Bistability in Atomic-Scale Antiferromagnets Loth, Sebastian; Baumann, Susanne; Lutz, Christopher P. ...
Science (American Association for the Advancement of Science),
01/2012, Volume:
335, Issue:
6065
Journal Article
Peer reviewed
Control of magnetism on the atomic scale is becoming essential as data storage devices are miniaturized. We show that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, ...exhibit two magnetic states, the Néel states, that are stable for hours at low temperature. For the smallest structures, we observed transitions between Néel states due to quantum tunneling of magnetization. We sensed the magnetic states of the designed structures using spin-polarized tunneling and switched between them electrically with nanosecond speed. Tailoring the properties of neighboring antiferromagnetic nanostructures enables a low-temperature demonstration of dense nonvolatile storage of information.
An unprecedented quantum tunneling effect has been observed in catalytic Si−H bond activations at room temperature. The cationic hydrido‐silyl‐iridium(III) complex, ...{IrSiMe(o‐C6H4SMe)2(H)(PPh3)(THF)}BArF4, has proven to be a highly efficient catalyst for the hydrolysis and the alcoholysis of organosilanes. When triethylsilane was used as a substrate, the system revealed the largest kinetic isotopic effect (KIESi−H/Si−D=346±4) ever reported for this type of reaction. This unexpectedly high KIE, measured at room temperature, together with the calculated Arrhenius preexponential factor ratio (AH/AD=0.0004) and difference in the observed activation energy (EaD
−EaH
)=34.07 kJ mol−1 are consistent with the participation of quantum tunneling in the catalytic process. DFT calculations have been used to unravel the reaction pathway and identify the rate‐determining step. Aditionally, isotopic effects were considered by different methods, and tunneling effects have been calculated to be crucial in the process.
A “giant” kinetic isotope effect (KIE=346) has been observed for the generation of hydrogen from the hydrolysis of silanes catalyzed by a silyl‐iridium(III) complex. This and other experimental evidence together with theoretical calculations have been used to demonstrate the participation of hydrogen quantum tunneling in this catalytic process.
In this work, we investigate the phase transition of the Schwarzschild black hole (SBH) inside an isothermal spherical cavity in the context of the non-commutative (NC) gauge theory of gravity, by ...using the Seiberg–Witten (SW) map and the star product. Firstly, we compute the NC correction to the Hawking temperature and derive the logarithmic correction to the entropy, then we derive the local temperature and local energy of NC SBH in isothermal cavity. Our results show that the non-commutativity removes the commutative divergence behavior of temperature, and prevents the SBH from the complete evaporation, which leads to a remnant black hole, and this geometry has predicted a minimal length in the order of Planck scale Θ∼lplanck. Therefore, the thermodynamic stability and phase transition is studied by analyzing the behavior of the local heat capacity and the Helmholtz free energy in the NC spacetime, where the results show that, the NC SBH has a two second-order phase transition and one first-order phase transition, with two Hawking–Page phase transition in the NC gauge theory.
In this Report we give a balanced account of the experimental and theoretical advances acquired over the last decade in the field of near-barrier fusion reactions induced by weakly bound stable and ...unstable nuclei. The elastic scattering and breakup reactions of these systems are also extensively reviewed as they play an important role in the fusion process. We review several theoretical tools used in the description of the data. The concepts of Complete Fusion (CF), Incomplete Fusion (ICF) and Total Fusion (TF), which is the sum of CF and ICF, are discussed and recent work on the calculation of these quantities is reviewed. The Continuum Discretized Coupled Channels (CDCC) method and its semiclassical version are described in detail and their limitations are pointed out. More importantly, we describe the salient features of the conclusions reached from the more than 40 measurements made, over a decade, of near-barrier fusion, elastic scattering and breakup reactions, and confront these data with the CDCC or other methods appropriate for these processes at the energy regime in question.
We show that the ground state tunnel splitting of the magnetization can be topologically quenched in a nanomagnetic Josephson φ0 junction. Due to the relative strength of the spin-orbit interaction ...the tunnel splitting is found to oscillate with the bias current applied to the junction as well as the magnetic field along the hard axis. Spin-coherent-state path integrals are used to obtain an analytic expression for the tunnel splitting controlled by topological interference of two tunnel paths of opposite windings and the condition in which the tunnel splitting is frozen. This feature is expected to be observable in existing experimental technique.
We simulate two recent matrix‐isolation experiments at cryogenic temperatures, in which a nitrene undergoes spin crossover from its triplet state to a singlet state via quantum tunnelling. We detail ...the failure of the commonly applied weak‐coupling method (based on a linear approximation of the potentials) in describing these deep‐tunnelling reactions. The more rigorous approach of semiclassical golden‐rule instanton theory in conjunction with double‐hybrid density‐functional theory and multireference perturbation theory does, however, provide rate constants and kinetic isotope effects in good agreement with experiment. In addition, these calculations locate the optimal tunnelling pathways, which provide a molecular picture of the reaction mechanism. The reactions involve substantial heavy‐atom quantum tunnelling of carbon, nitrogen and oxygen atoms, which unexpectedly even continues to play a role at room temperature.
The spin crossover from triplet to singlet in two nitrene reactions is accompanied by simultaneous heavy‐atom quantum tunnelling. Instanton theory was used to unveil the molecular mechanism of this tunnelling process and, in addition to excellent agreement with experiment, appreciable tunnelling effects were found even at room temperature.
With fluorobenzene as a neighboring molecule, the protons of the formic acid dimer can still switch through quantum tunneling as they do in the pure formic acid dimer. Using a combined experimental ...and theoretical approach, Weixing Li, Denis S. Tikhonov, and Melanie Schnell reveal the following mechanism in their Communication on page 25674: The neighboring fluorobenzene does not affect the reaction barrier but decreases the tunneling rate of the double‐proton transfer of the formic acid dimer. The molecular view on the environment reorganization in this proton‐transfer reaction is a step towards understanding these reactions in a complex environment.
An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low ...power dissipation in the OFF state. We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness. Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier. They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively. Such devices have potential for high-frequency operation and large-scale integration.
No dates for Dy3+: With a single‐ion magnet containing dysprosium, magnetic‐site dilution leads to a hysteresis loop that can be detected at 0.5 and 2 K. On cooling below 8 K, the relaxation ...mechanism of the undiluted complex changes from a thermally activated process to quantum tunneling. The quantum tunneling can be suppressed by applying a direct‐current field and by magnetic site dilution.
Abstract
Heavy‐atom tunneling limits the lifetime and observability of bicyclo4.1.0hepta‐2,4,6‐triene, a key intermediate in the rearrangement of phenylcarbene. Bicyclo4.1.0hepta‐2,4,6‐triene had ...been proposed as the primary intermediate of the rearrangement of phenylcarbene, but despite many efforts evaded its characterization even in cryogenic matrices. By introducing fluorine substituents into the
ortho
‐positions of the phenyl ring of phenylcarbene, the highly strained cyclopropene 1,5‐difluorobicyclo4.1.0hepta‐2,4,6‐triene becomes stable enough to be characterized in argon matrices. However, even at 3 K this cyclopropene is only metastable and rearranges via heavy‐atom tunneling to the corresponding cycloheptatetraene. Calculations suggest that fluorination is necessary to slow down the tunneling rearrangement of the bicycloheptatriene. The parent bicycloheptatriene rapidly rearranges via heavy‐atom tunneling and therefore cannot be detected under matrix isolation conditions.
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