1,392 research outputs found
Investigating ultrafast quantum magnetism with machine learning
We investigate the efficiency of the recently proposed Restricted Boltzmann
Machine (RBM) representation of quantum many-body states to study both the
static properties and quantum spin dynamics in the two-dimensional Heisenberg
model on a square lattice. For static properties we find close agreement with
numerically exact Quantum Monte Carlo results in the thermodynamical limit. For
dynamics and small systems, we find excellent agreement with exact
diagonalization, while for systems up to N=256 spins close consistency with
interacting spin-wave theory is obtained. In all cases the accuracy converges
fast with the number of network parameters, giving access to much bigger
systems than feasible before. This suggests great potential to investigate the
quantum many-body dynamics of large scale spin systems relevant for the
description of magnetic materials strongly out of equilibrium.Comment: 18 pages, 5 figures, data up to N=256 spins added, minor change
Ultrafast Quenching of the Exchange Interaction in a Mott Insulator
We investigate how fast and how effective photocarrier excitation can modify
the exchange interaction in the prototype Mott-Hubbard
insulator. We demonstrate an ultrafast quenching of both by
evaluating exchange integrals from a time-dependent response formalism and by
explicitly simulating laser-induced spin precession in an antiferromagnet that
is canted by an external magnetic field. In both cases, the electron dynamics
is obtained from nonequilibrium dynamical mean-field theory. We find that the
modified emerges already within a few electron hopping times
after the pulse, with a reduction that is comparable to the effect of chemical
doping.Comment: 8 pages, 4 figure
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
330 mK, which we interpret as an indication of a second phase
transition involving paramagnetic U. The rise in between 7 K and
330 mK and the absence of a large entropy liberated at may be due to a
combination of Kondo screening effects and frustration that strongly modifies
the low T transition.Comment: 4 pages, 4 figure
Ultrafast and reversible control of the exchange interaction in Mott insulators
The strongest interaction between microscopic spins in magnetic materials is
the exchange interaction . Therefore, ultrafast control of
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 on ultrafast
timescales in extended antiferromagnetic Mott insulators. In the regime of weak
driving strength, we find that can be enhanced and reduced for
frequencies below and above the Mott gap, respectively. Moreover, for strong
driving strength, even the sign of 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
Optical control of competing exchange interactions and coherent spin-charge coupling in two-orbital Mott insulators
In order to have a better understanding of ultrafast electrical control of
exchange interactions in multi-orbital systems, we study a two-orbital Hubbard
model at half filling under the action of a time-periodic electric field. Using
suitable projection operators and a generalized time-dependent canonical
transformation, we derive an effective Hamiltonian which describes two
different regimes. First, for a wide range of non-resonant frequencies, we find
a change of the bilinear Heisenberg exchange that is
analogous to the single-orbital case. Moreover we demonstrate that also the
additional biquadratic exchange interaction can be enhanced,
reduced and even change sign depending on the electric field. Second, for
special driving frequencies, we demonstrate a novel spin-charge coupling
phenomenon enabling coherent transfer between spin and charge degrees of
freedom of doubly ionized states. These results are confirmed by an exact
time-evolution of the full two-orbital Mott-Hubbard Hamiltonian.Comment: 3 pages, 6 figure
Parametrically driven THz magnon-pairs: predictions towards ultimately fast and minimally dissipative switching
Findings ways to achieve switching between magnetic states at the fastest
possible time scale that simultaneously dissipates the least amount of energy
is one of the main challenges in magnetism. Antiferromagnets exhibit intrinsic
dynamics in the THz regime, the highest among all magnets and are therefore
ideal candidates to address this energy-time dilemma. Here we study
theoretically THz-driven parametric excitation of antiferromagnetic
magnon-pairs at the edge of the Brillouin zone and explore the potential for
switching between two stable oscillation states. Using a semi-classical theory,
we predict that switching can occur at the femtosecond time scale with an
energy dissipation down to a few zepto Joule. This result touches the
thermodynamical bound of the Landauer principle, and approaches the quantum
speed limit up to 5 orders of magnitude closer than demonstrated with magnetic
systems so far.Comment: 8 pages, 4 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
Quantum many-body dynamics of the Einstein-de Haas effect
In 1915, Einstein and de Haas and Barnett demonstrated that changing the
magnetization of a magnetic material results in mechanical rotation, and vice
versa. At the microscopic level, this effect governs the transfer between
electron spin and orbital angular momentum, and lattice degrees of freedom,
understanding which is key for molecular magnets, nano-magneto-mechanics,
spintronics, and ultrafast magnetism. Until now, the timescales of
electron-to-lattice angular momentum transfer remain unclear, since modeling
this process on a microscopic level requires addition of an infinite amount of
quantum angular momenta. We show that this problem can be solved by
reformulating it in terms of the recently discovered angulon quasiparticles,
which results in a rotationally invariant quantum many-body theory. In
particular, we demonstrate that non-perturbative effects take place even if the
electron--phonon coupling is weak and give rise to angular momentum transfer on
femtosecond timescales.Comment: 15 pages, 5 figure
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