纳米尺度下超导体和拓扑绝缘体的电输运特性

超导体和拓扑绝缘体研究是当前凝聚态物理领域中的重大课题.文章重点介绍了作者所在实验室在纳米超导和拓扑绝缘体电输运领域的实验进展,其中包括金属和铁磁纳米线中的超导近邻效应、半金属纳米线中的新奇超导特性、拓扑绝缘体薄膜中的量子输运以及超导态-拓扑量子态的相互作用等,并对该领域的进一步发展进行了展望.     

Possible relationship between Cu-NQR frequencies and Cu valence states in CuO2-planes for high-T c superconductors

We report on⁶³Cu-NQR frequency ν_NQR measurements on various high-T _c superconductors. An empirical relationship betweenT _c and ν_NQR is presented. We attribute this finding to variations in the valence state of the in-plane Cu ions which, due to the unusual electric polarizability of the O^2? ions, depends sensitively on the Cu?O distance.     

Anisotropic diamagnetic response in type-II superconductors with gap and Fermi-surface anisotropies

The effects of anisotropic gap structures on a diamagnetic response are investigated in order to demonstrate that the field-angle-resolved magnetization [ML(chi)] measurement can be used as a spectroscopic method to detect gap structures. Our microscopic calculation based on the quasiclassical Eilenberger formalism reveals that ML(chi) in a superconductor with a fourfold gap displays a fourfold oscillation reflecting the gap and Fermi-surface anisotropies, and the sign of this oscillation changes at a field between Hc1 and Hc2. As a prototype of unconventional superconductors, magnetization data for borocarbides are also discussed.     

Pseudogaps,impurity states,and pairing polarons in superconductors

The origin of the pseudo gap in low carrier density superconductors is discussed. The role of phase fluctuations of the order parameter lead to destruction of long range order, preserving the short range order observed in tunneling and photoemission spectra. The electronic structure of magnetic impurities in s-wave superconductors is presented. One finds a first order quantum phase transition as a function of the exchange coupling in which a spin one half is trapped at the impurity. In addition, the possibility of polarons involving the amplitude of the gap parameter is explored. One finds rings of quasi particles form with gap states occurring on the ring wall.     

Electron exchanges in nuclear spin conversion of hydrogen physisorbed on diamagnetic insulators

Models are provided and discussed to interpret new experiments on the ortho-paraconversion of hydrogen “physisorbed” on dielectric and diamagnetic surfaces.Electro-static and dynamical molecule-surface interactions complemented by hyperfinecontacts are shown to be generally more effective than the magnetic ones. Coulombrepulsion induces exchanges of molecular and surface electrons and excites triplet spinstates which are effective in the angular momenta transfers to the catalyst. Theconversion time is obtained as the square of a ratio of two energies: the exchange andexcitation ones. The main channel is found composed of triplet excitations of the order ofthe eV, induced by molecule-surface exchanges of about a hundred of meV. It explains thezinc and oxygen rates of about one minute observed on the MOF samples as well as the aboutten times slower ones on the ASW. The same mechanism is also shown to occur in thetransient regime, but faster. Finally it explains also the conversion of a few hoursobserved for interstitial hydrogen in silicium by transitions to the conduction bandinduced by about 10 meVelectron exchanges. The molecule-surface orbital geometries of the MOF and ASWconfigurations are displayed and the quantum path when unfolded exhibits the successivebroken symmetries.     

Vortex states in mesoscopic superconductors

The results of theoretical investigations of the electronic structure and transport properties of vortex states in mesoscopic superconductors with sizes of several coherence lengths are reviewed. The features of the electronic spectra of multiquantum vortices and vortex molecules, as well as mechanisms of thermal transport along vortex lines, are considered by taking into account boundary effects.     

Spin and charge order in Mott insulators and d-wave superconductors

We argue that aspects of the anomalous, low temperature, spin and charge dynamics of the high temperature superconductors can be understood by studying the corresponding physics of undoped Mott insulators. Such insulators display a quantum transition from a magnetically ordered Néel state to a confining paramagnet with a spin gap; the latter state has bond-centered charge order, a low energy S=1 spin exciton, confinement of S=1/2 spinons, and a free S=1/2 moment near non-magnetic impurities. We discuss how these characteristics, and the quantum phase transitions, evolve upon doping the insulator into a d-wave superconductor. This theoretical framework was used to make a number of predictions for STM measurements and for the phase diagram of the doped Mott insulator in an applied magnetic field.     

Appearance and disappearance of edge states in topological insulators

We investigate the relationship between spin Chern numbers and edge state properties in general situations, where the time-reversal symmetry may be broken. As an example, we consider a thin film of three-dimensional topological insulators sandwiched between two ferromagnetic insulators with an antiparallel magnetization configuration. A topological quantum spin Hall phase with quantized spin Chern numbers C _± =  ±1, and a trivial insulator with C _± = 0 are found in different parameter regions. With tuning parameters, the quantum phase transition between the two phases can occur through closing of the spin spectrum gap rather than energy gap. It is further shown that for a junction between samples with different parameters, appearance of edge states at the interface is always related to the mismatch of spin Chern numbers, independent of symmetries.     

On the valence band photoemission of highT c superconductors

The valence band photoemission spectrum of highT _c -superconductors is discussed based on the half-filled single band Hubbard Hamiltonian with the strong Coulomb interaction. We discuss how to analyze these valence band and deep core level excitation spectra, concerning particularly with which orbital, Cud or Op state, a hole will occupy in the CuO₂ plane.     

Mean energies of hybridized valence states

A method of determining the relative signs of J _AB and J _BX in nuclear spin systems of the type AB₂X _q is described and applied to the AB₂X₃ proton spin system of 2-butyn-1-ol. Variation of the relative chemical shift v _A—v _B (through changes in temperature or in the composition of the solvent) causes certain pairs of ‘labile’ transitions in the X spectrum to coalesce on one side or other of the X spectrum. The information about the relative signs of the coupling constants is obtained by noting the two different critical values of v _A-v _B at which these coalescences occur. Since the method remains applicable even if the third coupling constant J _AX is vanishingly small, it is particularly useful in the determination of the signs of long-range spin coupling constants where the usual double resonance methods often break down. It is concluded that the long-range coupling J(H-C-C≡C-C-H) of 2-butyn-1-ol has a positive sign.