The lattice thermal conductivity of normal and superconducting niobium

The lattice thermal conductivity of superconducting and normal Nb as limited by the interaction of phonons with electrons has been deduced from measurements in the superconducting state. The results indicate that the mean free paths of transverse and longitudinal phonons are similar, ～4×10^?5 T ^?1 (cm K), in the normal state. Comparison is made with measurements on other metals. A compilation is included of the ratio of lattice conduction in the normal state to that in the superconducting state, based on the BCS theory of superconductivity.     

Resonant tunneling and perpendicular conduction in cuprate conductors with varying O-content

Anisotropy is inherent to layered cuprates with conduction mainly confined to the CuO₂-planes, claimed to be the source of superconductivity. Resonant tunnel exchange for conduction parallel and perpendicular to the CuO₂-planes shows different normal and superconducting properties by charging energies and localizations at the in-plane perturbations being atomic-like and between the CuO₂-planes, being extended. By overdoping the wave function overlap in perpendicular direction via the extended states grows, especially in the superconducting state. The counteraction of overlap and charging energies yield activated resonant tunneling, i.e. the pseudo gap, in the normal state and Josephson tunneling in the superconducting state.     

Conductive dense hydrogen

Molecular hydrogen is expected to exhibit metallic properties under megabar pressures. This metal is predicted to be superconducting with a very high critical temperature, T(c), of 200-400 K, and it may acquire a new quantum state as a metallic superfluid and a superconducting superfluid. It may potentially be recovered metastably at ambient pressures. However, experiments carried out at low temperatures, T<100 K, showed that at record pressures of 300 GPa, hydrogen remains in the molecular insulating state. Here we report on the transformation of normal molecular hydrogen at room temperature (295 K) to a conductive and metallic state. At 200 GPa the Raman frequency of the molecular vibron strongly decreased and the spectral width increased, evidencing a strong interaction between molecules. Deuterium behaved similarly. Above 220 GPa, hydrogen became opaque and electrically conductive. At 260-270 GPa, hydrogen transformed into a metal as the conductance of hydrogen sharply increased and changed little on further pressurizing up to 300 GPa or cooling to at least 30 K; and the sample reflected light well. The metallic phase transformed back at 295 K into molecular hydrogen at 200 GPa. This significant hysteresis indicates that the transformation of molecular hydrogen into a metal is accompanied by a first-order structural transition presumably into a monatomic liquid state. Our findings open an avenue for detailed and comprehensive studies of metallic hydrogen.     

Observation of a Possible Metallic State Induced by a Parallel Magnetic Field in Superconducting Au0.7In0.3 Samples With Very Low Normal-state Sheet Resistance

We report the observation of an apparent metallic state induced by a parallel magnetic field in Au_0.7In_0.3 samples with very low normal-state sheet resistance. These samples can be modeled as a random array of superconductor-normal metal-superconductor (SNS) junctions. For both the thin planar and cylindrical films, the magnetic field was applied parallel to the substrate of the sample and measurement currents. For the mesoscopic rings, however, a perpendicular field was used. Our electrical transport and tunneling measurements suggest that the samples consist of superconducting In-rich islands not linked by Josephson coupling in the metallic state. The physical origin of the metallic state is discussed.      

Tunneling Through Metallic Quantum Dots

We discuss single-electron tunneling measurements at dilutionrefrigerator temperatures on metallic grains, sufficientlysmall that the quantum levels of the conduction electrons canbe resolved. These measurements directly reveal the energyeigenvalues of the electrons in a grain that typicallycontains a few thousand conduction electrons. Suchmeasurements were first carried out a few years ago by Ralph,et al. on nanograins of Al. More recently, this workhas been extended to measurements on nanoparticles of theheavy metal Au by Davidovi and on nanoparticles ofalloys of Al and Au by Salinas, et al. This morerecent work has pointed up the need to go beyond the simplestindependent-electron model, to include the Coulomb interactionbetween electrons and also nonequilibrium electronpopulations. These interactions cause the energy levels tomerge into a continuum above the Thouless energy and can causea single quasiparticle level to show up as a cluster ofresonances. The strong spin-orbit interaction in Au cancause levels to split in magnetic fields with a g-factor of0.3, instead of the free electron g=2. In addition,there is evidence for a proliferation of low-lying energylevels suggestive of system spin values greater than1/2 induced by the exchangeinteraction. This paper will review the evolving progress thathas been made in interpreting these observations.     

The Mott-Hubbard insulating state and orbital degeneracy in the superconducting C60(3-) fulleride family

Electron correlation controls the properties of important materials such as superconducting and magnetoresistive transition metal oxides and heavy fermion systems. The role of correlation in driving metal-to-insulator transitions assumes further importance because many superconducting materials are located close to such transitions. The nature of the insulating ground state often reveals the dominant interactions in the superconductor, as shown by the importance of the properties of La2CuO4 in understanding the high-temperature-superconducting cuprates. The A3C60 alkali metal fullerides are superconducting systems in which the role of correlation in both the normal state and the superconducting pairing mechanism is controversial, because no magnetic insulator comparable to the superconducting materials has been identified. We describe the first example of a cubic C60(3-) system with degenerate orbitals that adopts the Mott-Hubbard insulating localized electron ground state. Electron repulsion is identified as the interaction that is suppressed on the transition to metallic and superconducting behaviour in the fullerides. This observation is combined with ab initio calculations to demonstrate that it is the orbital degeneracy that allows the superconducting cubic A3C60 fullerides to remain metallic while provoking electron localization in systems with lower symmetry.     

Evidence for Effective Weakening of Electron-Phonon Interaction in Superconducting Tantalum, Niobium, Lead and Aluminum

It is sporadically documented in the literature that the electron-phonon interaction is much weaker than expected in the superconducting state in metals. Owing to a tendency to explain it away this observation has so far gone under-recognized. By applying a straightforward numerical procedure to normal state resistivity and superconducting tunneling conductance experimental data the atomic potentials in tantalum, niobium, lead and aluminum are revealed to be consistently weaker in the superconducting state than in the normal state.     

Fluctuations and the superconducting phase transition: II. Onset of Josephson tunneling and paraconductivity of a junction

When one metallic film of a tunneling junction is in the superconducting state but the other is above its transition temperature, fluctuations in the pair field of the second film cause instantaneous Josephson currents to flow. Application of the Kubo formula to these current fluctuations yields a conductivity with a strong temperature dependence, tending to infinity at the transition. The numerical magnitude of this "paraconductivity" is sufficiently large to be observable for junctions of very low normal resistance (and may possibly be compared with some recent measurements of Lipsonet al.). High-frequency studies would determine directly the pair relaxation rate. An experimental advantage of tunneling paraconductivity is that it is not sensitive to the electron mean free path and therefore does not require amorphous films.This work has been supported in part by the Air Force Office of Scientific Research and by the Office of Naval Research.National Science Foundation Senior Postdoctoral Fellow.     

Quantum diffusion of positive muons and muonium atoms

The diffusion properties of the positive muon (μ⁺) and muonium (Mu) as light isotopes of protons and atomic hydrogen in crystalline solids are reported. It is demonstrated that they are excellently described as quantum tunneling of small polarons interacting with a phonon/electron bath. As one of the most dramatic manifestations of the quantum nature, the tunneling probability increases with decreasing temperature, which is in sharp contrast with the case of thermally activated diffusion. Moreover, the recent experimental study at very low temperature (below 0.1 K) strongly suggests that Mu is in a delocalized state analogous to the Bloch state for conduction electrons.     

Dephasing of Coherent Echoes by Nuclear Quadrupoles

Recent experiments on tunneling systems in insulating glasses showed an unexpected magnetic-field dependence of spontaneous polarization echoes. Besides a strong overall increase with the magnetic field, the echo amplitude slowly oscillates with a frequency that is proportional to B. We develop a theory that couples the tunneling motion to nuclear quadrupoles and obtain the beat frequency and dephasing in terms of the nuclear magnetic and quadrupolar moments, the waiting time, and the magnetic field. Our results compare favorably with measurements on various amorphous solids. PACS numbers: 77.22Ch, 61.43.Fs, 64.90.+b     

Quantum Coherence of Charge States in the Single Electron Box

A metallic electrode connected to electron reservoirs by tunnel junctions has a series of charge states corresponding to the number of excess electrons in the electrode. In contrast to the charge state of an atomic or molecular ion, the charge states of such an island are mesoscopic states involving all the conduction electrons of the island. Island charge states bear some resemblance to the photon number states of the cavity in cavity QED, the phase conjugate to the number of electrons being analogous to the phase of the field in the cavity. For a normal island, charge states decay irreversibly into charge states of lower energies. However, the gound state of a superconducting island connected to superconducting reservoirs can be a coherent superposition of charge states differing by two electrons (i.e., a Cooper pair). We describe an experiment in which this Josephson effect involving only one Cooper pair is measured.     

Inverse ductile–brittle transition in metallic glasses?

There is evidence that metallic glasses can show increased plasticity as the temperature is lowered. This behaviour is the opposite to what would be expected from phenomena such as the ductile–brittle transition in conventional alloys. Data collected for the plasticity of different metallic–glass compositions tested at room temperature and below, and at strain rates from rate 10^?5 to 10³ s^?1, are reviewed. The analogous effects of low temperature and high strain rate, as observed in conventional alloys, are examined for metallic glasses. The relevant plastic flow in metallic glasses is inhomogeneous, sharply localised in thin shear bands. The enhanced plasticity at lower temperature is attributed principally to a transition from shear on a single dominant band to shear on multiple bands. The origins of this transition and its links to shear bands operating ‘hot’ or ‘cold’ are explored. The stress drop on a shear band after initial yielding is found to be a useful parameter for analysing mechanical behaviour. Schematic failure mode maps are proposed for metallic glasses under compression and tension. Outstanding issues are identified, and design rules are considered for metallic glasses of improved plasticity.     

Thermal fluctuation-induced tunneling conduction through metal nanowire contacts

The temperature behavior of how electrons propagate through an insulating electronic contact formed at the interface between a submicron Cr/Au electrode and a metallic RuO(2) nanowire (NW) has been studied between 300 and 1?K. The NWs are typically of ～70?nm in diameter and a few microns long. The submicron electrodes were fabricated by the standard electron-beam lithography technique. By employing the two-probe method, the electronic contact resistances, R(c)(T), have been determined. We found that, in general, R(c) increases rapidly with decreasing temperature but eventually saturates at liquid-helium temperatures. Such a temperature behavior can be well described by a thermal fluctuation-induced tunneling (FIT) conduction process which considers the crossover feature from thermal activation conduction at high temperatures to simple elastic tunneling conduction at low temperatures. The wide applicability of this FIT model has further been established by employing metallic IrO(2) and Sn-doped In(2)O(3-x) NWs. This work demonstrates that the underlying physics for the charge transport properties of an insulating electronic contact can be well understood.     

Proximity effect sandwiches containing local spin fluctuations

A proximity effect sandwich that contains local spin fluctuations in the normal layer is studied. The renormalization group approach is used to treat the Coulomb repulsion between thed electrons of opposite spins that are localized on the transition metal impurities present in the normal layer. The perturbations due to the tunneling of electrons from one layer to the other and the mixing of the normal-layer electrons with the impurityd electrons are treated using the standard Born approximation. A decrease of the transition temperature for both thin and thick, superconducting layers is obtained. It is found that there is a critical concentration at which the superconductivity in the thin superconducting layer can be quenched. For the thick-superconducting-layer sandwiches, no critical concentration is found.     

Carrier transport mechanisms and photovoltaic characteristics of Au/toluidine blue/n-Si/Al heterojunction solar cell

Toluidine blue (TB)/n-silicon heterojunction solar cell was fabricated by depositing TB film on n-silicon wafer using thermal deposition technique. X-ray diffraction patterns of the TB film show presence of crystals with size 30 nm dispersed in amorphous matrix. The current–voltage–temperature performance of Au/TB/n-Si/Al device was studied in dark and under illumination conditions. The device showed diode behavior. The diode parameters such as ideality factor, barrier height, series and shunt resistance were determined using a conventional I–V–T characteristics. The analysis of the diode characteristics in forward bias direction confirmed that the transport mechanisms of the Au/TB/n-Si/Al solar cell at applied potential?<?0.1 V is thermionic emission and at high electric field?>?0.1 V is Ohmic conduction. The operating conduction mechanisms in reverse bias direction are Pool–Frenkel effect followed by Schootky field lowering mechanism. The small value of activation energy in reverse bias direction indicates that the conduction process is expected to be by tunneling of electrons between nearest-neighbor sites and it is temperature independent. The photo conduction characteristics of the diode suggests its application as a solar cell.     

Electroluminescence from a single-nanocrystal transistor

We report the fabrication and characterization of light-emitting transistors incorporating individual cadmium selenide (CdSe) nanocrystals. Electrical measurements conducted at low bias voltage and low temperature show clear evidence of Coulomb blockade behavior, indicating that electrons pass through the nanocrystal by single-electron tunneling. Once the bias voltage exceeds the band gap of CdSe, devices with asymmetric tunnel barriers emit linearly polarized light. Combined analyses of the electrical and optical data indicate that the tunnel couplings between the nanorod and the metallic electrodes change significantly as a function of bias voltage and light emission results from the inelastic scattering of tunneling electrons.     

The effect of the superconducting transition on the acoustic losses in audiofrequency niobium resonators

The mechanical Q of Nb disk samples has been measured as a function of temperature with different annealing treatments. A rapid increase in Q is seen on cooling through the superconducting transition temperature. This is shown to be consistent with the predictions of the theory of acoustic attenuation due to conduction electrons, and the measurements are believed to be the first audiofrequency observation of attenuation by this mechanism. Residual losses in the sample limit the possibility of a quantitative comparison with theory in the superconducting state. Maximum Q values obtained were 1.8 × 10⁷ at 690 Hz, 3.8 × 10⁷ at 1.6 kHz, and 4.8 × 10⁷ at 4.2 kHz. Optimization of the annealing treatment may increase the Q still further, thus enabling improved measurements of the electronic attenuation to be made.Work supported in part by the Australian Research Grants Committe.     

Superconducting state parameters of binary metallic glasses

Ashcroft’s empty core (EMC) model potential is used to study the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ^*, transition temperature T_C, isotope effect exponent and effective interaction strength N₀V of some binary metallic glasses based on the superconducting (S), conditional superconducting (S′) and non-superconducting (NS) elements of the periodic table. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used for the first time with EMC potential in the present investigation to study the screening influence on the aforesaid properties. The T_C obtained from IU-local field correction function are found an excellent agreement with available theoretical or experimental data. In the present computation, the use of pseudo-alloy-atom model (PAA) is proposed and found successful. The present work yield results in qualitative agreement with such earlier reported values either theoretical or experimental which confirm the superconducting phase in all metallic glasses. A strong dependency of the SSP of the metallic glasses on the valence Z is found.     

Phonon dispersion in metallic glasses

The phonon frequencies of longitudinal and transverse waves for metallic glasses are derived and computed for ternary Pd77.5Si16.5Cu6 glasses. The dispersion of phonon frequency with respect to the wavenumber is found to be influenced by the dielectric screening due to conduction electrons. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electron emissions in InAs quantum dots containing a nitrogen incorporation induced defect state: the influence of thermal annealing

With the incorporation of nitrogen (N) into InAs quantum dots (QDs), the carrier distribution near the QD displays electron emissions from a localized N-induced defect state at 0.34?eV and a weak emission at 0.15?eV from the QD. This defect state causes drastic carrier depletion in the neighboring GaAs bottom layer near the QD, which can effectively suppress tunneling emission for the QD excited states. As a result, electrons escape from the QD ground state through thermal emission to near the GaAs conduction band, rather than through thermal emission to the QD first excited state and a subsequent tunneling to the GaAs conduction band, as observed in InAs QDs without N incorporation. Thermal annealing can weaken the defect emission and enhance the QD emission, suggesting a removal of the defect state and a recovery of carriers in the QD. Increasing annealing temperature can significantly decrease the emission time and energy of the QD emission, which is explained by a weakening of tunneling suppression due to the removal of the defect state.