Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: We report nanoscale inter-particle magnetic orderings in self-assemblies of Fe3O4 nanoparticles (NPs), and the emergence of inter-particle antiferromagnetic (AF) (super-antiferromagnetic) correlations near the coercive field at low temperature. The magnetic ordering is probed via x-ray resonant magnetic scattering (XRMS), with the x-ray energy tuned to the Fe-L3 edge and using circular polarized light. By exploiting dichroic effects, a magnetic scattering signal is isolated from the charge scattering signal. The magnetic signal informs about nanoscale spatial orderings at various stages throughout the magnetization process and at various temperatures throughout the superparamagnetic blocking transition, for two different sizes of NPs, 5 and 11 nm, with blocking temperatures TB of 28 K and 170 K, respectively. At 300 K, while the magnetometry data essentially shows superparamagnetism and absence of hysteresis for both particle sizes, the XRMS data reveals the presence of non-zero (up to 9/100) inter-particle AF couplings when the applied field is released to zero for the 11 nm NPs. These AF couplings are drastically amplified when the NPs are cooled down below TB and reach up to 12/100 for the 5 nm NPs and 48/100 for the 11 nm NPs, near the coercive point. The data suggests that the particle size affects the prevalence of the AF couplings: compared to ferromagnetic (F) couplings, the relative prevalence of AF couplings at the coercive point increases from a factor ~ 1.6 to 3.8 when the NP size increases from 5 to 11 nm.

Abstract: We have developed a microscopic many-body theory of two-dimensional coherent spectroscopy (2DCS) for a model of polarons in one-dimensional (1D) materials. Our theory accounts for contributions from all three processes: excited-state emission (ESE), ground-state bleaching (GSB), and excited-state absorption (ESA). While the ESE and GSB contributions can be accurately described using a Chevy's ansatz with one particle-hole excitation, the ESA process requires information about the many-body eigenstates involving two impurities. To calculate these double polaron states, we have extended the Chevy's ansatz with one particle-hole excitation. The validity of this ansatz was verified by comparing our results with an exact calculation using Bethe's ansatz. Our numerical results reveal that in the weak interaction limit, the ESA contribution cancels out the total ESE and GSB contributions, resulting in less significant spectral features. However, for strong interactions, the features of the ESA contribution and the combined ESE and GSB contributions remain observable in the 2DCS spectra. These features provide valuable information about the interactions between polarons. Additionally, we have investigated the mixing time dynamics, which characterize the quantum coherences of the polaron resonances. Overall, our theory provides a comprehensive framework for understanding and interpreting the 2DCS spectra of polarons in 1D materials, shedding light on their interactions and coherent dynamics.

Abstract: The performance of semiconductor devices can be enhanced by understanding and characterizing the bound states of deep defects. However, simulating these states using conventional $\textit{ab initio}$ techniques, which are limited by small cell volumes, proves challenging due to their extended nature spanning tens of nanometers. Here, we present a semi-$\textit{ab initio}$ method for modeling the bound states of deep defects in semiconductors, applied to the negatively charged nitrogen vacancy (NV$^-$) center in diamond. We employ density functional theory (DFT) calculations to construct accurate potentials for an effective mass model and allowing us to unveil the structure of the bound hole states. We also develop a model to calculate the non-radiative capture cross-sections, which agrees with experiment within one order of magnitude. Finally, we present the first attempt at constructing the photoionization spectrum of NV$^0\rightarrow$ NV$^-$ + bound hole, showing that the electronic transitions of the bound holes can be distinguished from phonon sidebands. This approach can be adapted to other deep defects in semiconductors.