2,688 research outputs found
Two-dimensional single-valley exciton qubit and optical spin magnetization generation
Creating and manipulating coherent qubit states are actively pursued in
two-dimensional (2D) materials research. Significant efforts have been made
towards the realization of two-valley exciton qubits in monolayer
transition-metal dichalcogenides (TMDs), based on states from their two
distinct valleys in k-space. Here, we propose a new scheme to create qubits in
2D materials utilizing a novel kind of degenerate exciton states in a single
valley. Combining group theoretic analysis and ab initio GW plus Bethe-Salpeter
equation (GW-BSE) calculations, we demonstrate such novel qubit states in
substrate-supported monolayer bismuthene -- which has been successfully grown
using molecular beam epitaxy. In each of the two distinct valleys in the
Brillouin zone, strong spin-orbit coupling along with symmetry leads
to a pair of degenerate 1s exciton states with opposite spin configurations.
Specific coherent linear combinations of the two degenerate excitons in a
single valley can be excited with specific light polarizations, enabling full
manipulation of the exciton qubits and their spin configurations. In
particular, a net spin magnetization can be generated. Our finding opens new
routes to create and manipulate qubit systems in 2D materials.Comment: 27 pages, 5 figure
NNLO Computational Techniques: the Cases H -> gamma gamma and H -> g g
A large set of techniques needed to compute decay rates at the two-loop level
are derived and systematized. The main emphasis of the paper is on the two
Standard Model decays H -> gamma gamma and H -> g g. The techniques, however,
have a much wider range of application: they give practical examples of general
rules for two-loop renormalization; they introduce simple recipes for handling
internal unstable particles in two-loop processes; they illustrate simple
procedures for the extraction of collinear logarithms from the amplitude. The
latter is particularly relevant to show cancellations, e.g. cancellation of
collinear divergencies. Furthermore, the paper deals with the proper treatment
of non-enhanced two-loop QCD and electroweak contributions to different
physical (pseudo-)observables, showing how they can be transformed in a way
that allows for a stable numerical integration. Numerical results for the
two-loop percentage corrections to H -> gamma gamma, g g are presented and
discussed. When applied to the process pp -> gg + X -> H + X, the results show
that the electroweak scaling factor for the cross section is between -4 % and +
6 % in the range 100 GeV < Mh < 500 GeV, without incongruent large effects
around the physical electroweak thresholds, thereby showing that only a
complete implementation of the computational scheme keeps two-loop corrections
under control.Comment: LaTeX, 70 pages, 8 eps figure
Automatic Computation of Feynman Diagrams
Quantum corrections significantly influence the quantities observed in modern
particle physics. The corresponding theoretical computations are usually quite
lengthy which makes their automation mandatory. This review reports on the
current status of automatic calculation of Feynman diagrams in particle
physics. The most important theoretical techniques are introduced and their
usefulness is demonstrated with the help of simple examples. A survey over
frequently used programs and packages is provided, discussing their abilities
and fields of applications. Subsequently, some powerful packages which have
already been applied to important physical problems are described in more
detail. The review closes with the discussion of a few typical applications for
the automated computation of Feynman diagrams, addressing current physical
questions like properties of the and Higgs boson, four-loop corrections to
renormalization group functions and two-loop electroweak corrections.Comment: Latex, 62 pages. Typos corrected, references updated and some
comments added. Vertical offset changed. The complete paper is also available
via anonymous ftp at ftp://ttpux2.physik.uni-karlsruhe.de/ttp98/ttp98-41/ or
via www at http://www-ttp.physik.uni-karlsruhe.de/Preprints
Dynamically Triangulating Lorentzian Quantum Gravity
Fruitful ideas on how to quantize gravity are few and far between. In this
paper, we give a complete description of a recently introduced non-perturbative
gravitational path integral whose continuum limit has already been investigated
extensively in d less than 4, with promising results. It is based on a
simplicial regularization of Lorentzian space-times and, most importantly,
possesses a well-defined, non-perturbative Wick rotation. We present a detailed
analysis of the geometric and mathematical properties of the discretized model
in d=3,4. This includes a derivation of Lorentzian simplicial manifold
constraints, the gravitational actions and their Wick rotation. We define a
transfer matrix for the system and show that it leads to a well-defined
self-adjoint Hamiltonian. In view of numerical simulations, we also suggest
sets of Lorentzian Monte Carlo moves. We demonstrate that certain pathological
phases found previously in Euclidean models of dynamical triangulations cannot
be realized in the Lorentzian case.Comment: 41 pages, 14 figure
- …