94 research outputs found
Hall effect at a tunable metal-insulator transition
Journals published by the American Physical Society can be found at http://journals.aps.org/Using a rotating magnetic field, the Hall effect in three-dimensional amorphous GdxSi1-x has been measured in the critical regime of the metal-insulator transition for a constant total magnetic field. The Hall coefficient R-0 is negative, indicating electronlike conductivity, with a magnitude that increases with decreasing conductivity. R-0 diverges at the metal-insulator transition, and displays critical behavior with exponent -1 [R(0)similar to(H-H-C)(-1)]. This dependence is interpreted as a linear decrease in the density of mobile carriers nsimilar toR(0)(-1)similar toH-H-C, indicative of the dominant influence of interaction effects
Observation of shell effects in superconducting nanoparticles of Sn
In a zero-dimensional superconductor, quantum size effects(QSE) not only set
the limit to superconductivity, but are also at the heart of new phenomena such
as shell effects, which have been predicted to result in large enhancements of
the superconducting energy gap. Here, we experimentally demonstrate these QSE
through measurements on single, isolated Pb and Sn nanoparticles. In both
systems superconductivity is ultimately quenched at sizes governed by the
dominance of the quantum fluctuations of the order parameter. However, before
the destruction of superconductivity, in Sn nanoparticles we observe giant
oscillations in the superconducting energy gap with particle size leading to
enhancements as large as 60%. These oscillations are the first experimental
proof of coherent shell effects in nanoscale superconductors. Contrarily, we
observe no such oscillations in the gap for Pb nanoparticles, which is ascribed
to the suppression of shell effects for shorter coherence lengths. Our study
paves the way to exploit QSE in boosting superconductivity in low-dimensional
systems
Nanoscale phase-engineering of thermal transport with a Josephson heat modulator
Macroscopic quantum phase coherence has one of its pivotal expressions in the
Josephson effect [1], which manifests itself both in charge [2] and energy
transport [3-5]. The ability to master the amount of heat transferred through
two tunnel-coupled superconductors by tuning their phase difference is the core
of coherent caloritronics [4-6], and is expected to be a key tool in a number
of nanoscience fields, including solid state cooling [7], thermal isolation [8,
9], radiation detection [7], quantum information [10, 11] and thermal logic
[12]. Here we show the realization of the first balanced Josephson heat
modulator [13] designed to offer full control at the nanoscale over the
phase-coherent component of thermal currents. Our device provides
magnetic-flux-dependent temperature modulations up to 40 mK in amplitude with a
maximum of the flux-to-temperature transfer coefficient reaching 200 mK per
flux quantum at a bath temperature of 25 mK. Foremost, it demonstrates the
exact correspondence in the phase-engineering of charge and heat currents,
breaking ground for advanced caloritronic nanodevices such as thermal splitters
[14], heat pumps [15] and time-dependent electronic engines [16-19].Comment: 6+ pages, 4 color figure
Rectification of electronic heat current by a hybrid thermal diode
We report the realization of an ultra-efficient low-temperature hybrid heat
current rectifier, thermal counterpart of the well-known electric diode. Our
design is based on a tunnel junction between two different elements: a normal
metal and a superconducting island. Electronic heat current asymmetry in the
structure arises from large mismatch between the thermal properties of these
two. We demonstrate experimentally temperature differences exceeding mK
between the forward and reverse thermal bias configurations. Our device offers
a remarkably large heat rectification ratio up to and allows its
prompt implementation in true solid-state thermal nanocircuits and
general-purpose electronic applications requiring energy harvesting or thermal
management and isolation at the nanoscale.Comment: 8 pages, 6 color figure
Primordial magnetic fields at preheating
Using lattice techniques we investigate the generation of long range
cosmological magnetic fields during a cold electroweak transition. We will show
how magnetic fields arise, during bubble collisions, in the form of magnetic
strings. We conjecture that these magnetic strings originate from the alignment
of magnetic dipoles associated with EW sphaleron-like configurations. We also
discuss the early thermalisation of photons and the turbulent behaviour of the
scalar fields after tachyonic preheating.Comment: 7 pages. Talk presented at Lattice200
Magnetic effects in sulfur-decorated graphene
The interaction between two different materials can present novel phenomena that are quite different from the physical properties observed when each material stands alone. Strong electronic correlations, such as magnetism and superconductivity, can be produced as the result of enhanced Coulomb interactions between electrons. Two-dimensional materials are powerful candidates to search for the novel phenomena because of the easiness of arranging them and modifying their properties accordingly. In this work, we report magnetic effects in graphene, a prototypical non-magnetic two-dimensional semi-metal, in the proximity with sulfur, a diamagnetic insulator. In contrast to the well-defined metallic behaviour of clean graphene, an energy gap develops at the Fermi energy for the graphene/sulfur compound with decreasing temperature. This is accompanied by a steep increase of the resistance, a sign change of the slope in the magneto-resistance between high and low fields, and magnetic hysteresis. A possible origin of the observed electronic and magnetic responses is discussed in terms of the onset of low-temperature magnetic ordering. These results provide intriguing insights on the search for novel quantum phases in graphene-based compounds.open1165sciescopu
Are men well served by family planning programs?
Although the range of contraceptives includes methods for men, namely condoms, vasectomy and withdrawal that men use directly, and the Standard Days Method (SDM) that requires their participation, family planning programming has primarily focused on women. What is known about reaching men as contraceptive users? This paper draws from a review of 47 interventions that reached men and proposes 10 key considerations for strengthening programming for men as contraceptive users. A review of programming shows that men and boys are not particularly well served by programs. Most programs operate from the perspective that women are contraceptive users and that men should support their partners, with insufficient attention to reaching men as contraceptive users in their own right. The notion that family planning is womenâs business only is outdated. There is sufficient evidence demonstrating menâs desire for information and services, as well as menâs positive response to existing programming to warrant further programming for men as FP users. The key considerations focus on getting information and services where men and boys need it; addressing gender norms that affect menâs attitudes and use while respecting womenâs autonomy; reaching adolescent boys; including men as users in policies and guidelines; scaling up successful programming; filling gaps with implementation research and monitoring & evaluation; and creating more contraceptive options for men
Enhanced superconductivity in surface-electron-doped iron pnictide Ba(Fe1.94Co0.06)2As2
The superconducting transition temperature (TC) in a FeSe monolayer on SrTiO3 is enhanced up to 100âK (refs ,,,). High TC is also found in bulk iron chalcogenides with similar electronic structure to that of monolayer FeSe, which suggests that higher TC may be achieved through electron doping, pushing the Fermi surface (FS) topology towards leaving only electron pockets. Such an observation, however, has been limited to chalcogenides, and is in contrast to the iron pnictides, for which the maximum TC is achieved with both hole and electron pockets forming considerable FS nesting instability. Here, we report angle-resolved photoemission characterization revealing a monotonic increase of TC from 24 to 41.5âK upon surface doping on optimally doped Ba(Fe1-xCox)2As2. The doping changes the overall FS topology towards that of chalcogenides through a rigid downward band shift. Our findings suggest that higher electron doping and concomitant changes in FS topology are favourable conditions for the superconductivity, not only for iron chalcogenides, but also for iron pnictides
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