Resent Research and Education Highlights

highlight 2019

Interfacial Charge Engineering in Ferroelectric-Gated Mott Transistors


X. Chen, X. Zhang, M. A. Koten, Z. Xiao, L. Zhang, Jeffrey E. Shield, Peter A. Dowben and Xia Hong

Nebraska MRSEC

Complex oxide materials possess a plethora of technology-critical functionalities that span the entire electronic and magnetic spectra. The structural similarity among them makes it possible to achieve functional design of oxide heterostructures, which can be used as the building blocks for energy efficient nanoelectronics with performance transcending the existing semiconducting technology. Nebraska MRSEC researchers have fabricated such complex oxide heterostructures with atomic precision, exploiting them to build a prototype nonvolatile Mott transistor. Working with a ferroelectric Pb(Zr,Ti)O3 (PZT) gate, they have achieved room temperature on-off switching in a couple of nanometer-thick Mott channel Sm0.5Nd0.5NiO3 (SNNO). Significant enhancement in the channel resistance switching has been achieved by interfacing SNNO with an ultrathin La0.67Sr0.33MnO3 (LSMO) layer, leveraging the interfacial charge transfer effect between SNNO and LSMO. This study points to a new material strategy to achieve bulk-inaccessible functionalities via atomistic design of complex oxide hetero-interfaces, bridging its gap with practical applications. (Chen et al., Adv. Mater. 29, 1701385 (2017).

Picture: PZT-gated SNNO/LSMO composite channels exhibit significantly enhanced field effect switching due to interfacial charge transfer between SNNO and LSMO.

highlight 2019

New Hybrid Heterostructure Nanophotonic Materials

Christos Argyropoulos, Mathias Schubert, and Eva Schubert
Nebraska MRSEC

The inherently weak light-matter interaction at the nanoscale can be enhanced by new metal-dielectric hybrid nanomaterials. This enhancement can enrich some of the quantum and nonlinear features of light, leading to new nanophotonic applications. Nebraska MRSEC researchers have designed new hybrid heterostructure nanophotonic materials composed of plasmonic metals and dielectrics to manipulate photons at optical frequencies. They demonstrated tunable plasmonic resonant responses with narrowband spectra that can be used for nanosensing applications. The proposed new nanomaterial platforms are envisioned to provide a rich material basis for studying new physics at the nanoscale. Of special interest is the coupling between quantum and nonlinear excitations in these new materials with their enhanced photonic fields. These interesting properties are expected to be useful for photonic and electronic devices as well as thermoelectric, photocatalytic, and energy storage applications.
U. Kilic, A. Mock, R. Feder, D. Sekora, M. Hilfiker, R. Korlacki, E. Schubert, C. Argyropoulos, and M. Schubert, “Tunable plasmonic resonances in Si-Au slanted columnar heterostructure thin films,” Scientific Reports 9, 71 (2019).

Picture: Top panel: Cross-section image of the designed new hybrid heterostructure nanophotonic material. Bottom panel: The computed electric field distribution, indicating a substantial field enhancement along its metallic nanoscale regions.

highlight 2019

A Viable Material for Topological Antiferromagnetic Spintronics

Ding-Fu Shao, Gautam Gurung, and Evgeny Tsymbal
Nebraska MRSEC

Topological antiferromagnetic spintronics is an emerging field of research where topological properties of a material are coupled to the antiferromagnetic ordering. Topological properties involve non-trivial electronic states, such as Dirac nodal lines, which are protected by the structural and magnetic symmetry of the material.
Nebraska MRSEC researchers have theoretically predicted that antiferromagnetic metal MnPd2 supports an electrically controlled Dirac nodal line at room temperature. They showed that MnPd2 exhibits the required symmetry, which allows the reorientation of the antiferromagnetic order parameter by electrical current, and simultaneously holds a Dirac nodal line across the required energy range needed to affect functional behavior of this material. This reorientation changes the magnetic space group symmetry, producing an energy gap in the Dirac nodal line and thus changing spin-dependent transport properties. This prediction offers, for the first time, a viable material which can be employed in topological antiferromagnetic spintronics.
D.-F. Shao, G. Gurung, S.-H. Zhang, and E. Y. Tsymbal, “Dirac nodal line metal for topological antiferromagnetic spintronics,” Physical Review Letters 122, 077203 (2019).

Picture: Switching of the Dirac nodal line state from degenerate to gapped (bottom panels) of by reorientation of the antiferromagnetic order parameter (indicated by arrows on the top panels).

highlight 2019

Capturing Structural Dynamics of Materials with Ultrafast Electron Diffraction

Martin Centurion
Nebraska MRSEC

“Phase transition” is a term which is commonly used to describe transformations between solid, liquid, and gaseous states of matter. However, even in solids, phase transitions may occur between different structural phases, resulting in a discontinuous change of certain material properties, such as electrical conductivity and heat capacity, which can be used in technological applications. Many phase transitions in solids involve ultrafast motion in the atomic positions towards a new equilibrium configuration. Nebraska MRSEC supports a seed project focusing on capturing these changes, on femtosecond time scales, with atomic resolution. The MRSEC researchers have setup an instrument to perform ultrafast electron diffraction measurements on condensed matter samples. The performance of the instrument was demonstrated in a first measurement of the ultrafast changes in the diffraction pattern of photoexcited single-crystal gold. A femtosecond laser pulse is used to heat the sample on a very fast timescale, and a femtosecond electron pulse probes the structure a short time after. This instrument has the capability to resolve ultrafast structural changes with femtosecond resolution. 

Picture:

Changes in the diffraction pattern of gold 10 picoseconds after photoexcitation (1 picosecond = 1 trillionth of a second). The intensity of the diffraction peaks decrease (dark blue regions) due to the heating induced disorder, and the intensity around the peaks increases (yellow regions).

highlight 2019

Tenth Annual Conference for Undergraduate Women in Physical Sciences (WoPhyS)

Rebecca Lai and Jocelyn Bosley
Nebraska MRSEC

At Nebraska MRSEC’s Conference for Undergraduate Women in Physical Sciences (WoPhyS), participants present research accomplishments, attend keynote talks, participate in graduate school preparation workshops, and tour UNL facilities and labs.
The tenth annual WoPhyS conference, held October 11-13, 2018, set a new attendance record, drawing 113 registered participants from across the United States, including 14 invited student speakers and 78 poster presentations. In addition, the conference hosted 11 keynote speakers from academia, industry, and national labs, including a special talk by WoPhyS founder Prof. Axel Enders.
The WoPhyS ‘18 conference theme, Standing Out on the Shoulders of Giants, celebrated the history of the conference and the larger history of women’s contributions to the physical sciences. A video commemorating the conference’s tenth year, with commentary from WoPhyS alumnae, can be found at https://youtu.be/7TiSEyaywmY.

Figure: University of Puerto Rico, Rio Piedras undergraduate Shyline Santana gives an invited talk at the 2018 WoPhyS conference.

highlight 2019

Control of Spin State at the Molecular Level

Xiaoshan Xu, Jian Zhang, and Peter Dowben
Nebraska MRSEC

Axel Enders
University of Bayreuth, Germany

Spin-crossover molecules exhibit two non-equivalent spin states, known as low-spin and high-spin states, which can be controlled by external stimuli. This property makes the spin-crossover molecules candidate building blocks for molecular spintronic devices. An additional functionality comes from the coupling between electric and magnetic dipole moments in these molecules.
Nebraska MRSEC researchers have discovered that this coupling allows stabilizing a specific spin state in an iron-based spin-crossover complex. Using interface interactions arising from strongly dipolar zwitterion molecules, they showed a reversible control of the spin state at room temperature. This is finding is important because it may provide a route to molecular electronics that is printable, flexible inexpensive, and nonvolatile. Such molecular electronics could have a great societal impact, by permitting high density memory on something as small and compact as a piece of plastic the shape of a credit card. Especially if the memory aspects are retained in the absence of power.
P. Costa, G. Hao, A. T. N'Diaye, L. Routaboul, P. Braunstein, X. Zhang, J. Zhang, B. Doudin, A. Enders, and P. A. Dowben, “Perturbing the Spin Crossover Transition Activation Energies in Fe(H2B(pz)2)2(bipy) with Zwitterionic Additions,” J. Phys.: Condens. Matter 30, 305503 [5pp] (2018).

Picture: A schematics of a reversible control of the spin state of the iron-based spin-crossover complex [Fe{H2B(pz)2}2(bipy)] in the presence of a strongly dipolar zwitterionic molecule.

highlight 2019

Defect-Assisted Tunneling across Ferroelectric Tunnel Junctions

Konstantin Klyukin, Lingling Tao, Vitaly Alexandrov, and Evgeny Tsymbal
Nebraska MRSEC

Ferroelectric materials possess switchable electric polarization which makes them useful for novel electronic devices, such as ferroelectric tunnel junctions (FTJs). The latter employ an ultrathin ferroelectric layer between two metal electrodes. This layer is so thin that it allows quantum-mechanical electron tunneling across it. Switching its ferroelectric polarization leads to a significant change in electrical resistance of the FTJ, which makes it useful as a memory element.
Nebraska MRSEC researchers have demonstrated that strontium titanate can be used as a ferroelectric tunnel barrier in these devices. By performing state-of-the-art theoretical modeling, they predicted that titanium atoms substituting strontium form localized electronic states in the energy gap of this material. These antisite defects enhance electron tunneling conductance which strongly depends on ferroelectric polarization orientation. This prediction explains the intrinsic mechanism of electron tunneling across strontium titanate and shows a possibility of using this material in FTJs.
K. Klyukin, L. L. Tao, E. Y. Tsymbal, and V. Alexandrov, “Defect-assisted tunneling electroresistance effect in ferroelectric tunnel junctions,” Physical Review Letters 121, 056601 (2018) [PRL Cover page].

Picture: Electron tunneling across a ferroelectric tunnel junction with strontium titanate (SrTiO3) as a barrier layer for two polarization orientations (pointing to left – top panel and pointing to right – bottom panel). Color contrast indicates the amplitude of electron wave propagating from left to right (red – high; blue – low). The antisite TiSr defect enhances electron transmission across the junction which is seen from the red contrast on that site, indicating the enhanced amplitude for one polarization state but not for the other.

highlight 2019

Giant Electrostriction of Halide Perovskites Discovered

Jinsong Huang, Alexei Gruverman, and Stephen Ducharme
Nebraska MRSEC

Halide perovskites are promising materials for efficient conversion of solar energy to electricity. They also exhibit other functional properties, which make them useful as radiation detectors and light-emitting diodes. However, the electromechanical properties of these materials so far have remained largely unexplored.
Nebraska MRSEC researchers have discovered a giant electrostriction effect in methylammonium lead triiodide (MAPbI3) – one the well known halide perovskites. They observed that when an electric field is applied to a MAPbI3 crystal, it becomes compressed along the field direction. This property is a signature of electrostriction, which refers to the strain in a material under applied electric field irrespective of the field sign. The researchers found that the electrostriction energy of halide perovskites is highest among all existing electrostrictive materials, and comparable to that of human muscles. This discovery may lead to new potential applications of halide perovskites as actuators, sonar and micro-electromechanical systems.
B. Chen, T. Li, Q. Dong, E. Mosconi, J. Song, Z. Chen, Y. Deng, Y. Liu, S. Ducharme, A. Gruverman, F. De Angeles, and J. Huang, “Large electrostrictive response in lead halide perovskites,” Nature Materials 17, 1020 (2018).

Picture: Top: Electrostriction of halide perovskite MAPbI3. An electric bias produces an electric field which contracts the MAPbI3 single crystal along the field direction and expands it in the direction perpendicular to the field.

highlight 2019

Science Slams: Seeding a Community of Science Communicators

Rebecca Lai and Jocelyn Bosley
Nebraska MRSEC

Nebraska MRSEC’s signature Science Slams program, now in its third year, has given rise to a robust, cross-disciplinary community of early-career scholars who are cultivating the skills to communicate their work effectively to a broad audience. New in 2018, Science Slam finalists received professional coaching during a half-day science communication workshop presented by the American Association for the Advancement of Science. In addition, past Slammers provided one-on-one mentoring to current finalists. 2018 Science Slam finalists were Karl Ahrendsen, physics; Andrew Conner, mathematics; Kenneth Hipp, chemistry; Alice MillerMacPhee, sociology; Hernán Vázquez Miranda, natural resources; and Kelly Willemssens, natural resources.

Picture: Current and former Science Slam finalists participate in a science communication workshop presented by the AAAS Center for Public Engagement.

113 highlight 2019

Decoupling of Magnetization and Electric Polarization in Hexagonal Ferrites

Xiaoshan Xu and Peter Dowben
Nebraska MRSEC

Alpha T. N'Diaye 
Lawrence Berkeley National Laboratory

Magnetoelectric multiferroic materials exhibit multiple switchable properties such as electric and magnetic polarizations. Hexagonal rare earth ferrites (h-RFeO3, R: rare earth) are multiferroics with ferroelectric, antiferromagnetic, and weak ferromagnetic orders. The coupling between these degrees of freedoms are believed to be promising for application of energy-efficient information storage and processing. Nebraska MRSEC researchers, in collaboration with the Advanced Light Source (ALS) at the Lawrence Berkeley National Lab (LBL), have studied the coupling between the ferromagnetic and ferroelectric orders in h-YbFeO3, using advanced thin film growth and synchrotron x-ray magnetometry. Decoupling between the magnetization and electric polarization has been observed, suggesting strong interplay between the antiferromagnetic and the ferroelectric orders. These finding advance our understanding of the magnetoelectric coupling in hexagonal ferrites and effects of topology.

Picture: A multilayer structure for the measurement of coupling between electric and magnetic degrees of freedom in hexagonal ferrites.

113 highlight 2019

Ferroelectric Domain Wall as a Memristor

Alexei Gruverman, Alexander Sinitskii, and Jeffrey Shield
Nebraska MRSEC

Marty Gregg 
Queen's University Belfast

Ferroelectric materials consist of domains that are spontaneously polarized along the directions allowed by symmetry. Domain walls are boundaries between the domains, where the polarization orientation changes abruptly. Ferroelectric domain walls have recently aroused significant scientific interest due to their electrical conductivity that may be used in future nanoelectronics.
Nebraska MRSEC researchers in collaboration with their colleagues at Queen's University Belfast have discovered the electrical modulation of domain-wall conductivity in ferroelectric lithium niobate thin films down to the nanoscale. They showed that the domain-wall conductivity strongly depends on the preset voltage pulse polarity, amplitude, and duration, giving rise to multilevel conducting states that depend on the history of the pulses applied to the domain wall. Such a behavior is intrinsic to a memristor – a novel, not yet employed logic element. Memristors are important because they are non-volatile, i.e. retain memory without power, and have multiple resistance states. These results open a new functionality of a ferroelectric domain wall which may serve as a memristor in future nanoelectronics.

Picture: Electrical control of domain wall conductivity in lithium niobate thin films: (left panel) images of two domains and a domain wall between them; (right panel) a color map showing local domain-wall conductivity for different voltage pulses. It is seen that the local conductivity along the domain wall changes depending on magnitude and duration of the voltage pulse.

 

All Research and Education Highlights: