Engineering higher order Van Hove singularities in two dimensions: the example of the surface layer of Sr2_2RuO4_4

The properties of correlated electron materials are often intricately linked to Van Hove singularities (VHs) in the vicinity of the Fermi energy. The class of these VHs is of great importance, with higher order ones -- with power-law divergence in the density of states -- leaving frequently distinct signatures in physical properties. We use a new theoretical method to detect and analyse higher order Van Hove singularities (HOVHs) in two-dimensional materials and apply it to the electronic structure of the surface layer of Sr$_2$RuO$_4$. We then constrain a low energy model of the VHs of the surface layer of Sr$_2$RuO$_4$ against angle-resolved photoemission spectroscopy and quasiparticle interference data to analyse the VHs near the Fermi level. We show how these VHs can be engineered into HOVHs.Comment: 8 pages including Supplemental Material, 5 figure

Charge doping into spin minority states mediates doubling of TCT_\mathrm{C} in ferromagnetic CrGeTe3_3

The recent discovery of the persistence of long-range magnetic order when van der Waals layered magnets are thinned towards the monolayer limit has provided a tunable platform for the engineering of novel magnetic structures and devices. Here, we study the evolution of the electronic structure of CrGeTe$_3$ as a function of electron doping in the surface layer. From angle-resolved photoemission spectroscopy, we observe spectroscopic fingerprints that this electron doping drives a marked increase in $T_\mathrm{C}$, reaching values more than double that of the undoped material, in agreement with recent studies using electrostatic gating. Together with density functional theory calculations and Monte Carlo simulations, we show that, surprisingly, the increased $T_\mathrm{C}$ is mediated by the population of spin-minority Cr $t_{2g}$ states, forming a half-metallic 2D electron gas at the surface. We show how this promotes a novel variant of double exchange, and unlocks a significant influence of the Ge -- which was previously thought to be electronically inert in this system -- in mediating Cr-Cr exchange.Comment: 10 pages including supplementary informatio

Potent virucidal activity in vitro of photodynamic therapy with Hpericum extract as photosensitizer and white light against human coronavirus HCoV-229E

The emergent human coronavirus SARS-CoV-2 and its high infectivity rate has highlighted the strong need for new virucidal treatments. In this sense, the use of photodynamic therapy (PDT) with white light, to take advantage of the sunlight, is a potent strategy for decreasing the virulence and pathogenicity of the virus. Here, we report the virucidal effect of PDT based on Hypericum extract (HE) in combination with white light, which exhibits an inhibitory activity of the human coronavirus HCoV-229E on hepatocarcinoma Huh-7 cells. Moreover, despite continuous exposure to white light, HE has long durability, being able to maintain the prevention of viral infection. Given its potent in vitro virucidal capacity, we propose HE in combination with white light as a promising candidate to fight against SARS-CoV-2 as a virucidal compoundThis research was funded by Fundación Universidad Autónoma de Madrid, grant number PI21/00315 and by Instituto de Salud Carlos III, grant number PI21/00953. Institutional Review Board Statement: Not applicabl

Superconducting penetration depth through a Van Hove singularity: Sr₂RuO₄ under uniaxial stress

A plethora of experiments in Sr2⁢RuO4 have reached conflicting conclusions about the symmetry of the superconducting gap. To probe the gap's structure in k space, we use strain to continuously tune the band structure through a Van Hove singularity (VHS) while imaging the superconductivity with scanning superconducting quantum interference device microscopy. We find that the superfluid density peaks at the VHS and that the temperature dependence of the penetration depth is 2 quadratic over the entire measured range of strain ɛ. These results are consistent with a gap structure that has vertical line nodes, experimentally confirming that nonlocal effects in the Meissner screening can lead to 2 behavior and clarifying the nature of the low-energy excitations in Sr2⁢RuO4

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 perovskiteSr2RuO4 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.PostprintPeer 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

Direct observation of a uniaxial stress-driven Lifshitz transition in Sr2RuO4

Funding: We gratefully acknowledge support from the European Research Council (Grant No. ERC-714193-QUESTDO), the Royal Society, EPSRC for PhD studentship support through grant number EP/L015110/1 (VS).Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems, and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr2RuO4, leading to a pronounced peak in Tc versus strain whose origin is still under active debate. Here we develop a simple and compact method to passively apply large uniaxial pressures in restricted sample environments, and utilise this to study the evolution of the electronic structure of Sr2RuO4 using angle-resolved photoemission. We directly visualise how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in Tc. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr2RuO4. More generally, our experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.Publisher PDFPeer reviewe