arXiv subjects

Nazir Khan

Publications and source records attributed to Nazir Khan.

6 recordsLinked to original sources

arXiv2026

Collapse of Jahn-Teller Phonons in La$_{1-x}$Sr$_{x}$MnO$_3$ with Weak Magnetoresistance

Perovskite manganites are quantum materials exhibiting competing interactions inducing colossal magnetoresistance (CMR). The prevailing theory of CMR highlights the essential role of electron-phonon coupling (EPC), but mounting evidence suggests the underlying mechanism is more complicated. Here, we investigate phonons and spin-phonon coupling in ferromagnetic CMR manganites La$_{1-x}$Sr$_x$MnO$_3$ ($x$=0.2,0.3) with relatively small CMR associated with melting of the magnetic order above room temperature. High-resolution neutron scattering experiments combined with density functional theory (DFT) show that the low-temperature ferromagnetic phase is conventional: neutron scattering from phonons agrees with DFT predictions and magnons follow sinusoidal dispersions. Fluctuating magnetic moments and low-energy phonons remain conventional in the high temperature paramagnetic phase, indicating the Mn and La/Sr sublattices are not strongly perturbed by melting of ferromagnetism. In contrast, the Jahn-Teller-active optical oxygen vibrations collapse entirely above the Curie temperature, despite low CMR in these compositions, with some of the lost spectral weight reappearing as quasielastic scattering. We attribute this highly anomalous behavior to giant EPC in the charge and/or orbital channel. It drives cooperative diffusive motion of quasistatic carrier-trapping oxygen sublattice distortions once ferromagnetism disappears. We hypothesize the magnitude of magnetoresistance correlates with the rate of diffusion rather than with the strength of Jahn-Teller EPC.

cond-mat.mtrl-sci↗

arXiv2025

Manipulation of the orbital angular momentum of soft x-ray beams by consecutive diffractive optics

Production and manipulation of orbital angular momentum (OAM) of coherent soft x-ray beams is demonstrated utilizing consecutive diffractive optics. OAM addition is observed upon passing the beam through consecutive fork gratings. The OAM of the beam was found to be decoupled from its spin angular momentum (SAM). Practical implementation of angular momentum control by consecutive devices in the x-ray regime opens new experimental opportunities, such as direct measurement of OAM beams without resorting to phase sensitive techniques, including holography. OAM analyzers utilizing fork gratings can be used to characterize the beams produced by synchrotron and free electron lasers sources; they can also be used in scattering experiments.

physics.optics↗

arXiv2022

A combined inelastic neutron scattering and \textit{ab initio} lattice dynamics study of FeSi

The phonon renormalization across the semiconductor-to-metal crossover in FeSi is investigated by inelastic neutron scattering combined with \textit{ab-initio} lattice dynamical calculations. A significant part of reciprocal space with a particular focus on the 110$-$001 scattering plane is mapped by the time-of-flight inelastic neutron scattering data taken below and above the crossover. Individual momentum values are investigated in more detail as a function of temperature. The data reveal that the anomalous phonon softening upon metallization is not exclusive to the high symmetry $R$ and $Γ$ points. Several other phonon modes around the $R$-point as well as the phonon modes at the $M$ and $X$ points of the Brillouin zone exhibit anomalous phonon softening with magnitudes comparable to that observed at the $R$-point. The momentum dependence of the phonon softening is reproduced by the lattice dynamical calculation based on the density functional perturbation theory. We discuss our findings with respect to the nature of the semiconductor-to-metal crossover in FeSi, for which different microscopic origins have been proposed, i.e., lattice thermal disorder and electronic correlation effects.

cond-mat.mtrl-sci↗

arXiv2021

Toward cubic symmetry for Ir$^{4+}$: structure and magnetism of antifluorite K$_2$IrBr$_6$

Crystal structure, electronic state of Ir$^{4+}$, and magnetic properties of the antifluorite compound K$_2$IrBr$_6$ are studied using high-resolution synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS), thermodynamic and transport measurements, and ab initio calculations. The crystal symmetry is reduced from cubic at room temperature to tetragonal below 170 K and eventually to monoclinic below 122 K. These changes are tracked by the evolution of the non-cubic crystal-field splitting $Δ$ measured by RIXS. Non-monotonic changes in $Δ$ are ascribed to the competing effects of the tilt, rotation, and deformation of the IrBr$_6$ octahedra as well as tetragonal strain on the electronic levels of Ir$^{4+}$. The Néel temperature of $T_N=11.9$ K exceeds that of the isostructural K$_2$IrCl$_6$, and the magnitude of frustration on the fcc spin lattice decreases. We argue that the replacement of Cl by Br weakens electronic correlations and enhances magnetic couplings.

cond-mat.str-el↗

arXiv2019

Cubic symmetry and magnetic frustration on the $fcc$ spin lattice in K$_2$IrCl$_6$

Cubic crystal structure and regular octahedral environment of Ir$^{4+}$ render antifluorite-type K$_2$IrCl$_6$ a model fcc antiferromagnet with a combination of Heisenberg and Kitaev exchange interactions. High-resolution synchrotron powder diffraction confirms cubic symmetry down to at least 20 K, with a low-energy rotary mode gradually suppressed upon cooling. Using thermodynamic and transport measurements, we estimate the activation energy of $Δ\simeq 0.7$ eV for charge transport, the antiferromagnetic Curie-Weiss temperature of $θ_{\rm CW}\simeq -43$ K, and the extrapolated saturation field of $H_s\simeq 87$ T. All these parameters are well reproduced \textit{ab initio} using $U_{\rm eff}=2.2$ eV as the effective Coulomb repulsion parameter. The antiferromagnetic Kitaev exchange term of $K\simeq 5$ K is about one half of the Heisenberg term $J\simeq 13$ K. While this combination removes a large part of the classical ground-state degeneracy, the selection of the unique magnetic ground state additionally requires a weak second-neighbor exchange coupling $J_2\simeq 0.2$ K. Our results suggest that K$_2$IrCl$_6$ may offer the best possible cubic conditions for Ir$^{4+}$ and demonstrates the interplay of geometrical and exchange frustration in a high-symmetry setting.

cond-mat.str-el↗

arXiv2019

Complex exchange mechanism driven ferromagnetism in half-metallic Heusler Co$_{2}$TiGe: Evidence from critical behavior

We have investigated the critical phenomenon associated with the magnetic phase transition in the half-metallic full-Heusler Co$_2$TiGe. The compound undergoes a continuous ferromagnetic to paramagnetic phase transition at the Curie temperature $T_{C}$=371.5 K. The analysis of magnetization isotherms in the vicinity of $T_{c}$, following modified Arrott plot method, Kouvel-Fisher technique, and critical isotherm plot, yields the asymptotic critical exponents $β$=0.495, $γ$=1.324, and $δ$=3.67. The self-consistency and reliability of the obtained exponents are further verified by the Widom scaling relation and scaling equation of states. The mean-field-like value of the critical exponent $β$ suggests long-range nature of the exchange interactions, whereas the values of the critical exponents $γ$ and $δ$, imply sizeable critical spin fluctuations. The half-metallic itinerant character of Co$_{2}$TiGe in the presence of magnetic inhomogeneity may result in such a strong deviation from the three-dimensional Heisenberg values ($β$=0.369, $γ$=1.38 and $δ$=4.8) of the critical exponents towards the mean field values ($β$=0.5, $γ$=1 and $δ$=3). The results suggest complex nature of exchange couplings that stabilize the long-range ferromagnetic ordering in the system and are consistent with the earlier theoretical studies on the exchange mechanism in Co$_2$TiGe.

cond-mat.mtrl-sci↗