150 research outputs found
Quenched crystal field disorder and magnetic liquid ground states in Tb2Sn2-xTixO7
Solid-solutions of the "soft" quantum spin ice pyrochlore magnets Tb2B2O7
with B=Ti and Sn display a novel magnetic ground state in the presence of
strong B-site disorder, characterized by a low susceptibility and strong spin
fluctuations to temperatures below 0.1 K. These materials have been studied
using ac-susceptibility and muSR techniques to very low temperatures, and
time-of-flight inelastic neutron scattering techniques to 1.5 K. Remarkably,
neutron spectroscopy of the Tb3+ crystal field levels appropriate to at high
B-site mixing (0.5 < x < 1.5 in Tb2Sn2-xTixO7) reveal that the doublet ground
and first excited states present as continua in energy, while transitions to
singlet excited states at higher energies simply interpolate between those of
the end members of the solid solution. The resulting ground state suggests an
extreme version of a random-anisotropy magnet, with many local moments and
anisotropies, depending on the precise local configuration of the six B sites
neighboring each magnetic Tb3+ ion.Comment: 6 pages, 6 figure
Total Angular Momentum Conservation During Tunnelling through Semiconductor Barriers
We have investigated the electrical transport through strained
p-Si/Si_{1-x}Ge_x double-barrier resonant tunnelling diodes. The confinement
shift for diodes with different well width, the shift due to a central
potential spike in a well, and magnetotunnelling spectroscopy demonstrate that
the first two resonances are due to tunnelling through heavy hole levels,
whereas there is no sign of tunnelling through the first light hole state. This
demonstrates for the first time the conservation of the total angular momentum
in valence band resonant tunnelling. It is also shown that conduction through
light hole states is possible in many structures due to tunnelling of carriers
from bulk emitter states.Comment: 4 pages, 4 figure
Design and Implementation of a Scalable Membership Service for Supercomputer Resiliency-Aware Runtime
As HPC systems and applications get bigger and more complex, we are approaching an era in which resiliency and run-time elasticity concerns become paramount. We offer a building block for an alternative resiliency approach in which computations will be able to make progress while components fail, in addition to enabling a dynamic set of nodes throughout a computation lifetime. The core of our solution is a hierarchical scalable membership service providing eventual consistency semantics. An attribute replication service is used for hierarchy organization, and is exposed to external applications. Our solution is based on P2P technologies and provides resiliency and elastic runtime support at ultra large scales. Resulting middleware is general purpose while exploiting HPC platform unique features and architecture. We have implemented and tested this system on BlueGene/P with Linux, and using worst-case analysis, evaluated the service scalability as effective for up to 1M nodes
Emergence of long-range order in sheets of magnetic dimers
Quantum spins placed on the corners of a square lattice can dimerize and form singlets, which then can be transformed into a magnetic state as the interactions between dimers increase beyond threshold. This is a strictly 2D transition in theory, but real-world materials often need the third dimension to stabilize long-range order. We use high pressures to convert sheets of Cu^2+ spin 1/2 dimers from local singlets to global antiferromagnet in the model system SrCu_2(BO_3)_2. Single-crystal neutron diffraction measurements at pressures above 5 GPa provide a direct signature of the antiferromagnetic ordered state, whereas high-resolution neutron powder and X-ray diffraction at commensurate pressures reveal a tilting of the Cu spins out of the plane with a critical exponent characteristic of 3D transitions. The addition of anisotropic, interplane, spin–orbit terms in the venerable Shastry–Sutherland Hamiltonian accounts for the influence of the third dimension
Open peer-to-peer systems over blockchain and ipfs: An agent oriented framework
In recent years, the increasing concerns around the centralized cloud web services (e.g. privacy, governance, surveillance, security) have triggered the emergence of new distributed technologies, such as IPFS or the Blockchain. These innovations have tackled technical challenges that were unresolved until their appearance. Existing models of peer-to-peer systems need a revision to cover the spectrum of potential systems that can be now implemented as peer-to-peer systems. This work presents a framework to build these systems. It uses an agent-oriented approach in an open environment where agents have only partial information of the system data. The proposal covers data access, data discovery and data trust in peer-to-peer systems where different actors may interact. Moreover, the framework proposes a distributed architecture for these open systems, and provides guidelines to decide in which cases Blockchain technology may be required, or when other technologies may be sufficient
The magnetic phase diagram of the frustrated spin chain compound linarite, PbCuSO(OH), as seen by neutron diffraction and H-NMR
We report on a detailed neutron diffraction and H-NMR study on the
frustrated spin-1/2 chain material linarite, PbCuSO(OH), where
competing ferromagnetic nearest neighbor and antiferromagnetic next-nearest
neighbor interactions lead to frustration. From the magnetic Bragg peak
intensity studied down to 60 mK, the magnetic moment per Cu atom is obtained
within the whole magnetic phase diagram for axis. Further, we
establish the detailed configurations of the shift of the SDW propagation
vector in phase V with field and temperature. Finally, combining our neutron
diffraction results with those from a low-temperature/high-field NMR study we
find an even more complex phase diagram close to the quasi-saturation field
suggesting that bound two-magnon excitations are the lowest energy excitations
close to and in the quasi-saturation regime. Qualitatively and
semi-quantitatively, we relate such behavior to exchange anisotropy and
contributions from the Dzyaloshinsky-Moriya interaction to affect the magnetic
properties of linarite
From Spin Glass to Quantum Spin Liquid Ground States in Molybdate Pyrochlores
We present new magnetic heat capacity and neutron scattering results for two
magnetically frustrated molybdate pyrochlores: oxide LuMoO
and oxynitride LuMoON. LuMoO
undergoes a transition to an unconventional spin glass ground state at K. However, the preparation of the corresponding oxynitride tunes
the nature of the ground state from spin glass to quantum spin liquid. The
comparison of the static and dynamic spin correlations within the oxide and
oxynitride phases presented here reveals the crucial role played by quantum
fluctuations in the selection of a ground state. Furthermore, we estimate an
upper limit for a gap in the spin excitation spectrum of the quantum spin
liquid state of the oxynitride of meV or
, in units of its antiferromagnetic Weiss
constant K.Comment: 8 pages, 9 figure
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