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Review
. 2018 Aug 6;376(2125):20150001.
doi: 10.1098/rsta.2015.0001.

Andreev reflection spectroscopy in transition metal oxides

Affiliations
Review

Andreev reflection spectroscopy in transition metal oxides

Karen A Yates et al. Philos Trans A Math Phys Eng Sci. .

Abstract

Here we review the literature concerning measurement of the Andreev reflection between a superconductor (S) and ferromagnet (F), with particular attention to the case where the ferromagnet is a transition metal oxide. We discuss the practicality of utilization of the current models for determination of the transport current spin polarization and examine the evidence for Andreev bound states.This article is part of the theme issue 'Andreev bound states'.

Keywords: point contact spectroscopy; transition metal oxide.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
(a) PZ relation for the ballistic and diffusive limit fitting for a Pb contact onto a CrO2 (film). Reproduced with permission from Woods [12] (Copyright © 2004 American Physical Society).
Figure 2.
Figure 2.
Parameters as a function of Z extracted via the M-BTK model for Pb contacts onto CrO2 thin films, Reproduced with permission from Yates et al. [21] (Copyright © 2007 AIP Publishing LLC). (a) Polarization, (b) Δ, (c) broadening and from the literature ref [20], (d) P, (e) Δ/Δ0 where Δ0 is the expected gap of the superconductor and (f) the spreading resistance rs. Symbols are for the data from (filled square) [1], (filled triangle) [36], (filled circle) [13], (filled inverted triangle) [8], (open inverted triangle) [37], (filled diamond) [38], (open diamond) [39]. (Online version in colour.)
Figure 3.
Figure 3.
(a) G0/Gn in the MBTK model and (b) the SMM for P = 100% as a function of spin mixing angle, θ. (c) the excess current in the MBTK model as a function of Z, (d) in the SMM. Reproduced with permission from Löfwander et al. [20] (Copyright © 2010 American Physical Society). (Online version in colour.)
Figure 4.
Figure 4.
Observation of Andreev Bound States in tunnelling spectra between Fe and Al. Note the scale difference between figures (a) and (b). Reproduced with permission from Hübler et al. [50] (Copyright © 2012 American Physical Society). (Online version in colour.)
Figure 5.
Figure 5.
Temperature dependence of a Pb contact onto a CrO2 thin film, adapted from [49] (b) extracted G0/Gn data for CrO2 films onto TiO2 (dark blue symbols, [49], circle with line [14]), Al2O3 (light green, [21]). G0/Gn(T) for the SMM, with θ = 0, Z = 0.1 (lower curve—solid red line), 0.5 (dotted red line), θ = π/2, Z = 0.1 (upper curve—solid black line), Z = 0.5 (dotted black line). Also shown are data taken on La1.15Sr1.85Mn2O7 (boxed crosses) [51] and from the literature on LSMO3 (red crosses) [4] and (*) on HgCr2Se4 [52]. Reprinted with permission from [51] (Copyright © (2017) by the American Physical Society. https://doi.org/10.1103/PhysRevB.95.094516). (Online version in colour.)

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