Christian Pfleiderer

Topology of Correlated Systems

Technical University of Munich

Department of Physics

James-Franck-Str. 1

85748 Garching

christian.pfleiderer[at]tum.de

Group Webpage

Description

Research focus: quantum phase transitions, electronic order, topology of correlated systems

The systematic search for quantum order in bulk materials, comprising the synthesis of high-purity single crystals, the experimental exploration of their thermodynamic and transport properties under extreme conditions, as well as microscopic studies using advance neutron and x-ray scattering form the basis for harvesting quantum phenomena in real materials.


phase-plot
Quantum phase transitions

The notion of particle-like elementary excitations represents a cornerstone of present day condensed matter physics. The putative breakdown of this concept in the vicinity of so-called quantum phase transitions, driven by quantum rather than thermal fluctuations, has been attracting tremendous interest for many decades. In turn the materials properties in the vicinity of quantum phase transitions offer an important starting point for an understanding of a wide range of materials with anomalous normal state properties, as well as emergent novel electronic phases such as magnetically mediated superconductivity or partial spin and charge order.


Selected publications

- Formation of a Topological Non-Fermi Liquid in MnSi; Nature, 497, 231 (2013)

- Spin Dynamics and Spin Freezing at Ferromagnetic Quantum Phase Transitions, European Journal of Physics: Special Topics, 224, 1041 (2015)


fermi-surfaces
Complex forms of electronic order

Complex forms of electronic order are intimately connected with the electronic structure and the topological character of the Fermi surface as well as their modification in the presence of strong quantum correlations. The experimental investigation of the electronic structure of materials with complex forms of electronic order permits to track the effects of many body quantum effects and their relationship with thermodynamic and transport properties close to and far from equilibrium.

Selected publications

- Spin-resolved Fermi surface of the localized ferromagnetic Heusler compound Cu2MnAl measured with spin-polarized positron annihilation, Phys. Rev. Lett., 115, 206404 (2015)

- Parasitic small-moment-antiferromagnetism and non-linear coupling of hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction, Phys. Rev. Lett., 104, 106406 (2010)

- Superconducting phases of f-electron compounds, Rev. Mod. Phys., 81, 1551 (2009)


topsquirm
Topological phenomena

The notions of symmetry breaking and generalized rigidities have let to a remarkably comprehensive account of complex forms of magnetic order in condensed matter systems. In recent years a new facet of magnetism research receives increasing attention that concerns the topological character of magnetically ordered systems, notably those properties that remain unchanged under elastic deformations. Important examples include skyrmions, vortices and monopoles in chiral or frustrated magnets. These topological aspects of magnetic order are not only appealing from an esthetical and conceptual point of view, but offer strikingly simple explanations for materials properties that may seem to be surprising and hideously complicated at first sight.

Selected publications

- Universal Helimagnon and Skyrmion Excitations in Metallic, Semiconducting, and Insulating Chiral Magnets, Nature Materials 14, 478 (2015)

- Unwinding of a Skyrmion Lattice by Magnetic Monopoles, Science 340, 1076 (2013)

- First Order Metamagnetic Transition in Ho2Ti2O7 observed by Vibrating Coil Magnetometery at Milli- Kelvin Temperatures, Phys. Rev. Lett. 108, 257204 (2012)

- Spin Transfer Torques in MnSi at Ultra-low Current Densities, Science 330, 1648 (2010)

- Skyrmion Lattice in a Chiral Magnet, Science 323, 915 (2009)

Publications

Microwave Spectroscopy of the Low-Temperature Skyrmion State in Cu2OSeO3

A. Aqeel, J. Sahliger, T. Taniguchi, S. Maendl, D. Mettus, H. Berger, A. Bauer, M. Garst, C. Pfleiderer, C.H. Back.

Physical Review Letters 126 (1), 017202 (2021).

Show Abstract

In the cubic chiral magnet Cu2OSeO3 a low-temperature skyrmion state (LTS) and a concomitant tilted conical state are observed for magnetic fields parallel to h100i. Here, we report on the dynamic resonances of these novel magnetic states. After promoting the nucleation of the LTS by means of field cycling, we apply broadband microwave spectroscopy in two experimental geometries that provide either predominantly in-plane or out-of-plane excitation. By comparing the results to linear spin-wave theory, we clearly identify resonant modes associated with the tilted conical state, the gyrational and breathing modes associated with the LTS, as well as the hybridization of the breathing mode with a dark octupole gyration mode mediated by the magnetocrystalline anisotropies. Most intriguingly, our findings suggest that under decreasing fields the hexagonal skyrmion lattice becomes unstable with respect to an oblique deformation, reflected in the formation of elongated skyrmions.

DOI: 10.1103/PhysRevLett.126.017202

Ferromagnetic Resonance with Magnetic Phase Selectivity by Means of Resonant Elastic X-Ray Scattering on a Chiral Magnet

S. Pollath, A. Aqeel, A. Bauer, C. Luo, H. Ryll, F. Radu, C. Pfleiderer, G. Woltersdorf, C.H. Back

Physical Review Letters 123 (16), 167201 (2019).

Show Abstract

Cubic chiral magnets, such as Cu2OSeO3, exhibit a variety of noncollinear spin textures, including a trigonal lattice of spin whirls, the so-called skyrmions. Using magnetic resonant elastic x-ray scattering (REXS) on a crystalline Bragg peak and its magnetic satellites while exciting the sample with magnetic fields at gigahertz frequencies, we probe the ferromagnetic resonance (FMR) modes of these spin textures by means of the scattered intensity. Most notably, the three eigenmodes of the skyrmion lattice are detected with large sensitivity. As this novel technique, which we label REXS FMR, is carried out at distinct positions in reciprocal space, it allows us to distinguish contributions originating from different magnetic states, providing information on the precise character, weight, and mode mixing as a prerequisite of tailored excitations for applications.

DOI: 10.1103/PhysRevLett.123.167201

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