arXiv subjects

Matthieu Le Tacon

Publications and source records attributed to Matthieu Le Tacon.

40 records · Page 3Linked to original sources

arXiv2019

Orbitally resolved superconductivity in real space: FeSe

Multi-orbital superconductors combine unconventional pairing with complex band structures, where different orbitals in the bands contribute to a multitude of superconducting gaps. We here demonstrate a fresh approach using low-temperature scanning tunneling microscopy (LT-STM) to resolve the contributions of different orbitals to superconductivity. This approach is based on STM's capability to resolve the local density of states (LDOS) with a combined high energy and sub unit-cell resolution. This technique directly determines the orbitals on defect free crystals without the need for scatters on the surface and sophisticated quasi-particle interference (QPI) measurements. Taking bulk FeSe as an example, we directly resolve the superconducting gaps within the units cell using a 30 mK STM. In combination with density functional theory calculations, we are able to identify the orbital character of each gap.

cond-mat.supr-con↗

arXiv2019

Complex magnetic order in nickelate slabs

Magnetic ordering phenomena have a profound influence on the macroscopic properties of correlated-electron materials, but their realistic prediction remains a formidable challenge. An archetypical example is the ternary nickel oxide system RNiO3 (R = rare earth), where the period-four magnetic order with proposals of collinear and non-collinear structures and the amplitude of magnetic moments on different Ni sublattices have been subjects of debate for decades. Here we introduce an elementary model system - NdNiO3 slabs embedded in a non-magnetic NdGaO3 matrix - and use polarized resonant x-ray scattering (RXS) to show that both collinear and non-collinear magnetic structures can be realized, depending on the slab thickness. The crossover between both spin structures is correctly predicted by density functional theory and can be qualitatively understood in a low-energy spin model. We further demonstrate that the amplitude ratio of magnetic moments in neighboring NiO6 octahedra can be accurately determined by RXS in combination with a correlated double cluster model. Targeted synthesis of model systems with controlled thickness and synergistic application of polarized RXS and ab-initio theory thus provide new perspectives for research on complex magnetism, in analogy to two-dimensional materials created by exfoliation.

cond-mat.str-el↗

arXiv2017

On the phonon dispersion relation of single-crystalline $β$--FeSe

We report on the phonon spectrum probed at the $β$--FeSe(001) surface by means of high-resolution electron energy-loss spectroscopy (HREELS). Single crystals of $β$--FeSe are cleaved under ultra-high vacuum conditions and are subsequently measured below and above the nematic transition temperature. In total we observe five phonon modes and a phonon cutoff energy of about 40 meV. We identify the origin of each phonon mode based on the selection rules of HREELS and by comparing the experimental results to the ones of \emph{ab initio} density functional calculations. The most prominent phonon modes $A_{1g}$, $B_{1g}$, and $A_{2u}$ appear at energies of about $20.5$ and $25.6$ and $40$ meV, respectively. These phonon modes disperse rather weakly while changing the momentum from zero up to the zone boundary, indicating that they are mainly of optical nature. A comparison between our results and the results of \emph{ab initio} calculations indicates that there must be a mutual interplay between magnetism and lattice dynamics in this compound, similar to the other Fe-based superconductors. Finally, we comment on the role of temperature on the phonon modes probed at the $\barΓ$--point. It is observed that both the $B_{1g}$ and $A_{2u}$ phonon modes undergo a downward shift while increasing the temperature from 15 to 300 K. In the case of the $A_{2u}$ mode this shift is about 1.5 meV.

cond-mat.supr-con↗

arXiv2017

Raman scattering from Higgs mode oscillations in the two-dimensional antiferromagnet Ca$_2$RuO$_4$

We present and analyze Raman spectra of the Mott insulator Ca$_2$RuO$_4$, whose quasi-two-dimensional antiferromagnetic order has been described as a condensate of low-lying spin-orbit excitons with angular momentum $J_{eff}=1$. In the $A_g$ polarization geometry, the amplitude (Higgs) mode of the spin-orbit condensate is directly probed in the scalar channel, thus avoiding infrared-singular magnon contributions. In the $B_{1g}$ geometry, we observe a single-magnon peak as well as two-magnon and two-Higgs excitations. Model calculations using exact diagonalization quantitatively agree with the observations. Together with recent neutron scattering data, our study provides strong evidence for excitonic magnetism in Ca$_2$RuO$_4$ and points out new perspectives for research on the Higgs mode in two dimensions.

cond-mat.str-el↗