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See detailThermoelectric properties of heavily-doped Fe2YZ full-Heusler compounds
Lemal, Sébastien ULg; Ricci, Fabio ULg; Verstraete, Matthieu ULg et al

Poster (2017, July 31)

Fe2YZ full-Heusler compounds were recently predicted to exhibit very large thermoelectric power factors [Phys. Rev. Lett. 114, 136601 (2015)]. Combining first-principles density functional theory ... [more ▼]

Fe2YZ full-Heusler compounds were recently predicted to exhibit very large thermoelectric power factors [Phys. Rev. Lett. 114, 136601 (2015)]. Combining first-principles density functional theory calculations within a hybrid functional approach and Boltzmann semi-classical transport theory, we investigate the limits of the rigid band doping approximation, and the properties of heavily n-type doped full-Heusler Fe2YZ1-xAx compounds. Using a supercell approach and including explicit dopant impurities, we recover in some cases giant thermoelectric power factors, as previously predicted within the rigid band approximation. In others the effect on the electronic structure is very different from rigid shifts. Interestingly, we find that some systems present a magnetic instability consistent with the Stoner model, and evolve towards a ferromagnetic half-metallic ground state, with a strongly modified power factor. Our results show the promise of the heavily-doped phases of the studied Fe2YZ1-xAx compounds for Seebeck and spin-dependent Seebeck applications. [less ▲]

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See detailThermoelectric properties of layered calcium cobaltite Ca3Co4O9 from hybrid functional first-principles calculations
Lemal, Sébastien ULg; Varignon, Julien ULg; Bilc, Daniel ULg et al

in Physical Review B (2017), 95

Using a combination of first-principles calculations based on density functional theory and Boltzmann semiclassical transport theory, we compute and study the properties of pristine layered calcium ... [more ▼]

Using a combination of first-principles calculations based on density functional theory and Boltzmann semiclassical transport theory, we compute and study the properties of pristine layered calcium cobaltite Ca3Co4O9. We model the system with the B1WC hybrid functional. Two supercells of increasing size which approximate the incommensurate crystallographic structure of the compound are studied and we determine their structural, magnetic, and electronic properties. It is found that the B1WC hybrid functional is appropriate to reproduce the structural, electronic, and magnetic properties, which are then extensively discussed. From the electronic band structure, the Seebeck (S) and electrical resistivity (ρ) tensors are computed using Boltzmann transport theory within the constant relaxation-time approximation. The differences between the diagonal components are detailed and reveal a strong in-plane anisotropy of the properties. The qualitative behavior of the averaged in-plane properties, S // and ρ//, is consistent with the measurements reported in the literature. Our calculation clarifies and provides a broad picture of the evolution of the thermoelectric properties with both carrier density and temperature, and suggests that the change in S// and ρ// around 100 K is not necessarily related to the magnetic transitions occurring around 100 K. [less ▲]

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See detailA photocatalyst of sulphur depleted monolayered molybdenum sulfide nanocrystals for dye degradation and hydrogen evolution reaction
Lin, Liangxu; Miao, Naihua; Huang, Juntong et al

in Nano Energy (2017), 38

Molybdenum disulfide (MoS2) has a theoretical catalytic activity comparable to Pt but in practice is a poor catalyst in bulk form due to the scarcity of metal edge sites and low electrical conductivity ... [more ▼]

Molybdenum disulfide (MoS2) has a theoretical catalytic activity comparable to Pt but in practice is a poor catalyst in bulk form due to the scarcity of metal edge sites and low electrical conductivity. Recent developments on MoS2 monolayers (MLs) are more encouraging in developing cheap and efficient catalysts, but the majority metal atoms are on the basal plane and catalytically inactive. The rapid recombination of the electron-hole pairs and electronic band structure of the most stable 2H-MoS2 MLs are also unsuitable for efficient photocatalysis, especially for solar-driven water splitting. Here, we show that reducing the lateral size and creating sulphur (S) vacancies of MoS2 MLs not only increases dramatically the density of catalytically active sites, but also adjusts the band structure to become highly suitable for solar-driven catalysis. In addition, this preparation efficiently avoids fast charge recombination associated with MoS2, improves light harvesting, and gives a newly formed metallic state to transfer electrons for photocatalytic reactions. By way of example, we have demonstrated remarkable photocatalytic degradation of methylene blue (MB) and methylene orange (MO) dyes using the Sdepleted Mo-S nanocrystals (NCs, 2–25 nm). The NCs are also promising to efficiently generate hydrogen (H2) from water with sacrificial reagents and solar light irradiation. Our study shows how careful design and modification of materials can result in highly efficient photocatalysts, which give considerable opportunities of the transition metal dichalcogenides (TMDs) beyond just MoS2 to develop highly efficient and economic catalysts. [less ▲]

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See detailChemical Strain Engineering of Magnetism in Oxide Thin Films
Copie, Olivier; Varignon, Julien ULg; Rotella, Hélène et al

in Advanced Materials (2017)

Transition metal oxides having a perovskite structure form a wide and technologically important class of compounds. In these systems, ferroelectric, ferromagnetic, ferroelastic, or even orbital and charge ... [more ▼]

Transition metal oxides having a perovskite structure form a wide and technologically important class of compounds. In these systems, ferroelectric, ferromagnetic, ferroelastic, or even orbital and charge orderings can develop and eventually coexist. These orderings can be tuned by external electric, magnetic, or stress field, and the cross-couplings between them enable important multifunctional properties, such as piezoelectricity, magneto-electricity, or magneto-elasticity. Recently, it has been proposed that additional to typical fields, the chemical potential that controls the concentration of ion vacancies in these systems may reveal an efficient alternative parameter to further tune their properties and achieve new functionalities. In this study, concretizing this proposal, the authors show that the control of the content of oxygen vacancies in perovskite thin films can indeed be used to tune their magnetic properties. Growing PrVO3 thin films epitaxially on an SrTiO3 substrate, the authors reveal a concrete pathway to achieve this effect. The authors demonstrate that monitoring the concentration of oxygen vacancies through the oxygen partial pressure or the growth temperature can produce a substantial macroscopic tensile strain of a few percent. In turn, this strain affects the exchange interactions, producing a nontrivial evolution of Néel temperature in a range of 30 K. [less ▲]

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See detailAb initio study of hydrogenic effective mass impurities in Si nanowires
Peelaers, Hartwin; Durgun, Engin; Partoens, Bart et al

in Journal of Physics : Condensed Matter (2017), 29

The effect of B and P dopants on the band structure of Si nanowires is studied using electronic structure calculations based on density functional theory. At low concentrations a dispersionless band is ... [more ▼]

The effect of B and P dopants on the band structure of Si nanowires is studied using electronic structure calculations based on density functional theory. At low concentrations a dispersionless band is formed, clearly distinguishable from the valence and conduction bands. Although this band is evidently induced by the dopant impurity, it turns out to have purely Si character. These results can be rigorously analyzed in the framework of effective mass theory. In the process we resolve some common misconceptions about the physics of hydrogenic shallow impurities, which can be more clearly elucidated in the case of nanowires than would be possible for bulk Si. We also show the importance of correctly describing the effect of dielectric confinement, which is not included in traditional electronic structure calculations, by comparing the obtained results with those of G0W0 calculations. [less ▲]

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See detailFirst-principles reinvestigation of bulk WO3
Hamdi, Hanen ULg; Salje, Ekhard K. H.; Ghosez, Philippe ULg et al

in Physical Review B (2016), 94

Using first-principles calculations, we analyze the structural properties of tungsten trioxide WO3. Our calculations rely on density functional theory and the use of the B1-WC hybrid functional, which ... [more ▼]

Using first-principles calculations, we analyze the structural properties of tungsten trioxide WO3. Our calculations rely on density functional theory and the use of the B1-WC hybrid functional, which provides very good agreement with experimental data. We show that the hypothetical high-symmetry cubic reference structure combines several ferroelectric and antiferrodistortive (antipolar cation motions, rotations, and tilts of oxygen octahedra) structural instabilities. Although the ferroelectric instability is the largest, the instability related to antipolar W motions combines with those associated to oxygen rotations and tilts to produce the biggest energy reduction, yielding a P21/c ground state. This nonpolar P21/c phase is only different from the experimentally reported P c ground state by the absence of a very tiny additional ferroelectric distortion. The calculations performed on a stoichiometric compound so suggest that the low-temperature phase of WO3 is not intrinsically ferroelectric and that the experimentally observed ferroelectric character might arise from extrinsic defects such as oxygen vacancies. Independently, we also identify never observed R3m and R3c ferroelectric metastable phases with large polarizations and low energies close to the P 21 /c ground state, which makes WO3 a potential antiferroelectric material. The relative stability of various phases is discussed in terms of the anharmonic couplings between different structural distortions, highlighting a very complex interplay. [less ▲]

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See detailFirst principles study of heavily doped full Heusler Fe2YZ for high thermoelectric power factor
Lemal, Sébastien ULg; Ghosez, Philippe ULg; Bilc, Daniel ULg

Poster (2016, May 02)

Using a combination of first-principles calculations based on density functional theory and Boltzmann semi-classical transport theory, we investigate the properties of heavily $n$-type doped full Heusler ... [more ▼]

Using a combination of first-principles calculations based on density functional theory and Boltzmann semi-classical transport theory, we investigate the properties of heavily $n$-type doped full Heusler Fe2YZ1-xAx compounds. Using a supercell approach and including explicitly the dopant impurities, we recover in some cases giant thermoelectric power factors as previously predicted under doping within the rigid band approximations [Phys. Rev. Lett. 114, 136601 (2015)]. In other cases, however, we highlight that the system evolves toward a ferromagnetic half-metallic ground state so that the power factor is strongly modified. We rationalize the appearance of this magnetic instability, showing that it consistent with the Stoner model. The uncovered properties of the heavily doped phases of the studied Fe2YZ1-xAx compounds appear promising for Seebeck and spin-Seebeck applications. [less ▲]

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See detailLarge elasto-optic effect and reversible electrochromism in multiferroic BiFeO3
Sando, D.; Yang, Yurong; Bousquet, Eric ULg et al

in Nature Communications (2016), 7

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous ... [more ▼]

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology—a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption—reminiscent of piezochromism—which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses. [less ▲]

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See detailSulfur-Depleted Monolayered Molybdenum Disulfide Nanocrystals for Superelectrochemical Hydrogen Evolution Reaction
Lin, Liangxu; Miao, Naihua; Wen, Yan et al

in ACS Nano (2016), 10

Catalytically driven electrochemical hydrogen evolution reaction (HER) of monolayered molybdenum disulfide (MoS2) is usually highly suppressed by the scarcity of edges and low electrical conductivity ... [more ▼]

Catalytically driven electrochemical hydrogen evolution reaction (HER) of monolayered molybdenum disulfide (MoS2) is usually highly suppressed by the scarcity of edges and low electrical conductivity. Here, we show how the catalytic performance of MoS2 monolayers can be improved dramatically by catalyst size reduction and surface sulfur (S) depletion. Monolayered MoS2 nanocrystals (NCs) (2−25 nm) produced via exfoliating and disintegrating their bulk counterparts showed improved catalysis rates over monolayer sheets because of their increased edge ratios and metallicity. Subsequent S depletion of these NCs further improved the metallicity and made Mo atoms on the basal plane become catalytically active. As a result, the S-depleted NCs with low mass (∼1.2 μg) showed super high catalytic performance on HER with a low Tafel slope of ∼29 mV/decade, overpotentials of 60−75 mV, and high current densities jx (where x is in mV) of j150 = 9.64 mA·cm−2 and j200 = 52.13 mA·cm−2. We have found that higher production rates of H2 could not be achieved by adding more NC layers since HER only happens on the topmost surface and the charge mobility decreases dramatically. These difficulties can be largely alleviated by creating a hybrid structure of NCs immobilized onto three-dimensional graphene to provide a very high surface exposure of the catalyst for electrochemical HER, resulting in very high current densities of j150 = 49.5 mA·cm−2 and j200 = 232 mA·cm−2 with ∼14.3 μg of NCs. Our experimental and theoretical studies show how careful design and modification of nanoscale materials/structures can result in highly efficient catalysis. There may be considerable opportunities in the broader family of transition metal dichalcogenides beyond just MoS2 to develop highly efficient atomically thin catalysts. These could offer cheap and effective replacement of precious metal catalysts in clean energy production. [less ▲]

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See detailFirst-Principles Study of the Thermoelectric Properties of SrRuO3
Miao, Naihua; Xu, Bin ULg; Bristowe, Nicholas ULg et al

in Journal of Physical Chemistry (2016), 120

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See detailElectric Field Control of Jahn-Teller Distortions in Bulk Perovskites
Varignon, Julien ULg; Bristowe, Nicholas ULg; Ghosez, Philippe ULg

in Physical Review Letters (2016), 116

The Jahn-Teller distortion, by its very nature, is often at the heart of the various electronic properties displayed by perovskites and related materials. Despite the Jahn-Teller mode being nonpolar, we ... [more ▼]

The Jahn-Teller distortion, by its very nature, is often at the heart of the various electronic properties displayed by perovskites and related materials. Despite the Jahn-Teller mode being nonpolar, we devise and demonstrate, in the present Letter, an electric field control of Jahn-Teller distortions in bulk perovskites. The electric field control is enabled through an anharmonic lattice mode coupling between the Jahn-Teller distortion and a polar mode. We confirm this coupling and quantify it through first-principles calculations. The coupling will always exist within the Pb21m space group, which is found to be the favored ground state for various perovskites under sufficient tensile epitaxial strain. Intriguingly, the calculations reveal that this mechanism is not only restricted to Jahn-Teller active systems, promising a general route to tune or induce novel electronic functionality in perovskites as a whole. [less ▲]

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See detailPredictions of Pinning
Paruch, Patrycja; Ghosez, Philippe ULg

in Nature (2016), 534

A multiscale model has been implemented that provides accurate predictions of the behaviour of ferroelectric materials in electric fields, and might aid efforts to design devices such as sensors and ... [more ▼]

A multiscale model has been implemented that provides accurate predictions of the behaviour of ferroelectric materials in electric fields, and might aid efforts to design devices such as sensors and digital memory. [less ▲]

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See detailThermoelectric properties of the unfilled skutterudite FeSb3 from first principles and Seebeck local probes
Lemal, Sébastien ULg; Ngoc, Nguyen; de Boor, Johannes et al

in Physical Review. B: Condensed Matter and Materials Physics (2015), 92

Using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite FeSb3. We employ the hybrid ... [more ▼]

Using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite FeSb3. We employ the hybrid functional approach for exchange-correlation. The ground state is determined to be anti-ferromagnetic with an atomic magnetic moment of 1.6 μB/Fe. The Néel temperature Tn is estimated at 6 K, in agreement with experiments which found a paramagnetic state down to 10 K. The ground state is semiconducting, with a small electronic gap of 33 meV, also consistent with previous experiments on films. Charge carrier concentrations are estimated from Hall resistance measurements. The Seebeck coefficient is measured and mapped using a scanning probe at room temperature that yields an average value of 38.6 μV/K, slightly lower than the theoretical result. The theoretical conductivity is analyzed as a function of temperature and concentration of charge carriers. [less ▲]

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See detailCoupling and electrical control of structural, orbital and magnetic orders in perovskites
Varignon, Julien ULg; Bristowe, Nicholas ULg; Bousquet, Eric ULg et al

in Scientific Reports (2015), 5

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional ... [more ▼]

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional electronic properties exhibited by perovskites is often the so-called Jahn-Teller distortion. For applications, an electrical control of the Jahn-Teller distortions, which is so far out of reach, would therefore be highly desirable. Based on universal symmetry arguments, we determine new lattice mode couplings that can provide exactly this paradigm, and exemplify the effect from firstprinciples calculations. The proposed mechanism is completely general, however for illustrative purposes, we demonstrate the concept on vanadium based perovskites where we reveal an unprecedented orbital ordering and Jahn-Teller induced ferroelectricity. Thanks to the intimate coupling between Jahn-Teller distortions and electronic degrees of freedom, the electric field control of Jahn-Teller distortions is of general relevance and may find broad interest in various functional devices. [less ▲]

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See detailOptimization of Thermoelectric Properties of MgAgSb-Based Materials: A First-Principles Investigation
Miao, Naihua ULg; Ghosez, Philippe ULg

in Journal of Physical Chemistry C (2015)

Recently, MgAgSb-based materials (MAS) have been proposed as promising candidates for room-temperature thermoelectric applications with a ZT larger than unity. In this work, we present a comprehensive ... [more ▼]

Recently, MgAgSb-based materials (MAS) have been proposed as promising candidates for room-temperature thermoelectric applications with a ZT larger than unity. In this work, we present a comprehensive theoretical study of the structural, electronic, and thermoelectric properties of MAS by combining first-principles calculations and Boltzmann transport theory. The predicted Seebeck coefficients are compared with available experimental data. The effects of crystal structure and volume on the electronic and thermoelectric properties of MAS are discussed. The thermoelectric quantities are optimized with respect to the chemical potential tuned by doping carriers. It is suggested that the thermoelectric performance of the α phase of MAS can be enhanced by hole doping and strain engineering. Our work intends to provide a theoretical support for future improvement on the thermoelectric performance of MAS and related materials. [less ▲]

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See detailFerromagnetism induced by entangled charge and orbital orderings in ferroelectric titanate perovskites
Bristowe, Nicholas ULg; Varignon, Julien ULg; Fontaine, Denis et al

in Nature Communications (2015), 6

In magnetic materials, the Pauli exclusion principle typically drives anti-alignment between electron spins on neighbouring species resulting in antiferromagnetic behaviour. Ferromagnetism exhibiting ... [more ▼]

In magnetic materials, the Pauli exclusion principle typically drives anti-alignment between electron spins on neighbouring species resulting in antiferromagnetic behaviour. Ferromagnetism exhibiting spontaneous spin alignment is a fairly rare behaviour, but once materialized is often associated with itinerant electrons in metals. Here we predict and rationalize robust ferromagnetism in an insulating oxide perovskite structure based on the popular titanate series. In half-doped layered titanates, the combination of Jahn–Teller and oxygen breathing motions opens a band gap and creates an unusual charge and orbital ordering of the Ti d electrons. It is argued that this intriguingly intricate electronic network favours the elusive inter-site ferromagnetic (FM) ordering, on the basis of intra-site Hund's rules. Finally, we find that the layered oxides are also ferroelectric with a spontaneous polarization approaching that of ​BaTiO3. The concepts are general and design principles of the technologically desirable FM ferroelectric multiferroics are presented. [less ▲]

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See detailNovel magneto-electric multiferroics from first-principles calculations
Varignon, Julien ULg; Bristowe, Nicholas ULg; Bousquet, Eric ULg et al

in Comptes Rendus Physique (2015), 16

Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed ... [more ▼]

Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed behaviours, the trend has evolved and, nowadays, density functional theory calculations have become also an essential predicting tool for identifying original rules to achieve multiferroism and design new magneto-electric compounds. This chapter aims at highlighting the key advances in the field of multiferroics, to which first-principles methods have contributed significantly. The essential theoretical developments that made this research possible are also briefly presented. [less ▲]

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See detailUnderstanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments
Benedek, Nicole; Rondinelli, James; Djani-Ait, Hania ULg et al

in Dalton Transactions (Cambridge, England : 2003) (2015), 44(23), 10527-10890

ABO3 perovskites have fascinated solid-state chemists and physicists for decades because they display a seemingly inexhaustible variety of chemical and physical properties. However, despite the diversity ... [more ▼]

ABO3 perovskites have fascinated solid-state chemists and physicists for decades because they display a seemingly inexhaustible variety of chemical and physical properties. However, despite the diversity of properties found among perovskites, very few of these materials are ferroelectric, or even polar, in bulk. In this Perspective, we highlight recent theoretical and experimental studies that have shown how a combination of non-polar structural distortions, commonly tilts or rotations of the BO6 octahedra, can give rise to polar structures or ferroelectricity in several families of layered perovskites. We discuss the crystal chemical origin of the polarization in each of these families – which emerges through a so-called ‘trilinear coupling’ or ‘hybrid improper’ mechanism – and emphasize areas in which further theoretical and experimental investigation is needed. We also consider how this mechanism may provide a generic route for designing not only new ferroelectrics, but also materials with various other multifunctionalities, such as magnetoelectrics and electric field-controllable metal-insulator transitions. [less ▲]

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See detailModel of two-dimensional electron gas formation at ferroelectric interfaces
Aguado-Puente, P.; Bristowe, Nicholas ULg; Yin, B. et al

in Physical Review. B : Condensed Matter (2015), 92

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it ... [more ▼]

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it gives rise to. Here, we studyunderwhatcircumstancessimilarprocessescanalsotakeplaceunderneathferroelectricthinfilms.Weusea simpleLandaumodeltodemonstratethatintheabsenceofextrinsicscreeningmechanisms,amonodomainphase can be stabilized in ferroelectric films by means of an electronic reconstruction. Unlike in the LaAlO3/SrTiO3 heterostructure, the emergence with thickness of the free charge at the interface is discontinuous. This prediction is confirmed by performing first-principles simulations of free-standing slabs of PbTiO3. The model is also used to predict the response of the system to an applied electric field, demonstrating that the two-dimensional electron gas can be switched on and off discontinuously and in a nonvolatile fashion. Furthermore, the reversal of the polarization can be used to switch between a two-dimensional electron gas and a two-dimensional hole gas, which should, in principle, have very different transport properties. We discuss the possible formation of polarization domains and how such configuration competes with the spontaneous accumulation of free charge at the interfaces. [less ▲]

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See detailLow-Dimensional Transport and Large Thermoelectric Power Factors in Bulk Semiconductors by Band Engineering of Highly Directional Electronic States
Bilc, Daniel ULg; Hautier, Geoffroy; Waroquiers, David et al

in Physical Review Letters (2015), 114

Thermoelectrics are promising for addressing energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but ... [more ▼]

Thermoelectrics are promising for addressing energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but less by maximizing the power factor. The latter imposes apparently conflicting requirements on the band structure: a narrow energy distribution and a low effective mass. Quantum confinement in nanostructures andtheintroductionofresonantstatesweresuggestedaspossiblesolutionstothisparadox,butwithlimited success. Here, we propose an original approach to fulfill both requirements in bulk semiconductors. It exploits the highly directional character of some orbitals to engineer the band structure and produce a type of low-dimensional transport similar to that targeted in nanostructures, while retaining isotropic properties. Using first-principle calculations, the theoretical concept is demonstrated in Fe2YZ Heusler compounds, yielding power factors 4 to 5 times larger than in classical thermoelectrics at room temperature. Our findings are totally generic and rationalize the search of alternative compounds with similar behavior. Beyond thermoelectricity, these might be relevant also in the context of electronic, superconducting, or photovoltaic applications. [less ▲]

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