Thermal boundary resistance from transient nanocalorimetry: a multiscale modeling approach

The Thermal Boundary Resistance at the interface between a nanosized Al film and an Al_{2}O_{3} substrate is investigated at an atomistic level. A room temperature value of 1.4 m^{2}K/GW is found. The thermal dynamics occurring in time-resolved thermo-reflectance experiments is then modelled via macro-physics equations upon insertion of the materials parameters obtained from atomistic simulations. Electrons and phonons non-equilibrium and spatio-temporal temperatures inhomo- geneities are found to persist up to the nanosecond time scale. These results question the validity of the commonly adopted lumped thermal capacitance model in interpreting transient nanocalorimetry experiments. The strategy adopted in the literature to extract the Thermal Boundary Resistance from transient reflectivity traces is revised at the light of the present findings. The results are of relevance beyond the specific system, the physical picture being general and readily extendable to other heterojunctions.Comment: 12 pages, 8 figure

Non-linearity observed in the direct sub-ps photoemission regime in Mo

The total charge emitted from a polycrystalline Mo surface by 500 fs-264 nm laser pulses has been measured. Though a one-photon photoelectric effect is expected, a non-linear increase of the photoelectric yield was observed as a function of laser peak intensity, confirming earlier observations on Au, W and Zr. The threshold intensity for this non-linearity is 2 between 0.1 and 0.2 GW/ cm . The linear and non-linear regimes were clearly discerned in the experimental data. The non-equilibrium heating of the conduction electrons is considered as the cause of the observed non-linear behaviour. © 1999 Elsevier Science B.V. All rights reserved

Linear and nonlinear total-yield photoemission observed in the subpicosecond regime in Mo

The total charge emitted from a polycrystalline Mo sample by 500 fs laser pulses at normal incidence is measured as a function of the laser peak intensity. Total yield data are taken at wavelengths of 527 and 264 nm. In both cases, a nonlinearity higher than expected is measured. A thermally enhanced regime is clearly observed when using 264 nm pulses for laser peak intensity larger than 0.1--0.2 ${\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$. This effect is interpreted on the basis of the nonequilibrium heating of the conduction electrons. Pump and probe photoemission data at 527 nm show a significant enhancement of the photoelectric sensitivity when the probe pulse is delayed by 1 ps from the pump. This enhancement is related to the growth of the available electron density induced by the nonequilibrium heating. Single pulse photoemission at this wavelength is not properly explained by a thermally assisted photoemission regime. This may indicate that other processes have a role in determining the photoemission yield

Temperature dependence of the thermal boundary resistivity of glass-embedded metal nanoparticles

The temperature dependence of the thermal boundary resistivity is investigated in glass-embedded Ag particles of radius 4.5 nm, in the temperature range from 300 to 70 K, using all-optical time-resolved nanocalorimetry. The present results provide a benchmark for theories aiming at explaining the thermal boundary resistivity at the interface between metal nanoparticles and their environment, a topic of great relevance when tailoring thermal energy delivery from nanoparticles as for applications in nanomedicine and thermal management at the nanoscaleComment: 4 pages, 3 figure

Tracking local magnetic dynamics via high-energy charge excitations in a relativistic Mott insulator

We use time- and energy-resolved optical spectroscopy to investigate the coupling of electron-hole excitations to the magnetic environment in the relativistic Mott insulator Na$_2$IrO$_3$. We show that, on the picosecond timescale, the photoinjected electron-hole pairs delocalize on the hexagons of the Ir lattice via the formation of quasi-molecular orbital (QMO) excitations and the exchange of energy with the short-range-ordered zig-zag magnetic background. The possibility of mapping the magnetic dynamics, which is characterized by typical frequencies in the THz range, onto high-energy (1-2 eV) charge excitations provides a new platform to investigate, and possibly control, the dynamics of magnetic interactions in correlated materials with strong spin-orbit coupling, even in the presence of complex magnetic phases.Comment: 5 pages, 4 figures, supplementary informatio

Strong enhancement of d-wave superconducting state in the three-band Hubbard model coupled to an apical oxygen phonon

We study the hole binding energy and pairing correlations in the three-band Hubbard model coupled to an apical oxygen phonon, by exact diagonalization and constrained-path Monte Carlo simulations. In the physically relevant charge-transfer regime, we find that the hole binding energy is strongly enhanced by the electron-phonon interaction, which is due to a novel potential-energy-driven pairing mechanism involving reduction of both electronic potential energy and phonon related energy. The enhancement of hole binding energy, in combination with a phonon-induced increase of quasiparticle weight, leads to a dramatic enhancement of the long-range part of d-wave pairing correlations. Our results indicate that the apical oxygen phonon plays a significant role in the superconductivity of high-$T_c$ cuprates.Comment: 5 pages, 5 figure

Quasi-particles dynamics in underdoped Bi2212 under strong optical perturbation.

In this work an optical pump-probe set-up is used to study the photo-induced non-equilibrium dynamics of a superconducting underdoped Bi2212 single crystal in a strong excitation regime (10<<600 \ub5J/cm2). The use of a tunable repetition rate 120 fs pulsed laser source allows us to avoid significant average heating of the sample and to optimize the signal-to-noise ratio in the detection of the transient reflectivity variation. A discontinuity of the transient reflectivity is observed at high excitation intensities (~70 \ub5J/cm2). Numerical simulations of the heat diffusion problem indicate that, in this regime, the local temperature of the sample is lower than TC, confirming the impulsive nature of this phenomenon. The quasi-particles (QP) dynamics in the strongly perturbed superconducting state (10<<70 \ub5J/cm2) is analysed within the framework of the Rotwarf-Taylor model. The picture emerging from the data is consistent with a dynamics governed by high-frequency phonon (HFP) population, which causes a bottleneck effect in the QP recombinatio

Disentangling thermal and nonthermal excited states in a charge-transfer insulator by time- and frequency-resolved pump-probe spectroscopy

Time- and frequency-resolved pump-probe optical spectroscopy is used to investigate the effects of the impulsive injection of delocalized excitations through a charge-transfer process in insulating CuGeO3. A large broadening of the charge-transfer edge is observed on the sub-ps time scale. The modification of this spectral feature cannot be attributed to the local increase in the effective temperature, as a consequence of the energy absorbed by the pump pulse. The measured modifications of the optical properties of the system are consistent with the creation of a nonthermal state, metastable on the picosecond time scale, after the pump-induced impulsive modification of the electron interactions

Non-thermal light-assisted resistance collapse in a V2_2O3_3-based Mott-insulator device

The insulator-to-metal transition in Mott insulators is the key mechanism for a novel class of electronic devices, belonging to the Mottronics family. Intense research efforts are currently devoted to the development of specific control protocols, usually based on the application of voltage, strain, pressure and light excitation. The ultimate goal is to achieve the complete control of the electronic phase transformation, with dramatic impact on the performance, for example, of resistive switching devices. Here, we investigate the simultaneous effect of external voltage and excitation by ultrashort light pulses on a single Mottronic device based on a V$_2$O$_3$ epitaxial thin film. The experimental results, supported by finite-element simulations of the thermal problem, demonstrate that the combination of light excitation and external electrical bias drives a volatile resistivity drop which goes beyond the combined effect of laser and Joule heating. Our results impact on the development of protocols for the non-thermal control of the resistive switching transition in correlated materials