Search

Excitations in disordered systems are typically categorized as localized or delocalized, depending on whether they entail disturbances extending throughout the system or are confined to small, generally nanometer scale, subsystems. Such categorization is impossible to achieve using traditional spectroscopy where the response to a weak oscillating (ac) electromagnetic probe is measured as a function of frequency. However, the localized excitations can be separated from each other as well as the delocalized continuum by measuring a spectral "hole" in the ordinary response while a large amplitude pump is imposed at a fixed frequency. Localized excitations will result in a very sharp "hole," and any residual couplings to other excitations, both localized and extended, will determine its detailed shape. This technique probes incoherent lifetime effects as well as coherent mixing or quantum interference phenomena, describable in terms of the Fano effect. Here we show that in a disordered Ising magnet, LiHo0.045Y0.955F4, the quality factor Q for spectral holes, the ratio of the drive frequency to their width, can be as high as 100,000. In addition, we can tune the dynamics of the quantum degrees of freedom by sweeping the quantum mixing parameter through zero via the amplitude of the ac pump as well as a static external transverse field. The zero-crossing is associated with a dissipationless response at the drive frequency. The identification of such a point where localized degrees of freedom are minimally mixed with their environment in a dense and disordered dipolar coupled spin system implies control over the bath coupling of qubits emerging from strongly interacting many-body systems.

We discuss techniques for performing continuous measurements across a wide range of pressure-fielderature phase space, combining the milli-Kelvin temperatures of a helium dilution refrigerator with the giga-Pascal pressures of a diamond anvil cell and the Tesla magnetic fields of a superconducting magnet. With a view towards minimizing remnant magnetic fields and background magnetic susceptibility, we characterize high-strength superalloy materials for the pressure cell assembly, which allows high fidelity measurements of low-field phenomena such as superconductivity below 100 mK at pressures above 10 GPa. In situ tunability and measurement of the pressure permit experiments over a wide range of pressure, while at the same time making possible precise steps across abrupt phase transitions such as those from insulator to metal.

Traditional simulated annealing uses thermal fluctuations for convergence in optimization problems. Quantum tunneling provides a different mechanism for moving between states, with the potential for reduced time scales and different outcomes. Thermal and quantum annealing are compared in two concentration regimes of a model disordered magnet, where the effects of quantum mechanics can be tuned both by varying an applied magnetic field and by controlling the strength of thermal coupling between the magnet and an external heat bath. The results indicate that quantum annealing hastens convergence to the final state, and that the quantum character of the final state can be engineered thermodynamically.

Background: Genomic disorders are caused by copy number changes that may exhibit recurrent breakpoints processed by nonallelic homologous recombination. However, region-specific disease-associated copy number changes have also been observed which exhibit non-recurrent breakpoints. The mechanisms underlying these non-recurrent copy number changes have not yet been fully elucidated. Results: We analyze large NF1 deletions with non-recurrent breakpoints as a model to investigate the full spectrum of causative mechanisms, and observe that they