Johan Juul Chang

Related publications (68)

Search for relativistic fractionally charged particles in space

Johan Juul Chang, Jie Chen, Jennifer Maria Frieden, Ming Gao, Wei Li, Xuan Li, Junqiu Liu, Hao Liu, Shuai Liu, Chiara Perrina, Arshia Ruina, Jian Wang, Siyuan Wang, Yi Wang, Xiao Wang, Songmei Wu, Zhirui Xu, Yong Yang, Yi Zhang, Yan Zhang

More than a century after the performance of the oil drop experiment, the possible existence of fractionally charged particles (FCPs) still remains unsettled. The search for FCPs is crucial for some extensions of the Standard Model in particle physics. Most of the previously conducted searches for FCPs in cosmic rays were based on experiments underground or at high altitudes. However, there have been few searches for FCPs in cosmic rays carried out in orbit other than AMS-01 flown by a space shuttle and BESS by a balloon at the top of the atmosphere. In this study, we conduct an FCP search in space based on on -orbit data obtained using the Dark Matter Particle Explorer (DAMPE) satellite over a period of five years. Unlike underground experiments, which require an FCP energy of the order of hundreds of GeV, our FCP search starts at only a few GeV. An upper limit of 6.2 x 10-10 cm-2 sr-1 s-1 is obtained for the flux. Our results demonstrate that DAMPE exhibits higher sensitivity than experiments of similar types by three orders of magnitude that more stringently restricts the conditions for the existence of FCP in primary cosmic rays.

2022

Decoupling of lattice and orbital degrees of freedom in an iron-pnictide superconductor

Johan Juul Chang, László Forró, Sergiy Katrych, Qian Wang

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 electrondoped iron-pnictide superconductor Pr4Fe2As2Te0.88O4 (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 similar to 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.

AMER PHYSICAL SOC2021

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We present a combined oxygen K-edge x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) study of the bilayer ruthenate Ca3Ru2O7. Our RIXS experiments on Ca3Ru2O7 were carried out on the overlapping planar and interplanar oxygen resonances, which are distinguishable from the apical one. Comparison to equivalent oxygen K-edge spectra recorded on band-Mott insulating Ca2RuO4 is made. In contrast to Ca2RuO4 spectra, which contain excitations linked to Mott physics, Ca3Ru2O7 spectra feature only intra-t(2g) ones that do not directly involve the Coulomb energy scale. As found in Ca2RuO4, we resolve two intra-t(2g) excitations in Ca3Ru2O7. Moreover, the lowest lying excitation in Ca3Ru2O7 shows a significant dispersion, revealing a collective character different from what is observed in Ca2RuO4. Theoretical modeling supports the interpretation of this lowest energy excitation in Ca3Ru2O7 as a magnetic transverse mode with multiparticle character, whereas the corresponding excitation in Ca2RuO4 is assigned to combined longitudinal and transverse spin modes. These fundamental differences are discussed in terms of the inequivalent magnetic ground-state manifestations in Ca2RuO4 and Ca3Ru2O7.

AMER PHYSICAL SOC2020

Orbitally selective breakdown of Fermi liquid quasiparticles in Ca1.8Sr0.2RuO4

Johan Juul Chang, Gianmarco Gatti, Marco Grioni

We present a comprehensive angle-resolved photoemission spectroscopy study of Ca1.8Sr0.2RuO4. Four distinct bands are revealed and along the Ru-O bond direction their orbital characters are identified through a light polarization analysis and comparison to dynamical mean-field theory calculations. Bands assigned to d(xz),d(yz) orbitals display Fermi liquid behavior with fourfold quasiparticle mass renormalization. Extremely heavy fermions-associated with a predominantly d(xy) band character-are shown to display non-Fermi-liquid behavior. We thus demonstrate that Ca1.8Sr0.2RuO4 is a hybrid metal with an orbitally selective Fermi liquid quasiparticle breakdown.

AMER PHYSICAL SOC2019

Strain-engineering Mott-insulating La2CuO4

Johan Juul Chang, Henrik Moodysson Rønnow, Thorsten Schmitt, Noore Elahi Shaik, Yi Tseng

The transition temperature T-c of unconventional superconductivity is often tunable. For a monolayer of FeSe, for example, the sweet spot is uniquely bound to titanium-oxide substrates. By contrast for La2-xSrxCuO4 thin films, such substrates are sub-optimal and the highest T-c is instead obtained using LaSrAlO4. An outstanding challenge is thus to understand the optimal conditions for superconductivity in thin films: which microscopic parameters drive the change in T-c and how can we tune them? Here we demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of La2CuO4 thin films can be enhanced by compressive strain. Our experiments and theoretical calculations establish that the substrate producing the largest T-c under doping also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest optimising the parent Mott state as a strategy for enhancing the superconducting transition temperature in cuprates.

2019

Direct observation of orbital hybridisation in a cuprate superconductor

Johan Juul Chang, Claudia Giuseppina Fatuzzo, Martin Mansson, Thorsten Schmitt

Nature Publishing Group2018

Dispersive magnetic and electronic excitations in iridate perovskites probed by oxygen K-edge resonant inelastic x-ray scattering

Johan Juul Chang, Sara Fatale, Henrik Moodysson Rønnow, Thorsten Schmitt

2018

Spin-Orbital Excitations in Ca2RuO4 Revealed by Resonant Inelastic X-Ray Scattering

Johan Juul Chang, Claudia Giuseppina Fatuzzo, Henrik Moodysson Rønnow, Thorsten Schmitt, Yi Tseng

2018

Time-Reversal Symmetry Breaking in Re-Based Superconductors

Johan Juul Chang, Joël Mesot

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation and relaxation (mu SR) studies of superconducting noncentrosymmetric Re0.82Nb0.18 (T-c = 8.8 K) and centrosymmetric Re (T-c = 2.7 K). In Re0.82Nb0.18, the low-temperature superfluid density and the electronic specific heat evidence a fully gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electronphonon coupling. In both Re0.82Nb0.18 and pure Re, the spontaneous magnetic fields revealed by zero-field mu SR below Tc indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric ReT (T = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg10Ir19B16 and Nb0.5Os0.5, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and ReT. We discuss the superconducting order parameter symmetries that are compatible with the experimental observations.

AMER PHYSICAL SOC2018