Quantum spin singlet ground state with large excitation gap in the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>S</mi><mo>=</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></mrow></math> quasi-one-dimensional van der Waals materials <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>Mo</mi><msub><mi>X</mi><mn>3</mn></msub><mrow><mo>(</mo><mi>X</mi><mo>=</mo><mi mathvariant="normal">I</mi><mo>,</mo><mi>Br</mi><mo>)</mo></mrow></mrow></math>

Jordan Teeter; Topojit Debnath; Harshil Goyal; Md. Sabbir Hossen Bijoy; Maedeh Taheri; Nicholas R. Sesing; Clayton Mantz; Josemaria Garcia; Mathew B. Stone; Colin Sarkis; Do Hoon Kiem; Sohee Kwon; Mina Yoon; Fariborz Kargar; Kirill Shtengel; Tina T. Salguero; Alexander A. Balandin; Roger K. Lake

doi:10.1103/zgqr-8z72

Quantum spin singlet ground state with large excitation gap in the $S = \frac{3}{2}$ quasi-one-dimensional van der Waals materials $Mo X_{3} (X = I, Br)$

Jordan Teeter ^1,2, Topojit Debnath ³, Harshil Goyal ⁴, Md. Sabbir Hossen Bijoy⁵, Maedeh Taheri ^1,2, Nicholas R. Sesing ⁶, Clayton Mantz ⁶, Josemaria Garcia ⁶, Mathew B. Stone ⁷ et al.

Colin Sarkis ⁷, Do Hoon Kiem ⁸, Sohee Kwon ⁸, Mina Yoon ⁸, Fariborz Kargar ⁵, Kirill Shtengel ⁹, Tina T. Salguero ⁶, Alexander A. Balandin ^1,2,10,*, and Roger K. Lake ^3,†

¹Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA
²California NanoSystems Institute, University of California, Los Angeles, California 90095, USA
³Department of Electrical and Computer Engineering, University of California, Riverside, California 92521, USA
⁴Department of Electrical and Computer Engineering, Auburn University, Auburn, Alabama 36849, USA
⁵Materials Research and Education Center, Department of Mechanical Engineering, Auburn University, Auburn, Alabama 36849, USA
⁶Department of Chemistry, University of Georgia, Athens, Georgia 30602, USA
⁷Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
⁸Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
⁹Department of Physics and Astronomy, University of California, Riverside, California 92521, USA
¹⁰Center for Quantum Science and Engineering, University of California, Los Angeles, California 90095, USA

^*Contact author: balandin@seas.ucla.edu
^†Contact author: rlake@ucr.edu

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Phys. Rev. B 114, 264416 – Published 13 November, 2026

DOI: https://doi.org/10.1103/zgqr-8z72

Abstract

We report the first experimental evidence for a quantum-disordered spin-singlet ground state and gapped triplon excitations in quasi-one-dimensional van der Waals materials. The insulating van der Waals trihalides ${MoI}_{3}$ and ${MoBr}_{3}$ are analyzed in a combined thermodynamic, spectroscopic, and theoretical investigation. These materials contain structurally dimerized atomic chains of face-sharing octahedra with alternating short and long Mo-Mo separations along the chain direction. Temperature-dependent bulk magnetic susceptibility measurements reveal a broad thermally activated response consistent with a large spin gap. Inelastic neutron scattering (INS) spectra provide direct evidence of a collective spin gap at an exceptionally large energy transfer of $Δ \approx 30 meV$ ( $\approx 348 K$ ) for ${MoBr}_{3}$ . The absence of low-energy magnetic spectral weight and the activated susceptibility are inconsistent with a conventional classical antiferromagnetic ground state. Instead, they point to a spin-singlet ground state with a large singlet-triplet excitation gap. To identify the microscopic origin of this gap, we perform density matrix renormalization group (DMRG) calculations on one-dimensional spin chains using exchange constants derived from density functional theory (DFT). The dominance of the single dimer exchange term allows the interdimer coupling along the chain and between chains to be treated using perturbation theory. Intrachain exchange lowers the isolated-dimer triplet energy by producing a dispersive triplon band. INS data show the triplon bandwidth to be $\sim 20 meV$ . Interchain coupling produces negligible additional bandwidth on the order of 1 meV. The INS measurements directly establish quantum spin-singlet and triplon physics in ${MoBr}_{3}$ with a large singlet-triplet gap on the scale of room temperature, while the susceptibility and theoretical results indicate closely analogous behavior in ${MoI}_{3}$ .

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Quantum spin singlet ground state with large excitation gap in the $S = \frac{3}{2}$ quasi-one-dimensional van der Waals materials $Mo X_{3} (X = I, Br)$

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Quantum spin singlet ground state with large excitation gap in the S=32 quasi-one-dimensional van der Waals materials MoX3(X=I,Br)

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Quantum spin singlet ground state with large excitation gap in the $S = \frac{3}{2}$ quasi-one-dimensional van der Waals materials $Mo X_{3} (X = I, Br)$