Gapped Collective Charge Excitations and Interlayer Hopping in Cuprate Superconductors.

We use resonant inelastic x-ray scattering to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr_{0.9}La_{0.1}CuO_{2}. We detect a plasmon gap of ∼120 meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in Sr_{0.9}La_{0.1}CuO_{2} are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered t-J-V model calculations. A similar analysis performed on recent resonant inelastic x-ray scattering data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping t_{z}. Our work signifies the three dimensionality of the charge dynamics in layered cuprates and provides a new method to determine t_{z}.

Charge Density Waves in YBa_{2}Cu_{3}O_{6.67} Probed by Resonant X-Ray Scattering under Uniaxial Compression.

We report a comprehensive Cu L_{3}-edge resonant x-ray scattering (RXS) study of two- and three-dimensional (2D and 3D) incommensurate charge correlations in single crystals of the underdoped high-temperature superconductor YBa_{2}Cu_{3}O_{6.67} under uniaxial compression up to 1% along the two inequivalent Cu─O─Cu bond directions (a and b) in the CuO_{2} planes. We confirm the strong in-plane anisotropy of the 2D charge correlations and observe their symmetric response to pressure: pressure along a enhances correlations along b, and vice versa. Our results imply that the underlying order parameter is uniaxial. In contrast, 3D long-range charge order is only observed along b in response to compression along a. Spectroscopic RXS measurements show that the 3D charge order resides exclusively in the CuO_{2} planes and may thus be generic to the cuprates. We discuss implications of these results for models of electronic nematicity and for the interplay between charge order and superconductivity.

Re-entrant charge order in overdoped (Bi,Pb)2.12Sr1.88CuO6+δ outside the pseudogap regime.

In the underdoped regime, the cuprate high-temperature superconductors exhibit a host of unusual collective phenomena, including unconventional spin and charge density modulations, Fermi surface reconstructions, and a pseudogap in various physical observables. Conversely, overdoped cuprates are generally regarded as conventional Fermi liquids possessing no collective electronic order. In partial contradiction to this widely held picture, we report resonant X-ray scattering measurements revealing incommensurate charge order reflections for overdoped (Bi,Pb)2.12Sr1.88CuO6+δ (Bi2201), with correlation lengths of 40-60 lattice units, that persist up to temperatures of at least 250 K. The value of the charge order wavevector decreases with doping, in line with the extrapolation of the trend previously observed in underdoped Bi2201. In overdoped materials, however, charge order coexists with a single, unreconstructed Fermi surface without nesting or pseudogap features. The discovery of re-entrant charge order in Bi2201 thus calls for investigations in other cuprate families and for a reconsideration of theories that posit an essential relationship between these phenomena.

Fate of charge order in overdoped La-based cuprates.

In high-temperature cuprate superconductors, stripe order refers broadly to a coupled spin and charge modulation with a commensuration of eight and four lattice units, respectively. How this stripe order evolves across optimal doping remains a controversial question. Here we present a systematic resonant inelastic x-ray scattering study of weak charge correlations in La2-xSrxCuO4 and La1.8-xEu0.2SrxCuO4. Ultra high energy resolution experiments demonstrate the importance of the separation of inelastic and elastic scattering processes. Long-range temperature-dependent stripe order is only found below optimal doping. At higher doping, short-range temperature-independent correlations are present up to the highest doping measured. This transformation is distinct from and preempts the pseudogap critical doping. We argue that the doping and temperature-independent short-range correlations originate from unresolved electron-phonon coupling that broadly peaks at the stripe ordering vector. In La2-xSrxCuO4, long-range static stripe order vanishes around optimal doping and we discuss both quantum critical and crossover scenarios.

Chronic denervation of rat hemidiaphragm: maintenance of fiber heterogeneity with associated increasing uniformity of myosin isoforms.

During several months of denervation, rat mixed muscles lose slow myosin, though with variability among animals. Immunocytochemical studies showed that all the denervated fibers of the hemidiaphragm reacted with anti-fast myosin, while many reacted with anti-slow myosin as well. This has left open the question as to whether multiple forms of myosin co-exist within individual fibers or a unique, possibly embryonic, myosin is present, which shares epitopes with fast and slow myosins. Furthermore, one can ask if the reappearance of embryonic myosin in chronically denervated muscle is related both to its re-expression in the pre-existing fibers and to cell regeneration. To answer these questions we studied the myosin heavy chains from individual fibers of the denervated hemidiaphragm by SDS PAGE and morphologically searched for regenerative events in the long term denervated muscle. 3 mo after denervation the severely atrophic fibers of the hemidiaphragm showed either fast or a mixture of fast and slow myosin heavy chains. Structural analysis of proteins sequentially extracted from muscle cryostat sections showed that slow myosin was still present 16 mo after denervation, in spite of the loss of the selective distribution of fast and slow features. Therefore muscle fibers can express adult fast myosin not only when denervated during their differentiation but also after the slow program has been expressed for a long time. Light and electron microscopy showed that the long-term denervated muscle maintained a steady-state atrophy for the rat's life span. Some of the morphological features indicate that aneural regeneration events continuously occur and significantly contribute to the increasing uniformity of the myosin gene expression in long-term denervated diaphragm.

Dynamical charge density fluctuations pervading the phase diagram of a Cu-based high-T c superconductor.

Charge density modulations have been observed in all families of high-critical temperature (T c) superconducting cuprates. Although they are consistently found in the underdoped region of the phase diagram and at relatively low temperatures, it is still unclear to what extent they influence the unusual properties of these systems. Using resonant x-ray scattering, we carefully determined the temperature dependence of charge density modulations in YBa2Cu3O7-δ and Nd1+ x Ba2- x Cu3O7-δ for several doping levels. We isolated short-range dynamical charge density fluctuations in addition to the previously known quasi-critical charge density waves. They persist up to well above the pseudogap temperature T*, are characterized by energies of a few milli-electron volts, and pervade a large area of the phase diagram.

Magnetic bistability of a TbPc2 submonolayer on a graphene/SiC(0001) conductive electrode.

The alteration of the properties of single-molecule magnets (SMMs) due to the interaction with metallic electrodes is detrimental to their employment in spintronic devices. Conversely, herein we show that the terbium(iii) bis-phthalocyaninato complex, TbPc2, maintains its SMM behavior up to 9 K on a graphene/SiC(0001) substrate, making this alternative conductive layer highly promising for molecular spintronic applications.

Myosin light chains and muscle pathology.

We evaluated the isoform composition of heavy and light chains of myosin in single muscle fibers from patients with Duchenne dystrophy, myotonic dystrophy, or polymyositis. In all myopathic muscles, there was an increase in the proportion of intermediate fibers which, by analysis of myosin isoforms, fell into two subpopulations, one that contained both fast and slow myosin and another that contained myosin molecular hybrids. The increased proportion of intermediate (or transitional) fibers suggests changes in the equilibrium between fast and slow motor units. These changes could result from regeneration and subsequent maturation of fibers or from direct transformation of mature fibers of one type into the opposite.

Effects of fatigue on sarcoplasmic reticulum and myofibrillar properties of rat single muscle fibers.

Force decline during fatigue in skeletal muscle is attributed mainly to progressive alterations of the intracellular milieu. Metabolite changes and the decline in free myoplasmic calcium influence the activation and contractile processes. This study was aimed at evaluating whether fatigue also causes persistent modifications of key myofibrillar and sarcoplasmic reticulum (SR) proteins that contribute to tension reduction. The presence of such modifications was investigated in chemically skinned fibers, a procedure that replaces the fatigued cytoplasm from the muscle fiber with a normal medium. Myofibrillar Ca(2+) sensitivity was reduced in slow-twitch muscle (for example, the pCa value corresponding to 50% of maximum tension was 6.23 +/- 0.03 vs. 5.99 + 0.05, P < 0.01, in rested and fatigued fibers) and not modified in fast-twitch muscle. Phosphorylation of the regulatory myosin light chain isoform increased in fast-twitch muscle. The rate of SR Ca(2+) uptake was increased in slow-twitch muscle fibers (14.2 +/- 1.0 vs. 19.6 +/- 2. 5 nmol. min(-1). mg fiber protein(-1), P < 0.05) and not altered in fast-twitch fibers. No persistent modifications of SR Ca(2+) release properties were found. These results indicate that persistent modifications of myofibrillar and SR properties contribute to fatigue-induced muscle force decline only in slow fibers. These alterations may be either enhanced or counteracted, in vivo, by the metabolic changes that normally occur during fatigue development.

Effects of modulators of sarcoplasmic Ca2+ release on the development of skeletal muscle fatigue.

The reduced release of Ca2+ from sarcoplasmic reticulum (SR) is considered a major determinant of muscle fatigue. In the present study, we investigated whether the presence of dantrolene, an established inhibitor of SR Ca2+ release, or caffeine, a drug facilitating SR Ca2+ release, modifies muscle fatigue development. Accordingly, the effects of Ca2+ release modulators were analyzed in vitro in mouse fast-twitch [extensor digitorum longus (EDL)] and slow-twitch (soleus) muscles, fatigued by repeated short tetani (40 Hz for 300 ms, 0.5 s(-1) in soleus and 60 Hz for 300 ms, 0.3 s(-1) in EDL, for 6 min). Caffeine produced a substantial increase of tetanic tension of both EDL and soleus muscles, whereas dantrolene decreased tetanic tension only in EDL muscle. In both EDL and soleus muscles, 5 microM dantrolene did not affect fatigue development, whereas 20 microM dantrolene produced a positive staircase during the first 3 min of stimulation in EDL muscle and a slowing of fatigue development in soleus muscle. The development of the positive staircase was abolished by the addition of 15 microM ML-7, a selective inhibitor of myosin light chain kinase. On the other hand, caffeine caused a larger and faster loss of tension in both EDL and soleus muscles. The results seem to indicate that the changes in fatigue profile induced by caffeine or dantrolene are mainly due to the changes in the initial tetanic tension caused by the drugs, with the resulting changes in the level of contraction-dependent factors of fatigue, rather than to changes in the SR Ca2+ release during fatigue development.

[Effects of calcium, ethanol and hyperosmolarity on the distribution of intervals between MEPP's]. / Effetti del calcio, dell'etanolo e dell'iperosmolarita' sulla distribuzione degli intervalli tra i MEPP's.

Distributions of intervals between MEPP's, recorded from the frog sartorius muscle in various experimental conditions, were compared, graphically and by X2, with the exponential distributions expected for a Poisson process. For each experiment, 200 intervals were measured. In isotonic Ringer solutions containing no Ca++, MEPP's fitted an exponential distribution; by adding Ca++ (1 mM, 2,5 MM, 15 mM), the fit between empirical and expected distributions became poorer, with an excess of short intervals. MEPP's showed the tendency to occur in bursts, and this was particularly evident at 15 mM Ca++. In the presence of 0,5% ethanol, also at high Ca++ the intervals between MEPP's showed a fair fit to the exponential distribution. In Ringer solutions made hypertonic by the addition of 50 mM sucrose, the fit was poor and there was a relative lack of short intervals. These different patterns of MEPP distribution could be explained by supposing that Ca++, ethanol and hypertonicity activate different mechanisms of transmitter release.

Effects of the spinal cord section and of subsequent denervation on the mechanical properties of fast and slow muscles.

The Soleus muscle of the rat, 3--6 months old, becomes significantly faster than in the controls, if the spinal cord is cut at birth. Mechanical properties of Extensor Digitorum Longus (EDL) muscle are not altered by spinal cord section. In cordotomized animals Soleus muscle always remains slower than EDL muscle. Denervation, performed 3--6 months after birth, has the same slowing effects in the Soleus and EDL muscles, both in cordotomized and in the control animals.

[Variations in the resting membrane potential of denervated and tenotomized-denervated rat skeletal muscles]. / Variazioni del potenziale di membrana a riposo in muscoli scheletrici denervati e tenotomizzati-denervati di ratto.

Resting membrane potential (RMP) was studied in denervated (D), tenotomized (T) and tenotomized-denervated (TD) Soleus (S) and Tibialis Anterior (TA) muscles of the rat. In TD muscles, denervation was performed 1 week after tenotomy. The measurements were performed "in vitro" at 20 degrees C, 1-7 days after denervation in D and TD muscles, and 7 days after tenotomy in T muscles. In T muscles there were not differences in comparison with the controls. In D muscles RMP decreased earlier in S than in TA. The time course of membrane depolarization was similar in TD and D muscles until the 2nd day after denervation. At the 3rd day, RMP showed a further, significant reduction in TD muscles, both in S and TA, but not in D muscles. In D muscles, the depolarization increased very slowly up to the 7th day.

Combined effects of tenotomy and denervation on the dynamic properties of soleus muscle of the rat.

Isometric contraction time (CT), half relaxation time (1/2 RT), tetanus fusion frequency (TFF) and tetanus: twitch ratio (T : t ratio) were measured in the denervated (D) and tenotomized-denervated (TD) Soleus muscle of the rat. In D muscle there was an apparent speeding effect at the 2nd day after denervation, with a significant decrease of CT, which was followed by the usual slowing process of denervated muscle. In TD muscle, denervation was performed a week after tenotomy. Tenotomy "per se" was ineffective in modifying dynamic properties of muscle, but it accentuated the early shortening of CT caused by denervation, while reducing and delaying the subsequent slowing process. The results are discussed in the light of the hypothesis that muscle disuse has a speeding effect which counteracts the slowing effect of denervation, and/or that tenotomy modifies the effects of denervation by changing the pattern of fibrillation development.

Fast to slow transition induced by experimental myotonia in rat EDL muscle.

Experimental myotonia was induced by feeding rats with 20,25-diazacholesterol for up to 8 months. Histochemical analysis of myotonic extensor digitorum longus (EDL) muscle showed a progressive decrease of type IIB fibres and a concomitant increase of type IIA and type I fibres. A transient hypertrophy of type IIA fibres was observed 6 months after beginning the treatment. Analysis of the pattern of myosin light chains of single fibres from EDL showed that myotonia caused a progressive decrease of fibres showing a pure fast myosin light chain pattern and an increase of fibres showing coexistence of fast and slow myosin light chains (intermediate fibres). Only a small percentage of intermediate fibres showed coexistence of fast and slow myosin heavy chains. Myotonic fibres presented an increased sensitivity to caffeine which approached that of normal soleus fibres. Furthermore, sarcoplasmic reticulum (SR) vesicles isolated from hind limb fast muscles of myotonic rats demonstrated a decrease of Ca2+-dependent ATPase and Ca2+-transport activities as well as a decrease of immunoreactivity with anti-rabbit SR fast Ca2+-ATPase antibody. These results suggest that the increased electrical activity brought about by 20,25-diazacholesterol-induced myotonia, caused a fast to slow transition in the phenotypic expression of myosin and sarcoplasmic reticulum proteins.

Type 1, 2A, and 2B myosin heavy chain electrophoretic analysis of rat muscle fibers.

Mammalian skeletal muscles are mixture of three type of fibers: type 1, type 2A, and type 2B fibers. Immunological studies and proteolytic analysis of myosin heavy chains from the three type of fibers have demonstrated the presence of distinct myosin isoforms. By using typed single muscle fibers and improving an electrophoretic method we are able to resolve three distinct polypeptides which are demonstrate to correspond to type 1, 2A and 2B myosin heavy chain isoforms by using specific monoclonal antibodies. The analysis of single muscle fibers shows that different myosin heavy chain isoforms are frequently coexpressed in the same muscle fiber.

Myofibrillar-protein isoforms and sarcoplasmic-reticulum Ca2+-transport activity of single human muscle fibres.

In this study the polymorphism of myofibrillar proteins and the Ca2+-uptake activity of sarcoplasmic reticulum were analysed in single fibres from human skeletal muscles. Two populations of histochemically identified type-I fibres were found differing in the number of light-chain isoforms of the constituent myosin, whereas the pattern of light chains of fast myosin of type-IIA and type-IIB fibres was indistinguishable. Regulatory proteins, troponin and tropomyosin, and other myofibrillar proteins, such as M- and C-proteins, showed specific isoforms in type-I and type-II fibres. Furthermore, tropomyosin presented different stoichiometries of the alpha- and beta-subunits between the two types of fibres. Sarcoplasmic-reticulum volume, as indicated by the maximum capacity for calcium oxalate accumulation, was almost identical in type-I and type-II fibres, whereas the rate of Ca2+ transport was twice as high in type-II as compared with type-I fibres. It is concluded that, in normal human muscle fibres, there is a tight segregation of fast and slow isoforms of myofibrillar proteins that is very well co-ordinated with the relaxing activity of the sarcoplasmic reticulum. These findings may thus represent a molecular correlation with the differences of the twitch-contraction time between fast and slow human motor units. This tight segregation is partially lost in the muscle fibres of elderly individuals.

Polymorphism of myofibrillar proteins of rabbit skeletal-muscle fibres. An electrophoretic study of single fibres.

Rabbit predominantly fast-twitch-fibre and predominantly slow-twitch-fibre skeletal muscles of the hind limbs, the psoas, the diaphragm and the masseter muscles were fibre-typed by one-dimensional polyacrylamide-gel electrophoresis of the myofibrillar proteins of chemically skinned single fibres. Investigation of the distribution of fast-twitch-fibre and slow-twitch-fibre isoforms of myosin light chains and the type of myosin heavy chains, based on peptide 'maps' published in Cleveland. Fischer, Kirschner & Laemmli [(1977) J. Biol. Chem. 252, 1102-1106], allowed a classification of muscle fibres into four classes, corresponding to histochemical types I, IIA, IIB and IIC. Type I fibres with a pure slow-twitch-type of myosin were found to be characterized by a unique set of isoforms of troponins I, C and T, in agreement with the immunological data of Dhoot & Perry [(1979) Nature (London) 278, 714-718], by predominance of the beta-tropomyosin subunit and by the presence of a small amount of an additional tropomyosin subunit, apparently dissimilar from fast-twitch-fibre alpha-tropomyosin subunit. The myofibrillar composition of type IIB fast-twitch white fibres was the mirror image of that found for slow-twitch fibres in that the fast-twitch-fibre isoforms only of the troponin subunits were present and the alpha-tropomyosin subunit predominated. Type IIA fast-twitch red fibres showed a troponin subunit composition identical with that of type IIB fast-twitch white fibres. On the other hand, a unique type of myosin heavy chains was found to be associated with type IIA fibres. Furthermore, the myosin light-chain composition of these fibres was invariably characterized by a small amount of LC3F light chain and by a pattern that was either a pure fast-twitch-fibre light-chain pattern or a hybrid LC1F/LC2F/LC3F/LC1Sb light-chain pattern. By these criteria type IIA fibres could be distinguished from type IIC intermediate fibres, which showed coexistence of fast-twitch-fibre and slow-twitch-fibre forms of myosin light chains and of troponin subunits.

Effects of disuse and nerve stump length on the development of fibrillation in denervated soleus muscle.

Both in normal (control) and in cordotomized (disused) rats, the soleus muscle was denervated either by cutting the sciatic nerve near the trochanter (proximal denervation) or by cutting the soleus nerve near the insertion into the muscle (distal denervation). In the control muscles, the development of fibrillation was not dependent on the level of nerve section. In disused muscles, the development of fibrillation was greater following distal denervation that following the proximal one.

Cordotomy-denervation interactions on contractile and myofibrillar properties of fast and slow muscles in the rat.

Cordotomy-denervation interactions were studied on contractile and myofibrillar properties of slow (soleus) and fast (extensor digitorum longus) muscles of the rat. The spinal cord was transected midthoracically in neonatal (2-day-old) animals. Two months after birth, a unilateral transection of the sciatic nerve was carried out in both cordotomized and control animals. Five weeks after denervation, contractile properties were tested isometrically in vitro; myofibrillar properties were assessed by histochemical staining of the muscle fibers and by electrophoretic analysis of the myosin heavy chain composition. The following results were obtained: (i) In cordotomized animals the contraction time of the soleus was significantly shorter (-23.3% on average) than that in the control animals and this shortening was accompanied by a proportional slow-to-fast shift in myofibrillar properties. (ii) The extensor digitorum longus properties were not significantly different in the control and cordotomized animals. (iii) Denervation in control animals was followed by a marked increase of contraction and half-relaxation times in the extensor digitorum longus, whereas in the soleus only the half-relaxation time was significantly increased; myofibrillar properties in the soleus showed an appreciable slow-to-fast shift, whereas in the fast muscle the main change was an increase in type 2A fibers to the detriment of type 2B. (iv) In cordotomized animals, denervation caused the soleus contraction time to increase to control values, whereas myofibrillar properties shifted to an even faster pattern; in the extensor digitorum longus denervation caused the same changes seen in the control animals. The results showed that cordotomy at birth caused the soleus to develop as a faster muscle than in the control animals. The concurrent effects of cordotomy and denervation on the myofibrillar properties of the soleus suggest that the slow-to-fast change in these properties is a common consequence of the reduction in the level of motor activity. The opposite effects of the two experimental conditions in the soleus contraction time support the view that the contractile alterations that follow denervation mainly reflect alterations in the muscle activation process.