The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Role of Extracellular Matrix in Adaptation of Tendon and Skeletal Muscle to Mechanical Loading

In this article we review the experimental and theoretical investigations of the linear and nonlinear optical properties of semiconductor quantum well structures, including the effects of electrostatic fields, extrinsic carriers and real or virtual photocarriers.

Linear and nonlinear optical properties of semiconductor quantum wells

This paper describes the electronic energy levels of semiconductor heterostructures within the envelope function scheme. Quantum well and superlattice electronic states are calculated and discussed, especially the in-plane dispersion relations. The Coulombic bound states in heterostructures (impurities, excitons) are then discussed. Finally, we present a brief overview of the effect of a static electric field on the carrier and exciton energy levels in semiconductor quantum wells.

Electronic states in semiconductor heterostructures

Two-dimensional (2D) semiconductors, such as transition-metal dichalcogenide monolayers (TMD 1Ls), have attracted increasing attention owing to the underlying fundamental physics (e.g., many body effects) and the promising optoelectronic applications such as light-emitting diodes. Though much progress has been made, intrinsic excitonic states of TMD 1Ls are still highly debated in theory, which thirsts for direct experimental determination. Here, we report unconventional emission and excitonic fine structure in 1L WS2 revealed by electrical doping and photoexcitation, which reflects the interplay of exciton, trion, and other excitonic states. Tunable excitonic emission has been realized in a controllable manner via electrical and/or optical injection of charge carriers. Remarkably enough, the superlinear (i.e., quadratic) emission is unambiguously observed which is attributed to biexciton states, indicating the strong Coulomb interactions in such a 2D material. In a nearly neutral 1L WS2, trions and biexc...

/pdf/observation-of-excitonic-fine-structure-in-a-2d-transition-2yv8sk1jom.pdf

Observation of Excitonic Fine Structure in a 2D Transition-Metal Dichalcogenide Semiconductor

We have confirmed the presence of a two‐dimensional electron gas (2DEG) in a wide band‐gap GaN‐AlxGa1−xN heterojunction by observing steplike features in the quantum Hall effect. The 2DEG mobility for a GaN‐Al0.13Ga0.87N heterojunction was measured to be 834 cm2/V s at room temperature. It monotonically increased and saturated at a value of 2626 cm2/V s at 77 K. The 2DEG mobility remained nearly constant for temperatures ranging from 77 to 4.2 K. Using Shubnikov–de Haas (SdH) measurements the two‐dimensional carrier concentration was estimated to be 1×1011 cm−2. The peak mobility for the 2DEG was found to decrease with the heterojunction aluminum compositions in excess of 13%.

Observation of a two‐dimensional electron gas in low pressure metalorganic chemical vapor deposited GaN‐AlxGa1−xN heterojunctions

We have investigated the band-gap renormalization due to many-body effects in electron-hole plasmas in 2D GaAS-GaAlAs multiple quantum-well structures. A comparison of these data with corresponding 3D data and calculations for both dimensionalities shows that the band-gap shift increases absolutely but decreases in effective Rydberg units with decreasing dimensionality. The dimensionality dependence of the band-gap shift is traced to different screening efficiencies in 3D and 2D systems.

Dimensionality dependence of the band-gap renormalization in two- and three-dimensional electron-hole plasmas in GaAs

We have analyzed the dependence of the band-gap renormalization due to many-body effects on the carrier density in electron-hole plasmas in quasi-two-dimensional systems. A comparison of the data obtained at low lattice temperatures in various $\mathrm{GaAs}/{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$, GaSb/AlSb, and ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}/\mathrm{InP}$ multiple-quantum-well structures shows that the band-gap shift, if scaled in effective excitonic units, is independent of the material properties. This new general relation governing the dependence of the band-gap renormalization on the plasma density in two-dimensional systems is confirmed by theoretical considerations.

General relation between band-gap renormalization and carrier density in two-dimensional electron-hole plasmas.

InP, GaInAs and quantum well structures grown by adduct MOVPE

Size-induced direct-to-indirect gap transition in GaSb/AlSb multiple quantum well structures

Using reactive ion etching with CCl2F2/He a selectivity in etch rates between GaAs and AlGaAs of larger than 3000 has been obtained for low dc self‐bias voltages. Under isotropic etch conditions at low‐bias values this selectivity is used for the lateral etching of thin GaAs layers sandwiched between AlGaAs layers. The range of lateral etching is proportional to the square roots of the etch time and the thickness of the etched GaAs layer. Lateral etch widths of 4 μm for a GaAs layer thickness of 50 nm and larger than 20 μm for a thickness of 1 μm are achieved.

G. Tränkle

Papers

Dimensionality dependence of the band-gap renormalization in two- and three-dimensional electron-hole plasmas in GaAs

General relation between band-gap renormalization and carrier density in two-dimensional electron-hole plasmas.

InP, GaInAs and quantum well structures grown by adduct MOVPE

Size-induced direct-to-indirect gap transition in GaSb/AlSb multiple quantum well structures

Selective lateral dry etching of GaAs in AlGaAs/GaAs heterostructures with CCl2F2/He