376 research outputs found
The effect of Fermi surface curvature on low-energy properties of fermions with singular interactions
We discuss the effect of Fermi surface curvature on long-distance/time
asymptotic behaviors of two-dimensional fermions interacting via a gapless mode
described by an effective gauge field-like propagator. By comparing the
predictions based on the idea of multi-dimensional bosonization with those of
the strong- coupling Eliashberg approach, we demonstrate that an agreement
between the two requires a further extension of the former technique.Comment: Latex, 4+ pages. Phys. Rev. Lett., to appea
A radiological assessment of nuclear power and propulsion operations near Space Station Freedom
Scenarios were identified which involve the use of nuclear power systems in the vicinity of Space Station Freedom (SSF) and their radiological impact on the SSF crew was quantified. Several of the developed scenarios relate to the use of SSF as an evolutionary transportation node for lunar and Mars missions. In particular, radiation doses delivered to SSF crew were calculated for both the launch and subsequent return of a Nuclear Electric Propulsion (NEP) cargo vehicle and a Nuclear Thermal Rocket (NTR) personnel vehicle to low earth orbit. The use of nuclear power on co-orbiting platforms and the storage and handling issues associated with radioisotope power systems were also explored as they relate to SSF. A central philosophy in these analyses was the utilization of a radiation dose budget, defined as the difference between recommended dose limits from all radiation sources and estimated doses received by crew members from natural space radiations. Consequently, for each scenario examined, the dose budget concept was used to identify and quantify constraints on operational parameters such as launch separation distances, returned vehicle parking distances, and reactor shutdown times prior to vehicle approach. The results indicate that realistic scenarios do not exist which would preclude the use of nuclear power sources in the vicinity of SSF. The radiation dose to the SSF crew can be maintained at safe levels solely by implementing proper and reasonable operating procedures
Dynamic scaling in the vicinity of the Luttinger liquid fixed point
We calculate the single-particle spectral function A (k, omega) of a
one-dimensional Luttinger liquid by means of a functional renormalization group
(RG) approach. Given an infrared energy cutoff Lambda = Lambda_0 e^{- l}, our
approach yields the spectral function in the scaling form, A_{\Lambda} (k_F +
p, omega) = tau Z_l tilde{A}_l (p xi, omega tau), where k_F is the Fermi
momentum, Z_l is the wave-function renormalization factor, tau = 1 / \Lambda is
the time scale and xi = v_F / \Lambda is the length scale associated with
Lambda. At the Luttinger liquid fixed point (l rightarrow infty) our RG result
for A (k, omega) exhibits the correct anomalous scaling properties, and for k =
\pm k_F agrees exactly with the well-known bosonization result at weak
coupling. Our calculation demonstrates that the field rescaling is essential
for obtaining the crossover from Fermi liquid behavior to Luttinger liquid
behavior from a truncation of the hierarchy of exact RG flow equations as the
infrared cutoff is reduced.Comment: 15 pages, 5 figure
Dynamic structure factor of Luttinger liquids with quadratic energy dispersion and long-range interactions
We calculate the dynamic structure factor S (omega, q) of spinless fermions
in one dimension with quadratic energy dispersion k^2/2m and long range
density-density interaction whose Fourier transform f_q is dominated by small
momentum-transfers q << q_0 << k_F. Here q_0 is a momentum-transfer cutoff and
k_F is the Fermi momentum. Using functional bosonization and the known
properties of symmetrized closed fermion loops, we obtain an expansion of the
inverse irreducible polarization to second order in the small parameter q_0 /
k_F. In contrast to perturbation theory based on conventional bosonization, our
functional bosonization approach is not plagued by mass-shell singularities.
For interactions which can be expanded as f_q = f_0 + f_0^{2} q^2/2 + O (q^4)
with finite f_0^{2} we show that the momentum scale q_c = 1/ | m f_0^{2} |
separates two regimes characterized by a different q-dependence of the width
gamma_q of the collective zero sound mode and other features of S (omega, q).
For q_c << q << k_F we find that the line-shape in this regime is
non-Lorentzian with an overall width gamma_q of order q^3/(m q_c) and a
threshold singularity at the lower edge.Comment: 33 Revtex pages, 17 figure
Probabilistic seismic hazard map for Bulgaria as a basis for a new building code
A seismic hazard map proposed as part of a new building code for Bulgaria is presented here on basis of the recommendations in EUROCODE 8. <P> Seismic source zones within an area of about 200 km around Bulgaria were constructed considering seismicity, neotectonic and geological development. The most time consuming work was to establish a homogeneous earthquake catalogue out of different catalogues. <P> The probabilistic seismic hazard assessment in terms of intensities is performed following Cornell (1968) with the program EQRISK (see McGuire, 1976), modified by us for use of intensities. To cope with the irregular isoseismals of the Vrancea intermediate depth earthquakes a special attenuation factor is introduced (Ardeleanu et al., 2005), using detailed macroseismic maps of three major earthquakes. <P> The final seismic hazard is the combination of both contributions, of zones with crustal earthquakes and of the Vrancea intermediate depth earthquakes zone. Calculations are done for recurrence periods of 95, 475 and 10 000 years
Functional renormalization group in the broken symmetry phase: momentum dependence and two-parameter scaling of the self-energy
We include spontaneous symmetry breaking into the functional renormalization
group (RG) equations for the irreducible vertices of Ginzburg-Landau theories
by augmenting these equations by a flow equation for the order parameter, which
is determined from the requirement that at each RG step the vertex with one
external leg vanishes identically. Using this strategy, we propose a simple
truncation of the coupled RG flow equations for the vertices in the broken
symmetry phase of the Ising universality class in D dimensions. Our truncation
yields the full momentum dependence of the self-energy Sigma (k) and
interpolates between lowest order perturbation theory at large momenta k and
the critical scaling regime for small k. Close to the critical point, our
method yields the self-energy in the scaling form Sigma (k) = k_c^2 sigma^{-}
(k | xi, k / k_c), where xi is the order parameter correlation length, k_c is
the Ginzburg scale, and sigma^{-} (x, y) is a dimensionless two-parameter
scaling function for the broken symmetry phase which we explicitly calculate
within our truncation.Comment: 9 pages, 4 figures, puplished versio
The Randomized Shortened Dental Arch Study: Tooth Loss
The evidence concerning the management of shortened dental arch (SDA) cases is sparse. This multi-center study was aimed at generating data on outcomes and survival rates for two common treatments, removable dental prostheses (RDP) for molar replacement or no replacement (SDA). The hypothesis was that the treatments lead to different incidences of tooth loss. We included 215 patients with complete molar loss in one jaw. Molars were either replaced by RDP or not replaced, according to the SDA concept. First tooth loss after treatment was the primary outcome measure. This event occurred in 13 patients in the RDP group and nine patients in the SDA group. The respective Kaplan-Meier survival rates at 38 months were 0.83 (95% CI: 0.74-0.91) in the RDP group and 0.86 (95% CI: 0.78-0.95) in the SDA group, the difference being non-significant
Ferromagnetic Luttinger Liquids
We study weak itinerant ferromagnetism in one-dimensional Fermi systems using
perturbation theory and bosonization. We find that longitudinal spin
fluctuations propagate ballistically with velocity v_m << v_F, where v_F is the
Fermi velocity. This leads to a large anomalous dimension in the spin-channel
and strong algebraic singularities in the single-particle spectral function and
in the transverse structure factor for momentum transfers q ~ 2 Delta/v_F,
where 2 Delta is the exchange splitting.Comment: 4 pages, 3 figure
Collective fields in the functional renormalization group for fermions, Ward identities, and the exact solution of the Tomonaga-Luttinger model
We develop a new formulation of the functional renormalization group (RG) for
interacting fermions. Our approach unifies the purely fermionic formulation
based on the Grassmannian functional integral, which has been used in recent
years by many authors, with the traditional Wilsonian RG approach to quantum
systems pioneered by Hertz [Phys. Rev. B 14, 1165 (1976)], which attempts to
describe the infrared behavior of the system in terms of an effective bosonic
theory associated with the soft modes of the underlying fermionic problem. In
our approach, we decouple the interaction by means of a suitable
Hubbard-Stratonovich transformation (following the Hertz-approach), but do not
eliminate the fermions; instead, we derive an exact hierarchy of RG flow
equations for the irreducible vertices of the resulting coupled field theory
involving both fermionic and bosonic fields. The freedom of choosing a momentum
transfer cutoff for the bosonic soft modes in addition to the usual band cutoff
for the fermions opens the possibility of new RG schemes. In particular, we
show how the exact solution of the Tomonaga-Luttinger model emerges from the
functional RG if one works with a momentum transfer cutoff. Then the Ward
identities associated with the local particle conservation at each Fermi point
are valid at every stage of the RG flow and provide a solution of an infinite
hierarchy of flow equations for the irreducible vertices. The RG flow equation
for the irreducible single-particle self-energy can then be closed and can be
reduced to a linear integro-differential equation, the solution of which yields
the result familiar from bosonization. We suggest new truncation schemes of the
exact hierarchy of flow equations, which might be useful even outside the weak
coupling regime.Comment: 27 pages, 15 figures; published version, some typos correcte
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