Xiaoxing Xi - US grants

9404579 Venkatesan Technical abstract: Focus will be on the study of electric-field effects in high Tc superconducting thin films, a topic closely related to the development of three-terminal superconducting devices. The objective is to obtain a comprehensive assessment of the prospects for high Tc Supercccconducting Field-Effect Transistors (SuFET). Based on the existing data, two material problems have been identified as the limiting factors for the current device performance: the quality of the SrTiO3 (STO) dielectric layer and the quality of YBCO ultrathin films. In support of the project objective, work will be done in these two directions to improve the device properties. In addition to the study and optimization of the device characteristics experiments on unique SuFET applications, such as in SQUID circuits and microwave resonators will be continued and also extended to new device concepts. At the same time, the basic study of the physics of field effects in high Tc superconductors will continue. Non-technical abstract: The aim of the program is to investigate the effect of electric fields on the properties of the high temperature superconductors. It has been shown that by use of electric fields the transition temperature and the maximum current carrying ability of superconductors can be modified. Thus, one may be able to design novel devices where the device properties are modified by the use of electric fields. In order to understand the science as well as the full device potential of this phenomenon it is essential to fabricate high quality thin film based structures. One of the goals of this program is to understand and optimize the materials growth process. It is expected that a better understanding for the potential of three terminal superconducting devices will emerge from this program. ***

9702632 Xiaoxing Xi The research of this CAREER project studies the dielectric nonlinearity and microwave losses of ferroelectric thin films deposited by Pulsed Laser Deposition. Compared to bulk single crystals, the nonlinearity of ferroelectric thin films is smaller and the loss is higher, the cause of which is not well understood. This project aims to address these problems by focusing on the effects of four factors: (1) Stoichiometry; (2) Crystallinity; (3) Interface layers and space charges; and (4) Soft-mode phonons. The goal of the research is to reveal the fundamental mechanisms so as to reduce the losses while maintaining a high nonlinearity in ferroelectric thin films. The education plan is to develop a "Ferroelectric Thin Films" experiment for the Physics undergraduate laboratory course, which includes film deposition, structural and electrical characterizations and data analysis. %%% This integrated CAREER research and education project focuses on ferroelectric thin films used in voltage-tunable microwave devices. Such application requires large dielectric nonlinearity and low microwave losses in ferroelectric thin films. This research aims to reveal the fundamental mechanisms that determine these properties in order to improve them thus impacting the development of ferroelectric tunable microwave device technologies. The education plan attempts to incorporate research experience into the physics undergraduate curriculum. A "Ferroelectric Thin Films" experiment will be developed and included in the regular laboratory course so that students can get real-life research experience in this course. ***

9802900
Xi
This award provides partial support for the acquisition of a Microwave Network Analyzer System to support basic materials research by faculty members in the Physics, Chemistry, and Materials Science and Engineering Departments at the Pennsylvania State University. This equipment will enhance research in a number of areas including the following: (1) Dielectric nonlinearity and loss in ferroelectric thin films for tunable microwave devices; (2) characterization of high temperature superconductors by microwave penetration depth measurement to study insulator-superconductor transition by electrostatic charging; (3) study of two dimensional properties of high temperature superconductors concerning dynamics of vortex-antivortex pairs by frequency dependent measurements; (4) effects of spin injection on superconductivity in colossal magnetoresistance/high temperature superconductor multilayers; (5) single electron tunneling system produced by chemical self-assembly; (6) growth of metastable oxides by epitaxial stabilization using molecular bean epitaxy, including new artificial materials with ferroelectric and superconducting properties. Besides the four faculty members, these research projects currently involve 31 graduate students, postdoctoral fellows, and research associates. The microwave network analyzer system enables researchers to measure the magnitude, phase, and group delay of two-port networks to characterize their linear behavior from 45MHz-26.5GHz. It is a versatile system that can be used and configured for many different types of microwave measurements required by the various research projects. For example, it can be used for coplanar microstrip resonator, near-field microwave microscope, and normal metal or superconductor cavity measurements. In the proposed research, the network analyzer will be used to measure dielectric loss of ferroelectric thin films at microwave frequencies, the superfluid density in two-dimensional superconductors, and a full impedance analysis of single electron tunneling junction arrays to test quantum effects in these devices. In each of the research projects, the microwave frequency capability is crucially needed, but is not available at Penn State to these projects. The proposed network analyzer will tremendously enhance the effectiveness for the faculty members to carry out their projects and make big impact in their respective research fields. The instrument will also enhance the education activities in an NSF/CAREER project which aims to incorporate research experiences into the undergraduate curriculum by developing an experiment, Ferroelectric Thin Films, for the undergraduate laboratory course. This is part of the effort by the Physics Department at Penn State University, in particular the newly-established Center for Materials Physics, to reform physics education. Adding a microwave network analyzer to the existing facilities will expend the capability to offer students materials research experience through the undergraduate curriculum.
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This research focuses on the in situ deposition of epitaxial MgB2 superconductor thin films and multilayers by hybrid physical-chemical vapor deposition (HPCVD). MgB2 is a conventional superconductor with a high transition temperature of 39 K, promising Josephson junctions like the low-Tc superconductor junctions but operating at 25 K. The PIs have developed the HPCVD technique and grown epitaxial MgB2 films with excellent superconducting and transport properties. This project will further improve the HPCVD technique to make it more controllable and compatible to multilayer deposition for Josephson junctions. The research will model the HPCVD growth conditions, test various reactor configurations, and address various materials issues in the growth mechanism, the suitable tunnel barrier material, and the deposition of multilayer structures. A demonstration of a 25-K Josephson junction suitable for superconducting integrated circuits and a large scale processing technique will have a significant impact on the microelectronics industry. Students trained in this project will be exposed to both physics and materials science and acquire skills important for their careers in materials research in both academic and industrial environments.

Non-Technical Abstract
This research focuses on the thin film deposition of the newly-discovered MgB2 superconductor. Unlike the unconventional high-Tc superconductors, MgB2 is a conventional superconductor with a high transition temperature of 39 K. This makes it more promising to produce MgB2 Josephson junctions, the most elemental devices for superconducting integrated circuits for ultrafast digital processing. To achieve MgB2
Josephson junctions and integrated circuits, a thin film technique compatible with multilayer depositions is required. The PIs have developed an innovative technique, hybrid physical-chemical vapor deposition (HPCVD), to grow epitaxial MgB2 films with excellent superconducting and transport properties. However, the technique is in its very early stage of development and needs improvements. This project will use theoretical modeling to guide the HPCVD improvement, address various materials issues in the film growth, and develop multilayers for Josephson junctions. The success of the project will have a significant impact on the microelectronics industry. Students trained in this project will be exposed to both physics and materials science and acquire skills important for their careers in materials research in both academic and industrial environments.

NON-TECHNICAL DESCRIPTION: For many years molecular beam epitaxy (MBE) has been used to build layered semiconductor nanostructures atom-by-atom to investigate and improve our understanding of semiconductor physics and create new devices. These devices (which include laser diodes, high-performance transistors, and magnetic field sensors) have advanced healthcare, national security, communications, entertainment, and transportation-resulting in significant improvements in the quality of life for all Americans. Recent progress in research has demonstrated that this same atom-by-atom synthesis technique can be used to build nanostructures of oxides, including ferroelectrics, with comparable nanometer-scale layering control. Since ferroelectric materials exhibit a wide variety of electrical, optical, and electromechanical properties, they are extensively used in healthcare (e.g., medical ultrasound), national defense (e.g., night vision and sonar systems), and communications (e.g., miniature capacitors for cell phones and computers). The ability to customize the layering of ferroelectric materials at the atomic-layer level and strain them opens exciting possibilities to dramatically enhance their properties. The improved understanding gained via this research will be applied to the development of improved optical and acoustic devices. Future scientists in a highly interdisciplinary research environment in a technologically significant area of national importance will be trained and educated within this program. Professors from Pennsylvania State University, University of Wisconsin, University of Michigan and Rutgers University will run hands-on workshops during the summers at each of the campuses involved in this research team to expose K-12 students to the thrill of science.

TECHNICAL DETAILS: The technical objective is to understand the fundamental science underlying the electric, magnetic, and optical responses of strained nanoscale ferroelectrics and multiferroics. An integrated theoretical and experimental effort will be taken. Specifically, "first-principles effective Hamiltonian" approaches based on lattice Wannier functions and Landau-Ginzburg type phenomenological methods will be used to identify ferroelectric and multiferroic materials and heterostructures in which large enhancements in properties are expected with strain. Films will be grown by MBE and laser-MBE, patterned by focused ion beams, and characterized using a combination of x ray diffraction, analytical and transmission electron microscopy, Raman spectroscopy, second harmonic generation, and ferroelectric measurements, all as a function of temperature. Strain is utilized in many semiconductor device structures to improve the transport properties of thin semiconductor layers. Within this project, it will be used to enhance the properties of ferroelectrics. Ferroelectrics are very sensitive to strain and a distinct advantage of thin ferroelectric materials over their bulk counterparts is that they may be strained well beyond where their bulk counterparts would crack. For nanoscale ferroelectrics, huge strains become accessible. This feature combined with the ability to precisely integrate and engineer oxides at the atomic level provides a means to investigate, develop, and exploit the properties of oxides for optical modulators, two-dimensional photonic bandgap structures, and phonon-confining piezoelectric structures relevant to the long-term realization of a phonon laser.

NON-TECHNICAL DESCRIPTION: The mechanism that leads to the high temperature superconductivity in a family of layered copper oxides has been an active area of research since its discovery in 1986. Recently, it has been predicted theoretically that the electronic structure in the high temperature superconducting cuprates can be realized in lanthanum nickelate when it is sandwiched between insulating oxide such as lanthanum aluminate in the so-called superlattices. Both the lanthanum nickelate and lanthanum aluminate layers need to be very thin, just two atomic layers. Proving or disproving this prediction can not only help understand the mechanism of high temperature superconductivity, but potentially lead to discovery of new high temperature superconductors in cuprates, nickelates, and other material systems. The goal of this project is to fabricate the nickelate superlattices by atomic layer-by-layer growth using laser molecular beam epitaxy and measure their electronic structure properties and superconductivity to test the theoretical prediction. Success of the project can significantly advance the knowledge in the areas of strongly correlated transition metal oxides, materials by design, and nanoscale engineering of oxide heterostructures. The project supports a female graduate student for her Ph.D. degree, thus directly broadens the participation of underrepresented groups.

TECHNICAL DETAILS: This EAGER grant focuses on tuning orbital order in nickelate superlattices by atomic layer-by-layer growth using laser molecular beam epitaxy. Recently, it has been predicted theoretically that using reduced dimensionality and epitaxial strain the electronic structure in the high-Tc superconducting cuprates can be realized in lanthanum nickelate by sandwiching it between insulating oxide such as lanthanum aluminate in superlattices where each period contains one unit cell of each materials. Doping could then induce superconductivity. No experimental proof has been reported despite numerous efforts and the validity of the theoretical prediction has been questioned. This project uses a new film deposition technique, laser molecular beam epitaxy from separate oxide targets, to achieve the atomic layer-by-layer growth of the nickelate superlattices. This approach is more appropriate than the growth techniques that have been attempted in tuning the orbital order and inducing superconductivity in the nickelate superlattices. The success of the project can significantly advance the knowledge in the areas of strongly correlated transition metal oxides, materials design, and nanoscale engineering of oxide heterostructures. The project provides multidisciplinary training for a female graduate student, directly broadening the participation of an underrepresented group.

****Technical Abstract****
This project will use heterojunctions between the electron-doped and hole-doped iron pnictide superconductors to perform the phase-sensitive test of the sign-changing s-wave symmetry and use high quality tunnel junctions to measure the values and distributions of the multiple energy gaps of these superconductors. The iron pnictides and chalcogenides, with Tc up to 55 K, are a new family of high temperature superconductors with unconventional mechanism and unusual properties. The disconnected electron and hole Fermi surfaces in the iron pnictides give rise to unconventional pairing symmetries and gap structures, with the sign-changing s-wave pairing widely believed to be the correct one among several possibilities. Proving this pairing symmetry unambiguously with phase-sensitive measurements is the "Holy Grail" of the iron pnictide research. Josephson effect between the electron- and hole-doped iron pnictides has been suggested as the most promising technique to probe the gap symmetry and gap structures. This project will fabricate the electron/hole heterojunctions by depositing epitaxial thin films of the electron-doped pnictide on the single crystals of the hole-doped pnictide and use them to perform the phase-sensitive measurement. This basic research on iron pnictides could lead to the understanding of the mechanism of high-temperature superconductivity in general and lead to the discovery of more new superconductors with even higher Tc and better properties. This project will provide multidisciplinary training for a PhD student and provide research experiences for undergraduate students.

****Non-Technical Abstract****
The iron-containing superconductors, including the pnictides that contain arsenic and the chalcogenides that contain selenium, were discovered in 2008 to have superconducting transition temperatures up to 55 degrees Kelvin. They are a new family of high temperature superconductors with unconventional mechanism and unusual properties, one of which is the unique symmetry of the characteristic superconducting order parameter in the momentum space. It is widely believed, but not yet unambiguously proven experimentally, that both positive and negative order parameters exist in the iron-containing superconductors, but the signs do not change with respect to the directions in the momentum space. This project attempts to prove this symmetry using Josephson junctions between an electron-doped and a hole-doped pnictide; the junction will be formed by growing an electron-doped pnictide thin film on a hole-doped pnictide single crystal. Understanding the symmetry of the superconducting order parameter in the iron-containing superconductors can help to understand the mechanism of high-temperature superconductivity in general, which in turn could lead to the discovery of more new superconductors with even higher transition temperatures and better properties. This project will provide multidisciplinary training for one graduate student for his/her PhD degree, and provide research experiences for undergraduate students at Temple University, an urban university with a diverse student population.