Electrically Induced Angular Momentum Flow between Separated Ferromagnets
R. Schlitz, M. Grammer, T. Wimmer, J. Gückelhorn, L. Flacke, S. T. B. Goennenwein, R. Gross, H. Huebl, A. Kamra, and M. Althammer
Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.256701

In this experiment-theory collaboration led by the Althammer group in Munich, we found that the spin information can be transmitted from one ferromagnet to another without a direct contact. This perplexing observation has been understood in terms of magnons in one ferromagnet being able to communicate with their counterparts in the other magnet despite the large separation. The underlying long-range interaction between the magnons is consistent with magnetic dipole-dipole interaction, amplified by a “superradiance” effect due to magnons being collective excitations of the magnet. “Spin currents carried by magnetic waves called magnons can be sent across a device without using insulating magnets—a result that could lead to spintronic devices compatible with silicon electronics.” opines Ryan Wilkinson in his synopsis published in the Physics magazine.

Unconventional magnetism mediated by spin-phonon-photon coupling
P. A. Pantazopoulos, J. Feist, F. J. García-Vidal and A. Kamra
Nature Communications (2024). DOI: 10.1038/s41467-024-48404-z

In this work, we theoretically demonstrated the emergence of a biquadratic long-range interaction between spins mediated by their coupling to phonons hybridized with vacuum photons into polaritons. The resulting ordered state enabled by the exchange of virtual polaritons between spins is reminiscent of superconductivity mediated by the exchange of virtual phonons. The biquadratic nature of the spin-spin interaction makes the emergence of magnetic order a first-order phase transition. Consequently, a large magnetization develops abruptly on cooling the material. This feature could enable magnetic memories admitting low-power thermally-assisted writing while maintaining a high data stability. This and a few other features make the predicted spin-spin interaction and magnetism highly unconventional paving the way for new scientific and technological opportunities.

Resolving nonclassical magnon composition of a magnetic ground state via a qubit
A.-L. E. Römling, A. Vivas-Viaña, C. Sánchez Muñoz, and A. Kamra
Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.143602

Magnets have been predicted to host squeezed states – quantum states with reduced quantum fluctuations in one observable and harboring entanglement - in equilibrium. The ground state manifests itself as a quantum superposition of states with different number of magnons – the bosonic particles that can exist in the magnet. Its unique equilibrium nature requires new innovative ways of utilizing it. This work presents a first theoretical proposal for detecting this robust equilibrium quantum superpositions harbored by ferromagnets. It proposes and employs a new direct dispersive interaction between the magnon mode and a spin qubit which may stem from the exchange interaction between spins. We predict that the magnetic ground state reveals itself in qubit spectroscopy as multiple nontrivial peaks, each of which corresponds to a different contribution to the quantum superposition. This first protocol opens doors towards utilizing the entanglement that exists naturally in several quantum materials for carrying out useful quantum computing.

Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons
F. Dirnberger, J. Quan, R. Bushati, G. M. Diederich, M. Florian, J. Klein, K. Mosina, Z. Sofer, X. Xu, A. Kamra, F. J. García-Vidal, A. Alù, and V. M. Menon
Nature (2023). DOI: 10.1038/s41586-023-06275-2

In this experiment-theory collaboration led by the Menon group at CUNY, the team reports the properties of a layered van der Waals magnet (CrSBr) that hosts strongly bound excitons — quasiparticles with particularly strong optical interactions. Because of that, the material is capable of trapping light — all by itself. Experiments find that the optical responses of this material to magnetic phenomena are orders of magnitude stronger than those in typical magnets. As an example, when an external magnetic field is applied, the near-infrared reflection of light is altered such that the material basically changes its color. Ordinarily, light does not respond so strongly to magnetism. This is why technological applications based on magneto-optic effects often require the implementation of sensitive optical detection schemes. Given this discovery of such strong interactions between magnetism and light, there is a hope to create magnetic lasers and need to reconsider concepts for optically controlled magnetic memories. Image credit: Rezlind Bushati.

Ubiquitous Superconducting Diode Effect in Superconductor Thin Films
Y. Hou, F. Nichele, H. Chi, A. Lodesani, Y. Wu, M. F. Ritter, D. Z. Haxell, M. Davydova, S. Ilić, O. Glezakou-Elbert, A. Varambally, F. S. Bergeret, A. Kamra, L. Fu, P. A. Lee, and J. S. Moodera
Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.027001

The flow of electrons in metals underlies electricity thereby forming the foundation of modern technology. Semiconductor devices, such as diodes and transistors, provide the electrical switches that allow current flow in one direction while forbidding it in the reverse direction. These are at the heart of modern digital logic. The flow of electrical current is accompanied by heating, which forms a major bottleneck for the computing industry. In this experiment-theory collaboration led by the Moodera group at MIT, the team demonstrates superconducting diodes which allow nondissipative supercurrent flow, without any heating, in one direction but not in the opposite. The physical mechanism underlying the effect is found to be governed by vortex surface barriers, in consistence with our previous collaborative theoretical work [PRB 104, 184512 (2021)]. The demonstrated design is a simple two-layer system, employs widely available materials, highly scalable to smaller sizes, and ready to be integrated in industrial fabrication facilities. Image credit: A. Varambally, Y. Hou, and H. Chi.

Observation of the Nonreciprocal Magnon Hanle Effect
J. Gückelhorn, S. de-la-Peña, M. Grammer, M. Scheufele, M. Opel, S. Geprägs, J. C. Cuevas, R. Gross, H. Huebl, A. Kamra, and M. Althammer
Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.216703

The relatively young paradigm of magnonics employs spin excitations in ordered magnets for information and energy transport. The carriers – magnons – promise various advantageous possibilities due to their bosonic character, in contrast with the fermionic character of electrons. In this experiment-theory collaboration led by the Althammer group in Munich, we found that the spin information carried by these magnons evolves differently when moving forwards and backwards. The theoretical efforts led by Sebastian de-la-Peña, who was a bachelor student in our group at the time, predicted that such an effect should arise when magnons experience different pseudofields in the forward and backward directions. Fitting the data to the theoretical expressions, we could confirm and extract the degree of nonreciprocity in the pseudofield. However, a microscopic origin of this surprising observation in terms of material properties has so far eluded the scientific community. Do you have the answer for us?

Two-qubit logic gates based on the ultrafast spin transfer in π-conjugated graphene nanoflakes
Y. Zhang, J. Liu, W. Jin, G. Lefkidis, W. Hübner, and C. Li
Carbon (2022). DOI:10.1016/j.carbon.2022.03.012

In this theoretical work we looked into the possibility of using small magnetic atoms deposited on graphene nanoflakes for quantum computing. Using state-of-art quantum chemical methods we calculated the quantum states of the whole system (substrate plus adatoms) and by emplying specially designed laser pulses we were able to make the magnetic atoms take the role of quantum bits (qubits). 

Quantum magnonics: When magnon spintronics meets quantum information science
H. Y. Yuan, Y. Cao, A. Kamra, R. A. Duine, and P. Yan
Physics Reports (2022). DOI: 10.1016/j.physrep.2022.03.002

Tiny magnets have played an instrumental role in the modern data-heavy information technology based on the digital computers that we use everyday. Intense research efforts are currently underway in creating a new class of computers that would exploit the “spooky” nature of quantum mechanics to solve problems deemed impossible otherwise. Can the tiny nanomagnets be as fruitful for this new class of quantum computers as they have been for data storage in digital computers? A growing community of researchers asks and tries to answer this and related questions. This review article presents the state of the art in this young field of inquiry, highlighting some opportunities and challenges that lie ahead.

Control of Nonlocal Magnon Spin Transport via Magnon Drift Currents
R. Schlitz, S. Vélez, A. Kamra, C.-H. Lambert, M. Lammel, S. T. B. Goennenwein, and P. Gambardella
Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.257201

Theory of drift-enabled control in nonlocal magnon transport
S. de-la-Peña, R. Schlitz, S. Vélez, J. C. Cuevas, and A. Kamra
Journal of Physics: Condensed Matter (2022). DOI: 10.1088/1361-648X/ac6d9a

The flow of electrons in metals underlies electricity and electric currents thereby forming the foundation of modern technology. The negative charge of electrons means that they are attracted towards the positive terminal of a battery. Using this property, they can be steered in any chosen direction. But what if we had uncharged particles in our devices? The emerging paradigm of magnonics is based on such chargeless particles because they manifest unique bosonic features, not admitted by electrons. Will it be possible to steer these uncharged particles? These articles answer the question with a resounding “yes” and provides the necessary theory as well as the experimental demonstration.

Large Enhancement of Critical Current in Superconducting Devices by Gate Voltage
M. Rocci, D. Suri, A. Kamra, G. Vilela, Y. Takamura, N. M. Nemes, J. L. Martinez, M. G. Hernandez, and J. S. Moodera
Nano Letters (2021). DOI: 10.1021/acs.nanolett.0c03547

Interfacial control of vortex-limited critical current in type II superconductor films
M. K. Hope, M. Amundsen, D. Suri, J. S. Moodera, and A. Kamra
Physical Review B (2021). DOI: 10.1103/PhysRevB.104.184512

Our contemporary digital computers and electronics use as a basic building block a "switch" that connects or disconnects a circuit depending on the voltage applied at a gate electrode. This is achieved by realizing the circuit with silicon constrictions, which are electrically insulating. Application of a gate voltage creates charge carriers in these constrictions and makes them conducting thereby closing the circuit. Using similar devices made out of superconducting films, our collaborators at MIT, Cambridge, USA have succeeded in increasing the maximum supercurrent that can flow through the constrictions. Besides presenting fresh opportunities for applications based on the control, and especially an increase, of the dissipationless supercurrent, this achievement defies decades of research suggesting that such a gate voltage should not affect a good conductor. In an oversimplified and intuitive comparison, the observed effect seems like turning the water of an entire lake orange by adding a bucketful of orange juice.

Long-distance ultrafast spin transfer over a zigzag carbon chain structure
J. Liu, C. Li, W. Jin, G. Lefkidis, and W. Hübner
Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.037402

In this theoretical work we investigated the possibility of transferring the spin over large distances via carbon chains. This is an important step in nanospintronics, that is in computational elements in which the spin rather than the charge of the electron is used as an information carrier. Here we used high-level quantum chemical methods to exploit the conjugated bonds of carbon chains and showed that their delocalized nature can help couple far-apart magnetic centers. Additionally we discovered that by using double laser pulses one gains a much finer control of the whole process. So it is possible to integrate small magnetic molecular systems into large circuits.