Disorder-Induced Entanglement in Spin Ice Pyrochlores

We propose that in a certain class of magnetic materials, known as non-Kramers 'spin ice,' disorder induces quantum entanglement. Instead of driving glassy behavior, disorder provokes quantum superpositions of spins throughout the system, and engenders an associated emergent gauge structure and set of fractional excitations. More precisely, disorder transforms a classical phase governed by a large entropy, classical spin ice, into a quantum spin liquid governed by entanglement. As the degree of disorder is increased, the system transitions between (i) a "regular" Coulombic spin liquid, (ii) a phase known as "Mott glass," which contains rare gapless regions in real space, but whose behavior on long length scales is only modified quantitatively, and (iii) a true glassy phase for random distributions with large width or large mean amplitude.Comment: 6+2 pages, 2+1 figure

Coulombic Quantum Liquids in Spin-1/2 Pyrochlores

We develop a non-perturbative "gauge Mean Field Theory" (gMFT) method to study a general effective spin-1/2 model for magnetism in rare earth pyrochlores. gMFT is based on a novel exact slave-particle formulation, and matches both the perturbative regime near the classical spin ice limit and the semiclassical approximation far from it. We show that the full phase diagram contains two exotic phases: a quantum spin liquid and a coulombic ferromagnet, both of which support deconfined spinon excitations and emergent quantum electrodynamics. Phenomenological properties of these phases are discussed.Comment: 4+ pages, 6+ pages of Supplementary Material, 4 figures, 1 tabl

Spin Liquid Regimes at Nonzero Temperature in Quantum Spin Ice

Quantum spin liquids are highly entangled ground states of quantum systems with emergent gauge structure, fractionalized spinon excitations, and other unusual properties. While these features clearly distinguish quantum spin liquids from conventional, mean-field-like states at zero temperature (T), their status at T>0 is less clear. Strictly speaking, it is known that most quantum spin liquids lose their identity at non-zero temperature, being in that case adiabatically transformable into a trivial paramagnet. This is the case for the U(1) quantum spin liquid states recently proposed to occur in the quantum spin ice pyrochlores. Here we propose, however, that in practical terms, the latter quantum spin liquids can be regarded as distinct phases from the high temperature paramagnet. Through a combination of gauge mean field theory calculations and physical reasoning, we argue that these systems sustain both quantum spin liquid and thermal spin liquid phases, dominated by quantum fluctuations and entropy, respectively. These phases are separated by a first order "thermal confinement" transition, such that for temperatures below the transition, spinons and emergent photons are coherently propagating excitations, and above it the dynamics is classical. Even for parameters for which the ground state is magnetically ordered and not a quantum spin liquid, this strong first order transition occurs, pre-empting conventional Landau-type criticality. We argue that this picture explains the anomalously low temperature phase transition observed in the quantum spin ice material Yb2Ti2O7.Comment: 15 pages (including 7 pages of appendices), 3 figures, 1 tabl

Spin pumping and magnetization dynamics in ferromagnet-Luttinger liquid junctions

We study spin transport between a ferromagnet with time-dependent magnetization and a conducting carbon nanotube or quantum wire, modeled as a Luttinger liquid. The precession of the magnetization vector of the ferromagnet due for instance to an outside applied magnetic field causes spin pumping into an adjacent conductor. Conversely, the spin injection causes increased magnetization damping in the ferromagnet. We find that, if the conductor adjacent to the ferromagnet is a Luttinger liquid, spin pumping/damping is suppressed by interactions, and the suppression has clear Luttinger liquid power law temperature dependence. We apply our result to a few particular setups. First we study the effective Landau-Lifshitz-Gilbert (LLG) coupled equations for the magnetization vectors of the two ferromagnets in a FM-LL-FM junction. Also, we compute the Gilbert damping for a FM-LL and a FM-LL-metal junction.Comment: 7 pages, 3 figures, RevTex