THE difficulty of reconciling line spectra with the kinetic theory of gases, has been referred to by Prof. Fitzgerald (NATURE, January 3, p. 221). The following considerations show that it is possible under certain suppositions to have a number of spectral rays with a very restricted number of degrees of freedom. Most of us, I believe, now accept a definite atomic charge of electricity, and if each charge is imagined to be capable of moving along the surface of an atom, it would represent two degrees of freedom. If a molecule is capable of sending out a homogeneous vibration, it means that there must be a definite position of equilibrium of the “electron.” If there are several such positions, the vibrations may take place in several periods. Any one molecule may perform for a certain time a simple periodic oscillation about one position of equilibrium, and owing to some impact the electron may be knocked over into a new position. The vibrations under these circumstances would not be quite homogeneous, but if the electron oscillates about any one position sufficiently long to perform a few thousand oscillations, we should hardly notice the want of homogeneity. Each electron at a given time would only send out vibrations which in our instruments would appear as homogeneous. Each molecule could thus successively give rise to a number of spectral rays, and at any one time the electron in the different molecules would, by the laws of probability, be distributed over all possible positions of equilibrium, so that we should always see all the vibrations which any one molecule of the gas is capable of sending out. The probability of an electron oscillating about one of its positions of equilibrium need not be the same in all cases. Hence a line may be weak not because the vibration has a smaller amplitude, but because fewer molecules give rise to it. The fact that the vibrations of a gas are not quite homogeneous, is borne out by experiment. If impacts become more frequent by increased pressure, we should expect from the above views that the time during which an electron performs a certain oscillation is shortened; hence the line should widen, which is the case. I have spoken, for the sake of simplicity, as if an electron vibrating about one position of equilibrium could only do so in one period. If the forces called into play, by a displacement, depend on the direction of the displacement, there would be two possible frequencies. If the surface is nearly symmetrical, we should have double lines.

/pdf/the-kinetic-theory-of-gases-13lklkm24y.pdf

The Kinetic Theory of Gases

Using a dual-mode STM-AFM microscope operating below 50 mK we measured the local density of states along small normal wires connected at both ends to superconductors with different phases. We observe that a uniform minigap can develop in the whole normal wire and in the superconductors near the interfaces. The minigap depends periodically on the phase difference. The quasiclassical theory of superconductivity applied to a simplified 1D model geometry accounts well for the data.

https://hal.archives-ouvertes.fr/hal-00273401/document

Phase controlled superconducting proximity effect probed by tunneling spectroscopy.

We have performed an experimental characterization of the dynamics of oscillating quartz tuning forks which are being increasingly used in scanning probe microscopy as force sensors. We show that tuning forks can be described as a system of coupled oscillators. Nevertheless, this description requires knowledge of the elastic coupling constant between the prongs of the tuning fork, which has not yet been measured. Therefore, tuning forks have usually been described within the single oscillator or the weakly coupled oscillators approximation that neglects the coupling between the prongs. We propose three different procedures to measure the elastic coupling constant: an opto-mechanical method, a variation of the Cleveland method and a thermal noise based method. We find that the coupling between the quartz tuning fork prongs has a strong influence on the dynamics and the measured motion is in remarkable agreement with a simple model of coupled harmonic oscillators. The precise determination of the elastic coupling between the prongs of a tuning fork allows us to obtain a quantitative relation between the resonance frequency shift and the force gradient acting at the free end of a tuning fork prong.

https://iopscience.iop.org/article/10.1088/0957-4484/20/21/215502/pdf

Dynamics of quartz tuning fork force sensors used in scanning probe microscopy

We present a quantitative characterization of an electrically tunable Josephson junction defined in an InAs nanowire proximitized by an epitaxially-grown superconducting Al shell. The gate-dependence of the number of conduction channels and of the set of transmission coefficients are extracted from the highly nonlinear current–voltage characteristics. Although the transmissions evolve non-monotonically, the number of independent channels can be tuned, and configurations with a single quasi-ballistic channel achieved.

/pdf/conduction-channels-of-an-inas-al-nanowire-josephson-weak-3im0iy7x3g.pdf

Conduction channels of an InAs-Al nanowire Josephson weak link

We report high resolution simultaneous measurements of electrical conductance and force gradient between two sharp gold tips as their separation is varied from the tunneling distance to atomic-size contact. The use of atomically sharp tips minimizes van der Waals interaction, making it possible to identify the short-range metallic adhesion contribution to the total force.

http://absimage.aps.org/image/MAR05/MWS_MAR05-2004-003939.pdf

Metallic Adhesion in Atomic-Size Junctions

The mechanical properties of the reduced number of atoms forming the apex of a tip are interesting both from a fundamental point of view and for the interpretation of experiments related to scanning local probe methods. These mechanical properties can be studied by establishing a very small contact, a nanocontact, between a tip and a surface. The elasticity and fracture events during the controlled breaking of a nanocontact as the tip is separated from the surface provide information about the mechanical properties of the tip apex. In the case of metallic tips, electron transport through the nanocontact also provides information on its mechanical properties, because at the scale of a few atoms forming the nanocontact the mechanical and electron transport properties are strongly related.
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Mechanical Properties of Metallic Nanocontacts

Mechanical Properties of Metallic Nanojunctions

In this article, we describe and test a novel way to extend a low temperature scanning tunneling microscope with the capability to measure forces. The tuning fork that we use for this is optimized to have a high quality factor and frequency resolution. Moreover, as this technique is fully compatible with the use of bulk tips, it is possible to combine the force measurements with the use of superconductive or magnetic tips, advantageous for electronic spectroscopy. It also allows us to calibrate both the amplitude and the spring constant of the tuning fork easily, in situ and with high precision.

A low temperature scanning tunneling microscope for electronic and force spectroscopy

We present an experimental determination of the conduction channel distribution in lead nanoscale contacts with total conductances ranging from 1 to 15 G0, where G0 = 2e2/h. It is found that even for contacts having a cross section much smaller than the mean free path the distribution tends to be remarkably close to the universal diffusive limit. With the help of theoretical calculations we show that this behavior can be associated with the specific band structure of lead which produces a significant contribution of partially open channels even in the absence of atomic disorder.

http://iopscience.iop.org/article/10.1209/epl/i2005-10028-0/pdf

Distribution of conduction channels in nanoscale contacts: Evolution towards the diffusive limit

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Mechanical Properties of Metallic Nanocontacts

Mechanical Properties of Metallic Nanojunctions

A low temperature scanning tunneling microscope for electronic and force spectroscopy

Distribution of conduction channels in nanoscale contacts: Evolution towards the diffusive limit