Superconductor-altermagnet memory functionality without stray fields

A novel class of antiferromagnets, dubbed altermagnets, exhibit a nonrelativistically spin-split band structure reminiscent of the order parameter phase $d$-wave superconductors, despite the absence of net magnetization. This unique characteristic enables utilization in cryogenic stray-field-free memory devices, offering the possibility of achieving high storage densities. We here determine how a proximate altermagnet influences the critical temperature $T_c$ of a conventional $s$-wave singlet superconductor. Considering both a bilayer and trilayer, we show that such hybrid structures may serve as stray-field-free memory devices where the critical temperature is controlled by rotating the Néel vector of one altermagnet, providing infinite magnetoresistance. Furthermore, our study reveals that altermagnetism can coexist with superconductivity up to a critical strength of the altermagnetic order as well as robustness of the altermagnetic influence on the conduction electrons against nonmagnetic impurities, ensuring the persistence of the proximity effect under realistic experimental conditions.

Figure 1

The (a) order parameter and (b) critical temperature as a function of the altermagnetic and ferromagnetic strength in a $N_x=N_y=20a_0$ structure with coexisting superconductivity and altermagnetic/ferromagnetic spin splitting. The termperature in (a) is set to $T=0.01t/k_B$, where $k_B$ is the Boltzmann constant.

Figure 2

(a) The straight junction and (b) the skewed junction geometries. (c) The hopping term for a spin-up (spin-down) electron (hole). For a spin-down (spin-up) electron (hole), the signs are reversed. (d) The critical temperature for the straight (St) and skewed (Sk) geometries with $N_y=20a_0$, $N_x^{AM}=10a_0$, $N_x^{SC}=6a_0$, and $N_{\mathrm{\Delta }}=50$.

Figure 3

(a) The AM-SC-AM system with P and AP alignment of the altermagnets. (b) The critical temperatures in a system where the length of the SC is $12a_0$, and $N_y=20a_0$. The altermagnets on either side have a length of $10a_0$.

Figure 4

$\mathrm{\Delta }{C}_{\uparrow }$ as a function of $m$ and $N_i$, for impurity strength of (a) $w_i=1t$ and (b) $w=3t$. (c) $\mathrm{\Delta }{C}_{\uparrow }$ as a function of $m$ and $\mu$ in a system without impurities., The system size is $N_x=N_y=20a_0$.

Figure 5

The impurity average plotted together with the clean system, which is the same as the straight system in Fig. 2. In the bottom pane, the temperatures are normalized to the zero-impurity and zero-magnetism critical temperature $T_{c,0}(m=0)$, showing that $T_c$ is slightly higher in the presence of impurities. In the top pane, the temperatures are normalized to unity for $m=0$, i.e., the two curves are normalized by a different factor, which illustrates that the variation in $T_c$ with $m$ is of similar magnitude in both cases.