Zero-bias Resistance Research Articles

Bistable memory effect in chromium oxide junctions

ABSTRACTMagnetotransport properties of granular CrO2 /Cr2O3 films made of CrO2 crystals covered by 1– 2 nm native insulating Cr2O3 are presented. Electrical properties of a limited number of grains measured in series and parallel (10 to 15 grains) reveal intergrain tunneling characteristics. At lowest temperatures, a well pronounced zero bias anomaly indicates that impurities in the junctions block the electronic flow.Hysteresis in the IV curves are observed at intermediate temperatures on zero-field cooled samples. Changing the polarity of a short excitation pulse (100ns) of amplitude smaller than 1 V triggers a change in the zero-bias resistance by 10–50%. These states are stable and well reproducible in the temperature interval ranging from 100K to 250K. Applying an external magnetic field cancels the IV hysteresis. The resistance of the devices in the kΩ range, the potential high-speed for writing and reading the resistance sate, make these systems interesting candidates for magnetic non-volatile memories.

Reflectionless tunneling due to Andreev reflection in a gated superconductor–semiconductor junction

We have studied the differential resistance in a gated superconductor–semiconductor junction in the ballistic transport regime. The zero-bias resistance shows a minimum as a function of the gate voltage, while the junction normal resistance measured at a high bias voltage shows a monotonic increase. This is explained as follows: the conductance enhancement due to coherent Andreev reflection is controlled by the gate voltage which changes the transparency of the gate barrier. We have also fabricated a two-gated junction with a high transparency at the superconductor–semiconductor interface and measured its resistance as functions of two gate voltages. An increase in one gate voltage results in the resistance minimum as a function of the other gate voltage. The obtained results agree well with the so-called reflectionless tunneling theory.

Control of the electromagnetic environment for single Josephson junctions using arrays of dc SQUIDs

We have measured the current–voltage characteristics of small-capacitance single Josephson junctions at low temperatures T ≤ 0.04 K, where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc SQUIDs. When the zero-bias resistance of the SQUID arrays is much higher than the quantum resistance h/e2 ≈ 26 kΩ, a transition to a clear Coulomb blockade of Cooper-pair tunnelling is induced in the single junction. The measured blockade voltage agrees with the theoretical prediction.

Coulomb blockade and coherent single-cooper-pair tunneling in single Josephson junctions.

We have measured the current-voltage characteristics of small-capacitance single Josephson junctions at low temperatures ( T< or =0.04 K), where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc SQUIDs. We have clearly observed Coulomb blockade of Cooper-pair tunneling and even a region of negative differential resistance, when the zero-bias resistance of the SQUID arrays is much higher than the quantum resistance h/e(2) approximately 26 kOmega. The negative differential resistance is evidence of coherent single-Cooper-pair tunneling in the single Josephson junction.

Infrared detector performance in an area array

The performance of an IR detector in an area array is studied by numerically solving the 2-D diffusion equation for thermal and photo- generated carriers. The zero-bias resistance area product R0A, quan- tum efficiencyh, and the noise equivalent temperature difference NETD for diodes of different size and junction depth are calculated for long wavelength infrared (LWIR) HgCdTe n 1 -on-p diffusion-limited diodes in the backside illuminated configuration. The 2-D calculations incorporate thermally generated- and photocarriers that originate under the junction (the ''normal'' current), as well as those that originate from around the junction (the lateral current). The calculation of the diffusion currents— both optical and thermally generated carriers—is made using a trapezoi- dal grid, which better fits the symmetry of diodes in an area array. The present results are compared with previous calculations in which a uni- form grid was used. The results of R0A and h calculated by the uniform and trapezoidal grids differ significantly, especially for diodes with deep junctions or diodes that are small compared to the center-to-center dis- tance between diodes. Both the uniform and trapezoidal grid calculations have been compared with spot scan measurements and Semicad simu- lation in a stripe diode array in the literature. © 2001 Society of Photo-Optical

Interfacial density of states in magnetic tunnel junctions.

Large zero-bias resistance anomalies as well as a collapse of magnetoresistance were observed in Co/Al2O3/Co magnetic tunnel junctions with thin Cr interfacial layers. The tunnel magnetoresistance decays exponentially with nominal Cr interlayer thickness with a length scale of approximately 1 A more than twice as fast as for Cu interlayers. The strong suppression of magnetoresistance, as well as the zero-bias anomalies, can be understood by considering a strong spin-dependent modification of the density of states at Co/Cr interfaces. The role of the interfacial density of states is shown by the use of specially engineered structures. Similar effects are predicted and observed in junctions with Ru interfacial layers.

Resistance and tunneling spectra of aligned multiwalled carbon nanotube arrays

The resistance and tunneling spectra of samples formed by depositing silver electrodes at the two ends of aligned, template-grown, carbon nanotube arrays were measured in the temperature range 0.67–440 K. Two types of samples were fabricated, one with small oxide tunnel junctions separating the carbon nanotubes from the metal electrodes, the other with a significant Al2O3 tunnel barrier. The measurements indicate the presence of three regimes for dI/dV(V). For T&amp;gt;220 K, dI/dV(V) and the zero-bias conductivity show a broad minimum and an activation temperature dependence suggesting semiconductor behavior. In the temperature range 10&amp;lt;T&amp;lt;140 K, the zero-bias conductivity shows a square-root temperature dependence. For T&amp;lt;2 K, a very steep rise in the zero-bias tunneling resistance is observed with a strong simultaneous suppression of the tunneling conductivity near the Fermi energy. Coulomb blockade is suggested as a plausible explanation of the observed behavior.

Zero-bias resistance anomalies in normal metal/charge-density wave interfaces

A number of reduced dimensional conductors, such as K 0.3MoO 3, NbSe 3, and TaS 3, exhibit collective charge density wave (CDW) transport. We report on the observation of zero-bias anomalies in the differential resistance of NbSe 3 crystals near an interface between strongly pinned and weakly pinned regions of a CDW, where current is being converted from normal metallic to CDW current. We observe a dramatic drop in zero-bias resistance, suggesting a new phase transition, when the temperature is cooled below 44 K.

Reliability of normal-state current–voltage characteristics as an indicator of tunnel-junction barrier quality

We demonstrate that one of the most commonly used criteria to ascertain that tunneling is the dominant conduction mechanism in magnetic tunnel junctions—fits of current–voltage (I–V) data—is far from reliable. Using a superconducting electrode and measuring the differential conductance below Tc, we divide samples into junctions with an integral barrier and junctions having metallic shorts through the barrier. Despite the clear difference in barrier quality, equally reasonable fits to the I–V data are obtained above Tc. Our results further suggest that the temperature dependence of the zero-bias resistance is a more solid criterion, which could therefore be used to rule out possible pinholes in the barrier.

Excess conductance in normal-metal/Anderson insulator/superconductor junctions

The current-voltage characteristics of ${\mathrm{A}\mathrm{u}/\mathrm{I}\mathrm{n}\mathrm{O}}_{x}/\mathrm{Pb}$ tunnel junctions exhibit peculiar zero-bias anomalies. At low temperatures the zero-bias resistance attains saturation values that are smaller than the normal-state resistance by a factor that often exceeds 2. This zero-bias anomaly (ZBA) is also characterized by voltage $\ensuremath{\delta}&lt;~\ensuremath{\Delta},$ where \ensuremath{\Delta} is the energy gap of lead. \ensuremath{\delta} depends on the thickness of the ${\mathrm{InO}}_{x}$ layer. In the presence of a microwave field the ZBA features are affected in a nontrivial way. The possibility that these features arise from the nature of the barrier being an Anderson insulator is discussed.

Proximity effects and Andreev reflection in a mesoscopic SNS junction with perfect NS interfaces

Low temperature transport measurements on superconducting film - normal metal wire - superconducting film (SNS) junctions fabricated on the basis of 6 nm thick superconducting polycrystalline PtSi films are reported. The structures with the normal metal wires of two different lengths L=1.5 $\mu$m and L=6$\mu$m and the same widths W=0.3$\mu$m are studied. Zero bias resistance dip related to pair current proximity effect is observed for all junctions whereas the subharmonic energy gap structure originating from phase coherent multiple Andreev reflections have occurs only in the SNS junctions with short wires.

Splitting of the subgap resistance peak in superconductor/two-dimensional electron gas contacts at high magnetic fields

The differential resistance of NbN/two-dimensional electron gas (2DEG) contact is measured at high magnetic fields. In zero magnetic field the contact shows a pronounced resistance peak at zero bias due to a high barrier at the NbN/2DEG interface, which decreases if a magnetic field is applied in the plane of the 2DEG. For a magnetic field oriented perpendicular to the plane of the 2DEG, not only the zero-bias resistance decreases but so does the normal-state resistance which drops to vanishingly small values. A pronounced substructure due to a splitting of the resistance peak into a three-peak structure is observed at high perpendicular fields. We suggest that the appearance of this substructure can be explained by multiple Andreev reflections due to skipping orbits of electrons and holes accompanied by inelastic scattering in the 2DEG near the interface.

Laser doping in Si, InP and GaAs

Experimental results on laser solid-phase doping of Si, GaAs and InP are presented. Using 1 kW CO 2 laser, we fabricated ultra-shallow p–n junctions and Ohmic contacts with these semiconductors. The impurity distribution into the depth of the doped layers has been studied through the Auger electron spectroscopy (AES). The zero-bias resistance of formed GaAs p–n junctions was ∼10 10 Ω and a typical resistance of nonrectifying contacts with GaAs and InP was 5·10 −7 Ω·cm 2 and 5·10 −5 Ω·cm 2, respectively.

Phase slip scaling relationship for the 59 K and 143 K charge-density-waves in NbSe3

Selective area irradiation was used to create irradiated/unmodified/irradiated CDW heterostructures with well-defined interfaces on a single Nbse 3 crystal. The temperature dependence of the extra voltage required for carrier conversion (phase slip voltage, V Ps 0 is extracted from length dependent studies. We find that the temperature dependence of V Ps 0 for the 143 K and 59 K CDW transitions are identical if properly scaled by the transition temperature. The V ps 0 temperature dependence is not thermally activated, but contains an upper and lower temperature branch. The crossover temperature for the two branches is 0.75Tp. For the 59 K CDW we observe a zero-bias resistance anomaly near a single irradiated/unmodified interface. This anomaly abruptly changes with temperature near 44 K suggesting a qualitative change in the phase slip mechanism near 0.75Tp.

Magneto-Coulomb oscillations in ferromagnetic single electron transistors

Oscillatory behavior of the zero-bias resistance as a function of the external magnetic fields as well as of the gate voltage is found in a Ni/Co/Ni single electron transistor (SET). We show that the variations of the chemical potentials for the conduction electrons in the Co island and in the Ni leads in magnetic fields cause changes in the free energy of the island electrode and induce the oscillations of the resistance. Experimental results of other structures such as Co/Ni/Co-, Al/Co/Al-and Al/Al/Al- SETs are explained consistently by this mechanism.

Proximity effect, Andreev reflections, and charge transport in mesoscopic superconducting/semiconducting heterostructures

In the quasi-two-dimensional (Q2D) electron gas of an InAs channel between an AlSb substrate and superconducting niobium layers, the proximity effect induces a pair potential so that a Q2D mesoscopic superconducting/normal/superconducting (SNS) junction forms in the channel. The pair potential is calculated with quasiclassical Green’s functions in the clean limit. For such a junction, alternating Josephson currents and current–voltage characteristics (CVCs) are computed, using the nonequilibrium quasiparticle wavefunctions which solve the time-dependent Bogoliubov–de Gennes equations. The CVCs exhibit features found experimentally by the Kroemer group: a steep rise of the current at small voltages (‘foot’) changes at a ‘corner current’ to a much slower increase of current with higher voltages, and the zero-bias differential resistance increases with temperature. Phase-coherent multiple Andreev reflections and the associated Cooper pair transfers are the physical mechanisms responsible for the oscillating Josephson currents and the CVCs. Additional experimental findings not reproduced by the theory require model improvements, especially a consideration of the external current leads which should give rise to hybrid quasiparticle/collective-mode excitations.

Tunneling Spectroscopy in AlNiCo Decagonal Quasicrystals

Tunneling spectroscopy of ${\mathrm{Al}}_{73}{\mathrm{Ni}}_{17}{\mathrm{Co}}_{10}$ decagonal single quasicrystals has been measured at ultralow temperatures. In addition to the wide pseudogap observed in previous photoemission experiments, very rich fine structures in the density of states around the Fermi energy were discovered. The fine structures strongly depend on the applied magnetic field and disappear above $\ensuremath{\sim}4\mathrm{T}$. The zero-bias resistance of the junction varies with magnetic fields in a complicated way, which is suggested to be related to the hierarchy structure of quasicrystals. A puzzling hysteretic phenomenon was observed in some bias ranges in zero magnetic field, the origin of which needs to be clarified in the future.

Observation of strong Coulomb blockade in resistively isolated tunnel junctions

We report measurements of the Coulomb-blockade current in resistively isolated ( R Isol ≫ h/ e 2) tunnel junctions for the temperature range 60 mK <T<230 mK where the charging energy E c is much greater than the thermal energy. A zero-bias resistance R 0 of up to 10 4 R T (the tunnel resistance of the bare junction) is obtained. For eV≪ E c , the I- V curves for a given R Isol scale as a function of V/ T, with I∝ V α( R Isol ) over a range of V. The data agree well with numerical calculations of the tunneling rate that include environmental effects.

Length-Scale Dependence of the Superconductor-to-Insulator Quantum Phase Transition in One Dimension

One-dimensional (1D) arrays of small-capacitance Josephson junctions demonstrate a sharp transition, from Josephson-like behavior to the Coulomb blockade of Cooper-pair tunneling, as the effective Josephson coupling between nearest neighbors is tuned with an externally applied magnetic field. Comparing the zero-bias resistance of three arrays with 255, 127, and 63 junctions, we observe a critical behavior where the resistance, extrapolated to $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$, is independent of length at a critical magnetic field. Comparison is made with a theory of this $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ quantum phase transition, which maps to the 2D classical $\mathrm{XY}$ model.

Electrical properties of HgCdTe epilayers doped with silver using an AgNO3 solution

We employed AgNO3 solutions for doping Ag in liquid phase epitaxy (LPE) grown Hg0.78Cd0.22Te epilayers and found that the minority carrier lifetimes became longer so that the diode properties improved. After annealing LPE grown Hg(1-x)Cd(x)Te layers (x=0.22) in Hg atmosphere, the epilayers were immersed in an AgNO3 solution at room temperature. The typical carrier concentrations of holes was 3 × 1016 cm−3 at 77K. These values were almost the same as for the nondoped wafers. Also, its acceptor level was 3 to 4 meV. This shows that the Ag was activated. The doped crystals have lifetimes several times longer than those of the nondoped crystals. Numerical fitting showed the lifetime was limited mostly by the Auger 7 process. The Shockley-Read-Hall recombination process was not effective. To examine the Ag-doped wafer, we fabricated photodiodes using standard planar technology. The diodes have an average zero-bias resistance of several MΩ and a shunt resistance of about 1 GΩ for a 10 µm cutoff wavelength at 78K. These values are about four times higher than those of nondoped diodes. The photo current is also two times higher at the same pixel size. This shows that the quantum efficiency is increased. The extension of the lifetime contributes to the high resistance and the high quantum efficiency of the photodiode.