), or ii) weak distributions of electric charges and currents (case 2)). The particles and the dynamical EM field are described, for any time t > 0, by the classical phase‐space probability distribution functional (CPSPDF) f and, at the initial time (t = 0), by the initial CPSPDF f_in. The CPSPDF f and f_in, multiplied by suitable Hermite polynomials (for particles and field) and integrated over all canonical momenta, yield new moments. The Liouville equation and f_in imply a new nonequilibrium linear infinite hierarchy for the moments. In case 1), f_in describes local equilibrium but global nonequilibrium, and we propose a long‐time approximation in the hierarchy, which introduces irreversibility and relaxation towards global thermal equilibrium. In case 2), the statistical system, having been at global thermal equilibrium, without external influences, for t ≤ 0, is subject to weak external charge‐current distributions: then, new hierarchies for moments and their long‐time behaviours are discussed in outline. As examples, approximate mean‐field (Vlasov) approximations are treated for both cases 1) and 2).     

Coaxial Discharge and Transverse Magnetic Field

The study deals with the effect of an applied transverse magnetic field on the dynamics and parameters of the focused and expanded plasma in a coaxial discharge. The experimental results were found with a 3 kJ Plasma focus device of a Mather geometry. The discharge takes place in hydrogen gas with base pressure of 0.5 Torr. The experiments are conducted with a 10 kV bank voltage, which corresponds to 100 kA peak discharge current with rise time 8 μs. Helmholtz magnetic coils are placed outside the expansions chamber to produce a transverse magnetic field with intensity 280 G perpendicular to the plasma expanded from the coaxial electrodes. The investigations have shown that the plasma flow along the expansion chamber axis is restricted when applying the externally transverse magnetic field and the maximum axial velocity of the expanded plasma is decreased by 33%. X-ray probe has been used to measure the focused plasma electron temperature (T_e). The experimental results and the calculations showed that T_e is decreased from 2.2 keV to 800 eV with the application of a transverse magnetic field. The expanded plasma electron temperature and density have been measured by an electric double probe, the results cleared that the expanded plasma electron temperature is decreased by 2.6 times while its density is increased by 9 times, when a transverse magnetic field is applied.     

Anfachung und Dämpfung von Ionisationswellen in MHD-Kanälen und ihr Einfluß auf die effektive elektrische Leitfähigkeit,den effektiven Hall-Parameter und die mittlere Elektronentemperatur

The phase velocity, the amplification rate and the critical Hall parameter are theoretically determined for ionization waves in a weakly ionized plasma streaming across a strong external magnetic field and bearing a current flowing perpendicular to both the magnetic field and the stream velocity. The investigations hold for seeded rare gases at any degree of seed ionization. The critical Hall parameter β_c depends on the degree of ionization, the ionization energy and the temperatures of electron gas T₀ and neutral gas T_g · β_c is always greater than one, if 0 < T₀ — T_g ? T₀ holds. The three-dimensional treatment indicates the existence of waves with a nonvanishing wave vector component in the direction of the magnetic field. The influence of ionization waves on mean current density, mean Hall field intensity and mean electron temperature is determined up to second order terms in the relative fluctuations of the electron temperature. The amplification of ionization waves reduces the effective electric conductivity, the effective Hall parameter and the mean electron temperature compared to the undisturbed state. Similar results are also obtained for steady state homogeneous isotropic turbulence and a special case of axially symmetric turbulence. Furthermore, a component of the electric field in direction or in opposite direction to the magnetic field vector may be generated by non isotropic and non homogeneous turbulence.     

Numerical investigation of the theta pinch in intrinsic indiumantimonide

The electron-hole plasma in intrinsic InSb at 300 K can be compressed by electric and magnetic fields. In the theta-pinch configuration one applies, by a capacitor discharge over a one-winding coil around the sample, a magnetic field (0, 0,B _z), which induces an electric field (0,E _φ, 0); both fields cause the ambipolar motion (v _r, 0, 0) of the plasma. We describe the electron-hole plasma by means of a magnetohydrodynamical two-fluid model. The resulting system of differential equations is solved numerically under the assumptions of scalar hydrodynamic pressure and field-independent mobilities. The influence of the plasma properties on the theta pinch is studied as well as the dependence on the experimental conditions like radius of the sample, maximum value and risetime of the magnetic field.     

Mass and dual mass: A gravitational analogue of the Dirac quantization rule and its physical implications

It is shown that (asymptotically multi-NUT) gravitational magnetic monopoles, which can be described by anS ³/Z _N principal Hopf-bundle structure at conformal null infinity (Z _N is a cyclic subgroup of orderN ofZ). provide a gravitational analogue of the Dirac quantization rule, which involves the total magnetic (dual) mass of the space-time-a measurement of the first Chern class of the bundle-and the mass of a test particle located in the rest frame defined at infinity by the Bondi (or dual Bondi) 4-momentum. It is shown thatSU ₂/U ₁ preserves the asymptotic structure. A definition of the angular momentum operator which extends that available for test electric charges in the field of a (Maxwellian) Yang-Wu magnetic monopole is presented. The commutation relations are dictated by the quantization rule. Various physical consequences are mentioned. SinceSU ₂ is a double covering ofSO ₃, gravitational magnetic monopoles provide a topological explanation for the existence of particles with half-integer spin. Abelian (U ₁), non-AbelianSU ₁ asymptotic degrees of freedom of the gravitational field could be related to suitable nontrivial cohomology classes; Penrose's nonlinear graviton modes could emerge as self (antiself) adjoint (Yang-Mills) gauge connections.     

Thermal expansion and magnetostriction in an anomalous Gd intermetallic compound

Thermal expansion and forced magnetostriction measurements are reported on two Gd intermetallic compounds which order magnetically below 10 K. The relative influence of the electronic, lattice and magnetic degrees of freedom was determined using results obtained on a non-magnetic isostructural compound. A Grüneisen analysis revealed that whilst the magnetic contribution to the specific heat is similar for both Pd₂GdIn and Cu₂GdIn the spontaneous magnetostriction was significantly smaller in the Pd compound. Forced magnetostriction measurements suggest that the thermal expansion in Pd₂GdIn is primarily associated with spin fluctuations in the Pd 4d band. It is suggested that these additional degrees of freedom give rise to the enhanced specific heat observed in Pd₂GdIn. Received 28 July 1999     

Thermodynamic analysis of the quantum critical behavior of Ce-lattice compounds

A systematic analysis of low-temperature magnetic phase diagrams of Ce compounds is performed in order to recognize the thermodynamic conditions to be fulfilled by those systems to reach a quantum critical regime or, alternatively, to identify other kinds of low-temperature behavior. Based on specific heat (C _m ) and entropy results, three different types of phase diagrams are recognized: (i) with the entropy involved in the ordered phase (S _MO) decreasing proportionally to the ordering temperature (T _MO); (ii) those showing a transference of degrees of freedom from the ordered phase to a non-magnetic component, with their C _m (T _MO) jumps (ΔC _m ) vanishing at finite temperature; and (iii) those ending at a critical point at finite temperature because their ΔC _m do not decrease sufficiently with T _MO, producing an entropy accumulation at low temperature.

Only those systems belonging to the first case, i.e. with S _MO?→?0 as T _MO?→?0, can be regarded as candidates for quantum critical behavior. Their magnetic phase boundaries deviate from the classical negative curvature below T?≈?2.5?K, denouncing monotonic misleading extrapolations down to T?=?0. Different characteristic concentrations are recognized and analyzed for Ce-ligand alloyed systems. In particular, a pre-critical region is identified where the nature of the magnetic transition undergoes significant modifications, with its ?C _m /?T discontinuity strongly affected by the magnetic field and showing an increasing remnant entropy at T?→?0. Physical constraints arising from the third law at T?→?0 are discussed and recognized from experimental results.     

Spectroscopical Observation of a Relaxing High Speed Nitrogen Plasma Jet

A near sonic nitrogen plasma jet operating at pressures between 100 Torr and 1 atm has been investigated spectroscopically. From the absolute emission coefficient of a NI spectral line, local values for the electron temperature T_e have been derived. For pressures above 200 Torr, T_g was found to coincide within the limits of experimental error with the gas temperature T_g. The latter quantity has been determined via the relative emission coefficient of selected rotational line components of the N₂⁺ molecular band at 3914 Å. The results of these measurements together with control data for the electron density derived from the continuous emission coefficient indicate that at pressures above 200 Torr the existence of a thermal equilibrium between the degrees of freedom corresponding to particle translation, electron excitation, and ionization can be accepted, at least for the inner zone of the plasma jet. To the contrary, the data for the absolute emission coefficient of N₂⁺ molecular band lines show that the degree of dissociation in the plasma jet is much in excess of that corresponding to equilibrium. This phenomenon can be explained as a result of the rapid temperature decay in the plasma from initially 13000 K in the arc heating zone to T ≦ 9000 K in the plasma jet zone proceeding in a time interval of 10^?5 s which is much shorter than the time necessary for adjustment of dissociation equilibrium. In the outer cool zone of the plasma jet, an unusual high intensity of the N₂⁺ radiation was found thus indicating the existence of a nonequilibrium excitation mechanism typical for a decaying nitrogen plasma. From the supernormal high degree of dissociation in the high-speed subatmospheric nitrogen plasma jet, conclusions are drawn with respect to its applicability as source of reactive particles in plasma-chemical experiments.     

Magnetized Stiff Fluid Cylindrically Symmetric Universe with Two Degrees of Freedom in General Relativity

A magnetized stiff fluid cylindrically symmetric universe with two degrees of freedom for perfect fluid distribution, is investigated. The magnetic field is due to an electric current produced along x-axis. The distribution consists of an electrically neutral perfect fluid with an infinite electrical conductivity. The behaviour of the model in presence and absence of magnetic field is discussed. The other physical aspects of the model related to the observations are also discussed.     

Behaviour of gluons at high temperature

The screening of colour electric and magnetic fields and plasma oscillations in a high-temperature gluon plasma are investigated using linear response theory and self-consistent nonperturbative solutions to the Schwinger-Dyson equation. Static electric fields are screened, with

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. This result is proven to be gauge invariant in two ways: by computing π₀₀ in temporal axial, Coulomb and covariant gauges, and by computing the physical free energy of a heavy quark pair in the plasma in temporal axial gauge. To order g³ static magnetic fields are not screened.     

On the Direct Generation of Electricity from Heat in Plasmas via a Self-Organization Process

By combining the Nernst effect with the law of Ampere, it is shown that if a temperature gradient is present in a plasma, an electric current can be self-sustained due to the transformation of thermal into electromagnetic field energy. We call the configuration in which the above phenomenon occurs the “thermomagnetic battery” and study its steady state operation for an infinite slab geometry assuming that the plasma is fully ionized and the magnetic field is weak (ω_eτ_e ? 1) so that the heat flow is mainly transferred by electrons.     

Charmonium states in quark-gluon plasma

We discuss how the spectral changes of quarkonia at T _c can reflect the ‘critical’ behaviour of QCD phase transition. Starting from the temperature dependencies of the energy density and pressure from lattice QCD calculation, we extract the temperature dependencies of the scalar and spin-2 gluon condensates near T _c. We also parametrize these changes into the electric and magnetic condensate near T _c. While the magnetic condensate hardly changes across T _c, we find that the electric condensate increases abruptly above T _c. Similar abrupt change is also seen in the scalar condensate. Using the QCD second-order Stark effect and QCD sum rules, we show that these sudden changes induce equally abrupt changes in the mass and width of J/ψ, both of which are larger than 100 MeV at slightly above T _c.      

在Tokamak装置中用中空超导环段冻结环向磁场的设想

本文提出一个设想:即将n个中空超导圆柱体放置于n饼纵场线圈之中,分段冻结环向磁场,则其所需要的电功率将比常规线圈降低到2/n倍。我们知道超导材料的冻结场可近似地写为。H_fj(r,T)=H_fj(r,0)[1-(T/(T_c))²。因此选定T=T₁,当轴向场H在大圆环的内壁达到H≥H_fj(r,T₁)时,中空体的内边缘达到临界场,则场进入中空区,再使H降到零,中     

Stationary collisionless shock waves in an initial plasma with high ion temperature

Stationary collisonless shock waves propagating perpendicularly to an initial magnetic field are produced by the fast-rising magnetic field \((\dot B = 7 \cdot 10^{10} G/sec)\) of a theta pinch (coil diameter 16 cm, coil length 60 cm). The initial plasma is produced by a fast theta pinch discharge (810 kHz). At filling pressures between 5 and 15 mtorr H₂ or D₂ the degree of ionization is about 50%. By choosing the filling pressure properly it is possible to trap a homogeneous magnetic field. The ions of this plasma have a temperature of a few 10 eV. This value is much higher than the electron temperature and results in a local plasmaβ between 0.3 and 5. In this initial plasma stationary collisionless shock waves with Mach numbers between 1.5 and 5 are observed. The snow-plough model is used to derive conditions for the stationary state, attainable Mach number, and velocity of the front which relate the external magnetic field and the parameters of the initial plasma. Strong collisionless dissipation can be demonstrated by measuring the profiles of magnetic field, density, and electron temperature of the shock waves. For the electrons this dissipation mechanism can be described by an effective collison frequency. This phenomenologically introduced frequency determines the width of the shock front at least for subcritical shock waves. It exceeds the classical electron-ion collision frequency by 1–2 orders of magnitude and is roughly equal to one-third of the ion plasma frequency. The ion temperature can be estimated from the steady state conservation relations. The ions are heated in the two degrees of freedom perpendicular to the magnetic field. For shock waves with Mach numbers below the critical one the ions seem to be heated merely adiabatically. In strong shock waves this heating is considerably exceeded, and for high Mach numbers it yields ion temperatures up to about 500 eV. Finally, semi-empirical formulas are derived to estimate the possible temperatures of electrons and ions behind the shock front.     

Multiglass order and magnetoelectricity in Mn<Superscript>2+</Superscript> doped incipient ferroelectrics

“Multiglass” materials with simultaneous occurrence of two different glassy states extend the frame of conventional multiferroicity, which is devoted to crystalline materials with coexisting uniform long-range electric and magnetic ordering. The concept applies to Sr_0.98Mn_0.02TiO₃ ceramics, where A-site substituted Mn²⁺ ions are at the origin of both a polar and a spin cluster glass. Spin freezing is initiated below the dipolar glass temperature, T_g ^e ≈ 38 K, which is seemingly indicated by a divergence of the nonlinear susceptibility, χ₃. Below T_g ^m ≈ 34 K both glass phases are independently verified by memory and rejuvenation effects. Biquadratic interaction of the Mn²⁺ spins with ferroelectric correlations of their off-center pseudospins in the incipient ferroelectric host crystal SrTiO₃ explains the high spin glass temperature and comparably strong third-order magnetoelectric coupling between the polar and the magnetic degrees of freedom. Preliminary results on the related compound K_0.97Mn_0.03TaO₃ favorably comply with the magnetoelectric multiglass concept.     

Zeeman and orbital limiting magnetic fields in cuprates: The pseudogap connection

In cuprates, in a view where pairing correlations set in at the pseudogap energy scale T* and acquire global coherence at a lower temperature T_c, the regionT _c⪯ T ⪯ T* is a vast fluctuation regime.T _c andT* vary differently with doping and the question remains about the doping trends of the relevant magnetic field scales: the field H_c2 bounding the superconducting response and the pseudogap closing field H_pg. In-plane thermal (Nernst) and our interlayer (tunneling) transport experiments in Bi₂Sr₂CaCu₂O_8+y report hugely different limiting magnetic fields. Here, based on pairing (and the uncertainty principle) combined with the definitions of the Zeeman energy and the magnetic length, we show that both fields convert to the same pseudogap scaleT* upon transformation as orbital and Zeeman critical fields, respectively. The region of superconducting coherence is confined to the ‘dome’ that coincides with the usual unique upper critical field H_c2 on the strongly overdoped side. We argue that the distinctly different orbital and the Zeeman limiting fields can co-exist owing to charge and spin degrees of freedom separated to different parts of the strongly anisotropic Fermi surface.     

Neutron diffraction study of multipole order in light rare-earth hexaborides

Multipole interactions are known to play a central role in the unconventional properties of light rare-earth hexaborides and especially of CeB₆. Substituting Pr at the Ce sites has the effect of enhancing exchange interactions and changing the symmetry of the local 4f charge distribution, while suppressing the octupole moment. The (T, H) magnetic phase diagrams of the Ce _x Pr_{1 − x} B₆ compounds display a large variety of ordered phases involving magnetic and/or charge degrees of freedom. Here we focus on the compound Ce_0.7Pr_0.3B₆, which is located slightly beyond the Pr concentration where the antiferroquadrupolar phase of pure CeB₆ is suppressed in zero field. The different magnetic structures have been characterized by neutron diffraction and their origin is discussed in connection with recent non-resonant X-ray results by Tanaka et al.      

Temperature dependence of the critical magnetic field of the structural transition in MBBA-based ferronematics

We studied the structural transitions in ferronematics based on the thermotropic nematic liquid crystal MBBA (4^′ -methoxybenzylidene-4-n-butylaniline) having a nematic-to-isotropic transition temperature T _N–I?=?48.0^○C and in MBBA-based ferronematics doped with a magnetic suspension consisting of Fe₃O₄ particles (10?nm in diameter) coated with oleic acid as a surfactant. The ferronematic samples were prepared with different volume concentrations of magnetic particles φ?=,1× 10^?4, 2× 10^?4 and 5×10^?4. The temperature dependences of the critical magnetic fields in a bias electric field under strong applied magnetic fields are presented. We calculated the surface density of anchoring energy W at the nematic–magnetic particle boundary. Scaling of the structural transition in the MBBA and MBBA-based ferronematics with the temperature of the nematic-to-isotropic transition was observed.     

Hot electron plasmas trapped in helical magnetic surfaces

Experimental studies on nonneutral (pure electron) plasmas of finite temperature, trapped in helical closed magnetic surfaces have been conducted. The helical electron plasmas are produced with thermal electrons launched from the outside of the last closed flux surface (LCFS). About 150 μs after the electron injection, the plasmas reach equilibrium state. Around the LCFS, a steep gradient of plasma space potential φ _s is formed. The corresponding radial electric field is about 2.5 kV/m. On the other hand, around the magnetic axis of helical magnetic surfaces, φ _s is almost constant, indicating that there are little electrons there. The volume-averaged electron density is on the order of 10¹³ m^–3, smaller than the Brillouin density limit. The confinement time seems to be limited by a disruptive instability, and is so far about 1.5 ms.