981 research outputs found
Exact, constraint-based structure prediction in simple protein models
Die Arbeit untersucht die exakte Vorhersage der Struktur von Proteinen in dreidimensionalen, abstrakten Proteinmodellen; insbesondere wird ein exakter Ansatz zur Strukturvorhersage in den HP-Modellen (Lau und Dill, ACS, 1989) des kubischen und kubisch-flächenzentrierten Gitters entwickelt und diskutiert. Im Gegensatz zu heuristischen Methoden liefert das vorgestellte exakte Verfahren beweisbar korrekte Strukturen. HP-Modelle (Hydrophob, Polar) repräsentieren die Rückgratkonformation eines Proteins durch Gitterpunkte und berücksichti\-gen ausschließlich die hydrophobe Wechselwirkung als treibende Kraft bei der Ausbildung der Proteinstruktur. Wesentlich für die erfolgreiche Umsetzung des vorgestellten Verfahrens ist die Verwendung von constraint-basierten Techniken. Im Zentrum steht die Berechnung und Anwendung hydrophober Kerne für die Strukturvorhersage
Coherent quench dynamics in the one-dimensional Fermi-Hubbard model
Recently, it has been shown that the momentum distribution of a metallic
state of fermionic atoms in a lattice Fermi-Bose mixture exhibits coherent
oscillations after a global quench that suppresses tunneling. The oscillation
period is determined by the Fermi-Bose interaction strength. Here we show that
similar dynamics occurs in the fermionic Hubbard model when we quench a
noninteracting metallic state by introducing a Hubbard interaction and
suppressing tunneling. The period is determined primarily by the interaction
strength. Conversely, we show that one can accurately determine the Hubbard
interaction strength from the oscillation period, taking into account
corrections from any small residual tunneling present in the final Hamiltonian.
Such residual tunneling shortens the period and damps the oscillations, the
latter being visible in the Fermi-Bose experiment.Comment: 4 pages + 4 pages for supplementary material, 2 figures. Published
versio
Two-Photon Pathway to Ultracold Ground State Molecules of NaK
We report on high-resolution spectroscopy of ultracold fermionic
\nak~Feshbach molecules, and identify a two-photon pathway to the rovibrational
singlet ground state via a resonantly mixed \Bcres intermediate state.
Photoassociation in a Na-K atomic mixture and one-photon
spectroscopy on \nak~Feshbach molecules reveal about 20 vibrational levels of
the electronically excited \ctrip state. Two of these levels are found to be
strongly perturbed by nearby \Bsing states via spin-orbit coupling, resulting
in additional lines of dominant singlet character in the perturbed complex
{}, or of
resonantly mixed character in {}. The dominantly singlet level is used to locate
the absolute rovibrational singlet ground state via Autler-Townes spectroscopy. We demonstrate coherent
two-photon coupling via dark state spectroscopy between the predominantly
triplet Feshbach molecular state and the singlet ground state. Its binding
energy is measured to be 5212.0447(1) \cm, a thousand-fold improvement in
accuracy compared to previous determinations. In their absolute singlet ground
state, \nak~molecules are chemically stable under binary collisions and possess
a large electric dipole moment of Debye. Our work thus paves the way
towards the creation of strongly dipolar Fermi gases of NaK molecules.Comment: 23 pages, 8 figure
Ultracold Dipolar Gas of Fermionic NaK Molecules in their Absolute Ground State
We report on the creation of an ultracold dipolar gas of fermionic
NaK molecules in their absolute rovibrational and hyperfine
ground state. Starting from weakly bound Feshbach molecules, we demonstrate
hyperfine resolved two-photon transfer into the singlet ground state, coherently bridging a binding energy
difference of 0.65 eV via stimulated rapid adiabatic passage. The
spin-polarized, nearly quantum degenerate molecular gas displays a lifetime
longer than 2.5 s, highlighting NaK's stability against two-body chemical
reactions. A homogeneous electric field is applied to induce a dipole moment of
up to 0.8 Debye. With these advances, the exploration of many-body physics with
strongly dipolar Fermi gases of NaK molecules is in experimental
reach.Comment: 5 pages, 5 figure
Coherent Interaction of a Single Fermion with a Small Bosonic Field
We have experimentally studied few-body impurity systems consisting of a
single fermionic atom and a small bosonic field on the sites of an optical
lattice. Quantum phase revival spectroscopy has allowed us to accurately
measure the absolute strength of Bose-Fermi interactions as a function of the
interspecies scattering length. Furthermore, we observe the modification of
Bose-Bose interactions that is induced by the interacting fermion. Because of
an interference between Bose-Bose and Bose-Fermi phase dynamics, we can infer
the mean fermionic filling of the mixture and quantify its increase (decrease)
when the lattice is loaded with attractive (repulsive) interspecies
interactions.Comment: 4+ pages, 5 figures, updated to <a
href="http://dx.doi.org/10.1103/PhysRevLett.106.115305">published version</a
A Purely Gravitational Origin for Einstein-Proca Theory
We construct a theory of gravity in which a propagating massive vector field
arises from a quadratic curvature invariant. The Einstein-Cartan formulation
and a partial suppression of torsion ensure the absence of ghost and
strong-coupling problems, as we prove with nonlinear Lagrangian and Hamiltonian
analysis. Augmenting General Relativity with a propagating torsion vector, our
theory provides a purely gravitational origin of Einstein-Proca models and
constrains their parameter space. As an outlook to phenomenology, we discuss
the gravitational production of fermionic dark matter.Comment: 13 pages, 5 figures, 3 appendice
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