arXiv subjects

G. Sangiovanni

Publications and source records attributed to G. Sangiovanni.

At least 55 records · Page 3Linked to original sources

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

Correlation effects in transport properties of interacting nanostructures

We discuss how to apply many-body methods to correlated nanoscopic systems, and provide general criteria of validity for a treatment at the dynamical mean field theory (DMFT) approximation level, in which local correlations are taken into account, while non-local ones are neglected. In this respect, we consider one of the most difficult cases for DMFT, namely for a quasi-one-dimensional molecule such as a benzene ring. The comparison against a numerically exact solution shows that non-local spatial correlations are relevant only in the limit of weak coupling between the molecule and the metallic leads and of low inter-atomic connectivity, otherwise DMFT provides a quantitative description of the system. As an application we investigate the role of correlations on electronic transport in quantum junctions, and we show that a local Mott-Hubbard crossover is a robust phenomenon in sharp nanoscopic contacts.

cond-mat.str-el↗

arXiv2013

Exploring the Dark Side of Cuprate Superconductors: s-wave Symmetry of the Pseudogap

We reveal the full energy-momentum structure of the pseudogap of underdoped high-Tc cuprate superconductors. Our combined theoretical and experimental analysis explains the spectral-weight suppression observed in the B2g Raman response at finite energies in terms of a pseudogap appearing in the single-electron excitation spectra above the Fermi level in the nodal direction of momentum space. This result suggests an s-wave pseudogap (which never closes in the energy-momentum space), distinct from the d-wave superconducting gap. Recent tunneling and photoemission experiments on underdoped cuprates also find a natural explanation within the s-wave-pseudogap scenario.

cond-mat.str-el↗

arXiv2013

Mott-Hubbard transition in V2O3 revisited

The isostructural metal-insulator transition in Cr-doped V2O3 is the textbook example of a Mott-Hubbard transition between a paramagnetic metal and a paramagnetic insulator. We review recent theoretical calculations as well as experimental findings which shed new light on this famous transition. In particular, the old paradigm of a doping-pressure equivalence does not hold, and there is a microscale phase separation for Cr-doped V2O3.

cond-mat.str-el↗

arXiv2013

Divergent Precursors of the Mott-Hubbard Transition at the Two-Particle Level

Identifying the fingerprints of the Mott-Hubbard metal-insulator transition may be quite elusive in correlated metallic systems if the analysis is limited to the single particle level. However, our dynamical mean-field calculations demonstrate that the situation changes completely if the frequency dependence of the two-particle vertex functions is considered: The first non-perturbative precursors of the Mott physics are unambiguously identified well inside the metallic regime by the divergence of the local Bethe-Salpeter equation in the charge channel. At low temperatures this occurs in the region where incoherent high-energy features emerge in the spectral function, while at high temperatures it is traceable up to the atomic-limit.

cond-mat.str-el↗

arXiv2012

Double Exchange model for nanoscopic clusters

We solve the double exchange model on nanoscopic clusters exactly, and specifically consider a six-site benzene-like nanocluster. This simple model is an ideal testbed for studying magnetism in nanoclusters and for validating approximations such as the dynamical mean field theory (DMFT). Non-local correlations arise between neighboring localized spins due to the Hund's rule coupling, favoring a short-range magnetic order of ferro- or antiferromagnetic type. For a geometry with more neighboring sites or a sufficiently strong hybridization between leads and the nanocluster, these non-local correlations are less relevant, and DMFT can be applied reliably.

cond-mat.str-el↗

arXiv2012

Spin state of negative charge-transfer material SrCoO3

We employ the combination of the density functional and the dynamical mean-field theory (LDA+DMFT) to investigate the electronic structure and magnetic properties of SrCoO3, monocrystal of which were prepared recently. Our calculations lead to a ferromagnetic metal in agreement with experiment. We find that, contrary to some suggestions, the local moment in SrCoO3 does not arise from intermediate spin state, but is a result of coherent superposition of many different atomic states. We discuss how attribution of magnetic response to different atomic states in solids with local moments can be quantified.

cond-mat.str-el↗

arXiv2012

Quantum dynamical screening of the local magnetic moment in Fe-based superconductors

We have calculated the local magnetic susceptibility of one of the prototypical Fe-based superconductors (LaFeAsO) by means of the local density approximation + dynamical mean field theory as a function of both (imaginary) time and real frequencies with and without vertex corrections. Vertex corrections are essential for obtaining the correct $ω$-dependence, in particular a pronounced low-energy peak at $ω\sim 0.2 $eV, which constitutes the hallmark of the dynamical screening of a large instantaneous magnetic moment on the Fe atoms. In experiments, however, except for the case of x-ray absorption spectroscopy (XAS), the magnetic moment or the susceptibility represent typically the average over long time scales. In this respect, the frequency range of typical neutron experiments would be too limited to directly estimate the magnitude of the short-time moment.

cond-mat.str-el↗

arXiv2012

Orbitally resolved lifetimes in Ba(Fe0.92Co0.08)2As2 measured by ARPES

Despite many ARPES investigations of iron pnictides, the structure of the electron pockets is still poorly understood. By combining ARPES measurements in different experimental configurations, we clearly resolve their elliptic shape. Comparison with band calculation identify a deep electron band with the dxy orbital and a shallow electron band along the perpendicular ellipse axis with the dxz/dyz orbitals. We find that, for both electron and hole bands, the lifetimes associated with dxy are longer than for dxz/dyz. This suggests that the two types of orbitals play different roles in the electronic properties and that their relative weight is a key parameter to determine the ground state.

cond-mat.supr-con↗

arXiv2012

Enhancement of the effective disorder potential and the thermopower in Na$_x$CoO$_2$ through the electron-phonon coupling

The effects of an electron-phonon ($e$-ph) interaction on the thermoelectric properties of Na$_x$CoO$_2$ are analyzed. By means of dynamical mean field theory calculations we find that the $e$-ph coupling acts in a cooperative way with the disorder, enhancing the effective binary disorder potential strength on the Co sites, which stems from the presence or absence of a neighboring Na atom. Hence, the inclusion of the $e$-ph coupling allows us to assume smaller values of the binary disorder potential strength -- which for Na$_x$CoO$_2$ are in fact also more reasonable. More generally, we can conclude that the interplay between disorder effects and coupling to the lattice can be exploited to engineer more efficient thermoelectric materials.

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

Atomic and itinerant effects at the transition metal x-ray absorption K-pre-edge exemplified in the case of V$_2$O$_3$

X-ray absorption spectroscopy is a well established tool for obtaining information about orbital and spin degrees of freedom in transition metal- and rare earth-compounds. For this purpose usually the dipole transitions of the L- (2p to 3d) and M- (3d to 4f) edges are employed, whereas higher order transitions such as quadrupolar 1s to 3d in the K-edge are rarely studied in that respect. This is due to the fact that usually such quadrupolar transitions are overshadowed by dipole allowed 1s to 4p transitions and, hence, are visible only as minor features in the pre-edge region. Nonetheless, these features carry a lot of valuable information, similar to the dipole L-edge transition, which is not accessible in experiments under pressure due to the absorption of the diamond anvil pressurecell. We recently performed a theoretical and experimental analysis of such a situation for the metal insulator transition of (V(1-x)Crx)2O3. Since the importance of the orbital degrees of freedom in this transition is widely accepted, a thorough understanding of quadrupole transitions of the vanadium K-pre-edge provides crucial information about the underlying physics. Moreover, the lack of inversion symetry at the vanadium site leads to onsite mixing of vanadium 3d- and 4p- states and related quantum mechanical interferences between dipole and quadrupole transitions. Here we present a theoretical analysis of experimental high resolution x-ray absorption spectroscopy at the V pre-K edge measured in partial fluorescence yield mode for single crystals. We carried out density functional as well as configuration interaction calculations in order to capture effects coming from both, itinerant and atomic limits.

cond-mat.str-el↗

arXiv2011

Evolution of the electronic structure of a Mott system across its phase diagram: an X-ray absorption spectroscopy study of (V(1-x)Crx)2O3

V2O3 is an archetypal system for the study of correlation induced, Mott-Hubbard metal-insulator transitions. Despite decades of extensive investigations, the accurate description of its electronic properties remains an open problem in the physics of strongly correlated materials, also because of the lack of detailed experimental data on its electronic structure over the whole phase diagram. We present here a high resolution X-ray absorption spectroscopy study at the V K-edge of (V(1-x)Crx)2O3 to probe its electronic structure as a function of temperature, doping and pressure, providing an accurate picture of the electronic changes over the whole phase diagram. We also discuss the relevance of the parallel evolution of the lattice parameters, determined with X-ray diffraction. This allows us to draw two conclusions of general interest: first, the transition under pressure presents peculiar properties, related to a more continuous evolution of the lattice and electronic structure; second, the lattice mismatch is a good parameter describing the strength of the first order transition, and is consequently related to the tendency of the system towards the coexistence of different phases. Our results show that the evolution of the electronic structure while approaching a phase transition, and not only while crossing it, is also a key element to unveil the underlying physical mechanisms of Mott materials .

cond-mat.str-el↗

arXiv2011

Size control of Charge-Orbital Order in Half-Doped Manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$

Motivated by recent experimental results, we study the effect of size reduction on half-doped manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$, using the combination of density functional theory (DFT) and dynamical mean field theory (DMFT). We find that upon size reduction, the charge-ordered antiferromagnetic phase, observed in bulk, to be destabilized, giving rise to the stability of a ferromagnetic metallic state. Our theoretical results, carried out on defect-free nanocluster in isolation, establish the structural changes that follow upon size reduction to be responsible for this. Our study further points out the effect of size reduction to be distinctively different from application of hydrostatic pressure. Interestingly, our DFT+DMFT study, additionally, reports the correlation-driven stability of charge-orbitally ordered state in bulk La$_{0.5}$Ca$_{0.5}$MnO$_3$, even in absence of long range magnetic order.

cond-mat.str-el↗

arXiv2011

Isotope effect in the pseudogap of high-temperature superconducting copper oxides

We study cuprates within Dynamical Cluster Approximation and find the pseudogap displays an isotope effect of the same sign as observed experimentally. Notwithstanding the non-phononic origin of the pseudogap the interplay between electronic repulsion and retarded phonon-mediated attraction gives rise to an isotope dependence of the antinodal spectra. Due to the strong momentum differentiation, such interplay is highly non-trivial and leads to the simultaneous presence of heavier quasiparticles along the nodal direction. We predict an isotope effect in electron-doped materials.

cond-mat.str-el↗

arXiv2010

Analytical continuation of imaginary axis data for optical conductivity

We compare different methods for performing analytical continuation of spectral data from the imaginary time or frequency axis to the real frequency axis for the optical conductivity sigma(omega). We compare the maximum entropy (MaxEnt), singular value decomposition (SVD), sampling and Pade methods for analytical continuation. We also study two direct methods for obtaining sigma(0). For the MaxEnt approach we focus on a recent modification. The data are split up in batches, a separate MaxEnt calculation is done for each batch and the results are averaged. For the problems studied here, we find that typically the SVD, sampling and modified MaxEnt methods give comparable accuracy, while the Pade approximation is usually less reliable.

cond-mat.str-el↗

arXiv2010

Fourier transformation and response functions

We improve on Fourier transforms (FT) between imaginary time $τ$ and imaginary frequency $ω_n$ used in certain quantum cluster approaches using the Hirsch-Fye method. The asymptotic behavior of the electron Green's function can be improved by using a "sumrule" boundary condition for a spline. For response functions a two-dimensional FT of a singular function is required. We show how this can be done efficiently by splitting off a one-dimensional part containing the singularity and by performing a semi-analytical FT for the remaining more innocent two-dimensional part.

cond-mat.str-el↗

arXiv2010

A Microscopic View on the Mott transition in Chromium-doped V2O3

V2O3 is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal to insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator (PI). This or related MITs have a high technological potential, among others for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V2O3 with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure which drives a genuine Mott transition to an eventually homogeneous metallic state.

cond-mat.str-el↗