arXiv subjects

T. Jaouen

Publications and source records attributed to T. Jaouen.

At least 19 recordsLinked to original sources

arXiv2024

Photoelectron diffraction of twisted bilayer graphene

Photoelectron diffraction (PED) is a powerful spectroscopic technique that combines elemental resolution with a high sensitivity to the local atomic arrangement at crystal surfaces, thus providing unique fingerprints of selected atomic sites in matter. Stimulated by the rapid innovation in the development of various analysis methods for probing the atomic and electronic structures of van der Waals (vdW) heterostructures of two-dimensional materials, we present a theoretical assessment of the capacity of PED for extracting structural properties such as stacking, twist angles and interlayer distances. We provide a complete description of PED for the benchmark vdW heterostructure bilayer graphene (BLG), by calculating and analyzing the PED of BLG in Bernal and AA-stacking as well as twisted BLG for a wide range of the twist angle.

cond-mat.mtrl-sci↗

arXiv2023

Carrier-Density Control of the Quantum-Confined 1$T$-TiSe$_2$ Charge-Density-Wave

Using angle-resolved photoemission spectroscopy, combined with first principle and coupled self-consistent Poisson-Schrödinger calculations, we demonstrate that potassium (K) atoms adsorbed on the low-temperature phase of 1$T$-TiSe$_2$ induce the creation of a two-dimensional electron gas (2DEG) and quantum confinement of its charge-density-wave (CDW) at the surface. By further changing the K coverage, we tune the carrier-density within the 2DEG that allows us to nullify, at the surface, the electronic energy gain due to exciton condensation in the CDW phase while preserving a long-range structural order. Our study constitutes a prime example of a controlled exciton-related many-body quantum state in reduced dimensionality by alkali-metal dosing.

cond-mat.mtrl-sci↗

arXiv2021

Insensitivity of the striped charge-orders in IrTe$_2$ to alkali surface doping implies their structural origin

We present a combined angle-resolved photoemission spectroscopy and low-energy electron diffraction (LEED) study of the prominent transition metal dichalcogenide IrTe$_2$ upon potassium (K) deposition on its surface. Pristine IrTe$_2$ undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Supported by density functional theory calculations, we first show that the K atoms dope the topmost IrTe$_2$ layer with electrons, therefore strongly decreasing the work function and shifting only the electronic surface states towards higher binding energy. We then follow the evolution of its electronic structure as a function of temperature across the charge-ordered phase transitions and observe that their critical temperatures are unchanged for K coverages of $0.13$ and $0.21$~monolayer (ML). Using LEED, we also confirm that the periodicity of the related stripe phases is unaffected by the K doping. We surmise that the charge-ordered phase transitions of IrTe$_2$ are robust against electron surface doping, because of its metallic nature at all temperatures, and due to the importance of structural effects in stabilizing charge order in IrTe$_2$.

cond-mat.str-el↗

arXiv2021

Ultrafast Dynamics of the Surface Photovoltage in Potassium Doped Black Phosphorus

Black phosphorus is a quasi-two-dimensional layered semiconductor with a narrow direct band gap of 0.3 eV. A giant surface Stark effect can be produced by the potassium doping of black phosphorus, leading to a semiconductor to semimetal phase transition originating from the creation of a strong surface dipole and associated band bending. By using time- and angle-resolved photoemission spectroscopy, we report the partial photoinduced screening of this band bending by the creation of a compensating surface photovoltage. We further resolve the detailed dynamics of this effect at the pertinent timescales and the related evolution of the band structure near the Fermi level. We demonstrate that after a fast rise time, the surface photovoltage exhibits a plateau over a few tens of picoseconds before decaying on the nanosecond timescale. We support our experimental results with simulations based on drift-diffusion equations.

cond-mat.mtrl-sci↗

arXiv2021

Observation of a uniaxial strain-induced phase transition in the 2D topological semimetal IrTe$_2$

Strain is ubiquitous in solid-state materials, but despite its fundamental importance and technological relevance, leveraging externally applied strain to gain control over material properties is still in its infancy. In particular, strain control over the diverse phase transitions and topological states in two-dimensional (2D) transition metal dichalcogenides (TMDs) remains an open challenge. Here, we exploit uniaxial strain to stabilize the long-debated structural ground state of the 2D topological semimetal IrTe$_2$, which is hidden in unstrained samples. Combined angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) data reveal the strain-stabilized phase has a 6x1 periodicity and undergoes a Lifshitz transition, granting unprecedented spectroscopic access to previously inaccessible type-II topological Dirac states that dominate the modified inter-layer hopping. Supported by density functional theory (DFT) calculations, we show that strain induces a charge transfer strongly weakening the inter-layer Te bonds and thus reshaping the energetic landscape of the system in favor of the 6x1 phase. Our results highlight the potential to exploit strain-engineered properties in layered materials, particularly in the context of tuning inter-layer behavior.

cond-mat.str-el↗

arXiv2020

Nature of native atomic defects in ZrTe$_5$ and their impact on the low-energy electronic structure

Over the past decades, investigations of the anomalous low-energy electronic properties of ZrTe$_5$ have reached a wide array of conclusions. An open question is the growth method's impact on the stoichiometry of ZrTe$_5$ samples, especially given the very small density of states near its chemical potential. Here we report on high resolution scanning tunneling microscopy and spectroscopy measurements performed on samples grown via different methods. Using density functional theory calculations, we identify the most prevalent types of atomic defects on the surface of ZrTe$_5$, namely Te vacancies and intercalated Zr atoms. Finally, we precisely quantify their density and outline their role as ionized defects in the anomalous resistivity of this material.

cond-mat.mtrl-sci↗

arXiv2020

Unveiling the complete dispersion of the giant Rashba split surface states of ferroelectric $α$-GeTe(111) by alkali doping

$α$-GeTe(111) is a non-centrosymmetric ferroelectric material, for which a strong spin-orbit interaction gives rise to giant Rashba split states in the bulk and at the surface. The detailed dispersions of the surface states inside the bulk band gap remains an open question because they are located in the unoccupied part of the electronic structure, making them inaccessible to static angle-resolved photoemission spectroscopy. We show that this difficulty can be overcome via in-situ potassium doping of the surface, leading to a rigid shift of 80 meV of the surface states into the occupied states. Thus, we resolve in great detail their dispersion and highlight their crossing at the $\barΓ$ point, which, in comparison with density functional theory calculations, definitively confirms the Rashba mechanism.

cond-mat.mtrl-sci↗

arXiv2020

Examining the surface phase diagram of IrTe$_2$ with photoemission

In the transition metal dichalcogenide IrTe$_2$, low-temperature charge-ordered phase transitions involving Ir dimers lead to the occurrence of stripe phases of different periodicities, and nearly degenerate energies. Bulk-sensitive measurements have shown that, upon cooling, IrTe$_2$ undergoes two such first-order transitions to $(5\times1\times5)$ and $(8\times1\times8)$ reconstructed phases at $T_{c_1}\sim 280$~K and $T_{c_2}\sim 180$~K, respectively. Here, using surface sensitive probes of the electronic structure of IrTe$_2$, we reveal the first-order phase transition at $T_{c_3}=165$~K to the $(6\times1)$ stripes phase, previously proposed to be the surface ground state. This is achieved by combining x-ray photoemission spectroscopy and angle-resolved photoemission spectroscopy, which give access to the evolution of stripe domains and a particular surface state, the energy of which is dependent on the Ir dimer length. By performing measurements over a full thermal cycle, we also report the complete hysteresis of all these phases.

cond-mat.str-el↗

arXiv2019

Unveiling the Semimetallic Nature of 1$T$-TiSe$_2$ by Doping its Charge Density Wave

The semimetallic or semiconducting nature of the transition metal dichalcogenide 1$T$-TiSe$_2$ remains under debate after many decades mainly due to the fluctuating nature of its 2 $\times$ 2 $\times$ 2 charge-density-wave (CDW) phase at room-temperature. In this letter, using angle-resolved photoemission spectroscopy, we unambiguously demonstrate that the 1$T$-TiSe$_2$ normal state is semimetallic with an electron-hole band overlap of $\sim$110 meV by probing the low-energy electronic states of the perturbed CDW phase strongly doped by alkali atoms. Our study not only closes a long-standing debate but also supports the central role of the Fermi surface for driving the CDW and superconducting instabilities in 1$T$-TiSe$_2$.

cond-mat.str-el↗

arXiv2019

Phase Separation in the Vicinity of Fermi Surface Hot Spots

Spatially inhomogeneous electronic states are expected to be key ingredients for the emergence of superconducting phases in quantum materials hosting charge-density-waves (CDWs). Prototypical materials are transition-metal dichalcogenides (TMDCs) and among them, 1$T$-TiSe$_2$ exhibiting intertwined CDW and superconducting states under Cu intercalation, pressure or electrical gating. Although it has been recently proposed that the emergence of superconductivity relates to CDW fluctuations and the development of spatial inhomogeneities in the CDW order, the fundamental mechanism underlying such a phase separation (PS) is still missing. Using angle-resolved photoemission spectroscopy and variable-temperature scanning tunneling microscopy, we report on the phase diagram of the CDW in 1$T$-TiSe$_2$ as a function of Ti self-doping, an overlooked degree of freedom inducing CDW texturing. We find an intrinsic tendency towards electronic PS in the vicinity of Fermi surface (FS) "hot spots", i.e. locations with band crossings close to, but not at the Fermi level. We therefore demonstrate an intimate relationship between the FS topology and the emergence of spatially textured electronic phases which is expected to be generalizable to many doped CDW compounds.

cond-mat.mtrl-sci↗

arXiv2018

Semimetal-to-semiconductor transition and charge-density-wave melting in $1T$-TiSe$_{2-x}$S$_x$ single crystals

The transition metal dichalcogenide $1T$-TiSe$_2$ is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge density wave (CDW) at a critical temperature T$_{c}\approx 200$K. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the non-reconstructed Fermi surface topology, remains elusive. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we investigate $1T$-TiSe$_{2-x}$S$_x$ single crystals. Using STM, we first show that the long-range phase coherent CDW state is stable against S substitutions with concentrations at least up to $x=0.34$. The ARPES measurements then reveal a slow but continuous decrease of the overlap between the electron and hole ($e$-$h$) bands of the semimetallic normal-state well reproduced by DFT and related to slight reductions of both the CDW order parameter and $T_c$. Our DFT calculations further predict a semimetal-to-semiconductor transition of the normal state at a higher critical S concentration of $x_c$=0.9 $\pm$0.1, that coincides with a melted CDW state in TiSeS as measured with STM. Finally, we rationalize the $x$-dependence of the $e$-$h$ band overlap in terms of isovalent substitution-induced competing chemical pressure and charge localization effects. Our study highlights the key role of the $e$-$h$ band overlap for the CDW instability.

cond-mat.mtrl-sci↗

arXiv2018

Local Real-Space View of the Achiral 1$T$-TiSe$_2$ 2 $\times$ 2 $\times$ 2 Charge Density Wave

The transition metal dichalcogenide 1$T$-TiSe$_2$ is a quasi-two-dimensional layered material undergoing a commensurate 2 $\times$ 2 $\times$ 2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below $\approx$ 200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows to go beyond the usual spatial resolution of STM and to intimately probe the three-dimensional character of the PLD. Furthermore, the inversion-symmetric, achiral nature of the CDW in the $z$-direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral order. This study paves the way to a simultaneous real-space probing of both charge and structural reconstructions in CDW compounds.

cond-mat.mtrl-sci↗

arXiv2018

Robustness of the charge-ordered phases in IrTe$_2$ against photoexcitation

We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe$_2$, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral function occuring within a few hundreds of femtoseconds and persisting up to several picoseconds, which we interpret as a partial photoinduced phase transition (PIPT). The necessary time for photoinducing these spectral changes increases with increasing photoexcitation density and reaches timescales longer than the rise time of the transient electronic temperature. We conclude that the PIPT is driven by a transient increase of the lattice temperature following the energy transfer from the electrons. However, the photoinduced changes of the spectral function are small, which indicates that the low temperature phase is particularly robust against photoexcitation. We suggest that the system might be trapped in an out-of-equilibrium state, for which only a partial structural transition is achieved.

cond-mat.str-el↗

arXiv2017

Selective probing of hidden spin-polarized states in inversion-symmetric bulk MoS2

Spin- and angle-resolved photoemission spectroscopy is used to reveal that a large spin polarization is observable in the bulk centrosymmetric transition metal dichalcogenide MoS2. It is found that the measured spin polarization can be reversed by changing the handedness of incident circularly-polarized light. Calculations based on a three-step model of photoemission show that the valley and layer-locked spin-polarized electronic states can be selectively addressed by circularly-polarized light, therefore providing a novel route to probe these hidden spin-polarized states in inversion-symmetric systems as predicted by Zhang et al. [Nature Physics 10, 387 (2014)].

cond-mat.mtrl-sci↗

arXiv2016

Local resilience of the $1T$-TiSe$_2$ charge density wave to Ti self-doping

In Ti-intercalated self-doped $1T$-TiSe$_2$ crystals, the charge density wave (CDW) superstructure induces two nonequivalent sites for Ti dopants. Recently, it has been shown that increasing Ti doping dramatically influences the CDW by breaking it into phase-shifted domains. Here, we report scanning tunneling microscopy and spectroscopy experiments that reveal a dopant-site dependence of the CDW gap. Supported by density functional theory, we demonstrate that the loss of the longrange phase coherence introduces an imbalance in the intercalated-Ti site distribution and restrains the CDW gap closure. This local resilient behavior of the $1T$-TiSe$_2$ CDW reveals a novel mechanism between CDW and defects in mutual influence.

cond-mat.mtrl-sci↗

arXiv2016

Short-range phase coherence and origin of the 1T-TiSe2 charge density wave

The impact of variable Ti self-doping on the 1T-TiSe2 charge density wave (CDW) is studied by scanning tunneling microscopy. Supported by density functional theory we show that agglomeration of intercalated-Ti atoms acts as preferential nucleation centers for the CDW that breaks up in phaseshifted CDW domains whose size directly depends on the intercalated-Ti concentration and which are separated by atomically-sharp phase boundaries. The close relationship between the diminution of the CDW domain size and the disappearance of the anomalous peak in the temperature dependent resistivity allows to draw a coherent picture of the 1T-TiSe2 CDW phase transition and its relation to excitons.

cond-mat.mtrl-sci↗

arXiv2015

Excited States at Interfaces of a Metal-Supported Ultrathin Oxide Film

We report layer-resolved measurements of the \textit{unoccupied} electronic structure of ultrathin MgO films grown on Ag(001). The metal-induced gap states at the metal/oxide interface, the oxide band gap as well as a surface core exciton involving an image-potential state of the vacuum are revealed through resonant Auger spectroscopy of the Mg $KL_{23}L_{23}$ Auger transition. Our results demonstrate how to obtain new insights on \textit{empty} states at interfaces of metal-supported ultrathin oxide films.

cond-mat.mtrl-sci↗

arXiv2014

Induced Work Function Changes at Mg-doped MgO/Ag(001) Interfaces: a Combined Auger Electron Diffraction and Density Functional Study

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL_23 L_23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atoms incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2- ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.

cond-mat.mtrl-sci↗