Showing papers on "Electronic band structure published in 2012"
TL;DR: In this paper, differential reflectance and photoluminescence spectra of mono- to few-layer Molybdenum disulphide (MoS2) and WSe2 were analyzed.
Abstract: Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion with decreasing layer thickness. Molybdenum disulphide (MoS2) was recently found to exhibit indirect to direct gap transition when the thickness is reduced to a single monolayer. This leads to remarkable enhancement in the photoluminescence efficiency, which opens up new opportunities for the optoelectronic applications of the material. Here we report differential reflectance and photoluminescence (PL) spectra of mono- to few-layer WS2 and WSe2 that indicate that the band structure of these materials undergoes similar indirect to direct transition when thinned to a single monolayer. Strong enhancement in PL quantum yield is observed for monoayer WS2 and WSe2 due to exciton recombination at the direct band edge. In contrast to natural MoS2 crystals extensively used in recent studies, few-layer WS2 and WSe2 show comparatively strong indirect gap emission along with distinct direct gap hot electron emission, suggesting high quality of synthetic crystals prepared by chemical vapor transport method. Fine absorption and emission features and their thickness dependence suggest strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe2.
1,424 citations
TL;DR: In this article, self-consistent calculations of the band structures and related effective mass parameters are carried out for bulk, monolayer, and bilayer MoS, including excitonic effects within the Mott-Wannier theory.
Abstract: Quasiparticle self-consistent $GW$ calculations of the band structures and related effective-mass parameters are carried out for bulk, monolayer, and bilayer MoS${}_{2}$. Including excitonic effects within the Mott-Wannier theory, quantitative agreement is obtained between the $A$, $B$ excitons, measured by absorption [Phys. Rev. Lett. 105, 136805 (2010)], and the calculated exciton gap energies at $K$. The $A$-$B$ splitting arises from the valence-band splitting which in the monolayer is entirely due to spin-orbit coupling and leads to spin-split states, while in the bilayer it is a combined effect of interlayer and spin-orbit coupling.
1,158 citations
TL;DR: The O-doping in the g-C(3)N(4) lattice could induce intrinsic electronic and band structure modulation, resulting in its absorbance edge up to 498 nm and enhanced visible-light photoactivity, consequently.
887 citations
TL;DR: It is remarkable that a topological quantum phase transition can be induced simply by changing electric field in a single silicene sheet.
Abstract: Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb lattice, which shares almost every remarkable property with graphene. The low-energy structure of silicene is described by Dirac electrons with relatively large spin-orbit interactions due to its buckled structure. The key observation is that the band structure is controllable by applying electric field to silicene. We explore the phase diagram of silicene together with exchange field M and by applying electric field E(z). A quantum anomalous Hall (QAH) insulator, valley polarized metal (VPM), marginal valley polarized metal (M-VPM), quantum spin Hall insulator, and band insulator appear. They are characterized by the Chern numbers and/or by the edge modes of a nanoribbon. It is intriguing that electrons have been moved from a conduction band at the K point to a valence band at the K' point for E(z) > 0 in the VPM. We find in the QAH phase that almost flat gapless edge modes emerge and that spins form a momentum-space Skyrmion to yield the Chern number. It is remarkable that a topological quantum phase transition can be induced simply by changing electric field in a single silicene sheet.
858 citations
TL;DR: The coupling of the spin of electrons to their motional state lies at the heart of recently discovered topological phases of matter and the spin-orbit gap is revealed via spin-injection spectroscopy, which characterizes the energy-momentum dispersion and spin composition of the quantum states.
Abstract: The coupling of the spin of electrons to their motional state lies at the heart of recently discovered topological phases of matter. Here we create and detect spin-orbit coupling in an atomic Fermi gas, a highly controllable form of quantum degenerate matter. We directly reveal the spin-orbit gap via spin-injection spectroscopy, which characterizes the energy-momentum dispersion and spin composition of the quantum states. For energies within the spin-orbit gap, the system acts as a spin diode. We also create a spin-orbit coupled lattice and probe its spinful band structure, which features additional spin gaps and a fully gapped spectrum. In the presence of s-wave interactions, such systems should display induced p-wave pairing, topological superfluidity, and Majorana edge states.
664 citations
TL;DR: It is theoretically and experimentally demonstrated that carbon self-doping could induce intrinsic electronic and band structure change of g-C(3)N(4)via the formation of delocalized big π bonds to increase visible light absorption and electrical conductivity as well as surface area and thus enhance both photooxidation and photoreduction activities.
632 citations
TL;DR: In this article, the band structure of a two-dimensional honeycomb lattice of silicon atoms is investigated and the authors show that the gap closes at a certain critical electric field, and that it is possible to generate helical zero modes anywhere in a silicene sheet by adjusting the electric field.
Abstract: Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb lattice, which shares almost every remarkable property with graphene. The low energy structure of silicene is described by Dirac electrons with relatively large spin-orbit interactions due to its buckled structure. The key observation is that the band structure is controllable by applying the electric field to a silicene sheet. In particular, the gap closes at a certain critical electric field. Examining the band structure of a silicene nanoribbon, we demonstrate that a topological phase transition occurs from a topological insulator to a band insulator with the increase of the electric field. We also show that it is possible to generate helical zero modes anywhere in a silicene sheet by adjusting the electric field locally to this critical value. The region may act as a quantum wire or a quantum dot surrounded by topological and/or band insulators. We explicitly construct the wave functions for some simple geometries based on the low-energy effective Dirac theory. These results are applicable also to germanene, that is a two-dimensional honeycomb structure of germanium.
593 citations
TL;DR: The comparison of the present calculations with measured optical response data of rutile indicate that discrepancies discussed in numerous earlier studies are due to the measurements rather than related to an insufficient theoretical description.
Abstract: In this study, we present a combined density functional theory and many-body perturbation theory study on the electronic and optical properties of TiO2 brookite as well as the tetragonal phases rutile and anatase. The electronic structure and linear optical response have been calculated from the Kohn‐Sham band structure applying (semi)local as well as nonlocal screened hybrid exchange‐correlation density functionals. Single-particle excitations are treated within the GW approximation for independent quasiparticles. For optical response calculations, two-particle excitations have been included by solving the Bethe‐Salpeter equation for Coulomb correlated electron‐hole pairs. On this methodological basis, gap data and optical spectra for the three major phases of TiO2 are provided. The common characteristics of brookite with the rutile and anatase phases, which have been discussed more comprehensively in the literature, are highlighted. Furthermore, the comparison of the present calculations with measured optical response data of rutile indicate that discrepancies discussed in numerous earlier studies are due to the measurements rather than related to an insufficient theoretical description. (Some figures may appear in colour only in the online journal)
575 citations
TL;DR: In this article, the wave functions for simple geometries based on the low-energy effective Dirac theory were constructed for a silicene nanoribbon and showed that a topological phase transition occurs from a topologically topological insulator to a band insulator with an increase of electric field.
Abstract: Silicene is a monolayer of silicon atoms forming a two-dimensional (2D) honeycomb lattice and shares almost all the remarkable properties of graphene. The low-energy structure of silicene is described by Dirac electrons with relatively large spin–orbit interactions owing to its buckled structure. A key observation is that the band structure can be controlled by applying an electric field to a silicene sheet. In particular, the gap closes at a certain critical electric field. Examining the band structure of a silicene nanoribbon, we show that a topological phase transition occurs from a topological insulator to a band insulator with an increase of electric field. We also show that it is possible to generate helical zero modes anywhere in a silicene sheet by adjusting the electric field locally to this critical value. The region may act as a quantum wire or a quantum dot surrounded by topological and/or band insulators. We explicitly construct the wave functions for some simple geometries based on the low-energy effective Dirac theory. These results are also applicable to germanene, which is a 2D honeycomb structure of germanium.
573 citations
TL;DR: The experimental results show that at a finite Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi(1-x)Sb(x).
Abstract: A topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction-driven band inversion. The topological phase in the Bi(1-x)Sb(x) system is due to an odd number of band inversions. A related spin-orbit system, the Pb(1-x)Sn(x)Te, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb(1-x)Sn(x)Te class of materials that has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi(1-x)Sb(x). Our observation of the spin-polarized Dirac surface states in the inverted Pb(1-x)Sn(x)Te and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.
536 citations
TL;DR: A systematic Raman study of unconventionally stacked double-layer graphene finds that the spectrum strongly depends on the relative rotation angle between layers, and reveals changes in electronic band structure due to the interlayer interaction are responsible for the observed spectral features.
Abstract: We present a systematic Raman study of unconventionally stacked double-layer graphene, and find that the spectrum strongly depends on the relative rotation angle between layers. Rotation-dependent trends in the position, width and intensity of graphene 2D and G peaks are experimentally established and accounted for theoretically. Our theoretical analysis reveals that changes in electronic band structure due to the interlayer interaction, such as rotational-angle dependent Van Hove singularities, are responsible for the observed spectral features. Our combined experimental and theoretical study provides a deeper understanding of the electronic band structure of rotated double-layer graphene, and leads to a practical way to identify and analyze rotation angles of misoriented double-layer graphene.
TL;DR: In this article, hybrid density functional theory is used to explore the band structure and effective masses of MoS and the effects of strain on the electronic properties, and the transition in band structure from bulk to monolayer is investigated.
Abstract: We use hybrid density functional theory to explore the band structure and effective masses of MoS${}_{2}$, and the effects of strain on the electronic properties. Strain allows engineering the magnitude as well as the nature (direct versus indirect) of the band gap. Deformation potentials that quantify these changes are reported. The calculations also allow us to investigate the transition in band structure from bulk to monolayer, and the nature and degeneracy of conduction-band valleys. Investigations of strain effects on effective masses reveal that small uniaxial stresses can lead to large changes in the hole effective mass.
TL;DR: This study of PbSe illustrates the importance of the deformation potential coefficient of the charge-carrying band as one of several key parameters to consider for band structure engineering and the search for high performance thermoelectric materials.
Abstract: PbSe is a surprisingly good thermoelectric material due, in part, to its low thermal conductivity that had been overestimated in earlier measurements. The thermoelectric figure of merit, zT, can exceed 1 at high temperatures in both p-type and n-type PbSe, similar to that found in PbTe. While the p-type lead chalcogenides (PbSe and PbTe) benefit from the high valley degeneracy (12 or more at high temperature) of the valence band, the n-type versions are limited to a valley degeneracy of 4 in the conduction band. Yet the n-type lead chalcogenides achieve a zT nearly as high as the p-type lead chalcogenides. This effect can be attributed to the weaker electron–phonon coupling (lower deformation potential coefficient) in the conduction band as compared with that in the valence band, which leads to higher mobility of electrons compared to that of holes. This study of PbSe illustrates the importance of the deformation potential coefficient of the charge-carrying band as one of several key parameters to consider for band structure engineering and the search for high performance thermoelectric materials.
TL;DR: It is demonstrated that mixed MoS2/MoSe 2/MoTe2 compounds are thermodynamically stable at room temperature, so that such materials can be manufactured using chemical-vapor deposition technique or exfoliated from the bulk mixed materials.
Abstract: Using density-functional theory calculations, we study the stability and electronic properties of single layers of mixed transition metal dichalcogenides (TMDs), such as MoS2xSe2(1–x), which can be referred to as two-dimensional (2D) random alloys. We demonstrate that mixed MoS2/MoSe2/MoTe2 compounds are thermodynamically stable at room temperature, so that such materials can be manufactured using chemical-vapor deposition technique or exfoliated from the bulk mixed materials. By applying the effective band structure approach, we further study the electronic structure of the mixed 2D compounds and show that general features of the band structures are similar to those of their binary constituents. The direct gap in these materials can continuously be tuned, pointing toward possible applications of 2D TMD alloys in photonics.
TL;DR: In this article, the authors investigated the electronic structure and the quantum Hall effect in twisted bilayer graphenes with various rotation angles in the presence of magnetic field and computed the energy spectrum and quantized Hall conductivity in a wide range of magnetic fields.
Abstract: We investigate the electronic structure and the quantum Hall effect in twisted bilayer graphenes with various rotation angles in the presence of magnetic field. Using a low-energy approximation, which incorporates the rigorous interlayer interaction, we computed the energy spectrum and the quantized Hall conductivity in a wide range of magnetic field from the semiclassical regime to the fractal spectrum regime. In weak magnetic fields, the low-energy conduction band is quantized into electronlike and holelike Landau levels at energies below and above the van Hove singularity, respectively, and the Hall conductivity sharply drops from positive to negative when the Fermi energy goes through the transition point. In increasing magnetic field, the spectrum gradually evolves into a fractal band structure called Hofstadter's butterfly, where the Hall conductivity exhibits a nonmonotonic behavior as a function of Fermi energy. The typical electron density and magnetic field amplitude characterizing the spectrum monotonically decrease as the rotation angle is reduced, indicating that the rich electronic structure may be observed in a moderate condition.
TL;DR: In this paper, quasi-particle electronic properties of these materials are investigated by many-body perturbation theory in the GW approximation, currently the most accurate first-principles approach for electronic band structure of extended systems.
Abstract: Molybdenum and tungsten dichalcogenides, MX2 (M = Mo and W; X = S and Se), characterized by their quasi-two-dimensional layered structure, have attracted intensive interest due to their intriguing physical and chemical properties. In this work, quasi-particle electronic properties of these materials are investigated by many-body perturbation theory in the GW approximation, currently the most accurate first-principles approach for electronic band structure of extended systems. It is found that the fundamental band gaps of all of these materials can be well described by the GW approach, and the calculated density of states from GW quasi-particle band energies agree very well with photoemission spectroscopy data. Ionization potentials of these materials are also studied by combining the slab model using density functional theory and GW correction. On the basis of our theoretical findings, we predict that none of the materials in MX2 (M = Zr, Hf, Mo, and W; X = S and Se) in their bulk form can be directly use...
TL;DR: In this article, a spin liquid ground state in a wide region of the phase diagram, between a semi-metal (SM) and an antiferromagnetic insulator (AFMI), was found.
Abstract: A spin liquid is a novel quantum state of matter with no conventional order parameter where a finite charge gap exists even though the band theory would predict metallic behavior. Finding a stable spin liquid in two or higher spatial dimensions is one of the most challenging and debated issues in condensed matter physics. Very recently, it has been reported that a model of graphene, i.e., the Hubbard model on the honeycomb lattice, can show a spin liquid ground state in a wide region of the phase diagram, between a semi-metal (SM) and an antiferromagnetic insulator (AFMI). Here, by performing numerically exact quantum Monte Carlo simulations, we extend the previous study to much larger clusters (containing up to 2592 sites), and find, if any, a very weak evidence of this spin liquid region. Instead, our calculations strongly indicate a direct and continuous quantum phase transition between SM and AFMI.
TL;DR: The robust p-type doping observed for quasi-free-standing graphene on hexagonal silicon carbide is explained by the spontaneous polarization of the substrate, and models based on hypothetical acceptor-type defects as they are discussed so far are obsolete.
Abstract: We explain the robust p-type doping observed for quasi-free-standing graphene on hexagonal silicon carbide by the spontaneous polarization of the substrate. This mechanism is based on a bulk property of SiC, unavoidable for any hexagonal polytype of the material and independent of any details of the interface formation. We show that sign and magnitude of the polarization are in perfect agreement with the doping level observed in the graphene layer. With this mechanism, models based on hypothetical acceptor-type defects as they are discussed so far are obsolete. The n-type doping of epitaxial graphene is explained conventionally by donorlike states associated with the buffer layer and its interface to the substrate that overcompensate the polarization doping. The basis for the unique electronic and optical properties of graphene is the linear dispersion relation of the � electrons, which is responsible for Dirac-type quasiparticles with many unusual properties. The band structure in the relevant energy range is made up by double cones in the corners of the two-dimensional hexagonal Brillouin zone; their opening angle is determined by the slope vF ¼ d! dk of the dispersion relation called the Fermi velocity, which is an intrinsic material parameter. The origin of these so-called Dirac cones defines the Fermi energy in an isolated and intrinsic graphene layer. At finite temperatures, the reservoir of mobile charge carriers is due to thermal excitation of equal concentrations n0 and p0 of electrons and holes. Evaluation of the Fermi statistics yields a value of n0 ¼ �k 2 B
TL;DR: In this article, a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states.
Abstract: Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial Z2 topology. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure whose origin and properties have proved controversial. Here we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states. In combination with circular dichroism in angle-resolved photoemission experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study sheds new light on the surface-bulk connectivity in topological insulators, and reveals how this is modified by quantum confinement.
TL;DR: In this article, the electronic band structures of bulk Ge1-xSnx alloys were investigated using the empirical pseudopotential method (EPM) for Sn composition x varying from 0 to 0.2.
Abstract: This work investigates the electronic band structures of bulk Ge1-xSnx alloys using the empirical pseudopotential method (EPM) for Sn composition x varying from 0 to 0.2. The adjustable form factors of EPM were tuned in order to reproduce the band features that agree well with the reported experimental data. Based on the adjusted pseudopotential form factors, the band structures of Ge1-xSnx alloys were calculated along high symmetry lines in the Brillouin zone. The effective masses at the band edges were extracted by using a parabolic line fit. The bowing parameters of hole and electron effective masses were then derived by fitting the effective mass at different Sn compositions by a quadratic polynomial. The hole and electron effective mass were examined for bulk Ge1-xSnx alloys along specific directions or orientations on various crystal planes. In addition, employing the effective-mass Hamiltonian for diamond semiconductor, band edge dispersion at the Γ-point calculated by 8-band k.p. method was fitted...
TL;DR: In this paper, a set of epitaxial layers of GaBixAs1−x (2.3%), of thickness 30-40nm, were grown compressively strained onto GaAs (100) substrates.
Abstract: The GaBixAs1−x bismide III-V semiconductor system remains a relatively underexplored alloy particularly with regards to its detailed electronic band structure. Of particular importance to understanding the physics of this system is how the bandgap energy Eg and spin-orbit splitting energy Δo vary relative to one another as a function of Bi content, since in this alloy it becomes possible for Δo to exceed Eg for higher Bi fractions, which occurrence would have important implications for minimising non-radiative Auger recombination losses in such structures. However, this situation had not so far been realised in this system. Here, we study a set of epitaxial layers of GaBixAs1−x (2.3% ≤ x ≤ 10.4%), of thickness 30–40 nm, grown compressively strained onto GaAs (100) substrates. Using room temperature photomodulated reflectance, we observe a reduction in Eg, together with an increase in Δo, with increasing Bi content. In these strained samples, it is found that the transition energy between the conduction an...
TL;DR: In this article, the correlation between the surface chemistry and electronic structure is studied for SrTi1−xFexO3 (STF), as a model perovskite system, to explain the impact of Sr segregation on the oxygen reduction activity of cathodes in solid oxide fuel cells.
Abstract: The correlation between the surface chemistry and electronic structure is studied for SrTi1−xFexO3 (STF), as a model perovskite system, to explain the impact of Sr segregation on the oxygen reduction activity of cathodes in solid oxide fuel cells. Dense thin films of SrTi0.95Fe0.05O3 (STF5), SrTi0.65Fe0.35O3 (STF35) and SrFeO3 (STF100) were investigated using a coordinated combination of surface probes. Composition, chemical binding, and valence band structure analysis using angle-resolved X-ray photoelectron spectroscopy showed that Sr enrichment increases on the STF film surfaces with increasing Fe content. In situ scanning tunnelling microscopy/spectroscopy results proved the important and detrimental impact of this cation segregation on the surface electronic structure at high temperature and in an oxygen environment. While no apparent band gap was found on the STF5 surface due to defect states at 345 °C and 10−3 mbar of oxygen, the surface band gap increased with Fe content, 2.5 ± 0.5 eV for STF35 and 3.6 ± 0.6 eV for STF100, driven by a down-shift in energy of the valence band. This trend is opposite to the dependence of the bulk STF band gap on the Fe fraction, and is attributed to the formation of a Sr-rich surface phase in the form of SrOx on the basis of the measured surface band structure. The results demonstrate that Sr segregation on STF can deteriorate oxygen reduction kinetics through two mechanisms – inhibition of electron transfer from bulk STF to oxygen species adsorbing onto the surface and the smaller concentration of oxygen vacancies available on the surface for incorporating oxygen into the lattice.
TL;DR: In this article, the spectral element (SE) method and the Bloch theorem were combined with the spectral equation for complex band structure calculation in metamaterial-based elastic rods with periodically attached multi-degree-of-freedom spring mass resonators.
Abstract: Wave propagation and vibration transmission in metamaterial-based elastic rods containing periodically attached multi-degree-of-freedom spring–mass resonators are investigated. A methodology based on a combination of the spectral element (SE) method and the Bloch theorem is developed, yielding an explicit formulation for the complex band structure calculation. The effects of resonator parameters on the band gap behavior are investigated by employing the attenuation constant surface plots, which display information on the location, the width and the attenuation performance of all band gaps. It is found that Bragg-type and resonance-type gaps co-exist in these systems. In some special situations, exact coupling between Bragg and resonance gaps occurs, giving rise to super-wide coupled gaps. The advantage of multi-degree-of-freedom resonators in achieving multiband and/or broadband gaps in metamaterial-based rods is demonstrated. Band gap formation mechanisms are further examined by analytical and physical models, providing explicit formulae to locate the band edge frequencies of all the band gaps.
TL;DR: This work applies soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2 and identifies pronounced 3D warping of the Fermi surface and shows that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe 2.
Abstract: The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular to) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular to) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure.
TL;DR: In this article, the electronic and optical properties of 1 H -MoS 2 have been calculated and compared with existing experimental results. But the results are in very good agreement with experimental results and the results of the calculated results are also consistent with the theoretical results.
TL;DR: In this article, a capacitance study of dual gated bilayer graphene was performed to probe the electronic compressibility as a function of carrier density, temperature, and applied perpendicular electrical displacement.
Abstract: We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied perpendicular electrical displacement $\overline{D}$. As a band gap is induced with increasing $\overline{D}$, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a one-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. For $\overline{D}g0$, the additional peak is observed only for electrons, while for $\overline{D}l0$ the peak appears only for holes. This asymmetry can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization.
TL;DR: In this article, it was shown that the density of conduction neutrons, i.e., neutrons that are effectively free, can be much smaller than that of unbound neutrons due to Bragg scattering.
Abstract: Even though the ``free'' neutrons in the inner crust of a neutron star are superfluid, they are still strongly coupled to nuclei due to nondissipative entrainment effects. These effects have been systematically studied in all regions of the inner crust in the framework of the band theory of solids. Using concepts from solid-state physics, it is shown that the density of conduction neutrons, i.e., neutrons that are effectively free, can be much smaller than the density of unbound neutrons (by an order of magnitude in some layers) due to Bragg scattering. These results suggest that a revision of the interpretation of various observable neutron-star phenomena might be necessary.
TL;DR: In this article, the energy band alignment at interfaces of conducting oxides, which have been experimentally determined using photoelectron spectroscopy with in situ sample preparation, is presented. And a good estimate of the band alignment is derived by considering the density of states of the materials involved.
TL;DR: It has been found that the O(2) molecule acts in two ways: as a reversibly adsorbed electron-acceptor molecule and as an irreversibly Adsorbed molecule that heals natural oxygen vacancy defects in the near-surface region.
Abstract: The photoluminescence (PL) of TiO2 at 529.5 nm (2.34 eV) has been found to be a sensitive indicator of UV-induced band structure modification. As UV irradiation occurs, the positive surface potential changes and shifts the depth of the depletion layer. In addition, reversible band bending due to the adsorption of the electron-donor NH3 and CO molecules has been observed in measurements combining PL with FTIR surface spectroscopy. It has been found that the O2 molecule acts in two ways: as a reversibly adsorbed electron-acceptor molecule and as an irreversibly adsorbed molecule that heals natural oxygen vacancy defects in the near-surface region.
TL;DR: In this article, a two-step solid state reaction followed by a spark plasma sintering process was adopted to prepare a series of Sb-doped Mg2Si0.4Sn0.6 solid solutions.
Abstract: Due to the rich reserves of the raw materials, along with their low cost and nontoxic nature, Mg2Si1−xSnx-based compounds have generated intense attention from the international thermoelectric community for their application in thermoelectric power generation within the intermediate temperature range. In this work, we have adopted a two-step solid state reaction followed by a spark plasma sintering process to prepare a series of Sb-doped Mg2.16(Si0.4Sn0.6)1−ySby (0 ≤ y ≤ 0.055) solid solutions. We discuss the influence of Sb doping and the microstructure on the thermoelectric properties. Our results confirm that Sb acts as an effective n-type dopant and we estimate the maximum amount of Sb the Mg2Si0.4Sn0.6 structure can accommodate to be ∼2.3% by XRD, DSC and EPMA analyses. The electron transport properties and low-temperature electronic heat capacity measurements reveal that both the light conduction band and the heavy conduction band contribute to the transport in n-type Mg2Si0.4Sn0.6 solid solutions. The highest density-of-states effective mass and power factor were observed for Mg2.16(Si0.4Sn0.6)0.985Sb0.015 with an electron concentration of n ≈ 1.67 × 1020 cm−3, which is likely to be due to the Fermi level positioned within ∼2kBT of both the heavy and light conduction bands providing contributions from both bands. In addition, doping with Sb does not seem to affect the lattice thermal conductivity above room temperature. TEM analysis indicates the presence of Sn-rich precipitates with the size of several tens of nanometers dispersed in the Mg2Si0.4Sn0.6 matrix. Such a nanophase may enhance the boundary scattering of phonons and contribute to a low lattice thermal conductivity. Owing to the above characteristics of the band structure and the microstructure, the Mg2.16(Si0.4Sn0.6)0.985Sb0.015 solid solution with n = 1.67 × 1020 cm−3 possessed excellent thermoelectric properties and achieved a high ZT value of 1.3 at 740 K. Further reductions in the lattice thermal conductivity could be achieved via optimization of the nanophase inclusions, leading to a further enhancement of the figure of merit for Mg2Si0.4Sn0.6-based solid solutions.