Periodic Reporting for period 4 - FIRSTORM (Modeling first-order Mott transitions)
Reporting period: 2021-03-01 to 2023-02-28
The Mott transition is a fascinating phenomenon, especially since it is frequently surrounded by a wealth of different phases with intriguing physical properties. Furthermore, Mott insulators are also of potential interest for technological applications. Indeed, a Mott insulator is essentially an “unsuccessful metal” that possesses a large density of carriers whose motion at equilibrium is however impeded by Coulomb repulsion, but which could, in principle, become all of a sudden available for transport in non-equilibrium conditions.
It’s a given that Mott transitions in real materials have all first order character, frequently quite a strong one, thus featuring broad metal-insulator coexistence that
offers unique opportunities unattainable in more conventional band insulators. Its potentials revealed themselves in a series of remarkable experiments.
For instance, the dielectric breakdown of narrow gap Mott insulators resembles more a genuine resistive transition, rather than a Landau-Zener tunnelling breakdown as in conventional semiconductors. This unusual behaviour may be consequence of the electric field stabilising a metal phase that existed also in absence of the field but only as a metastable state.
Even more, there are abundant evidences that non-thermal states of matter with notable properties can be reached by shooting with properly designed laser pulses correlated materials, both metallic and insulating.
We just mention the striking series of pump-probe experiments by Cavalleri’s group in Hamburg, see Mitrano et al., Nature 530, 461 (2016), and Budden et al., Nature Physics 17, 611 (2021), which report a superconducting-like optical response well above the equilibrium critical temperature in K3C60 molecular conductor shot by a mid-infrared laser pulse. Such transient non-thermal phase can be stabilized for several nanoseconds by properly tailoring the laser pulse. This phenomenon is truly unexpected since, in common experience, shooting a material by a laser should heat it, whereas the experiments detect a behaviour that is observed only at much lower temperatures.
More recently, a new platform to investigate strong correlations has been discovered in small angle twisted-bilayer graphene, see Cao et al., Nature 556, 43 and 80 (2018). Surprisingly, this simple system made of carbon atoms displays a wealth of phases upon doping the flat bands around the charge neutrality point, including Chern insulators that hint at the intriguing possibility of topological Mott insulators.
Finally, a new inroad in the ever-surprising physics of correlated systems is the evidence that several Mott insulators display quasiparticle excitations like those of metals, see, e.g. Sato et al., Nature Physics 15, 954 (2019), and Ruan et al., Nature Physics 17, 1154 (2021).
The goal of this project has been to theoretically piece together this blooming experimental evidence that has just one thing in common: the materials under investigation are all correlated ones.
It’s a given that Mott transitions in real materials have all first order character, frequently quite a strong one, thus featuring broad metal-insulator coexistence that
offers unique opportunities unattainable in more conventional band insulators. Its potentials revealed themselves in a series of remarkable experiments.
For instance, the dielectric breakdown of narrow gap Mott insulators resembles more a genuine resistive transition, rather than a Landau-Zener tunnelling breakdown as in conventional semiconductors. This unusual behaviour may be consequence of the electric field stabilising a metal phase that existed also in absence of the field but only as a metastable state.
Even more, there are abundant evidences that non-thermal states of matter with notable properties can be reached by shooting with properly designed laser pulses correlated materials, both metallic and insulating.
We just mention the striking series of pump-probe experiments by Cavalleri’s group in Hamburg, see Mitrano et al., Nature 530, 461 (2016), and Budden et al., Nature Physics 17, 611 (2021), which report a superconducting-like optical response well above the equilibrium critical temperature in K3C60 molecular conductor shot by a mid-infrared laser pulse. Such transient non-thermal phase can be stabilized for several nanoseconds by properly tailoring the laser pulse. This phenomenon is truly unexpected since, in common experience, shooting a material by a laser should heat it, whereas the experiments detect a behaviour that is observed only at much lower temperatures.
More recently, a new platform to investigate strong correlations has been discovered in small angle twisted-bilayer graphene, see Cao et al., Nature 556, 43 and 80 (2018). Surprisingly, this simple system made of carbon atoms displays a wealth of phases upon doping the flat bands around the charge neutrality point, including Chern insulators that hint at the intriguing possibility of topological Mott insulators.
Finally, a new inroad in the ever-surprising physics of correlated systems is the evidence that several Mott insulators display quasiparticle excitations like those of metals, see, e.g. Sato et al., Nature Physics 15, 954 (2019), and Ruan et al., Nature Physics 17, 1154 (2021).
The goal of this project has been to theoretically piece together this blooming experimental evidence that has just one thing in common: the materials under investigation are all correlated ones.
The project has proceeded in many directions bringing about several results. I will herein present what I repute the main ones.
1. We have proposed [1] that the photoinduced superconductivity observed in K3C60 realizes a novel laser cooling mechanism made possible by the dual nature, partly metallic and partly insulating, of the correlated molecular conductor K3C60. Our transient cooling mechanism is quite general, as we discuss in [2] and pictorially explained in Fig. 1, and it might be realized in other correlated metals, too. It is sufficient that the metal hosts localized excitations, which can serve as entropy sink when the laser is on, and which very gradually release back that entropy when the laser is off.
2. Upon lowering temperature, V2O3 undergoes a first order transition from a rhombohedral corundum metal into a monoclinic antiferromagnetic Mott insulator. The structural transition is martensitic and leads to the coexistence of monoclinic twins. It is therefore worth exploring the entangled dynamics of electronic and structural degrees of freedom across the transition. Our experimental colleagues designed a nice experiment to detect through the dichroic signal of photoemission microscopy the real space dynamic of monoclinic and corundum domains by increasing T or by photoexciting the sample. At the meantime, we built a Landau-Ginzburg free-energy functional that describes both the shear strain driving the structural transition and the electronic parameter distinguishing the metal from the insulator. This functional reproduces well the experimental evidence and suggests that the presence of domains can be exploited to gain full control of the insulator-metal transition in V2O3, see Ref. [3] and Fig. 2.
3. Small-angle twisted bilayer graphene form a moiré superstructure with a cell containing tens of thousand of C atoms. We found that the normal modes of the superlattice include very special ones, almost nondispersive and modulated on the large supercell length scale, see Fig.3 and Ref. [4]. These modes were found to be extremely efficient in breaking the emergent valley symmetry leading to topological insulating phases, see Fig. 4 and Ref. [5], not in disagreement with those observed experimentally.
4. There is evidence in several Mott insulators of quasiparticle excitations like those of metals, and great interest in spinon Fermi surfaces within spin-liquid insulators. We have shown in Refs. [6] that these properties are consistent with Landau’s Fermi liquids within the Luttinger surface scenario, which can also explain the existence of quantum oscillations in insulators.
[1] Cooling quasiparticles in A3C60 fullerides by excitonic mid-infrared absorption, A. Nava, C. Giannetti, A. Georges, E. Tosatti and M. Fabrizio, Nature Physics 14, 154 (2018).
[2] Selective transient cooling by impulse perturbations in a simple toy model, M. Fabrizio, Physical Review Letters 120, 22061 (2018).
[3] Nanoscale self-organization and metastable non-thermal metallicity in Mott insulators, A. Ronchi et. al, Nature Communications 13, 3730 (2022).
[4] Valley Jahn-Teller effect in twisted bilayer graphene, M. Angeli, E. Tosatti, and M. Fabrizio, Physical Review X 9, 041010 (2019).
[5] Local Kekulé distortion turns twisted bilayer graphene into topological Mott insulators and superconductors, A. Blason and M. Fabrizio, Physical Review B 106, 235112 (2022).
[6] Emergent quasiparticles at Luttinger surfaces, M. Fabrizio, Nature Communications 13, 1561 (2022); Spin liquid insulators can be Fermi liquids, M. Fabrizio, Physical Review Letters 130, 156702 (2023).
1. We have proposed [1] that the photoinduced superconductivity observed in K3C60 realizes a novel laser cooling mechanism made possible by the dual nature, partly metallic and partly insulating, of the correlated molecular conductor K3C60. Our transient cooling mechanism is quite general, as we discuss in [2] and pictorially explained in Fig. 1, and it might be realized in other correlated metals, too. It is sufficient that the metal hosts localized excitations, which can serve as entropy sink when the laser is on, and which very gradually release back that entropy when the laser is off.
2. Upon lowering temperature, V2O3 undergoes a first order transition from a rhombohedral corundum metal into a monoclinic antiferromagnetic Mott insulator. The structural transition is martensitic and leads to the coexistence of monoclinic twins. It is therefore worth exploring the entangled dynamics of electronic and structural degrees of freedom across the transition. Our experimental colleagues designed a nice experiment to detect through the dichroic signal of photoemission microscopy the real space dynamic of monoclinic and corundum domains by increasing T or by photoexciting the sample. At the meantime, we built a Landau-Ginzburg free-energy functional that describes both the shear strain driving the structural transition and the electronic parameter distinguishing the metal from the insulator. This functional reproduces well the experimental evidence and suggests that the presence of domains can be exploited to gain full control of the insulator-metal transition in V2O3, see Ref. [3] and Fig. 2.
3. Small-angle twisted bilayer graphene form a moiré superstructure with a cell containing tens of thousand of C atoms. We found that the normal modes of the superlattice include very special ones, almost nondispersive and modulated on the large supercell length scale, see Fig.3 and Ref. [4]. These modes were found to be extremely efficient in breaking the emergent valley symmetry leading to topological insulating phases, see Fig. 4 and Ref. [5], not in disagreement with those observed experimentally.
4. There is evidence in several Mott insulators of quasiparticle excitations like those of metals, and great interest in spinon Fermi surfaces within spin-liquid insulators. We have shown in Refs. [6] that these properties are consistent with Landau’s Fermi liquids within the Luttinger surface scenario, which can also explain the existence of quantum oscillations in insulators.
[1] Cooling quasiparticles in A3C60 fullerides by excitonic mid-infrared absorption, A. Nava, C. Giannetti, A. Georges, E. Tosatti and M. Fabrizio, Nature Physics 14, 154 (2018).
[2] Selective transient cooling by impulse perturbations in a simple toy model, M. Fabrizio, Physical Review Letters 120, 22061 (2018).
[3] Nanoscale self-organization and metastable non-thermal metallicity in Mott insulators, A. Ronchi et. al, Nature Communications 13, 3730 (2022).
[4] Valley Jahn-Teller effect in twisted bilayer graphene, M. Angeli, E. Tosatti, and M. Fabrizio, Physical Review X 9, 041010 (2019).
[5] Local Kekulé distortion turns twisted bilayer graphene into topological Mott insulators and superconductors, A. Blason and M. Fabrizio, Physical Review B 106, 235112 (2022).
[6] Emergent quasiparticles at Luttinger surfaces, M. Fabrizio, Nature Communications 13, 1561 (2022); Spin liquid insulators can be Fermi liquids, M. Fabrizio, Physical Review Letters 130, 156702 (2023).
Besides the above main achievements obtained within the project, many other works have been performed on related topics, each of them representing an advance beyond the state of the art as testified by the over sixty papers authored by the team members and published on renowned scientific journals.