This award supports theoretical and computational research and educational activities related to the problem of inhomogeneous competing order in metals. An increasing number of novel materials exhibit emergent inhomogeneous phases, which are states of matter where the ground state is modulated and may display various types of long-range order. The theory of such states, which represents a fundamental departure from the basic momentum-space one-particle paradigms of condensed matter physics, is still in its infancy. Along this direction, the PI will study the general question of how inhomogeneous order emerges and how the particular choice taken by the system depends on disorder, interactions, and its electronic structure. Models will be studied which are applicable to a diverse class of interacting systems, appropriate for heavy fermion, iron-pnictide, cuprate, and other oxide materials. An improved understanding of the role of disorder in the presence of competing ordered states will facilitate the identification of the most important instabilities in various systems, and enable researchers to focus on the intrinsic aspects of exotic ordering phenomena such as high temperature superconductivity.
This award supports research, which is ultimately expected to have an impact on the design and development of oxide materials for applications. It is noteworthy that the entire electronics industry is built on doped semiconductor devices at present, and may be said to have been so successful in part because solutions were found to prevent electron mobilities from being limited by disorder which accompanies the doping process. Since in correlated systems and novel oxides in particular, impurities have a much more important impact, specifically through their nucleation of inhomogeneous phases, these problems represent important and also practical hurdles to be surmounted before oxide electronics can succeed in the marketplace.
The PI will train two graduate students during the course of the project. The PI will also engage in outreach activities to educate school children in the Gainesville area about superconductivity and notions of order and disorder which are important in many disciplines of science.
NONTECHNICAL SUMMARY
This award supports theoretical and computational research and educational activities related to the problem of "inhomogeneous competing order" in metals. This fascinating situation occurs when a complex material is close to making a transition from one electronic phase to another, and two or more different quantum mechanical states become so close in energy as to be essentially indistinguishable. Rather than deciding to form in one or the other of these states, in the presence of small amounts of impurities or imperfections, the material sometimes forms an inhomogeneous state, which varies in composition from point to point in space. Materials that exhibit such exotic behavior can be potentially tuned for use in so-called smart materials which can improve the way many different electronic devices work. In this project, the PI will perform theoretical calculations and computer simulations on a variety of novel materials where disorder is known to play a role of this type, to understand how superconductivity, an electronic state that can conduct electricity without resistance, and magnetism can be influenced by impurities.
This is fundamental research aimed to advance our understanding of materials that are very sensitive to impurities and imperfections leading to unusual quantum mechanical states of electrons. Many of these states appear to be connected to superconductivity. This research contributes to understanding key properties of materials that lead to superconductivity.
The PI will train two graduate students during the course of the project. The P.I. will also engage in outreach activities to educate schoolchildren in the Gainesville area about superconductivity and notions of order and disorder which are important in many disciplines of science.
Metals where electron-electron interactions are poorly screened are often unstable to a variety of inhomogneous phases like charge and spin density waves (SDW). Impurities are known to be able to "freeze" fluctuations of these ordered phases around themselves, creating an emergent defect state. Recently, the PI and group performed simulations of a model including antiferromagnetic spin fluctuations in a multiband metal appropriate for the iron-based superconductors (FeSC), a new class of high-temperature superconducting materials. They found that impurities can create unusual "emergent" defect structures due to the proximity to magnetic stripe order. These "nematogens" take the form shown in Figure 1: they elongate and grow above the transition, then freeze below into remarkably long defect structures. Defect structures very similar to the latter have been seen in many scanning tunneling microscopy (STM) experiments on the FeSC. In addition, the PI and co-workers noted that such defects should scatter conduction electrons anisotropically, and predicted the difference to be expected between the a and b crystal axes of an orthorhombic system of this type. With this model, they were able to explain a number of puzzling aspects of transport experiments on the commonly studied FeSC material BaFe2As2. The PI and collaborators also studied the general problem of the instability of a SDW metal to Cooper pairing, and showed that progress can be made on this problem by utilizing the fact that when the magnetic state is formed, the Fermi surface of the metal is "reconstructed" into pockets which become quite small for a system near half-filling. The interactions between the fermionic quasiparticles of the SDW is complicated, involving exchange of spin waves, longitudinal spin and charge fluctuations, as well as particle-hole excitations at various wavelengths. However in the limit when the pockets are small, this interaction can be analyzed and decomposed into component parts with simple physical interpretations. This method and results should lead to be better understanding of the phase diagram of the cuprates, FeSC and other unconventional superconducting systems where superconductivity and magnetism coexist. Finally, the PI and co-workers investigated the effect of impurities on the pairing interaction in strongly spin fluctuating systems. Within Hubbard-type models for metals with strong local Coulomb, they showed that impurities lowered the energies of spin fluctuations locally in a small region around themselves, of order the antiferromagnetic correlations length. This "freezing" of spin fluctuations, if it is not quite complete, can lead to a significant enhancement of the pairing of two electrons near the impurity site. Thus the correct picture of the pairing in a disordered strongly correlated electron system is one in which the pairing interaction itself is not a constant, but a landscape with local atomic scale regions of enhanced pairing, as deduced by the P.I.’s group from phenomenological analyses of STM data.
