zhangj

Supported by NSF DOE ONR	1. New Phases at the Surfaces/Interfaces of Transition Metal Oxides
Transition-metal oxides (TMOs) are characterized by physical complexity resulting from the coexistence and competition between different kinds of interactions involving charge, lattice, orbital, and spin degrees of freedom. The relationship between these degrees of freedom is often synergistic and nonlinear. The balance between competing phases is subtle and small changes can create new phenomena. It is both their complexity and tunability that make TMOs attractive for fundamental studies and applications. The complexity of TMOs is directly responsible for their tunability. Creating surfaces, interfaces, ultrathin films, and artificial superstructures add the additional twists of ‘man-made’ dimensions, approaching the quantum phenomena of correlated materials with broken symmetry and reduced dimensionality. New physics and new functionality appear, thus offering an opportunity for new discovery and new technological implication. We have every expectation that the phenomena awaiting discovery at the surfaces/interfaces of correlated electron materials will prove as intellectually stimulating as the bulk phenomena in these materials
The objective of this project is to take advantage of these “man-made” twists to explore new physics of TMOs in which the exotic properties will respond to broken symmetry, dimensional confinement, lattice strain, or the interaction of the adjacent nanometer scale entities. These entities can arise either spontaneously from the inherent nanophase separation that occurs in TMOs or as a product of careful engineering, e.g. a heterostructure of closely coupled alternating layers of dissimilar ordered phases. Our theme is science-driven fabrication aided by advanced characterization. As pointed out by Anderson, “At each new level of complexity, entirely new properties appear, and the understanding of these behaviors requires research which I think is as fundamental in its nature as any other."
We employ the state-of-art surface sensitive techniques such as variable temperature scanning tunneling microscope (VT-STM), electron energy loss spectroscopy, as well as photoelectron spectroscopy, to probe the structural and electronic properties in the proximity of surface/interface. We also use newly designed laser-assisted molecular beam epitaxy (MBE) system to fabricate artificial superstructures, and then characterize them in situ. We collaborate with Prof. E. W. Plummer's Group at the University of Tennessee-Knoxville (UTK), Prof. Hong Ding's Group at Boston College, the Correlated Electron Materials Group, the Low-Dimensional Materials by Design Group, and the Center for Nanophase Materials Science at Oak Ridge National Laboratory.
Research Highlights: ---- The Surface Phases of the p-wave superconductor Sr₂RuO₄ [.pdf file] ---- Unusual Surface Mott Metal-Insulator Transition [.pdf file] ---- Surface Structure of Layered Perovskites: A LEED-I(V) Study [.pdf file] ---- Surface Lattice Dynamics of Layered Transition-Metal Oxides: Sr₂RuO₄ and La_0.5Sr_1.5MnO₄ [.pdf file] ---- An Imperfection-Driven Phase Transition at 120 K in Cd₂Re₂O₇ [.pdf file] ---- Surfaces: A Playground for Physics with Broken Symmetry in Reduced Dimensionality [.pdf file] ---- Dopant-Induced Nanoscale Electronic Inhomogeneities in Ruthenates [.pdf file]

Supported by

NSF

DOE

ONR

1. New Phases at the Surfaces/Interfaces of Transition Metal Oxides

Transition-metal oxides (TMOs) are characterized by physical complexity resulting from the coexistence and competition between different kinds of interactions involving charge, lattice, orbital, and spin degrees of freedom. The relationship between these degrees of freedom is often synergistic and nonlinear. The balance between competing phases is subtle and small changes can create new phenomena. It is both their complexity and tunability that make TMOs attractive for fundamental studies and applications. The complexity of TMOs is directly responsible for their tunability.

Creating surfaces, interfaces, ultrathin films, and artificial superstructures add the additional twists of ‘man-made’ dimensions, approaching the quantum phenomena of correlated materials with broken symmetry and reduced dimensionality. New physics and new functionality appear, thus offering an opportunity for new discovery and new technological implication. We have every expectation that the phenomena awaiting discovery at the surfaces/interfaces of correlated electron materials will prove as intellectually stimulating as the bulk phenomena in these materials

The objective of this project is to take advantage of these “man-made” twists to explore new physics of TMOs in which the exotic properties will respond to broken symmetry, dimensional confinement, lattice strain, or the interaction of the adjacent nanometer scale entities. These entities can arise either spontaneously from the inherent nanophase separation that occurs in TMOs or as a product of careful engineering, e.g. a heterostructure of closely coupled alternating layers of dissimilar ordered phases. Our theme is science-driven fabrication aided by advanced characterization. As pointed out by Anderson, “At each new level of complexity, entirely new properties appear, and the understanding of these behaviors requires research which I think is as fundamental in its nature as any other."

We employ the state-of-art surface sensitive techniques such as variable temperature scanning tunneling microscope (VT-STM), electron energy loss spectroscopy, as well as photoelectron spectroscopy, to probe the structural and electronic properties in the proximity of surface/interface. We also use newly designed laser-assisted molecular beam epitaxy (MBE) system to fabricate artificial superstructures, and then characterize them in situ. We collaborate with Prof. E. W. Plummer's Group at the University of Tennessee-Knoxville (UTK), Prof. Hong Ding's Group at Boston College, the Correlated Electron Materials Group, the Low-Dimensional Materials by Design Group, and the Center for Nanophase Materials Science at Oak Ridge National Laboratory.

Research Highlights:

---- The Surface Phases of the p-wave superconductor Sr₂RuO₄ [.pdf file]

---- Unusual Surface Mott Metal-Insulator Transition [.pdf file]

---- Surface Structure of Layered Perovskites: A LEED-I(V) Study [.pdf file]

---- Surface Lattice Dynamics of Layered Transition-Metal Oxides: Sr₂RuO₄ and La_0.5Sr_1.5MnO₄ [.pdf file]

---- An Imperfection-Driven Phase Transition at 120 K in Cd₂Re₂O₇ [.pdf file]

---- Surfaces: A Playground for Physics with Broken Symmetry in Reduced Dimensionality [.pdf file]

---- Dopant-Induced Nanoscale Electronic Inhomogeneities in Ruthenates [.pdf file]

Supported by ACS NSF	2. Lattice Structure and Electronic Properties of Polymer Materials
Manipulating/controlling a single atom or molecule has been a compelling research goal in the field of molecular electronics for more than half of a century. After the invention of scanning tunneling microscopy and atomic force microscopy, single atom or molecule manipulation were achieved both on inorganic atoms (molecules) or organic molecules. In addition, there is now an increasing body of evidence for reversible conductance transition associated with a molecular reorientation induced by the tip of scanning probe microscope. The ultrafast and/or ultrasensitive molecular devices and ultrahigh density data storages have being developed rapidly based upon the understanding of local structural, electronic, and optical properties at the atomic scale. One of the attractive materials for molecular electronics is the class of polymers or copolymers in which monomer or monomer clusters may be manipulated with potential novel properties. Strong dipoles as are present in ferroelectric polymer materials server as the “toys” for molecular manipulation. Vinylidene fluoride [P(VDF)] and the copolymer of vinylidene fluoride with trifluoroethylene [P(VDF-TrFE)] are those exhibiting distinctive ferroelectric properties and drawing much of attention. In particular, the copolymer of P(VDF-TrFE) with the molar content of VDF larger than 50% has been well studied on its optical, electrical properties as well as its quasi long-range ordered structure.	Ferroelectric P(VDF-TrFE) Copolymer
The objective of this project is to explore the microscopic structure, the local electronic properties and the fundamental mechanism of nanoscale manipulation for novel functionalities in these polymer materials. The related issues include: · Understanding of the microscopic mechanism of the ferroelectric phase transition in reduced dimensionality such as film thickness. · Study of the stability of local dipole polarization and dynamic flip/flop rate of dipole moments in nanoscale. · Exploring the tunability of local switching property for future application of memory and sensor devices. We collaborate with Dr. Dowben's Group and Dr. Ducharme group at the University of Nebraska-Lincoln.
Research Highlights: ---- Nanoscale Polarization Manipulation and Conductance Switching in Copolymer Films [.pdf file] ---- Microscopic Characterization of the Lattice Structure on P(VDF-TrFE) Copolymer Surface [.pdf file]

Supported by

ACS

NSF

2. Lattice Structure and Electronic Properties of Polymer Materials

Manipulating/controlling a single atom or molecule has been a compelling research goal in the field of molecular electronics for more than half of a century. After the invention of scanning tunneling microscopy and atomic force microscopy, single atom or molecule manipulation were achieved both on inorganic atoms (molecules) or organic molecules. In addition, there is now an increasing body of evidence for reversible conductance transition associated with a molecular reorientation induced by the tip of scanning probe microscope. The ultrafast and/or ultrasensitive molecular devices and ultrahigh density data storages have being developed rapidly based upon the understanding of local structural, electronic, and optical properties at the atomic scale.

One of the attractive materials for molecular electronics is the class of polymers or copolymers in which monomer or monomer clusters may be manipulated with potential novel properties. Strong dipoles as are present in ferroelectric polymer materials server as the “toys” for molecular manipulation. Vinylidene fluoride [P(VDF)] and the copolymer of vinylidene fluoride with trifluoroethylene [P(VDF-TrFE)] are those exhibiting distinctive ferroelectric properties and drawing much of attention. In particular, the copolymer of P(VDF-TrFE) with the molar content of VDF larger than 50% has been well studied on its optical, electrical properties as well as its quasi long-range ordered structure.

Ferroelectric P(VDF-TrFE) Copolymer

The objective of this project is to explore the microscopic structure, the local electronic properties and the fundamental mechanism of nanoscale manipulation for novel functionalities in these polymer materials. The related issues include:

· Understanding of the microscopic mechanism of the ferroelectric phase transition in reduced dimensionality such as film thickness.

· Study of the stability of local dipole polarization and dynamic flip/flop rate of dipole moments in nanoscale.

· Exploring the tunability of local switching property for future application of memory and sensor devices.

We collaborate with Dr. Dowben's Group and Dr. Ducharme group at the University of Nebraska-Lincoln.

Research Highlights:

---- Nanoscale Polarization Manipulation and Conductance Switching in Copolymer Films [.pdf file]

---- Microscopic Characterization of the Lattice Structure on P(VDF-TrFE) Copolymer Surface [.pdf file]

Supported by DOE NSF	3. Collective Phenomena in Highly Correlated Electron Materials
Neutrons, as one of the most powerful probes for the properties of condensed matter, have played an important role in the development of the physics for many complex materials. The reasons for this include: 1) Neutrons carry a magnetic moment that interacts with unpaired electrons in the system. 2) Neutron scattering permits access to nearly any part of energy and momentum space. 3) Neutrons also interact with nuclei, so neutrons peek what the atoms are doing (phonons). Neutron scattering can therefore be used to study both static and dynamic behaviors of lattice or spin degrees of freedom. It can also be used for the study of the charge/orbital orderings or fluctuations indirectly through the effects on either structural or magnetic properties. The objective of this project is to use both elastic and inelastic neutron scattering as the probe to address the fundamental issues in highly correlated materials, especially the emerging collective phenomena due to the strong coupling between different degrees of freedom. We are particularly interested in the novel behaviors of phonon and magnon, correlation between lattice and magnetic phase transitions. the study of the doped ruthenates and manganites is our current focus. Two kinds of samples will be used: single crystals and multilyer films. Single crystals will be acquired through our fruitful collaborations with the Correlated Electron Materials Group at ORNL as well as Prof. Y. Tokura’s group at the University of Tokyo and the Correlated Electron Research Center (CERC) of Japan. Multilayer films will be fabricated with our laser-assisted MBE system or through the collaboration with Prof. E.W. Plummer's Group at the UTK/ORNL and Low-Dimensional Materials by Design Group at ORNL. We have a close collaboration with Prof. P. Dai Group at UTK/ORNL and HFIR Center for Neutron Scattering at ORNL.
The figure shows: (top) the ferromagnetic magnon and optical phonon (open symbols) dispersion of La_0.7Ca _0.3MnO₃ single crystal at 10K along different directions in reciprocal space; (Bottom) the magnon line width as a function of magnon energy of La_0.7Ca _0.3MnO₃ single crystal at 10K. In both [100] and [110] directions which is associated with the MnO plane in crystal structure. It is clear that broadening of magnon occurs when it merge with phonons but only in MnO plane (i.e., not in [111] direction), indicating a strong but anisotropic magnon-phonon coupling.
Research Highlights: ---- John-Teller Phonon Anomaly and Dynamic Phase Fluctuations in La_0.7Ca_0.3MnO₃ [.pdf file] ---- Magnon Damping by Magnon-Phonon Coupling in Manganese Perovskites [.pdf file] ---- Experimental Evidence for the Dynamic Jahn-Teller Effect in La_0.65Ca_0.35MnO₃[.pdf file] ---- Evolution of Spin-Wave Excitations in Ferromagnetic Metallic Manganites [.pdf file]

Supported by

DOE

NSF

3. Collective Phenomena in Highly Correlated Electron Materials

Neutrons, as one of the most powerful probes for the properties of condensed matter, have played an important role in the development of the physics for many complex materials. The reasons for this include: 1) Neutrons carry a magnetic moment that interacts with unpaired electrons in the system. 2) Neutron scattering permits access to nearly any part of energy and momentum space. 3) Neutrons also interact with nuclei, so neutrons peek what the atoms are doing (phonons). Neutron scattering can therefore be used to study both static and dynamic behaviors of lattice or spin degrees of freedom. It can also be used for the study of the charge/orbital orderings or fluctuations indirectly through the effects on either structural or magnetic properties.

The objective of this project is to use both elastic and inelastic neutron scattering as the probe to address the fundamental issues in highly correlated materials, especially the emerging collective phenomena due to the strong coupling between different degrees of freedom. We are particularly interested in the novel behaviors of phonon and magnon, correlation between lattice and magnetic phase transitions. the study of the doped ruthenates and manganites is our current focus. Two kinds of samples will be used: single crystals and multilyer films. Single crystals will be acquired through our fruitful collaborations with the Correlated Electron Materials Group at ORNL as well as Prof. Y. Tokura’s group at the University of Tokyo and the Correlated Electron Research Center (CERC) of Japan. Multilayer films will be fabricated with our laser-assisted MBE system or through the collaboration with Prof. E.W. Plummer's Group at the UTK/ORNL and Low-Dimensional Materials by Design Group at ORNL. We have a close collaboration with Prof. P. Dai Group at UTK/ORNL and HFIR Center for Neutron Scattering at ORNL.

The figure shows: (top) the ferromagnetic magnon and optical phonon (open symbols) dispersion of La_0.7Ca _0.3MnO₃ single crystal at 10K along different directions in reciprocal space; (Bottom) the magnon line width as a function of magnon energy of La_0.7Ca _0.3MnO₃ single crystal at 10K. In both [100] and [110] directions which is associated with the MnO plane in crystal structure. It is clear that broadening of magnon occurs when it merge with phonons but only in MnO plane (i.e., not in [111] direction), indicating a strong but anisotropic magnon-phonon coupling.

Research Highlights:

---- John-Teller Phonon Anomaly and Dynamic Phase Fluctuations in La_0.7Ca_0.3MnO₃ [.pdf file]

---- Magnon Damping by Magnon-Phonon Coupling in Manganese Perovskites [.pdf file]

---- Experimental Evidence for the Dynamic Jahn-Teller Effect in La_0.65Ca_0.35MnO₃[.pdf file]

---- Evolution of Spin-Wave Excitations in Ferromagnetic Metallic Manganites [.pdf file]


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