2,912 research outputs found
Systematic Low-Energy Effective Field Theory for Magnons and Holes in an Antiferromagnet on the Honeycomb Lattice
Based on a symmetry analysis of the microscopic Hubbard and t-J models, a
systematic low-energy effective field theory is constructed for hole-doped
antiferromagnets on the honeycomb lattice. In the antiferromagnetic phase,
doped holes are massive due to the spontaneous breakdown of the
symmetry, just as nucleons in QCD pick up their mass from spontaneous chiral
symmetry breaking. In the broken phase the effective action contains a
single-derivative term, similar to the Shraiman-Siggia term in the square
lattice case. Interestingly, an accidental continuous spatial rotation symmetry
arises at leading order. As an application of the effective field theory we
consider one-magnon exchange between two holes and the formation of two-hole
bound states. As an unambiguous prediction of the effective theory, the wave
function for the ground state of two holes bound by magnon exchange exhibits
-wave symmetry.Comment: 33 pages, 6 figure
Microscopic Model versus Systematic Low-Energy Effective Field Theory for a Doped Quantum Ferromagnet
We consider a microscopic model for a doped quantum ferromagnet as a test
case for the systematic low-energy effective field theory for magnons and
holes, which is constructed in complete analogy to the case of quantum
antiferromagnets. In contrast to antiferromagnets, for which the effective
field theory approach can be tested only numerically, in the ferromagnetic case
both the microscopic and the effective theory can be solved analytically. In
this way the low-energy parameters of the effective theory are determined
exactly by matching to the underlying microscopic model. The low-energy
behavior at half-filling as well as in the single- and two-hole sectors is
described exactly by the systematic low-energy effective field theory. In
particular, for weakly bound two-hole states the effective field theory even
works beyond perturbation theory. This lends strong support to the quantitative
success of the systematic low-energy effective field theory method not only in
the ferromagnetic but also in the physically most interesting antiferromagnetic
case.Comment: 34 pages, 1 figur
Multiple-Bridged Bis-Tetrathiafulvalenes: New Synthetic Protocols and Spectroelectrochemical Investigations
Synthetic strategies for preparing dimeric tetrathiafulvalenes (TTFs) linked by either one, two, or four bridges have been developed. In particular, we report efficient few-step protocols for the preparation of face-to-face overlapped quadruple-bridged bis-TTFs. The ready interconversion of cis and trans TTFs in the presence of catalytic amounts of acid was implemented in one synthetic protocol as a way to control the isomeric outcome. The compounds were characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. Moreover, the X-ray crystal structure of the macrocycle 4b is presented and compared to semiempirical (PM3) geometry optimizations. Cyclic voltammetry and spectroelectrochemistry were used to describe the interactions established between two TTF units upon oxidation, that is, their ability to form mixed-valence complexes and π-dimers either intra- or intermolecularly. The length, flexibility, and number of bridging units in a bis-TTF, as well as the specific TTF positions being connected, determine the extent of these interactions. Thus, rigid linkers enhance the formation of intermolecular mixed-valence complexes. For 4b, the absorption spectrum of this mixed-valence state of TTF in solution has been recorded for the first time. Finally, preliminary complexation experiments with different electron-deficient molecules are described
Coupling a single atomic quantum bit to a high finesse optical cavity
The quadrupole S -- D optical transition of a single trapped
Ca ion, well suited for encoding a quantum bit of information, is
coherently coupled to the standing wave field of a high finesse cavity. The
coupling is verified by observing the ion's response to both spatial and
temporal variations of the intracavity field. We also achieve deterministic
coupling of the cavity mode to the ion's vibrational state by selectively
exciting vibrational state-changing transitions and by controlling the position
of the ion in the standing wave field with nanometer-precision
Global and regional left ventricular myocardial deformation measures by magnetic resonance feature tracking in healthy volunteers: comparison with tagging and relevance of gender
This work was funded by a grant from the Engineering and Physical Sciences Research Council (EP/G030693/1) and supported by the Oxford British Heart Foundation Centre of Research Excellence and the National Institute for Health Research Oxford Biomedical Research Centr
Lattice formulation of super Yang-Mills theory
We construct a lattice action for super Yang-Mills theory in
four dimensions which is local, gauge invariant, free of spectrum doubling and
possesses a single exact supersymmetry. Our construction starts from the
observation that the fermions of the continuum theory can be mapped into the
component fields of a single real anticommuting Kahler-Dirac field. The
original supersymmetry algebra then implies the existence of a nilpotent scalar
supercharge and a corresponding set of bosonic superpartners. Using this
field content we write down a -exact action and show that, with an
appropriate change of variables, it reduces to a well-known twist of super Yang-Mills theory due to Marcus. Using the discretization
prescription developed in an earlier paper on the theory in two
dimensions we are able to translate this geometrical action to the lattice.Comment: 15 pages. 1 reference correcte
Precision measurement and compensation of optical Stark shifts for an ion-trap quantum processor
Using optical Ramsey interferometry, we precisely measure the laser-induced
AC-stark shift on the -- "quantum bit" transition near 729
nm in a single trapped Ca ion. We cancel this shift using an
additional laser field. This technique is of particular importance for the
implementation of quantum information processing with cold trapped ions. As a
simple application we measure the atomic phase evolution during a rotation of the quantum bit.Comment: 4 pages, 4 figure
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