Effects of Boron Doping on the Bulk and Surface Acoustic Phonons in Single-Crystal Diamond.

We report the results of the investigation of bulk and surface acoustic phonons in the undoped and boron-doped single-crystal diamond films using the Brillouin-Mandelstam light scattering spectroscopy. The evolution of the optical phonons in the same set of samples was monitored with Raman spectroscopy. It was found that the frequency and the group velocity of acoustic phonons decrease nonmonotonically with the increasing boron doping concentration, revealing pronounced phonon softening. The change in the velocity of the shear-horizontal and the high-frequency pseudo-longitudinal acoustic phonons in the degenerately doped diamond, as compared to that in the undoped diamond, was as large as ∼15% and ∼12%, respectively. As a result of boron doping, the velocity of the bulk longitudinal and transverse acoustic phonons decreased correspondingly. The frequency of the optical phonons was unaffected at low boron concentration but experienced a strong decrease at the high doping level. The density-functional-theory calculations of the phonon band structure for the pristine and highly doped samples confirm the phonon softening as a result of boron doping in diamond. The obtained results have important implications for thermal transport in heavily doped diamond, which is a promising material for ultra-wide-band-gap electronics.

High sensitivity pump-probe measurements of magnetic, thermal, and acoustic phenomena with a spectrally tunable oscillator.

We describe an optical pump/probe system for sensitive measurements of time-resolved optical measurements of material dynamics. The instrument design is optimized for time-resolved magneto-optic Kerr effect (TR-MOKE) measurements of dynamics in magnetic materials. The system also allows for time-domain thermoreflectance (TDTR) measurements of thermal transport properties and picosecond acoustic measurements of film thickness and/or elastic constants. The system has several advantages over the conventional designs for TR-MOKE and/or TDTR systems. Measurements of pump-induced changes to the probe beam intensity are shot-noise limited. The system's design allows for MOKE and/or thermoreflectance measurements of both sides of a sample. Pumping and probing the sample on opposite sides allows nanoscale flash diffusivity measurements of transport properties. The wavelengths of the pump and probe beams are straightforward to tune between 350-525 nm and 690-1050 nm. A tunable wavelength allows for optical resonances in a wide array of materials to be excited and/or probed. Finally, the setup is calibrated to allow for the real and imaginary components of Kerr signals to be separately quantified.

Phonon and Thermal Properties of Quasi-Two-Dimensional FePS3 and MnPS3 Antiferromagnetic Semiconductors.

We report results of investigation of the phonon and thermal properties of the exfoliated films of layered single crystals of antiferromagnetic FePS3 and MnPS3 semiconductors. Raman spectroscopy was conducted using three different excitation lasers with wavelengths of 325 nm (UV), 488 nm (blue), and 633 nm (red). UV-Raman spectroscopy reveals spectral features which are not detectable via visible Raman light scattering. The thermal conductivity of FePS3 and MnPS3 thin films was measured by two different techniques: the steady-state Raman optothermal and transient time-resolved magneto-optical Kerr effect. The Raman optothermal measurements provided the orientation-average thermal conductivity of FePS3 to be 1.35 ± 0.32 W m-1 K-1 at room temperature. The transient measurements revealed that the through-plane and in-plane thermal conductivity of FePS3 are 0.85 ± 0.15 and 2.7 ± 0.3 W m-1 K-1, respectively. The films of MnPS3 have higher thermal conductivity of 1.1 ± 0.2 W m-1 K-1 through-plane and 6.3 ± 1.7 W m-1 K-1 in-plane. The data obtained by the two techniques are in agreement and reveal strong thermal anisotropy of the films and the dominance of phonon contribution to heat conduction. The obtained results are important for the interpretation of electric switching experiments with antiferromagnetic materials as well as for the proposed applications of the antiferromagnetic semiconductors in spintronic devices.

Ultrafast magnetization reversal by picosecond electrical pulses.

The field of spintronics involves the study of both spin and charge transport in solid-state devices. Ultrafast magnetism involves the use of femtosecond laser pulses to manipulate magnetic order on subpicosecond time scales. We unite these phenomena by using picosecond charge current pulses to rapidly excite conduction electrons in magnetic metals. We observe deterministic, repeatable ultrafast reversal of the magnetization of a GdFeCo thin film with a single sub-10-ps electrical pulse. The magnetization reverses in ~10 ps, which is more than one order of magnitude faster than any other electrically controlled magnetic switching, and demonstrates a fundamentally new electrical switching mechanism that does not require spin-polarized currents or spin-transfer/orbit torques. The energy density required for switching is low, projecting to only 4 fJ needed to switch a (20 nm)3 cell. This discovery introduces a new field of research into ultrafast charge current-driven spintronic phenomena and devices.

Thermal conductivity of mechanically joined semiconducting/metal nanomembrane superlattices.

The decrease of thermal conductivity is crucial for the development of efficient thermal energy converters. Systems composed of a periodic set of very thin layers show among the smallest thermal conductivities reported to-date. Here, we fabricate in an unconventional but straightforward way hybrid superlattices consisting of a large number of nanomembranes mechanically stacked on top of each other. The superlattices can consist of an arbitrary composition of n- or p-type doped single-crystalline semiconductors and a polycrystalline metal layer. These hybrid multilayered systems are fabricated by taking advantage of the self-rolling technique. First, differentially strained nanomembranes are rolled into three-dimensional microtubes with multiple windings. By applying vertical pressure, the tubes are then compressed and converted into a planar hybrid superlattice. The thermal measurements show a substantial reduction of the cross-sectional heat transport through the nanomembrane superlattice compared to a single nanomembrane layer. Time-domain thermoreflectance measurements yield thermal conductivity values below 2 W m(-1) K(-1). Compared to bulk values, this represents a reduction of 2 orders of magnitude by the incorporation of the mechanically joined interfaces. The scanning thermal atomic force microscopy measurements support the observation of reduced thermal transport on top of the superlattices. In addition, small defects with a spatial resolution of ∼100 nm can be resolved in the thermal maps. The low thermal conductivity reveals the potential of this approach to fabricate miniaturized on-chip solutions for energy harvesters in, e.g., microautonomous systems.