Skyrmion lattice creep at ultra-low current densities

Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities $j_c$ to depin from the underlying atomic lattice. Above $j_c$ skyrmions exhibit well-controlled motion, making them prime candidates for race-track memories. In thin films thermally-activated creep motion of isolated skyrmions was observed below $j_c$ as predicted by theory. Uncontrolled skyrmion motion is detrimental for race-track memories and is not fully understood. Notably, the creep of skyrmion lattices in bulk materials remains to be explored. Here we show using resonant ultrasound spectroscopy--a probe highly sensitive to the coupling between skyrmion and atomic lattices--that in the prototypical skyrmion lattice material MnSi depinning occurs at $j_c^*$ that is only 4 percent of $j_c$. Our experiments are in excellent agreement with Anderson-Kim theory for creep and allow us to reveal a new dynamic regime at ultra-low current densities characterized by thermally-activated skyrmion-lattice-creep with important consequences for applications.

Elasticity in the skyrmion phase unveils depinning at ultra-low current densities

Controlled movement of nano-scale stable magnetic objects has been proposed as the foundation for a new generation of magnetic storage devices. Magnetic skyrmions, vortex-like spin textures stabilized by their topology are particularly promising candidates for this technology. Their nanometric size and ability to be displaced in response to an electrical current density several orders of magnitude lower than required to induce motion of magnetic domain walls suggest their potential for high-density memory devices that can be operated at low power. However, to achieve this, skyrmion movement needs to be controlled, where a key question concerns the coupling of skyrmions with the underlying atomic lattice and disorder (pinning). Here, we use Resonant Ultrasound Spectroscopy (RUS), a probe highly sensitive to changes in the elastic properties, to shed new light on skyrmion elasticity and depinning in the archetypal skyrmion material MnSi. In MnSi, skyrmions form a lattice that leads to pronounced changes in the elastic properties of the atomic lattice as a result of magneto-crystalline coupling. Without an applied current, the shear and compressional moduli of the underlying crystal lattice exhibit an abrupt change in the field-temperature range where skyrmions form. For current densities exceeding $j_c^*$ the changes of elastic properties vanish, signaling the decoupling of skyrmion and atomic lattices. Interestingly, $j_c^*$, which we identify as the onset of skyrmion depinning, is about 20 times smaller than $j_c$ previously measured via non-linear Hall effect. Our results suggest the presence of a previously-undetected intermediate dynamic regime possibly dominated by skyrmion-creep motion with important consequences for potential applications.

Decoupling and tuning competing effects of different types of defects on flux creep in irradiated YBa$_{2}$Cu$_{3}$O$_{7-{delta}}$ coated conductors

YBa$_{2}$Cu$_{3}$O$_{7-{delta}}$ coated conductors (CCs) have achieved high critical current densities ($textit{J}_{c}$) that can be further increased through the introduction of additional defects using particle irradiation. However, these gains are accompanied by increases in the flux creep rate, a manifestation of competition between the different types of defects. Here, we study this competition to better understand how to design pinning landscapes that simultaneously increase $textit{J}_{c}$ and reduce creep. CCs grown by metal organic deposition show non-monotonic changes in the temperature-dependent creep rate, $textit{S}(textit{T})$. Notably, in low fields, there is a conspicuous dip to low $textit{S}$ as temperature ($textit{T}$) increases from ~20 K to ~65 K. Oxygen-, proton-, and Au-irradiation substantially increase $textit{S}$ in this temperature range. Focusing on an oxygen-irradiated CC, we investigate the contribution of different types of irradiation-induced defects to the flux creep rate. Specifically, we study $textit{S}(textit{T})$ as we tune the relative density of point defects to larger defects by annealing both an as-grown and an irradiated CC in O$_{2}$ at temperatures $textit{T}_{A}$ = 250${deg}$C to 600${deg}$C. We observe a steady decrease in $textit{S}$($textit{T}$ > 20 K) with increasing $textit{T}_{A}$, unveiling the role of pre-existing nanoparticle precipitates in creating the dip in $textit{S}(textit{T})$ and point defects and clusters in increasing $textit{S}$ at intermediate temperatures.

Toward superconducting critical current by design

We present the new paradigm of critical current by design. Analogous to materials by design, it aims at predicting the optimal defect landscape in a superconductor for targeted applications by elucidating the vortex dynamics responsible for the bulk critical current. To highlight this approach, we demonstrate the synergistic combination of critical current measurements on commercial high-temperature superconductors containing self-assembled and irradiation tailored correlated defects by using large-scale time-dependent Ginzburg-Landau simulations for vortex dynamics.

Quasi-bound states and continuum absorption background of polar Al0.5Ga0.5N/GaN quantum dots

A theoretical interpretation of the photoluminescence excitation spectra of self-organized polar GaN/(Al,Ga)N quantum dots is proposed. A numerical method assuming a realistic shape of the dots and including the built-in electric field effects is developed to calculate their energy structure and hence their optical absorption. The electron and hole spectra show the existence of a set of quasi-bound states that does not originate from the wetting layer, and plays a crucial role in the observed absorption spectrum of the GaN/(Al,Ga)N dots. Transitions involving these quasi-bound states and wetting layer states give a sufficient explanation for the observed continuum absorption background. The properties of this absorption band, especially its extension, depend strongly on the dots size. Our simulation provides a natural explanation of the experimental luminescence excitation spectra of ensembles of dots of different heights. Our theoretical model can be extended to cases where the confinement potentials are complicated by the presence of a stronger electric field.

Doubling the Critical Current Density of High Temperature Superconducting Coated Conductors through Proton Irradiation

The in-field critical current of commercial YBa2Cu3O7 coated conductors can be substantially enhanced by post-fabrication irradiation with 4 MeV protons. Irradiation to a fluence of 8x10^16 p/cm^2 induces a near doubling of the critical current in fields of 6 T || c at a temperature of 27 K, a field and temperature range of interest for applications such as rotating machinery. A mixed pinning landscape of preexisting precipitates and twin boundaries and small, finely dispersed irradiation induced defects may account for the improved vortex pinning in high magnetic fields. Our data indicate that there is significant head-room for further enhancements.