Spin excitations in fluctuating stripe phases of doped cuprate superconductors

Using a phenomenological lattice model of coupled spin and charge modes, we determine the spin susceptibility in the presence of fluctuating stripe charge order. We assume the charge fluctuations to be slow compared to those of the spins, and combine Monte Carlo simulations for the charge order parameter with exact diagonalization of the spin sector. Our calculations unify the spin dynamics of both static and fluctuating stripe phases and support the notion of a universal spin excitation spectrum in doped cuprate superconductors.     

Collective excitations in superconductors

The excitations connected with quantum condensation of a fermion gas are calculated quantitatively along the lines of a previous paper. We find exact agreement with earlier results: excitations of the phonon type in the absence of Coulomb interaction, strong indications for their nonexistence in the presence of this interaction.     

Macroscopic excitations in superconductors

The dynamical density functional theory (DDFT) as an alternative approach to the mode coupling theory (MCT) for supercooled liquids and glass transitions was recast in the path integral form, which can then be analyzed by the methods of equilibrium statistical mechanics. The hope is to avoid drastic approximations entering the original MCT such as factorization of the multiparticle correlation function. The renormalized perturbation theory is developed and the freezing is interpreted as a result of the life-time renormalization. The multicomponent extension of the theory is proposed, which could be treated exactly in the limit of infinite number of the components.     

自旋超导态

文章综述了近几年来自旋超导领域的研究进展。自旋超导态是由电荷为零自旋非零的玻色子在低温下凝聚成的超流态。文章将介绍自旋超导态的类伦敦方程、类金兹堡—朗道方程及其BCS(Bardeen-Cooper-Schrieffer)型哈密顿量。然后利用这些方程和理论,导出自旋超导体的一些有趣现象,如：零自旋阻现象、电迈斯纳效应和自旋流约瑟夫森效应。此外,作者也讨论了一些很可能是自旋超导体的候选体系,自旋超导体的应用前景,以及该子领域的发展前景展望。     

Bound-pair excitations in superconductors

Properties of bound-pair excitations as correlated states of two Fermi quasiparticles in superconductors are calculated within the BCS theory of superconductivity. The partition function of the interacting electron system is expressed as a product of the partition function of a free Fermi gas and the partition function of a free Bose gas. The Bose gas partition function determines energy levels of the bound-pair excitations and their degeneracies. The calculated properties of the bound-pair excitations perfectly respect generally proven theorems concerning the BCS theory of superconductivity. This work received financial support from the Grant Agency of the Slovak Republic, Grant No V2 F20-G and 2/4109/97.     

Spin-wave excitations of paramagnetic impurities in superconductors

The existence of spin-wave excitations of paramagnetic impurities in a superconductor in an external magnetic field due to the long-range exchange interactions is predicted. The damping has been calculated and possible manifestations of this collective excitations in ESR experiments are analysed.     

Spin susceptibility divergence in high-temperature superconductors

Neutron scattering experiments at low frequency reveal susceptibility peaks in La_2−xSr_xCuO₄ which diverge as the temperature is lowered toward the superconducting transition. This divergence at a nesting momentum stands out from the conventional Pauli susceptibility of this cuprate at long wavelengths. We explain the peak singularity by calculating many-body Coulomb correlations which originate from Fermi surface nesting and dominant vertex corrections. We sum the leading singularities using the parquet method and find that the susceptibility diverges as T^−α, where α depends on the Coulomb interaction. Our theory also explains a similar susceptibility surge in the metallic V₂O₃.     

Collective excitations in superconductors with structural transition

In this paper we have found new low-frequency collective excitations in superconductors with structural transformation. The connection between our results and the experiments on Raman scattering in 2H-NbSe₂ cDw- superconductor is discussed.     

Relevance of 3d multiplet structure in nickelate and cuprate superconductors

The recent discovery of superconductivity in doped rare-earth infinite-layer nickelates RNiO_2, R = Nd, Pr as a new family of unconventional superconductors has inspired extensive research on their intriguing properties. One of the major motivation to explore the nickelate superconductors originated from their similarities with and differences from the cuprate superconductors, which have been extensively studied over the last decades but are still lack of the thorough understanding.In this short review, we summarized our recent investigation of the relevance of Ni/Cu-3 d multiplet structure on the hole doped spin states in cuprate and recently discovered nickelate superconductors via an impurity model incorporating all the 3d orbitals. Further plausible explorations to be conducted are outlined as well. Our presented work provides an insightful framework for the investigation of the strongly correlated electronic systems in terms of the multiplet structure of transition metal compounds.     

Spin wave excitations in amorphous itinerant ferromagnet

The Izuyama, Kim and Kubo theory of spin waves in crystalline itinerant ferromagnets is extended to the amorphous itinerant ferromagnetic materials. The qualitative features of this theory are similar to that of the generalized Landau-Lifshitz theory discussed by Henderson and de Graaf.     

Spin triplet pairing in high temperature superconductors

Various superconductivity mechanisms show that the electron pair is in spin singlet (S=0) belowTc. In contrast with these, our EPR measurements show that the electron pair is in spin triplet (S=1) rather than the spin singlet. Since an intense zero field absorption was observed in the high temperature superconductors of YBa₂Cu₃O_7–y , and YBa₂Cu₃O_7–y F₁, which violated various spin-pairing singlet superconductivity mechanisms.     

Spin and orbital excitations in perovskite Mn oxides

An effective Hamiltonian of perovskite type LaMnO₃ is derived by taking into account the degeneracy ofe _g orbitals, Hund coupling and strong Coulomb interactions betweend-electrons in Mn ions. Spin and orbital excitations are calculated by using the effective Hamiltonian.     

Spin excitations in granular structures with ferromagnetic nanoparticles

Spin excitations and relaxation in a granular structure which contains metallic ferromagnetic nanoparticles in an insulating amorphous matrix are studied in the framework of the s-d exchange model. As the d system, we consider the granule spins, and the s system is represented by localized electrons in the amorphous matrix. In the one-loop approximation with respect to the s-d exchange interaction for a diagram expansion of the spin Green’s function, the spin excitation spectrum is found, which consists of spin-wave excitations in the granules and of polarized spin excitations. In polarized spin excitations, a change in the granule spin direction is accompanied by an electron transition with a spin flip between two sublevels of a split localized state in the matrix. We considered polarized spin relaxation (relaxation of the granule spins occurring by means of polarized spin excitations) determined by localized deep energy states in the matrix and the thermally activated electronic cloud of the granule. It is found that polarized spin relaxation is efficient over a wide frequency range. Estimates made for structures with cobalt granules showed that this relaxation could be observed in centimetric, millimetric, and submillimetric wavelength ranges.     

Low-lying excitations induced by non-magnetic impurities in d-wave superconductors

We have performed numerical calculations for low-lying excitations induced by a single non-magnetic impurity in a d-wave superconductor on the basis of two different frameworks. One is the Bogoliubov–de Gennes theory in real coordinates, while the other is the Born approximation in wave-number coordinates. This paper shows that these results for low-lying excitations are identical by using Fourier transformations.