Influence of Magnetic Order on Proximity-Induced Superconductivity in Mn Layers on Nb(110) from First Principles

TL;DR

This work addresses how magnetic order in ultrathin Mn overlayers on Nb(110) modulates proximity-induced superconductivity. It employs a fully relativistic Dirac-BdG solver within the SKKR framework to compute normal-state DOS, induced Nb moments, momentum-resolved BSF, SC DOS, and singlet/triplet order parameters for single and double Mn layers under FM and AFM configurations. Key findings include magnetism-dependent in-gap DOS features and BSF crossings, a small Mn singlet order parameter (max ~4.44% of Nb bulk) with finite IAT triplet mixing, and qualitative agreement with experimental in-gap states. The results provide a microscopic, first-principles understanding of magnetism–superconductivity interplay in Mn/Nb heterostructures, with implications for SC spintronics and potential topological superconductivity.

Abstract

We investigate the influence of magnetic order on the proximity-induced superconducting state in the Mn layers of a Mn-Nb(110) heterostructure by using a first-principles method. For this study, we use the recently developed Bogoliubov-de Gennes (BdG) solver for superconducting heterostructures [Csire et al., Phys. Rev. B 97, 024514 (2018)] within the first-principles calculations based on multiple scattering theory and the screened Korringa-Kohn-Rostoker (SKKR) Green's function method. In our calculations, we first study the normal-state density of states (DOS) in the single- and double-Mn-layer heterostructures, and calculate the induced magnetic moments in the Nb layers. Next, we compute the momentum-resolved spectral functions in the superconducting state for the heterostructure with a single Mn layer, and find bands crossing the Fermi level within the superconducting (SC) gap. We also study the SC state DOS in the single- and double-Mn-layer heterostructures and compare some of our results with experimental findings, revealing secondary gaps, plateau-like regions, and central V-shaped in-gap states within the bulk SC Nb gap that are magnetic-order-dependent. Finally, we compute the singlet and internally antisymmetric triplet (IAT) order parameters for each layer for both heterostructures, and find an order of magnitude difference in the induced singlet part of the SC order parameter in the Mn layer/s between the FM and AFM cases in favor of the AFM pairing with the maximum still being only 4.44% of the bulk Nb singlet order parameter value. We also find a negligible induced triplet part, yet comparable to the induced singlet values, indicating some singlet-triplet mixing in the Mn layer/s.

Figures (16)

• Figure 1: (a) A single Mn atomic layer with FM ordering over a Nb(110) substrate. The two-dimensional unit cell is centered-rectangular with $\vec{a_1}=4.6675$ Å along the $[\overline{1}10]$ direction and $\vec{a_2}=2.86$ Å along the $[\overline{1}11]$ direction. The arrows represent the Mn spin moments and are pointing into the page (Red). (b) A single Mn atomic layer with AFM c(2$\times$2) ordering on a Nb(110) substrate. The two-dimensional unit cell is primitive-rectangular with $\vec{a_1}=4.6675$ Å along the $[\overline{1}10]$ direction and $\vec{a_2}=3.3004$ Å along the $[001]$ direction. The arrows represent the Mn spin moments and are pointing into the page (Red) and out of the page (Blue). (c) A schematic diagram of Mn layers on a Nb(110) substrate with the middle interface region consisting of (1-2) Mn layers, some (10-12) Nb layers, as well as several vacuum layers.
• Figure 2: Magnetic ordering cases of the Mn layer(s) in the Mn-Nb heterostructure. The first column shows the single Mn layer cases (FM - AFM), while the second and third columns show the double Mn layer cases. The second column shows the cases with interlayer FM ordering, while the third row shows the cases with interlayer AFM ordering. The first row shows all cases with intralayer FM ordering, while the second row shows all cases with intralayer AFM ordering.
• Figure 3: Normal-state DOS for the double-Mn-layer heterostructure showing element contributions and spin contributions where relevant in all four magnetic cases.
• Figure 4: DOS projected onto the Mn $3d$-orbitals for the four magnetic cases of the double-Mn-layer- heterostructure.
• Figure 5: BSF for the Mn monolayer heterostructure in the SC state - FM case (a) along the $x$-direction in $k$-space in units of inverse Bohr radius or $a_B^{-1}$ and (b) along the $y$-direction in $k$-space, within the bulk Nb SC gap. The inset in (a) shows a zoom-in for 0.07 $a_B^{-1}$$< k_{\text{OX}} <$ 0.09 $a_B^{-1}$, while the inset in (b) shows the 2D irreducible BZ in the FM case Cunningham_1974.