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Researchers bring the theory of thermoelectricity in superconductors and experiment into agreement., For almost a century, thermoelectricity in superconductors has been one of the most intriguing topics in physics. During its early stages in the 1920s, the mere existence of thermoelectric effects in superconductors was questioned. In 1944, it was demonstrated that the effects may occur in inhomogeneous superconductors. Theoretical breakthrough followed in the 1970s, when the generation of a measurable thermoelectric magnetic flux in superconducting loops was predicted; however, a major crisis developed when experiments showed puzzling discrepancies with the theory. Moreover, different experiments were inconsistent with each other. This led to a stalemate in bringing theory and experiment into agreement. With this work, we resolve this stalemate, thus solving this long-standing “paradox,” and open prospects for exploration of novel thermoelectric phenomena predicted recently.

We report on the experimental study of the phase periodic resistance oscillations in an Andreev-type interferometer consisting of a mesoscopic normal metal ring (Ag) connected to a superconductor (Al). We observe a spectacular modulation of the oscillations due to an interplay of the phase transfer from the superconducting condensate to quasiparticles and interference phenomena in the normal ring. We discuss the results within the framework of recently developed theory of the long-range proximity effect.

We studied a mesoscopic cross-like normal metal structure connected to two superconducting (S) and two normal (N) reservoirs. We observed the Josephson effect under unusual conditions when there is no current through one of the two S/N interfaces. The potential difference between the S reservoirs was zero unless the voltage applied between S and N reservoirs exceeded a critical value although the electric potential in the N wire connecting the superconductors varied in a nonmonotonic way. The observed effects are discussed theoretically., Comment: 5 pages, 4 figures, to be published in PRB Rapid Communication

The proximity effect in mesoscopic ferromagnet/superconductor(FS) Ni/Al structures of various geometrieswas studied experimentally on both F- and S-sides of thestructures. Samples with a wide range of interfacetransparency were fabricated. The dependence of the effect onFS interface transparency was investigated. The amplitude ofthis effect was found to be larger than expected fromclassical theory of proximity effect. Preliminary experimentsshowed no phase-sensitive oscillations in Andreevinterferometer geometry. Various theoretical models arediscussed.

Normal (N) metallic (Ag) mesoscopic conductors with two superconducting (S) faces (Al), arranged mirror-symmetrically relative to the streamlines of the current, periodically switch into the normal state as the superconducting phase difference Δϕ between the NS boundaries approaches the values Δϕ =(2n+1)π, n=0,1,2,..., irrespective of temperature and applied voltage. For Δϕ =2nπ and low applied voltages the conductance passes through a maximum and approaches the normal value as temperature decreases (reentrance). As the voltage subsequently increases, the conductance increases and passes through a maximum. As the phase difference moves away from the values Δϕ=2nπ, the maxima shift in the direction of low temperatures and voltages. The latter result shows unequivocally that in our metal structures it is necessary to take into account the next-order corrections to the “weak” proximity effect approximation.

The investigation of the growth-temperature effect on electrical conductivity, measured at room and helium temperatures, as well as on structure, defined by reflection high-energy electron diffraction measurement, of thin tungsten and niobium films, grown by laser ablation deposition on a sapphire R-plane under ultra high vacuum, was made. It was shown that for the films with thickness of 60–80 nm the residual resistance ratio (RRR), which is used as the film quality parameter, is in limits of 12–80 and 7–20 for Nb and W, respectively. It increases rapidly if the substrate temperature, at which the film is grown, exceeds 350°C due to the growth of a perfect monocrystallinc epitaxial film, which has a bec structure with the [100] axis perpendicular to the substrate plane and with the [111] axis parallel to the c axis of the substrate. The film resistivity is close to the bulk monocrystal resistivity and smoothly depends on film growth temperature while the residual film resistivity diminishes as temperature increases. Both RRR and residual film resistivity are saturated at substrate temperature greater than 400–500 °C. Such dependence can be explained by the presence of an imperfect “bulk” phase, whose concentration is low at a high film growth temperature and rapidly increases with its diminishing to lower than 400–500°C. As a result, the scattering of conducting electrons is defined by film surface scattering under low concentration of the bulk phase at high and by bulk scattering on “static” defects of the imperfect bulk phase at low growth temperatures.

We report measurements of the temperature dependence of the amplitude of phase-periodic conductance oscillations in disordered (diffusive) normal metal (Ag) structures attached to a superconducting (Pb) wire at two points. The oscillation amplitude exceeds the value of e 2/h by orders of magnitude and reaches its maximum at a temperature corresponding to the Thouless energy. The results support the recent theory of Nazarov and Stoof, [Phys. Rev. Lett. 76, 823 (1996)], which takes into account the characteristic energy dependence of the change in the diffusion coefficient of quasiparticles in a normal conductor due to Andreev reflections. The line shape of the oscillations as a function of the superconducting phase difference is found to be extremely sensitive to the quality of N/S interfaces and shows hysteretic behavior in interferometers with specially prepared clean interfaces.

An investigation was made of electrical properties of tungsten films of thickness in the range of 30–140 nm, grown by laser ablation deposition in ultrahigh vacuum on [1012] sapphire substrate. From the data on the size effect and the temperature dependence of the resistivity r, supported with reflection high energy electron diffraction measurements, we find that the films, deposited onto clean substrates kept at temperatures higher than 500 °C, grow epitaxially with high quality crystalline structure. The effective electron mean free path changes from 300 to 1400 nm while the residual resistance ratio RRR=r (295 K)/r (4.2 K) changes from 7 to 35. At low temperatures we find the temperature dependent part ρ(T)≊ATn, with A=1.56 mΩ cm K−n, n=3–3.4 for the temperatures T

Aharonov-Bohm resistance oscillations of mesoscopic silver rings are found to be enhanced ~ 100 times when contact to the ring is made with bismuth rather than silver or gold electrodes. An argument based on the idea that a mismatch of the Fermi surfaces at the interfaces leads to a reduced effective electrode size is shown to be inadequate, as the resistance oscillations exceed the maximum theoretical value of Altshuler, Aronov and Spivak (1991) by an order of magnitude.

We argue that odd-frequency triplet superconductivity can be conveniently realized in hybrid superconductor-ferromagnet (SF) structures with a ferromagnetic vortex. We demonstrate that due to proximity-induced long-range triplet pairing such SFS junctions can sustain appreciable supercurrent which can be directly measured in experiments., Comment: 4 pages, 3 figures

We report on results of computer micromodelling of anti-vortex states in asymmetrical cross-like ferromagnetic nanostructures and their practical realization. The arrays of cobalt crosses with 1 mkm branches, 100 nm widths of the branches and 40 nm thicknesses were fabricated using e-beam lithography and ion etching. Each branch of the cross was tapered at one end and bulbous at the other. The stable formation of anti-vortex magnetic states in these nanostructures during magnetization reversal was demonstrated experimentally using magnetic force microscopy., 19 pages, 9 figures

We study the variation of the differential conductance $G=dj/dV$ of a normal metal wire in a Superconductor/Normal metal heterostructure with a cross geometry under external microwave radiation applied to the superconducting parts. Our theoretical treatment is based on the quasiclassical Green's functions technique in the diffusive limit. Two limiting cases are considered: first, the limit of a weak proximity effect and low microwave frequency, second, the limit of a short dimension (short normal wire) and small irradiation amplitude., Comment: 11 pages, 10 figures

As the dimensions of a normal (N) metallic conductor are reduced, the resistance of the conductor begins to increase when the contacts go into the superconducting (S) state. Likewise, the amplitude of the Aharonov-Bohm effect is strongly enhanced in mesoscopic wires and rings with superconducting contacts. We propose an explanation based on a quantization of electron energies resulting from Andreev reflections at N/S interfaces.

Large time-independent conduction fluctuations were observed as a function of transverse electric field in thin (25 nm) and narrow (60 nm) bismuth wires. The conduction of leads far away from a gate capacitor was influenced by changes in the gate voltage. The effects are interpreted as being due to a variation in the Fermi wavelength caused by gate-induced changes in the charge concentration of the leads rather than an electrostatic Aharonov-Bohm-type interference. The screening of charge is strongly reduced in narrow wires

We study a mesoscopic normal-metal structure with four superconducting contacts, two of which are joined into a loop. The structure undergoes transitions between three (meta)stable states, with different phase configurations triggered by nonequilibrium conditions. These transitions result in spectacular changes in the magnetoresistance. We find a qualitative agreement between the experiments and a theory based on the quasiclassical Keldysh formalism.

We report superconducting phase-periodic conductance oscillations in ferromagnetic wires with interfaces to conventional superconductors. The ferromagnetic wires were made of Ho, a conical ferromagnet. The distance between the interfaces was much larger than the singlet superconducting penetration depth. We explain the observed oscillations as due to the long-range penetration of an unusual "helical" triplet component of the order parameter that is generated at the superconductor/ferromagnet interfaces and maintained by the intrinsic rotating magnetization of Ho., 4 pages, 3 figures

We present a detailed study of the magnetic states in sub-micron nickel disks in the diameter range of 300–900 nm and thickness of 15–55 nm using magnetic force microscopy and micromagnetic simulations. In this dimensional range, we find three main magnetic states, namely: the single domain state, the vortex state, and the stripe domain state. We identify the crossover dimensions between these magnetic states, demonstrating that the disk thickness is the dominant parameter, which is further supported by the fact that the magnetocrystalline anisotropy of the Ni films is thickness dependant and plays a dominant role at large thicknesses. Experimental results are in a very good agreement with micromagnetic modelling using the out-of-plane effective anisotropy constant. Using in-situ high resolution Magnetic Force Microscopy techniques, we investigate in detail the motion of the vortex core under applied in-plane magnetic field and find that vortex nucleates and annihilates at ∼100 Oe.

The influence of electron irradiation on the resistance of the NOVER-1 resist to ion-beam etching is studied. Etching is carried out by argon ions with energies between 300 and 2500 eV. It is found that, depending on the energy and angle of incidence of the ions on the surface of the resist, electron irradiation may either speed up or slow down the NOVER-1 etching. A clear correlation is observed between the penetration depth of the ions in the resist and the influence of the electron irradiation on the resistance of the resist to etching. At ion energies higher than 500 eV (ion penetration depth ≳3.5 nm) the resistance decreases, passes through a minimum at low electron irradiation doses, and returns to the etching rate of the initial resist at high doses. For glancing etching angles (∼ 70° to the surface normal) and low ion energies (300 eV), i.e., small ion penetration depths (≲2.5 nm), an electron-irradiated resist is etched more slowly than the initial resist at all the electron irradiation doses studied. This effect may be used to enhance the resistance of resist structures whose height exceeds their width, which in this case is determined mostly by the rate of etching of the inclined facets.

The thermopower of mesoscopic normal metal/superconductor structures has been measured at low temperatures. Effect of supercurrent present in normal part of the structure was studied in two cases: when it was created by applied external magnetic field and when it was applied directly using extra superconducting electrodes. Temperature and magnetic field dependencies of thermopower are compared to the numerical simulations based on the quasiclassical theory of the superconducting proximity effect., 21 pages, 12 figures. To be published in the proceedings of the ULTI conference organized in Lammi, Finland (2006)

The dependence of the superconducting proximity effect on the amount of magnetic impurities in the normal part of Andreev interferometers has been studied experimentally. The dephasing rates obtained from fitting experimental data to quasiclassical theory of the proximity effect are consistent with the spin flip scattering from Cr impurities forming a local moment in the Cu host. In contrast, Ni impurities do not form a local moment in Cu and as a result there is no extra dephasing from Ni as long as $\mathrm{Cu}∕\mathrm{Ni}$ alloy remain paramagnetic.

We report measurements of spin valve effect in hybrid Normal metal/Ferromagnet nanostructures. We compare signals from structures where ferromagnetic electrodes have single magnetic domain with those having multiple domains. We find that spin valve effects are present in both cases. However, the width of transition from parallel to antiparallel state of ferromagnetic electrodes makes multiple domain electrodes unsuitable for some applications.

A new technique for in-vacuum 3D nanofabrication of multicomponent devices with smallest features less than 10 nm of controlled geometry is suggested and tested.

A new class of Negative Organic Vacuum Electron-beam Resists (NOVER) [1] has been synthesised for invacuum fabrication of devices with dimensions down to 25 nm. Here we report the results of tests of NOVER-1, a dry vacuum resist having gross formula C18H17BrN2O2. NOVER-1 can be applied for fabrication of samples for which regular wet resists cannot be used. Examples are fragile nanostructures fabricated on small substrates such as membranes, whiskers, small wires, etc., and on substrates with surfaces having complicated relief. X-ray lenses and ballistic mesoscopic structures of refractory metals, including epitaxial Nb films, have been fabricated using NOVER-1. The films of NOVER-1 can be used for experiments with low energy exposure using, for example, a Scanning Tunnelling Microscope in the cold emission regime.

The thermopower of a normal ring with one superconducting contact has been found to oscillate in a magnetic field with a period corresponding to the magnetic flux quantum through the area of the ring. The oscillation symmetry is the same as that of the magnetoresistance. The absolute value of thermopower is of the order of 2 nV/K. These two facts suggest that the observed thermopower is due to quasiparticle thermoelectric currents in the ring rather than the giant thermopower in the superconductor-normal loops reported earlier.

Using the extraordinary sensitivity of Andreev interferometers to the superconducting phase difference associated with currents, we measure the persistent current quantum states in superconducting loops interrupted by Josephson junctions. Straightforward electrical resistance measurements of the interferometers give continuous read-out of the states, allowing us to construct the energy spectrum of the quantum circuit. The probe is estimated to be more precise and faster than previous methods, and can measure the local phase difference in a wide range of superconducting circuits., Changes made in light of referees comments; to appear in PRL

We report measurements of phase-periodic thermopower in a diffusive Andreev interferometer. Upon the increase of the dc current applied to the heater electrodes, the amplitude of the thermopower oscillations first increases then goes to zero as one would expect. Surprisingly, the oscillations reappear at yet higher heater currents being inverted compared to low current values. The dependence of the amplitude of the oscillations on temperature strongly correlates with that of the resistance derivative $dR/dT.$

A strong mutual influence of superconductors (S) and ferromagnetic (F) conductors in hybrid F/S (Ni/Al) nanostructures is observed. The magnitude of a proximity-induced conductance on the F-side is more than two orders larger than that predicted by theory. Re-entrance of the superconductors to the normal state reciprocated by changes on the F-side has been found in low applied magnetic fields with new peaks in the differential resistance as an effect of the saturation magnetisation. An analysis has been developed providing a base for a numerical description of the system. PACS numbers: 74.50.+r, 74.80.Fp, 85.30St, 7 pages in html format, 3 figures in ps format. Submitted to Phys. Rev. Lett

We observe large amplitude changes in the resistance of ferromagnetic (F) wires at the onset of superconductivity of adjacent superconductors (S). New sharp peaks of large amplitude are found in the magnetoresistance of the F-wires. We discuss a new mechanism for the long-range superconducting proximity effect in F/S nanostructures based on the analysis of the topologies of actual Fermi-surfaces in ferromagnetic metals., Comment: 7 pages in LaTeX, 5 eps figures. Submitted to the Proceedings of MS2000

We measure the resistance of a normal mesoscopic sample with two Superconducting mirrors and find two regimes with qualitatively different behavior. At temperatures below 90 mK peaks in the conductance were found when the phase difference between the two superconductors is an odd multiple of rr. The peak heights increase with decreasing temperature. Above 100 mK the observed peaks give way to dips in the conductance. While the high-temperature behavior can be explained in terms of the thermal effect [Phys. Rev. Lett, 76, 823 (1996)], we propose that the low-temperature behavior is: a manifestation of resonant transmission of low-energy quasiparticles through Andreev states. [S0163-1829(99)11641-2].

The conductance (G) of mirror-symmetric, disordered normal (N) metal mesoscopic structures with two interfaces to superconductors (S) has been studied experimentally with applied condensate phase differences Delta phi between the N/S interfaces. At Delta phi = 2n pi(n = 0,1,2,3,...) the conductance showed reentrance to the normal state below the temperature corresponding to the Thouless energy. The current-voltage characteristics were found to be: strongly nonlinear even at distances between the N/S interfaces largely exceeding the normal-metal coherence length. An influence of superconductors almost completely disappeared at Delta phi = (2n + 1) pi where the structures showed normal behavior. Calculations based on a quasiclassical theory have been performed offering a quantitative explanation of such a phase-periodic reentrance. The value of the superconducting gap Delta(eff) at the Ag/Al interface has been obtained. We find that Delta(eff)(T,V-->0) = beta .Delta(BCS)(T) with beta = 0.2 independent of temperature in the temperature interval of 0.1 K < T < 1.6 K; Delta(BCS)(T) is the BCS gap vs T function in Al. [S0163-1829(98)01746-9].

The charging energy is important for a mesoscopic body with small capacitance to the environment. Connecting the small island via low capacitance, high resistance tunnel junctions, the current through the resulting structure will depend upon the charge of the island - charge that can be applied via a gate electrode. For a normal metal island, the current varies periodically with the charge; the periodicity is the electron charge e. If the island is superconducting, the period, e or 2e, depends upon whether the superconducting energy gap is less than or larger than the charging energy. This parity effect can be used to study possible nodes in the gap function of a high-Tc superconductor.

The conductance of a normal $\left(N\right)$ mesoscopic silver or antimony conductor, to which a superconducting $\left(S\right)$ aluminum wire is attached at two points, oscillates when a control current in the superconductor is varied. The amplitude of oscillations ranges from $2\ifmmode\times\else\texttimes\fi{}{10}^{2}({e}^{2}/h)$ for Ag/Al to $5.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}({e}^{2}/h)$ for Sb/Al structures at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.02\phantom{\rule{0ex}{0ex}}\mathrm{K}$. The oscillations are attributed to phase transfer from the superconducting condensate to normal electrons via Andreev reflections at the $N\ensuremath{-}S$ interfaces (``mirrors''). The influence of the magnetic field and of the Aharonov-Bohm effect on the oscillations is studied.

An investigation was made of conductance oscillations in normal metal mesoscopic structrures to which superconducting wires were attached. The oscillations were due to phase transfer from the superconducting condensate to normal electrons via Andreev reflections at the normal metal/superconductor interfaces. The phase difference between the interfaces was controlled in a wide range by varying a sub-critical current through the superconductor and, alternatively, by applied magnetic field. This technique may be used to measure phase gradients in superconductors directly, or to control dissipative currents by a non-dissipative supercurrent. Drastic difference was found between the spectra of oscillations in the structures with normal rings and the structures with normal cross-like conductors.

The phase-periodic thermopower in diffusive Sb/Al nanostructures (Andreev interferometers) has been measured in the temperature range 0.3

We report anomalies in the weak localisation (wl) effect and strong deviations from the Kohler rule (DKR) in the classical magnetoresistance of antimony wires of cross-sections down to 30×100 nm 2 in the fields up to 5 T in the temperature interval of 0.27–10 K. A strong anisotropy of the Fermi surface generic to semi-metals and reflections at grain boundaries are suggested to account for DKR. There is probably a link between the DKR and the wl anomalies.

We present low-temperature measurements of Co/Al spin valves with a double-injector source of spin polarized current. Using an in-plane magnetic field, the injector electrodes were magnetized in opposite directions. The spin polarization of the injected current was controlled by changing the ratio of currents through the two electrodes and was continuously varied from zero up to the maximum spin polarization of Co. This result was verified by measuring the spin valve signal, using the detector electrode magnetized to align with one of the injectors. This source can be used for spintronic applications as well as in research on hybrid ferromagnet/superconductor structures.

We report measurements of the temperature dependence of the amplitude of phase-periodic conductance oscillations in disordered normal metal (Ag) structures, attached to a superconducting (Al) wire at two points. The amplitude of oscillations reaches its maximum at temperatureT *, when the Thouless energy is of the order ofk B T. The results are in agreement with recent calculations by Nazarov and Stoof [Phys. Rev. Lett. 76 (1996) 823].

We present an experimental study of electron phase memory effects in disordered mesoscopic rings of a normal metal (Ag) with superconducting boundaries. Magneto resistance oscillations of two periods, h/2e and h/4e, were observed. The measurement current, and the position of the superconducting “mirrors” with respect to the ring, had a drastic influence on the h/4e component, while the h/2e one was much less sensitive to these parameters. Such behaviour is explained in terms of influence of the macroscopic phase of the superconductor on the microscopic phase of the normal electron. In this context we discuss the cross-over from the adiabatic to the non-adiabatic regime in our system.

By employing an electron cyclotron resonance plasma‐enhanced chemical vapor deposition technique, we report the successful growth of cubic boron nitride films on single‐crystal (100) silicon wafer without external rf or dc substrate biasing. Ammonia and boron trifluoride gases were used for the deposition of cubic boron nitride. The substrate temperature during deposition was about 675 °C. The films were characterized by infrared spectroscopy and ellipsometry. The existence of cubic boron nitride was identified by the characteristic boron nitride infrared signal at 1110 cm−1. The film thickness was about 1000 A, with a growth rate of 100 A/min.

Helicon damping in indium is investigated in magnetic fields extending to 8 T. A fine structure, observed for field directions within about 1 degrees of (100), is associated with magnetic breakdown. A critical breakdown field B0

This review is devoted to helicons-electromagnetic waves propagating in the electron-hole plasma in metals at low temperatures. The review is divided into two parts. In the first part the author presents the theory of, and the experimental data on, helicons in the absence of effects caused by Landau quantisation of conduction electron energy in a strong magnetic field. The author emphasises the peculiar features of helicon propagation which are due to the anisotropy of the electron spectrum of metals, effects not observable in a gaseous plasma. An analysis of data on the anisotropy of the collision damping, the Landau damping and on dopplerons-a new branch of electromagnetic excitations in the vicinity of the Doppler-shifted cyclotron resonance-is given. He also discusses the results of studies of the acoustic satellites of helicon resonance and the peculiarities of helicon propagation in the intermediate state of type-I superconductors. In the second part he presents the results of observations of new effects caused by Landau quantisation, namely oscillations of the phase velocity of helicons, non-linear helicon resonance and the anomalous attenuation of helicons in metals with diamagnetic domains. Examples are discussed in which the helicon resonances are employed to investigate transport relaxation in high magnetic fields, the production of defects by plastic deformation, open orbits and the absolute amplitude of the de Haas-van Alphen effect.

100‐keV e‐beam lithography and electroplating of gold have been used to fabricate a device for the measurement of quantum changes in magnetic flux. The structure is defined in a 200‐nm‐thick layer of a copolymer of methyl methacrylate and methacrylic acid (PMMA/MAA) on top of a Si wafer covered by 150 nm of Si3N4. The device is a square frame of 2×2 μm2 and 100‐nm linewidth. Gold was electroplated to a thickness of 100 nm. This technique allows the fabrication of high resolution, high aspect ratio gold structures.