Unconventional superconductivity is widely believed to arise from pairing mechanisms involving magnetic fluctuations rather than conventional electron-phonon coupling [1]–[4]. For spin-triplet superconductivity, a phase of broad interest both for its fundamental novelty and for its potential relevance to topological quantum computing [5]–[14], ferromagnetic (FM) fluctuations have long been considered a critical ingredient for spin-triplet pairings. FM fluctuations have been observed in many candidate materials including K\(_2\)Cr\(_3\)As\(_3\) and UTe\(_2\) [5], [8], [9], [11], [15]–[19]. Identifying the interactions between such FM fluctuations and low-energy electronic states in candidate materials is therefore central to understanding spin-triplet pairing.

CeSb\(_2\) has recently been identified as a candidate for pressure-induced spin-triplet superconductivity [20], [21]. INS measurements on CeSb\(_2\) resolve well-defined quasi-one-dimensional (q1D) FM excitations that persist well above the magnetic ordering temperature, offering a possible FM channel relevant for triplet pairing [21]. However, the microscopic origin of these q1D FM excitations is unclear. CeSb\(_2\) crystallizes in a quasi-two-dimensional (q2D) lattice with nearly identical inter- and intra-ladder Ce-Ce distances [Fig. 1(a)], so the lattice geometry cannot explain the dimensional reduction of the magnetic response. Moreover, the system does not condense into simple FM order but goes through four successive magnetic transitions below \(T\) \(\sim\)​16 K [22]–[24], including a commensurate AFM order with propagation vectors (-1, \(\pm\) \(\frac{1}{6}\), 0) [21], [25]. Together, the q1D FM excitations in a near-isotropic q2D lattice and the multiple magnetic orders raise the possibility that the effective dimensionality of the magnetic interactions is determined not by the lattice geometry itself. This calls for a microscopic mechanism rooted in the electronic structure.

Here, we address this issue by directly probing the electronic structure of CeSb\(_2\) using angle-resolved photoemission spectroscopy (ARPES). We uncover coexisting near \(C_4\)-distributed and \(C_2\)-distributed Fermi surfaces. High-resolution measurements at low temperatures reveal no detectable spin-density wave (SDW) gap, which disfavors an SDW-driven scenario for the magnetic excitations. Instead, resonant ARPES suggests band-selective Kondo coupling on a subset of small \(C_2\)-distributed pockets aligned with the Ce ladder, generating intrinsically anisotropic magnetic exchange and competing FM/AFM tendencies. This band-selective magnetic interaction provides a natural microscopic basis for the emergence of q1D ferromagnetic fluctuations in CeSb\(_2\), demonstrating how such coupling can generate q1D spin fluctuations in a q2D system.

High-quality single crystals of CeSb\(_2\) were grown by the self-flux method within a graphite resistance heating system [26]. Samples were cleaved \(in\)-\(situ\) below 30 K along the (001) plane. ARPES measurements were performed at the synchrotron radiation facilities-Advanced Light Source (ALS), Shanghai Synchrotron Radiation Facility (SSRF), and BESSY-II under optimized experimental conditions. More details on sample growth [26] and characterization [22], [24], [25], [27]–[29], dynamical mean-field theory (DMFT) calculations[30]–[34], and ARPES measurements were provided in Supplemental Materials (SM) Sections I-IV [35].

The electronic structure of CeSb\(_2\) shows q2D character, as revealed by photon-energy dependent measurements [Fig. S3]. The representative in-plane Fermi surfaces consist of multiple features [Figs. 1(c) and 1(d)]. An inner square-like \(\beta\) pocket and an outer diamond-shaped \(\gamma\) pocket are centered around the \(\Gamma\) point [Figs. 1(c) and 1(d)], both preserving a nearly \(C_4\) symmetry. These two Fermi surfaces cross at the X and Y high-symmetry points in a complex manner according to calculations [Fig. S2] [23]; the details of these crossings are not well resolved in ARPES. In addition, two small \(\alpha\) pockets and two small \(\eta\) pockets are primarily located along the \(\Gamma\)S\(_1\) direction [Figs. 1(c)-1(e)], reflecting the quasi-one-dimensional character of the Ce ladder and its intrinsic \(C_2\) symmetrical distribution [Figs. 1(a)-1(b)]. Polarization-dependent measurements show that the \(C_2\)-distributed feature persists under both linear horizontal (LH) and linear vertical (LV) geometries [Fig. S4], ruling out matrix-element effects. Along both \(\Gamma\)S\(_1\) and \(\Gamma\)S\(_2\) [Figs. 1(e) and 1(f)], the \(\beta\) and \(\gamma\) bands are highly dispersive and cross \(E_{\rm{F}}\) with a relatively large Fermi velocity, indicating relatively weak electronic correlation. In contrast, the \(\alpha\) and \(\eta\) bands show broader spectral features [Fig. 1(e)] and reduced velocities near \(E_{\rm{F}}\) along \(\Gamma\)S\(_1\) [Figs. 2(a) and 2(b)], characteristic of strong correlations, well reproduced by DMFT calculations [Figs. 1(e) and 1(f)]. In the ARPES spectra along \(\Gamma\)S\(_2\) [Fig. 1(f)], the residual weight of the \(\eta\) band originates from the superposition of 90\(^{\circ}\)-rotated structural twin domains [25]. Micro-ARPES measurements on single-domain [Fig. S4] confirm the absence of \(\eta\) pocket along \(\Gamma\)S\(_2\), supporting its intrinsic \(C_2\)-distributed character.

Given the parallel segments of the \(\beta\) and \(\gamma\) pockets [Fig. 1(d)], a possible Fermi surface nesting condition might give rise to SDW order and contribute to the competing magnetic orders. To test this scenario, high-resolution ARPES measurements were conducted at low temperature down to 2 K, yielding a total energy resolution of 3 meV. Symmetrized energy-momentum intensity plots along the \(\Gamma\)S\(_1\) and \(\Gamma\)S\(_2\) directions show no indication of gap opening at the temperature below any ordering transitions, with both \(\beta\) and \(\gamma\) bands dispersing to \(E_{\rm{F}}\) without suppression of spectral weight. Upon cooling from 20 K to 2 K, the \(\beta\), \(\gamma\), and \(\alpha\) bands disperse through \(E_{F}\) along \(\Gamma\)S\(_1\), with no detectable gap across any of the magnetic transitions [Figs. 2(e)-2(g)], within an energy resolution of 3 meV. While we cannot exclude a sub-meV gap or a partial gap, the dominance of well-defined Fermi crossings disfavors nesting as the primary driver of the complex magnetic orders below 16 K.

Resonant ARPES measurements were performed along the \(\Gamma\)S\(_1\) (Ce-ladder) direction by tuning the photon energy across the Ce \(M\)-edge. Off-resonance spectra resolve the dispersive \(\alpha\), \(\beta\), \(\gamma\) and \(\eta\) bands crossing \(E_{\rm{F}}\) with no observable hybridization gap [Fig. 3(a)]. Upon approaching resonance [Fig. 3(b)] and at resonance [Fig. 3(c)], the spectral weight of Ce 4\(f\)-derived states is strongly enhanced. Distinct flat features emerge near \(E_{\rm{F}}\) and at binding energy of \(\sim\) 270 meV, which are attributed to the Ce 4\(f^1_{5/2}\) and 4\(f^1_{7/2}\) states, respectively. The position of the 4\(f^1_{5/2}\) feature \(\sim\)​24 meV below \(E_{\rm{F}}\), differs from that in typical Ce-based heavy-fermion systems such as CeCoIn\(_5\), where the analogous feature lies \(\sim\)​2 meV above \(E_{\rm{F}}\) [36]. It is instead reminiscent of CeSb, where the Ce 4\(f\) feature appears \(\sim\)​50 meV below \(E_{\rm{F}}\), and has been interpreted as a mirrored crystal electric field (CEF) excitation [37]. One possible interpretation of CeSb\(_2\), analogous to CeSb, is illustrated in Fig. 3(d): the primary Ce 4\(f\) level (CEF1) lies at \(E_{\rm{F}}\), accompanied by a CEF excitation at \(\sim\)​24 meV above \(E_{\rm{F}}\) (CEF2) and its mirrored counterpart below \(E_{\rm{F}}\). (Independent confirmation of the CEF level from INS and specific heat in CeSb\(_2\) is currently lacking.) In the ARPES spectra, no hybridization gap is resolved at 24 meV for any of the dispersive bands [Figs. 3(a)-3(c)], indicating that hybridization with the CEF2 state is relatively weak. The resonant enhancement is most pronounced for the hole-like \(\alpha\) and \(\eta\) bands [Figs. 3(b)-3(c)], indicating band-selective Kondo hybridization/coherence and suggesting that Kondo coupling at \(E_{\rm{F}}\) with the CEF1 state is finite and occurs on the Ce-ladder-aligned \(C_2\)-distributed Fermi pockets.

The 4\(f^1_{5/2}\) peak intensity grows monotonically upon cooling [Figs. 3(e) and 3(f)], reflecting the buildup of Kondo coherence. The flat band feature persists up to 160 K [see also Fig. S5], indicating that heavy-quasiparticle formation develops well above the coherence temperature (\(T_{K}\sim 100~K\)), as commonly seen in Ce-based heavy-fermion systems [36], [38]–[41]. The 4\(f^1_{5/2}\) weight follows a typical Kondo coherence evolution above \(T_0\) \(\sim\)​30 K but saturates below \(T_0\) [Fig. 3(g)]. The sizable residual moment of \(\sim\)​1.4 \(\mu_B\)/Ce at 2 K [25], approaching the free Ce\(^{3+}\) Hund’s-rule value (2.54 \(\mu_B\)) and exceeding that of most Ce-based heavy fermions [42]–[45], indicates incomplete Kondo screening and the persistence of substantial 4\(f\) local-moment character on the Ce sites at low temperature. This relocalization-like behavior, similar to that reported in several other Ce-based heavy fermions [46]–[50], allows partial Kondo coherence to coexist with magnetic order.

The band-selective Kondo coupling has direct consequences for the momentum-space structure of the effective magnetic susceptibility. The resonance-enhanced Fermi surface map highlights the momentum-space distribution of Ce 4\(f\) spectral weight [Fig. 4(a)]. Besides a generally enhanced intensity in the first Brillouin zone with no obvious correspondence with Fermi surfaces, the \(\alpha\) and \(\eta\) pockets exhibit clear band-specific enhancement as also observed in Figs. 3(b)-3(c). Since these pockets are intrinsically \(C_2\)-distributed, appearing along the Ce-ladder direction (\(b\)-axis) and absent along \(\Gamma\) \(S_2\) [Figs. 1(c)-(f), Fig. S4], the enhanced Kondo coupling is geometrically confined to the ladder-aligned subset of the Fermi surface. The enhanced Kondo-derived spectral weight at the Fermi wave vectors (\(k_{\rm{F}}\)) of the \(\alpha\) and \(\eta\) pockets [Figs. 4(a)-4(b)] naturally gives rise to a larger effective exchange constant between the itinerant electrons and the residual Ce 4\(f\) local moments at these momenta. This band-selective anisotropy provides a direct electronic basis for highly anisotropic magnetic exchange parameters, as observed by INS [21]. Specifically, the intra-ladder exchange \(J_{\rm{\parallel}}\) dominates over the inter-ladder coupling \(J_{\rm{\perp}}\) as illustrated in Fig. 4(c), despite the quasi-2D crystal structure.

One plausible manifestation of the band-selective Kondo coupling could be Ruderman-Kittel-Kasuya-Yosida (RKKY)-type magnetic exchange \(J(r)\) along the Ce-ladder direction \(b\). Through a simplified simulation in Supplementary Materials Sec. IX [35], [51]–[53], the \(\alpha\) and \(\eta\) pockets favor AFM and FM nearest-neighbor exchange channels, respectively, suggesting competing magnetic interactions and the origin of q1D FM interactions. More intriguingly, within the experimental uncertainty on \(k_\alpha\), the contributions from \(\alpha\) and \(\eta\) pockets can also produce a significant AFM channel at the third-nearest neighbor, consistent with the experimentally observed commensurate AFM order with propagation vectors (-1, \(\pm\) \(\frac{1}{6}\), 0) [21], [25]. These coexisting FM and AFM tendencies provide an electronic mechanism for both the dominant FM excitations and the complex magnetic phase diagram of CeSb\(_2\) that cannot be accounted for by a single FM exchange alone [21].

The \(f\)-electron spectral weight saturates below \(T_0\) \(\sim\)​30 K, indicating incomplete Kondo screening, leaving residual Ce 4\(f\) moments that remain available for magnetic ordering. The small size of the \(\alpha\) and \(\eta\) Fermi pockets suggests a low density of itinerant carriers, which may partially contribute to the incomplete screening of the Ce 4\(f\) moments. In addition, CEF effects further reduce the \(f\)-electron spectral weight near \(E_{\rm F}\). The combined effect may account for the relatively small effective mass observed in CeSb\(_2\) [29], [54]. Notably, the saturation of \(f\)-electron spectral weight occurs above the highest magnetic transition temperature \(T_1\) (\(\sim\) 15 K) [Fig. 3(e)], suggesting the presence of short-range magnetic correlations that persist into the paramagnetic regime. This is consistent with neutron scattering results showing well-defined q1D paramagons [21]. The coexistence of band-selective Kondo coherence and persistent spin fluctuations in the paramagnetic regime may be relevant to the emergence of pressure-induced superconductivity [9], [55]–[57].

In summary, we have identified band-selective Kondo coupling as a microscopic electronic mechanism underlying the q1D FM excitations in CeSb\(_2\). The selective hybridization between Ce 4\(f\) local moments and the \(C_2\)-distributed Fermi pockets aligned with the Ce ladder gives rise to anisotropic Kondo-mediated exchange interactions. Unlike conventional RKKY scenarios in which all conduction-electron pockets contribute uniformly to the exchange, our results reveal a band-selective mechanism in which only the ladder-aligned, Kondo-coupled pockets dominate the magnetic interactions. More broadly, our results suggest that band-selective Kondo coupling can effectively reduce the dimensionality of magnetic interactions independently of the underlying crystal lattice, providing a general electronic route to emergent q1D magnetism in correlated \(f\)-electron systems.

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