arXiv subjects

Mukul S. Laad

Publications and source records attributed to Mukul S. Laad.

At least 19 recordsLinked to original sources

arXiv2026

Field driven Metal-Insulator transition in rhombohedral Bismuth and Arsenic crystals

The metal to insulator (MIT) transition is accompanied by huge changes in physical responses by the control and tuning of experimental parameters like doping, pressure, chemical composition, and magnetic field. Here, we study the magnetic field-driven MIT for two pnictides in their elemental form, namely Arsenic and Bismuth. At low temperatures, Bismuth shows an unusual behaviour of a re-entrant IMT at high fields in addition to a higher temperature MIT at smaller fields. However, Arsenic shows the commonly observed single MIT. The Shubnikov de Haas (SdH) oscillations are observed for both As and Bi below 10 K. Giant magneto-resistance of the order of ~105 (MR%) is observed for both crystals at 2 K and 14 Tesla transverse magnetic field. The unusual Kohler scaling behaviour of MR at low temperature indicate the presence of increased carrier density attributed to the melting of excitons. Based on a microscopic model, the microscopic processes underpinning the unusual features of a field-driven MIT and re-entrant IMT, along with the relevance of both excitonic and Bose metal correlations near these incipient instabilities, are qualitatively described in the framework of field-driven excitonic condensate and Das-Doniach preformed pair scenarios in one single picture.

cond-mat.mtrl-sci↗

arXiv2020

Fascinating interplay between Charge Density Wave Order and magnetic field in Non-magnetic Rare-Earth Tritelluride LaTe$_{3}$

Charge density wave (CDW) states in solids bear an intimate connection to underlying fermiology. Modification of the latter by a suitable perturbation provides an attractive handle to unearth novel CDW states. Here, we combine extensive magnetotransport experiments and first-principles electronic structure calculations on a non-magnetic tritelluride LaTe$_{3}$ single crystal to uncover phenomena rare in CDW systems: $(i)$ hump-like feature in the temperature dependence of resistivity at low temperature under application of magnetic field, which moves to higher temperature with increasing field strength, $(ii)$ highly anisotropic large transverse magnetoresistance (MR) upon rotation of magnetic field about current parallel to crystallographic c-axis, (iii) anomalously large positive MR with spike-like peaks at characteristic angles when the angle between current and field is varied in the bc-plane, (iv) extreme sensitivity of the angular variation of MR on field and temperature. Moreover, our Hall measurement reveals remarkably high carrier mobility $\sim$ 33000 cm$^{2}$/Vs, which is comparable to that observed in some topological semimetals. These novel observations find a comprehensive explication in our density functional theory (DFT) and dynamical mean field theory (DMFT) calculations that capture field-induced electronic structure modification in LaTe$_{3}$. The band structure theory together with transport calculations suggest the possibility of a second field-induced CDW transition from the field-reconstructed Fermi surface, which qualitatively explains the hump in temperature dependence of resistivity at low temperature. Thus, our study exposes the novel manifestations of the interplay between CDW order and field-induced electronic structure modifications in LaTe$_{3}$, and establishes a new route to tune CDW states by perturbations like magnetic field.

cond-mat.mtrl-sci↗

arXiv2018

Emergent Strange Nodal Metallicity from Orbital-Selective Mott Physics

While a specific kind of strange metal is increasingly found to be the "normal" states in a wide variety of unconventional superconductors, its microscopic origin is presently a hotly debated enigma. Using dynamical mean-field theory (DMFT) based on hybridization expansion of continuous-time quantum Monte-Carlo (CTQMC) solver for an extended two-band Hubbard model (2BHM), we investigate the conditions underlying the emergence of such a metal. Specifically, we tie strange metallicity to an orbital-selective Mottness in 2BHM or momentum-selective Mott phase (OSMP) in 2D Hubbard models inspired by a cluster-to-orbital mapping. We find $(i)$ disparate spin and charge responses, $(ii)$ fractional power-law behavior and $ω/T$-scaling in the charge and spin fluctuation responses, and $(iii)$ very good accord with optical conductivity and nuclear magnetic relaxation rates in the slightly underdoped normal states of cuprates and Fe-arsenides. We analyze the local problem using bosonization to show that such anomalous responses arise from a lattice orthogonality catastrophe specifically in the OSMP. Our work establishes the intimate link between strange metallicity and selective Mottness in quantum matter.

cond-mat.str-el↗

arXiv2018

Microscopic Description of Unconventional Nodal Superconductivity in FeSe

Finding of unconventional superconductivity (USC) in FeSe in an electronic "normal" state with broken $C_{4v}$ rotational symmetry testifies to the diversity of pairing states in Fe-based superconductors. Moreover, such USC emerges as a direct instability of a normal state without Landau Fermi liquid quasiparticles, increasingly dubbed a `strange' metal. Here, we combine inputs from a first-principles correlated electronic structure method (LDA+DMFT) and symmetry analyses to propose a novel mechanism for unconventional nodal superconductivity as a direct instability of an incoherent bad-metal without Landau Fermi-liquid quasiparticles. We find that a ferro-quadrupolar order, with novel spin quadrupoar correlations enhances orbital-selective Mottness in FeSe, and competes with unconventional nodal superconductivity with $s_{\pm}$-pair symmetry. We support our proposal by demonstrating good accord with spectral and magnetic fluctuation data, and rationalize the strain and pressure dependence of $T_{c}$ by appealing to competition between superconductivity and electronic ferro-quadrupolar order.

cond-mat.supr-con↗

arXiv2018

Can disorder act as a chemical pressure? An optical study of the Hubbard model

The optical properties have been studied using the dynamical mean-field theory (DMFT) on a disordered Hubbard model. Despite the fact that disorder turns a metal to an insulator in high dimensional correlated materials, we notice that it can enhance certain metallic behavior as if a chemical pressure is applied to the system resulting in an increase of the effective lattice bandwidth (BW). We study optical properties in such a scenario and compare results with experiments where the BW is changed through chemical doping and obtain remarkable similarities vindicating our claim. We also make a point that these similarities differ from some other forms of BW tuned optical effects.

cond-mat.str-el↗

arXiv2015

Mott transitions and Novel Orders in Multi-Orbital Models: The Relevance of Structural "Double Exchange"

In real transition-metal oxides, the so-called GdFeO3 octahedral tilt is long known to be a relevant control parameter influencing the range of orbital and magnetic ordered states found across families of cubic perovskite families. Their precise role in the interplay between itinerance and localisation, long known to underpin Mottness in d-band oxides, has however received much less attention. We analyse the relevance of the GdFeO3 tilt in detail in a representative setting of a partially-filled e g orbital system. We identify a new generalised principle of broad relevance, namely, that this tilt acts like a structural "double exchange" and acts contrary to the well-known Anderson-Hasegawa double exchange. As a function of this tilt, therefore, a phase transition from a Mott-Hubbard insulator to an incoherent bad-metal occurs as an effective band-width-controlled Mott transition. We analyse the incoherent metal in detail by studying one- and two-particle spectral responses and propose that this selective-metal is a novel orbital analogue of the FL* state with fractionalised orbitons. Finally, we apply these ideas to qualitatively discuss the effect of strain on thin films of such d-band oxides on suitable substrates, and discuss the exciting possibility of engineering novel ordered phases, such as unconventional circulating currents, nematics and superconductors, by suitable strain engineering in TMO thin films.

cond-mat.str-el↗

arXiv2015

Fermiology in a Local Quantum Critical Metal

Recent experimental work has brought the twin issues of the origin of non-Lifshitz-Kosevich scaling in de Haas van Alphen (dHvA) and its precise relation to anomalously broad non-quasiparticle spectral features in "strange" metals, to the forefront. Here, we revisit these issues in the specific context of a "local" quantum critical phase in an extended periodic Anderson model (EPAM). In contrast to the famed Kondo-RKKY scenarios for local quantum criticality, strong local valence fluctuations cause Kondo destruction in the EPAM. We uncover a common underlying element, namely, the Kondo-destruction-driven infra-red continuum branch-cut behavior in the one-electron propagator, as the relevant feature that governs both non-Lifshitz-Kosevich scaling in dHvA and anomalously broad non-quasiparticle spectral responses in such a "strange" metal. Employing a non-perturbative scheme to treat effects of non-magnetic disorder in this version of a local strange metal, we propose a modified Dingle scaling that can also be used to test "local" criticality scenarios. Thus, our findings potentially afford an internally consistent description of novel fermiology expected to manifest in strange metals arising as a result of Kondo-destruction coming from an underlying orbital-selective Mott transition.

cond-mat.str-el↗

arXiv2013

Orbital-Selective Mottness in Layered Iron Oxychalcogenides: The Case of Na_{2}Fe_{2}OSe_{2}

Using a combination of th local-density approximtion (LDA) and dynamicla mean-field theory (DMFT) calculations, we explore the correlated electronic structure of a member of the layered Iron oxychalcogenide Na_{2}Fe_{2}OSe_{2}. We find that the parent compund is a multi-orbital Mott insulator. Surprisingly, and somewhat reminiscent of the doped high-T_{c} cuprates, carrier localization is found to persist upon weak hole doping because the chemical potential lies in a gap structure wit almost vanishing density-of-states (DOS). On the other hand, in remarkable contrast, electron doping drives an orbital-selective metallic phase (OSMP) with co-existing pseudogapped (Mott localized) and itinerant carriers. These remarkably contrasting behaviors in a single system thus stem from drastic electronic reconstruction caused by large-scale transfe of dynamical spectral weightinvolving states with distinct orbital character at low energies, putting the Fe oxychalcogenidesneatly into the increasingly visible tendency of Fe-baed systems as ones in orbital-selective Mott phases. We detail the implications of our analysis, and discuss the nature and symmetry of the superconductive states that may arise upon proper doping or pressurizing Na_{2}Fe_{2}OSe_{2}.

cond-mat.str-el↗

arXiv2010

Theory of Orbital Nematicity in Underdoped Iron Arsenides

Recent finding of an {\it unusual} in-plane resistivity anisotropy in the underdoped 122-family at high temperature ($T$) suggests an orbital nematic (ON) order, posing a challenge to extant theories. The {\it sign} of the anisotropy contradicts expectations from weakly correlated as well as pure spin-only nematic views. Here, we show how such an ON order with accompanying structural distortion arises from {\it residual}, intersite and inter-orbital two-body interactions in an incoherent "bad metal" close to Mottness. Enhancement of orbital-selective incoherence is shown to be necessary for understanding transport anisotropy. Our results suggest that ON order, with subsequent antiferromagnetic order might be the {\it primary} competitor to superconductivity in Fe arsenides.

cond-mat.str-el↗

arXiv2010

Effect of Short-Range Fluctuations on Thermodynamic and Resistive Properties: The case of Ising Order

We consider the effects of the non-local Ising-like "core spin" correlations on the order-parameter fluctuation contribution to the resistivity and thermodynamics of metals showing Ising-like order at finite temperature. We employ the well-known cluster-variation method, and present explicit results in the pair approximation for short-range order. Our calculation generalizes earlier works, where such effects were considered in the mean-field (Ornstein-Zernicke) approximation. The mean-field (MF) transition temperature T_{c}^{MF}, is corrected to O(1/d), and the effect of the Ising spin fluctuations on the $dc$ resistivity and magnetothermal responses is analyzed in detail. The method can be applied straightforwardly to lattices in arbitrary d, and, as an appealing feature, it reproduces the exact correlation length and T_{c}^{1d}=0 for the 1d Ising model. We apply our results for two interesting physical cases: (i) the double-exchange model with J_{H}>>t, where the core-spins can be approximated quite well by Ising spins, and (ii) a model of band electrons coupled to a {\it localized} subsystem which undergoes a nematic ordering transition coupled to an appropriate structural transition.

cond-mat.supr-con↗

arXiv2009

Theory of Multiband Superconductivity in Iron Pnictides

The precise nature of unconventional superconductivity in Iron Pnictides is presently a hotly debated issue. Here, using insights from normal state electronic structure and symmetry arguments, we show how an unconventional SC emerges from the bad metal "normal" state. Short-ranged, multi-band spin- and charge correlations generates nodeless SC in the active planar $d_{xz,yz}$ bands, and an inter-band proximity effect induces out-of-plane gap nodes in the passive $d_{3z^{2}-r^{2}}$ band. While very good quantitative agreement with various key observations in the SC state and reconciliation with NMR and penetration depth data in the same picture are particularly attractive features of our proposal, clinching evidence would be an experimental confirmation of c-axis nodes in future work.

cond-mat.str-el↗

arXiv2008

Valence Fluctuation Driven Non-Fermi LIquid Behavior and Unconventional Superconductivity in Pressurized CeCu_{2}Si_{2}

I study an Extended Periodic Anderson Model (EPAM) with non-local hybridisation, $V_{fc}$, and a coulomb interaction, $U_{fc}$, between localised $f$ electrons and wide band conduction ($c$) electrons. Within DMFT, a quantum phase transition (QPT) driven by soft $f$-valence fluctuations, accompanied by a discontinuous jump in the Fermi volume, is found as $V_{fc}$ is tuned across a critical value. Near the associated QCP, an unconventional superconductive instability, driven by these extended, singular valence fluctuations, naturally emerges. {\it All} these phenomena are representative of observations in pressurised $CeCu_{2}Si_{2}$.

cond-mat.str-el↗

arXiv2007

Ordering from frustration in a strongly correlated one-dimensional system

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 (CO) ground states as the strength of the longer range interactions is varied. At lower energies, these CO 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 QCP associated with T=0 charge order. For a ladder system relevant to $Sr_{14}Cu_{24}O_{41}$, we find in-chain CO, rung-dimer, and orbital antiferromagnetic ordered phases with varying interchain couplings and superconductivity with hole-doping. RPA studies of many chains (ladders) coupled reveal a phase diagram with the ordered phase extended to finite temperatures and a phase boundary ending at a quantum critical point (QCP). Critical quantum fluctuations at the QCP are found to enhance the transverse dispersion, leading to a dimensional crossover and a T=0 decofinement transition.

cond-mat.str-el↗

arXiv2007

From frustrated insulators to correlated anisotropic metals: charge ordering and quantum criticality in coupled chain systems

A recent study revealed the dynamics of the charge sector of a one-dimensional quarter-filled electronic system with extended Hubbard interactions to be that of an effective pseudospin transverse-field Ising model (TFIM) in the strong coupling limit. With the twin motivations of studying the co-existing charge and spin order found in strongly correlated chain systems and the effects of inter-chain couplings, we investigate the phase diagram of coupled effective (TFIM) systems. A bosonisation and RG analysis for a two-leg TFIM ladder yields a rich phase diagram showing Wigner/Peierls charge order and Neel/dimer spin order. In a broad parameter regime, the orbital antiferromagnetic phase is found to be stable. An intermediate gapless phase of finite width is found to lie in between two charge-ordered gapped phases. Kosterlitz-Thouless transitions are found to lead from the gapless phase to either of the charge-ordered phases. A detailed analysis is also carried out for the dimensional crossover physics when many such pseudospin systems are coupled to one another. Importantly, the analysis reveals the key role of critical quantum fluctuations in driving the strong dispersion in the transverse directions, as well as a T=0 deconfinement transition. Our work is potentially relevant for a unified description of a class of strongly correlated, quarter-filled chain and ladder systems.

cond-mat.str-el↗

arXiv2007

Mott Transition and Strange Metal in Two Dimensions: A View from Cellular Dynamical Cluster Approximation

We introduce a Cellular Dynamical Cluster Approximation (CDCA) to study the nature of the Mott insulator-metal transition in the extended Hubbard model on a square lattice. At strong coupling, a d-wave Mott insulator is obtained. Hole doping drives a first order Mott transition to a non-Fermi (nFL) liquid metal. Remarkably, this nFL is caused by an Anderson orthogonality catastrophe at low energies due to the non-trivial competition between strong, non-local interactions and hopping. This constitutes the first explicit realisation of Anderson's Luttinger liquid idea in two dimensions. Many experimental responses in the ``strange metal'' phase found around optimal doping in cuprates are understood naturally within our approach.

cond-mat.str-el↗

arXiv2005

Charge ordering in quasi-one-dimensional systems with frustrating interactions

Motivated by the co-existing charge and spin order found in strongly correlated ladder systems, we study an effective pseudospin model on a coupled two-leg ladder. A bosonisation analysis yields a rich phase diagram showing Wigner/Peierls charge order and Neel/dimer spin order. In a broad parameter regime, the orbital antiferromagnetic phase is found to be stable. An intermediate gapless phase of finite width is found to lie in between two charge-ordered gapped phases. Our work is potentially relevant for a unified description of a class of strongly correlated, quarter-filled ladder systems.

cond-mat.str-el↗

arXiv2001

On the problem of Yb_{4}As_{3}

Based on a perusal of available experimental data, we propose a possible explanation for the heavy-fermion Fermi liquid state coexisting with quasi-one dimensional antiferromagnetism in the low-carrier pnictide Yb_{4}As_{3}. Both of these low temperature features are shown to originate from one single fact - the charge ordered state forming at high-T is imperfect, due to quantum fluctuations in the CO state. The quasi-1D magnetism is understood in terms of the physics of the squeezed Heisenberg chain, while the heavy fermion-like behavior arises from the compensation of the fraction of the Yb moments (which are disordered) by the 2p band of As. The proposed picture shows how the seemingly strange features observed experimentally can be reconciled as a result of this single hypothesis.

cond-mat.str-el↗

arXiv2000

Electron Energy Loss Spectroscopy of strongly correlated systems in infinite dimensions

We study the electron-energy loss spectra of strongly correlated electronic systems doped away from half-filling using dynamical mean-field theory ($d=\infty$). The formalism can be used to study the loss spectra in the optical (${\bf q=0}$) limit, where it is simply related to the optical response, and hence can be computed in an approximation-free way in $d=\infty$. We apply the general formalism to the one-band Hubbard model off $n=1$, with inclusion of site-diagonal randomness to simulate effects of doping. The interplay between the coherence induced plasmon feature and the incoherence-induced high energy continuum is explained in terms of the evolution in the local spectral density upon hole doping. Inclusion of static disorder is shown to result in qualitative changes in the low-energy features, in particular, to the overdamping of the plasmon feature, resulting in a completely incoherent response. The calculated EELS lineshapes are compared to experimentally observed EELS spectra for the normal state of the high-$T_{c}$ materials near optimal doping and good qualitative agreement is found.

cond-mat.str-el↗