arXiv subjects

M. S. Laad

Publications and source records attributed to M. S. Laad.

At least 55 records · Page 3Linked to original sources

arXiv2008

Mottness underpins the anomalous optical response of Iron Pnictides

The recent discovery of high-temperature superconductivity (HTSC) in doped Iron pnictides is the latest example of unanticipated behavior exhibited by $d$- and $f$-band materials. The symmetry of the SC gap, along with the mechanism of its emergence from the ``normal'' state is a central issue in this context. Here, motivated by a host of experimental signatures suggesting strong correlations in the Fe-pnictides, we undertake a detailed study of their normal state. Focussing on symmetry-unbroken phases, we use the correlated band structure method, LDA+DMFT, to study the one-particle responses of both ${\rm LaO_{1-x}FeAsF_{x}}$ and ${\rm SmO_{1-x}FeAsF_{x}}$ in detail. Basing ourselves on excellent quantitative agreement between LDA+DMFT and key experiments probing the one-particle responses, we extend our study, undertaking the first detailed study of their normal state electrodynamic response. In particular, we propose that near-total normal state incoherence, resulting from strong, {\it local} correlations in the Fe $d$-shell in Fe-pnictides, underpins the incoherent normal state transport found in these materials, and discuss the specific electronic mechanisms leading to such behavior. We also discuss the implications of our work for the multi-band nature of the SC by studying the pairing ``glue'' function, which we find to be an overdamped, electronic continuum. Similarities and differences between cuprates and Fe-pnictides are also touched upon. Our study supports the view that SC in Fe-pnictides arises from a bad metallic, incoherent ``normal'' state that is proximate to a Mott insulator.

cond-mat.supr-con↗

arXiv2008

GdI_2: A New Ferromagnetic Excitonic Solid?

The two-dimensional, colossal magnetoresistive system GdI_2 develops an unusual metallic state below its ferromagnetic transition and becomes insulating at low temperatures. It is argued that this geometrically frustrated, correlated poor metal is a possible candidate for a ferromagnetic excitonic liquid. The renormalized Fermi surface supports a further breaking of symmetry to a charge ordered, excitonic solid ground state at lower temperatures via order by disorder mechanism. Several experimental predictions are made to investigate this unique orbitally correlated ground state.

cond-mat.str-el↗

arXiv2008

Hidden Orbital Liquid State Within Ferromagnetically Ordered Metallic SrRuO3

We have experimentally found related anomalies in electrical resistivity, dc and ac magnetic susceptibility, appearing deeply within ferromagnetically ordered state in SrRuO3. Lack of Jahn-Teller distortion in this regime rules out conventional orbital order, forcing one to describe these in terms of an orbital liquid ground state coexisting with ferromagnetic spin order. We suggest that weak spin-orbit coupling in such an unusual state underpins the observed anomalies.

cond-mat.str-el↗

arXiv2008

Normal State Correlated Electronic Structure of Iron Pnictides

We describe the correlated electronic structure of a prototype Fe-pnictide superconductor, $SmO_{1-x}F_{x}FeAs$, using LDA+DMFT. Strong, multi-orbital electronic correlations generate a low-energy pseudogap in the undistorted phase, giving a bad, incoherent metal in qualitative agreement with observations. Very good semi-quantitative agreement with the experimental spectral functions is seen, and interpreted, within a correlated, multi-orbital picture. Our results show that Fe-pnictides should be understood as low-carrier density, incoherent metals, in resemblance to the underdoped cuprate superconductors.

cond-mat.str-el↗

arXiv2008

Fermiology of Cuprates from First Principles: From Small Pockets to the Luttinger Fermi surface

Fermiology, the shape and size of the Fermi surface, underpins the low-temperature physical properties of a metal. Recent investigations of the Fermi surface of high-Tc superconductors, however, show a most unusual behavior: upon addition of carriers, ``Fermi'' pockets appear around nodal (hole doping) and antinodal (electron doping) regions of the Brillouin zone in the ``pseudogap'' state. With progressive doping, p, these evolve into well-defined Fermi surfaces around optimal doping (p_opt), with no pseudogap. Correspondingly, various physical responses, including d-wave superconductivity, evolve from highly anomalous, up to p_opt, to more conventional beyond. Describing this evolution holds the key to understanding high-temperature superconductivity. Here, we present ab initio quantum chemical results for cuprates, providing a quantitative description of the evolution of the Fermi surface with doping. Our results constitute an ab initio justification for several, hitherto proposed semiphenomenological theories, offering an unified basis for understanding of various, unusual physical responses of doped cuprates.

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↗

arXiv2007

Two-dimensional Ising model with competing interactions and its application to clusters and arrays of $π$-rings and adiabatic quantum computing

We study planar clusters consisting of loops including a Josephson $π$-junction ($π$-rings). Each $π$-ring carries a persistent current and behaves as a classical orbital moment. The type of particular state associated with the orientation of orbital moments at the cluster depends on the interaction between these orbital moments and can be easily controlled, i.e. by a bias current or by other means. We show that these systems can be described by the two-dimensional Ising model with competing nearest-neighbor and diagonal interactions and investigate the phase diagram of this model. The characteristic features of the model are analyzed based on the exact solutions for small clusters such as a 5-site square plaquette as well as on a mean-field type approach for the infinite square lattice of Ising spins. The results are compared with spin patterns obtained by Monte Carlo simulations for the 100 $\times$ 100 square lattice and with experiment. We show that the $π$-ring clusters may be used as a new type of superconducting memory elements. The obtained results may be verified in experiments and are applicable to adiabatic quantum computing where the states are switched adiabatically with the slow change of coupling constants.

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

On charge and spin ordering in a one-dimensional model with frustrating interactions

We study a one-dimensional extended Hubbard model with longer-range Coulomb interactions at quarter-filling in the strong coupling limit. We find two different charge-ordered ground states as the strength of the longer range interactions is varied. At lower energies, these charge ordered states drive two different spin-ordered ground states. A variety of response functions computed here bear a remarkable resemblance to recent experimental observations for organic TMTSF systems, and so we propose that these systems are proximate to a quantum critical point (QCP) associated with T=0 charge order. For a ladder system relevant to Sr_{14}Cu_{24}O_{41}, we find in-chain charge order, rung-dimer, and orbital antiferromagnetic ordered phases with varying interchain couplings and superconductivity with hole-doping.

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↗