4Hb-TaS2 is a superconductor that exhibits unique characteristics such as time-reversal symmetry breaking, hidden magnetic memory, and topological edge modes. It is a naturally occurring heterostructure comprising of alternating layers of 1H-TaS2 and 1T-TaS2. The former is a well-known superconductor, while the latter is a correlated insulator with a possible non- trivial magnetic ground state. In this study, we use angle resolved photoemission spectroscopy to investigate the normal state electronic structure of this unconventional superconductor. Our findings reveal that the band structure of 4Hb-TaS2 fundamentally differs from that of its constituent materials. Specifically, we observe a significant charge transfer from the 1T layers to the 1H layers that drives the 1T layers away from half-filling. In addition, we find a substantial reduction in inter-layer coupling in 4Hb-TaS2 compared to the coupling in 2H-TaS2 that results in a pronounced spin-valley locking within 4Hb-TaS2.
The doping of metal oxides is an interesting route to increase catalyst activity and lower activation temperatures in H2 dissociation to replace Pt in catalysts for electrochemical devices. In this process, the roles of both the matrix and dopant cations are fundamental to understanding and designing more efficient catalysts. In this work, we have investigated the reduction process in pure and doped CeO2 films. We followed the oxidation states of Ce and dopants (Cu and Fe) during H2 exposure at ambient pressure by combining X-ray absorption spectroscopy and gas chromatography on 5 nm films in the temperature range of 300–620 K. We have observed that Cu doping (at concentrations of 5 and 14 at. %) promotes the ceria reduction, while the addition of Fe seems to have a limited impact on the oxide chemical reactivity only at low temperatures. Moreover, thanks to the chemical sensitivity of operando X-ray absorption spectroscopy, we were able to follow simultaneously the evolution of Ce and Cu oxidation states during the reaction, which has permitted to identify two distinct reduction processes taking place above and below 500 K. These measurements show that at low temperatures, the H2 dissociation takes place at the Cu1+ sites, thus explaining the higher reactivity of the Cu-doped samples. The described mechanism can help in the design of Pt-free catalysts with enhanced performances.
This booklet collects the results of my work as a doctoral student of the Ph.D. School in Physics, Astrophysics and Applied Physics at Universit`a degli Studi di Milano, that has been carried out since November 2020 at Istituto Officina dei Materiali of Consiglio Nazionale delle Ricerche (IOM-CNR) and within the framework of Nanoscale Foundries and Fine Analysis (NFFA) consortium.
My experimental activity addressed the coupling of magnetic and acoustic degrees of freedom in transition-metal ferromagnetic systems. Within the NFFA-SPRINT laboratory, hosted in the premises of the facility FERMI@Elettra (Elettra-Sincrotrone Trieste), I developed a setup to perform optical Transient-Grating spectroscopy, and correlative time-resolved optical spectroscopies (time-resolved reflectivity and polarimetry). Via sub-picosecond optical pulses, acoustic and magnetic transients are impulsively generated: their intertwined evolution and decay are monitored via time-resolved optical probing.
In a first experiment, a Ni thin film was investigated via Transient-Grating spectroscopy. Acoustically-driven magnetization precession was observed at the condition of crossing of phononic and magnonic dispersions, at finite wavevector. With the aid of correlative ferromagnetic resonance measurements the boundary of applicability of the proposed experimental approach was established.
In a second experiment, time-resolved magneto-optical polarimetry was employed to investigate magneto-acoustic waves excited in a ferromagnetic nanostructured array. The details of the magnon-phonon mode crossing allowed to identify experimental features which qualify the degree of coherence in the coupling; a Hamiltonian model was proposed to account for the observations.
The combination of the ability to absorb most of the solar radiation and simultaneously suppress infrared re-radiation allows selective solar absorbers (SSAs) to maximize solar energy to heat conversion, which is critical to several advanced applications. The intrinsic spectral selective materials are rare in nature and only a few demonstrated complete solar absorption. Typically, intrinsic materials exhibit high performances when integrated into complex multilayered solar absorber systems due to their limited spectral selectivity and solar absorption. In this study, we propose CoSbx (2 < x < 3) as a new exceptionally efficient SSA. Here we demonstrate that the low bandgap nature of CoSbx endows broadband solar absorption (0.96) over the solar spectral range and simultaneous low emissivity (0.18) in the mid-infrared region, resulting in a remarkable intrinsic spectral solar selectivity of 5.3. Under 1 sun illumination, the heat concentrates on the surface of the CoSbx thin film, and an impressive temperature of 101.7 °C is reached, demonstrating the highest value among reported intrinsic SSAs. Furthermore, the CoSbx was tested for solar water evaporation achieving an evaporation rate of 1.4 kg m−2 h−1. This study could expand the use of narrow bandgap semiconductors as efficient intrinsic SSAs with high surface temperatures in solar applications.
Mn3Si2Te6 is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in- to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experimental electronic structure, and the role of correlations in shaping this, are little explored to date. Here, we combine x-ray and photoemission-based spectroscopies with first-principles calculations to probe the elemental-selective electronic structure and magnetic order in Mn3Si2Te6. Through these, we identify a marked Mn-Te hybridization, which weakens the electronic correlations and enhances the magnetic anisotropy. We demonstrate how this strengthens the magnetic frustration in Mn3Si2Te6, which is key to stabilizing its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbors and antisymmetric exchange in dictating its ordering temperature. Together, our results demonstrate a powerful methodology of using experimental electronic structure probes to constrain the parameter space for first-principles calculations of magnetic materials, and through this approach, reveal a pivotal role played by covalency in stabilizing the ferrimagnetic order in Mn3Si2Te6.
Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI3 van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and x-ray magnetic circular dichroism spectra acquired at the V $L_{2,3}$ edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI6 unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V3+ ion due to the unbalanced hybridization between the vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of the $t_{2g}^2$ ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn–Teller mechanism. Our results clarify the role of the distortion in VI3 and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.
Transition metal dichalcogenides exhibit many fascinating properties including superconductivity, magnetic orders, and charge density wave. The combination of these features with a non-trivial band topology opens the possibility of additional exotic states such as Majorana fermions and quantum anomalous Hall effect. Here, we report on photon-energy and polarization dependent spin-resolved angle-resolved photoemission spectroscopy experiments on single crystal 1T-VSe2, revealing an unexpected band inversion and emergent Dirac nodal arc with spin-momentum locking. Density functional theory calculations suggest a surface lattice strain could be the driving mechanism for the topologically nontrivial electronic structure of 1T-VSe2.
Magnesium chloride is a prototypical deliquescent material whose surface properties, although central for Ziegler–Natta cataysis, have so far remained elusive to experimental characterization. In this work, we use surface-selective X-ray absorption spectroscopy (XAS) at ambient pressure in combination with multivariate curve resolution, molecular dynamics, and XAS theoretical methods to track in real time and accurately describe the interaction between water vapor and the MgCl2 surface. By exposing MgCl2 to water vapor at temperatures between 595 and 391 K, we show that water is preferentially adsorbed on five-coordinated Mg2+ sites in an octahedral configuration, confirming previous theoretical predictions, and find that MgCl2 is capable of retaining a significant amount of adsorbed water even under prolonged heating to 595 K. As a consequence, our work provides first experimental insights into the unique surface affinity of MgCl2 for atmospheric water. The developed technique is proven highly sensitive to the modifications induced by adsorbates on a given low-Z metal based surface and may be useful in the toolbox required to disentangle the mechanisms of interfacial chemical processes.
Given the urgency of achieving the forthcoming zero emission targets, the research of green fuels and efficient catalysts able to easily convert them in other valuable compounds is fundamental. The work presented in this thesis is focused on the application of an innovative spectroscopic technique, the operando Soft X-Rays NEXAFS spectroscopy, in order to investigate the surface reactivity of heterogeneous catalysts. In fact, it is well known the importance that operando characterizations have acquired in recent years, allowing to study a material at its working conditions. Since the technique requires the use of Synchrotron Radiation and a specific experimental setup, all the measurements reported in this thesis have been performed exploiting a home made reaction cell developed at the APE-HE beamline, at Elettra Synchrotron (Trieste). In this thesis work, we investigated the possibility of coupling the operando NEXAFS technique with other in situ spectroscopies, together with standard ex situ characterizations and computational simulations. This multitechnique approach allowed to extract the maximum potential of the technique, addressing its role as a key tool in the optic of speeding up the design of efficient heterogeneous catalysts.
The catalytic reactions investigated in this thesis are focused on methanol valorization, given its great potential in numerous applications related to the energy transition. In detail, we focused our first investigation on methanol production through the direct partial oxidation of methane, catalysed by a CeO2/CuO composite synthesized using a scalable and green milling process. We exploited the combination of in situ DRIFT and operando Soft X-Ray NEXAFS spectroscopies to monitor at the same time the electronic structure modifications occurring at the catalyst surface and the adsorbates evolution during the different reaction steps.
The operando analysis of the Cu L2,3 and Ce M4,5 edges during the catalyst thermal activation allowed us to detect a charge transfer from Ce3+ surface sites to Cu2+ atoms, resulting in the formation of reactive sites close to the CeO2/CuO interface. When the sample was exposed to CH4 at 250°C and at a pressure of 1 bar, a Cu2+ → Cu+ reduction was observed, indicating that the catalyst is able to activate the methane molecule. At the same time, DRIFT spectra shown the formation of methoxy and formate species, that are products of methane activation on the surface. Adding an oxidizing agent (O2), Cu+ sites were re-oxidized to Cu2+, together with the disappearing of the methoxy and formate related structures in the DRIFT spectra. The results indicated the reversibility of the chemical modifications occurring at the catalyst surface. During the operando NEXAFS experiment, the reaction products were monitored with an online micro-GC: the main products observed during the reaction were CO2, H2O, CH2O and CH3OH, indicating that total and partial oxidation of methane were occurring. As a comparison, an equivalent experiment has been conducted on a similar CeO2/CuO catalyst synthetized with a conventional impregnation method. In this case, no spectroscopic modification were observed with both NEXAFS and DRIFT techniques, confirming that the synthetic method used is crucial in creating specific active sites for methane activation and oxidation. The experimental results have been validated through DFT calculations, which confirmed that when CuO and CeO2 surfaces merge during the synthesis, a net charge transfer from Ce to Cu atoms occurs in proximity of the CeO2 − CuO interface. Another promising route to valorize methanol is represented by its catalytic decomposition to syngas mixture (H2 + CO), whose reaction mechanism was investigated in the second part of the thesis. Indeed, one major challenge for this reaction is related to the quest for stable, cost-effective, and selective catalysts operating below 400 °C. In the present study, we illustrate a surface reactivity study of a Ni3Sn4 catalyst working at 250 °C, by combining density functional theory (DFT), operando NEXAFS at ambient pressure, in situ XPS and high-resolution transmission electron microscopy (HR-TEM). For Ni3Sn4, we discovered that the catalytic reaction is driven by surface tin-oxide phases, able to protect the underlying Ni atoms from irreversible chemical modifications, increasing the catalyst durability. Moreover, exploiting the online micro-GC connected to the operando NEXAFS reaction cell and by comparing the results with a Ni3Sn2 compound, we found that Sn content plays a key role in enhancing the H2 selectivity, with respect to secondary products such as CO2. These findings open new perspectives for the engineering of scalable and low-cost catalysts for hydrogen production.
Interfaces between water and materials are ubiquitous and are crucial in materials sciences and in biology, where investigating the interaction of water with the surface under ambient conditions is key to shedding light on the main processes occurring at the interface. Magnesium oxide is a popular model system to study the metal oxide–water interface, where, for sufficient water loadings, theoretical models have suggested that reconstructed surfaces involving hydrated Mg2+ metal ions may be energetically favored. In this work, by combining experimental and theoretical surface-selective ambient pressure X-ray absorption spectroscopy with multivariate curve resolution and molecular dynamics, we evidence in real time the occurrence of Mg2+ solvation at the interphase between MgO and solvating media such as water and methanol (MeOH). Further, we show that the Mg2+ surface ions undergo a reversible solvation process, we prove the dissolution/redeposition of the Mg2+ ions belonging to the MgO surface, and we demonstrate the formation of octahedral [Mg(H2O)6]2+ and [Mg(MeOH)6]2+ intermediate solvated species. The unique surface, electronic, and structural sensitivity of the developed technique may be beneficial to access often elusive properties of low-Z metal ion intermediates involved in interfacial processes of chemical and biological interest.
Motivated by the recent wealth of exotic magnetic phases emerging in two-dimensional frustrated lattices, we investigate the origin of possible magnetism in the monolayer family of triangular lattice materials MX2 (M=V, Mn, Ni and X=Cl, Br, I). We first show that consideration of general properties such as filling and hybridization enables to formulate the trends for the most relevant magnetic interaction parameters. In particular, we observe that the effects of spin-orbit coupling (SOC) can be effectively tuned through the ligand elements as the considered 3d transition metal ions do not strongly contribute to the anisotropic component of the intersite exchange interaction. Consequently, we find that the corresponding SOC matrix elements differ significantly from the atomic limit. In the next step and by using two ab initio based complementary approaches, we extract realistic effective spin models and find that in the case of heavy ligand elements, SOC effects manifest in anisotropic exchange and single-ion anisotropy only for specific fillings.
This work presents an original approach to preparing pure and Ni-doped CeO2 nanoparticles (NPs) that can be directly drop-casted on a substrate or calcined to form powders. The reduction of the NPs in H2 is very different than the one usually anticipated for supported Ni–CeO2 catalysts. In situ soft X-ray absorption and infrared spectroscopies revealed that the reduction of Ce4+ into Ce3+ in H2 proceeds via simultaneous oxidation of Ni2+ ions into Niδ+ (2<δ<3). Comparison with reference samples indicates that Ce4+ ions reduction is promoted over Ni-doped CeO2 NPs, whereas that of Ni2+ is hindered. Theoretical simulation of Ni L-edge spectra suggested that Ni dopant into ceria is in a square planar four-coordinate environment, in contrast to the familiar octahedral symmetry of bulk nickel oxides. Our results reveal that the surface chemistry of Ni-doped CeO2 is quite distinct as compared to that of the individual bulk oxides, which potentially can lead to a different performance of this material, notably in catalytic applications.
Curved magnets attract considerable interest for their unusually rich phase diagram, often encompassing exotic (e.g., topological or chiral) spin states. Micromagnetic simulations are playing a central role in the theoretical understanding of such phenomena; their predictive power, however, rests on the availability of reliable model parameters to describe a given material or nanostructure. Here we demonstrate how noncollinear-spin polarized density-functional theory can be used to determine the flexomagnetic coupling coefficients in real systems. By focusing on monolayer CrI3, we find a crossover as a function of curvature between a magnetization normal to the surface to a cycloidal state, which we rationalize in terms of effective anisotropy and Dzyaloshinskii-Moriya contributions to the magnetic energy. Our results reveal an unexpectedly large impact of spin-orbit interactions on the curvature-induced anisotropy, which we discuss in the context of existing phenomenological models
Multiferroic materials have attracted wide interest because of their exceptional static1,2,3 and dynamical4,5,6 magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order that directly induces ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii–Moriya effect3,7. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects3,8. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of magnetoelectric coupling in nanoelectronic devices1,9,10. Here we report the discovery of type-II multiferroic order in a single atomic layer of the transition-metal-based van der Waals material NiI2. The multiferroic state of NiI2 is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Combining birefringence and second-harmonic-generation measurements with theoretical modelling and simulations, we detect a highly anisotropic electronic state that simultaneously breaks three-fold rotational and inversion symmetry, and supports polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic polar state that persists down to the ultrathin limit of monolayer NiI2. These observations establish NiI2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.
Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform for exploring, on the fundamental side, new microscopic mechanisms and for developing, on the technological side, ultracompact spintronic applications. So far, bilinear spin Hamiltonians have been commonly adopted to investigate the magnetic properties of 2D magnets, neglecting higher order magnetic interactions. However, we here provide quantitative evidence of giant biquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), by combining first-principles calculations and the newly developed machine learning method for constructing Hamiltonian. Interestingly, we show that the ferromagnetic ground state within NiCl2 single layers cannot be explained by means of the bilinear Heisenberg Hamiltonian; rather, the nearest-neighbor biquadratic interaction is found to be crucial. Furthermore, using a three-orbitals Hubbard model, we propose that the giant biquadratic exchange interaction originates from large hopping between unoccupied and occupied orbitals on neighboring magnetic ions. On a general framework, our work suggests biquadratic exchange interactions to be important in 2D magnets with edge-shared octahedra.
Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of La0.65Sr0.35MnO3 (LSMO), a landmark system for spintronics applications, a description based solely on Mn 3d and O 2p electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La 5p, Mn 4s, and O 2s orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.
The magnetic properties of the two-dimensional VI3 bilayer are the focus of our first-principles analysis, highlighting the role of t2g orbital splitting and carried out in comparison with the CrI3 prototypical case, where the splitting is negligible. In VI3 bilayers, the empty a1g state is found to play a crucial role in both stabilizing the insulating state and in determining the interlayer magnetic interaction. Indeed, an analysis based on maximally localized Wannier functions allows one to evaluate the interlayer exchange interactions in two different VI3 stackings (labeled AB and AB′), to interpret the results in terms of the virtual-hopping mechanism, and to highlight the strongest hopping channels underlying the magnetic interlayer coupling. Upon application of electric fields perpendicular to the slab, we find that the magnetic ground state in the AB′ stacking can be switched from antiferromagnetic to ferromagnetic, suggesting the VI3 bilayer as an appealing candidate for electric-field-driven miniaturized spintronic devices.
The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.
We study the 2×2 charge density wave (CDW) in epitaxially-grown monolayer TiSe2. Our temperature-dependent angle-resolved photoemission spectroscopy measurements indicate a strong-coupling instability, but reveal how not all states couple equally to the symmetry-breaking distortion, with an electron pocket persisting to low temperature as a non-bonding state. We further show how the CDW order can be suppressed by a modest doping of around 0.06(2) electrons per Ti. Our results provide an opportunity for quantitative comparison with a realistic tight-binding model, which emphasises a crucial role of structural aspects of the phase transition in understanding the hybridisation in the ground state. Together, our work provides a comprehensive understanding of the phenomenology of the CDW in TiSe2 in the 2D limit.
Out-of-plane Ga2Se3 nanowires are grown by molecular beam epitaxy via Au-assisted heterovalent exchange reaction on GaAs substrates in the absence of Ga deposition. It is shown that at a suitable temperature around 560 degrees C the Audecorated GaAs substrate releases Ga atoms, which react with the incoming Se and feed the nanowire growth. The nanowire composition, crystal structure, and morphology are characterized by Raman spectroscopy and electron microscopy. The growth mechanism is investigated by X-ray photoelectron spectroscopy. We explore the growth parameter window and find an interesting effect of shortening of the nanowires after a certain maximum length. The nanowire growth is described within a diffusion transport model, which explains the nonmonotonic behavior of the nanowire length versus the growth parameters. Nanowire shortening is explained by the blocking of Ga supply from the GaAs substrate by thick, in-plane worm-like Ga2Se3 structures, which grow concomitantly with the nanowires, followed by backward diffusion of Ga atoms from the nanowires down to the substrate surface.
We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe3. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through TC. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te 5p and the Cr eg orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr t2g states that carry the majority of the spin moment. The t2g states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.
The present thesis work has been performed within a new-born laboratory called Spin Polar-ization Research Instrument in the Nanoscale and Time domain (SPRINT laboratory), as apart of the research infrastructures circuit NFFA-Trieste (Nano Foundries and Fine Analysis -belonging to the wider NFFA-Europe circuit) and hosted in the experimental hall of the freeelectron laser FERMI@Elettra.The SPRINT laboratory rises as an answer to the urgent request of the scientific communityof extension of photoemission spectroscopies (PES), not only energy-, but possibly also angle-and spin-resolved, to the time domain in the sub-picosecond regime. The integration of a PESapparatus within a setup for stroboscopic measurements (that is in a pump-probe scheme) pavesthe way to time resolved study of the relaxation of optically populated electronic states, thusenabling the study the ultrafast dynamics of the excitations inside the materials, with greatbenefit from both the fundamental and the technological point of view.
Band inversions are key to stabilising a variety of novel electronic states in solids, from topological surface states to the formation of symmetry-protected three-dimensional Dirac and Weyl points and nodal-line semimetals. Here, we create a band inversion not of bulk states, but rather between manifolds of surface states. We realise this by aliovalent substitution of Nb for Zr and Sb for S in the ZrSiS family of nonsymmorphic semimetals. Using angle-resolved photoemission and density-functional theory, we show how two pairs of surface states, known from ZrSiS, are driven to intersect each other near the Fermi level in NbGeSb, and to develop pronounced spin splittings. We demonstrate how mirror symmetry leads to protected crossing points in the resulting spin-orbital entangled surface band structure, thereby stabilising surface state analogues of three-dimensional Weyl points. More generally, our observations suggest new opportunities for engineering topologically and symmetry-protected states via band inversions of surface states.
The tetragonal phase of chromium (III) oxide, although unstable in the bulk, can be synthesized in epitaxial heterostructures. Theoretical investigation by density-functional theory predicts an antiferromagnetic ground state for this compound. We demonstrate experimentally antiferromagnetism up to 40 K in ultrathin films of t−Cr2O3 by electrical measurements exploiting interface effect within a neighboring ultrathin Pt layer. We show that magnetotransport in Pt is affected by both spin-Hall magnetoresistance and magnetic proximity effect while we exclude any role of magnetism for the low-temperature resistance anomaly observed in Pt.
The band inversions that generate the topologically non-trivial band gaps of topological insulators and the isolated Dirac touching points of three-dimensional Dirac semimetals generally arise from the crossings of electronic states derived from different orbital manifolds. Recently, the concept of single orbital-manifold band inversions occurring along high-symmetry lines has been demonstrated, stabilising multiple bulk and surface Dirac fermions. Here, we discuss the underlying ingredients necessary to achieve such phases, and discuss their existence within the family of transition metal dichalcogenides. We show how their three-dimensional band structures naturally produce only small k z projected band gaps, and demonstrate how these play a significant role in shaping the surface electronic structure of these materials. We demonstrate, through spin- and angle-resolved photoemission and density functional theory calculations, how the surface electronic structures of the group-X TMDs PtSe2 and PdTe2 are host to up to five distinct surface states, each with complex band dispersions and spin textures. Finally, we discuss how the origin of several recently-realised instances of topological phenomena in systems outside of the TMDs, including the iron-based superconductors, can be understood as a consequence of the same underlying mechanism driving k z -mediated band inversions in the TMDs.
We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor
PdTe2 by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe2 with its sister compound PtSe2, we demonstrate how enhanced interlayer hopping in the Te-based material drives a band inversion within the antibonding p-orbital manifold well above the Fermi level. We show how this mediates spin-polarized topological surface states which form rich multivalley Fermi surfaces with complex spin textures. Scanning tunneling spectroscopy reveals type-II superconductivity at the surface, and moreover shows no evidence for an unconventional component of its superconducting order parameter, despite the presence of topological surface states.
Interfaces play a crucial role in the study of novel phenomena emerging at heterostructures comprising metals and functional oxides. For this reason, attention should be paid to the interface chemistry, which can favor the interdiffusion of atomic species and, under certain conditions, lead to the formation of radically different compounds with respect to the original constituents. In this work, we consider Cr/
BaTiO3 heterostructures grown on SrTiO3 (100) substrates. Chromium thin films (1–2 nm thickness) are deposited by molecular beam epitaxy on the
BaTiO3 layer, and subsequently annealed in vacuum at temperatures ranging from 473 to 773 K. A disordered metallic layer is detected for annealing temperatures up to 573 K, whereas, at higher temperatures, we observe a progressive oxidation of chromium, which we relate to the thermally activated migration of oxygen from the substrate. The chromium oxidation state is +3 and the film shows a defective rocksalt structure, which grows lattice matched on the underlying BaTiO3 layer. One out of every three atoms of chromium is missing, producing an uncommon tetragonal phase with Cr2O3 stoichiometry. Despite the structural difference with respect to the ordinary corundum α-Cr2O3 phase, we demonstrate both experimentally and theoretically that the electronic properties of the two phases are, to a large extent, equivalent.
Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin- and angle-resolved photoemission, we find that these generically host a co-existence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics.