Ultrafast and reversible control of the exchange interaction in Mott insulators

The strongest interaction between microscopic spins in magnetic materials is the exchange interaction $J_\text{ex}$. Therefore, ultrafast control of $J_\text{ex}$ holds the promise to control spins on ultimately fast timescales. We demonstrate that time-periodic modulation of the electronic structure by electric fields can be used to reversibly control $J_\text{ex}$ on ultrafast timescales in extended antiferromagnetic Mott insulators. In the regime of weak driving strength, we find that $J_\text{ex}$ can be enhanced and reduced for frequencies below and above the Mott gap, respectively. Moreover, for strong driving strength, even the sign of $J_\text{ex}$ can be reversed and we show that this causes time reversal of the associated quantum spin dynamics. These results suggest wide applications, not only to control magnetism in condensed matter systems, for example, via the excitation of spin resonances, but also to assess fundamental questions concerning the reversibility of the quantum many-body dynamics in cold atom systems.Comment: 9 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

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

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

Specific Heat of URu2_{2}Si2_{2} in Fields to 42 T: Clues to the 'Hidden Order'

The large $\Delta$C observed at 17.5 K in URu$_{2}$Si$_{2}$ is inconsistent with the small, 0.04 $\mu_{B}$ moment measured for the antiferromagnetism observed starting (perhaps coincidentally) at the same temperature. We report measurements of this specific heat transition, thought to be due to some 'hidden order', in magnetic fields between 24 and 42 T, i. e. through the field region where three metamagnetic transtions are known to occur at 35.8, 37.3, and 39.4 T. The response of $\Delta$C in single crystal URu$_{2}$Si$_{2}$ to magnetic field, which includes a change to $\Delta$C being possibly associated with a first order phase transition for high fields, is analyzed to shed further light on the possible explanations of this unknown ordering process. At fields above 35 T, a new high field phase comes into being; the connection between this high field phase revealed by the specific heat and earlier magnetization data is discussed

Characterisation of a multi-channel multiplexed EMG recording system: towards realising variable electrode configurations

First steps towards osseointegrated myoelec-trically-controlled prostheses: Bone anchor conduit conveys EMG signals from implanted electrodes [1]. In vivo selection of electrode configurations would improve signal-to-noise ratio (SNR) of EMG recordings [2]; optimal electrode config-urations are not known before implantation. The CAPITel system: Control of Active Prostheses using Implant-able Telemetry [3,4]. Implantable EMG amplifier with a novel multi-plexed frontend. In vivo selection of monopolar, bipolar or tripolar configurations. Designed using commercially available com-ponents for use in animal models. After further research design will be imple-mented as an ASIC