Inelastic Relaxation Research Articles

In situ neutron diffraction study of residual stress development in MgO/SiC ceramic nanocomposites during thermal cycling

Ceramic nanocomposites often contain large residual stresses due to differing thermal contraction between phases upon cooling from processing temperatures. Their role in affecting the mechanical properties is not fully understood, but is certainly of importance. This investigation used neutron diffraction to quantify the residual stresses in MgO/SiC nanocomposites throughout a thermal cycle to 1550°C. The results showed that average stresses in 10vol.% SiC samples at 100°C approached −4GPa in the particles and were +560MPa in the matrix. The stresses showed good agreement with an elastic model with a stress-free temperature of 1600°C. A small amount of inelastic relaxation (15%) was observed after cooling back to room temperature. Modelling suggested that this was due to relaxation of the stresses in grain boundary particles at a rate limited by diffusional processes in the MgO/SiC interface. The effect of particle size on stress level is discussed.

The Mechanical, Thermal and Electronic Properties of Nanoscale Materials by Statistical Moment and Ab Initio Molecular Dynamics Methods

The mechanical, thermal and electronic properties of the nanoscale materials are studied using an ab initio molecular dynamics (TBMD) method and statistical moment method. We investigate the strength and fracture behaviors of nanoscale materials like carbon nanotubes (CNT), graphens and nanowires in comparison with those of corresponding bulk materials. The thermal lattice expansion coefficients, linear elastic parameters, nonlinear elastic instabilities and bucking, inelastic relaxation, yield strength and fracture behaviors are studied in detail, including the anharmonicity of thermal lattice vibrations. We will show that the thermodynamic and strength properties of the nanoscale materials are quite different from those of the corresponding bulk materials. The electronic density of states and electronic transports of the nanoscale materials, with and without the atomistic defects are also discussed.

Coherent Probing of Excited Quantum Dot States in an Interferometer

Measurements of elastic and inelastic cotunneling currents are presented on a two-terminal Aharonov-Bohm interferometer with a Coulomb-blockaded quantum dot embedded in each arm. Coherent current contributions, even in a magnetic field, are found in the nonlinear regime of inelastic cotunneling at a finite-bias voltage. The phase of the Aharonov-Bohm oscillations in the current exhibits phase jumps of pi at the onsets of inelastic processes. We suggest that additional coherent elastic processes occur via the excited state. Our measurement technique allows the detection of such processes on a background of other inelastic current contributions and contains qualitative information about the ratio of transport and inelastic relaxation rates.

Inelastic Relaxation in Tin Dioxide Thin Films

Internal friction results obtained in thin SnO2 films produced by reactive magnetron sputtering (thickness of the film ~ 1 μm) and by the dehydration of water solution of tin salts (thickness of the film ~ 10 μm) are reported. It is suggested that internal friction peak observed in SnO2 films at around 170 oC is caused by relaxation processes on grain boundaries (average grain size is 20 nm). The investigation of internal friction in SnO2 films can yield new information about the structure of thin films.

Inelastic cotunneling-induced decoherence and relaxation, charge, and spin currents in an interacting quantum dot under a magnetic field

We present a theoretical analysis of several aspects of nonequilibirum cotunneling through a strong Coulomb-blockaded quantum dot (QD) subject to a finite magnetic field in the weak coupling limit. We carry this out by developing a generic quantum Heisenberg-Langevin equation approach leading to a set of Bloch dynamical equations which describe the nonequilibrium cotunneling in a convenient and compact way. These equations describe the time evolution of the spin variables of the QD explicitly in terms of the response and correlation functions of the free reservoir variables. This scheme not only provides analytical expressions for the relaxation and decoherence of the localized spin induced by cotunneling, but it also facilitates evaluations of the nonequilibrium magnetization, the charge current, and the spin current at arbitrary bias-voltage, magnetic field, and temperature. We find that all cotunneling events produce decoherence, but relaxation stems only from {\em inelastic} spin-flip cotunneling processes. Moreover, our specific calculations show that cotunneling processes involving electron transfer (both spin-flip and non-spin-flip) contribute to charge current, while spin-flip cotunneling processes are required to produce a net spin current in the asymmetric coupling case. We also point out that under the influence of a nonzero magnetic field, spin-flip cotunneling is an energy-consuming process requiring a sufficiently strong external bias-voltage for activation, explaining the behavior of differential conductance at low temperature: in particular, the splitting of the zero-bias anomaly in the charge current and a broad zero-magnitude "window" of differential conductance for the spin current near zero-bias-voltage.

Anisotropy of Spin Splitting and Spin Relaxation in Lateral Quantum Dots

Inelastic spin relaxation and spin splitting epsilon(s) in lateral quantum dots are studied in the regime of strong in-plane magnetic field. Because of both the g-factor energy dependence and spin-orbit coupling, epsilon(s) demonstrates a substantial nonlinear magnetic field dependence similar to that observed by Hanson et al. [Phys. Rev. Lett. 91, 196802 (2003)]. It also varies with the in-plane orientation of the magnetic field due to crystalline anisotropy of the spin-orbit coupling. The spin relaxation rate is also anisotropic, the anisotropy increasing with the field. When the magnetic length is less than the "thickness" of the GaAs dot, the relaxation can be an order of magnitude faster for B ||[100] than for B || [110].

Theory of microwave-induced oscillations in the magnetoconductivity of a two-dimensional electron gas

We develop a theory of magneto-oscillations in the photoconductivity of a two-dimensional electron gas observed in recent experiments. The effect is governed by a change of the electron distribution function induced by the microwave radiation. We analyze a nonlinearity with respect to both the dc field and the microwave power, as well as the temperature dependence determined by the inelastic relaxation rate.

Spontaneous Dissociation ofR85bFeshbach Molecules

The spontaneous dissociation of 85Rb dimers in the highest lying vibrational level has been observed in the vicinity of the Feshbach resonance which was used to produce them. The molecular lifetime shows a strong dependence on magnetic field, varying by three orders of magnitude between 155.5 G and 162.2 G. Our measurements are in good agreement with theoretical predictions in which molecular dissociation is driven by inelastic spin relaxation. Molecule lifetimes of tens of milliseconds can be achieved close to resonance.

Spontaneous Dissociation of Long-Range Feshbach Molecules

We study the spontaneous dissociation of diatomic molecules produced in cold atomic gases via magnetically tunable Feshbach resonances. We provide a universal formula for the lifetime of these molecules that relates their decay to the scattering length and the loss rate constant for inelastic spin relaxation. Our universal treatment as well as our exact coupled channels calculations for 85Rb dimers predict a suppression of the decay over several orders of magnitude when the scattering length is increased. Our predictions are in good agreement with recent measurements of the lifetime of 85Rb(2).

Scattering properties of weakly bound dimers of fermionic atoms

We consider weakly bound diatomic molecules (dimers) formed in a two-component atomic Fermi gas with a large positive scattering length for the interspecies interaction. We develop a theoretical approach for calculating atom-dimer and dimer-dimer elastic scattering and for analyzing the inelastic collisional relaxation of the molecules into deep bound states. This approach is based on the single-channel zero-range approximation, and we find that it is applicable in the vicinity of a wide two-body Feshbach resonance. Our results draw prospects for various interesting manipulations of weakly bound dimers of fermionic atoms.

Theory of the oscillatory photoconductivity of a two-dimensional electron system

We develop a theory of magnetooscillations in the photoresistivity of a two-dimensional electron gas observed in recent experiments. According to our theory, the effect is governed by a change of the electron distribution function induced by the microwave radiation. We analyze a nonlinearity with respect to both the DC field and the microwave power, as well as the temperature dependence determined by the inelastic relaxation rate.

Asteroid 1620 Geographos: III. Inelastic Relaxation in the Vicinity of the Poles

The rate of inelastic relaxation of asteroid 1620 Geographos in the vicinity of the poles, corresponding to rotation about the axes of minimal and maximal principal moment of inertia, was considered on the basis of the theory stated by Efroimsky and Lazaryan. The low bound of the total relaxation time is estimated as 4 × 106 years.

Inelastic electron relaxation rates caused by spin-M/2Kondo impurities

We study a spin S=M/2--Kondo system coupled to electrons in an arbitrary nonequilibrium situation above Kondo temperature. Coupling to hot electrons leads to an increased inverse lifetime of pseudo particles, related to the Korringa width. This in turn is responsible for the increased inelastic relaxation rates of the electronic system. The rates are related to spin--spin correlation functions which are determined using a projection operator formalism. The results generalize recent findings for S=1/2--Kondo impurities which have been used to describe energy relaxation experiments in disordered mesoscopic wires.

Viscous motion of vortices in superconducting niobium films

Motion of vortices at high velocity in niobium thin films is presented for the first time and treated as a viscous flux flow for both dirty and clean samples. The critical velocity v * , at which the abrupt transition occurs, yields a temperature determination of the inelastic relaxation time τ in of quasiparticules outside the core, which turns to be temperature independent for the dirty film.

Aharonov-bohm magnetization of mesoscopic rings caused by inelastic relaxation.

The magnetization of a system of many mesoscopic rings under nonequilibrium conditions is considered. The corresponding disorder-averaged current in a ring I(phi) is shown to be a sum of the "thermodynamic" and "kinetic" contributions both resulting from the electron-electron interaction. The thermodynamic part can be expressed through the diagonal matrix elements J(micro micro) of the current operator in the basis of exact many-body eigenstates and is a generalization of the equilibrium persistent current. The novel kinetic part is present only out of equilibrium and is governed by the off-diagonal matrix elements J(micro nu). It has drastically different temperature and magnetic field behavior.

Inelastic relaxation and noise temperature in S/N/S junctions

We studied electronic relaxation in long diffusive superconductor / normal metal / superconductor (S/N/S) junctions by means of current noise and transport measurements down to very low temperature (100mK). Samples with normal metal lengths of 4, 10 and 60 micrometer have been investigated. In all samples the shot noise increases very rapidly with the voltage. This is interpreted in terms of enhanced heating of the electron gas confined between the two S/N interfaces. Experimental results are analyzed quantitatively taking into account electron-phonon interaction and heat transfer through the S/N interfaces. Transport measurements reveal that in all samples the two S/N interfaces are connected incoherently, as shown by the reentrance of the resistance at low temperature. The complementarity of noise and transport measurements allows us to show that the energy dependence of the reentrance at low voltage is essentially due to the increasing effective temperature of the quasiparticles in the normal metal.

Weak Localization and Antilocalization in the Two-Dimensional Electron System on p-Type InAs

We have performed low temperature magnetoresistance measurements of the two-dimensional electron system (2DES) in an inversion layer on p-type InAs. In high magnetic fields perpendicular to the 2DES beating patterns in Shubnikov-de Haas oscillations are observed from which the Rashba spin-orbit interaction parameter is determined. In the low-field regime we perceive weak localization and antilocalization. By fitting to the gathered localization data the elastic, inelastic, and spin relaxation times as well as coherence lengths for diffusive transport are determined for different applied gate-voltages. The Rashba parameter determined from these relaxation times is compared to the values obtained from the high-field Shubnikov-de Haas oscillations.

Vibrational spectroscopy of a harmonic oscillator system nonlinearly coupled to a heat bath

Vibrational relaxation of a harmonic oscillator nonlinearly coupled to a heat bath is investigated by the Gaussian–Markovian quantum Fokker–Planck equation approach. The system–bath interaction is assumed to be linear in the bath coordinate, but linear plus square in the system coordinate modeling the elastic and inelastic relaxation mechanisms. Interplay of the two relaxation processes induced by the linear–linear and square–linear interactions in Raman or infrared spectra is discussed for various system–bath couplings, temperatures, and correlation times for the bath fluctuations. The one-quantum coherence state created through the interaction with the pump laser pulse relaxes through different pathways in accordance with the mechanisms of the system–bath interactions. Relations between the present theory, Redfield theory, and stochastic theory are also discussed.

Strain in cracked AlGaN layers

The strain relaxation due to cracks of different depths in AlGaN layers grown on GaN template layers has been investigated using spatially resolved cathodoluminescence spectroscopy, high-resolution x-ray diffraction and two-dimensional finite element simulations. The experimental data consistently show that the relief of tensile stress increases with decreasing crack spacing. The measured strain profiles between the cracks are well described by the theoretical calculations for small crack spacings; whereas, deviations for larger crack spacings have been found. The latter is discussed in terms of inelastic strain relaxation mechanisms, the reliability of the deformation potential for AlGaN employed in this article, and the spatial variations in the properties of the AlGaN, e.g., its composition.

Resonance and reversibility of vibrational relaxation of HF in high temperature Ar bath gas

The Boltzmann averaged rate constants for total vibrational relaxation of HF(v=1) in collisions with Ar are computed in the range of temperatures between 100 and 1500 K. The computed rate constants overestimate the experimental measurements at high temperatures by a large factor. It is concluded that the deviation between theory and experiment cannot be explained by inaccuracy of the PES or dynamical approximations made. It is shown that increasing initial rotational energy enhances a resonant character of the vibrational energy transfer to a great extent. An assumption is made that total vibrational relaxation of HF(v=1) at high temperatures is determined by competition between vibrational relaxation to a resonant level (v=0,jres), vibrational excitation from the resonant level, and purely rotational relaxation of HF(v=0,jres). It is demonstrated that at high temperatures the latter process can be significantly slower than vibrationally inelastic transitions and rotational relaxation of HF(v=0,jres) may in fact be a rate-limiting stage of vibrational relaxation.