Possibility of two-stage flux penetration in antiferromagnetic superconductors

An induced ferromagnetism in antiferromagnetic superconductors is possible, caused by a magnetic structure of vortex lines appearing in an external magnetic field strong enough to “flip over” the spins in the vortex core from their antiferromagnetic configuration. If the magnetic field is less than the “flip over” field the vortex line is entirely in the antiferromagnetic phase. Therefore the vortex interaction with the surface of a sample is altered when an applied magnetic field exceeds the “flip over” field. This mechanism makes the appearance of a new energy barrier that strongly influences the flux penetration possible. An estimation of the second critical entry field is made for DyMo₆S₈.     

Surface Superconductivity¶in Applied Magnetic Fields Above HC2

In this paper we study the surface superconductivity phenomenon of type 2 superconductors under applied magnetic fields above the critical field . We show that, for a cylindrical superconductor of infinite height placed in a homogeneous applied magnetic field, H e ₃, with H lying in between and keeping away from and , superconductivity persists uniformly at a thin sheath surrounding the entire lateral surface of the sample. As the applied field decreases gradually, superconductivity in the surface sheath becomes strong and develops into a surface superconducting state, while the interior of the sample remains close to the normal state. Received: 21 May 2001 / Accepted: 11 February 2002     

Electronic correlation effects on the properties of an half-filled octahedron cluster in a magnetic field

The present study focuses on electronic correlation effects on magnetic energy, the spin-spin correlation function of an octahedron cluster in the (3↑, 3 ↓) electronic configuration threaded by a magnetic field. Some other spin configurations are also discussed and various field directions are considered. An accurate diagonalisation technique has been used to solve the Hubbard Hamiltonian. A result is analysed on a linear energy stabilisation at low magnetic flux. Moreover, two types of antiferromagnetic transition versus the flux occurring for a correlation term larger than a critical one have been observed, i.e. the likelihood of a charge excitation before the antiferromagnetic transition. Finally, a comparison between the results obtained from the exact diagonalisation and the Gutzwiller method has been carried out, leading to a suggested modification of the Gutzwiller approach in order to improve it. Received 23 June 1999 and Received in final form 28 July 2000     

CEF nature of the magnetic excitations in ordered HoNi B C

The magnetic excitations in the antiferromagnetic phase of HoNi₂B₂C are studied by inelastic neutron scattering on single crystals for the first time. Spectra measured at constant T = 2 K along symmetry directions of the reciprocal space are well explained in terms of crystal electric field (CEF) magnetic excitons within the J = 8 ground state multiplet of Ho³⁺. Very modest bandwidth with planar energy dispersion describes the magnetic exciton dynamics. A perturbative model approach consisting of the CEF states in the effective exchange mean-field provides a simple but applicable characterization of the experimental observations. The microscopic determination of the relevant exchange parameters is discussed in connection with previous works on the subject. Received 25 February 2002 / Received in final form 13 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: nordal.cavadini@psi.ch     

Zeeman effects on d-wave superconductor and tunneling spectrum in normal-metal/d-wave superconductor tunnel junction

We study the Zeeman effect on the d-wave superconductor and tunneling spectrum in normal-metal(N)/d-wave superconductor(S) junction by applying a Zeeman magnetic field to the S. It is shown that: (1) the Zeeman magnetic field can lead to the S gap decreasing, and with the increase in Zeeman energy, the superconducting state is changed to the normal state, exhibiting a first-order phase transition; (2) the Zeeman magnetic field may make the zero-bias conductance peak split into two peaks, and the energy difference between the two splitting peaks in the conductance spectrum is equal to 2h ₀ (h ₀ is the Zeeman energy); (3) both the barrier strength of interface scattering and the temperature can lower the magnitudes of splitting peaks, of which the barrier strength can lead to the splitting peaks becoming sharp and the temperature can smear out the peaks, however, neither of them can influence the Zeeman effect.     

Surface impedance of a superconducting slab in the mixed state: Effect of the surface barrier

The effect of the surface barrier on the surface impedance Z of a type-II superconductor slab with a finite thickness is investigated in dc magnetic fields H ₀, which are aligned parallel or perpendicular to the slab plane. It is demonstrated that, in a perpendicular geometry, the surface resistivity ρ_s=ReZ has a maximum when the depth of penetration of the ac magnetic field is of the order of the slab thickness (size effect). For a parallel orientation of the magnetic field H ₀, the effect of the Bean-Livingston surface barrier manifests itself as a decrease in the dissipative loss and a change in the field dependence of the surface resistivity characterized by a magnetic hysteresis. It is shown for the first time that, under the conditions of persistent trapped magnetic flux, the dependence ρ_s(H ₀) is a decreasing function, which is associated, in particular, with a nontrivial suppression of the size effect.     

Dynamics of the magnetic flux component normal to the layers of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8</Subscript> in a tilted field

High-frequency losses in the strongly anisotropic layered superconductor Bi₂Sr₂CaCu₂O₈ are measured at 600 MHz under a magnetic field rocking about the ab plane. Anomalies in losses and hysteretic phenomena are found while performing periodic rocking, i.e., cycling the magnetic field component normal to the sample surface. Based on these observations, conclusions are drawn about the nature of magnetic-flux penetration into the superconductor. It is found that, in the range between 60 K and T _c , the dynamics of magnetic-flux vortex lines normal to the ab plane in the presence of a constant magnetic field applied parallel to this plane is governed by the critical penetration field H _c ^⊥* and the surface barrier in the presence of thermally activated vortex motion (giant flux creep). The dependences of H _c1 ^⊥* and the characteristic field of the surface barrier on the magnitude of the parallel magnetic field are measured.     

Magnetic transition and polaron crossover in a two-site single polaron model including double exchange interaction

A two-site double exchange model with a single polaron is studied using a perturbation expansion based on the modified Lang-Firsov transformation. The antiferromagnetic to ferromagnetic transition and the crossover from small to large polaron are investigated for different values of the antiferromagnetic interaction (J) between the core spins and the hopping (t) of the itinerant electron. Effect of the external magnetic field on the small to large polaron crossover and on the polaronic kinetic energy are studied. When the magnetic transition and the small to large polaron crossover coincide for some suitable range of J/t, the magnetic field has very pronounced effect on the dynamics of polarons. Received 1 June 2000     

Magnetic phase transition and magnetic structure of ground state in CsDyW O

Magnetic phase transition in the CsDyW₂O₈ magnet has been studied by means of low temperature specific heat C ( T ) measurements. The magnetic ordering temperature of the Dy³⁺ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions between Dy³⁺ ions. The behavior of the C ( T ) dependencies above and below T _N is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure of the ground state. Received 7 January 2002 / Received in final form 15 May 2002 Published online 7 September 2002     

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.     

Magnetic flux penetration into high-temperature superconductors in the vortex liquid state under the blow-up conditions

The problem of magnetic field penetration into a type-II high-temperature superconductor that is in the weakly pinned vortex-liquid phase is considered. A magnetic field on the superconductor boundary rises with time in the blow-up regime. A model hydrodynamic equation describing the magnetic induction distribution in the vortex-liquid phase for thermomagnetic motion of the flux is derived. Analytical expressions for the depth and rate of magnetic field penetration into the superconductor are found. It is demonstrated that these quantities depend on parameters of the problem: index of power n in the boundary regime characterizing the penetration rate of vortices into the superconducting half-space and a parameter describing the effect of random pinning forces and thermal fluctuations on the magnetic flux distribution.     

Vortex lattice symmetry in hexagonal superconductor CaAlSi probed by small angle neutron scattering

The small angle neutron scattering diffraction patterns from the flux line lattice state in the layered hexagonal superconductor CaAlSi are observed. Under an applied magnetic field (H) parallel to the crystalline c-axis, a hexagonal vortex structure is observed over the entire temperature/field regions. On the other hand, the vortex configuration under H∥a shows an ellipsoidal arrangement of the first-order Bragg peaks due to the anisotropic penetration depth. It was inferred from these results that the vortex state characterized by penetration depth and coherence length in CaAlSi may be described by that of anisotropic uniaxial superconductor using London theory.     

Antiferromagnetism and superconductivity in a model with extended pairing interactions

The competition between antiferromagnetism and the d + id superconducting state is studied in a model with near and next near neighbour interactions in the absence of any on-site repulsion. A mean field study shows that it is possible to have simultaneous occurrence of an antiferromagnetic and a singlet d + id superconducting state in this model. In addition, such a coexistence generates a triplet d + id superconducting order parameter with centre of mass momentum Q = (π,π) dynamically having the same orbital symmetry as the singlet superconductor. Inclusion of next nearest neighbour hopping in the band stabilises the d_xy superconducting state away from half filling, the topology of the phase diagram, though, remains similar to the near neighbour model. In view of the very recent observation of a broad region of coexistence of antiferromagnetic and unconventional superconducting states in organic superconductors, the possibility of observation of the triplet state has been outlined. Received 30 November 2000 and Received in final form 27 March 2001     

The propagation of magnetostatic surface waves in ferrite/superconductor structures

An electrodynamic model that describes the dispersion of magnetostatic surface waves in ferrite/superconductor structures is suggested. On its basis, a new approach to determining the microwave sheet resistance R _S of superconducting films in a magnetic field is elaborated. The values calculated (R _S =0.20–0.96 mΩ) agree with results obtained by the Tauber method. For YIG/YBCO structures, the controllable phase shift is about 1.5π when the depth of magnetostatic wave penetration into the YBCO film varies from 2.0 to 0.8 μm.     

Quantum fluctuations and ground state of weakly interacting helix spin chains in a magnetic field

Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J₁ develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J₂ is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit. Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997     

The first principle study on the electronic structure and spin ordering of the rare earth palladium sulfide, EuPd3S4

We have performed relativistic first-principles full-potential linearized augmented plane wave (FLAPW) calculation for rare earth palladium sulfide EuPd₃S₄ in the ferromagnetic and antiferromagnetic states. The density of 4f electrons of Eu is taken from a local-spin-density approximation self-interaction correction (LSDA-SIC) atomic calculation. EuPd₃S₄ is found to exhibit antiferromagnetic ordering in its ground state. The charge, orbital, magnetic moment and spin ordering are explained with the electronic structure, the orbital-projected density of states and the total energy study. EuPd₃S₄ is found to be stable in the body-centered Type-I antiferromagnetic state, in agreement with experimental results. Different Eu states are found in antiferromagnetic ordering. The magnetic moments of different states obtained through spin-polarized calculation are also in good agreement with experimental results. The phenomena observed are explained by the orbital hybridization of Eu and Pd ions as compared with the free ions.     

Ferrimagnetic ground state of a phenylene polymer with organic radicals

A ferrimagnetic polymer with m-phenylene skeleton as coupling unit is studied with the Hubbard model in the self-consistent mean-field theory. The ferrimagnetic ground state with a total spin S = 1 per unit cell is obtained and originates from the antiferromagnetic correlations between the nearest neighbors. If the on-site electron-electron repulsions at the radical sites and at the phenylene ring sites are different, the gap in energy band structure may disappear and the ferrimagnetic ground state becomes unstable. The charge density and spin density can transfer between the radical sites and the phenylene ring sites due to the competition between the hopping integral and the on-site repulsion at different sites. Received 15 July 2002 Published online 31 December 2002     

Characteristics of transmission of electrons in a quantum wire tangentially attached to a superconductor ring threaded by magnetic flux

We present numerical investigations of the transmission properties of electrons in a normal quantum wire tangentially attached to a superconductor ring threaded by magnetic flux. A point scatterer with a δ -function potential is placed at node to model scattering effect. We find that the transmission characteristics of electrons in this structure strongly depend on the normal or superconducting state of the ring. The transmission probability as a function of the energy of incident electrons, in the case of a superconductor ring threaded by one quantum magnetic flux, emerges one deep dip, imposed upon the first broad bump in spectrum. This intrinsic conductance dip originates from the superconductor state of the ring. When increasing the magnetic flux from one quantum magnetic flux to two, the spectrum shifts toward higher energy region in the whole. This conductance dip accordingly shifts and appears in the second bump. In the presence of a point-scatterer at the node, the spectrum is substantially modified. Based on the condition of the formation of the standing wave functions in the ring and the broken of the time-reserve symmetry of Schr?dinger equation after switching magnetic flux, the characteristics of transmission of electrons in this structure can be well understood. Received 6 November 2001     

Phase transition in a vortice lattice in a Bi2212:Pb single crystal

The magnetooptical method was used to investigate the penetration of a magnetic flux into a single crystal of a high-temperature superconductor (Bi_0.84Pb_0.16)_2.2Sr₂CaCu₂O₈ in crossed magnetic fields. It is shown that at low temperatures the penetration of the magnetic flux is anisotropic: the flux moves preferentially along the magnetic field applied in the plane of the sample, and the anisotropy grows as the temperature increases. At a temperature T_m = 54±2 K, there occurs a sharp change in the character of penetration of the magnetic field into the superconductor; the direction of the flux ceases be dependent on the direction and magnitude of the magnetic field applied in the plane of the sample. In this case, the transition temperature T_m is independent of the applied magnetic field. The effect is interpreted in terms of the concepts of a phase transition in the system of vortices, which is related to a sharp decrease in the correlations in the position of vortices in various CuO planes, i.e., with the transition from three-dimensional to two-dimensional behavior of the vortex structure.