Charge Carriers In Systems Research Articles

Influence of shallow traps on the kinetics of Optically Stimulated Luminescence of the Li and Ce ions co-doped MgB4O7 compound

The presence of point defects associated with shallow traps has been identified as a limiting factor in the design of materials with desirable characteristics for applications such as scintillation and luminescence dosimetry, among others. In this context, this study aims to elucidate the dynamics inherent to the charge carriers in systems containing multiple traps and one single recombination centre. To unravel this complexity, we employed a synergetic approach combining Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) measurements with kinetic models. Specifically, for the Li and Ce co-doped MgB4O7, we verified that the electron population trapped in shallower traps exhibits a slower decay rate under optical stimulation compared to deeper traps. This behaviour led to a delay in the luminescence emission, as a result of charge re-trapping or even competition with other centres. This phenomenon can be attributed to its low photoionization cross-section associated with a high re-trapping rate, providing a rationale for these remarks. In addition, we observed that the shallow traps have a greater optical than thermal probability of being detrapped, indicating that they were not solely detrapped through thermal stimulation. Also, the magnitude of the thermal probability leads us to believe that there is an electron population different from zero in the conduction band at t = 0 (before optical stimulation). Lastly, this study paves the way for a deeper understanding of charge carrier dynamics in systems of multiple traps, improving the performance of existing materials for scintillation and dosimetry.

Nanoelectromechanical Switches by Controlled Switchable Cracking

Nanoelectromechanical (NEM) switches could surmount the Boltzmann Tyranny in the current charge-carrier systems. However, thus far, practical implementations of the NEM systems have been hindered by the complicated fabrication processes of forming the extremely small air gap. Here, we realize a very simple NEM switch by exploiting a switchable nanocrack controlled by an electric field in a metallic alloy-ferroelectric heterostructure. The crack is formed in a controllable manner in terms of its initiation, location, and orientation through a bridge-like structure. The open and closed states of the crack are programmed under a cyclic electric field. In addition, an abrupt switching behavior with a nonvolatile high ON/OFF current ratio (>10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ) is measured owing to the near-zero OFF-state leakage current across the crack. This simple nanocrack switch presents a novel opportunity in the NEM systems, which can be used as a new nonvolatile random-access memory and logic.

Photocatalytic activity of ZnO nanopowders: The role of production techniques in the formation of structural defects

The effect of the type of structural defect in zinc oxide on its photocatalytic properties was studied for phenol photodegradation under UV-irradiation. It was shown that the use of different types of precursors (zinc oxalate and zinc hydroxide) for the production of zinc oxide leads to the formation of a material with the same phase composition and equal energy of the forbidden band, but different photocatalytic activities. Simultaneously, the peculiarities of the luminescence and electron spin resonance spectra indicate the formation of different types of defects in the structure of the material, namely, oxygen vacancies (Vo) in the anionic and zinc vacancies (VZn) in the cationic sublattices of zinc oxide synthesised from the zinc oxalate and hydroxide, respectively. Also, the different characteristics of the luminescence decays reveal the different recombination paths for the free charge carriers in the systems synthesised from the different precursors. The different times of the luminescence decay also confirmed the different methods of recombination of free charge carriers in systems synthesised from different precursors. It was shown that the appearance of defects in the cationic sublattice leads to a decrease in the photocatalytic activity of the material relative to phenol degradation.

Ambipolar quantum transport in few-layer black phosphorus

We have investigated the quantum transport properties of high-mobility electrons and holes in atomically thin black phosphorus ambipolar devices. The two-dimensional hole system exhibits unambiguously the quantum Hall effect in a magnetic field up to 30 T, while the electron system shows clearly developing Hall plateaus at integer Landau level filling factors accompanied by ${R}_{xx}$ oscillations, signaling the onset of the quantum Hall effect. By bringing the spin-resolved Landau levels of the electron system to a coincidence, we determine an electron spin susceptibility to be ${\ensuremath{\chi}}_{se}={m}^{*}{g}^{*}=1.1\ifmmode\pm\else\textpm\fi{}0.03$, which, combined with the electron mass ${m}^{*}=0.39\phantom{\rule{0.16em}{0ex}}{m}_{0}$, yields a Land\'e $g$ factor ${g}^{*}=2.8\ifmmode\pm\else\textpm\fi{}0.2$. The enhancement of spin susceptibility in the black phosphorus two-dimensional electron system is around 50% compared with band susceptibility, which agrees well with various two-dimensional charge-carrier systems with weak spin-orbit coupling, suggesting the important role played by the exchange interaction.

Universal peak conductivities at the transition between insulating and integer quantum Hall states

Experimental data available in the literature for the peak values, σ xx max , of the diagonal conductivity at the transition between the insulating state and the lowest integer quantum Hall states are compared with the universal values of 1 2 and 1 (in units e 2 / h ). A good agreement is found for different two-dimensional charge-carrier systems by taking account for the presence of electronic valley or spin degeneracy in the appropriate cases.

Effective temperature for hopping transport in a Gaussian density of states

For hopping transport in disordered materials, the mobility of charge carriers is strongly dependent on the temperature and the electric field. Our numerical study shows that both the energy distribution and the mobility of charge carriers in systems with a Gaussian density of states, such as organic disordered semiconductors, can be described by a single parameter---effective temperature, which is dependent on the magnitude of the electric field. Furthermore, this effective temperature does not depend on the concentration of charge carriers, while the mobility does depend on the charge carrier concentration. The concept of the effective temperature is shown to be valid for systems with and without space-energy correlations in the distribution of localized states.

Distribution of charge carriers in a system of large radius polarons and bipolarons and delocalized carriers

Statistical properties of polarons and bipolarons of large radius have not been studied yet with the limitations of their momentums imposed by the spatial dispersion of ionic polarizability taken into account. We derive a distribution function for charge carriers in systems where autolocalized and delocalized carriers can occur. Properties of such a distribution turn out to be unusual. It shows that the concentration of polarons in the system is limited and the temperature region where they can occur is restricted from the top. The chemical potential in the system of autolocalized fermions and bosons and delocalized fermions can increase with the temperature, which makes possible bipolaron Bose-condensation, even if the bipolaron is not in the most energetically profitable state. A phase diagram Bose-vapour—Bose-condensate is constructed with the limitation on momentums of bipolarons allowed for. It shows, that owing to a sharp decrease in the number of states in the Bose-vapour of bipolarons caused by the momentum limitation, the temperature of the bipolaron Bose-condensation can attain 100-200K at typical values of the bipolaron effective mass.

Nearest-neighbor correlation effects in spinless one-dimensional conductors

Certain anisotropic conductors such as tetracyanoquinodimethane (TCNQ) salts can be modeled as one-dimensional systems with spinless electrons. This theoretical study covered nearest-neighbor correlation effects on the ground state and the soliton excitations of these models. The results described are the first solutions of the many-body Hamiltonian for all values of the correlation strength, obtained by finding eigenvectors of very large matrices. For finite systems, the ground state has a conditional Peierls transition for all values of the correlation energy $V$. The trends with increasing size indicate a critical transition. The infinite systems will dimerize for Hubbard $V l2t$ and the degree of dimerization increases with $V$. The dimerization abruptly terminates at $V=2t$. Similar results on the $\mathrm{XXZ}$ spin model are confirmed by behavior predicted using scaling relationships. The trends in soliton energy, width, and charge distribution have been calculated. Solitons are the lowest-energy charge carriers in systems which dimerize. The solitons in the spinless conductor carry fractional charge and those in the $\mathrm{XXZ}$ spin model have fractional spin. The charge on the most depleted site provides a useful measure of $V$. The solitons do not have a significant activation energy and should be highly mobile.