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Entanglement Randomness and Gapped Itinerant Carriers in a Frustrated Quantum Magnet

Yuanqi Lyu (吕源祺)1,*,†, Luke Pritchard Cairns1,*, Josue Rodriguez1,2, Chunxiao Liu (刘春骁)1, Kenneth Ng (吴子建)1, John Singleton3, and James G. Analytis1,2,4,5,‡

  • *These authors contributed equally to this work.
  • †Contact author: yuanqilyu@berkeley.edu
  • ‡Contact author: analytis@berkeley.edu
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Phys. Rev. X 15, 041035 – Published 21 November, 2025

DOI: https://doi.org/10.1103/tx6t-gbxy

Abstract

The quantum spin liquid is a state manifesting extraordinary many-body entanglement, and the material NaYbSe2 is thought to be one of the most promising candidates for its realization. Through low-temperature heat capacity and thermal conductivity measurements, we identify an apparent contradiction familiar to many quantum spin liquid candidates: While entropy is stored by apparently gapless excitations, the itinerant carriers of entropy are gapped. By studying the compositional series NaYbxLu1−xSe2 across a percolation transition of the magnetic lattice, we suggest that this contradiction can be resolved by the presence of entanglement scales of random sizes. Moreover, as we truncate the scale of entanglement by magnetic dilution, we show that the itinerant magnetic entropy carrier in NaYbSe2 does not arise from a uniform globally entangled spin ground state but rather materializes through the stochastic propagation of boundaries between locally entangled spin objects.

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Popular Summary

Frustrated quantum magnets often show a puzzling tension: They can store an enormous amount of heat but are unable to transport it, as if their excitations are frozen and localized rather than liquid and mobile. This tension is especially evident in NaYbSe2, a leading quantum spin liquid candidate. We suggest that this conflict arises from the false notion that a material must be either perfectly ordered or perfectly entangled. In reality, quenched disorder limits how far entanglement can spread. Our results show that in NaYbSe2 mobile magnetic excitations emerge not from a uniform spin liquid but from the random motion of boundaries between locally entangled spin regions.

We uncover this behavior through low-temperature heat capacity and thermal conductivity measurements on NaYbxLu1−xSe2, where nonmagnetic Lu3+ ions replace magnetic Yb3+ ions. Heat capacity data reveal that most spins pair into two-spin bonds, or dimers, and form larger clusters of entangled spins that freeze into a disordered “valence bond glass” rather than a regular crystal-like pattern. Thermal conductivity data show that a gapped magnetic excitation appears near the point where the magnetic lattice remains connected but becomes irregular. The number of these excitations peaks there, confirming they originate from short-range, disordered connectivity rather than long-range entanglement.

Our findings resolve the paradox of localized entanglement giving rise to mobile excitations in NaYbSe2. The spin pairs and clusters store entropy, while fluctuating boundaries between them carry it. Because structural disorder is common in real materials, this mechanism may explain similar puzzles in other quantum spin liquid candidates.

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Phys. Rev. X 15, 041035– Published 21 November, 2025
  • Received 29 April 2025
  • Revised 19 September 2025
  • Accepted 16 October 2025
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