Showing papers on "Effective mass (solid-state physics) published in 2014"

Optimum Carrier Concentration in n-Type PbTe Thermoelectrics

Abstract: Taking La- and I-doped PbTe as an example, the current work shows the effects of optimizing the thermoelectric figure of merit, zT, by controlling the doping level. The high doping effectiveness allows the carrier concentration to be precisely designed and prepared to control the Fermi level. In addition to the Fermi energy tuning, La-doping modifies the conduction band, leading to an increase in the density of states effective mass that is confirmed by transport, infrared reflectance and hard X-ray photoelectron spectroscopy measurements. Taking such a band structure modification effect into account, the electrical transport properties can then be well-described by a self-consistent single non-parabolic Kane band model that yields an approximate (m*T)^(1.5) dependence of the optimal carrier concentration for a peak power factor in both doping cases. Such a simple temperature dependence also provides an effective approximation of carrier concentration for a peak zT and helps to explain, the effects of other strategies such as lowering the lattice thermal conductivity by nanostructuring or alloying in n-PbTe, which demonstrates a practical guide for fully optimizing thermoelectric materials in the entire temperature range. The principles used here should be equally applicable to other thermoelectric materials.

Electronic and thermoelectric properties of few-layer transition metal dichalcogenides.

Abstract: The electronic and thermoelectric properties of one to four monolayers of MoS2, MoSe2, WS2, and WSe2 are calculated For few layer thicknesses, the near degeneracies of the conduction band K and Σ valleys and the valence band Γ and K valleys enhance the n-type and p-type thermoelectric performance The interlayer hybridization and energy level splitting determine how the number of modes within kBT of a valley minimum changes with layer thickness In all cases, the maximum ZT coincides with the greatest near-degeneracy within kBT of the band edge that results in the sharpest turn-on of the density of modes The thickness at which this maximum occurs is, in general, not a monolayer The transition from few layers to bulk is discussed Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses

Strain and Orientation Modulated Bandgaps and Effective Masses of Phosphorene Nanoribbons

Abstract: Passivated phosphorene nanoribbons, armchair (a-PNR), diagonal (d-PNR), and zigzag (z-PNR), were investigated using density functional theory. Z-PNRs demonstrate the greatest quantum size effect, tuning the bandgap from 1.4 to 2.6 eV when the width is reduced from 26 to 6 A. Strain effectively tunes charge carrier transport, leading to a sudden increase in electron effective mass at +8% strain for a-PNRs or hole effective mass at +3% strain for z-PNRs, differentiating the (mh*/me*) ratio by an order of magnitude in each case. Straining of d-PNRs results in a direct to indirect band gap transition at either -7% or +5% strain and therein creates degenerate energy valleys with potential applications for valleytronics and/or photocatalysis.

Modulation of the Electronic Properties of Ultrathin Black Phosphorus by Strain and Electrical Field

Abstract: The structural and electronic properties of the bulk and ultrathin black phosphorus and the effects of in-plane strain and out-of-plane electrical field on the electronic structure of phosphorene are investigated using first-principles methods. The computed results show that the bulk and few-layer black phosphorus from monolayer to six-layer demonstrates inherent direct bandgap features ranging from 0.5 to 1.6 eV. Interestingly, the band structures of the bulk and few-layer black phosphorus from X point via A point to Y point present degenerate distribution, which shows totally different partial charge dispersions. Moreover, strong anisotropy in regard to carrier effective mass has been observed along different directions. The response of phosphorene to in-plane strain is diverse. The bandgap monotonically decreases with increasing compressive strain, and semiconductor-to-metal transition occurs for phosphorene when the biaxial compressive reaches −9%. Tensile strain first enlarges the gap until the strai...

Self-consistent relativistic band structure of the CH3NH3PbI3 perovskite

Abstract: The electronic structure and properties of the orthorhombic phase of the $\mathrm{CH}{}_{3}\mathrm{NH}{}_{3}\mathrm{PbI}{}_{3}$ perovskite are computed with density functional theory. The structure, optimized using a van der Waals functional, reproduces closely the unit cell volume. The experimental band gap is reproduced accurately by combining spin-orbit effects and a hybrid functional in which the fraction of exact exchange is tuned self-consistently to the optical dielectric constant. Including spin-orbit coupling strongly reduces the anisotropy of the effective mass tensor, predicting a low electron effective mass in all crystal directions. The computed binding energy of the unrelaxed exciton agrees with experimental data, and the values found imply a fast exciton dissociation at ambient temperature. Also polaron masses for the separated carriers are estimated. The values of all these parameters agree with recent indications that fast dynamics and large carrier diffusion lengths are key in the high photovoltaic efficiencies shown by these materials.

Holographic lattices give the graviton an effective mass.

Abstract: We discuss the DC conductivity of holographic theories with translational invariance broken by a background lattice. We show that the presence of the lattice induces an eective mass for the graviton via a gravitational version of the Higgs mechanism. This allows us to obtain, at leading order in the lattice strength, an analytic expression for the DC conductivity in terms of the size of the lattice at the horizon. In locally critical theories this leads to a power law resistivity that is in agreement with an earlier eld theory analysis of Hartnoll and Hofman.

Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state

Abstract: Applying a very large magnetic field to charge-neutral monolayer graphene produces a symmetry-protected quantum spin Hall state with helical edge states whose properties can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability. In the search for new electronic states made robust against disturbances by their topological features, Pablo Jarillo-Herrero and colleagues have identified graphene edge states that result from strong interactions between electrons. In contrast to well-studied topological insulators in which time reversal symmetry breaking has an essential role, these newly discovered graphene states are protected by symmetry rules. The novel electronic states, which separate electrons by their spin, appear when graphene is subjected to a large magnetic field angled with respect to the plane. The dependence on electron–electron interactions makes it possible to control the features with a voltage, revealing a fundamentally new electronic system with tunable electronic band gap and associated spin properties. Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires and graphene. Recently, a new method has emerged with the recognition that symmetry-protected topological (SPT) phases1,2, which occur in systems with an energy gap to quasiparticle excitations (such as insulators or superconductors), can host robust surface states that remain gapless as long as the relevant global symmetry remains unbroken. The nature of the charge carriers in SPT surface states is intimately tied to the symmetry of the bulk, resulting in one- and two-dimensional electronic systems with novel properties. For example, time reversal symmetry endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, fixing the orientation of their spin relative to their momentum3,4. Weakly breaking this symmetry generates a gap on the surface5, resulting in charge carriers with finite effective mass and exotic spin textures6. Analogous manipulations have yet to be demonstrated in two-dimensional topological insulators, where the primary example of a SPT phase is the quantum spin Hall state7,8. Here we demonstrate experimentally that charge-neutral monolayer graphene has a quantum spin Hall state9,10 when it is subjected to a very large magnetic field angled with respect to the graphene plane. In contrast to time-reversal-symmetric systems7, this state is protected by a symmetry of planar spin rotations that emerges as electron spins in a half-filled Landau level are polarized by the large magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability11,12,13, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with a tunable bandgap and an associated spin texture14.

Trion-Induced Negative Photoconductivity in Monolayer MoS 2

Abstract: Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We, however, observe a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS(2)), a two-dimensional (2D) semiconductor, using ultrafast optical-pump terahertz-probe spectroscopy. In particular, the conductivity is reduced to only 30% of its equilibrium value at high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the 2D system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the conductivity.

Crystal Structures, Optical Properties, and Effective Mass Tensors of CH3NH3PbX3 (X = I and Br) Phases Predicted from HSE06.

Abstract: The crystal structures are successfully established for tetragonal and orthorhombic CH3NH3PbX3 (X = I and Br). The equilibrium lattice parameters are computed by the DFT+D2 method, and the results are compared to experimental values. The band dispersions and electronic densities of states are calculated by HSE06, showing that their band gaps are in the range from 1.63 to 2.3 eV. Although the calculated dielectric functions of MAPbX3 compounds are similar to other semiconductors, the absorption spectra of their bulk crystals are drifted away from visible light spectrum. The effective mass tensors of holes and electrons are also evaluated in three principal directions at the Γ point. The anisotropies in the effective masses of the hole and electron are illustrated for two orthorhombic phases.

DFT and k · p modelling of the phase transitions of lead and tin halide perovskites for photovoltaic cells

Abstract: 3D hybrid organic perovskites, CH3NH3PbX3 (X = halogen), have recently been used to strongly improve the efficiency of dye sensitized solar cells (DSSC) leading to a new class of low-cost mesoscopic solar cells. CsSnI3 perovskite can also be used for hole conduction in DSSC. Density functional theory and GW corrections are used to compare lead and tin hybrid and all-inorganic perovskites. The room temperature optical absorption is associated to electronic transitions between the spin–orbit split-off band in the conduction band and the valence band. Spin–orbit coupling is about three times smaller for tin. Moreover, the effective mass of relevant band edge hole states is small (0.17). The high temperature phase sequence of CsSnI3 leading to the room temperature orthorhombic phase and the recently reported phases of CH3NH3MI3 (where M = Pb, Sn) close to the room temperature, are also studied. Tetragonal distortions from the ideal cubic phase are analysed by a k · p perturbation, including spin–orbit effect. In addition, the non-centrosymmetric phases of CH3NH3MI3 exhibit a splitting of the electronic bands away from the critical point. The present work shows that their physical properties are more similar to conventional semiconductors than to the absorbers used in DSSC. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum confinement in Si and Ge nanostructures: Theory and experiment

Abstract: The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters.

Large Enhancement and Tunable Band Gap in Silicene by Small Organic Molecule Adsorption

Abstract: Adsorption of eight organic molecules (acetone, acetonitrile, ammonia, benzene, methane, methanol, ethanol, and toluene) onto silicene has been investigated using van der Waals density functional theory calculations (DFT-D). The calculated values of the adsorption energies vary from −0.11 to −0.95 eV. Quantitatively, these values are higher than the corresponding adsorption energies of the molecules adsorbed on graphene. In addition, electronic structure calculations have been performed. The obtained values of the band gap range from 0.006 to 0.35 eV for acetonitrile to acetone, respectively. Furthermore, the effective mass of the electron is estimated and found to be comparatively small, which is expected to result in high electron mobility. In addition, we study the effect of Li atoms doped in pristine and acetone adsorbed silicene. In particular, we focus on the variation of the adsorption energy with respect to the number of Li atoms in the systems. Our results suggest new approaches for the use of si...

Neutrino quantum kinetics

Abstract: We present a formulation of the quantum kinetic equations (QKEs), which govern the evolution of neutrino flavor at high density and temperature. Here, the structure of the QKEs is derived from the ground up, using fundamental neutrino interactions and quantum field theory. We show that the resulting QKEs describe coherent flavor evolution with an effective mass when inelastic scattering is negligible. The QKEs also contain a collision term. This term can reduce to the collision term in the Boltzmann equation when scattering is dominant and the neutrino effective masses and density matrices become diagonal in the interaction basis. We also find that the QKEs include equations of motion for a new dynamical quantity related to neutrino spin. This quantity decouples from the equations of motion for the density matrices at low densities or in isotropic conditions. However, the spin equations of motion allow for the possibility of coherent transformation between neutrinos and antineutrinos at high densities and in the presence of anisotropy. Although the requisite conditions for this exist in the core collapse supernova and compact object merger environments, it is likely that only a self-consistent incorporation of the QKEs in a sufficiently realistic model could establish whether or not significant neutrino-antineutrino conversion occurs.

Inertia, diffusion, and dynamics of a driven skyrmion

Abstract: Skyrmions recently discovered in chiral magnets are a promising candidate for magnetic storage devices because of their topological stability, small size ($\ensuremath{\sim}3--100$ nm), and ultralow threshold current density ($\ensuremath{\sim}{10}^{6}$ ${A/m}^{2}$) to drive their motion. However, the time-dependent dynamics has hitherto been largely unexplored. Here, we show, by combining the numerical solution of the Landau-Lifshitz-Gilbert equation and the analysis of a generalized Thiele's equation, that inertial effects are almost completely absent in skyrmion dynamics driven by a time-dependent current. In contrast, the response to time-dependent magnetic forces and thermal fluctuations depends strongly on frequency and is described by a large effective mass and a (anti-) damping depending on the acceleration of the skyrmion. Thermal diffusion is strongly suppressed by the cyclotron motion and is proportional to the Gilbert damping coefficient $\ensuremath{\alpha}$. This indicates that the skyrmion position is stable, and its motion responds to the time-dependent current without delay or retardation even if it is fast. These findings demonstrate the advantages of skyrmions as information carriers.

Variational study of polarons in Bose-Einstein condensates

Abstract: We use a class of variational wave functions to study the properties of an impurity in a Bose-Einstein condensate, i.e., the Bose polaron. The impurity interacts with the condensate through a contact interaction, which can be tuned by a Feshbach resonance. We find a stable attractive polaron branch that evolves continuously across the resonance to a tight-binding diatomic molecule deep in the positive scattering length side. A repulsive polaron branch with finite lifetime is also observed and it becomes unstable as the interaction strength increases. The effective mass of the attractive polaron also changes smoothly across the resonance connecting the two well-understood limits deep on both sides.

How Does Chemistry Influence Electron Effective Mass in Oxides? A High-Throughput Computational Analysis

Abstract: Many technologies require oxides with high electronic conductivity or mobility (e.g., transparent conducting oxides, oxide photovoltaics, or photocatalysis). Using high-throughput ab initio computing, we screen more than 4000 binary and ternary oxides to identify the compounds with the lowest electron effective mass. We identify 74 promising oxides and suggest a few novel potential n-type transparent conducting oxides combining a large band gap to a low effective mass. Our analysis indicates that it is unlikely to find oxides with electron effective masses significantly lower than the current high-mobility binary oxides (e.g., ZnO and In2O3). Using the large data set, we extract chemical rules leading to low electron effective masses in oxides. Main group elements with (n–1)d10ns0np0 cations in the rows 4 and 5 and groups 12–15 of the periodic table (i.e., Zn2+, Ga3+, Ge4+, Cd2+, In3+, Sn4+, and Sb5+) induce the lowest electron effective masses because of their s orbitals hybridizing adequately with oxyge...

Mechanically induced metal-insulator transition in carbyne.

Abstract: First-principles calculations for carbyne under strain predict that the Peierls transition from symmetric cumulene to broken-symmetry polyyne structure is enhanced as the material is stretched. Interpretation within a simple and instructive analytical model suggests that this behavior is valid for arbitrary 1D metals. Further, numerical calculations of the anharmonic quantum vibrational structure of carbyne show that zero-point atomic vibrations eliminate the Peierls distortion in the mechanically free chain, preserving the cumulene symmetry. The emergence and increase of Peierls dimerization under tension then implies a qualitative transition between the two forms, which our computations place around 3% strain. Thus, the competition between the zero-point vibrations and mechanical strain determines a switch in symmetry resulting in the transition from metallic state to a dielectric, with a small effective mass and a high carrier mobility. In any practical realization, it is important that the effect is a...

Transition from parabolic to ring-shaped valence band maximum in few-layer GaS, GaSe, and InSe

Abstract: By performing first-principles electronic structure calculations in frames of density functional theory we study the dependence of the valence band shape on the thickness of few-layer III-VI crystals (GaS, GaSe, and InSe). We estimate the critical thickness of transition from the bulklike parabolic to the ring-shaped valence band. Direct supercell calculations show that the ring-shaped extremum of the valence band appears in $\ensuremath{\beta}$-GaS and $\ensuremath{\beta}$-GaSe at a thickness below 6 tetralayers $(\ensuremath{\sim}4.6nm)$ and 8 tetralayers $(\ensuremath{\sim}6.4nm)$, respectively. Zone-folding calculations estimate the $\ensuremath{\beta}$-InSe critical thickness to be equal to 28 tetralayers $(\ensuremath{\sim}24.0nm)$. The origin of the ring-shaped valence band maximum can be understood in terms of $\text{k}\ifmmode\cdot\else\textperiodcentered\fi{}\text{p}$ theory, which provides a link between the curvature of the energy bands and the distance between them. We explain the dependence of the band shape on the thickness, as well as the transition between two types of extremes, by the $k$-dependent orbital composition of the topmost valence band. We show that in the vicinity of critical thickness the effective mass of holes in III-VI compounds depends strongly on the number of tetralayers.

Thermoelectric properties of Sn-doped p-type Cu3SbSe4: a compound with large effective mass and small band gap

Abstract: Cu_3SbSe_4-based compounds composed of earth-abundant elements have been found to exhibit good thermoelectric performance at medium temperatures. High zT values were achieved in previous studies, but further insight into the transport mechanism as well as some key material parameters is still needed. In this work, we studied the electrical and thermal transport properties of Sn-doped Cu_3SbSe_4 between 300 K and 673 K. It was found that the single parabolic band model explains the electrical transport very well. Experimentally, we determined the band gap to be around 0.29 eV. The density-of-state effective mass was found to be about 1.5 me for the doped samples. The transport properties suggested degeneracy splitting near the valence band maximum that was not captured by previous band structure calculations. The maximum zT ~0.70 was obtained at 673 K, and the optimized carrier density was ~1.8 × 10^20 cm^(−3), and the potential for further improvement of zT via material engineering is briefly discussed.

Advanced thermoelectrics governed by a single parabolic band: Mg2Si0.3Sn0.7, a canonical example

Abstract: The well-known single parabolic band (SPB) model has been useful in providing insights into the understanding of transport properties of numerous thermoelectric materials. However, the conduction and valence bands of real semiconductors are rarely truly parabolic which limits the predictive power of the SPB model. The coincidence of the band edges of two parabolic bands, a situation arising in Mg2Si1−xSnx solid solutions when x ∼ 0.7, naturally makes the SPB approximation applicable to evaluate all transport parameters. We demonstrate this in the case of Bi-doped Mg2Si0.3Sn0.7 where the minima of the two conduction bands at the X-point of the Brillouin zone coincide. The combination of a large density-of-states effective mass m* ∼ 2.6 me arising from the enhanced valley degeneracy Nv, high mobility μd due to low deformation potential Ed (8.77–9.43 eV), and ultra-low alloy scattering parameter Ea (0.32–0.39 eV) leads to an outstanding power factor, PFmax ∝ (m*)3/2μd, of up to 4.7 mW m−1 K−2 at around 600 K. The specification and improved understanding of scattering parameters using the SPB model are important and instructive for further optimization of the thermoelectric performance of n-type Mg2Si0.3Sn0.7.

Metaconcrete: designed aggregates to enhance dynamic performance

Abstract: We propose a new type of concrete for the attenuation of elastic waves induced by dynamic excitation In this metamaterial, which we call metaconcrete, the stone, sand, and gravel aggregates of standard concrete are replaced with spherical inclusions consisting of a heavy metal core coated with a soft outer layer These engineered aggregates can be tuned so that particular frequencies of a propagating blast wave will activate resonant oscillations of the heavy mass within the inclusions The resonant behavior causes the system to exhibit negative effective mass, and this interaction between the wave motion and the resonant aggregates results in the attenuation of the applied dynamic loading We introduce the concept of negative mass by deriving the effective momentum mass for the system and we define the geometrical and material parameters for the design of resonant aggregates We develop finite element models for the analysis of metaconcrete behavior, defining a section of slab containing a periodic arrangement of inclusions By computing the energy histories for the system when subject to a blast load, we show that there is a transfer of energy between the inclusions and the surrounding mortar The inclusions are able to absorb a significant portion of the applied energy, resulting in a reduction in the amount of stress carried by the mortar phase and greatly improving the ability of the material to resist damage under explosive dynamic loading

Quantum Yield of Polariton Emission from Hybrid Light-Matter States.

Abstract: The efficiency of light-matter strong coupling is tuned by precisely varying the spatial position of a thin layer of cyanine dye J-aggregates in Fabry–Perot microcavities, and their photophysical properties are determined. Placing the layer at the cavity field maximum affords an interaction energy (Rabi splitting) of 503 meV, a 62% increase over that observed if the aggregates are simply spread evenly through the cavity, placing the system in the ultrastrong coupling regime. The fluorescence quantum yield of the lowest polaritonic state P– integrated over k-space is found to be ∼10–2. The same value can be deduced from the 1.4 ps lifetime of P– measured by femtosecond transient absorption spectroscopy and the calculated radiative decay rate constant. Thus, the polariton decay is dominated by nonradiative processes, in contrast with what might be expected from the small effective mass of the polaritons. These findings provide a deeper understanding of hybrid light-molecule states and have implications for ...

Quantum droplets of electrons and holes

Abstract: Fast optical pulses create a plasma of electrons and holes in a semiconductor in which excitons (pairs of holes and electrons) and combinations of two excitons emerge; now a stable liquid-like droplet of electrons and holes has been detected and called a ‘dropleton’. Excitons, plasmons and phonons are some of the better known quasiparticles — exotic entities that act in some respects like ordinary particles. New types do not come along all that often but here is one — a fundamentally new many-body particle named the 'dropleton'. Mackillo Kira and colleagues have identified this new quantum entity, a quantum droplet created when four or more electrons and holes (electronic vacancies) form a tiny correlation bubble via the Coulomb attraction, in direct-gap semiconductors such as gallium arsenide. The cover illustrates the pair-correlation function g(r) of quantum droplets — the central peak of the correlation function shows that electrons and holes are likely to be co-located and the ripples show that otherwise they form regularly spaced shells. Interacting many-body systems are characterized by stable configurations of objects—ranging from elementary particles to cosmological formations1,2,3—that also act as building blocks for more complicated structures. It is often possible to incorporate interactions in theoretical treatments of crystalline solids by introducing suitable quasiparticles that have an effective mass, spin or charge4,5 which in turn affects the material’s conductivity, optical response or phase transitions2,6,7. Additional quasiparticle interactions may also create strongly correlated configurations yielding new macroscopic phenomena, such as the emergence of a Mott insulator8, superconductivity or the pseudogap phase of high-temperature superconductors9,10,11. In semiconductors, a conduction-band electron attracts a valence-band hole (electronic vacancy) to create a bound pair, known as an exciton12,13, which is yet another quasiparticle. Two excitons may also bind together to give molecules, often referred to as biexcitons14, and even polyexcitons may exist15,16. In indirect-gap semiconductors such as germanium or silicon, a thermodynamic phase transition may produce electron–hole droplets whose diameter can approach the micrometre range17,18. In direct-gap semiconductors such as gallium arsenide, the exciton lifetime is too short for such a thermodynamic process. Instead, different quasiparticle configurations are stabilized dominantly by many-body interactions, not by thermalization. The resulting non-equilibrium quantum kinetics is so complicated that stable aggregates containing three or more Coulomb-correlated electron–hole pairs remain mostly unexplored. Here we study such complex aggregates and identify a new stable configuration of charged particles that we call a quantum droplet. This configuration exists in a plasma and exhibits quantization owing to its small size. It is charge neutral and contains a small number of particles with a pair-correlation function that is characteristic of a liquid. We present experimental and theoretical evidence for the existence of quantum droplets in an electron–hole plasma created in a gallium arsenide quantum well by ultrashort optical pulses.

Tunable Electronic Properties of Two-Dimensional Transition Metal Dichalcogenide Alloys: A First-Principles Prediction

Abstract: We investigated the composition-dependent electronic properties of two-dimensional transition-metal dichalcogenide alloys (WxMo1–xS2) based on first-principles calculations by applying the supercel...

Anisotropic two-dimensional electron gas at SrTiO3(110)

Abstract: Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG––the SrTiO3(110)-(4 × 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising “semiheavy” band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system.

Active acoustic metamaterials with tunable effective mass density by gradient magnetic fields

Abstract: Magnetically controlled acoustic metamaterials are designed and experimentally studied. Magneto-acoustic metamaterials are fabricated by covering an aluminum circular ring with magnetorheological elastomer. The resonant frequency of the structured elastomer is actively tunable by external gradient magnetic field, allowing for values of effective mass density of metamaterials to be adjusted in the low-frequency region. A prestressed plate theory is proposed to explain the shifting of the resonant frequency induced by the magnetic field and coincides very well with the experimental results. It is found that the tunability of magneto-acoustic metamaterials is attributed to the competition between the magnetic-field-induced prestress and the structural flexural rigidity. The proposed magneto-acoustic metamaterials realize the dynamic tuning of effective mass density with non-contact and fast-response gradient magnetic fields, providing a degree of freedom for full control of sound.

Acoustic cloaking by a near-zero-index phononic crystal

Abstract: Zero-refractive-index materials may lead to promising applications in various fields. Here, we design and fabricate a near Zero-Refractive-Index (ZRI) material using a phononic crystal (PC) composed of a square array of densely packed square iron rods in air. The dispersion relation exhibits a nearly flat band across the Brillouin zone at the reduced frequency f = 0.5443c/a, which is due to Fabry-Perot (FP) resonance. By using a retrieval method, we find that both the effective mass density and the reciprocal of the effective bulk modulus are close to zero at frequencies near the flat band. We also propose an equivalent tube network model to explain the mechanisms of the near ZRI effect. This FP-resonance-induced near ZRI material offers intriguing wave manipulation properties. We demonstrate both numerically and experimentally its ability to shield a scattering obstacle and guide acoustic waves through a bent structure.

Thermoelectric alloys between PbSe and PbS with effective thermal conductivity reduction and high figure of merit

Abstract: The n-type alloys between PbSe and PbS are studied. The effect of alloy composition on transport properties is evaluated and the results are interpreted with theories based on random atomic site substitution. The alloying in PbSe1−xSx brings thermal conductivity reduction, carrier mobility reduction as well as change of effective mass. When all these factors are evaluated, both experimentally and theoretically, the optimized thermoelectric performance is found to change gradually with alloy composition. High zT can be found in all PbSe1−xSx alloys. The possibility of achieving significant improvement of zT through alloying is also discussed.

Improved Photoelectrochemical Water Splitting Performance of Cu2O/SrTiO3 Heterojunction Photoelectrode

Abstract: Nanostructured thin films of Cu2O modified by overlayering SrTiO3 with varying thickness have been studied for the first time as photoelectrode in photoelectrochemical (PEC) water splitting. Effective mass calculations for electrons and holes in bulk SrTiO3 and Cu2O using DFT first-principles have also been attempted to explain the enhanced charge separation at Cu2O/SrTiO3 interface. All samples were characterized using XRD, SEM, and UV–vis spectrometry. The influence of surface modification of Cu2O with varying thickness of SrTiO3 on PEC performance has been investigated. Photocurrent density for Cu2O/SrTiO3 heterojunction with overall thickness of 343 nm at 0.8 V/SCE was found to be 2.52 mA cm–2 which is 25 times higher than that of pristine Cu2O (0.10 mA cm–2 at 0.8 V/SCE). Theoretical studies showed that the electrons in SrTiO3 had large effective masses as compared to electrons in Cu2O at conduction band minima indicating weak mobility of photogenerated electrons in SrTiO3 and strong mobility in Cu2O...