The Diffraction Model and its Applicability for Wakefield Calculations

The operation of a Free Electron Laser (FEL) in the ultraviolet or in the X-ray regime requires the acceleration of electron bunches with an rms length of 25 to 50 micro meters. The wakefields generated by these sub picosecond bunches extend into the frequency range well beyond the threshold for Cooper pair breakup (about 750 GHz) in superconducting niobium at 2 K. It is shown, that the superconducting cavities can indeed be operated with 25 micro meter bunches without suffering a breakdown of superconductivity (quench), however at the price of a reduced quality factor and an increased heat transfer to the superfluid helium bath. This was first shown by wakefield calculations based on the diffraction model. In the meantime a more conventional method of computing wake fields in the time domain by numerical methods was developed and used for the wakefield calculations. Both methods lead to comparable results: the operation of TESLA with 25 micro meter bunches is possible but leads to an additional heat load due to the higher order modes (HOMs). Therefore HOM dampers for these high frequencies are under construction. These dampers are located in the beam pipes between the 9-cell cavities. So it is of interest, if there are trapped modes in the cavity due to closed photon orbits. In this paper we investigate the existence of trapped modes and the distribution of heat load over the surface of the TESLA cavity by numerical photon tracking.Comment: Linac2000 conference paper ID No. MOE0

Star-shaped Local Density of States around Vortices in a Type II Superconductor

The electronic structure of vortices in a type II superconductor is analyzed within the quasi-classical Eilenberger framework. The possible origin of a sixfold ``star'' shape of the local density of states, observed by scanning tunneling microscope experiments on NbSe$_2$, is examined in the light of the three effects; the anisotropic pairing, the vortex lattice, and the anisotropic density of states at the Fermi surface. Outstanding features of split parallel rays of this star are well explained in terms of an anisotropic $s$-wave pairing. This reveals a rich internal electronic structure associated with a vortex core.Comment: 4 pages, REVTeX, 3 figures available upon reques

Possible solution of the Coriolis attenuation problem

The most consistently useful simple model for the study of odd deformed nuclei, the particle-rotor model (strong coupling limit of the core-particle coupling model) has nevertheless been beset by a long-standing problem: It is necessary in many cases to introduce an ad hoc parameter that reduces the size of the Coriolis interaction coupling the collective and single-particle motions. Of the numerous suggestions put forward for the origin of this supplementary interaction, none of those actually tested by calculations has been accepted as the solution of the problem. In this paper we seek a solution of the difficulty within the framework of a general formalism that starts from the spherical shell model and is capable of treating an arbitrary linear combination of multipole and pairing forces. With the restriction of the interaction to the familiar sum of a quadrupole multipole force and a monopole pairing force, we have previously studied a semi-microscopic version of the formalism whose framework is nevertheless more comprehensive than any previously applied to the problem. We obtained solutions for low-lying bands of several strongly deformed odd rare earth nuclei and found good agreement with experiment, except for an exaggerated staggering of levels for K=1/2 bands, which can be understood as a manifestation of the Coriolis attenuation problem. We argue that within the formalism utilized, the only way to improve the physics is to add interactions to the model Hamiltonian. We verify that by adding a magnetic dipole interaction of essentially fixed strength, we can fit the K=1/2 bands without destroying the agreement with other bands. In addition we show that our solution also fits 163Er, a classic test case of Coriolis attenuation that we had not previously studied.Comment: revtex, including 7 figures(postscript), submitted to Phys.Rev.

Local density of states in the vortex lattice in a type II superconductor

Local density of states (LDOS) in the triangular vortex lattice is investigated based on the quasi-classical Eilenberger theory. We consider the case of an isotropic s-wave superconductor with the material parameter appropriate to NbSe_2. At a weak magnetic field, the spatial variation of the LDOS shows cylindrical structure around a vortex core. On the other hand, at a high field where the core regions substantially overlap each other, the LDOS is sixfold star-shaped structure due to the vortex lattice effect. The orientation of the star coincides with the experimental data of the scanning tunneling microscopy. That is, the ray of the star extends toward the nearest-neighbor (next nearest-neighbor) vortex direction at higher (lower) energy.Comment: 10 pages, RevTex, 32 figure

A Contour Integral Representation for the Dual Five-Point Function and a Symmetry of the Genus Four Surface in R6

The invention of the "dual resonance model" N-point functions BN motivated the development of current string theory. The simplest of these models, the four-point function B4, is the classical Euler Beta function. Many standard methods of complex analysis in a single variable have been applied to elucidate the properties of the Euler Beta function, leading, for example, to analytic continuation formulas such as the contour-integral representation obtained by Pochhammer in 1890. Here we explore the geometry underlying the dual five-point function B5, the simplest generalization of the Euler Beta function. Analyzing the B5 integrand leads to a polyhedral structure for the five-crosscap surface, embedded in RP5, that has 12 pentagonal faces and a symmetry group of order 120 in PGL(6). We find a Pochhammer-like representation for B5 that is a contour integral along a surface of genus five. The symmetric embedding of the five-crosscap surface in RP5 is doubly covered by a symmetric embedding of the surface of genus four in R6 that has a polyhedral structure with 24 pentagonal faces and a symmetry group of order 240 in O(6). The methods appear generalizable to all N, and the resulting structures seem to be related to associahedra in arbitrary dimensions.Comment: 43 pages and 44 figure

Echo of the Quantum Phase Transition of CeCu6−x_{6-x}Aux_x in XPS: Breakdown of Kondo Screening

We present an X-ray photoemission study of the heavy-fermion system CeCu$_{6-x}$Au$_x$ across the magnetic quantum phase transition of this compound at temperatures above the single-ion Kondo temperature $T_K$. In dependence of the Au concentration $x$ we observe a sudden change of the $f$-occupation number $n_f$ and the core-hole potential $U_{df}$ at the critical concentration $x_c=0.1$. We interpret these findings in the framework of the single-impurity Anderson model. Our results are in excellent agreement with findings from earlier UPS measurements %\cite{klein08qpt} and provide further information about the precursors of quantum criticality at elevated temperatures.Comment: 5 pages, 3 figures; published version, references updated, minor changes in wordin

High-temperature signatures of quantum criticality in heavy fermion systems

We propose a new criterion for distinguishing the Hertz-Millis (HM) and the local quantum critical (LQC) mechanism in heavy fermion systems with a magnetic quantum phase transition (QPT). The criterion is based on our finding that the spin screening of Kondo ions can be completely suppressed by the RKKY coupling to the surrounding magnetic ions even without magnetic ordering and that, consequently, the signature of this suppression can be observed in spectroscopic measurements above the magnetic ordering temperature. We apply the criterion to high-resolution photoemission (UPS) measurements on CeCu$_{6-x}$Au$_{x}$ and conclude that the QPT in this system is dominated by the LQC scenario.Comment: Inveted paper, International Conference on Magnetism, ICM 2009, Karlsruhe. Published version, added discussions of the relevance of Fermi-surface fluctuations and of a structural transition near the QC

25 kHz narrow spectral bandwidth of a wavelength tunable diode laser with a short waveguide-based external cavity

We report on the spectral properties of a diode laser with a tunable external cavity in integrated optics. Even though the external cavity is short compared to other small-bandwidth external cavity lasers, the spectral bandwidth of this tunable laser is as small as 25 kHz (FWHM), at a side-mode suppression ratio (SMSR) of 50 dB. Our laser is also able to access preset wavelengths in as little as 200 us and able to tune over the full telecom C-band (1530 nm - 1565 nm).Comment: 8 pages, 7 figure

Detection of OD towards the low-mass protostar IRAS16293-2422

Although water is an essential and widespread molecule in star-forming regions, its chemical formation pathways are still not very well constrained. Observing the level of deuterium fractionation of OH, a radical involved in the water chemical network, is a promising way to infer its chemical origin. We aim at understanding the formation mechanisms of water by investigating the origin of its deuterium fractionation. This can be achieved by observing the abundance of OD towards the low-mass protostar IRAS16293-2422, where the HDO distribution is already known. Using the GREAT receiver on board SOFIA, we observed the ground-state OD transition at 1391.5 GHz towards the low-mass protostar IRAS16293-2422. We also present the detection of the HDO 111-000 line using the APEX telescope. We compare the OD/HDO abundance ratio inferred from these observations with the predictions of chemical models. The OD line is detected in absorption towards the source continuum. This is the first detection of OD outside the solar system. The SOFIA observation, coupled to the observation of the HDO 111-000 line, provides an estimate of the abundance ratio OD/HDO ~ 17-90 in the gas where the absorption takes place. This value is fairly high compared with model predictions. This may be reconciled if reprocessing in the gas by means of the dissociative recombination of H2DO+ further fractionates OH with respect to water. The present observation demonstrates the capability of the SOFIA/GREAT instrument to detect the ground transition of OD towards star-forming regions in a frequency range that was not accessible before. Dissociative recombination of H2DO+ may play an important role in setting a high OD abundance. Measuring the branching ratios of this reaction in the laboratory will be of great value for chemical models.Comment: 6 pages, 6 figures, 3 tables, accepted for publication in A&A SOFIA/GREAT special issu