Dmitry Yarotski

Conducting interface in oxide homojunction: Understanding of superior properties in black TiO2

Black TiO2 nanoparticles with a crystalline core and amorphous-shell structure exhibit superior optoelectronic properties in comparison with pristine TiO2. The fundamental mechanisms underlying these enhancements, however, remain unclear, largely due to the inherent complexities and limitations of powder materials. Here, we fabricate TiO2 homojunction films consisting of an oxygen-deficient amorphous layer on top of a highly crystalline layer, to simulate the structural/functional configuration of black TiO2 nanoparticles. Metallic conduction is achieved at the crystalline-amorphous homointerface via electronic interface reconstruction, which we show to be the main reason for the enhanced electron transport of black TiO2. This work not only achieves an unprecedented understanding of black TiO2 but also provides a new perspective for investigating carrier generation and transport behavior at oxide interfaces, which are of tremendous fundamental and technological interest.

Role of scaffold network in controlling strain and functionalities of nanocomposite films

The tuning of functional properties in thick oxide films via nanoscaffolds induced large vertical lattice strain. Strain is a novel approach to manipulating functionalities in correlated complex oxides. However, significant epitaxial strain can only be achieved in ultrathin layers. We show that, under direct lattice matching framework, large and uniform vertical strain up to 2% can be achieved to significantly modify the magnetic anisotropy, magnetism, and magnetotransport properties in heteroepitaxial nanoscaffold films, over a few hundred nanometers in thickness. Comprehensive designing principles of large vertical strain have been proposed. Phase-field simulations not only reveal the strain distribution but also suggest that the ultimate strain is related to the vertical interfacial area and interfacial dislocation density. By changing the nanoscaffold density and dimension, the strain and the magnetic properties can be tuned. The established correlation among the vertical interface—strain—properties in nanoscaffold films can consequently be used to tune other functionalities in a broad range of complex oxide films far beyond critical thickness.

Observation of 200th harmonic with fractional linewidth of 10−10 in a microwave frequency comb generated in a tunneling junction

A microwave frequency comb with up to 200th harmonic of a laser repetition rate is generated by nonlinear intermodal mixing of 15 fs laser pulses in the junction of a scanning tunneling microscope. The highest harmonic has an output power of −146 dBm at 14.85 GHz with a signal/noise ratio of 20 dB and a measured linewidth of 1.2 Hz, which is still larger than the actual linewidth due to phase noise of the spectrum analyzer. Theory suggests that the harmonics have comparable magnitude up to terahertz frequencies, while the observed roll-off is caused by a shunting capacitance in detection circuitry.

Linewidth of the harmonics in a microwave frequency comb generated by focusing a mode-locked ultrafast laser on a tunneling junction

Previous analyses suggest that microwave frequency combs (MFCs) with harmonics having extremely narrow linewidths could be produced by photodetection with a mode-locked ultrafast laser. In the MFC generated by focusing a passively mode-locked ultrafast laser on a tunneling junction, 200 harmonics from 74.254 MHz to 14.85 GHz have reproducible measured linewidths approximating the 1 Hz resolution bandwidth (RBW) of the spectrum analyzer. However, in new measurements at a RBW of 0.1 Hz, the linewidths are distributed from 0.12 to 1.17 Hz. Measurements and analysis suggest that, because the laser is not stabilized, the stochastic drift in the pulse repetition rate is the cause for the distribution in measured linewidths. It appears that there are three cases in which the RBW is (1) greater than, (2) less than, or (3) comparable with the intrinsic linewidth. The measured spectra in the third class are stochastic and may show two or more peaks at a single harmonic.

Magnetic, electronic, and optical properties of double perovskite Bi2FeMnO6

Double perovskite Bi2FeMnO6 is a potential candidate for the single-phase multiferroic system. In this work, we study the magnetic, electronic, and optical properties in BFMO by performing the density functional theory calculations and experimental measurements of magnetic moment. We also demonstrate the strain dependence of magnetization. More importantly, our calculations of electronic and optical properties reveal that the onsite local correlation on Mn and Fe sites is critical to the gap opening in BFMO, which is a prerequisite condition for the ferroelectric ordering. Finally, we calculate the x-ray magnetic circular dichroism spectra of Fe and Mn ions (L2 and L3 edges) in BFMO.

Temperature-tunable Fano resonance induced by strong coupling between Weyl fermions and phonons in TaAs

Strong coupling between discrete phonon and continuous electron–hole pair excitations can induce a pronounced asymmetry in the phonon line shape, known as the Fano resonance. This effect has been observed in various systems. Here we reveal explicit evidence for strong coupling between an infrared-active phonon and electronic transitions near the Weyl points through the observation of a Fano resonance in the Weyl semimetal TaAs. The resulting asymmetry in the phonon line shape, conspicuous at low temperatures, diminishes continuously with increasing temperature. This behaviour originates from the suppression of electronic transitions near the Weyl points due to the decreasing occupation of electronic states below the Fermi level (EF) with increasing temperature, as well as Pauli blocking caused by thermally excited electrons above EF. Our findings not only elucidate the mechanism governing the tunable Fano resonance but also open a route for exploring exotic physical phenomena through phonon properties in Weyl semimetals.

Electric polarization observed in single crystals of multiferroic Lu2MnCoO6

We report electric polarization and magnetization measurements in single crystals of double perovskite Lu2MnCoO6 using pulsed magnetic fields and optical second harmonic generation in dc magnetic fields. We observe well-resolved magnetic field-induced changes in the electric polarization in single crystals and thereby resolve the question about whether multiferroic behavior is intrinsic to these materials or is an extrinsic feature of polycrystals. We find electric polarization along the crystalline b axis, that is suppressed by applying a magnetic fields along the c axis, and advance a model for the origin of magnetoelectric coupling. We furthermore map the phase diagram using both capacitance and electric polarization to identify regions of ordering and regions of magnetoelectric hysteresis. This compound is a rare example of coupled hysteretic behavior in the magnetic and electric properties. Furthermore, the ferromagneticlike magnetic hysteresis loop that couples to hysteretic electric polarization can be attributed not to ordinary ferromagnetic domains, but to the rich physics of magnetic frustration of Ising-like spins in the axial next-nearest-neighbor interaction model.

Site-mixing effect on the XMCD spectrum in double perovskite Bi2FeMnO6

We investigate magnetization in double perovskite multiferroic Bi2FeMnO6 (BFMO) thin film using density functional theory (DFT) simulations, and X-ray magnetic circular dichroism (XMCD) measurements. The exchange interaction between Fe and Mn sites gives rise to a ferrimagnetic ordering in BFMO. When grown without structural defects, distinct XMCD signal is expected from this system. The site resolved magnetization, thus, can be extracted using XMCD sum rules. Although our theoretical calculations are consistent with this expectation for the ideal BFMO system, experimental measurements find evidence of anomalous peak for the L2 and L3 edges of XMCD signals, and thus, the XMCD sum rules are no longer valid. We theoretically explain this phenomenon by considering both tetragonal (near interface), and monoclinic (bulk) phases of BFMO system, with Fe and Mn ions interchanged between their respective sites. Such site-mixing between magnetic cations are commonly found during the synthesis process. Our DFT calculations of XMCD for site interchanged Fe and Mn ions in the bulk phase (monoclinic) of BFMO are in good agreement with experimental XMCD signal and reproduce the anomalous peak features at L2/L3 edges.

Characterization of irradiation damage distribution near TiO2/SrTiO3 interfaces using coherent acoustic phonon interferometry

We apply ultrafast coherent acoustic phonon interferometry to characterize the distribution of the radiation damage near the TiO2/SrTiO3 interfaces. We show that the optical and mechanical properties of anatase TiO2 remain unaffected by the radiation dosages in the 0.1÷5 dpa (displacements per atom) range, while the degraded optical response indicates a significant defect accumulation in the interfacial region of SrTiO3 at 0.1 dpa and subsequent amorphization at 3 dpa. Comparison between the theoretical simulations and the experimental results reveals an almost threefold reduction of the sound velocity in the irradiated SrTiO3 layer with peak damage levels of 3 and 5 dpa.

Manipulating multiple order parameters via oxygen vacancies: The case of Eu0.5Ba0.5TiO3−δ

Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (V-O) is a key issue for the future development of transition-metal oxides. Progress in this field is currently addressed through V-O variations and their impact on mainly one order parameter. Here we reveal a mechanism for tuning bothmagnetism and ferroelectricity simultaneously by using V-O. Combining experimental and density-functional theory studies of Eu0.5Ba0.5TiO3-delta , we demonstrate that oxygen vacancies create Ti3+ 3d(1) defect states, mediating the ferromagnetic coupling between the localized Eu 4f(7) spins, and increase an off-center displacement of Ti ions, enhancing the ferroelectric Curie temperature. The dual function of Ti sites also promises a magnetoelectric coupling in the Eu0.5Ba0.5TiO3-delta.