arXiv subjects

Shantanu Pathak

Publications and source records attributed to Shantanu Pathak.

6 recordsLinked to original sources

arXiv2026

A Universal Crystal-Field Design Principle for Orbital-Order-Driven Altermagnetism

Altermagnets combine collinear antiferromagnetic order with nonrelativistic spin splitting, enabling spintronic functionalities without relying on spin--orbit coupling. While staggered orbital ordering has recently emerged as an alternative route to altermagnetism, its generality has remained unexplored. Here, we establish a universal crystal-field design principle for orbital-order-driven altermagnetism. We show that structural relaxation consistently reconstructs the crystal-field landscape, activating a common $d_{xz}/d_{yz}$ orbital manifold that drives spontaneous staggered orbital ordering and robust $d$-wave nonrelativistic spin splitting across transition-metal compounds spanning electron fillings from $d^1$ to $d^7$. By introducing a unified symmetry framework based on layer-dependent magnetic and orbital order parameters, we demonstrate how interlayer stacking determines whether the system realizes a bulk altermagnetic state or a globally compensated antialtermagnetic phase. Furthermore, we reveal that this symmetry-protected spin-split texture gives rise to highly anisotropic spin-polarized conductivities. Our results establish crystal-field engineering as a predictive design strategy for discovering and engineering orbital-order-driven altermagnets.

cond-mat.mtrl-sci↗

arXiv2026

Lone-Pair-Induced Lattice Softness Enables Ultralow Thermal Conductivity in Hybrid Organic-Inorganic Perovskite GuaPbI$_3$

Thermal conductivity ($\kappa$) minimization of inorganic thermoelectrics can only be achieved to a certain extent via nanostructural engineering. Here, we introduce a lone-pair-driven materials design strategy based on chemically induced lattice softness to develop hybrid organic-inorganic perovskites with ultra-low $\kappa$. A physics-guided symbolic-regression machine-learning framework identifies a lone-pair-dominated compositional regime statistically associated with suppressed lattice $\kappa$ and selects $\text{GuaPbI}_3$ as a candidate material. Mechanochemical synthesis yields $\text{GuaPbI}_3$ with an ultra-low room-temperature $\kappa \approx 0.088\text{ W m}^{-1}\text{ K}^{-1}$. Electrical measurements reveal electronically active, bias-dependent bulk conduction pathways despite strong phonon suppression, while impedance spectroscopy confirms predominantly bulk-dominated transport. Density functional theory calculations indicate weakly dispersive valence bands, pronounced valence-conduction asymmetry, and localized electrostatic microenvironments arising from lattice charge redistribution. Calculated transport coefficients suggest strong sensitivity of carrier transport to chemical potential, while Lorenz-number analysis reveals deviations from conventional Wiedemann-Franz behavior near band edges. These observations indicate that lone-pair-rich hybrid frameworks generate intrinsically soft and electronically heterogeneous lattice environments capable of strongly suppressing phonon transport while preserving electronically accessible states. This work establishes chemically induced lattice softness as a viable design principle for identifying ultralow-thermal-conductivity hybrid materials without relying on nanostructuring or extrinsic disorder engineering.

cond-mat.mtrl-sci↗

arXiv2026

Strain- and Field-Tunable Nonrelativistic Spin Splitting and Wave-Symmetry-Dependent Spin Transport in Twisted Bilayer Altermagnets

Magnetism-driven nonrelativistic spin splitting (NRSS) provides a pathway toward efficient, spin-orbit-free spintronics. In centrosymmetric two-dimensional antiferromagnets, spin-polarized transport is symmetry-forbidden due to the combined space-time inversion ($PT$) symmetry. Here, by employing first-principles density functional theory and spin-group symmetry analysis, we demonstrate that twisting two antiferromagnetic or ferromagnetic monolayers of CoCl$_2$, AX$_2$ (A = Mn, V; X = Cl, Br, I), NiF$_2$, NiBr$_2$, FeS, CoS, MnTe$_2$, MnSe$_2$, and RuSe induces finite NRSS even in the absence of spin-orbit coupling. The relative twist breaks $[C_2||P]$ and $[E||C_{nz}]$ symmetries, giving rise to momentum-dependent spin polarization with distinct $d$-, $g$-, and $i$-wave altermagnetic patterns across the Brillouin zone. Using symmetry-invariant $k\cdot p$ modeling, we extract linear spin-splitting coefficients $\alpha^{(1)}$ ranging from 800-1100 meV\AA{}, comparable to SOC-induced Rashba-Dresselhaus strengths observed in noncentrosymmetric semiconductors. An out-of-plane electric field ($\mathcal{E}_z$) introduces Zeeman-type band splitting up to 110 meV at 10 MV/cm, while biaxial strain tunes the NRSS magnitude nearly linearly without altering symmetry. Crucially, the strain $u_{xx-yy}$ reduces the spin point group symmetry and drives reversible $g/i \rightarrow d$ wave-type transitions, resulting in finite spin conductivity and an enhanced spin-splitter angle (up to 18$^\circ$). These results extend the concept of altermagnetism to twisted bilayer geometries and establish a general route for realizing exchange-driven, nonrelativistic spin currents through symmetry engineering without requiring heavy elements or spin-orbit coupling.

cond-mat.mtrl-sci↗

arXiv2025

Imaging Quantum Well States of Dirac Electrons in Exfoliated 3D Topological Insulators

We present a controlled mechanical exfoliation technique for bulk 3D topological insulators that yields atomically clean ultrathin flakes, enabling quantum well states (QWS) of Dirac electrons to be clearly resolved. Achieving reliable fabrication of pristine, high-quality two-dimensional layers suitable for atomic-scale spectroscopy remains a central experimental challenge in uncovering their emergent quantum states and realizing device-relevant functionalities. Atomically resolved scanning probe microscopy and micro-Raman spectroscopy reveal a strong correlation between Raman intensity and film thickness, enabling rapid identification of (Bi\textsubscript{0.1}Sb\textsubscript{0.9})\textsubscript{2}Te\textsubscript{3} flakes with desired thickness. High resolution scanning tunneling spectroscopy on exfoliated flakes with atomically flat terraces reveals QWS, driven by quantum confinement of Dirac electrons. This effect is rarely observed due to the electrons resistance to electrostatic confinement caused by Klein tunneling. The standard phase accumulation model accurately captures the characteristics of QWS and extracts the electronic band dispersion, showing excellent agreement with density functional theory calculations. Band structure calculation reveals that with increasing quantum-layer thickness, the interlayer coupling enhances the electronic dispersion, progressively reducing subband splitting and giving rise to bulk-like continuous bands. Spatially resolved spectroscopy around surface defects further confirms that QWS of Dirac electrons in topological insulators remains robust against defect scattering. This work paves the way for exploring diverse quantum phenomena and device applications through quantum confinement, surface-state engineering, and tunable topological phases.

cond-mat.mes-hall↗

arXiv2025

Strain-Induced Modulation of Spin Splitting and Persistent Spin Textures in Low-Symmetry 2D Hybrid Perovskites: A case study of RP phase

We report the observation of a persistent spin texture (PST) in pseudo-2D hybrid perovskite, characterized by significant spin splitting strength on the order of $3 \, \text{eV} \cdot \text{\AA}$. Using first-principles density functional theory (DFT) calculations, complemented by a $\mathbf{k} \cdot \mathbf{p}$ model analysis, we validate the presence of PST and its robustness under various conditions. The material's non-centrosymmetric nature and strong spin-orbit coupling ensure uniform spin orientation in momentum space, enabling long spin lifetimes and promising spintronic applications. Furthermore, we demonstrate the tunability of the spin splitting via the application of external strain and stress, offering a versatile approach to control spin configurations. Our results highlight the potential of this perovskite system for next-generation spintronic devices, where external perturbations can be used to precisely modulate electronic properties.

cond-mat.mtrl-sci↗

arXiv2013

Ber Performance Analysis of WiMAX PHY Layer under different channel conditions

This paper gives an introduction on the IEEE 802.16 standard WIMAX or Worldwide Interoperability for Microwave Access. The different parts give details on the architectural specifications of WiMAX networks and also on the working principle of WiMAX networks including its services provided. It also provides brief descriptions on its salient features of this technology and how it benefits the networking industry. A brief outline of the basic building blocks or equipment of WiMAX architecture is also provided. This paper also evaluates the simulation performance of IEEE 802.16 OFDM PHY layer. The Stanford University Interim (SUI) channel model under varying parameters is selected for the wireless channel in the simulation. The performance measurements and analysis was done in simulation developed in MATLAB.

cs.NI↗