Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet α − RuCl 3
Not Available
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Engineering topics
Geck, J.
Publications and source records attributed to Geck, J..
Site-selective spectroscopy with depth resolution using resonant x-ray reflectometry
Combining dissimilar transition metal oxides (TMOs) into artificial heterostructures enables to create electronic interface systems with new electronic properties that do not exist in bulk. A detailed understanding of how such interfaces can be used to tailor physical properties requires characterization techniques capable to yield interface sensitive spectroscopic information with monolayer resolution. In this regard resonant x-ray reflectivity (RXR) provides a unique experimental tool to achieve exactly this. It yields the element specific electronic depth profiles in a non-destructive manner. Here, using a YBa 2 Cu 3 O 7-δ (YBCO) thin film, we demonstrate that RXR is further capable to deliver site selectivity. By applying a new analysis scheme to RXR, which takes the atomic structure of the material into account, together with information of the local charge anisotropy of the resonant ions, we obtained spectroscopic information from the different Cu sites (e.g., chain and plane) throughout the film profile. As a result, while most of the film behaves bulk-like, we observe that the Cu-chains at the surface show characteristics of electron doping, whereas the Cu-planes closest to the surface exhibit an orbital reconstruction similar to that observed at La 1-x Ca x MnO 3 /YBCO interfaces.
36 MATERIALS SCIENCE↗
Orbital symmetry of charge-density-wave order in La 1.875 Ba 0.125 CuO 4 and YBa 2 Cu 3 O 6.67
Recent theories of charge density wave (CDW) order in high temperature superconductors have predicted a primarily d CDW orbital symmetry. Here, we report on the orbital symmetry of CDW order in the canonical cuprate superconductors La 1.875 Ba 0.125 CuO 4 (LBCO) and YBa 2 Cu 3 O 6.67 (YBCO), using resonant soft x-ray scattering and a model mapped to the CDW orbital symmetry. From measurements sensitive to the O sublattice, we conclude that LBCO has predominantly s0 CDW orbital symmetry, in contrast to the d orbital symmetry recently reported in other cuprates. Additionally, we show for YBCO that the CDW orbital symmetry differs along the a and b crystal axes and that these both differ from LBCO. This work highlights CDW orbital symmetry as an additional key property that distinguishes the di erent cuprate families.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Nematicity in stripe ordered cuprates probed via resonant x-ray scattering
We found that in underdoped cuprate superconductors, a rich competition occurs between superconductivity and charge density wave (CDW) order. Whether rotational symmetry-breaking (nematicity) occurs intrinsically and generically or as a consequence of other orders is under debate. Here, we employ resonant x-ray scattering in stripe-ordered superconductors (La,M) 2 CuO 4 to probe the relationship between electronic nematicity of the Cu 3d orbitals, structure of the (La,M) 2 O 2 layers, and CDW order. We find distinct temperature dependences for the structure of the (La,M) 2 O 2 layers and the electronic nematicity of the CuO 2 planes, with only the latter being enhanced by the onset of CDW order. Our results identify electronic nematicity as an order parameter that is distinct from a purely structural order parameter in underdoped striped cuprates.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Influence of Ti doping on the incommensurate charge density wave in 1 T –TaS 2
Here, we report temperature-dependent transport and x-ray diffraction measurements of the influence of Ti hole doping on the charge density wave (CDW) in 1T–Ta 1–x Ti x S 2 . Confirming past studies, we find that even trace impurities eliminate the low-temperature commensurate (C) phase in this system. Surprisingly, the magnitude of the in-plane component of the CDW wave vector in the nearly commensurate (NC) phase does not change significantly with Ti concentration, as might be expected from a changing Fermi surface volume. Instead, the angle of the CDW in the basal plane rotates, from 11.9° at x=0 to 16.4° at x=0.12. Ti substitution also leads to an extended region of coexistence between incommensurate (IC) and NC phases, indicating heterogeneous nucleation near the transition. Finally, we explain a resistive anomaly originally observed by Di Salvo [F. J. Di Salvo et al., Phys. Rev. B 12, 2220 (1975)] as arising from pinning of the CDW on the crystal lattice. Our study highlights the importance of commensuration effects in the NC phase, particularly at x ~ 0.08.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Femtosecond time-resolved MeV electron diffraction
We report the experimental demonstration of femtosecond electron diffraction using high-brightness MeV electron beams. High-quality, single-shot electron diffraction patterns for both polycrystalline aluminum and single-crystal 1T-TaS 2 are obtained utilizing a 5 fC (~3 × 10 4 electrons) pulse of electrons at 2.8 MeV. The high quality of the electron diffraction patterns confirms that electron beam has a normalized emittance of ~50 nm rad. The transverse and longitudinal coherence length is ~11 and ~2.5 nm, respectively. The timing jitter between the pump laser and probe electron beam was found to be ~100 fs (rms). The temporal resolution is demonstrated by observing the evolution of Bragg and superlattice peaks of 1T-TaS 2 following an 800 nm optical pump and was found to be 130 fs. Lastly, our results demonstrate the advantages of MeV electrons, including large elastic differential scattering cross-section and access to high-order reflections, and the feasibility of ultimately realizing below 10 fs time-resolved electron diffraction.
74 ATOMIC AND MOLECULAR PHYSICS↗