Fariborz Kargar

Acoustic phonon spectrum and thermal transport in nanoporous alumina arrays

We report results of a combined investigation of thermal conductivity and acoustic phonon spectra in nanoporous alumina membranes with the pore diameter decreasing from D = 180 nm to 25 nm. The samples with the hexagonally arranged pores were selected to have the same porosity ϕ ≈ 13%. The Brillouin-Mandelstam spectroscopy measurements revealed bulk-like phonon spectrum in the samples with D = 180-nm pores and spectral features, which were attributed to spatial confinement, in the samples with 25-nm and 40-nm pores. The velocity of the longitudinal acoustic phonons was reduced in the samples with smaller pores. Analysis of the experimental data and calculated phonon dispersion suggests that both phonon-boundary scattering and phonon spatial confinement affect heat conduction in membranes with the feature sizes D < 40 nm.

Direct observation of confined acoustic phonon polarization branches in free-standing semiconductor nanowires

Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries. However, despite strong scientific and practical importance, conclusive experimental evidence of confined acoustic phonon polarization branches in individual free-standing nanostructures is lacking. Here we report results of Brillouin—Mandelstam light scattering spectroscopy, which reveal multiple (up to ten) confined acoustic phonon polarization branches in GaAs nanowires with a diameter as large as 128 nm, at a length scale that exceeds the grey phonon mean-free path in this material by almost an order-of-magnitude. The dispersion modification and energy scaling with diameter in individual nanowires are in excellent agreement with theory. The phonon confinement effects result in a decrease in the phonon group velocity along the nanowire axis and changes in the phonon density of states. The obtained results can lead to more efficient nanoscale control of acoustic phonons, with benefits for nanoelectronic, thermoelectric and spintronic devices.

Variable-temperature inelastic light scattering spectroscopy of nickel oxide: Disentangling phonons and magnons

We report the results of an investigation of the temperature dependence of the magnon and phonon frequencies in NiO. A combination of Brillouin-Mandelstam and Raman spectroscopies allowed us to elucidate the evolution of the phonon and magnon spectral signatures from the Brillouin zone center (GHz range) to the second-order peaks from the zone boundary (THz range). The temperature-dependent behavior of the magnon and phonon bands in the NiO spectrum indicates the presence of antiferromagnetic (AF) order fluctuation or a persistent AF state at temperatures substantially above the Neel temperature (TN=523 K). Tuning the intensity of the excitation laser provides a method for disentangling the features of magnons from acoustic phonons in AF materials without the application of a magnetic field. Our results are useful for the interpretation of the inelastic-light scattering spectrum of NiO and add to the knowledge of its magnon properties important for THz spintronic devices.We report results of an investigation of the temperature dependence of the magnon and phonon frequencies in NiO. A combination of Brillouin - Mandelstam and Raman spectroscopies allowed us to elucidate the evolution of the phonon and magnon spectral signatures from the Brillouin zone center (GHz range) to the second-order peaks from the zone boundary (THz range). The temperature-dependent behavior of the magnon and phonon bands in the NiO spectrum indicates the presence of antiferromagnetic (AF) order fluctuation or a persistent AF state at temperatures above the Neel temperature (T=523 K). Tuning the intensity of the excitation laser provides a method for disentangling the features of magnons from acoustic phonons without the application of a magnetic field. Our results are useful for interpretation of the inelastic-light scattering spectrum of NiO, and add to the knowledge of its magnon properties important for THz spintronic devices.

Brillouin-Mandelstam spectroscopy of standing spin waves in a ferrite waveguide

This article reports results of experimental investigation of the spin wave interference over large distances in the Y3Fe2(FeO4)3 waveguide using Brillouin-Mandelstam spectroscopy. Two coherent spin waves are excited by the micro-antennas fabricated at the edges of the waveguide. The amplitudes of the input spin waves are adjusted to provide approximately the same intensity in the central region of the waveguide. The relative phase between the excited spin waves is controlled by the phase shifter. The change of the local intensity distribution in the standing spin wave is monitored using Brillouin-Mandelstam light scattering spectroscopy. Experimental data demonstrate the oscillation of the scattered light intensity depending on the relative phase of the interfering spin waves. The oscillations of the intensity, tunable via the relative phase shift, are observed as far as 7.5 mm away from the spin-wave generating antennas at room temperature. The obtained results are important for developing techniques fo...

Spin-phonon coupling in antiferromagnetic nickel oxide

We report the results of ultraviolet Raman spectroscopy of NiO, which allowed us to determine the spin-phonon coupling coefficients in this important antiferromagnetic material. The use of the second-order phonon scattering and ultraviolet laser excitation (λ = 325 nm) was essential for overcoming the problem of the optical selection rules and dominance of the two-magnon band in the visible Raman spectrum of NiO. We established that the spins of Ni atoms interact more strongly with the longitudinal than transverse optical phonons and produce opposite effects on the phonon energies. The peculiarities of the spin-phonon coupling are consistent with the trends given by density functional theory. The obtained results shed light on the nature of the spin-phonon coupling in antiferromagnetic insulators and can help in developing spintronic devices.

Acoustic phonon spectrum engineering in bulk crystals via incorporation of dopant atoms

We report results of Brillouin—Mandelstam spectroscopy of transparent Al2O3 crystals with Nd dopants. The ionic radius and atomic mass of Nd atoms are distinctively different from those of the host Al atoms. Our results show that even a small concentration of Nd atoms incorporated into the Al2O3 samples produces a profound change in the acoustic phonon spectrum. The velocity of the transverse acoustic phonons decreases by ∼600 m/s at the Nd density of only ∼0.1%. Interestingly, the decrease in the phonon frequency and velocity with the doping concentration is non-monotonic. The obtained results, demonstrating that modification of the acoustic phonon spectrum can be achieved not only by traditional nanostructuring but also by low-concentration doping, have important implications for thermal management as well as thermoelectric and optoelectronic devices.

Thermal Percolation Threshold and Thermal Properties of Composites with High Loading of Graphene and Boron Nitride Fillers

We investigated thermal properties of the epoxy-based composites with the high loading fraction-up to f ≈ 45 vol %-of the randomly oriented electrically conductive graphene fillers and electrically insulating boron nitride fillers. It was found that both types of the composites revealed a distinctive thermal percolation threshold at the loading fraction fT > 20 vol %. The graphene loading required for achieving thermal percolation, fT, was substantially higher than the loading, fE, for electrical percolation. Graphene fillers outperformed boron nitride fillers in the thermal conductivity enhancement. It was established that thermal transport in composites with high filler loadings, f ≥ fT, is dominated by heat conduction via the network of percolating fillers. Unexpectedly, we determined that the thermal transport properties of the high loading composites were influenced strongly by the cross-plane thermal conductivity of the quasi-two-dimensional fillers. The obtained results shed light on the debated mechanism of the thermal percolation, and facilitate the development of the next generation of the efficient thermal interface materials for electronic applications.

A comparative study of the thermal interface materials with graphene and boron nitride fillers

We report the results of an experimental study that compares the performance of graphene and boron nitride flakes as fillers in the thermal interface materials. The thickness of both fillers varied from a single atomic plane to about a hundred. The measurements have been conducted using a standard TIM tester. Our results show that the addition of a small fraction of graphene (f=4 wt%) to a commercial thermal interface material increases the resulting apparent thermal conductivity substantially stronger than the addition of boron nitride. The obtained data suggest that graphene and fewlayer graphene flakes couple better to the matrix materials than the boron nitride fillers. A combination of both fillers can be used to increase the thermal conductivity while controlling the electrical conduction.