This is part one in a series of two papers dedicated to the notion that the destruction of the topological order associated with stripe phases is about the simplest theory controlled by local symmetry: Ising gauge theory. This first part is intended to be a tutorial- we will exploit the simple physics of the stripes to vividly display the mathematical beauty of the gauge theory. Stripes, as they occur in the cuprates, are clearly topological' in the sense that the lines of charges are at the same time domain walls in the antiferromagnet. Imagine that the stripes quantum melt so that all what seems to be around is a singlet superconductor. What if this domain wall-ness is still around in a delocalized form? This turns out to be exactly the kind of matter' which is described by the Ising gauge theory. The highlight of the theory is the confinement phenomenon, meaning that when the domain wall-ness gives up it will do so in a meat-and-potato phase transition. We suggest that this transition might be the one responsible for the quantum criticality in the cuprates. In part two, we will become more practical, arguing that another phase is possible according to the theory. It might be that this quantum spin-nematic has already been observed in strongly underdoped LSCO.
We construct a theory of continuous stripe melting quantum phase transitions in two-dimensional metals and the associated Fermi surface reconstruction. Such phase transitions are strongly coupled but yet theoretically tractable in situations where the stripe ordering is destroyed by proliferating doubled dislocations of the charge stripe order. The resulting non-Landau quantum critical point has strong stripe fluctuations which we show decouple dynamically from the Fermi surface even though static stripe ordering reconstructs the Fermi surface. We discuss connections to various stripe phenomena in the cuprates. We point out several puzzling aspects of old experimental results [G. Aeppli et al., Science 278, 1432 (1997)] on singular stripe fluctuations in the cuprates, and provide a possible explanation within our theory. These results may thus have been the first observation of non-Landau quantum criticality in an experiment.
We attempt to access the regime of strong coupling between charge carriers and transverse dynamics of an isolated conducting ``stripe,'' such as those found in cuprate superconductors. A stripe is modeled as a partially doped domain wall in an antiferromagnet
(AF), introduced in the context of two different models: the $t\ensuremath{-}J$ model with strong Ising anisotropy, and the Hubbard model in the Hartree-Fock approximation. The domain walls with a given linear charge density are supported artificially by boundary conditions. In both models we find a regime of parameters where doped holes lose their spin and become holons (charge
Q=1, spin
S3=0), which can move along the stripe without frustrating AF environment. One aspect in which the holons on the AF domain wall differ from those in an ordinary one-dimensional electron gas is their transverse degree of freedom: a mobile holon always resides on a transverse kink (or antikink) of the domain wall. This gives rise to two holon flavors and to a strong coupling between doped charges and transverse fluctuations of a stripe.
This thesis is devoted to the study of strongly correlated phases of quantum matter in two spatial dimensions. In presence of strong interactions, the electron can split up into separate spin and charge degrees of freedom. The diagnosis of such fractionalized excitations in recent experiments, as well as finding new probes for their detection, comprise the two major themes of this thesis.\nThe first part of the thesis, comprising chapters 2-6, discusses the relevance of fractionalization to the pseudogap phase of the underdoped high Tc cuprate superconductors. This is motivated by transport experiments that show a violation of Luttinger's theorem, possible only in the presence of topological order that can arise naturally as a consequence of fractionalization. Chapters 2 and 3 focus on phenomenological models with bosonic charge carriers and fermionic spin carriers, and investigate transport properties as well as various confinement transitions to symmetry broken phases. Chapters 4, 5 and 6 deal with a different framework of electron fractionalization, where the charge carriers are fermionic and spin carriers are bosonic. We find that these models can better explain the thermal and electrical transport properties in chapter 5. In chapters 6 and 7, we show how such models arise naturally from quantum fluctuations of antiferromagnetism, and can simultaneously intertwine the anti-nodal spectral gap with the discrete broken symmetries that are observed in the cuprates. \nThe second part of the thesis, comprising chapters 7 and 8, proposes new experimental probes to study the behavior of insulating two dimensional quantum magnets. Aided by geometric frustration and strong quantum fluctuations, they may realize a spin liquid ground state which can be extremely difficult to detect via conventional probes. Chapter 7 discusses how novel spin-transport probes from the spintronics community can be used to detect fractionalized excitations in a quantum spin liquid. Chapter 8 studies how the temperature dependent anisotropy of in-plane versus out-of-plane thermal conductivities can serve as an explicit signature of fractionalization in layered two-dimensional materials.
We develop a new theoretical framework for describing and analyzing exotic phases of strongly correlated electrons which support excitations with fractional quantum numbers. Starting with a class of microscopic models believed to capture much of the essential physics of the cuprate superconductors, we derive a new gauge theory---based upon a discrete Ising or
Z2 symmetry---which interpolates naturally between an antiferromagnetic Mott insulator and a conventional d-wave superconductor. We explore the intervening regime, and demonstrate the possible existence of an exotic fractionalized insulator, the nodal liquid, as well as various more conventional insulating phases exhibiting broken lattice symmetries. A crucial role is played by vortex configurations in the
Z2 gauge field. Fractionalization is obtained if they are uncondensed. Within the insulating phases, the dynamics of these
Z2 vortices in two dimensions is described, after a duality transformation, by an Ising model in a transverse field, the Ising spins representing the
Z2 vortices. The presence of an unusual Berry's phase term in the gauge theory leads to a doping-dependent ``frustration'' in the dual Ising model, being fully frustrated at half filling. The
Z2 gauge theory is readily generalized to a variety of different situations, in particular, it can also describe three-dimensional insulators with fractional quantum numbers. We point out that the mechanism of fractionalization for
d>1 is distinct from the well-known one-dimensional spin--charge separation. Other interesting results include a description of an exotic fractionalized superconductor in two or higher dimensions.
For a gapped disordered many-body system with both internal and translation symmetry, one can define the corresponding weak and strong symmetry protected topological (SPT) phases. A strong SPT phase is protected by the internal symmetry
G only while a weak SPT phase, fabricated by alignment of a strong SPT state in a lower dimension, requires additional discrete translation symmetry protection. In this paper, we construct a phase transition between weak and strong SPT phase in a strongly interacting boson system. The starting point of our construction is the superconducting Dirac fermions with pair density wave (PDW) order in
2d. We first demonstrate that the nodal line of the PDW contains a
1d boson SPT phase. We further show that melting the PDW stripe and condensing the nodal line provoke the transition from weak to strong SPT phase in
2d. The phase transition theory contains an O(4) nonlinear-$\ensuremath{\sigma} \mathrm{model}\phantom{\rule{0.28em}{0ex}}(\mathrm{NL}\ensuremath{\sigma}\mathrm{M})$ with topological $\mathrm{\ensuremath{\Theta}}$ term emerging from the proliferation of domain walls bound to an SPT chain. A similar scheme also applies to weak-strong SPT transition in other dimensions and predicts possible phase transition from
2d to
3d topological order.
Recent experiments have shown the existence of an ordered stripe phase in the cuprates, suggesting that the nearby metallic phase is best described as a domain-wall fluid. As a minimal model to describe the melting of charged domain walls we introduce quantum lattice strings. This model is related to a generalized quantum spin chain. The string exhibits a rich phase diagram, containing several rough phases with low-lying excitations as well as gapped ordered phases. In this scenario the occurrence of the stripe phase is related to a transition from the quantum-rough (metal) to the classical flat phase.