In two-layer graphene rotated through a "magic" angle, a rare linear dependence of the electrical resistance on temperature near absolute zero was found. This feature makes double-layer graphene related to an unusual class of substances called strange metals. It includes, for example, cuprates, including the record holders for superconductivity temperature at normal pressure, as well as ruthenates, pnictides, and some other materials. The discovery confirms the presence of a new fundamental mechanism of charge and heat transfer in such compounds, write the authors in the journal Physical Review Letters.
Graphene is a two-dimensional allotropic modification of carbon, consisting of atoms arranged in the form of hexagons, united in sheets of atomic thickness. Graphene has many unusual properties that are potentially useful in science and technology. However, scientists continue to discover new unusual characteristics of this material.
One of the important discoveries of the last two years has been the discovery of superconductivity in bilayer graphene. Rotating the sheets by a small angle creates a periodic moiré hexagonal superlattice with a much longer period than that of graphene itself. If the angle takes one of the "magic" values, the smallest of which is close to 1.1 degrees, then at low temperatures the substance goes into a superconducting state. Detailed studies have shown that such graphene in some properties, in particular, the phase diagram, is similar to cuprates - compounds, with the discovery of which the term high-temperature superconductivity appeared.
Pablo Jarillo-Herrero from the Massachusetts Institute of Technology and his colleagues from the United States and Japan have discovered another feature that makes double-layer graphene rotated by a "magic" angle similar to cuprates: the presence of a strange metal phase with a linear dependence of resistance on temperature near absolute zero. Such a regularity is not observed for ordinary metals, in which, as a rule, a sharp increase in resistance occurs after the superconducting phase. Moreover, at the moment there is no full theoretical explanation for this phenomenon.
For a long time, electron transport in metals was successfully described by the Drude theory, formulated in 1900, which relates the conductivity to the density of electrons considered as a gas, their mass and the average time τ between scattering by ions. With quantum corrections that replaced the mass of real particles by the effective mass of charge carriers and linked the time between scattering at low temperatures by a proportionality τ ∼ T-2, this model successfully described most of the experimental data up to the 1980s.
The discovery of cuprates in 1986 demonstrated the limitations of the theory, which could not explain the phase of a strange metal observed in them with a linear dependence of resistance on temperature. This behavior suggests that the time between scatterings is inversely proportional to the first power of the temperature, and not to the square, as in the Drude model. The discovery of the strange metal phase in bilayer graphene further indicates the need to develop a new theoretical approach to transport phenomena and speaks of the possibility of such a phase in many different systems.
If we calculate the time between scattering in strange metals using the Drude formula (which is poorly substantiated from a theoretical point of view), then we get the expression τ = Cℏ ∕ kT, where ℏ is Planck's constant, T is temperature, k is Boltzmann's constant, and C is a numerical coefficient proportionality. It is believed that the scattering rate must be related to the strength of the electron-electron interactions (which are completely ignored in the original Drude model), and they are very different in various strange metals.
However, observations show that the C coefficient is close to unity for a wide variety of strange metals and, as it turns out, for two-layer graphene as well: in the new work, the measured C values fell in the range from 1.1 to 1.6. This universality leads theorists to believe that there is a new fundamental mechanism for transport phenomena in strange metals. Scientists associate this situation with Planckian dissipation, that is, the state of quantum entanglement of many electrons, in which the maximum rate of energy dissipation allowed by the laws of physics is reached.
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Two-layer graphene may turn out to be a convenient system for continuing experiments in this field. Its main advantage lies in the ability to control the filling factor of the superlattice, that is, in fact, the density of charge carriers, by applying an electric voltage, while other strange metals must be manufactured anew with other impurities.
Timur Keshelava
