Triplet supercurrents in clean and disordered half-metallic ferromagnets

Eschrig, Matthias; Löfwander, Tomas

doi:10.1038/nphys831

Cited by 401 publications

(468 citation statements)

References 16 publications

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“…3d), suggesting significant Andreev reflection. This conundrum may be resolved by spin-flip Andreev reflections resulting from spin-orbit coupling in the superconductor 32 , dephasing 33 , magnetization gradients 34 , and/or impurities 35 . Such processes would restore a BTK-like lineshape for split peaks on the e 2 /h plateau (Supplementary Information).…”

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confidence: 99%

Gate-tunable superconducting weak link and quantum point contact spectroscopy on a strontium titanate surface

et al. 2014

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Two-dimensional electron systems in gallium arsenide and graphene have enabled ground-breaking discoveries in condensed-matter physics, in part because they are easily modulated by voltages on nanopatterned gate electrodes. Electron systems at oxide interfaces hold a similarly large potential for fundamental studies of correlated electrons and novel device technologies 1-3 , but typically have carrier densities too large to control by conventional gating techniques. Here we present a quantum transport study of a superconducting strontium titanate (STO) interface, enabled by a combination of electrolyte 4-7 and metal-oxide gating. Our structure consists of two superconducting STO banks flanking a nanoscale STO weak link, which is tunable at low temperatures from insulating to superconducting behaviour by a local metallic gate. At low gate voltages, our device behaves as a quantum point contact that exhibits a minimum conductance plateau of e 2 /h in zero applied magnetic field, half the expected value for spin-degenerate electrons, but consistent with predictions 8-12 and experimental signatures 13-18 of a magnetically ordered ground state. The quantum point contact mediates tunnelling between normal and superconducting regions, enabling lateral tunnelling spectroscopy of the local superconducting state. Our work provides a generic scheme for quantum transport studies of STO and other surface electron liquids.Transition metal oxides exhibit electronic ground states not seen in conventional semiconductors 2,3 . For instance, the interface between lanthanum aluminate and strontium titanate 19 -two non-magnetic band insulators-hosts superconductivity 20 and magnetism [13][14][15][16][17][18] . Rashba spin-orbit coupling at conductive STO interfaces 21 has led to predictions of unconventional superconducting states 10 , and of helical wires and Majorana fermions in nanoscale STO channels 11 . Furthermore, nanoscale lateral control of carriers could allow fabrication of gate-tunable, single-material Josephson junctions and superconducting quantum interference devices, as well as mesoscopic devices. However, few experiments on nanostructures in STO systems exist: superconductivity has been demonstrated in submicron regions tunable with a global back gate 22 , and channels sketched by a nanoscale tip show superconducting features 23 . In this work, we demonstrate the first realization of an all-STO, gate-tunable superconducting weak link. We tune our device to the ballistic 1D limit and perform superconducting spectroscopy that agrees with BCS weak-coupling expectations and previous work 24 .Undoped STO is a band insulator. Inducing a two-dimensional (2D) electron density near 10 13 cm −2 creates a metal, and several times that density creates an optimally doped superconducting state 4 . To modulate the density on this scale, we use the electric double-layer transistor technique [4][5][6][7] . A schematic of our sample is shown in Fig. 1a. A single crystal of TiO 2 -terminated (100) SrTiO 3 is patterned with ohmic conta...

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confidence: 99%

Gate-tunable superconducting weak link and quantum point contact spectroscopy on a strontium titanate surface

et al. 2014

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“…In strong ferromagnets is large and is of the order of ~1 nm at low temperature ; however, theories have emerged recently suggesting the existence of a novel proximity effect where spin-triplet pairing is generated at the S/F interface resulting in a greatly extended decay length, . This exotic proximity effect named "the long range spin triplet proximity effect" (LRSTPE) 4,5 only exists if either some form of inhomogeneous magnetization is present at the S/F interface or if there is a spin active region (such as spin scattering in a strong spin orbit coupled medium) between the S and F layers 4,5 . Current thinking suggests that the inhomogeneous magnetic state could either be provided by a magnetic system that offers intrinsic inhomogeneity due to a non co-linear spin arrangement or could be artificially created in a number of ways including the presence of domain walls (although domain wall density may prove an issue), or through a thin film multilayer arrangement using different types of ferromagnets 6 .…”

Section: Introductionmentioning

confidence: 99%

Evidence for spin mixing in holmium thin film and crystal samples

et al. 2011

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In a number of recent experiments, holmium has been shown to promote spin triplet pairing when in proximity to a spin singlet superconductor. The condition for the support of spin triplet pairing is that the ferromagnet should have an inhomogeneous magnetic state at the interface with the superconductor. Here we use Andreev reflection spectroscopy to study the properties of single ferromagnet/superconductor interfaces formed of holmium and niobium, as a function of the contact resistance of the junction between them. We find that both single crystal and c-axis oriented thin film holmium show unusual behavior for low junction contact resistance, characteristic of spin mixing type properties, which are thought necessary to underpin spin triplet formation. We also explore whether this signature is observed when the junction is formed of Ni 19 Pd 81 and niobium.

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“…23,24 In these studies SFS-Josephson junctions with weakly spin polarized ferromagnetic barriers like coppernickel or palladium-nickel alloys were used to avoid strong suppression of the proximity effect. Recent observation of a supercurrent across the half-metal chromium dioxide [25][26][27] triggered intense activity to study triplet supercurrents (i.e., long range proximity effects) both theoretically [13][14][15] and experimentally. [16][17][18]28 Therefore the successful fabrication of oxide SFS-structures may help to investigate and understand the long-range proximity effects between d-wave high-T c superconductors and half-metal ferromagnetic LCMO layers.…”

Section: Introductionmentioning

confidence: 99%

Preparation of a ferromagnetic barrier in YBa2Cu3O7–δ thinner than the coherence length

Soltan

Albrecht

Goering

et al. 2015

Journal of Applied Physics

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Thin ferromagnetic La2∕3Ca1∕3MnO3 layers are deposited in between two layers of the high-Tc superconductor YBa2Cu3O7–δ (YBCO) by pulsed laser deposition. Using a tailored buffer layer allows the change of the orientation of the CuO2 planes inside the superconducting layers. With this technique it is possible to produce ferromagnetic barrier layers with high ferromagnetic transition temperatures that are thinner than the coherence length of the adjacent superconductors. Magnetization and electric transport measurements suggest that this geometry might be successful for the generation of ferromagnetic Josephson junctions in high temperature superconducting YBCO.

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Triplet supercurrents in clean and disordered half-metallic ferromagnets

Cited by 401 publications

References 16 publications

Gate-tunable superconducting weak link and quantum point contact spectroscopy on a strontium titanate surface

Gate-tunable superconducting weak link and quantum point contact spectroscopy on a strontium titanate surface

Evidence for spin mixing in holmium thin film and crystal samples

Preparation of a ferromagnetic barrier in YBa2Cu3O7–δ thinner than the coherence length

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