A New State of Matter –“Black Hole Physics of Strange Metals”

A New State of Matter --"The Black Hole Physics of Strange Metals"

 

“Not only does God play dice but… he sometimes throws them where they cannot be seen,” said Stephen Hawking about the paradoxical physics of black Holes. Welcome to the bizarre quantum world of “strange metals” –a new state of matter.

“The fact that we call them strange metals should tell you how well we understand them. Strange metals share remarkable properties with black holes, opening exciting new directions for theoretical physics,” says Olivier Parcollet, a senior research scientist at the Flatiron Institute’s Center for Computational Quantum Physics (CCQ), about the quantum world of metals that dissipate energy as fast as they’re allowed to under the laws of quantum mechanics. The electrical resistivity of a strange metal, unlike that of ordinary metals, is proportional to the temperature.

Even by the standards of quantum physicists, reports the Flatiron Institute, strange metals are just plain odd. Generating a theoretical understanding of strange metals is one of the biggest challenges in condensed matter physics. Now, using cutting-edge computational techniques, researchers from the Flatiron Institute and Cornell University have solved the first robust theoretical model of strange metals. The work reveals that strange metals are a new state of matter, the researchers report July 22 in the Proceedings of the National Academy of Sciences.

In the quantum mechanical world, electrical resistance is a byproduct of electrons bumping into things. As electrons flow through a metal, they bounce off other electrons or impurities in the metal. The more time there is between these collisions, the lower the material’s electrical resistance.

“Shape-Shifting Cosmos” — Physicists Seek the Question to Which the Universe is the Answer

For typical metals, electrical resistance increases with temperature, following a complex equation. But in unusual cases, such as when a high-temperature superconductor is heated just above the point where it stops superconducting, the equation becomes much more straightforward. In a strange metal, electrical conductivity is linked directly to temperature and to two fundamental constants of the universe: Planck’s constant and Boltzmann’s constant.

Planck’s fundamental physical constant (H) links the amount of energy a photon carries with the frequency of its electromagnetic wave. Boltzmann’s constant of physics (k) occurs in nearly every statistical formulation of both classical and quantum physics. The Planck scale sets the universe’s minimum limit, beyond which the laws of physics break down.

Also Known as Planckian Metals

“In the late 1890s,” observes Symmetry, “physicist Max Planck proposed a set of units to simplify the expression of physics laws. Using just five constants in nature (including the speed of light and the gravitational constant), you, me and even aliens from Alpha Centauri could arrive at these same Planck units.”

Consequently, strange metals are also known as Planckian metals.

“Quantum Sapiens” –Measuring Quantum Effects at the Human Scale

Models of strange metals have existed for decades, but accurately solving such models proved out of reach with existing methods. Quantum entanglements between electrons mean that physicists can’t treat the electrons individually, and the sheer number of particles in a material makes the calculations even more daunting.

Cornell doctoral candidate Peter Cha and his colleagues employed two different methods to crack the problem. First, they used a quantum embedding method based on ideas developed by CCQ director Antoine Georges in the early ’90s. With this method, instead of performing detailed computations across the whole quantum system, physicists perform detailed calculations on only a few atoms and treat the rest of the system more simply. They then used a quantum Monte Carlo algorithm (named for the Mediterranean casino), which uses random sampling to compute the answer to a problem. The researchers solved the model of strange metals down to absolute zero (minus 273.15 degrees Celsius), the unreachable lower limit for temperatures in the universe.

Strange Metals as a New State of Matter

The resulting theoretical model reveals the existence of strange metals as a new state of matter bordering two previously known phases of matter: Mott insulating spin glasses and Fermi liquids.

“We found there is a whole region in the phase space that is exhibiting a Planckian behavior that belongs to neither of the two phases that we’re transitioning between,” says Cornell physics professor Eun-Ah Kim. “This quantum spin liquid state is not so locked down, but it’s also not completely free. It is a sluggish, soupy, slushy state. It is metallic but reluctantly metallic, and it’s pushing the degree of chaos to the limit of quantum mechanics.”

“Ringing Black Holes”

The new work could help physicists better understand the physics of higher-temperature superconductors. Perhaps surprisingly, the work has links to astrophysics. Like strange metals, black holes exhibit properties that depend only on temperature and the Planck and Boltzmann constants, such as the amount of time a black hole ‘rings’ after merging with another black hole. “The fact that you find this same scaling across all these different systems, from Planckian metals to black holes, is fascinating,” Parcollet says.

In addition to Parcollet, the research team consisted  Cha, CCQ associate data scientist Nils Wentzell, CCQ director Georges, and Cornell physics professor Eun-Ah Kim.

Source: Peter Cha et al, Linear resistivity and Sachdev-Ye-Kitaev (SYK) spin liquid behavior in a quantum critical metal with spin-1/2 fermions, Proceedings of the National Academy of Sciences (2020). DOI: 10.1073/pnas.2003179117

The Daily Galaxy, Max Goldberg via Symmetry and The Simons Foundation

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