LQ11600
Atomic Scale Force Vector Fields
In this work experimental data and theoretical simulations about the
magnitude and directionality of interatomic forces are presented. Excellent
agreement between both approaches delivers new insight into the fundamental
interaction mechanisms of single atoms. This is important to better
understand molecular adsorption and diffusion as well as friction processes.
The experiments were performed with an atomic force microscope (AFM). An
atomically sharp tip (AFM-tip), which is attached to a small and flexible
cantilever, is utilized to sense very small forces between the tip-end and
the surface of a sample. Atomically resolved surface scans are routinely
obtained with AFM's under ultra high vacuum conditions. Furthermore, forces
acting between the AFM-tip and the sample can be measured as a function of
the relative distance.
In the present paper, this distance dependent force data was used to model
the force field acting right above a potassium bromide (KBr) sample surface
with atomic resolution. Comparison of these experimental results and
theoretical models supports the notion that the atomic contrast in AFM
experiments on ionic crystals is usually determined by small amounts of
sample material being picked up during the measurements. This strongly
impacts the measured forces, since they depend on the sample as well as the
tip material. Moreover, the analysis allows to identify the chemical nature
of each ion on the KBr surface, which is usually impossible in AFM
experiments
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AV10245
Tunneling of a diatomic molecule with many bound states
In this paper we calculate the probability that a diatomic molecule will
tunnel through a physical barrier in one dimension. The tunneling of two
particle systems (such as a diatomic molecule) have been studied only very
recently due to the accessibility of high powered computation. We
consider molecules with various numbers of bound states and investigate
the corresponding effects on the probability of tunneling. We find that
the tunneling behavior of a molecule with a large number of bound states
is both quantitatively and qualitatively different as compared to a
molecule with relatively few bound states. This is due to enhancements
and reductions in the probability of tunneling caused by transitions to
lower and higher energy bound states respectively. Furthermore, we
observe an interesting effect due to the presence of many states: Use of
higher energy bound states as intermediary states during molecular state
transitions leads to much larger reductions in the probability of
tunneling than would otherwise be expected.
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LN11250
Intense light gets into the groove!
Imagine the desperation of a boxer unable to pack all his strength into a punch? This is precisely the quandary faced by an intense, femtosecond laser pulse impinging on a piece of matter- it pulls its punches, typically delivering only about half the whack it is capable of! For long, this has bothered researchers using intense light to create high energy densities in matter, say in laser fusion. The reason such light cannot deliver a clean knock out is that it gets turned away by the plasma it inevitably creates in the interaction. As most undergraduates know, plasmas love to reflect light and that is the reason metals shine and radio waves are reflected by the ionosphere. A breakthrough experiment at the Tata Institute of Fundamental Research has now reduced such reflection and coupled in a whopping 93% of such light! They used the tiny wiggles on the surface of a ‘sub-lambda’ diffraction grating, to enable near-complete absorption of a 50 femtosecond, intense Ti-Sapphire laser pulse by the high density plasma. They measure the severely lowered reflection, facilitated by the induced electron waves on the surface (surface plasmons), and simultaneously find that the extra absorption creates more abundant and more energetic ‘hot’ electrons in the plasma. Hot electrons are important for igniting laser fusion in the much touted ‘fast ignition’ scheme. And they are brilliant emitters of femtosecond, hard x-ray pulses. The Tata group teamed up with theorists at Institute of Plasma Research in India and Chinese Academy of Sciences to pave the way (albeit a wiggly one!) for brighter radiation sources at multi-kilohertz repetition rates.
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ES10439
Molecular Phonons Are Essential for Describing Water Evaporation
Traditionally, it is assumed that during steady-state water evaporation the interfacial temperature in the liquid and vapor phases are equal, but that view has been challenged by recent measurements that were made with micro-thermocouples. When water evaporates in the environment, the vapor is heated by the sun. If this circumstance is mimicked in the lab, by heating the vapor electrically, the interfacial temperature of the vapor has been reported to be as much as 27.83 ¡C larger than that of the liquid. Since these temperature discontinuities are so large, and they have only been reported from measurements made with micro-thermocouples, the question arises of whether they could be artifacts of the measurement method. This possibility has been examined by using the quantum-mechanically based statistical rate theory (SRT) to determine if the temperature discontinuities can be predicted. It is found that provided the molecular phonons (quantized molecular-vibration energies) are included in the SRT calculations, the observed temperature discontinuities up to 15.69 ¡C are predicted. Larger temperature discontinuities cannot be confirmed because of uncertainties in the pressure measurements. If the classical limit is taken of the SRT equations in which the phonon energy divided by the Boltzmann constant times the temperature is set equal to zero, it is found that the predicted temperature discontinuities no longer agree with those measured. Thus, the phonon structure of the water molecule must be included in the description of water evaporation kinetics.
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LS11758B
SPIN FILTERING BY A PERIODIC NANO-SPINTRONIC DEVICE
Future device technology and quantum information processing may
be based on spintronics, where one manipulates the electron's spin
(an not only its charge). In this paper we propose a spin valve
(or spin filter), which generates a tunable spin-polarized current
out of unpolarized sources. Our device, which consists of a periodic
array of diamond-like loops, is based on quantum interference of
the electronic spinor wave functions in each `diamond'. We use the
interplay between the Aharonov-Bohm effect, in which the orbital
wave function gains a phase due to a magnetic field perpendicular
to the electron's trajectory, and the Rashba spin-orbit effect,
which causes the electron spin to rotate when it moves in a
narrow-gap semiconductor heterostructure perpendicular to an
asymmetric electric field. For broad ranges of energy, magnetic
field and electric field, we find that there is only one propagating
wave-like solution through the device; other solutions decay due to
destructive interference. The single mode which survives carries a
unique spin polarization, in a direction which can be tuned by
varying the energy and the fields.
