1,236 research outputs found
Second Low Temperature Phase Transition in Frustrated UNi_4B
Hexagonal UNi_4B is magnetically frustrated, yet it orders
antiferromagnetically at T_N = 20 K. However, one third of the U-spins remain
paramagnetic below this temperature. In order to track these spins to lower
temperature, we measured the specific heat C of \unib between 100 mK and 2 K,
and in applied fields up to 9 T. For zero field there is a sharp kink in C at
330 mK, which we interpret as an indication of a second phase
transition involving paramagnetic U. The rise in between 7 K and
330 mK and the absence of a large entropy liberated at may be due to a
combination of Kondo screening effects and frustration that strongly modifies
the low T transition.Comment: 4 pages, 4 figure
Kondo Screening and Magnetic Ordering in Frustrated UNi4B
UNi4B exhibits unusual properties and, in particular, a unique
antiferromagnetic arrangement involving only 2/3 of the U sites. Based on the
low temperature behavior of this compound, we propose that the remaining 1/3 U
sites are nonmagnetic due to the Kondo effect. We derive a model in which the
coexistence of magnetic and nonmagnetic U sites is the consequence of the
competition between frustration of the crystallographic structure and
instability of the 5f moments.Comment: 4 pages, 2 figure
Optical control of competing exchange interactions and coherent spin-charge coupling in two-orbital Mott insulators
In order to have a better understanding of ultrafast electrical control of
exchange interactions in multi-orbital systems, we study a two-orbital Hubbard
model at half filling under the action of a time-periodic electric field. Using
suitable projection operators and a generalized time-dependent canonical
transformation, we derive an effective Hamiltonian which describes two
different regimes. First, for a wide range of non-resonant frequencies, we find
a change of the bilinear Heisenberg exchange that is
analogous to the single-orbital case. Moreover we demonstrate that also the
additional biquadratic exchange interaction can be enhanced,
reduced and even change sign depending on the electric field. Second, for
special driving frequencies, we demonstrate a novel spin-charge coupling
phenomenon enabling coherent transfer between spin and charge degrees of
freedom of doubly ionized states. These results are confirmed by an exact
time-evolution of the full two-orbital Mott-Hubbard Hamiltonian.Comment: 3 pages, 6 figure
Supervised learning of an opto-magnetic neural network with ultrashort laser pulses
The explosive growth of data and its related energy consumption is pushing
the need to develop energy-efficient brain-inspired schemes and materials for
data processing and storage. Here, we demonstrate experimentally that Co/Pt
films can be used as artificial synapses by manipulating their magnetization
state using circularly-polarized ultrashort optical pulses at room temperature.
We also show an efficient implementation of supervised perceptron learning on
an opto-magnetic neural network, built from such magnetic synapses.
Importantly, we demonstrate that the optimization of synaptic weights can be
achieved using a global feedback mechanism, such that the learning does not
rely on external storage or additional optimization schemes. These results
suggest there is high potential for realizing artificial neural networks using
optically-controlled magnetization in technologically relevant materials, that
can learn not only fast but also energy-efficient.Comment: 9 pages, 4 figure
Inflection point in the magnetic field dependence of the ordered moment of URu2Si2 observed by neutron scattering in fields up to 17 T
We have measured the magnetic field dependence of the ordered
antiferromagnetic moment and the magnetic excitations in the heavy-fermion
superconductor URu2Si2 for fields up to 17 Tesla applied along the tetragonal c
axis, using neutron scattering. The decrease of the magnetic intensity of the
tiny moment with increasing field does not follow a simple power law, but shows
a clear inflection point, indicating that the moment disappears first at the
metamagnetic transition at ~40 T. This suggests that the moment m is connected
to a hidden order parameter Phi which belongs to the same irreducible
representation breaking time-reversal symmetry. The magnetic excitation gap at
the antiferromagnetic zone center Q=(1,0,0) increases continuously with
increasing field, while that at Q=(1.4,0,0) is nearly constant. This field
dependence is opposite to that of the gap extracted from specific-heat data.Comment: 10 pages, 5 figures, submitted to PR
Simultaneous power transfer and bidirectional serial communication for implantable electronics
Laser-driven quantum magnonics and THz dynamics of the order parameter in antiferromagnets
The impulsive generation of two-magnon modes in antiferromagnets by
femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent
longitudinal oscillations of the antiferromagnetic order parameter that cannot
be described by a thermodynamic Landau-Lifshitz approach. We argue that this
dynamics is triggered as a result of a laser-induced modification of the
exchange interaction. In order to describe the oscillations we have formulated
a quantum mechanical description in terms of magnon pair operators and coherent
states. Such an approach allowed us to} derive an effective macroscopic
equation of motion for the temporal evolution of the antiferromagnetic order
parameter. An implication of the latter is that the photo-induced spin dynamics
represents a macroscopic entanglement of pairs of magnons with femtosecond
period and nanometer wavelength. By performing magneto-optical pump-probe
experiments with 10 femtosecond resolution in the cubic KNiF and the
uniaxial KNiF collinear Heisenberg antiferromagnets, we observed
coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The
detected frequencies as a function of the temperature ideally fit the
two-magnon excitation up to the N\'eel point. The experimental signals are
described as dynamics of magnetic linear dichroism due to longitudinal
oscillations of the antiferromagnetic vector.Comment: 25 pages, 10 figure
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