Decrease Of Spectral Weight Research Articles

Disorder effects on hot spots in electron-doped cuprates

Antiferromagnetic fluctuations in two dimensions cause a decrease in spectral weight at so-called hot spots associated with the pseudogap in electron-doped cuprates. In the $2\text{D}$ Hubbard model, these hot spots occur when the Vilk criterion is satisfied, namely, when the spin correlation length exceeds the thermal de Broglie wavelength. Using the two-particle self-consistent approach, here we show that this criterion may be violated near the antiferromagnetic quantum critical point when sufficient disorder is added to the model. Static disorder decreases inelastic scattering, in contradiction with Matthiessen's rule, leading to a shift in the position of the quantum critical point and a modification of the conditions for the appearance of hot spots. This opens the road to a study of the interplay between disorder, antiferromagnetic fluctuations, and superconductivity in electron-doped cuprates.

Optical signature of the pressure-induced dimerization in the honeycomb iridate α−Li2IrO3

We studied the effect of external pressure on the electrodynamic properties of $\alpha$-Li$_2$IrO$_3$ single crystals in the frequency range of the phonon modes and the Ir $d$-$d$ transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure $P_c$=3.8 GPa. With increasing pressure an overall decrease in spectral weight of the Ir $d$-$d$ transitions is found up to $P_c$. Above $P_c$, the local (on-site) $d$-$d$ excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The non-local (intersite) Ir $d$-$d$ transitions show a monotonic blue-shift and decrease in spectral weight. The changes observed for the non-local excitations are most prominent well above $P_c$, namely for pressures $\geq$12 GPa, and only small changes occur for pressures close to $P_c$. The profile of the optical conductivity at high pressures ($\sim$20 GPa) appears to be indicative for the dimerized state in iridates.

Correlation between Fermi arc and charge order resulting from the momentum-dependent self-energy correction in cuprates

We calculate momentum-dependent self-energies due to self-consistent spin and charge-density fluctuations within a single-band Hubbard model for hole-doped cuprates. We find that the dynamical density instability (mainly paramagnonlike spin fluctuations) arising from the van Hove singularity at the antinodal point produces strongly anisotropic mass renormalization, increasing from the nodal direction to the antinodal one. This gives rise to a coherent Fermi arc pattern in which although the self-energy dressed Green's function has poles at all Fermi momenta, the corresponding spectral weight decreases from the nodal to the antinodal direction. We study the feedback effects of the anisotropic self-energy and Fermi arc on the charge-density wave (CDW) instability by direct computations of charge susceptibility and the quasiparticle interference pattern. We find that that due to the loss of spectral weight at the antinodal points, the CDW nesting weakens, and the corresponding hot spot shifts from the antinodal region to the tip of the Fermi arc below the magnetic Brillouin zone. The results are in good agreement with experiments, reproducing the observed slow doping dependence of the CDW wave vector. Our investigation therefore provides a mechanism for the Fermi arc in cuprates, with various testable predictions such as a sharp, first-order-like phase transition of the CDW order on the higher-doping side.

Role of electron-electron interactions in the charge dynamics of rare-earth-dopedCaFe2As2

We have investigated the charge dynamics and the nature of many-body interactions in La- and Pr- doped CaFe2As2. From the infrared part of the optical conductivity, we discover that the scattering rate of mobile carriers above 200 K exhibits saturation at the Mott-Ioffe-Regel limit of metallic transport. However, the dc resistivity continues to increase with temperature above 200 K due to the loss of Drude spectral weight. The loss of Drude spectral weight with increasing temperature is seen in a wide temperature range in the uncollapsed tetragonal phase, and this spectral weight is recovered at energy scales about one order of magnitude larger than the Fermi energy scale in these semimetals. The phenomena noted above have been observed previously in other correlated metals in which the dominant interactions are electronic in origin. Further evidence of significant electron-electron interactions is obtained from the presence of quadratic temperature and frequency-dependent terms in the scattering rate at low temperatures and frequencies in the uncollapsed tetragonal structures of La-doped and Pr-doped CaFe2As2. For temperatures below the structure collapse transition in Pr-doped CaFe2As2 at 70 K, the scattering rate decreases due to weakening of electronic correlations, and the Drude spectral weight decreases due to modification of the low-energy electronic structure.

Tuning the Interface Conductivity of LaAlO3/SrTiO3 Using Ion Beams: Implications for Patterning

Patterning of the two-dimensional electron gas formed at the interface of two band insulators such as LaAlO3/SrTiO3 is one of the key challenges in oxide electronics. The use of energetic ion beam exposure for engineering the interface conductivity has been investigated. We found that this method can be utilized to manipulate the conductivity at the LaAlO3/SrTiO3 interface by carrier localization, arising from the defects created by the ion beam exposure, eventually producing an insulating ground state. This process of ion-beam-induced defect creation results in structural changes in SrTiO3 as revealed by the appearance of first-order polar TO2 and TO4 vibrational modes which are associated with Ti-O bonds in the Raman spectra of the irradiated samples. Furthermore, significant observation drawn from the magnetotransport measurements is that the irradiated (unirradiated) samples showed a negative (positive) magnetoresistance along with simultaneous emergence of first-order (only second order) Raman modes. In spectroscopic ellipsometry measurements, the optical conductivity features of the irradiated interface are broadened because of the localization effects, along with a decrease of spectral weight from 4.2 to 5.4 eV. These experiments allow us to conclude that the interface ground state (metallic/insulating) at the LaAlO3/SrTiO3 can be controlled by tailoring the defect structure of the SrTiO3 with ion beam exposure. A resist-free, single-step direct patterning of a conducting LaAlO3/SrTiO3 interface has been demonstrated. Patterns with a spatial resolution of 5 μm have been fabricated using a stencil mask, while nanometer scale patterns may be possible with direct focused ion beam writing.

Renormalization-group analysis of the one-dimensional extended Hubbard model with a single impurity

We analyze the one-dimensional extended Hubbard model with a single static impurity by using a computational technique based on the functional renormalization group. This extends previous work for spinless fermions to spin-1/2 fermions. The underlying approximations are devised for weak interactions and arbitrary impurity strengths, and have been checked by comparing with density-matrix renormalization-group data. We present results for the density of states, the density profile and the linear conductance. Two-particle backscattering leads to striking effects, which are not captured if the bulk system is approximated by its low-energy fixed point, the Luttinger model. In particular, the expected decrease of spectral weight near the impurity and of the conductance at low energy scales is often preceded by a pronounced increase, and the asymptotic power laws are modified by logarithmic corrections.

Effect of a staggered magnetic field on the S=1 Haldane chain with single-ion anisotropy

We analyze the gaps in the excitation spectrum of a Haldane chain with single-ion anisotropy in a staggered field. We show that the gap along the direction of the field increases at a faster rate than the others, while its spectral weight decreases, being transferred to a two-magnon continuum.

Interplay between electron-electron interaction and electron-phonon coupling near the Fermi surface of1T−TaS2

We present a detailed high-resolution angle-resolved photoemission study of the electronic band structure of the room-temperature quasicommensurate charge-density-wave phase of $1T\ensuremath{-}{\mathrm{TaS}}_{2}.$ In particular, we show that no crossings of the Fermi level are visible in the complete Brillouin zone, indicating that an electron-electron correlation-induced pseudogap in the Ta $5d$ derived band exists already above the Mott localization-induced transition at 180 K. Moreover, we find that the electronic structure is governed by at least two quasiparticle peaks, which can be assigned to electrons from starlike shells of Ta atoms within the distorted crystal lattice. These peaks show quasilocalized (dispersionless) behavior in parts of the Brillouin zone where the one-particle band is unoccupied and they follow the one-particle dispersion in the occupied part. In order to address the question of possible Fermi-surface (FS) nesting, we scanned the remaining remnant FS and found regions with a considerable decrease of spectral weight. However, we find no clear evidence for FS nesting.

Origin of superconductivity suppression in (Dy1−xPrx)Ba2Cu3O7studied by soft-x-ray absorption spectroscopy

O K-edge and Cu ${\mathrm{L}}_{23}$-edge x-ray-absorption near-edge structure (XANES) spectra for the series of (${\mathrm{Dy}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Pr}}_{\mathrm{x}}$)${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ compounds (x=0--0.7) were measured to investigate how the variation of hole states related to the superconductivity. Near the O 1s edge, prepeaks at \ensuremath{\sim}527.6 and \ensuremath{\sim}528.4 eV are assigned to transitions into O 2p holes located in the ${\mathrm{CuO}}_{3}$ ribbons and ${\mathrm{CuO}}_{2}$ planes, respectively. Both the O 2p hole concentration in the ${\mathrm{CuO}}_{2}$ planes and ${\mathrm{CuO}}_{3}$ ribbons decrease monotonically with increasing Pr doping, giving evidence in support of the hole depletion model proposed by Liechtenstein and Mazin. It clearly demonstrates that the suppression of superconductivity with Pr-doping results predominantly from the hole depletion effect. In the Cu 2p absorption edge, the high-energy shoulders originating from Cu ${3\mathrm{d}}^{9}$L defected states show a linear decrease in spectral weight as the Pr doping increases, where L denotes the ligand O 2p hole on the ${\mathrm{CuO}}_{3}$ ribbons and the ${\mathrm{CuO}}_{2}$ planes.

Spectral properties of the t-J model in the presence of hole-phonon interaction.

We examine the effects of electron-phonon interaction on the dynamics of the charge carriers doped in two-dimensional (2D) Heisenberg antiferromagnet. The $t$-$J$ model Hamiltonian with a Fr\"ohlich term which couples the holes to a dispersionless (optical) phonon mode is considered for low doping concentration. The evolution of the spectral density function, the density of states, and the momentum distribution function of the holes with an increase of the hole-phonon coupling constant $g$ is studied numerically. As the coupling to a phonon mode increases the quasiparticle spectral weight decreases and a ``phonon satellite'' feature close to the quasi-particle peak becomes more pronounced. Furthermore, strong electron-phonon coupling smears the multi-magnon resonances (``string states'') in the incoherent part of the spectral function. The jump in the momentum distribution function at the Fermi surface is reduced without changing the hole pocket volume, thereby providing a numerical verification of Luttinger theorem for this strongly interacting system. The vertex corrections due to electron- phonon interaction are negligible in spite of the fact that the ratio of the phonon frequency to the effective bandwidth is not small.

Temperature dependence of 4f PES features and hole occupancy in yterbium heavy fermions

Abstract We have measured the temperature dependence between 80 and 300 K and spectral weight of the 4f features in clean single crystal Yb heavy fermions, cleaved in a vacuum of 5 × 10−11 Torr. A small (∼ 15%) decrease in spectral weight differs substantially from the reported ∼ 50% decrease over this range. All divalent features are much larger than expected, yielding nf values far too small. The intensity of the divalent features in YbAuCu4 is about two orders of magnitude larger than expected.

Electron spectroscopy studies of scaling behaviours in heavy fermions

We present in this paper photoemission and inverse photoemission measurements of several Ce- and Yb-based compounds. Scaling behaviours characteristic of the Kondo effect are clearly observed: in the CeSix series, the modification of the Kondo temperature with stoichiometry is accompanied by a decrease of spectral weight near EF in photoemission and inverse photoemission. Moreover, the 4f spectral density of several Ce- and Yb-based compounds exhibits the temperature dependence expected in the Anderson model. These measurements show that the Anderson model contains the fundamental mechanisms accounting for the spectroscopic properties of Kondo systems.