Strange metals

Cool a metal, pass a small current through it, and measure its opposition to that current. In a 2021 study of YbRh2Si2, a compound of ytterbium, rhodium and silicon, its resistivity—resistance with the sample’s shape factored out—varied almost linearly with temperature in its nonsuperconducting state near a particular magnetic-field setting. Equal temperature steps gave roughly equal changes. The usual low-temperature picture of collisions between electron-like carriers instead gives a contribution proportional to temperature squared. What collective electron motion produces this strange-metal response?[2] [3]

A metal’s resistivity measures its opposition to current with sample geometry factored out. In the usual metal description, an electron and its surroundings act as a long-lived entity called a quasiparticle. Some strongly interacting metals have a low-temperature resistivity contribution that varies almost linearly with temperature rather than quadratically, challenging this description. Which collective processes undermine those carriers or replace their transport picture?

Write the measured resistivity as a temperature-independent residual value plus a changing part. For an illustrative comparison within a valid regime, doubling the absolute temperature, measured in kelvin, doubles a part proportional to T, but quadruples one proportional to T². This arithmetic is not an additional measurement; it shows why the temperature dependence is informative. In ordinary low-temperature Fermi-liquid transport, the square-law contribution arises from scattering between well-defined quasiparticles.

In heavy-fermion materials, the carriers respond as if they had unusually large masses. The Hu–Chen–Si review develops an explanation for these systems. Localized electron spins interact with the mobile electrons; the Kondo effect helps assemble heavy composite carriers. Competing interactions between those localized spins can undermine that assembly. At the proposed Kondo-destruction quantum critical point—a continuous phase change at zero temperature—the collective charge response and the disappearance of heavy quasiparticles are linked. Temperature-dependent transport, changes in the Hall response—a transverse voltage in a magnetic field—and spectroscopic probes offer different checks on that account.[2] [3]

The resistivity curve is a clue, not a complete description of the moving electrons. Some theories explain particular materials, but researchers still need to establish when the usual electron-like carriers break down and whether similar curves in copper oxides and other metals have the same cause.[1] [3]

Linearity alone does not show that quasiparticles have vanished: lattice vibrations can also produce a linear contribution in ordinary metals. The review notes how hard direct spectral tests of quasiparticle destruction are in heavy-fermion systems. A mechanism must explain several responses together and their change with field, pressure or composition. Even success in one heavy-fermion compound would leave open which ingredients, if any, transfer to cuprates.[2] [3]

Electrical resistance depends on how electrons lose directed motion. These metals can reveal how strong interactions reshape current flow. In some of them superconductivity also develops nearby, making the normal current-carrying state a useful clue to how electrons cooperate in pairs.[2] [3]

This is a test of the building blocks of many-electron theory. If long-lived carriers fail, calculations based on their scattering can misidentify how a material transports charge. Connecting transport with the competing magnetic interactions and superconducting pairing would explain how one system changes from dissipating current to supporting a collective superconducting state. Whether that connection extends between material families is itself a useful test of the explanation.[2] [3]