Andreev reflection spectroscopy in transition metal oxides

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.

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.

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 r_s. 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.

(a) G₀/G_n 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.

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.

Temperature dependence of a Pb contact onto a CrO₂ thin film, adapted from [49] (b) extracted G₀/G_n data for CrO₂ films onto TiO₂ (dark blue symbols, [49], circle with line [14]), Al₂O₃ (light green, [21]). G₀/G_n(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 La_1.15Sr_1.85Mn₂O₇ (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.)