Extreme Strain Controlled Correlated Metal-Insulator Transition in the Altermagnet CrSb

Abstract

Correlated flat bands and altermagnetism are two important directions in quantum materials, centred respectively on interaction-dominated phases and symmetry-enforced spin-textured states, yet both derive from lattice symmetry and orbital hybridization. This common origin implies that extreme crystal distortion, by narrowing bandwidths, enhancing correlations and reshaping the symmetries of altermagnetic spin splittings, could unify flat-band and altermagnetic physics in a single material; in practice, however, achieving such large distortions in a crystalline altermagnet is a formidable challenge. Here we combine a dedicated strain device with a tailored single-crystal mounting scheme to impose a highly tensile strain gradient in bulk CrSb, a prototypical altermagnet, creating a near-surface layer in which the in-plane lattice is strongly distorted relative to the weakly strained bulk, while the average bulk distortion remains small. Angle-resolved photoemission reveals a reversible regime at moderate strain, where a deeper flat-band feature, attributed to a strain-gradient-driven suppression of Cr-Sb hybridization, coexists with a correlation-enhanced Cr 3d flat band, and an irreversible regime at larger strain where partial bond decoupling drives a predominantly insulating spectral response. Density-functional calculations show that an orbital-selective altermagnetic spin texture persists across this correlated regime despite strong bandwidth renormalisation. These results define a strain-symmetry-correlation map for CrSb and establish extreme tensile strain as a route to co-engineer flat-band tendencies and spin-textured, zero-net-moment correlated states in altermagnets, pointing toward strain-adaptive, spin-selective Mott filtering and related device concepts.

Figures (4)

• Figure 1: Crystal structure of CrSb and strain induced flat bands on the (0001) plane. (a) The crystal structure of CrSb with the space group $P6_3/mmc$ (no. 194). (b) The side and top views of the opposite-spin sublattices. (c) The 3D BZ of the original unit cell of CrSb, and the corresponding two-dimensional Brillouin zone (BZ) projected on the (0001) plane (green lines), (1$\overline{1}$00) plane (red lines) and (11$\overline{2}$0) plane (orange lines). (d) The XPS measurements of CrSb were recorded with a photon energy of 200 eV under applied strain (purple line), without strain (red line), and after strain release (green line). All measurements were normalized over the range -140 eV to -110 eV. The inset presents an enlarged view of the Si 3p orbital, Sb 4d orbital and value band (VB). The inset in the upper right corner shows the sample holder with strain functionality. The black arrows mark the flat band. (e-g) Fermi surfaces of CrSb measured at a photon energy of 102 eV: (e) without applied strain, (f) under applied strain along [11$\overline{2}$0] ($\Gamma$-M) direction, and (g) after strain release. Cyan dots indicate the six vertices of the unstrained Star-of-David Fermi surface. (h-j) Corresponding band structures of CrSb measured at a photon energy of 200 eV along the $\Gamma$-M directon [the red line in (e)]: (h) without strain, (i) with applied strain along [11$\overline{2}$0] ($\Gamma$-M) direction, and (j) after strain release.
• Figure 2: Strain-induced evolution of the electronic structure near the Fermi level. (a-d) Fermi surface evolution under applied strain measured with photon energy of 102 eV: (a) shows the Fermi surface without strain; (b–d) correspond to increasing levels of applied strain along the [1$\overline{1}$00] ($\Gamma$-K) directions. Cyan solid and open dots indicate the six vertices of the unstrained Star-of-David Fermi surface. The red and green double-headed arrows indicate the momentum-space distances along k$_x$ and k$_y$ between equivalent Fermi-surface positions in the first and second BZs. (e) The DFT calculated Fermi surface at k$_z$ = 0.8 $\pi$/c. (f, h) Band structures along Cut1 [cyan line in (a), f] and Cut2 [red line in (a), h], measured without strain with photon energy of 102 eV. (g, i) The density functional theory (DFT) calculated band structures along the $\widetilde{K}$-$\widetilde{\Gamma}$-$\widetilde{K}$ (g) and $\widetilde{M}$-$\widetilde{\Gamma}$-$\widetilde{M}$ (i) directions at k$_z$ = 0.8 $\pi$/c. (j-m) Band structures measured with photon energy of 102 eV along Cut3-Cut6 [blue, orange, green and purple lines in (d)] after the third applied strain. The red arrows marks the surface states (SSs). (n-o) The extracted MDCs from band cut1-cut6 at Fermi level. Black ticks indicate the bulk band k$_F$ positions and red ticks indicate the SSs.
• Figure 3: Spatial evolution of strain-induced flat bands and irreversible spectral response. (a) Spatial map of the integrated spectral intensity within $\pm$0.75 eV around -1 eV, acquired across a near-surface region with microcracks that delineate a strong strain gradient. (b) Ratio map of the integrated spectral intensity within $\pm$0.75 eV around -6 eV and -1 eV, defining measurement points P1-P5 along the strian gradient. The colored dots mark the beam positions; their sizes approximately represent the beam-spot diameter. (c-g) Fermi-surface mappings at P1-P5 measured with photon energy of 102 eV. (h-l) The corresponding band dispersions along the $\Gamma$-K direction at P1-P5 measured with photon energy of 102 eV. As the measurement position moves from P1 to P5, the spectral weight near the Fermi level gradually decreases, and a distinct flat band emerges around -2 eV, accompanied by the development of a broadened feature near -4.5 eV. Notably, a residual Fermi-surface signal remains visible even where the -2 eV flat band dominates, which likely arises from the photoemission contribution of the beam spot partially overlapping with less-strained metallic regions. (m) XPS spectra measured at P1-P5. The inset presents an enlarged view of the Si 3p orbital, Sb 4d orbital and VB. (n-o) CD-ARPES, plotted as ($I_{CR}$-$I_{CL}$)/($I_{CR}$+$I_{CL}$), for a moderately strained region in the reversible flat-band regime (n) and for a highly strained region in the irreversible regime (o); the irreversible regime at the most strongly strained positions (e.g. P5) likely reflects both defects introduced when cleaving the pre-loaded crystal and additional local damage from the highly non-uniform near-surface stress during tensile strain. (p) Spin-resolved DFT band structure along $\widetilde{M}$-$\widetilde{\Gamma}$-$\widetilde{M}$ under 20% tensile strain at $k_{z}$ = 0.8 $\pi/c$, with colors indicating opposite $S_z$ projections.
• Figure 4: Strain-controlled evolution of electronic states and correlated phases in CrSb. (a) Illustration of the near-surface strain-gradient geometry: direct tensile deformation mainly stretches the top several Cr–Sb layers, while the underlying bulk remains weakly distorted. (b) Real-space charge density distributions for the unstrained (left) and 20% tensile-strained (right) lattices. The isosurfaces, primarily derived from Cr 3$d$ and Sb 5$p$ states from -6 eV to -2 eV, illustrate that tensile strain elongates Cr-Sb bonds and weakens $p$-$d$ hybridization while modulating Cr-Cr $d$-$d$ coupling, leading to enhanced orbital localization. (c) Schematic illustration of the strain-dependent evolution of the electronic density of states (DOS). With increasing tensile strain, CrSb evolves from an altermagnetic metal with dispersive bands to a distorted metallic phase featuring a reversible -6 eV flat band originating from a $p$-$d$ hybridization collapse. Further strain enhances correlations and reduces the bandwidth of the Cr 3$d$ manifold, producing a correlated Mott-like flat-band regime in which the -6 eV and -2 eV features coexist. Excessive strain causes bond rupture and disorder in the near-surface region, leading to a largely irreversible insulating spectral regime dominated by the -2 eV and -4.5 eV flat bands. (d) Schematic strain-interaction-distortion diagram showing the evolution from an altermagnetic metal through a correlated Mott-like flat-band regime to a disorder-dominated insulating regime with increasing strain.