All publications from Michele Fabrizio
Mott resistive switching initiated by topological defects
Milloch A., Figueruelo-Campanero I., Hsu W.F., Mor S., Mellaerts S., Maccherozzi F., Veiga L.S.I., Dhesi S.S., Spera M., Seo J.W., Locquet J.P., Fabrizio M., Menghini M., Giannetti C.
Avalanche resistive switching is the fundamental process that triggers the sudden change of the electrical properties in solid-state devices under the action of intense electric fields. Despite its relevance for information processing, ultrafast electronics, neuromorphic devices, resistive memories and brain-inspired computation, the nature of the local stochastic fluctuations that drive the formation of metallic regions within the insulating state has remained hidden. Here, using operando X-ray nano-imaging, we have captured the origin of resistive switching in a V2O3-based device under working conditions. V2O3 is a paradigmatic Mott material, which undergoes a first-order metal-to-insulator phase transition together with a lattice transformation that breaks the threefold rotational symmetry of the rhombohedral metallic phase. We reveal a new class of volatile electronic switching triggered by nanoscale topological defects appearing in the shear-strain based order parameter that describes the insulating phase. Our results pave the way to the use of strain engineering approaches to manipulate such topological defects and achieve the full dynamical control of the electronic Mott switching. Topology-driven, reversible electronic transitions are relevant across a broad range of quantum materials, comprising transition metal oxides, chalcogenides and kagome metals.
Order from disorder phenomena in BaCoS2
Lenz B., Fabrizio M., Casula M.
At T N ≃ 300K the layered insulator BaCoS2 transitions to a columnar antiferromagnet that signals non-negligible magnetic frustration despite the relatively high T N, all the more surprising given its quasi two-dimensional structure. Here, we show, by combining ab initio and model calculations, that the magnetic transition is an order-from-disorder phenomenon, which not only drives the columnar magnetic order, but also the inter-layer coherence responsible for the finite Néel transition temperature. This uncommon ordering mechanism, actively contributed by orbital degrees of freedom, hints at an abundance of low energy excitations above and across the Néel transition, in agreement with experimental evidence.
Exciton condensation driven by bound states of Green's function zeros
Pasqua I., Blason A., Fabrizio M.
The interaction-driven transition between quantum spin Hall and Mott insulators in the Bernevig, Hughes, and Zhang model is studied by dynamical cluster approximation, and found to be accompanied by the emergence of Green's function zeros already in the quantum spin Hall regime. The nontrivial interplay between Green's function poles and zeros leads to an exotic quantum spin Hall insulator exhibiting two chiral branches of edge Green's function poles and one of zeros. When symmetry breaking is allowed, a nontopological excitonic insulator is found to intrude between quantum spin Hall and Mott insulators. We find evidence that excitons in the Mott insulator, which become soft at the transition to the excitonic insulator, are actually bound states between valence and conduction bands of Green's function zeros, rather than between lower and upper Hubbard bands.
Unified role of Green's function poles and zeros in correlated topological insulators
Blason A., Fabrizio M.
Green's function zeros, which can emerge only if correlation is strong, have been for long overlooked and believed to be devoid of any physical meaning, unlike Green's function poles. Here, we prove that Green's function zeros instead contribute on the same footing as poles to determine the topological character of an insulator. The key to the proof, worked out explicitly in two dimensions but easily extendable in three dimensions, is to express the topological invariant in terms of a quasiparticlethermal Green's function matrix G∗(iϵ,k)=1/[iϵ-H∗(ϵ,k)], with Hermitian H∗(ϵ,k), by filtering out the positive-definite quasiparticle residue. In that way, the topological invariant is easily found to reduce to the Thouless, Kohmoto, Nightingale, and den Nijs formula for quasiparticles described by the noninteracting Hamiltonian H∗(0,k). Since the poles of the quasiparticle Green's function G∗(ϵ,k) on the real frequency axis correspond to poles and zeros of the physical-particle Green's function G(ϵ,k), both of them equally determine the topological character of an insulator.
Bending stiffness collapse, buckling, topological bands of freestanding twisted bilayer graphene
Wang J., Khosravi A., Silva A., Fabrizio M., Vanossi A., Tosatti E.
The freestanding twisted bilayer graphene (TBG) is unstable, below a critical twist angle θc∼3.7∘, against a moiré (2×1) buckling distortion at T=0. Realistic simulations reveal the concurrent unexpected collapse of the bending rigidity, an unrelated macroscopic mechanical parameter. An analytical model connects bending and buckling anomalies at T=0, but as temperature rises the former fades, while buckling persists further. The (2×1) electronic properties are also surprising. The magic twist angle narrow bands, now eight in number, fail to show zone boundary splittings despite the different periodicity. Symmetry shows how this is dictated by an effective single-valley physics. These structural, critical, and electronic predictions promise to make the freestanding state of TBG especially interesting.
Efficient computational screening of strongly correlated materials: Multiorbital phenomenology within the ghost Gutzwiller approximation
Mejuto-Zaera C., Fabrizio M.
The theoretical description of strongly correlated materials relies on the ability to simultaneously capture, on equal footing, the different competing energy scales. Unfortunately, existing approaches are either typically extremely computationally demanding, making systematic screenings of correlated materials challenging or are limited to a subset of observables of interest. The recently developed ghost Gutzwiller ansatz (gGut) has shown great promise to remedy this dichotomy. It is based on a self-consistency condition around the comparatively simple static one-particle reduced density matrix, yet has been shown to provide accurate static and dynamical observables in one-band systems. In this work, we investigate its potential role in the modeling of correlated materials, by applying it to several multiorbital lattice models. Our results confirm the accuracy at lower computational cost of the gGut, and show promise for its application to materials research.
Coherent control of the orbital occupation driving the insulator-to-metal Mott transition in V2 O3
Franceschini P., Policht V.R., Milloch A., Ronchi A., Mor S., Mellaerts S., Hsu W.F., Pagliara S., Ferrini G., Banfi F., Fabrizio M., Menghini M., Locquet J.P., Dal Conte S., Cerullo G., Giannetti C.
Managing light-matter interactions on timescales faster than the loss of electronic coherence is key for achieving full quantum control of the final products in solid-solid transformations. In this Letter, we demonstrate coherent optical control of the orbital occupation that determines the insulator-to-metal transition in the prototypical Mott insulator V2O3. Selective excitation of a specific interband transition with two phase-locked light pulses manipulates the occupation of the correlated bands in a way that depends on the coherent evolution of the photoinduced superposition of states. A comparison between experimental results and numerical solutions of the optical Bloch equations provides an electronic coherence time on the order of 5 fs. Temperature-dependent experiments suggest that the electronic coherence time is enhanced in the vicinity of the insulator-to-metal transition critical temperature, thus highlighting the role of fluctuations in determining the electronic coherence. These results open different routes to selectively switch the functionalities of quantum materials and coherently control solid-solid electronic transformations.
Spin-Liquid Insulators Can Be Landau's Fermi Liquids
Fabrizio M.
The long search for insulating materials that possess low-energy quasiparticles carrying electron's quantum numbers except charge - inspired by the neutral spin-1/2 excitations, the so-called spinons, exhibited by Anderson's resonating-valence-bond state - seems to have reached a turning point after the discovery of several Mott insulators displaying the same thermal and magnetic properties as metals, including quantum oscillations in a magnetic field. Here, we show that such anomalous behavior is not inconsistent with Landau's Fermi liquid theory of quasiparticles at a Luttinger surface. That is the manifold of zeros within the Brillouin zone of the single-particle Green's function at zero frequency, and which thus defines the spinon Fermi surface conjectured by Anderson.
Exciton condensation in strongly correlated quantum spin Hall insulators
Amaricci A., Mazza G., Capone M., Fabrizio M.
Time-reversal symmetric topological insulators are generically robust with respect to weak local interaction unless symmetry-breaking transitions take place. Using dynamical mean-field theory, we solve an interacting model of quantum spin Hall insulators and show the existence at intermediate coupling of a symmetry-breaking transition to a nontopological insulator characterized by exciton condensation. This transition is of first order. For a larger interaction strength, the insulator evolves into a Mott one. The transition is continuous if magnetic order is prevented, and notably, for any finite Hund's exchange, it progresses through a Mott localization before the condensate coherence is lost. We show that the correlated excitonic state corresponds to a magneto-electric insulator, which allows for direct experimental probing. Finally, we discuss the fate of the helical edge modes across the excitonic transition.
Wetting critical behavior in the quantum Ising model within the framework of Lindblad dissipative dynamics
Artiaco C., Nava A., Fabrizio M.
We investigate the critical behavior, both in space and time, of the wetting interface within the coexistence region around the first-order phase transition of a fully connected quantum Ising model in slab geometry. For that, we employ the Lindblad master equation formalism in which temperature is inherited by the coupling to a dissipative bath, rather than being a functional parameter as in the conventional Cahn's free energy. Lindblad's approach gives not only access to the dissipative dynamics and steady-state configuration of the quantum wetting interface throughout the whole phase diagram but also shows that the wetting critical behavior can be successfully exploited to characterize the phase diagram as an alternative to the direct evaluation of the free energies of the competing phases.
Kondo nanomechanical dissipation in the driven Anderson impurity model
Kohn L., Santoro G.E., Fabrizio M., Tosatti E.
The cyclic sudden switch of a magnetic impurity from a Kondo to a non-Kondo state and back was recently proposed to involve an important dissipation of the order of several kBTK per cycle. The possibility to reveal this and other electronic processes through nanomechanical dissipation by, e.g., ultrasensitive atomic force microscope (AFM) tools would represent an unusual and interesting form of spectroscopy. Here, we explore the dependence on the switching time of the expected dissipation, a quantity whose magnitude is physically expected to drop from maximum to zero between sudden and slow switching, respectively. As an application of a recently established matrix-product-state-based time-dependent variational algorithm, we study the magnetic-field-induced Kondo switching in an Anderson model of the magnetic impurity. We find, quite reasonably, that dissipation requires switching within the Kondo timescale (kBTK)-1 or faster. While such a fast switching seems problematic for current AFM setups, the challenge remains open for future means to detect this dissipation by time-dependent magnetic fields, an electrostatic impurity level shift, or hybridization switching. The technical aspects revealed by this approach will be of interest for future nonequilibrium calculations.
Local Kekulé distortion turns twisted bilayer graphene into topological Mott insulators and superconductors
Blason A., Fabrizio M.
Magic-angle twisted bilayer graphene displays at different fillings of the four flat bands lying around the charge neutrality point a wealth of notable phases that include magnetic Chern insulators, whose magnetization is mostly of an orbital nature and contiguous superconducting domes. Such a rich phase diagram is explained through the positive interplay of Coulomb repulsion and the electron coupling to a twofold optical mode that corresponds to Kekulé distortions localized into the small AA stacked regions of the moiré supercells. A static distortion stabilizes, at any integer filling of the flat bands, valence-bond insulators that carry finite Chern number away from charge neutrality. Similarly, a dynamic distortion that resonates between the two lattice vibrations leads to resonating-valence-bond topological insulators with built-in chiral d-wave pairs that have finite Chern number equal to the angular momentum, and thus are prone to turn superconducting upon doping away from integer filling.
Nanoscale self-organization and metastable non-thermal metallicity in Mott insulators
Ronchi A., Franceschini P., De Poli A., Homm P., Fitzpatrick A., Maccherozzi F., Ferrini G., Banfi F., Dhesi S.S., Menghini M., Fabrizio M., Locquet J.P., Giannetti C.
Mott transitions in real materials are first order and almost always associated with lattice distortions, both features promoting the emergence of nanotextured phases. This nanoscale self-organization creates spatially inhomogeneous regions, which can host and protect transient non-thermal electronic and lattice states triggered by light excitation. Here, we combine time-resolved X-ray microscopy with a Landau-Ginzburg functional approach for calculating the strain and electronic real-space configurations. We investigate V2O3, the archetypal Mott insulator in which nanoscale self-organization already exists in the low-temperature monoclinic phase and strongly affects the transition towards the high-temperature corundum metallic phase. Our joint experimental-theoretical approach uncovers a remarkable out-of-equilibrium phenomenon: the photo-induced stabilisation of the long sought monoclinic metal phase, which is absent at equilibrium and in homogeneous materials, but emerges as a metastable state solely when light excitation is combined with the underlying nanotexture of the monoclinic lattice.
Emergent quasiparticles at Luttinger surfaces
Fabrizio M.
In periodic systems of interacting electrons, Fermi and Luttinger surfaces refer to the locations within the Brillouin zone of poles and zeros, respectively, of the single-particle Green’s function at zero energy and temperature. Such difference in analytic properties underlies the emergence of well-defined quasiparticles close to a Fermi surface, in contrast to their supposed non-existence close to a Luttinger surface, where the single-particle density-of-states vanishes at zero energy. We here show that, contrary to such common belief, dispersive ‘quasiparticles’ with infinite lifetime do exist also close to a pseudo-gapped Luttinger surface. Thermodynamic and dynamic properties of such ‘quasiparticles’ are just those of conventional ones. For instance, they yield well-defined quantum oscillations in Luttinger surface and linear-in-temperature specific heat, which is striking given the vanishing density of states of physical electrons, but actually not uncommon in strongly correlated materials.
Disentangling Structural and Electronic Properties in V2O3Thin Films: A Genuine Nonsymmetry Breaking Mott Transition
Mazzola F., Chaluvadi S.K., Polewczyk V., Mondal D., Fujii J., Rajak P., Islam M., Ciancio R., Barba L., Fabrizio M., Rossi G., Orgiani P., Vobornik I.
Phase transitions are key in determining and controlling the quantum properties of correlated materials. Here, by using the combination of material synthesis and photoelectron spectroscopy, we demonstrate a genuine Mott transition undressed of any symmetry breaking side effects in the thin films of V2O3. In particular and in contrast with the bulk V2O3, we unveil the purely electronic dynamics approaching the metal-insulator transition, disentangled from the structural transformation that is prevented by the residual substrate-induced strain. On approaching the transition, the spectral signal evolves slowly over a wide temperature range, the Fermi wave-vector does not change, and the critical temperature is lower than the one reported for the bulk. Our findings are fundamental in demonstrating the universal benchmarks of a genuine nonsymmetry breaking Mott transition, extendable to a large array of correlated quantum systems, and hold promise of exploiting the metal-insulator transition by implementing V2O3 thin films in devices.
Luttinger's theorem in the presence of Luttinger surfaces
Skolimowski J., Fabrizio M.
Breakdown of Landau's hypothesis of adiabatic continuation from noninteracting to fully interacting electrons is commonly believed to bring about a violation of Luttinger's theorem. Here, we elucidate what may go wrong in the proof of Luttinger's theorem. The analysis provides a simple way to correct Luttinger's expression of the electron number in single-band models where perturbation theory breaks down through the birth of a Luttinger surface without symmetry breaking. In those cases, we find that the Fermi volume only accounts for the doping away from half-filling. In the hypothetical circumstance of a non-symmetry-breaking Mott insulator with a Luttinger surface, our analysis predicts the noteworthy existence of quasiparticles whose Fermi surface is just the Luttinger one. Therefore, those quasiparticles can be legitimately regarded as spinons, and the Mott insulator with a Luttinger surface as realization of a spin-liquid insulator.
Dissipative cooling induced by pulse perturbations
Nava A., Fabrizio M.
We investigate the dynamics brought on by an impulse perturbation in two infinite-range quantum Ising models coupled to each other and to a dissipative bath. We show that, if dissipation is faster the higher the excitation energy, the pulse perturbation cools down the low-energy sector of the system, at the expense of the high-energy one, eventually stabilising a transient symmetry-broken state at temperatures higher than the equilibrium critical one. Such non-thermal quasi-steady state may survive for quite a long time after the pulse, if the latter is properly tailored.
Moving Dirac nodes by chemical substitution
Nilforoushan N., Casula M., Amaricci A., Caputo M., Caillaux J., Khalil L., Papalazarou E., Simon P., Perfetti L., Vobornik I., Das P.K., Fujii J., Barinov A., Santos-Cottin D., Klein Y., Fabrizio M., Gauzzi A., Marsi M.
Dirac fermions play a central role in the study of topological phases, for they can generate a variety of exotic states, such as Weyl semimetals and topological insulators. The control and manipulation of Dirac fermions constitute a fundamental step toward the realization of novel concepts of electronic devices and quantum computation. By means of Angle-Resolved Photo-Emission Spectroscopy (ARPES) experiments and ab initio simulations, here, we show that Dirac states can be effectively tuned by doping a transition metal sulfide, BaNiS2, through Co/Ni substitution. The symmetry and chemical characteristics of this material, combined with the modification of the charge-transfer gap of BaCo1−xNixS2 across its phase diagram, lead to the formation of Dirac lines, whose position in k-space can be displaced along the Γ − M symmetry direction and their form reshaped. Not only does the doping x tailor the location and shape of the Dirac bands, but it also controls the metal-insulator transition in the same compound, making BaCo1−xNixS2 a model system to functionalize Dirac materials by varying the strength of electron correlations.
Landau-Fermi liquids without quasiparticles
Fabrizio M.
Landau-Fermi liquid theory is conventionally believed to hold whenever the interacting single-particle density of states develops a δ-like component at the Fermi surface, which is associated with quasiparticles. Here we show that a microscopic justification can be actually achieved under more general circumstances, even in the case where coherent quasiparticles are totally missing and the interacting single-particle density of states vanishes at the chemical potential as a consequence of a pole singularity in the self-energy.
Jahn–Teller coupling to moiré phonons in the continuum model formalism for small-angle twisted bilayer graphene
Angeli M., Fabrizio M.
We show how to include the Jahn–Teller coupling of moiré phonons to the electrons in the continuum model formalism which describes small-angle twisted bilayer graphene. These phonons, which strongly couple to the valley degree of freedom, are able to open gaps at most integer fillings of the four flat bands around the charge neutrality point. Moreover, we derive the full quantum mechanical expression of the electron–phonon Hamiltonian, which may allow accessing phenomena such as the phonon-mediated superconductivity and the dynamical Jahn–Teller effect.

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