can allow the discovery of basic new physical phenomena and the development of new device concepts. [1] The discovery of new vdW quantum materials and their heterostructures starting from graphene, insulators, semiconductors, superconductors, and topological materials has revolutionized both fundamental and applied research. [2,3] The most recent addition to this vdW family is magnets, which have offered various advantages over conventional magnets and opened new perspectives in vdW heterostructure designs. [4][5][6] In addition to the atomically thin and flat nature of vdW magnets, flexibility, gate tunability, strong proximity interactions, and twist angle between the layers can offer a unique degree of freedom and an innovative platform for device functionalities. [4,5] Recently, several vdW magnets have emerged with the discovery of insulating Cr 2 Ge 2 Te 6 , [7] semiconducting (CrI 3 , [8] CrBr 3 [9] ), and metallic Fe x GeTe 2 . [10,11] The insulating vdW magnets are useful for spin-filter tunneling [9,12] and proximityinduced magnetism, [13][14][15] whereas the metallic magnets can be used as electrodes in magnetic tunnel junctions, [16] observationThe discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe 5 GeTe 2 in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe 5 GeTe 2 are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe 5 GeTe 2 /graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe 5 GeTe 2 along with the presence of negative spin polarization at the Fe 5 GeTe 2 / graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

One-atom-thick rare-earth/noble metal (RE-NM) compounds are attractive materials to investigate two-dimensional magnetism, since they are easy to synthesize into a common RE-NM2 structure with high crystal perfection. Here we perform...

Bulk insulators with strong spin orbit coupling exhibit metallic surface states possessing topological order protected by the time reversal symmetry. However, experiments show vulnerability of topological states to aging and impurities. Different studies show contrasting behavior of the Dirac states along with plethora of anomalies, which has become an outstanding problem in material science. Here, we probe the electronic structure of Bi2Se3 employing high resolution photoemission spectroscopy and discover the dependence of the behavior of Dirac particles on surface terminations. The Dirac cone apex appears at different binding energies and exhibits contrasting shift on Bi and Se terminated surfaces with complex time dependence emerging from subtle adsorbed oxygen-surface atom interactions. These results uncover the surface states behavior of real systems and the dichotomy of topological and normal surface states important for device fabrication as well as realization of novel physics such as Majorana Fermions, magnetic monopole, etc.

CaFe2As2 exhibits collapsed tetragonal (cT) structure and varied exotic behaviour under pressure at low temperatures that led to debate on linking the structural changes to its exceptional electronic properties like superconductivity, magnetism, etc. Here, we investigate the electronic structure of CaFe2As2 forming in different structures employing density functional theory. The results indicate that the stability of the cT phase under pressure arises from the enhancement in hybridization induced effects and shift of the energy bands towards lower energies. The Fermi surface centered around Γ point gradually vanishes with the increase in pressure. Consequently, the nesting between the hole and electron Fermi surfaces associated to the spin density wave state disappears indicating a pathway to achieve the proximity to quantum fluctuations. The magnetic moment at the Fe sites diminishes in the cT phase consistent with the magnetic susceptibility results. Notably, the hybridization of Ca 4s states (Ca-layer may be treated as a charge reservoir layer akin to those in cuprate superconductors) is significantly enhanced in the cT phase revealing its relevance in its interesting electronic properties.

Graphene nanoribbons (GNRs) can be synthesized with atomic precision through on-surface chemistry of self-assembled organic precursors on metal surfaces. Here we examine the growth of 7-armchair GNRs (7-AGNRs) on the Au(16 14 15) vicinal surface, namely, a surface vicinal to Au(111) that features kinked steps. During the thermal activation of the polymerization and cyclodehydrogenation processes that produce the GNRs, the kinked substrate undergoes a strong step-edge reshaping, accompanied by a massive missing-row reconstruction within (111) terraces that aligns GNRs preferentially along two equivalent [110] directions. Using angle-resolved photoemission we are able to detect the occupied frontier band of the 7-AGNR at the center of the first Brillouin zone, as predicted by theoretical calculations. This allows to unambiguously determine the relevant 7-AGNRs band properties, namely energy and effective mass.

We investigate the origin of exoticity in Fe-based systems via studying the Fermiology of CaFe2As2 employing Angle Resolved Photoemission spectroscopy (ARPES). While the Fermi surfaces (FSs) at 200 K and 31 K are observed to exhibit two dimensional (2D) and three dimensional (3D) topology, respectively, the FSs at intermediate temperatures reveal emergence of the 3D topology at much lower temperature than the structural & magnetic phase transition temperature (170 K, for the sample under scrutiny). This leads to the conclusion that the evolution of FS topology is not directly driven by the structural transition. In addition, we discover the existence in ambient conditions of energy bands related to the cT phase. These bands are distinctly resolved in the highphoton energy spectra exhibiting strong Fe 3d character. They gradually move to higher binding energies due to thermal compression with cooling, leading to the emergence of 3D topology in the Fermi surface. These results reveal the so-far hidden existence of a cT phase in ambient conditions, which is argued to lead to quantum fluctuations responsible for the exotic electronic properties in Fe-pnictide superconductors.The parent compounds of Fe-based superconductors are paramagnetic metals and undergo structural & magnetic transitions exhibiting spin density wave (SDW) state as the magnetic ground state [1] with Fe atoms possessing magnetic moment close to a Bohr magneton [2][3][4][5][6]. These materials exhibit varied unusual phenomena involving competing interactions related to magnetic order, superconductivity [7,8], etc. Superconductivity in the Fe-based compounds is believed to appear due to spin fluctuations. Recent studies, however, revealed mysterious superconductivity in pressure induced non-magnetic phase [9].The finding of superconductivity under pressure raises concern over the applicability of spin-fluctuation theory of superconductivity [10,11]. CaFe 2 As 2 is an archetypical test case for the investigation of such puzzles. It has paramagnetic tetragonal structure at room temperature and undergoes a transition to the orthorhombic antiferromagnetic (AFM) phase below 170 K [2-4]. Application of small pressure (> 0.35 GPa) collapses the system in its tetragonal symmetry; this is called collapsed tetragonal (cT) phase and does not exhibit magnetic order [12]. Extensive studies have been carried out on this system resulting into conflicting conclusions [13,14]. Some studies find T c as high as 45 K [15] in doped CaFe 2 As 2 under pressure and attributed the superconductivity to cT phase [16,17]. Some other studies do not find superconductivity in cT phase, which is interpreted as a support of the spin-fluctuation theory of superconductivity [18]. Evidently, the link between superconductivity, structural phase and spin fluctuations is an outstanding issue. Here, we study the Fermiology of CaFe 2 As 2 at different temperatures and discover evidence of cT phase hidden within the structural phase at atmospheric pressure, which plays important role in deri...

Understanding exotic solids is a difficult task as interactions are often hidden by the symmetry of the system. Here, we study the electronic properties of a noncentrosymmetric solid, BiPd, which is a rare material exhibiting both superconductivity and topological phase of matter. Employing high resolution photoemission spectroscopy with photon energies ranging from hard x-ray to extreme ultraviolet regime, we show that hard x-ray spectroscopy alone is not enough to reveal surface-bulk differences in the electronic structure. We derived the escape depths close to the extreme surface sensitivity and find that the photon energies used for high resolution measurements such as ARPES fall in the surface sensitive regime. In addition, we discover deviation of the branching ratio of Bi core level features derived from conventional quantum theories of the core hole final states. Such paradigm shift in core level spectroscopy can be attributed to the absence of center of symmetry and spin-orbit interactions.

We investigate the electronic structure of a specially prepared highly dense conventional high temperature superconductor, MgB2 employing high resolution photoemission spectroscopy. The spectral evolution close to the Fermi energy is commensurate to BCS descriptions as expected. However, the spectra in the wider energy range reveal emergence of a pseudogap much above the superconducting transition temperature indicating apparent departure from the BCS scenario. The energy scale of the pseudogap is comparable to the energy of E2g phonon mode responsible for superconductivity in MgB2 and the pseudogap can be attributed to the effect of electron-phonon coupling on the electronic structure. These results reveal a scenario of the emergence of the superconducting gap within an electron-phonon coupling induced pseudogap and have significant implication in the study of high temperature superconductors.