A tiny squeeze can reveal which hidden quantum states are making electrons behave as if they are enormously heavy. In this crystal, that pressure exposes a six-state secret. Some metals have electrons that behave as though they are unusually heavy, and these materials can host unusual forms of superconductivity and other striking quantum behavior.

The difficulty is that several important energy scales are all small and close together, making it hard to determine which one controls the unusually high electronic mass. An everyday comparison is a crowded room where several quiet conversations overlap: the challenge is to identify which voice belongs to which person.

YbPtBi is one of the materials with the largest electronic mass, but the ordering of its important energy levels had remained debated. The study identifies a four-state level as the first excited state, only about one and a half energy units above the two-state ground level, and places it below the proposed energy scale associated with heavy-electron behavior.

That matters because the unusually heavy electrons may be connected to several nearly available states, rather than only the lowest two. The key idea is to squeeze the crystal in one direction and watch whether its temperature changes as the squeeze changes. Unlike a magnetic field, which can affect every pair of states protected by time-reversal symmetry, a one-direction squeeze can lift only the degeneracies protected by the crystal’s shape.

That selective action lets the pressure probe the crystal’s internal levels and separate their effects from the other energy scale associated with electron binding. The measurement applies a small repeating change in pressure and records the resulting temperature change with a thermometer attached to the sample.

A steady background strain can also be added, allowing the temperature response to be tracked as that strain is varied in the experiment. The arrangement establishes the point of zero pressure precisely, which is essential for using the crystal’s symmetry to interpret the response.

The temperature response changes strongly as the squeeze changes, while the response right at zero squeeze is small. This means the main effect comes from a pressure-induced splitting of the first excited level. That level must be the four-state level, because the two-state levels are protected by time-reversal symmetry and cannot be split by this kind of squeeze.

Squeezing the crystal creates a sizable temperature swing, while two distinct temperature landmarks respond differently: one shifts strongly, near three kelvin, while the other stays near eight kelvin. That contrast points to pressure splitting an excited energy level rather than simply moving it.

The analysis places the four-state level as the first excited state, separated from the two-state ground level by about one and a half energy units. But one feature remains that this single-ion picture cannot explain, and it persists at every applied pressure.

The remaining response is therefore attributed to energy scales other than the four-state level. The study finds that the gap to the first excited four-state level is much smaller than the proposed energy scale associated with heavy-electron behavior, placing both levels below it.

That leaves open the possibility that the conduction electrons interact with a combined group of six nearby states. This gives researchers a clearer way to identify hidden crystal-field levels and establish the hierarchy of energy scales in YbPtBi. It also helps explain why electrons in this material can behave as extraordinarily heavy, because enhanced degeneracy of the crystal-field levels can support that large electron mass.

The crystal’s first excited group of states sits far below the energy scale that binds electrons into heavy behavior, so several states may work together to create that unusual mass. A gentle squeeze makes those hidden levels measurable.