233 research outputs found
Stabilization of helical magnetic structures in thin multilayers
Based on micromagnetic simulations, we report on a novel helical magnetic
structure in a soft magnetic film that is sandwiched between and
exchange-coupled to two hard magnetic layers. Confined between antiparallel
hard magnetic moments, a helix with a turn of 180 is stable without
the presence of an external magnetic field. The magnetic stability is
determined by the energy minimization and is a result of an internal field
created by exchange interaction and anisotropy. Since the internal field stores
magnetic energy, the helix can serve as an energy-storing element in spin-based
nanodevices. Due to the significantly different magnetic resonance frequencies,
the ferromagnetic and helical ground states are easy to distinguish in a
broadband ferromagnetic resonance experiment.Comment: 4 pages, 3 figure
Coherent storage and phase modulation of single hard x-ray photons using nuclear excitons
Coherent storage and phase modulation of x-ray single-photon wave packets in
resonant scattering of light off nuclei is investigated theoretically. We show
that by switching off and on again the magnetic field in the nuclear sample,
phase-sensitive storage of photons in the keV regime can be achieved.
Corresponding phase modulation of the stored photon can be accomplished
if the retrieving magnetic field is rotated by . The development
of such x-ray single-photon control techniques is a first step towards
forwarding quantum optics and quantum information to shorter wavelengths and
more compact photonic devices.Comment: 12 pages, 6 figures; v2 modified to match the published version,
condensed to 4 figures, results unchange
Spin reorientation in TlFe1.6Se2 with complete vacancy ordering
The relationship between vacancy ordering and magnetism in TlFe1.6Se2 has
been investigated via single crystal neutron diffraction, nuclear forward
scattering, and transmission electron microscopy. The examination of chemically
and structurally homogenous crystals allows the true ground state to be
revealed, which is characterized by Fe moments lying in the ab-plane below
100K. This is in sharp contrast to crystals containing regions of order and
disorder, where a competition between c-axis and ab-plane orientations of the
moments is observed. The properties of partially-disordered TlFe1.6Se2 are
therefore not associated with solely the ordered or disordered regions. This
contrasts the viewpoint that phase separation results in independent physical
properties in intercalated iron selenides, suggesting a coupling between
ordered and disordered regions may play an important role in the
superconducting analogues.Comment: Minor changes; updated references and funding acknowledgemen
Tunable sub-luminal propagation of narrowband x-ray pulses
Group velocity control is demonstrated for x-ray photons of 14.4 keV energy
via a direct measurement of the temporal delay imposed on spectrally narrow
x-ray pulses. Sub-luminal light propagation is achieved by inducing a steep
positive linear dispersion in the optical response of Fe M\"ossbauer
nuclei embedded in a thin film planar x-ray cavity. The direct detection of the
temporal pulse delay is enabled by generating frequency-tunable spectrally
narrow x-ray pulses from broadband pulsed synchrotron radiation. Our
theoretical model is in good agreement with the experimental data.Comment: 8 pages, 4 figure
Coherent control of the cooperative branching ratio for nuclear x-ray pumping
Coherent control of nuclear pumping in a three level system driven by x-ray
light is investigated. In single nuclei, the pumping performance is determined
by the branching ratio of the excited state populated by the x-ray pulse. Our
results are based on the observation that in ensembles of nuclei, cooperative
excitation and decay leads to a greatly modified nuclear dynamics, which we
characterize by a time-dependent cooperative branching ratio. We discuss
prospects of steering the x-ray pumping by coherently controlling the
cooperative decay. First, we study an ideal case with purely superradiant decay
and perfect control of the cooperative emission. A numerical analysis of x-ray
pumping in nuclear forward scattering with coherent control of the cooperative
decay via externally applied magnetic fields is presented. Next, we provide an
extended survey of nuclei suitable for our scheme, and propose
proof-of-principle implementations already possible with typical M\"ossbauer
nuclear systems such as . Finally, we discuss the application
of such control techniques to the population or depletion of long-lived nuclear
states.Comment: 11 pages, 8 figures; updated to the published versio
Interferometric phase detection at x-ray energies via Fano resonance control
Modern x-ray light sources promise access to structure and dynamics of matter
in largely unexplored spectral regions. However, the desired information is
encoded in the light intensity and phase, whereas detectors register only the
intensity. This phase problem is ubiquitous in crystallography and imaging, and
impedes the exploration of quantum effects at x-ray energies. Here, we
demonstrate phase-sensitive measurements characterizing the quantum state of a
nuclear two-level system at hard x-ray energies. The nuclei are initially
prepared in a superposition state. Subsequently, the relative phase of this
superposition is interferometrically reconstructed from the emitted x-rays. Our
results form a first step towards x-ray quantum state tomography, and provide
new avenues for structure determination and precision metrology via x-ray Fano
interference.Comment: 5 pages, 3 figures, plus supplementary informatio
Synchrotron Mössbauer spectroscopy using high-speed shutters
A new method of performing Mössbauer spectroscopy using a fast shutter in combination with microfocused synchrotron radiation is demonstrated
Coherent control of collective nuclear quantum states via transient magnons
Ultrafast and precise control of quantum systems at x-ray energies involves photons with oscillation periods below 1 as. Coherent dynamic control of quantum systems at these energies is one of the major challenges in hard x-ray quantum optics. Here, we demonstrate that the phase of a quantum system embedded in a solid can be coherently controlled via a quasi-particle with subattosecond accuracy. In particular, we tune the quantum phase of a collectively excited nuclear state via transient magnons with a precision of 1 zs and a timing stability below 50 ys. These small temporal shifts are monitored interferometrically via quantum beats between different hyperfine-split levels. The experiment demonstrates zeptosecond interferometry and shows that transient quasi-particles enable accurate control of quantum systems embedded in condensed matter environments
Cooperative effects in nuclear excitation with coherent x-ray light
The interaction between super-intense coherent x-ray light and nuclei is
studied theoretically. One of the main difficulties with driving nuclear
transitions arises from the very narrow nuclear excited state widths which
limit the coupling between laser and nuclei. In the context of direct
laser-nucleus interaction, we consider the nuclear width broadening that occurs
when in solid targets, the excitation caused by a single photon is shared by a
large number of nuclei, forming a collective excited state. Our results show
that for certain isotopes, cooperative effects may lead to an enhancement of
the nuclear excited state population by almost two orders of magnitude.
Additionally, an update of previous estimates for nuclear excited state
population and signal photons taking into account the experimental advances of
the x-ray coherent light sources is given. The presented values are an
improvement by orders of magnitude and are encouraging for the future prospects
of nuclear quantum optics.Comment: 22 pages, 4 figures, 5 tables; updated to the published version, one
additional results tabl
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