Towards analytic description of a transition from weak to strong coupling regime in correlated electron systems. I. Systematic diagrammatic theory with two-particle Green functions

We analyze behavior of correlated electrons described by Hubbard-like models at intermediate and strong coupling. We show that with increasing interaction a pole in a generic two-particle Green function is approached. The pole signals metal-insulator transition at half filling and gives rise to a new vanishing ``Kondo'' scale causing breakdown of weak-coupling perturbation theory. To describe the critical behavior at the metal-insulator transition a novel, self-consistent diagrammatic technique with two-particle Green functions is developed. The theory is based on the linked-cluster expansion for the thermodynamic potential with electron-electron interaction as propagator. Parquet diagrams with a generating functional are derived. Numerical instabilities due to the metal-insulator transition are demonstrated on simplifications of the parquet algebra with ring and ladder series only. A stable numerical solution in the critical region is reached by factorization of singular terms via a low-frequency expansion in the vertex function. We stress the necessity for dynamical vertex renormalizations, missing in the simple approximations, in order to describe the critical, strong-coupling behavior correctly. We propose a simplification of the full parquet approximation by keeping only most divergent terms in the asymptotic strong-coupling region. A qualitatively new, feasible approximation suitable for the description of a transition from weak to strong coupling is obtained.Comment: 17 pages, 4 figures, REVTe

Bone intake by vultures in Namibia

The use of bones by vultures was assessed during early 2005 in the Otjiwarongo area in north-central Namibia. Bone fragments were utilized by all species, especially by the African White-backed Vulture Gyps africanus and the Lappet-faced Vulture Torgos tracheliotos. There was an overall increase in bone fragment consumption from May onwards (taken as the beginning of the breeding period). A rough estimate of bone fragment use for all vultures of 2.49 g/vulture (consumption/total number of vultures observed) and 60.31 g/vulture (consumption/individuals of vultures observed) was determined. The results suggest that bone fragments should be added as a supplement at vulture restaurants.Vulture News Vol. 57 2007: pp. 17-2

Microwave growth from the beam breakup instability in long‐pulse electron beam experiments

The beam breakup (BBU) instability has been investigated in high‐current, long‐pulse electron beams propagating through microwave cavities. Experiments are performed using a relativistic electron‐beam generator with diode parameters: 0.7–0.8 MV, 1–15 kA, and 0.5–1.5 μs. The magnitude of the solenoidal magnetic field places these experiments in an intermediate regime between strong focusing and weak focusing. The electron‐beam transport system consists of ten identical pillbox cavities each containing a small microwave loop antenna designed to detect the TM110 beam breakup mode. The TM110 microwave mode is primed in the first cavity by a magnetron tuned to the resonance frequency of 2.5 GHz. The BBU instability growth is measured through the amplification of the 2.5 GHz microwaves between the second and tenth cavities. Strong growth (25–38 dB) of the TM110 microwave signal is observed when the initial cavity is primed exactly on resonance, with a rapid decrease of the growth rate off‐resonance. The magnitude of microwave growth is consistent with the predictions of BBU theory.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69350/2/APPLAB-61-6-642-1.pd

The beam breakup instability in quadrupole and solenoidal electron‐beam transport systems

Dispersion relations are derived to determine the growth rate, dominant wavelength, and group velocity of disturbances caused by the beam breakup instability. Considerations include weak and strong focusing, x‐y coupling in solenoidal transport, the spacing of accelerator cavities, and periodically pulsed beams. Beam breakup growth is minimum when the cavity spacing equals an integral number of half‐betatron wavelengths for quadrupole focusing, and an integral number of betatron wavelengths for solenoidal focusing. Minimum growth is also found for periodic pulses separated by an integral number of half‐periods of the TM110 cavity mode. Expressions for beam breakup growth at the minima are obtained.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71286/2/JAPIAU-71-7-3091-1.pd

Beam breakup growth and reduction experiments in long‐pulse electron beam transport

The results of an experimental program whose sole objective is to investigate the cumulative beam breakup instability (BBU) in electron beam accelerators are presented. The BBU growth rate scalings are examined with regard to beam current, focusing field, cavity Q, and propagation distance. A microwave cavity array was designed and fabricated to excite and measure the cumulative BBU resulting from beam interactions with the deflecting TM110 cavity mode. One phase of this experiment used high Q(≊1000) cavities with relatively large frequency spread (Δf/f0≊0.1%). The observed TM110 mode microwave growth between an upstream (second) and a downstream (tenth) cavity indicated BBU growth of 26 dB for an electron beam of kinetic energy of 750 keV, 45 A, and focused by a 1.1 kG solenoidal field. At beam currents of less than 100 A the experiments agreed well with a two‐dimensional continuum theory; the agreement was worse at higher beam currents (≳100 A) due to beam loading. The second‐phase experiments used lower Q(≊200) cavities with relatively low frequency spread (Δf/f0≊0.03%). Theory and experiment agreed well for beam currents up to 220 A. Distance scaling experiments were also performed by doubling the propagation length. Instability growth reduction experiments using the technique of external cavity coupling resulted in a factor of four decrease in energy in BBU growth when seven internal beam cavities were coupled by microwave cable to seven identical external dummy cavities. A theory invoking power sharing between the internal beam cavities and the external dummy cavities was used to explain the experimental reduction with excellent agreement using an equivalent circuit model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71080/2/JAPIAU-75-3-1258-1.pd

An exploration of impaired walking dynamics and fatigue in Multiple Sclerosis

BACKGROUND: Physical disability in multiple sclerosis (MS) is frequently characterized by impaired ambulation. Although walking tests have been successfully employed to assess walking ability in MS patients, data analytic procedures have predominantly relied on result-oriented parameters (e.g. total distance covered during a given amount of time), whereas process-oriented, dynamic walking patterns have mostly been ignored. This is striking, since healthy individuals have been observed to display a stereotypical U-shaped pattern of walking speed during timed walking, characterized by relatively high speed during the initial phase, subsequent slowing and final acceleration. Objective of the current study was to test the utility of the 6 min Walk (6MW) and the 12 min Walk (12MW) for revealing putatively abnormal temporal dynamic features of walking in MS. METHODS: A group of 37 MS patients was divided into subgroups with regard to their level of disability analyzed with the Expanded Disability Status Scale (EDSS; Mild MS Group, n = 20, EDSS 0 – 3.5; Moderate MS Group, n = 17, EDSS 4 – 5). Subsequently, both groups were compared to age-matched healthy controls (n = 25) on both tests with regard to result-oriented characteristics (mean walking speed), as well as dynamic features (mean decline in walking speed, degree of observed U-shape). RESULTS: Both MS groups showed a significantly lower mean walking speed than healthy controls, independent of test duration. Compared to controls, the Moderate MS Group also slowed down more rapidly throughout both tests. The same pronounced decline in walking speed was observed for the Mild MS Group in case of the 12MW. Additionally, for both MS groups an attenuated U-shaped velocity pattern was observed relative to controls in the 6MW. Patients' subjective fatigue scores were more strongly correlated with the decline in walking speed than with the common parameter of mean walking speed in the 6MW. CONCLUSIONS: MS patients display abnormal dynamics in their walking patterns. A pronounced linear decline in walking speed can be identified with the 12MW even in MS patients with seemingly mild disability. Similarly, the 6MW can be used to assess an abnormal walking profile. Particularly the linear decline in walking speed on this test shows a more robust association with subjective fatigue than mean walking speed. Dynamic walking parameters may hence represent valuable clinical features, serving as surrogate measures of motor fatigue. Future studies are needed to verify their prognostic value

Absence of Antisperm Antibodies in Anejaculatory Men

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96435/1/j.1939-4640.1990.tb00162.x.pd