Magnetic field effects on total and partial conductance histograms in Cu and Ni nanowires

Conductance histograms have become a powerful tool for studying transport properties of metallic nanowires. However, the individual conductance curves display a very rich structure that might be concealed by the statistical procedure of finding preferred conductance values by building conductance occurrence histograms using consecutive nanocontact breakage experiments. This is particularly true when it comes to discerning 1/2G₀=e²/hquantization in magnetic nanowires. The effect of disorder, added to possible magnetic sources of scattering, and different magnetic states of different nanowires, might hide its appearance as histogram peaks. This work analyzes and compares Ni and Cu nanowire experimental histograms at room temperature (RT). Those obtained with no curve selection criteria are basically unaffected by the presence of a magnetic field. A selection of particular sets of conductance curves shows that conductance quantization could occur in steps of e²/h and 2e²/h in Ni as well as in Cu in the presence or absence of a magnetic field. Sorting out curves in sets that present conductance plateaus at half integer and integer values, and compiling statistics on the number of such curves that appear, depending on the applied magnetic field, results in differences between the behaviour of Cu and Ni. While for Cu, the magnetic field keeps the ratio of curves that present plateaus at 1/2G₀with respect those presenting G₀ plateaus unchanged; for Ni, the number of curves which exhibit plateaus at just G₀ almost disappears with the applied field. This experimental fact might indicate that the magnetic field removes spin degeneracy in these magnetic nanowires. PACS 72.25.Ba; 73.40.Jn; 73.63.Rt; 75.75.+a     

Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ≈ 3G₀ to G ≈ 1G₀ (G₀ = 2e/h²). Formation of a true one-atom point contact, with G ≈ 1G₀ and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 ≤ E-E_F ≤ 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear “gold molecule" between the electrodes, and a weak coupling of this “molecule" to the tips.     

Theoretical study of conductance in stretched monatomic nanowires

Recent experiments showed that the last, single channel conductance step in monatomic gold contacts exhibits significant fluctuations as a function of stretching. From simulations of a stretched gold nanowire linked to deformable tips, we determine the distribution of the bond lengths between atoms forming the nanocontact and analyze its influence on the electronic conductance within a simplified single channel approach. We show that the inhomogeneous distribution of bond lengths can explain the occurrence and the 5% magnitude of conductance fluctuations below the quantum conductance unit g₀=2e²/h.     

Ballistic transport in PbTe-based nanostructures

We describe our study of ballistic transport in nanostructures of lead telluride, PbTe. Submicron devices have been fabricated by electron beam lithography and chemical etching of 50 nm wide PbTe single quantum wells embedded between Pb_0.92Eu_0.08Te barriers grown by MBE on BaF₂. The electron concentration in the devices was tuned by the gate voltage applied across an interfacial p–n junction. The most important observation was zero-magnetic field conductance quantization (in multiplies of 2e²/h) in narrow constrictions of dimensions comparable to electron mean free path calculated from transport mobility. This indicates considerable relaxation of requirements for quantum ballistic transport in comparison with other materials. We argue that the huge static dielectric constant of PbTe (₀=1350 at 4.2 K) leads to suppression of the long-range Coulomb potentials of charged impurities and, thus, provides favorable conditions for the conductance quantization.     

Transport through a double barrier for interacting quasi one-dimensional electrons in a quantum wire in the presence of a transverse magnetic field

We discuss the Luttinger liquid behaviour of a semiconducting quantum wire. We show that the measured value of the bulk critical exponent, α_bulk, for the tunneling density of states can be easily calculated. Then, the problem of the transport through a quantum dot formed by two quantum point contacts along the quantum wire, weakly coupled to spinless Tomonaga-Luttinger liquids is studied, including the action of a strong transverse magnetic field B. The known magnetic dependent peaks of the conductance, G(B), in the ballistic regime at a very low temperature, T, have to be reflected also in the transport at higher T and in different regimes. The temperature dependence of the maximum G_max of the conductance peak, according to the Correlated Sequential Tunneling theory, yields the power law G_max∝T^{2α end-1}, with the critical exponent, α_end, strongly reduced by B. This behaviour suggests the use of a similar device as a magnetic field modulated transistor.     

Nonmagnetic impurities induced magnetism in SnO2

A density functional study is performed to investigate the magnetism induced by the nonmagnetic impurity substitution for the cation in SnO₂. The calculated results show that the K impurity substitution leads to a robust magnetism in SnO₂, and the induced magnetic moments are mainly attributed to the first shell of oxygen atoms surrounding the impurity atom. Meanwhile, no magnetism is observed in SnO₂ doped with Ca which implies a decreasing tendency of induced magnetic moments for Sn substituted by vacancy, K, and Ca. It is also demonstrated that the magnetic coupling constant oscillates as a function of K-K separation distance, and the Curie temperature above room temperature can be obtained in K-doped SnO₂.     

Magnetic anisotropy and anisotropic ballistic conductance of thin magnetic wires

The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μ_B, while the orbital moment as high as 0.5 μ_B. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment.     

The effect of water vapor on the electrical properties and sensitivity of thin-film gas sensors based on tin dioxide

The results of theoretical and experimental studies into the effect of water vapor on the electrical conductance of a gas sensor and the sensor response to hydrogen action are discussed. A relation describing the dependence of electrical conductance G₀ on absolute humidity in the pure air is derived using a hypothesis of the presence of space-charge regions depleted of electrons between the SnO ₂ grains in a polycrystalline tin dioxide film. Due to dissociative chemisorption of water molecules, the energy-band bending at the SnO ₂ grain interfaces decreases and the oxygen-vacancy concentration in the grains increases, resuling in an increase in G₀. An equation for the sensor response to hydrogen action is derived (the G₁/G₀, ratio, where G₁ is the sensor conductance in a gas mixture containing molecular hydrogen). The expression describes the dependence of G₁/G₀ on the hydrogen concentration in the interval 50–6·10³ ppm, band bending at the SnO ₂ grain interface, and sensor temperature. The dependences of the sensor conductance, highest possible conductance, and energy-band bending on temperature and absolute humidity resulting from processing of the experimental data are in good agreement with the theoretical predictions. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 50–56, August, 2008.     

Spin-flip conductance and magnetoresistance of magnetic nanocontacts

The quantized conductance of nanocontacts with atomic sizes is calculated with allowance made for the conduction-electron spin flip in terms of the quantum scattering theory. The exact solution of the Schrödinger equation describing the electron motion in a piecewise-smooth potential is used as the zeroth-order approximation of the perturbation theory. The probabilities of electron transmission (reflection) through a magnetic domain wall, as well as the spin-conserving and spin-flip conductances of the nanocontact, are calculated. It is demonstrated that the spin-flip conductance imposes the natural limitation on the formally infinite increase in the ballistic magnetoresistance of the nanocontact when its cross-sectional area tends to zero.     

Break conductance of Al nanocontacts

We measured a break conductance, the last conductance of a contact before its complete break, for Al nanocontacts of 0–200G₀ (G₀≡2e²/h is the quantum unit of conductance) in ultrahigh vacuum at room temperature. We found that the distribution of the break conductance shows a broad single peak, the position of which shifts with the contact current. From the observed current dependence of the break conductance peak, it is suggested that Al nanocontacts break up most likely when the contact current density reaches a critical value 5×10¹⁰ A/cm².     

Evidence for charging effects in an open dot at zero magnetic field

We have measured the low-temperature transport properties of an open quantum dot formed in a clean one-dimensional channel. At zero magnetic field, continuous and periodic oscillations superimposed upon ballistic conductance steps are observed when the conductance through the dot G exceeds 2e²/h. We ascribe the observed conductance oscillations to evidence for charging effects in an open dot. This is supported by the evolution of the oscillating features for G>2e²/h as a function of both temperature and barrier transparency.     

Mechanical fabrication of metal nano-contacts showing conductance quantization under electrochemical potential control

We have mechanically fabricated Ni and Cu nano-constrictions in solution to study their quantized conductance behavior under electrochemical potential control. Conductance quantization was observed at both metals in solution at room temperature for the first time. The conductance of Cu nano-constriction was quantized in units of G₀(=2e²/h). A sharp 1G₀ peak was observed in the conductance histogram. For Ni, a rather broad peak at 1–1.5G₀ was observed in the histogram. The conductance quantization behavior was discussed by comparing previously documented results of nano-constrictions fabricated in air or ultra-high vacuum conditions, with those fabricated in solution.     

Electrochemical fabrication and characterization of nanocontacts and nm-sized gaps

Copper nanocontacts and molecular-sized nanogaps were prepared and characterized at electrified solid/liquid interfaces employing lithographic and electrochemical techniques. A dedicated four-electrode potentiostat was developed for controlling the electrochemical fabrication process and for monitoring the electrical characteristics of the nanostructures created. The formation and breaking of Cu nanocontacts exhibits conductance quantization characteristics. The statistical analysis of conductance histograms revealed a preferential stability of nanocontacts with integer values of G₀, with a clear preference for 1 G₀, 2 G₀ and 3 G₀. The growth of molecular-sized gaps shows quantized tunneling current, which is attributed to the discrete nature of Cu atoms, water molecules, and specifically adsorbed ions. PACS 73.23Ad; 73.63.Rt; 82.45.Yz; 85.35.-p     

Quantum corrections to the conductance of n-GaAs films in a strong magnetic field

The conductance of doped n-GaAs films is studied experimentally as a function of magnetic field and temperature in strong magnetic fields right up to the quantum limit (ħω_c = E _F). The Hall conductance G _xy is virtually independent of temperature T until the transverse conductance G _xx is quite large compared with e ²/h. In strong fields, when G _xx becomes comparable to e ²/h, G _xy starts to depend on T. The difference between the conductances G _xx at the two temperatures 4.2 and 0.35 K depends only weakly on the magnetic field H over a wide range of magnetic fields, while the conductances G _xx themselves vary strongly. The results can be explained by quantum corrections to the conductance as a result of the electron-electron interaction in the diffusion channel. The possibility of quantization of the Hall conductance as a result of the electron-electron interaction is discussed. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 201–206 (10 February 1998)     

Fractional quantum conductance values in Au nanoelectrodes due to hydrogen adsorption

Hydrogen adsorption in gold nanocontact electrodes in electrochemical solution is experimentally discerned. This is performed with gold nanocontact conductance histograms in an electrochemical environment in which both the electrochemical potential and the electrolyte type are varied. Different salts, acids, and hydrogen peroxide electrolytes are studied. Salts and acids exhibit at negative electrochemical potentials different fractional quantum conductance histograms peaks associated to extra stable structures due to H adsorption while these peaks do not appear for H₂O₂ where electron transfer between solution and electrodes occurs without hydrogen formation or hydrogen adsorption on the gold electrode.     

Single-molecule conductance measurement of self-assembled organic monolayers using scanning tunneling spectroscopy in combination with statistics analysis

Based on ambient atmosphere scanning tunneling microscope (STM) technique, scanning tunneling spectroscopy (STS) combined with statistics analysis was developed to investigate the single-molecule conductance of various kinds of molecules which were self-assembled on the Au (1 1 1). Conductance histograms obtained from current-voltage curves revealed well-defined peaks at integer multiples of a fundamental conductance and were used to identify the conductance of a single molecule. The conductances of saturated molecules like 1,8-octanedithol and hexanethiocyanate were found to be 0.072 × 10⁻⁴G₀ and 0.06 × 10⁻⁴G₀ respectively and 0.23 × 10⁻⁴G₀ and 0.13 × 10⁻⁴G₀ for unsaturated molecules like 5,5′-dithiol- 2,2′,5′,2″-terthiophene and 4,4′-dithio-tert(phenylene ethylene).     

Rashba spin–orbit coupling effect on a diluted magnetic semiconductor cylinder surface and ballistic transport

We have studied the Rashba spin–orbital effect on a diluted magnetic semiconductor (DMS) cylinder surface in the presence of a magnetic field parallel to the cylinder axis, taking into account the Zeeman coupling and the s–d exchange interaction between the carriers and the magnetic ions. We have obtained an analytical expression for the electron energy spectrum, which depends on the magnetic ion concentration, temperature and strength of magnetic field. The results are used to obtain the conductance of the cylinder at finite temperature. It is shown that the presence of additional local extremum points in the subbands of the electronic spectrum leads to a nonmonotonic dependence of the ballistic conductance of the system on the chemical potential and magnetic field. In the presence of anomalous Zeeman terms with taking into account the Rashba splitting, each minimum of subband contributes _G0/2$G_0/2$ to conductance and each local maxima in the subband, actually reduce the conductance by _G0/2$G_0/2$ compared with the value _G0$G_0$, without the anomalous Zeeman splitting. The effect of finite temperature on the DMS cylinder conductance is a smearing out the sharp steps in the zero-temperature conductance, and shifting the peaks due to the temperature dependence of the s–d exchange interaction term.     

Characterization of quantum conducting channels in metal/molecule/metal devices using pressure-modulated conductance microscopy

Nanoscale switches will play a crucial role in the design of future nanoelectronic circuits. An interesting candidate involves metal/molecule/metal structures that operate via modulation of nanoscale conducting channels. When the conductance falls in the ballistic regime between 1∼2G _Q (where G _Q =2e ²/h or ≈80 μS), resonant electron transport was observed in such devices at room temperature. By performing pressure-modulated conductance microscopy, we have characterized the quantum conducting channels in terms of the wave vector of the electrons. We also observed two-level fluctuations in conductance, with each level showing opposite pressure responses, confirming the existence of resonant electron transport. These observations could lead to a new type of high speed quantum switching device based on electron wave interference.     

Electric conductance of metal nanowires at mechanically controllable break junctions under electrochemical potential control

We have developed the mechanically controllable break junction setup with an electrochemical cell (EC-MCBJ) to measure the electric conductance of metal nanowires under electrochemical potential control. The electric conductance of Au nanowires was investigated in 0.1 M Na₂SO₄ solution using EC-MCBJ. The conductance of the Au nanowires was quantized in units of G₀ (=2e²/h), showing clear features in the conductance histogram. The atomic contact with a specific conductance value was kept for >5 s, indicating the relatively high stability of the present EC-MCBJ system.     

A first principle study of encapsulated and functionalized silicon nanotube of chirality (6,6) with monoatomically thin metal wires of Ag,Au and Cu

First principle calculations have been performed to study the influence of interaction of monoatomically thin metal nanowires of Ag, Au and Cu placed inside (encapsulation) and outside (functionalization) the silicon nanotube having armchair conformation with chirality (6,6). The cohesive energy for all the encapsulated and functionalized systems under study was found to be almost same. In comparison to the pristine silicon nanotube (SiNT) which is found to be semiconducting in nature, all the encapsulated and functionalized systems of SiNT are found to be metallic in nature. The calculated electronic band structures show that the conductance in case of Ag, Au and Cu nanowires encapsulation is 2G₀. However, its value for functionalized Ag, Au and Cu nanowires is found to be 1G₀, 2G₀ and 4G₀ for the outside positioning of nanowires respectively. Optical properties of all the encapsulated and functionalized SiNTs have been studied. All the systems under study show reflectivity in the infrared (IR) region and behave as non-absorbing transparent conductors in the visible region.