Distribution Of Two-level Systems Research Articles

Probing of the spectral diffusion in doped glasses by fluctuations of dichroism

A method of studying spectral diffusion using temporal fluctuations of the dichroism of an isotropic sample is described. Unlike conventional methods of absorption spectroscopy of single molecules, this method is easier to realize, provides direct information about the distribution of two-level systems over the relaxation rate, and does not require that the absorption lines be Stark modulated in frequency. The latter fact means that this method is applicable to conducting matrices (e.g., for studying current-induced relaxations in polymer materials). In combination with the probing of spectral diffusion using fluorescence intensity fluctuations, this method allows one to determine the degree of correlation between the distributions of impurity centers over the relaxation rate and over the fluorescence quantum yield.

Experimental study of Raman-active two-level systems and the boson peak inLaF3-doped fluorite mixed crystals

A broad spectral distribution of two-level systems (TLS) displaying only ${A}_{1g}$ symmetry, not ${E}_{g}$ or ${T}_{2g},$ is observed in the low-frequency, low-temperature Raman spectra of fluorite single crystal hosts doped with large amounts of ${\mathrm{LaF}}_{3}.$ The observed range of each TLS spectrum is found to be independent of the dopant concentration although each of the three crystalline hosts, ${\mathrm{CaF}}_{2},$ ${\mathrm{SrF}}_{2},$ and ${\mathrm{BaF}}_{2},$ displays a slightly different cutoff frequency. In a somewhat higher frequency range but still far below the Brillouin Zone TA phonon mode frequency, a boson peak, showing ${E}_{g}$ symmetry, is also observed and its strength is proportional to the defect-induced vibrational Raman activity for the three crystal lattices. A surprising property is that for all three host crystals the boson peak center frequency increases with increasing lattice disorder. The strengths of the TLS spectra show distinctively different ${\mathrm{La}}^{3+}$ concentration dependencies in each host, which is also different from that observed for the boson peaks. By comparing the density of state of TLS derived from the Raman scattering to the $\overline{P}$ density of states value determined from the earlier specific heat measurements, the number density of Raman-active TLS is separated from the coupling coefficient. When the total number density of Raman-active TLS is examined versus different physical properties it is found that lattice disorder, not the ${\mathrm{LaF}}_{3}$ concentration per se, is the important variable. The experimental results are consistent with a model where the TLS number density is controlled by the law of mass action with the ``effective'' temperature associated with the production of TLS determined by the frozen-in lattice disorder. Good agreement is found for the number density of TLS versus concentration as determined from the present Raman scattering data and the previous far infrared measurements when the far infrared dipole moment is used as a single fitting parameter. However, in contrast with the far infrared studies, no TLS excited state transitions are observed in Raman scattering. The overall findings from this study suggest that the Raman and infrared active TLS distributions, which extends up to about 2 meV, are associated with extended entities.

Probing the spectral dynamics of single terrylenediimide molecules in low-temperature solids

Fluorescence excitation lines of single terrylenediimide (TDI) molecules were recorded in the matrices polyethylene (PE) and hexadecane (HD) in the temperature range between 1.4 and 13 K. From line width distributions at 2.5 K in both matrices it was concluded that the disorder, theoretically modeled by a distribution of two-level systems (TLSs), is about three times stronger in PE. Temperature-dependent measurements of the line shape of single chromophores showed a reversible broadening and shift of the zero-phonon lines. We attributed this behavior to dephasing caused by pseudolocal phonons and to spectral diffusion caused by fluctuating TLSs of the disordered host. Following these assumptions, the data could be well approximated with the pertinent expressions. The specific environments of the individual molecules are reflected by different behaviors of their `homogeneous' lines.

Glass-like properties observed in low-frequency Raman scattering of mixed fluorite crystals

Abstract The low-frequency Raman spectra of (CaF 2 ) l− x (LaF 3 ) x mixed crystals in the concentration range from 0.05 ≤ x ≤ 0.4 have been measured as a function of temperature from 4.2 K to 295 K. Concentration dependent quasi-elastic Raman scattering is found for frequencies below 10 cm −1 for A 1g + E g scattering but not for T 2g symmetry scattering. The temperature dependence of Raman intensity suggests that this effect is dominated by scattering from a distribution of two-level systems between 4.2 K and 20 K. The lowest impurity-induced vibrational Boson scattering peak at 82 cm −1 , which shifts to higher frequency with increasing dopant concentration, also only appears in A 1g + E g symmetry, indicating that the two different kinds of glass-like excitations in these mixed crystals may have a common origin.

Spectral Hole-Burning Investigations in the Restricted Geometry of Molecular Sieves

Abstract Low-temperature spectroscopy and persistent spectral hole-burning have been used to characterize several aspects of the dynamics of physiadsorbed chromophores and amorphous phases confined to the A-lengthscale voids of molecular sieves. It is shown that subtle differences of geometric factors, i.e. the relationship between molecular size and form and the confining void play a decisive role in the mobility and dynamics of thiazine dyes encapsulated in solvent-free faujasite structures. Additional solvent in the pores forms an amorphous phase which allows the formation of persistent spectral holes. Spectral diffusion in some of the solvent/molecular sieve systems is largely reduced due to the effect of the restricted geometry on the structure of the amorphous phase and/or the spatial distribution of two-level systems. For a hydrated AIPO4-5 host with phthalocyanine-zinc derivates efficient hole formation has been demonstrated up to 80 K, the highest temperature for which stable non-photochemical ho...

Symmetry and distribution of two-level systems in glasses.

The low-temperature anomalous properties of amorphous solids have been explained by the two-level-system (TLS) model. But there are some disagreements between thermal and acoustic experimental results. An anisotropic structure for these TLS's has been proposed, but no investigation has been made to date into the possibility of any specific structure. This paper considers a generalized distribution of the TLS parameters, takes into account all variations with external strain, and investigates the property of amorphous materials for specific symmetry in the structure of the TLS's. It is found that the present model with a single set of five parameters is capable of providing a unified description of about eleven different observed physical properties of vitreous silica, including phonon-echo experiments at low temperatures. The model indicates that all experiments are consistent with one another and reduces the disagreement regarding the number of TLS's taking part in acoustic and specific-heat experiments.

Activation energy spectrum of induced anisotropy in Co70.4Fe4.6Si15B10

Isothermal annealing curves of the magnetic anisotropy, induced for H=0 in non-magnetostrictive Co70.4Fe4.6Si15B10, have been obtained for as-quenched and pre-annealed samples. Utilizing a micromagnetic theory of induced anisotropy based upon a distribution of two-level systems, a spectrum of activation energies has been obtained for reversible processes. The activation energy spectrum also accounts for the increase in the coercive field upon annealing.

Low-temperature calorimetric investigation of Co-Ga spin-glass

Low-temperature specific-heat measurements are presented for the spin-glass alloys ${\mathrm{Co}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}$ with $x$ between 0.49 and 0.58. For these compositions the antistructure defects are the principal magnetic entities with concentrations in the range 0.9-8.0%. For the high-concentration samples, nuclear Schottky and spin-wave contributions to the specific heat have been identified. Over the entire composition range a term with a linear temperature dependence has been found and is identified with the spin-glass state. Measurements have been taken in magnetic field from 0 to 2.8 T. These data can be understood in terms of a model with a distribution of two-level systems.

Impurity dephasing by optical modes in polymers and glasses

The contribution of low-lying optical and librational oscillators to the homogeneous linewidth of organic impurities in organic glasses and polymers is calculated in a specific model. The disordered (amorphous) nature of the host system is described by a distribution of two-level systems (TLS) following earlier work. We demonstrate that at low temperatures the contribution to the dephasing from optical modes can dominate that arising from coupling to acoustic phonons due to the different phase-space distributions of these modes.

Dynamics of the dielectric absorption in the disordered solid RbCl:OH at low temperatures

The dielectric dissipation in two crystals of RbCl containing 100- and 9-ppm O${\mathrm{H}}^{\ensuremath{-}}$ dipolar impurities was measured at 930 MHz for temperatures between 15 and 500 mK. The power and temperature dependence of ${\ensuremath{\epsilon}}^{\ensuremath{'}\ensuremath{'}}$ are similar to those observed in glasses and indicate saturable resonant absorption of a broad distribution of two-level systems at low energies. From a study of the dynamics of the saturation, the tunneling energy of 7 mK is determined, along with the longitudinal and transverse relaxation rates, both of which scale linearly with temperature.

Ultrasonic velocities in amorphousAs2S3andAs2Se3between 1.5 and 296 K

The velocities of ultrasonic longitudinal and transverse (shear) waves have been measured in amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ and ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ between 1.5 K and room temperature. No dependence of frequency was found in the 10- to 90-MHz range employed. The velocities increased monotonically with decreasing $T$. The increase down to about 40 K is interpreted as being due to the anharmonicity of atomic vibrations. Using simplified versions of expressions developed by others for crystalline solids, we deduce ($T$-dependent) ultrasonic $\ensuremath{\gamma}'\mathrm{s}$ from our longitudinal-wave data, which allow successful calculation of shear-wave velocities versus temperature down to 40 K. The ultrasonic $\ensuremath{\gamma}'\mathrm{s}$ are consistent with Gr\"uneisen, elastic, and Raman-mode $\ensuremath{\gamma}'\mathrm{s}$. At lower $T$ the velocities are more temperature dependent than implied by the quasiharmonic model. We interpret them in terms of relaxation processes involving thermal phonon-assisted tunneling in two-level systems. Curves calculated from theoretical expressions from the literature are fitted to our data below 7 K, yielding information about the distribution of two-level systems and values for deformation potentials.