Mechanical relaxation in glasses

The basic properties of glasses and the characteristics of mechanical relaxation in glasses were briefly reviewed, and then our studies concerned were presented. Experimental methods adopted were viscosity, internal friction, ultrasonic attenuation, and Brillouin scattering measurements. The specimens used were several kinds of inorganic, organic, and metallic glasses. The measurements were mainly carried out from the room temperature up to the glass transition temperature, and the relaxation time was determined as a function of temperature. The “double relaxation” composed of two Arrhenius-type relaxations was observed in many materials. In both relaxations, the “compensation effect” showing a correlation of the pre-exponential factor and the activation energy was observed. These results were explained by considering the “complex relaxation” due to cooperative motions of atoms or group of atoms. Values of activation energy near the glass transition determined by the various experimental methods were compared with each other.     

Slow hot-carrier relaxation in colloidal graphene quantum dots

Reducing hot-carrier relaxation rates is of great significance in overcoming energy loss that fundamentally limits the efficiency of solar energy utilization. Semiconductor quantum dots are expected to have much slower carrier cooling because the spacing between their discrete electronic levels is much larger than phonon energy. However, the slower carrier cooling is difficult to observe due to the existence of many competing relaxation pathways. Here we show that carrier cooling in colloidal graphene quantum dots can be 2 orders of magnitude slower than in bulk materials, which could enable harvesting of hot charge carriers to improve the efficiency of solar energy conversion.     

Slow relaxation and compaction of granular systems

Granular materials are of substantial importance in many industrial and natural processes, yet their complex behaviours, ranging from mechanical properties of static packing to their dynamics, rheology and instabilities, are still poorly understood. Here we focus on the dynamics of compaction and its 'jamming' phenomena, outlining recent statistical mechanics approaches to describe it and their deep correspondence with thermal systems such as glass formers. In fact, granular media are often presented as ideal systems for studying complex relaxation towards equilibrium. Granular compaction is defined as an increase of the bulk density of a granular medium submitted to mechanical perturbation. This phenomenon, relevant in many industrial processes and widely studied by the soil mechanics community, is simple enough to be fully investigated and yet reveals all the complex nature of granular dynamics, attracting considerable attention in a broad range of disciplines ranging from chemical to physical sciences.     

A relaxation trend in the electron spin resonance phenomena in copper tellurite glasses containing NiO,CoO and Lu2O3

Copper tellurite glasses containing NiO, CoO and Lu₂O₃ were prepared by the melt-quenching technique. The composition used was 65TeO₂-(35-x)CuO-xTMO (mol%), where TMO indicates NiO, CoO, Lu₂O₃, and for NiO- and CoO-doped glasses,x has the values 0, 0.5, 1 to 4, and for Lu₂O₃ doped glasses x=0 to 4. Electron spin resonance (ESR) spectra of all glasses were recorded at room temperature. The results on glasses doped with NiO, CoO and Lu₂O₃ are discussed in terms of oxidation-reduction, cross-relaxation and interelectronic repulsion processes, respectively. Cobalt oxide is found to be more effective in relaxing the ESR spectrum than nickel and lutetium oxides when substituted in copper tellurite glasses.     

Hole spin relaxation in Ge-Si core-shell nanowire qubits

Controlling decoherence is the biggest challenge in efforts to develop quantum information hardware. Single electron spins in gallium arsenide are a leading candidate among implementations of solid-state quantum bits, but their strong coupling to nuclear spins produces high decoherence rates. Group IV semiconductors, on the other hand, have relatively low nuclear spin densities, making them an attractive platform for spin quantum bits. However, device fabrication remains a challenge, particularly with respect to the control of materials and interfaces. Here, we demonstrate state preparation, pulsed gate control and charge-sensing spin readout of hole spins confined in a Ge-Si core-shell nanowire. With fast gating, we measure T(1) spin relaxation times of up to 0.6 ms in coupled quantum dots at zero magnetic field. Relaxation time increases as the magnetic field is reduced, which is consistent with a spin-orbit mechanism that is usually masked by hyperfine contributions.     

Observation of extremely long spin relaxation times in an organic nanowire spin valve

Organic semiconductors that are pi-conjugated are emerging as an important platform for 'spintronics', which purports to harness the spin degree of freedom of a charge carrier to store, process and/or communicate information. Here, we report the study of an organic nanowire spin valve device, 50 nm in diameter, consisting of a trilayer of ferromagnetic cobalt, an organic, Alq3, and ferromagnetic nickel. The measured spin relaxation time in the organic is found to be exceptionally long-between a few milliseconds and a second-and it is relatively temperature independent up to 100 K. Our experimental observations strongly suggest that the primary spin relaxation mechanism in the organic is the Elliott-Yafet mode, in which the spin relaxes whenever a carrier scatters and its velocity changes.     

Structural relaxation and embrittlement in Fe-Ni based metallic glasses

The fracture strain, changes in electrical resistivity and Curie temperature, and the volume change (the amount of annealed-out excess volume) were measured as a function of annealing temperature in some Fe-Ni based metallic glasses (Fe₂₇Ni₅₃P₁₄B₆, Fe₂₉Ni₄₉P₁₄B₆Si₂, Fe₄₀Ni₄₀P₁₄B₆, Fe₄₀Ni₃₈Si₈B₁₄ and Fe₆₃Ni₁₅Si₈B₁₄), in order to clarify the embrittlement behaviour during structural relaxation. A close relationship between the ductile-brittle transition temperature and the resistivity change was observed in these metallic glasses. Particularly, in Fe₂₇Ni₅₃P₁₄B₆ metallic glass, it was found that the ductile-brittle transition temperature is well consistent with the annealing temperature at which the changes in resistivity and Curie temperature are maximum. The results obtained in the present study indicate that the embrittlement behaviour during structural relaxation in these Fe-Ni based metallic glasses is closely related to the formation of more stable short range ordered structure.     

Slow dynamics and internal stress relaxation in bundled cytoskeletal networks

Crosslinked and bundled actin filaments form networks that are essential for the mechanical properties of living cells. Reconstituted actin networks have been extensively studied not only as a model system for the cytoskeleton, but also to understand the interplay between microscopic structure and macroscopic viscoelastic properties of network-forming soft materials. These constitute a broad class of materials with countless applications in science and industry. So far, it has been widely assumed that reconstituted actin networks represent equilibrium structures. Here, we show that fully polymerized actin/fascin bundle networks exhibit surprising age-dependent changes in their viscoelastic properties and spontaneous dynamics, a feature strongly reminiscent of out-of-equilibrium, or glassy, soft materials. Using a combination of rheology, confocal microscopy and space-resolved dynamic light scattering, we demonstrate that actin networks build up stress during their formation and then slowly relax towards equilibrium owing to the unbinding dynamics of the crosslinking molecules.     

Nuclear spin lattice relaxation rate in the excitonic state

The nuclear spin lattice relaxation rate in the excitonic state is studied. Use is made of the two-band model, where the Fermi radii of the electron band and the hole band are assumed to be of the same size. It is important to distinguish two cases; the spin singlet case (i.e., the nonmagnetic excitonic state) and the spin triplet case (i.e., the antiferromagnetic state). In the case of the spin singlet excitonic state the nuclear spin lattice relaxation rate first increases in the excitonic state just below the transition temperature and then decreases rapidly as the temperature decreases. In the case of the spin triplet excitonic state the relaxation rate depends on whether the nuclear spin is polarized parallel or perpendicular to the spin-density wave describing the excitonic state. In the parallel case the relaxation rate decreases monotonically in the excitonic state as the temperature decreases, while in the transverse case it has a small peak just below the transition temperature.     

Universal secondary relaxation and unusual brittle-to-ductile transition in metallic glasses

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Electron spin resonance and electrical conductivity of vanadate glasses

Electron spin resonance (ESR) and d.c. conductivity were measured for a series of vanadium borophosphate glasses before and after heat treatment. The ESR spectra showed the presence of vanadium in the V⁴⁺ state in all untreated and heat-treated samples free from iron. The variable temperature ESR and d.c. conductivity results obtained on the sample free from iron showed an inflexion at about 140°C. The electrical conductivity was found to decrease on substitution of 1 mol.% V₂O₅ by 1 mol.% Fe₂O₃ which may be due to a decrease in the V⁴⁺/V ratio. However, the electrical conductivity was found to increase on addition of more than 1 mol.% Fe₂O₃ which may be due to possible hopping conduction between Fe²⁺−Fe³⁺, V⁴⁺−Fe³⁺ and Fe²⁺−V⁵⁺. The increase in conductivity in the sample heat treated at 350°C relative to those heat treated at 300°C and 400°C may be due to the variation in the V⁴⁺/V total ratio. The activation energy values for untreated and heat-treated samples were calculated and were found to depend on the variation in the V⁴⁺/V ratio and the microstructure.     

A study of electron spin resonance in copper-phosphate glasses containing praseodymium

The e.s.r. spectra of glasses of compositions (P₂O₈)₆₅(CuO)_35–x (Pr₆O₁₁) and (P₂O₅)65(CuO)₂₅-(CaO)_10–x (Pr₆O₁₁) _x , wherex varied from 0 to 5 mol %, were measured. From the results and the chemical analyses of the samples it is found that the reduced valency ratioC of the copper in the glasses investigated generally increases with increasing Pr₆O₁₁ content. In our samples it is believed that enhanced chemical reduction of the cupric (Cu²⁺) ion is a consequence of its interaction with a reduced ion of the added praseodymium oxide.     

Anomalous nuclear spin relaxation of adsorbed helium-3

Within the framework of a general theory of two-dimensional NMR we are able to elucidate certain anomalous features of spin relaxation in adsorbed and porous systems. We explain the linear dependence of relaxation time on applied magnetic field, and this is demonstrated to be related to the relative insensitivity ofT ₁to temperature. We also show thatT ₁remains field dependent on the “fast” side of theT ₁minimum.     

Electron spin resonance in phosphate glasses containing mixed transition metal ions

Phosphate glasses containing mixed Cu²⁺/Ni²⁺ and Cu²⁺/Co²⁺ oxides have been examined. A pronounced decrease in the optical absorption at 830 nm due to the Cu²⁺ ions is observed as the CuO in the glasses is gradually replaced by NiO or CoO and the decrease is accompanied by a pronounced decrease in the strength of the electron spin resonance (ESR) signal at 9.52 GHz. By combining the ESR and optical absorption data it is concluded that the decrease in concentration of Cu²⁺ ions in phosphate glasses may be due to an oxidation-reduction mechanism between two valency states of the two different transition metals, of the form Cu²⁺+Ni⁺Cu⁺+Ni²⁺ and Cu²⁺+Co⁺Cu⁺+Co²⁺.     

Spin relaxation in the channel of a spin field-effect transistor

We examine the major spin relaxation mechanism (D'yakonov-Perel') in the channel of a spin field-effect transistor (SPINFET) and show analytically that it can be completely eliminated if the channel is a quantum wire and transport is strictly single moded (only the lowest subband is occupied). Single-moded transport also produces the largest "on" to "off" conductance ratio and the largest transconductance of the transistor.     

Ultrasonic relaxation in CoO-P2O5 glasses at high temperatures

The attenuation of ultrasonic compressional waves in CoO-P₂O₅ glasses has been measured over the temperature and frequency ranges 250 to 640 K and 15 to 75 MHz, respectively. Twin loss peaks in the attenuation against temperature plots at constant frequency, were attributed to a relaxation loss of the standard linear solid type, with low dispersion and two, discrete, Arrhenius relaxation times. Unlike the case of low-temperature loss peaks in glasses, it was not found necessary to assume a continuum of relaxation times to explain the width of the loss peaks. Activation energies and attempt frequencies were in the range, respectively, almost one and three orders of magnitude higher than corresponding values typifying the low-temperature loss peaks in the same glass system. This result suggests the existence of 2-well systems with dimensions of the order of the diameter of a structural unit or atomic ring rather than the width of one or two atoms, and perhaps relaxing particles the size of a structural unit rather than a single oxygen atom. Activation energies in the various glasses were found to be proportional to the product of the mean force constant and compressibility. The calculated number of two-well systems (loss centres) per unit volume is small and comparable with what obtains in the low-temperature loss case. The composition dependence of several other properties of the relaxation process is discussed and a common feature is a discontinuity in the composition dependence of all of these properties at metaphosphate composition.