arXiv subjects

Brian M. Andersen

Publications and source records attributed to Brian M. Andersen.

137 records · Page 8Linked to original sources

arXiv2005

Superconducting Junctions with Ferromagnetic, Antiferromagnetic or Charge-Density-Wave Interlayers

Spectra and spin structures of Andreev interface states and the Josephson current are investigated theoretically in junctions between clean superconductors (SC) with ordered interlayers. The Josephson current through the ferromagnet-insulator-ferromagnet interlayer can exhibit a nonmonotonic dependence on the misorientation angle. The characteristic behavior takes place if the pi state is the equilibrium state of the junction in the particular case of parallel magnetizations. We find a novel channel of quasiparticle reflection (Q reflection) from the simplest two-sublattice antiferromagnet (AF) on a bipartite lattice. As a combined effect of Andreev and Q reflections, Andreev states arise at the AF/SC interface. When the Q reflection dominates the specular one, Andreev bound states have almost zero energy on AF/ s-wave SC interfaces, whereas they lie near the edge of the continuous spectrum for AF/d-wave SC boundaries. For an s-wave SC/AF/s-wave SC junction, the bound states are found to split and carry the supercurrent. Our analytical results are based on a novel quasiclassical approach, which applies to interfaces involving itinerant antiferromagnets. Similar effects can take place on interfaces of superconductors with charge density wave materials (CDW), including the possible d-density wave state (DDW) of the cuprates.

cond-mat.supr-con↗

arXiv2005

0-pi transitions in Josephson junctions with antiferromagnetic interlayers

We show that the dc Josephson current through superconductor-antiferromagnet-superconductor (S/AF/S) junctions manifests a remarkable atomic scale dependence on the interlayer thickness. At low temperatures the junction is either a 0- or pi-junction depending on whether the AF interlayer consists of an even or odd number of atomic layers. This is associated with different symmetries of the AF interlayers in the two cases. In the junction with odd AF interlayers an additional pi-0 transition can take place as a function of temperature. This originates from the interplay of spin-split Andreev bound states. Experimental implications of these theoretical findings are discussed.

cond-mat.supr-con↗

arXiv2005

Disorder effects on the intrinsic nonlinear current density in YBa$_2$Cu$_3$O$_{7-δ}$

We present harmonic generation measurements of the intrinsic nonlinear current density $j_2$ of YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films at temperatures close the $T_c$. Experiments on a range of different quality samples allow us to extract the dependence of $j_2$ on the penetration depth of the superconductor. In order to model these results, we calculate the intrinsic nonlinear current response of $d_{x^2-y^2}$-wave superconductors in the Meissner regime in the presence of nonmagnetic impurities within the self-consistent T-matrix approximation.

cond-mat.supr-con↗

arXiv2005

Fourier transform spectroscopy of d-wave quasiparticles in the presence of atomic scale pairing disorder

The local density of states power spectrum of optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO) has been interpreted in terms of quasiparticle interference peaks corresponding to an "octet'' of scattering wave vectors connecting k-points where the density of states is maximal. Until now, theoretical treatments have not been able to reproduce the experimentally observed weights and widths of these "octet'' peaks; in particular, the predominance of the dispersing "q$_1$'' peak parallel to the Cu-O bond directions has remained a mystery. In addition, such theories predict "background'' features which are not observed experimentally. Here, we show that most of the discrepancies can be resolved when a realistic model for the out-of-plane disorder in BSCCO is used. Weak extended potential scatterers, which are assumed to represent cation disorder, suppress large-momentum features and broaden the low-energy "q$_7$''-peaks, whereas scattering at order parameter variations, possibly caused by a dopant-modulated pair interaction around interstitial oxygens, strongly enhances the dispersing "q$_1$''-peaks.

cond-mat.supr-con↗

arXiv2005

Andreev states near short-ranged pairing potential impurities

We study Andreev states near atomic scale modulations in the pairing potential in both $s$- and d-wave superconductors with short coherence lengths. For a moderate reduction of the local gap, the states exist only close to the gap edge. If one allows for local sign changes of the order parameter, however, resonances can occur at energies close to the Fermi level. The local density of states (LDOS) around such pairing potential defects strongly resembles the patterns observed by tunneling measurements around Zn impurities in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO). We discuss how this phase impurity model of the Zn LDOS pattern can be distinguished from other proposals experimentally.

cond-mat.supr-con↗

arXiv2005

Dopant-modulated pair interaction in cuprate superconductors

Comparison of recent experimental STM data with single-impurity and many-impurity Bogoliubov-de Gennes calculations strongly suggests that random out-of-plane dopant atoms in cuprates modulate the pair interaction locally. This type of disorder is crucial to understanding the nanoscale electronic structure inhomogeneity observed in BSCCO-2212, and can reproduce observed correlations between the positions of impurity atoms and various aspects of the local density of states such as the gap magnitude and the height of the coherence peaks. Our results imply that each dopant atom modulates the pair interaction on a length scale of order one lattice constant.

cond-mat.supr-con↗

arXiv2004

Spin dynamics in the stripe phase of the cuprates

Within a model that supports stripe spin and charge order coexisting with a d$_{x^2-y^2}$-wave superconducting phase, we study the self-consistently obtained electronic structure and the associated transverse dynamical spin susceptibility. In the coexisting phase of superconducting and static stripe order, the resulting particle-hole continuum can strongly damp parts of the low-energy spin wave branches. This provides insight into recent inelastic neutron scattering data revealing the dispersion of the low-energy collective magnetic modes of lanthanum based cuprate superconductors.

cond-mat.supr-con↗

arXiv2003

Quantum interference between multiple impurities in anisotropic superconductors

We study the quantum interference between impurities in d-wave superconductors within a potential scattering formalism that easily applies to multiple impurities. The evolution of the low-energy local density of states for both magnetic and nonmagnetic short-ranged scatterers are studied as a function of the spatial configuration of the impurities. Further we discuss the influence of subdominant bulk superconducting order parameters on the interference pattern from multiple impurities.

cond-mat.supr-con↗

arXiv2003

Two nonmagnetic impurities in the DSC and DDW state of the cuprate superconductors as a probe for the pseudogap

The quantum interference between two nonmagnetic impurities is studied numerically in both the d-wave superconducting (DSC) and the d-density wave (DDW) state. In all calculations we include the tunnelling through excited states from the CuO$_2$ planes to the BiO layer probed by the STM tip. Compared to the single impurity case, a systematic study of the modulations in the two-impurity local density of states can distinguish between the DSC or DDW states. This is important if the origin of the pseudogap phase is caused by preformed pairs or DDW order. Furthermore, in the DSC state the study of the LDOS around two nonmagnetic impurities provide further tests for the potential scattering model versus more strongly correlated models.

cond-mat.supr-con↗

arXiv2002

Electronic checkerboard pattern in striped racetrack domains: a consistent picture of recent neutron and STM experiments

We discuss recent elastic neutron scattering and scanning tunneling experiments on high-T_c cuprates exposed to an applied magnetic field. In particular we show that a physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. Further, we calculate the electronic structure in such a region using a T-matrix method, and find a checkerboard interference pattern consistent with recent scanning tunneling experiments.

cond-mat.supr-con↗

arXiv2002

Andreev Bound States at the Interface of Antiferromagnets and d-wave Superconductors

We set up a simple transfer matrix formalism to study the existence of bound states at interfaces and in junctions between antiferromagnets and d-wave superconductors. The well-studied zero energy mode at the {110} interface between an insulator and a d-wave superconductor is spin split when the insulator is an antiferromagnet. This has as a consequence that any competing interface induced superconducting order parameter that breaks the time reversal symmetry needs to exceed a critical value before a charge current is induced along the interface.

cond-mat.supr-con↗