arXiv subjects

M. S. Laad

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

At least 37 records · Page 2Linked to original sources

arXiv2016

Manifestations of Strange Metallicity in Inelastic Neutron Studies

Emergence of an orbital-selective Mott phase (OSMP) found in multi-band correlated systems leads to a non-perturbative obliteration of the Landau Fermi liquid in favor of a novel metallic state exhibiting anomalous infra-red (branch-cut) continuum features in one- and two-particle responses. We use a combination of $(1)$ dynamical mean-field theory (DMFT) using the continuous-time-quantum Monte-Carlo (CTQMC) solver for a two-band Hubbard model and $(2)$ analytic arguments from an effective bosonized description to investigate strange metal features in inelastic neutron scattering studies for cuprates. Specifically, restricting our attention to symmetry-unbroken metallic phase, we study how emergence of an OSMP leads to qualitatively novel features in $(i)$ the dynamical spin and charge susceptibilities, and $(ii)$ phonon response in the strange metal, in detail. Extinction of the Landau quasiparticle pole in the one-electron propagator in the OSMP mirrors the emergence of critical liquid-like features in the dynamical spin response. This novel finding also underpins truly anomalous features in phonon dynamics, which we investigate by coupling half-breathing phonons in the specific context of cuprates to such a multi-electronic continuum. We find good understanding of various anomalies encountered in experimental inelastic neutron scattering studies in the near-optimally doped cuprates. We also extend these results in a phenomenological way to argue how modification of phonon spectra in underdoped cuprates can be reconciled with proposals for a nematic-plus-d-wave charge modulation order in the pseudogap state. We also study the issue of the dominant "pair glue" contributions to superconductivity, allowing us to interpret recent pump-probe results within a strange metal scenario.

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arXiv2015

Quantum Criticality in the 122 Iron Pnictide Superconductors Emerging from Orbital-Selective Mottness

The twin issues of the nature of the normal state and competing order(s) in the iron arsenides are central to understanding their unconventional, high-Tc superconductivity. We use a combination of transport anisotropy measurements on detwinned Sr(Fe(1-x)Co(x))2As2 single crystals and local density approximation plus dynamical mean field theory (LDA + DMFT) calculations to revisit these issues. The peculiar resistivity anisotropy and its evolution with x are naturally interpreted in terms of an underlying orbital-selective Mott transition (OSMT) that gaps out the dxz or dyz states. Further, we use a Landau-Ginzburg approach using LDA + DMFT input to rationalize a wide range of anomalies seen up to optimal doping, providing strong evidence for secondary electronic nematic order. These findings suggest that strong dynamical fluctuations linked to a marginal quantum-critical point associated with this OSMT and a secondary electronic nematic order constitute an intrinsically electronic pairing mechanism for superconductivity in Fe arsenides.

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arXiv2015

Emergent Quantum Coherence from Instabilities of a Perturbed Fractionalized Spin Liquid

Effects of perturbations on topologically ordered (TO) states with fractionalized excitations is very intriguing. While small perturbations are not expected to affect a robust TO, sizable perturbations must lead to onset of novel routes to order(s). Observability of such emergent orders in real condensed matter systems continues to be an open issue of paramount interest. Here, we detail how a featureless Kitaev spin liquid with rigorous TO gives way to partial topological ordered state(s) coexisting with a wide staircase of novel `excitonic' orders of fractionalized entities in a Zeeman field. We provide quasi-exact and novel realization for a range of strange features, from nodal spin-metals, fractionalization in selective-Mott states, to Luttinger surfaces and hidden coherence, all characteristics of the influential resonating- valence-bond as well as fractionalized-Fermi-Liquid views in the context of strange metals. As a novel application, we show how Josephson-junction array may exhibit fractional Josephson oscillations upon flux tuning as a particularly novel manifestation of emergent quantum coherence.

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arXiv2015

Mott-Kondo Insulator Behavior in the Iron Oxychalcogenides

We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeO$\emph{Ch}$) series La$_{2}$O$_{2}$Fe$_{2}$O\emph{M}$_{2}$ (\emph{M}=S, Se), which is the latest among the Fe-based materials with the potential \ to show unconventional high-T$_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard features in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectra, as well as resitivity data, reveal that the parent FeO$\emph{Ch}$ are correlation-driven insulators. To uncover microscopics underlying these findings, we perform local density approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that unravel a Mott-Kondo insulating state. Based upon good agreement between theory and a range of data, we propose that FeO$\emph{Ch}$ may constitute a new, ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.

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arXiv2012

Preformed Excitons, Orbital Selectivity, and Charge-Density-Wave Order in 1T-TiSe_2

Traditional routes to Charge-Density-Wave in transition metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities have recently been brought into question. While this calls for exploration of alternative views, paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between CDW and superconductive orders in transition metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic structure calculations, to an excitonic-CDW order in 1T-TiSe$_{2}$. We show that orbital-selective dynamical localisation arising from preformed excitonic liquid correlations is somewhat reminiscent to states proposed for $d$- and $f$-band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition metal dichalcogenides on the border of CDW. This brings the transition metal dichalcogenides closer to the bad actors in $d$- and $f$-band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism.

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arXiv2011

Quantum Critical Phase and Lifshitz Transition in an Extended Periodic Anderson Model

We study the quantum phase transition in $f$-electron systems as a quantum Lifshitz transition driven by selective Mott localization in a realistic extended Anderson lattice model. Using DMFT, we find that a quantum critical {\it phase} with anomalous $ω/T$ scaling separates a heavy Landau-Fermi liquid from ordered phase(s). Fermi surface reconstruction occurs via the interplay between, and penetration of the Green function zeros to the poles, leading to violation of Luttinger's theorem in the selective-Mott phase . We show how this naturally leads to scale-invariant responses in transport. Our work is represents a specific (DMFT) realization of the hidden-FL and FL$^{*}$ theories, and holds promise for study of "strange" metal phases in quantum matter.

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arXiv2011

Unconventional Mott Transition in KxFe2-ySe2

Whether the newly discovered KxFe2-ySe2 systems are doped Mott or band insulators is key to how superconductivity emerges at lower temperature. With extant theoretical studies supporting conflicting scenarios, a more realistic approach is urgently called for. Here, we use LDA+DMFT to study this issue in KxFe2-ySe2. We find that the undoped KFe1.6Se2 system is a new kind of Mott-Kondo insulator (MKI). Electron doping this MKI drives a Mott transition to an orbital-selective non-Fermi liquid metal. Good agreement with spectral and transport responses supports our view, implying that superconductivity arises from a doped Mott insulator, as in the high-Tc cuprates.

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arXiv2011

Preformed excitonic liquid route to a charge density wave in 2H-TaSe$_2$

Recent experiments on 2H-TaSe$_2$ contradict the long-held view of the charge density wave arising from a nested band structure. An intrinsically strong coupling view, involving a charge density wave state arising as a Bose condensation of preformed excitons emerges as an attractive, albeit scantily investigated alternative. Using local density approximation plus multiorbital dynamic mean-field theory, we show that this scenario agrees with a variety of normal state data for 2H-TaSe$_{2}$. Based thereupon, the ordered states in a subset of dichalcogenides should be viewed as instabilities of a correlated, preformed excitonic liquid.

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arXiv2010

Theory of Normal State Pseudogap Behavior in FeSe(1-x)$Te(x)

The normal state of the recently discovered Iron Selenide (FeSe)-based superconductors shows a range of inexplicable features. Along with bad-metallic resistivity, characteristic pseudogap features and proximity to insulating states, reminiscent of the underdoped high-Tc cuprates, mark these systems as strongly correlated non-Fermi Liquid metals. Here, using the first-principles LDA+DMFT method, we show how strong multi-orbital correlation-induced orbital-selective Mott-like physics leads to an orthogonality catastrophe underpinning these inexplicable incoherent features. Excellent agreement with a range of resistivity and Seebeck data strongly support our proposal. We discuss pseudogap regime microscopically, along with implications for the nature of the instability at lower T, and propose that related systems could be of use in thermoelectric devices.

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arXiv2009

$α$-FeSe as an orbital-selective incoherent metal: An LDA+DMFT study

$α$-FeSe, a prototype iron-chalcogenide superconductor, shows clear signatures of a strange incoherent normal state. Motivated thereby, we use LDA+DMFT to show how strong multi-band correlations generate a low-energy pseudogap in the normal state, giving an incoherent metal in very good semi-quantitative agreement with observations. We interpret our results in terms of $α$-FeSe being close to Mottness. A wide range of anomalous responses in the "normal" state are consistently explained, lending strong support for this view. Implications for superconductivity arising from such an anomalous state are touched upon.

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arXiv2009

Towards A Possible Charge-Kondo effect in Optical Lattices

The Kondo effect underpins a large body of recent developments in the physics of $d$- and $f$-band compounds. Although its {\it charge} analog is a rarity in solids, the recent observations of the charge Kondo effect and the consequent rise in superconducting T$_c$ encourage a search for other accessible systems. Motivated by the possibility of wilfully tuning the {\it sign} of the inter-electronic interaction in optical lattices, we study conditions for the elusive {\it charge Kondo liquid} (CKL) state to manifest. We propose that a combination of Feshbach resonances and sequentially controlled laser pulses may produce the CKL. We show that the CKL is {\it never} a stable ground state, appearing only when the ordered ground states are destabilized. Finally, we discuss interesting analogies with nuclear matter.

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arXiv2009

Anomalous Magnetic Susceptibility in Iron Pnictides: Paramagnetic Phase

Observation of an anomalous temperature dependence of the spin susceptibility, along with a spin gap in NMR, in the quantum paramagnetic normal state of Iron Pnictides is a signature of an unusual metallic state. We argue that both these anomalous features are associated with a wide fluctuational regime dominated by dynamical, short-ranged and frustrated spin correlations in a strongly correlated metal. Using LDA+DMFT, we show that both these features can be quantitatively undertstood in the doped Fe-pnictides. We argue that such spin correlations naturally arise in a Mottness scenario, where an effective, dualistic description involves coexisting renormalized quasiparticles and effectively localized moments, arising from the same set of d bands.

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arXiv2009

Theory of Magnetic Fluctuations in Iron Pnictides

Magnetic fluctuations in an unconventional superconductor (U-SC) can distinguish between distinct proposals for the symmetry of the order parameter. Motivated thereby, we undertake a study magnetic fluctuations in Iron pnictides, tracking their evolution from the incoherent normal, pseudogapped metal, to the U-SC state. Within our proposal of extended-s-plus s_{xy} inplane gap with proximity-induced out-of-plane line nodes, (i) we describe the evolution of the spin-lattice relaxation rate, from a non-Korringa form in the normal state, to a power-law form in the U-SC in good agreement with experiment, and (ii) we predict a sharp resonance in the U-SC state along (π,π), but not along (π/2,0), along with modulated c-axis intensity in inelastic neutron scattering work as a specific and testable manifestation of our proposal.

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arXiv2009

A Quantum Chemistry Plus Dynamical Mean Field Approach for Correlated Insulators: Application to La_{2}CuO_{4}

While the traditional local-density approximation (LDA) cannot describe Mott insulators, {\it ab-initio} determination of the Hubbard $U$, for example, limits LDA-plus dynamical mean field theory (DMFT) approaches. Here, we attempt to overcome these bottlenecks by achieving fusion of the quantum chemistry (QC) approach with DMFT. QC+DMFT supplants the LDA bandstructure by its QC counterpart as an input to DMFT. Using QC+DMFT, we show that undoped $La_{2}CuO_{4}$ is a $d$-Mott insulator, and qualitatively discuss the circulating current- and incoherent metal phase, at small but finite hole doping. Very good quantitative agreement with experimental photoemission- and optical spectra constitutes strong support for efficacy of QC+DMFT. Our work thus opens a new avenue for truly {\it ab-initio} correlation-based approaches to describe correlated electronic systems in general.

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arXiv2009

Self-Doping Induced Orbital-Selective Mott Transition in Hg2Ru2O7

Pyrochlore oxides are fascinating systems where strong, multi-orbital correlations in concert with geometrical frustration give rise to unanticipated physical properties. The detailed mechanism of the insulator-metal transitions (IMT) underpinning these phenomena is, however, ill-understood in general. Motivated thereby, we study the IMT in the pyrochlore ${\rm Hg_{2}Ru_{2}O_{7}}$ using LDA+DMFT. In contrast to the well-known examples of Mott transitions in TMO, we show that, in the negative charge-transfer situation characteristic of \hg, self-doping plays a crucial role in the emergence of an orbital-selective IMT. We argue that this mechanism has broader relevance to other correlated pyrochlore oxides.

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arXiv2008

Ferromagnetism in the Extended Periodic Anderson Model Near Selective Mott Localization

Motivated by the experimental finding of Ferromagnetism (FM) near non-FL quantum phase transitions (QPT), I investigate FM in an Extended Periodic Anderson Model (EPAM). Using the Frobenius-Perron theorem, a FM {\it metallic} phase near the selective Mott localization is rigorously found. Interesting connections with Tasaki's flat-band ferromagnetism, as well as with the well-known double exchange (DE) mechanism are elucidated. This finding is a higher dimensional generalization of numerical work on the D=1 PAM, and constitutes a robust way to FM metallic behavior in $D>1$. Finally, we discuss a concrete manifestation of our results to $UGe_{2}$.

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arXiv2008

On Field-Induced Quantum Criticality in YbRh_{2}Si_[2}

The quantum critical point (QCP) in $YbRh_{2}Si_{2}$ is an enigma for the itinerant view of QCP. In an alternative view, this QCP is intimately linked to the selective Mott localization of the heavy $f$ electrons. Following a perusal of this unusual QCP, I study an Extended Periodic Anderson Model (EPAM) within DMFT. A quantum phase transition (FQPT), accompanied by a rapid change in the Fermi volume, is found near the quantum-critical end-point of the selective Mott transition in the $f$-electron sector. The theory accounts for a wide range of unusual, singular non-Fermi liquid features exhibited at this QCP in $YbRh_{2}Si_{2}$ in a natural way.

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arXiv2008

On the Impossibility of Electronic Phase Separation in the $D=1,2$ Dimensional Hubbard Model at T=0

We show explicitly that the one- and two-dimensional Hubbard model does not show phase separation at any filling at T=0. Apart from a single plausible assumption, only known exact results and symmetry properties for the one-band Hubbard model are used. Implications for usage of the simple one-band Hubbard model for analysing the physics of cuprate superconductors are discussed. We also discuss models where electronic phase separation might generically occur, in the specific context of colossal magneto-resistance manganites.

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