In this thesis, the physics of two systems is investigated, both of which support unconventional phases of matter. We investigate a frustrated system which has a classical spin liquid regime in the paramagnetic phase. We also consider a periodically-driven, disordered system and explore nonequilibrium regimes. Both systems exhibit macroscopic behaviour which differs strongly from the behaviour in standard scenarios. First, the properties of frustrated magnets and related systems are discussed. We introduce three-dimensional generalisations of a well-studied two-dimensional frustrated system. These generalisations consist of coupled layers of two-dimensional triangular lattice Ising antiferromagnets. We study three stackings that have nearest-neighbour interactions: two frustrated stackings (abc and abab) and the unfrustrated stacking (aaa). We use a combination of methods, including numerics and analytics, to show that these generalisations exhibit a strongly correlated, highly fluctuating regime known as a classical spin liquid. We show this by investigating the structure factor and correlations directly and by describing a mapping to a continuum field theory that gives further clarity about the classical spin liquid regime using a renormalisation group analysis and consideration of low-lying excitations in these systems. Second, we study the response of a disordered system to a time-periodic driving potential. We start in the context of Mott's work on a.c. conductivity in Anderson insulators and move beyond this to study the Floquet (long-time) regimes beyond linear response. Using Landau-Zener physics, we construct a description of the various regimes which are possible. We also present the results of thorough numerical studies of these systems.
In this thesis, the physics of two systems is investigated, both of which support unconventional phases of matter. We investigate a frustrated system which has a classical spin liquid regime in the paramagnetic phase. We also consider a periodically-driven, disordered system and explore nonequilibrium regimes. Both systems exhibit macroscopic behaviour which differs strongly from the behaviour in standard scenarios. First, the properties of frustrated magnets and related systems are discussed. We introduce three-dimensional generalisations of a well-studied two-dimensional frustrated system. These generalisations consist of coupled layers of two-dimensional triangular lattice Ising antiferromagnets. We study three stackings that have nearest-neighbour interactions: two frustrated stackings (abc and abab) and the unfrustrated stacking (aaa). We use a combination of methods, including numerics and analytics, to show that these generalisations exhibit a strongly correlated, highly fluctuating regime known as a classical spin liquid. We show this by investigating the structure factor and correlations directly and by describing a mapping to a continuum field theory that gives further clarity about the classical spin liquid regime using a renormalisation group analysis and consideration of low-lying excitations in these systems. Second, we study the response of a disordered system to a time-periodic driving potential. We start in the context of Mott's work on a.c. conductivity in Anderson insulators and move beyond this to study the Floquet (long-time) regimes beyond linear response. Using Landau-Zener physics, we construct a description of the various regimes which are possible. We also present the results of thorough numerical studies of these systems.
Strongly correlated systems show a variety of intriguing properties and have been extensively studied over the years. In this thesis two strongly correlated systems are discussed: the two-dimensional antiferromagnet and the Luttinger liquid. The Luttinger liquid has attracted much interest as a paradigm of non-Fermi liquid behavior. In the first part of the thesis, basic properties of Fermi liquids are briefly described while those of Luttinger liquids are considered in more detail. The differences in spectral properties of Luttinger liquids and Fermi liquids are discussed, as are the consequences for photoemission experiments. In this context Luttinger liquids with boundaries, treated in Papers I, II, and III, are introduced. The exact single-electron Green's function at finite temperature is calculated with bosonization methods, for both periodic and finite open-boundary systems. The corresponding local spectral density is analyzed. For a system with open boundaries, the well-known zero temperature bulk behavior always crosses over to a boundary dominated regime for small energies, while at finite temperature the low energy behavior becomes quadratic. For a periodic finite-size system we also calculate the Coulomb-blockade-like oscillations and study their dependence on interaction and temperature. The two-dimensional antiferromagnet has attracted much attention due to its connection to the undoped phase of the ceramic high-temperature superconductors. In the second part of the thesis including Papers IV and V, the two-dimensional Heisenberg antiferromagnet defined on the triangular and the honeycomb lattices is studied. We use the Schwinger-boson mean-field theory to investigate the stability of the ground state with respect to frustration. For the honeycomb lattice model we find an increased Néel stability as compared to the classical case. The dynamic structure factor is considered for various temperatures and the spin-wave velocity and the susceptibility are presented as functions of frustration for spin S=1/2. In the case of the intrinsically frustrated triangular lattice model we obtain, in the limit of infinite spin, results in complete agreement with the linear spin-wave theory. In contrast to the latter, however, two of the three modes acquire a mass for finite spin. The origin of this effect is discussed.
The strong interest in strongly correlated systems in condensed matter physics has continued unabated for the past few decades. In recent years, the number of novel, exotic quantum phases found in theoretical studies has seen a phenomenal rise. Among those interesting quantum states are bose liquids and spin liquids, where strong quantum fluctuations have prevented the systems from developing a long range order. Our work in this thesis seeks to further the understanding of frustrated systems. In the study of a hard-core boson model with ring-only exchange interactions on a square lattice, we obtain concrete numerical realization of the unconventional Exciton Bose Liquid (EBL) phase, which possesses interesting properties such as a "Bose surface" which resembles the Fermi surface in a metal, as well as unusual thermodynamic properties such as a T Log T dependence for specific heat. An equally important result from this work is the demonstration that the widely used Gutzwiller projection on slave-particle wave functions may generally fail to capture the correct long wavelength physics in the respective systems. For the Heisenberg antiferromagnet on the kagome lattice, which is a promising candidate for realizing a spin-disordered ground state, our variational study shows that the projected Schwinger boson wave function is energetically better than the Dirac spin liquid wave function when a small antiferromagnetic second-neighbor spin coupling is added to the nearest-neighbor model. We also study the anisotropic triangular Heisenberg antiferromagnetic in magnetic field, and find simple, yet accurate wave functions for various regions of the surprisingly rich phase diagram, thus providing insights into the energetics of the competing phases in this interesting model. Finally, our work also highlights permanent-type wave functions as potentially useful constructions in variational studies of systems with short-ranged correlations, e.g., a Mott insulator and a gapped spin liquid.
The quantum nature of electrons emerges in various exotic ways when they are put together with quantum many-body interactions. In Mott insulating systems with strong electron correlation, frustration and strong quantum fluctuations promote localized electron spins to form nontrivial quantum paramagnetic phases such as quantum spin liquid and valence bond solid preventing the development of conventional magnetic orders. In this thesis, we study such exotic quantum phases in various different frustrated quantum magnets. First, encouraged by the spin liquid phenomenology observed in Ba3XSb2O9 (X=Cu, Ni), we investigate Z2 spin liquids on a three dimensional triangular lattice by employing the Schwinger boson mean-field theory with projective symmetry group analysis. It is found that a nematic spin liquid is energetically favorable on this lattice structure rather than symmetric spin liquids. Next, we study various valence bond solid phases that can appear in kagome lattice antiferromagnets by generalizing the recently proposed phenomenological Z2 gauge theory for the topological Z2 spin liquid state found in the nearest-neighbor kagome lattice Heisenberg model. This work provides a bridge between the topological spin liquid and valence bond solid phases previously proposed for kagome lattice antiferromagnets via vison condensation in the gauge theory. Then, we consider optical conductivity in Mott insulators as an experimental probe to detect quantum paramagnetic phases like valence bond solid. Applying this idea to deformed kagome lattice antiferromagnet Rb2Cu3SnF12 with a 12-site valence bond solid state, we estimate the size of optical conductivity in this quantum magnet. After this, another quantum paramagnet BiCu2PO6 is carefully investigated to clarify its puzzling ground state nature and quasiparticle excitations. Utilizing recent high resolution neutron scattering data, we construct a microscopic spin model and also an effective theory for the low energy spin excitations in the compound. We find significant influences of magnetic anisotropies on the ground state and quasiparticle dynamics. Lastly, we investigate emergent quantum phases in the thin film geometries of the pyrochlore iridates, where a number of exotic quantum ground states are proposed to occur in bulk materials as a result of the interplay between electron correlation and strong spin-orbit coupling. The fate of these bulk phases, as well as the novel quantum states that may arise only in a thin film setting, are studied via a theoretical model that allows layer-dependent magnetic structures.
In this thesis, we investigate strongly correlated phenomena in quantum spin systems. In the first part of this work, we study geometrically frustrated antiferromagnets (AFMs). Generalizing the SU(2) Heisenberg Hamiltonian to Sp(N) symmetry, we obtain, in the large-N limit, the mean-field phase diagrams for the planar pyrochlore and cubic AFMs. We then use gauge theories to consider fluctuation effects about their respective mean-field configurations. We find, in addition to conventional Neel states, a plethora of novel magnetically disordered phases: two kinds of spin liquids, Z2 in 2+1D and U(1)in 3+1D, and several valence bond solids such as two and three-dimensional plaquette and columnar singlet states. We use the same approach to study the diamond lattice AFM which possesses extended classical ground state degeneracy. We demonstrate that quantum and entropic fluctuations lift this degeneracy in different ways. In the second part of the thesis, we study ultracold spinor atoms confined in optical lattices. We first demonstrate the feasibility of experimental realization of rotor models using ultracold spin-one Bose atoms in a spin-dependent and disordered optical lattice. We show that the ground state of such disordered rotor models with quadrupolar interactions can exhibit biaxial nematic ordering in the disorder-averaged sense, and suggest an imaging experiment to detect the biaxial nematicity in such systems. Finally, using variational wavefunction methods, we study the Mott phases and superfluid-insulator transition of spin-three bosons in an optical lattice with an anisotropic two dimensional optical trap. We chart out the phase diagrams for Mott states with n = 1 and n = 2 atoms per lattice site. We show that the long-range dipolar interaction stabilizes a state characterized by antiferromagnetic chains made of ferromagnetically aligned spins. We also obtain the mean-field phase boundary for the superfluid-insulator transition, and show that inside the superfluid phase and near the superfluid-insulator phase boundary, the system undergoes a first order antiferromagnetic-ferromagnetic spin ordering transition.
The study of various types of orders and phases of matter in magnetic systems is extremely popular theme in condensed matter physics. Not only frequently applicable in real world, quantum magnetism provides a fruitful and at the same time frequently feasible framework to search for new phases of matter. In this work we study quantum and associated classical phase transitions of spin systems by combinations of numerical and analytical methods.In Chapter 1 we study quantum paramagnets that are a generalization of antiferromagnetic AKLT [2] construction for the spin group SU(N). We map the so called simplex solid state ansatz wavefunction into a partition function of the classical system on the same lattice using coherent state representation. Then we study phase transitions of that classical models via classical Monte Carlo methods to determine properties of the original quantum simplex solid state [3].In Chapter 2 we are interested in the O(2) model in (2+1)-dimensional system with the dilution, i.e. when some sites are removed from the system. This relativistic O(2) model originates from the description of a transition between superfluid and Mott insulator phases. We study critical properties of the transition using a recently invented [4] worm algorithm.In Chapter 3 we are studying large-N expansion for the symplectic spin group Sp(N). When the original SU(2) spin model proves to be too difficult to be solved, for example at low temperatures, one of the alternative ways is to consider a system based on a more general spin group, for example SU(N) or Sp(N). Results often remain relevant for the original SU(2) group. We are using the mean-field theory approach to find phase transitions associated with the cubic Sp(N) system at large N.Another fundamental question of interest is how different systems behave when they are periodically driven by some external field. Such systems are called Floquet systems, and we will study them in Chapter 4. We will consider interacting and non-interacting systems, that might have a non-conventional type of ordering, topological order.
The present thesis adresses three different topics that are all in direct connection to the statistical physics of disordered systems. The first chapter, which is the main part of this work, is devoted to driven interfaces in random media. The main focus is on the case of an ac driving. Our investigations start with the associated mean field theory. At the beginning we consider the case of a constant driving force and discuss the critical behaviour close to the depinning transition. Subsequently, we examine the velocity hysteresis curve for small frequencies by use of analytical and numerical methods. It turns out that all observables under consideration exhibit scaling behaviour on approaching the adiabatic case. Next, we analyse perturbation theory for ac-driven interfaces. Tt turns out, that perturbation theory can only work for sufficiently high interface dimensionalities D>4. The failure of perturbation theory in low dimensions is discussed. The second part of this thesis is concerned with the influence of electroelastic coupling on the behaviour of domain walls in ferroelectrics. For the case of electrostrictive coupling in the high temperature phase we find, that merely the interface tension coefficients are changed in an anisotropic fashion. On the contrary, a piezo effect in the paraphase entails a long range term. After deriving the interface hamiltonian in the harmonic approximation, we determine the roughness of domain walls due to random field disorder using an Imry-Ma type argument. On large length scales, domain walls turn out to be flat. In the last chapter, we consider one-dimensional electronic systems, so-called Luttinger liquids. The subject of the first part is the competition of two phases of one-dimensional disordered fermionic systems, namely the Mott and the Anderson insulator. By relating both, the compressibility as well as a finite optical conductivity to a vanishing mass of charged excitations, the existence of an intermediate Mott glass phase can be excluded. Combining informations about the renormalisation group flow and the energy of topological excitations, as well as using simple scaling arguments, we construct the phase diagram. In the second part we study the replica trick to describe the quantum creep and the linear conductivity at finite temperatures for Luttinger liquids with relevant disorder. We provide approximate solutions to the Euler-Lagrange equations for the replica action. The replica limit turns out to be problematic. At the example of a toy model we illustrate a method for the replica limit and demonstrate the emergent difficulties.