Research

Latest:

Imaging the ultra-small with x-ray nano vision: first observation of 3D topological vortex in a single ferroelectric nanoparticle

Research Directions

Do you want to know about the read-head inside your computer works? Why are some butterflies blue, yellow and multi-colored? The arrangement of spins in magnetic nanostructures and how they fluctuate? How your TV remote control works? How is information stored and read in your “pen drives” and flash memory? The quantum fluctuations of atoms in single crystals of solid Helium at temperatures near absolute zero? Or the structure and dynamics of stripe domains and puddles in ferroelectric, ferromagnetic and superconducting phases? These topics and more are investigated by the Fohtung Group, primarily using the interaction of radiation (photons, neutrons, and ions) with condensed matter systems. The focus of our research is the study of the structure and dynamics of condensed matter.

We investigate the quantum interaction between electromagnetic radiation (photons and neutrons) with the condensed and soft matter at the nanoscale. To this end, we are interested in developing novel optical metrology (lens-less microscopy, coherent diffraction imaging with photons at synchrotrons and XFELs) and neutron scattering techniques to probe spatial and temporal dynamics due to competing charge-, spin-, orbital and lattice degree of freedoms at the nanoscale. Applications of our research include Magneto-electric based nano-mechanical oscillators, sensing, quantum photonics, Magneto-electric conversion of optical energy to electricity, transparent ceramics and biomimetics.  We also focus on training the next generation of innovative students/scientists. We aim to foster an environment that is conducive to learning, creativity, and personal development.

Our group is broadly engaged in the study of the structure and dynamics of condensed matter using the techniques of optical, x-ray and neutron scattering. For this purpose, we use in-house x-ray sources and diffractometers, Optical Metrology, while most of our studies require an intensity that can only be provided at synchrotron sources and XFELs.

Imaging the ultra-small with x-ray nano vision: first observation of 3D topological vortex in a single ferroelectric nanoparticle

vortexBTO

(Left) The volume of a ferroelectric nanoparticle with the topological vortex structural phase. (Right) Projections of the toroidal moment of the ferroelectric displacement and 3D rendering of ferroelectric polarization under the application of an external electric field. The lack of mirror symmetry of each spatial component of the toroidal moment and their reproducible evolution under external electric field indicates the electrically controllable chirality which suggests potential applications in novel nanoelectronic devices.

Looking for future technology for integrated electronics? Maybe topology can help. Researchers at Los Alamos National Laboratory (LANL) have observed a three-dimensional vortex structure in a single ferroelectric nanoparticle of Barium Titanate. These results were published in Nature Communications, and exhibit topological structure formed by subtle but long-ordered shifts in the displacement fields. The group led by LANL and New Mexico State University (NMSU) Rosen Professor Edwin Fohtung and his group were able to show that the topological vortex can be controlled with an external electric field, which opens exciting opportunities in next-generation integrated electronics and nanoscale devices.

Dr. Fohtung stated: ‘the visualization of the whole volume behavior under applied perturbations is extremely challenging due to such fundamental limitations of the probes as penetrability, resolution, noninvasiveness etc. But we took on this challenge by combining the unique technique of Bragg coherent x-ray diffractive imaging, available at synchrotron radiation sources and X-ray free electron laser facilities, and a functional sample environment. It all provides us with greater control of the experimental conditions while the high flux of coherent x-rays propagating through and scattering from the sample’s extremities permits to study dynamical responses.’

The experimental work was done at station 34-ID-C of the Advanced Photon Source, which is a synchrotron radiation facility located at Argonne National Laboratory. Bragg coherent diffractive imaging (BCDI) is a lensless imaging technique where the physical lens is replaced by computational phase retrieval algorithms serving as a virtual lens. Only, this virtual lens in not diffraction-limited. When the coherent waves of X-rays are scattered by the sample it forms a reciprocal image in the far-field. The diffraction intensities can be iteratively inverted using algorithms based on Fourier optics and kinematic diffraction theory. A major benefit of this approach is the ability to achieve diffraction limited resolution. In the case of the current published paper, the authors were able to achieve up to 18 nm spatial resolution which allowed them to track the subtle structural changes in the nanoparticle undergoing phase transitions.

The interpretation of experimental results was possible through the aid of theoretical simulations provided by the group of Dr. Turab Lookman in the Theoretical Division of LANL. The group used Landau theory and phase-field models to confirm the experimentally observed behavior of the topological vortex-structure, allowing to decipher complex interactions of the crystal structure, structural phase transitions and the competing energy contributions to the formation of the stable vortex structural phase within a single nanoparticle. Dr. Lookman is also working with Dr. Fohtung to design a form of a theoretically guided phase retrieval algorithm. Dmitry Karpov graduate student from NMSU-TPU, who performed the experiments, said that “this approach may help in providing physically realistic solutions to a host of nanoscale material systems studied using BCDI.”

Published results demonstrate that the vortex core is mobile under an external field and represents 2D projections of a 3D paraelectric nanorod spanning the volume of the nanoparticle. This 3D nanorod can be displaced, erased and created within the monolith of an individual Barium Titanate nanoparticle. Thus making the nanorod a conductive channel in an otherwise insulating nanocrystal.

Both Dr. Fohtung and Dr. Lookman share their anticipation that this research direction will likely open a new avenue in information technology where quantum bits can be designed by quantizing the energy spectra of such nanorod and using not only the orientation of the axial polarization under an applied electric field but also the chirality of the 3D toroidal moment. Other applications include nanomotors, nanoswitches, and materials where controllable optical chiral behavior is desired. The results are also crucial for future studies of the dynamical behavior of material properties under external perturbations such as electromagnetic waves, shock waves especially in light of current LANL plans for its signature XFEL facility known as MaRIE (Matter-Radiation Interactions in Extremes).

Source: http://www.nature.com/articles/doi:10 1038/s41467-017-00318-9.

Biophysics and Biomedical Imaging: from a single cell to a collective behavior of cells

Understanding of morphology and collective behavior of progenitor cells, spindle cells, dynamics during different phases from gastrulation to formation of neoplasm are all crucial in developmental biology, artificial life, regenerative medicine, cell signaling and many other fields encompassed by Life Sciences. One crucial requirement for these studies is the availability of instrumentation capable of imaging complex systems non-invasively with high spatial and temporal resolution, of course preferably at various land scales. To make the task even more challenging it is highly desirable to be able to differentiate the information acquired in the imaging experiment by the types of tissues or elements of a cell, i.e. to control the contrast mechanisms post hoc.

Despite the tremendous progress achieved in microscopic techniques in the last decades, a general approach for simultaneously imaging non-emissive specimen at broad spatial range from deep cellular level to microscopic collective dynamics with sub-wavelength resolution remains elusive. The most persistent problems of super-resolution microscopy today are: (i) limited applicability due to the requirement to use fluorescent dyes for imaging, (ii) limited temporal resolution due to the requirement for spatial or temporal scanning of the object, (iii) limited imaging depth due to unfavorable object-probe interactions.

interfacial magnetism

The physical properties of interfaces between a ferromagnetic layer and insulating ferroelectric layer or a substrate are very important for device applications. For example, a magnetic “dead” layer close to an interface in a tunnel junction can be problematical.  Such a layer is often present in perovskite heterostructures such as BaTiO3 or Pb[ZrxTi1-x]O3 (0≤x≤1), also called PZT. Previously, degradation of the magnetic interface has been attributed to charge discontinuity across the interface.

Here, we utilize polarized neutron reflectometry in consort with ab initio based density functional theory calculations to study magnetoelectric coupling at the interface of a ferroelectric PbZr0.2Ti0.8O3and magnetic La0.67Sr0.33MnO3heterostructure. Magnetoelectric multiferroics, having the simultaneous occurrence of two ferroic order parameters (i.e. ferroelectric and anti/ferromagnetic), exhibit novel and exotic functionalities due to the coupling between these parameters and show promise for new device application in magnetic random access memory, sensors, and spintronic devices. However, single-phase multiferroics with large magnetoelectric coupling of the FM and FE order parameters that can operate well above room temperatures are rare. This is because of the mutually exclusive chemistry required for the existence of two order parameters.

For more information see  our artcile: Binod Paudel, Igor Vasiliev, Mahmoud Hammouri, Dmitry Karpov, Aiping Chen and Edwin Fohtung*. “Strain vs. charge mediated magnetoelectric coupling across the magnetic oxide/ferroelectric interfaces.Royal Society of Chemistry: RSC Advances 9 (29), 13033-13041, 2019.

 Bragg Coherent X-ray Diffraction Imaging

We have recently developed a technique called Bragg Coherent Diffraction Imaging (BCDI).  BCDI is a technique that enables us to capture dynamical and evolving processes such as the transformation path of a vortex(vortices), domain walls, etc. in ferroelectric, magnetic, and multiferroic nanostructures and bulk materials. This technique allows us to elucidate the complex topologies of polarizations, FE displacements, structural phase transitions and mechanical responses of device relevant materials when subjected to an external perturbation such as electric, magnetic fields, and heat. BCDI takes advantage of improvements incoherence at third and fourth generation light sources. Advancements in phase retrieval algorithms enable the visualization of whole-volume information of the electron density distribution, atomic and ionic displacement fields within individual nanoparticles with nanoscale resolution.

 Illustration of the experimental scheme with incident coherent X-ray beam being scattered from the extremities of the nanoparticle under applied electric field. Detector is used to record the diffraction patterns and is mounted on motorized arm to properly position the detector around the diffraction sphere for fine tuning into specific peak. b, Diffraction patterns recorded at different applied electric fields with numbering indicating evolution of the pattern from (1) for 0V of applied field, (2) 10V of applied field, (3) again 0V of applied field, and (4) for -10V of applied field. As given by phase field simulation, 10V corresponds to an electric field of 223 kV cm-1across the single BTO nanoparticle. Scale bar corresponds to 0.1 Å-1

Illustration of the experimental scheme with incident coherent X-ray beam being scattered from the extremities of the nanoparticle under an applied electric field. The detector is used to record the diffraction patterns and is mounted on the motorized arm to properly position the detector around the diffraction sphere for fine-tuning into a specific peak. b, Diffraction patterns recorded at different applied electric fields with numbering indicating an evolution of the pattern from (1) for 0V of an applied field, (2) 10V of an applied field, (3) again 0V of an applied field, and (4) for -10V of an applied field. As given by phase field simulation, 10V corresponds to an electric field of 223 kV cm-1across the single BTO nanoparticle. Scale bar corresponds to 0.1 Å-1.

In BCDI experiments the sample is illuminated with focused coherent X-ray beam ( See Figure above). A random orientation of nanoparticles or nanowires along with the experimental geometry allows us to isolate and record a given (hkl) Bragg reflections from a single nanoparticle on an area detector. By applying an electric field in cycles and monitoring changes in the diffraction pattern, we can select FE nanoparticles acting as nanocapacitor (Supplementary Fig. 3) from the particles which are electrically insulated in the dielectric matrix.

Birefringent Coherent Diffraction Imaging

The directional dependence of the index of refraction contains a wealth of information about anisotropic optical properties in semiconducting and transparent insulating materials. Here we develop a novel high-resolution lens-less technique that uses birefringence as a contrast mechanism to map the index of refraction and dielectric permittivity in optically anisotropic materials.

Schematic of Birefringent Coherent Diraction Imaging.(a) The collimated Polarized 633 nm laser is focused on a pinhole to generate the incident illumination on the Birefringent liquid crystal polymer thin lm. A detector placed 200mm from the sample is utilized to collect coherent diraction patterns for a given incident polarization of the photon beam. We use a tunable polarizer to prepare dierent polarization state of the incident beam. (b) Frequency dependence of refractive index along two axes in a Birefringent material. Birefringence is manifested as broken symmetry in the optical axes. Regions where the slope is negative corresponds to absorption band. Adapted from 42. (c)The Electron Oscillator/Lorentz atom model is used to describe the response of an atom in an optically anisotropic media. Quantum mechanics suggests we can describe the atom as a point-like nucleus bounded to an electron charged cloud by springs of dierent spring constant.

Schematic of Birefringent Coherent Di raction Imaging.(a) The collimated Polarized 633 nm laser is
focused on a pinhole to generate the incident illumination on the Birefringent liquid crystal polymer thin lm. A detector
placed 200mm from the sample is utilized to collect coherent di raction patterns for a given incident polarization of the
photon beam. We use a tunable polarizer to prepare di erent polarization state of the incident beam. (b) Frequency
dependence of refractive index along two axes in a Birefringent material. Birefringence is manifested as broken symmetry
in the optical axes. Regions, where the slope is negative, corresponds to absorption band. Adapted from 42. (c)The
Electron Oscillator/Lorentz atom model is used to describe the response of an atom in an optically anisotropic media.
Quantum mechanics suggests we can describe the atom as a point-like nucleus bounded to an electron charged cloud by
springs of di erent spring constant.

We have applied this approach successfully to a liquid crystal polymer film using polarized light from a helium-neon laser. This approach is scalable to imaging with diffraction-limited resolution, a prospect rapidly becoming a reality in view of emergent brilliant X-ray sources. Applications of this novel imaging technique are in disruptive technologies, including novel electronic devices, in which both charge and spin carry information as in multiferroic materials and photonic materials such as light modulators and optical storage.