Spectral Function Changes Research Articles

Engineering the spectral profile of photon pairs by using multi-stage nonlinear interferometers

Using the quantum interference of photon pairs in N-stage nonlinear interferometers (NLIs), the contour of the joint spectral function can be modified into an islands pattern. We perform two series of experiments. One is that all of the nonlinear fibers in pulse pumped NLIs are identical; the other is that the lengths of N pieces of nonlinear fibers are different. We not only demonstrate how the pattern of spectral function changes with the stage number N, but also characterize how the relative intensity of island peaks varies with N. The results well agree with theoretical predictions, revealing that the NLI with lengths of N pieces of nonlinear fibers following binomial distribution can provide a better active filtering function. Our investigation shows that the active filtering effect of multi-stage NLI is a useful tool for efficiently engineering the factorable two-photon state—a desirable resource for quantum information processing.

Itinerant magnetism in URhGe revealed by angle-resolved photoelectron spectroscopy

The electronic structure of the ferromagnetic superconductor URhGe in the paramagnetic phase has been studied by angle-resolved photoelectron spectroscopy using soft x rays (hn=595-700 eV). Dispersive bands with large contributions from U 5f states were observed in the ARPES spectra, and form Fermi surfaces. The band structure in the paramagnetic phase is partly explained by the band-structure calculation treating all U 5f electrons as being itinerant, suggesting that an itinerant description of U 5f states is a good starting point for this compound. On the other hand, there are qualitative disagreements especially in the band structure near the Fermi level (E_B < 0.5 eV). The experimentally observed bands are less dispersive than the calculation, and the shape of the Fermi surface is different from the calculation. The changes in spectral functions due to the ferromagnetic transition were observed in bands near the Fermi level, suggesting that the ferromagnetism in this compound has an itinerant origin.

Correlation between Raman sum and optical conductivity sum in La2−xSrxCuO4

In a strongly correlated electron system, the single-particle spectral function changes into a coherent peak and incoherent humps which extend over 1 eV. The incoherent parts lose the symmetry and k dependence, so that the Raman spectra with different symmetries become identical and they are expressed by the optical conductivity. We found that the B1g and B2g spectra in La2−xSrxCuO4 become identical above 2000 cm−1 in the underdoped phase, if Fleury–Loudon type B1g two-magnon scattering is removed. The first Raman susceptibility moment correlates with the generalized optical conductivity moment. The good correlation arises from the incoherent states of a hump from 1000 to 4000 cm−1. The hump is the only structure of the incoherent electronic states in the mid-infrared absorption spectra below 1.4 eV at low carrier densities. The energy is twice the separated dispersion segments of the spin wave in the k⊥ stripe direction. The incoherent state is formed by the magnetic excitations created by the hole hopping in the antiferromagnetic spin stripes in the real space picture.

Positivity issues for the pinch-technique gluon propagator and their resolution

Although gauge-boson propagators in asymptotically-free gauge theories satisfy a dispersion relation, they do not satisfy the K\"allen-Lehmann (KL) representation because the spectral function changes sign. We argue that this is a simple consequence of asymptotic freedom. On the basis of the QED-like Ward identities of the pinch technique (PT) we claim that the product of the coupling ${g}^{2}$ and the scalar part $\stackrel{^}{d}({q}^{2})$ of the PT propagator, which is both gauge invariant and renormalization-group invariant, can be factored into the product of the running charge ${\overline{g}}^{2}({q}^{2})$ and a term $\stackrel{^}{H}({q}^{2})$ both of which satisfy the KL representation although their product does not. We show that this behavior is consistent with some simple analytic models that mimic the gauge-invariant PT Schwinger-Dyson equations (SDE), provided that the dynamic gauge-boson mass is sufficiently large. The PT SDEs do not depend directly on the PT propagator through $\stackrel{^}{d}$ but only through $\stackrel{^}{H}$.

Width of the quasiparticle spectral function in a two-dimensional electron gas with spin-orbit interaction

The results of an investigation of the width of the electron and hole spectral functions in an isotropic electron gas with the Rashba spin-orbit interaction as a function of the wave vector within the G0W0 approximation are reported. This kind of electron gas is used to simulate two-dimensional systems formed by electrons in InxGa1 − xAs layers of various heterostructures and by surface electron states on Au(111). It is demonstrated how the width of the spectral function changes as a result of the spin-orbit interaction and how it depends on the branch index of the energy spectrum split by this interaction.

Systematic change of spectral function observed by controlling electron correlation in Ca 1−xSr xVO 3 with fixed 3 d1 configuration.

By using ultra-violet photoemission spectroscopy (UPS) and soft x-ray absorption spectroscopy (XAS), we have observed a systematic change of the spectral function around the Fermi level in the perovskite-type conductive 3 d 1 system, Ca 1−xSr xVO 3. When we decrease the Sr concentration (x) from 1 to 0 in Ca 1−xSr xVO 3, UPS and XAS intensities near the Fermi level decrease, and the reduced spectral weight is transferred to high energy part about 2 eV below and above Fermi level. The high energy features are thought to correspond to the remnants of lower and upper Hubbard bands, and the spectral weight within ∼1eV of Fermi level is assigned to the quasiparticle excitations or renormalized 3 d-band. That is, the observed systematic spectral change indicates the opening of Mott-Hubbard gap.

Degenerate Anderson impurity model in the presence of spin-orbit and crystal-field splitting.

Using a quantum Monte Carlo technique combined with the method of maximum entropy, we calculated the magnetic susceptibility and spectral properties of the degenerate single-impurity Anderson model in the presence of spin-orbit and crystal-field splitting. At small hybridization and moderate temperatures, the magnetic susceptibility displays a nonuniversal behavior. Under the influence of spin-orbit splitting, the magnetic susceptibility decreases due to the quenching of a sixfold-degenerate state to an effectively doubly degenerate state. The effect of crystal-field splitting on the magnetic susceptibility is less pronounced. The temperature dependence of the spectral function changes dramatically under the influence of spin-orbit splitting. In the accessible temperature regime, the central peak at \ensuremath{\omega}\ensuremath{\sim}0 decreases with decreasing temperature.

Spectral function of the Anderson model: A quantum Monte Carlo calculation.

We have obtained the spectral function of the asymmetric Anderson model at low temperature through analytic continuation of quantum Monte Carlo data. While not quantitatively exact, the qualitative features of the spectral function clearly allow us to distinguish between the local-moment, mixed-valent, and empty-orbital regimes. We show how the spectral function changes as a function of particle-hole asymmetry and the Coulomb potential {ital U}.

Dynamical approach to analytic continuation of quantum Monte Carlo data.

A new method of analytic continuation from a Matsubara single-particle Green's function to a spectral function is presented. We recast this problem onto a new one where we dynamically minimize a suitably defined potential. Our method allows the imposition of physical constraints such as smoothness and adherence to the sum rule. The method is applied to the symmetric Anderson impurity model. We show how the spectral function changes with the Kondo temperature ${T}_{K}$, the hybridization witdth \ensuremath{\Delta}, and the Coulomb potential U.