Symmetric behavior of vortex states of superconducting networks in a magnetic field

We present magnetic field dependence of phase transition temperature and vortex configuration of superconducting networks based on theoretical study. The applied magnetic field is called “filling field” that is defined by applied magnetic flux (in unit of the flux quantum) per unit loop of the superconducting network. If a superconducting network is composed of very thin wires whose thicknesses are less than coherence length, the de Gennes–Alexander (dGA) theory is applicable. We have already shown that field dependences of transition temperature curves have symmetric behavior about the filling field of 1/2 by solving the dGA equation numerically in square lattices, honeycomb lattices, cubic lattices and those with randomly lack of wires networks. Many experimental studies also show the symmetric behavior. In this paper, we make an explicit theoretical explanation of symmetric behaviors of superconducting network respect to the applied field.     

Low-field magnetic response of multi-junction superconducting quantum interference devices

The magnetic states of multi-junction superconducting quantum interference device containing 2N identical conventional Josephson junctions are studied by means of a perturbation analysis of the non-linear first-order ordinary differential equations governing the dynamics of the Josephson junctions in these devices. In the zero-voltage state, persistent currents are calculated in terms of the externally applied magnetic flux Φ_ex . The resulting d.c. susceptibility curves show that paramagnetic and diamagnetic states are present, depending on the value of Φ_ex . The stability of these states is qualitatively studied by means of the effective potential notion for the system.     

On superconducting pairing in the two-band Hubbard model

A two-band Hubbard model for highly hybridized copper 3d(x ²–y ²) and oxygen 2p(x,y) electrons in CuO₂ plane with strong Coulomb repulsion at copper sites is considered. A real-space pairing mechanism induced by antiferromagnetic interaction of Kondo type is treated. To avoid the Gutzwiller approximation the projection technique for two-time Green functions for Hubbard operators is employed. It is shown that due to rigorous restriction of no double occupancy at copper sites the exchange-mediatedp-d pairing with a site-independent gap function isabsent.     

Two superconducting phases in the d=3 Hubbard model

The phase diagram of the d=3 Hubbard model is calculated as a function of temperature and electron density 〈n_i〉, in the full range of densities between 0 and 2 electrons per site, using renormalization-group theory. An antiferromagnetic phase occurs at lower temperatures, at and near the half-filling density of 〈n_i〉= 1. The antiferromagnetic phase is unstable to hole or electron doping of at most 15%, yielding to two distinct“τ" phases: for large coupling U/t, one such phase occurs between 30–35% hole or electron doping, and for small to intermediate coupling U/t another such phase occurs between 10–18% doping. Both τ phases are distinguished by non-zero hole or electron hopping expectation values at all length scales. Under further doping, the τ phases yield to hole- or electron-rich disordered phases. We have calculated the specific heat over the entire phase diagram. The low-temperature specific heat of the weak-coupling τ phase shows an exponential decay, indicating a gap in the excitation spectrum, and a cusp singularity at the phase boundary. The strong-coupling τ phase, on the other hand, has a critical exponent α≈-1, and an additional peak in the specific heat above the transition temperature possibly indicating pair formation. In the limit of large Coulomb repulsion, the phase diagram of the tJ model is recovered.     

Metal-insulator transition in the Hubbard model with incommensurate magnetic structures

The metal-insulator transition for the square, simple cubic, and body centered cubic lattices has been studied within the Hubbard model at half-filling taking into account nearest- and next-nearest-neighbor electron hopping. Both staggered antiferromagnetic and incommensurate magnetic states (spin-spiral wave) have been considered. The inclusion of the latter states for the three-dimensional lattices does not change the general pattern of the metal-insulator transition, but opens the fundamentally new possibility of the metal-insulator transition of the first order between the magnetically ordered states for the square lattice.     

Hubbard regime for the mixed-valence systems

We consider the hybridization of atomic and nearly free states in the mixed-valence systems within the procedure which allows to separate in special cases the Hubbard and Fermi branches of elementary excitations. We discuss the physical consequences of this separation and propose a mechanism of phase transition with the valence change where the localized moments of the f-states disappear in the low-temperature phase due to the Mott-Hubbard transition.     

A superconducting vibrator with a trapped magnetic field

A design of a superconducting vibrator of bending oscillations is proposed. The magnetic-field dependence of the vibrator natural frequency is calculated. It is shown that, for the suggested construction, the natural frequency is tens of times more sensitive to the magnetic field than for all the resonators used before. It is proposed to use such a resonator to study the magnetic field penetrating into superconductors.     

Novel magnetic properties of the Hubbard chain with an attractive interaction

The magnetic properties of an attractive Hubbard chain are considered. Based on the Bethe Ansatz equations of the problem, exact analytic expressions are derived for the magnetization and susceptibility. These formulae can be evaluated after solving certain derivatives of the Bethe Ansatz equations. These derivative equations are also given. We give the magnetization and susceptibility curves for several values of the interaction-strength and bandfilling. We find that the susceptibility at the onset of magnetization (at the critical field) isfinite for all bandfillings, except for the cases of half filled and empty bands, and in the limit of vanishing interaction. We argue that the finiteness of the initial susceptibility is due to the fermion-like behavior of the bound pairs. We also give the gap (what is equal to the critical field) and the initial susceptibility as functions of the interaction-strength and bandfilling for the cases of nearly half filled and almost empty bands as a function of the interaction, and in the weak coupling limit as a function of the bandfilling. To our knowledge, this is the first Bethe Ansatz calculation for the gap in this latter limit.     

超导磁分离技术的应用研究

首先介绍超导磁分离技术的基本原理、磁选设备的发展概况,其次重点阐述超导磁分离技术在高岭土提纯、矿石选矿、燃煤脱硫、污水处理等领域的应用情况,分析超导磁分离技术应用在上述领域的独特优势,最后展望超导磁分离技术的应用趋势。     

Symmetric and asymmetric vortex-antivortex molecules in a fourfold superconducting geometry

In submicron superconducting squares in a homogeneous magnetic field, Ginzburg-Landau theory may admit solutions of the vortex-antivortex type, conforming to the symmetry of the sample [L. F. Chibotaru, Nature (London) 408, 833 (2000)10.1038/35048521]. Here we show that these fascinating, but never experimentally observed states, can be enforced by artificial fourfold pinning, with their diagnostic features enhanced by orders of magnitude. The second-order nucleation of vortex-antivortex molecules can be driven by either temperature or an applied magnetic field, with stable asymmetric vortex-antivortex equilibria found on its path.     

Momentum dependence of magnetic response in heavy-fermion systems

The dynamical magnetic response of a heavy-fermion system CeCu₆ is analyzed with the concept of nearly localized quasi-particles. It is explained why the product χ(q)Г(q) of the susceptibility χ(q) and the magnetic relaxation rate Г(q) is independent of the momentum q although both χ(q) and Г(q) depend on q. A generalization of the Korringa relation between χ(q) and Г(q) is formulated.     

Itinerant Topological Magnons in SU (2) Symmetric Topological Hubbard Models with Nearly Flat Electronic Bands

We show that a suitable combination of flat-band ferromagnetism,geometry and nontrivial electronic band topology can give rise to itinerant topological magnons.An SU(2) symmetric topological Hubbard model with nearly flat electronic bands,on a Kagome lattice,is considered as the prototype.This model exhibits ferromagnetic order when the lowest electronic band is half-filled.Using the numerical exact diagonalization method with a projection onto this nearly flat band,we can obtain the magnonic spectra.In the flat-band limit,the spectra exhibit distinct dispersions with Dirac points,similar to those of free electrons with isotropic hoppings,or a local spin magnet with pure ferromagnetic Heisenberg exchanges on the same geometry.Significantly,the non-flatness of the electronic band may induce a topological gap at the Dirac points,leading to a magnonic band with a nonzero Chern number.More intriguingly,this magnonic Chern number changes its sign when the topological index of the electronic band is reversed,suggesting that the nontrivial topology of the magnonic band is related to its underlying electronic band.Our work suggests interesting directions for the further exploration of,and searches for,itinerant topological magnons.     

Symmetric states of composite systems

Størmer proved a theorem on the integral decomposition of symmetric states on a C ^*-algebra . Motivated by problems in statistical mechanics, we define summetric states on a composite algebra A ( )and extend Størmer's theorem to this situation. Applications to spin-boson models are sketched.     

Single-particle excitation spectrum of the Hubbard model with magnetic frustration at finite temperature

The single-particle excitation spectrum of the Hubbard model with magnetic frustration at finite temperature is examined using numerical exact diagonalization techniques. The magnetic frustration is introduced by a proper choice of the Hamiltonian parameters, which lead to rich low-energy spin excitation behavior, resembling those observed in heavy fermion systems. At finite temperature, the low-lying excited states become thermally populated with significant weight. As a result, the calculated spectrum shows interesting temperature dependent evolution. The calculated results are presented and discussed in a many-body picture to gain insight into the photoelectron spectroscopy of strongly correlated electron systems.