Thermodynamic investigation of an insulator irradiated by a low-energy electron beam

The surface of an insulating material irradiated by a beam of low energy electrons charges positively if the yield of secondary electron is greater than unity. For such a dynamical equilibrium, the thermodynamic properties have been investigated by measuring the surface potential in response to a temperature oscillation of the material. It is shown that an oscillation amplitude of 0.4 K at 530 K induces an oscillation of the surface potential of about 0.5 volts. The frequency dependence indicates a monotonous decrease in the response with decreasing frequency, extrapolating to zero at zero frequency. We propose that this modification of the surface charge is driven by the temperature dependence of a gas of charged particles in equilibrium with the vacuum level

Design of an experimental setup to achieve sinusoidal temperature oscillation in ultrahigh vacuum

We designed an experimental setup to produce a temperature oscillation at the surface of a sample, in ultrahigh vacuum. The heating device, a tungsten wire, uses infrared radiation to heat the sample. A regular sine wave cannot produce a harmonic power oscillation, due to the nonlinear character of the Stefan-Boltzmann law of radiation. To achieve it we generate a complex wave form to take into account the thermal behavior of the heating filament as well as its electronic transport properties. An example of temperature oscillation is shown in the case of a tantalum sample, at a 0.12 Hz frequency. It exhibits harmonic behavior with an oscillation amplitude of about 1 K. This method opens the field to new experiments in surface science, to study reversible surface phase transitions

Improving the coherence of a low-energy electron beam by modulation

In a standard low-energy electron generator the beam is formed by particles traveling close to the axis of symmetry. However some electron trajectories are unstable and strongly dependent on the initial conditions. Numerical ray tracing shows that ultimate beam coherence is limited by these trajectories that pass far from the symmetry axis. This contribution can be partly eliminated by modulating the initial conditions and selecting the modulated response. This is exemplified with a low-energy electron beam used for electron diffraction, where the beam current modulation produces a modulated diffraction pattern that displays noteworthy improvement ͑sevenfold͒ in wave vector resolution

Interplay between the glassy transition and granular superconductivity in organic materials

It is known that some (BEDT-TTF)2X layered organic superconductors undergo a glassy transition near 80 K. Our purpose is to exploit quenched disorder to get new insights on both the superconducting state (T < 12 K) and the glassy transition by studying the superconducting properties as functions of annealing time (ta) and temperature (Ta) around 80 K. The main results on the fully deuterated kappa-(BEDT-TTF)2Cu[N(CN)2]Br compound are: 1) The data can be described by a percolation cluster model. 2) At short time scales, the clusters grow with ta following a power law. 3) At large time scales the clusters grow toward a thermodynamic state following a stretched exponential law in (1 - exp(-(t/tau)beta)with beta varying from about 0.5 to 1 in our Ta range (65 - 110 K). 4) The relaxation time follows an Arrhenius law tau(T)=tau0exp(U/T) with U around 2660 K and 1/tau0 around 2x1013 s-1. 5) The asymptotic magnetization fits with a scaling law with Tg around 55K and n around 3.2. The results are consistent with a Ising spin-glass-like model.Comment: 17 pages; 4 figure

Boson gas in a periodic array of tubes

We report the thermodynamic properties of an ideal boson gas confined in an infinite periodic array of channels modeled by two, mutually perpendicular, Kronig-Penney delta-potentials. The particle's motion is hindered in the x-y directions, allowing tunneling of particles through the walls, while no confinement along the z direction is considered. It is shown that there exists a finite Bose- Einstein condensation (BEC) critical temperature Tc that decreases monotonically from the 3D ideal boson gas (IBG) value $T_{0}$ as the strength of confinement $P_{0}$ is increased while keeping the channel's cross section, $a_{x}a_{y}$ constant. In contrast, Tc is a non-monotonic function of the cross-section area for fixed $P_{0}$. In addition to the BEC cusp, the specific heat exhibits a set of maxima and minima. The minimum located at the highest temperature is a clear signal of the confinement effect which occurs when the boson wavelength is twice the cross-section side size. This confinement is amplified when the wall strength is increased until a dimensional crossover from 3D to 1D is produced. Some of these features in the specific heat obtained from this simple model can be related, qualitatively, to at least two different experimental situations: $^4$He adsorbed within the interstitial channels of a bundle of carbon nanotubes and superconductor-multistrand-wires Nb$_{3}$Sn.Comment: 9 pages, 10 figures, submitte

Linear-T scattering and pairing from antiferromagnetic fluctuations in the (TMTSF)_2X organic superconductors

An exhaustive investigation of metallic electronic transport and superconductivity of organic superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4 in the Pressure-Temperature phase diagram between T=0 and 20 K and a theoretical description based on the weak coupling renormalization group method are reported. The analysis of the data reveals a high temperature domain (T\approx 20 K) in which a regular T^2 electron-electron Umklapp scattering obeys a Kadowaki-Woods law and a low temperature regime (T< 8 K) where the resistivity is dominated by a linear-in temperature component. In both compounds a correlated behavior exists between the linear transport and the extra nuclear spin-lattice relaxation due to antiferromagnetic fluctuations. In addition, a tight connection is clearly established between linear transport and T_c. We propose a theoretical description of the anomalous resistivity based on a weak coupling renormalization group determination of electron-electron scattering rate. A linear resistivity is found and its origin lies in antiferromagnetic correlations sustained by Cooper pairing via constructive interference. The decay of the linear resistivity term under pressure is correlated with the strength of antiferromagnetic spin correlations and T_c, along with an unusual build-up of the Fermi liquid scattering. The results capture the key features of the low temperature electrical transport in the Bechgaard salts

Signatures of granular superconductivity and Josephson effects in macroscopic measurements: the case of new superconductors

We report systematic investigations of the magnetic superconducting properties of the new superconducting materials (NS): New high temperature superconductors (HTS), Organic superconductors (OS), fullerenes, carbon nanotubes, MgB2 etc. We show that, contrary to conventional superconductors where the superconducting state can be coherent over several tenths of km, the macroscopic coherence range lc of the NS is often as short as 0.1 to 10 µm typically. As a consequence, the magnetic properties are dominated by granular-like effects as well as Josephson coupling between grains. Here, we concentrate on HTS ceramics and organic superconductors exclusively. In the first case we observe three distinct regimes: (i) At very low field (H &lt; 5 Oe to say) all the grains are coupled via Josephson effect and lc can be considered as infinite. (2) At intermediate field (5 &lt; H &lt; 50 Oe, typically) the grains are gradually decoupled by H and/or T. (iii) At higher fields all the grains are decoupled and lc roughly coincides with the diameter of the metallurgical grains. The case of OS is more subtle and is connected with a kind of order-disorder transition that occurs in most of them. For instance, in this study, we exploit quenched disorder (after crossing such a transition) in the -(BEDT-TTF)2Cu[N(CN)2]Br layered organic superconductor to get new insights on both the superconducting state (T £ 11.6 K) and the glassy transition at Tg, by studying the superconducting properties as functions of annealing time and annealing temperature around the glassy transition. Our main result is that the data can be described by a percolation molecular cluster model in which the topology and the growth of the molecular clusters obey an Ising spin-glass-like model with Tg ≈ 80 K for the hydrogenated compound and Tg ≈ 55 K for the fully deuterated one

NEW ASPECTS OF THE PHASE DIAGRAM OF THE FIELD INDUCED SPIN DENSITY WAVES IN (TMTSF)2 ClO4

Thermodynamic properties of the field induced spin density waves have been investigated by magnetocalorimetric measurements. In the relaxed state a treelike phase diagram is observed. These new experimental results are discussed in term of fractional quantization of the nesting vector

Mise en évidence expérimentale et théorique de deux causes de dissymétrie dans la raie d'émission d'un laser à gaz

The Gaussian light beam of a non diaphragmed laser is characterized by its intensity E2 on the axis and by its diameter W. These two quantities vary with frequency and E2 is asymmetric and has its maximum on the low frequency side of the émission line. In a diaphragmed laser, W-variations result in the « Garside effect » which is also asymmetric and adds to the preceding one. In this paper we calculate and compare the two effects and describe an expérimental vérification.Le faisceau lumineux gaussien d'un laser non diaphragmé est caractérisé par son intensité sur l'axe E2 et par son diamètre W. Ces deux quantités varient avec la fréquence et E2 est dissymétrique et maximum du côté des basses fréquences dans le cas d'une raie d'émission. Par ailleurs, dans un laser diaphragmé, la variation de W conduit à « l'effet Garside » qui donne lieu également à une dissymétrie qui renforce la précédente. Dans cet article, on calcule et on compare les deux effets et on en donne une vérification expérimentale