Second Low Temperature Phase Transition in Frustrated UNi_4B

Hexagonal UNi_4B is magnetically frustrated, yet it orders antiferromagnetically at T_N = 20 K. However, one third of the U-spins remain paramagnetic below this temperature. In order to track these spins to lower temperature, we measured the specific heat C of \unib between 100 mK and 2 K, and in applied fields up to 9 T. For zero field there is a sharp kink in C at $T^\ast\approx$ 330 mK, which we interpret as an indication of a second phase transition involving paramagnetic U. The rise in $\gamma = C/T$ between 7 K and 330 mK and the absence of a large entropy liberated at $T^\ast$ may be due to a combination of Kondo screening effects and frustration that strongly modifies the low T transition.Comment: 4 pages, 4 figure

Kondo Screening and Magnetic Ordering in Frustrated UNi4B

UNi4B exhibits unusual properties and, in particular, a unique antiferromagnetic arrangement involving only 2/3 of the U sites. Based on the low temperature behavior of this compound, we propose that the remaining 1/3 U sites are nonmagnetic due to the Kondo effect. We derive a model in which the coexistence of magnetic and nonmagnetic U sites is the consequence of the competition between frustration of the crystallographic structure and instability of the 5f moments.Comment: 4 pages, 2 figure

Stable and fast semi-implicit integration of the stochastic Landau-Lifshitz equation

We propose new semi-implicit numerical methods for the integration of the stochastic Landau-Lifshitz equation with built-in angular momentum conservation. The performance of the proposed integrators is tested on the 1D Heisenberg chain. For this system, our schemes show better stability properties and allow us to use considerably larger time steps than standard explicit methods. At the same time, these semi-implicit schemes are also of comparable accuracy to and computationally much cheaper than the standard midpoint implicit method. The results are of key importance for atomistic spin dynamics simulations and the study of spin dynamics beyond the macro spin approximation.Comment: 24 pages, 5 figure

Supervised learning of an opto-magnetic neural network with ultrashort laser pulses

The explosive growth of data and its related energy consumption is pushing the need to develop energy-efficient brain-inspired schemes and materials for data processing and storage. Here, we demonstrate experimentally that Co/Pt films can be used as artificial synapses by manipulating their magnetization state using circularly-polarized ultrashort optical pulses at room temperature. We also show an efficient implementation of supervised perceptron learning on an opto-magnetic neural network, built from such magnetic synapses. Importantly, we demonstrate that the optimization of synaptic weights can be achieved using a global feedback mechanism, such that the learning does not rely on external storage or additional optimization schemes. These results suggest there is high potential for realizing artificial neural networks using optically-controlled magnetization in technologically relevant materials, that can learn not only fast but also energy-efficient.Comment: 9 pages, 4 figure

CAPITEL: Design and Implementation of a wireless 6 channel EMG measurement system for permanent in vivo use: in vitro results

Introduction Surface Electromyography of partial limb amputees is used to control prostheses. Implantable EMG systems offer a higher Signal to Noise Ratio (SNR) as well as improved muscle specificity, and a more convenient daily use. Material and methods We present a design for an implantable device (“implant”) with 6 channels, each suitable for an electrode array with 3 electrodes. The implant uses an ADS1298 analog front end with ADG2188 multiplexers for versatile electrode array configuration. The analog input is filtered with a balanced analog bandpass filter with corner frequencies of 30 and 800 Hz. The ADC sample rate is 2 KHz per channel, with 9 bits resolution. The dimensions of the PCB implant are 17.2 x 14.1x2.15 mm. To measure the SNR a sinusoidal signal with a peak to peak amplitude of 7 mV and a frequency of 200 Hz was applied to each input. To simulate muscle impedance, an equivalent muscle impedance model (Figure 1) was placed between the generator and each channel of the implant [1]. We have implemented two data transmission methods: wired duplex communication and wireless inductive link. The wired link is used to transfer raw data, while only the 6 EMG envelopes, with an update rate of 20 Hz, are sent via the wireless link. Results Each analog input channel performed with a SNR better than 52 dB, both for wired and wireless operation. Wired data was received successfully at 115200 bps and wireless data at 1080 bps. Discussion Our design achieves a high SNR and data rate. These early results are promising and we are packaging the PCBs for in-vivo testing. Conclusion We have demonstrated a very compact design suitable for the monitoring of 6 EMG channels, with options for raw data or EMG envelope transmission. [1] Kalvoy, H, 2009, doi: 10.1088/0967-3334/30/2/002

Inflection point in the magnetic field dependence of the ordered moment of URu2Si2 observed by neutron scattering in fields up to 17 T

We have measured the magnetic field dependence of the ordered antiferromagnetic moment and the magnetic excitations in the heavy-fermion superconductor URu2Si2 for fields up to 17 Tesla applied along the tetragonal c axis, using neutron scattering. The decrease of the magnetic intensity of the tiny moment with increasing field does not follow a simple power law, but shows a clear inflection point, indicating that the moment disappears first at the metamagnetic transition at ~40 T. This suggests that the moment m is connected to a hidden order parameter Phi which belongs to the same irreducible representation breaking time-reversal symmetry. The magnetic excitation gap at the antiferromagnetic zone center Q=(1,0,0) increases continuously with increasing field, while that at Q=(1.4,0,0) is nearly constant. This field dependence is opposite to that of the gap extracted from specific-heat data.Comment: 10 pages, 5 figures, submitted to PR

Heat treatment optimizations for Wind-and-React Bi-2212 racetrack coils

Lawrence Berkeley National Laboratory (LBNL) is developing Wind-and-React (W&R) Bi sr cacu o +δ (Bi-2212) accelerator magnet technology for insert coils, to surpass the intrinsic limitations of Nb-based magnets, and eventually develop hybrid systems that can approach 20 T dipole fields. The Bi-2212 technology is being developed in close collaboration with industry, and has been partly supported by the US Very High Field Superconducting Magnet Collaboration (VHFSMC). Steady improvements were made over the last several years, with coil HTS-SC08 reaching 2636 A, or about 85% of its witness sample critical current (Ic). Though this is still a factor 3 to 4 too low to be competitive with Nb-based materials, it is expected that the required Ic can be achieved through further conductor optimizations. Recent developments include the commissioning of infrastructure for the reaction of coils at LBNL. Earlier coils were fabricated and tested at LBNL, but were reacted at the wire manufacturer. We describe in detail the furnace calibrations and heat treatment optimizations that enable coil reactions at temperatures approaching 890 °C with a homogeneity of ± 1 °C in a pure oxygen flow. We reacted two new coils at LBNL, and tested the performance of coil HTS-SC10 at 4.2 K in self-field using a superconducting transformer system. We find that its performance is consistent with witness samples, and comparable to coil HTS-SC08, which is an identical coil that was reacted at Oxford Instruments Superconductor Technology (OST), thereby validating the in-house reaction process. 2 2 2