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

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

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

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

Molecular dynamics simulations of polarization domains and flexoelectricity in BaTiO3

This thesis is a part of a project started in December 2016 in collaboration with Prof. Jorge Alcalá Cabrelles (UPC, Barcelona) and Dr. Jan Ocenasek (ZCU institute, Czech Republic) aiming to perform piezoelectric and flexoelectric studies in barium titanate by means of molecular dynamics simulations. The specific objective of this work is to gain expertise on molecular dynamics simulations of piezoelectric response, develop several analytical tools and characterize the basic thermodynamic and electric properties through a core-shell model of barium titanate fitted from first principle calculations. With the emergence of increasingly powerful and cheap supercomputing, atomistic simulations are quickly becoming a very attractive and reliable method for testing materials at the nanoscale and mesoscale. This thesis provides a literature review regarding the basics of molecular dynamics simulations performed to investigate dielectric properties of barium titanate, which is a key archetypal material used in sensors and memory storage devices as well as in the development of new supercapacitors. This work comprises a series of 13 simulations of barium titanate aimed to characterize the spontaneous polarization developing in the ferroelectric phases, study the effect of externally applied electric fields, build the temperature versus pressure phase diagram, estimate the piezoelectric coefficients of the tetragonal phase, calculate the hysteresis loops, identify ferroelectric switching and analyze the paraelectricity of the cubic phase. Furthermore, the limitations of simulating barium titanate using periodic boundary conditions are described and alternatives are given in the context of the investigation of flexoelectric response. Phonon dispersion is introduced as a key aspect to elucidate the underlying mechanisms in phase transformations. Finally, considerable amount of software has been developed to facilitate tracking, measuring and analysis of the physical properties in each simulatio

Angle-resolved photoemission studies of uniaxial stress-driven Lifshitz transitions in the bulk and surface electronic structure of Sr₂RuO₄

In experimental condensed matter physics, the utilisation of momentum-resolved probes has proven valuable in disentangling the underpinning effects driving the formation of rich collective states in quantum materials. Furthermore, the ability to tune relevant features in the electronic structure and control the breaking of particular symmetries, comprises a powerful route to stabilise phases and electronic states that are not available naturally to equilibrium chemistry. In this work, I show how one can simultaneously benefit from both approaches, specifically, with the development of a technique combining angle-resolved photoemission spectroscopy (ARPES) with the application of uniaxial stress. After a thorough discussion of the experimental method, I show its capabilities in the normal state of the unconventional superconductor Sr₂RuO₄, where the application of uniaxial pressure has recently been shown to more than double the transition temperature, leading to a peak in Tc versus strain. We directly visualise how uniaxial stress drives a Lifshitz transition of one of its three Fermi surfaces, which is in close proximity to a van Hove singularity (vHS), and we point to the key role of strain-tuning the vHS to the Fermi level in mediating the peak in Tc. Our measurements also provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr₂RuO₄. In the bilayer sister compound Sr₃Ru₂O₇, in-plane rotations of the RuO₆ octahedra and the corresponding doubling of the in-plane unit cell turn the vHS into higher (4th) order. Tuning this extended vHS to the Fermi level with large magnetic fields is thought to drive an exotic nematic state to emerge, which exhibits signa- tures of quantum criticality. Interestingly, the octahedra rotations that characterise Sr₃Ru₂O₇ are also found in the surface layer of Sr₂RuO₄, potentially making such states accessible also at the surface of the single-layer compound. In this work, I show the evolution of the electronic structure at the surface layer of Sr₂RuO₄ under large in-plane uniaxial stress. From ARPES, we show how the induced strain drives a sequence of Lifshitz transitions, fundamentally reshaping the low-energy electronic structure and the rich spectrum of vHSs that the surface layer of Sr₂RuO₄ hosts. From comparison of tight-binding modelling to our measured dispersions, I show that, surprisingly, the strain is accommodated predominantly by bond-length changes rather than modifications of the octahedral tilt and rotation angles, thus shedding 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 surface of Sr₂RuO₄."I gratefully acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials. Also, I gratefully acknowledge support from the European Research Council (Grant no. ERC-714193-QUESTDO), the Royal Society, the Max-Planck Society, the International Max-Planck partnership for Measurement and Observation at the Quantum Limit, the Engineering and Physical Sciences Research Council (Grant nos,. EP/T02108X/1 and EP/R031924/1), and the Leverhulme Trust (Grant No. RL-2016-006)."--Acknowledgement