Implantation and spectroscopy of metal atoms in solid helium

The first implantation of neutral Ba and Cs atoms into solid⁴He is reported. We discuss details of the experimental setup and techniques used to load the helium with atoms at concentrations of 10⁸ cm^–3. If photodissociation of Cs molecules and clusters is performed twice per hour this atomic concentration can be kept without a second implantation for almost a day. From the optical spectra of Ba in solid helium we infer no significant difference in the trapping site with respect to that in liquid helium.     

Nearly frictionless transport of glassy solid helium at low temperature regime

Glassy matter, as subjected to high shear rates, exhibit shear-thinning: i.e., the viscosity diminishes with increasing shear rate. One possible outcome for almost vanishing viscosity is the nearly frictionless transport in micropores subjected to some surface conditions. Here we show that, by treating the glass as a shear-thinning matter and using the transition rate dependent model together with the boundary perturbation method, we can observe a sudden change of the shearing characteristics (which directly relates to the resistance and the time or rate dependent shear modulus) at low temperature regime (ranged from 30 to 50 mK) for corresponding physical activation energy. If the solid helium could be treated as glassy matter at very low temperature regime then our results resemble those recently reported by Day and Beamish [J. Day, J. Beamish, Nature 450 (2007) 853].     

Spin physics in solid helium: Experimental results and applications

We have observed optical pumping signals from Cs atoms trapped in solid⁴He. While the longitudinal electronic spin relaxation timeT ₁ is found to be in the range of 1–2 s, the transverse relaxation timeT ₂, as inferred from magnetic resonance linewidths has a lower bound of 150 s, and is determined by magnetic field inhomogeneities. We present a quantitative discussion of how paramagnetic species trapped in solid He might be used in a highly sensitive search for permanent atomic electric dipole moments.     

Pressure shift of atomic resonance lines in liquid and solid helium

We show that for atoms trapped in a liquid He matrix the excitation and emission spectra can be quantitatively described by the bubble model which treats the matrix as an incompressible continuous medium. Comparison with experimental results shows that the model also holds for a solid matrix. The model is only valid in the case where the attractive He–He interaction dominates the attractive part of the foreign atom-helium interaction. Its failure in the case of implanted Na atoms is discussed.     

Coulomb interaction between conduction electrons andf-electrons in the Anderson impurity model

The influence of the Coulomb interaction between localf-electrons and conduction electrons on the dynamical properties off-electrons is investigated for the Anderson impurity model. An equation-of-motion technique is used to treat simultaneously the Coulomb interaction and the hybridization. We find that the Abricosov-Suhl resonance remains sharp but is reduced in size while the charge excitation peak of thef-electrons is broadened.     

Spectroscopic studies on Yb+ ions in liquid helium

An emission spectrum of the - (D1) transition and excitation spectra of the - (J=1/2 (D1) and 3/2 (D2)) transitions of Yb⁺ ions in liquid helium have been observed by means of a laser-induced fluorescence method. It has been found that all these spectra are blue-shifted from the ones in the free space, and also that the D2 excitation spectrum has double peaks. To understand these spectra, theoretical calculations have also been carried out based on a vibrating bubble model, in which the bubble surface is assumed to vibrate in the spherical (breathing), dipolar and quadrupolar modes. As a result, it has been found that the blue shifts are well understood with this bubble model, and also that the dynamic Jahn-Teller effect due to the quadrupole vibration of the bubble plays an important role for the double-peaked profile of the D2 excitation spectrum. Received 27 December 1999 and Received in final form 31 July 2000     

Novel states of electrons on a helium film

Three novel states of electrons on a helium film are described. The first is a diplon, an electron trapped by a positive charge located on a substrate beneath the film, the second is a variation of the first leading to the possibility of a two-dimensional metal-insulator transition, and the third is a ripplonic polaron, an electron self-trapped in a surface dimple. From a survey of experimental searches for the ripplonic polaron state I conclude that no polaron transition occurs on films thicker than 14 nm at T>1.5 K.     

Bistable heating of solid samples immersed in liquid helium

A bistable heating of samples is found in liquid helium. Using the strong temperature dependence of the no-phonon doublet of the ⁴ T ₁(G) ⁶ A ₁(S) emission of the ZnSe:Cr⁺ centre, the temperature can be determined in the excited volume. Temperature jumps of about 5 K are recorded. The cause for the bistable crystal heating lies in the transition from the nucleate boiling to the film boiling state of the helium. In this process, the heat transfer is drastically reduced and thus the sample is further heated.     

Atomic and molecular defects in solid He

The studies of defects formed by impurity particles (atoms, molecules, exciplexes, clusters, free electrons, and positive ions) embedded in liquid and solid ⁴He are reviewed. The properties of free electrons and neutral particles in condensed helium are described by the electron (atomic) bubble model, whereas for the positive ions a snowball structure is considered. We compare the properties of the defects in condensed helium with those of metal atoms isolated in heavier rare gas matrices.     

CARS study of deuterium clusters stabilized in solid helium

Solid deuterium clusters that for the first time have been isolated in a matrix of solid helium have been investigated at T=1.3 K and P=3 MPa by the coherent anti-Stokes Raman spectroscopy (CARS) technique. The vibronic Q₁((J=0) and Q₁(J=1) line intensity, shape, and positions have been studied as functions of ortho and para content in the solid, as well as of the size of clusters. The strong effect of Raman scattering cross-section sensitivity to the molecular environment nuclear spin state has been found in CARS: the ratio of probabilities for the scattering by para(J=1) and ortho(J=0) deuterium, which is equal to 1 in a gas, is as high as 10000 in nearly pure ortho deuterium, whereas it is about 50 in spontaneous Raman scattering. This effect has been shown to occur starting from a cluster size corresponding to the onset of the phonon band.     

Influence of substrate in photothermal measurements of thin film absorption

In photothermal measurements of low optical absorption coefficients, as in the case of thin films of amorphous semiconductors, the substrate absorption can play a non-negligible role. In this paper, the substrate influence is discussed and generalized on the basis of a theoretical model of a two-layer absorbing sample, and the interaction of the thermal and optical parameters which control the photothermal process in the sample is shown. Some experimental results obtained on thin films of amorphous silicon carbide deposited on different substrates are presented and discussed.     

Pinning mode resonances of new phases of 2D electron systems in high magnetic fields

A striking rf or microwave resonance is a generic feature of electron solid phases of two-dimensional electron systems. These resonances have served to identify and characterize such solids, in the insulator that terminates the series of fractional quantum Hall effects at high magnetic field, in the range of the integer quantum Hall effect, and in bubble phases in the first excited and higher Landau levels.     

Opto-acoustic study of tinuvin-P and rhodamine 6G in solid polymeric matrices

Laser-induced opto-acoustic calorimetry has been used to examine the thermo-elastic properties of two polymer matrices doped with tinuvin-P and the radiation-less decay processes of rhodamine 6G (Rh6G) in them. The matrices assayed were methyl methacrylate (MMA) homopolymers P(MMA), and methyl methacrylate and 2-hydroxyethyl methacrylate (HEMA) copolymers P(MMA:HEMA). A slight difference in the thermo-elastic properties of the polymers, namely their adiabatic expansion coefficients, has been detected and found to correlate with the long-term stability of the laser material. This suggests a key role of the heat-dissipation processes in the photostability of these polymeric materials. On the other hand, the fluorescence quantum yield of Rh6G is shown to vary with the polymer-matrix composition in a way consistent with its lasing efficiency. The Rh6G absorption and fluorescence quantum yield also change dramatically upon increasing its concentration, which suggests the co-existence of different Rh6G forms with different photophysical properties. Received: 10 February 2000 / Revised version: 12 June 2000 / Published online: 10 January 2001     

Thermal lens investigation in amorphous SiN

In this work we report the use of thermal lens (TLs) technique to determine the thermo-optical parameters, such as thermal conductivity (K), thermal diffusivity (D) and the optical path dependence with temperature (ds/dT) of amorphous SiN. Our results indicate that ds/dT is positive at room temperature. Moreover, the obtained values of thermal diffusivity and thermal conductivity are in good agreement with that found in the literature.     

Mechanism of Th, Zr doping in PbWO4 crystals

To check the doping mechanism of trivalent ion doping in PbWO₄ (PWO) and verify the formation of dipoles [2M_Pb³⁺-V_Pb], the researches on an investigation of tetravalent ion (Th⁴⁺, Zr⁴⁺) doped PWO was conducted by dielectric loss spectroscopy, thermoluminescence and optical absorption spectroscopy. The doping mechanism of tetravalent ion doping is similar to that of trivalent ion doping, while the dipole defects should be formed more easily due to the convenience for the two defects V_Pb and M_Pb⁴⁺ to get close to each other. However, in the case of Zr⁴⁺ doping, Zr was found to enter interstitial sites because of its small ionic radius, with the introduced charges compensated by V_Pb far away and hence does not exert an obvious influence on the performance of this material. Thus, no dielectric relaxation was found in the DS (dielectric spectra) experiment.     

Photothermal detection of surface states in amorphous silicon films

A new method for the determination of surface states in amorphous silicon films is proposed. The method is based on the observation of enhancement of the interference fringe amplitude in photothermal deflection spectra, due to the presence of surface states.A theoretical approach is presented, together with the experimental results and an evaluation of the density of surface states. The method proves to be adequate, in most cases, to yield results in good agreement with those reported in literature.     

Condensation of injected electrons and holes in silicon

We report that the condensed electron—hole phase in silicon has been produced by electrical carrier injection. The condensed phase recombination radiation occured at 1.082 ± 0.001 eV with a linewidth of 0.012 eV. Hence the line position and linewidth appear to be independent of whether the semiconductor is excited by optical of electrical injection. Joule heating is shown to be important by analyses of time resolved recombination radiation spectra and double pulse experiments.     

Laser resonance external photoelectric effect in F 2 − color centers of LiF crystals

The laser-resonance two-step external photoelectric effect is revealed in the F ₂ ^– color centers of LiF crystals exposed to nanosecond laser pulses. The first photoionization step uses IR radiation tunable within the resonance absorption band of the color centers and the second step the second harmonic of a Nd:YAG laser. The photoionization cross sections of the F ₂ ^– centers are estimated.