Properties of Bose-Einstein condensates with altermagnetism

Abstract

We investigate a weakly interacting two-component Bose--Einstein condensate in the miscible regime in the presence of \emph{altermagnetism}, i.e., a collinear and globally compensated magnetic order that breaks spin-rotation symmetry while maintaining zero net magnetization. Within Bogoliubov theory, we derive the quasiparticle spectrum and coherence factors and show that altermagnetic order generically induces an angular dependence of the low-energy excitations. As a result, the sound velocity, momentum-resolved magnetization in the quantum depletion, and density--spin response functions acquire anisotropic components. We show that these anisotropic contributions vanish after angular averaging, consistent with the defining feature of altermagnetism: nontrivial local spin polarization without a global magnetization. Finally, we evaluate the Lee--Huang--Yang correction to the ground-state energy in the altermagnetic phase. Our results should be testable with ultracold-atom experiments in the foreseeable future.

Figures (4)

• Figure 1: Angular dependence of the sound velocities in the altermagnetic Bose condensate. The color and radial distance of each point represent the sound speeds $c_{\pm}(\theta,\phi;\lambda)$ for propagation along the direction $(\theta,\phi)$, normalized by the isotropic reference values $\tilde{c}_{\pm}\equiv c_{\pm}(\lambda=0)$. Parameters: $na^3= 0.002$, $a_{\uparrow\downarrow}=0.1a$, and $\lambda=0.8$.
• Figure 2: (a) Momentum-resolved magnetization of the quantum depletion at $k=0.1 a^{-1}$ in the $x$--$y$ plane ($\theta=\pi/2$) as a function of the azimuthal angle $\phi$. (b) Total quantum depletion as a function of $\lambda$ for different interspecies scattering lengths $a_{\uparrow\downarrow}$. The density is fixed to $na^3= 0.002$.
• Figure 3: Mixed density--spin structure factor in the $x$--$y$ plane. Angular dependence of the static mixed structure factor $S_{sd}(\mathbf{q})$ at fixed $q=0.1a^{-1}$ and $\theta=\pi/2$ as a function of the azimuthal angle $\phi$ for various $\lambda$.
• Figure 4: Lee--Huang--Yang (LHY) energy correction per particle as a function of density for several altermagnetic strengths $\lambda$. Circle, square, and diamond symbols show our numerical evaluation for $\lambda=0.0$, $0.4$, and $0.8$, respectively; crosses indicate the known analytic result in the non-altermagnetic limit $\lambda=0$. Parameters: $a_{\uparrow\downarrow}=0.1a$.