arXiv subjects

Matteo Michiardi

Publications and source records attributed to Matteo Michiardi.

30 records · Page 2Linked to original sources

arXiv2019

Intrinsic Insulating Ground State in Transition Metal Dichalcogenide TiSe2

The transition metal dichalcogenide TiSe$_2$ has received significant research attention over the past four decades. Different studies have presented ways to suppress the 200~K charge density wave transition, vary low temperature resistivity by several orders of magnitude, and stabilize magnetism or superconductivity. Here we give the results of a new synthesis technique whereby samples were grown in a high pressure environment with up to 180~bar of argon gas. Above 100~K, properties are nearly unchanged from previous reports, but a hysteretic resistance region that begins around 80~K, accompanied by insulating low temperature behavior, is distinct from anything previously observed. An accompanying decrease in carrier concentration is seen in Hall effect measurements, and photoemission data show a removal of an electron pocket from the Fermi surface in an insulating sample. We conclude that high inert gas pressure synthesis accesses an underlying nonmetallic ground state in a material long speculated to be an excitonic insulator.

cond-mat.str-el↗

arXiv2018

Quasi-free-standing single-layer WS2 achieved by intercalation

Large-area and high-quality single-layer transition metal dichalcogenides can be synthesized by epitaxial growth on single-crystal substrates. An important advantage of this approach is that the interaction between the single-layer and the substrate can be strong enough to enforce a single crystalline orientation of the layer. On the other hand, the same interaction can lead to hybridization effects, resulting in the deterioration of the single-layer's native properties. This dilemma can potentially be solved by decoupling the single-layer from the substrate surface after the growth via intercalation of atoms or molecules. Here we show that such a decoupling can indeed be achieved for single-layer WS2 epitaxially grown on Ag(111) by intercalation of Bi atoms. This process leads to a suppression of the single-layer WS2-Ag substrate interaction, yielding an electronic band structure reminiscent of free-standing single-layer WS2.

cond-mat.mtrl-sci↗

arXiv2018

Growth and Structure of Singly-Oriented Single-Layer Tungsten Disulfide on Au(111)

We present a complete characterisation at the nanoscale of the growth and structure of single-layer tungsten disulfide (WS$_2$) epitaxially grown on Au(111). Following the growth process in real time with fast x-ray photoelectron spectroscopy, we obtain a singly-oriented layer by choosing the proper W evaporation rate and substrate temperature during the growth. Information about the morphology, size and layer stacking of the WS$_2$ layer were achieved by employing x-ray photoelectron diffraction and low-energy electron microscopy. The strong spin splitting in the valence band of WS$_2$ coupled with the single-orientation character of the layer make this material the ideal candidate for the exploitation of the spin and valley degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2018

Epitaxial Growth of Single-Orientation High-Quality MoS$_2$ Monolayers

We present a study on the growth and characterization of high-quality single-layer MoS$_2$ with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS$_2$ layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements. Spin- and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials.

cond-mat.mtrl-sci↗

arXiv2017

Electronic Structure of Fe$_{1.08}$Te bulk crystals and epitaxial FeTe thin films on Bi$_2$Te$_3$

The electronic structure of thin films of FeTe grown on Bi$_2$Te$_3$ is investigated using angle-resolved photoemission spectroscopy, scanning tunneling microscopy and first principles calculations. As a comparison, data from cleaved bulk \FeTe taken under the same experimental conditions is also presented. Due to the substrate and thin film symmetry, FeTe thin films grow on Bi$_2$Te$_3$ in three domains, rotated by 0$^{\circ}$, 120$^{\circ}$, and 240$^{\circ}$. This results in a superposition of photoemission intensity from the domains, complicating the analysis. However, by combining bulk and thin film data, it is possible to partly disentangle the contributions from three domains. We find a close similarity between thin film and bulk electronic structure and an overall good agreement with first principles calculations, assuming a p-doping shift of 65~meV for the bulk and a renormalization factor of around 2. By tracking the change of substrate electronic structure upon film growth, we find indications of an electron transfer from the FeTe film to the substrate. No significant change of the film's electronic structure or doping is observed when alkali atoms are dosed onto the surface. This is ascribed to the film's high density of states at the Fermi energy. This behavior is also supported by the ab-initio calculations.

cond-mat.str-el↗

arXiv2017

Contact-Induced Semiconductor-to-Metal Transition in Single-Layer WS$_2$

Low-resistance ohmic contacts are a challenge for electronic devices based on two-dimensional materials. We show that an atomically precise junction between a two-dimensional semiconductor and a metallic contact can lead to a semiconductor-to-metal transition in the two-dimensional material--a finding which points the way to a possible method of achieving low-resistance junctions. Specifically, single-layer WS$_2$ undergoes a semiconductor-to-metal transition when epitaxially grown on Ag(111), while it remains a direct band gap semiconductor on Au(111). The metallicity of the single layer on Ag(111) is established by lineshape analysis of core level photoemission spectra. Angle-resolved photoemission spectroscopy locates the metallic states near the Q point of the WS$_2$ Brillouin zone. Density functional theory calculations show that the metallic states arise from hybridization between Ag bulk bands and the local conduction band minimum of WS$_2$ near the Q point.

cond-mat.mtrl-sci↗

arXiv2016

Reconstruction-induced trefoil knot Fermi contour of Au(111)

Using angle-resolved photoemission spectroscopy (ARPES), we study the effect of the so-called herringbone reconstruction of Au(111) on the dispersion of the free electron-like surface state. While earlier ARPES investigations have only reported a minor interplay of the surface state dispersion and the underlying reconstruction, we show that the uniaxial lattice distortion and the thereby changed reciprocal lattice for the first atomic layer leads to distinct surface state dispersions around the first order reciprocal lattice points of the three domains, creating a constant energy surface resembling a trefoil knot. The findings resolve the long-standing discrepancy between, on one hand, the reconstruction-induced surface state modifications reported in scanning tunnelling microscopy and first principle calculations and, on the other hand, their conspicuous absence in photoemission.

cond-mat.mes-hall↗

arXiv2016

Absence of superconductivity in ultra-thin layers of FeSe synthesized on a topological insulator

The structural and electronic properties of FeSe ultra-thin layers on Bi$_{2}$Se$_{3}$ have been investigated with a combination of scanning tunneling microscopy and spectroscopy and angle-resolved photoemission spectroscopy. The FeSe multi-layers, which are predominantly 3-5 monolayers (ML) thick, exhibit a hole pocket-like electron band at \barΓ and a dumbbell-like feature at \bar{M}, similar to multi-layers of FeSe on SrTiO$_{3}$. Moreover, the topological state of the Bi2Se3 is preserved beneath the FeSe layer, as indicated by a heavily \it{n}-doped Dirac cone. Low temperature STS does not exhibit a superconducting gap for any investigated thickness down to a temperature of 5 K.

cond-mat.supr-con↗

arXiv2016

Band gap engineering by Bi intercalation of graphene on Ir(111)

We report on the structural and electronic properties of a single bismuth layer intercalated underneath a graphene layer grown on an Ir(111) single crystal. Scanning tunneling microscopy (STM) reveals a hexagonal surface structure and a dislocation network upon Bi intercalation, which we attribute to a $\sqrt{3}\times\sqrt{3}R30°$ Bi structure on the underlying Ir(111) surface. Ab-initio calculations show that this Bi structure is the most energetically favorable, and also illustrate that STM measurements are most sensitive to C atoms in close proximity to intercalated Bi atoms. Additionally, Bi intercalation induces a band gap ($E_g=0.42\,$eV) at the Dirac point of graphene and an overall n-doping ($\sim 0.39\,$eV), as seen in angular-resolved photoemission spectroscopy. We attribute the emergence of the band gap to the dislocation network which forms favorably along certain parts of the moiré structure induced by the graphene/Ir(111) interface.

cond-mat.mtrl-sci↗

arXiv2015

Growth and electronic structure of epitaxial single-layer WS$_2$ on Au(111)

Large-area single-layer WS$_2$ is grown epitaxially on Au(111) using evaporation of W atoms in a low pressure H$_2$S atmosphere. It is characterized by means of scanning tunneling microscopy, low-energy electron diffraction and core-level spectroscopy. Its electronic band structure is determined by angle-resolved photoemission spectroscopy. The valence band maximum at $\bar{K}$ is found to be significantly higher than at $\barΓ$. The observed dispersion around $\bar{K}$ is in good agreement with density functional theory calculations for a free-standing monolayer, whereas the bands at $\barΓ$ are found to be hybridized with states originating from the Au substrate. Strong spin-orbit coupling leads to a large spin-splitting of the bands in the neighborhood of the $\bar{K}$ points, with a maximum splitting of 419(11)~meV. The valence band dispersion around $\bar{K}$ is found to be highly anisotropic with spin-branch dependent effective hole masses of $0.40(02)m_e$ and $0.57(09)m_e$ for the upper and lower split valence band, respectively. The large size of the spin-splitting and the low effective mass of the valence band maximum make single-layer WS$_2$ a promising alternative to the widely studied MoS$_2$ for applications in electronics, spintronics and valleytronics.

cond-mat.str-el↗

arXiv2014

Strongly anisotropic spin-orbit splitting in a two-dimensional electron gas

Near-surface two-dimensional electron gases on the topological insulator Bi$_2$Te$_2$Se are induced by electron doping and studied by angle-resolved photoemission spectroscopy. A pronounced spin-orbit splitting is observed for these states. The $k$-dependent splitting is strongly anisotropic to a degree where a large splitting ($\approx 0.06$ Å$^{-1}$) can be found in the $\barΓ\bar{M}$ direction while the states are hardly split along $\barΓ\bar{K}$. The direction of the anisotropy is found to be qualitatively inconsistent with results expected for a third-order anisotropic Rashba Hamiltonian. However, a $\mathbf{k} \cdot \mathbf{p}$ model that includes the possibility of band structure anisotropy as well as both isotropic and anisotropic third order Rashba splitting can explain the results. The isotropic third order contribution to the Rashba Hamiltonian is found to be negative, reducing the energy splitting at high $k$. The interplay of band structure, higher order Rashba effect and tuneable doping offers the opportunity to engineer not only the size of the spin-orbit splitting but also its direction.

cond-mat.str-el↗

arXiv2014

Bulk Band Structure of Bi$_2$Te$_3$

The bulk band structure of Bi$_2$Te$_3$ has been determined by angle-resolved photoemission spectroscopy and compared to first-principles calculations. We have performed calculations using the local density approximation (LDA) of density functional theory and the one-shot $GW$ approximation within the all-electron full-potential linearized augmented-plane-wave (FLAPW) formalism, fully taking into account spin-orbit coupling. Quasiparticle effects produce significant changes in the band structure of \bite~when compared to LDA. Experimental and calculated results are compared in the spectral regions where distinct differences between the LDA and $GW$ results are present. Overall a superior agreement with $GW$ is found, highlighting the importance of many-body effects in the band structure of this family of topological insulators.

cond-mat.mtrl-sci↗