643 research outputs found
Breakdown of Migdal's theorem and intensity of electron-phonon coupling in high-Tc superconductors
In this article we quantify the possible effects of the breakdown of Migdal's
theorem on the electron-phonon (e-p) coupling constant, on the critical
temperature and on the superconducting gap by examining different kinds of
superconducting materials either with low and high critical temperature. We use
the theoretical approach developed by Grimaldi, Pietronero and Strassler [PRB
52, 10516 & 10530 (1995)] on experimental data taken both from literature and
from our recent break-junction tunneling experiments in Bi_2Sr_2CaCu_2O_{8+x}
(BSCCO) [Physica C 275, 162 (1997) and cond-mat/9612220]. The results show that
a large violation of the Migdal's theorem (as in BSCCO) yields to a large
increase of the observed e-p coupling constant and, in a first approximation
and by direct solution of the Eliashberg equations, to large but different
increases of Tc and gap. The same theory gives no modifications when applied to
low-Tc conventional superconductors. In BSCCO, the renormalized values of the
e-p coupling constant and of the ratio 2Delta/kTc (1.85 and 4.98, respectively)
are compatible with a strong-coupling electron-phonon origin of
superconductivity.Comment: 11 pages REVTeX file, 3 PostScript figures, to be published in Phys.
Rev. B (Rapid Commun.
Intrinsic surface depression of the order parameter under mixed (s+id)-wave pair symmetry and its effect on the critical current of high-Tc SIS Josephson junctions
An intrinsic gap depression at the Superconductor-Insulator interface due to
the very short value of the coherence length in High-Tc Superconductors [HTSs]
is considered, in the framework of a mixed (s+id)-wave pair symmetry for the
order parameter ranging from pure s to pure d-wave. This gap depression acts as
the main physical agent causing the relevant reduction of IcRn(T) values with
respect to BCS expectations in HTS SIS Josephson junctions. Good agreement with
various experimental data is obtained with both pure s-wave and pure d-wave
symmetries of the order parameter, but with amounts of gap depression depending
on the pair symmetry adopted. Regardless of the pair symmetry considered, these
results prove the importance of the surface order-parameter depression in the
correct interpretation of the Ic(T)Rn(T) data in HTS SIS junctions. In a case
of planar YBCO-based junction the use of the de Gennes condition allowed us to
tentatively obtain an upper limit for the amount of d-wave present in the order
parameter of YBCO.Comment: 11 pages REVTeX file, 6 PostScript figures, to be published in J.
Superconductivit
Obesity and fracture risk.
Obesity and osteoporosis are two common diseases with an increasing prevalence and a high impact on morbidity and mortality. Obese women have always been considered protected against osteoporosis and osteoporotic fractures. However, several recent studies have challenged the widespread belief that obesity is protective against fracture and have suggested that obesity is a risk factor for certain fractures. Fat and bone are linked by many pathways, which ultimately serve the function of providing a skeleton appropriate to the mass of adipose tissue it is carrying. Leptin, adiponectin, adipocytic estrogens and insulin/amylin are involved in this connection. However, excessive body fat, and particularly abdominal fat, produces inflammatory cytokines which may stimulate bone resorption and reduce bone strength. This review aimed to examine the literature data on the relationships of BMI and fat mass with factures in adult and elderly subjects. Even though the more recent studies have shown conflicting results, there is growing evidence that obesity, and particularly severe obesity, may be related to an increased risk of fracture at different skeletal sites which is partially independent from BMD. Moreover, the relationship between obesity and fracture appears to be markedly influenced by ethnicity, gender and fat distribution. Even though the incidence and the pathogenesis of fracture in obese individuals has not yet been clearly defined, the growing evidence that obesity may be related to an increased risk of fracture has important public health implications and emphasizes the need to develop effective strategies to reduce fracture risk in obese subject
Divergent effects of obesity on fragility fractures
Obesity was commonly thought to be advantageous for maintaining healthy bones due to the higher bone mineral density observed in overweight individuals. However, several recent studies have challenged the widespread belief that obesity is protective against fracture and have suggested that obesity is a risk factor for certain fractures. The effect of obesity on fracture risk is site-dependent, the risk being increased for some fractures (humerus, ankle, upper arm) and decreased for others (hip, pelvis, wrist). Moreover, the relationship between obesity and fracture may also vary by sex, age, and ethnicity. Risk factors for fracture in obese individuals appear to be similar to those in nonobese populations, although patterns of falling are particularly important in the obese. Research is needed to determine if and how visceral fat and metabolic complications of obesity (type 2 diabetes mellitus, insulin resistance, chronic inflammation, etc) are causally associated with bone status and fragility fracture risk. Vitamin D deficiency and hypogonadism may also influence fracture risk in obese individuals. Fracture algorithms such as FRAXÂź might be expected to underestimate fracture probability. Studies specifically designed to evaluate the antifracture efficacy of different drugs in obese patients are not available; however, literature data may suggest that in obese patients higher doses of the bisphosphonates might be required in order to maintain efficacy against nonvertebral fractures. Therefore, the search for better methods for the identification of fragility fracture risk in the growing population of adult and elderly subjects with obesity might be considered a clinical priority which could improve the prevention of fracture in obese individual
Multi-Valley Superconductivity In Ion-Gated MoS2 Layers
Layers of transition metal dichalcogenides (TMDs) combine the enhanced
effects of correlations associated with the two-dimensional limit with
electrostatic control over their phase transitions by means of an electric
field. Several semiconducting TMDs, such as MoS, develop superconductivity
(SC) at their surface when doped with an electrostatic field, but the mechanism
is still debated. It is often assumed that Cooper pairs reside only in the two
electron pockets at the K/K' points of the Brillouin Zone. However,
experimental and theoretical results suggest that a multi-valley Fermi surface
(FS) is associated with the SC state, involving 6 electron pockets at the Q/Q'
points. Here, we perform low-temperature transport measurements in ion-gated
MoS flakes. We show that a fully multi-valley FS is associated with the SC
onset. The Q/Q' valleys fill for dopingcm, and the
SC transition does not appear until the Fermi level crosses both spin-orbit
split sub-bands Q and Q. The SC state is associated with the FS
connectivity and promoted by a Lifshitz transition due to the simultaneous
population of multiple electron pockets. This FS topology will serve as a
guideline in the quest for new superconductors.Comment: 12 pages, 7 figure
Carrier mobility and scattering lifetime in electric double-layer gated few-layer graphene
We fabricate electric double-layer field-effect transistor (EDL-FET) devices
on mechanically exfoliated few-layer graphene. We exploit the large capacitance
of a polymeric electrolyte to study the transport properties of three, four and
five-layer samples under a large induced surface charge density both above and
below the glass transition temperature of the polymer. We find that the carrier
mobility shows a strong asymmetry between the hole and electron doping regime.
We then employ ab-initio density functional theory (DFT) calculations to
determine the average scattering lifetime from the experimental data. We
explain its peculiar dependence on the carrier density in terms of the specific
properties of the electrolyte we used in our experiments.Comment: 6 pages, 3 figure
Probing the current-phase relation in Josephson point-contact junctions between Pb 0.6 In 0.4 and Ba 0.6 K 0.4 (FeAs) 2 superconductors
The Josephson effect in point contacts between an âordinaryâ superconductor Pb 0.6In0.4 (Tcâ6.6K) and single crystals of the Fe-based superconductor Ba0.6K0.4(FeAs)2 (Tcâ38.5K), was investigated. In order to shed light on the order parameter symmetry of Ba0.6K0.4(FeAs)2, the dependence of the Josephson supercurrent Is on the temperature and on sin (dÏ) with d= 1 , 2 was studied. The dependencies of the critical current on temperature Ic(T) and of the amplitudes of the first current steps of the currentâvoltage characteristic inexp(P)(n= 0 , 1 , 2) on the power of microwave radiation with frequency f=(1.5Ă·8)GHz were measured. It is shown that the dependencies Ic(T) are close to the well-known AmbegaokarâBaratoff (AB) dependence for tunnel contacts between âordinaryâ superconductors and to the dependence calculated by Burmistrova et al. (Phys Rev B 91, 214501 (2015)) for microshorts between an âordinaryâ superconductor and a two-band superconductor with s± order parameter symmetry at certain values of the transparency of boundaries and thickness of the transition layer. It is found that the dependencies inexp(P) cannot be approximated within the resistively shunted model using the normalized microwave frequencies Ω = 2 Ïf/ (2 eVc/ ħ) with characteristic voltages Vc= IcRN, (RNânormal resistance of the contact) found from the low-voltage parts of the currentâvoltage characteristics. The reasons for this failure are discussed and a method is proposed for accurately determining the value of Ω , which takes into account all the features of the point contact affecting the period of the dependence inexp(P). An analysis of the Ic(T) and inexp(P) dependencies shows that the superconducting current of the Josephson contacts under investigation is proportional to the sin of the phase difference Ï, Is= Icsin(Ï). The implications of these results on the symmetry of the order parameter are also discussed
Undiagnosed vertebral fractures influence quality of life in postmenopausal women with reduced ultrasound parameters.
Osteoporosis, a multifactorial systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue leading to increased bone fragility, is a worldwide public health problem. Vertebral fractures affect approximately 20% of postmenopausal women and are a hallmark of osteoporosis, but they may pass unnoticed, although they may lead to long-term immobility and disability.
The aims of the present study were (1) to determine the prevalence and the severity of vertebral fractures in a large cohort of Italian women aged 60 years or older with reduced values of quantitative ultrasound parameters; and (2) to assess whether vertebral fractures and other variables may be associated with health-related quality of life.
A total of 2450 women without back pain aged 60 years or older, after the completion of the Quality of Life Questionnaire of the European Foundation for Osteoporosis QUALEFFO, underwent quantitative ultrasound evaluation of the calcaneus; in those with a stiffness t-score of a parts per thousand currency sign -2 (n = 1194), radiographic evaluation of the thoracic and lumbar spine was carried out and then quantitative morphometry was performed by dedicated software (MorphoXpress). The radiographic analysis was carried out on 885 women who presented films of adequate quality. Multivariate regression was used to adjust for confounding variables.
Of those who underwent radiographic analysis, 681 had no vertebral fractures, and 204 women (23.1%) had one or more previously undiagnosed vertebral fractures. The prevalence of previously undiagnosed vertebral fractures increased with advancing age with more than 30% of women older than 75 years having at least one fracture. Older age, body mass index, and severe vertebral fractures were independently associated with a worse total QUALEFFO score.
We found that approximately one in four women showed evidence of undiagnosed vertebral fractures, and there was a strong age effect trend. Moreover, the severity grade of vertebral fractures, more than the number of fractures, was associated with a worsening of health-related quality of life as assessed by QUALEFFO. These findings confirm the clinical relevance of an early diagnosis of vertebral fractures and seem to support the usefulness of quantitative ultrasound measurements in the stratification of postmenopausal women at increased fracture risk
Geometry-based tunability enhancement of flexible thin-film varactors
In this letter, flexible voltage-controlled capacitors (varactors) based on an amorphous IndiumâGalliumâZincâOxide (a-IGZO) semiconductor are presented. Two different varactor designs and their influence on the capacitance tuning characteristics are investigated. The first design consists of a top electrode finger structure which yields a maximum capacitance tunability of 6.9 at 10 kHz. Second, a novel interdigitated varactor structure results in a maximum tunability of 93.7 at 100 kHz. The design- and frequency-dependencies of the devices are evaluated through CâV measurements. Furthermore, we show bending stability of the devices down to a tensile radius of 6 mm without altering the performance. Finally, a varactor is combined with a thin-film resistor to demonstrate a tunable RC-circuit for impedance matching and low-pass filtering applications. The device fabrication flow and material stack are compatible with standard flexible thin-film transistor fabrication which enables parallel implementation of analog or logic circuitry and varactor devices
Control of bulk superconductivity in a BCS superconductor by surface charge doping via electrochemical gating
The electrochemical gating technique is a powerful tool to tune the surface conduction properties
of various materials by means of pure charge doping, but its efficiency is thought to be hampered in
materials with a good electronic screening. We show that, if applied to a metallic superconductor
(NbN thin films), this approach allows observing reversible enhancements or suppressions of the bulk
superconducting transition temperature, which vary with the thickness of the films. These results
are interpreted in terms of proximity effect, and indicate that the effective screening length depends
on the induced charge density, becoming much larger than that predicted by standard screening
theory at very high electric fields
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