873 research outputs found
Positivity violation for the lattice Landau gluon propagator
We present explicit numerical evidence of reflection-positivity violation for
the lattice Landau gluon propagator in three-dimensional pure SU(2) gauge
theory. We use data obtained at very large lattice volumes (V = 80^3, 140^3)
and for three different lattice couplings in the scaling region (beta = 4.2,
5.0, 6.0). In particular, we observe a clear oscillatory pattern in the
real-space propagator C(t). We also verify that the (real-space) data show good
scaling in the range t \in [0,3] fm and can be fitted using a Gribov-like form.
The violation of positivity is in contradiction with a stable-particle
interpretation of the associated field theory and may be viewed as a
manifestation of confinement.Comment: 5 pages, 6 figures; minor modifications in the text and in the
bibliograph
Inequalities for trace anomalies, length of the RG flow, distance between the fixed points and irreversibility
I discuss several issues about the irreversibility of the RG flow and the
trace anomalies c, a and a'. First I argue that in quantum field theory: i) the
scheme-invariant area Delta(a') of the graph of the effective beta function
between the fixed points defines the length of the RG flow; ii) the minimum of
Delta(a') in the space of flows connecting the same UV and IR fixed points
defines the (oriented) distance between the fixed points; iii) in even
dimensions, the distance between the fixed points is equal to
Delta(a)=a_UV-a_IR. In even dimensions, these statements imply the inequalities
0 =< Delta(a)=< Delta(a') and therefore the irreversibility of the RG flow.
Another consequence is the inequality a =< c for free scalars and fermions (but
not vectors), which can be checked explicitly. Secondly, I elaborate a more
general axiomatic set-up where irreversibility is defined as the statement that
there exist no pairs of non-trivial flows connecting interchanged UV and IR
fixed points. The axioms, based on the notions of length of the flow, oriented
distance between the fixed points and certain "oriented-triangle inequalities",
imply the irreversibility of the RG flow without a global a function. I
conjecture that the RG flow is irreversible also in odd dimensions (without a
global a function). In support of this, I check the axioms of irreversibility
in a class of d=3 theories where the RG flow is integrable at each order of the
large N expansion.Comment: 24 pages, 3 figures; expanded intro, improved presentation,
references added - CQ
Continuum Limit of Spin Models with Continuous Symmetry and Conformal Quantum Field Theory
According to the standard classification of Conformal Quantum Field Theory
(CQFT) in two dimensions, the massless continuum limit of the model at
the Kosterlitz-Thouless (KT) transition point should be given by the massless
free scalar field; in particular the Noether current of the model should be
proportional to (the dual of) the gradient of the massless free scalar field,
reflecting a symmetry enhanced from to . More
generally, the massless continuum limit of a spin model with a symmetry given
by a Lie group should have an enhanced symmetry . We point out
that the arguments leading to this conclusion contain two serious gaps: i) the
possibility of `nontrivial local cohomology' and ii) the possibility that the
current is an ultralocal field. For the model we give analytic
arguments which rule out the first possibility and use numerical methods to
dispose of the second one. We conclude that the standard CQFT predictions
appear to be borne out in the model, but give an example where they
would fail. We also point out that all our arguments apply equally well to any
symmetric spin model, provided it has a critical point at a finite
temperature.Comment: 19 page
AdS/CFT correspondence in the Euclidean context
We study two possible prescriptions for AdS/CFT correspondence by means of
functional integrals. The considerations are non-perturbative and reveal
certain divergencies which turn out to be harmless, in the sense that
reflection-positivity and conformal invariance are not destroyed.Comment: 20 pages, references and two remarks adde
Instantons of M(atrix) Theory in PP-Wave Background
M(atrix) theory in PP-wave background possesses a discrete set of classical
vacua, all of which preserves 16 supersymmetry and interpretable as collection
of giant gravitons. We find Euclidean instanton solutions that interpolate
between them, and analyze their properties. Supersymmetry prevents direct
mixing between different vacua but still allows effect of instanton to show up
in higher order effective interactions, such as analog of v^4 interaction of
flat space effective theory. An explicit construction of zero modes is
performed, and Goldstone zero modes, bosonic and fermionic, are identified. We
further generalize this to massive M(atrix) theory that includes fundamental
hypermultiplets, corresponding to insertion of longitudinal fivebranes in the
background. After a brief comparison to their counterpart in AdS\times S, we
close with a summary.Comment: 25 pages, LaTeX, references added, section 5 update
Trust and privacy in distributed work groups
Proceedings of the 2nd International Workshop on Social Computing, Behavioral Modeling and PredictionTrust plays an important role in both group cooperation and economic exchange. As new technologies emerge for communication and exchange, established mechanisms of trust are disrupted or distorted, which can lead to the breakdown of cooperation or to increasing fraud in exchange. This paper examines whether and how personal privacy information about members of distributed work groups influences individuals' cooperation and privacy behavior in the group. Specifically, we examine whether people use others' privacy settings as signals of trustworthiness that affect group cooperation. In addition, we examine how individual privacy preferences relate to trustworthy behavior. Understanding how people interact with others in online settings, in particular when they have limited information, has important implications for geographically distributed groups enabled through new information technologies. In addition, understanding how people might use information gleaned from technology usage, such as personal privacy settings, particularly in the absence of other information, has implications for understanding many potential situations that arise in pervasively networked environments.Preprin
First observation of spin-helical Dirac fermions and topological phases in undoped and doped Bi2Te3 demonstrated by spin-ARPES spectroscopy
Electron systems that possess light-like dispersion relations or the conical
Dirac spectrum, such as graphene and bismuth, have recently been shown to
harbor unusual collective states in high magnetic fields. Such states are
possible because their light-like electrons come in spin pairs that are
chiral,which means that their direction of propagation is tied to a quantity
called pseudospin that describes their location in the crystal lattice. An
emerging direction in quantum materials research is the manipulation of atomic
spin-orbit coupling to simulate the effect of a spin dependent magnetic
field,in attempt to realize novel spin phases of matter. This effect has been
proposed to realize systems consisting of unpaired Dirac cones that are
helical, meaning their direction of propagation is tied to the electron spin
itself, which are forbidden to exist in graphene or bismuth. The experimental
existence of topological order can not be determined without spin-resolved
measurements. Here we report a spin-and angle-resolved photoemission study of
the hexagonal surface of the Bi2Te3 and Bi{2-x}MnxTe3 series, which is found to
exhibit a single helical Dirac cone that is fully spin-polarized. Our
observations of a gap in the bulk spin-degenerate band and a spin-resolved
surface Dirac node close to the chemical potential show that the low energy
dynamics of Bi2Te3 is dominated by the unpaired spin-helical Dirac modes. Our
spin-texture measurements prove the existence of a rare topological phase in
this materials class for the first time, and suggest its suitability for novel
2D Dirac spin device applications beyond the chiral variety or traditional
graphene.Comment: 13 pages, 4 figure
- …