arXiv subjects

Céline Barreteau

Publications and source records attributed to Céline Barreteau.

11 recordsLinked to original sources

arXiv2024

Doping Tunable CDW Phase Transition in Bulk 1T-ZrSe$_2$

Tuneable electronic properties in transition metal dichalcogenides (TMDs) are essential to further their use in device applications. Here, we present a comprehensive scanning tunnelling microscopy and spectroscopy study of a doping-induced charge density wave (CDW) in semiconducting bulk 1T-ZrSe$_2$. We find that atomic impurities which locally shift the Fermi level ($E_F$) into the conduction band trigger a CDW reconstruction concomitantly to the opening of a gap at $E_F$. Our findings shed new light on earlier photoemission spectroscopy and theoretical studies of bulk 1T-ZrSe$_2$, and provide a local understanding of the electron-doping mediated CDW transition observed in semiconducting TMDs.

cond-mat.str-el↗

arXiv2023

Large saturation moment and high ferromagnetic transition temperature in a structurally disordered inverse Heusler alloy Fe2RuGe

We report the successful synthesis of a new 4$d$ based polycrystalline inverse Heusler alloy Fe$_2$RuGe by an arc melting process and have studied in detail its structural, magnetic and transport properties complemented with first principle calculations. X-ray and neutron diffraction, Extended X-ray Absorption Fine Structure and $^{57}$Fe Mössbauer spectroscopic studies confirm the single phase nature of the system where the Fe and Ru atoms are randomly distributed in the 4$c$ and 4$d$ Wyckoff positions in a ratio close to 50:50. The formation of the disordered structure is also confirmed by the theoretical energy minimization calculation. Despite the random cross-site disorder of Fe and Ru atoms, magnetic measurements suggest not only a high Curie temperature of $\sim$860\,K, but also a large saturation magnetic moment $\sim$4.9\,$μ_B$ per formula unit at 5\,K, considerably exceeding the theoretical limit (4\,$μ_B$ per formula unit) predicted by the Slater-Pauling rule. Only a few Fe-based inverse Heusler alloys are known to exhibit such high Curie temperatures. Neutron diffraction analysis coupled with the isothermal magnetization value indicates that the magnetic moments in Fe$_2$RuGe are associated with Fe-atoms only, which is also confirmed by Mössbauer spectrometry. Interestingly, in comparison to the cubic or hexagonal phase of the parent compound, Fe$_3$Ge, the Curie temperature of Fe$_2$RuGe has increased significantly despite the substitution of the nonmagnetic, yet isoelectronic element Ru in this structurally disordered compound. Our theoretical calculation reveals that the large Fe moment ($\sim2.8μ_B$/Fe) on the 4$b$ site can be attributed to a charge transfer from this Fe site towards its Ru neighbours. Such a substantial increase in magnetic moment due to electron charge transfer has not previously been reported in a Heusler alloy system.

cond-mat.mtrl-sci↗

arXiv2021

Multiband charge density wave exposed in a transition metal dichalcogenide

In the presence of multiple bands, well-known electronic instabilities may acquire new complexity. While multiband superconductivity is the subject of extensive studies, the possibility of multiband charge density waves (CDWs) has been largely ignored so far. Here, combining energy dependent scanning tunnelling microscopy (STM) topography with a simple model of the charge modulations and a self-consistent calculation of the CDW gap, we find evidence for a multiband CDW in 2H-NbSe$_2$. This CDW not only involves the opening of a gap on the inner band around the K-point, but also on the outer band. This leads to spatially out-of-phase charge modulations from electrons on these two bands, which we detect through a characteristic energy dependence of the CDW contrast in STM images.

cond-mat.str-el↗

arXiv2020

Synthetic Semimetals with van der Waals Interfaces

The assembly of suitably designed van der Waals (vdW) heterostructures represents a new approach to produce artificial systems with engineered electronic properties. Here, we apply this strategy to realize synthetic semimetals based on vdW interfaces formed by two different semiconductors. Guided by existing ab-initio calculations, we select WSe$_2$ and SnSe$_2$ mono and multilayers to assemble vdW interfaces, and demonstrate the occurrence of semimetallicity by means of different transport experiments. Semimetallicity manifests itself in a finite minimum conductance upon sweeping the gate over a large range in ionic liquid gated devices, which also offer spectroscopic capabilities enabling the quantitative determination of the band overlap. The semimetallic state is additionally revealed in Hall effect measurements by the coexistence of electrons and holes, observed by either looking at the evolution of the Hall slope with sweeping the gate voltage or with lowering temperature. Finally, semimetallicity results in the low-temperature metallic conductivity of interfaces of two materials that are themselves insulating. These results demonstrate the possibility to implement a state of matter that had not yet been realized in vdW interfaces, and represent a first step towards using these interfaces to engineer topological or excitonic insulating states.

cond-mat.mes-hall↗

arXiv2018

Microfocus laser-ARPES on encapsulated mono-, bi-, and few-layer 1T'-WTe$_2$

Two-dimensional crystals of semimetallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few layer 1T'-WTe$_2$ and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiment about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T'-WTe$_2$ by laser-based micro-focus angle resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe$_2$ comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T'-WTe$_2$ and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.

cond-mat.mtrl-sci↗

arXiv2018

Holographic imaging of the complex charge density wave order parameter

The charge density wave (CDW) in solids is a collective ground state combining lattice distortions and charge ordering. It is defined by a complex order parameter with an amplitude and a phase. The amplitude and wavelength of the charge modulation are readily accessible to experiment. However, accurate measurements of the corresponding phase are significantly more challenging. Here we combine reciprocal and real space information to map the full complex order parameter based on topographic scanning tunneling microscopy (STM) images. Our technique overcomes limitations of earlier Fourier space based techniques to achieve distinct amplitude and phase images with high spatial resolution. Applying this analysis to transition metal dichalcogenides provides striking evidence that their CDWs consist of three individual charge modulations whose ordering vectors are connected by the fundamental rotational symmetry of the crystalline lattice. Spatial variations in the relative phases of these three modulations account for the different contrasts often observed in STM topographic images. Phase images further reveal topological defects and discommensurations, a singularity predicted by theory for a nearly commensurate CDW. Such precise real space mapping of the complex order parameter provides a powerful tool for a deeper understanding of the CDW ground state whose formation mechanisms remain largely unclear.

cond-mat.str-el↗

arXiv2017

Dimensional cross-over of the charge density wave order parameter in thin exfoliated 1T-VSe$_2$

The capability to isolate one to few unit-cell thin layers from the bulk matrix of layered compounds opens fascinating prospects to engineer novel electronic phases. However, a comprehensive study of the thickness dependence and of potential extrinsic effects are paramount to harness the electronic properties of such atomic foils. One striking example is the charge density wave (CDW) transition temperature in layered dichalcogenides whose thickness dependence remains unclear in the ultrathin limit. Here we present a detailed study of the thickness and temperature dependences of the CDW in VSe$_2$ by scanning tunnelling microscopy (STM). We show that mapping the real-space CDW periodicity over a broad thickness range unique to STM provides essential insight. We introduce a robust derivation of the local order parameter and transition temperature based on the real space charge modulation amplitude. Both quantities exhibit a striking non-monotonic thickness dependence that we explain in terms of a 3D to 2D dimensional crossover in the FS topology. This finding highlights thickness as a true tuning parameter of the electronic ground state and reconciles seemingly contradicting thickness dependencies determined in independent transport studies.

cond-mat.str-el↗

arXiv2016

Direct Observation of Long-range field-effect from gate tuning of non-local conductivity

We report the observation of an unexpected, long-range field-effect in WTe$_2$ devices, leading to large gate-induced changes of the transport properties of crystals much thicker than the electrostatic screening length. The phenomenon --which manifests itself very differently from the conventional field-effect-- originates from the non-local nature of transport in the devices that are thinner than the carrier mean free path, because of the dominant role of surface scattering. We reproduce theoretically the gate dependence of the measured classical and quantum magneto-transport in all detail, and show that the phenomenon is caused by the gate-tuning of the bulk carrier mobility by changing the scattering at the surface. Our results demonstrate the possibility to gate tune the electronic properties deep in the interior of conducting materials, avoiding limitations imposed by electrostatic screening.

cond-mat.mes-hall↗

arXiv2016

High-pressure melt growth and transport properties of SiP, SiAs, GeP, and GeAs 2D layered semiconductors

Silicon and Germanium monopnictides SiP, SiAs, GeP and GeAs form a family of 2D layered semiconductors. We have succeeded in growing bulk single crystals of these compounds by melt-growth under high pressure (0.5-1 GPa) in a cubic anvil hot press. Large (mm-size), shiny, micaceous crystals of GeP, GeAs and SiAs were obtained, and could be exfoliated into 2D flakes. Small and brittle crystals of SiP were yielded by this method. High-pressure sintered polycrystalline SiP and GeAs have also been successfully used as a precursor in the Chemical Vapor Transport growth of these crystals in the presence of I$_{2}$ as a transport agent. All compounds are found to crystallize in the expected layered structure and do not undergo any structural transition at low temperature, as shown by Raman spectroscopy down to T=5K. All materials exhibit a semiconducting behavior. The electrical resistivity of GeP, GeAs and SiAs is found to depend on temperature following a 2D-Variable Range Hopping conduction mechanism. The availability of bulk crystals of these compounds opens new perspectives in the field of 2D semiconducting materials for device applications.

cond-mat.mtrl-sci↗

arXiv2015

Tuning Magnetotransport in a Compensated Semimetal at the Atomic Scale

Either in bulk form, or when exfoliated into atomically thin crystals, layered transition metal dichalcogenides are continuously leading to the discovery of new phenomena. The latest example is provided by 1T'-WTe$_2$, a semimetal recently found to exhibit the largest known magnetoresistance in bulk crystals, and predicted to become a two-dimensional topological insulator in strained monolayers. Here, we show that reducing the thickness through facile exfoliation provides an effective experimental knob to tune the electronic properties of WTe$_2$, which allows us to identify the microscopic mechanisms responsible for the observed classical and quantum magnetotransport down to the ultimate atomic scale. We find that the longitudinal resistance and the very unconventional B-dependence of the Hall resistance are reproduced quantitatively in terms of a classical two-band model for crystals as thin as six monolayers, and that for thinner crystals a crossover to an insulating, Anderson-localized state occurs. Besides establishing the origin of the very large magnetoresistance of bulk WTe$_2$, our results represent the first, complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe$_2$ layers remain gapless semimetals, from which we conclude that searching for a topological insulating state by straining monolayers is a challenging, but feasible experiment.

cond-mat.mes-hall↗

arXiv2015

Indirect-to-direct band-gap crossover in few-layer MoTe$_2$

We study the evolution of the band-gap structure in few-layer MoTe$_2$ crystals, by means of low-temperature micro-reflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that, in complete analogy with other semiconducting transition metal dichalchogenides (TMDs), the dominant PL emission peaks originate from direct transitions associated to recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe$_2$ behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer and it starts decreasing for trilayers. A quantitative analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe$_2$ being direct band-gap semiconductors, with tetralayer MoTe$_2$ being an indirect gap semiconductor, and with trilayers having nearly identical direct and indirect gaps.This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides, for which only monolayers are found to be direct band-gap semiconductors, with thicker layers having indirect band gaps that are significantly smaller, by hundreds of meV, than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications.

cond-mat.mes-hall↗