Quantum Spin Singlet Ground State with Large Excitation Gap in the S = 3/2 quasi-1D van der Waals materials MoX3 (X = I, Br)

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Kai: We're looking at the paper: "Quantum Spin Singlet Ground State with Large Excitation Gap in the S = 3/2 quasi-1D van der Waals materials MoX3 (X = I, Br)".

Mira: Here's the gist of the paper. The gist MoX3 (X = I, Br) are rare van der Waals materials that exhibit signatures of both quantum spin chains with a spin singlet ground state and classical Néel order.

Kai: Experimental Signatures and Transition Bulk single crystals grown by chemical vapor transport exhibit classical antiferromagnetic ground states with a transition temperature of ∼ 40 K as revealed by susceptibility and specific heat measurements The magnetic susceptibility measurements of MoI3 and MoBr3 indicate low-temperature Néel order supported by weak interchain coupling with a transition temperature of ∼ 40 K The specific heat measurements also show features at ∼ 40 K consistent with the susceptibility measurements These features around 40 K will be revisited in the context of specific heat measurements The magnetic susceptibility and specific heat capacity measurements of the MoX3 samples exhibit features of a classical Néel ordered lowtemperature ground state stabilized by weak interchain coupling that transitions above TNN ∼ 40 K into a phase dominated by single-chain physics with a gapped quantum spin singlet ground state with a large singlet-triplet gap Δ The magnetic susceptibility and specific heat capacity measurements of the MoX3 samples exhibit features of a classical Néel ordered lowtemperature ground state stabilized by weak interchain coupling that transitions above TNN ∼ 40 K into a phase dominated by single-chain physics with a gapped quantum spin singlet ground state with a large singlet-triplet gap Δ The transition temperature is qualitatively consistent with our classical MC calculations using values of the exchange couplings extracted from DFT calculations The predicted transition temperatures TNN are 5 K – 20 K lower than the observed ∼ 40 K features in the susceptibilities An increase of the interchain exchange constants by ∼ 0.2 meV move the calculated transition temperatures to 40 K The specific heat measurements also show small features at ∼ 80 K, which we cannot explain from our calculations A recent classical MC study of single-chain MoBr3 found TNN to be 80 K although, as we show below, the classical Néel transition is governed by the interchain coupling At temperatures above 40 K, the susceptibility measurements show a large broad peak expected from the singlet-triplet gap of a quantum spin chain with a maximum at TT ∼ Δ The gaps extracted from the susceptibility measurements are 21 meV and 25 meV for MoI3 and MoBr3, respectively The magnitude of these gaps can only be reproduced using exchange values extracted from DFT calculations assuming S=3/2 A matching of the calculated and experimentally estimated gaps was obtained using U values of 0.6 eV and 1.2 eV for MoI3 and MoBr3, respectively The ratios Δ/J1 being 0.75 and 0.67 for MoI3 and MoBr3, respectively The small peak in the susceptibility of MoI3 at 40 K is consistent with an AFM transition, and an AFM transition indicates the presence of a classical Néel-type ground state The interchain coupling (J3) plays a critical role in determining the Néel temperature, as a stronger J3 drives the system toward more three-dimensional (bulk-like) magnetic behavior Increasing J3 by a factor of 2 increases TNN from 35 K to 52 K in MoI3 and from 26 K to 43 K in MoBr3 The dependence on the dominant exchange term J1 is weak, since its unperturbed value is already 10 times greater than J2 TNN is most sensitive to J3, then J2, and it is relatively insensitive to J1 These findings highlight how small differences in interchain and intrachain bonding geometry can markedly influence the magnetic dimensionality and the magnitude of T in quasi-1D Mo halides The specific heat measurements also show features at ∼ 80 K, which we cannot explain from our calculations The low-temperature anomalies at 37–38 K for both materials are likely attributed to interchain long-range ordering and are consistent with the susceptibility cusp and kink observed in MoI3 and MoBr3 These findings suggest a classical Néel-type transition resulting from the interchain coupling in both MoI3 and MoBr3 The magnetic susceptibility and specific heat capacity measurements of the MoX3 samples exhibit features of a classical Néel ordered lowtemperature ground state stabilized by weak interchain coupling that transitions above TNN ∼ 40 K into a phase dominated by single-chain physics with a gapped quantum spin singlet ground state with a large singlet-triplet gap Δ The gaps extracted from the susceptibility measurements are 21 meV and 25 meV for MoI3 and MoBr3, respectively The magnitude of these gaps can only be reproduced using exchange values extracted from DFT calculations assuming S=3/2 A matching of the calculated and experimentally estimated gaps was obtained using U values of 0.6 eV and 1.2 eV for MoI3 and MoBr3, respectively The experimental spin gap, the DFT calculated values of J1 and Δ, and the quantum spin calculations only lead to consistent results for a S=3/2 spin chain Because of the strong crystallographic dimerization and resulting exchange dimerization, the magnitudes of the singlet-triplet gaps are close to the values of J1 with the ratios Δ/J1 being 0.75 and 0.67 for MoI3 and MoBr3, respectively The combination of the 1D van der Waals motif, quantum spin chain behavior, and classical AFM order provides a unique platform for exploring quantum magnetism and magnetic excitations at the atomic chain limit One can expect that the integration of 1D AFM materials into spintronic platforms could enable progress in memory, logic, interconnects, and quantum technologies The data that support the findings of this study are available from the corresponding author upon reasonable request The work at UCLA was supported by the Vannevar Bush Faculty Fellowship (VBFF) to A.A.B.

Mira: under the Office of Naval Research (ONR) contract N00014-21-1-2947 on One-Dimensional Quantum Materials The work at UCR and the University of Georgia was supported, in part, via the subcontracts of the ONR project N00014-21-1-2947 DFT calculations were performed on STAMPEDE3 at TACC and EXPANSE at SDSC under allocation DMR130081 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program

38: The data that support the findings of this study are available from the corresponding author upon reasonable request The work at UCLA was supported by the Vannevar Bush Faculty Fellowship (VBFF) to A.A.B.

Kai: under the Office of Naval Research (ONR) contract N00014-21-1-2947 on One-Dimensional Quantum Materials The work at UCR and the University of Georgia was supported, in part, via the subcontracts of the ONR project N00014-21-1-2947 DFT calculations were performed on STAMPEDE3 at TACC and EXPANSE at SDSC under allocation DMR130081 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program

38: The data that support the findings of this study are available from the corresponding author upon reasonable request The work at UCLA was supported by the Vannevar Bush Faculty Fellowship (VBFF) to A.A.B.

Paper summary: Kai: The thesis of this paper is that MoXthree materials have signatures of both quantum spin chains with a spin singlet ground state and classical Néel order. It really suggests that these materials are more complex than just simple 1D systems <ref:2510.12613#pg1>.

Mira: They claim that bulk single crystals grown by chemical vapor transport exhibit classical antiferromagnetic ground states with a transition temperature of about forty K as revealed by susceptibility and specific heat measurements <ref:2510.12613#pg2,bulk single crystals grown by chemical vapor transport exhibit classical antiferromagnetic ground>.

Lev: That's the starting point, but what is interesting is what happens above that temperature, because the paper says there are these large broad peaks in the susceptibilities.

Kai: Those peaks are associated with a quantum spin singlet ground state and large singlet-triplet gaps of twenty-one meV and twenty-five meV for MoI3 and MoBr3 <ref:2510.12613#pg2,ground state and large singlet-triplet gaps of 21 meV and 25>.

Mira: The paper also states that Monte Carlo simulations, density matrix renormalization group calculations for finite spin-three/two chains, and density functional theory reproduce the experimental behavior <ref:2510.12613#pg1>.

Lev: That's the validation step we need; it means the theoretical models align with what you'd expect from a real system, which is good because it tells us what kind of physics we should be looking for when we build real hardware.

Kai: The paper confirms this interplay between strong onedimensional intrachain and weak three-dimensional interchain couplings in these MoXthree materials.

Mira: They highlight that MoXthree offers a unique platform for exploring quantum magnetism and magnetic excitations at the atomic chain limit, because they combine a one-dimensional van der Waals motif, spin chain behavior, and classical interchain order.

Conclusion: Kai: So the title itself points to this big idea about a quantum spin singlet ground state with a large excitation gap in these MoXthree materials. It’s basically saying they have some weird quantum behavior layered on top of their classical magnetic ordering.

Mira: The authors use Monte Carlo simulations and DFT calculations to reproduce what we see experimentally, which gives the results real backing. They confirm that the interplay between 1D chain physics and weak interchain coupling is key here <ref:2510.12613#pg1>.

Lev: For me, what this means for hardware is that understanding how these gaps form could inform how we design materials for quantum devices where you want to control spin states precisely.

Kai: It’s about taking a system that looks like a simple chain and seeing how those weak couplings change the entire magnetic landscape into something complex.

Mira: The implication is that MoXthree provides a clean system where you can study these 1D quantum phenomena while still having the possibility of some classical ordering effects from the interchain coupling <ref:2510.12613#pg1>.

Lev: It shows that even in materials that look simple, like these van der Waals structures, you can find rich magnetic physics at the atomic chain limit.

Kai: The work on this paper really opens up a new way to think about how we can engineer magnetic properties in these quasi-1D systems for future quantum technologies <ref:2510.12613#pg1>.