Search

We report the observation of enhanced interfacial two-component superconductivity possessing a dominant triplet component in nonmagnetic CoSi2/TiSi2 superconductor/normal-metal planar heterojunctions.

We present 1/$f$ noise measurements of IrO$_2$ nanowires from 1.7 to 350 K. Results reveal that the noise magnitude (represented by Hooge parameter $\gamma$) increases at low temperatures, indicating low-frequency resistance noise from universal conductance fluctuations. The cause of this noise is determined to be due to oxygen vacancies in the rutile structure of IrO$_2$. Additionally, the number density of these mobile defects can be calculated from the $\sqrt{T}$ resistance rise caused by the orbital two-channel Kondo effect in the Dirac nodal line metal IrO$_2$., Comment: 5 pages, 4 figures

Monolayer transition metal dichalcogenides offer an appropriate platform for developing advanced electronics beyond graphene. Similar to two-dimensional molecular frameworks, the electronic properties of such monolayers can be sensitive to perturbations from the surroundings; the implied tunability of electronic structure is of great interest. Using scanning tunneling microscopy/spectroscopy, we demonstrated a bandgap engineering technique in two monolayer materials, MoS

We reanalyze the seminal work by Dolan and Osheroff [Phys. Rev. Lett. $\textbf{43}$, 721 (1979)] which reported anomalous low-temperature conduction of high-resistivity thin-film metal strips. We argue that the observed logarithmic increase of resistance with decreasing temperature in their 3-nm-thick Au-Pd strips be ascribed to the granularity effect on electron conduction in discontinuous metal films. This reanalysis is further supported by our measurements on conducting Pb$_x$(SiO$_2$)$_{1-x}$ nanogranular films, where $x$ is the volume fraction of Pb., 7 pages, 2 figures

Two-dimensional (2D) semiconducting monolayers such as transition metal dichalcogenides (TMDs) are promising channel materials to extend Moore's Law in advanced electronics. Synthetic TMD layers from chemical vapor deposition (CVD) are scalable for fabrication but notorious for their high defect densities. Therefore, innovative endeavors on growth reaction to enhance their quality are urgently needed. Here, we report that the hydroxide W species, an extremely pure vapor phase metal precursor form, is very efficient for sulfurization, leading to about one order of magnitude lower defect density compared to those from conventional CVD methods. The field-effect transistor (FET) devices based on the proposed growth reach a peak electron mobility ~200 cm

We have measured low-temperature variation of resistance and nonlinear current-voltage behavior in antidot graphene in the vicinity of the charge neutrality point. The data are found to be consistent with the manifestations of a variable-range hopping electronic density of states (DOS) with a small hard gap of $\ensuremath{\sim}1$ meV around the Fermi level, in conjunction with a parallel tunneling conduction channel that exists at the center of the gap. The hard gap is confirmed by the appearance of a low-conductive plateau at low-bias electric field, whereas the parallel tunneling conduction channel, with temperature-independent conductance, is manifest through the nonlinear electric field variation. Unified good agreement between the temperature and electric field dependencies of conductance, for both channels, is obtained with the predictions of a proposed DOS model. An increase in the gap size with applied magnetic field is observed.

Unconventional superconductivity and in particular triplet superconductivity have been front and center of topological materials and quantum technology research. Here we report our observation of triplet superconductivity in nonmagnetic CoSi$_2$/TiSi$_2$ heterostructures on silicon. CoSi$_2$ undergoes a sharp superconducting transition at a critical temperature $T_c \approx$ 1.5 K, while TiSi$_2$ is a normal metal. We investigate conductance spectra of both two-terminal CoSi$_2$/TiSi$_2$ tunnel junctions and three-terminal T-shaped CoSi$_2$/TiSi$_2$ superconducting proximity structures. We report an unexpectedly large spin-orbit coupling in CoSi$_2$ heterostructures. Below $T_c$, we observe (1) a narrow zero-bias conductance peak on top of a broad hump, accompanied by two symmetric side dips in the tunnel junctions, (2) a narrow zero-bias conductance peak in T-shaped structures, and (3) hysteresis in the junction magnetoresistance. These three independent and complementary observations are indicative of chiral $p$-wave pairing in CoSi$_2$/TiSi$_2$ heterostructures. This chiral triplet superconductivity and the excellent fabrication compatibility of CoSi$_2$ and TiSi$_2$ with present-day silicon integrated-circuit technology facilitate full scalability for potential use in quantum-computing devices., 38 page (main text: 17 pages, supplementary information: 21 pages)

We investigate the dynamical interaction between an electron crystal trapped above the surface of liquid helium and surface waves (`ripplons') excited by its motion. At rest, the electron system is `dressed' by static ripplons to form `ripplopolaron' states. As the electrons move, resonant ripplon scattering results in a growth of the ripplopolaron effective mass, on timescales comparable with the inverse of the ripplon frequency ($\ensuremath{\sim}100$ ns). Under sufficient driving force, the electron system decouples from the surface waves and moves at high velocity, before decelerating sharply when the electron solid and surface excitations recouple to form a `new' ripplopolaron system. The mass of the newly formed ripplopolarons is similar to that in the initial static case.

The present-day nanodevice dimensions continuously shrink, with the aim to prolong Moore's law. As downsizing meticulously persists, undesirable dynamic defects, which cause low-frequency noise and structural instability, play detrimental roles on limiting the ultimate performance and reliability of miniaturized devices. A good understanding and a meaningful control of the defect kinetics then become fundamental and urgent issues. Here we report observations of thermally activated atomic defect motion as well as nanocrystalline defect motion through electrical noise processes in metallic ${\mathrm{Ru}\mathrm{O}}_{2}$ rutile nanowires around room temperature. First, we extract the energy-distribution function and the number density of mobile atomic defects (oxygen vacancies). Second, we obtain the geometrical size, grain-boundary bonding strength, and relaxation times of dynamic nanocrystallites. Our results show clearly a powerful probe for effective and noninvasive characterizations of nanostructures and nanomaterials for which quantitative information about mechanical hardness, breakdown current density, and/or resistance noise is essential.

We demonstrate two-step electron-beam lithographic fabrication of transparent disilicide superconductor/normal-metal CoSi2/TiSi2 contact junctions, where CoSi2 is a superconductor (S) with a transition temperature Tc ≈ 1.5 K, and TiSi2 is a normal metal (N). The lateral S/N contact junction is embedded in a silicon substrate. The fabrication processes are compatible with the state-of-the-art silicon integrated-circuit technology. We show potential advantages of these junctions over conventional (tunnel) junctions where S and N are vertically stacked on a substrate. We propose that epitaxial CoSi2/Si heterostructures may be used as a basis in superconducting devices and qubits.

Strong electron correlations have long been recognized as driving the emergence of novel phases of matter. A well recognized example is high-temperature superconductivity which cannot be understood in terms of the standard weak-coupling theory. The exotic properties that accompany the formation of the two-channel Kondo effect including the emergence of an unconventional metallic state in the low-energy limit also originate from strong electron interactions. Despite its paradigmatic role for the formation of non-standard metal behavior, the stringent conditions required for its emergence have made the observation of the nonmagnetic, orbital two-channel Kondo effect in real quantum materials difficult, if not impossible. We report the observation of orbital one- and two-channel Kondo physics in the symmetry-enforced Dirac nodal line metals IrO2 and RuO2 nanowires and show that the symmetries that enforce the existence of Dirac nodal lines also promote the formation of nonmagnetic Kondo correlations. Rutile oxide nanostructures thus form a versatile quantum matter platform to engineer and explore intrinsic, interacting topological states of matter., 40 pages including supplement

We revisit the classical problem of granular hopping conduction's temperature dependence, and offer a straightforward and simple explanation on a phenomenon that was widely observed over diverse material systems, but which has remained a puzzle in spite of the various efforts for its explanation.

New ternary phase of Bi 3 Se 2 Te thin films were successfully grown through pulsed laser deposition with a single Bi 2 Se 2 Te crystal as the target. Bi-rich and Se- and Te-deficient compositions deposited at a substrate temperature ( T s ) of 250 °C and helium gas pressures ( P He ) ranging from 2.0 × 10 −5 to 6.5 × 10 −1 Torr aided the formation of the dominant Bi-rich phase: Bi 3 Se 2 Te. The films were grown epitaxially on Al 2 O 3 (001) substrates with Bi 3 Se 2 Te [001]//Al 2 O 3 [001] and Bi 3 Se 2 Te [110]//Al 2 O 3 [210]. An increase in P He remarkably enhanced the hardness and Young's modulus of the films, primarily because of the decrease in nanograin size, following the Hall–Petch relationship. Moreover, the Bi 3 Se 2 Te films present linear magnetoresistance under a high perpendicular magnetic field ( B ≥ 4 T) and two-dimensional weak antilocalization effect under a low B (±1 T) which may be attributed to topological insulator surface state (TSS). Clearly, further theoretical calculations and experiments are needed to confirm the suggested interpretation.

Ruthenium dioxide is an important material, widely used in nanoelectronic devices and interconnects, supercapacitors, and as a catalyst. A good understanding of the origin of the low-frequency $1/f$ noise in RuO${}_{2}$'s resistivity will advance the design and efficiency of these applications. The authors demonstrate that the flicker noise in RuO${}_{2}$ films originates from fluctuating oxygen vacancies, which act as dynamical structural defects, and they quantify the activation-energy distribution function of these dynamical defects and calculate the oxygen-vacancy density from the measured temperature dependence of the $1/f$ noise.

We report here the correlation of electrical properties on Schottky barrier contacts and atomic scale imperfections based on the degree of lattice distortion and moire fringes at the interfaces of CoSi2 and Si. The I-V measurements showed that the higher levels of perfection of the interfaces gave better Schottky barrier characteristics and we discuss the implications of the transport property at Schottky barrier contacts.

Epitaxial $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) films have been grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$ (001) substrates via pulsed laser deposition. In a 22-nm-thick LSMO film with a low residual resistivity of ${\ensuremath{\rho}}_{0}\ensuremath{\approx}59\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$, we found a zero-field dip in the magnetoresistance (MR) below 10 K, manifesting the weak antilocalization (WAL) effect due to strong spin-orbit coupling (SOC). We have analyzed the MR data by including the D'yakonov-Perel' spin-relaxation mechanism in the WAL theory. We explain that the delocalized spin-down electron sub-band states play a crucial role for facilitating marked SOC in clean LSMO. Moreover, we find that the SOC strength and gate voltage tunability are similar to those in a two-dimensional electron gas at the $\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ interface, indicating the presence of an internal electric field near the LSMO/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface. In a control measurement on a 5-nm-thick high resistivity $({\ensuremath{\rho}}_{0}\ensuremath{\approx}280\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm})$ LSMO film, we observe only a small zero-field peak in MR from weak localization effect, indicating negligible SOC.

Dynamical structural defects exist naturally in a wide variety of solids. They fluctuate temporally, and hence can deteriorate the performance of many electronic devices. Thus far, the entities of such dynamic objects have been identified to be individual atoms. On the other hand, it is a long-standing question whether a nanocrystalline grain constituted of a large number of atoms can switch, as a whole, reversibly like a dynamical atomic defect (i.e., a two-level system). This is an emergent issue considering the current development of nanodevices with ultralow electrical noise, qubits with long quantum coherence time, and nanoelectromechanical system (NEMS) sensors with ultrahigh resolution. Here we demonstrate experimental observations of dynamic nanocrystalline grains which repeatedly switch between two or more metastable coordinate states. We study temporal resistance fluctuations in thin ruthenium dioxide (RuO2) metal nanowires and extract microscopic parameters including relaxation time scales, mobile grain sizes, and the bonding strengths of nanograin boundaries. Such material parameters are not obtainable by other experimental approaches. When combined with previous in-situ high-resolution transmission electron microscopy (HRTEM), our electrical method can be used to infer rich information about the structural dynamics of a wide variety of nanodevices and new 2D materials., Comment: main text (28 pages) + supplementary material (4 pages)

Indium tin oxide (Sn-doped In$_2$O$_{3-\delta}$ or ITO) is an interesting and technologically important transparent conducting oxide. This class of material has been extensively studied for decades, with research efforts focusing on the application aspects. The fundamental issues of the electronic conduction properties of ITO from 300 K down to low temperatures have rarely been addressed. Studies of the electrical-transport properties over a wide range of temperature are essential to unraveling the underlying electronic dynamics and microscopic electronic parameters. We show that one can learn rich physics in ITO material, including the semi-classical Boltzmann transport, the quantum-interference electron transport, and the electron-electron interaction effects in the presence of disorder and granularity. To reveal the opportunities that the ITO material provides for fundamental research, we demonstrate a variety of charge transport properties in different forms of ITO structures, including homogeneous polycrystalline films, homogeneous single-crystalline nanowires, and inhomogeneous ultrathin films. We not only address new physics phenomena that arise in ITO but also illustrate the versatility of the stable ITO material forms for potential applications. We emphasize that, microscopically, the rich electronic conduction properties of ITO originate from the inherited free-electron-like energy bandstructure and low-carrier concentration (as compared with that in typical metals) characteristics of this class of material. Furthermore, a low carrier concentration leads to slow electron-phonon relaxation, which causes ($i$) a small residual resistance ratio, ($ii$) a linear electron diffusion thermoelectric power in a wide temperature range 1-300 K, and ($iii$) a weak electron dephasing rate. We focus our discussion on the metallic-like ITO material., Comment: 17 figures, 1 table, Topical Review Article

We present time-resolved transport measurements of a Wigner solid (WS) on the surface of liquid Helium confined in a micron-scale channel. At rest, the WS is `dressed' by a cloud of quantised capillary waves (ripplons). Under a driving force, we find that repeated WS-ripplon decoupling leads to stick-slip current oscillations, the frequency of which can be tuned by adjusting the temperature, pressing electric field, or electron density. The WS on liquid He is a promising system for the study of polaron-like decoupling dynamics., 5 pages, 5 figures, and supplemental

Electrons on helium substrates form a model Coulomb system in which the transition from classical electron liquid to Wigner crystal is readily observed. However, attempts to increase the electron density in order to observe the ‘quantum melting’ of the system to a Fermi degenerate gas are hindered by an instability of the helium surface. Here we describe experimental efforts to reach the degenerate regime on thin helium films and microstructured substrates, for which the surface instability is suppressed. We demonstrate that, although the electron densities obtained exceed those for bulk helium substrates, observation of quantum melting remains challenging. We discuss possible solutions to the technical challenges involved. published

We investigate the influence of confinement on the positional order of a quasi-1D electron system trapped on the surface of liquid helium. We find evidence that the melting of the Wigner solid (WS) depends on the confinement strength, as well as electron density and temperature. A reentrant solid-liquid-solid transition is observed for increasing electron density under constant electrostatic confinement. As the electron row number ${N}_{y}$ changes, varying commensurability results in a modulation of the WS order, even when ${N}_{y}$ is large (several tens). This is confirmed by Monte Carlo simulations.

We have measured the resistance and thermopower of a series of RF sputtered and annealed indium tin oxide (ITO) thin films from 300 K down to liquid-helium temperatures. Thermal annealing was performed to modulate the levels of disorder (i.e., resistivity) of the samples. The measured resistances are well described by the Bloch–Gruneisen law between 150 and 300 K, suggesting that our thin films are metallic. At lower temperatures, a resistance rise with decreasing temperature was observed, which can be quantitatively ascribed to the two-dimensional electron–electron interaction and weak-localization effects. The thermopowers in all samples are negative and reveal fairly linear temperature dependence over the whole measurement temperature range, strongly indicating free-electron like conduction characteristics in ITO thin films. As a result, the carrier concentration in each film can be reliably determined. This work demonstrates that ITO films as thin as 15 nm thick can already possess high metallic conductivity.

Zn0.92Co0.08O nanowires with two different average diameters, 19 and 38 nm, have been made by the vapor transport and Co ion implantation method. The as-implanted nanowires were thermally annealed through multiple steps in a high vacuum. The morphology and crystal structure of the nanowires were inspected by use of scanning and transmission electron microscopes. Electron microscopy analysis was used to ensure the absence of Co nanocrystals in the annealed nanowires. Measurements of temperature-dependent hysteresis were carried out to demonstrate a strong magnetic state in the high-vacuum annealed samples. The coercive field obeying a square-root temperature dependence and the room-temperature ferromagnetism have been discovered. Moreover, by comparison of magnetic properties between different-diameter nanowires, field-dependent magnetizations reveal considerably stronger ferromagnetism in the smaller diameter (19 nm) Zn0.92Co0.08O nanowires than that in the larger diameter (38 nm) nanowires. We argue that...

We measured electron transport state spectra of an In0.05Ga0.95As vertical double quantum dot. Both the ground and excited states of transport spectra from two electrons to three electrons are measured using a large source–drain voltage. In the obtained transition spectrum, the ground state transition from the 1S2 singlet state to the 1S2P triplet state is observed at 5 T. Zeeman splitting with a g-factor of 0.36 is clearly observed at magnetic fields higher than 5 T. The observation of two Zeeman sublevels instead of three for the triplet state is explained by the spin selection rules for the SZ components between the two-electron and three-electron spin states. Transition with the total spin difference between the initial and final states larger than 1/2 is forbidden. Because the transition between doublet states is much longer than electron tunneling, both spin up and spin down can be the initial states from spin transitions. There are four transitions contributing to tunneling processes, but only two energy differences lead to the two Zeeman sublevels in the excitation spectra.

Diameter controllable ZnO nanowires have been fabricated by thermal evaporation (vapor transport) with various sizes of gold nanoparticles as catalysts. Diluted magnetic semiconductor (DMS) Zn1−xCoxO nanowires were then made by high energy Co ion implantation. The as-implanted and the argon-annealed Zn1−xCoxO nanowires displayed weak ferromagnetism while the high-vacuum annealed nanowires exhibited strong ferromagnetic ordering at room temperature. Size dependent behavior has been observed in the magnetic field and temperature dependences of magnetization. The shrinkage of the nanowire diameter reduced the spontaneous magnetization as well as the hysteresis loops. Field cooled and zero-field cooled magnetization and coercivity measurements were performed between 2 and 300 K to study the evolution of magnetism from the weak to the strong ferromagnetic states. In particular, superparamagnetic features were observed and shown to be intrinsic characteristics of the DMS Zn1−xCoxO nanowires. The room-temperature spontaneous magnetization of individual Zn1−xCoxO nanowires was also established by using magnetic force microscope measurements.

This paper demonstrates the applicability of electron-spectroscopic imaging (ESI) for valence-state mapping of the iron oxide system. We have previously developed a set of signal-processing methods for an ESI series, to allow mapping of sp 2 /sp 3 ratio, dielectric function and energy bandgap. In this study, these methods are applied to generate a valence-state map of an iron oxide thin film (Fe/α-Fe 2 O 3 ). Two problems, data undersampling and a convolution effect associated with extraction of the image-spectrum from the core loss image series, were overcome by using cubic-polynomial interpolation and maximum-entropy deconvolution. As a result, the reconstructed image-spectrum obtained from the ESI series images has a quality as good as that of conventional electron energy-loss spectra. The L 3 / L 2 ratio of the reconstructed ESI spectrum is determined to be 3.30 ± 0.30 and 5.0 ± 0.30 for Fe and α-Fe 2 O 3 , respectively. Our L 3 / L 2 ratio mapping shows an accurate correspondence across the Cu/Fe/α-Fe 2 O 3 region. The effect of delocalization and chromatic aberration on the ESI resolution is discussed and estimated to be about 2 nm for the case of L 3 / L 2 ratio mapping.

We study the temperature behaviors of the Hall coefficient, ${R}_{H}$, and longitudinal conductivity, $\ensuremath{\sigma}$, of two series of ${\mathrm{Ag}}_{x}({\mathrm{SnO}}_{2}){}_{1\ensuremath{-}x}$ $(x$ being the Ag volume fraction) granular films lying in the metallic regime. The first (second) series of films are $\ensuremath{\sim}500$ $(\ensuremath{\sim}9)$ nm thick and constitute a three- (two-) dimensional granular array. In the high-$x$ regime with ${g}_{T}\phantom{\rule{0.16em}{0ex}}\ensuremath{\gg}\phantom{\rule{0.16em}{0ex}}{g}_{T}^{c}$ $({g}_{T}$ being the dimensionless intergrain tunneling conductance, and ${g}_{T}^{c}$ being the critical tunneling conductance at the percolation threshold), we observe a ${R}_{H}\ensuremath{\propto}lnT$ law from $\ensuremath{\sim}6$ to 300 K. This $lnT$ behavior is independent of array dimensionality. We also observe a $\ensuremath{\sigma}\ensuremath{\propto}lnT$ law in the temperature range $\ensuremath{\sim}20$--100 K. Below $\ensuremath{\sim}10$ K, the temperature behavior of $\ensuremath{\sigma}$ changes to a $\sqrt{T}$ dependence in thick films, while it changes to a different $lnT$ dependence in thin films. The overall ${R}_{H}$ and $\ensuremath{\sigma}$ characteristics can be explained by the electron-electron interaction (EEI) effects in the presence of granularity. As $x$ is reduced and approaches the percolation threshold, we found that the $lnT$ dependences of ${R}_{H}$ and $\ensuremath{\sigma}$ still hold for a wide temperature range. We propose an explanation for the long-standing puzzle of the $\ensuremath{\sigma}\ensuremath{\propto}lnT$ dependence, which has previously been frequently observed in composite systems near the quantum percolation threshold, as arising from the same EEI effect considered in the recent theory of granular metals.

Diluted magnetic semiconductor Zn1−xCoxO (x≤0.11) nanowires with average diameter of ~40 nm were prepared by thermal evaporation, followed by high-energy Co ion implantation. Bombardment by Co ions produced a good number of structural defects (stacking faults and orientational variations) in the nanowires. The as-implanted nanowires were paramagnetic. We performed two types of thermal annealing, one in 1 atm argon flow and the other in a high vacuum, at 600 °C, and studied the effects of annealing on the magnetic properties of these nanowires. Argon annealing removed structural defects in the nanowires and the nanowires then revealed ferromagnetic ordering. This result suggests that structure defects are harmful to the occurrence of ferromagnetism in the Co-implanted ZnO. The structure of the as-implanted and the annealed nanowires was inspected in detail by using scanning electron microscopy, energy dispersive x-ray spectroscopy, maps of electron energy loss spectra, x-ray diffraction, and high-resolution transmission electron microscopy. Taken together, these studies suggested that no second phase existed on the scale down to the spatial resolution of ~0.5 nm. Noticeably, the nanowires even displayed largely enhanced ferromagnetism after annealing in a high vacuum. A subsequent annealing in oxygen has also been performed on those vacuum-annealed nanowires to study the roles played by the O vacancies in determining the ferromagnetic properties of the nanowires. Our results indicate that both the improved structural quality and the increased number of O vacancies are key factors for the occurrence of ferromagnetic ordering in the Zn1−xCoxO nanowires.

We examine the transverse magnetoresistance of three-dimensional bi-component composites Aup–(PrBa2Cu3O7)1−p. This property, seldom measured on percolation systems, has been measured below 5 K and up to 1.5 T. Contrarily to well known transport properties, the magnetoresistance of these compounds exhibits a metal volume fraction's threshold pc linked to the onset of disorder rather than to percolation.

The electrical resistivity, thermal conductivity, and thermopower of α -TiAl alloy samples with Al content from 0 at.% to 10 at.% are measured from room temperature down to liquid helium temperature. It is found that with the increase in Al content the single phonon scattering contribution to the resistivity remains nearly constant, but the multi-phonon scattering contribution monotonously increases. This provides an alternative explanation of resistivity saturation to the shunt-resistor model. The Wiedemann–Franz law holds for the whole temperature range, allowing the separation of the electron and phonon contributions to the thermal conductivity. The data from samples with different doping levels enable us to separate the band and the scattering terms in the thermopower.

Sb1−x(SiO2)x granular films were prepared by the co-sputtering method with the volume fraction of SiO2, x, ranging from 0 (i.e. pure Sb) to about 30%. Systematic electronic transport studies, including resistivity, magnetoresistance, Hall effect and Seebeck effect, were carried out against the temperature, magnetic field and volume fraction x of SiO2. With the gradual increase of the SiO2 content, the mean grain size of the Sb decreases, and eventually the film becomes amorphous, as illustrated by the changes of the x-ray diffraction patterns. The temperature coefficient of resistivity also changes its sign from positive to negative, indicating a semimetal–semiconductor or insulator transition. Magnetoresistance studies using the weak localization theory revealed that the electron dephasing time follows approximately a T−2 law. This behaviour indicates that the electron–phonon (e–ph) scattering still dominates the electron dephasing processes in these granular systems with a fair number of SiO2 inclusions. The Hall coefficient decreases monotonically with temperature and with the volume fraction of SiO2. The giant Hall effect is absent in these granular films. Finally, an interesting but rather complicated behaviour of the Seebeck coefficient versus temperature was observed when the volume content of the SiO2 exceeded 18%.

We have systematically measured the electrical transport properties of several RuO2 and IrO2 single crystals over a wide temperature range from 300 K down to 0.3 K to study the conduction mechanisms in these oxides. Our measured resistivities are in close agreement with the recent band-theory calculations for these materials. The characteristic temperatures for the acoustic-mode and optical-mode phonons are determined. Our measured magnetoresistances are positive and follow the Kohler rule, indicating that the transport properties of these oxides exhibit normal behaviour as described by the Boltzmann equation. In contrast, we do not find any signature of superconductivity down to 0.3 K, though the band-theory calculations predict a superconducting transition temperature of K. Magnetization measurements suggest a very low level of paramagnetic impurities in our crystals.

In this Letter we report on the electrical resistivity and magneto-resistivity of disordered V 80 Al 20− x Fe x alloys in the temperature range 1.5⩽ T ⩽300 K and analyze them in the light of weak localization and electron–electron interaction. The low temperature zero field resistivity obeys a T 1/2 law, which is explained by electron–electron interaction. The low field magneto-resistivity is described by weak localization theory under strong spin–orbit interaction. The electron–phonon scattering rate obeys a quadratic temperature dependence. This observation is interpreted by the existing theories of electron–phonon interaction.

Structural, magnetic, and transport properties of (La 1−x Ndx )0.75Na0.25MnO3 (0 x 1) are studied. The system exhibits a rhombohedrally distorted perovskite structure for x 0.2 and it convert to an orthorhombic structure for x 0.4. Re-entrant ferromagnetic type charge ordering transitions are observed in the narrow bandwidth samples (x 0.6), while charge ordering transition is not observed for these samples with rhombohedral structure. Combining with the published results related to the charge ordering transition, we argue that the rhombohedral structure likely favor double exchange interaction and suppress charge ordering interaction. The compounds show metal to insulator transitions except Nd0.75Na0.25MnO3. The resistivity data above the metal to insulator transition temperature for the charge ordered sample are discussed in the frame work of variablerange hopping model.  2004 Elsevier B.V. All rights reserved.

Resistivity and magnetoresistance measurements have been performed on insulating icosahedral AlPdRe quasicrystal (QC) bar samples. At temperatures in the range , the resistivities follow a simple inverse temperature law: ρ(T) = ρ0/T(1.0 ± 0.1). Below 1 K, the resistivity of a weakly insulating sample exhibited a simple inverse temperature law where ρ(T) = ρ0/T0.33 and not an activated variable-range hopping (VRH) law. Strongly insulating samples exhibit saturation of their resistivities to finite values as . These saturation resistivity values are believed to arise from the presence of a second metallic phase located within the quasicrystal's structure. By extrapolating the measured resistivities at 22 mK to absolute zero, the saturation conductivity values were estimated at T = 0 K and subtracted from the conductivity data points. These 'corrected' data, corresponding only to the QC phase, were found to follow activated VRH laws, having hopping exponents y that vary in the range . The activated VRH behaviours are observed only below 1 K. The magnetoresistances (MRs) of these samples are also anomalous. The MRs can be explained by including contributions from both the saturation conductivity values and from the QC MR ratios, estimated using the wavefunction shrinkage model.

In this review, we discuss the results of recent experimental studies of the low-temperature electron dephasing time (τφ) in metal and semiconductor mesoscopic structures. A major focus of this review is on the use of weak localization, and other quantum-interference-related phenomena, to determine the value of τφ in systems of different dimensionality and with different levels of disorder. Significant attention is devoted to a discussion of three-dimensional metal films, in which dephasing is found to predominantly arise from the influence of electron–phonon (e–ph) scattering. Both the temperature and electron mean free path dependences of τφ that result from this scattering mechanism are found to be sensitive to the microscopic quality and degree of disorder in the sample. The results of these studies are compared with the predictions of recent theories for the e–ph interaction. We conclude that, in spite of progress in the theory for this scattering mechanism, our understanding of the e–ph interaction remains incomplete. We also discuss the origins of decoherence in low-diffusivity metal films, close to the metal–insulator transition, in which evidence for a crossover of the inelastic scattering, from e–ph to ‘critical’ electron–electron (e–e) scattering, is observed. Electron– electron scattering is also found to be the dominant source of dephasing in experimental studies of semiconductor quantum wires, in which the effects of both large- and small-energy-transfer scattering must be taken into account. The latter, Nyquist, mechanism is the stronger effect at a few kelvins, and may be viewed as arising from fluctuations in the electromagnetic background, generated by the thermal motion of electrons. At higher temperatures, however, a crossover to inelastic e–e scattering typically occurs; and evidence for this large-energy-transfer process has been found at temperatures as high as 30 K. Electron–electron interactions are also thought to play an important role in dephasing in ballistic quantum dots, and the results of recent experiments in this area are reviewed. A common feature of experiments, in both dirty metals 3 Authors to whom any correspondence should be addressed.

We have studied the effect of thermal annealing on electron dephasing times $\tau_\phi$ in three-dimensional polycrystalline metals. Measurements are performed on as-sputtered and annealed AuPd and Sb thick films, using weak-localization method. In all samples, we find that $\tau_\phi$ possesses an extremely weak temperature dependence as $T \to 0$. Our results show that the effect of annealing is non-universal, and it depends strongly on the amount of disorder quenched in the microstructures during deposition. The observed "saturation" behavior of $\tau_\phi$ cannot be easily explained by magnetic scattering. We suggest that the issue of saturation can be better addressed in three-dimensional, rather than lower-dimensional, structures.

Magnetoconductance (MC) measurements have been performed on a 2140 A thick tungsten carbide film at temperatures very close to the superconducting transition temperature Tc of the film. The data are dominated by superconducting fluctuations. A novel three-dimensional phenomenological model is proposed to explain the MC data, yielding good fits. The Larkin beta factor, βLarkin, appeared as a fitting parameter. An expression proposed by Larkin for βLarkin failed badly for temperatures extremely close to Tc. But at intermediate and high temperatures compared to Tc, Larkin's expression gave very good agreement in fits to the MC data. At temperatures very close to Tc, a crossover from three dimensions to two dimensions was observed in the behaviour of the MC of the film.

We have measured the low-temperature resistivities of a series of bulk crystalline disordered Ti 73− x Al 27 Sn x alloys ( x ≲5) as well as the sheet resistances of a number of thin ferromagnetic Ni films (≈120 A thick) sandwiching an ultrathin Ag or Au (≲5 A) layer. The level of impurities (concentration of Sn in the former case, and thickness of Ag or Au in the latter case) is progressively increased in order to enhance the spin–orbit scattering in a controllable manner. The influence of the spin–orbit scattering on the electron–electron interaction effects is studied from the temperature dependence of resistivities (sheet resistance) at low temperatures. We find that the electron–electron interaction contribution to the resistivities (sheet resistances) increases slightly with increasing spin–orbit scattering. Our observation is discussed in terms of the current theoretical concept for the electron–electron interactions in disordered metals.

Nearly 3 orders of magnitude enhancement in the Hall coefficient is observed in $\mathrm{Cu}{}_{x}\ensuremath{-}(\mathrm{SiO}{}_{2}{)}_{1\ensuremath{-}x}$ granular films. This large enhancement of the Hall coefficient not only is significantly larger than the prediction of the classical percolation theory, but also occurs at a metal concentration identified to be the quantum percolation threshold. Measurements of the electron dephasing length and magnetoresistance, plus the TEM characterization of microstructures, yield a physical picture consistent with the mechanism of the local quantum interference effect.

We have systematically investigated the low-temperature electron dephasing times $\tau_\phi$ in more than 40 three-dimensional polycrystalline impure metals with distinct material characteristics. In all cases, a saturation of the dephasing time is observed below about a (few) degree(s) Kelvin, depending on samples. The value of the saturated dephasing time $\tau_0$ [$\equiv \tau_\phi (T \to 0 {\rm K})$] falls basically in the range 0.005 to 0.5 ns for all samples. Particularly, we find that $\tau_0$ scales with the electron diffusion constant $D$ as $\tau_0 \sim D^{- \alpha}$, with $\alpha$ close to or slightly larger than 1, for over two decades of $D$ from about 0.1 to 10 cm$^2$/s. Our observation suggests that the saturation behavior of $\tau_\phi$ is universal and intrinsic in three-dimensional polycrystalline impure metals. A complete theoretical explanation is not yet available.

The upper critical field H-c2 of alpha-Ti1-xAlx(0 less than or equal to x less than or equal to 0.1) alloys has been investigated in detail. The behaviors can well be described by the theory of type-II superconductors. The electronic specific heat deduced from the superconducting properties is in excellent agreement with previous calorimetric measurements. However, the Fermi velocity obtained here is substantially lower than predicted by band calculation. The present work gives clear evidence that pure Ti is an intrinsic type-II superconducting element due to its very low renormalized Fermi velocity.