Abstract
Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems, and as such has proved of key importance in the study of quantum ...materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr
2
RuO
4
, leading to a pronounced peak in
T
c
versus strain whose origin is still under active debate. Here we develop a simple and compact method to passively apply large uniaxial pressures in restricted sample environments, and utilise this to study the evolution of the electronic structure of Sr
2
RuO
4
using angle-resolved photoemission. We directly visualise how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in
T
c
. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr
2
RuO
4
. More generally, our experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.
The Skyrme-particle, the skyrmion, was introduced over half a century ago in the context of dense nuclear matter. But with skyrmions being mathematical objects--special types of topological ...solitons--they can emerge in much broader contexts. Recently skyrmions were observed in helimagnets, forming nanoscale spin-textures. Extending over length scales much larger than the interatomic spacing, they behave as large, classical objects, yet deep inside they are of quantum nature. Penetrating into their microscopic roots requires a multi-scale approach, spanning the full quantum to classical domain. Here, we achieve this for the first time in the skyrmionic Mott insulator Cu2OSeO3. We show that its magnetic building blocks are strongly fluctuating Cu4 tetrahedra, spawning a continuum theory that culminates in 51 nm large skyrmions, in striking agreement with experiment. One of the further predictions that ensues is the temperature-dependent decay of skyrmions into half-skyrmions.
Abstract
Applying in-plane uniaxial pressure to strongly correlated low-dimensional systems has been shown to tune the electronic structure dramatically. For example, the unconventional ...superconductor Sr
2
RuO
4
can be tuned through a single Van Hove point, resulting in strong enhancement of both
T
c
and
H
c2
. Out-of-plane (
c
axis) uniaxial pressure is expected to tune the quasi-two-dimensional structure even more strongly, by pushing it towards two Van Hove points simultaneously. Here, we achieve a record uniaxial stress of 3.2 GPa along the
c
axis of Sr
2
RuO
4
.
H
c2
increases, as expected for increasing density of states, but unexpectedly
T
c
falls. As a first attempt to explain this result, we present three-dimensional calculations in the weak interaction limit. We find that within the weak-coupling framework there is no single order parameter that can account for the contrasting effects of in-plane versus
c
-axis uniaxial stress, which makes this new result a strong constraint on theories of the superconductivity of Sr
2
RuO
4
.
There has been widespread interest in using interfaces of transition-metal oxides as a platform to control not only their electronic structure, as in semiconductor heterostructures, but also to tune ...between different collective phases. A major goal is to realize states of the quantum many-body system that are not found in the bulk phase diagrams of the constituent materials. Here, we perform a combined experimental and theoretical study of the delafossite oxide metals PdCoO
2
and PdCrO
2
, finding how electronic reconstructions at their polar surfaces drive instabilities to itinerant surface ferromagnetism. Neither compound supports ferromagnetism in bulk, with PdCrO
2
a bulk antiferromagnet, demonstrating how a delicate competition of magnetic correlations can be engineered by intrinsic self-doping at a polar surface or interface.
The ability to modulate the collective properties of correlated electron systems at their interfaces and surfaces underpins the burgeoning field of “designer” quantum materials. Here, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability to itinerant ferromagnetism at the Pd-terminated surface of the nonmagnetic delafossite oxide metal PdCoO
2
. Combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we show how this leads to a rich multiband surface electronic structure. We find similar surface state dispersions in PdCrO
2
, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.
The interplay between spin–orbit coupling and structural inversion symmetry breaking in solids has generated much interest due to the nontrivial spin and magnetic textures which can result. Such ...studies are typically focused on systems where large atomic number elements lead to strong spin–orbit coupling, in turn rendering electronic correlations weak. In contrast, here we investigate the temperature-dependent electronic structure of Ca3Ru2O7, a 4d oxide metal for which both correlations and spin–orbit coupling are pronounced and in which octahedral tilts and rotations combine to mediate both global and local inversion symmetry-breaking polar distortions. Our angle-resolved photoemission measurements reveal the destruction of a large hole-like Fermi surface upon cooling through a coupled structural and spin-reorientation transition at 48 K, accompanied by a sudden onset of quasiparticle coherence. We demonstrate how these result from band hybridization mediated by a hidden Rashba-type spin–orbit coupling. This is enabled by the bulk structural distortions and unlocked when the spin reorients perpendicular to the local symmetry-breaking potential at the Ru sites. We argue that the electronic energy gain associated with the band hybridization is actually the key driver for the phase transition, reflecting a delicate interplay between spin–orbit coupling and strong electronic correlations and revealing a route to control magnetic ordering in solids.
Abstract
The single-band, quasi-two dimensional metals PdCoO
2
and PtCoO
2
have recently come to prominence because of their extremely long mean free paths, which establish them as some of the most ...electronically pure materials known, and as potential hosts of previously unobservable regimes of electronic transport. To fully establish their magnetotransport properties, we have studied the magnetoresistance and Hall effect in bulk single crystals to which electrical contacts have been made with high precision using focused ion beam machining. We observe a strong temperature dependence of the Hall resistivity in small applied fields, linked to a large violation of Kohler’s rule in the magnetoresistance. We discuss the extent to which these observations can be accounted for by standard transport theory.
The first binary phase in the system Co‐Bi was obtained by high‐pressure high‐temperature synthesis. The NiBi3‐type structure motif comprises cobalt‐centered monocapped bismuth prisms with covalent ...Co‐Bi interactions. The polyhedra are condensed into infinite columns (see picture), which are arranged in the form of a distorted hexagonal rod packing. The new compound is a superconductor with a Tc slightly below 0.5 K.
Since the discovery of the Fe‐based superconductors (FeSC) a lot of efforts has been applied to elucidate the microscopic mechanism behind cooper pairing and the symmetry of the order parameter. ...Examination of effective interactions of electrons with spin‐fluctuations in these systems yield that couplings are not weak and the materials have to be considered in a strong coupling scheme. In the present article, we review some aspects of the description of iron‐based superconductors in the frame of the Eliashberg approach for multiband systems. The application of the Eliashberg scheme is demonstrated with two compounds Ba0.6K0.4Fe2As2 and Ca0.32Na0.68Fe2As2. In both cases, it was found that these materials are in the intermediate coupling regime. Further, we consider effects of impurities on physical properties of multiband superconductors. It is demonstrated that the impurities affect nontrivially the superconducting properties. In particular, they may trigger a symmetry change of superconducting order parameters. This effect can be used for determination of the superconducting order parameter. Finally, we consider KFe2As2 and show that its thermodynamic properties are consistent with a nodal superconducting order parameter.
Multiband superconductors differ considerably from single‐band superconductors. In particular, the effect of disorder on superconductivity has no analog in single‐band superconductors. Above all, disorder may trigger a symmetry change of superconducting order parameters. Therefore, a systematic study of the evolution of superconducting properties with impurity concentration using phase‐insensitive techniques may give information about the superconducting order parameters including both amplitude and phase values in the clean materials.
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