Normal Metal Insulator Research Articles

Tunneling conductance in graphene-based normal metal–insulator–superconductor junctions

Based on the transfer-matrix method, we have investigated the coherent quantum transport in a graphene-based normal metal–insulator–superconductor ( NG / IG / SG ) junctions, in the limit of a thin barrier. It is found that the conductance spectra of such system, in contrast to a π periodic oscillatory behavior is shown for aligned Fermi surfaces of the normal and superconducting regions, exhibit a new π / 2 periodic oscillatory behavior as a function of barrier strength ( χ) for a large Fermi surface mismatch.

Cold-electron bolometer arrays

Superconductor bolometers such as transition edge sensors, hotelectron bolometers, bolometers on the kinetic inductance of a superconductor, and cold� electron bolometers are the most sensitive receivers for the submillimeter band. The radiation power in such bolometers is absorbed in an absorber made of a thin normalmetal film connected to superconductor- insulator-normal metal (SIN) tunnel junctions. These SIN junctions serve for electron cooling (simi� larly to the Peltier effect in semiconductors) and the output signal proportional to the absorbed power is measured on them. The maximum electron cooling and maximum current response are observed in a bias region somewhat lower than the energy gap of the superconductor in the region of the maximum nonlin� earity of the current-voltage characteristic. Electron cooling makes it possible not only to improve the sen� sitivity, but also to expand the dynamic range due to an increase in the saturation power, because the absorbed power is removed from the absorber by a cooling cur� rent. However, for applications in atmospheric radio astronomy, the power of cosmic microwave back� ground radiation is usually higher than the saturation power of a single bolometer of a superconductor- insulator-normal metal-insulator-superconductor (SINIS) structure, where the normal metal is an absorber film at whose edges SIN junctions are formed. When arrays of such structures are used, the signal power is distributed between individual bolom� eters; this makes it possible to expand the dynamic range in proportion to the number of bolometers in an array. To improve electron cooling, expand the dynamic range, and match the input and output impedances, series/parallel arrays of coldele ctron bolometers have been developed, manufactured, and measured at temperatures from 50 to 350 mK. The bolometers are integrated into crossslot antennas to analyze the polarization of cosmic microwave background radiation at a frequency of 345 GHz. The maximum tem� perature response is 6.5 µV/mK. The noiseequivalent electric powe r at a temperature of 300 mK is 1.2 × 10 -17 W/Hz 1/2 .

Quantized current of a hybrid single-electron transistor with superconducting leads and a normal-metal island

We discuss the operation of the superconductor–insulator–normal-metal–insulator–superconductor (SINIS) turnstile. This voltage-biased hybrid single-electron transistor (SET) provides current quantization even with only one radio-frequency (rf) control parameter, namely the gate voltage of the single island. We give an overview of the main error mechanisms of the turnstile and consider its feasibility as a quantum current standard. We also present experimental results of pumping with the SINIS structure which show decreased leakage current compared to earlier measurements with the opposite NISIN structure.

Quantum digital ac waveform synthesizer based on pulse-width modulation method

The paper describes the theory and the experimental results for the synthesis of precision waveforms by using the pulse-width modulation of the power of the microwave, which drives the superconductor–insulator–normal metal–insulator–superconductor Josephson array. The synthesized sine waveform has a high precision. Its harmonic content was determined to be less than 0.003%. This degree of harmonic content is in agreement with the theory.

Differential conductance through a NINS junction on graphene

The differential conductance through a normal metal–insulator–normal metal–superconductor(NINS) junction on graphene is obtained via numerical calculation of the Dirac–Bogoliubov–deGennes equation. A series of sub-gap peaks of differential conductance can befound no matter whether the Andreev reflection is specular or standard. In themeantime, the differential conductance displays an oscillatory behaviour as a functionof the effective barrier strength even if the Fermi surface mismatch is large. Inthe two limiting situations with the superconducting gap much smaller or muchlarger than the Fermi energy, all of the oscillations are in the same phase, and thenumber and positions of the sub-gap peaks are determined only by the distancebetween the NI and NS interfaces. In the intermediate regime with the Andreevretro-reflection crossing over to a specular one, however, a phase shift appears betweenany two different oscillations. The number and positions of the sub-gap peaksare related not only to that distance but also to the effective barrier strength.

Toward the optimization of electronic refrigerators

Normal metal–insulator–superconductor (NIS) junction can act as refrigerator of electron gas in a normal metal electrode under certain conditions. If majority of high energy electrons in a normal metal is moved with a bias voltage to energy levels above the superconductor's energy gap, their extraction from a normal metal is enabled by tunnelling. Their substitution with lower energy electrons results in lowering of the normal metal electron gas energy. Due to excess quasiparticle density in the superconductive electrode in vicinity of the junction, which is a result of tunnelling of quasiparticles from the normal metal and low capability of removal of these particles from the junction area the cooling capability of junctions deteriorates at lower environment temperatures. This is mainly the results of back tunnelling of excess quasiparticles to a normal metal and of dissipated energy, released by Coooper pair formation in proximity of the junction. Quantitative description of quasiparticle behavior in the superconductive electrode of the NIS junction is given. Energy dependent diffusion of quasiparticles in superconductor, their consumption due to Cooper pair creation and inflow of additional quasiparticles from a normal metal are considered. The model results in position dependence of quasiparticles' density in the junction proximity.

Conductance enhancement due to resonant tunneling into the subgap vortex core states in normal metal/superconductor ballistic junctions

We investigate the low-energy quantum transport in the ballistic normal metal-insulator -superconductor junction in the presence of magnetic field creating Abrikosov vortices in the superconductor. Within the Bogolubov- de Gennes theory we show that the presence of the subgap quasiparticle states localized within the vortex cores near the junction interface leads to the strong resonant enhancement of the Andreev reflection probability, and the normal-to supercurrent conversion. The corresponding increase of the charge conductance of the junction is determined by the distance from the vortex chain to the junction interface, which can be controlled by the applied magnetic field. The effect that we study provides a tool for probing the vortex core states by the measurements of charge transport across the applied magnetic field.

An array of 100 Al–Al2O3–Cu SIN tunnel junctions in direct-write trilayer technology

We present superconductor–insulator–normal metal (SIN) tunnel junction thermometers made of arrays of4–100 Al–Al2O3–Cu SIN tunnel junctions fabricated in direct-write technology. The technology is based onin situ evaporation of the superconductive electrode followed by the oxidation and thenormal counter-electrode as a first step and deposition of normal metal absorber as asecond one.This approach allows one to realize any geometry of the tunnel junctions and of theabsorber with no limitation related to the size of the junctions or the absorber, which is notpossible using the shadow evaporation technique.Measurements performed at 300 mK showed the high quality ofthe fabricated tunnel junctions, low leakage currents, and that anRd/Rn ratio of 500 has been achieved at that temperature.The junctions were characterized as temperature sensors, andvoltage versus temperature dependence measurements showed adV/dT of0.5 mV K−1 for each single junction, which is typical for this kind of tunnel junction. A temperature resolutionof ± 5 µK has been achieved which is much better than the previously reported value of ± 30 µK for this type of thermometer.

Ti–TiO2–Al normal metal–insulator–superconductor tunnel junctions fabricated in direct-writetechnology

We present a novel Ti-based direct-write technology for fabricatingTi–TiO2–Al tunnel junctions for bolometer and thermometry applications. The goal of our research is todevelop simple and efficient technology for fabricating SIS tunnel junctions between Ti and Al withTiO2 as an insulating barrier. The key point of this technology is the deposition of a Ti filmas a base electrode and deposition of an Al electrode after oxidation of the Ti.This approach allows one to realize any geometry of the tunnel junctions and of theabsorber with no limitation related to the area of the junctions or the thickness of theabsorber. In particular, a very thin and completely flat absorber can be created with nobending parts, which is not possible using the shadow evaporation technique or standardtrilayer technology. Besides, the proposed new approach does not require one-cycleevaporation for deposition of tunnel junctions which gives us more freedom in the geometryof the counter-electrodes.The junctions are to be used for bolometer applications, such as the fabrication ofmicrowave receivers for sensitive measurements in new generation telescopes,e.g. CLOVER and BOOMERANG projects including polarization cosmic microwavebackground radiation measurements, and the OLIMPO balloon telescope project whichis dedicated to measuring the Sunyaev–Zeldovich effect in clusters of galaxies.As the first step, SIN tunnel junctions have been fabricated and characterized.

Influence of Temperature Gradients on Tunnel Junction Thermometry below 1 K: Cooling and Electron–Phonon Coupling

We have studied thermal gradients in thin Cu and AlMn wires, both experimentally and theoretically. In the experiments, the wires were Joule heated non-uniformly at sub-Kelvin temperatures, and the resulting temperature gradients were measured using normal metal–insulator–superconducting tunnel junctions. The data clearly shows that even in reasonably well-conducting thin wires with a short (~10 μm) non-heated portion, significant temperature differences can form. In most cases, the measurements agree well with a model which includes electron–phonon interaction and electronic thermal conductivity by the Wiedemann–Franz law.

Properties of ${\hbox{MgB}}_{2}$ Thin Films and SIS Junctions Fabricated Under Various Fabrication Conditions

We investigated the optimal conditions to fabricate MgB2 superconductor-insulator-superconductor junctions. By investigating thin film qualities obtained under various deposition conditions, we found that the critical temperature depended strongly on the Mg sputtering power. Although a higher Mg sputtering power increased the MgB2 thin film's critical temperature, the dependence of the zero resistance transition width suggested that the higher Mg deposition power produced Mg-rich thin Alms. To evaluate the gap voltage of MgB2 thin films on c-plane sapphire substrates, we fabricated superconductor- insulator-normal metal junctions and determined that the gap voltage of MgB2 was 2.0 mV. Using these MgB2 thin Alms, we produced an MgB2/SiC/MgB2 junction and an MgB2/AlN/MgB2 junction. For both of these SIS junctions, we obtained clear gap structures, but at this stage, the AlN insulator appeared to be better than the SiC insulator based on current-voltage characteristics. We found that the current-voltage characteristics depend on the MgB2 thin film quality when using an AlN insulator. Our TEM measurements revealed epitaxial growth of the lower MgB2 thin films and amorphous-like growth of the upper MgB2 thin films. We obtained their depth profile with Auger electron spectroscopy, and the profile indicated that the trilayer has a nitrided and oxidized region at the insulator portion.

Subgap conductivity in SIN-junctions of high barrier transparency

We investigate the current–voltage characteristics of high-transparency superconductor–insulator–normal metal (SIN) junctions with the specific tunnel resistance ρ≲30Ωμm2. The junctions were fabricated from different superconducting and normal conducting materials, including Nb, Al, AuPd and Cu. The subgap leakage currents were found to be appreciably larger than those given by the standard tunnelling model. We explain our results using the model of two-electron tunnelling in the coherent diffusive transport regime. We demonstrate that even in the high-transparency SIN-junctions, a noticeable reduction of the subgap current can be achieved by splitting a junction into several submicron sub-junctions. These structures can be used as nonlinear low-noise shunts in rapid-single-flux-quantum (RSFQ) circuitry for controlling Josephson qubits.

Shot Noise in Ferromagnetic Superconductor Tunnel Junctions

In this paper, the superconducting order parameter andthe energy spectrum of the Bogoliubov excitations are obtainedfrom the Bogoliubov-de Gennes (BdG) equation for a ferromagneticsuperconductor (FS). Taking into account the rough interfacescattering effect, we calculate the shot noise and the differentialconductance of the normal-metal insulator ferromagnetic superconductorjunction. It is shown that the exchange energyEh in FS can lead to splitting of the differentialshot noise peaks and the conductance peaks. The energydifference between the two splitting peaks is equal to2Eh. The rough interface scattering strengthresults in descent of conductance peaks and the shotnoise-to-current ratio but increases the shot noise.

Electron–phonon interaction in a thin Al–Mn film

Aluminum doped with manganese is an interesting novel material with applications in normal metal–insulator–superconductor (NIS) tunnel junction devices and transition-edge sensors at sub-Kelvin temperatures. We have studied the electron–phonon (e–p) coupling in a thin aluminum film doped with 1% manganese, with a measuring technique based on DC hot-electron effect. The electron temperature was measured with the help of symmetric normal metal–insulator–superconductor tunnel-junction pairs (SINIS-thermometers). Measurements show that the temperature dependence of the e–p interaction is not consistent with existing theories for disordered metals, but follows a higher power law.

Normal metal–insulator–superconductor junction technology for bolometers

Normal metal–insulator–superconductor tunnel junctions are candidates for large-format arrays of ultra-low noise equivalent power bolometers. Using a wafer scale process we have fabricated devices with the required sub μ m 3 volumes and ohmic superconductor–normal metal contacts for delivering power loads. Additionally, we demonstrate simultaneous tunnel junction thermometry and SQUID-based Johnson current noise thermometry.

A superconductor–insulator–normal metal bolometer with microwave readout suitable for large-format arrays

We demonstrate high bandwidth and low-noise readout of a superconductor–insulator–normal metal–insulator–superconductor hot-electron bolometer element. We measure a noise equivalent temperature of better than 0.6μK∕Hz1∕2 and infer an electrical noise equivalent power of 7×10−17W∕Hz1∕2 for a normal-metal volume of 4.5μm3 at an operating temperature of 270mK. Using a pulsed bias technique we measure a thermal time constant of 1.2μs at 270mK. The microwave readout method we employ is compatible with large-format arrays of 103–105pixels required by future far-infrared space and ground-based observatories.

Fabrication and characterization of scanning tunneling microscopy superconducting Nb tips having highly enhanced critical fields

We report a simple method for the fabrication of Niobium superconducting (SC) tips for scanning tunneling microscopy which allow atomic resolution. The tips, formed in situ by the mechanical breaking of a niobium wire, reveal a clear SC gap of 1.5 meV and a critical temperature T c = 9.2 ± 0.3 K, as deduced from Superconductor Insulator Normal metal (SIN) and Superconductor Insulator Superconductor (SIS) spectra. These match the values of bulk Nb samples. We systematically find an enhanced value of the critical magnetic field in which superconductivity in the tip is destroyed (around 1 T for some tips) up to five times larger than the critical field of bulk Nb (0.21 T). Such enhancement is attributed to a size effect at the tip apex.

Mesoscopic Supercurrent Transistor Controlled by Nonequilibrium Cooling

The distinctive quasiparticle distribution existing under nonequilibrium in a superconductor—insulator—normal metal—insulator—superconductor (SINIS) mesoscopic line is proposed as a novel tool to control the supercurrent intensity in a long Josephson weak link. We present a description in this system in the framework of the diffusive-limit quasiclassical Green-function theory and take into account the effects of inelastic scattering with arbitrary strength. Supercurrent enhancement and suppression, including a marked transition to a π-junction are striking features leading to a fully tunable structure. The role of the degree of nonequilibrium, temperature, and materials choice as well as feature like noise, switching time, and current and power gain are also addressed. PACS numbers: 74.50.+r, 73.12.-b, 74.40.+k.

Effective electron microrefrigeration by superconductor–insulator–normal metal tunneljunctions with advanced geometry of electrodes and normal metal traps

We demonstrate effective electron cooling of the normal metal strip bysuperconductor–insulator–normal metal (SIN) tunnel junctions. The improvement wasachieved by two methods: first, by using an advanced geometry of the superconductingelectrodes for more effective removal of the quasiparticles; and second, by adding a normalmetal trap just near the cooling junctions. With simple cross geometry and without normalmetal traps, the decrease in electron temperature is 56 mK. With the advanced geometry ofthe superconducting electrodes, the decrease in electron temperature is 129 mK. With theaddition of the normal metal traps, the decrease in electron temperature is 197 mK.

MgB2 energy gap determination by scanning tunnelling spectroscopy

We report scanning tunnelling spectroscopy (STS) measurements of the gap properties of both ceramic MgB2 and c-axis oriented epitaxial MgB2 thin films. Both show a temperature dependent zero bias conductance peak and evidence for two superconducting gaps. We report tunnelling spectroscopy of superconductor–insulator–superconductor (S-I-S) junctions formed in two ways in addition to normal metal–insulator–superconductor (N-I-S) junctions. We find a gap Δ = 2.2–2.8 meV, with spectral features and temperature dependence that are consistent between S-I-S junction types. In addition, we observe evidence of a second, larger gap, Δ = 7.2 meV, consistent with a proposed two-band model.