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Intrinsic Spin Filter Effect in a d-wave altermagnet KV 2 Se 2 O with Open Fermi Surface Bin Liu, 1 Pei-Hao Fu, 1, 2, # Yu-Xuan Sun, 1 Xiao-Lin Zhang, 3 Si-Cong Zhu, 4 Xiang-Long Yu, 1, * Hua
Wu, 5, † Yuan-Zhi Shao 1, 6, ‡ 1 2 3 School of Science, Sun Yat-sen University, Shenzhen 518107, China Department of Physics and Astronomy, University of Florence, I-50019 Sesto Fiorentino, Italy Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics
and Technology, Wuhan University, Wuhan 430072, China 4 The State Key Laboratory for Refractories and Metallurgy, Wuhan University of Science and
Technology, Wuhan 430081, China 5 Laboratory for Computational Physical Sciences (MOE), State Key Laboratory of Surface Physics,
and Department of Physics, Fudan University, Shanghai 200433, China 6 Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices, Sun Yat-sen
University, Guangzhou 510275, China # * Corresponding author: [email protected] Corresponding author: [email protected]
† ‡ Corresponding author: [email protected] Corresponding author: [email protected] ABSTRACT Altermagnets offer a unique pathway to functional spintronics by combining vanishing
magnetization with large spin splitting. Here, we demonstrate that the canonical d-wave
altermagnet KV 2 Se 2 O can deliver giant tunneling magnetoresistance through orientation-
dependent spin filtering. By analyzing the crystallographic spin segregation, we show that
transport along specific crystallographic axes is nearly fully spin-polarized within the
symmetry-protected ballistic channels. We implement this mechanism in a lattice-matched
KV 2 Se 2 O/Bi 2 O 2 Se/KV 2 Se 2 O magnetic tunnel junction, which achieves a robust half-metallic
transport regime. The symmetry-protected spectral gap in the parallel/anti-parallel
configuration ensures a high tunneling magnetoresistance ratio, resulting in substantial
tunneling magnetoresistance, robust thermally driven spin filtering, and spin Seebeck effect at
room temperature. These findings provide a path of altermagnetic heterostructures as a high-
performance platform for scalable, field-free, and thermally stable spin logic.
1 / 24
Spin-polarized charge currents constitute the fundamental information carrier in spintronic
technologies, underpinning key functionalities such as nonvolatile memory, logic operations,
and spin-torque-driven switching [1] . Achieving robust spin polarization without sacrificing
device scalability and stability remains a central challenge. Ferromagnets (FM) naturally
generate longitudinal spin-polarized currents but suffer from stray magnetic fields that limit
device density and introduce cross-talk [2-5] . Antiferromagnets (AFM) eliminate stray fields and
exhibit ultrafast spin dynamics, yet their compensated spin symmetry typically enforces spin-
degenerate electronic bands, resulting in negligible longitudinal spin polarization and severely
restricting their applicability in spin-charge conversion and transport [6-9] . Altermagnetism (AM)
has recently emerged as a distinct magnetic order that bridges this long-standing dichotomy by
unifying key attributes of ferromagnets and antiferromagnets [10-12] . Altermagnets host large,
exchange-driven spin splitting comparable to ferromagnets and unconventional spin Hall
responses enabled by magnetic crystal symmetries, while maintaining a strictly vanishing net
magnetization enforced by combined crystal rotation and time-reversal symmetries [13-17] .
Owing to the absence of net magnetization, AM avoids the stray fields that challenge FM-based
devices. At the same time, the presence of spin splitting enables spin-dependent transport, in
contrast to AFM-based systems in which the spin information is largely hidden [18-23] .
However, the symmetry protection that stabilizes altermagnetism also imposes a fundamental
constraint: spin textures related by combined crystalline and time-reversal symmetries, exhibit
an orientation-dependent compensation that leads to an exact cancellation of spin polarization
when integrated over the Brillouin-zone, thereby prohibiting the emergence of sizable
macroscopic spin-polarized charge currents in equilibrium. In this sense, the defining advantage
of altermagnetism simultaneously constitutes its central obstacle for spintronic functionality [24-
27] . From a symmetry perspective, both strain- and electric-field-based approaches rely on
explicit symmetry breaking to enable spin polarization in altermagnets. Uniaxial strain induces
fully spin-polarized transport by breaking mirror and rotational symmetries, analogous to
electric-field-based schemes that rely on strong gate fields to lift symmetry constraints.
Crucially, both mechanisms yield momentum-dependent and valley-confined spin responses [28,
2 / 24 29] . Such strong external perturbations raise concerns regarding robustness, scalability, and
experimental feasibility. In terms of physical mechanisms, these strategies rely on explicit
symmetry breaking to lift the symmetry-enforced momentum-space cancellation of spin
polarization. The resulting spin-polarized transport can be interpreted as arising from an
effective Zeeman-like field that induces a finite net magnetization [30-34] . While this route enables
longitudinal spin polarization, it effectively drives the system back to a
ferromagnetic/ferrimagnetic-like regime, undermining the symmetry-protected nature of
altermagnetism and potentially reintroducing familiar limitations such as stray-field coupling
and reduced device robustness. These considerations underscore a critical open challenge in
altermagnetic spintronics: achieving intrinsic longitudinal spin-polarized charge transport
without resorting to external symmetry-breaking fields or net magnetization.
In this work, we propose an intrinsic spin-filter effect in AM driven by open Fermi-surface
geometry, enabling highly spin-polarized charge transport without net magnetization. Focusing
on KV 2 Se 2 O as a representative material platform, we first establish the microscopic
mechanism underlying the intrinsic spin-filter effect using a simple low-energy effective model.
We show that the closed-to-open Fermi surface transition selectively suppresses one spin
channel in a direction-selective manner, resulting in ideal spin polarization in longitudinal
transport. We then provide a proof of concept by investigating KV 2 Se 2 O—a material exhibiting
strong crystalline and magnetic anisotropy that enables pronounced directional modulation of
spin-dependent charge transport. Owing to its quasi-two-dimensional lattice, strong in-plane
anisotropy, and intrinsic magnetic order, KV 2 Se 2 O supports symmetry-constrained, direction-
dependent spin textures, enabling field-free spin-polarized transport within a realistic materials
platform [14, 35] . By combining first-principles electronic-structure calculations with device-level
transport modeling, we elucidate the symmetry origin of the anisotropic spin splitting, track the
evolution of momentum-resolved spin textures under different transport geometries, and
identify specific crystallographic directions along which nearly approaching fully longitudinal
spin polarization emerges despite the absence of net magnetization. Guided by the Density
Functional Theory (DFT) results, we further propose lattice-matched tunneling heterostructures
based on KV 2 Se 2 O. The sharp contrast between the conducting parallel-configuration (PC) state
3 / 24 and the insulating antiparallel-configuration (APC) state yields a giant tunneling
magnetoresistance (TMR) ratio of approximately 2.24 × 10 3 % in the absence of any external
magnetic field. Moreover, we predict a colossal thermal tunneling magnetoresistance exceeding
750% at low temperatures, which remains substantial even at room temperature. Our findings
demonstrate that altermagnets with open Fermi surfaces enable a distinct class of spintronic
functionalities, including nearly fully spin filters and giant tunneling magnetoresistance,
without requiring magnetic fields, spin--orbit coupling, or ferromagnetic components.
2.1 Low-Energy Effective Model for Fermi Surface Controlled Spin Transport In this section, we use a low-energy effective model for a two-dimensional d-wave altermagnet
to elucidate the mechanism of the geometry of the Fermi surface enabling theoretically perfect
spin-polarized charge transport. The purpose of this effective model is to capture, in a minimal
but analytically transparent manner, how the closed-to-open Fermi-surface transition in
altermagnets controls (i) the density of states (DOS) and (ii) the direction-dependent spin-
resolved carrier velocity, which together dictate the spin polarization of the longitudinal
conductance.
The effective Hamiltonian for two-dimensional 𝑑 𝑥 2 −𝑦 2 -wave altermagnet is [10, 14]
where 𝑎 is the lattice constant and 𝐸 𝐹 denotes the Fermi level, 𝜎 = +/- for spin-up/down
states. The d-wave altermagnetic anisotropic spin splitting is encoded in the spin- and direction-
nearest-neighbor hopping amplitude and 𝑡 𝐴𝑀 the anisotropic spin-dependent hopping that
captures the d-wave altermagnetic spin splitting. This anisotropic spin splitting is directly
manifested in the spin-resolved equal-energy contours and dispersions of Eq. (1), shown in Fig.
with spin- and direction-dependent curvature, leading to two spin-dependent closed elliptical
Fermi surfaces whose principal axes are mutually orthogonal [see the insert of Fig. 1(a)]. For
4 / 24 𝑡 𝐴𝑀 /𝑡 0 > 1 [Fig. 1(b)], the dispersion consists of two parabolic bands with opposite opening
directions (along one momentum axis) and spin-dependent curvature. This yields open Fermi
surfaces: within a finite momentum window, only a single spin species is present at the Fermi
level as exhibited in the insert of Fig. 1(b). Moreover, the band structure hosts a saddle point at
the spin-degenerate momentum (𝑘 𝑥 , 𝑘 𝑦 ) = (0, 0) [36] , where the curvature is positive along one
direction and negative along the other. Notably, at the critical point 𝑡 𝐴𝑀 /𝑡 0 = 1 [Fig. S1], a
flat band emerges: along one direction, only one spin sub-band remains dispersive, while the
In the altermagnet with the open Fermi surface, the occurrence of the flat band and saddle
point gives rise to a van Hove singularity in the DOS near the corresponding energy [36] . A
straightforward calculation yields the spin-resolved DOS [37]
where 𝐸 cut = 8𝑡 0 is a bandwidth cutoff, Θ(𝑥) is the Heaviside step function. Importantly,
summarize the energy dependence of 𝐷(𝐸) for representative 𝑡 AM /𝑡 0 . For 𝑡 AM /𝑡 0 < 1 ,
𝐷(𝐸) is essentially the constant DOS of a two-dimensional parabolic band [37] , with
modification induced by the altermagnetism via 𝑡 AM , but no singular behavior. By contrast,
when 𝑡 AM /𝑡 0 ≥ 1, the flat band and saddle point generate a logarithmic van Hove singularity
near 𝐸 ≃ 𝐸 𝐹 . Notably, in the vicinity of the critical point 𝑡 AM /𝑡 0 = 1, 𝐷(𝐸) → ∞ within the
present effective description, implying a strong enhancement of the transport response. This
sharp contrast between the DOS of closed and open Fermi surfaces is a direct signature of the
distinct Fermi-surface geometries.
The impact of Fermi-surface geometry is also encoded in the carrier velocity. For Eq. (1), the
where 𝑣 𝛼𝜎 denotes the quantum-mechanical expectation value in the spin-𝜎 eigenstate. The dwave altermagnetic symmetry implies 𝑣 𝑥𝜎 = 𝑣 𝑦−𝜎 , inherited from Eq. (1). As illustrated in Fig.
1(c), for 𝑡 AM < 𝑡 0 (closed Fermi surfaces), both spin channels have finite velocities along both
directions, so spin-up and spin-down electrons contribute comparably to transport. In contrast,
for 𝑡 AM ≳ 𝑡 0 [Figs. 1(d)], one spin channel becomes nearly blocked along a given direction while the other channel remains highly conductive. In particular, 𝑣 𝑥↑ = 𝑣 𝑦↓ ∝ 𝑡 0 + 𝑡 AM ,whereas
providing the kinematic basis for perfectly spin-polarized currents.
The effect of the Fermi-surface geometry on the DOS [Eq. (2)] and the velocity [Eq. (3)] is
6 / 24 then manifested in the longitudinal conductance. To demonstrate how the Fermi-surface
geometry controls transport, we evaluate the longitudinal conductance within semiclassical
Boltzmann theory using the relaxation-time approximation. In the linear-response regime,
𝐼 𝛼 = 𝐺 𝛼𝛽 𝑉 𝛽 , and we focus on the longitudinal component 𝛼 = 𝛽 . At zero temperature, the
conductance can be written as
where 𝜏 is the momentum relaxation time. For the 𝑥direction, substituting Eq. (3) into Eq. (4)
yields the scaling form
which explicitly contains 𝑡 AM /𝑡 0 through the spin- and direction-dependent velocity. For ∣
AM /𝑡 0 ∣< 1 , both 𝐺 𝑥𝑥 and 𝐺 𝑥𝑥 are comparable, and the net conductance contains
↑ contributions from both spins. For ∣ 𝑡
AM /𝑡 0 ∣≳ 1 , however, 𝐺 𝑥𝑥 is strongly enhanced
↓ (particularly near the van Hove singularity at 𝐸 ≃ 𝐸
𝐹 ), whereas 𝐺 𝑥𝑥 becomes negligible,
yielding a nearly perfect spin polarization
Repeating the same analysis for transport along 𝑦gives the opposite polarization, 𝑃 𝑦𝑦 ≃ −1 =
as illustrated in Fig. 1.
We emphasize that Eq. (1) is a minimal effective description. In realistic materials such as
RuO 2[38-41] , La 2 O 3 Mn 2 Se 2 , and Ba 2 CaOsO 6[42] , 𝑑 -wave altermagnetism often involves
additional orbital and sublattice degrees of freedom (e.g., two-sublattice tetragonal [38-40] or
Lieb-lattice models [42] ). Here we isolate the universal role of Fermi-surface geometry, for which
Eq. (1) already captures the key distinction between closed and open Fermi surfaces. We also
note that the divergence in Eq. (2) is regularized in real systems by disorder and finite
quasiparticle lifetimes [43] . A standard phenomenological broadening amounts to the
𝜏 ′ due to impurity scattering [44-46] . Finally, the Boltzmann treatment in Eq. (4) assumes a 7 / 24 constant (isotropic) relaxation time 𝜏, weak disorder, zero temperature, and a small bias 𝑉 𝛽 .
Despite these approximations, the analysis demonstrates the central point: open Fermi surfaces
naturally suppress one spin channel in a direction-dependent manner, enabling giant and nearly
perfectly spin-polarized longitudinal conductance when the chemical potential is tuned close to
the flat-band or saddle-point energies. In the next section, we substantiate this mechanism using
material-based calculations.
2.2 Nearly Perfectly Directional Spin Polarization in KV 2 Se 2 O To validate the mechanism proposed in Eq. (1), in this section and the following we perform
DFT calculations for the altermagnetic candidate material KV 2 Se 2 O. The calculations were
performed using the nonequilibrium Green’s function formalism combined with density
functional theory (DFT+NEGF), as implemented in QuantumATK (version T-2022.03,
Synopsys QuantumATK) [47-49] , A Hubbard 𝑈 value of 1 eV was applied to the V 3𝑑 orbitals of
one pair of V atoms with opposite spin orientations [50-52] , thereby explicitly treating spin
polarization. The electrically-driven spin transport performance can be quantitatively analyzed from the Landauer-Buttiker equation [53] :
where 𝑓 𝐿(𝑅) (𝐸, 𝜇)is the equilibrium Fermi distribution of the left (right) electrode, and 𝜇 is
the electrochemical potential of the electron. Fig. 2(a) shows the crystal structure of KV 2 Se 2 O,
where the magnetic V atoms form a square lattice separated by Se and O ligands. The
corresponding spin-resolved Fermi surfaces are shown in Fig. 2(b). Two bands (labeled as
Bands 35 and 36) cross the Fermi energy and exhibit pronounced anisotropy: one spin sub band
is dispersive along one direction (e.g. spin-up is dispersive along Γ-X direction, spin-down is
dispersive along Γ-Y direction), it becomes flat band without dispersion along the orthogonal
direction. As a result, the Fermi surfaces are open, extending across the Brillouin zone rather
than forming closed contours. As what is expected in other altermagnetic candidate materials
such as MnTe, Nb 2 FeB 2 , and V 2 Se 2 O [54-56] . This geometry closely resembles the open-Fermi8 / 24
The origin of this open Fermi surface can be traced back to the orbital character and bonding
geometry of KV 2 Se 2 O. As a results, in Fig. 2(c), the band structure near the Fermi level consists
of two parabolic bands with opposite opening directions (along one momentum axis) and spin-
dependent curvature, as shown in Fig. 1(b) via the effective model [Eq. (1)]. Using a
dimensionless parametrization with 𝑡 0 = 1, the band evolution shown in Fig. 1 corresponds to
𝑡 am = 0.5, 1.0, 1.5 , representing the closed-Fermi-surface, critical, and open-Fermi-surface
9 / 24 regimes, respectively, and the DFT bands of KV 2 Se 2 O are consistent with the 𝑡 am ≳ 𝑡 0 . In
particular, one spin channel remains dispersive along one in-plane direction while becoming
nearly flat along the orthogonal direction. In particular, the presence of Se atoms strongly
suppresses the effective V-V hopping along one in-plane direction, whereas hopping along the
perpendicular direction remains sizable. This anisotropic hybridization leads to an almost
dispersionless band along one momentum axis, giving rise to a quasi-flat band and a saddle-
point-like structure near the Fermi energy. This band anisotropy is further reflected in the spin-
resolved DOS, shown in Fig. 2(d). While the total DOS remains spin compensated, consistent
with the absence of net magnetization, a pronounced enhancement of the DOS appears near E F ,
originating from the flat-band and saddle-point features. This behavior is qualitatively
consistent with the logarithmic van Hove singularity predicted by the effective model in Eq. (2)
for the open-Fermi-surface regime. Taken together, Figs. 2(a-d) demonstrate that KV 2 Se 2 O
realizes, at the material level, the key ingredients of the low-energy d-wave altermagnetic model:
direction-dependent hopping, open spin-resolved Fermi surfaces, and a strong DOS
enhancement near the Fermi level. These features provide the microscopic foundation for the
giant and nearly perfectly spin-polarized transport discussed in the following sections.
2.3 Tunneling Magnetoresistance in Lattice-Matched Heterostructures In the following, we investigate two fundamental spintronic phenomena: tunneling
magnetoresistance and spin-dependent thermoelectric transport, that are strongly enhanced by
the nearly perfect spin polarization realized in altermagnetic spintronic devices constructed
from KV 2 Se 2 O -based heterostructures.
To transform the crystal spin filtering of KV 2 Se 2 O into a functional device, we construct a
fully epitaxial magnetic tunnel junction (MTJ) in which a Bi 2 O 2 Se barrier is sandwiched
between two KV 2 Se 2 O electrodes in Fig. S2. Owing to the small lattice mismatch between
Bi 2 O 2 Se and KV 2 Se 2 O, Bi 2 O 2 Se is chosen as the tunnel barrier, which preserves the intrinsic d-
wave spin texture at the interface without introducing significant momentum scattering. Here,
the parallel (PC) and antiparallel (APC) configurations refer to the relative orientation of the 10 / 24 alternating magnetic sublattice spin textures projected along the transport direction, rather than
a net magnetization alignment. In both configurations, the net magnetization remains zero. The
detailed device geometry and the local spin-resolved electron density are shown in Fig. S3. Fig.
3 presents the spin-dependent transport characteristics by directly linking the microscopic
energy-resolved state, described by the projected local density of states (PLDOS) [Figs. 3(a)-
(d)], to the macroscopic momentum-resolved transmission [(Figs. 3(e)-(h)]. In the PC
configuration, the junction exhibits a half-metallic-like transport behavior. For the spin-down
channel, the PLDOS [Fig. 3(b)] reveals a continuous electronic-state bridge connecting the
source and drain across the Bi 2 O 2 Se barrier. This transportation channel connectivity gives rise
to a robust transmission peak near the Fermi energy, as indicated by the red curve, which
directly corresponds to a high-transparency window (bright region) in momentum space [Fig.
3(f)]. In contrast, the spin-up channel [Fig. 3(a)] is strictly blocked due to a symmetry-enforced
spectral gap along the transport direction, resulting in a vanishing transmission coefficient [T(E)]
and a dark momentum-resolved map [Fig. 3(e)]. As a consequence, the tunnel current becomes
highly spin polarized, with a spin polarization reaching approximately 92.5% in the absence of
any external field. This performance significantly surpasses that of the benchmark altermagnet
RuO 2[22] , and rivals the efficiency of conventional ferromagnetic spin injectors [57-59] . In the APC
configuration, the crystal spin filter of the drain electrode is opposite to the spin-polarized
current injected from the source. As shown in Figs. 3(c) and 3(d), although transport states may
exist in one electrode, they lack matching counterparts in the other electrode, leading to rapid
decay of the wave function at the interface. As a result, the total transmission for both spin
channels is strongly suppressed, with the red curves remaining close to zero, and the
momentum-resolved spectra [Figs. 3(g) and (h)] show no propagating channels. The sharp
contrast between the conducting PC state and the insulating APC state gives rise to a giant
tunneling magnetoresistance (TMR) ratio of approximately 2.24 × 10 3 % in the absence of any
external magnetic field, which is more than an order of magnitude larger than the previously
reported TMR of about 93% in RuO 2 -based devices. [22] . These results establish the KV 2 Se 2 O
/Bi 2 O 2 Se heterostructure as a promising platform for field-free spintronic logic applications.
11 / 24
2.4 Giant Spin-Dependent Thermoelectricity Benefiting from the high-temperature altermagnetic nature of KV₂Se₂O, which stabilizes spin
order under thermal gradients, we can explore thermally generated spin currents in
KV 2 Se 2 O/Bi 2 O 2 Se/KV 2 Se 2 O heterostructures. Here, we explore the potential of
KV 2 Se 2 O/Bi 2 O 2 Se heterostructures for spin caloritronic by investigating thermally generated
spin currents. The thermal transport properties were evaluated under two distinct regimes: (i)
fixed temperature difference (ΔT) with varying base left electrode temperature (T L ), and (ii)
fixed base left electrode temperature near room temperature with varying ΔT. Fig. 4(a)
illustrates the thermally induced currents under a fixed ΔT. In the parallel configuration (PC),
the device operates as a highly efficient thermal spin generator. The spin-down current (red)
rises significantly with temperature, driven by the Seebeck effect in the metallic minority
12 / 24 channel. Conversely, the spin-up current (blue) remains suppressed which blocks thermally
excited carriers just as effectively as electrically injected ones. This asymmetry results in a
colossal thermal tunneling magnetoresistance, exceeding 750% at low temperatures and
remaining substantial even at 300 K [Fig. 4(b)]. The robustness of this effect for ambient
applications is demonstrated in Figs. 4(c) and 4(d), where the T L is fixed near room temperature
(T L in [290, 300, 310] K). A nearly linear dependence of the spin-polarized current on ΔT is
observed in the PC state, confirming the diffusive nature of the thermal transport. Most notably,
the spin filtration efficiency remains remarkably high (>80%) across the entire temperature
range tested [Fig. 4(d)], which indicates that the crystallographic spin segregation mechanism
is energetically robust against thermal broadening, positioning KV 2 Se 2 O as a promising
3 DISCUSSION
Our investigation establishes a unified framework for realizing high-performance spintronics
in compensated magnetic systems, governed by the principle of crystallographic spin
segregation. By bridging the symmetry-resolved toy model (Fig. 1) with specific material
realizations (Figs. 2-4), we demonstrate that robust spin filtering can be rigorously enforced by
magnetic crystal symmetries that allow a d-wave spin splitting, providing a deterministic route
to generate pure spin currents without net magnetization.
Our approach is conceptually based on the distinction between direction-selective spin
transport and symmetry-compensated spin splitting. While previous studies have classified
altermagnets primarily based on global Fermi surface topology, our work highlights that
functional spin selectivity is fundamentally dictated by local momentum-space connectivity. In
KV 2 Se 2 O, the characteristic d-wave symmetry imposes a strict orthogonal segregation of spin
channels along the principal axes. Consequently, ballistic transport along the Γ-X (or Γ-Y)
direction is populated exclusively by a single spin species. This crystallographic locking
effectively utilizes the anisotropic band structure to "switch on" high spin polarization simply
by aligning the device geometry with the symmetry-broken crystal axes. The translation of this
intrinsic quantization into macroscopic device performance is validated by the giant tunneling
magnetoresistance observed in the all-epitaxial KV 2 Se 2 O/Bi 2 O 2 Se heterostructure. The
calculated TMR ratio of 2.24 × 10 3 % represents an order-of-magnitude improvement over
previously reported values in RuO 2 -based devices.
Crucially, our results extend the utility of altermagnets into the domain of spin caloritronic.
The persistence of a high spin filtration efficiency (SFE> 80%) and substantial thermal TMR
at room temperature indicates that the energy scale of the symmetry-protected spin splitting
significantly exceeds the thermal broadening. This resilience is a direct consequence of the
large spectral gaps identified in the PLDOS analysis, which effectively block thermally excited
carriers in the forbidden channel. The ability to harvest pure spin currents from waste heat
without external magnetic fields positions KV 2 Se 2 O as a versatile platform for energy-efficient,
zero-bias spin logic.
14 / 24 We emphasize that the orientation-dependent filtering mechanism identified here is generic
to the d-wave symmetry class. By exploiting the synergy between altermagnetic ordering and
epitaxial engineering, our work paves the way for a new generation of scalable, field-free, and
room-temperature spintronic devices that combine the ultrafast dynamics of antiferromagnets
with the high readout fidelity of ferromagnets.
DATA AVAILABILITY
The data supporting the findings of this study are available from the corresponding authors
upon reasonable request.
ACKNOWLEDGEMENTS
This work is supported by the Shenzhen Science and Technology Program (Grant No.
JCYJ20250604174400001), the Guangdong Basic and Applied Basic Research Foundation
(Grant No. 2023A1515011852), and the Guangdong Science and Technology Innovation
Strategy Special Fund (Grant No. pdjh2025bk012). P.-H. F. acknowledges the financial support
from the Fondazione Cariplo under the grant 2023-2594. The authors would like to thank
National Supercomputer Center in Guangzhou for providing high performance computational
resources.
REFERENCE [1] [2] [3] [4] [5] Song Cheng, Bai Hua, Zhou Zhiyuan, et al. Altermagnets as a new class of functional materials [J]. Nature Reviews Materials, 2025, 10(6): 473-85. 10.1038/s41578-025- 00779-1 Wolf S. A., Awschalom D. D., Buhrman R. A., et al. Spintronics: A Spin-Based Electronics Vision for the Future [J]. Science, 2001, 294(5546): 1488-95. 10.1126/science.1065389 Žutić Igor, Fabian Jaroslav, Das Sarma S. Spintronics: Fundamentals and applications [J]. Reviews of Modern Physics, 2004, 76(2): 323-410. 10.1103/RevModPhys.76.323 Chappert Claude, Fert Albert, Van Dau Frédéric Nguyen. The emergence of spin electronics in data storage [J]. Nature Materials, 2007, 6(11): 813-23. 10.1038/nmat2024 Bader S.D., Parkin S.S.P. Spintronics [J]. 2010, 1(Volume 1, 2010): 71-88.
15 / 24 [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] https://doi.org/10.1146/annurev-conmatphys-070909-104123 Jungwirth T., Marti X., Wadley P., et al. Antiferromagnetic spintronics [J]. Nature Nanotechnology, 2016, 11(3): 231-41. 10.1038/nnano.2016.18 Baltz V., Manchon A., Tsoi M., et al. Antiferromagnetic spintronics [J]. Reviews of Modern Physics, 2018, 90(1): 015005. 10.1103/RevModPhys.90.015005 Šmejkal Libor, Mokrousov Yuriy, Yan Binghai, et al. Topological antiferromagnetic spintronics [J]. Nature Physics, 2018, 14(3): 242-51. 10.1038/s41567-018-0064-5 Chen Hongyu, Liu Li, Zhou Xiaorong, et al. Emerging Antiferromagnets for Spintronics [J]. Advanced Materials, 2024, 36(14): 2310379. https://doi.org/10.1002/adma.202310379 Šmejkal Libor, Sinova Jairo, Jungwirth Tomas. Emerging Research Landscape of Altermagnetism [J]. Physical Review X, 2022, 12(4): 040501. 10.1103/PhysRevX.12.040501 Fukaya Yuri, Lu Bo, Yada Keiji, et al. Superconducting phenomena in systems with unconventional magnets [J]. Journal of Physics: Condensed Matter, 2025, 37(31): 313003. 10.1088/1361-648X/adf1cf Ashcroft Neil W., Mermin N. David. Solid State Physics [M]. New York, NY: Holt, Rinehart and Winston, 1976. Šmejkal Libor, Hellenes Anna Birk, González-Hernández Rafael, et al. Giant and Tunneling Magnetoresistance in Unconventional Collinear Antiferromagnets with Nonrelativistic Spin-Momentum Coupling [J]. Physical Review X, 2022, 12(1): 011028. 10.1103/PhysRevX.12.011028 Šmejkal Libor, Sinova Jairo, Jungwirth Tomas. Beyond Conventional Ferromagnetism and Antiferromagnetism: A Phase with Nonrelativistic Spin and Crystal Rotation Symmetry [J]. Physical Review X, 2022, 12(3): 031042. 10.1103/PhysRevX.12.031042 Chen Xiaobing, Liu Yuntian, Liu Pengfei, et al. Unconventional magnons in collinear magnets dictated by spin space groups [J]. Nature, 2025, 640(8058): 349-54. 10.1038/s41586-025-08715-7 Bai H., Han L., Feng X. Y, et al. Observation of Spin Splitting Torque in a Collinear Antiferromagnet RuO2 [J]. Physical Review Letters, 2022, 128(19): 197202. 10.1103/PhysRevLett.128.197202 Karube Shutaro, Tanaka Takahiro, Sugawara Daichi, et al. Observation of Spin-Splitter Torque in Collinear Antiferromagnetic RuO2 [J]. Physical Review Letters, 2022, 129(13): 137201. 10.1103/PhysRevLett.129.137201 Lai Junwen, Yu Tianye, Liu Peitao, et al. d-Wave Flat Fermi Surface in Altermagnets Enables Maximum Charge-to-Spin Conversion [J]. Physical Review Letters, 2025, 135(25): 256702. 10.1103/bf1n-sxdl Zhu Haiyuan, Li Jiayu, Chen Xiaobing, et al. Magnetic geometry induced quantum geometry and nonlinear transports [J]. Nature Communications, 2025, 16(1): 4882. 10.1038/s41467-025-60128-2 Peng Rui, Fang Shibo, Ho Pin, et al. Ferroelastic altermagnetism [J]. npj Quantum Materials, 2025. 10.1038/s41535-025-00835-7 16 / 24 [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] Yang Zongmeng, Yang Xingyue, Wang Jianhua, et al. Unconventional thickness scaling of coherent tunnel magnetoresistance in altermagnets [J]. Physical Review B, 2025, 112(20): 205202. 10.1103/2thy-fzzj Shao Ding-Fu, Jiang Yuan-Yuan, Ding Jun, et al. N'eel Spin Currents in Antiferromagnets [J]. Physical Review Letters, 2023, 130(21): 216702. 10.1103/PhysRevLett.130.216702 Shao Ding-Fu, Tsymbal Evgeny Y. Antiferromagnetic tunnel junctions for spintronics [J]. npj Spintronics, 2024, 2(1): 13. 10.1038/s44306-024-00014-7 Fu Pei-Hao, Lv Qianqian, Xu Yong, et al. All-electrically controlled spintronics in altermagnetic heterostructures [J]. npj Quantum Materials, 2025, 10(1): 111. 10.1038/s41535-025-00827-7 Gonzalez Betancourt R. D, Zubáč J., Gonzalez-Hernandez R., et al. Spontaneous Anomalous Hall Effect Arising from an Unconventional Compensated Magnetic Phase in a Semiconductor [J]. Physical Review Letters, 2023, 130(3): 036702. 10.1103/PhysRevLett.130.036702 Chi Boyuan, Jiang Leina, Zhu Yu, et al. Crystal-facet-oriented altermagnets for detecting ferromagnetic and antiferromagnetic states by giant tunneling magnetoresistance [J]. Physical Review Applied, 2024, 21(3): 034038. 10.1103/PhysRevApplied.21.034038 Yan Zhi, Yang Ruixia, Fang Cheng, et al. Giant electrode effect on tunneling magnetoresistance and electroresistance in van der Waals intrinsic multiferroic tunnel junctions using VS2 [J]. Physical Review B, 2024, 109(20): 205409. 10.1103/PhysRevB.109.205409 Wu Yanzhao, Deng Li, Yin Xiang, et al. Valley-Related Multipiezo Effect and Noncollinear Spin Current in an Altermagnet Fe2Se2O Monolayer [J]. Nano Letters, 2024, 24(34): 10534-9. 10.1021/acs.nanolett.4c02554 Zhang Run-Wu, Cui Chaoxi, Li Runze, et al. Predictable Gate-Field Control of Spin in Altermagnets with Spin-Layer Coupling [J]. Physical Review Letters, 2024, 133(5): 056401. 10.1103/PhysRevLett.133.056401 Fernandes Rafael M., De Carvalho Vanuildo S., Birol Turan, et al. Topological transition from nodal to nodeless Zeeman splitting in altermagnets [J]. Physical Review B, 2024, 109(2): 024404. 10.1103/PhysRevB.109.024404 Zhai Yahui, Yu Longju, Lv Jian, et al. Zeeman-type spin splittings in strained $d$-wave altermagnets [J]. Physical Review B, 2025, 112(17): 174411. 10.1103/7q2d-jcqg Sumita Shuntaro, Naka Makoto, Seo Hitoshi. Phase-modulated superconductivity via altermagnetism [J]. Physical Review B, 2025, 112(14): 144510. 10.1103/3k12-2467 Fu Pei-Hao, Mondal Sayan, Liu Jun-Feng, et al. Light-induced Floquet spin-triplet Cooper pairs in unconventional magnets [J]. 2025. Fu Pei-Hao, Mondal Sayan, Liu Jun-Feng, et al. Floquet engineering spin triplet states in unconventional magnets [J]. Physical Review Letters, 2026. 10.1103/lkf9-jgv6 Yang Xin-Ren, Fang Shibo, Yang Zongmeng, et al. Altermagnetic Flatband-Driven Fermi Surface Geometry for Giant Tunneling Magnetoresistance, F, 2025 [C]. Ziletti A., Huang S. M., Coker D. F., et al. Van Hove singularity and ferromagnetic 17 / 24 [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] instability in phosphorene [J]. Physical Review B, 2015, 92(8): 085423. 10.1103/PhysRevB.92.085423 Datta Supriyo. Electronic Transport in Mesoscopic Systems [M]. Cambridge: Cambridge University Press, 1995. Liu Jiayu, Zhan Jie, Li Tongrui, et al. Absence of Altermagnetic Spin Splitting Character in Rutile Oxide RuO2 [J]. Physical Review Letters, 2024, 133(17): 176401. 10.1103/PhysRevLett.133.176401 Feng Zexin, Zhou Xiaorong, Šmejkal Libor, et al. An anomalous Hall effect in altermagnetic ruthenium dioxide [J]. Nature Electronics, 2022, 5(11): 735-43. 10.1038/s41928-022-00866-z Tschirner Teresa, Keßler Philipp, Gonzalez Betancourt Ruben Dario, et al. Saturation of the anomalous Hall effect at high magnetic fields in altermagnetic RuO2 [J]. APL Materials, 2023, 11(10): 101103. 10.1063/5.0160335 Li Yu-Xin, Chen Yiyuan, Pan Liqing, et al. Exploration of Altermagnetism in RuO2 [J]. 2025. Jaeschke-Ubiergo Rodrigo, Bharadwaj Venkata-Krishna, Campos Warlley, et al. Atomic altermagnetism [J]. 2025. https://doi.org/10.48550/arXiv.2503.10797 Von N. W. Ashcroft Und N. D. Mermin; Holt Rinehart and Winston. Solid State Physics. New York 1976, [J]. Physik in unserer Zeit, 1978, 9(1): 33-. https://doi.org/10.1002/piuz.19780090109 Borda László, Fritz Lars, Andrei Natan, et al. Theory of inelastic scattering from quantum impurities [J]. Physical Review B, 2007, 75(23): 235112. 10.1103/PhysRevB.75.235112 Zaránd Gergely, Borda László, Von Delft Jan, et al. Theory of Inelastic Scattering from Magnetic Impurities [J]. Physical Review Letters, 2004, 93(10): 107204. 10.1103/PhysRevLett.93.107204 Quang Doan Nhat, Tung Nguyen Huyen, Tuoc Vu Ngoc, et al. Quantum and transport lifetimes due to roughness-induced scattering of a two-dimensional electron gas in wurtzite group-III-nitride heterostructures [J]. Physical Review B, 2006, 74(20): 205312. 10.1103/PhysRevB.74.205312 Smidstrup Søren, Markussen Troels, Vancraeyveld Pieter, et al. QuantumATK: an integrated platform of electronic and atomic-scale modelling tools [J]. Journal of Physics: Condensed Matter, 2020, 32(1): 015901. 10.1088/1361-648X/ab4007 Smidstrup Søren, Stradi Daniele, Wellendorff Jess, et al. First-principles Green'sfunction method for surface calculations: A pseudopotential localized basis set approach [J]. Physical Review B, 2017, 96(19): 195309. 10.1103/PhysRevB.96.195309 Brandbyge Mads, Mozos José-Luis, Ordejón Pablo, et al. Density-functional method for nonequilibrium electron transport [J]. Physical Review B, 2002, 65(16): 165401. 10.1103/PhysRevB.65.165401 Jiang Bei, Hu Mingzhe, Bai Jianli, et al. A metallic room-temperature d-wave altermagnet [J]. Nature Physics, 2025, 21(5): 754-9. 10.1038/s41567-025-02822-y Anisimov Vladimir I., Zaanen Jan, Andersen Ole K. Band theory and Mott insulators:
18 / 24 [52] [53] [54] [55] [56] [57] [58] [59] Hubbard U instead of Stoner I [J]. Physical Review B, 1991, 44(3): 943-54. 10.1103/PhysRevB.44.943 Dudarev S. L., Botton G. A., Savrasov S. Y., et al. Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study [J]. Physical Review B, 1998, 57(3): 1505-9. 10.1103/PhysRevB.57.1505 Büttiker M., Imry Y., Landauer R., et al. Generalized many-channel conductance formula with application to small rings [J]. Physical Review B, 1985, 31(10): 6207-15. 10.1103/PhysRevB.31.6207 Li Shuyi, Zhang Yu, Bahri Adrian, et al. Altermagnetism and strain induced altermagnetic transition in Cairo pentagonal monolayer [J]. npj Quantum Materials, 2025, 10(1): 83. 10.1038/s41535-025-00793-0 Ma Hai-Yang, Hu Mengli, Li Nana, et al. Multifunctional antiferromagnetic materials with giant piezomagnetism and noncollinear spin current [J]. Nature Communications, 2021, 12(1): 2846. 10.1038/s41467-021-23127-7 Zhu Yu-Peng, Chen Xiaobing, Liu Xiang-Rui, et al. Observation of plaid-like spin splitting in a noncoplanar antiferromagnet [J]. Nature, 2024, 626(7999): 523-8. 10.1038/s41586-024-07023-w Moodera Jagadeesh S., Mathon George. Spin polarized tunneling in ferromagnetic junctions [J]. Journal of Magnetism and Magnetic Materials, 1999, 200(1): 248-73. https://doi.org/10.1016/S0304-8853(99)00515-6 Manno M., Frakie R., Bolon B., et al. Spin-dependent intergranular transport in highly spin-polarized Co1−xFexS2 thin films [J]. Applied Physics Letters, 2009, 95(18): 182510. 10.1063/1.3262958 Yang Zhi, Qin Shen-Ao, Ruan Bao-Fu, et al. Van der Waals multiferroic tunnel junction with giant tunneling electroresistance and magnetoresistance toward bidirectional photoresponse and photoassisted memory [J]. Physical Review B, 2025, 112(11): 115312. 10.1103/9m1x-q7dn 19 / 24 Supplemental Material of “Intrinsic Spin Filter Effect in a d-wave altermagnet KV 2 Se 2 O
with Open Fermi Surface” Bin Liu, 1 Pei-Hao Fu, 1, 2, # Yu-Xuan Sun, 1 Xiao-Lin Zhang, 3 Si-Cong Zhu, 4 Xiang-Long Yu, 1, * Hua
Wu, 5, † Yuan-Zhi Shao 1, 6, ‡ 1 2 3 School of Science, Sun Yat-sen University, Shenzhen 518107, China Department of Physics and Astronomy, University of Florence, I-50019 Sesto Fiorentino, Italy Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics
and Technology, Wuhan University, Wuhan 430072, China 4 The State Key Laboratory for Refractories and Metallurgy, Wuhan University of Science and
Technology, Wuhan 430081, China 5 Laboratory for Computational Physical Sciences (MOE), State Key Laboratory of Surface Physics,
and Department of Physics, Fudan University, Shanghai 200433, China 6 Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices, Sun Yat-sen
University, Guangzhou 510275, China # * Corresponding author: [email protected] Corresponding author: [email protected]
† ‡ Corresponding author: [email protected] Corresponding author: [email protected] 20 / 24
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Fig. S2 displays the spin-resolved local density of states (LDOS) projected onto the scattering region of the magnetic tunnel junction (MTJ). The heterostructure is constructed using Bi 2 O 2 Se as the tunnel barrier, selected for its excellent lattice matching and chemical compatibility with the KV 2 Se 2 O electrodes, which minimizes interface scattering and preserves the intrinsic dwave spin texture. In the parallel configuration (PC), a striking dichotomy in wavefunction penetration is observed. The spin-down channel [Fig. S2(b)] exhibits continuous, delocalized states extending from the source electrode, through the Bi 2 O 2 Se barrier, and into the drain, confirming the high-transparency state predicted by the orientation-dependent spin filter model. In contrast, the spin-up channel [Fig. S2(a)] shows strong localization within the electrodes with negligible spectral weight inside the barrier region, indicative of the symmetry-forbidden transport gap along the stacking direction. The blocking mechanism is further visualized in the anti-parallel configuration (APC). Here, the mismatch between the spin-projected pass-bands of the two electrodes leads to a suppression of wavefunction overlap. As shown in
22 / 24 Supplementary Figs. S2(c, d), electronic states are confined to the injection side of the junction, decaying rapidly within the barrier without coupling to the collector. This spatial segregation of states in the APC limit, contrasted with the robust tunneling channel in the PC spin-down limit, visually corroborates the giant magnetoresistance effect arising from the crystallographic spin filtering.
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