Dr.jens.brede

Observation of dipolar spin-exchange interactions with lattice-confined polar molecules

Nature 501, 7468 (2013). doi:10.1038/nature12483

Authors: Bo Yan, Steven A. Moses, Bryce Gadway, Jacob P. Covey, Kaden R. A. Hazzard, Ana Maria Rey, Deborah S. Jin & Jun Ye

With the production of polar molecules in the quantum regime, long-range dipolar interactions are expected to facilitate understanding of strongly interacting many-body quantum systems and to realize lattice spin models for exploring quantum magnetism. In ordinary atomic systems, where contact interactions require wavefunction overlap, effective spin interactions on a lattice can be mediated by tunnelling, through a process referred to as superexchange; however, the coupling is relatively weak and is limited to nearest-neighbour interactions. In contrast, dipolar interactions exist even in the absence of tunnelling and extend beyond nearest neighbours. This allows coherent spin dynamics to persist even for gases with relatively high entropy and low lattice filling. Measured effects of dipolar interactions in ultracold molecular gases have been limited to the modification of inelastic collisions and chemical reactions. Here we use dipolar interactions of polar molecules pinned in a three-dimensional optical lattice to realize a lattice spin model. Spin is encoded in rotational states of molecules that are prepared and probed by microwaves. Resonant exchange of rotational angular momentum between two molecules realizes a spin-exchange interaction. The dipolar interactions are apparent in the evolution of the spin coherence, which shows oscillations in addition to an overall decay of the coherence. The frequency of these oscillations, the strong dependence of the spin coherence time on the lattice filling factor and the effect of a multipulse sequence designed to reverse dynamics due to two-body exchange interactions all provide evidence of dipolar interactions. Furthermore, we demonstrate the suppression of loss in weak lattices due to a continuous quantum Zeno mechanism. Measurements of these tunnelling-induced losses allow us to determine the lattice filling factor independently. Our work constitutes an initial exploration of the behaviour of many-body spin models with direct, long-range spin interactions and lays the groundwork for future studies of many-body dynamics in spin lattices.

Time for change

Nature 501, 7468 (2013). doi:10.1038/501461b

Angela Merkel needs to tackle the issue of Germany’s uneven university funding.

Interface superconductivity found in single crystal

Nature 501, 7468 (2013). http://www.nature.com/doifinder/10.1038/501474a

Author: Eugenie Samuel Reich

Iron-based compound revives search for room-temperature superconductors.

Article

If single molecules are to be used in spintronic devices, it is necessary to interlink molecular spin states and charge transport. Here, the authors approach this goal by directly accessing highly spin-polarized hybrid states of a molecular complex of an early lanthanide on a metal surface.

Nature Communications doi: 10.1038/ncomms3425

Authors: Sarah Fahrendorf, Nicolae Atodiresei, Claire Besson, Vasile Caciuc, Frank Matthes, Stefan Blügel, Paul Kögerler, Daniel E. Bürgler, Claus M. Schneider

Author(s): Christian F. Hermanns, Matthias Bernien, Alex Krüger, Waldemar Walter, Yin-Ming Chang (張銀銘), Eugen Weschke, and Wolfgang Kuch

Using x-ray magnetic circular dichroism, we show that Co porphyrin molecules adsorbed on graphene-covered Ni surfaces possess a huge in-plane (IP) orbital magnetic moment of the Co ions, despite their fourfold coordination, even comparable in size to the spin moment. Both are antiferromagnetically c...

[Phys. Rev. B 88, 104420] Published Mon Sep 23, 2013

Nature advance online publication 18 September 2013. doi:10.1038/nature12483

Authors: Bo Yan, Steven A. Moses, Bryce Gadway, Jacob P. Covey, Kaden R. A. Hazzard, Ana Maria Rey, Deborah S. Jin & Jun Ye

With the production of polar molecules in the quantum regime, long-range dipolar interactions are expected to facilitate understanding of strongly interacting many-body quantum systems and to realize lattice spin models for exploring quantum magnetism. In ordinary atomic systems, where contact interactions require wavefunction overlap, effective spin interactions on a lattice can be mediated by tunnelling, through a process referred to as superexchange; however, the coupling is relatively weak and is limited to nearest-neighbour interactions. In contrast, dipolar interactions exist even in the absence of tunnelling and extend beyond nearest neighbours. This allows coherent spin dynamics to persist even for gases with relatively high entropy and low lattice filling. Measured effects of dipolar interactions in ultracold molecular gases have been limited to the modification of inelastic collisions and chemical reactions. Here we use dipolar interactions of polar molecules pinned in a three-dimensional optical lattice to realize a lattice spin model. Spin is encoded in rotational states of molecules that are prepared and probed by microwaves. Resonant exchange of rotational angular momentum between two molecules realizes a spin-exchange interaction. The dipolar interactions are apparent in the evolution of the spin coherence, which shows oscillations in addition to an overall decay of the coherence. The frequency of these oscillations, the strong dependence of the spin coherence time on the lattice filling factor and the effect of a multipulse sequence designed to reverse dynamics due to two-body exchange interactions all provide evidence of dipolar interactions. Furthermore, we demonstrate the suppression of loss in weak lattices due to a continuous quantum Zeno mechanism. Measurements of these tunnelling-induced losses allow us to determine the lattice filling factor independently. Our work constitutes an initial exploration of the behaviour of many-body spin models with direct, long-range spin interactions and lays the groundwork for future studies of many-body dynamics in spin lattices.

Author(s): A. Szilva, M. Costa, A. Bergman, L. Szunyogh, L. Nordström, and O. Eriksson

We derive ab inito exchange parameters for general noncollinear magnetic configurations, in terms of a multiple scattering formalism. We show that the general exchange formula has an anisotropiclike term even in the absence of spin-orbit coupling, and that this term is large, for instance, for colli...

[Phys. Rev. Lett. 111, 127204] Published Wed Sep 18, 2013

Author(s): M. G. House, Ming Xiao, GuoPing Guo, HaiOu Li, Gang Cao, M. M. Rosenthal, and HongWen Jiang

A quantum point contact was used to observe single-electron fluctuations of a quantum dot in a GaAs heterostructure. The resulting random telegraph signals (RTS) contain statistical information about the electron spin state if the tunneling dynamics are spin dependent. We develop a statistical metho...

[Phys. Rev. Lett. 111, 126803] Published Wed Sep 18, 2013

We extend the orbital-dependent electron tunneling model implemented within the three-dimensional (3D) Wentzel-Kramers-Brillouin (WKB) atom-superposition approach to simulate spin-polarized scanning tunneling microscopy (SP-STM) above magnetic surfaces. The tunneling model is based on the electronic structure data of the magnetic tip and surface obtained from first principles. Applying our method, we analyze the orbital contributions to the tunneling current, and study the nature of atomic contrast reversals occurring on constant-current SP-STM images above the Fe(110) surface. We find an interplay of orbital-dependent tunneling and spin-polarization effects responsible for the contrast inversion, and we discuss its dependence on the bias voltage, on the tip-sample distance, and on the tip orbital composition.

Author(s): B. Bryant, A. Spinelli, J. J. T. Wagenaar, M. Gerrits, and A. F. Otte

Individual Fe atoms on a Cu₂N/Cu(100) surface exhibit strong magnetic anisotropy due to the crystal field. We show that we can controllably enhance or reduce this anisotropy by adjusting the relative position of a second nearby Fe atom, with atomic precision, in a low-temperature scanning tunneling ...

[Phys. Rev. Lett. 111, 127203] Published Tue Sep 17, 2013

We present a detailed analysis of the spin-flip excitations induced by a periodic time-dependent electric field in the Rashba prototype Au(111) noble metal surface. Our calculations incorporate the full spinor structure of the spin-polarized surface states and employ a Wannier-based scheme for the spin-flip matrix elements. We find that the spin-flip excitations associated with the surface states exhibit an important angular modulation which is completely absent in the standard Rashba model \cite{rashba}. Furthermore, we demonstrate that the maximum of the calculated spin-flip absorption rate is about twice the model prediction. These results show that although the Rashba model accurately describes the spectrum and spin polarization, it does not fully account for the dynamical properties of the surface states.

Article

The use of spider silk in electronic devices is dependent on its compatibility with electrically conductive materials. Here the authors modify spider silk with carbon nanotubes to produce a strong, flexible and electrically conductive thread.

Nature Communications doi: 10.1038/ncomms3435

Authors: Eden Steven, Wasan R. Saleh, Victor Lebedev, Steve F. A. Acquah, Vladimir Laukhin, Rufina G. Alamo, James S. Brooks

Nature Nanotechnology 8, 612 (2013). doi:10.1038/nnano.2013.177

Authors: Christoph Boehme & John M. Lupton

In recent years, there has been a proliferation of models for spin-dependent electronic processes in organic semiconductors. Researchers aiming to utilize these processes for new organic spintronics devices should focus more on scrutinizing these models experimentally by embracing spectroscopy.

Nature Nanotechnology 8, 611 (2013). doi:10.1038/nnano.2013.187

Progress in the field of organic spintronics has, to a degree, stagnated, but what can the community do about it?

Author(s): Martin Callsen, Vasile Caciuc, Nikolai Kiselev, Nicolae Atodiresei, and Stefan Blügel

We reveal for the first time through a theoretical first-principles study that the adsorption of a nonmagnetic π-conjugated organic molecule on a ferromagnetic surface locally increases the strength of the magnetic exchange interaction between the magnetic atoms binding directly to the molecule. Thi...

[Phys. Rev. Lett. 111, 106805] Published Wed Sep 04, 2013

We propose a mechanism of coherent coupling between distant spin qubits interacting dipolarly with a ferromagnet. We derive an effective two-spin interaction Hamiltonian and estimate the coupling strength. We discuss the mechanisms of decoherence induced solely by the coupling to the ferromagnet and show that there is a regime where it is negligible. Finally, we present a sequence for the implementation of the entangling CNOT gate and estimate the corresponding operation time to be a few tens of nanoseconds. A particularly promising application of our proposal is to atomistic spin-qubits such as silicon-based qubits and NV-centers in diamond to which existing coupling schemes do not apply.

Crystallites of magnetic charges in artificial spin ice

Nature 500, 7464 (2013). doi:10.1038/nature12399

Authors: Sheng Zhang, Ian Gilbert, Cristiano Nisoli, Gia-Wei Chern, Michael J. Erickson, Liam O’Brien, Chris Leighton, Paul E. Lammert, Vincent H. Crespi & Peter Schiffer

Artificial spin ice is a class of lithographically created arrays of interacting ferromagnetic nanometre-scale islands. It was introduced to investigate many-body phenomena related to frustration and disorder in a material that could be tailored to precise specifications and imaged directly. Because of the large magnetic energy scales of these nanoscale islands, it has so far been impossible to thermally anneal artificial spin ice into desired thermodynamic ensembles; nearly all studies of artificial spin ice have either treated it as a granular material activated by alternating fields or focused on the as-grown state of the arrays. This limitation has prevented experimental investigation of novel phases that can emerge from the nominal ground states of frustrated lattices. For example, artificial kagome spin ice, in which the islands are arranged on the edges of a hexagonal net, is predicted to support states with monopolar charge order at entropies below that of the previously observed pseudo-ice manifold. Here we demonstrate a method for thermalizing artificial spin ices with square and kagome lattices by heating above the Curie temperature of the constituent material. In this manner, artificial square spin ice achieves unprecedented thermal ordering of the moments. In artificial kagome spin ice, we observe incipient crystallization of the magnetic charges embedded in pseudo-ice, with crystallites of magnetic charges whose size can be controlled by tuning the lattice constant. We find excellent agreement between experimental data and Monte Carlo simulations of emergent charge–charge interactions.

Author(s): D. L. R. Santos, P. Venezuela, R. B. Muniz, and A. T. Costa

The magnetic behavior of ultrathin ferromagnetic films deposited on substrates is strongly affected by the properties of the substrate. We investigate the spin pumping rate, interlayer exchange coupling, and dynamic exchange coupling between ultrathin ferromagnetic films through palladium, a nonmagn...

[Phys. Rev. B 88, 054423] Published Mon Aug 26, 2013

We herein formulate the concept of a generalized lock-in amplifier for the precision measurement of high frequency signals based on digital cavities. Accurate measurement of signals higher than 200 MHz using the generalized lock-in is demonstrated. The technique is compared with a traditional lock-in and its advantages and limitations are discussed. We also briefly point out how the generalized lock-in can be used for precision measurement of giga-hertz signals by using parallel processing of the digitized signals.

Author(s): A. N. Rudenko, F. J. Keil, M. I. Katsnelson, and A. I. Lichtenstein

Magnetism in single-side hydrogenated (C₂H) and fluorinated (C₂F) graphene is analyzed in terms of the Heisenberg model with parameters determined from first principles. We predict a frustrated ground state for both systems, which means the instability of collinear spin structures and sheds light on...

[Phys. Rev. B 88, 081405] Published Thu Aug 22, 2013