2,549 research outputs found
Electronic Orders Induced by Kondo Effect in Non-Kramers f-Electron Systems
This paper clarifies the microscopic nature of the staggered scalar order,
which is specific to even number of f electrons per site. In such systems,
crystalline electric field (CEF) can make a singlet ground state. As exchange
interaction with conduction electrons increases, the CEF singlet at each site
gives way to Kondo singlets. The collective Kondo singlets are identified with
itinerant states that form energy bands. Near the boundary of itinerant and
localized states, a new type of electronic order appears with staggered Kondo
and CEF singlets. We present a phenomenological three-state model that
qualitatively reproduces the characteristic phase diagram, which have been
obtained numerically with use of the continuous-time quantum Monte Carlo
combined with the dynamical mean-field theory. The scalar order observed in
PrFe_4P_{12} is ascribed to this staggered order accompanying charge density
wave (CDW) of conduction electrons. Accurate photoemission and tunneling
spectroscopy should be able to probe sharp peaks below and above the Fermi
level in the ordered phase.Comment: 7 pages, 8 figure
Electron And Positron Scattering From 1,1- C2 H2 F2
1,1-difluoroethylene (1,1- C2 H2 F2) molecules have been studied for the first time experimentally and theoretically by electron and positron impact. 0.4-1000 eV electron and 0.2-1000 eV positron impact total cross sections (TCSs) were measured using a retarding potential time-of-flight apparatus. In order to probe the resonances observed in the electron TCSs, a crossed-beam method was used to investigate vibrational excitation cross sections over the energy range of 1.3-49 eV and scattering angles 90Ā° and 120Ā° for the two loss energies 0.115 and 0.381 eV corresponding to the dominant C-H (2 and 9) stretching and the combined C-F (3) stretching and C H2 (11) rocking vibrations, respectively. Electron impact elastic integral cross sections are also reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range from 0.5 to 50 eV in the static-exchange approximation and from 0.5 to 20 eV in the static-exchange plus polarization approximation. Resonance peaks observed centered at about 2.3, 6.5, and 16 eV in the TCSs have been shown to be mainly due to the vibrational and elastic channels, and assigned to the B2, B1, and A1 symmetries, respectively. The Ļ* resonance peak at 1.8 eV in C2 H4 is observed shifted to 2.3 eV in 1,1- C2 H2 F2 and to 2.5 eV in C2 F4; a phenomenon attributed to the decreasing CC bond length from C2 H4 to C2 F4. For positron impact a conspicuous peak is observed below the positronium formation threshold at about 1 eV, and other less pronounced ones centered at about 5 and 20 eV. 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Acceleration in perpendicular relativistic shocks for plasmas consisting of leptons and hadrons
We investigate the acceleration of light particles in perpendicular shocks
for plasmas consisting of a mixture of leptonic and hadronic particles.
Starting from the full set of conservation equations for the mixed plasma
constituents, we generalize the magneto-hydrodynamical jump conditions for a
multi-component plasma, including information about the specific adiabatic
constants for the different species. The impact of deviations from the standard
model of an ideal gas is compared in theory and particle-in-cell simulations,
showing that the standard-MHD model is a good approximation. The simulations of
shocks in electron-positron-ion plasmas are for the first time
multi-dimensional, transverse effects are small in this configuration and 1D
simulations are a good representation if the initial magnetization is chosen
high. 1D runs with a mass ratio of 1836 are performed, which identify the
Larmor frequency \omega_{ci} as the dominant frequency that determines the
shock physics in mixed component plasmas. The maximum energy in the non-thermal
tail of the particle spectra evolves in time according to a power-law
proportional to t^\alpha with \alpha in the range 1/3 < \alpha < 1, depending
on the initial parameters. A connection is made with transport theoretical
models by Drury (1983) and Gargate & Spitkovsky (2011), which predict an
acceleration time proportional to \gamma and the theory for small wavelength
scattering by Kirk & Reville (2010), which predicts a behavior rather as
proportional to \gamma^2. Furthermore, we compare different magnetic field
orientations with B_0 inside and out of the plane, observing qualitatively
different particle spectra than in pure electron-ion shocks
Anisotropic heating and magnetic field generation due to Raman scattering in laser-plasma interactions
We identify a mechanism for magnetic field generation in the interaction of intense electromagnetic waves and
underdense plasmas. We show that Raman scattered plasma waves trap and heat the electrons preferentially in
their propagation direction, resulting in a temperature anisotropy. In the trail of laser pulse, we observe magnetic
field growth that matches the Weibel mechanism due to the temperature anisotropy. We discuss the role of the
initial electron temperature in our results. The predictions are confirmed with multidimensional particle-in-cell
simulations. We show how this configuration is an experimental platform to study the long-time evolution of the
Weibel instabilityinfo:eu-repo/semantics/publishedVersio
Electron and positron scattering from 1,1-CāHāFā
1,1-difluoroethylene (1,1-CāHāFā) molecules have been studied for the first time experimentally and theoretically by electron and positron impact. 0.4-1000 eV electron and 0.2-1000 eV positron impact total cross sections (TCSs) were measured using a retarding potential time-of-flight apparatus. In order to probe the resonances observed in the electron TCSs, a crossed-beam method was used to investigate vibrational excitation cross sections over the energy range of 1.3-49 eV and scattering angles 90 degrees and 120 degrees for the two loss energies 0.115 and 0.381 eV corresponding to the dominant C-H (Ī½ā and Ī½ā) stretching and the combined C-F (Ī½ā) stretching and CHā (Ī½āā) rocking vibrations, respectively. Electron impact elastic integral cross sections are also reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range from 0.5 to 50 eV in the static-exchange approximation and from 0.5 to 20 eV in the static-exchange plus polarization approximation. Resonance peaks observed centered at about 2.3, 6.5, and 16 eV in the TCSs have been shown to be mainly due to the vibrational and elastic channels, and assigned to the Bā, Bā, and Aā symmetries, respectively. The pi* resonance peak at 1.8 eV in CāHā is observed shifted to 2.3 eV in 1,1-CāHāFā and to 2.5 eV in CāFā; a phenomenon attributed to the decreasing C=C bond length from CāHā to CāFā. For positron impact a conspicuous peak is observed below the positronium formation threshold at about 1 eV, and other less pronounced ones centered at about 5 and 20 eV.The work was supported in part by a Grant-in-Aid, the
Ministry of Education, Science, Technology, Sport and Culture,
Japan, the Japan Society for the Promotion of Science
JSPS, and the Japan Atomic Energy Research Institute
JAERI. One of the authors C.M. is also grateful to the
JSPS for financial support under Grant No. P04064. Another
author H.T. acknowledges Dr. T. Ozeki of the JAERI for
his encouragement and support during this work. This work
was also done under the International Atomic Energy Agency
IAEA project for three of the authors C.M., M.H., and
H.T.. Two of the authors M.H.F.B. and M.A.P.L. acknowledge
support from the Brazilian agency Conselho Nacional
de Desenvolvimento CientĆfico e TecnolĆ³gico CNPq.
MHFB also acknowledges support from the ParanĆ” state
agency FundaĆ§Ć£o AraucĆ”ria and from FINEP ( under Project
No. CT-Infra 1)
Electronic Order with Staggered Kondo and Crystalline Electric Field Singlets
Novel electronic order is found theoretically for a system where even number
of localized electrons per site are coupled with conduction electrons. For
precise quantitative study, a variant of the Kondo lattice model is taken with
crystalline electric field (CEF) singlet and triplet states for each site.
Using the dynamical mean-field theory combined with the continuous-time quantum
Monte Carlo method, a staggered order with alternating Kondo and CEF singlets
is identified for a case with one conduction electron per site being
distributed in two conduction bands each of which is quarter-filled. This
electronic order accompanies a charge density wave (CDW) of conduction
electrons that accumulate more on Kondo-singlet sites than on CEF-singlet
sites. Possible relevance of the present order to the scalar order in
PrFeP is discussed.Comment: 11 pages, 17 figure
Nonthermal Electron Acceleration at Collisionless Quasi-perpendicular Shocks
Shock waves propagating in collisionless heliospheric and astrophysical
plasmas have been studied extensively over the decades. One prime motivation is
to understand the nonthermal particle acceleration at shocks. Although the
theory of diffusive shock acceleration (DSA) has long been the standard for
cosmic-ray acceleration at shocks, plasma physical understanding of particle
acceleration remains elusive. In this review, we discuss nonthermal electron
acceleration mechanisms at quasi-perpendicular shocks, for which substantial
progress has been made in recent years. The discussion presented in this review
is restricted to the following three specific topics. The first is stochastic
shock drift acceleration (SSDA), which is a relatively new mechanism for
electron injection into DSA. The basic mechanism, related in-situ observations
and kinetic simulations results, and how it is connected with DSA will be
discussed. Second, we discuss shock surfing acceleration (SSA) at very high
Mach number shocks relevant to young supernova remnants (SNRs). While the
original proposal under the one-dimensional assumption is unrealistic, SSA has
now been proven efficient by a fully three-dimensional kinetic simulation.
Finally, we discuss the current understanding of the magnetized
Weibel-dominated shock. Spontaneous magnetic reconnection of self-generated
current sheets within the shock structure is an interesting consequence of
Weibel-generated strong magnetic turbulence. We argue that high Mach number
shocks with both Alfven and sound Mach numbers exceeding 20-40 will likely
behave as a Weibel-dominated shock. Despite a number of interesting recent
findings, the relative roles of SSDA, SSA, and magnetic reconnection for
electron acceleration at collisionless shocks and how the dominant particle
acceleration mechanisms change depending on shock parameters remain to be
answered.Comment: To appear in Reviews of Modern Plasma Physics as an invited revie
Substitution effects in elastic electron collisions with CHāX (X=F, Cl, Br, I) molecules
We report absolute elastic differential, integral, and momentum transfer cross sections for electron interactions with the series of molecules CHāX (X=F, Cl, Br, I). The incident electron energy range is 50ā200 eV, while the scattered electron angular range for the differential measurements is 15Ā°ā150Ā°. In all cases the absolute scale of the differential cross sections was set using the relative flow method with helium as the reference species. Substitution effects on these cross sections, as we progress along the halomethane series CHāF, CHāCl, CH3Br, and CHāI, are investigated as a part of this study. In addition, atomic-like behavior in these scattering systems is also considered by comparing these halomethane elastic cross sections to results from other workers for the corresponding noble gases Ne, Ar, Kr, and Xe, respectively. Finally we report results for calculations of elastic differential and integral cross sections for electrons scattering from each of the CHāX species, within an optical potential method and assuming a screened corrected independent atom representation. The level of agreement between these calculations and our measurements was found to be quite remarkable in each case.This work was conducted under the support of the Japanese
Ministry of Education, Sport, Culture and Technology
and also by the Ministerio de EducaciĆ³n Ciencia e InnovaciĆ³n
Plan Nacional de Fisica, Project No. FIS2006-
00702, the Consejo de Seguridad Nuclear and the European
Science Foundation COST Action No. CM0601. Additional
support from the Australian Research Council, through its
Centres of Excellence Program, and the Korea Science and
Engineering Foundation Grant No. 2009-0052415 is further
noted. One of us H.K. also acknowledges the Japan Society
for the Promotion of Science JSPS for his fellowships as
grants-in-aid for scientific research and, most recently, to facilitate
his visit to Flinders University and the ANU
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