Double-barrier Junctions Research Articles

Intrinsic supercurrent non-reciprocity coupled to the crystal structure of a van der Waals Josephson barrier

Non-reciprocal electronic transport in a spatially homogeneous system arises from the simultaneous breaking of inversion and time-reversal symmetries. Superconducting and Josephson diodes, a key ingredient for future non-dissipative quantum devices, have recently been realized. Only a few examples of a vertical superconducting diode effect have been reported and its mechanism, especially whether intrinsic or extrinsic, remains elusive. Here we demonstrate a substantial supercurrent non-reciprocity in a van der Waals vertical Josephson junction formed with a Td-WTe2 barrier and NbSe2 electrodes that clearly reflects the intrinsic crystal structure of Td-WTe2. The Josephson diode efficiency increases with the Td-WTe2 thickness up to critical thickness, and all junctions, irrespective of the barrier thickness, reveal magneto-chiral characteristics with respect to a mirror plane of Td-WTe2. Our results, together with the twist-angle-tuned magneto-chirality of a Td-WTe2 double-barrier junction, show that two-dimensional materials promise vertical Josephson diodes with high efficiency and tunability.

Gate-Controlled NiO/Graphene/4H-SiC Double Schottky Barrier Heterojunction Based on a Metal-Oxide-Semiconductor Structure for Dual-Mode and Wide Range Ultraviolet Detection

Based on a metal-oxide-semiconductor (MOS) structure, a double Schottky barrier junction (SBJ) made of NiO/graphene/4H-SiC is built and employed in ultraviolet (UV) detection. The hole concentration of NiO can be modulated as depleted or accumulated states with gate voltages, which allows the device to work in dual-mode when used as a photodetector. In this work, a negative gate bias causes the device to operate as a photoconductive detector with gain due to the negligible Schottky barrier, whereas a zero or positive gate bias makes it work as a Schottky photodiode. The device has a high responsivity of 103.3 A/W and a gain of 490.8 despite the low light intensity (261 nm laser @ 30.19 μW/cm2) at VDS = 5 V and VGS = −3 V. The NiO layer and SiC substrate both serve as UV absorption materials and produce photogenerated carriers, and the device has a wide UV response range from 240 to 400 nm with a gain of 80.34 when VDS = −3 V and VGS = 0 V at 240 nm. The above findings suggest that this MOS-based NiO/graphene/4H-SiC double SBJ has a great prospect in practical UV detection.

Directional Excitation of Surface Plasmon Polaritons via Molecular Through-Bond Tunneling across Double-Barrier Tunnel Junctions.

Directional excitation of surface plasmon polaritons (SPPs) by electrical means is important for the integration of plasmonics with molecular electronics or steering signals toward other components. We report electrically driven SPP sources based on quantum mechanical tunneling across molecular double-barrier junctions, where the tunneling pathway is defined by the molecules' chemical structure as well as by their tilt angle with respect to the surface normal. Self-assembled monolayers of S(CH2)nBPh (BPh = biphenyl, n = 1-7) on Au, where the alkyl chain and the BPh units define two distinct tunnel barriers in series, were used to demonstrate and control the geometrical effects. The tilt angle of the BPh unit with respect to the surface normal depends on the value of n, and is 45° when n is even and 23° when n is odd. The tilt angle of the alkyl chain is fixed at 30° and independent of n. For values of n = 1-3, SPPs are directionally launched via directional tunneling through the BPh units. For values of n > 3, tunneling along the alkyl chain dominates the SPP excitation. Molecular level control of directionally launching SPPs is achieved without requiring additional on-chip optical elements, such as antennas, or external elements, such as light sources. Using the molecular tunneling junctions, we provide the first direct experimental demonstration of molecular double-barrier tunneling junctions.

Spin dependent resonant electron tunneling through planar graphene barriers

We study spin-dependent electron transport properties of two dimensional graphene double and triple barrier junctions via first-principles calculations. The double barrier junction consists of two graphene leads, a quantum well of zigzag graphene nanoribbon (ZGNR) in the center, and two vacuum barriers separating the ZGNR from the two leads. Resonant electron tunneling occurs when the energy bands of graphene and ZGNR are well aligned in energy and wavevector. Highly spin-polarized electron transmission arises in such junctions when the two edges of the center ZGNR are in the ferromagnetic configuration. The spin polarization of the electron transmission at the Fermi energy can be tuned by gate voltage. We further investigate the dependence of the electron transmission on the width of the ZGNR, effects on barrier height when replacing vacuum by h-BN, and the consequence of replacing a double barrier by a triple barrier.

Coherent charge transport in ferromagnet/semiconductor nanowire/ferromagnet double barrier junctions with the interplay of Rashba spin–orbit coupling, induced superconducting pair potential, and external magnetic field

By solving the Bogoliubov–de Gennes equation, the influence of the interplay of Rashba spin–orbit coupling, induced superconducting pair potential, and external magnetic field on the spin-polarized coherent charge transport in ferromagnet/semiconductor nanowire/ferromagnet double barrier junctions is investigated based on the Blonder–Tinkham–Klapwijk theory. The coherence effect is characterized by the strong oscillations of the charge conductance as a function of the bias voltage or the thickness of the semiconductor nanowire, resulting from the quantum interference of incoming and outgoing quasiparticles in the nanowire. Such oscillations can be effectively modulated by varying the strength of the Rashba spin–orbit coupling, the thickness of the nanowire, or the strength of the external magnetic field. It is also shown that two different types of zero-bias conductance peaks may occur under some particular conditions, which have some different characteristics and may be due to different mechanisms.

Conductance switching of a phthalocyanine molecule on an insulating surface

We study the electron transport through the double-barrier junction consisted of the phthalocyanine molecule adsorbed on a NaCl bilayer on a metal substrate and the STM tip from first principles. The hydrogen tautomerization reaction happened in the molecule changes the spatial extensions of the molecular π orbitals under the tip, leading to junction conductance switching. Shifting the molecule to locate on different ions also varies the conductance. The transport channels of the tautomers on different adsorbed sites are identified.

Temperature dependence of enhanced spin relaxation time in metallic nanoparticles: Experiment and theory

We study the enhanced spin relaxation time of Au nanoparticles in nanopillar-shaped double-barrier junction devices with a stacked Fe/MgO/Au-nanoparticle/MgO/Fe structure. The size of Au nanoparticles located in a current path is deduced from a transmission electron micrograph and the Coulomb blockade behavior in the current-voltage characteristics of the devices. A finite tunnel magnetoresistance (TMR) is observed above a critical current and is attributable to spin accumulation in Au nanoparticles. Based on a simple model of TMR due to spin accumulation in a nanoparticle, the spin relaxation time ${\ensuremath{\tau}}_{s}$ is estimated from the magnitude of the critical current. The temperature and bias-voltage region where TMR appears are determined from systematic observations, showing that the appearance of TMR is not associated with the Coulomb blockade but with spin accumulation. We find that the obtained ${\ensuremath{\tau}}_{s}$ is anomalously extended (\ensuremath{\sim}800 ns) at low temperatures and abruptly decreases above a critical temperature. Interestingly, the critical temperature strongly depends on the size of the Au nanoparticles and is much lower than the effective temperature corresponding to the discrete energy spacing. A theoretical analysis for the spin relaxation of electrons with discrete energy levels shows that not only the anomalously extended spin relaxation time, but also the strong temperature dependence of ${\ensuremath{\tau}}_{s}$ arise from the broadening of discrete energy levels due to coupling with phonons in the surrounding matrix. Numerical calculations using reasonable parameter values well reproduce the observed temperature and size dependence of the spin relaxation time in Au nanoparticles.

Cu/ nano-CuO Thick Film/Cu Double Barrier Junction: Structural and Dielectric Study

In this paper, the structural and dielectric properties of CuO nanoparticles have been investigated by extended X-ray powder diffraction and impedance spectroscopy, respectively. The X-ray diffraction reveals the monoclinic phase structure of nanoparticles with preferential orientations parallel to (002) and (111) crystalline planes. The average crystallite size was calculated to be 18.3nm from the given line widths using Scherrer's equation. Impedance spectroscopy was performed on Cu/ nano- CuO thick film/Cu double barrier junction as a function of frequency in the range 1Hz - 106Hz, at room temperature. The complex impedance plots demonstrate that the dielectric characterizations are strongly frequency dependent.

Enhancement of tunnel magnetoresistance in magnetic tunnel junction by a superlattice barrier

Tunnel magnetoresistance of magnetic tunnel junction improved by a superlattice barrier composed of alternate layers of a nonmagnetic metal and an insulator is proposed. The forbidden band of the superlattice is used to predict the low transmission range in the superlattice barrier. By forbidding electron transport in the anti-parallel configuration, the tunnel magnetoresistance is enhanced in the superlattice junction. The results show that the tunnel magnetoresistance ratio for a superlattice magnetic tunnel junction is greater than that for traditional single or double barrier junctions.

Current-phase relation of double-barrier Josephson junctions with a two-gap superconductor as intermediate electrode

The current-phase (I-ϕ) relation of a double-barrier Josephson junction with a two-gap superconductor as intermediate electrode is derived by means of a simplified version of Ohta’s model. As in conventional double-barrier Josephson junctions, a marked skewness in the I-ϕ curves is present. Moreover, as in heterotic Josephson devices, a reduction of the maximum Josephson current is predicted. An appropriate experiment to verify the rich behavior of this type of Josephson device is suggested.

Tunneling characteristics of a double-barrier magnetic junction

The spin-polarized current through a planar double-barrier magnetic tunnel junction has been calculated using the quasi-classical model. The coefficients of electron transmission through the barriers have been calculated in terms of the quantum theory. The dependences of the transmission coefficients, spinpolarized currents, and tunneling magnetoresistance on the applied voltage under resonant conditions have been shown. Under non-resonant conditions, the tunneling magnetoresistance has been compared with the experimental data.

Admittance of Au/1,4-benzenedithiol/Au single-molecule junctions

Employing the admittance formula for double-barrier junctions [Fu and Dudley, Phys. Rev. Lett. 70, 65 (1993)], we have estimated an ac susceptance (imaginary part of admittance) of Au/1,4-benzenedithiol/Au single-molecule junctions from their current-voltage characteristics. In the MHz regime, we find that the junction susceptance shows a very small (∼0.1 aF) capacitive component that can be entirely masked by a larger electrode capacitance. Direct ac signal transmission measurements up to 1 GHz reveal no molecular signals and confirm the smallness of the molecular capacitance in the MHz regime.

Andreev bound state in a superconducting double barrier junction: The role of anisotropic dx2−y2-wave pairing potential

In this study, we have investigated the transport properties in the superconducting double barrier junction, considering unconventional anisotropic d-wave pairing potential. It is shown that the Andreev bound state and the spin conductance of such system exhibit essential different features, as compared to the results in the conventional isotropic s-wave superconducting double barrier junction.

Coherent transmission of nodal Dirac fermions through a graphene-based superconducting double barrier junction

Transport characteristics of relativistic electrons through graphene-based d-wave superconducting double barrier junction and ferromagnet/d-wave superconductor/normal metal double junction have been investigated based on the Dirac–Bogoliubov–de Gennes equation. We have first presented the results of superconducting double barrier junction. In the subgap regime, both the crossed Andreev and nonlocal tunneling conductance all oscillate with the bias voltage due to the formation of Andreev bound states in the normal metal region. Moreover, the critical voltage beyond which the crossed Andreev conductance becomes to zero decreases with increasing value of superconducting pair potential α. In the presence of the ferromagnetism, the MR through graphene-based ferromagnet/ d-wave superconductor/normal metal double junction has been investigated. It is shown that the MR increases from exchange splitting h0=0 to h0=EF (Fermi energy), and then it goes down. At h0=EF, MR reaches its maximum 100. In contrast to the case of a single superconducting barrier, Andreev bound states also manifest itself in the zero bias MR, which result in a series of peaks except the maximum one at h0=EF. Besides, the resonance peak of the MR can appear at certain bias voltage and structure parameter. Those phenomena mean that the coherent transmission can be tuned by superconducting pair potential, structure parameter, and external bias voltage, which benefits the spin-polarized electron device based on the graphene materials.

Spin-dependent tunneling spectroscopy in MgO-based double-barrier magnetic tunnel junctions

We investigated the dynamic conductance and inelastic electron tunneling spectroscopy in MgO-based double barrier magnetic tunnel junctions with Co50Fe50/Co40Fe40B20 hybrid free layers. The tunneling is coherent through the MgO (001) barriers but nonresonant, and the highest tunneling magnetoresistance reaches 260% at 2 K. Based on the detailed discussion of the tunneling mechanisms, the double-barrier junctions investigated here can be considered as two single-barrier junctions in series.

Reversible Switch Memory Effect in Hydrogen‐Terminated Ultrananocrystalline Diamond

AbstractInnovative memory switch devices require reliable bistable conductance properties. It would be desirable if such bistable characteristics were available in robust solid state materials, such as diamond, which benefit from outstanding physical properties. A bistable current with reversible switching effect from surface transfer doped ultrananocrystalline diamond thin films measured by electron field emission is reported. This switching is manifested by the appearance of huge jumps in the current emission, up to four orders of magnitude, that occur at specific extracting electric field values. Persistent hysteresis is exhibited whenever the field is ramped down. It is proposed that these phenomena are the result of resonant‐tunneling through a double barrier junction composed of tetrahedral amorphous carbon (ta‐C)/nanodiamond/adsorbent/vacuum. This finding may pave the way for the realization of novel types of memory switch devices with unprecedented performance.

Resonant transport through a carbon nanotube junction exposed to an ac field

The electron transport through a carbon nanotube (CNT) double barrier junction exposed to an external electromagnetic field is studied. The electron spectrum in the quantum well (QW) formed by the junction bears relativistic features. We examine how the ac field affects the level quantization versus the ac field parameters and chirality. We find that the transport through the junction changes dramatically versus the ac field frequency and amplitude. These changes are pronounced in the junction’s differential conductance, which allows judgment about the role of relativistic effects in the CNT QW structures.

Resonant tunneling structures based on epitaxial graphene on SiC

Recently some experiments have suggested that graphene epitaxially grown on SiC can exhibit an energy bandgap of 260 meV, which enhances the potential of this material for electronic applications. On this basis, we propose to use spatial doping to generate graphene-on-SiC double-barrier structures. The non-equilibrium Green's function technique for solving the massive Dirac model is applied to highlight typical transport phenomena such as the electron confinement and the resonant tunneling effects. The I–V characteristics of graphene resonant tunneling diodes were then investigated and the effect of different device parameters was discussed. It is finally shown that this kind of double-barrier junction provides an efficient way to confine the charge carriers in graphene and to design graphene resonant tunneling structures.

Dissipative charge transport in diffusive superconducting double-barrier junctions

We solve the coherent multiple Andreev reflection (MAR) problem and calculate current-voltage characteristics (IVCs) for Josephson SINIS junctions, where S are local-equilibrium superconducting reservoirs, I denotes tunnel barriers, and N is a short diffusive normal wire, the length of which is much smaller than the coherence length, and the resistance is much smaller than the resistance of the tunnel barriers. The charge transport regime in such junctions qualitatively depends on a characteristic value \gamma = \Delta \tau_d of relative phase shifts between the electrons and retro-reflected holes accumulated during the dwell time \tau_d. In the limit of small electron-hole dephasing \gamma << 1, our solution recovers a known formula for a short mesoscopic connector extended to the MAR regime. At large dephasing, the subharmonic gap structure in the IVC scales with 1/ \gamma, which thus plays the role of an effective tunneling parameter. In this limit, the even gap subharmonics are resonantly enhanced, and the IVC exhibits portions with negative differential resistance.

Nb / Al – AlO x – Nb superconducting heterostructures: A promising class of self-shunted Josephson junctions

The measurements of dc Josephson and quasiparticle current-voltage characteristics of four-layered Nb/Al–AlOx–Nb devices with a fixed Nb thickness of 270 nm and Al thicknesses ranging from 40 to 120 nm are reported and analyzed in the framework of a microscopic model developed to determine stationary properties of dirty limit double-barrier junctions. It is shown that the temperature dependence of the supercurrent as well as the values of characteristic voltages are well reproduced by the model calculations with only one fitting parameter. We have revealed a hysteretic-to-nonhysteretic transition in the current-voltage characteristics of our junctions at temperatures near 4.2 K and argue that this effect is caused by two factors: high-transparency insulating barrier with a broad distribution of the transmission coefficient and the temperature as a tuning parameter, which decreases the McCumber–Stewart parameter from values above unity at low temperatures to less than one above 4.2 K. Finally, we show how and why the temperature stability of the proposed Josephson devices can be significantly improved by choosing an appropriate Al thickness.