Experimental Physics V

Open postdoctoral and PhD positions

in the field of magnetic skyrmions and magnetoelectric materials!

For further information contact Prof. Dr. István Kézsmárki (istvan.kezsmarki@physik.uni-augsburg.de).


Magnetoelectric spectroscopy of spin excitations in LiCoPO4

fig 805
Magnetoelectric spectroscopy is a powerful tool to determine all off-diagonal elements of the magnetoelectric tensor in a contactless fashion. Our colleagues demonstrate the efficiency of this optical method by measuring the off-diagonal magnetoelectric response of LiCoPO4 in the GHz-THz regime. According to their finding, the magnetoelectric effect in this antiferromagnet is dominated by the symmetric (quadrupolar) part of the magnetoelectric tensor.
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Ferroelectricity in vectorchiral phases

fig 798
Chirality, that is the handedness of objects is acknowledged for its great importance in many fields of biology and chemistry. However, chirality also plays a huge role in physical phenomena, e. g. symmetry aspects of frustrated magnets. In case of noncollinear magnetic ground states, spin-spirals may emerge. It is predicted for these states, that even above the magnetic ordering temperature, so-called vectorchiral phases emerge, which feature an ordered spin rotation (either of clockwise or anticlockwise fashion) between neighbouring spins, still there is no explicit relation concerning the angle spanned by neighbouring spins. By means of magnetic field dependent polarization measurements, we are the first to provide proof for the emergence of this phenomenon in LiCuVO4, a one dimensional quantum magnet with concurrent ferromagnetic and antiferromagnetic exchange interactions (marked as "VC" in the attached phase diagram). This proof relies on the fact that the vectorchiral state implies a finite ferroelectric polarization at temperatures above the ordering of the three dimensional spin-spirals.
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Optical pumping of magnetic skyrmions

fig 796
GaV4S8 is a multiferroic semiconductor hosting magnetic cycloid (Cyc) and Néel-type skyrmion lattice (SkL) phases with a broad region of thermal and magnetic stability. Here, we use time-resolved magneto-optical Kerr spectroscopy to show the coherent generation of collective spin excitations in the Cyc and SkL phases. Our micromagnetic simulations reveal that these are driven by an optically induced modulation of uniaxial anisotropy. Our results shed light on spin dynamics in anisotropic materials hosting skyrmions and pave a new pathway for the optical manipulation of their magnetic order.
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Probing skyrmion-hosts by microwave magnetoelectric effect

directional dichroism GaV4S8
Magnetic skyrmions in multiferroic materials also carry an electric polarization. Consequently, their collective excitations, located in the microwave frequency region, can be excited via both the electric and magnetic fields of the microwave radiation. This microwave diode effect can provide a new tool for the ultrafast manipulation of skyrmions.
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Identification of AFM domains via the optical magnetoelectric effect

magnetoelectric spin-excitations
The ultimate goal of multiferroic research is the development of a new-generation of nonvolatile memory devices, where magnetic bits are controlled via electric fields. In a recent publication in PRL we demonstrate the optical identification of antiferromagnetic domains in LiCoPO4, exploiting the strong absorption difference at spin-wave excitations between the domains. The control and the optical readout of AFM/ME domains will likely promote the development of ME and spintronic devices based on AFM insulators.
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Current switching on the atomic scale

schematic drawing
Domain walls in ferroelectric semiconductors show promise as multifunctional two-dimensional elements for next-generation nanotechnology. In collaboration with the NTNU Trondheim (Norway) and ETH Zurich (Switzerland) we demonstrate diode-like alternating-to-direct current conversion based on neutral ferroelectric domain walls in ErMnO3.
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Tearing down the barriers to domain wall electronics

We report a detailed electric-field poling study of the geometrically-driven improper ferroelectric h-ErMnO3. Special emphasis is put on frequency dependent polarisation switching, which is explained in terms of domain-wall movement similar to proper ferroelectrics. This approach enables the control of domain walls in the improper ferroelectric manganites and brings us a step closer towards their utilization in domain-wall based electronics.
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The James Clerk Maxwell Writers Prize 2017...

GaV<sub>4</sub>S<sub>8</sub> phase diagram
... for the best article of a young researcher published in Philosophical Magazine was awarded to the Augsburg physicist Sebastian Widmann. His article "On the multiferroic skyrmion-host GaV4S8" is now freely accessible for one year.
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Magnetic control of cycloidal domains and electric polarization in BiFeO3

BiFeO3 SANS measurement
The cycloidal spin structure in the room-temperature multiferroic BiFeO3 can be rearranged via an external magnetic field. This phenomenon was studied using small-angle neutron scattering. The cycloid propagation vectors were observed to rotate, when magnetic fields applied perpendicular to the polar axis exceeded a threshold value. To explain these findings, a phenomenological model was proposed.
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Experimental evidence for the Gardner transition

gardner transition schema
According to theoretical predictions, deep in the glassy state of matter the Gardner transition should occur. It corresponds to a fractionalization of the energy landscape sensed by the molecules and is believed to be relevant for various disordered systems. Our detailed sub-Tg dielectric experiments on two glasses reveal significant anomalies, for the first time providing experimental hints of this transition in canonical structural glass formers.
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First experimental observation of Bethe strings

Bethe strings image
Bethe strings are excitations of strongly bound electron spins in one-dimensional quantum spin systems, which have first been described theoretically in 1931. Two physicists from Augsburg, Alois Loidl and Zhe Wang, in cooperation with researchers from Berlin, Dresden, Mumbai, Nijmegen and San Diego were able to observe these excitations experimetally for the first time. The results are published in the renowned international journal "Nature".
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Ferroelectric-ferroelastic domains in Néel-type skyrmion host GaV4S8

GaV4S8 PFM image
GaV4S8 is a multiferroic semiconductor hosting Néel-type magnetic skyrmions dressed with electric polarization. We used low-temperature piezoresponse force microscopy in order to investigate the local-scale ferroelectric domain distribution in GaV4S8. We expect that the control of ferroelectric domain size in polar skyrmion hosts can be exploited for the spatial confinement and manipulation of Néel-type skyrmions.
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Equilibrium Skyrmion Lattice Ground State in a Polar Easy-plane Magnet

GaV<sub>4</sub>Se<sub>8</sub> phase diagram
The skyrmion lattice state (SkL) gains its stability via thermal fluctuations in all bulk skyrmion host materials known to date. Therefore, its existence is limited to a narrow temperature region below the paramagnetic state. In a recent publication in Scientific Report we demon­strate, that a proper choice of material parameters alone guarantees the thermodynamic stability of the SkL down to zero kelvin. We found that GaV4Se8 hosts a robust Néel-type SkL even in its ground state.
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YMnO3 - Conductivity of vortex-like domain patterns

YMnO3 vortex cores
The multiferroic hexagonal manganites RMnO3 exhibit unusual topologically-protected, vortex-like domain patterns, where the ferroelectric domain walls (DWs) have significantly different conductivity than the domains. By an equivalent-circuit analysis of dielectric spectra, for the first time we have deduced absolute values of the conductivity in the DWs and revealed the true nature of the charge-transport mechanism. The conductivity contrast between bulk and DWs reaches up to a factor 500, much higher than previously thought. This is highly relevant, e.g., for possible applications in microelectronics that could make use of the nanoscale DWs instead of the domains themselves as active device elements.
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Supersolidity in MnCr2S4

Supersolid MnCr<sub>2</sub>S<sub>4</sub>
Solid, liquid and gaseous - these are the three states of matter, we are familiar with. When we consider our everyday experience, the idea, that a homogeneous material could exhibit properties of two of these states simultaneously, seems implausible. Even more difficult to imagine is a compound, that is solid and at the same time superfluid, i.e. without any viscosity. In such a supersolid the atoms are arranged in a crystalline pattern, while at the same time behaving like a superfluid, in which particles move without friction. Nevertheless, such a complex state of matter has been proposed theoretically for more than 50 years.
At the same time as two other international research groups, that used laser cooling of atoms in "atomic traps" for their experiments, physicists from Augsburg and Dresden now report on the experimental realization of supersolidity in spin systems at high magnetic fields in the renowned journal "Science Advances". In 1970 Antony Legget posed the question "Can a Solid be Superfluid”? In 2017 it seems that this question can clearly be answered with "Yes".
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A quantumfluid of fluctuating spin-spirals in MnSc2S4

Neutron Scattering and Simulation
An unconventional and exotic magnetic ground state has been detected in cooperation with an international group of scientists: Neutron-scattering experiments at the Heinz-Maier-Leibnitz Zentrum in Munich and at the Paul Scherrer Institut in Villingen (CH) provide experimental evidence, that at low temperatures the spins in MnSc2S4 do not exhibit long-range magnetic order, but rather form a quantum state of fluctuating spin spirals. These results have recently been published in "Nature Physics".
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How does the melt become glass?

glass forming
In a recently published paper in "Science", together with scientists from the University of Paris, we solve a long-standing controversy: We provide evidence that the solidification of glass is due to a phase transition, even though of unconventional nature, accompanied by an increase of molecular cooperativity.
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Research Topics



Our group covers a broad field of investigations in condensed matter physics. We focus on new materials for future electronics, on unconventional ground states, superconductors, the dynamics of disordered matter and biological materials.

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Experimental Methods



Aside of a large number of sample characterization methods, a strong point of our group is the combination of a variety of spectroscopic methods enabling deep insight into the microscopic properties of condensed matter. This not only includes dielectric, THz, and optical spectroscopy but also electron and nuclear magnetic resonance techniques.

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National and International Collaborative Research Projects

Our group participates in several specially funded collaborative research projects:

Sino-German Cooperation
TRR 80

graduate school
FOR 1394
  • Research Unit FOR 1394 "Nonlinear Response to Probe Vitrification"
    • Project P9 "Investigation of nonlinear effects in glassy matter using dielectric methods"
  • Research Unit FOR 960 "Quantum Phase Transitions"
    • Project P5 "Quantum criticality in itinerant transition-metal oxides and chalcogenides and in frustrated lattices"

  • DFG Priority Programme 1458 "High Temperature Superconductivity in Iron Pnictides"
    • Project "Itineranter und lokalisierter Magnetismus in Fe basierten Supraleitern – Elektronenspinresonanz-Spektroskopie und Einkristallzucht"