7 research outputs found
Valence bond glass -- A unified theory of electronic disorder and pseudogap phenomena in high temperature superconductors
We show that the low-energy fluctuations of the valence bond in underdoped
high-T_c cuprates, originating from quantum fluctuations of the superexchange
interaction, are pinned by the electronic disorder due to off-stoichiometric
dopants, leading to a valence bond glass (VBG) pseudogap phase. The antinodal
Fermi surface sections are gapped out, giving rise to a normal state Fermi arc
whose length shrinks with underdoping. Below T_c, the superexchange interaction
induces a d-wave superconducting gap that coexists with the VBG pseudogap. The
evolution of the local and momentum-space spectroscopy with doping and
temperature captures the salient properties of the pseudogap phenomena and the
electronic disorder. The unified theory elucidates the important interplay
between strong correlation and the intrinsic electronic disorder in doped
transition metal oxides.Comment: 5 pages and 4 figures,revised version; to appear in Phys. Rev. Let
Incommensurate Valence Bond Density Waves in the Glassy Phase of Underdoped Cuprates
Thesis advisor: Ziqiang WangOne of the most unconventional electronic states in high transition temperature cuprate superconductors is the pseudogap state. In the temperature versus doping phase diagram, the pseudogap state straddles across the antiferromagnetic (AF) state near half filling and the superconducting (SC) dome on the hole doped side above the transition temperature Tc. The relationship between the pseudogap state and these two well known states - the AF state and the SC state is believed to be very important for understanding superconductivity and the emergent quantum electronic matter in doped Mott insulators. The pseudogap is characterized by the emergence of a soft gap in the single-particle excitation spectrum in the normal state in the temperature range between Tc and a characteristic temperature T*, i.e. Tc < T < T*. The most puzzling feature of the pseudogap is the nodal-antinodal dichotomy. Observed by ARPES in momentum space, the Fermi surface is gapped out in the antinodal region leaving a Fermi arc of gapless excitations near the nodes. Whether the pseudogap is an incoherent superconducting gap (onegap scenario) or it is a different gap governed by other mechanisms, other than superconductivity, (two-gap scenario) is still under debate. In this thesis I study the particle-particle channel and the particle-hole channel of the valence bond fluctuations away from half filling. Based on a strong-coupling analysis of the t-J model, I argue that the superexchange interaction J induced incommensurate bond centered density wave order is the driving mechanism for the pseudogap state. Low energy density of states (DOS) are eliminated by multiple incommensurate scatterings in the antinodal region at the Fermi level. I show that the interplay between the incommensurate bond centered d-wave density wave instability and the intrinsic electronic inhomogeneity in real cuprate materials is responsible for the observed pseudogap phenomena. Utilizing the spatially unrestricted Gutzwiller approximation, I show that the off-stoichiometric doping induced electrostatic disorder pins the low-energy d-wave bond density fluctuations, resulting in a VBG phase. The antinodal Fermi surface (FS) sections are gapped out, giving rise to a genuine normal state Fermi arc. The length of the Fermi arc shrinks with underdoping below the temperature T* determined by thermal filling of the antinodal pseudogap. Below Tc, the d-wave superconducting gap due to singlet pairing coexists and competes with the VBG pseudogap. The spatial, momentum, temperature and doping dependence of these two gaps are consistent with recent ARPES and STM observations in underdoped and chemically substituted cuprates. The temperature versus doping phase diagram captures the salient properties of the pseudogap phenomena and provides theoretical support for the two-gap scenario. In addition to resolving the complexities of the quantum electronic states in hole-doped cuprates, my unified theory elucidates the important role of the interplay between the strong electronic correlation and the intrinsic electronic disorder in doped transition metal oxides.Thesis (PhD) — Boston College, 2011.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics
Quasispecies Theory for Evolution of Modularity
Biological systems are modular, and this modularity evolves over time and in
different environments. A number of observations have been made of increased
modularity in biological systems under increased environmental pressure. We
here develop a quasispecies theory for the dynamics of modularity in
populations of these systems. We show how the steady-state fitness in a
randomly changing environment can be computed. We derive a fluctuation
dissipation relation for the rate of change of modularity and use it to derive
a relationship between rate of environmental changes and rate of growth of
modularity. We also find a principle of least action for the evolved modularity
at steady state. Finally, we compare our predictions to simulations of protein
evolution and find them to be consistent.Comment: 21 pages, 4 figures; presentation reordered; to appear in Phys. Rev.
Physical Model of the Immune Response of Bacteria Against Bacteriophage Through the Adaptive CRISPR-Cas Immune System
Bacteria and archaea have evolved an adaptive, heritable immune system that
recognizes and protects against viruses or plasmids. This system, known as the
CRISPR-Cas system, allows the host to recognize and incorporate short foreign
DNA or RNA sequences, called `spacers' into its CRISPR system. Spacers in the
CRISPR system provide a record of the history of bacteria and phage
coevolution. We use a physical model to study the dynamics of this coevolution
as it evolves stochastically over time. We focus on the impact of mutation and
recombination on bacteria and phage evolution and evasion. We discuss the
effect of different spacer deletion mechanisms on the coevolutionary dynamics.
We make predictions about bacteria and phage population growth, spacer
diversity within the CRISPR locus, and spacer protection against the phage
population.Comment: 37 pages, 13 figure
Prediction of heart rate response to conclusion of spontaneous breathing trial by fluctuation dissipation theory
The non-equilibrium fluctuation dissipation theorem is applied to predict how critically ill patients
respond to treatment, based upon data currently collected by standard hospital monitoring devices.
This framework is demonstrated on a common procedure in critical care: the spontaneous
breathing trial. It is shown that the responses of groups of similar patients to the spontaneous
breathing trial can be predicted by the non-equilibrium fluctuation dissipation approach. This
mathematical framework, when fully formed and applied to other clinical interventions, may serve
as part of the basis for personalized critical care