Atomic tunneling from a scanning-tunneling or atomic-force microscope tip: dissipative quantum effects from phonons

Contactless doping characterization of [Formula: see text] using acceptor Cd probes

Finding suitable p-type dopants, as well as reliable doping and characterization methods for the emerging wide bandgap semiconductor [Formula: see text]-[Formula: see text] could strongly influence and contribute to the development of the next generation of power electronics. In this work, we combine easily accessible ion implantation, diffusion and nuclear transmutation methods to properly incorporate the Cd dopant into the [Formula: see text]-[Formula: see text] lattice, being subsequently characterized at the atomic scale with the Perturbed Angular Correlation (PAC) technique and Density Functional Theory (DFT) simulations. The acceptor character of Cd in [Formula: see text]-[Formula: see text] is demonstrated, with Cd sitting in the octahedral Ga site having a negative charge state, showing no evidence of polaron deformations nor extra point defects nearby. The possibility to determine the charge state of Cd will allow assessing the doping type, in particular proving p-type character, without the need for ohmic contacts. Furthermore, a possible approach for contactless charge mobility studies is demonstrated, revealing thermally activated free electrons for temperatures above [Formula: see text] 648 K with an activation energy of 0.54(1) and local electron transport dominated by a tunneling process between defect levels and the Cd probes at lower temperatures.

Atomic spin-sensitive dissipation on magnetic surfaces

We identify the mechanism of energy dissipation relevant to spin-sensitive nanomechanics including the recently introduced magnetic exchange force microscopy, where oscillating magnetic tips approach surface atomic spins. The tip-surface exchange couples spin and atom coordinates, leading to a spin-phonon problem with Caldeira-Leggett-type dissipation. In the overdamped regime, that can lead to a hysteretic flip of the local spin with a large spin-dependent dissipation, even down to the very low experimental tip oscillation frequencies, describing recent observations for Fe tips on NiO. A phase transition to an underdamped regime with dramatic drop of magnetic tip dissipation should in principle be possible by tuning tip-surface distance.

Dynamics of a tunneling magnetic impurity: Kondo effect induced incoherence

We study how the formation of the Kondo compensation cloud influences the dynamical properties of a magnetic impurity that tunnels between two positions in a metal. The Kondo effect dynamically generates a strong tunneling impurity-conduction electron coupling, changes the temperature dependence of the tunneling rate, and may ultimately result in the destruction of the coherent motion of the particle at zero temperature. We find an interesting two-channel Kondo fixed point as well for a vanishing overlap between the electronic states that screen the magnetic impurity. We propose experiments where the predicted features could be observed.

Stabilizing coherent destruction of tunneling

The localization of a tunneling particle by means of an oscillating external field is examined for an arbitrary doublet of tunneling states. The condition of degenerate Floquet levels, required for localization in a symmetric system, can be substantially relaxed for tunneling systems with broken symmetry. A synergistic effect of dynamic and static asymmetry is found, which extends the localization regime substantially. This generalization equally applies to tunneling systems coupled to a dissipative environment.