Impurity effects on the magnetization and magnetic susceptibility of an electron confined in a quantum ring under the presence of an external magnetic field

The magnetization and the magnetic susceptibility of a single electron confined in a two-dimensional (2D) parabolic quantum ring under the effect of external uniform magnetic field and in the presence of an acceptor impurity have been studied. The shifted 1/N expansion method was used to solve the Hamiltonian quantum ring within the effective mass approximation. The computed energy spectra, the magnetization and magnetic susceptibility have been displayed as a function of the quantum ring parameters: confinement strength ω₀, magnetic field strength (ω_c), and temperature (T). The obtained energy results show level-crossings, in the absence and presence of acceptor impurity, which are manifested as oscillations in the magnetization and magnetic susceptibility curves.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—I: Thermal equilibrium case

The far IR cyclotron resonance of conduction electrons is investigated in n-type indium antimonide in the quantum regimes, ?ω_ck_BT and ?ω_c?k_BT. The resonance peak position, half width, and the degree asymmetry in the line shape are studied as a function of temperature. In analyzing the experimental data, the three band model has been employed together with modern theoretical results of electron scattering by ionized impurities in the presence of a strong magnetic field. It is found that, for example for an experiment at 84 μm, the Une width depends very little on temperature between 4.2 and 45 K where the ionized impurity scattering is dominant, and increases rapidly with temperature above 45 K where the onset of phonon scattering is expected. Further details of the ionized impurity scattering were investigated by using three different laser wavelengths 84, 119 and 172μm. The line width at the phonon-limited temperature region depends very little on magnetic field and sample. The temperature dependence of the band gap was also determined by analysis of the resonance peak shift.     

CDW instability of electron gas in a strong magnetic field

It is shown that within the Hartree-Fock approximation the electron gas in the quantum limit of a strong magnetic field has an instability as the temperature is lowered toward the charge density wave state with an wave-vector which has finite components in the directions not only parallel but also perpendicular to the field. The critical temperature, T_c, is estimated under the assumption of T_c??_F?ω_c, where ?_F and ω_c are the Fermi energy of the non-interacting system and the cyclotron frequency respectively.     

Magnetic properties of (NdxY1-x)Co2(1⩾ x ⩾ 0)

The concentration dependence of T_c and T_R (T_c magnetic ordering temperature, T_R spin reorientation temperature) of the pseudobinary system (Nd, Y)Co₂ is reported. Furthermore the influence of an external magnetic field on the spin reorientation and the magnetization is studied. The observed variation of the magnetization in the vicinity of the spin reorientation is compared with theoretical results. For the calculation a Hamiltonian with terms describing a molecular field, a cubic crystal field, and an external field is used.     

The large polaron in a magnetic field: Possible existence of a phase transition

It is shown that the ground state energy of a polaron in a magnetic field (calculated in the Feynman approximation) exhibits a discontinuity in the first derivative with respect to α (the electron-phonon coupling constant) if ω_c/ω_o (cyclotron frequency to LO-phonon frequency) is larger than 2.24. This discontinuity has the characteristics of a first order phase transition if α is interpreted as an inverse temperature. For ω_c/ω_o = 2.24 the transition is second order. We found that below the transition point the phonon cloud cannot follow the electron in its motion in the magnetic field.     

Pressure effect on magnetization for (CoTm)90Zr10 (Tm = Cr,Mo) amorphous alloys

Temperature dependences of the forced volume magnetostriction dω/dH and the saturation magnetization σ_s for (CoTm)₉₀Zr₁₀ (Tm = Cr, Mo) amorphous alloys have recently been measured by the 3-terminal capacitance method and a vibrating sample magnetometer and magnetic balance at temperatures from 77 K to the Curie temperature T_c or the crystallization temperature. The pressure coefficient of σ_s0 at 0 K, d ln σ_s0/dp, is estimated from (dω/dH)₀ extrapolated to 0 K using the thermodynamical relation. The values of d ln σ_s0/dp increase in negative value with increasing Tm concentration. The relation between d ln σ_s0/dp and the pressure coefficient of T_c, d ln T_c/dp, estimated indirectly from dω/dH is discussed.     

Functional derivative of Tc with α2(ω)F(ω) for an anisotropic superconductor

The functional derivative δT_c/δα²(ω)F(ω) of the critical temperature (T_c) with the electron-phonon spectral density (α²(ω)F(ω)) gives information on the effectiveness of various phonon modes in enhancing T_c. For an anisotropic superconductor, it is found that δT_c/δα²(ω)F(ω) goes negative at some small but finite phonon energy. This contrasts with the isotropic case for which it is well known that the functional derivative is positive everywhere. Thus, very low energy phonons reduce T_c in an anisotropic superconductor which is similar to the known effects of static impurities that wash out anisotropy and hence reduce T_c.     

Coherent state and superconductivity in the free carrier-bipolaron interacting system

In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T¹ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T_c ∼ ω_c, where ω_c is the Debye frequency related with local electron-phonon coupling. In general we have T¹ > T_c ⪢ T_c0 (T_c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high-T_c superconductor.     

The sensitivity of the transition temperature to changes in α2 F(ω)

The transition temperature of a superconductor depends on α² F(ω), the spectral function of the effective interaction due to phonon exchange. We discuss how strongly the transition temperature is influenced by different frequency parts of α² F(ω). For this purpose the functional derivative δT _c /δα² F(ω) is calculated. It is shown that all frequency regions of α² F(ω) yield a positive contribution toT _c and that the most effective range covers frequencies, slightly above 2πT _c . The functional derivative is calculated numerically for several superconductors from their measured α² F(ω)-spectra. Finally, we discuss the change in transition temperature due to the softening of α² F(ω) which has been observed in amorphous superconductors.     

Influence of annealing on magnetic properties of Co-based metallic glasses

The influence of low-temperature annealing on the magnetic hysteresis loop parameters of magnetostrictive Co-Si-B and nonmagnetostrictive Co-Fe-Si-B glasses was studied. The dependence of Curie temperature (T_C), crystallization temperature (T_x), full-stress-relaxation temperature, saturation magnetostriction constant, saturation magnetization and coercive field H_c on metalloids contents for as quenched Co_100-x(Si_0.5B_0.5)_x glasses was determined.It was found that annealing enhances remanence magnetization for all investigated Co-Si-B glasses. The coercive field of these glasses is influenced by annealing owing to: stress relaxation (resulting in an H_c decrease) and domain structure stabilization (resulting in an H_c increase). Significant H_c reduction for both magnetostrictive and non-magnetostrictive glasses was observed, after annealing above T_C. For glasses with T_C<>T_x, it was necessary to apply an external magnetic field in ord er to decrease H_c.It was found that non-magnetostrictive metallic glasses with low Curie temperatures (T_C ? 450 K) exhibit the most stable magnetic hysteresis loop parameters.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Perturbative versus lattice QCD energy density correlators at high temperatures

Correlators of magnetic and electric field energy density are investigated for SU(N_c) gauge theory at high temperatures T. A separations z ⩽ 2/T the correlators are shown to be dominated by a power-law behavior even for finite gluon screening masses. This continuum behavior is well approximated on current 4 × 16³-lattices in the perturbative limit and leads to a considerable overestimate of screening masses deduced from fitting the lattice correlators with conventional exponential forms. The use of extended sources and sinks to enhance the signal improves the situation for screening masses m ⪢ T but leads to a largely uncontrolled error for masses less than T. In fact, we show that recent lattice QCD data of Grossmann et al., from which a magnetic screening mass m_M ∼ 2.9T was deduced, may even be consistent with a vanishing actual magnetic screening mass.     

Stability of bipolarons in the presence of a magnetic field

The stability of large Fröhlich bipolarons in the presence of a static magnetic field is investigated with the path integral formalism. We find that the application of a magnetic field (characterized by the cyclotron frequence ω _c) favors bipolaron formation: (i) the critical electronphonon coupling parameter α _c (above which the bipolaron is stable) decreases with increasing ω _c and (ii) the critical Coulomb repulsion strength U _c (below which the bipolaron is stable) increases with increasing ω _c. The binding energy and the corresponding variational parameters are calculated as a function of α, U and ω _c. Analytical results are obtained in various limiting cases. In the limit of strong electron-phonon coupling (α ? 1) we obtain for ω _c ? 1 that E _estim ? E _estim(ω _c = 0) + c(u)ω _c/α ⁴ with c(u) an explicitly calculated constant, dependent on the ratio u = U/α where U is the strength of the Coulomb repulsion. This relation applies both in 2D and in 3D, but with a different expression for c(u). For ω _c ? α ²? 1 we find in 3D E _estim ? ω _c - α ² A(u) ln²(ω _c/α ²), (also with an explicit analytical expression for A(u)) whereas in 2D E _estim ^2D ? ω _c - α√ω _cπ(u-2-√2)/2. The validity region of the Feynman-Jensen inequality for the present problem, bipolarons in a magnetic field, remains to be examined.     

The magnetic properties of GaAs parabolic quantum dot in the presence of donor impurity,magnetic and electric fields

The magnetization and the magnetic susceptibility quantities of a single electron moving in a two-dimensional (2D) parabolic quantum dot are studied under the influence of external uniform electric and magnetic fields, in the presence of a donor impurity. The Hamiltonian was solved using shifted 1/N expansion method within the effective mass approximation. The results have been displayed as a function of physical parameters: confinement strength ω₀, magnetic field strength ω_c, temperature T and electric field strength F.     

Role of inter-site exchange and hybrid interactions in itinerant ferromagnetism

In this work, we study the magnetic properties of itinerant electron systems using the Hubbard-like tight binding Hamiltonian along with inter-site exchange and hybrid interactions. We have used the mean-field approximation to deal with the exchange and hybrid interactions. It is found that hybrid interaction is more effective than exchange interaction for the on-set of ferromagnetic state. We have studied the effect of hybrid interaction on various physical quantities at different temperatures. The effective mass (m*/m) of up spin electrons increases slowly as the temperature decreases but below the critical temperature (T_c), it decreases rapidly. For down spin electrons effective mass increases slowly as the temperature decreases and below T_c, it increases more rapidly. Spectral weight (n/m*) for up spin electrons decreases slowly upto T_c and below T_c, it increases rapidly. For down spin electrons spectral weight decreases slowly upto T_c and below T_c, it decreases rapidly. Our results for both the effective mass and spectral weight are in good agreement with recently observed experimental behaviour in itinerant ferromagnet Ga_1−xMn_xAs [Phys. Rev. Lett. 89 (2002) 097203]. We have also studied variation of the spectral weight and optical absorption with temperature in presence of magnetic field. We found that these two quantities for up spin electrons increase as applied magnetic field increases at all temperatures (∼4T_c). For down spin electrons these two quantities decrease as applied magnetic field increases.     

Magnetic superconductors of HoMo6S8 type: The effect of magnetic field and supercurrent

We study the magnetic superconductors for which the second order phase transition to the ferro-magnetic state would take place at the temperature θ_c ? T_c1 if the superconductivity was absent. In this case the inhomogeneous magnetic structure of domain-like type (DS phase) appears in superconductors below T_M ≈ θ_c. The effect of the magnetic field H on DS phase is analysed and the region of DS phade existence in the plane (H, T) is found. The new peaks 2nQ (n is integer) should appear in the neutron scattering in the presence of the magnetic field. The wavevector Q of magnetic structure decreases with the growth of magnetic field or supercurrent.     

Domain structure of a thin single-crystal plate of terbium iron garnet near the magnetic compensation point

The domain structure of a thin single-crystal plate of the iron garnet Tb₃Fe₅O₁₂ has been investigated using the magneto-optical method in the temperature range near the magnetic compensation point of this ferrimagnet T _c = 248.6 K. It has been shown that, when the temperature of the sample approaches the magnetic compensation point, the domain width significantly increases, but remains finite at T = T _c . The magnetic H-T phase diagram, which determines the boundary between the multidomain and domain-free (uniformly magnetized) states of the sample, has been constructed using the data on visual observations of the transformation of the domain structure with variations in the temperature and external magnetic field. The results obtained have been interpreted in terms of the thermodynamic theory of stability of different magnetic phases of a two-sublattice cubic ferrimagnet near T _c .     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.     

Low-frequency electrical resistance of iron,cobalt, and nickel in the vicinity of their Curie temperatures

Alternating-current electrical resistance measurements between 17 Hz and 100 kHz were made on high purity Fe, Co, and Ni wires in the vicinity of their Curie temperatures (T _c). The electrical resistance was independent of frequency for temperatures (T) aboveT _c. As the temperature was lowered, however, there was an abrupt jump in the electrical resistance atT _c followed by a gradual decrease toward its dc value. The magnitude of the electrical resistance jump atT _c increased as the square root of the frequency. The enhancement of the electrical resistance forT≦T _c is produced by an abrupt decrease of the skin depth atT _c which, in turn, is due to the sudden increase in the initial magnetic permeability atT _c. Measurements of the ac electrical resistance in the vicinity of the Curie temperature of certain ferromagnetic metals can be utilized to 1) accurately determine the Curie temperature using frequencies as low as 17 Hz, and 2) quantitatively determine the initial magnetic permeability as a function of temperature and heat treatment.     

Enhancing of the critical temperature of an in-plane FFLO state in heterostructures by the orbital effect of the magnetic field

It is well-known that the orbital effect of the magnetic field suppresses superconducting T _c . We show that for a system, which is in the Larkin-Ovchinnikov-Fulde-Ferrell (FFLO) state at zero external magnetic field, the orbital effect of an applied magnetic field can lead to the enhancement of the critical temperature higher than T _c at zero field. We concentrate on two systems, where the in-plane FFLO state was predicted recently. These are equilibrium S/F bilayers and S/N bilayers under nonequilibrium quasiparticle distribution. However, it is suggested that such an effect can take place for any plane superconducting system, which is in the in-plane FFLO state (or is close enough to it) at zero applied field.