21,748 research outputs found
The Grothendieck Group of a Quantum Projective Space Bundle
We compute the Grothendieck group K_0 of non-commutative analogues of quantum
projective space bundles. Our results specialize to give the Grothendieck
groups of non-commutative analogues of projective spaces, and specialize to
recover the Grothendieck group of a usual projective space bundle over a
regular noetherian separated scheme. As an application we develop an
intersection theory for the quantum ruled surfaces defined by Van den Bergh.Comment: This paper is being replaced so I can correct the metadata, the
title! I (Paul) spelled Grothendieck's name incorrectly. The paper is being
reposted with the journal reference and doi added to the metadat
Transverse self-modulation of ultra-relativistic lepton beams in the plasma wakefield accelerator
The transverse self-modulation of ultra-relativistic, long lepton bunches in
high-density plasmas is explored through full-scale particle-in-cell
simulations. We demonstrate that long SLAC-type electron and positron bunches
can become strongly self-modulated over centimeter distances, leading to wake
excitation in the blowout regime with accelerating fields in excess of 20 GV/m.
We show that particles energy variations exceeding 10 GeV can occur in
meter-long plasmas. We find that the self-modulation of positively and
negatively charged bunches differ when the blowout is reached. Seeding the
self-modulation instability suppresses the competing hosing instability. This
work reveals that a proof-of-principle experiment to test the physics of bunch
self-modulation can be performed with available lepton bunches and with
existing experimental apparatus and diagnostics.Comment: 8 pages, 8 figures, accepted for publication in Physics of Plasma
Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors
We calculate the low-frequency optical conductivity sigma(w) of clean metals
and superconductors at zero temperature neglecting the effects of impurities
and phonons. In general, the frequency and temperature dependences of sigma
have very little in common. For small Fermi surfaces in three dimensions (but
not in 2D) we find for example that Re sigma(w>0)=const. for low w which
corresponds to a scattering rate Gamma proportional to w^2 even in the absence
of Umklapp scattering when there is no T^2 contribution to Gamma. In the main
part of the paper we discuss in detail the optical conductivity of d-wave
superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest
frequencies and the Umklapp processes typically set in smoothly above a finite
threshold w_0 smaller than twice the maximal gap Delta. In cases where the
nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among
them is possible, one obtains Re sigma(w) \propto w^2.Comment: 7 pages, 3 figure
Anomalous infrared spectra of hybridized phonons in type-I clathrate BaGaGe
The optical conductivity spectra of the rattling phonons in the clathrate
BaGaGe are investigated in detail by use of the terahertz
time-domain spectroscopy. The experiment has revealed that the lowest-lying
vibrational mode of a Ba(2) ion consists of a sharp Lorentzian peak at
1.2 THz superimposed on a broad tail weighted in the lower frequency regime
around 1.0 THz. With decreasing temperature, an unexpected linewidth broadening
of the phonon peak is observed, together with monotonic softening of the phonon
peak and the enhancement of the tail structure. These observed anomalies are
discussed in terms of impurity scattering effects on the hybridized phonon
system of rattling and acoustic phonons.Comment: Submitted to JPS
Quantum renormalization group of XYZ model in a transverse magnetic field
We have studied the zero temperature phase diagram of XYZ model in the
presence of transverse magnetic field. We show that small anisotropy (0 =<
Delta <1) is not relevant to change the universality class. The phase diagram
consists of two antiferromagnetic ordering and a paramagnetic phases. We have
obtained the critical exponents, fixed points and running of coupling constants
by implementing the standard quantum renormalization group. The continuous
phase transition from antiferromagnetic (spin-flop) phase to a paramagnetic one
is in the universality class of Ising model in transverse field. Numerical
exact diagonalization has been done to justify our results. We have also
addressed on the application of our findings to the recent experiments on
Cs_2CoCl_4.Comment: 5 pages, 5 figures, new references added to the present versio
Measurement of Magnetic-Field Structures in a Laser-Wakefield Accelerator
Experimental measurements of magnetic fields generated in the cavity of a
self-injecting laser-wakefield accelerator are presented. Faraday rotation is
used to determine the existence of multi-megagauss fields, constrained to a
transverse dimension comparable to the plasma wavelength and several plasma
wavelengths longitudinally. The fields are generated rapidly and move with the
driving laser. In our experiment, the appearance of the magnetic fields is
correlated to the production of relativistic electrons, indicating that they
are inherently tied to the growth and wavebreaking of the nonlinear plasma
wave. This evolution is confirmed by numerical simulations, showing that these
measurements provide insight into the wakefield evolution with high spatial and
temporal resolution
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