Control of Domain Structures in Multiferroic Thin Films through Defect Engineering

Domain walls (DWs) have become an essential component in nanodevices based on ferroic thin films. The domain configuration and DW stability, however, are strongly dependent on the boundary conditions of thin films, which make it difficult to create complex ordered patterns of DWs. Here, it is shown that novel domain structures, that are otherwise unfavorable under the natural boundary conditions, can be realized by utilizing engineered nanosized structural defects as building blocks for reconfiguring DW patterns. It is directly observed that an array of charged defects, which are located within a monolayer thickness, can be intentionally introduced by slightly changing substrate temperature during the growth of multiferroic BiFeO3 thin films. These defects are strongly coupled to the domain structures in the pretemperatureâ change portion of the BiFeO3 film and can effectively change the configuration of newly grown domains due to the interaction between the polarization and the defects. Thus, two types of domain patterns are integrated into a single film without breaking the DW periodicity. The potential use of these defects for building complex patterns of conductive DWs is also demonstrated.Engineered structural defects are used as nanosized building blocks for configuring domainâ wall patterns in multiferroic BiFeO3 thin films. By utilizing the interaction between the polarization and the defects, two types of twinning domain structures are integrated into a single film without breaking the domainâ wall periodicity.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146435/1/adma201802737-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146435/2/adma201802737_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146435/3/adma201802737.pd

Detection and characterization of spin-orbit resonances in the advanced gravitational wave detectors era

In this paper, we test the performance of templates in detection and characterization of Spin-orbit resonant (SOR) binaries. We use precessing SEOBNRv3 waveforms as well as {\it four} numerical relativity (NR) waveforms to model GWs from SOR binaries and filter them through IMRPhenomD, SEOBNRv4 (non-precessing) and IMRPhenomPv2 (precessing) approximants. We find that IMRPhenomD and SEOBNRv4 recover only $\sim70\%$ of injections with fitting factor (FF) higher than 0.97 (or 90\% of injections with ${\rm FF} >0.9$).However, using the sky-maxed statistic, IMRPhenomPv2 performs magnificently better than their non-precessing counterparts with recovering $99\%$ of the injections with FFs higher than 0.97. Interestingly, injections with $\Delta \phi = 180^{\circ}$ have higher FFs ($\Delta \phi$ is the angle between the components of the black hole spins in the plane orthogonal to the orbital angular momentum) as compared to their $\Delta \phi =0^{\circ}$ and generic counterparts. This implies that we will have a slight observation bias towards $\Delta \phi=180^{\circ}$ SORs while using non-precessing templates for searches. All template approximants are able to recover most of the injected NR waveforms with FFs $>0.95$. For all the injections including NR, the error in estimating chirp mass remains below $<10\%$ with minimum error for $\Delta \phi = 180^{\circ}$ resonant binaries. The symmetric mass ratio can be estimated with errors below $15\%$. The effective spin parameter $\chi_{\rm eff}$ is measured with maximum absolute error of 0.13. The in-plane spin parameter $\chi_p$ is mostly underestimated indicating that a precessing signal will be recovered as a relatively less precessing signal. Based on our findings, we conclude that we not only need improvements in waveform models towards precession and non-quadrupole modes but also better search strategies for precessing GW signals.Comment: 27 pages, 15 figures. Abstract shortened due to word limi

Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials

Confining materials to two-dimensional forms changes the behavior of electrons and enables new devices. However, most materials are challenging to produce as uniform thin crystals. Here, we present a new synthesis approach where crystals are grown in a nanoscale mold defined by atomically-flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mold made of hexagonal boron nitride (hBN), we grow ultraflat bismuth crystals less than 10 nanometers thick. Due to quantum confinement, the bismuth bulk states are gapped, isolating intrinsic Rashba surface states for transport studies. The vdW-molded bismuth shows exceptional electronic transport, enabling the observation of Shubnikov-de Haas quantum oscillations originating from the (111) surface state Landau levels, which have eluded previous studies. By measuring the gate-dependent magnetoresistance, we observe multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. Our vdW-mold growth technique establishes a platform for electronic studies and control of bismuth's Rashba surface states and topological boundary modes. Beyond bismuth, the vdW-molding approach provides a low-cost way to synthesize ultrathin crystals and directly integrate them into a vdW heterostructure

Detection and characterization of spin-orbit resonances in the advanced gravitational wave detectors era

Spin-orbit resonances have important astrophysical implications as the evolution and subsequent coalescence of supermassive black hole binaries in one of these configurations may lead to low recoil velocity of merger remnants. It has also been shown that black hole spins in comparable mass stellar-mass black hole binaries could preferentially lie in a resonant plane when their gravitational waves (GWs) enter the advanced LIGO frequency band [1]. Therefore, it is highly desirable to investigate the possibility of detection and subsequent characterization of such GW sources in the advanced detector era, which can, in turn, improve our perception of their high mass counterparts. The current detection pipelines involve only nonprecessing templates for compact binary searches whereas parameter estimation pipelines can afford to use approximate precessing templates. In this paper, we test the performance of these templates in detection and characterization of spin-orbit resonant binaries. We use fully precessing time-domain SEOBNRv3 waveforms as well as four numerical relativity (NR) waveforms to model GWs from spin-orbit resonant binaries and filter them through IMRPhenomD, SEOBNRv4 and IMRPhenomPv2 approximants. We find that the nonprecessing approximants IMRPhenomD and SEOBNRv4 recover only ∼70% of injections with fitting factor (FF) higher than 0.97 (or 90% of injections with FF>0.9). This loss in signal-to-noise ratio is mainly due to the missing physics in these approximants in terms of precession and nonquadrupole modes. However, if we use a new statistic, i.e., maximizing the matched filter output over the sky-location parameters as well, the precessing approximant IMRPhenomPv2 performs magnificently better than their nonprecessing counterparts with recovering 99% of the injections with FFs higher than 0.97. Interestingly, injections with Δϕ=180° have higher FFs (Δϕ is the angle between the components of the black hole spins in the plane orthogonal to the orbital angular momentum) as compared to their Δϕ=0° and generic counterparts. This is because Δϕ=180° binaries are not as strongly precessing as Δϕ=0° and generic binaries. This implies that we will have a slight observation bias towards Δϕ=180° and away from Δϕ=0° resonant binaries while using nonprecessing templates for searches. Moreover, all template approximants are able to recover most of the injected NR waveforms with FFs >0.95. For all the injections including NR, the systematic error in estimating chirp mass remains below <10% with minimum error for Δϕ=180° resonant binaries. The symmetric mass-ratio can be estimated with errors below 15%. The effective spin parameter χ_(eff) is measured with maximum absolute error of 0.13. The in-plane spin parameter χ_p is mostly underestimated indicating that a precessing signal will be recovered as a relatively less precessing signal. Based on our findings, we conclude that we not only need improvements in waveform models towards precession and nonquadrupole modes but also better search strategies for precessing GW signals

Phonon Imaging of Nanostructure Interfaces by Electron Microscopy

Thermal properties in nanostructured materials are dominated by atomic scale and nanoscale inhomogeneities in crystal lattices. However, tools for studying nanoscale thermal properties have relied on theoretical frameworks and optical measurements of defect aggregates. Vibrational spectroscopy in a transmission electron microscope offers unprecedented access into nanoscale phonon physics. We have studied a SiGe quantum dot (QD) superlattice which is a strong candidate for high temperature thermoelectrics. These dome shaped, SiGe alloy QDs are bounded by a sharp and gradual interface due to its nonuniform alloy distribution. With a 6mev (~50 cm-1) energy resolution via a monochromated beam, we map Si optical phonons in a single SiGe QD. We find a composition-induced strain in the alloy matrix in the QD nanostructure that manifests as a redshift in the phonon energy that is exactly consistent with the nonuniform composition distribution. More notably, we find an accumulation of non-equilibrium phonons below the sharp interface. To investigate the dynamics of this further, I developed a new technique called differential momentum mapping (DPM) that allows for the mapping of propagating phonon modes. With this, we mapped phonon group velocity vectors showing unequivocally a drastically larger reflection of phonons from the sharp interface than the gradual one. For the first time, dynamical thermal processes at the nanoscale have been imaged.This vibrational EELS capability is then applied to a ferroelectric-insulator system: BiFeO3/TbScO3 (BFO/TSO). Our goal for this study was to identify relationships between the phonon structure and ferroelectricity in BFO. Ferroelectric domain walls (DW) and the emergence of free charge gases at the interface offered us an excellent opportunity to also investigate nanoscale electron-phonon dynamics. We discovered that DW-localized shear strain due to alternating polarization, causes phonons to propagate anisotropically by utilizing the DPM technique developed in the previous work. Furthermore, the free charge gasses at the interface also break phonon symmetry but due to accumulated net charge rather than structural distortions. This work provides a nanoscale angle-resolved picture for the decrease in cross-DW and cross-interface thermal conductivity. Uncovering this physics is fundamental to the well-informed fabrication of ferroelectric-based materials and proper design of ferroelectric memories.To enable trustable data analysis of EELS data, a data workflow stack has been developed with the following key aspects in mind: accuracy, efficiency, transparency, and useability. With the advent of advanced detectors and electron spectroscopy techniques, demand for intelligent, high throughput, processing has sky-rocketed, but the lack of programming and data analysis expertise has been a high barrier to entry. The developed modules serve to mitigate this problem by taking advantage of relatively low-level APIs that interface with data and provide scientific analysis tools and wrapping them in high-level modules designed for user-specific workflows. An example workflow for vibrational EELS includes aligning of the zero-loss peak (ZLP), background subtracting the vibrational signal, using PCA to reconstruct the data in a lower dimensional representation, fitting prominent peaks, and finally visualizing the data in the form of line plots or contour maps. This framework also includes tools for analyzing the fine structure of core-loss edges, bandgap measurements, and a general set of tools including deconvolution, clustering, and filtering. Because entire blocks in the workflow are condensed into single lines of code, these high-level modules can aid non-coders to process their data using innovative data analysis technique

Nanoscale imaging of phonon dynamics by electron microscopy

Spatially resolved vibrational mapping of nanostructures is indispensable to the development and understanding of thermal nanodevices1, modulation of thermal transport2 and novel nanostructured thermoelectric materials3,4,5. Through the engineering of complex structures, such as alloys, nanostructures and superlattice interfaces, one can significantly alter the propagation of phonons and suppress material thermal conductivity while maintaining electrical conductivity2. There have been no correlative experiments that spatially track the modulation of phonon properties in and around nanostructures due to spatial resolution limitations of conventional optical phonon detection techniques. Here we demonstrate two-dimensional spatial mapping of phonons in a single silicon–germanium (SiGe) quantum dot (QD) using monochromated electron energy loss spectroscopy in the transmission electron microscope. Tracking the variation of the Si optical mode in and around the QD, we observe the nanoscale modification of the composition-induced red shift. We observe non-equilibrium phonons that only exist near the interface and, furthermore, develop a novel technique to differentially map phonon momenta, providing direct evidence that the interplay between diffuse and specular reflection largely depends on the detailed atomistic structure: a major advancement in the field. Our work unveils the non-equilibrium phonon dynamics at nanoscale interfaces and can be used to study actual nanodevices and aid in the understanding of heat dissipation near nanoscale hotspots, which is crucial for future high-performance nanoelectronics.Department of Energy (DOE)National Science Foundation (NSF