66 research outputs found
Spin dynamics of molecular nanomagnets fully unraveled by four-dimensional inelastic neutron scattering
Molecular nanomagnets are among the first examples of spin systems of finite
size and have been test-beds for addressing a range of elusive but important
phenomena in quantum dynamics. In fact, for short-enough timescales the spin
wavefunctions evolve coherently according to the an appropriate cluster
spin-Hamiltonian, whose structure can be tailored at the synthetic level to
meet specific requirements. Unfortunately, to this point it has been impossible
to determine the spin dynamics directly. If the molecule is sufficiently
simple, the spin motion can be indirectly assessed by an approximate model
Hamiltonian fitted to experimental measurements of various types. Here we show
that recently-developed instrumentation yields the four-dimensional
inelastic-neutron scattering function S(Q,E) in vast portions of reciprocal
space and enables the spin dynamics to be determined with no need of any model
Hamiltonian. We exploit the Cr8 antiferromagnetic ring as a benchmark to
demonstrate the potential of this new approach. For the first time we extract a
model-free picture of the quantum dynamics of a molecular nanomagnet. This
allows us, for example, to examine how a quantum fluctuation propagates along
the ring and to directly test the degree of validity of the
N\'{e}el-vector-tunneling description of the spin dynamics
Atomic spin chain realization of a model for quantum criticality
The ability to manipulate single atoms has opened up the door to constructing
interesting and useful quantum structures from the ground up. On the one hand,
nanoscale arrangements of magnetic atoms are at the heart of future quantum
computing and spintronic devices; on the other hand, they can be used as
fundamental building blocks for the realization of textbook many-body quantum
models, illustrating key concepts such as quantum phase transitions,
topological order or frustration. Step-by-step assembly promises an interesting
handle on the emergence of quantum collective behavior as one goes from one, to
few, to many constituents. To achieve this, one must however maintain the
ability to tune and measure local properties as the system size increases.
Here, we use low-temperature scanning tunneling microscopy to construct arrays
of magnetic atoms on a surface, designed to behave like spin-1/2 XXZ Heisenberg
chains in a transverse field, for which a quantum phase transition from an
antiferromagnetic to a paramagnetic phase is predicted in the thermodynamic
limit. Site-resolved measurements on these finite size realizations reveal a
number of sudden ground state changes when the field approaches the critical
value, each corresponding to a new domain wall entering the chains. We observe
that these state crossings become closer for longer chains, indicating the
onset of critical behavior. Our results present opportunities for further
studies on quantum behavior of many-body systems, as a function of their size
and structural complexity.Comment: published online on 18 Apr 2016 in Nature Physic
Antiferromagnetic Spin Coupling between Rare Earth Adatoms and Iron Islands Probed by Spin-Polarized Tunneling
High-density magnetic storage or quantum computing could be achieved using small magnets with large magnetic anisotropy, a requirement that rare-earth iron alloys fulfill in bulk. This compelling property demands a thorough investigation of the magnetism in low dimensional rare-earth iron structures. Here, we report on the magnetic coupling between 4f single atoms and a 3d magnetic nanoisland. Thulium and lutetium adatoms deposited on iron monolayer islands pseudomorphically grown on W(110) have been investigated at low temperature with scanning tunneling microscopy and spectroscopy. The spin-polarized current indicates that both kind of adatoms have in-plane magnetic moments, which couple antiferromagnetically with their underlying iron islands. Our first-principles calculations explain the observed behavior, predicting an antiparallel coupling of the induced 5d electrons magnetic moment of the lanthanides with the 3d magnetic moment of iron, as well as their in-plane orientation, and pointing to a non-contribution of 4f electrons to the spin-polarized tunneling processes in rare earths
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