arXiv subjects

Luca de' Medici

Publications and source records attributed to Luca de' Medici.

At least 19 recordsLinked to original sources

arXiv2025

Tunable Electronic Correlations in 135-Kagome Metals

Kagome metals exhibit rich correlated-electron physics, yet a systematic understanding of the degree of correlation across transition-metal species remains elusive. Using density-functional theory plus multi-orbital slave-spin mean-field theory, we investigate electronic correlations in the Ti-, V-, and Cr-based 135 compounds with Sb and Bi pnictogens. We find that the significantly stronger degree of correlation of the Cr-based materials compared to Ti and V can only be explained through the synergy of two effects: the larger electron filling of the $d$-shell and the reduced characteristic kinetic energy. We put forward that the substitution of Sb with Bi strengthens correlations in all compounds and make the prediction that the-yet-to-be-synthesized CsCr$_3$Bi$_5$ must be the most strongly correlated member of the entire family. These findings provide a quantitative, band-structure-based framework for understanding and predicting correlation strength in Kagome metals.

cond-mat.str-el↗

arXiv2025

Local-moment magnetism in Mn-based pnictides

We report a comprehensive study of electronic-correlation effects in Manganese-based antiferromagnetic pnictides BaMn$_2$Pn$_2$ (Pn=P,As,Sb,Bi). Our density functional theory plus slave-spin mean-field simulations indicate that all the compounds lie on the strong-coupling side of an itinerant-to-localized moment crossover, corresponding to the critical interaction strength for the Mott transition in the high-temperature paramagnetic phase. We also show that the experimental Néel temperature of each compound scales with the distance from this crossover.

cond-mat.str-el↗

arXiv2024

Dynamical correlations and order in magic-angle twisted bilayer graphene

In magic angle twisted bilayer graphene, transport, thermodynamic and spectroscopic experiments pinpoint at a competition between distinct low-energy states with and without electronic order. We use Dynamical Mean Field Theory (DMFT) on the topological heavy Fermion (THF) model of twisted bilayer graphene to investigate the emergence of electronic correlations and long-range order in the absence of strain. We contrast moment formation, Kondo screening and ordering on a temperature basis and explain the nature of emergent correlated states based on three central phenomena: (i) the formation of local spin and valley isospin moments around 100K, (ii) the ordering of the local isospin moments around 10K preempting Kondo screening, and (iii) a cascadic redistribution of charge between localized and delocalized electronic states upon doping. At integer fillings, we find that low energy spectral weight is depleted in the symmetric phase, while we find insulating states with gaps enhanced by exchange coupling in the zero-strain ordered phases. Doping away from integer filling results in distinct metallic states: a "bad metal" above the ordering temperature, where scattering off the disordered local moments suppresses electronic coherence, and a "good metal" in the ordered states with coherence of quasiparticles facilitated by isospin order. This finding reveals coherence from order as the microscopic mechanism behind the Pomeranchuk effect observed experimentally. Upon doping, there is a periodic charge reshuffling between localized and delocalized electronic orbitals leading to cascades of doping-induced Lifshitz transitions, local spectral weight redistributions and periodic variations of the electronic compressibility. Our findings provide a unified understanding of the most puzzling aspects of scanning tunneling spectroscopy, transport, and compressibility experiments.

cond-mat.str-el↗

arXiv2023

Paradigm for finding d-electron heavy fermions: the case of Cr-doped CsFe$_2$As$_2$

We define a general strategy for finding new heavy-fermionic materials without rare-earth elements: doping a Hund metal with pronounced orbital-selective correlations towards half-filling. We argue that in general band structures a possible orbital-selective Mott transition is frustrated by inter-orbital hopping into heavy-fermion behaviour - where d-orbitals provide both the heavy and the light electrons - which is enhanced when approaching half-filling. This phase ultimately disappears due to magnetic correlations, as in a standard Doniach diagram. Experimentally we have further hole doped CsFe$_2$As$_2$, a Hund metal with 0.5 electrons/Fe away from half-filling, and obtained a heavy fermionic state with the highest Sommerfeld coefficient for Fe-pnictides to date (270 mJ/mol K$^2$), before signatures of an antiferromagnetic phase set in.

cond-mat.str-el↗

arXiv2023

Slave-spin mean field for broken-symmetry states: Néel antiferromagnetism and its phase separation in multi-orbital Hubbard models

We introduce the generalization of the Slave-Spin Mean-Field method to broken-symmetry phases. Through a variational approach we derive the single-particle energy shift in the mean-field equations which generates the appropriate self-consistent field responsible for the stabilization of the broken symmetry. With this correction the different flavours of the slave-spin mean-field are actually the same method and they give identical results to Kotliar-Ruckenstein slave-bosons and to the Gutzwiller approximation. We apply our formalism to the Néel antiferromagnetic state and study it in multi-orbital models as a function of the number of orbitals and Hund's coupling strength, providing phase diagrams in the interaction-doping plane. We show that the doped antiferromagnet in proximity of half-filling is typically unstable towards insulator-metal and magnetic-non magnetic phase separation. Hund's coupling extends the range of this antiferromagnet, and favors its phase separation.

cond-mat.str-el↗

arXiv2023

Manipulation of the magnetic state of a porphyrin-based molecule on gold: From Kondo to quantum nanomagnet via the charge fluctuation regime

By moving individual Fe-Porphyrin-based molecules with the tip of Scanning Tunneling Microscope in the vicinity of a Br-atom containing elbow of the herringbone-reconstructed Au(111), we reversibly and continuously control their magnetic state. Several regimes are obtained experimentally and explored theoretically: from the integer spin limit, through intermediate magnetic states with renormalized magnetic anisotropy, until the Kondo-screened regime, corresponding to a progressive increase of charge fluctuations and mixed valency due to an increase in the interaction of the molecular Fe states with the substrate Fermi sea. Our results open a route for the realization, tuning and experimental studies of novel quantum magnetic states in molecule-surface hybrids.

cond-mat.mes-hall↗

arXiv2022

Rationalizing doping and electronic correlations in LaFe$_2$As$_2$

We compute the electronic properties of the normal state of uncollapsed LaFe$_2$As$_2$, taking into account local dynamical correlations by means of slave-spin mean-field+density-functional theory. Assuming the same local interaction strength used to model the whole electron- and hole-doped BaFe$_2$As$_2$ family, our calculations reproduce the experimental Sommerfeld specific heat coefficient, which is twice the value predicted by uncorrelated band theory. We find that LaFe$_2$As$_2$ has a reduced bare bandwidth and this solves the apparent paradox of its sizeable correlations despite its nominal valence d$^{6.5}$, which would imply extreme overdoping and uncorrelated behaviour in BaFe$_2$As$_2$. Our results yield a consistent picture of the whole 122 family and point at the importance of the correlation strength, rather than sheer doping, in the interpretation of the phase diagram of iron-based superconductors

cond-mat.supr-con↗

arXiv2022

Mott Quantum Critical Points at finite doping

Strongly correlated materials often undergo a Mott metal-insulator transition, which is tipically first-order, as a function of control parameters like pressure. Upon doping, rich phase diagrams with competing instabilities are found. Yet, the conceptual link between the interaction-driven Mott transition and the finite-doping behavior lacks a clear connection with the theory of critical phenomena. In a prototypical case of a first-order Mott transition the surface associated with the equation of state for the homogeneous system is "folded" so that in a range of parameters stable metallic and insulating phases exist and are connected by an unstable metallic branch. Here we show that tuning the chemical potential the zero-temperature equation of state gradually unfolds. Under general conditions, we find that the Mott transition evolves into a first-order transition between two metals, associated to a phase separation region ending in a quantum critical point (QCP) at finite doping. This scenario is here demonstrated solving a simple multi-orbital Hubbard model relevant for the Iron-based superconductors, but its origin - the splitting of the atomic ground state multiplet by a small energy scale, here Hund's coupling - is much more general. A strong analogy with cuprate superconductors is traced.

cond-mat.str-el↗

arXiv2021

Hund's metal crossover and superconductivity in the 111 family of iron-based superconductors

We study LiFeP, LiFeAs and NaFeAs in their paramagnetic metallic phase including dynamical electronic correlations within a density functional theory + slave-spin mean-field framework. The three compounds are found to lie next to the crossover between a normal and a Hund's metal, where a region of enhanced electronic compressibility that may boost superconductivity is systematically present in this type of systems. We find that LiFeP lies in the normal metallic regime, LiFeAs at the crossover, and NaFeAs is in the Hund's metal regime, which possibly explains the different experimental trends for the pressure- and doping-dependence of superconductivity in these compounds. Our picture captures the orbitally-resolved mass renormalizations measured in these materials, while an analysis of the Sommerfeld specific-heat coefficient highlights some limitations of currently used implementations of density-functional theory for the correct prediction of the details of band structures in the iron-based superconductors.

cond-mat.supr-con↗

arXiv2021

Accurate modeling of FeSe with screened Fock exchange and Hund's metal correlations

We reproduce the electronic properties of FeSe in the high-temperature phase within an ab initio framework that includes screened Fock exchange and local dynamical correlations. We robustly capture the experimental band structure, as long as the system is in the Hund's metal phase. In particular, we account for the shrinking of the Fermi pockets and the sinking below the Fermi level of the hole pocket with $xy$ orbital character. This entails the elusive correct estimate of the Sommerfeld coefficient, and supports the interpretation of non-compensated Fermi pockets seen in ARPES in terms of surface electron doping. More stringently, our modeling matches well the experimental interband optical spectrum, and captures qualitatively the temperature dependence of the thermoelectric power, extremely sensitive to the details of the bands around the Fermi level.

cond-mat.str-el↗

arXiv2020

Enhancement of charge instabilities in Hund's metals by the breaking of rotational symmetry

We analyze multi-orbital Hubbard models describing Hund's metals, focusing on the ubiquitous occurrence of a charge instability, signalled by a divergent/negative electronic compressibility, in a range of doping from the half-filled Mott insulator corresponding to the frontier between Hund's and normal metals. We show that the breaking of rotational invariance favors this instability: both spin-anisotropy in the interaction and crystal-field splitting among the orbitals make the instability zone extend to larger dopings, making it relevant for real materials like iron-based superconductors. These observations help us build a coherent picture of the occurrence and extent of this instability. We trace it back to the partial freezing of the local degrees of freedom in the Hund's metal, which reduces the allowed local configurations and thus the quasiparticle itinerancy. The abruptness of the unfreezing happening at the Hund's metal frontier can be directly connected to a rapid change in the electronic kinetic energy and thus to the enhancement and divergence of the compressibility.

cond-mat.str-el↗

arXiv2019

Nature of the Doping-Driven Metal-Insulator Transition in Correlated Electron Systems with strong Hund's Exchange Coupling

We study the doping-driven Mott metal-insulator transition for multi-orbital Hubbard models with Hund's exchange coupling at finite temperatures. As in the single-orbital Hubbard model, the transition is of first-order within dynamical mean field theory, with a coexistence region where two solutions can be stabilized. We find, that in the presence of finite Hund's coupling, the insulating phase is connected to a badly metallic phase, which extends to surprisingly large dopings. While fractional power-law behavior of the self-energies on the Matsubara axis is found on both sides of the transition, a regime with frozen local moments develops only on the branch connected to the insulating phase.

cond-mat.str-el↗

arXiv2019

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

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.

cond-mat.str-el↗

arXiv2018

Hund-enhanced electronic compressibility in FeSe and its correlation with T$_c$

We compute the compressibility of the conduction electrons in both bulk orthorhombic FeSe and monolayer FeSe on SrTiO$_3$ substrate, including dynamical electronic correlations within slave-spin mean-field + density-functional theory. Results show a zone of enhancement of the electronic compressibility crossing the interaction-doping phase diagram of these compounds in accord with previous simulations on iron pnictides and in general with the phenomenology of Hund's metals. Interestingly at ambient pressure FeSe is found slightly away from the zone with enhanced compressibility but moved right into it with hydrostatic pressure, while in monolayer FeSe the stronger enhancement region is realized on the electron-doped side. These findings correlate positively with the enhancement of superconductivity seen in experiments, and support the possibility that Hund's induced many-body correlations boost superconductive pairing when the system is at the frontier of the normal- to Hund's-metal crossover.

cond-mat.str-el↗

arXiv2017

Hund's metals, explained

A possible practical definition for a Hund's metal is given, as a metallic phase - arising consistently in realistic simulations and experiments in Fe-based superconductors and other materials - with three features: large electron masses, high-spin local configurations dominating the paramagnetic fluctuations and orbital-selective correlations. These features are triggered by, and increase with the proximity to, a Hund's coupling-favored Mott insulator that is realized for half-filled conduction bands. A clear crossover line is found where these three features get enhanced, departing from the Mott transition at half filling and extending in the interaction/doping plane, between a normal (at weak interaction and large doping) and a Hund's metal (at strong interaction and small doping). This phenomenology is found identically in models with featureless bands, highlighting the generality of this physics and its robustness by respect to the details of the material band structures. Some analytical arguments are also given to gain insight into these defining features. Finally the attention is brought on the recent theoretical finding of enhanced/diverging electronic compressibility near the Hund's metal crossover, pointing to enhanced quasiparticle interactions that can cause or boost superconductivity or other instabilities.

cond-mat.str-el↗

arXiv2016

Mott transitions with partially-filled correlated orbitals

We investigate the metal-insulator Mott transition in a generalized version of the periodic Anderson model, in which a band of itinerant electrons is hybridrized with a narrow and strongly correlated band. Using dynamical mean-field theory, we show that the precondition for a Mott transition is an integer total filling of the two bands, while for an integer constant occupation of the correlated band the system remains a correlated metal at arbitrary large interaction strength. We picture the transition at a non-integer filling of the correlated orbital as the Mott localization of the singlet states between itinerant and strongly interacting electrons, having occupation of one per lattice site. We show that the Mott transition is of the first-order and we characterize the nature of the resulting insulating state with respect to relevant physical parameters, such as the charge-transfer energy.

cond-mat.str-el↗

arXiv2016

Chromium analogues of Iron-based superconductors

We theoretically investigate the $d^4$ (Cr$^{2+}$) compound BaCr$_2$As$_2$ and show that, despite non-negligible differences in the electronic structure, its many-body physics mirrors that of BaFe$_2$As$_2$, which has instead a $d^6$ (Fe$^{2+}$) configuration. This reflects a symmetry of the electron correlation effects around the half-filled $d^5$ Mott insulating state. The experimentally known metallic antiferromagnetic phase is correctly modeled by dynamical mean-field theory, and for realistic values of the interaction it shows a moderate mass enhancement of order $\sim$2. This value decreases if the ordered moment grows as a result of a stronger interaction. The antiferromagnetic phase diagram for this $d^4$ shows similarities with that calculated for the $d^6$ systems. Correspondingly, in the paramagnetic phase the influence of the half-filled Mott insulator shows up as a crossover from a weakly correlated to an orbitally differentiated "Hund's metal" phase which reflects an analogous phenomenon in $d^6$ iron compounds including a strong enhancement of the compressibility in a zone just inside the frontier between the normal and the Hund's metal. The experimental evidence and our theoretical description place BaCr$_2$As$_2$ at interaction strength slightly below the crossover which implies that negative pressures and/or electron doping (e.g. Cr $\rightarrow$ Mn,Fe or Ba $\rightarrow$ Sc,Y,La) might strongly enhance the compressibility, thereby possibly inducing a pairing instability in this non-superconducting compound.

cond-mat.str-el↗

arXiv2016

Hund's induced Fermi-liquid instabilities and enhanced quasiparticle interactions

Hund's coupling is shown to generally favor, in a doped half-filled Mott insulator, an increase in the compressibility culminating in a Fermi-liquid instability towards phase separation. The largest effect is found about the frontier between an ordinary and an orbitally-decoupled ("Hund's") metal. The increased compressibility implies an enhancement of quasiparticle scattering, thus favoring other possible symmetry breakings. This physics is shown to happen in simulations of the 122 Fe-based superconductors, possibly implying the relevance of this mechanism in the enhancement of the the critical temperature for superconductivity.

cond-mat.str-el↗