arXiv subjects

G. Di Santo

Publications and source records attributed to G. Di Santo.

6 recordsLinked to original sources

arXiv2024

Interfacing Quantum Spin Hall and Quantum Anomalous Hall insulators: Bi bilayer on MnBi$_2$Te$_4$-family materials

Meeting of non-trivial topology with magnetism results in novel phases of matter, such as Quantum Anomalous Hall (QAH) or axion insulator phases. Even more exotic states with high and tunable Chern numbers are expected at the contact of intrinsic magnetic topological insulators (IMTIs) and 2D topological insulators (TIs).Here we synthesize a heterostructures composed of 2D TI and 3D IMTIs, specifically of bismuth bilayer on top of MnBi$_2$Te$_4$-family of compounds and study their electronic properties by means of angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT). The epitaxial interface is characterized by hybridized Bi and IMTI electronic states. The Bi bilayer-derived states on different members of MnBi$_2$Te$_4$-family of materials are similar, except in the region of mixing with the topological surface states of the substrate. In that region, the new, substrate dependent interface Dirac state is observed. Our \emph{ab initio} calculations show rich interface phases with emergence of exchange split 1D edge states, making the Bi/IMTI heterostructures promising playground for observation of novel members in the family of quantum Hall effects.

cond-mat.other↗

arXiv2023

Observation of Dirac-like surface state bands on the top surface of BiSe

Two quintuple layers of strong topological insulator Bi2Se3 are coupled by a Bi bilayer in BiSe crystal. We investigated its electronic structure using angle resolved photoelectron spectroscopy to study its topological nature. Dirac like linearly dispersive surface state bands are observed on the 001 surface of BiSe and Sb doped BiSe, similar to Bi2Se3. Moreover, the lower part of the SSBs buries deep in the bulk valence band. Overlap region between the SSBs and BVB is large in Sb doped system and the SSBs deviate from the Dirac like linear dispersion in this region. These results highlight the role of interlayer coupling between the Bi bilayer and the Bi2Se3 QLs. Furthermore, we observed a large intensity imbalance in the SSBs located at the positive and negative k parallel directions. This asymmetry pattern gradually reverses as the excitation energy scans from low 14eV to high 34eV value. However, we did not observe signal of surface magnetization resulting from the intensity imbalance in SSBs due to hole-generated uncompensated spin accumulation in the photoexcitation process. The main reason for this could be the faster relaxation process for photo hole due to the presence of the Bi bilayer between the adjacent Bi2Se3 QLs. The observed photon energy dependent intensity variation could be a signature of the mixing between the spin and the orbit texture of the SSBs.

cond-mat.mtrl-sci↗

arXiv2020

Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates

High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. Here, we combine equilibrium and time-resolved ARPES to track the temperature dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and double-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-Liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates.

cond-mat.supr-con↗

arXiv2019

Variety of magnetic topological phases in the (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ family

Quantum states of matter combining non-trivial topology and magnetism attract a lot of attention nowadays; the special focus is on magnetic topological insulators (MTIs) featuring quantum anomalous Hall and axion insulator phases. Feasibility of many novel phenomena that \emph{intrinsic} magnetic TIs may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here, using angle- and spin-resolved photoemission spectroscopy along with \emph{ab initio} calculations we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ with $m=0, ..., 6$. Magnetic, electronic and, consequently, topological properties of these materials depend strongly on the $m$ value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi2Te4 (m=0) to MnBi4Te7 (m=1), changes to ferromagnetic (FM) one in MnBi6Te10 (m=2) and disappears with further increase in m. In this way, the AFM and FM TI states are respectively realized in the $m=0,1$ and $m=2$ cases, while for $m \ge 3$ a novel and hitherto-unknown topologically-nontrivial phase arises, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the \MBT\, building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.

cond-mat.mtrl-sci↗

arXiv2019

Origin of the flat band in heavily Cs doped graphene

A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we introduce a method to induce a flat band in two-dimensional (2D) materials. We show that the flat band can be achieved by sandwiching the 2D material by two cesium (Cs) layers. We apply this method to monolayer graphene and investigate the flat band by a combination of angle-resolved photoemission spectroscopy experiment and the calculation. Our work highlights that charge transfer, zone folding of graphene bands and the covalent bonding between C and Cs atoms are at the origin of the flat energy band formation. The presented approach is an alternative route for obtaining flat band materials to twisting bilayer graphene which yields thermodynamically stable flat band materials in large areas.

cond-mat.mes-hall↗

arXiv2017

Synchrotron radiation induced magnetization in magnetically-doped and pristine topological insulators

Quantum mechanics postulates that any measurement influences the state of the investigated system. Here, by means of angle-, spin-, and time-resolved photoemission experiments and ab initio calculations we demonstrate how non-equal depopulation of the Dirac cone (DC) states with opposite momenta in V-doped and pristine topological insulators (TIs) created by a photoexcitation by linearly polarized synchrotron radiation (SR) is followed by the hole-generated uncompensated spin accumulation and the SR-induced magnetization via the spin-torque effect. We show that the photoexcitation of the DC is asymmetric, that it varies with the photon energy, and that it practically does not change during the relaxation. We find a relation between the photoexcitation asymmetry, the generated spin accumulation and the induced spin polarization of the DC and V 3d states. Experimentally the SR-generated in-plane and out-of-plane magnetization is confirmed by the $k_{\parallel}$-shift of the DC position and by the splitting of the states at the Dirac point even above the Curie temperature. Theoretical predictions and estimations of the measurable physical quantities substantiate the experimental results.

cond-mat.mtrl-sci↗