Controlling the charge density wave transition in single-layer TiTe2xSe2(1−x) alloys by band gap engineering

Funding: We gratefully acknowledge support from the Leverhulme Trust via Grant No. RL-2016-006 and the UK Royal Society. The MBE growth facility was funded through an EPSRC strategic equipment grant: EP/M023958/1.Closing the band gap of a semiconductor, into a semimetallic state, gives a powerful potential route to tune the electronic energy gains that drive collective phases like charge density waves (CDW) and excitonic insulator states. We explore this approach for the controversial CDW material monolayer (ML) TiSe2 by engineering its narrow band gap to the semimetallic limit of ML-TiTe2. Using molecular beam epitaxy, we demonstrate the growth of ML-TiTe2xSe2(1−x) alloys across the entire compositional range, and unveil how the (2 × 2) CDW instability evolves through the normal state semiconductor-semimetal transition via in situ angle-resolved photoemission spectroscopy. Through model electronic structure calculations, we identify how this tunes the relative strength of excitonic and Peierls-like coupling, demonstrating band gap engineering as a powerful method for controlling the microscopic mechanisms underpinning the formation of collective states in two-dimensional materials.Publisher PDFPeer reviewe

Hierarchy of Lifshitz transitions in the surface electronic structure of Sr2RuO4 under uniaxial compression

Funding: We gratefully acknowledge support from the Engineering and Physical Sciences Research Council (Grant Nos. EP/T02108X/1 and EP/R031924/1), the European Research Council (through the QUESTDO project, 714193), and the Leverhulme Trust (Grant No. RL-2016-006). E.A.M., A.Z., and I.M. gratefully acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials. N.K. is supported by a KAKENHI Grants-in-Aids for Scientific Research (Grant Nos.18K04715, and 21H01033), and Core-to-Core Program (No. JPJSCCA20170002) from the Japan Society for the Promotion of Science (JSPS) and by a JST-Mirai Program (Grant No. JPMJMI18A3). APM and CWH acknowledge support from the Deutsche Forschungsgemeinschaft - TRR 435 288 - 422213477 (project A10). We thank Diamond Light Source for access to Beamline I05 (Proposals SI27471 and SI28412), which contributed to the results presented here.We report the evolution of the electronic structure at the surface of the layered perovskite Sr2RuO4 under large in-plane uniaxial compression, leading to anisotropic B1g strains of εxx − εyy = −0.9 ± 0.1%. From angle-resolved photoemission, we show how this drives a sequence of Lifshitz transitions, reshaping the low-energy electronic structure and the rich spectrum of van Hove singularities that the surface layer of Sr2RuO4 hosts. From comparison to tight-binding modelling, we find that the strain is accommodated predominantly by bond-length changes rather than modifications of octahedral tilt and rotation angles. Our study sheds new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of states singularities to the Fermi level, in turn paving the way to the possible realisation of rich collective states at the Sr2RuO4 surface.Peer reviewe

Giant valley-Zeeman coupling in the surface layer of an intercalated transition metal dichalcogenide

Funding: We gratefully acknowledge support from the Leverhulme Trust (Grant No. RL-2016-006 [P.D.C.K., B.E., T.A., A.R., C.B.]), the European Research Council (through the QUESTDO project, 714193 [P.D.C.K., G.R.S.]), the Engineering and Physical Sciences Research Council (Grant Nos. EP/T02108X/1 [P.D.C.K., P.A.E.M.] and EP/N032128/1 [D.A.M., G.B.]), and the Center for Computational Materials Science at the Institute for Materials Research for allocations on the MASAMUNE-IMR supercomputer system (Project No. 202112-SCKXX-0510 [R.B.V., M.S.B.]). S.B., E.A.M. and A.Z. gratefully acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.Spin–valley locking is ubiquitous among transition metal dichalcogenides with local or global inversion asymmetry, in turn stabilizing properties such as Ising superconductivity, and opening routes towards ‘valleytronics’. The underlying valley–spin splitting is set by spin–orbit coupling but can be tuned via the application of external magnetic fields or through proximity coupling. However, only modest changes have been realized to date. Here, we investigate the electronic structure of the V-intercalated transition metal dichalcogenide V1/3NbS2 using microscopic-area spatially resolved and angle-resolved photoemission spectroscopy. Our measurements and corresponding density functional theory calculations reveal that the bulk magnetic order induces a giant valley-selective Ising coupling exceeding 50 meV in the surface NbS2 layer, equivalent to application of a ~250 T magnetic field. This energy scale is of comparable magnitude to the intrinsic spin–orbit splittings, and indicates how coupling of local magnetic moments to itinerant states of a transition metal dichalcogenide monolayer provides a powerful route to controlling their valley–spin splittings.PostprintPeer reviewe

Spin-orbit coupling induced Van Hove singularity in proximity to a Lifshitz transition in Sr4Ru3O10

Funding: CAM, MN and PW gratefully acknowledge funding from the Engineering and Physical Sciences Research Council through EP/R031924/1 and EP/S005005/1, IB through the International Max Planck Research School for Chemistry and Physics of Quantum Materials and LCR from a fellowship from the Royal Commission of the Exhibition of 1851. RA, RF and AV thank the EU’s Horizon 2020 research and innovation program under Grant Agreement No. 964398 (SUPERGATE).Van Hove singularities (VHss) in the vicinity of the Fermi energy often play a dramatic role in the physics of strongly correlated electron materials. The divergence of the density of states generated by VHss can trigger the emergence of new phases such as superconductivity, ferromagnetism, metamagnetism, and density wave orders. A detailed understanding of the electronic structure of these VHss is therefore essential for an accurate description of such instabilities. Here, we study the low-energy electronic structure of the trilayer strontium ruthenate Sr4Ru3O10, identifying a rich hierarchy of VHss using angle-resolved photoemission spectroscopy and millikelvin scanning tunneling microscopy. Comparison of k-resolved electron spectroscopy and quasiparticle interference allows us to determine the structure of the VHss and demonstrate the crucial role of spin-orbit coupling in shaping them. We use this to develop a minimal model from which we identify a new mechanism for driving a field-induced Lifshitz transition in ferromagnetic metals.Peer reviewe

Chemical trends of the bulk and surface termination-dependent electronic structure of metal-intercalated transition metal dichalcogenides

Funding: VINNOVA - 2018-04969; H2020 Research Infrastructures - 730872; Engineering and Physical Sciences Research Council - EP/L01548X/1, EP/N032128/1, EP/T02108X/1; H2020 European Research Council - 714193; Svenska Forskningsrådet Formas - 2019-02496; Leverhulme Trust - RL-2016-006, RPG-2023-253; Max-Planck-Gesellschaft.The addition of metal intercalants into the van der Waals gaps of transition metal dichalcogenides has shown great promise as a method for controlling their functional properties. For example, chiral helimagnetic states, current-induced magnetization switching, and a giant valley-Zeeman effect have all been demonstrated, generating significant renewed interest in this materials family. Here, we present a combined photoemission and density-functional theory study of three such compounds: V1/3NbS2, Cr1/3NbS2, and Fe1/3NbS2, to investigate chemical trends of the intercalant species on their bulk and surface electronic structure. Our resonant photoemission measurements indicate increased hybridization with the itinerant NbS2-derived conduction states with increasing atomic number of the intercalant, leading to pronounced mixing of the nominally localized intercalant states at the Fermi level. Using spatially and angle-resolved photoemission spectroscopy, we show how this impacts surface-termination-dependent charge transfers and leads to the formation of new dispersive states of mixed intercalant-Nb character at the Fermi level for the intercalant-terminated surfaces. This provides an explanation for the origin of anomalous states previously reported in this family of compounds and paves the way for tuning the nature of the magnetic interactions in these systems via control of the hybridization of the magnetic ions with the itinerant states.Peer reviewe

Spin-orbit coupled spin-polarised hole gas at the CrSe2-terminated surface of AgCrSe2

Funding: We gratefully acknowledge support from the European Research Council (through the QUESTDO project, 714193), the Engineering and Physical Sciences Research Council (Grant No. EP/T02108X/1), and the Leverhulme Trust (Grant No. RL-2016-006). S.-J.K., E.A.M., A.Z., and I.M. gratefully acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.In half-metallic systems, electronic conduction is mediated by a single spin species, offering enormous potential for spintronic devices. Here, using microscopic-area angle-resolved photoemission, we show that a spin-polarised two-dimensional hole gas is naturally realised in the polar magnetic semiconductor AgCrSe2 by an intrinsic self-doping at its CrSe2-terminated surface. Through comparison with first-principles calculations, we unveil a striking role of spin-orbit coupling for the surface hole gas, unlocked by both bulk and surface inversion symmetry breaking, suggesting routes for stabilising complex magnetic textures in the surface layer of AgCrSe2.Publisher PDFPeer reviewe

Studying the electronic structure of the polar surfaces of delafossites using μ-ARPES

This thesis presents the results of my angle-resolved photoemission (ARPES) study of the delafossite oxides PdCoO₂, PtCoO₂ and PdCrO₂ as well as delafossite-like AgCrSe₂. When cleaving the sample, their layered ABX₂ structure results in distinct surface terminations that are spatially distributed across the sample. These are either electron (A-termination) or hole doped (BX₂-termination) with respect to the bulk, resulting in their markedly different surface electronic structures. These electronic surface structures host a delicate interplay between the spin-, charge-, and orbital-degrees of freedom, driving the formation of new phases. The understanding of this interplay is the central concept of this thesis. Probing defined surface domains has often been challenging due to the beam spot of the incoming light being larger than a single domain. Here, I show that by using μ-ARPES, where the beam spot is focused to 4 μm, it is possible to probe the electronic structure of a distinct surface termination and study the extent of spatial variations in the electronic structure across the sample. Identifying areas of pristine terminations greatly increased the effective resolution, enabling the detailed study of the surface electronic structure. I show that PdCrO₂ undergoes a charge-order driven surface reconstruction that protects the insulating nature of the CrO₂ states at the surface. For PdCoO₂, I demonstrate strong electron-phonon coupling on both the CoO₂- and Pd-terminated surfaces, with an unusually strong coupling on the Pd-termination driving the formation of two distinct polaron modes. I conclude by presenting my study on non-centrosymmetric delafossite-like AgCrSe₂. Here, a delicate interplay between bulk and surface inversion symmetry breaking results in a Rashba-type splitting of the states derived from different magnetic domains. I will also revise the role of the subsurface layer for these systems which has thus far been widely neglected but is crucial to achieve an accurate description of the underlying physics

Studying the electronic structure of the polar surfaces of delafossites using mu-ARPES (thesis data)

All research data used within the thesis is saved as .nc files and ordered by Chapter. The data files are embargoed until 30/04/202

Hierarchy of Lifshitz transitions in the surface electronic structure of Sr₂RuO₄ under uniaxial compression

We report the evolution of the electronic structure at the surface of the layered perovskite Sr2RuO4 under large in-plane uniaxial compression, leading to anisotropic B1g strains of εxx − εyy = −0.9 ± 0.1%. From angle-resolved photoemission, we show how this drives a sequence of Lifshitz transitions, reshaping the low-energy electronic structure and the rich spectrum of van Hove singularities that the surface layer of Sr2RuO4 hosts. From comparison to tight-binding modelling, we find that the strain is accommodated predominantly by bond-length changes rather than modifications of octahedral tilt and rotation angles. Our study sheds new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of states singularities to the Fermi level, in turn paving the way to the possible realisation of rich collective states at the Sr2RuO4 surface