Direct Phonon Research Articles

Optical decoherence and energy level structure of 0.1%Tm3+:LiNbO3

We report the energy level structure of the ${}^{3}$H${}_{6}$ and ${}^{3}$H${}_{4}$ multiplets for Tm${}^{3+}$ doped congruent LiNbO${}_{3}$, as well as the decoherence properties and their temperature dependencies for the ${}^{3}$H${}_{6}(1){\ensuremath{\leftrightarrow}}^{3}$H${}_{4}(1a)$ transition at 794 nm. It is shown that this material provides very significant improvements in bandwidth, time-bandwidth product, and sensitivity for spatial-spectral holographic signal processing devices and quantum memories based on spectral hole burning. The available signal processing bandwidth for 0.1$%$Tm${}^{3+}$:LiNbO${}_{3}$ is 300 GHz versus 20 GHz for Tm${}^{3+}$YAG. The peak absorption coefficient for 0.1$%$Tm${}^{3+}$:LiNbO${}_{3}$ is 15 cm${}^{\ensuremath{-}1}$ at 794.5 nm compared with 1.7 cm${}^{\ensuremath{-}1}$ for 0.1$%$Tm:YAG at 793 nm, and the total absorption strength is eighty times stronger. The oscillator strength for Tm${}^{3+}$:LiNbO${}_{3}$ is about twenty-five times larger than that for Tm${}^{3+}$:YAG, making the material five times more sensitive for processing high-bandwidth analog signals. The homogeneous linewidth, which determines processing time or spectrum analyzer resolution, is 30 kHz at 1.6 K and 350 kHz at 6 K, as measured by photon echoes. Those values establish potential time-bandwidth products of 10${}^{7}$ and 7 $\ifmmode\times\else\texttimes\fi{}$10${}^{5}$, respectively. The temperature dependence of the homogeneous linewidth was explained by observation of a 7.8 cm${}^{\ensuremath{-}1}$ crystal field level in the ground multiplet and direct phonon coupling. The excited state ${}^{3}$H${}_{4}$ lifetime ${T}_{1}$ is 152 $\ensuremath{\mu}$s and the bottleneck lifetime of the lowest ${}^{3}$F${}_{4}$ level is 7 ms from photon echo measurements. These factors combine to provide a surprisingly large increase in key parameters that determine material performance for spatial-spectral holography, quantum information, and other spectral hole burning applications.

Dynamic response of graphene to thermal impulse

A transient molecular dynamics technique is developed to characterize the thermophysical properties of two-dimensional graphene nanoribbons (GNRs). By directly tracking the thermal-relaxation history of a GNR that is heated by a thermal impulse, we are able to determine its thermal diffusivity quickly and accurately. We study the dynamic thermal conductivity of various length GNRs of 1.99 nm width. Quantum correction is applied in all of the temperature calculations and is found to have a critical role in the thermal-transport study of graphene. The calculated specific heat of GNRs agrees well with that of graphite at 300.6 and 692.3 K, showing little effect of the unique graphene structure on its ability to store thermal energy. A strong size effect on GNR's thermal conductivity is observed and its theoretical values for an infinite-length limit are evaluated by data fitting and extrapolation. With infinite length, the 1.99-nm-wide GNR has a thermal conductivity of 149 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$ at 692.3 K, and 317 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$ at 300.6 K. Our study of the temperature distribution and evolution suggests that diffusive transport is dominant in the studied GNRs. Non-Fourier heat conduction is observed at the beginning of the thermal-relaxation procedure. Thermal waves in GNR's in-plane direction are observed only for phonons in the flexural direction (ZA mode). The observed propagation speed ($c$ = 4.6 km s${}^{\ensuremath{-}1}$) of the thermal wave follows the relation of $c={v}_{g}/\sqrt{2}$ (${v}_{g}$ is the ZA phonon group velocity). Our thermal-wave study reveals that in graphene, the ZA phonons transfer thermal energy much faster than longitudinal (LA) and transverse (TA) modes. Also, ZA\ensuremath{\leftrightarrow}ZA energy transfer is much faster than the ZA\ensuremath{\leftrightarrow}LA/TA phonon energy transfer.

Direct acoustic phonon excitation by intense and ultrashort terahertz pulses

We report on the direct and resonant excitation of acoustic phonons in an AlGaAs intrinsic semiconductor using intense coherent and single cycle terahertz pulses created by two-color femtosecond laser pulse filamentation in air. While the electrons are left unperturbed, we follow the lattice dynamics with time-delayed optical photons tuned to the interband transition.

Phonon spectroscopy through the electronic density of states in graphene

We study how phonon structure manifests itself in the electronic density of states of graphene. A procedure for extracting the value of the electron-phonon renormalization $\lambda$ is developed. In addition, we identify direct phonon structures. With increasing doping, these structures, along with $\lambda$, grow in amplitude and no longer display particle-hole symmetry.

Experimental determination of the elasticity of iron at high pressure

We present a multitechnique approach to experimentally determine the elastic anisotropy of polycrystalline hcp Fe at high pressure. Directional phonon measurements from inelastic X‐ray scattering on a sample with lattice preferred orientation at 52 GPa in a diamond anvil cell were coupled with X‐ray diffraction data to determine the elastic tensor. Comparison of the results from this new method with the elasticity determined by lattice strain analysis of radial X‐ray diffraction measurements showed significant differences, highlighting the importance of strength anisotropy in hcp Fe. At 52 GPa, we found that a method which combines results from inelastic scattering and pressure‐volume measurements gives a shape in the velocity anisotropy close to sigmoidal (with a faster c and slower a axis) a smaller magnitude in the anisotropy and compared to velocities based on the lattice strain method which gives a bell shape velocity distribution with the fast direction between the c and a axes. We used additional results from nuclear resonant inelastic X‐ray scattering to constrain errors and provide additional validation of the accuracy of our results.

Mechanisms causing thermal rectification: The influence of phonon frequency, asymmetry, and nonlinear interactions

We have determined the mechanisms leading to thermal rectification by applying the nonequilibrium Green's function method to study thermal transport through a one-dimensional asymmetric lattice. Thermal rectification occurs in the presence of nonlinear interactions and a nonuniform phonon frequency distribution. We show that thermal rectification results from the contribution of high-frequency phonons; a thermal rectifier can thus be realized by filtering out high-frequency phonons in a single direction. Using these results, we present methods of increasing the level of rectification, and also discuss possible limitations on thermal rectifier performance.

Ca ( Al H 4 ) 2 , CaAlH5, and CaH2+6LiBH4: Calculated dehydrogenation enthalpy, including zero point energy, and the structure of the phonon spectra

The dehydrogenation enthalpies of Ca(AlH(4))(2), CaAlH(5), and CaH(2)+6LiBH(4) have been calculated using density functional theory calculations at the generalized gradient approximation level. Harmonic phonon zero point energy (ZPE) corrections have been included using Parlinski's direct method. The dehydrogenation of Ca(AlH(4))(2) is exothermic, indicating a metastable hydride. Calculations for CaAlH(5) including ZPE effects indicate that it is not stable enough for a hydrogen storage system operating near ambient conditions. The destabilized combination of LiBH(4) with CaH(2) is a promising system after ZPE-corrected enthalpy calculations. The calculations confirm that including ZPE effects in the harmonic approximation for the dehydrogenation of Ca(AlH(4))(2), CaAlH(5), and CaH(2)+6LiBH(4) has a significant effect on the calculated reaction enthalpy. The contribution of ZPE to the dehydrogenation enthalpies of Ca(AlH(4))(2) and CaAlH(5) calculated by the direct method phonon analysis was compared to that calculated by the frozen-phonon method. The crystal structure of CaAlH(5) is presented in the more useful standard setting of P2(1)c symmetry and the phonon density of states of CaAlH(5), significantly different to other common complex metal hydrides, is rationalized.

Phonon bottleneck in the low-excitation limit

The phonon-bottleneck problem in the relaxation of two-level systems (spins) via direct phonon processes is considered numerically in the weak-excitation limit where the Schroedinger equation for the spin-phonon system simplifies. The solution for the relaxing spin excitation p(t), emitted phonons n_k(t), etc. is obtained in terms of the exact many-body eigenstates. In the absence of phonon damping Gamma_{ph} and inhomogeneous broadening, p(t) approaches the bottleneck plateau p_\infty > 0 with strongly damped oscillations, the frequency being related to the spin-phonon splitting Delta at the avoided crossing. For any Gamma_{ph} > 0 one has p(t) -> 0 but in the case of strong bottleneck the spin relaxation rate is much smaller than Gamma_{ph} and p(t) is nonexponential. Inhomogeneous broadening exceeding Delta partially alleviates the bottleneck and removes oscillations of p(t). The line width of emitted phonons, as well as Delta, increase with the strength of the bottleneck, i.e., with the concentration of spins.

The fabrication and characterization of granular aluminium/palladium bilayer microbolometers

Superconducting granular aluminium/palladium bilayer microbolometers with subnanosecond response can be easily fabricated using an all lift-off process with image-reversal lithography. The palladium capping layer allows for convenient and reproducible Ohmic contacts to be made to the device. We report their fabrication, resistance–current calibration, responsivity and response time. Finally, a deconvolution algorithm is used to improve the time resolution to approximately 1 ns while obtaining the incident photon and phonon fluxes in both direct photoexcitation and phonon heat pulse experiments.

Dynamics of quasi-1D topological soliton in 2D strongly anisotropic crystal

We investigate a 2D strongly anisotropic crystal: 2D system of coupled linear chains of particles with strong intrachain and weak interchain interactions: a generalization of the Frenkel–Kontorova (FK) model giving better approximation to polymer crystals consisting of all moving chains. We study shape and dynamics of topological solitons located mainly on one chain of the crystal. It turns out that at weak interactions between chains there exist exact propagating soliton-like solutions. We have found that the upper boundary of their velocity spectrum is less than unity (the upper boundary of soliton velocity spectrum in the corresponding FK model). The reason of this new restriction is appearance of Cherenkov-type radiation emitted by the kinks moving at higher velocities. In the case of stronger interchain interactions the exact soliton-like solutions are absent. Kinks emit phonons at any velocity. This emission originates from resonances between the kink and phonon modes touched in the second or the third Brillouin zones and so this effect is a pure result of discreteness of the crystal. Deceleration of kink falls exponentially with its velocity (as in the FK model) at medium velocities and is nonzero constant at low velocities (contrary to the FK model where deceleration at low velocities is zero), so at all velocities there is no generally expected direct proportion between deceleration and velocity. Spatial pattern of phonons emitted proves to be in accordance with the conception that kink emits packets of corresponding resonant phonons in direction of their group velocities.

11B nuclear magnetic resonance study of boron nitride nanotubes prepared by mechano-thermal method

We reported 11B nuclear magnetic resonance studies of boron nitride (BN) nanotubes prepared by mechano-thermal route. The NMR lineshape obtained at 192.493 MHz (14.7 T) was fitted with two Gaussian functions, and the 11B nuclear magnetization relaxations were satisfied with the stretched–exponential function, exp [ - ( t / T 1 ) ( D + 1 ) / 6 ] ( D: space dimension) at all temperatures. In addition, the temperature dependence of spin–lattice relaxation rates was well described by T 1 − 1 = a T ( a: constant, T: temperature) and could be understood in terms of direct phonon process. All the 11BNMR results were explained by considering the inhomogeneous distribution of the paramagnetic metal catalysts, such as α-Fe, Fe–N, and Fe 2 B, that were incorporated during the process of high-energy ball milling of boron powder and be synthesized during subsequent thermal annealing. X-ray powder diffraction as well as electron paramagnetic resonance (EPR) on BN nanotubes were also conducted and the results obtained supported these conclusions.

Phonon drag thermopower in doped bismuth

The low-temperature (2<T<80 K) thermopower in bismuth doped by tellurium, a donor impurity (0<c≤0.07 at. % Te), is dominated by the phonon component, which shifts to higher temperatures with increasing dopant concentration. The temperature and concentration dependences of the phonon thermopower of doped bismuth are satisfactorily described by the theory of phonon drag of electrons. The theory is developed for a strongly anisotropic electron spectrum and includes both direct and two-step phonon drag.

Adiabatic Landau-Zener-Stückelberg transition with or without dissipation in the low-spin molecular systemV15

The spin one half molecular system V15 shows no barrier against spin reversal. This makes possible direct phonon activation between the two levels. By tuning the field sweeping rate and the thermal coupling between sample and thermal reservoir we have control over the phonon-bottleneck phenomena previously reported in this system. We demonstrate adiabatic motion of molecule spins in time dependent magnetic fields and with different thermal coupling to the cryostat bath. We also discuss the origin of the zero-field tunneling splitting for a half-integer spin.

Decoherence of electron spin qubits in Si-based quantum computers

Direct phonon spin-lattice relaxation of an electron qubit bound by a donor impurity or quantum dot in SiGe heterostructures is investigated. The aim is to evaluate the importance of decoherence from this mechanism in several important solid-state quantum computer designs operating at low temperatures. We calculate the relaxation rate $1/T_1$ as a function of [100] uniaxial strain, temperature, magnetic field, and silicon/germanium content for Si:P bound electrons. The quantum dot potential is much smoother, leading to smaller splittings of the valley degeneracies. We have estimated these splittings in order to obtain upper bounds for the relaxation rate. In general, we find that the relaxation rate is strongly decreased by uniaxial compressive strain in a SiGe-Si-SiGe quantum well, making this strain an important positive design feature. Ge in high concentrations (particularly over 85%) increases the rate, making Si-rich materials preferable. We conclude that SiGe bound electron qubits must meet certain conditions to minimize decoherence but that spin-phonon relaxation does not rule out the solid-state implementation of error-tolerant quantum computing.

Magnetostriction and angular dependence of ferromagnetic resonance linewidth in Tb-doped Ni0.8Fe0.2 thin films

We present the dependence of the magnetostriction in Ni0.8Fe0.2 films on Tb and Gd doping concentration and compare with the measured doping dependence of the high-frequency damping. While the magnetostriction and the high-frequency damping are correlated, the dependence is complicated. In particular, the high-frequency damping parameter α increases rapidly (α=0.008–0.84) with a modest increase in the magnetostriction (λs=−0.6×10−6 to 5.7×10−6) for Tb doping concentrations up to 10%. For Gd doping, the high-frequency damping changes slowly (α=0.008–0.02) as the doping concentration is increased to 10%, whereas the increase in magnetostriction is similar to that observed in the Tb-doped films. Further, it is possible to achieve low magnetostriction (λs=2×10−6) near the region of critical damping. Measurements of the angular dependence of the ferromagnetic resonance linewidth in Tb-doped Ni0.8Fe0.2 films show little change similar to the behavior observed in undoped Ni0.8Fe0.2 films, although the linewidths are considerably larger. This is in contrast to systems such as Ni0.8Fe0.2 on NiO, which have a large angular dependence indicating that the relaxation process proceeds through the generation of spin waves. The enhanced damping in the Tb-doped films appears, therefore, to be mediated through direct phonon generation.

Dynamics of wild-type HiPIPs: a Cys77Ser mutant and a partially unfolded HiPIP.

The temperature dependence of the mean square displacement of the (57)Fe nuclei due to motion faster than 100 ns are measured by temperature-dependent Mössbauer spectroscopy for oxidized and reduced HiPIPs from Ectothiorhodospira halophila, Chromatium vinosum WT and a Cys77Ser mutant. The behaviour is interpretable in the frame of the general model of protein dynamics distinguishing two temperature intervals. The character of harmonic and quasi-diffusional modes in HiPIPs is discussed. Dynamic information obtained from Mössbauer spectroscopy and Fe K-edge EXAFS are compared. Structure dynamics of the iron-sulfur cluster in the partially unfolded reduced HiPIP from C. vinosum was investigated by Mössbauer spectroscopy and EXAFS, indicating an intact metal centre and a protein backbone with a largely collapsed secondary structure. The role of the cofactor during protein folding is discussed. Differences in the dynamics between the native protein and the molten globule are found at physiological temperatures only. The structure and dynamic behaviour of the [Fe(4)S(4)]Cys(3)Ser cluster in the Cys77Ser mutant of the HiPIP from C. vinosum are analysed. The temperature dependence of electron relaxation in oxidized HiPIPs is investigated by Mössbauer spectroscopy and analysed theoretically, considering spin-spin and spin-lattice relaxation. The latter consists of contributions from direct phonon bottleneck and Orbach mechanisms. The data agree with former pulsed EPR results. Orbach relaxation is interpreted as due to transitions between electronic isomers of oxidized HiPIPs. With this interpretation, the energetic difference between both isomers equals the energy gap estimated from the temperature dependence of the Orbach relaxation.

Absorption of nonequilibrium acoustic phonons by low-mobility electrons in GaN

We report direct phonon absorption experiments in n-type GaN epitaxial layers grown by molecular beam epitaxy. Nonequilibrium phonons with characteristic energies up to 15 meV (3.5 THz) are injected from a constantan heater. They propagate ballistically through the sapphire substrate, reach the GaN layer, and are absorbed by the degenerate three-dimensional electron gas. The phonon absorption is studied as a function of heater temperature through the effects of phonon induced changes of the device resistivity. The experimental results lead us to the conclusion that in low-mobility GaN, the momentum conservation cutoff for electron-phonon transitions is shifted to higher energy than predicted by standard theory.

Optical band gap in tungsten diselenide single crystals intercalated by indium

Optical absorption in single crystals of In x WSe 2 ( x=0, 0.33, 0.50) has been measured at room temperature near the fundamental absorption edge. Results have been analyzed on the basis of two- and three-dimensional models. Both direct and indirect transitions are involved in the absorption process. The indirect transition was found to be allowed with two phonons involved in the process. The direct and indirect energy gaps and phonon energies for all the crystals have been estimated. The three-dimensional model and not the two-dimensional model could be used to describe the optical properties of In x WSe 2 single crystals.

Phonon group velocity and thermal conduction in superlattices

With the use of a face-centered cubic model of lattice dynamics we calculate the group velocity of acoustic phonons in the growth direction of periodic superlattices. Comparing with the case of bulk solids, this component of the phonon group velocity is reduced due to the flattening of the dispersion curves associated with Brillouin-zone folding. The results are used to estimate semiquantitatively the effects on the lattice thermal conductivity in Si/Ge and GaAs/AlAs superlattices. For a Si/Ge superlattice an order of magnitude reduction is predicted in the ratio of superlattice thermal conductivity to phonon relaxation time [consistent with the results of P. Hyldgaard and G. D. Mahan, Phys. Rev. B 56, 10 754 (1997)]. For a GaAs/AlAs superlattice the corresponding reduction is rather small, i.e., a factor of 2--3. These effects are larger for the superlattices with larger unit period, contrary to the recent measurements of thermal conductivity in superlattices.

Direct phonon transmission across wafer-bonded crystals

Several factors play an important role in reducing the transport of phonons across a solid–solid interface: the largest factors are the presence of subsurface disorder and imperfect physical contact between the solids. When bonding crystals, micrometer size contaminants of the surface can easily cause voids at the interface which strongly scatter phonons. Recent improvements in wafer-bonding techniques, however, have produced extremely clean interfaces with strong phonon transmission. We have studied such a GaAs–GaAs interface using the techniques of phonon imaging. We observe unprecedented phonon transmission through these bonded samples. The high quality of these interfaces makes them suitable for a variety of phonon applications, including potential phonon optics.