Disorder 

      

Disorder-induced structure on the mesoscopic scale can have profound macroscopic consequences, leading to materials with novel electronic, optical, and magnetic properties. This structure can result directly from gross inhomogeneities in a material's composition or, more subtly, by self-organization on scales from nanometers to microns in the presence of quenched, atomic disorder. The penchant for self-organization is enabled by strong correlations and fluctuations in the near vicinity of a phase transition and can be found, for example, in puddles of electrons that remain on the insulating side of a metal-insulator transition or variegated charge order in high-temperature superconductors. We pick a particularly simple system of randomly-distributed magnetic dipoles to probe the ability of clusters of spins to coherently self organize and decouple from the local environment ("spin bath"). Coherent quantum oscillations of hundreds of spins labeled by frequency in the rare earth tetrafluorides permit the encoding of information and fundamental studies of quantum decoherence mechanisms.

 

At higher dipole concentrations, the system is well described by the Ising model in transverse magnetic field, where the transverse field acts as a “knob” in the laboratory to tune the quantum mechanics.. A combination of multi-axis AC susceptometry, DC magnetometry, noise measurements, hole burning, non-linear Fano experiments, and neutron diffraction as functions of temperature, transverse field, frequency, dipole concentration, and excitation amplitude, address issues of stability, overlap, coherence, and random field effects. This research promises insights into fundamental quantum physics, including entanglement and coherence.  It also addresses the question of how best to use complex systems, such as magnetic solids, to process quantum information. Extensions to a spin liquid stabilized by geometrical frustration rather than quantum fluctuations, gadolinium gallium garnet, and a quantum antiferromagnet (versus a quantum ferromagnet), promise tests of universality and new models of self-organized collective behavior. 
More reading: Entangled Quantum State of Magnetic Dipoles" 
Coherent Spin Oscillations in a Disordered Magnet” 
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A ferromagnet in a continuously tunable random field
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Switchable hardening of a ferromagnet at fixed temperature"
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Reversible Disorder in a Room Temperature Ferromagnet" 

 

Magnetoresistive Compounds

The subtle manipulation of stoichiometry in the silver chalcogenides can produce a profound macroscopic response. Although non-magnetic, the materials become highly sensitive to magnetic field when local fluctuations in the conductivity become amplified by the proximity of a band-crossing instability.  The usual quadratic magnetoresistance can be converted to a linear dependence with applied field, and the response does not saturate even up to at least a MegaGauss. We have also extended the ability to create a linear and non-saturating magnetoresistance  to a new, more commercially potent system. By tailoring the microstructure of the familiar semiconductor, InSb, we find a linear response with a magnitude of tens of thousands of percent at Tesla fields.  Moreover, properly introduced disorder can even circumvent the usual tendency of a sensor to lose its sensitivity at high temperatures because of phonon scattering, with a magnetoresistance that now grows with increasing temperature!
More reading: Quantum and Classical Routes to Linear Magnetoresistance
Non-Saturating Magnetoresistance of Inhomogeneous Conductors: Comparison of Experiment and Simulation"