Celine Barreteau

Institut de Chimie et des Matériaux de Paris Est · Chimie Metallurgique des Terres Rares CMTR

In this paper, we report the successful synthesis of a Fe-based highly spin-polarized quaternary Heusler alloy FeMnVGa and its structural, magnetic, and transport properties probed through different experimental methods and theoretical techniques. Density functional theory (DFT) calculations performed on different types of structures reveal that th...

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 M\"{o}ssb...

The pursuit of efficient spin-polarization in quaternary Heusler alloys with the general formula $XX'YZ$ (where X, $X'$, and Y are transition metals and Z is a p-block element), has been a subject of significant scientific interest. While previous studies shows that isoelectronic substitution of 4d element in place of 3d element in quaternary Heusl...

In this work, we report the successful synthesis of a Fe-based novel half-metallic quaternary Heusler alloy FeMnVGa and its structural, magnetic and transport properties probed through different experimental methods and theoretical technique. Density functional theory (DFT) calculations performed on different types of structure reveal that Type-2 o...

Half-metallic ferromagnets (HMF) are on one of the most promising materials in the field of spintronics due to their unique band structure consisting of one spin sub-band having metallic characteristics along with another sub-band with semiconductor-like behavior. In this work, we report the synthesis of a novel quaternary Heusler alloy FeMnVAl and...

Half-metallic ferromagnets (HMFs) are one of the most promising materials in the field of spintronics due to their unique band structure, consisting of one spin subband having metallic characteristics along with another subband with semiconductor-like behavior. In this work, we report the synthesis of a novel quaternary Heusler alloy—FeMnVAl—and we...

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 si...

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 si...

Chalcogenide semiconductors and semimetals are a fertile class of efficient thermoelectric materials, which, in most cases, exhibit very low lattice thermal conductivity κ ph despite lacking a complex crystal structure such as the tetragonal binary compound InTe. Our measurements of κ ph (T) in single-crystalline InTe along the c axis show that κ p...

High-throughput calculations can be applied to a large number of compounds, in order to discover new useful materials. In the present work, ternary intermetallic compounds are investigated, to find new potentially interesting materials for thermoelectric applications. The screening of stable nonmetallic compounds required for such applications is p...

Optical selection rules in monolayers of transition metal dichalcogenides and of their heterostructures are determined by the conservation of the z-component of the total angular momentum - J Z = L Z +S Z - associated with the C3 rotational lattice symmetry which assumes half integer values corresponding, modulo 3, to distinct states. Here we show,...

Identifying quantum numbers to label elementary excitations is essential for the correct description of light-matter interaction in solids. In monolayer semiconducting transition metal dichalcogenides (TMDs) such as MoSe$_2$ or WSe$_2$, most optoelectronic phenomena are described well by labelling electron and hole states with the spin projection a...

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...

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 WSe2 a...

High-throughput calculations are a very promising tool for screening a large number of compounds in order to discover new useful materials. Ternary intermetallic are thus investigated in the present work to find new compounds potentially interesting for thermoelectric applications. The screening of the stable non-metallic compounds required for suc...

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 si...

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...

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, 1T−TiSe2 exhibiting intertwined CDW and superconducting states under Cu intercalation,...

The BiCuSeO-based compound with very low thermal conductivity are among the most promising thermoelectric materials recently discovered. Their lattice dynamics, which remains mostly unexplored experimentally, is investigated in the present paper. We report Raman experiments on BiCuSe1-xSxO solid solutions and infrared experiments on BiCuSeO and BiC...

Spatially inhomogeneous electronic states are expected to be key ingredients for the emergence of superconducting phases in quantum materials hosting charge-density-waves (CDWs). Prototypi-cal materials are transition-metal dichalcogenides (TMDCs) and among them, 1T-TiSe2 exhibiting intertwined CDW and superconducting states under Cu intercalation,...

The transition-metal dichalcogenide 1T−TiSe2 is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge-density wave (CDW) at a critical temperature Tc≈200K. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the nonrecons...

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 kno...

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., w...

In quantum materials, doping plays a central role in determining the electronic ground state among competing phases such as charge order and superconductivity. In addition, the real-space distribution of dopants has been found to induce quenched disorder and lead to phase separation (PS). However, an open question remains as to the extent to which...

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 kno...

Within 4 different crystal structures, 2280 ternary intermetallic configurations have been investigated via high-throughput density functional theory calculations in order to discover new semiconducting materials. The screening is restricted to intermetallics with the equimolar composition TMX, where T is a transition metal from the Ti, V, Cr colum...

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 si...

The transition metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material undergoing a commensurate 2 x 2 x 2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ~ 200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows to go beyond the usual...

Isolating single unit-cell thin layers from the bulk matrix of layered compounds offers tremendous opportunities to design novel functional electronic materials. However, a comprehensive thickness dependence study is paramount to harness the electronic properties of such atomic foils and their stacking into synthetic heterostructures. Here we show...

Due to an ultra-low thermal expansion, Zr2O(PO4)2 could find many applications as a thermal shock resistant material. To this end, ceramic processing is a key step in order to reach best properties. In this work, Zr2O(PO4)2 was sintered by conventional sintering and by the spark plasma sintering technique (SPS) with and without additive. Samples ma...

Evolution of the CDW in Sulfur substituted 1T-TiSe2: A combined ARPES and STM/STS study

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 exam...

In Ti-intercalated self-doped 1T−TiSe2 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...

We report the influence on the superconducting critical temperature $T_c$ in doped SrTiO$_3$ of the substitution of the natural $^{16}$O atoms by the heavier isotope $^{18}$O. We observe that for a wide range of doping this substitution causes a strong ($\sim 50 \%$) enhancement of $T_c$. Also the magnetic critical field $H_{c2}$ is increased by a...

We report the direct observation of a long-range field effect in WTe2 devices, leading to large gate-induced changes of transport through crystals much thicker than the electrostatic screening length. The phenomenon—which manifests itself very differently from the conventional field effect—originates from the nonlocal nature of transport in the dev...

We report angle-resolved photoemission experiments resolving the distinct electronic structure of the inequivalent top and bottom (001) surfaces of WTe2. On both surfaces, we identify a surface state that forms a large Fermi arc emerging out of the bulk electron pocket. Using surface electronic structure calculations, we show that these Fermi arcs...

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...

We find that crystals of semi-metallic tungsten ditelluride (WTe$_2$) exhibit a strong, unusual gate dependence of magneto-transport, originating from gate-induced variations in the mobility ($\mu_{e,h}$) of bulk electrons and holes. The phenomenon occurs if the crystal thickness is smaller than the mean free path of bulk carriers in WTe$_2$. The g...

We report a combined experimental and theoretical study of the candidate type-II Weyl semimetal MoTe2. Using laser-based angle-resolved photoemission we resolve multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces. All surface states observed experimentally are reproduced by an electronic structure calculation for the expe...

We report angle-resolved photoemission experiments identifying an arc-like surface state connecting the bulk electron and hole pockets of the candidate type-II Weyl semimetal WTe2. This surface state can be isolated clearly on one of two distinct surface terminations observed experimentally, which we associate with the in-equivalent top and bottom...

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 phase-shifted CDW domains whose size directly depends on the in...

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 phase-shifted CDW domains whose size directly depends on the in...

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 exfoliate...

Supplementary Figures 1-6, Supplementary Notes 1-4 and Supplementary References.

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-dimension...

We study the evolution of the band-gap structure in few-layer MoTe2 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 emissi...

Thermoelectric p-type Pb-free polycrystalline BiCuSeO is prepared by a two-step solid state reaction using stoichiometric amounts of Bi2O3, Bi, Cu, and Se (silica tubes, 700 °C, 1 week) followed by densification via pressurized spark plasma sintering under Ar (675 °C, 10 min, 100 MPa).

BiCuSeO based materials have been reported as very promising thermoelectric materials, with thermoelectric figure of merit values that make them among the best p-type Pb-free polycrystalline thermoelectric materials, and they could be suitable for applications in thermal to electrical power conversion modules in a 300–650 °C temperature range. In t...

Since 2010, we have evidenced the very promising thermoelectric properties of layered oxychalcogenides, with parent compound BiCuSeO, which could be used in thermoelectric conversion systems in the 300–600 °C temperature range. These materials, that were first studied in the early 2000s for their optoelectronic properties, exhibit thermoelectric fi...

During the past two years, we have underlined the great potential of p-type oxychalcogenides, with parent compound BiCuSeO, for thermoelectric applications in the medium temperature range (400–650°C). These materials, which do not contain lead and are less expensive than Te containing materials, exhibit large thermoelectric figure of merit, exceedi...

This thesis addresses the issues of the elaboration and the characterization of the chemical and physical properties of a new family of thermoelectric materials, the oxychalcogenides with the general formula BiCuSeO. This compound, called 1111, cristallises in the ZrCuSiAs structure-type. One feature of this structure lies in the fact that the laye...

LaFe1-xZnxAsO0.85F0.15 composites were synthesized by solid state reaction in order to improve the mechanical properties while trying to keep the superconductivity. XRD results indicate that LaFe1-xZnxAsO0.85F0.15 are series of superconducting composites composed of LaFeAsO0.85F0.15 and LaZnAsO0.85F0.15. LaFe1-xZnxAsO0.85F0.15 still exhibits superc...

BiAgSeS shows intrinsically low thermal conductivity in the temperature range from 300 K (�0.46 W m�1 K�1) to 823 K (�0.29 W m�1 K�1). Low thermal conductivity coupling with enhanced electrical transport properties achieved through partially substituting S2� by Cl� leads to a high ZTof �1.0 at 823 K for BiAgSeS0.97Cl0.03, indicating that the BiAgSe...

We report on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive (Cu2Se2)2− and insulating (Bi2O2)2+ layers. The electrical transport properties of BiCuSeO materials can be significantly improved by substituting Bi3+ with Ca2+. The resulting materials exhibit a large positi...

We report on the influence of tellurium substitution on the structural and electronic properties of the thermoelectric oxychalcogenide BiCuSeO. Our results show that a complete solid solution exists between the two end members BiCuSeO and BiCuTeO. However, a complex evolution of the properties has been observed, with both a monotonous evolution of...

We report on the effect of Mg doping on the thermoelectric properties of p-type BiCuSeO oxyselenide, with layer structure composed of conductive (Cu2Se2)(2) layers alternately stacked with insulating (Bi2O2)(2+) layers. The substitution of Bi3+ by Mg2+ leads to an enhancement of the electrical conductivity and a decrease of the thermal conductivity...

The effect of Pb doping on the thermoelectric properties of p-type BiCuSeO from 25 K to 873 K has been studied. The electrical resistivity and Seebeck coefficient of Bi1−xPbxCuSeO both decreased monotonically in all temperature range with increasing Pb content due to the increased carrier concentration. The power factor of Bi1−xPbxCuSeO (x = 0.03)...

The discovery of a 26K superconductivity in the tetragonal iron-based pnictide LaFeAsO1-xFx (x = 0.05-0.12) has stimulated a tremendous search for superconductivity in a wide class of materials with similar structure. Beside superconductivity, promising thermoelectric properties have been discovered in this family in the cryogenic temperature range...

A high ZT value of similar to 1.1 at 923 K in the BiCuSeO system is achieved via heavily doping with Ba and refining grain sizes (200-400 nm), which is higher than any thermoelectric oxide. Excellent thermal and chemical stabilities up to 923 K and high thermoelectric performance confirm that the BiCuSeO system is promising for thermoelectric power...

We report on the structural and electronic transport properties of BiCuSeO based compounds, that have recently been reported as promising thermoelectric materials with figure of merit ZT > 0.8 at 923 K, and share the same crystal structure as the high-Tc iron based 1111 oxypnictides. We show that the substitution of Bi3+ by Sr2+ induces a strong de...