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Instruments / Technical Area	Publication Details	Published
I21-Resonant Inelastic X-ray Scattering (RIXS)	Charge order lock-in by electron-phonon coupling in La1.675Eu0.2Sr0.125CuO4 Qisi Wang , Karin Von Arx , Masafumi Horio , Deepak John Mukkattukavil , Julia Kuespert , Yasmine Sassa , Thorsten Schmitt , Abhishek Nag , Sunseng Pyon , Tomohiro Takayama , Hidenori Takagi , Mirian Garcia-Fernandez , Ke-Jin Zhou , Johan Chang Science Advances 7 DOI: 10.1126/sciadv.abg7394 Diamond Proposal Number(s): [24481] Open Access Show Abstract ▼ Abstract: Charge order is universal to all hole-doped cuprates. Yet, the driving interactions remain an unsolved problem. Electron-electron interaction is widely believed to be essential, whereas the role of electron-phonon interaction is unclear. We report an ultrahigh-resolution resonant inelastic x-ray scattering (RIXS) study of the in-plane bond-stretching phonon mode in stripe-ordered cuprate La1.675Eu0.2Sr0.125CuO4. Phonon softening and lifetime shortening are found around the charge ordering wave vector. In addition to these self-energy effects, the electron-phonon coupling is probed by its proportionality to the RIXS cross section. We find an enhancement of the electron-phonon coupling around the charge-stripe ordering wave vector upon cooling into the low-temperature tetragonal structure phase. These results suggest that, in addition to electronic correlations, electron-phonon coupling contributes substantially to the emergence of long-range charge-stripe order in cuprates.	Jun 2021
I05-ARPES	Decoupling of lattice and orbital degrees of freedom in an iron-pnictide superconductor C. E. Matt , O. Ivashko , M. Horio , J. Choi , Q. Wang , D. Sutter , N. Dennler , M. H. Fischer , S. Katrych , L. Forro , J. Ma , B. Fu , B. Q. Lv , M. V. Zimmermann , T. K. Kim , N. C. Plumb , N. Xu , M. Shi , Johan Chang Physical Review Research 3 DOI: 10.1103/PhysRevResearch.3.023220 Diamond Proposal Number(s): [16104] Open Access Show Abstract ▼ Abstract: The interplay between structural and electronic phases in iron-based superconductors is a central theme in the search for the superconducting pairing mechanism. While electronic nematicity is competing with superconductivity, the effect of purely structural orthorhombic order is unexplored. Here, using x-ray diffraction and angle-resolved photoemission spectroscopy, we reveal a structural orthorhombic phase in the electron-doped iron-pnictide superconductor Pr 4 Fe 2 As 2 Te 0.88 O 4 ( T c = 25 K), which is distinct from orthorhombicity in the nematic phase in underdoped pnictides. Despite the high electron doping we find an exceptionally high orthorhombic onset temperature ( T ort ∼ 250 K), no signatures of phase competition with superconductivity, and absence of electronic nematic order as the driving mechanism for orthorhombicity. Combined, our results establish a high-temperature phase in the phase diagram of iron-pnictide superconductors and impose strong constraints for the modeling of their superconducting pairing mechanism.	Jun 2021
I21-Resonant Inelastic X-ray Scattering (RIXS)	Evolution of spin excitations from bulk to monolayer FeSe Jonathan Pelliciari , Seher Karakuzu , Qi Song , Riccardo Arpaia , Abhishek Nag , Matteo Rossi , Jiemin Li , Tianlun Yu , Xiaoyang Chen , Rui Peng , Mirian Garcia-Fernandez , Andrew C. Walters , Qisi Wang , Jun Zhao , Giacomo Ghiringhelli , Donglai Feng , Thomas A. Maier , Ke-Jin Zhou , Steven Johnston , Riccardo Comin Nature Communications 12 DOI: 10.1038/s41467-021-23317-3 Diamond Proposal Number(s): [18883] Open Access Show Abstract ▼ Abstract: In ultrathin films of FeSe grown on SrTiO3 (FeSe/STO), the superconducting transition temperature Tc is increased by almost an order of magnitude, raising questions on the pairing mechanism. As in other superconductors, antiferromagnetic spin fluctuations have been proposed to mediate SC making it essential to study the evolution of the spin dynamics of FeSe from the bulk to the ultrathin limit. Here, we investigate the spin excitations in bulk and monolayer FeSe/STO using resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to its bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems revealing a remarkable reconfiguration of spin excitations in FeSe/STO, essential to understand the role of spin fluctuations in the pairing mechanism.	May 2021
I05-ARPES	Electronic reconstruction forming a C2-symmetric Dirac semimetal in Ca3Ru2O7 M. Horio , Q. Wang , V. Granata , K. P. Kramer , Y. Sassa , S. Jöhr , D. Sutter , A. Bold , L. Das , Y. Xu , R. Frison , R. Fittipaldi , T. K. Kim , C. Cacho , J. E. Rault , P. Le Fèvre , F. Bertran , N. C. Plumb , M. Shi , A. Vecchione , M. H. Fischer , J. Chang Npj Quantum Materials 6 DOI: 10.1038/s41535-021-00328-3 Diamond Proposal Number(s): [20259] Open Access Show Abstract ▼ Abstract: Electronic band structures in solids stem from a periodic potential reflecting the structure of either the crystal lattice or electronic order. In the stoichiometric ruthenate Ca3Ru2O7, numerous Fermi surface-sensitive probes indicate a low-temperature electronic reconstruction. Yet, the causality and the reconstructed band structure remain unsolved. Here, we show by angle-resolved photoemission spectroscopy, how in Ca3Ru2O7 a C2-symmetric massive Dirac semimetal is realized through a Brillouin-zone preserving electronic reconstruction. This Dirac semimetal emerges in a two-stage transition upon cooling. The Dirac point and band velocities are consistent with constraints set by quantum oscillation, thermodynamic, and transport experiments, suggesting that the complete Fermi surface is resolved. The reconstructed structure—incompatible with translational-symmetry-breaking density waves—serves as an important test for band structure calculations of correlated electron systems.	Mar 2021
I21-Resonant Inelastic X-ray Scattering (RIXS)	High-temperature charge-stripe correlations in La1.675Eu0.2Sr0.125CuO4 Q. Wang , M. Horio , K. Von Arx , Y. Shen , D. John Mukkattukavil , Y. Sassa , O. Ivashko , C. E. Matt , S. Pyon , T. Takayama , H. Takagi , T. Kurosawa , N. Momono , M. Oda , T. Adachi , S. M. Haidar , Y. Koike , Y. Tseng , W. Zhang , J. Zhao , K. Kummer , M. Garcia-Fernandez , K. Zhou , N. B. Christensen , H. M. Ronnow , T. Schmitt , J. Chang Physical Review Letters 124 DOI: 10.1103/PhysRevLett.124.187002 Diamond Proposal Number(s): [20828, 24481] Show Abstract ▼ Abstract: We use resonant inelastic x-ray scattering to investigate charge-stripe correlations in La 1.675 Eu 0.2 Sr 0.125 CuO 4 . By differentiating elastic from inelastic scattering, it is demonstrated that charge-stripe correlations precede both the structural low-temperature tetragonal phase and the transport-defined pseudogap onset. The scattering peak amplitude from charge stripes decays approximately as T − 2 towards our detection limit. The in-plane integrated intensity, however, remains roughly temperature independent. Therefore, although the incommensurability shows a remarkably large increase at high temperature, our results are interpreted via a single scattering constituent. In fact, direct comparison to other stripe-ordered compounds ( La 1.875 Ba 0.125 CuO 4 , La 1.475 Nd 0.4 Sr 0.125 CuO 4 , and La 1.875 Sr 0.125 CuO 4 ) suggests a roughly constant integrated scattering intensity across all these compounds. Our results therefore provide a unifying picture for the charge-stripe ordering in La-based cuprates. As charge correlations in La 1.675 Eu 0.2 Sr 0.125 CuO 4 extend beyond the low-temperature tetragonal and pseudogap phase, their emergence heralds a spontaneous symmetry breaking in this compound.	May 2020
	Coexistence of ferromagnetic and stripe-type antiferromagnetic spin fluctuations in YFe 2 Ge 2 H. Wo , Q. Wang , Y. Shen , X. Zhang , Y. Hao , Y. Feng , S. Shen , Z. He , B. Pan , W. Wang , K. Nakajima , S. Ohira-kawamura , P. Steffens , M. Boehm , K. Schmalzl , T. R. Forrest , M. Matsuda , Y. Zhao , J. W. Lynn , Z. Yin , J. Zhao Physical Review Letters 122 DOI: 10.1103/PhysRevLett.122.217003 Show Abstract ▼ Abstract: We report neutron scattering measurements of single-crystalline YFe 2 Ge 2 in the normal state, which has the same crystal structure as the 122 family of iron pnictide superconductors. YFe 2 Ge 2 does not exhibit long-range magnetic order but exhibits strong spin fluctuations. Like the iron pnictides, YFe 2 Ge 2 displays anisotropic stripe-type antiferromagnetic spin fluctuations at ( π , 0, π ). More interesting, however, is the observation of strong spin fluctuations at the in-plane ferromagnetic wave vector (0, 0, π ). These ferromagnetic spin fluctuations are isotropic in the ( H , K ) plane, whose intensity exceeds that of stripe spin fluctuations. Both the ferromagnetic and stripe spin fluctuations remain gapless down to the lowest measured energies. Our results naturally explain the absence of magnetic order in YFe 2 Ge 2 and also imply that the ferromagnetic correlations may be a key ingredient for iron-based materials.	May 2019

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