Sr₂RuO₄ pairing symmetry
Sr₂RuO₄ is a superconductor, but experiments still do not select one consistent symmetry for its paired-electron state.
Physics
Herbertsmithite ground state
Herbertsmithite is a leading candidate for a quantum spin liquid. Which low-temperature state is intrinsic to its kagome layers?
Mercury’s formation
Mercury has an unusually large metal core. Which formation history produced it while leaving the planet’s other observed properties?
Air-shower muon puzzle
Cosmic-ray air showers produce more muons than standard collision models predict. Which particle-production physics explains the excess?
Common-envelope ejection
Two stellar cores can spiral through one shared envelope. What determines whether that gas escapes and how close the surviving binary becomes?
Collectivity in small collision systems
Particles from some proton collisions have coordinated motion. Which interactions create those patterns in such small systems?
Inner-core seismic anisotropy
Seismic waves travel through Earth’s inner core at different speeds in different directions. What produces the pattern and its changes with depth?
Martian methane
A rover reports methane near the surface; an orbiter finds none. Even the reliability of the rover detections is disputed.
Planet Nine
Some distant orbits suggest an unseen planet beyond Neptune. Searches have narrowed the possibilities without finding it.
Exoplanet radius valley
Close-in small planets cluster into two size groups. Which formation and atmospheric processes produce the gap between them?
Fermi bubbles
Giant gamma-ray lobes rise above and below the Milky Way’s center. Their formation history and radiation mechanisms remain debated.
Chiral magnetic effect in nuclear collisions
Hot quark matter may separate electric charges along a magnetic field. Similar particle patterns can also arise from ordinary collision dynamics.
Polymer contact electrification
Plastic surfaces can acquire charge when they touch and separate. Which carriers dominate depends on material chemistry and contact conditions.
Marine ice-cliff instability
A tall Antarctic ice cliff could break up faster than ice flows forward. Whether this can sustain accelerating retreat depends on competing deformation and failure processes.
Phonon thermal Hall effect
Heat carried by atomic vibrations can flow sideways in a magnetic field, even in a nonmagnetic electrical insulator. What microscopic process produces this response in strontium titanate?
Two-level systems in glasses
Very cold glasses absorb heat and sound through low-energy motions beyond ordinary vibrations. Tunneling between two arrangements describes many measurements, but which microscopic structures supply these states?
Gallium anomaly
Gallium detectors exposed to intense laboratory neutrino sources make fewer germanium atoms than standard capture calculations predict. Is the missing yield a nuclear-calculation problem, a measurement effect or new neutrino physics?
Bilayer nickelate pairing mechanism
Nickel-oxide bilayers superconduct under high pressure and in suitably prepared ambient-pressure films. What interaction binds the electron pairs, and can one quantitative account explain the measured gaps and material dependence?
Saturn’s tidal dissipation
Saturn transfers spin to its moons, moving them outward. The strength and frequency pattern of this transfer point to unusually effective internal energy loss. Which interior processes set those rates and their evolution?
Magnetotail reconnection energy partition
When Earth’s stretched magnetic field reconnects, released energy heats particles and creates fast-moving tails in their energy distributions. What determines the ion/electron and thermal/nonthermal shares under measured magnetotail conditions?
Gamma-ray burst prompt emission
The first flash of a long gamma-ray burst reveals an ultra-relativistic jet releasing energy. Which radiation processes produce the photons, and how do their contributions change during the burst?
Mid-Pleistocene transition
Why did the dominant rhythm of ice ages change about a million years ago?
Dansgaard–Oeschger events
What drove Greenland’s repeated rapid warmings during the last ice age?
Great Oxidation Event
What controlled the rise and fluctuations of atmospheric oxygen about 2.4 billion years ago?