Various aspects of phonon spectrum changes in nanostructured phonon-mediated superconductors are considered. It is shown how, with the development of experimental techniques and, accordingly, ...obtaining new results, the understanding of the influence of the surface and nanoscale on the magnitude of the electron-phonon interaction and the critical temperature
changed and deepened. The review is organized as follows. After the
, in the
part we give the quick theoretical background for the description of superconductivity within the framework of various formalisms. In the
part we describe the properties of nanostructured (granular) thin films paying attention to the impact of grain sizes and methods of deposition on the
value. The role of material parameters is underlined and different aspects of the behavior of granular thin films are discussed. In the
section the impact of external sources of modification of the phonon spectra like noble gases and organic molecules are considered. Problems and progress in this area are discussed. The
part is dedicated to the phonon modification and related quantum size effects in nanostructured superconductors. In the
part we review the results of direct evidence of phonon softening in nanostructured superconductors and in the
section we discuss a possible alternative description of the superconducting properties of nanostructured superconductors related to the concept of metamaterials. In the
and
parts we review the impact of substrates with lattice mismatched parameters and graphene sheets, respectively, on the modification of the phonon spectrum and enhancement of superconductivity in various superconducting thin films. Finally, in the last
section we consider the nonequilibrium superconductivity driven by femtosecond pulses of light, which leads to generation of coherent phonons and to a significant increase in the critical temperature in a number of superconducting materials.
We investigate the influence of carbon nanotubes (CNT) aligned array on the magnetic properties of ensemble of densely packed Co nanoparticles (NPs) embedded inside CNT. Each CNT contains only one ...nanosized Co. Such a special structure was formed by catalyst chemical vapor deposition (CCVD) activated by current discharge plasma and hot filament. The Co NPs, previously deposited onto SiO2/Si substrate, acted as a catalyst. By varying the parameters of the CCVD process, we were able to also sputter the substrate instead of CNT growth. Co NPs were used as a mask and the structure of Si-based nanocones with Co NPs on the top of each cone was formed. Exhaustive investigation of the structural, morphology and crystalline properties of Co nanoparticles were performed. The magnetic properties of two kinds of samples, Co on the Si-based nanocone and Co inside CNT, were differ drastically. In the former case, the magnetic anisotropy of thin-film-type has been observed with large magnetic domains. Whereas for the Co-CNT samples ferromagnetic NPs were magnetically isolated. It was established that the magnetic anisotropy of nanosized Co plays more dominant role than the dipole interaction between Co NPs. The role of the CNT container in this is discussed.
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In this work, we investigate magnetic properties of iron based nanoparticles (NP) intercalated into carbon nanotube (CNT) aligned arrays synthesized by injection chemical vapor deposition. We have ...analyzed the temperature (T) and the ferrocene concentration (CF) dependences of the macroscopic magnetic parameters. From these experiments a weaker interaction between magnetic moments of NP was obtained for low CF values. The random anisotropy model for the experimental data analysis was applied and micromagnetic parameters were evaluated. The law of the approach to magnetic saturation (LAS) was analyzed using the general expression with the correlation function C(r=x/Ra) of magnetic axes, Ra being the magnetic anisotropy correlation length. We obtained that, while for CF=0.5% C(r) is a step-like (C(r<10)=1, C(r>10)=0), for CF⩾1% C(r) decays rapidly on a short range, (2-3)Ra. Such extended correlations for CF=0.5% could be associated with the dominant role of the coherent anisotropy, which is caused by the influence of the alignment of CNT. When the aligned CNTs for CF=0.5% are destroyed into powder, the LAS is changed to H−1/2, which means the dominant role of the exchange mechanism.
To take advantage of the graphene appealing electronic properties, in this work we present a photodetector (PD) based on graphene/n-silicon heterojunction (GSH). In this device, graphene acts as ...light transmitter, counter electrode junction element and photocarrier collector. The photodetector has been provided with metal contacts allowing either photovoltaic or photoconductive operation mode. We investigated the response of GSH PD to a 35-femtosecond laser pulse. In the photovoltaic configuration, the PD exhibits rise times of some tens of nanoseconds, detecting light from ultraviolet (275 nm) to infrared (1150 nm). In photoconductive mode applying a gate voltage VG, the external quantum efficiency hugely increases, from a value of 2% up to 200%. Together with the observation of a rise time, that decreases down to a minimum value of about 1 ns, this makes our device even more competitive and comparable with commercial photodetectors.
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Structural and magnetic properties of Ni nanowires electrochemically deposited into pores of mesoporous silicon template under the stationary galvanostatic regime have been investigated. Samples have ...been exhaustively studied by using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and specific magnetization measurements. SEM analysis revealed the formation of porous silicon/nickel nanocomposite at the initial stages of Ni deposition with the characteristic dimension of Ni nanoparticles in the range of 40–60nm. After 60min of deposition Ni continuous nanowires of 10μm length have been formed. XRD analysis confirmed the polycrystalline structure of Ni in the mesoporous silicon template with the preferential orientation along 111 axis. Also some amount of silicide Ni2Si was formed, which diffraction peak at 2Θ≈33° was especially pronounced for low deposition times. The possible mechanism of nickel silicide formation during the electrochemical process has been discussed. It was supposed that, the presence of amorphous silicon on pore walls facilitates the diffusion of Ni inside silicon matrix with subsequent nickel silicide formation without heating. The idea has been confirmed by the fact that on crystalline silicon the formation of nickel silicide was not observed.
The magnetic properties have been investigated by studying the temperature dependence (77K–700K) of the specific magnetization σ. The measured σ values were lower with respect to that of bulk Ni. The effect has been explained by the influence of uncontrolled formation of nickel silicide, which causes, after heating, larger irreversibility of σ(T) curves for samples with less deposition time. The obtained σ(T) dependencies allowed us to determine the Curie temperature, TC, which for low deposition times of Ni was lower (575K) with respect to the bulk Ni (630K). This is caused by the influence of dimensional effects on TC value.
► Ni nanowires were electrochemically grown in mesoporous Si templates. ► At the initial stage of deposition magnetic nanocomposite Ni/Porous Silicon is formed. ► Ni nanowires were polycrystalline with preferential orientation along 111 axis. ► The magnetic properties were studied by static ponderomotive method. ► The Curie temperature and the specific magnetization depend on the deposition time.
The influence of the magnetic medium can lead to peculiar interaction between ferromagnetic nanoparticles (NPs). Most research in this area involves analysis of the interplay between magnetic ...anisotropy and exchange coupling. Increasing the average interparticle distance leads to the dominant role of the random magnetic anisotropy. Here we study the interparticle interaction in a carbon nanotube (CNT) matrix with low ferromagnetic NP content. Samples were synthesized by floating catalyst chemical vapor deposition. We found that below some critical NP concentration, when NPs are intercalated only inside CNTs, and at low temperatures, the extended magnetic order, of up to 150 nm, presents in our samples. It is shown by analyzing the correlation functions of the magnetic anisotropy axes that the extended order is not simply due to random anisotropy but is associated with the coherent magnetic anisotropy, which is strengthened by the CNT alignment. With increasing temperature the extended magnetic order is lost. Above the critical NP concentration, when NPs start to be intercalated not only into inner CNT channels, but also outside CNTs, the coherent anisotropy weakens and the exchange coupling dominates in the whole temperature range. We can make a connection with the various correlation functions using the generalized expression for the law of the approach to saturation and show that these different correlation functions reflect the peculiarities in the interparticle interaction inside CNTs. Moreover, we can extract such important micromagnetic parameters like the exchange field, local fields of random and coherent anisotropies, as well as their temperature and NP concentration dependencies.
We report on the electric transport properties of Si heavily doped with Sb at concentration just below the insulator-to-metal transition in the temperature range 1.9-3.0 K for current density J < ...0.2 A cm
. The change in the sign of the temperature dependence of the differential resistivity Formula: see text was observed: the dFormula: see text/dT is positive if J < 0.045 A cm
whereas it becomes negative at J > 0.045 A cm
. The effect is explained assuming the exchange by electrons between the upper Hubbard band (UHB) and the conduction band. The obtained J dependencies of the activation energy, nonequilibrium concentration, mobility and scattering time of the conduction electrons correspond well to this hypothesis. The reason for charge instability is the Coulomb repulsion between electrons occupying states both in the UHB and conduction band. The estimated J dependencies of the conduction electrons lifetime and concentration of the D
states in the UHB strongly supports this assumption.
Superconducting NbN nanonetworks with a very small number of interconnected nanowires, with diameter of the order of 4 nm, are fabricated combining a bottom-up (use of porous silicon nanotemplates) ...with a top-down technique (high-resolution electron beam lithography). The method is easy to control and allows the fabrication of devices, on a robust support, with electrical properties close to a one-dimensional superconductor that can be used fruitfully for novel applications.
Magnetoresistance oscillations were observed on networks of superconducting ultrathin Nb nanowires presenting evidence of either thermal or quantum activated phase slips. The magnetic transport data, ...discussed in the framework of different scenarios, reveal that the system behaves coherently in the temperature range where the contribution of the fluctuations is important.
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