arXiv subjects

M. Capone

Publications and source records attributed to M. Capone.

At least 37 records · Page 2Linked to original sources

arXiv2015

Electronic correlation assisted ferroelectric metallic state in LiOsO$_3$

LiOsO$_3$ has been recently identified as the first unambiguous "ferroelectric metal", experimentally realizing a prediction from 1965 by Anderson and Blount. In this work, we investigate the metallic state in LiOsO$_3$ by means of infrared spectroscopy supplemented by Density Functional Theory and Dynamical Mean Field Theory calculations. Our measurements and theoretical calculations clearly show that LiOsO$_3$ is a very bad metal with a small quasiparticle weight, close to a Mott-Hubbard localization transition. The agreement between experiments and theory allows us to ascribe all the relevant features in the optical conductivity to strong electron-electron correlations within the $t_{2g}$ manifold of the osmium atoms.

cond-mat.str-el↗

arXiv2015

First order character and observable signatures of topological quantum phase transitions

Topological quantum phase transitions are characterised by changes in global topological invariants. These invariants classify many body systems beyond the conventional paradigm of local order parameters describing spontaneous symmetry breaking. For non-interacting electrons, it is well understood that such transitions are continuous and always accompanied by a gap-closing in the energy spectrum, given that the symmetries protecting the topological phase are maintained. Here, we demonstrate that sufficiently strong electron-electron interaction can fundamentally change the situation: we discover a topological quantum phase transition of first order character in the genuine thermodynamic sense, that occurs without gap closing. Our theoretical study reveals the existence of a quantum critical endpoint associated with an orbital instability on the transition line between a 2D topological insulator and a trivial band insulator. Remarkably, this phenomenon entails unambiguous signatures associated to the orbital occupations that can be detected experimentally.

cond-mat.str-el↗

arXiv2015

Snapshots of the retarded interaction of charge carriers with ultrafast fluctuations in cuprates

One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive since electronic correlations are expected to dramatically speed up the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and the frequency range of transient reflectivity measurements up to an unprecedented level that enables us to directly observe the 16 fs build-up of the effective electron-boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t-J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations.

cond-mat.supr-con↗

arXiv2014

Electronic transport and dynamics in correlated heterostructures

We investigate by means of the time-dependent Gutzwiller approximation the transport properties of a strongly-correlated slab subject to Hubbard repulsion and connected with to two metallic leads kept at a different electrochemical potential. We focus on the real-time evolution of the electronic properties after the slab is connected to the leads and consider both metallic and Mott insulating slabs. When the correlated slab is metallic, the system relaxes to a steady-state that sustains a finite current. The zero-bias conductance is finite and independent of the degree of correlations within the slab as long as the system remains metallic. On the other hand, when the slab is in a Mott insulating state, the external bias leads to currents that are exponentially activated by charge tunneling across the Mott-Hubbard gap, consistent with the Landau-Zener dielectric breakdown scenario.

cond-mat.str-el↗

arXiv2014

Correlation-driven electronic multiferroicity in (TMTTF)$_2$-$X$ organic crystals

Using a combination of density functional theory and dynamical mean field theory we show that electric polarization and magnetism are strongly intertwined in (TMTTF)$_2$-$X$ (X$=$PF$_6$, As$F_6$, and SbF$_6$) organic crystals and they originate from short-range Coulomb interactions. Electronic correlations induce a charge-ordered state which, combined with the molecular dimerization, gives rise to a finite electronic polarization and to a ferroelectric state. We predict that the value of the electronic polarization is enhanced by the onset of antiferromagnetism showing a sizable magnetoelectric leading to a multiferroic behavior of (TMTTF)$_2$-$X$ compounds.

cond-mat.str-el↗

arXiv2014

Cooperative effects of Jahn-Teller distortion, magnetism and Hund's coupling in the insulating phase of BaCrO$_3$

We employ a combination of density functional theory and dynamical mean-field theory to investigate the electronic structure of the recently synthesized insulator BaCrO$_3$. Our calculations show that Hund's coupling is responsible for strong correlation effects, which are however not sufficient to turn the system insulating. A finite Jahn-Teller distortion lifting the orbital degeneracy is necessary to stabilize an insulating state with orbital ordering and consequent magnetic ordering.

cond-mat.str-el↗

arXiv2013

Local moment dynamics and screening effects in doped charge-transfer insulators

By means of Dynamical Mean-Field Theory we investigate the spin response function of a model for correlated materials with d- or f-electrons hybridized with more delocalized ligand orbitals. We point out the existence of two different processes responsible for the dynamical screening of local moments of the correlated electrons. Studying the local spin susceptibility we identify the contribution of the "direct" magnetic exchange and of an "indirect" one mediated by the itinerant uncorrelated orbitals. In addition, we characterize the nature of the dynamical screening processes in terms of different classes of diagrams in the hybridization-expansion contributing to the density-matrix. Our analysis suggests possible ways of estimating the relative importance of these two classes of screening processes in realistic calculations for correlated materials.

cond-mat.str-el↗

arXiv2013

Inhomogeneous BCS-BEC crossover for trapped cold atoms in optical lattices

The BCS-BEC crossover in a lattice is a powerful paradigm to understand how a superconductor deviates from the Bardeen-Cooper-Schrieffer physics as the attractive interaction increases. Optical lattices loaded with binary mixtures of cold atoms allow to address it experimentally in a clean and controlled way. We show that, however, the possibility to study this phenomenon in actual cold-atoms experiments is limited by the effect of the trapping potential. Real-space Dynamical Mean-Field Theory calculations show indeed that interactions and the confining potential conspire to pack the fermions in the center of the trap, which approaches a band insulator when the attraction become sizeable. We show how this physics reflects in several observables, and we propose an alternative strategy to disentangle the effect of the harmonic potential and measure the intrinsic properties resulting from the interaction strength.

cond-mat.str-el↗

arXiv2013

IRIDE White Book, An Interdisciplinary Research Infrastructure based on Dual Electron linacs&lasers

This report describes the scientific aims and potentials as well as the preliminary technical design of IRIDE, an innovative tool for multi-disciplinary investigations in a wide field of scientific, technological and industrial applications. IRIDE will be a high intensity 'particle factory', based on a combination of a high duty cycle radio-frequency superconducting electron linac and of high energy lasers. Conceived to provide unique research possibilities for particle physics, for condensed matter physics, chemistry and material science, for structural biology and industrial applications, IRIDE will open completely new research possibilities and advance our knowledge in many branches of science and technology. IRIDE will contribute to open new avenues of discoveries and to address most important riddles: What does matter consist of? What is the structure of proteins that have a fundamental role in life processes? What can we learn from protein structure to improve the treatment of diseases and to design more efficient drugs? But also how does an electronic chip behave under the effect of radiations? How can the heat flow in a large heat exchanger be optimized? The scientific potential of IRIDE is far reaching and justifies the construction of such a large facility in Italy in synergy with the national research institutes and companies and in the framework of the European and international research. It will impact also on R&D work for ILC, FEL, and will be complementarity to other large scale accelerator projects. IRIDE is also intended to be realized in subsequent stages of development depending on the assigned priorities.

physics.ins-det↗

arXiv2013

Linear-Response Dynamics from the Time-Dependent Gutzwiller Approximation

Within a Lagrangian formalism we derive the time-dependent Gutzwiller approximation for general multi-band Hubbard models. Our approach explicitly incorporates the coupling between time-dependent variational parameters and a time-dependent density matrix from which we obtain dynamical correlation functions in the linear response regime. Our results are illustrated for the one-band model where we show that the interacting system can be mapped to an effective problem of fermionic quasiparticles coupled to "doublon" (double occupancy) bosonic fluctuations. The latter have an energy on the scale of the on-site Hubbard repulsion $U$ in the dilute limit but becomes soft at the Brinkman-Rice transition which is shown to be related to an emerging conservation law of doublon charge and the associated gauge invariance. Coupling with the boson mode produces structure in the charge response and we find that a similar structure appears in dynamical mean-field theory.

cond-mat.str-el↗

arXiv2012

Orbital Selectivity in Hund's metals: The Iron Chalcogenides

We show that electron correlations lead to a bad metallic state in chalcogenides FeSe and FeTe despite the intermediate value of the Hubbard repulsion $U$ and Hund's rule coupling $J$. The evolution of the quasi particle weight $Z$ as a function of the interaction terms reveals a clear crossover at $U \simeq$ 2.5 eV. In the weak coupling limit $Z$ decreases for all correlated $d$ orbitals as a function of $U$ and beyond the crossover coupling they become weakly dependent on $U$ while strongly depend on $J$. A marked orbital dependence of the $Z$'s emerges even if in general the orbital-selective Mott transition only occurs for relatively large values of $U$. This two-stage reduction of the quasi particle coherence due to the combined effect of Hubbard $U$ and the Hund's $J$, suggests that the iron-based superconductors can be referred to as Hund's correlated metals.

cond-mat.mtrl-sci↗

arXiv2012

Non-equilibrium dynamics of the driven Hubbard model

We investigate the dynamics of a two-dimensional Hubbard model in a static electric field in order to identify the conditions to reach a non-equilibrium stationary state. For a generic electric field, the convergence to a stationary state requires the coupling to a thermostating bath absorbing the work done by the external force. Following the real-time dynamics of the system, we show that a non-equilibrium stationary state is reached for essentially any value of the coupling to the bath. We map out a phase diagram in terms of dissipation and electric field strengths and identify the dissipation values in which steady current is largest for a given field.

cond-mat.str-el↗

arXiv2012

Augmented hybrid exact-diagonalization solver for dynamical mean field theory

We present a new methodology to solve the Anderson impurity model, in the context of dynamical mean-field theory, based on the exact diagonalization method. We propose a strategy to effectively refine the exact diagonalization solver by combining a finite-temperature Lanczos algorithm with an adapted version of the cluster perturbation theory. We show that the augmented diagonalization yields an improved accuracy in the description of the spectral function of the single-band Hubbard model and is a reliable approach for a full d-orbital manifold calculation.

cond-mat.str-el↗

arXiv2012

A path to poor coherence in heavy fermions from Mott physics and hybridization

We investigate the anomalous metal arising by hole doping the Mott insulating state of the periodic Anderson model. Using Dynamical Mean-Field Theory we show that, as opposed to the electron-doped case, in the hole-doped regime the hybridization between localized and delocalized orbitals leads to the formation of composite quasi-particles reminiscent of the Zhang-Rice singlets. We compute the coherence temperature of this state, showing its exponentially small value at low dopings. As a consequence the weakly-doped Mott state deviates from the predictions of Fermi-liquid theory already at small temperatures. The onset of the Zhang-Rice state and of the consequent poor coherence is due to the electronic structure in which both localized and itinerant carriers have to be involved in the formation of the conduction states and to the proximity to the Mott state. By investigating the magnetic properties of this state, we discuss the relation between the anomalous metallic properties and the behavior of the magnetic degrees of freedom.

cond-mat.str-el↗

arXiv2012

Electronic correlations stabilize the antiferromagnetic Mott state in Cs$_3$C$_{60}$

Cs$_3$C$_{60}$ in the A15 structure is an antiferromagnet at ambient pressure in contrast with other superconducting trivalent fullerides. Superconductivity is recovered under pressure and reaches the highest critical temperature of the family. Comparing density-functional calculations with generalized gradient approximation to the hybrid functional HSE, which includes a suitable component of exchange, we establish that the antiferromagnetic state of Cs$_3$C$_{60}$ is not due to a Slater mechanism, and it is stabilized by electron correlation. HSE also reproduces the pressure-driven metalization. Our findings corroborate previous analyses suggesting that the properties of this compound can be understood as the result of the interplay between electron correlations and Jahn-Teller electron-phonon interaction.

cond-mat.supr-con↗

arXiv2012

Mott transition of fermionic mixtures with mass imbalance in optical lattices

We investigate the effect of mass imbalance in binary Fermi mixtures loaded in optical lattices. Using dynamical mean-field theory, we study the transition from a fluid to a Mott insulator driven by the repulsive interactions. For almost every value of the parameters we find that the light species with smaller bare mass is more affected by correlations than the heavy one, so that their effective masses become closer than their bare masses before a Mott transition occurs. The strength of the critical repulsion decreases monotonically as the mass imbalance grows so that the minimum is realized when one of the species is localized. The evolution of the spectral functions testifies that a continuous loss of coherence and a destruction of the Fermi liquid occur as the imbalance grows. The two species display distinct properties and experimentally-observable deviations from the behavior of a balanced Fermi mixture.

cond-mat.str-el↗

arXiv2011

Signature of antiferromagnetic long-range order in the optical spectrum of strongly correlated electron systems

We show how the onset of a non-Slater antiferromagnetic ordering in a correlated material can be detected by optical spectroscopy. Using dynamical mean-field theory we identify two distinctive features: The antiferromagnetic ordering is associated with an enhanced spectral weight above the optical gap, and well separated spin-polaron peaks emerge in the optical spectrum. Both features are indeed observed in LaSrMnO_4 [Gössling et al., Phys. Rev. B 77, 035109 (2008)]

cond-mat.str-el↗

arXiv2011

Pressure induced magnetic phase separation in La$_{0.75}$Ca$_{0.25}$MnO$_{3}$ manganite

The pressure dependence of the Curie temperature T$_{C}(P)$ in La$_{0.75}$Ca$_{0.25}$MnO$_{3}$ was determined by neutron diffraction up to 8 GPa, and compared with the metallization temperature T$_{IM}(P)$ \cite{irprl}. The behavior of the two temperatures appears similar over the whole pressure range suggesting a key role of magnetic double exchange also in the pressure regime where the superexchange interaction is dominant. Coexistence of antiferromagnetic and ferromagnetic peaks at high pressure and low temperature indicates a phase separated regime which is well reproduced with a dynamical mean-field calculation for a simplified model. A new P-T phase diagram has been proposed on the basis of the whole set of experimental data.

cond-mat.str-el↗