Brian M. Andersen

Imaging the impact on cuprate superconductivity of varying the interatomic distances within individual crystal unit cells

Many theoretical models of high-temperature superconductivity focus only on the doping dependence of the CuO2-plane electronic structure. However, such models are manifestly insufficient to explain the strong variations in superconducting critical temperature, Tc, among cuprates that have identical hole density but are crystallographically different outside of the CuO2 plane. A key challenge, therefore, has been to identify a predominant out-of-plane influence controlling the superconductivity, with much attention focusing on the distance dA between the apical oxygen and the planar copper atom. Here we report direct determination of how variations in interatomic distances within individual crystalline unit cells affect the superconducting energy-gap maximum Δ of Bi2Sr2CaCu2O8+δ. In this material, quasiperiodic variations of unit cell geometry occur in the form of a bulk crystalline “supermodulation.” Within each supermodulation period, we find ≈9 ± 1% cosinusoidal variation in local Δ that is anticorrelated with the associated dA variations. Furthermore, we show that phenomenological consistency would exist between these effects and the random Δ variations found near dopant atoms if the primary effect of the interstitial dopant atom is to displace the apical oxygen so as to diminish dA or tilt the CuO5 pyramid. Thus, we reveal a strong, nonrandom out-of-plane effect on cuprate superconductivity at atomic scale.

How grain boundaries limit supercurrents in high-temperature superconductors

S. Graser, 2, ∗ P. J. Hirschfeld, T. Kopp, R. Gutser, B. M. Andersen, and J. Mannhart Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany Department of Physics, University of Florida, Gainesville, FL 32611, USA Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark (Dated: December 21, 2009)

Dopant-Modulated Pair Interaction in Cuprate Superconductors

A 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 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.

Discovery of orbital-selective Cooper pairing in FeSe

A deeper look into iron selenide In the past 10 years, iron-based superconductors have created more puzzles than they have helped resolve. Some of the most fundamental outstanding questions are how strong the interactions are and what the electron pairing mechanism is. Now two groups have made contributions toward resolving these questions in the intriguing compound iron selenide (FeSe) (see the Perspective by Lee). Gerber et al. used photoemission spectroscopy coupled with x-ray diffraction to find that FeSe has a very sizable electron-phonon interaction. Quasiparticle interference imaging helped Sprau et al. determine the shape of the superconducting gap and find that the electron pairing in FeSe is orbital-selective. Science, this issue p. 71, p. 75; see also p. 32 Cooper pairing in iron selenide predominantly occurs between electrons from dyz orbitals of iron atoms. The superconductor iron selenide (FeSe) is of intense interest owing to its unusual nonmagnetic nematic state and potential for high-temperature superconductivity. But its Cooper pairing mechanism has not been determined. We used Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the electronic bands surrounding the Γ = (0, 0) and X = (π/aFe, 0) points of FeSe and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless and that they exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique, we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing that, in FeSe, is based preferentially on electrons from the dyz orbitals of the iron atoms.

Kondo-enhanced andreev tunneling in InAs nanowire quantum dots

We report measurements of the nonlinear conductance of InAs nanowire quantum dots coupled to superconducting leads. We observe a clear alternation between odd and even occupation of the dot, with subgap peaks at |V(sd)| = Delta/e markedly stronger (weaker) than the quasiparticle tunneling peaks at |V(sd)| = 2Delta/e for odd (even) occupation. We attribute the enhanced Delta peak to an interplay between Kondo correlations and Andreev tunneling in dots with an odd number of spins, and we substantiate this interpretation by a poor mans scaling analysis.

Competing magnetic double- Q phases and superconductivity-induced reentrance of C2 magnetic stripe order in iron pnictides

We perform a microscopic theoretical study of the generic properties of competing magnetic phases in iron pnictides. As a function of electron filling and temperature, the magnetic stripe (single-Q) order forms a dome, but competing non-collinear and non-uniform double-Q phases exist at the foot of the dome in agreement with recent experiments. We compute and compare the electronic properties of the different magnetic phases, investigate the role of competing superconductivity, and show how disorder may stabilize double-Q order. Superconductivity is shown to compete more strongly with double-Q magnetic phases, which can lead to re-entrance of the C2 (single-Q) order in agreement with recent thermal expansion measurements on K-doped Ba-122 crystals.

Spin reorientation driven by the interplay between spin-orbit coupling and Hund's rule coupling in iron pnictides

In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal non-uniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microscopic electronic model that respects all the symmetry properties of a single FeAs plane. We find that the interplay between the spin-orbit coupling and the Hunds rule coupling can account for the observed spin anisotropies, including the spin reorientation in hole-doped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron- and hole-doped compounds, with only the latter displaying tetragonal magnetic states.

Low-temperature ferroelectric phase and magnetoelectric coupling in underdoped La2CuO4+x

We report the discovery of ferroelectricity below 4.5 K in highly underdoped La2CuO4+x accompanied by slow charge dynamics which develop below T similar to 40 K. An anisotropic magnetoelectric response has also been observed, indicating considerable spin-charge coupling in this lightly doped parent high-temperature copper-oxide superconductor. The ferroelectric state is proposed to develop from polar nanoregions, in which spatial inversion symmetry is locally broken due to nonstoichiometric carrier doping.

Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots

PACS numbers: 73.21.La, 73.23.Hk, 73.63.Fg, 74.50.r Superconducting electrodes provide a useful means of sharpening the spectroscopic features observed in tunneling experiments. In the superconducting phase, an otherwise nearly constant density of states DOS acquires a gap of width 2 centered at the Fermi level and characteristic sharp coherence peaks at the gap edges . These peaks transform a featureless metallic electrode into a high-resolution tunneling probe. This widely used investigative tool 1 was demonstrated already by Giaever’s seminal work 2 from 1960 and more recently used to obtain a high-resolution bias spectrum of the levels in a metallic Al nanoparticle. 3 Here we report low-temperature transport measurements in which this type of BCS focusing promotes an otherwise featureless elastic EL cotunneling conductance to sharp peaks at bias voltages Vsd= 2 /e, corresponding to the onset of quasiparticle QP cotunneling. In the same way, inelastic INEL cotunneling processes involving transitions between two subbands in the nanotube are revealed as sharp peaks rather than steps or cusps in the nonlinear conductance. This sharpening of cotunneling lines inside the Coulomb diamonds allows us to investigate more closely the tunneling-induced gate voltage dependence of the orbital splitting. 4 Finally, we discuss an unusual subgap structure observed in a particularly well-coupled device signaling the importance of both multiple Andreev reflections MARs and dynamically generated bound states in spinful dots. A number of experiments have already investigated interesting aspects of quantum dots with superconducting electrodes, such as supercurrent, 5–7 MAR, 8 and effects of size and charge quantization in the Fabry-Perot, 9–12 Kondo, 13–18

Disorder-induced static antiferromagnetism in cuprate superconductors

Using model calculations of a disordered d-wave superconductor with on-site Hubbard repulsion, we show how dopant disorder can stabilize novel states with antiferromagnetic order. We find that the critical strength of correlations or impurity potential necessary to create an ordered magnetic state in the presence of finite disorder is reduced compared to that required to create a single isolated magnetic droplet. This may explain why, in cuprates such as La2-xSrxCuO4, low-energy probes have identified a static magnetic component which persists well into the superconducting state, whereas, in cleaner systems such as YBa(2)Cu(3)O(6+delta), it is absent or minimal.