3,256 research outputs found
Enhanced quantum coherence in exchange coupled spins via singlet-triplet transitions
Manipulation of spin states at the single-atom scale underlies spin-based
quantum information processing and spintronic devices. Such applications
require protection of the spin states against quantum decoherence due to
interactions with the environment. While a single spin is easily disrupted, a
coupled-spin system can resist decoherence by employing a subspace of states
that is immune to magnetic field fluctuations. Here, we engineered the magnetic
interactions between the electron spins of two spin-1/2 atoms to create a clock
transition and thus enhance their spin coherence. To construct and electrically
access the desired spin structures, we use atom manipulation combined with
electron spin resonance (ESR) in a scanning tunneling microscope (STM). We show
that a two-level system composed of a singlet state and a triplet state is
insensitive to local and global magnetic field noise, resulting in much longer
spin coherence times compared with individual atoms. Moreover, the spin
decoherence resulting from the interaction with tunneling electrons is markedly
reduced by a homodyne readout of ESR. These results demonstrate that
atomically-precise spin structures can be designed and assembled to yield
enhanced quantum coherence
Averaged Energy Inequalities for the Non-Minimally Coupled Classical Scalar Field
The stress energy tensor for the classical non-minimally coupled scalar field
is known not to satisfy the point-wise energy conditions of general relativity.
In this paper we show, however, that local averages of the classical stress
energy tensor satisfy certain inequalities. We give bounds for averages along
causal geodesics and show, e.g., that in Ricci-flat background spacetimes, ANEC
and AWEC are satisfied. Furthermore we use our result to show that in the
classical situation we have an analogue to the phenomenon of quantum interest.
These results lay the foundations for analogous energy inequalities for the
quantised non-minimally coupled fields, which will be discussed elsewhere.Comment: 8 pages, RevTeX4. Minor typos corrected; version to appear in Phys
Rev
Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220
We report on Herschel/PACS observations of absorption lines of OH+, H2O+ and
H3O+ in NGC 4418 and Arp 220. Excited lines of OH+ and H2O+ with E_lower of at
least 285 and \sim200 K, respectively, are detected in both sources, indicating
radiative pumping and location in the high radiation density environment of the
nuclear regions. Abundance ratios OH+/H2O+ of 1-2.5 are estimated in the nuclei
of both sources. The inferred OH+ column and abundance relative to H nuclei are
(0.5-1)x10^{16} cm-2 and \sim2x10^{-8}, respectively. Additionally, in Arp 220,
an extended low excitation component around the nuclear region is found to have
OH+/H2O+\sim5-10. H3O+ is detected in both sources with
N(H3O+)\sim(0.5-2)x10^{16} cm-2, and in Arp 220 the pure inversion, metastable
lines indicate a high rotational temperature of ~500 K, indicative of formation
pumping and/or hot gas. Simple chemical models favor an ionization sequence
dominated by H+ - O+ - OH+ - H2O+ - H3O+, and we also argue that the H+
production is most likely dominated by X-ray/cosmic ray ionization. The full
set of observations and models leads us to propose that the molecular ions
arise in a relatively low density (\gtrsim10^4 cm-3) interclump medium, in
which case the ionization rate per H nucleus (including secondary ionizations)
is zeta>10^{-13} s-1, a lower limit that is severalx10^2 times the highest rate
estimates for Galactic regions. In Arp 220, our lower limit for zeta is
compatible with estimates for the cosmic ray energy density inferred previously
from the supernova rate and synchrotron radio emission, and also with the
expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray
ionization due to an AGN is responsible for the molecular ion production.Comment: 24 pages, 13 figures. Accepted for publication in Astronomy &
Astrophysic
Single-Atom Gating of Quantum State Superpositions
The ultimate miniaturization of electronic devices will likely require local
and coherent control of single electronic wavefunctions. Wavefunctions exist
within both physical real space and an abstract state space with a simple
geometric interpretation: this state space--or Hilbert space--is spanned by
mutually orthogonal state vectors corresponding to the quantized degrees of
freedom of the real-space system. Measurement of superpositions is akin to
accessing the direction of a vector in Hilbert space, determining an angle of
rotation equivalent to quantum phase. Here we show that an individual atom
inside a designed quantum corral can control this angle, producing arbitrary
coherent superpositions of spatial quantum states. Using scanning tunnelling
microscopy and nanostructures assembled atom-by-atom we demonstrate how single
spins and quantum mirages can be harnessed to image the superposition of two
electronic states. We also present a straightforward method to determine the
atom path enacting phase rotations between any desired state vectors. A single
atom thus becomes a real space handle for an abstract Hilbert space, providing
a simple technique for coherent quantum state manipulation at the spatial limit
of condensed matter.Comment: Published online 6 April 2008 in Nature Physics; 17 page manuscript
(including 4 figures) + 3 page supplement (including 2 figures);
supplementary movies available at http://mota.stanford.ed
Formation of the BiAg2 surface alloy on lattice-mismatched interfaces
We report on the growth of a monolayer-thick BiAg2 surface alloy on thin Ag films grown on Pt(111) and Cu(111). Using low energy electron diffraction (LEED), angle resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) we show that the surface structure of the 13 ML Bi/x-ML Ag/Pt(111) system (x≥2) is strongly affected by the annealing temperature required to form the alloy. As judged from the characteristic (3×3)R30 LEED pattern, the BiAg2 alloy is partially formed at room temperature. A gentle, gradual increase in the annealing temperatures successively results in the formation of a pure BiAg2 phase, a combination of that phase with a (2×2) superstructure, and finally the pure (2×2) phase, which persists at higher annealing temperatures. These results complement recent work reporting the (2×2) as a predominant phase, and attributing the absence of BiAg2 alloy to the strained Ag/Pt interface. Likewise, we show that the growth of the BiAg2 alloy on similarly lattice-mismatched 1 and 2 ML Ag-Cu(111) interfaces also requires a low annealing temperature, whilst higher temperatures result in BiAg2 clustering and the formation of a BiCu2 alloy. The demonstration that the BiAg2 alloy can be formed on thin Ag films on different substrates presenting a strained interface has the prospect of serving as bases for technologically relevant systems, such as Rashba alloys interfaced with magnetic and semiconductor substrates.This work was supported by the Spanish Gouvernment (Grant No. MAT2013-46593-C6-4-P), the Basque Gouvernment (Grant No. IT621-13), and the Spanish Research Council (Grant No. CSIC-201560I022). Z.M.A. would like to acknowledge funding from DAAD and DIPC. P.L. would also like to acknowledge funding from the Deutsche
Forschungsgemeinschaft via Project No.RE 1469/8-1.Peer Reviewe
Feeding Nucleotides with Corn Germ Meal or Dried Corn Distillers Grains Does Not Promote Growth Performance of Receiving and Growing Calves
Corn germ meal is often used in swine and poultry diets, but very little information exists on the effects on beef cattle and on newly arrived stressed cattle. When formulating receiving and growing diets, calf health and stress are important factors to consider. Including nucleotides, an immune-boosting feed additive may aid in gastrointestinal health of an animal and furthermore improve growth performance. The objective of these experiments was to determine: 1) the effects of corn germ meal in comparison to dried corn distillers grains on growth performance and 2) the effects of nucleotides on growth performance, by receiving and growing cattle
Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution
Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resolution of spin relaxation measurements to the atomic scale has been a challenge. We show how a scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers were found to vary between 50 and 250 nanoseconds. Our method can in principle be generalized to monitor the temporal evolution of other dynamical systems
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
Utilizing a single atom magnet and oscillating electric fields to coherently drive magnetic resonance in single atoms
Scanning tunneling microscopes (STM) equipped with pulsed electron spin
resonance (ESR) have paved a way to coherently control individual atomic and
molecular spins on surfaces. A recent breakthrough was to drive ESR of a spin
outside the tunnel junction by locating a single atom magnet in proximity to a
qubit, composing a 'spin-magnet pair'. Here we present a combined experimental
and model study on the ESR driving mechanism in such a spin-magnet pair. Pulsed
ESR of a single hydrogenated Ti atom on MgO with an Fe atom located between 6
and 8 {\AA} away showed a non-vanishing Rabi rate even when the tip is
substantially retracted, comparable in strength with that driven by the
interaction with the tip's magnetic moment under normal tunnel conditions. We
reveal that this ESR driving field is contributed by Fe through the spin-spin
interaction in the pair and show its tunability using a vector magnetic field.
The spin-magnet pair therefore expands ESR-STM to address and coherently
control on-surface atomic and molecular spins independent of the tip's magnetic
apex. Together with existing atom manipulation techniques in STM, our study
establishes a feasible method to design spin-based multi-qubit systems on
surfaces
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
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