3,979 research outputs found
Non-Perturbative U(1) Gauge Theory at Finite Temperature
For compact U(1) lattice gauge theory (LGT) we have performed a finite size
scaling analysis on lattices for fixed by
extrapolating spatial volumes of size to . Within the
numerical accuracy of the thus obtained fits we find for , 5 and~6
second order critical exponents, which exhibit no obvious
dependence. The exponents are consistent with 3d Gaussian values, but not with
either first order transitions or the universality class of the 3d XY model. As
the 3d Gaussian fixed point is known to be unstable, the scenario of a yet
unidentified non-trivial fixed point close to the 3d Gaussian emerges as one of
the possible explanations.Comment: Extended version after referee reports. 6 pages, 6 figure
Wither the sliding Luttinger liquid phase in the planar pyrochlore
Using series expansion based on the flow equation method we study the zero
temperature properties of the spin-1/2 planar pyrochlore antiferromagnet in the
limit of strong diagonal coupling. Starting from the limit of decoupled crossed
dimers we analyze the evolution of the ground state energy and the elementary
triplet excitations in terms of two coupling constants describing the inter
dimer exchange. In the limit of weakly coupled spin-1/2 chains we find that the
fully frustrated inter chain coupling is critical, forcing a dimer phase which
adiabatically connects to the state of isolated dimers. This result is
consistent with findings by O. Starykh, A. Furusaki and L. Balents (Phys. Rev.
B 72, 094416 (2005)) which is inconsistent with a two-dimensional sliding
Luttinger liquid phase at finite inter chain coupling.Comment: 6 pages, 4 Postscript figures, 1 tabl
On the stability problem in the O(N) nonlinear sigma model
The stability problem for the O(N) nonlinear sigma model in the 2+\epsilon
dimensions is considered. We present the results of the 1/N^{2} order
calculations of the critical exponents (in the 2<d<4 dimensions) of the
composite operators relevant for this problem. The arguments in the favor of
the scenario with the conventional fixed point are given.Comment: 9 pages, revtex, 1 Postscript figur
A Gravitational Redshift Determination of the Mean Mass of White Dwarfs. DBA and DB Stars
We measure apparent velocities (v_app) of absorption lines for 36 white
dwarfs (WDs) with helium-dominated atmospheres -- 16 DBAs and 20 DBs -- using
optical spectra taken for the European Southern Observatory SN Ia progenitor
survey (SPY). We find a difference of 6.9+/-6.9 km/s in the average apparent
velocity of the H-alpha lines versus that of the HeI 5876AA for our DBAs. This
is a measure of the blueshift of this He line due to pressure effects. By using
this as a correction, we extend the gravitational redshift method employed by
Falcon et al. (2010) to use the apparent velocity of the HeI 5876AA line and
conduct the first gravitational redshift investigation of a group of WDs
without visible hydrogen lines. We use biweight estimators to find an average
apparent velocity, _BI, (and hence average gravitational redshift,
_BI) for our WDs; from that we derive an average mass, _BI. For the
DBAs, we find _BI = 40.8+/-4.7 km/s and derive _BI = 0.71 +0.04 -0.05
Msun. Though different from of DAs (32.57 km/s) at the 91% confidence
level and suggestive of a larger DBA mean mass than that for normal DAs derived
using the same method (0.647 +0.013 -0.014 Msun; Falcon et al. 2010), we do not
claim this as a stringent detection. Rather, we emphasize that the difference
between _BI of the DBAs and of normal DAs is no larger than 9.2
km/s, at the 95% confidence level; this corresponds to roughly 0.10 Msun. For
the DBs, we find ^He_BI = 42.9+/-8.49 km/s after applying the blueshift
correction and determine _BI = 0.74 +0.08 -0.09 Msun. The difference between
^He_BI of the DBs and of DAs is less than or equal to 11.5 km/s
(~0.12 Msun), at the 95% confidence level. The gravitational redshift method
indicates much larger mean masses than the spectroscopic determinations of the
same sample by Voss et al. (2007)...Comment: Accepted to the Astrophysical Journal, 10 pages double-column, 3
figures, 5 table
Block Diagonalization using SRG Flow Equations
By choosing appropriate generators for the Similarity Renormalization Group
(SRG) flow equations, different patterns of decoupling in a Hamiltonian can be
achieved. Sharp and smooth block-diagonal forms of phase-shift equivalent
nucleon-nucleon potentials in momentum space are generated as examples and
compared to analogous low-momentum interactions ("v_lowk").Comment: 4 pages, 9 figures (pdfLaTeX
A metal-insulator transition as a quantum glass problem
We discuss a recent mapping of the Anderson-Mott metal-insulator transition
onto a random field magnet problem. The most important new idea introduced is
to describe the metal-insulator transition in terms of an order parameter
expansion rather than in terms of soft modes via a nonlinear sigma model. For
spatial dimensions d>6 a mean field theory gives the exact critical exponents.
In an epsilon expansion about d=6 the critical exponents are identical to those
for a random field Ising model. Dangerous irrelevant quantum fluctuations
modify Wegner's scaling law relating the conductivity exponent to the
correlation or localization length exponent. This invalidates the bound s>2/3
for the conductivity exponent s in d=3. We also argue that activated scaling
might be relevant for describing the AMT in three-dimensional systems.Comment: 10 pp., REvTeX, 1 eps fig., Sitges Conference Proceedings, final
version as publishe
Surface-electronic structure of La(0001) and Lu(0001)
Most spectroscopic methods for studying the electronic structure of metal
surfaces have the disadvantage that either only occupied or only unoccupied
states can be probed, and the signal is cut at the Fermi edge. This leads to
significant uncertainties, when states are very close to the Fermi level. By
performing low-temperature scanning tunneling spectroscopy and ab initio
calculations, we study the surface-electronic structure of La(0001) and
Lu(0001), and demonstrate that in this way detailed information on the
surface-electronic structure very close to the Fermi energy can be derived with
high accuracy.Comment: 6 pages, 4 figures, 1 table submitted to PR
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