Mott transition

About: Mott transition is a research topic. Over the lifetime, 2444 publications have been published within this topic receiving 78401 citations.

The role of 1-D finite size Heisenberg chains in increasing the metal to insulator transition temperature in hole rich VO2

Abstract: VO2 samples are grown with different oxygen concentrations leading to different monoclinic, M1, and triclinic, T, insulating phases which undergo a first order metal to insulator transition (MIT) followed by a structural phase transition (SPT) to the rutile tetragonal phase. The metal insulator transition temperature (Tc) was found to be increased with increasing native defects. Vanadium vacancy (VV) is envisaged to create local strains in the lattice which prevents twisting of the V-V dimers promoting metastable monoclinic, M2 and T phases at intermediate temperatures. It is argued that MIT is driven by strong electronic correlation. The low temperature insulating phase can be considered as a collection of one-dimensional (1-D) half-filled bands, which undergo a Mott transition to 1-D infinitely long Heisenberg spin ½ chains leading to structural distortion due to spin-phonon coupling. The presence of VV creates localized holes (d0) in the nearest neighbor, thereby fragmenting the spin ½ chains at the nanoscale, which in turn increases the Tc value more than that of an infinitely long one. The Tc value scales inversely with the average size of the fragmented Heisenberg spin ½ chains following a critical exponent of ⅔, which is exactly the same as predicted theoretically for the Heisenberg spin ½ chain at the nanoscale undergoing SPT (spin-Peierls transition). Thus, the observation of MIT and SPT at the same time in VO2 can be explained from our phenomenological model of reduced 1-D Heisenberg spin ½ chains. The reported increase (decrease) in the Tc value of VO2 by doping with metals having valency less (more) than four can also be understood easily with our unified model, for the first time, considering finite size scaling of Heisenberg chains.

Effects of the orbital self-interaction in both strongly and weakly correlated systems

Abstract: The orbital occupation, which is the centerpiece of both self-interaction and several metal-insulator transition analyses, as well as of the local density or generalized gradient approximation with a Hubbard term, is not well defined, in the sense that it is partially ambiguous. A general treatment can be applied to both strongly and weakly correlated systems. When it is applied to an intermediate- and partially filled band within of the host semiconductor gap whose width is less than the semiconductor gap, the original single band can either split as in a Mott transition or not. The former situation is usual and almost always generalized. However the latter also takes place and results from a dilution effect of the self-interaction where a large orbital correlation is reduced if there are other orbital contributions with lower self-interaction in the band. The key is in the choice of the subspace of correlated orbitals. This effect can neither be ignored nor discarded for those systems where there is a substantial mix of states. Examples of these behaviors will be presented and compared to other results. Moreover, the combination of different Hubbard terms acting on different atomic state subspaces can also be used to correct the spurious self-interaction of the bands and the gap underestimation. The relationship between these terms applied to different subspaces of correlated electrons will be presented.

Many-body effects on excitonic optical properties of photoexcited semiconductor quantum wire structures

Abstract: We study carrier interaction induced many-body effects on the excitonic optical properties of highly photoexcited one-dimensional semiconductor quantum wire systems by solving the dynamically screened Bethe-Salpeter equation using realistic Coulomb interaction between carriers. Including dynamical screening effects in the electron/hole self-energy and in the electron-hole interaction vertex function, we find that the excitonic absorption is essentially peaked at a constant energy for a large range of photoexcitation density ($n= 0-6\times 10^5$ cm$^{-1}$), above which the absorption peak disappears without appreciable gain i.e., \textit{no} exciton to free electron-hole plasma Mott transition is observed, in contrast to previous theoretical results but in agreement with recent experimental findings. This absence of gain (or the non-existence of a Mott transition) arises from the strong inelastic scattering by one-dimensional plasmons or charge density excitations, closely related to the non-Fermi liquid nature of one-dimensional systems. Our theoretical work demonstrates a transition or a crossover in one-dimensional photoexcited electron-hole system from an effective Fermi liquid behavior associated with a dilute gas of noninteracting excitons in the low density region ($n 10^5$ cm$^{-1}$). The conventional quasi-static approximation for this problem is also carried out to compare with the full dynamical results. Numerical results for exciton binding energy and absorption spectra are given as functions of carrier density and temperature.

Superfluidity of indirect excitons and biexcitons in coupled quantum wells and superlattices

Abstract: The collective properties of indirect excitons in coupled quantum wells (CQWs) are considered. The energy of the ground state of the exciton liquid as a function of the density of electrons e and holes h at different separations D between e and h layers is analysed. The quantum gas–liquid transition as D decreases is studied. The superfluidity appearance temperatures in the system (Kosterlitz–Thouless transition temperatures) have been estimated at different separations D between e and h layers. For the anisotropic two-dimensional e–h system in CQWs the Mott metal–insulator quantum transition is considered. The instability of the ground state of the system of interacting two-dimensional indirect excitons in a slab of superlattice with alternating e and h layers is established. The stable system of indirect quasi-two-dimensional biexcitons, consisting of indirect excitons with opposite directed dipole moments, is considered. The radius and the binding energy of the indirect biexciton are calculated. The collective spectrum of a rare system of two-dimensional indirect biexcitons, interacting as electrical quadrupoles, is studied. The density of the superfluid component ns (T) and the Kosterlitz–Thouless phase transition to the superfluid state in this system are analysed.