Researchers at Nara Women’s University have revealed a surprising shift in the behavior of liquid helium-4, demonstrating a transition between two distinct liquid states over a temperature range of 0.1, 3.3 K and a pressure range of 1, 60 bar. Mika Tanabe, Momoko Tsujimoto, and Kenichi Kinugawa identify a transport crossover marked by simultaneous minima in shear and kinematic viscosities, thermal conductivity, and thermal diffusivity as the liquid transforms from a low quantum-dispersion liquid (LQDL) to a high quantum-dispersion liquid (HQDL). Counterintuitively, the HQDL state exhibits gas-like transport characterized by superdiffusion and ultralow viscosity as temperature decreases, a phenomenon driven by nuclear quantum fluctuations rather than thermal effects. This transition defines a second Frenkel line, and these results demonstrate that nuclear quantum fluctuations alone can induce gas-like behavior in a liquid, providing a unified understanding of transport in helium-4 without superfluidity.
Path integral centroid molecular dynamics simulations reveal a surprising fluidity in helium-4, even when atomic exchange is absent, challenging conventional understanding of quantum liquids. Mika Tanabe, Momoko Tsujimoto, and Kenichi Kinugawa utilized these simulations, spanning temperatures from 0.1, 3.3 K and pressures from 1, 60 bar, to identify two distinct liquid states: the low quantum-dispersion liquid (LQDL) and the high quantum-dispersion liquid (HQDL). This counterintuitive emergence of gas-like dynamics at lower temperatures highlights the dominant role of nuclear quantum fluctuations, contrasting with thermal fluctuations at higher temperatures. LQDL closely resembles real He I, while HQDL presents a unique quantum liquid state without a direct analogue in conventional helium-4.
Recent path integral centroid molecular dynamics simulations over 0.1, 3.3 K and 1, 60 bar have revealed a surprising fluidity crossover within liquid helium-4, even when considering distinguishable atoms obeying Boltzmann statistics. This emergence of two states challenges prior assumptions about helium’s behavior, particularly regarding the role of atomic exchange. Mika Tanabe, Momoko Tsujimoto, and Kenichi Kinugawa state that “LQDL is a heat-transport-dominated dissipative fluid, whereas HQDL is a momentum-dominated inertial fluid.” The transition between these states is not a traditional thermodynamic phase change, but rather a transport crossover marked by a qualitative change in the velocity autocorrelation function (VAF) and a transition in the Prandtl number.
The conventional understanding of liquids shifting to gaseous states relies on increased thermal energy overcoming intermolecular forces; however, recent simulations of helium-4 reveal a surprising driver: quantum fluctuations. This emergence of gas-like dynamics isn’t driven by heat, but differs from the thermal fluctuations dominant at higher temperatures.
The current understanding of liquid helium-4 increasingly focuses on the interplay between quantum fluctuations and atomic behavior, revealing complexities beyond traditional models of superfluidity. Using path integral centroid molecular dynamics simulations over 0.1, 3.3 K and 1, 60 bar, Mika Tanabe, Momoko Tsujimoto, and Kenichi Kinugawa found that “the HQDL is characterized by a pronounced spatial delocalization of atoms,” a key factor distinguishing it from the LQDL. Visualizations of these states reveal significant differences in atomic necklace configurations; snapshots show HQDL necklaces exhibiting substantial overlap, indicating greater atomic delocalization than those in the LQDL.
Spanning 0.1, 3.3 K and 1, 60 bar, our simulations investigate the transport properties of two distinct liquid states: the low quantum-dispersion liquid (LQDL) and the high quantum-dispersion liquid (HQDL). While LQDL exhibits conventional liquid behavior consistent with the Stokes, Einstein (SE) relation, HQDL emerges at lower temperatures and displays anomalous gas-like transport characterized by superdiffusion and ultralow viscosity, accompanied by a breakdown of the SE relation. Across the LQDL, HQDL boundary, Mika Tanabe, Momoko Tsujimoto, and Kenichi Kinugawa identify a transport crossover marked by a qualitative change in the velocity autocorrelation function (VAF), a transition in the Prandtl number, and the emergence of transport minima in shear and kinematic viscosities, thermal conductivity, and thermal diffusivity. These minima reflect a crossover from liquid-like to gas-like transport upon cooling in the low-temperature subcritical region, and the transition from oscillatory to monotonic VAF defines a second Frenkel line, distinct from those observed in the supercritical regime.
The peculiar behavior of helium-4, even when its atoms are treated as distinguishable rather than identical, is revealing unexpected insights into quantum polyamorphism, the existence of multiple distinct liquid states arising from quantum fluctuations. Mika Tanabe, Momoko Tsujimoto, and Kenichi Kinugawa, in conversation with the author, revealed a surprising shift spanning 0.1, 3.3 K and 1, 60 bar of pressure. This work builds on previous findings demonstrating similar states in amorphous, or glassy, forms, the low and high quantum dispersion amorphous solids. This quantum polyamorphism, driven by changes in atomic quantum delocalization, may be a general feature of quantum systems lacking atomic exchange, offering a new lens through which to view the behavior of matter at extremely low temperatures.
Source: https://arxiv.org/abs/2607.23127
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