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Physics Today

From black holes to strange metals

Hong Liu

Published Jun 1, 2012• 35 citations
Publisher

Abstract

String theory relates gravity to the physics of a novel phase of matter observed above the superconducting transition temperature.
DOI
10.1063/pt.3.1616

Related papers

Strange Metal Transport Realized by Gauge/Gravity Duality

Thomas Faulkner, Nabil Iqbal, Hong Liu +2 more

Black Holes as Tools When confronted with a difficult problem, physicists often resort to mapping it to a more tractable one. A good example of this strategy is provided by new developments linking string theory and condensed-matter physics to make theoretical connections between gravity and complex systems of interacting electrons. This theoretical convergence provides a description of Fermi liquids, which can be thought of as interacting systems of electrons whose excitations can be expressed in terms of non-interacting quasiparticles. Several interesting systems elude quasiparticle description, but Faulkner et al. (p. 1043 , published online 5 August) have now developed a mathematical framework that describes the non-Fermi liquid represented by the strange metal phase of cuprate high-temperature superconductors. They calculate the electronic response and, for a particular value of a tunable parameter, recover the linear resistivity. Further development of this framework may allow elucidation of other exotic properties of the cuprates and similar complex systems.

Science•Aug 6, 2010•163 citations

Strange metals and quantum phase transitions from gauge/gravity duality

Hong Liu

Bulletin of the American Physical Society•Mar 21, 2011•0 citations

Strange connections to strange metals

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Liu replies: While many ingenious ideas have been advanced to explain the strange-metal phase, the community has not reached a consensus, and many of the phase’s properties remain mysterious. Philip W. Anderson’s hidden Fermi liquid theory, arguably a most ambitious attempt, has provided a useful lens to view various aspects of the phase, but further developments are worthwhile.As described in my Quick Study, a key obstacle in the pursuit to explain the strange metal phase is the lack of a suitable mathematical framework to characterize “quantum soups”—strongly interacting, dense quantum matter whose constituents have lost their individuality. Holographic duality has now provided such a framework, although only for the class of systems that have gravity duals. It does for those quantum soups what Boltzmann equations do for a dilute gas of quasiparticles: It enables one to calculate reliably all equilibrium and nonequilibrium properties.At short distances, holographic systems are conformal, Lorentz invariant, and often supersymmetric, all of which make them seem poor models for real-life condensed-matter systems. Those symmetries, however, are all destroyed at long distances if a temperature or a chemical potential is turned on. The surprise of the past few years is that despite significant differences in short-distance details, at long distances most states of matter that have been understood—superfluids, Fermi liquids, magnets, stripes, and so on—can be described in terms of dual gravity systems. Such insensitivity to microscopic detail gives hope that the striking parallels in macroscopic behavior found between black holes and strange metals are not accidents, and gravity may help us decipher the mysteries of strange metals.I thank Anderson for emphasizing other anomalies exhibited by the strange-metal phase of cuprates that I did not have space to mention. He is correct that the holographic approach has not yet produced a model that could account for all anomalous properties. However, the power-law dependence of photoemission spectra on energy, his last bullet point, is a hallmark of holographic strange metals. I discussed that point in the last part of the Quick Study, in terms of the power-law temperature dependence of scattering rates. Such power laws follow from the semilocal property emphasized in the second-to-last paragraph.Whether or not one finds a “conventional” explanation for strange metals, connections between the physics of strange metals and black holes are worth exploring. They hint at a new paradigm for thinking about strongly correlated quantum soups. As an added bonus, we may also obtain new insights into quantum gravity from advances in condensed-matter physics.© 2013 American Institute of Physics.

Physics Today•Mar 29, 2013•1 citation

A predictive framework for quantum gravity and black hole to white hole transition

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Planckian dissipation, minimal viscosity and the transport in cuprate strange metals

Jan Zaanen

Could it be that the matter from the electrons in high Tc superconductors is of a radically new kind that may be called "many body entangled compressible quantum matter"? Much of this text is intended as an easy to read tutorial, explaining recent theoretical advances that have been unfolding at the cross roads of condensed matter- and string theory, black hole physics as well as quantum information theory. These developments suggest that the physics of such matter may be governed by surprisingly simple principles. My real objective is to present an experimental strategy to test critically whether these principles are actually at work, revolving around the famous linear resistivity characterizing the strange metal phase. The theory suggests a very simple explanation of this "unreasonably simple" behavior that is actually directly linked to remarkable results from the study of the quark gluon plasma formed at the heavy ion colliders: the "fast hydrodynamization" and the "minimal viscosity". This leads to high quality predictions for experiment: the momentum relaxation rate governing the resistivity relates directly to the electronic entropy, while at low temperatures the electron fluid should become unviscous to a degree that turbulent flows can develop even on the nanometre scale.

•Jan 1, 2019•146 citations
Black Holes as Tools When confronted with a difficult problem, physicists often resort to mapping it to a more tractable one. A good example of this strategy is provided by new developments linking string theory and condensed-matter physics to make theoretical connections between gravity and complex systems of interacting electrons. This theoretical convergence provides a description of Fermi liquids, which can be thought of as interacting systems of electrons whose excitations can be expressed in terms of non-interacting quasiparticles. Several interesting systems elude quasiparticle description, but Faulkner et al. (p. 1043 , published online 5 August) have now developed a mathematical framework that describes the non-Fermi liquid represented by the strange metal phase of cuprate high-temperature superconductors. They calculate the electronic response and, for a particular value of a tunable parameter, recover the linear resistivity. Further development of this framework may allow elucidation of other exotic properties of the cuprates and similar complex systems.
Liu replies: While many ingenious ideas have been advanced to explain the strange-metal phase, the community has not reached a consensus, and many of the phase’s properties remain mysterious. Philip W. Anderson’s hidden Fermi liquid theory, arguably a most ambitious attempt, has provided a useful lens to view various aspects of the phase, but further developments are worthwhile.As described in my Quick Study, a key obstacle in the pursuit to explain the strange metal phase is the lack of a suitable mathematical framework to characterize “quantum soups”—strongly interacting, dense quantum matter whose constituents have lost their individuality. Holographic duality has now provided such a framework, although only for the class of systems that have gravity duals. It does for those quantum soups what Boltzmann equations do for a dilute gas of quasiparticles: It enables one to calculate reliably all equilibrium and nonequilibrium properties.At short distances, holographic systems are conformal, Lorentz invariant, and often supersymmetric, all of which make them seem poor models for real-life condensed-matter systems. Those symmetries, however, are all destroyed at long distances if a temperature or a chemical potential is turned on. The surprise of the past few years is that despite significant differences in short-distance details, at long distances most states of matter that have been understood—superfluids, Fermi liquids, magnets, stripes, and so on—can be described in terms of dual gravity systems. Such insensitivity to microscopic detail gives hope that the striking parallels in macroscopic behavior found between black holes and strange metals are not accidents, and gravity may help us decipher the mysteries of strange metals.I thank Anderson for emphasizing other anomalies exhibited by the strange-metal phase of cuprates that I did not have space to mention. He is correct that the holographic approach has not yet produced a model that could account for all anomalous properties. However, the power-law dependence of photoemission spectra on energy, his last bullet point, is a hallmark of holographic strange metals. I discussed that point in the last part of the Quick Study, in terms of the power-law temperature dependence of scattering rates. Such power laws follow from the semilocal property emphasized in the second-to-last paragraph.Whether or not one finds a “conventional” explanation for strange metals, connections between the physics of strange metals and black holes are worth exploring. They hint at a new paradigm for thinking about strongly correlated quantum soups. As an added bonus, we may also obtain new insights into quantum gravity from advances in condensed-matter physics.© 2013 American Institute of Physics.
Could it be that the matter from the electrons in high Tc superconductors is of a radically new kind that may be called "many body entangled compressible quantum matter"? Much of this text is intended as an easy to read tutorial, explaining recent theoretical advances that have been unfolding at the cross roads of condensed matter- and string theory, black hole physics as well as quantum information theory. These developments suggest that the physics of such matter may be governed by surprisingly simple principles. My real objective is to present an experimental strategy to test critically whether these principles are actually at work, revolving around the famous linear resistivity characterizing the strange metal phase. The theory suggests a very simple explanation of this "unreasonably simple" behavior that is actually directly linked to remarkable results from the study of the quark gluon plasma formed at the heavy ion colliders: the "fast hydrodynamization" and the "minimal viscosity". This leads to high quality predictions for experiment: the momentum relaxation rate governing the resistivity relates directly to the electronic entropy, while at low temperatures the electron fluid should become unviscous to a degree that turbulent flows can develop even on the nanometre scale.