arXiv subjects

Caleb Stamper

Publications and source records attributed to Caleb Stamper.

4 recordsLinked to original sources

arXiv2025

Tuning the Surface States of $Fe_3O_4$ Nanoparticles for Enhanced Magnetic Anisotropy and Induction Efficacy

Magnetite ($Fe_3O_4$) nanoparticles are crucial for biomedical applications, including magnetic hyperthermia, targeted drug delivery, and MRI contrast enhancement, due to their biocompatibility and unique physicochemical properties. Here, we investigate how surface states influence their induction performance. Heat treatment removes surface water and FeOOH, forming a ${\gamma}$-$Fe_2O_3$ shell, as confirmed by synchrotron powder diffraction, neutron powder diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and time-of-flight inelastic neutron spectroscopy. AC magnetic susceptibility measurements reveal that this surface modification enhances magnetic anisotropy and reduces the spin relaxation time, leading to a 140% increase in the specific absorption rate. Additionally, the increased anisotropy suppresses the low-temperature clustered spin-glass transition and raises the blocking temperature. These findings highlight surface-state engineering as a powerful approach to optimizing $Fe_3O_4$ nanoparticles for biomedical applications.

cond-mat.mes-hall↗

arXiv2024

Thermal Nanoquakes: Terahertz Frequency Surface Rayleigh Waves in Diamond Nanocrystals

Mechanical THz vibrations in nanocrystals have recently been harnessed for quantum sensing and thermal management. The free boundaries of nanocrystals introduce new surface wave solutions, analogous to the seismic waves on Earth, yet the implications of these surface waves on nanocrystals have remained largely unexplored. Here, we use atomistic molecular dynamics simulations and experimental neutron spectroscopy to elucidate these THz-scale features in nanodiamond. Our key insight is that thermally induced Rayleigh surface phonons, which have a low group velocity and an amplitude that decays exponentially away from the surface, are responsible for the previously observed but unexplained linear scaling of the low-energy vibrational density of states in nanocrystals. Large thermal atomic displacements, relative to the nanoparticle radius, induce perpetual surface quakes, even at ambient conditions. Normalised to the radius, the surface displacement ratio in diamond nanocrystals exceeds that of the largest recorded earthquakes by a factor of $10^{5\pm1}$. We explicate how these dramatic Rayleigh waves coexist with other distinctive features including confined lattice phonons, soft surface modes, the acoustic gap, Love waves, and Lamb modes, thereby offering a complete framework for the vibrational dynamics of nanocrystals.

cond-mat.mes-hall↗

arXiv2023

On the temperature dependence of the density of states of liquids at low energies

We report neutron-scattering measurements of the density of states (DOS) of water and liquid Fomblin in a wide range of temperatures. In the liquid phase, we confirm the presence of a universal low-energy linear scaling of the experimental DOS as a function of the frequency, $g(\omega)= a(T) \omega$, which persists at all temperatures. The low-frequency scaling of the DOS exhibits a sharp jump at the melting point of water, below which the standard Debye's law, $g(\omega) \propto \omega^2$, is recovered. On the contrary, in Fomblin, we observe a continuous transition between the two exponents reflecting its glassy dynamics, which is confirmed by structure measurements. More importantly, in both systems, we find that the slope $a(T)$ grows with temperature following an exponential Arrhenius-like form, $a(T) \propto \exp(-\langle E \rangle /T)$. We confirm this experimental trend using molecular dynamics simulations and show that the prediction of instantaneous normal mode (INM) theory for $a(T)$ is in qualitative agreement with the experimental data.

cond-mat.soft↗

arXiv2022

Experimental Confirmation of the Universal Law for the Vibrational Density of States of Liquids

An analytical model describing the vibrational phonon density of states (VDOS) of liquids has long been elusive, mainly due to the difficulty in dealing with the imaginary modes dominant in the low-energy region, as described by the instantaneous normal mode (INM) approach. Nevertheless, Zaccone and Baggioli have recently developed such a model based on overdamped Langevin liquid dynamics. The model was proposed to be the universal law for the vibrational density of states of liquids. Distinct from the Debye law, g({\omega}) ~ {\omega}2, for solids, the universal law for liquids reveals a linear relationship, g({\omega}) ~ {\omega}, in the low-energy region. The universal law has been successfully verified with computer simulated VDOS for Lennard-Jones liquids. We further confirm this universal law with experimental VDOS measured by inelastic neutron scattering on real liquid systems including water, liquid metal, and polymer liquids. We have applied this model and extracted the effective relaxation rate for the short time dynamics for each liquid. The model has been further evaluated in the predication of the specific heat. The results have been compared with the existing experimental data as well as with values obtained by different approaches.

cond-mat.soft↗