arXiv subjects

Dheeraj Kumar Singh

Publications and source records attributed to Dheeraj Kumar Singh.

At least 19 recordsLinked to original sources

arXiv2026

Role of atomic vacancies and second-neighbor antiferromagnetic-exchange coupling in a ferromagnetic nanoparticle

Several factors may be responsible for disorder and frustration in a magnetic nanoparticle, including atomic vacancies on the surface and inside, impurity atoms, long-range magnetic exchange coupling, etc. We use Monte-Carlo simulations within the Heisenberg model to examine the role of randomly distributed atomic vacancies and long-range magnetic-exchange coupling on the temperature-dependent magnetic properties of ferromagnetic nanoparticles. In particular, we study the role of the second-neighbor antiferromagnetic exchange coupling and missing atoms inside the particle resulting in broken nearby bonds. We find that both factors may enhance the superparamagnetic behaviors of such particles.

cond-mat.mtrl-sci↗

arXiv2026

Weyl semimetallic state with antiferromagnetic order in Rashba-Hubbard model

We study the phase diagram of Rashba-Hubbard model by employing the Hartree-Fock meanfield theory, and thereby establish the existence of an antiferromagnetically ordered Weyl semimetallic state with in-plane magnetic moments. This phase is found to be sandwiched in between the antiferromagnetic insulator and Rashba metal in the interaction vs spin-orbit coupling phase diagram. The antiferromagnetically-ordered topological semimetallic state exists in the presence of combined time-reversal and inversion symmetry though individually both are broken. The study of the static magnetic susceptibility indicates the robustness of the antiferromagnetic order within a realistic range of interaction and spin-orbit coupling parameters. In addition to the edge states associated with the Weyl points, we also investigate the spin-resolved quasiparticle interference, which provides important insight into the possible spin texture of the bands especially in the vicinity of Weyl points.

cond-mat.str-el↗

arXiv2026

Auditing and Mitigating Bias in Gender Classification Algorithms: A Data-Centric Approach

Gender classification systems often inherit and amplify demographic imbalances in their training data. We first audit five widely used gender classification datasets, revealing that all suffer from significant intersectional underrepresentation. To measure the downstream impact of these flaws, we train identical MobileNetV2 classifiers on the two most balanced of these datasets, UTKFace and FairFace. Our fairness evaluation shows that even these models exhibit significant bias, misclassifying female faces at a higher rate than male faces and amplifying existing racial skew. To counter these data-induced biases, we construct BalancedFace, a new public dataset created by blending images from FairFace and UTKFace, supplemented with images from other collections to fill missing demographic gaps. It is engineered to equalize subgroup shares across 189 intersections of age, race, and gender using only real, unedited images. When a standard classifier is trained on BalancedFace, it reduces the maximum True Positive Rate gap across racial subgroups by over 50% and brings the average Disparate Impact score 63% closer to the ideal of 1.0 compared to the next-best dataset, all with a minimal loss of overall accuracy. These results underline the profound value of data-centric interventions and provide an openly available resource for fair gender classification research.

cs.CV↗

arXiv2025

Non-reciprocal spin-wave excitations in Rashba-Hubbard ferromagnets

We explore the nonreciprocity of spin-wave excitations in the Rashba-Hubbard ferromagnet on a square lattice. Our study reveals that the propagation of spin-wave excitations exhibit non-reciprocal behavior, i.e., spin waves traveling in opposite directions display asymmetry in energy dispersion $ω({\bf q}) \ne ω(-{\bf q)}$, which also results in an asymmetric behavior of group velocity, spin stiffness, etc. We find that this asymmetric behavior arises only when the magnetic moments are aligned inside the atomic plane, while the excitations remain symmetric for out-of-plane magnetization. The first dominating term in the low-energy dispersion is linear. However, if the magnetic moments are out-of-plane, then the first dominant term is quadratic instead. The low-energy non-quadratic behavior is examined in the intermediate-to-strong coupling regime for various strengths of Rashba spin-orbit coupling.

cond-mat.str-el↗

arXiv2025

Orbital correlations in bilayer nickelates: roles of doping and interlayer coupling

We study the nature of orbital correlations present in the bilayer nickelate within a minimal two-orbital tight-binding model to gain insights into their possible role in stabilizing the less-known weakly-insulating state. The latter has been observed experimentally at ambient pressure. In order to achieve this objective, we examine the static orbital susceptibilities within the random-phase approximation. Our study highlights the sensitivity of orbital correlations to various factors including the interlayer coupling, carrier concentration, band-structure details such as the orbital contents, the number of bands contributing at the Fermi level etc. We relate this sensitiveness to the modification of the Fermi surfaces as well as their orbital contents dependent on aforementioned factors.

cond-mat.str-el↗

arXiv2025

Insight into interplay between bandstructure and Coulomb interaction via quasiparticle interference

Quasiparticle interference has been used frequently for the purpose of unraveling the electronic states in the vicinity of the Fermi level as well as the nature of superconducting gap in the unconventional superconductors. Using the metallic spin-density wave state of iron pnictides as an example, we demonstrate that the quasiparticle interference can also be used as a probe to provide crucial insight into the interplay of the electronic bandstructure and correlation effects in addition to bringing forth the essential features of electronic states in the vicinity of the Fermi level. Our study reveals that the features of quasiparticle interference pattern can help us narrowing down the interaction parameter window and choose a more realistic tight-binding model.

cond-mat.supr-con↗

arXiv2025

Role of Dirac cones in the anisotropic properties associated with the spin-density wave state of iron pnictides

The origin of unusual anisotropic electronic properties in the spin-density wave state of iron pnictides has conventionally been attributed to the breaking of four-fold rotational symmetry associated with the collinear magnetic order. By using a minimal two-orbital model, we show that a significant portion of the contribution to the anisotropy may come from the Dirac cones, which are not far away from the Fermi level. We demonstrate this phenomenon by examining optical conductivity and quasiparticle interference in the Dirac-semimetallic state with spin-density wave order, and the latter can be obtained by choosing appropriate interaction parameters and orbital splitting between the $d_{xz}$ and $d_{yz}$ orbitals. We further extend this study to investigate the low-energy spin-wave excitations in the Dirac-semimetallic state with spin-density wave order.

cond-mat.str-el↗

arXiv2024

Possible pairing states in the superconducting bilayer nickelate

We examine various possibilities for the pairing mechanisms in the recently discovered bilayer-nickelate superconductor within the Bardeen-Cooper-Schrieffer framework. Unlike earlier studies, where only a pure $d$-wave or sign-changing $s$-wave superconductivity instability was investigated, our study explores the possibilities of mixed-state superconducting instability such as the one involving both $d$- and sign-changing $s$-waves. While assuming that the superconductivity arises because of the magnetic correlations, we examine the nature of the superconducting gap function associated density of states with various possible magnetic correlation wavevectors arising out as a result of multiple pockets owing to the multiple orbitals and bilayer splitting. We also explore the effect of differences in the nature of Fermi surfaces suggested by various studies.

cond-mat.supr-con↗

arXiv2024

Antiferromagnetically ordered Dirac semimetal in Hubbard model with spin-orbit coupling

We examine the possible existence of Dirac semimetal with magnetic order in a two-dimensional system with a nonsymmorphic symmetry by using the Hartree-Fock mean-field theory within the Hubbard model. We locate the region in the second-neighbor spin-orbit coupling vs Hubbard interaction phase diagram, where such a state is stabilized. The edge states for the ribbons along two orthogonal directions concerning the orientation of in-plane magnetic moments are obtained. Finally, the effect of the in-plane magnetic field, which results in the stabilization of the Weyl semimetallic state, and the nature of the edge states corresponding to the Weyl semimetallic state for ribbon geometries are also explored.

cond-mat.str-el↗

arXiv2023

Effect of next-nearest neighbor hopping on the single-particle excitations

In the half-filled one-orbital Hubbard model on a square lattice, we study the effect of next-nearest neighbor hopping on the single-particle spectral function at finite temperature using an exact-diagonalization + Monte-Carlo based approach to the simulation process. We find that the pseudogap-like dip, existing in the density of states in between the Néel temperature $T_N$ and a relatively higher temperature $T^*$, is accompanied with a significant asymmetry in the hole- and particle-excitation energy along the high-symmetry directions as well as along the normal-state Fermi surface. On moving from ($π/2, π/2$) toward $(π, 0)$ along the normal state Fermi surface, the hole-excitation energy increases, a behavior remarkably similar to what is observed in the $d$-wave state and pseudogap phase of high-$T_c$ cuprates, whereas the particle-excitation energy decreases. The quasiparticle peak height is the largest near ($π/2, π/2$) whereas it is the smallest near $(π, 0)$. These spectral features survive beyond $T_N$. The temperature window $T_N \lesssim T \lesssim T^*$ shrinks with an increase in the next-nearest neighbor hopping, which indicates that the next-nearest neighbor hopping may not be supportive to the pseudogap-like features.

cond-mat.str-el↗

arXiv2023

Unidirectional charge correlations in hole-doped cuprates

We examine charge correlations and instabilities in the pseudogap phase of high-$T_c$ cuprates modeled by $d$-density wave ordering. The latter has a gap symmetry similar to the one observed in the $d$-wave superconductor. We use $t$-$J$ model to describe the charge correlations in the presence of electron-phonon interaction. Our finding suggest that the charge instability in the normal state is dominating at an incommensurate wavevector along (1, 1) instead of (1, 0) for a realistic interaction parameter. The dominance at the diagonal wavevector is further enhanced if the coupling between electron and bond-buckling $B_{1g}$ phonon is incorporated. On the other hand, a dominating charge-density correlation develops along (1, 0) at an incommensurate wavevector in the $d$-density wave ordered state, which shows a qualitative agreement with the experiments. The correlation becomes robust only in the presence of $B_{1g}$ phonon.

cond-mat.str-el↗

arXiv2022

Temperature dependence of quasiparticle interference in $d$-wave superconductors

We investigate the temperature dependence of quasiparticle interference in the high $T_c$-cuprates using an Exact-Diagonalization + Monte-Carlo based scheme to simulate the $d$-wave superconducting order parameter. The quasiparticle interference patterns have features largely resulting from the scattering vectors of the octet model at lower temperature. Our findings suggest that the features of quasiparticle interference in the pseudogap region of the phase diagram are also dominated by the set of scattering vectors belonging to the octet model because of the persisting antinodal gap beyond the superconducting transition $T_c$. However, beyond a temperature when the antinodal gap becomes very small, a set of scattering vectors different from those belonging to the octet model are responsible for the quasiparticle interference patterns. With a rise in temperature, the patterns are increasingly broadened.

cond-mat.supr-con↗

arXiv2022

Thermal evolution of single-particle spectral function in the half-filled Hubbard model and pseudogap

In the half-filled one-orbital Hubbard model on a square lattice, we find pseduogap features in the form of two-peak structures associated with the momentum-resolved spectral function, which exists within the temperature window $T_N \lesssim T \lesssim T^*$. $T^*$ is the temperature below which there exists a well-formed dip in the density of state. Inside the window $T_N \lesssim T \lesssim T^*$, the peak-to-peak separation in the two-peak structure of the momentum-resolved spectral function rises on moving away from the point ($π/2, π/2$) along the normal state Fermi surface towards $(π, 0)$, a behavior remarkably similar to what is observed in the pseudogap phase. We unveil these features by using a parallelized cluster-based Monte-Carlo method for simulating the magnetic order parameter fields on a superlattice, which enables us to access the momentum-resolved single-particle spectral function corresponding to a lattice size of $\sim$ 240 $\times$ 240 with almost negligible finite-size effects.

cond-mat.str-el↗

arXiv2022

Spin-orbit coupling and magnetism in $\rm Sr_2CrO_4$

With octahedrally coordinated $t_{\rm 2g}$ orbitals which are active at filling $n=2$, the $\rm Sr_2CrO_4$ compound exhibits rich interplay of spin-orbital physics with tetragonal distortion induced crystal field tuning by external agent such as pressure. Considering both reversed and restored crystal field regimes, collective spin-orbital excitations are investigated in the antiferromagnetic state using the generalized self consistent + fluctuations approach including spin-orbit coupling (SOC). A transition is found from staggered to entangled orbital order at critical SOC value in the realistic regime. Behavior of the calculated energy scales of collective excitations with crystal field is in striking similarity to that of the transition temperatures with pressure as obtained from susceptibility and resistivity anomalies in high-pressure studies.

cond-mat.str-el↗

arXiv2022

Semimetallic spin-density wave state in iron pnictides

We examine the existence of semimetallic spin-density wave states in iron pnictides. In the experimentally observed metallic spin-density wave state, the symmetry-protected Dirac cones are located away from the Fermi surface giving rise to tiny pockets and there are also additional Fermi pockets such as one around $Γ$. We find that the location of a pair of Dirac points with respect to the Fermi surface exhibits significant sensitivity to the orbital splitting between the $d_{xz}$ and $d_{yz}$ orbitals. Besides, in the presence of orbital splitting, the Fermi pockets not associated with the Dirac cones, can be suppressed so that a semimetallic spin-density wave state can be realized. We explain these finding in terms of difference in the slopes and orbital contents of the bands which form the Dirac cone, and obtain the necessary conditions dependent on these two and other parameters for the coexisting Dirac semimetallic and spin-density wave states. Additionally, the topologically protected edge states are studied in the ribbon geometry when the same are oriented either along $x$ or $y$ axes.

cond-mat.str-el↗

arXiv2022

Orbital entangled antiferromagnetoc order and spin-orbit-distortion exciton in $\rm Sr_2VO_4$

With electron filling $n=1$ in the $\rm Sr_2VO_4$ compound, the octahedrally coordinated $t_{\rm 2g}$ orbitals are strongly active due to tetragonal distortion induced crystal field tuning by external agent such as pressure. Considering the full range of crystal field induced tetragonal splitting in a realistic three-orbital model, collective spin-orbital excitations are investigated using the generalized self consistent and fluctuation approach. The variety of self consistent states obtained including orbital entangled ferromagnetic and antiferromagnetic orders reflects the rich spin-orbital physics resulting from the interplay between the band, spin-orbit coupling, crystal field, and Coulomb interaction terms. The behavior of the calculated energy scales of collective excitations with crystal field is consistent with that of the transition temperatures with pressure as obtained from susceptibility and resistivity anomalies in high-pressure studies.

cond-mat.str-el↗

arXiv2022

Hund's coupling and electronic anisotropy in the spin-density wave state of iron pnictides

In the multiband systems, Hund's coupling ($J$) plays a significant role in the spin and charge excitations. We study the dependence of electronic anisotropy on $J$ in terms of Drude-weight along different directions as well as the orbital order in the four-fold symmetry broken spin-density wave state of iron pnictides. A robust behavior of the Drude-weight anisotropy within a small window around $J \sim 0.25U$ with $U$ as intraorbital Coulomb interaction is described in terms of orbital-weight distribution along the reconstructed Fermi surfaces. We also find that the ferro-orbital order increases with $J$ in the widely accepted regime for the latter, which is explained as a consequence of rising exchange field with an increase in magnetization.

cond-mat.str-el↗

arXiv2021

Fermi arcs and pseudogap phase in a minimal microscopic model of $d$-wave superconductivity

We show conclusively that a pseudogap state can arise at $T > T_c$, for reasonable pairing interaction strength, from order parameter fluctuations in a two dimensional minimal model of $d$-wave superconductivity. The occurrence of the pseudogap requires neither strong correlation nor the presence of competing order. We study a model with attractive nearest neighbor interaction and establish our result using a combination of cluster based Monte Carlo for the order parameter field and a twisted-boundary scheme to compute the momentum-resolved spectral function. Apart from a dip in the density of states that characterizes the pseudogap, the momentum and frequency resolution on our effective lattice size $\sim 160 \times 160$ allows two major conclusions: (i)~at $T < T_c$, despite the presence of thermal phase fluctuations the superconductor has only nodal Fermi points while all non nodal points on the normal state Fermi surface show a two peak spectral function with a dip at $ω=0$, and (ii)~for $T > T_c$ the Fermi points develops into arcs, characterized by a single quasiparticle peak, and the arcs connect up to recover the normal state Fermi surface at a temperature $T^* > T_c$. We show the variation of $T_c$ and $T^*$ with coupling strength and provide detailed spectral results at a coupling where $T^* \sim 1.5T_c$.

cond-mat.supr-con↗