arXiv subjects

Dheeraj Kumar Singh

Publications and source records attributed to Dheeraj Kumar Singh.

35 records · Page 2Linked to original sources

arXiv2020

Effect of strain-induced orbital splitting on the magnetic excitations in undoped cuprates

We investigate the magnetic excitations in view of the recent reports suggesting that the spin-wave energy may exhibit a significant dependence on the in-plane strain of a thin film of La$_2$CuO$_4$. The nature of dependence, as we find, can be explained naturally within a two-orbital model based on the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals. In particular, as the orbital-splitting energy between the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals increases with compressive strain, the zone-boundary spin-wave energy hardens. However, the hardening persists only until the orbital splitting reaches $\sim$ 2eV, beyond which there is no significant change. The behavior of zone-boundary spin-wave energy is explained in terms of the extent of hybridization between one of the exchange-split $d_{x^2-y^2}$ band which is nearly half filled and the $d_{3z^2-r^2}$ band. The role of second-order antiferromagnetic superexchange process involving the inter-orbital hopping is also discussed.

cond-mat.str-el↗

arXiv2018

Drude weight anisotropy in the doped iron pnictides: the primary role of orbital weight redistribution along the reconstructed Fermi surfaces

We explore the anisotropy in low frequency conductivity as a function of hole doping in multiorbital models of pnictides by analyzing the Drude weight in the $x$ and $y$ directions of a ($π$, 0) spin-density wave state. A reduction in the conductivity anisotropy with increased hole doping, and subsequent sign reversal, both observed in experiments, is found to be a common trend in these models. This behavior arises due to the interplay of low energy orbitally-resolved density of states and the geometrical features of the Fermi surface. An understanding of anisotropy in the electron doped regime, however, would require additional ingredients.

cond-mat.str-el↗

arXiv2018

Quasi-one Dimensional Nanostructures as Sign of Nematicity in Iron Pnictides and Chalcogenides

Impurity scattering is found to lead to quasi-one dimensional nanoscale modulation of the local density of states in the iron pnictides and chalcogenides. This `quasiparticle interference' feature is remarkably similar across a wide variety of pnictide and chalcogenide phases, suggesting a common origin. We show that a unified understanding of the experiments can be obtained by simply invoking a four-fold symmetry breaking $d_{xz}-d_{yz}$ orbital splitting, of a magnitude already suggested by the experiments. This can explain the one-dimensional characteristics in the local density of states observed in the orthorhombic nematic, tetragonal paramagnetic, as well as the spin-density wave and superconducting states in these materials.

cond-mat.supr-con↗

arXiv2017

Highly anisotropic quasiparticle interference patterns in the spin-density wave state of the iron pnictides

We investigate the impurity scattering induced quasiparticle interference in the ($π, 0$) spin-density wave phase of the iron pnictides. We use a five orbital tight binding model and our mean field theory in the clean limit captures key features of the Fermi surface observed in angle-resolved photoemission. We use a t-matrix formalism to incorporate the effect of doping induced impurities on this state. The impurities lead to a spatial modulation of the local density of states about the impurity site, with a periodicity of $\sim 8a_{{\rm Fe}-{\rm Fe}}$ along the antiferromagnetic direction. The associated momentum space quasiparticle interference pattern is anisotropic, with major peaks located at $\sim (\pm π/4,0)$, consistent with spectroscopic imaging scanning tunneling microscopy. We trace the origin of this pattern to an elliptical contour of constant energy around momentum (0,0), with major axis oriented along the (0,1) direction, in the mean field electronic structure.

cond-mat.str-el↗

arXiv2017

Roles of anisotropic and unequal gaps in the quasiparticle interference of superconducting iron pnictides

We investigate the role of gap characteristics such as anisotropy and inequality of the gaps in the quasiparticle interferences of iron pnictides using a five-orbital tight-binding model. We examine how the difference in the sensitivities exhibited by the sign-changing and -preserving $s$-wave superconductivity in an annular region around ($π, 0$), which can be used to determine the sign change of the superconducting gap, gets affected when the gaps are unequal on the electron and hole pocket. In addition, we also discuss how robust these differentiating features are on changing the quasiparticle energy or when the gap is anisotropic.

cond-mat.supr-con↗

arXiv2017

Spin-wave excitations in the SDW state of iron pnictides: a comparison between the roles of interaction parameters

We investigate the role of Hund's coupling in the spin-wave excitations of the ($π, 0$) ordered magnetic state within a five-orbital tight-binding model for iron pnictides. To differentiate between the roles of intraorbital Coulomb interaction and Hund's coupling, we focus on the self-consistently obtained mean-field SDW state with a fixed magnetic moment obtained by using different sets of interaction parameters. We find that the Hund's coupling is crucial for the description of various experimentally observed characteristics of the spin-wave excitations including the anisotropy, energy-dependent behavior, and spin-wave spectral weight distribution.

cond-mat.str-el↗

arXiv2017

Spin-wave excitations in the SDW state of doped iron pnictides

We investigate the spin-wave excitations in the spin-density wave state of doped iron pnictides within a five-orbital model. We find that the excitations along ($π, 0$)$\rightarrow$($π, π$) are very sensitive to the doping whereas they do not exhibit a similar sensitivity along ($0, 0$) $\rightarrow$ ($π, 0$). Secondly, anisotropy in the excitations around ($π, 0$) with an elliptical shape grows on moving towards the hole-doped region for low energy, whereas it decreases for the high-energy excitations on the contrary. Thirdly, spin-wave spectral weight shifts towards the low-energy region on moving away from zero doping. We find these features to be in qualitative agreement with the inelastic neutron-scattering measurements for the doped pnictides.

cond-mat.str-el↗

arXiv2017

Two-orbital model for CeB$_6$

We describe a two-orbital tight-binding model with bases belonging to the $Γ_8$ quartet. The model captures several characteristics of the Fermiology unravelled by the recent angle-resolved photoemission spectroscopic (ARPES) measurements on cerium hexaboride CeB$_6$ samples cleaved along different high-symmetry crystallographic directions, which includes the ellipsoid-like Fermi surfaces (FSs) with major axes directed along $Γ$-X. We calculate various multipolar susceptibilities within the model and identify the susceptibility that shows the strongest divergence in the presence of standard onsite Coulomb interactions and discuss it's possible implication and relevance with regard to the signature of strong ferromagnetic correlations existent in various phases as shown by the recent experiments.

cond-mat.str-el↗

arXiv2015

Charge order induced in an orbital density-wave state

Motivated by the recent ARPES measurements (Evtushinsky et. al., PRL {\bf 105}, 147201 (2010)) and evidence for the density-wave state for the charge and orbital ordering (García et al., PRL {\bf 109}, 107202 (2012)) in La$_{0.5}$Sr$_{1.5}$MnO$_4$, the issue of charge and orbital ordering in a two orbital tight-binding model for layered manganite near half doping is revisited. We find that the charge order with an ordering wavevector 2${\bf Q}$ = $(π, π)$ is induced by the orbital order of $B_{1g}$ representation with a different ordering wavevector ${\bf Q}$, where the primary order parameter results from the strong Fermi-surface nesting. The orbital and charge order parameters develop according to $\sqrt{T_{CO}-T}$ and $T_{CO}-T$, respectively, by decreasing the temperature below the orbital ordering temperature $T_{CO}$. Moreover, the orbital order is found to stabilize the CE-type spin arrangement observed experimentally below $T_{CE} < T_{CO}$.

cond-mat.str-el↗

arXiv2015

Orbital-lattice coupling and orbital ordering instability in iron pnictides

Orbital-ordering instability arising due to the intrapocket nesting is investigated for the tight-binding models of pnictides in the presence of orbital-lattice coupling. The incommensurate instabilities with small momentum, which may play an important role in the nematic-ordering transition, vary from model to model besides being more favorable in comparison to the spin-density wave instability in the absence of good interpocket nesting. We also examine the doping dependence of such instabilities. The electron-phonon coupling parameter required to induce them are compared with the first-principle calculations.

cond-mat.str-el↗

arXiv2014

Orbital ordering transition in the single-layer manganites near half doping: a weak-coupling approach

The roles of crystal-field splitting and Jahn-Teller distortions on the orbital ordering transition are investigated in the single-layer manganites near half doping. Crystal-field splitting of energy levels favoring the $d_{3z^2-r^2}$ occupancy provides not only the correct Fermi surface topology for La$_{0.5}$Sr$_{1.5}$MnO$_4$ having a circular electron pocket around the $Γ$ point, but also enhances the flatness of the hole pocket around the M point thereby improving the nesting. In the presence of the circular electron pocket, Jahn-Teller distortions are found to be crucial for the transition to the transverse orbital ordering with ordering wavevector ($0.5π, 0.5π$). In the hole-doping regime $0.5 \leq x \leq 0.7$, the orbital ordering wavevector shows a linear dependence on the hole concentration in accordance with the experiments.

cond-mat.str-el↗

arXiv2014

On the origin of CE-type orbital fluctuations in the ferromagnetic metallic La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We investigate the orbital fluctuations in the ferromagnetic-metallic phase of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ by considering a two orbital model within a tight-binding description which reproduces the ARPES Fermi surface. We find strong antisymmetric transverse orbital fluctuations at wavevector ($ 0.5 π, 0.5 π$) resulting from the Fermi-surface nesting between the portions of bonding and antibonding bands instead of the widely believed nesting between the portions of bonding band despite their flat segments, which provide an insight into the origin of so called CE-type orbital fluctuations in the ferromagnetic-metallic phase. Subsequent renormalization of the phonons near wavevector ($0.5 π,0.5 π$) and the behavior of the phonon linewidth as a function of momentum are in agreement with the inelastic neutron scattering experiments.

cond-mat.str-el↗

arXiv2013

Magnon self energy in the correlated ferromagnetic Kondo lattice model: spin-charge coupling effects on magnon excitations in manganites

Magnon self energy due to spin-charge coupling is calculated for the correlated ferromagnetic Kondo lattice model using a diagrammatic expansion scheme. Systematically incorporating correlation effects in the form of self-energy and vertex corrections, the expansion scheme explicitly preserves the continuous spin rotation symmetry and hence the Goldstone mode. Due to a near cancellation of the correlation-induced quantum correction terms at intermediate coupling and optimal band filling relevant for ferromagnetic manganites, the renormalized magnon energies for the correlated FKLM are nearly independent of correlation term. Even at higher band fillings, despite exhibiting overall non-Heisenberg behavior, magnon dispersion in the Γ-X direction retains nearly Heisenberg form. Therefore, the experimentally observed doping dependent zone-boundary magnon softening must be ascribed to spin-orbital coupling effects.

cond-mat.str-el↗

arXiv2011

Magnetic excitations in iron pnictides

Spin wave dispersion and damping are investigated in the metallic SDW state of different itinerant electron models including a small interlayer hopping. Magnetic excitations in iron pnictides are shown to be well understood in terms of physical mechanisms characteristic of metallic magnets, such as carrier-induced ferromagnetic spin couplings, intra-band particle-hole excitations, and the spin-charge coupling mechanism, which is also important in ferromagnetic manganites.

cond-mat.str-el↗

arXiv2010

Onset and melting of local orbital order

The onset and melting of locally staggered charge/orbital correlations is investigated within a two-orbital correlated electron model with inter-orbital and inter-site Coulomb interactions. Beyond the critical doping concentration x_c ~ 0.2, the (pi,pi,pi) staggered orbital order in the ferromagnet is found to be stabilized at finite temperature. The CE-type orbital correlation exhibits a sharp onset close to the Curie temperature and rapid thermal melting, which provides quantitative understanding of the (pi/2,pi/2,0) feature observed near T_c in neutron scattering experiments. In the zig-zag AF state, the CE-type orbital correlations are found to be even more readily stabilized, but only within a narrow doping regime around x=0.5.

cond-mat.str-el↗

arXiv2010

Spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites

Correlation-induced spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites are calculated with realistic parameters in order to provide a quantitative comparison with experimental results for spin stiffness, magnon dispersion, magnon damping, anomalous zone-boundary magnon softening, and Curie temperature. The role of orbital degeneracy, orbital ordering, and orbital correlations on spin dynamics in different doping regimes is highlighted.

cond-mat.str-el↗

arXiv2010

Orbital fluctuations, spin-orbital coupling, and anomalous magnon softening in an orbitally degenerate ferromagnet

The correlated motion of electrons in the presence of strong orbital fluctuations and correlations is investigated with respect to magnetic couplings and excitations in an orbitally degenerate ferromagnet. Introduction of the orbital degree of freedom results in a class of diagrams representing spin-orbital coupling which become particularly important near the orbital ordering instability. Low-energy staggered orbital fluctuation modes, particularly with momentum near (π/2,π/2,0) (corresponding to CE-type orbital correlations), are shown to generically yield strong intrinsically non-Heisenberg (1-\cos q)^2 magnon self energy correction, resulting in no spin stiffness reduction, but strongly suppressed zone-boundary magnon energies in the Gamma-X direction. The zone-boundary magnon softening is found to be strongly enhanced with increasing hole doping and for narrow-band materials, which provides insight into the origin of zone-boundary anomalies observed in ferromagnetic manganites.

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