Skyrmions in a doped antiferromagnet

Magnetization and magnetoresistance have been measured in insulating antiferromagnetic La2Cu0.97Li0.03O4 over a wide range of temperatures, magnetic fields, and field orientations. The magnetoresistance step associated with a weak ferromagnetic transition exhibits a striking nonmonotonic temperature dependence, consistent with the presence of Skyrmions.     

Spin fluctuations and the superconducting state in doped insulators

We propose a model of electron pairing via spin fluctuations in doped insulators. The bare states for the superconducting condensate correspond to impurity bands in the original band gap of the undoped material. We obtain a complete set of equations for the superconducting state. We show that fermion pairing in impurity bands of extended states is possible, and thus so is superconductivity, if localized spin-0 bosons are produced. The latter are necessarily accompanied by localized spin-1 bosons, which are responsible for the relationship between singlet and triplet pairing channels of quasiparticles. Zh. éksp. Teor. Fiz. 114, 1765–1784 (November 1998)     

Local moment formation in the superconducting state of a doped Mott insulator

A microscopic theory is presented for the local moment formation near a nonmagnetic impurity or a copper defect in high-Tc superconductors. We use a renormalized mean-field theory of the t-J model for a doped Mott insulator and study the fully self-consistent, spatially unrestricted solutions of the d-wave superconducting (SC) state in both the spin S=0 and S=1/2 sectors. We find a transition from the singlet d-wave SC state to a spin doublet SC state when the renormalized exchange coupling exceeds a doping dependent critical value. The induced S=1/2 moment is staggered and localized around the impurity. It arises from the binding of an S=1/2 nodal quasiparticle to the impurity. The local density of states is calculated and connections to NMR and STM experiments are discussed.     

Coupling of electrons and phonons in a doped antiferromagnet

It is well-known that low-energy electronic excitations in high-T _c superconductors have energies of the order of the exchange couplingJ, i.e. of the same order as the phonon energies. Therefore, low-energy electronic excitations and phonons should strongly influence each other. To investigate this problem we consider a coupled electron-phonon system. For the electronic degrees of freedom we start from the three band Hubbard or Emery model. In analogy to the transformation of the three band Hubbard model to thet?J model, studied by Zhang and Rice, we derive an effective electron-phonon interaction. Its electronic degrees of freedom are those of thet?J model which couple to the phonons of the original system. The coupling of electrons and phonons is discussed by means of the phonon Green function for a breathing-like mode.     

On the Hall effect in a two-dimensional doped antiferromagnet

The conductivity and the Hall coefficient of a doped 2D antiferromagnet in the normal state are considered using the Kondo lattice model in the multimoment approximation. The anomalous temperature dependence of the kinetic coefficients is explained by the strong anisotropic charge-carrier scattering from the spin subsystem and found to be in qualitative agreement with the experimental data for the normal state of high-T _c superconductors.     

Boson wavefunction and condensation energy in the superconducting state of doped semiconductors

The superconducting state of doped semiconductors, in which a superconducting gap opens in the spectrum of one-particle states formed (upon doping) in the region of the initial energy gap, is studied within the framework of the Holstein-Anderson model. The wavefunction of the relative motion of a pair of particles in the bound state is determined. The dependence of the size of this wavefunction on the level of doping in the overdoping region of the phase diagram is consistent with the published experimental data for La_{2 ? x} Sr_xCuO₄. Variation of the temperature in the range of T < T _c only influences the natural normalization of the boson wavefunction (normalized to the boson density), while the size of the wavefunction is independent of the temperature. Theoretical values of the condensation energy and the heat capacity jump upon the superconductor-metal transition in the region of optimal doping agree with the experimental data for La_{2 ? x} Sr_xCuO₄.     

Hydrogen induced change from superconducting to magnetically ordered state in Th7Fe3

The Pauli paramagnet Th₇Fe₃ becomes a superconductor at temperatures below 1.86 K.¹ This compound readily hydrogenates to Th₇Fe₃H₃₀, giving one of the densest hydrogen media known. Magnetization measurements on the hydride show that it is magnetically ordered. It has a Curie temperature above 300 K and a saturated moment measured at 4.2 K of 1.4 μ_B/Fe. The differing behavior in the host metal and the hydride is ascribed, in the latter, to competition between H and Fe for the electrons provided by Th. In the host metal the Fe d-band is filled by electron transfer and the Fe moment is quenched. In the hydride some of the electrons provided by Th are absorbed by H, leaving vacancies in the Fe d-band. These, in turn, give rise to the magnetic order. The present instance in which a superconductor is converted into a ferromagnet appears to be unique as regards the effects of hydrogenation on the properties of metallic systems.     

Enhanced critical currents of superconducting ErNi2B2C in the ferromagnetically ordered state

We report on transport and magnetization studies of the critical current in single crystal ErNi2B2C for applied fields below 4 kG. Below T approximately 2.5 K superconductivity coexists with weak ferromagnetism. We find that the critical currents are strongly enhanced for all field orientations in this ferromagnetic regime, corresponding to a threefold increase of the pinning force of the flux line lattice. We speculate that this increase is due to strong pair breaking by the ferromagnetism.     

Crossover from a pseudogap state to a superconducting state

This paper deduces that the particular electronic structure of cuprate superconductors confines Cooper pairs to be first formed in the antinodal region which is far from the Fermi surface,and these pairs are incoherent and result in the pseudogap state.With the change of doping or temperature,some pairs are formed in the nodal region which locates the Fermi surface,and these pairs are coherent and lead to superconductivity.Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface.It also shows that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure,and non-s wave symmetry gap favours the high-temperature superconductivity.Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.     

Anomalous tunneling into magnetically ordered dilute alloys which have been induced into the superconducting state

Unusual tunneling characteristics for junctions consisting of Al, oxide and magnetic alloys, which are backed by Pb, are presented. The Pb induces the alloys to be superconducting. The alloys studied are 1500 ppm Fe in Au and 1200 ppm Co in Pd.     

Partially disordered antiferromagnetic state in two-dimensional triangular-lattice antiferromagnet

The partially disordered antiferromagnetic (PDA) state, as an exotic phase peculiar to the antiferromagnet with Ising spin in triangular lattice, is investigated by using Monte Carlo simulations of Wang-Landau algorithm and Glauber algorithm. It is revealed that PDA state, as the ground state of the triangular antiferromagnetic system, presents the complicated spin configuration due to geometrical frustration. The formation of multi-domain structure within the framework of honeycomb antiferromagnet results in the high degeneracy of PDA state. And this degeneracy of ground state can be lifted by a small magnetic field. Consequently the system shows the ferrimagnetic state, and the magnetization plateau of 1/3 saturate value (Ms) is observed in many experiments. Moreover, due to the multi-domain structure of PDA state and those spins on domain walls, the metastable steps may manifest themselves superposed on the 1/3Ms plateau in some special cases.