Reducing Government Secrecy: Finding What Works

Sunlight is the best disinfectant, Justice Brandeis famously declared, praising publicity as a remedy for corruption. But sunlight is more than that; it is an indispensable precondition of life. And to extend the Brandeis metaphor, sunlight in the form of robust public access to government information is essential to the vitality of democratic governance, even in the absence of corruption. Our political institutions cannot function properly without it

Developing Dynamical Probes of Quantum Spin Liquids Inspired by Techniques from Spintronics

We theoretically study low dimensional insulating spin systems using spin fluctuations as a probe of the spin dynamics. In some systems, low dimensionality in conjunction with other quantum fluctuation enhancing effects impedes spontaneous generation of long range magnetic ordering down to zero absolute temperature. In particular, we focus on exotic spin systems hosting mobile, fractionalized spin excitations above their ground state, and ultimately show that techniques already commonplace in spintronics can be utilized in the context of quantum magnetism to develop probes of exotic ground states. We initially consider quantum spin chains (QSCs), and examine a system of two exchange-coupled QSCs connected at their finite ends. Downstream, one QSC is driven out of equilibrium by both an overpopulation of spin excitations and temperature elevation, and the other QSC is coupled to a drain bath where electrical measurements are performed via spin Hall effects. We compare this low dimensional system hosting spin-half solitons to a coupled 3d magnon hosting system arranged in the same geometry and show that a quantity we call the spin Fano factor, defined as the spin current noise-to-signal ratio, differentiates between the two systems. However, we assume that the noise is measured electrically via the inverse spin Hall effect after conversion in the drain metal. Recognizing that this technique may lead to enhancements due to intrinsic conversion mechanisms, we then suggest an experimental method by which to measure spin current noise minimally invasively using an LC resonator coupled to a transmission line. In this way, we show that photon counting allows for directly accessing the spin current dc noise in a QSC, and thus for constructing the spin Fano factor. Moving on to 2d insulating magnets, we examine a bilayer system interfacing a spin-orbit coupled heavy metal film with an exotic quantum magnet. We develop the theoretical underpinnings of the proposed bilayer system for a general quantum magnet lacking long-range magnetic order, and then apply the theory to three QSL models. We select the Heisenberg kagom{\\u27e} lattice model, a model possessing gapless fermionic spinons and an emergent U(1) gauge field, and the Kitaev model on the honeycomb lattice, each with extant candidate materials in mind. Ultimately, a bilayer system of the type proposed can utilize interfacial spin fluctuations as a probe of the low energy density of states of QSL materials, and access quantities of interest in these systems such as gap energies and gauge field renormalizations. Additionally, in any multilayer system, the interface is of obvious interest, and so we close with a microscopic examination of a metal-to-quantum magnet interface. The metal contains short-range, spin-inert impurities, the insulator provides a bath of spin fluctuations, and the interface itself breaks inversion symmetry and results in interfacial Rashba spin-orbit coupling. We calculate the electrical conductivity in the metallic layer in the presence of all three of these factors, and show that corrections to the conductivity may themselves be utilized as a probe of the affixed insulating spin system

Probing quantum spin liquids in equilibrium using the inverse spin Hall effect

We propose an experimental method utilizing a strongly spin-orbit coupled metal to quantum magnet bilayer that will probe quantum magnets lacking long range magnetic order, e.g., quantum spin liquids, via examination of the voltage noise spectrum in the metal layer. The bilayer is held in thermal and chemical equilibrium, and spin fluctuations arising across the single interface are converted into voltage fluctuations in the metal as a result of the inverse spin Hall effect. We elucidate the theoretical workings of the proposed bilayer system, and provide precise predictions for the frequency characteristics of the enhancement to the ac electrical resistance measured in the metal layer for three candidate quantum spin liquid models. Application to the Heisenberg spin-$1/2$ kagom{\'e} lattice model should allow for the extraction of any spinon gap present. A quantum spin liquid consisting of fermionic spinons coupled to a $U(1)$ gauge field should cause subdominant \W^{4/3} scaling of the resistance of the coupled metal. Finally, if the magnet is well-captured by the Kitaev model in the gapless spin liquid phase, then the proposed bilayer can extract the two-flux gap which arises in spite of the gapless spectrum of the fermions. We therefore show that spectral analysis of the ac resistance in the metal in a single interface, equilibrium bilayer can test the relevance of a quantum spin liquid model to a given candidate material.Comment: 17 pages, 6 figure