Magnetic field dependence on transition temperature Tc in cuprate superconductors

We investigate the magnetic field dependence on T_c in the high transition temperature superconductors. It is shown that phonon-enhanced spin fluctuations drive this superconductivity once more suggested by us [Phys. Rev. B 61 (2001) 4289]. We know magnetic field dependence on our transition temperature is in good correspondence with experimental data. It is elucidated that the external field is closely related to the local internal field in order to influence spin fluctuations.     

Improvement to the exchange field acting upon Tb ions in TbIG

The spin fluctuations of the magnetic ions play an important role on the magnetic properties of the crystals and lead to a new mechanism for the Curie-Weiss susceptibility. The exchange field H_exch acting on the rare-earth ions in Tb:YIG is improved based on the temperature dependence of the spin fluctuations, which is expressed as H_exch=n₀(1+γT+βT⁻²)M_YIG. By means of the improved exchange field, the magnetic and magneto-optical properties of Tb³⁺ ions in Tb:YIG are calculated. The calculated results are in good agreement with the measured data in the temperature range from 40 to 300 K.     

Monte Carlo simulation of spin-wave excitation for the magnetic films with structure and interaction fluctuations

Monte Carlo simulation studies are performed to examine influence of structure and interaction fluctuations on magnetic properties of a ferromagnetic system modelled with a Heisenberg Hamiltonian. It is found that the spontaneous magnetization at low temperature for the multilayered films decreases with temperature in a Bloch law of spin-wave excitations. Both Bloch coefficient B and exponent b vary evidently because of a strong surface and size effect in the finite magnetic films with free boundaries. For the disordered bulk FCC magnet with periodic boundary, the Bloch T^3/2 law is followed at low temperature and B is greatly influenced by the structure and interaction fluctuations. At the same time, Bloch coefficient B of the amorphous magnet with the coordination and interaction fluctuations has been derived. The simulated results are in good agreement with the theoretical predictions of spin-wave excitation, and explain the experimental facts well.     

Radiative noise in circuits with inductance

Using the general idea of the fluctuation–dissipation theorem we study a new contribution to the voltage fluctuations which is associated with the presence of radiation resistance. We consider the particular case of a solenoid immersed in a cavity with equilibrium radiation at temperature T. We prove that these new fluctuations are generated by the random magnetic field present in the cavity. These magnetic voltage fluctuations are shown to be experimentally distinguishable from the voltage fluctuations associated with the well known Nyquist noise. Accordingly we suggest feasible experiments to measure this magnetic noise. All the calculations are made within the context of Stochastic Electrodynamics, a theory in which the vacuum zero-point field is taken as a real electromagnetic field. We also study the average energy of an RLC circuit in thermodynamic equilibrium with the radiation.     

Study on the competition between density waves, singlet, and triplet pairing superconductivity in organic conductors (TMTSF)2X

We study the effect of dimerization of TMTSF molecules and the effect of magnetic field (Zeeman splitting) on the phase competition in quasi one-dimensional organic superconductors (TMTSF)₂X by applying the random phase approximation method. As for the dimerization effect, we conclude that due to the decrease of the dimerization, which corresponds to applying the pressure and cooling, spin and charge density wave states are suppressed and give way to a superconducting state. As for the magnetic field effect, we find generally that spin-triplet pairing mediated by a coexistence of 2k_F spin and 2k_F charge fluctuations can be strongly enhanced by applying magnetic field rather than triplet pairing due to a ferromagnetic spin fluctuations. Applying the above idea to (TMTSF)₂X compounds, a magnetic field induced singlet-triplet transition is consistent with above mechanism in (TMTSF)₂ClO₄.     

Universal scaling properties of extreme type-II superconductors in magnetic fields

Anisotropic Ginzburg-Landau superconductors of extreme type-II are considered in an approximation where magnetic field fluctuations are neglected. A formulation of the scaling properties is presented for the singular part of the free energy density in the presence of a magnetic field. From the existence of a magnetization, a diamagnetic susceptibility and superconductivity we determine the limiting behavior of the scaling function in the vicinity of the zero field transition temperature, where critical fluctuations dominate. Our predictions for the temperature and field dependence of magnetization, magnetic torque and melting line etc., uncover the universal critical properties and provide an extension of hitherto used mean-field treatments. The results are consistent with experimental data. Received: 24 April 1998 / Accepted: 5 May 1998     

Clear experimental evidence for boundary and impurity scattering related crossover field scales in GaAs/AlGaAs split-gate wires

We study the magnetic field dependence of the correlation field ΔB_cand amplitude δgof the conductance fluctuations, observed in the low temperature magnetoresistance of GaAs/AlGaAs split-gate wires. Near zero field, universality of quantum interference is retained and the magnetoresistance shows universal conductance fluctuations. At high magnetic fields, although the discrete Landau level quantization becomes resolved. ΔB_cand δgare found to increase linearly with magnetic field, with a slope which depends upon the nature of electron scatterings in the wire.     

Magnetization studied on spin alignments in organic molecule-based ferrimagnetics

A physics picture of spin alignments in molecule-based ferrimagnets is presented from studying the temperature dependence of the effective sublattice magnetic moments and the total reduced magnetization by means of Green’s function theory combined with the Jordan-Wigner transformation. The ferrimagnetic chain includes an S=1 biradical and an S=1/2 monoradical with antiferromagnetic alternating interactions, and the S=1 site in the chain is composed of two S=1/2 spins coupled by a finite ferromagnetic interaction. From the calculations of the sublattice magnetic moments, the magnetic moment of the S=1 biradical is negative, while that of the S=1/2 monoradical is positive, leading to a ferrimagnetic ground state. With the different kinds of the elementary excitations and the competition between the magnetic interactions and thermal fluctuations, the temperature dependence of the magnetization displays rich thermodynamic properties. Meanwhile, the external magnetic field dependence of the magnetization has a clear plateau at one third of the saturation magnetization, which can be compared with the possible experimental findings.     

Quasi-two-dimensional conductivity and magnetoconductivity of graphite-like nanosize crystallites

The temperature dependence of electrical conductivity and magnetoconductivity of new type of carbon films composed of nanosize thin graphite-like crystallites were investigated at temperature interval of 4.2-300 K and in the magnetic field range of 0-12 kG at 4.2 K, respectively. The crystallites consist of several (5-50) graphene layers which have predominant orientation perpendicularly to a film surface. At temperature ≤30 K the logarithmic conductivity decreases linearly with temperature. The positive magnetoconductivity of the films was observed in a magnetic field directed perpendicularly to the film surface in all intervals of field values. In magnetic field B≥4 kG the logarithmic asymptotic of conductivity from magnetic field was observed. That is characteristic of the systems with two-dimensional quantum corrections to magnetoconductivity. In a magnetic field directed along a film surface, the crossover from negative to positive magnetoresistivity is observed at B≥8 kG.     

Magnetic Fluctuation Measurement in Sino United Spherical Tokamak Plasma

To investigate the magnetic fluctuations and for further transport study, the poloidal and radial magnetic field t is conducted on the Sino United Spherical Tokamak （SUNIST）. Auto-power spectral density indicares that the magnetic fiuctuation energy mainly concentrates in the frequency region lower than lO kttz. The magnetic field oscillations, which are characterized by harmonic frequencies of 40 kHz, are observed in the scrape- off layer; by contrast, in the plasma core, the magnetic fluctuations are of Gaussian type. The time-frequency profiles show that the poloidal magnetic fluctuations are temporally intermittent. The autocorrelation calculation indicates that the fluctuations in decorrelation time vary between the core and the edge.     

The effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system

We investigate the effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system by applying the concept of negativity. It is found that the nonlinear couplings favor the thermal entanglement creating. Only when the nonlinear couplings ∣K∣ are larger than a certain critical value does the entanglement exist. The dependence of the thermal entanglement in this system on the magnetic field and temperature is also presented. The critical magnetic field increases with the increasing nonlinear couplings constant ∣K∣. And for a fixed nonlinear couplings constant, the critical temperature is independent of the magnetic field B.     

Fluctuations in high field superconductors

We present calculations for the influence of fluctuations in high field superconductors where the critical field is limited by Pauli paramagnetism. Due to the fact that the critical field at the second order phase transition point as function of temperature may have a maximum atT≠0 the additional conductivity due to fluctuations may have a nonmonotonic temperature dependence. This way we can account for recent experimental findings by Tedrow, Meservey and Schwartz. We also calculate the additional tunneling density of states due to fluctuations. Under proper conditions it exhibits a maximum at zero frequency like in the gapless regime. Finally we show that our findings of a nonmonotonic resistivity should also apply to superconductors containing magnetic impurities such as La_3-x Gd _x In in an external field.     

Electron phonon induced cyclotron resonance

The optical absorption coefficient of free polarons in a magnetic field is calculated for weak coupling at zero temperature in the E//H configuration. Characteristic fluctuations in the absorption are reported.     

Theoretical investigation of magnetic property in paramagnetic neodymium gallium garnet under high magnetic field

The magnetic property in neodymium gallium garnet (NdGaG) is studied by the quantum theory. The ground configuration split states are calculated taking into account the spin–orbit interaction and crystal field effect. Taking account of the Nd–Nd exchange interaction, a good agreement between experimental and theoretical values can be obtained for the variation of the magnetic moment with the external magnetic field under “extreme” conditions (low temperature and high magnetic field). Moreover, the temperature dependence of magnetic moment and the magnetic susceptibility χ is also discussed. Above 30 K, the magnetization (M) shows a linear field (H_e) dependence.     

Magnetic field induced ordering in quasi-one-dimensional quantum magnets

Three-dimensional magnetic ordering transitions are studied theoretically in strongly anisotropic quantum magnets. An external magnetic field can drive quasi-one-dimensional subsystems with a spin gap into a gapless regime, thus inducing long-range three-dimensional magnetic ordering due to weak residual magnetic coupling between the subsystems. Compounds with higher spin degrees of freedom, such as N-leg spin-1/2 ladders, are shown to have cascades of ordering transitions. At high magnetic fields, zero-point fluctuations within the quasi-1D subsystems are suppressed, causing quantum corrections to the ordering temperature to be reduced. Received 24 March 2000     

Nature of the quantum spin correlations through the superconducting-normal phase transition in electron-doped superconducting Pr0.88LaCe0.12CuO4

We use neutron scattering and specific heat measurements to relate the response of the spin fluctuations and static antiferromagnetic (AF) order to the superconductivity in the electron-doped high-transition-temperature superconductor, Pr_.88LaCe_.12CuO_4−δ (PLCCO) (T_c=24 K), as the system is tuned via a magnetic field applied beyond the upper critical field (H_c2) and driven into the normal state. The strength of the collective magnetic excitation commonly termed “resonance” decreases smoothly with increasing field and vanishes in the normal state, paralleling the behavior of the superconducting condensation energy. The suppression of superconductivity is accompanied by a smooth reduction in the very low energy spin fluctuations, and the concomitant emergence of static AF order. Our results suggest an intimate connection between the resonance and the superconducting condensation energy.     

Investigation of magneto-anisotropy field for system of particles with near superparamagnetic volume

We obtained the temperature dependence for low-field boundary of the anisotropy field distribution in a system of barium hexaferrite nanocrystals in the temperature range from 300 to 700 K. We treated the experimental data taking into account the influence of thermal fluctuations on the anisotropy field and the transition of particles into the paramagnetic state, stimulated by external magnetic field. We showed that the dependence under consideration is formed by particles of different volume, which increased from 3.5×10⁻¹⁸ to 40×10⁻¹⁸ cm³ while the particles lost their magnetic stability with the temperature growth.     

Weak localization and electron-electron interactions in thin Pd and PdHx films

We investigate the temperature and magnetic field dependence of the resistance R of thin Pd and PdHx films. We show that the logarithmic behaviour R(T) ≈ A ln T is dominated in Pd by the interaction effect. Loading progressively with hydrogen shows first no change in A and then a decrease followed by a sign reversal when superconducting fluctuations prevail. The application of a magnetic field restores then a positive and large value of A. These data are analyzed within recent theories.     

Residual conductance fluctuations of tiny disordered conductors

Conductances of the equivalent samples differ randomly (Stone 1985). At zero temperature these fluctuations were found to be of the order ofe ²/h for samples of arbitrary size and form (Altshuler 1985; Lee and Stone 1985). Experimentally such fluctuations manifest themselves as e.g. the reproducible aperiodic oscillations of the given sample conductance in magnetic field (Webbet al 1985; Stone 1985). These oscillations can be understood in terms of the correlation function (Lee and Stone 1985; Altshuler and Khmel’nitskii 1985) of the conductances in different fields. The characteristic field scale of the aperiodic oscillations corresponds to the unit magnetic flux through the sample. Conductance fluctuations decrease with the growth of temperature if the sample size is larger than the diffusion length within the timeh/T (Stone 1985; Lee and Stone 1985; Webbet al 1984, 1985; Altshuler and Khmel’nitskii 1985). These fluctuations are proportional toT ^?1/4,T ^?1/2 logT, andT ^?1/2 in the 3-d, 2-d and 1-d cases, respectively (Altshuler and Khmel’nitskii 1985) (the experiments of Webbet al 1984, 1985 correspond to the latter case). Random potential in tiny samples breaks all space symmetries. All effects which are forbidden in the average by these symmetries should manifest themselves by (i) conductance anisotropy, (ii) its dependence on the electric field direction and (iii) giant generation of the second harmonic in the granular sample under light radiation (Altshuler and Khmel’nitskii 1985). Conductance changes aperiodically with variation of the chemical potential (Lee and Stone 1985). Because of this thermopower fluctuations are much larger than its average value (Altshuler and Khmel’nitskii 1985). Conductance fluctuations are very sensitive to the random impurity potential variations (Altshuler and Spivak 1985). For instance, the change of the film conductance due to the shift ofone impurity isfinite for any film size. This effect can be used for the super flow impurity diffusion investigations. Variations of the localized spins realization in spin glasses change the conductance. This can explain (Altshuler and Spivak 1985) the conductance dependence on the magnetic field direction observed by Webbet al (1984, 1985).