The mechanism hinges on a subtle piece of quantum field theory. The researchers considered a model in which a Dirac fermion, the class of matter particles that includes electrons and quarks, couples directly to the oscillating inflaton through a pseudoscalar Yukawa interaction. In plain terms, the coupling involves the gamma-five matrix, an object in the Dirac equation that distinguishes between the two possible spin orientations of a particle relative to its direction of motion. Because the inflaton field swings back and forth like a cosmic pendulum, the effective mass that the fermion experiences, which the authors call the pseudo-mass, repeatedly changes sign. Every time it crosses zero, the quantum vacuum becomes unstable and pairs of fermions are wrenched into existence, and crucially, the two helicity states are not created in equal numbers.

To make this statement precise, the team turned to the WKB approximation, a semiclassical technique borrowed from quantum mechanics in which wave functions are written as exponentials with slowly varying amplitude and phase. By matching the solutions on either side of each zero crossing of the pseudo-mass, they derived analytical expressions for the Bogoliubov coefficients, the mathematical objects that encode how many particles are produced in each quantum mode. The calculation, which involves parabolic cylinder functions, remains valid for an arbitrary number of successive production events, a significant advance over earlier treatments that could only handle a single pass of the oscillating field.

The key result is that the pseudoscalar coupling injects an imaginary contribution into the first-order adiabatic phase, denoted by the authors as the complex phase of the fermion mode. This imaginary piece depends on the helicity state of the particle, and it is solely responsible for breaking the symmetry between left- and right-handed fermions. When the coupling is switched off, the phase vanishes and the familiar helicity-symmetric result of standard fermionic preheating is recovered, with occupation numbers falling off as a clean Gaussian in momentum space. With the coupling present, that Gaussian is warped, and the two helicities peel apart in a way that can be computed exactly.

The researchers also identified a sharp boundary condition on where the asymmetry can live. Helical asymmetry is generated only for comoving momenta much smaller than a characteristic wavelength set by the fermion mass and the expansion rate of the Universe. Once a mode’s wavelength enters the corresponding horizon scale, the asymmetry is dramatically suppressed. This means the imbalance is fundamentally a large-scale phenomenon, imprinted on the longest wavelengths available in the early cosmos, precisely the scales that could matter for subsequent cosmological evolution.

Perhaps the most striking finding concerns coherence. In an expanding Universe, the authors show that the combined effects of parametric resonance and Pauli blocking, the quantum-mechanical exclusion principle that caps fermion occupation at one particle per mode, produce coherent enhancement in certain frequency bands and coherent suppression in others. Unlike bosons, whose occupation numbers grow exponentially through Bose enhancement, fermions can only be nudged up or down within strict limits. The real part of the complex WKB phase governs this coherent superposition, and the numerical simulations reveal that it significantly boosts the number density of each helicity state after roughly eight oscillations of the inflaton, a periodicity traced back to a numerical factor in the imaginary part of the phase.

The team validated their analytics with numerical simulations in both a static universe and an expanding one. In the static case, the helical asymmetry appears as isolated narrow resonance bands in momentum space, reminiscent of the instability bands familiar from bosonic preheating. When cosmic expansion is included, these bands wash out, but the deviation from Gaussian helicity distributions persists and grows with each successive production event. The authors also found that heavier fermions concentrate their production on smaller physical scales, and that massless fermions, consistent with earlier results on gravitational particle production, escape the mechanism entirely.

What elevates this work beyond a technical exercise is its potential connection to one of the deepest mysteries in cosmology: why the Universe contains vastly more matter than antimatter. Helicity-asymmetric fermion production is a natural ingredient in leptogenesis scenarios, in which an early asymmetry in neutrino-like particles is eventually converted into the baryon asymmetry we observe today. Because the mechanism identified by the authors does not depend on the precise value of the inflaton mass, it applies equally to low-scale inflationary models such as small-field and hybrid inflation, widening the range of theories in which such an asymmetry could have been seeded.

The framework is also extendable in directions that touch other observational frontiers. The authors note that the same WKB machinery can accommodate momentum-dependent pseudoscalar couplings, quantum chromodynamics effects, and electroweak interactions, and that helical fermion production may be linked to the generation of primordial magnetic fields, chiral gravitational waves, and even dark matter production in the early Universe. Each of these connections offers a potential observational handle on a process that unfolded when the cosmos was a fraction of a second old.

For now, the result remains theoretical, resting on analytical mathematics and numerical simulation rather than direct measurement. But it sharpens a growing realization among cosmologists that the transition from inflation to the hot Big Bang was not a passive fade but an active, symmetry-breaking crucible. If the inflaton really did couple to fermions through a pseudoscalar channel, the newborn Universe may have emerged from preheating with a slight but decisive twist, a handedness written into its matter content that echoes, in principle, all the way to the galaxies we see today.

Subject of Research: Helical asymmetry in fermionic preheating induced by a pseudoscalar Yukawa coupling between the inflaton and Dirac fermions after inflation

Article Title: Pseudoscalar Yukawa coupling induced helical asymmetry in fermionic preheating

Article References: Fu, K., Liu, L.-S., Wang, Y.-F., Gu, X.-Y., & Li, X.-B. (2026). Pseudoscalar Yukawa coupling induced helical asymmetry in fermionic preheating. The European Physical Journal C, 86(9), Article 1083. https://doi.org/10.1140/epjc/s10052-026-16298-4

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16298-4

Keywords: preheating, cosmic inflation, fermion production, helicity asymmetry, pseudoscalar Yukawa coupling, inflaton, WKB approximation, Bogoliubov coefficients, leptogenesis, baryogenesis, early universe, quantum field theory

The new work, published in the open-access journal iScience, describes a low-frequency bidirectional pulsed current heating method that operates at frequencies of one hertz or below. The central innovation is adaptivity: rather than applying fixed pulse parameters, the system continuously adjusts the pulse currents according to the battery’s real-time state, ensuring that the anode potential never crosses the lithium-plating threshold throughout the entire heating process. The result, according to the authors, is the fastest possible heating that a given battery chemistry can safely tolerate, a genuine non-destructive approach validated on both lithium iron phosphate and nickel-cobalt-manganese cells.

To understand why this matters, it helps to consider the landscape of existing solutions. External heating methods, which rely on thermal fluids, heat pipes, or phase change materials placed outside the cell, tend to produce uneven temperature distributions and accelerate aging. Internal heating, by contrast, exploits the battery’s own internal resistance to generate Joule heat directly where it is needed, minimizing losses. Alternating current heating and mutual pulse heating, in which battery modules alternately charge and discharge one another, have both been explored extensively. Bidirectional pulsed current strategies, in which positive and negative pulses alternate to keep the state of charge nearly constant, have shown particular promise, but earlier high-frequency implementations demanded costly power electronics that limited real-world adoption.

The research team began by systematically mapping how pulse parameters influence heat generation. Using 2 ampere-hour LFP soft-pack cells placed in a temperature chamber, they varied the ratio of positive to negative pulse amplitudes from 1:1 up to 1:5 while holding the pulse period fixed at six seconds. Across both low and high states of charge, a larger amplitude ratio produced a greater temperature rise, a consequence of the fundamental relationship that heat generation scales with the square of current. They then varied the pulse period from fractions of a second up to 36 seconds and found that longer periods heated the battery more effectively, although the benefit plateaued beyond roughly ten seconds. That plateau defined their operating point: a ten-second pulse period, well within the low-frequency regime that avoids expensive high-frequency excitation hardware.

The truly distinctive element of the study is its use of three-electrode cells to police the lithium-plating boundary in real time. The researchers modified commercial cells by implanting a lithium titanate reference electrode at the center of the electrode stack, allowing them to measure the anode potential directly during operation, something impossible with conventional two-terminal cells. Through hybrid pulse power characterization tests conducted at temperatures from minus ten to twenty-five degrees Celsius and across a range of states of charge, they constructed a three-dimensional map of the maximum charging current the battery could tolerate at each condition without lithium deposition. This boundary current map became the adaptive brain of the heating algorithm, dictating the positive pulse amplitude as temperature and state of charge evolved.

The preheating experiments themselves were conducted under conditions mimicking a real electric vehicle arriving at a charger on a frigid winter day. With cells adjusted to ten percent state of charge and stabilized at minus fifteen degrees Celsius, the bidirectional pulses were applied. For the LFP cell, heating from minus fifteen to ten degrees took 2,068 seconds, an average temperature rise of 0.73 degrees Celsius per minute. Crucially, the measured anode potential, corrected against the reference electrode’s equilibrium potential of 1.565 volts versus lithium, remained above zero volts throughout the entire process, the electrochemical signature that no metallic lithium was plating onto the graphite anode at any moment.

Durability is where many preheating schemes have quietly failed, so the team subjected cells to one hundred repeated heating cycles at fixed state of charge. Because the bidirectional pulses deliver equal charge and discharge capacity within each cycle, the state of charge never drifts, allowing every cycle to begin from identical conditions. The authors note that with typical weekly charging and a three-month winter, one hundred heating cycles correspond to roughly seven years of real-world service, comparable to the lifetime of a traction battery. Capacity measurements taken before the test, after fifty cycles, and after one hundred cycles showed essentially no degradation. Post-mortem analysis sealed the case: transmission electron microscopy revealed an intact, uncracked SEI film, scanning electron microscopy showed an orderly lamellar electrode structure with no dendritic deposits, and energy-dispersive spectroscopy confirmed the elemental composition was unchanged.

To test whether the framework generalizes beyond one chemistry, the researchers repeated the entire procedure with a 1.75 ampere-hour commercial NCM811 pouch cell, independently recalibrating its lithium-plating boundary rather than simply copying LFP parameters. The results were striking. The NCM811 cell heated from minus fifteen to ten degrees in just 865.8 seconds, an average rise of 1.75 degrees Celsius per minute, nearly double the LFP rate. The reason lies in the interplay of resistance and allowable current: the NCM cell’s internal resistance is roughly half that of the LFP cell, but its lithium-plating boundary current is nearly twice as large, so its heating power, proportional to current squared times resistance, is consistently higher across all temperatures. The NCM cell likewise survived one hundred heating cycles with no measurable capacity loss and no lithium plating detected.

The physics underlying these results rewards aggressive current within safe limits. Because heat generation is proportional to the square of current, the alternating charge and discharge pulses of the bidirectional strategy allow larger amplitudes than constant-current heating would, since the alternating polarity alleviates anode polarization and prevents the anode potential from collapsing below zero volts. The authors point out an encouraging trend for the future: as battery technology improves fast-charging capability, the allowable lithium-plating boundary current grows, which in turn permits even larger pulse currents and faster heating, even as falling internal resistance reduces the Joule heating contribution per ampere.

The study is candid about its limitations. The pulse period was optimized at zero degrees and extrapolated to minus fifteen, an assumption supported but not exhaustively characterized across the full temperature range. The long-term stability of the implanted reference electrodes under repeated pulsing remains an open question, the durability test did not combine heating with subsequent fast charging, and the hardware costs and system-level energy efficiency of the required bidirectional power electronics were not quantified. Even so, the demonstration that a low-frequency, adaptive, chemistry-agnostic pulse strategy can warm frozen batteries at up to 1.75 degrees per minute without any detectable damage represents a meaningful step toward electric vehicles and stationary storage that shrug off winter. For the millions of drivers who watch their range evaporate on cold mornings, the prospect of a battery that heats itself safely and swiftly may be the most welcome news of the season.

Subject of Research: Non-destructive low-temperature pulse preheating of lithium-ion batteries

Article Title: Low-frequency non-destructive adaptive fastest pulse preheating for lithium-ion batteries

Article References: Li, C., Mao, S., Gao, M., He, M., Sun, Y., Wang, D., Han, X., Xia, C.-J., Wang, X., Lu, Y., & Gao, W. (2026). Low-frequency non-destructive adaptive fastest pulse preheating for lithium-ion batteries. iScience, 29(10), Article 117715. https://doi.org/10.1016/j.isci.2026.117715

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117715

Keywords: lithium-ion batteries, preheating, bidirectional pulse current, lithium plating, cold climate, electric vehicles, anode potential, three-electrode cells, LFP, NCM811, fast charging, battery degradation