Charge Collective Modes Research Articles

Dispersive collective charge modes in a spin 1/2 cuprate ladder

We present the momentum evolution of collective charge modes in Sr 14Cu 24O 41 over the complete Brillouin zone using inelastic resonant X-ray scattering. The observed spectrum is found to be dispersive with periodicity corresponding to the ladder sublattice for momentum transfers along the ladder “leg” direction, and nondispersive along the “rung”. Details of the spectrum, including dispersion and linewidth, fall between characteristic charge spectra of single chain quasi-1D Mott insulators and quasi-2D cuprates.

Dispersive collective charge modes in an incommensurately modulated cuprate Mott insulator

We report measurement of collective charge modes of insulating Sr_(14)Cu_(24)O_(41) using inelastic resonant x-ray scattering over the complete Brillouin zone. Our results show that the intense excitation modes at the charge gap edge predominantly originate from ladder-containing planar substructures. The observed modes are found to be dispersive for momentum transfers along the “legs” (ℏQ ∥∥c ) but nearly localized along the “rungs” (ℏQ ∥∥a ). We show that the dispersion and peak width characteristics of the modes can be understood in the strong-coupling quantum limit (Hubbard U⪢t_(ladder)>t_(chain), where t is the hopping parameter). Quite generally, we demonstrate that the momentum tunability (Q resolution) of inelastic x-ray scattering can be utilized to resolve mode contributions in multicomponent incommensurate quantum electron systems.

Collective charge modes in quasi-1D Mott insulator SrCuO2

Inelastic resonant X-ray scattering is carried out on SrCuO2, a model quasi one-dimensional spin 1/2 Mott insulator. Tuning excitation energy to a previously unstudied resonance allows insight into the effect of the intermediate state coupling to the fundamental charge modes exhibited by the sample. Redistribution of momentum and energy resolved spectral weight at the new resonance permits identification of features resembling the strong coupling prediction for the modes. Despite expected resonance enhancement, no spinon-like modes are observed above 0.5eV (two-spinon threshold) in the measured scattering spectra.

Charge critical fluctuations in cuprates: Isotope effect, pseudogap, conductivity and Raman spectroscopy

Abstract Within the stripe quantum critical point theory for high T c superconductors, we point out that there is a direct contribution of charge collective fluctuations to the optical absorption and to the Raman spectra. In this latter case, we find that the critical charge collective modes can or cannot be excited depending on the direction of their wavevector and on the polarization of the incoming and outgoing photons. This indicates a marked distinction between quasiparticles which are strongly and weakly coupled to critical collective modes and provides a direct confirmation that the order associated to the quantum critical point near optimal doping of cuprates occurs at finite wavevectors.

Collective transport and optical absorption near the stripe criticality

Within the stripe quantum critical point scenario for high T c superconductors, we point out that in addition to the usual Drude term, there is a direct contribution of charge collective fluctuations to the optical absorption and to the DC resistivity. In particular, we find that the critical charge collective modes lead to T-linear resistivity, which extends to zero temperature near optimal doping.

Collective charge modes in a 1D electron liquid

A new behavior of the collective mode in a 1D electron liquid is found. The charge mode frequency goes to zero when the wave number is close to double Fermi wave number, i.e. the soft mode appears in addition to common long-wave plasmons. This mode is related to dynamic short-range electron correlations, which are adequately described in the frame of the Luttinger model. The soft mode is immanent in 1D and is absent in higher-dimensional systems. The results are valid for both Coulomb and short-range electron–electron interaction.

Strongly coupled quantum criticality with a Fermi surface in two dimensions: Fractionalization of spin and charge collective modes

We describe two dimensional models with a metallic Fermi surface which display quantum phase transitions controlled by strongly interacting critical field theories below their upper critical dimension. The primary examples involve transitions with a topological order parameter associated with dislocations in collinear spin density wave ("stripe") correlations: the gapping of the order parameter fluctuations leads to a fractionalization of spin and charge collective modes, and this transition has been proposed as a candidate for the cuprates near optimal doping. The coupling between the order parameter and long-wavelength volume and shape deformations of the Fermi surface is analyzed by the renormalization group, and a runaway flow to a non-perturbative regime is found in most cases. A phenomenological scaling analysis of simple observable properties of possible second order quantum critical points is presented, with results quite similar to those near quantum spin glass transitions and to phenomenological forms proposed by Schroeder et al. (cond-mat/0011002).

High-resolution pixel detectors for second generation digital mammography

Abstract Hybrid CdZnTe, CdTe, GaAs, selenium and PbI 2 pixel detector arrays with 50×50 μm 2 pixel sizes that convert X-rays directly into charge signals are under development at NOVA for application to digital mammography. These detectors have superior X-ray quantum efficiency compared to either emulsion-based film, phosphor-based detectors or other low-Z, solid-state detectors such as silicon. During this work, CdZnTe and CdTe pixel detectors gave the best results. The other detectors are at very early stages of development and need significant improvement. Among other detectors, selenium is showing the highest potential. The preliminary results show that single crystal CdZnTe detectors yield better results in Detective Quantum Efficiency (DQE) as well as in images obtained from phantoms, compared to the polycrystalline CdZnTe detectors. This is due to the non-uniformities in the polycrystaline CdZnTe that degrade the charge transport properties. In this paper, preliminary results from thin (0.15 to 0.2 mm) CdZnTe and CdTe detectors will be presented in terms of MTF, DQE and phantom images. Because of the charge-coupling limitation of the readout Application Specific Integrated Circuit (ASIC) that was originally designed for Si detectors, the detector is biased to collect holes from the input. This charge collection mode limits the CdZnTe detector performance. Their DQE measurements yield 25% and 65% for the polycrystal and single-crystal CdZnTe detectors, respectively. Polycrystal CdTe test detectors were also hybridized to the same type charge readout chip. Since CdTe has much longer hole-propagation lengths compared to CdZnTe, it shows better performance in the hole-collecting mode. However, it suffers from polarization. Excellent images were also obtained from the CdTe detectors. Future work to redesign the readout ASIC and thus improve the detector performance will be discussed. These detectors can also be used for other medical radiography with increased thickness and also for industrial imaging such as non-destructive evaluation and non-destructive inspection.

Anomalous optical absorption in the normal state of overdoped cuprates near the charge-ordering instability.

We argue that the hump observed in the optical conductivity at or below a few hundreds of cm(-1), in overdoped cuprates such as the electron-doped Nd(2-x)Ce(x)CuO(4-y) at x > or approximately equal to 0.15 and the hole-doped Bi2Sr2CuO6 and La2-xSrxCuO4, cannot be accounted for within a single-fluid description. We propose instead an interpretation based on the direct excitation of charge collective modes, which become nearly critical in the proximity to a charge-ordering instability. Their critical character entails a peculiar temperature dependence and a pseudoscaling form of the optical spectra, which agree with the experimental data.

Phonon anomalies due to collective stripe modes in high T(c) cuprates.

Phonon anomalies observed in various high T(c) cuprates by neutron experiments are analyzed theoretically in terms of the stripe concept. The phonon self-energy correction is evaluated by taking into account the charge collective modes of stripes, giving rise to dispersion gap, or kink and shadow phonon modes at twice the wave number of spin stripe. These features coincide precisely with observations. The gapped branches of the phonon are found to be in-phase and out-of-phase oscillations relative to the charge collective mode.

Direct conversion CdZnTe and CdTe detectors for digital mammography

Hybrid CdZnTe and CdTe pixel detector arrays with 50 /spl times/ 50 micron/sup 2/ pixel sizes that convert X-rays directly into charge signals are under development at NOVA for applications to digital mammography. CdZnTe and CdTe have superior X-ray quantum efficiency compared to either emulsion-based film, phosphor-based detectors or other low-Z, solid-state detectors such as silicon. In this paper, latest results from thin (0.15 to 0.2 mm) CdZnTe and CdTe detectors will be presented in terms of modulation transfer function (MTF), detective quantum efficiency (DQE), and phantom images. Single-crystal CdZnTe detectors yield better results in DQE as well as phantom images, compared to the polycrystalline CdZnTe detectors. This is due to the nonuniformities in the polycrystal that degrade the charge transport properties. Because of the charge-coupling limitation of the readout ASIC that was originally designed for Si detectors, the detector is biased to collect holes from the front side. This charge collection mode limits the CdZnTe detector performance. Their DQE measurements yield 25 % and 65 % for the poly-crystal and single-crystal CdZnTe detectors respectively. Poly-crystal CdTe test detectors were also hybridized to the same type charge readout chip. Since CdTe has much longer hole-propagation lengths compared to CdZnTe, it shows better performance in the hole-collecting mode. However, a severe polarization effect degrades performance of the present device. Excellent images were also obtained from the CdTe detectors. Future work to redesign the readout ASIC and, thus, improve the detector performance is discussed. Application to industrial imaging such as nondestructive evaluation (NDE) and nondestructive inspection (NDI) is a natural extension.

Collective Modes of Incommensurate Spin Density Wave in One-Dimensional Hubbard Model

Dynamical spin and charge susceptibility and collective modes are studied in the framework of the random phase approximation. They are calculated from the wave function of the selfconsistently obtained ground state, including all possible higher harmonics components of the ordering vector Q . We investigate a spin wave excitation in the spin transverse mode, and phason excitation related to the domain-wall motion in the spin longitudinal and charge modes. We consider the dispersion curve of the collective mode, and discuss the dependence on the on-site U interaction and the period of the incommensurate structure.

Non-quasiparticle microwave absorption in Bi2Sr2CaCu2O8+δ

We show that a non-quasiparticle charge collective mode, in parallel and coincident with the d-wave pair conductivity, leads to a quantitative understanding of microwave surface impedance measurements on superconducting Br 2 Sr 2 CaCu 2 O 8+δ. The analysis suggests an inhomogeneous chrge ground state in Bi 2 Sr 2 CaCu 2 O 8+δ and other HTS.

Observation of spin and charge collective modes in one-dimensional metallic chains

The many-body theory of interacting electrons in solids establishes the existence of elementary excitations, named quasi-particles, which show a one-to-one correspondence with non-interacting electrons. But this so-called Fermi liquid approach breaks down spectacularly in one-dimensional metals1. In this situation, which is described by the Luttinger liquid formalism, the quasiparticles are replaced by distinct collective excitations involving spin and charge, called spinons and holons, respectively2. This approach predicts power-law behaviour for the various properties of one-dimensional metals which is experimentally testable using a wide variety of methods, such as transport measurements3,4 and optical conductivity measurements5. Photoemission, on the other hand, provides a means by which the spin and charge excitations can be observed directly. Previous photoemission studies of quasi-one-dimensional metals have essentially revealed only the absence of any discontinuity of the spectral function at the Fermi energy6, consistent with theoretical expectations. Recently, signatures of the existence of spin-charge separation have been inferred from line-shape analyses in a metal with different bands7 and in an insulator8. Here we present photoemission data from a genuine one-dimensional metal constructed on an insulating substrate. The spectra contain structures indicative of the excitation of spin and charge collective modes.

Response of an [formula omitted] source mounted in a liquid argon ionization cell and read out in full charge collection mode

The total charge response curve of an 241 Am source placed in a liquid argon ionization cell has been measured. In a pure liquid argon sample (O 2 concentration <0.1 ppm), it can be described by a semi-empirical formula which is derived from a model formerly elaborated by Thomas and Imel. The validity of this new formula was also tested in the case of oxygen-polluted liquid argon. The agreement with the data seems to indicate that the initial ion–electron pair recombination factor is not affected when the O 2 concentration is smaller than 7 ppm. Further use of our recombination factor parameterization has been made to analyze data of liquid argon purity monitors utilizing 241 Am sources with a sensitivity to O 2 concentrations of ∼0.1 ppm.

Incommensurate charge and spin fluctuations ind-wave superconductors

We show analytic results for the irreducible charge and spin susceptibilities, $\chi_0 (\omega, {\bf Q})$, where ${\bf Q}$ is the momentum transfer between the nodes in d-wave superconductors. Using the BCS theory and a circular Fermi surface, we find that the singular behavior of the irreducible charge susceptibility leads to the dynamic incommensurate charge collective modes. The peaks in the charge structure factor occur at a set of wave vectors which form an ellipse around ${\bf Q}_{\pi}=(\pi,\pi)$ and ${\bf Q}_0=(0,0)$ in momentum space with momentum dependent spectral weight. It is also found that, due to the non-singular irreducible spin susceptibility, an extremely strong interaction via random phase approximation is required to support the magnetic peaks near ${\bf Q}_{\pi}$. Under certain conditions, the peaks in the magnetic structure factor occur near ${\bf Q}=(\pi,\pi (1 \pm \delta))$ and $(\pi (1 \pm \delta),\pi)$.

Collective excitations in a 2D electron system: Canted field geometry

We investigate the charge and spin collective modes induced in a 2D electron gas by a weak electromagnetic perturbation in the presence of a dc magnetic field which makes an angle θ with the electron layer. The excitation frequencies are determined within the framework of the Landau–Silin theory of Fermi liquids by solving a semi-classical equation for transport in the self-consistent electromagnetic field associated with particle density fluctuations. The quasiparticle interaction is spin dependent and varies parametrically with the degree of spin polarization. In the long wavelength limit, we obtain analytic results for the frequencies of the collective modes as functions of θ.

Spin and charge separation in a mesoscopic ring

Abstract We propose a simple model, i.e., a magnetic impurity embedded in a mesoscopic ring, which can be used to detect the spin and charge separation in a one-dimensional interacting electron system by measuring the basic period of the persistent current produced by the Aharonov-Bohm flux threading the ring and the low temperature behavior of the impurity susceptibility. The former only depends upon a dimensionless coupling strength parameter g which characterizes the interaction among the charge collective excitation modes, while the latter is completely controlled by another parameter g , which characterizes the interaction among the spin collective excitation modes.

Field-dependent photoelectron extraction from CsI in different gases

We present the results of our latest investigations, made in the laboratory and in a RICH detector, of the dependence of the UV quantum efficiency (QE) of CsI on the gas and the electric field. We confirm that in charge collection mode the QE in a gas is generally lower than in vacuum due to electron backscattering; in charge multiplication mode the QE increases with the field and reaches the vacuum value at high gas gains. The lowest photoemission under low electric field at the photocathode surface was observed in He-based gas mixtures; in Ar-hydrocarbon mixtures it approaches that of pure hydrocarbons. In CH 4 and in CH 4 i-C 4H 10 the QE value is practically independent of the field and is close to that in vacuum. The implications of CsI-based UV-detectors, particularly to fast RICH devices, are discussed.

Self-consistent charge dynamics and collective modes in a dusty plasma.

Charges on dust grains immersed in a plasma may vary as a consequence of plasma currents falling onto their surface. Such charge variations might lead to many collective phenomena in the dusty plasma. We have written a set of multifluid equations that describe the charge dynamics of the dust particles self-consistently, i.e., as a consequence of loss in the electron and ion number densities. Various collective phenomena related with dust- and ion-acoustic modes are then studied in the context of these equations. Consistency of the results with previous work and aspects arising due to the present self-consistent charge dynamics are discussed.