Phonon Echoes Research Articles

Generation of Longitudinal Ultrasound by Shear Strain Pulses in the Spin–Phonon Echo Regime

The possibility of generation of longitudinal ultrasonic pulses of gigahertz frequencies in the spin–phonon echo regime by acting on a magnetized isotropic paramagnet with nanosecond shear strain pulses is predicted. The frequency of generated longitudinal ultrasonic echo signals is twice as high as the frequency of pulses entered into the medium. This effect can be realized when the velocity of longitudinal ultrasound exceeds the velocity of shear strain waves by less than two times. This resonant mechanism of the pulsed regime of second harmonic generation, which is accompanied by a nonlinear transformation of transverse into longitudinal ultrasound, has no optical analog.

Generation of Longitudinal Ultrasound by Shear Strain Pulses in the Spin–Phonon Echo Regime

Generation of Longitudinal Ultrasound by Shear Strain Pulses in the Spin–Phonon Echo Regime

Echoes from anharmonic normal modes in model glasses.

Glasses display a wide array of nonlinear acoustic phenomena at temperatures T ≲ 1 K. This behavior has traditionally been explained by an ensemble of weakly coupled, two-level tunneling states, a theory that is also used to describe the thermodynamic properties of glasses at low temperatures. One of the most striking acoustic signatures in this regime is the existence of phonon echoes, a feature that has been associated with two-level systems with the same formalism as spin echoes in NMR. Here we report the existence of a distinctly different type of acoustic echo in classical models of glassy materials. Our simulations consist of finite-ranged, repulsive spheres and also particles with attractive forces using Lennard-Jones interactions. We show that these echoes are due to anharmonic, weakly coupled vibrational modes and perhaps provide an alternative explanation for the phonon echoes observed in glasses at low temperatures.

Phonon echo in high-temperature superconductors as a nonlinear magnetoacoustic phenomenon

The nonlinear behavior of a phonon echo signal in high-temperature superconductors is studied experimentally. It is demonstrated that conventional consideration of anharmonic effects is insufficient for comprehensive explanation of the echo signal behavior. It is shown that the nonlinear properties of the phonon echo should be taken into account when the phonon echo is used as a means for studying superconductor parameters. An increase in the elastic nonlinearity in iron-doped yttrium ceramics is found.

Phonon echo probing of high-temperature superconductors containing magnetic ions

Polycrystalline high-temperature superconductors (HTSCs) containing iron and gadolinium ions have been studied using the phonon echo technique, whereby a powdered sample is probed by a special sequence of RF pulses and the response having an acoustic nature is measured. It is shown that the behavior of this signal in the iron-containing HTSC materials exhibits some distinctive features in comparison to the response of a conventional yttrium-based ceramic. The superconducting gap estimated for the samples with magnetic ions does not differ significantly from the value for a usual HTSC. This fact is related to a spatial separation of the magnetic and superconducting phases in the materials studied.

Phonon Echo Study on Lithium Ionic Diffusion in LiNbO3 Powder

Two-pulse phonon echo measurements were performed from room temperature to 900 K in micro particles of polycrystalline and single crystalline LiNbO3 to investigate the relationship between lithium ionic diffusion and decay time, T2, in a phonon echo study. Decay time was dependent on temperature, showing a rapid decrease above 800 K in the polycrystalline sample. The origin of this anomaly is interpreted in terms of a simple Debye-type relaxation due to lithium ionic diffusion. The activation energy, which was evaluated as 0.93 eV in the polycrystalline samples, was comparable with reported values from ionic conductivity and NMR studies. On the other hand, single crystalline LiNbO3 showed no signs of lithium ionic diffusion up to 900 K. The contribution to T2 from lithium ionic diffusion would be dominant at higher temperatures than 900 K because of the much higher activation energy in a single crystalline sample. It is shown that phonon echo decay time is strongly connected with ionic diffusion in the particles.

Phonon Echo and Ionic Diffusion in Powdered Superionic Conductor

Phonon Echo and Ionic Diffusion in Powdered Superionic Conductor

B-2 Observation of ionic diffusion in LiNbO_3 by phonon echo method

B-2 Observation of ionic diffusion in LiNbO_3 by phonon echo method

Optical transitions of liquid phase epitaxy grown (Ga,Mn)As in magnetic fields

Low temperature (4.0 K) photoluminescence measurements of liquid phase epitaxy grown (Ga,Mn)As epilayers show distinctive donor–acceptor pair transitions. Below 0.3% Mn molar fractions, only one broad transition and its phonon echo were observed. However, exceeding 0.3%, three peaks at 1.409, 1.408 and 1.400 eV labeled as H, M and L, respectively, of acceptor related transitions were observed along with their phonon echoes. In the presence of magnetic field, the transitions are highly polarized to σ+ polarization and H peak shows free-carrier-like transition behavior whereas M and L peaks exhibit conventional donor–acceptor bound exciton transitions.

Acoustic solitons, phonon echoes, and sound amplification in Si:B at very low temperatures

Phonon echoes, sound amplification, and acoustic solitons are observed in Si:B at temperatures below about 25 mK depending on the density of states of two-level systems associated with the acceptor ground state. We report on the influence of acceptor concentration, external stress and magnetic field and we compare our results with those associated with the two-level systems in glasses.

Phonon echo in hydrogen-containing metals

The phonon echo due to the tunneling of the hydrogen atom between two equilibrium positions in hydrogen-containing niobium NbOxHy is investigated. The time dependence of the intensity and shape of the two-pulse echo signal is obtained.

Piezoelectric and pyroelectric properties of protein amino acids as basic materials of Soft State Physics

There are claims in the scientific literature that many biological materials are ferroelectrics, and that ferroelectricity, along with piezo-and pyroelectricity, are a fundamental property of biological materials. The best way to tackle this general problem is to start from the very beginning, namely from protein amino acids (20 in number) which are the basic elements in biology (basic elements of Soft State Physics). Dielectric, piezoelectric and pyroelectric properties of protein amino acids are discussed. As an example, electromechanical behavior of glycine and phonon echo experiments in L-alanine are considered.

Radio-frequency acoustic powder echoes in niobium: observation of both echoes in superconducting and normal states

Field dependence of intensity of radio-frequency acoustic powder echoes (phonon echoes) is studied for powdered samples of niobium at low temperatures. Both echoes inherent to the superconducting state of niobium and echoes in the normal state are successively observed with increasing applied field across the critical field H C2, proving the validity of our model presented before.

Quantum beats of nuclear exciton decay radiation probing single crystal Bragg scattering

The time structure and high brilliance of pulsed synchrotron radiation sources stimulate new experiments in the field of nuclear resonance fluorescence from an almost instantaneously excited nuclear ensemble. The decay radiation of this so-called nuclear exciton was used for scattering experiments from samples not containing resonance nuclei in order to study the interaction with condensed matter in the time domain. Ultrasound induced coherent rephasing of the decay radiation from two nuclear absorber targets caused phonon echo peaks and dips. Quantum beats result as a consequence of coherent (Bragg) scattering and superposition of nuclear excitons from two distant absorbers with Doppler shifted resonance energies between each other. In the case of inelastic scattering these quantum beats concentrate on the times of phonon induced echoes proving experimentally a periodic coherent rephasing of the involved decay radiation. In this article the theoretical background of such experiments will be outlined together with realistic numerical modelling of the scattering experiments. A comparison with our experimental work is also incorporated.

Phonon echo in L-alanine

Phonon echo signals have been observed in a finely-dispersed crystalline powder of the L-alanine amino acid. Measurements of the relaxation time T2 have revealed a phase transition in L-alanine crystals at a temperature of about 170 K.

New Phonon Echo-Signals Due to the Two-Level Systems in Glasses

A theory of the microwave tunnel phonon echo excited by long acoustic pulses in glass is developed. We take into account a large broadening of the two-level system (TLS) energy levels, a broad distribution of glassy parameters through their averaging with some distribution functions, TLS potential asymmetry as well as a tunneling parameter, dipole–dipole interaction between TLS and echo decay. For the first time new subsidiary phonon echo signals excited by long acoustic pulses are found. New echoes appear due to the asymmetry of the TLS potential. These signals are less intensive than basic echo. They have unusual orientational dependence. The intensities, form and moments of generation of echo signals are calculated. It is shown that the broad distribution of glassy parameters results in the change of echo amplitude, echo form and echo decay character. The amplitude and form of echoes in glass depend on the amplitude and length of external exciting pulses. The delay effects in time moments of echo generation are found. The estimations shown a possibility for the experimental display of the basic and subsidiary echo signals. A corresponding experiment relative to observing of the new echoes is proposed.

Echo in optical lattices: Stimulated revival of breathing oscillations

We analyze a stimulated revival (echo) effect for the breathing modes of the atomic oscillations in optical lattices. The effect arises from the dephasing due to the weak anharmonicity being partly reversed in time by means of additional parametric excitation of the optical lattice. The shape of the echo response is obtained by numerically simulating the equation of motion for the atoms with subsequent averaging over the thermal initial conditions. A qualitative analysis of the phenomenon shows that the suggested echo mechanism combines the features of both spin and phonon echoes.

Acoustic study of tunneling states in Kramers-ion-doped glasses: Wave propagation and phonon echoes.

We report on an acoustic study of insulating glasses doped with dysprosium, gadolinium, and europium ions, which have Kramers degeneracy. The magnetic-ion content is varied between 1 and 10 at. %. The measurements are performed down to 10 mK, using acoustic frequencies up to 900 MHz and in magnetic fields up to 50 kG. There is an effect of the magnetic-ion concentration and the magnetic field on the sound velocity at low temperature. This effect is explained by assuming the existence of two kinds of low-energy excitations: the usual elastic tunneling states and some magnetic tunneling states, which are magnetic moments whose local anisotropy axis is almost perpendicular to the local magnetic field. These magnetic tunneling states have relaxation times shorter than the elastic ones by several orders of magnitude at low temperature. They couple strongly to the latter whose dynamics is then drastically changed. This one is directly observed in saturation-recovery and phonon-echo experiments. The relaxation time ${\mathit{T}}_{1}$ of the elastic tunneling states is strongly shortened by the magnetic ions, whereas their dephasing time ${\mathit{T}}_{2}$ remains almost unchanged. This effect implies a relaxation process much more efficient than the usual one-phonon process at low temperature.

Phonon echoes in a dysprosium-doped glass

Spontaneous phonon echoes in dysprosium-doped glasses have been studied. A non-exponential decay of the echo is reported. It is explained by assuming the existence of tunneling states with very short relaxation times which attenuate strongly the phonon echo generated by the standard tunneling states. This behaviour is assigned to the Kramers ions which interact strongly with some tunneling states. The results of a hole-burning experiment are reported and the effect of a magnetic field is also considered.

Harmonic analysis of phonon echoes in liquids

We present, for the first time, a study comparing phonon quench echoes in supercooled liquid argon with a normal mode analysis of the echoes. The phonon echoes are produced via molecular dynamics simulation. We model the echoes by considering a collection of harmonic oscillators whose frequency distribution is defined by the density of normal modes in the liquid under consideration. We find for times less than 300 fs, the fluid behaves in a relatively harmonic manner. We also find that low temperatures and short times are best suited to harmonic analysis. We comment on the agreement and deviation between simulation and model particularly as dephasing processes are concerned. Future approaches to this phenomenon are addressed as well.