arXiv subjects

L. Craco

Publications and source records attributed to L. Craco.

At least 37 records · Page 2Linked to original sources

arXiv2007

Electronic phase transitions in the half-filled ionic Hubbard model

A detailed study of electronic phase transitions in the ionic Hubbard model at half filling is presented. Within the dynamical mean field approximation a series of transitions from the band insulator via a metallic state to a Mott-Hubbard insulating phase is found at intermediate values of the one-body potential $Δ$ with increasing the Coulomb interaction $U$. We obtain a critical region in which the metallic phase disappears and a {\it novel} coexistence phase between the band and the Mott insulating state sets in. Our results are consistent with those obtained at low dimensions, thus they provide a concrete description for the charge degrees of freedom of the ionic Hubbard model.

cond-mat.str-el↗

arXiv2007

Orbital Order, Stripe Phases and Mott Transition in a Planar Model for Manganites

Understanding orbital ordered (OO) Mott insulating states lies at the heart of a consistent resolution of the colossal magneto-resistance (CMR) observed in manganites, where its melting induces a low-$T$ insulator-metal transition for $0.25 \le x\le 0.45$. Motivated thereby, we study the OO states in a planar model for bilayer manganites using DMFT and finite-size diagonalisation methods. We derive the correct OO ground states observed in manganites for $x=0,{1/2},{2/3},{3/4}$ in exact agreement with observations, including the charge-orbital-magnetic ordered stripe phases for $x>{1/2}$. These OO states are {\it exactly} shown to be associated with an ``alloy'' ordering of the $d_{3x^{2}-r^{2}}/d_{3y^{2}-r^{2}}$ orbitals on each $Mn^{3+}$ site.

cond-mat.str-el↗

arXiv2005

Re-examining the Verwey transition in Fe3O4

Motivated by recent structural data questioning the adequacy of the charge order (CO)/disorder picture for the Verwey transition (at T=T_V) in magnetite, we re-investigate this issue within a new theoretical picture. Using the state-of-the-art LDA+DMFT method, we show that the non-trivial interplay between the B-site octahedral distortions and strong, multi-orbital electronic correlations in the half-metallic state is a necessary ingredient for a proper quantitative understanding of the physical responses across T_V. While weak CO is found to have very small effects on the low-T spectral function, the low-T charge gap and the resistivity jump across T_V are quantitatively reproduced only upon inclusion of CO in LSDA+DMFT scheme. Our results strongly suggest that the Verwey transition is dominantly driven by multi-orbital electronic correlations with associated JT distortions on the B-sublattice, and constitutes a non-trivial advance in attempts to understand the physics of Fe3O4.

cond-mat.str-el↗

arXiv2005

Critical behaviour of thin films with quenched impurities

The critical behaviour of thin films containing quenched random impurities and inhomogeneities is investigated by the renormalization-group method. The finite-size crossover in impure films has been considered on the basis of the fundamental relationship between the effective spatial dimensionality and the characteristic lengths of the system. The difference between the critical properties of infinite systems and films is demonstrated and investigated. A new critical exponent, describing the scaling properties of the thickness of films with extended impurities has been deduced and calculated. A special attention is paid to the critical behaviour of real impure films.

cond-mat.stat-mech↗

arXiv2005

Mott-Hubbard quantum criticality in paramagnetic CMR pyrochlores

We present a correlated {\it ab initio} description of the paramagnetic phase of Tl$_2$Mn$_2$O$_7$, employing a combined local density approximation (LDA) with multiorbital dynamical mean field theory (DMFT) treatment. We show that the insulating state observed in this colossal magnetoresistance (CMR) pyrochlore is determined by strong Mn intra- and inter-orbital local electron-electron interactions. Hybridization effects are reinforced by the correlation-induced spectral weight transfer. Our result coincides with optical conductivity measurements, whose low energy features are remarkably accounted for by our theory. Based on this agreement, we study the disorder-driven insulator-metal transition of doped compounds, showing the proximity of Tl$_2$Mn$_2$O$_7$ to quantum phase transitions, in agreement with recent measurements.

cond-mat.str-el↗

arXiv2005

Insulator-Metal Transition in the Rutile-based VO2

The metal-insulator transition (MIT) in paramagnetic VO2 is studied within LDA+DMFT(IPT), which merges the local density approximation (LDA) with dynamical mean field theory (DMFT). With a fixed value of the Coulomb U=5.0eV, we show how the MIT is understood in a new picture: spectral weight transfer accompanying the increase in the displacement of V ion ($\perp c$) within the strong correlation scenario. Within this new scenario, we find good quantitative agreement with (i) switch of the orbital occupation of (xy,yz+zx,yz-zx), (ii) thermodynamics, and (iii) the one-electron spectral function in the metallic phase of VO2. We also compare our results for the total spectral density with other approaches which use QMC to solve the impurity problem of DMFT.

cond-mat.str-el↗

arXiv2005

Orbital selective insulator-metal transition in V2O3 under external pressure

We present a detailed account of the physics of Vanadium sesquioxide (${\rm V_2O_3}$), a benchmark system for studying correlation induced metal-insulator transition(s). Based on a detailed perusal of a wide range of experimental data, we stress the importance of multi-orbital Coulomb interactions in concert with first-principles LDA bandstructure for a consistent understanding of the PI-PM MIT under pressure. Using LDA+DMFT, we show how the MIT is of the orbital selective type, driven by large changes in dynamical spectral weight in response to small changes in trigonal field splitting under pressure. Very good quantitative agreement with ($i$) the switch of orbital occupation and ($ii$) S=1 at each $V^{3+}$ site across the MIT, and ($iii$) carrier effective mass in the PM phase, is obtained. Finally, using the LDA+DMFT solution, we have estimated screening induced renormalisation of the local, multi-orbital Coulomb interactions. Computation of the one-particle spectral function using these screened values is shown to be in excellent quantitative agreement with very recent experimental (PES and XAS) results. These findings provide strong support for an orbital-selective Mott transition in paramagnetic ${\rm V_2O_3}$.

cond-mat.str-el↗

arXiv2004

Metallizing the Mott insulator TiOCl by electron doping: a view from LDA+DMFT

Based on recent experiments, we describe the Mott insulating, but undimerized state of $TiOCl$ using the local-density approximation combined with multi-orbital dynamical mean field theory (LDA+DMFT) for this $3d^{1}$ system.Good agreement with the high estimated value of the superexchange is obtained. The possibility of an electron-doped insulator-metal transition in $TiOCl$ is investigated in this scheme and a Mott-Hubbard transition with a jump in carrier density {\it inside} the metallic state is found. Clear, co-incident, discontinuous changes in orbital occupations are observed, showing that such a transition would involve strong, multi-orbital correlations. These results call for studies on suitably intercalated $TiOCl$ that may induce metallization and, possibly, unconventional superconductivity.

cond-mat.str-el↗

arXiv2004

Cluster coherent potential approximation for electronic structure of disordered alloys

We extend the single-site coherent potential approximation (CPA) to include the effects of non-local disorder correlations (alloy short-range order) on the electronic structure of random alloy systems. This is achieved by mapping the original Anderson disorder problem to that of a selfconsistently embedded cluster. This cluster problem is then solved using the equations of motion technique. The CPA is recovered for cluster size $N_{c}=1$, and the disorder averaged density-of-states (DOS) is always positive definite. Various new features, compared to those observed in CPA, and related to repeated scattering on pairs of sites, reflecting the effect of SRO are clearly visible in the DOS. It is explicitly shown that the cluster-CPA method always yields positive-definite DOS. Anderson localization effects have been investigated within this approach. In general, we find that Anderson localization sets in before band splitting occurs, and that increasing partial order drives a continuous transition from an Anderson insulator to an incoherent metal.

cond-mat.str-el↗

arXiv2004

VO2: a two-fluid incoherent metal?

We present {\it ab initio} LDA+DMFT results for the many-particle density of states of $VO_{2}$ on the metallic side of the strongly first-order ($T$-driven) insulator-metal transition. In strong contrast to LDA predictions, there is {\it no} remnant of even correlated Fermi liquid behavior in the correlated metal. Excellent quantitative agreement with published photoemission and X-ray absorption experiments is found in the metallic phase. We argue that the absence of FL-quasiparticles provides a natural explanation for the bad-metallic transport for $T > 340 K$. Based on this agreement, we propose that the I-M transition in $VO_{2}$ is an orbital-selective Mott transition, and point out the relevance of orbital resolved one-electron and optical spectroscopy to resolve this outstanding issue.

cond-mat.str-el↗

arXiv2003

Insulator-Metal transition in the Doped 3d1 Transition Metal Oxide LaTiO3

The doping induced insulator-metal transition in $La_{1-x}Sr_{x}TiO_{3}$ is studied using the ab-initio LDA+DMFT method. Combining the LDA bandstructure for the actual, distorted structure found recently with multi-orbital DMFT to treat electronic correlations, we find: $(i)$ ferro-orbital order in the Mott insulating state without orbital degeneracy, $(ii)$ a continuous filling induced transition to the paramagnetic metal (PM) with $x$, and $(iii)$ excellent quantitative agreement with published photoemission data for the case of 6% doping. Our results imply that this system can be described as a Mott-Hubbard system without orbital (liquid) degeneracy.

cond-mat.str-el↗

arXiv2003

Towards a Fully Ab-Initio Description of the Diluted Magnetic Semiconductor $Ga_{1-x}Mn_{x}As$. Ferromagnetism, Electronic Structure, and Optical Response

There are two competing views of itinerant ferromagnetism, the first viewing ferromagnetism as resulting from the indirect coupling between local moments via the itinerant carrier dynamics, the so-called RKKY mechanism, while in the alternative picture, ferromagnetism results from the spin polarization of itinerant carriers by the strong atomic Hund interaction - the so-called double exchange (DE) scenario. Which view describes the ferromagnetism in diluted magnetic semiconductors, materials with promise for spintronic applications, is still unclear. Here, we describe the detailed physical response of the prototype material $Ga_{1-x}Mn_{x}As$ using a combination of first-principles bandstructure with methods based on dynamical mean field theory to incorporate strong, dynamical correlations {\it and} intrinsic as well as extrinsic disorder in one single theoretical picture. We show how ferromagnetism is driven by DE, in agreement with very recent observations, along with a good quantitative description of the details of the electronic structure, as probed by scanning tunnelling microscopy (STM) and optical conductivity. Our results show how ferromagnetism can be driven by DE even in diluted magnetic semiconductors with small carrier concentration.

cond-mat.str-el↗

arXiv2002

Orbital Switching and the First-Order Insulator-Metal Transition in Paramagnetic V_2O_3

The first-order metal-insulator transition (MIT) in paramagnetic $V_{2}O_{3}$ is studied within the ab-initio scheme LDA+DMFT, which merges the local density approximation (LDA) with dynamical mean field theory (DMFT). With a fixed value of the Coulomb $U=6.0 eV$, we show how the abrupt pressure driven MIT is understood in a new picture: pressure-induced decrease of the trigonal distortion within the strong correlation scenario (which is not obtained within LDA). We find good quantitative agreement with $(i)$ switch of the orbital occupation of $(a_{1g},e_{g1}^π, e_{g2}^π)$ and the spin state S=1 across the MIT, $(ii)$ thermodynamics and $dc$ resistivity, and $(iii)$ the one-electron spectral function, within this new scenario.

cond-mat.str-el↗

arXiv2002

Orbital Kondo Effect in $CrO_{2}$: A LSDA+DMFT Study

Motivated by a collection of experimental results indicating the strongly correlated nature of the ferromagnetic metallic state of $CrO_{2}$, we present results based on a combination of the actual bandstructure [3] with dynamical mean-field theory (DMFT) for the multi-orbital case. In striking contrast with LSDA(+U) [3] and model many-body approaches [14], much better semiquantitative agreement with (i) recent photoemission results, (ii) domain of applicability of the half-metal concept, and (iii) thermodynamic and dc transport data, is obtained within a single picture. Our approach has broad applications for the detailed first principles investigation of other transition metal oxide-based half-metallic ferromagnets.

cond-mat.str-el↗

arXiv2002

Backbone-induced semiconducting behavior in short DNA wires

We propose a model Hamiltonian for describing charge transport through short homogeneous double stranded DNA molecules. We show that the hybridization of the overlapping pi orbitals in the base-pair stack coupled to the backbone is sufficient to predict the existence of a gap in the nonequilibrium current-voltage characteristics with a minimal number of parameters. Our results are in a good agreement with the recent finding of semiconducting behavior in short poly(G)-poly(C) DNA oligomers. In particular, our model provides a correct description of the molecular resonances which determine the quasi-linear part of the current out of the gap region.

cond-mat.soft↗

arXiv2002

On the heavy-fermion behavior of the pyrochlore transition-metal oxide $LiV_{2}O_{4}$

Motivated by the heavy fermion Fermi liquid (HFFL) features observed at low-$T$ in the pyrochlore $LiV_{2}O_{4}$, we consider a material-specific model that includes aspects of the local quantum chemistry, the geometrically frustrated lattice structure, and strong correlations in a {\it single} approach. In particular, we show how geometrical frustration (GF) gives rise to a crossover scale, $T^{*}<<J$, the intersite (AF) exchange, below which the metallic system shows HFFL features. Our scenario is a specific realization of the importance of GF effects in driving HFFL behavior in $LiV_{2}O_{4}$, and provides a natural understanding of various puzzling features observed experimentally.

cond-mat.str-el↗

arXiv2001

Dynamical correlations in half-metallic manganites

Motivated by the recent optical and photoemission measurements on half-metallic ferromagnetic three-dimensional manganites, we combine a tight-binding fit of the one-particle bandstructure with the dynamical mean-field theory, which treats the dynamical orbital correlations and the combination of Jahn-Teller and doping-induced disorder on the same footing. We show how all of the above effects are necessary to obtain good semiquantitative agreement with experimental features. As applications, we show how modest external magnetic fields drive drastic changes in the optical spectrum, demonstrating the {\it colossal ac magnetoconductivity}. The photoemission lineshape contribution is evaluated, and good agreement with published experimental work is found.

cond-mat.str-el↗

arXiv2001

Orbital Correlations in the Ferromagnetic Half-Metal $CrO_{2}

We deduce a model relevant for the description of the ferromagnetic half-metal Chromium dioxide ($CrO_{2}$), widely used in magnetic recording technology. The model describes the effect of dynamical, local orbital correlations arising from local quantum chemistry of the material. A finite temperature solution of the model in $d=\infty$ provides a natural explanation of the optical response, photoemission, resistivity and the large Woods-Saxon ratio observed in experiments. Our study confirms the important role of many body dynamical correlation effects for a proper understanding of the metallic phase of $CrO_{2}$.

cond-mat.str-el↗