Matthias Eschrig

Spin-polarized supercurrents for spintronics

A marriage between superconductivity and ferromagnetism is opening the door for new spin-based applications.

Triplet supercurrents in clean and disordered half-metallic ferromagnets

Interfaces between materials with differently ordered phases present unique opportunities to study fundamental problems in physics. One example is the interface between a singlet superconductor and a half-metallic ferromagnet, where Cooper pairing occurs between electrons with opposite spin on the superconducting side, whereas the other exhibits 100% spin polarization. The recent surprising observation of a supercurrent through half-metallic CrO2 therefore requires a mechanism for conversion between unpolarized and completely spin-polarized supercurrents. Here, we suggest a conversion mechanism based on electron spin precession together with triplet-pair rotation at interfaces with broken spin-rotation symmetry. In the diffusive limit (short mean free path), the triplet supercurrent is dominated by inter-related odd-frequency s-wave and even-frequency p-wave pairs. In the crossover to the ballistic limit, further symmetry components become relevant. The interface region exhibits a superconducting state of mixed-spin pairs with highly unusual symmetry properties that open up new perspectives for exotic Josephson devices.

Theoretical aspects of Andreev spectroscopy and tunneling spectroscopy in non-centrosymmetric superconductors : a topical review

Tunneling spectroscopy at surfaces of unconventional superconductors has proven an invaluable tool for obtaining information about the pairing symmetry. It is known that mid gap Andreev bound states manifest itself as a zero bias conductance peak in tunneling spectroscopy. The zero bias conductance peak is a signature for a non-trivial pair potential that exhibits different signs on different regions of the Fermi surface. Here, we review recent theoretical results on the spectrum of Andreev bound states near interfaces and surfaces in non-centrosymmetric superconductors. We introduce a theoretical scheme to calculate the energy spectrum of a non-centrosymmetric superconductor. Then, we discuss the interplay between the spin orbit vector field on the Fermi surface and the order parameter symmetry. The Andreev states carry a spin supercurrent and represent a helical edge mode along the interface. We study the topological nature of the resulting edge currents. If the triplet component of the order parameter dominates, then the helical edge mode exists. If, on the other hand, the singlet component dominates, the helical edge mode is absent. A quantum phase transition occurs for equal spin singlet and triplet order parameter components. We discuss the tunneling conductance and the Andreev point contact conductance between a normal metal and a non-centrosymmetric superconductor.Tunneling spectroscopy at surfaces of unconventional supe rconductors has proven an invaluable tool for obtaining information abo ut the pairing symmetry. It is known that mid gap Andreev bound states manifest itself as a zero bias conductance peak in tunneling spectroscopy. The zero bias conduct ance peak is a signature for a non-trivial pair potential that exhibits different si gns on different regions of the Fermi surface. Here, we review recent theoretical resul ts on the spectrum of Andreev bound states near interfaces and surfaces in non-ce ntrosymmetric superconductors. We introduce a theoretical scheme to calculate the nergy spectrum of a non-centrosymmetric superconductor. Then, we discuss th e interplay between the spin orbit vector field on the Fermi surface and the order para meter symmetry. The Andreev states carry a spin supercurrent and represent a hel ic l edge mode along the interface. We study the topological nature of the resulting ed e currents. If the triplet component of the order parameter dominates, then the helica l dge mode exists. If, on the other hand, the singlet component dominates, the heli cal dge mode is absent. A quantum phase transition occurs for equal spin singlet and triplet order parameter components. We discuss the tunneling conductance and the An dreev point contact conductance between a normal metal and a non-centrosymmetr ic superconductor. Matthias Eschrig (a) Fachbereich Physik, Universität Konstanz, D-78464 Ko nstanz, Germany, and (b) Institut für Theoretische Festkörperphysik and DFGCenter for Functional Nanostructures, Karlsruhe Institute of Technology D-76128 Karlsruhe, Germ any, e-mail:Matthias.Eschrig@kit.de Christian Iniotakis Institute for Theoretical Physics, ETH Zurich, 8093 Zurich , Switzerland, e-mail:iniotaki@phys.ethz.ch Yukio Tanaka Department of Applied Physics, Nagoya University, Nagoya, 464-8603, Japan e-mail:ytanaka@nuap.nagoya-u.ac.jp

Theory of half-metal/superconductor heterostructures

We investigate the Josephson coupling between two singlet superconductors separated by a half-metallic magnet. The mechanism behind the coupling is provided by the rotation of the quasiparticle spin in the superconductor during reflection events at the interface with the half metal. Spin rotation induces triplet correlations in the superconductor which, in the presence of surface spin-flip scattering, results in an indirect Josephson effect between the superconductors. We present a theory appropriate for studying this phenomenon and calculate physical properties for a superconductor/half-metal/superconductor heterostructure.

Spatially resolved electronic structure inside and outside the vortex cores of a high-temperature superconductor.

Puzzling aspects of high-transition-temperature (high-Tc) superconductors include the prevalence of magnetism in the normal state and the persistence of superconductivity in high magnetic fields. Superconductivity and magnetism generally are thought to be incompatible, based on what is known about conventional superconductors. Recent results, however, indicate that antiferromagnetism can appear in the superconducting state of a high-Tc superconductor in the presence of an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines, each of which represents a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. Here we report the results of a high-field nuclear-magnetic-resonance (NMR) imaging experiment in which we spatially resolve the electronic structure of near-optimally doped YBa2Cu3O7-δ inside and outside vortex cores. Outside the cores, we find strong antiferromagnetic fluctuations, whereas inside we detect electronic states that are rather different from those found in conventional superconductors.

Spin-polarized supercurrents for spintronics: a review of current progress

During the past 15 years a new field has emerged, which combines superconductivity and spintronics, with the goal to pave a way for new types of devices for applications combining the virtues of both by offering the possibility of long-range spin-polarized supercurrents. Such supercurrents constitute a fruitful basis for the study of fundamental physics as they combine macroscopic quantum coherence with microscopic exchange interactions, spin selectivity, and spin transport. This report follows recent developments in the controlled creation of long-range equal-spin triplet supercurrents in ferromagnets and its contribution to spintronics. The mutual proximity-induced modification of order in superconductor-ferromagnet hybrid structures introduces in a natural way such evasive phenomena as triplet superconductivity, odd-frequency pairing, Fulde-Ferrell-Larkin-Ovchinnikov pairing, long-range equal-spin supercurrents, [Formula: see text]-Josephson junctions, as well as long-range magnetic proximity effects. All these effects were rather exotic before 2000, when improvements in nanofabrication and materials control allowed for a new quality of hybrid structures. Guided by pioneering theoretical studies, experimental progress evolved rapidly, and since 2010 triplet supercurrents are routinely produced and observed. We have entered a new stage of studying new phases of matter previously out of our reach, and of merging the hitherto disparate fields of superconductivity and spintronics to a new research direction: super-spintronics.

Anomalous Enhancement of the Coupling to the Magnetic Resonance Mode in Underdoped Pb-Bi2212

High-resolution angle-resolved photoemission with variable excitation energies is used to disentangle bilayer splitting effects and intrinsic (self-energy) effects in the electronic spectral function near the (

Symmetries of Pairing Correlations in Superconductor-Ferromagnet Nanostructures

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Effect of the magnetic resonance on the electronic spectra of high T{sub c} superconductors.

,0)-point of differently doped (Pb,Bi)

Neutron Resonance in the Cuprates and its Effect on Fermionic Excitations

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