We determine the Dzyaloshinskii-Moriya interaction within and between two magnetic cobalt layers separated by a nonmagnetic spacer through ab initio calculations. We investigate different materials for the nonmagnetic layer, focusing on the experimentally realized Co/Ag/Co system. We laterally shift the atoms in the nonmagnetic layer to achieve the symmetry breaking required for the interlayer Dzyaloshinskii-Moriya interaction. We compare the resulting interactions with the Levy-Fert model and observe a good overall agreement between the model and the ab initio calculations for the dependence on the atomic positions. Additionally, we derive a formula for the strength of the interlayer isotropic exchange interaction depending on the position of the atoms in the nonmagnetic layer and compare it to the first-principles results. We investigate the limitations of the Levy-Fert model by turning off the spin-orbit coupling separately on the nonmagnetic and magnetic atoms and by studying the effect of band filling. Our work advances the understanding of the microscopic mechanisms of the interlayer Dzyaloshinskii-Moriya interaction and gives insight into possible new material combinations with strong interlayer Dzyaloshinskii-Moriya interaction.

Az 1930-as évek közepe táján Wigner Jenő és PhD-hallgatója, Frederick Seitz a fémek kötési energiájának problémáján kezdett dolgozni. Wigner elég hamar felvetette azt a kérdést, hogy a fémes kötést adó elektronfolyadék-állapot fennmarad-e nagyon kis sűrűségű elektronrendszerben, amikor az elektronok kinetikus energiájához képest az elektronok közötti Coulomb-taszítás válik dominánssá? Hiszen ilyenkor rögzített sűrűség mellett energetikaikag kedvezőbb lehet az az állapot, amelyben az elektronok szabályos kristályszerkezetet alkotnak. Wigner megmutatta, hogy a Coulomb-energia és a kinetikus energia hányadosaként definiált Γ paraméter értékétől függően megváltozik a rendszer viselkedése. Ha Γ értéke 1-nél jóval kisebb, az elektronok közötti korreláció is kicsi lesz a kinetikus energiához képest, ezért az elektronok majdnem szabadon mozognak, a rendszer fémes tulajdonságokkal rendelkezik. Az ellenkező esetben viszont, amikor a Coulomb-taszításból adódó potenciális energia dominál a kinetikus energiával szemben (Γ jóval nagyobb 1-nél), az elektronok kristályrácsba lokalizálódnak, a rendszer szigetelővé válik. Ezt nevezik azóta Wigner-kristálynak.

Márk Kondákor won the Best Poster Award. Congratulations. Our student Péter Kránitz’ report: “Since autumn 2021, I have been a Wigner intern under the supervision of Dr. Karlo Penc, with a semester break. My first motivation was to write a thesis for the Scientific Student Conference and to join cutting-edge research. My topic was to investigate a two-dimensional spin ice system, first numerically and then analytically. Working with my supervisor was a very positive experience, and I could also get to know the whole research group. The research eventually led to my B.Sc. thesis and a publication in Physical Review Research. I presented my results at a summer school in Dresden and an international conference in India, where I won a poster prize. I am still a Wigner intern, working on my M.Sc. thesis on the theory of superconductivity in elementary rhenium. The broad spectrum of the group allows me to widen my knowledge and learn about new phenomena and methods.”

Skyrmions are localized, topological spin structures that can be described as quasiparticles. Skyrmions in thin films are an ideal model system to study Brownian motion and lattice formation in two dimensions. They follow an equation of motion, the Thiele equation, which includes a topology-dependent chiral term, linear in velocity, causing a skyrmion Hall effect and a drastic reduction of the diffusion coefficient for individual skyrmions, as compared to normal Brownian particles. Using Brownian dynamics simulations, we show that this topological suppression of the diffusion can be partially lifted in two-dimensional lattices of skyrmions. Counterintuitively, this causes enhanced diffusive properties with increasing particle density, similar to odd-diffusive Brownian particles. We show how the topological charge of the skyrmions influences the dynamics of topological lattice defects, which also affects the dynamics of the phase formation.

Since autumn 2021, I have been a Wigner intern under the supervision of Dr. Karlo Penc, with a semester break. My first motivation was to write a thesis for the Scientific Student Conference and to join cutting-edge research. My topic was to investigate a two-dimensional spin ice system, first numerically and then analytically. Working with my supervisor was a very positive experience, and I could also get to know the whole research group. The research eventually led to my B.Sc. thesis and a publication in Physical Review Research. I presented my results at a summer school in Dresden and an international conference in India, where I won a poster prize. I am still a Wigner intern, working on my M.Sc. thesis on the theory of superconductivity in elementary rhenium. The broad spectrum of the group allows me to widen my knowledge and learn about new phenomena and methods.

Van a fizikának néhány olyan területe, ahol a kutatások rendkívül lassan haladnak, és kívülről könnyen úgy tűnhet, nem is érdemes ezeket folytatni, pénzzel támogatni, de ugyanakkor az is nyilvánvaló, hogy megvalósulásuk esetén olyan mértékű változást hoznának az emberiség életébe, hogy mégsem lehet felhagyni velük. Ilyen terület például a fúziós energiatermelés és a szobahőmérsékletű szupravezetés.

Our Students participated the Scientific Student Conference (Tudományos Diákköri Konferencia) at Budapest University of Technology and Economics in Session Theoretical Physics 1. Péter Kránitz was awarded second place, and László Rudner recieved a commendation. Congratulations.

Grain boundary energy (GBE) and its temperature dependence in body-centered cubic (bcc) metals are investigated using ab initio calculations. We reveal a scaling relationship between the GBEs of the same grain boundary structure in different bcc metals and find that the scaling factor can be best estimated by the ratio of the low-index surface energy. Applying the scaling relationship, the general GBEs of bcc metals at 0 K are predicted. Furthermore, adopting the Foiles’s method which assumes that the general GBE has the same temperature dependence as the elastic modulus c44 [Scr. Mater., 62 (2010) 231–234], the predicted general GBEs at elevated temperatures are found in good agreement with available experimental data. Reviewing two experimental methods for determining the general GBEs, we conclude that the two sets of experimental GBEs for bcc metals correspond to different GB structural spaces and differ by approximately a factor of 2. The present work puts forward an efficient methodology for predicting the general GBEs of metals, which has the potential to extend its application for homogeneous alloys without strong segregation of the alloying element and facilitates GB engineering for advanced alloy design.

Több mint egy hete lázban tartja az internetet az LK-99 nevű anyag, amiről dél-koreai szakemberek azt állítják, hogy szupravezetésre képes szobahőmérsékleten és normál légköri nyomáson.

Intrinsic stacking fault energy (SFE) values of γ-Fe and AISI 304 austenitic stainless steels were determined as a function of carbon and nitrogen content using ab initio calculations. In contrast to previous investigations, the analysis was conducted incorporating the paramagnetic state to account for the magnetic constitution of real austenitic stainless steels.

Spin chains proximitized by s-wave superconductors are predicted to enter a mini-gapped phase with topologically protected Majorana modes (MMs) localized at their ends. However, the presence of non-topological end states mimicking MM properties can hinder their unambiguous observation. Here, we report on a direct method to exclude the non-local nature of end states via scanning tunneling spectroscopy by introducing a locally perturbing defect on one of the chain’s ends. We apply this method to particular end states observed in antiferromagnetic spin chains within a large minigap, thereby proving their topologically trivial character. A minimal model shows that, while wide trivial minigaps hosting end states are easily achieved in antiferromagnetic spin chains, unrealistically large spin-orbit coupling is required to drive the system into a topologically gapped phase with MMs. The methodology of perturbing candidate topological edge modes in future experiments is a powerful tool to probe their stability against local disorder.

Egy rúdmágnes köré szórt vasreszelékhez hasonlóan az atomi mágneses momentumok is a környezetükben lévõ mágneses térnek megfelelõen állnak be. A kvantummechanikai eredetû kölcsönhatások sokszínûsége következtében ez gyakran egzotikus szerkezetek kialakulásához vezet atomi méretekben, amelyekben a mágneses momentumok egymással nem párhuzamosan, azaz nemkollineárisan rendezõdnek. A rengetegféle ilyen mágneses konfiguráció közötti eligazodásban nagy segítséget nyújt a topológia, és ez a fizikai tulajdonságokat is befolyásolja.

The newest research results of researchers of the Wigner Research Center for Physics, Krisztián Palotás and Levente Rózsa, in collaboration with the Technical University of Ilmenau have been published in the leading international scientific journal Nano Letters. They studied the interaction of the magnetic moment of a molecule with a superconducting lead substrate upon sequential chemical modifications of the molecule induced by the tip of a scanning tunneling microscope. A distance change of only 8 picometers, one ten-millionth of the diameter of the human hair, between the magnetic cobalt center of the molecule and the lead substrate resulted in a significant change of the measured tunneling spectrum of the molecule. The researchers of the Wigner RCP contributed to the understanding of the remarkable experimental results of the German partner by theoretical calculations. The results might lead to new developments and applications in various fields of surface physics and chemistry, e.g., in surface chemical reactions, in (molecular) nanoelectronic devices or in ultra-sensitive detectors. The illustration about the discovery was selected for a cover page of the journal.

In a German-Hungarian collaboration led by the University of Hamburg, Krisztián Palotás (Wigner RCP, MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Budapest University of Technology and Economics (BME)), András Lászlóffy (Wigner RCP, BME) and László Szunyogh (BME, MTA-BME Condensed Matter Research Group) from Hungary, and Levente Rózsa (University of Konstanz) contributed to the theoretical understanding of quantum states at superconducting surfaces induced by magnetic impurities, and published their results in Nature Communications.