29 research outputs found
Consequences of Kondo exchange on quantum spins
When individual quantum spins are placed in close proximity to conducting substrates, the localized spin is coupled to the nearby itinerant conduction electrons via Kondo exchange. In the strong coupling limit this can result in the Kondo effect â the formation of a correlated, many body singlet state â and a resulting renormalization of the density of states near the Fermi energy. However, even when Kondo screening does not occur, Kondo exchange can give rise to a wide variety of other phenomena. In addition to the well known renormalization of the g factor and the finite spin decoherence and relaxation times, Kondo exchange has recently been found to give rise to a newly discovered effect: the renormalization of the single ion magnetic anisotropy. Here we put these apparently different phenomena on equal footing by treating the effect of Kondo exchange perturbatively. In this formalism, the central quantity is ÏJ, the product of the density of states at the Fermi energy Ï and the Kondo exchange constant J. We show that perturbation theory correctly describes the experimentally observed exchange induced shifts of the single spin excitation energies, demonstrating that Kondo exchange can be used to tune the effective magnetic anisotropy of a single spin.This work was supported by the Engineering and Physical Sciences Research Council UK (EP/D063604/1 and EP/H002367/1); the Leverhulme Trust (RPG-2012-754); the Ministry of Science and Education Spain (FIS2010-21883-C02-01, MAT2010-19236, CONSOLIDER CSD2007-0010 and Programa de Movilidad Postdoctoral); the European Commission FP7 Programme (PER-GA-2009-251791) and the GV grant Prometeo 2012â11
Gating Classical Information Flow via Equilibrium Quantum Phase Transitions
The development of communication channels at the ultimate size limit of atomic scale physical dimensions will make the use of quantum entities an imperative. In this regime, quantum fluctuations naturally become prominent and are generally considered to be detrimental. Here, we show that for spin-based information processing, these fluctuations can be uniquely exploited to gate the flow of classical binary information across a magnetic chain in thermal equilibrium. Moreover, this information flow can be controlled with a modest external magnetic field that drives the system through different many-body quantum phases in which the orientation of the final spin does or does not reflect the orientation of the initial input. Our results are general for a wide class of anisotropic spin chains that act as magnetic cellular automata and suggest that quantum phase transitions play a unique role in driving classical information flow at the atomic scale.L. B. and S. B. have received funding for this research from the European Research Council under the European Unionâs Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement No. 308253 PACOMANEDIA; C. F. H. from the Leverhulme Trust (Grant No. RPG-2012-754) and EPSRC (Grant No. EP/M009564/1); J. F. R. from the UCLQ Visitors Program
The Force Needed to Move an Atom on a Surface
Manipulation of individual atoms and molecules by scanning probe microscopy offers the ability of controlled assembly at the single-atom scale. However, the driving forces behind atomic manipulation have not yet been measured. We used an atomic force microscope to measure the vertical and lateral forces exerted on individual adsorbed atoms or molecules by the probe tip. We found that the force that it takes to move an atom depends strongly on the adsorbate and the surface. Our results indicate that for moving metal atoms on metal surfaces, the lateral force component plays the dominant role. Furthermore, measuring spatial maps of the forces during manipulation yielded the full potential energy landscape of the tip-sample interaction
The role of magnetic anisotropy in the Kondo effect
In the Kondo effect, a localized magnetic moment is screened by forming a
correlated electron system with the surrounding conduction electrons of a
non-magnetic host. Spin S=1/2 Kondo systems have been investigated extensively
in theory and experiments, but magnetic atoms often have a larger spin. Larger
spins are subject to the influence of magnetocrystalline anisotropy, which
describes the dependence of the magnetic moment's energy on the orientation of
the spin relative to its surrounding atomic environment. Here we demonstrate
the decisive role of magnetic anisotropy in the physics of Kondo screening. A
scanning tunnelling microscope is used to simultaneously determine the
magnitude of the spin, the magnetic anisotropy and the Kondo properties of
individual magnetic atoms on a surface. We find that a Kondo resonance emerges
for large-spin atoms only when the magnetic anisotropy creates degenerate
ground-state levels that are connected by the spin flip of a screening
electron. The magnetic anisotropy also determines how the Kondo resonance
evolves in a magnetic field: the resonance peak splits at rates that are
strongly direction dependent. These rates are well described by the energies of
the underlying unscreened spin states.Comment: 14 pages, 4 figures, published in Nature Physic
Characterizing and optimizing qubit coherence based on SQUID geometry
The dominant source of decoherence in contemporary frequency-tunable
superconducting qubits is 1/ flux noise. To understand its origin and find
ways to minimize its impact, we systematically study flux noise amplitudes in
more than 50 flux qubits with varied SQUID geometry parameters and compare our
results to a microscopic model of magnetic spin defects located at the
interfaces surrounding the SQUID loops. Our data are in agreement with an
extension of the previously proposed model, based on numerical simulations of
the current distribution in the investigated SQUIDs. Our results and detailed
model provide a guide for minimizing the flux noise susceptibility in future
circuits.Comment: 14 pages, 6 figure
Characterization of superconducting through-silicon vias as capacitive elements in quantum circuits
The large physical size of superconducting qubits and their associated
on-chip control structures presents a practical challenge towards building a
large-scale quantum computer. In particular, transmons require a
high-quality-factor shunting capacitance that is typically achieved by using a
large coplanar capacitor. Other components, such as superconducting microwave
resonators used for qubit state readout, are typically constructed from
coplanar waveguides which are millimeters in length. Here we use compact
superconducting through-silicon vias to realize lumped element capacitors in
both qubits and readout resonators to significantly reduce the on-chip
footprint of both of these circuit elements. We measure two types of devices to
show that TSVs are of sufficient quality to be used as capacitive circuit
elements and provide a significant reductions in size over existing approaches
Visualizing the atomic-scale origin of metallic behavior in Kondo insulators
A Kondo lattice is often electrically insulating at low temperatures.
However, several recent experiments have detected signatures of bulk
metallicity within this Kondo insulating phase. Here we visualize the
real-space charge landscape within a Kondo lattice with atomic resolution using
a scanning tunneling microscope. We discover nanometer-scale puddles of
metallic conduction electrons centered around uranium-site substitutions in the
heavy-fermion compound URuSi, and around samarium-site defects in the
topological Kondo insulator SmB. These defects disturb the Kondo screening
cloud, leaving behind a fingerprint of the metallic parent state. Our results
suggest that the mysterious 3D quantum oscillations measured in SmB could
arise from these Kondo-lattice defects, although we cannot rule out other
explanations. Our imaging technique could enable the development of
atomic-scale charge sensors using heavy-fermion probes