Instability of square vortex lattice in d-wave superconductors is due to paramagnetic depairing

The effects of paramagnetic depairing on structural transitions between vortex lattices of a quasi-two-dimensional d-wave superconductor are examined. We find that, when the Maki parameter alphaM is of order unity, a square lattice induced by a d-wave pairing is destabilized with increasing fields, and that a reentrant rhombic lattice occurs in higher fields. Further, a weak Fermi surface anisotropy competitive with the pairing symmetry induces another structural transition near Hc2. These results are consistent with the structure changes of the vortex lattice in CeCoIn5 in H parallel c determined from recent neutron scattering data.     

Fermionic entropy of the vortex state in d-wave superconductors

In d-wave superconductors the electronic entropy associated with an isolated vortex diverges logarithmically with the size of the system even at low temperatures. In the vortex array the entropy per vortex per layer, S _V , is much larger than k _B and depends on the distribution of the velocity field v _s around the vortex. If there is a first-order transition upon a change of the velocity distribution, then there will be a big entropy jump ΔS _V ∼k _B at the transition. This entropy jump comes from the electronic degrees of freedom on the vortex background, which is modified by the vortex transition. This can explain the big jump in the entropy observed in the so-called vortex-melting transition [A. Junod, M. Roulin, J-Y. Genoud et al., Physica C, to be published], in which the vortex array and thus the velocity field are redistributed. The possibility of the Berezinskii-Kosterlitz-Thouless transition in the 3-dimensional d-wave superconductor due to the fermionic bound states in the vortex background is discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 465–469 (25 March 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Index theoretic characterization of d-wave superconductors in the vortex state

We study analytically the low energy spectrum of a lattice d-wave superconductor in the vortex lattice state. For an inversion symmetric hc/2e vortex lattice and in the presence of particle-hole symmetry we prove an index theorem that imposes a lower bound on the number of zero-energy modes. Generic cases are constructed in which this bound exceeds the number of zero modes of an equivalent lattice of hc/e vortices, despite the identical point group symmetries. The quasiparticle spectrum around the zero modes is doubly degenerate and exhibits a Dirac-like dispersion, with velocities that become universal functions of Delta(0)/t in the limit of low magnetic field. For weak particle-hole symmetry breaking, the gapped state can be characterized by a topological quantum number, related to spin-Hall conductivity, which generally differs in the cases of the hc/2e and hc/e vortex lattices.     

Singularity of the vortex density of states in d-wave superconductors

In d-wave superconductors, the electronic density of states (DOS) induced by a vortex exhibits a divergence at low energies: N _vortex(E) ∼1/|E|. This divergence is the result of gap nodes in the spectrum of excitation outside the vortex core. The heat capacity in two regimes, T ²/T _c ² ≫ B/B _c 2 and T ²/T _c ² ≪ B/B _c 2, is discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 641–645 (10 November 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Tunneling spectra of layered strongly correlated d-wave superconductors

Tunneling conductance experiments on cuprate superconductors exhibit a large diversity of spectra that appear in different nanosized regions of inhomogeneous samples. In this Letter, we use a mean-field approach to the tt't'J model in order to address the features in these spectra that deviate from the BCS paradigm, namely, the bias sign asymmetry at high bias, the generic lack of evidence for the van Hove singularity, and the absence of coherence peaks at low dopings. We conclude that these features can be reproduced in homogeneous layered d-wave superconductors solely due to a proximate Mott insulating transition. We also establish the connection between the above tunneling spectral features and the strong renormalization of the electron dispersion around (0, pi) and (pi, 0) and the momentum space anisotropy of electronic states observed in angle-resolved photoemission spectroscopy experiments.     

Tunable striped-patterns by lattice anisotropy and magnetic impurities in d-wave superconductors

The pattern transition induced by lattice anisotropy (LA) and magnetic impurities is computationally observed in near-optimally doped d-wave superconductors (DSCs). For the single impurity case, a transition from the checkerboard to stripe pattern can be induced by a very weak LA. For the two-impurity case, an orientation transition from the longitudinal stripe into transverse pattern is observed when the LA ratio reaches some critical value. At the critical point, the structures around impurities can restore checkerboard patterns. These results indicate that the formation of stripes in DSCs might induced by various effects, and could be tunable experimentally.     

Reentrant vortex lattice transformation in fourfold symmetric superconductors

The physics behind the rhombic-->square-->rhombic flux line lattice transformation in increasing fields is clarified on the basis of Eilenberger theory. We demonstrate that this reentrance observed in LuNi2B2C is due to intrinsic competition between the superconducting gap and Fermi surface anisotropies. The calculations not only reproduce it but also predict the not yet found lock-in transition to a square lattice with different orientation in a higher field. In view of the physical origin given, this sequence of transitions is rather generic to occur in fourfold symmetric superconductors.     

Impurity in a d-wave superconductor: Kondo effect and STM spectra

We present a theory for recent STM studies of Zn impurities in the superconductor Bi2Sr2CaCu2O8+delta, using insights from NMR experiments which show that there is a net S = 1/2 moment on the Cu ions near the Zn. We argue that the Kondo spin dynamics of this moment is the origin of the low bias peak in the differential conductance, rather than a resonance in a purely potential scattering model. The spatial and energy dependence of the STM spectra of our model can also fit the experiments.     

The structure of the vortex lattice in anisotropic superconductors

The structure of the vortex lattice in anisotropic superconductors at an arbitrary temperature in magnetic fields close to critical was studied. Generally, a rhombohedral structure with a vertex angle depending on temperature, magnetic field, and material constants is formed. An important factor is the small (2%) difference of the free energies of the triangular and square lattices in the Ginzburg-Landau approximation. This factor also persists in anisotropic superconductors.     

Superflow in d-wave superconductors

Superflow in a phenomenological tight-binding model for the superconducting state of some high-temperature superconductors is discussed thoroughly. The formalism used is explicitly gauge invariant and currents are computed exactly within the BCS theory, going therefore beyond linear response theory. The dependence of gap functions, current density, critical currents and free energy as a function of the superfluid velocity for different angles and doping concentrations is investigated. Different sources of anisotropy, like the dispersion relation of the model, the internal symmetry of the order parameter, and orthorhombic distortions of the lattice are also studied.     

Triangular vortex lattice in thin films of type I superconductors

By decoration-microscopy a triangular lattice of single vortices was observed on a 1500 Å thick Pb-crystal. The correlation between the crystal orientation of the vortex lattice was studied by selected area electron diffraction.     

高温超导混合配对态与磁通涡旋格子

建立在对基于Gor’kov方法而导出的微观GinzburgLandau方程的分析揭示了高温超导体YBa2Cu3O7配对态对称性和磁通涡旋格子结构.分析指出,存在一个格子转变温度T,当温度高于T时,超导基态显示dx2-y2波对称性特征;低于该温度s波沟道幅值成为可观的量级;超导基态为混合s-dx2-y2态.对应单分量波函数磁通涡旋格子为三角的结构;而稳定的斜格子反映出混合波特征.s与d沟道间耦合约束了磁场下dx2-x2波对称性自由度,而对高温超导反常输运行为如上临界磁场温度曲线上翘现象所负责 关键词： 高温超导 GinzburgLandau理论 磁通涡旋