Dr. Thorsten Schmitt

Thorsten Schmitt is head of the Spectroscopy of Quantum Materials / ADRESS, SIS Group at the Photon Science Division (PSD) of the Paul Scherrer Institut. He holds a PhD in Physics from Uppsala University, Sweden and a Dipl. Phys. degree (equivalent to MSc in Physics) from the Johannes Gutenberg-University of Mainz, Germany. He started his career working as undergraduate researcher at the Max-Planck-Institute for Polymer Research in Mainz, Germany, where he worked on “lateral decomposition in thin polymer blend films”. After his PhD on “Resonant Soft X-ray Emission Spectroscopy of Vanadium Oxides and Related Compounds”, for which he was awarded with the Silver Medal of the Ångström Premium for outstanding PhD thesis, he moved as a Gustafsson Postdoctoral fellow to the Royal Institute of Technology, Stockholm - Kista, Sweden working at MAX-Lab, Lund University, Lund, Sweden on semindonducting materials. Thorsten Schmitt moved then as staff scientist to the Spectroscopy of Novel Materials Group of the Paul Scherrer Institut, where he subsequently became group leader. He serves the scientific community in numerous review panels for beamtime proposal applications, beam line evaluations and project development advice. Thorsten Schmitt is member of the editorial board of the “Journal of Electron Spectroscopy and Related Phenomena”, referee for funding proposals, tenuring processes and a number of journals. He is frequently engaged in the organization and as committee member of conferences and workshops.

Line management as head of the Spectroscopy of Novel Materials Group (with ca. 20 scientists and technicians) at the Laboratory for Condensed Matter. Integrated planning and management of two synchrotron radiation spectroscopy beamlines at the Swiss Light Source (SIS and ADRESS beamline) including budget responsibility. This includes development and operation of all optical and mechanical components of the two beamline facilities and the corresponding end-stations, as well as scientific exploitation by in-house and external (peer-reviewed) users. Beamline scientist responsibility of the RIXS project at the ADRESS beamline and its RIXS spectrometer. Thorsten Schmitt is the PSD-representative in the working group “avoidable air travel, reduction CO₂ footprint”, a PSI visitor guide and member of various beamline scientist recruitment panels at PSI.

Thorsten Schmitt operates one of the world-wide most advanced soft X-ray RIXS facilities at the ADRESS beamline of the Swiss Light Source (SLS). His scientific research focuses on correlated-electron materials associated with phenomena such as superconductivity, metal-insulator transitions (MITs), charge order, magnetic order and low-dimensional magnetism. Thorsten Schmitt has developed momentum resolved soft X-ray RIXS at the SLS as a sensitive probe of the spin, charge, lattice and orbital dynamics in copper and iron based superconductors, quasi one-dimensional (1D) cuprate materials as well as in nickelates, vanadates, ruthenates and iridates. In particular, he has discovered a novel Spin-Orbital Separation mechanism in the 1D antiferromagnetic Heisenberg spin-1/2 chain Sr₂CuO₃ with RIXS at the Cu L₃ edge and demonstrated that four-spinon excitations can be accessed directly in a region of phase space clearly separated from the two-spinon continuum using RIXS at the O K-edge. He has applied RIXS to reveal the electronic configuration of NdNiO₃ as Ni 3d⁸ with holes in the O 2p band identifying the bond-disproportionation mechanism as the decisive driver for the MIT in rare-earth nickelates.  In recent efforts on time resolved RIXS he was demonstrating the great potential of RIXS spectroscopy to study the ultrafast orbital dynamics and magnetic fluctuations in strongly correlated materials.

For an extensive overview we kindly refer you to our publication repository DORA.

Orbital dynamics during an ultrafast insulator to metal transition, Sergii Parchenko, Eugenio Paris, Daniel McNally, Elsa Abreu, Markus Dantz, Elisabeth M. Bothschafter, Alexander H. Reid, William F. Schlotter, Ming-Fu Lin, Scott F. Wandel, Giacomo Coslovich, Sioan Zohar, Georgi L. Dakovski, J. J. Turner, S. Moeller, Yi Tseng, Milan Radovic, Conny Saathe, Marcus Agaaker, Joseph E. Nordgren, Steven L. Johnson, Thorsten Schmitt, and Urs Staub, Phys. Rev. Research 2, 023110 (2020)
We present ultrafast resonant inelastic x-ray scattering (RIXS) experiments performed at the vanadium L edge to track changes in the electronic structure of V₂O₃, a classical Mott-Hubbard material. The probed orbital excitations within the d shell of the V ion show a sub-ps time response, which evolves at later times to a state that appears electronically indistinguishable from the high-temperature metallic state. For low excitation fluences, a transient recovery or delay is observed, which could be related to a transient dimerization of the V-V bonds. Our results demonstrate the great potential for RIXS spectroscopy to study the ultrafast orbital dynamics in strongly correlated materials.

Electronic structure of the parent compound of superconducting infinite-layer nickelates, M. Hepting, D. Li, C. J. Jia, H. Lu, E. Paris, Y. Tseng, X. Feng, M. Osada, E. Been, Y. Hikita, Y.-D. Chuang, Z. Hussain, K. J. Zhou, A. Nag, M. Garcia-Fernandez, M. Rossi, H. Y. Huang, D. J. Huang, Z. X. Shen, T. Schmitt, H. Y. Hwang, B. Moritz, J. Zaanen, T. P. Devereaux and W. S. Lee, Nature Materials 19, 381-385 (2020)
The search continues for nickel oxide-based materials with electronic properties similar to cuprate high-temperature superconductors^1-10. The recent discovery of superconductivity in the doped infinite-layer nickelate NdNiO₂ (refs. ^11,12) has strengthened these efforts. Here, we use X-ray spectroscopy and density functional theory to show that the electronic structure of LaNiO₂ and NdNiO₂, while similar to the cuprates, includes significant distinctions. Unlike cuprates, the rare-earth spacer layer in the infinite-layer nickelate supports a weakly interacting three-dimensional 5d metallic state, which hybridizes with a quasi-two-dimensional, strongly correlated state with 3d_x²−y² symmetry in the NiO₂ layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare-earth intermetallics^13-15, which are well known for heavy fermion behaviour, where the NiO₂ correlated layers play an analogous role to the 4f states in rare-earth heavy fermion compounds. This Kondo- or Anderson-lattice-like ‘oxide-intermetallic’ replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.

Electronic localization in CaVO₃ films via bandwidth control, Daniel E. McNally, Xingye Lu, Jonathan Pelliciari, Sophie Beck, Marcus Dantz, Muntaser Naamneh, Tian Shang, Marisa Medarde, Christof W. Schneider, Vladimir N. Strocov, Ekaterina V. Pomjakushina, Claude Ederer, Milan Radovic and Thorsten Schmitt, npj Quantum Materials 4, 6 (2019)
Understanding and controlling the electronic structure of thin layers of quantum materials is a crucial first step towards designing heterostructures where new phases and phenomena, including the metal-insulator transition (MIT), emerge. Here, we demonstrate control of the MIT via tuning electronic bandwidth and local site environment through selection of the number of atomic layers deposited. We take CaVO₃, a correlated metal in its bulk form that has only a single electron in its V⁴⁺ 3d manifold, as a representative example. We find that thick films and ultrathin films (≤6 unit cells, u.c.) are metallic and insulating, respectively, while a 10 u.c. CaVO₃ film exhibits a clear thermal MIT. Our combined X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) study reveals that the thickness-induced MIT is triggered by electronic bandwidth reduction and local moment formation from V³⁺ ions, that are both a consequence of the thickness confinement. The thermal MIT in our 10 u.c. CaVO₃ film exhibits similar changes in the RIXS response to that of the thickness-induced MIT in terms of reduction of bandwidth and V 3d–O 2p hybridization.

Reciprocity between local moments and collective magnetic excitations in the phase diagram of BaFe₂(As_1-xP_x)₂, Jonathan Pelliciari, Kenji Ishii, Yaobo Huang, Marcus Dantz, Xingye Lu, Paul Olalde-Velasco, Vladimir N. Strocov, Shigeru Kasahara, Lingyi Xing, Xiancheng Wang, Changqing Jin, Yuji Matsuda, Takasada Shibauchi, Tanmoy Das and Thorsten Schmitt, Communications Physics 2, 139 (2019)
Unconventional superconductivity arises at the border between the strong coupling regime with local magnetic moments and the weak coupling regime with itinerant electrons, and stems from the physics of criticality that dissects the two. Unveiling the nature of the quasiparticles close to quantum criticality is fundamental to understand the phase diagram of quantum materials. Here, using resonant inelastic x-ray scattering (RIXS) and Fe K_β emission spectroscopy (XES), we visualize the coexistence and evolution of local magnetic moments and collective spin excitations across the superconducting dome in isovalently doped BaFe₂(As_1-xP_x)₂ (0.00 ≤ x ≤ 0.52). Collective magnetic excitations resolved by RIXS are gradually hardened, whereas XES reveals a strong suppression of the local magnetic moment upon doping. This relationship is captured by an intermediate coupling theory, explicitly accounting for the partially localized and itinerant nature of the electrons in Fe pnictides. Finally, our work identifies a local-itinerant spin fluctuations channel through which the local moments transfer spin excitations to the particle-hole (paramagnons) continuum across the superconducting dome.

Orbital-selective confinement effect of Ru 4d orbitals in SrRuO₃ ultrathin film, Soonmin Kang, Yi Tseng, Beom Hyun Kim, Seokhwan Yun, Byungmin Sohn, Bongju Kim, Daniel McNally, Eugenio Paris, Choong H. Kim, Changyoung Kim, Tae Won Noh, Sumio Ishihara, Thorsten Schmitt, and Je-Geun Park, Phys. Rev. B 99, 045113 (2019)
The electronic structure of SrRuO₃ thin film with a thickness from 1 to 50 unit cell (u.c.) is investigated via the resonant inelastic x-ray scattering (RIXS) technique at the O K edge to unravel the intriguing interplay of orbital and charge degrees of freedom. We found that the orbital-selective quantum confinement effect (QCE) induces the splitting of Ru 4d orbitals. At the same time, we observed a clear suppression of the electron-hole continuum across the metal-to-insulator transition occurring in the 4-u.c. sample. From these two clear observations we conclude that the QCE gives rise to a Mott insulating phase in ultrathin SrRuO₃ films. Our interpretation of the RIXS spectra is supported by the configuration interaction calculations of RuO₆ clusters.

Resolving the nature of electronic excitations in resonant inelastic x-ray scattering, M. Kang, J. Pelliciari, Y. Krockenberger, J. Li, D. E. McNally, E. Paris, R. Liang, W. N. Hardy, D. A. Bonn, H. Yamamoto, T. Schmitt, and R. Comin, Phys. Rev. B 99, 045105 (2019)
The study of elementary bosonic excitations is essential toward a complete description of quantum electronic solids. In this context, resonant inelastic x-ray scattering (RIXS) has recently risen to becoming a versatile probe of electronic excitations in strongly correlated electron systems. The nature of the radiation-matter interaction endows RIXS with the ability to resolve the charge, spin, and orbital nature of individual excitations. However, this capability has been only marginally explored to date. Here, we demonstrate a systematic method for the extraction of the character of excitations as imprinted in the azimuthal dependence of the RIXS signal. Using this approach, we resolve the charge, spin, and orbital nature of elastic scattering, (para-)magnon/bimagnon modes, and higher-energy dd excitations in magnetically ordered and superconducting copper oxide perovskites (Nd₂CuO₄ and YBa₂Cu₃O_6.75). Our method derives from a direct application of scattering theory, enabling us to deconstruct the complex scattering tensor as a function of energy loss. In particular, we use the characteristic tensorial nature of each excitation to precisely and reliably disentangle the charge and spin contributions to the low-energy RIXS spectrum. This procedure enables to separately track the evolution of spin and charge spectral distributions in cuprates with doping. Our results demonstrate a new capability that can be integrated into the RIXS toolset and that promises to be widely applicable to materials with intertwined spin, orbital, and charge excitations.

Probing multi-spinon excitations outside of the two-spinon continuum in the antiferromagnetic spin chain cuprate Sr₂CuO_3, J. Schlappa, U. Kumar, K.J. Zhou, S. Singh, M. Mourigal, V.N. Strocov, A. Revcolevschi, L. Patthey, H.M. Rønnow, S. Johnston and T. Schmitt, Nature Communications 9, 5394 (2018)
One-dimensional (1D) magnetic insulators have attracted significant interest as a platform for studying quasiparticle fractionalization, quantum criticality, and emergent phenomena. The spin-1/2 Heisenberg chain with antiferromagnetic nearest neighbour interactions is an important reference system; its elementary magnetic excitations are spin-1/2 quasiparticles called spinons that are created in even numbers. However, while the excitation continuum associated with two-spinon states is routinely observed, the study of four-spinon and higher multi-spinon states is an open area of research. Here we show that four-spinon excitations can be accessed directly in Sr₂CuO₃ using resonant inelastic x-ray scattering (RIXS) in a region of phase space clearly separated from the two-spinon continuum. Our finding is made possible by the fundamental differences in the correlation function probed by RIXS in comparison to other probes. This advance holds promise as a tool in the search for novel quantum states and quantum spin liquids.

Doping Evolution of Magnetic Order and Magnetic Excitations in (Sr_1−xLa_x)₃Ir₂O₇, Xingye Lu, D. E. McNally, M. Moretti Sala, J. Terzic, M. H. Upton, D. Casa, G. Ingold, G. Cao and T. Schmitt, Physical Review Letters 118, 027202 (2017)
We use resonant elastic and inelastic x-ray scattering at the Ir-L₃ edge to study the doping-dependent magnetic order, magnetic excitations, and spin-orbit excitons in the electron-doped bilayer iridate (Sr_1−xLa_x)₃Ir₂O₇ (0≤x≤0.065). With increasing doping x, the three-dimensional long range antiferromagnetic order is gradually suppressed and evolves into a three-dimensional short range order across the insulator-to-metal transition from x=0 to 0.05, followed by a transition to two-dimensional short range order between x=0.05 and 0.065. Because of the interactions between the J_eff=1/2 pseudospins and the emergent itinerant electrons, magnetic excitations undergo damping, anisotropic softening, and gap collapse, accompanied by weakly doping-dependent spin-orbit excitons. Therefore, we conclude that electron doping suppresses the magnetic anisotropy and interlayer couplings and drives(Sr_1−xLa_x)₃Ir₂O₇ into a correlated metallic state with two-dimensional short range antiferromagnetic order. Strong antiferromagnetic fluctuations of the J_eff=1/2 moments persist deep in this correlated metallic state, with the magnon gap strongly suppressed.