My name is Börge and I am a postdoc in the group of Prof. Ingrid Mertig in Halle, Germany.
I am a theoretical physicist working in the field of condensed matter physics with a
special focus on magnetic systems.
My research explores the interplay between chirality and transport of charge, spin and orbital angular momentum.
I investigate how topological spin textures, such as skyrmions, give rise to new, emergent phenomena.
In several collaborations with experimentalists we explore new systems with a focus on two-dimensional materials.
Besides doing fundamental research, I aim to contribute to the development of next-generation spintronic and orbitronic devices, with potential applications in quantum computing, memory storage, and neuromorphic computing.
If you want to get in contact, please feel free to write me an email
[41] B. Göbel, L. Schimpf, I. Mertig. Topological orbital Hall effect caused by skyrmions and antiferromagnetic skyrmions. arXiv preprint number: 2410.00820 (2024)
[39] I. Ribeiro de Assis, I. Mertig, B. Göbel. Circular motion of non-collinear spin textures in Corbino disks: Dynamics of Neel- versus Bloch-type skyrmions and skyrmioniums. Physical Review B 110, 064404 (2024) open
[38] R. Saha, H. Meyerheim, B. Göbel, I. Mertig, S. Parkin. High-temperature Neel skyrmions in Fe3GaTe2 stabilized by Fe intercalation into the van der Waals gap. npj Spintronics 2, 21 (2024) open
[37] G. Lazrak, B. Göbel, A. Barthelemy, I. Mertig, A. Johansson, M. Bibes Boosting the Edelstein effect of two-dimensional electron gases by ferromagnetic exchange. Physical Review Research 6, 023074 (2024) open
2023
[35] S. Mallik*, B. Göbel*, H. Witt*, L. Vicente-Arche, S. Varotto, J. Brehin, G. Menard, G. Saiz, D. Tamsaout, A. Santander-Syro, F. Fortuna, F. Bertran, P. Le Fevre, J. Rault, I. Boventer, I. Mertig, A. Barthelemy, N. Bergeal, A. Johansson, M. Bibes. Electronic band structure of superconducting KTaO3 (111) interfaces. APL Materials 11, 121108 (2023) open
[34] I. Ribeiro de Assis, I. Mertig, B. Göbel. Skyrmion motion in magnetic anisotropy gradients: Acceleration caused by deformation. Physical Review B 108, 144438 (2023) open
[31] B. Hazra, B. Pal, J.-C. Jeon, R. Neumann, B. Göbel, B. Grover, H. Deniz, A. Styervoyedov, H. Meyerheim, I. Mertig, S.-H. Yang, S. Parkin. Generation of out-of-plane polarized spin current by spin swapping. Nature Communications 14, 4549 (2023) open
2022
[29] S. Varotto*, A. Johansson*, B. Göbel*, L. Vicente-Arche, S. Mallik, J. Brehin, R. Salazar, F. Bertran, P. Fevre, N.Bergeal, J. Rault, I. Mertig, M. Bibes. Direct visualization of Rashba-split bands and spin/orbital-charge interconversion at KTaO3 interfaces. Nature Communications 13, 6165 (2022) open
[28] R. Saha, H. Meyereheim, B. Göbel, B. Hazra, H. Deniz, K. Mohseni, D. Knyazev, A. Bedoya-Pinto, I. Mertig, V. Antonov, A. Ernst, S. Parkin. Observation of Neel-type skyrmions in acentric self-intercalated Cr1+δTe2. Nature Communications 13, 3965 (2022) open
[27] B. Pal*, B. Hazra*, B. Göbel*, J.-C. Jeon, A. Pandeya, A. Chakraborty, O. Busch, A. Srivastava, H. Deniz, J. Taylor, H. Meyerheim, I. Mertig, S.-H. Yang, S. Parkin. Setting of the magnetic structure of chiral kagome antiferromagnets by a Seeded Spin-Orbit Torque. Science Advances 8, eabo5930 (2022) open
[26] J. Jena, B. Göbel, T. Hirosawa, S. Diaz, D. Wolf, T. Hinokihara, V. Kumar, I. Mertig, C. Felser, A. Lubk, D. Loss, S. Parkin. Observation of fractional spin textures in a Heusler material. Nature Communications 13, 2348 (2022) open
[23] M. Raju, A. Petrovic, A. Yagil, K. Denisov, N. Duong, B. Göbel, E. Sasioglu, O. Auslaender, I. Mertig, I. Rozhansky, C. Panagopoulos. Colossal topological Hall effect at the transition between isolated and lattice-phase interfacial skyrmions. Nature Communications 12, 2758 (2021) open
[22] A. Johansson, B. Göbel, J. Henk, M. Bibes, I. Mertig. Spin and orbital Edelstein effects in a two-dimensional electron gas: Theory and application to SrTiO3 interfaces. Physical Review Research 3, 013275 (2021) open
[21] B. Göbel, I. Mertig. Skyrmion ratchet propagation: Utilizing the skyrmion Hall effect in AC racetrack storage devices. Scientific Reports 11, 3020 (2021) open
[20] B. Göbel, I. Mertig, O. Tretiakov. Beyond skyrmions: Review and perspectives of alternative magnetic quasiparticles. Physics Reports 895, 1-28 (2021) open
2020
[19] J. Jena*, B. Göbel*, V. Kumar, I. Mertig, C. Felser, S. Parkin. Evolution and competition between chiral spin textures in nano-stripes with D2d symmetry. Science Advances 6, eabc0723 (2020) open
[18] P. Sivakumar, B. Göbel, E. Lesne, A. Markou, J. Gidugu, J. Taylor, H. Deniz, J. Jena, C. Felser, I. Mertig, S. Parkin. Topological Hall Signatures of Two Chiral Spin Textures Hosted in a Single Tetragonal Inverse Heusler Thin Film. ACS Nano 14, 13463 (2020) open
[15] J. Jena*, B. Göbel*, T. Ma, V. Kumar, R. Saha, I. Mertig, C. Felser, S. Parkin. Elliptical Bloch skyrmion chiral twins in an antiskyrmion system. Nature Communications 11, 1115 (2020) open
2019
[13] D. Vaz*, P. Noël*, A. Johansson*, B. Göbel*, F. Bruno, G. Singh, S. McKeown-Walker, F. Trier, L. Vicente-Arche, A. Sander, S. Valencia, P. Bruneel, M. Vivek, M. Gabay, N. Bergeal, F. Baumberger, H. Okuno, A. Barthélémy, A. Fert, L. Vila, I. Mertig, J. Attané, M. Bibes. Mapping spin-charge conversion to the band structure in a topological oxide two-dimensional electron gas. Nature Materials 18, 1187 (2019)
[12] B. Göbel*, A. F. Schäffer*, J. Berakdar, I. Mertig, S. S. P. Parkin. Electrical writing, reading, and moving of magnetic skyrmioniums in a racetrack device. Scientific Reports 9, 12119 (2019) open
[11] B. Göbel, J. Henk, I. Mertig. Forming magnetic biskyrmions by merging two skyrmions in a centrosymmetric nanodisk. Scientific Reports 9, 9521 (2019) open
[10] B. Göbel, A. Mook, J. Henk, I. Mertig, O. A. Tretiakov. Magnetic bimerons as skyrmion analogues in in-plane magnets. Physical Review B 99, 060407(R) (2019)
[9] B. Göbel, A. Mook, J. Henk, I. Mertig. Magnetoelectric effect and orbital magnetization in skyrmion crystals: Detection and characterization of skyrmions. Physical Review B 99, 060406(R) (2019)
[8] B. Göbel, A. Mook, J. Henk, I. Mertig. Overcoming the speed limit in skyrmion racetrack devices by suppressing the skyrmion Hall effect. Physical Review B 99, 020405(R) (2019)
2018
[7] B. Göbel, A. Mook, J. Henk, I. Mertig. Publisher Correction to: The family of topological Hall effects for electrons in skyrmion crystals. European Physical Journal B 91, 262 (2018) open
[5] A. Mook, B. Göbel, J. Henk, I. Mertig. Taking an electron-magnon duality shortcut from electron to magnon transport. Physical Review B 97, 140401(R) (2018)
2017
[4] B. Göbel, A. Mook, J. Henk, I. Mertig. Antiferromagnetic skyrmion crystals: Generation, topological Hall, and topological spin Hall effect. Physical Review B 96, 060406(R) (2017)
[2] B. Göbel, A. Mook, J. Henk, I. Mertig. Unconventional topological Hall effect in skyrmion crystals caused by the topology. Physical Review B 95, 094413 (2017)
[1] A. Mook, B. Göbel, J. Henk, I. Mertig. Magnon transport in noncollinear spin textures: Anisotropies and topological magnon Hall effects. Physical Review B 95, 020401(R) (2017)
The publications include collaborations with the groups of Prof. Stuart Parkin, Prof. Daniel Loss, Prof. Gen Tatara, Prof. Oleg Tretiakov and Prof. Christos Panagopoulos.
A magnetic skyrmion consists of non-collinear magnetic moments: the colored arrows in the figure above. It is topologically non-trivial,
since it cannot be continuously transformed into a ferromagnetic state. This property, characterized by its integer topological charge
Q = 1, gives
it an enourmous stability which appears to be favorable for data storage applications. In fact, skyrmions can be generated,
deleted, driven by currents and read by their unique Hall signature. Therefore, they can be considered the carriers of information
in racetrack storage devices where the presence and absence of a skyrmion at predefined positions in a magnetic stripe corresponds to
bits of 0 and 1.
"The non-trivial real-space topology of skyrmions also becomes apparent in the
current-driven motion of the skyrmions themselves. One typically discusses two scenarios:
the motion under spin-transfer torque (STT), where a spin- polarized current goes through
the spin texture and reorients the magnetic moments of a skyrmion (Fig.), and the motion
under spin-orbit torque (SOT), where a spin accumulation created by an electric current in
the presence of spin-orbit interaction exerts a torque on the skyrmion texture.
In the STT scenario, the torque is determined by the gradient of the magnetization density
which reorients the magnetic moments due to angular momentum conservation. This collective reorientation can be identified with a
motion of the skyrmions. The SOT mechanism turns out to be more efficient, since electron
spins and magnetic moments can have a large misalignment leading to larger torques.
Still, in both scenarios, it has been predicted and observed that the skyrmions do not move parallel to the
current, but experience a transverse deflection in this case. This phenomenon is called
the skyrmion Hall effect and originates in the topological charge of the skyrmions.
This effect is highly detrimental for a racetrack storage device based on magnetic skyrmion and
has to be suppressed.
Emergent electrodynamics of a skyrmion. The topological Hall effect and the skyrmion
Hall effect due to spin-transfer torque under application of an electric current are
visualized. The current electrons (blue) and the skyrmion itself experience a
transverse deflection, since the electron spin (colored arrows) aligns partially with
the texture and generates a torque."
"If an electron hops between two sites of the sample and reorients its spin, the original transfer integral
t accumulates a complex phase factor. This factor is the analogue of the Peierls phase,
which characterizes the magnetic field in the ordinary Hall effect. Therefore, in the case of
topological spin textures, it can be related to an effective vector potential. In an adiabatic
approximation, the corresponding field is called emergent field.
In two dimensions it is proportional to the topological charge density of the topological object.
This fictitious
field can be used to easily relate the transverse deflection and the generation of a
Hall effect with the presence of skyrmions. As long as the electron spin and the
texture are coupled strongly, the topological Hall effect is proportional to the number
of skyrmions in the sample. In this sense, the topological Hall effect is very similar to
the ordinary Hall effect. Consequently, since for small skyrmions the emergent field can
be as large as thousands of Tesla, there are theoretical predictions of a quantized
version of the topological Hall effect as well, very similar to the quantum Hall effect
[B. Göbel et al., Phys. Rev. B 95, 094413 (2017)].
Emergent electrodynamics of a skyrmion. The topological Hall effect and the skyrmion
Hall effect due to spin-transfer torque under application of an electric current are
visualized. The current electrons (blue) and the skyrmion itself experience a
transverse deflection, since the electron spin (colored arrows) aligns partially with
the texture and generates a torque."
A major research focus is on circumventing the short-comings of conventional magnetic skyrmions:
Skyrmions do not move parallel to the driving currents but instead experience
the skyrmion Hall effect that pushes them towards the racetrack edge.
Bit sequences, corresponding to present and absent skyrmions, may not be stable because
skyrmions interact with each other and experience thermal diffusion.
Both problems are critical as they will lead to a malfunction of the data storage due to a
loss or change of information.
(1) The transverse motion, identified as the skyrmion Hall effect, can be surpressed by using alternative magnetic objects.
This includes objects with a vanishing topological charge, like the antiferromagnetic (AFM) skyrmion, but also objects with
a broken rotational symmetry, like bimerons or antiskyrmions.
(2) To nullify the detrimental effect of irregular
distances between the bits, one could represent the bits by two topologically distinct objects.
We have shown that this is possible due to the coexistence of skyrmions and antiskyrmions in Heusler materials.
The field of orbitronics is concerned with the orbital angular momentum of electrons.
We investigate the following effects:
Orbital magnetization:
The equilibrium magnetization generated by the orbital angular momentum.
Orbital Edelstein effect:
The nonequilibrium orbital magnetic moment that is generated once an electric field is applied.
Orbital Hall effect:
The transverse orbital angular momentum current that is generated once an electric field is applied.
The orbital angular momentum can be generated at an atom by hybridization of atomic orbitals or by
inter-site hybridization.
In the image below, an orbital current is generated based on inter-site hybridization.
I am most interested in the orbital Hall effect. In a recent publication, we show that the famous quantum Hall effect is accompanied by an orbital Hall effect.
Our quantum mechanical calculations fit well the semiclassical interpretation in terms of "skipping orbits". Physical Review Letters (2024)
Likewise, the topological Hall effect of skyrmions is accompanied by a topological orbital Hall effect.
antiferromagnetic skyrmions and antiferromagnetic bimerons that have a compensated emergent field, exhibit a topological orbital Hall conductivity that is not accompanied by charge transport and can be orders of magnitude larger than the topological spin Hall conductivity.
arXiv (2024)
2d systems
The field of two-dimensional systems is closely related to many of the previously presented topics.
For example, skyrmions are two-dimensional object that are extended as tubes along the third dimension and the quantum Hall effect appears only in two-dimensional samples.
My research topics that have not been discussed so far are:
Chiral antiferromagnets:
These textures exhibit a finite anomalous Hall effect without a net magnetization and form in materials such as Mn3Sn consisting of kagome planes. We analyze their Hall effects and simulate how magnetic domains can be switched.
Two-dimensional electron gases:
At interfaces of perovskites the bulk conduction band drops below the Fermi level and a two-dimensional electron gas is formed. They give rise to the Rashba-Edelstein effect causing very large spin-to-charge and orbital-to-charge interconversion efficiencies.
Layered skyrmion materials:
Layered materials can host skyrmions due to the interfacial DMI or due to intercalation that leads to a bulk-type DMI.
We calculate the anomalous, spin and orbital Hall effect of coplanar spin textures in materials such as Mn3Sn.
In an experimental collaboration with the groups of Prof. Stuart Parkin and Prof. Claudia Felser we have simulated and observed the switching of the non-collinear magnetic domains. Science Advances (2022)
In experimental collaborations with the groups of Dr. Manuel Bibes, Prof. Albert Fert, Dr. Laurent Vila and others, we model the electronic structure of two-dimensional electron gases, such as STO/AlO and KTO/AlO, and calculate their spin- and orbital-to-charge conversion to understand the experimental observations.
In KTO/AlO we have experimentally resolved the Rashba splitting, as shown in the image below. Nature Communications (2022)
Furthermore, we have observed non-collinear spin textures stabilized due to intercalation in van der Waals materials such as CrTe2 and Fe3GaTe2.
Nature Communications (2022)
Contact
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