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PreprintCC BY 4.0arXiv

Occupancy Driven Phase Transitions in Gapped Systems

  • Ian Madden
  • Alex Kociubinski
  • Wouter Montfrooij
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Abstract

Systems characterized by an energy gap between the ground state and excited states support a growing number of thermal excitations upon raising the temperature. Provided these excitations interact with each other, their number affects the size of the energy gap akin to a harmonic oscillator where the oscillation frequency is damping dependent. Here we show that once a critical number of excitations is reached, the excitation population becomes thermodynamically unstable and the system transitions between two states. This transition is marked by a divergence in the specific heat, which does not have to be accompanied by an order parameter, nor by the full closure of the gap. We illustrate the thermodynamics using a simple two level system and superfluid 4He as examples, and we argue that the hidden order transition in URu2Si2 is an example of such an occupancy driven transition in the absence of an order parameter.

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Cite

BibTeXRIS
BibTeX
@misc{madden2026occupancy,
  title = {{Occupancy Driven Phase Transitions in Gapped Systems}},
  author = {Ian Madden and Alex Kociubinski and Wouter Montfrooij},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.31564},
  eprint = {2610.31564},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.31564},
  note = {CC BY 4.0},
}
RIS
TY  - UNPB
TI  - Occupancy Driven Phase Transitions in Gapped Systems
AU  - Ian Madden
AU  - Alex Kociubinski
AU  - Wouter Montfrooij
PY  - 2026
DA  - 2026/10/22
PB  - arXiv
DO  - 10.48550/arxiv.2610.31564
UR  - https://arxiv.org/abs/2610.31564
AB  - Systems characterized by an energy gap between the ground state and excited states support a growing number of thermal excitations upon raising the temperature. Provided these excitations interact with each other, their number affects the size of the energy gap akin to a harmonic oscillator where the oscillation frequency is damping dependent. Here we show that once a critical number of excitations is reached, the excitation population becomes thermodynamically unstable and the system transitions between two states. This transition is marked by a divergence in the specific heat, which does not have to be accompanied by an order parameter, nor by the full closure of the gap. We illustrate the thermodynamics using a simple two level system and superfluid ^4He as examples, and we argue that the hidden order transition in URu_2Si_2 is an example of such an occupancy driven transition in the absence of an order parameter.
ER  -