Showing papers on "Effective mass (solid-state physics) published in 2013"

Electrical control of neutral and charged excitons in a monolayer semiconductor

Abstract: Monolayer group-VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts), which have thus far been challenging to generate and control in the ultimate two-dimensional limit. Utilizing high-quality monolayer molybdenum diselenide, we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X+), neutral (Xo) and negatively charged (X−) excitons in field-effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, whereas the linewidth is narrow (5 meV) at temperatures <55 K. This is greater spectral contrast than in any known quasi-two-dimensional system. We also find the charging energies for X+ and X− to be nearly identical implying the same effective mass for electrons and holes. Single layers of group-VI transition metal dichalcogenides have emerged as direct bandgap semiconductors in the two-dimensional limit. The authors show that monolayer molybdenum diselenide is an ideal system enabling electrostatic tunability of charging effects in neutral and charged electron-hole pairs, so-called excitons.

Direct Measurement of the Thickness-Dependent Electronic Band Structure of MoS2 Using Angle-Resolved Photoemission Spectroscopy

Abstract: We report on the evolution of the thickness-dependent electronic band structure of the two-dimensional layered-dichalcogenide molybdenum disulfide (MoS2). Micrometer-scale angle-resolved photoemission spectroscopy of mechanically exfoliated and chemical-vapor-deposition-grown crystals provides direct evidence for the shifting of the valence band maximum from Γ to K, for the case of MoS2 having more than one layer, to the case of single-layer MoS2, as predicted by density functional theory. This evolution of the electronic structure from bulk to few-layer to monolayer MoS2 had earlier been predicted to arise from quantum confinement. Furthermore, one of the consequences of this progression in the electronic structure is the dramatic increase in the hole effective mass, in going from bulk to monolayer MoS2 at its Brillouin zone center, which is known as the cause for the decreased carrier mobility of the monolayer form compared to that of bulk MoS2.

Electronic band structure, phonons, and exciton binding energies of halide perovskites CsSnCl 3 , CsSnBr 3 , and CsSnI 3

Abstract: The halide perovskites CsSn${X}_{3}$, with $X=$ Cl, Br, I, are investigated using quasiparticle self-consistent $GW$ electronic structure calculations. These materials are found to have an ``inverted'' band structure from most semiconductors with a nondegenerate $s$-like valence band maximum (VBM) and triply degenerate $p$-like conduction band minimum (CBM). The small hole effective mass results in high hole mobility, in agreement with recent reports for CsSnI${}_{3}$. The relatively small band gap changes from Cl to Br to I result from the intra-atomic Sn $s$ and Sn $p$ characters of the VBM and CBM, respectively. The latter is also responsible for the high oscillator strength of the optical transition in these direct-gap semiconductors and hence a strong luminescence and absorption. The band gap change with lattice constant is also anomalous. It increases with increasing lattice constant, and this results from the decreasing valence band width due to the decreased Sn $s$ with anion $p$ interaction. It leads to an anomalous temperature dependence of the gap. The changes in band gap in different lower-symmetry crystallographic phases is studied. The exciton binding energy of the free exciton, estimated from the Wannier-Mott exciton theory and the calculated dielectric constants and effective masses, is found to be two orders of magnitude smaller than previously claimed in literature, or of the order of 0.1 meV. The photoluminescence peak previously assigned to the free exciton is instead ascribed to an acceptor bound exciton. The phonons at the $\ensuremath{\Gamma}$ point are calculated as well as the related enhancement of the dielectric constants.

Beneficial Contribution of Alloy Disorder to Electron and Phonon Transport in Half-Heusler Thermoelectric Materials

Abstract: Electron and phonon transport characteristics determines the potential of thermoelectric materials for power generation or refrigeration. This work shows that, different from most of high performance thermoelectric materials with dominant acoustic phonon scattering, the promising ZrNiSn based half-Heusler thermoelectric solid solutions exhibit an alloy scattering dominated charge transport. A low deformation potential and a low alloy scattering potential are found for the solid solutions, which is beneficial to maintain a relatively high electron mobility despite of the large effective mass, and can be intrinsic favorable features contributing to the noticeably high power factors of ZrNiSn based alloys. A quantitive description of the different phonon scattering mechanisms suggests that the point defect scattering is the most important mechanism that determines the phonon transport process of the solid solutions. The present results indicate that alloying can be an effective approach for such materials systems to enhance thermoelectric figure of merit ZT by reducing phonon thermal conductivity, while minimizing the deterioration of charge mobility due to the low alloy scattering potential.

Coupled Membranes with Doubly Negative Mass Density and Bulk Modulus

Abstract: We present a structurally and conceptually simple acoustic double negative metamaterial comprising two coupled membranes. Owing to its symmetry, the system can generate both monopolar and dipolar resonances that are separately tunable, thereby making broadband double negativity possible. A homogenization scheme is implemented that enables the exact characterization of our metamaterial by the effective mass density and bulk modulus even beyond the usual long-wavelength regime, with the measured displacement fields on the sample's surfaces as inputs. Double negativity is achieved in the frequency range of 520--830 Hz. Transmission and reflection predictions using effective parameters are shown to agree remarkably well with the experiment.

Massive Dirac quasiparticles in the optical absorbance of graphene, silicene, germanene, and tinene.

Abstract: We present first-principles studies of the optical absorbance of the group IV honeycomb crystals graphene, silicene, germanene, and tinene. We account for many-body effects on the optical properties by using the non-local hybrid functional HSE06. The optical absorption peaks are blueshifted due to quasiparticle corrections, while the influence on the low-frequency absorbance remains unchanged and reduces to a universal value related to the Sommerfeld fine structure constant. At the Dirac points spin-orbit interaction opens fundamental band gaps; parabolic bands with a very small effective mass emerge. Consequently, the low-frequency absorbance is modified with a spin-orbit-induced transparency region and an increase of the absorbance at the fundamental absorption edge.

Low Electron Scattering Potentials in High PerformanceMg_2Si_(0.45)Sn_(0.55) Based Thermoelectric Solid Solutions with Band Convergence

Abstract: Understanding the electron and phonon transport characteristics is crucial for designing and developing high performance thermoelectric materials. Weak scattering effects on charge carriers, characterized by deformation potential and alloy scattering potential, are favorable for thermoelectric solid solutions to enable high carrier mobility and thereby promising thermoelectric performance. Mg_2(Si,Sn) solid solutions have attracted much attention due to their low cost and environmental compatibility. Usually, their high thermoelectric performance with ZT ∼ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. In this work, both a low deformation potential Ξ = 13 eV and a low alloy scattering potential U = 0.7 eV are found for the thermoelectric alloys by characterizing and modeling of thermoelectric transport properties. The band convergence is also verified by the increased density-of-states effective mass. It is proposed that, in addition to band convergence and reduced lattice thermal conductivity, the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.

Colloidal graphene quantum dots with well-defined structures.

Abstract: When the size of a semiconductor crystal is reduced to the nanometer scale, the crystal boundary significantly modifies electron distribution, making properties such as bandgap and energy relaxation dynamics size dependent. This phenomenon, known as quantum confinement, has been demonstrated in many semiconductor materials, leading to practical applications in areas such as bioimaging, photovoltaics, and light-emitting diodes.Graphene, a unique type of semiconductor, is a two-dimensional crystal with a zero bandgap and a zero effective mass of charge carriers. Consequently, we expect new phenomena from nanometer-sized graphene, or graphene quantum dots (QDs), because the energy of charge carriers in graphene follows size-scaling laws that differ from those in other semiconductors. From a chemistry point of view, graphene is made of carbon, an element for which researchers have developed a whole branch of chemistry. Thus, it is possible to synthesize graphene QDs through stepwise, well-controlled organic c...

Linear and nonlinear optical properties of ZnO/ZnS and ZnS/ZnO core shell quantum dots: Effects of shell thickness, impurity, and dielectric environment

Abstract: In the present work, we investigated theoretically the linear, nonlinear, and total absorption coefficients and refractive index changes associated with intersubband transitions in ZnO/ZnS core shell quantum dot (CSQD) and ZnS/ZnO inverted CSQD (ICSQD), emphasizing on the influence of the shell thickness, impurity, and dielectric environment. The effect of the polarization charges due to the possible existence of the dielectric mismatch between the system and its surrounding matrix is considered. The electronic structures are numerically calculated by employing the potential morphing method in the framework of effective mass approximation. We find that in both impurity-free CSQD and ICSQD, increasing the shell thickness red shifts significantly the threshold energy and enhances drastically the nonlinear absorption coefficients and all the refractive index changes, independently on the dielectric environments. Similar behaviour has also been observed in most of the cases studied when the impurity is displaced from the core center to the shell center. In contrast, comparing to a dielectrically homogeneous system, dispersing the systems into a matrix with a lower dielectric constant blue shifts all the peak positions of the absorption coefficients and refractive index changes. However, the corresponding magnitudes (in absolute value) are substantially reduced. Finally, we find that the nonlinear properties are more sensitive to the external perturbations, while at a weak radiation intensity, the variation of the total quantities is generally dominated by that of the corresponding linear terms.

Heavy solitons in a fermionic superfluid

Abstract: Solitons—solitary waves that maintain their shape as they propagate—occur as water waves in narrow canals, as light pulses in optical fibres and as quantum mechanical matter waves in superfluids and superconductors. Their highly nonlinear and localized nature makes them very sensitive probes of the medium in which they propagate. Here we create long-lived solitons in a strongly interacting superfluid of fermionic atoms and directly observe their motion. As the interactions are tuned from the regime of Bose–Einstein condensation of tightly bound molecules towards the Bardeen–Cooper–Schrieffer limit of long-range Cooper pairs, the solitons’ effective mass increases markedly, to more than 200 times their bare mass, signalling strong quantum fluctuations. This mass enhancement is more than 50 times larger than the theoretically predicted value. Our work provides a benchmark for theories of non-equilibrium dynamics of strongly interacting fermions. Solitons — solitary waves that maintain their shape as they propagate — in a strongly interacting superfluid of fermionic lithium atoms are found to have an effective mass more than 50 times larger than the theoretically predicted value, a sign of strong quantum fluctuations. Solitons — solitary waves that maintain their shape as they propagate — occur in nonlinear systems ranging from shallow waterways to DNA and act as exquisite probes of the medium in which they propagate. These authors create long-lived solitons in a strongly interacting superfluid of fermionic lithium atoms and directly observe their motion. As the interactions are tuned, the effective mass of the solitons increases by a factor of at least 200, more than fifty times greater than the theoretically predicted value. The observed mass enhancement is a sign of strong quantum fluctuations and provides an important benchmark for theories on non-equilibrium dynamics of strongly interacting fermions.

Comprehensive study of the metal-insulator transition in pulsed laser deposited epitaxial VO2 thin films

Abstract: In this paper, we present a comprehensive, correlative study of the structural, transport, optical and thermoelectric properties of high-quality VO2 thin films across its metal-insulator phase transition. Detailed x-ray diffraction study shows that it's textured polycrystalline along [010]M1, with in-plane lattice orienting along three equivalent crystallographic directions. Across the metal-insulator transition, the conductivity increases by more than 3 orders of magnitude with a value of 3.8 × 103 S/cm in the metallic phase. This increase is almost entirely accounted for by a change in electron density, while the electron mobility changes only slightly between the two phases, yet shows strong domain boundary scattering when the two phases coexist. Electron effective mass was determined to be ∼65m0 in the insulating phase. From the optical and infrared reflection spectra in the metallic phase, we obtained the plasma edge of VO2, from which the electron effective mass was determined to be ∼23m0. The bandg...

Inverse relationship between carrier mobility and bandgap in graphene

Abstract: A frequently stated advantage of gapless graphene is its high carrier mobility. However, when a nonzero bandgap is opened, the mobility drops dramatically. The hardness to achieve high mobility and large on/off ratio simultaneously limits the development of graphene electronics. To explore the underlying mechanism, we investigated the intrinsic mobility of armchair graphene nanoribbons (AGNRs) under phonon scattering by combining first-principles calculations and a tight-binding analysis. A linear dependence of the effective mass on bandgap was demonstrated to be responsible for the inverse mobility-gap relationship. The deformation-potential constant was found to be determined by the strain dependence of the Fermi energy and the bandgap, resulting in two mobility branches, and is essential for the high mobility of AGNRs. In addition, we showed that the transport polarity of AGNRs can be switched by applying a uniaxial strain.

Hartree-Fock-Bogoliubov nuclear mass model with 0.50 MeV accuracy based on standard forms of Skyrme and pairing functionals

Abstract: We present a new Hartree-Fock-Bogoliubov nuclear mass model based on standard forms of Skyrme and pairing functionals, which corresponds to the most accurate mass model we ever achieved within the framework of the nuclear energy density functional theory. Our new mass model is characterized by a model standard deviation ${\ensuremath{\sigma}}_{\mathrm{mod}}=0.500$ MeV with respect to essentially all the 2353 available mass data for nuclei with neutron and proton numbers larger than 8. At the same time, the underlying Skyrme functional yields a realistic description of infinite homogeneous nuclear matter properties, as determined by realistic calculations and by experiments; these include in particular the incompressibility coefficient, the pressure in charge-symmetric nuclear matter, the neutron-proton effective mass splitting, the stability against spin and spin-isospin fluctuations, as well as the neutron-matter equation of state.

Tunable Band Gap and Doping Type in Silicene by Surface Adsorption: towards Tunneling Transistors

Abstract: Structural and electronic properties of silicene adsorbed by five kinds of transition metal atoms (Cu, Ag, Au, Pt, and Ir) are systematically studied by using first-principles calculations. We find that such adsorption can induce a band gap at the Dirac point of doped silicene. Doped silicene can reach a band gap up to 0.23 eV while keeping a relatively small effective mass of around 0.1 me, thus having high carrier mobility estimated to be 50000 cm2/Vs. P-type doping and neutral state is realized in silicene by Ir and Pt adsorption, respectively, while n-type doping is done by Cu, Ag, and Au adsorption. Based on the knowledge above, a silicene p-i-n tunneling field effect transistor (TFET) is proposed and simulated by both first-principles and semi-empirical approaches. Silicene TFET shows high performance with an on-off ratio of 10^3, a sub-threshold swing of 90 mV/dec, and an on-state current of 1 mA/{\mu}m. Such an on-state current is even larger than that of most other existing TFETs.

Origin of the superior conductivity of perovskite Ba(Sr)SnO3

Abstract: ASnO3 (A = Ba, Sr) are unique perovskite oxides in that they have superior electron conductivity despite their wide optical band gaps. Using first-principles band structure calculations, we show that the small electron effective masses, thus, good electron conductivity of ASnO3 can be attributed to the large size of Sn in this system that gives the conduction band edge with antibonding Sn and Os characters. Moreover, we show that ASnO3 can be easily doped by La with shallow LaA(+/0) donor level. Our results, therefore, explain why the perovskite BaSnO3, SrSnO3, and their alloys are promising candidates for transparent conducting oxides.

Large Enhancements of Thermopower and Carrier Mobility in Quantum Dot Engineered Bulk Semiconductors

Abstract: The thermopower (S) and electrical conductivity (σ) in conventional semiconductors are coupled adversely through the carriers’ density (n) making it difficult to achieve meaningful simultaneous improvements in both electronic properties through doping and/or substitutional chemistry. Here, we demonstrate the effectiveness of coherently embedded full-Heusler (FH) quantum dots (QDs) in tailoring the density, mobility, and effective mass of charge carriers in the n-type Ti0.1Zr0.9NiSn half-Heusler matrix. We propose that the embedded FH QD forms a potential barrier at the interface with the matrix due to the offset of their conduction band minima. This potential barrier discriminates existing charge carriers from the conduction band of the matrix with respect to their relative energy leading to simultaneous large enhancements of the thermopower (up to 200%) and carrier mobility (up to 43%) of the resulting Ti0.1Zr0.9Ni1+xSn nanocomposites. The improvement in S with increasing mole fraction of the FH-QDs aris...

Wigner crystallization of single photons in cold Rydberg ensembles.

Abstract: The coupling of weak light fields to Rydberg states of atoms under conditions of electromagnetically induced transparency leads to the formation of Rydberg polaritons which are quasiparticles with tunable effective mass and nonlocal interactions. Confined to one spatial dimension their low energy physics is that of a moving-frame Luttinger liquid which, due to the nonlocal character of the repulsive interaction, can form a Wigner crystal of individual photons. We calculate the Luttinger K parameter using density-matrix renormalization group simulations and find that under typical slow-light conditions kinetic energy contributions are too strong for crystal formation. However, adiabatically increasing the polariton mass by turning a light pulse into stationary spin excitations allows us to generate true crystalline order over a finite length. The dynamics of this process and asymptotic correlations are analyzed in terms of a time-dependent Luttinger theory.

Surface lattice resonances strongly coupled to Rhodamine 6G excitons: tuning the plasmon-exciton-polariton mass and composition

Abstract: We demonstrate the strong coupling of surface lattice resonances (SLRs) — hybridized plasmonic/photonic modes in metallic nanoparticle arrays — to excitons in Rhodamine 6G molecules. We investigate experimentally angle-dependent extinction spectra of silver nanorod arrays with different lattice constants, with and without the Rhodamine 6G molecules. The properties of the coupled modes are elucidated with simple Hamiltonian models. At low momenta, plasmon-exciton-polaritons — the mixed SLR/exciton states — behave as free-quasiparticles with an effective mass, lifetime, and composition tunable via the periodicity of the array. The results are relevant for the design of plasmonic systems aimed at reaching the quantum degeneracy threshold, wherein a single quantum state becomes macroscopically populated.

High-temperature surface superconductivity in rhombohedral graphite

Abstract: Surface superconductivity in rhombohedral graphite is a robust phenomenon which can exist even when higher order hoppings between the layers lift the topological protection of the surface flat band and introduce a quadratic dispersion of electrons with a heavy effective mass. We show that for weak pairing interaction, the flat-band character of the surface superconductivity transforms into a BCS-like relation with high critical temperature characterized by a higher coupling constant due to a much larger density of states than in the bulk. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high transition temperature.

Role of the effective mass and interfacial dipoles on exciton dissociation in organic donor-acceptor solar cells

Abstract: Efficient exciton dissociation at a donor-acceptor interface is the crucial, yet not fully understood, step for obtaining high efficiency organic solar cells. Recent theoretical work suggested an influence of polymer conjugation length and of interfacial dipoles on the exciton dissociation yield. This necessitates experimental verification. To this end, we measured the dissociation yield of several polymer/C${}_{60}$ planar heterojunction solar cells up to high electric fields. The results indeed prove that the yield of exciton dissociation depends strongly on the conjugation length of the polymers. Complementary photoemission experiments were carried out to assess the importance of dipoles at the donor-acceptor interfaces. Comparison of exciton dissociation models with experimental data shows that the widely used Onsager-Braun approach is unsuitable to explain photodissociation in polymer/C${}_{60}$ cells. Better agreement can be obtained using ``effective mass'' models that incorporate conjugation length effects by considering a reduced effective mass of the hole on the polymer and that include dielectric screening effects by interfacial dipoles. However, successful modeling of the photocurrent field dependence over a broad field range, in particular for less efficient solar cell compounds, requires that the dissociation at localized acceptor sites is also taken into account.

Two-dimensional excitons in three-dimensional hexagonal boron nitride

Abstract: The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8 A and 740 meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54mo, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.

Doping for higher thermoelectric properties in p-type BiCuSeO oxyselenide

Abstract: The low power factor (PF) of BiCuSeO oxyselenide inhibits further improvement on thermoelectric figure of merit in the moderate temperature range. In this Letter, we show that the electron transport properties of doped BiCuSeO oxyselenide can be accurately described in acoustic phonon scattering assumption within the framework of single parabolic band model. It is further found that the doping elements alter the electron transport properties by tuning the effective mass and deformation potential. Based on these understandings, we argue that the higher power factor can be achieved by choosing the doping element based on reducing deformation potential coefficient and decreasing effective mass.

Density of state effective mass and related charge transport properties in K-doped BiCuOSe

Abstract: We report the enhanced p-type conduction properties in BiCuOSe by doping of monovalent ions (K+). As compared with undoped BiCuOSe, simultaneous increase in both the carrier concentration and the Hall mobility was achieved in the K-doped BiCuOSe. The origin of the enhancement was discussed in terms of the two-band structure in the valence band of the BiCuOSe, and the density of state effective masses of the heavy (∼1.1 me) and light hole (∼0.18 me) were estimated by using Pisarenko relation.

Temperature dependence of the direct bandgap and transport properties of CdO

Abstract: Temperature-dependent optical absorption, Hall effect, and infrared reflectance measurements have been performed on as-grown and post-growth annealed CdO films grown by metal organic vapor phase epitaxy on sapphire substrates. The evolution of the absorption edge and conduction electron plasmon energy with temperature has been modeled, including the effects arising from the Burstein-Moss shift and bandgap renormalization. The zero-temperature fundamental direct bandgap and band edge effective mass have been determined to be 2.31+/-0.02 eV and 0.27+/-0.01m(0), respectively. The associated Varshni parameters for the temperature dependence of the bandgap are found to be a alpha = 8 x 10(-4) eV/K and beta = 260 K.

Effects of strain on the electron effective mass in GaN and AlN

Abstract: Stress is known to strongly alter the effective mass in semiconductors, changing the mobility of carriers. Transport measurements on AlGaN/GaN heterostructures indicated a large increase in mobility under tensile strain [M. Azize and T. Palacios, J. Appl. Phys. 108, 023707 (2010)]. Using first-principles methods, we calculate the variation of electron effective mass in GaN and AlN under hydrostatic and biaxial stress. Unexpected trends are found, which are explained within k·p theory through a variation of the interband momentum matrix elements. The magnitude of the effective-mass reduction is too small to explain the experimentally reported increase in mobility.

Negative mass density and ρ-near-zero quasi-two-dimensional metamaterials: Design and applications

Abstract: We report the design and the characterization of artificial structures made of periodical distributions of structured cylindrical scatterers embedded in a two-dimensional (2D) waveguide. For certain values of their geometrical parameters they show simultaneously negative effective bulk modulus and negative effective mass density. Here our analysis is focused on the frequencies where they behave like materials with negative density or density near zero (DNZ). The scattering units consist of a rigid cylindrical core surrounded by an anisotropic shell divided in angular sectors. The units are embedded in a 2D waveguide whose height is smaller than the length of the cylinders, which makes the structure quasi-2D. We have obtained the dispersion relation of the surface acoustic waves excited at frequencies with negative effective density. Also, we report phenomena associated with their DNZ behavior, such as tunneling through narrow channels, control of the radiation field, perfect transmission through sharp corners, and power splitting. Preliminary experiments performed on samples with millimeter-scale dimensions demonstrated their single-negative behavior, with the main drawback being the strong losses measured at the frequencies where the negative behavior is observed.

Cation composition effects on electronic structures of In-Sn-Zn-O amorphous semiconductors

Abstract: Based on density-functional theory calculations, the effects of cation compositions on electronic structures of In-Sn-Zn-O amorphous semiconductors were investigated. We considered various composition ratios of In, Sn, and Zn in O stoichiometric condition, and found that the conduction band minimum (CBM) energy level decreases and the valence band tail (VBT) energy level extent increases as the sum of In and Sn ratios (RIn+RSn) increases. The CBM lowering is attributed to the increased overlap of the In-5s and Sn-5s orbitals as the RIn+RSn increases, and correspondingly the electron effective masses (me*) are found to be reduced. The VBT increase is found to be due to the increased density of the In and Sn atoms, near which the O-2p inter-site ppσ* coupling is larger than that near the Zn atoms. The acute O-(In,Sn)-O angles are suggested to be structurally important, giving the stronger O-O ppσ* coupling.