In many-body quantum systems, especially those with strong electronic correlations, the collective behavior of quasiparticles can give rise to emergent gauge degrees of freedom that are not imposed externally but generated intrinsically by the dynamics of the system. This paper proposes a unified theoretical framework for describing such self-generated gauge fields, here denoted as Aμeff, that arise naturally in quantum matter. We formulate the conditions under which the internal degrees of freedom of a material can spontaneously form a gauge structure, and we derive the effective action for the emergent gauge field in terms of the microscopic variables. The formalism is applied to a variety of strongly correlated systems, including spin liquids, fractional quantum Hall states, and high-temperature superconductors, illustrating how Aμeff modifies the low-energy excitations and leads to novel observable phenomena. Furthermore, we discuss the potential of these emergent interactions to serve as platforms for discovering new artificial fundamental forces, thereby opening a route toward Nobel Prize–level physics.
This review gives a rather general discussion of high temperature superconductors as an example of a strongly correlated material. The argument is made that in view of the many examples of unconventional superconductors discovered in the past twenty years, we should no longer be surprised that superconductivity emerges as a highly competitive ground state in systems where Coulomb repulsion plays a dominant role. The physics of the cuprates is discussed, emphasizing the unusual pseudogap phase in the underdoped region. It is argued that the resonating valence bond picture, as formulated using gauge theory with fermionic and bosonic matter fields, gives an adequate physical understanding, even though many details are beyond the powers of current calculational tools. The recent discovery of quantum oscillations in a high magnetic field is discussed in this context. Meanwhile, the problem of the quantum spin liquid (a spin system with antiferromagnetic coupling which refuses to order even at zero temperature) is a somewhat simpler version of the high T c problem where significant progress has been made recently. It is understood that the existence of matter fields can lead to deconfinement of the U (1) gauge theory in 2 + 1 dimensions, and novel new particles (called fractionalized particles), such as fermionic spinons which carry spin and no charge, and gapless gauge bosons can emerge to create a new critical state at low energies. We even have a couple of real materials where such a scenario may be realized experimentally. The paper ends with answers to questions such as: What limits T c if pairing is driven by an electronic energy scale? Why is the high T c problem hard? Why is there no consensus? And why is the high T c problem important?
The class of relativistic spin particle models reveals the `quantization' of parameters already at the classical level. The special parameter values emerge if one requires the maximality of classical global continuous symmetries. The same requirement applied to a non-relativistic particle with odd degrees of freedom gives rise to supersymmetric quantum mechanics. Coupling classical non-relativistic superparticle to a `U(1) gauge field', one can arrive at the quantum dynamical supersymmetry. This consists in supersymmetry appearing at special values of the coupling constant characterizing interaction of a system of boson and fermion but disappearing in a free case. Possible relevance of this phenomenon to high-temperature superconductivity is speculated.
Gauge theories and topological phases play a fundamental role in different areas of physics. The first ones are at the basis of the Standard Model in the field of particle physics, describing the electroweak and strong interactions through a non-Abelian gauge theory. In condensed matter and statistical physics, gauge theories arise as low-energy effective descriptions of strongly correlated phenomena, such as quantum spin liquids and quantum Hall effect. In this realm there is a strong connection with topological phases and order, as emergent gauge fermions and bosons often describe collective excitations of new exotic states of matter of spin models. The discretization on a lattice is a possible way of dealing with the strongly coupled nature of these theories, due to the fact that this formulation at finite volume provides natural regularizing cut-offs, i.e. the lattice size and spacing. This allows for the investigation of different non-perturbative properties both numerically and analytically. Despite the success of these methods, there are various aspects which remain intractable due to the sign or complex action problems, like the out of equilibrium real time evolution or the analysis of quantum chromodynamics with finite chemical potential. In this respect, quantum simulators of many-body systems to simulate high energy physics arise as promising alternatives to face these problems in the near future. They are quantum systems that can be controlled and used to simulate more complicated systems, whose properties could not be analysed with classical computational, experimental or theoretical tools. In the last decades, there has been a huge development in the fields of quantum optics and atomic physics, allowing for the realization highly precise and controllable platforms by means of trapped ions, superconducting circuits, Rydberg atoms and ultracold atoms in optical lattices. This work of Thesis is part of these fields and has different purposes, all of them connected with the study of gauge theories and topological phases. Firstly, we want to develop a reformulation of lattice gauge theories in terms of gauge invariant fields, in a way to deal solely with physical variables directly in the action. Among several possible advantages, a crucial point is that this can be particularly helpful for the construction of consistent approximation schemes, such as mean-field theories, in order to understand and capture some of the physical features of the theory and make the mean-field approximation consistent with the Elitzur theorem, stating that a local gauge symmetry can not be spontaneously broken. As a second point, we face the problem of simulating higher dimensional gauge theories with ultracold atoms. One of the challenges in more than one dimension is indeed the realization of plaquette interaction terms in the Hamiltonian of lattice gauge theories: these can be engineered through four correlated hoppings in perturbation theory, or by means of constrained hoppings in the dual formulation. In this respect, our main target is to set up an ultracold atomic platform generating the plaquette term in two dimensions using only two correlated hoppings, and protecting gauge invariance through angular momentum conservation. The last point we address in the Thesis is related to the analysis of particles moving in static background gauge potentials, i.e. when the considered gauge field has no dynamical term in the action of the system. Indeed, the dynamics of quantum particles in the presence of static gauge fields gives rise to intriguing physical phenomena. In particular, using ultracold atom setups, the realization of artificial gauge potentials can be used to investigate the physics of topological semimetals, such as Weyl or Dirac type. In the last ten years, a lot of attention has been paid to their characterization, due to the appearence of clear theoretical predictions and very well-controlled experimental techniques. We investigate here the relation between topological phase transitions and van Hove singularities, i.e. the discontinuities in the energy derivative of the density of states, in three-dimensional gapless systems. In such materials, topological phase transitions can be defined by changes of the topological invariants of the Fermi sheets, happening at specific singular points. Moreover, Fermi surface singularities result in the presence of the so-called van Hove points. We then present a general argument to relate topological phase transitions and van Hove singularities, and show observable consequences that are related to the transport properties of the system. We exemplify our argument in Weyl systems by analyzing the three-dimensional Hofstadter model for various commensurate fluxes, which offers the opportunity to consider different kinds of Weyl metals and to understand the features of their density of states.
This thesis concerns two topics where symmetry and strong correlations in time and space are important. The first topic is the theory of dynamical systems, and this theory deals with generic phenomena which are often convenient to study in very simple models. Dynamical systems can be divided in two basic classes: dissipative and conservative systems. Dissipation often effectively reduces the degrees of freedom in a dynamical system, and it is often sufficient to study low-dimensional models. An example is the one- dimensional circle map. The transition from ordered to chaotic motion in a circle map with extra symmetry is studied in Paper I using renormalization group techniques. Special emphasis is put into the possible breakdown of the extra symmetry. An example of a conservative system is a particle moving in a potential field, and typical for the dynamics is the large number of periodic trajectories. A particularly simple class of conservative systems is the integrable systems. In such a system there are continuous manifolds of marginally stable periodic orbits, which are divided into discrete sets of stable and unstable periodic orbits if the potential of the system is nonlinearly perturbed. In Paper II we demonstrate how the stable and unstable orbits in a quartic potential exchange stability several times as the strength of the nonlinear perturbation is increased. The second topic of this thesis is the ceramic high-temperature superconductors. Apart from being superconducting these materials show several other phases dependent on the doping. The 2-dimensional Hubbard model has been suggested as a highly simplified model for the electronic properties of these materials, and we study an extended version of this model at arbitrary temperature and doping using a theory capable of describing a rich variety of states simultaneously. Important is that we use no a priori assumptions of the state of the system. The high degree of freedom in the theory requires a group- theoretical treatment in order to classify the possible states, and this classification relies on the basic symmetries of the Hubbard model. Examples of states that we find are antiferromagnetic, ferromagnetic, superconducting (s- and d-wave), charge density wave and spiral spin-wave states.