142 research outputs found
Tandem-pumped, tunable thulium-doped fiber laser in 21 μm wavelength region
We present a continuously tunable thulium(Tm)-doped fiber laser operating in the important 2.1 μm region, which is tandem-pumped by another Tm-doped fiber laser at 1908 nm. The advantages of pumping a Tm-doped fiber laser at the long-wavelength absorption tail (>1900 nm) of the fiber include a reduced quantum-defect, and efficient suppression of the amplified spontaneous noise (and potential parasitic lasing) at the short-wavelength region. This facilitates attainment of stable lasing operation in the long-wave emission tail of the Tm fiber at ~2.1 μm. By rotating a diffraction grating inside the Tm fiber laser cavity, we experimentally achieved a wavelength-tuning range of 2000-2172 nm. At central wavelengths of 2050 nm, 2150 nm, and 2172 nm, the slope efficiencies were 23%, 16%, and 9.9%, respectively. To the best of our knowledge, this is the first demonstration of long-wavelength operation of a Tm fiber laser system tandem-pumped at >1900 nm
Intertwined dipolar and multipolar order in the triangular-lattice magnet TmMgGaO
A phase transition is often accompanied by the appearance of an order
parameter and symmetry breaking. Certain magnetic materials exhibit exotic
hidden-order phases, in which the order parameters are not directly accessible
to conventional magnetic measurements. Thus, experimental identification and
theoretical understanding of a hidden order are difficult. Here we combine
neutron scattering and thermodynamic probes to study the newly discovered
rare-earth triangular-lattice magnet TmMgGaO. Clear magnetic Bragg peaks at
K points are observed in the elastic neutron diffraction measurements. More
interesting, however, is the observation of sharp and highly dispersive spin
excitations that cannot be explained by a magnetic dipolar order, but instead
is the direct consequence of the underlying multipolar order that is "hidden"
in the neutron diffraction experiments. We demonstrate that the observed
unusual spin correlations and thermodynamics can be accurately described by a
transverse field Ising model on the triangular lattice with an intertwined
dipolar and ferro-multipolar order.Comment: Published versio
Asynchronous Wireless Federated Learning with Probabilistic Client Selection
Federated learning (FL) is a promising distributed learning framework where
distributed clients collaboratively train a machine learning model coordinated
by a server. To tackle the stragglers issue in asynchronous FL, we consider
that each client keeps local updates and probabilistically transmits the local
model to the server at arbitrary times. We first derive the (approximate)
expression for the convergence rate based on the probabilistic client
selection. Then, an optimization problem is formulated to trade off the
convergence rate of asynchronous FL and mobile energy consumption by joint
probabilistic client selection and bandwidth allocation. We develop an
iterative algorithm to solve the non-convex problem globally optimally.
Experiments demonstrate the superiority of the proposed approach compared with
the traditional schemes.Comment: To appear in IEEE Transactions on Wireless Communication
Emergent criticality in fully frustrated quantum magnets
Phase transitions in condensed matter are often linked to exotic emergent
properties. We study the fully frustrated bilayer Heisenberg antiferromagnet to
demonstrate that an applied magnetic field creates a novel emergent
criticality. The quantum phase diagram contains four states, the DS (singlets
on every interlayer dimer bond), DTAF (all triplets with antiferromagnetic
order), TC (a singlet-triplet checkerboard) and FM (saturated ferromagnet). The
thermal phase diagram is dominated by a wall of discontinuities extending from
the zero-field DTAF-DS transition to a quantum critical endpoint where the
field drives the DTAF and TC into the FM. This first-order wall is terminated
at finite temperatures by a line of critical points, where the
Berezinskii-Kosterlitz-Thouless (BKT) transition of the DTAF and the thermal
Ising transition of the TC also terminate. We demonstrate by quantum Monte
Carlo simulations that the BKT transition does not change the Ising nature of
the DTAF-DS critical line. By contrast, the combination of symmetries merging
on the multicritical DTAF-TC line leads to a 4-state Potts universality not
contained in the microscopic Hamiltonian, which we associate with the
Ashkin-Teller model. Our results represent a systematic step in understanding
emergent phenomena in quantum magnetic materials including the
``Shastry-Sutherland compound'' SrCu(BO).Comment: 10+8 pages, 5+7 figure
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