Glass Defies Disorder as Heat Flows Near the Quantum Limit in Atomic Chains

Heat is the silent saboteur of modern electronics. As transistors shrink toward individual atoms and quantum devices squeeze into ever-smaller spaces, the ability to move waste heat away from critical junctions becomes a defining constraint on performance. For decades, engineers have assumed that disorder is the enemy of heat flow: crystalline materials conduct heat efficiently because their ordered lattices carry vibrations coherently, while glasses, with their chaotic atomic arrangements, scatter those vibrations and behave as thermal insulators. A new study published in Nature Nanotechnology upends that intuition at the smallest possible scale, showing that heat can pass through a junction of amorphous silicon dioxide, the archetypal glass, with remarkable efficiency, reaching forty percent of the fundamental quantum limit of thermal conductance at room temperature.

The quantum limit, first articulated in theoretical work by Pendry and Landauer in the 1980s, describes the maximum heat current that a single perfect channel can carry. Each independent mode of vibration, or phonon, contributes a fixed quantum of thermal conductance, and no material, however pure, can exceed this ceiling for a given channel. Measuring such tiny heat flows, on the order of picowatts per kelvin, has historically required cryogenic temperatures and exquisitely engineered nanostructures. Earlier landmark experiments measured the quantum of thermal conductance in suspended silicon nitride membranes at low temperatures, and subsequent work probed ballistic phonons in carbon nanotubes and superlattices. But the fundamental limits of phonon-mediated heat transport at the atomic scale, at ambient conditions, remained unknown, largely because the experimental tools did not exist.

A team led by Longji Cui at the University of Colorado Boulder, working with collaborators at the Instituto de Ciencia de Materiales de Madrid, the Universidad Autónoma de Madrid, and other institutions, set out to close that gap. Their solution was a custom-fabricated scanning thermal microscope probe of unusual stiffness and sensitivity. The probe, dubbed Hs-SThM for high-stiffness scanning thermal microscopy, incorporates a niobium nitride sensing element defined by electron-beam lithography and reactive sputtering, metallized with chromium and gold for electrical isolation. Finite element simulations reveal directional stiffnesses reaching 2,000 newtons per meter laterally and an extraordinary 50,000 newtons per meter along the pressing axis, values that allow the tip to form and hold stable contacts with surfaces at the atomic scale without buckling or drifting.

The measurement strategy borrows a page from the field of molecular electronics. For decades, researchers have used mechanically controllable break junctions and scanning tunneling microscopes to form and rupture single-atom electrical contacts, capturing the discrete conductance signatures of individual atoms and molecules. The Colorado-led team adapted this concept to heat, creating what they call phononic break junctions. The probe is brought into contact with a surface and then slowly withdrawn at a controlled speed of roughly 0.11 nanometers per second. Thermal feedback, monitored through the probe’s temperature response, signals the transition from near-field radiative heat transfer across a vanishing gap to solid contact, marked by a sharp jump in measured conductance. As the tip retracts further, the contact narrows through a nanocontact to a single atomic bridge, and the thermal conductance is recorded throughout in real time.

The material under scrutiny was amorphous silicon dioxide, the glass that forms the insulating layers of virtually every integrated circuit ever made. In bulk form, this glass is a poor thermal conductor: its disordered network of silicon and oxygen atoms scatters vibrational energy into diffusive modes known as diffusons, alongside localized modes called locons and only a minority of truly propagating modes. Macroscopic thermal conductivity is correspondingly low. Yet when the researchers pulled their probe away from a silica surface and the contact thinned to just a few atoms, the measured thermal conductance did not collapse. Instead, it settled onto reproducible plateaus, with the finest junctions supporting a phononic thermal conductance of up to forty percent of the quantum limit at room temperature.

That figure is all the more striking when compared with crystalline metals. The atomically thin glass junctions conducted heat roughly an order of magnitude better, per phonon channel, than the phonon thermal conductance of crystalline gold atomic contacts, which the team measured in parallel as a reference system. Gold, despite its perfect lattice, is a poor phonon conductor because its heavy atoms vibrate at low frequencies. Silica, with its light oxygen atoms, supports much higher vibrational frequencies, and this difference proved decisive. Through molecular dynamics simulations performed with non-equilibrium methods and spectral heat current decomposition, the researchers found that the phononic thermal conductance of an atomic junction is dictated not by the degree of structural order but by the Debye frequency, the maximum vibrational frequency the material can support.

The simulations, run with the LAMMPS package using Tersoff-parameterized potentials for the silicon-oxygen system and embedded-atom-method potentials for gold, reproduced the experimental conductance traces in striking detail. Spectral decomposition of the heat current across a one-dimensional silica contact revealed that the energy distribution entering the junction through one plane exits essentially unperturbed at another, a hallmark of coherent transport. Most of the transmitted heat lies below the Debye frequency cutoff, and the vibrational spectrum is preserved across the junction rather than scrambled by the amorphous structure. In other words, at the atomic limit, the glass behaves not as a diffusive insulator but as a ballistic waveguide for coherent phonons, with structural disorder failing to impose the fundamental penalty that dominates its macroscopic behavior.

Statistical rigor was essential to rule out artifacts. Because each rupture event samples a different local atomic configuration, the team recorded thousands of traces and applied an unsupervised machine-learning pipeline combining principal component analysis with Gaussian mixture model clustering. The clustering, performed on jump-independent feature vectors without any manual labels, identified five distinct trace families, and multiple independently identified clusters showed a peak or shoulder near the dominant conductance maximum of approximately 1.2 times the single-mode quantum of thermal conductance. This repeated spectral weight across clusters indicates that the characteristic conductance is not produced by one anomalous trace subclass but reflects a statistically favored atomic-scale junction configuration in the amorphous network, with cluster-dependent variations arising from local bonding geometry and rupture pathway.

The implications extend well beyond fundamental physics. Heat dissipation is a first-order problem in nanoscale electronic and quantum devices, and the finding that intrinsic disorder does not fundamentally constrain atomic-scale heat flow opens a new design space. If the Debye frequency, rather than crystallinity, is the governing parameter, then materials can be selected and engineered for atomic-scale thermal links on the basis of their vibrational spectra alone. Light-atom glasses and other disordered solids, long dismissed as thermal dead ends, may serve as efficient phononic interconnects where crystalline channels are impractical. The break-junction platform itself offers an experimental testbed for studying wave-like coherent phonon transport at room temperature, complementing cryogenic quantum-coherence experiments on phononic crystals and superlattices.

The researchers anticipate that their approach will enable the engineering of high-performance thermal phononic devices, including thermal diodes that pass heat preferentially in one direction and thermal transistors that modulate heat currents, the functional analogs of their electronic counterparts. Earlier work has demonstrated phonon-based routing and rectification in nanostructures such as a graphite thermal Tesla valve driven by hydrodynamic phonon transport, and coherent phonon control has been used to manage interactions between high-frequency phonons and single quantum systems. An atomic-scale platform that works at ambient temperature, with picowatt resolution and reproducible single-atom contacts, brings those device concepts closer to practical realization. The data and code underlying the study have been released openly, and the team suggests that the same thermal-feedback break-junction technique can be extended to other disordered materials, molecular junctions, and hybrid inorganic-organic contacts, charting a path toward a technology in which heat itself is guided, switched, and computed with at the scale of individual atoms.

Subject of Research: Coherent phonon heat transport through atomic-scale amorphous silicon dioxide junctions

Article Title: Thermal conductance of atomic-scale glass reaches 40% of quantum limit

Article References: Thermal conductance of atomic-scale glass reaches 40% of quantum limit. (n.d.). https://doi.org/10.1038/s41565-026-02308-9

Image Credits: AI Generated

DOI: 10.1038/s41565-026-02308-9

Keywords: phonon transport, thermal conductance, quantum limit, amorphous silicon dioxide, break junction, scanning thermal microscopy, ballistic transport, Debye frequency, nanoscale heat dissipation, thermal diode, molecular dynamics simulation, glass