Publications

Publications

Here you can find and download the publications related to the ERC PROJECT - FIRSTORM.

Emergent D6 symmetry in fully-relaxed magic-angle twisted bilayer graphene

Journal: 

Phys. Rev. B 98, 235137

Date: 

October, 2018

We present a tight-binding calculation of a twisted bilayer graphene at magic angle θ∼1.08°, allowing for full, in- and out-of-plane, relaxation of the atomic positions. The resulting band structure displays as usual four narrow mini bands around the neutrality point, well separated from all other bands after the lattice relaxation. A thorough analysis of the mini-bands Bloch functions reveals an emergent D6 symmetry, despite the lack of any manifest point group symmetry in the relaxed lattice. The Bloch functions at the Γ point are degenerate in pairs, reflecting the so-called valley degeneracy. Moreover, each of them is invariant under C3z, i.e., transforming like one-dimensional, in-plane symmetric irreducible representation of an "emergent" D6 group. Out of plane, the lower doublet is even under C2x, while the upper doublet is odd, which implies that at least eight Wannier orbitals, two s-like and two pz-like for each of the two supercell sublattices AB and BA are necessary, probably not sufficient, to describe the four mini bands. This unexpected one-electron complexity is likely to play an important role in the still unexplained metal-insulator-superconductor phenomenology of this system.

Charge disproportionation, mixed valence, and Janus effect in multiorbital systems: A tale of two insulators

Journal: 

Phys. Rev. Lett. 122, 186401

Date: 

October, 2018

Multiorbital Hubbard models host strongly correlated 'Hund's metals' even for interactions much stronger than the bandwidth. We characterize this interaction-resilient metal as a mixed-valence state. In particular it can be pictured as a bridge between two strongly correlated insulators: a high-spin Mott insulator and a charge-disproportionated insulator which is stabilized by a very large Hund's coupling. This picture is confirmed comparing models with negative and positive Hund's coupling for different fillings. Our results provide a characterization of the Hund's metal state and connect its presence with charge disproportionation, which has indeed been observed in chromates and proposed to play a role in iron-based superconductors.

Exciton Mott transition revisited

Date: 

October, 2018

The dissociation of excitons into holes and electrons in photoexcited semiconductors, despite being one of the first recognized examples of a Mott transition, still defies a complete understanding, especially regarding the character of the transition, which is first order in some cases and second order in others. Here we consider an idealized model of photoexcited semiconductors that can be mapped onto a spin-polarised half-filled Hubbard model, whose phase diagram reproduces most of the phenomenology of photoexcited semiconductors and uncovers the key role of the exciton binding energy in determining the order of the exciton Mott transition.

Nanoscale dynamics across the Mott transition in V2O3

Date: 

July, 2018

The phase diagrams of 3d metal oxides provide rich landscapes to explore the non-equilibrium degrees of freedoms during an insulator-to-metal transition (IMT). In these materials, the dynamics of nano-textured insulating and metallic phases is characterized by an unexplored complexity than enables manipulation of phase separation to control the properties of quantum materials on ultrafast timescales. Here, we combine X-ray photoemission electron microscopy and non-equilibrium optical spectroscopy to link the temporal and spatial dynamics of the IMT in the Mott insulator V2O3. We show that metallic droplets, which form at the boundaries of striped insulating domains, act as seeds for the non-equilibrium expansion of the metallic phase triggered by the photo-induced change in the 3d-orbital occupation. We demonstrate that the growth of the metallic phase can be controlled by properly tailoring the light-excitation protocol. Our results unveil the coupled electronic and structural dynamics during an ultrafast IMT and open up the possibility of controlling the ultrafast dynamics of Mott transitions in a way that is inaccessible by thermal means.

Correlation-driven Lifshitz transition and orbital order in a two-band Hubbard model

Journal: 

Phys. Rev. B 98, 045105

Date: 

July, 2018

We study by dynamical mean field theory the ground state of a quarter-filled Hubbard model of two bands with different bandwidths. At half-filling, this model is known to display an orbital selective Mott transition, with the narrower band undergoing Mott localisation while the wider one being still itinerant. At quarter-filling, the physical behaviour is different and to some extent reversed. The interaction generates an effective crystal field splitting, absent in the Hamiltonian, that tends to empty the narrower band in favour of the wider one, which also become more correlated than the former at odds with the orbital selective paradigm. Upon increasing the interaction, the depletion of the narrower band can continue till it empties completely and the system undergoes a topological Lifshitz transition into a half-filled single-band metal that eventually turns insulating. Alternatively, when the two bandwidths are not too different, a first order Mott transition intervenes before the Lifshitz's one. The properties of the Mott insulator are significantly affected by the interplay between spin and orbital degrees of freedom.

Ultrafast orbital manipulation and Mott physics in multi-band correlated materials

Journal: 

Proc. SPIE 10530, 105300V

Date: 

January, 2018

Multiorbital correlated materials are often on the verge of multiple electronic phases (metallic, insulating, super- conducting, charge and orbitally ordered), which can be explored and controlled by small changes of the external parameters. The use of ultrashort light pulses as a mean to transiently modify the band population is leading to fundamentally new results. In this paper we will review recent advances in the field and we will discuss the pos- sibility of manipulating the orbital polarization in correlated multi-band solid state systems. This technique can provide new understanding of the ground state properties of many interesting classes of quantum materials and offers a new tool to induce transient emergent properties with no counterpart at equilibrium. We will address: the discovery of high-energy Mottness in superconducting copper oxides and its impact on our understanding of the cuprate phase diagram; the instability of the Mott insulating phase in photoexcited vanadium oxides; the manipulation of orbital-selective correlations in iron-based superconductors; the pumping of local electronic excitons and the consequent transient effective quasiparticle cooling in alkali-doped fullerides. Finally, we will discuss a novel route to manipulate the orbital polarization in a a k-resolved fashion.

Selective cooling by impulse perturbations in a simple toy model

Journal: 

Phys. Rev. Lett. 120, 220601

Date: 

January, 2018

We show in a simple exactly-solvable toy model that a properly designed impulse perturbation can transiently cool down low-energy degrees of freedom at the expenses of high-energy ones that heat up. The model consists of two infinite-range quantum Ising models, one, the high-energy sector, with a transverse field much bigger than the other, the low-energy sector. The finite-duration perturbation is a spin-exchange that couples the two Ising models with an oscillating coupling strength. We find a cooling of the low-energy sector that is optimised by the oscillation frequency in resonance with the spin-exchange excitation. After the perturbation is turned off, the Ising model with low transverse field can even develop spontaneous symmetry-breaking despite being initially above the critical temperature.

Interplay of charge and spin dynamics after an interaction quench in the Hubbard model

Journal: 

Phys. Rev. B 96, 201115(R)

Date: 

November, 2017

We investigate the unitary dynamics following a sudden increase ΔU>0 of repulsion in the paramagnetic sector of the half-filled Hubbard model on a Bethe lattice, by means of a variational approach that combines a Gutzwiller wavefunction with a partial Schrieffer-Wolff transformation, both defined through time-dependent variational parameters. Besides recovering at ΔUc the known dynamical transition linked to the equilibrium Mott transition, we find pronounced dynamical anomaly at larger ΔU∗>ΔUcmanifested in a singular behaviour of the long-time average of double occupancy. Although the real-time dynamics of the variational parameters at ΔU∗ strongly resembles the one at ΔUc, careful frequency spectrum analysis suggests a dynamical crossover, instead of a dynamical transition, separating regions of a different behaviour of the spin-exchange.

Mott physics beyond Brinkman-Rice scenario

Journal: 

Phys. Rev. B 95, 161106

Date: 

October, 2017

The main flaw of the well-known Brinkman-Rice description, obtained through the Gutzwiller approximation, of the paramagnetic Mott transition in the Hubbard model is in neglecting high-energy virtual processes that generate for instance the antiferromagnetic exchange Jt2/U. Here we propose a way to capture those processes by combining the Brinkman-Rice approach with a variational Schrieffer-Wolff transformation, and apply this method to study the single-band metal-to-insulator transition in a Bethe lattice with infinite coordination number, where the Gutzwiller approximation becomes exact. We indeed find for the Mott transition a description very close to the real one provided by dynamical mean-field theory; an encouraging result in view of possible applications to more involved models.

Many-body breakdown of indirect gap in topological Kondo insulators

Journal: 

Phys. Rev. B 94, 121102(R)

Date: 

October, 2017

We show that the inclusion of nonlocal correlation effects in a variational wave function for the ground state of a topological Anderson lattice Hamiltonian is capable of describing both topologically trivial insulating phases and nontrivial ones characterized by an indirect gap, as well as its closure at the transition into a metallic phase. The method, though applied to an oversimplified model, thus captures the metallic and insulating states that are indeed observed in a variety of Kondo semiconductors, while accounting for topologically nontrivial band structures.

Manumitting slave-spins in the Anderson impurity model

Journal: 

Phys. Rev. B 96, 201106

Date: 

July, 2017

We show that a generic single-orbital Anderson impurity model, lacking for instance any kind of particle-hole symmetry, can be exactly mapped without any constraint onto a resonant level model coupled to two Ising variables, which reduce to one if the hybridisation is particle-hole symmetric. The mean-field solution of this model is found to be stable to unphysical spontaneous magnetisation of the impurity, unlike the saddle-point solution in the standard slave-boson representation. Remarkably, the mean-field estimate of the Wilson ratio approaches the exact value RW=2 in the Kondo regime.We show that a generic single-orbital Anderson impurity model, lacking for instance any kind of particle-hole symmetry, can be exactly mapped without any constraint onto a resonant level model coupled to two Ising variables, which reduce to one if the hybridisation is particle-hole symmetric. The mean-field solution of this model is found to be stable to unphysical spontaneous magnetisation of the impurity, unlike the saddle-point solution in the standard slave-boson representation. Remarkably, the mean-field estimate of the Wilson ratio approaches the exact value RW=2 in the Kondo regime.

Two-Band s± Strongly Correlated Superconductivity in K3 p-Terphenyl?

Date: 

May, 2017

A new organic superconductor, possibly with formula K3 p-terphenyl, has been discovered by Wang, Gao, Huang and Chen, reaching a very high Tc of about 120~K. Besides a clear diamagnetic signal, most other details such as stoichiometry and structure are yet unknown. However, pristine p-terphenyl has a familiar P21/a staggered bimolecular structure, and it can be reasonably assumed that a similar bimolecular structure could be retained by hypothetical K3 p-terphenyl. We point out that the resulting 2-narrow band metal would support the same s± superconductivity recently proposed for doped polycyclic aromatic hydrocarbons such as La-phenanthrene or K3-picene. In that model, narrow bands, a large Hubbard U and the neighbourhood of a Mott transition enhance superconductivity rather than damaging it.

Cooling quasiparticles in A3C60 fullerides by excitonic mid-infrared absorption

Journal: 

Nature Physics DOI:10.1038/nphys4288

Date: 

April, 2017

Long after its discovery superconductivity in alkali fullerides A3C60 still challenges conventional wisdom. The freshest inroad in such ever-surprising physics is the behaviour under intense infrared (IR) excitation. Signatures attributable to a transient superconducting state extending up to temperatures ten times higher than the equilibrium T∼ 20 K have been discovered in K3C60 after ultra-short pulsed IR irradiation - an effect which still appears as remarkable as mysterious. Motivated by the observation that the phenomenon is observed in a broad pumping frequency range that coincides with the mid-infrared electronic absorption peak still of unclear origin, rather than to TO phonons as has been proposed, we advance here a radically new mechanism. First, we argue that this broad absorption peak represents a "super-exciton" involving the promotion of one electron from the t1u half-filled state to a higher-energy empty t1g state, dramatically lowered in energy by the large dipole-dipole interaction acting in conjunction with Jahn Teller effect within the enormously degenerate manifold of (t1u)2(t1g)1 states. Both long-lived and entropy-rich because they are triplets, the IR-induced excitons act as a sort of cooling mechanism that permits transient superconductive signals to persist up to much larger temperatures. 

Quantum fluctuations beyond the Gutzwiller approximation

Journal: 

Phys. Rev. B 95, 075156

Date: 

February, 2017

We present a simple scheme to evaluate linear response functions including quantum fluctuation corrections on top of the Gutzwiller approximation. The method is derived for a generic multi-band lattice Hamiltonian without any assumption about the dynamics of the variational correlation parameters that define the Gutzwiller wavefunction, and which thus behave as genuine dynamical degrees of freedom that add on those of the variational uncorrelated Slater determinant. We apply the method to the standard half-filled single-band Hubbard model. We are able to recover known results, but, as by-product, we also obtain few novel ones. In particular, we show that quantum fluctuations can reproduce almost quantitatively the behaviour of the uniform magnetic susceptibility uncovered by dynamical mean field theory, which, though enhanced by correlations, is found to be smooth across the paramagnetic Mott transition. By contrast, the simple Gutzwiller approximation predicts that susceptibility to diverge at the transition. 

Field-Driven Mott Gap Collapse and Resistive Switch in Correlated Insulators

Journal: 

Phys. Rev. Lett. 117, 176401

Date: 

October, 2016

Mott insulators are “unsuccessful metals” in which Coulomb repulsion prevents charge conduction despite a metal-like concentration of conduction electrons. The possibility to unlock the frozen carriers with an electric field offers tantalizing prospects of realizing new Mott-based microelectronic devices. Here we unveil how such unlocking happens in a simple model that shows the coexistence of a stable Mott insulator and a metastable metal. Considering a slab subject to a linear potential drop, we find, by means of the dynamical mean-field theory, that the electric breakdown of the Mott insulator occurs via a first-order insulator-to-metal transition characterized by an abrupt gap collapse in sharp contrast to the standard Zener breakdown. The switch on of conduction is due to the field-driven stabilization of the metastable metallic phase. Outside the region of insulator-metal coexistence, the electric breakdown occurs through a more conventional quantum tunneling across the Hubbard bands tilted by the field. Our findings rationalize recent experimental observations and may offer a guideline for future technological research.

Ultrafast evolution and transient phases of the prototype out-of-equilibrium Mott-Hubbard material V2O3

Journal: 

Nature Communications 8, 13917

Date: 

August, 2016

The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behavior, including non-thermal phases and photoinduced phase transitions. 

Non-Fermi-liquid behavior in quantum impurity models with superconducting channels

Journal: 

Phys. Rev. B 95, 085121

Date: 

June, 2016

We study how the non-Fermi-liquid nature of the overscreened multi-channel Kondo impurity model affects the response to a BCS pairing term that, in the absence of the impurity, opens a gap ∆. We find that nonFermi liquid features do persist even at finite ∆: the local density of states lacks coherence peaks, the states in the continuum above the gap are unconventional, and the boundary entropy is a non-monotonic function of temperature. Even more surprisingly, we also find that the low-energy spectrum in the limit ∆ → 0 actually does not correspond to the spectrum strictly at ∆ = 0.