Charge Ordering Gap Research Articles

Optical Conductivity in a Two-Dimensional Extended Hubbard Model for an Organic Dirac Electron System α-(BEDT-TTF)2I3

The optical conductivity in the charge order phase is calculated in the two-dimensional extended Hubbard model describing an organic Dirac electron system α -(BEDT-TTF) 2 I 3 using the mean field theory and the Nakano-Kubo formula. Because the interband excitation is characteristic in a two-dimensional Dirac electron system, a peak structure is found above the charge order gap. It is shown that the peak structure originates from the Van Hove singularities of the conduction and valence bands, where those singularities are located at a saddle point between two Dirac cones in momentum space. The frequency of the peak structure exhibits drastic change in the vicinity of the charge order transition.

Doping dependence of charge order in electron-doped cuprate superconductors

In the recent studies of the unconventional physics in cuprate superconductors, one of the central issues is the interplay between charge order and superconductivity. Here the mechanism of the charge-order formation in the electron-doped cuprate superconductors is investigated based on the t-J model. The experimentally observed momentum dependence of the electron quasiparticle scattering rate is qualitatively reproduced, where the scattering rate is highly anisotropic in momentum space, and is intriguingly related to the charge-order gap. Although the scattering strength appears to be weakest at the hot spots, the scattering in the antinodal region is stronger than that in the nodal region, which leads to the original electron Fermi surface is broken up into the Fermi pockets and their coexistence with the Fermi arcs located around the nodal region. In particular, this electron Fermi surface instability drives the charge-order correlation, with the charge-order wave vector that matches well with the wave vector connecting the hot spots, as the charge-order correlation in the hole-doped counterparts. However, in a striking contrast to the hole-doped case, the charge-order wave vector in the electron-doped side increases in magnitude with the electron doping. The theory also shows the existence of a quantitative link between the single-electron fermiology and the collective response of the electron density.

Theoretical study of the role of charge ordering in antiferromagnetically ordered manganites

We address the interplay of charge and magnetic orderings in colossal magnetoresistive material manganese oxides. We propose here on-site double exchange spin–spin interaction in the presence of Heisenberg-type spin–spin interaction in localized t2g core electrons. We consider charge-density wave (CDW) interaction in the crystal lattice as an extra mechanism in the itinerant eg band, to take into account of colossal magnetoresistance (CMR) in the system. We calculate electron Green’s functions by Zubarev’s Green’s function technique and hence calculate the charge-ordering gap and magnetic gap in the conduction band as well as core electron states. These orders are solved self-consistently for different model parameters of the system. We observe that the induced magnetic gap in the conduction band exists near the antiferromagnetic Néel temperature, which accounts for the CMR in the system. For all values of temperature, the CDW coupling lies in the range of g = 0.04 to 0.06, where the induced magnetic gap exists. The temperature-dependent specific heat exhibits anomalous jumps near charge-ordering and magnetic-ordering temperatures. The eg electron density of states exhibits a two-gap structure which explains tunneling conductance spectra measurements.

Evidence of mobile carriers with charge-ordering gap in epitaxialPr0.625Ca0.375MnO3thin films

Epitaxial thin films of charge-ordered Pr$_{0.625}$Ca$_{0.375}$MnO$_{3}$ have been studied using variable temperature Scanning tunneling microscopy and spectroscopy (STM/STS). The as grown films were found to be granular while the annealed films show atomic terraces at all temperatures and are found to be electronically homogeneous in 78-300K temperature range. At high temperatures (T$>$T$_{CO}\approx$ 230 K) the local tunnel spectra of the annealed films show a depression in the density of states (DOS) near Fermi energy implying a pseudogap with a significant DOS at E$_F$. The gap feature becomes more robust with cooling with a sharp jump in DOS at E$_F$ at T$_{CO}$ and with a gap value of $\sim$0.3 eV at 78K. At low temperatures we find a small but finite DOS at E$_F$ indicative of some delocalized carriers in the CO phase together with an energy gap. This is consistent with bulk transport, which shows weakening of the activation gap with cooling below 200K, and indicates the presence of two types of carriers at low temperatures.

Spin and charge order in the CE phase ofLa1−xCaxMnO3

The CE phase of ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ is stable for $x\ensuremath{\gtrsim}0.5$ and displays long-range magnetic, charge, and orbital orders. The magnetic order of the Mn spins arises from the competition of the superexchange and double-exchange interactions and the checkerboard charge and the orbital order is the consequence of the Jahn-Teller coupling of the ${e}_{g}$ orbitals to the lattice. Using a mean-field slave-boson approach for the ${e}_{g}$ electrons in two orbitals per site with excluded multiple occupancy and Hund's rule coupling between the ${e}_{g}$ and ${t}_{2g}$ states, we obtain the tight-binding band structure of the CE phase. The unit cell of the CE phase consists of 16 sites. The 32 ${e}_{g}$ bands in the Brillouin zone are grouped into two sets of 16 bands separated by a charge order gap. The phase diagram including the A, B, C, CE, and G phases reproduces the phase sequence observed in ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$.

Mean-field model for the CE-phase of manganites

The CE phase of La0.5Ca0.5MnO3 displays long-range magnetic, charge and orbital order of the eg states of the Mn ions. The magnetic order of the t2g spins of Mn is determined by the antiferromagnetic superexchange and the ferromagnetic double-exchange. The checkerboard charge and the orbital order is the consequence of the Jahn-Teller coupling of the eg orbitals to the lattice. Using a mean-field slave-boson approach for the eg electrons in two orbitals per site with excluded multiple occupancy and Hund's rule coupling between the eg and t2g states, we obtain the tight-binding band structure of the CE phase. The unit cell of the CE phase consists of 16 sites. The 32 eg bands in the Brillouin zone are grouped into two sets of 16 bands separated by a charge order gap. The charge gap does not directly affect the properties of the compound. If long-range orbital order is included an additional gap opens at the Fermi level.

Near Fermi level electronic structure ofPr1−xSrxMnO3: Photoemission study

In this study, we report the observation of a pseudogap associated with the insulator-metal transition in compositions of ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ system with no charge ordering. Our valence-band photoemission study shows that the observed shifts in the near Fermi level density of states are abrupt at the Curie transition and occur over an energy scale of $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, strongly suggesting that the charge-ordering gap observed earlier in other manganites and the pseudogap observed here may indeed have the same origin. These results could be understood within the framework of models based on electronic phase separation where it has been shown that the pseudogap is a generic feature of the mixed-phase compositions. Also, our band-structure calculations on ${\mathrm{Pr}}_{0.75}{\mathrm{Sr}}_{0.25}\mathrm{Mn}{\mathrm{O}}_{3}$ show the possible existence of half metallicity in this system.

The nature of the charge-ordered state in with a very small average radius of the A-site cations

A detailed investigation of with a very small radius of the A-site cations reveals the occurrence of a charge-ordering transition in the paramagnetic state, at a relatively high temperature of 260 K. The orthorhombic lattice distortion, as measured by the dimensionless index D, is large over the entire 300-100 K range, but the antiferromagnetic interactions become prominent only at low temperatures (< 160 K). The charge-ordering gap in , measured by low-temperature vacuum tunnelling spectroscopy, is large and the charge-ordered state is unaffected by the application of a magnetic field of 6 T. The study indicates that the nature of charge-ordering in which is dominated by the cooperative Jahn-Teller effect and the associated lattice distortion is distinctly different from analogous manganates with larger .

Reentrant transition from an incipient charge-ordered state to a ferromagnetic metallic state in a rare-earth manganate

A reentrant transition from an incipient charge-ordered (CO) state to a charge-delocalized ferromagnetic (CDFM) state has been established in the manganate ${\mathrm{Nd}}_{0.25}{\mathrm{La}}_{0.25}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3},$ in which the average $A$-site ionic radius is 1.19 \AA{}. The reentrant CDFM phase is associated with a first-order phase transition that reduces the orthorhombic distortion of the lattice, in contrast to the CO transition in other manganates where the orthorhombically distorted CO state is stabilized at low temperatures. At the CO-CDFM transition, there is a collapse of the charge-ordering gap as measured by vacuum tunneling spectroscopy.