Zero-bias Anomaly Research Articles

Coexistence of the Kondo effect and a ferromagnetic phase in magnetic tunnel junctions

Planar macroscopic magnetic tunnel junctions exhibit well defined zero-bias anomalies when a thin layer of ferromagnetic CoFe(B) nanodots is inserted within a MgO based tunnel barrier. The conductance curves exhibit a single and a double peak, respectively, for antiparallel and parallel alignment of the magnetizations of the electrodes that sandwich the tunnel barrier. This leads to a suppression of the tunneling magnetoresistance near zero bias. We show that the double-peak structure indicates that the zero-bias anomaly is spin split due to a magnetic exchange interaction between the magnetic nanodots and the ferromagnetic electrodes. Using a model based on an Anderson quantum dot coupled to ferromagnetic leads, we show that these results imply the coexistence of a Kondo effect and ferromagnetism.

Temperature dependence of the zero-bias anomaly in the Anderson–Hubbard model:insights from an ensemble of two-site systems

Motivated by experiments on doped transition metal oxides, this paper considers theinterplay of interactions, disorder, kinetic energy and temperature in a simple system. Anensemble of two-site Anderson–Hubbard model systems has already been shown to displaya zero-bias anomaly (Wortis and Atkinson 2010 Phys. Rev. B 82 073107) whichshares features with that found in the two-dimensional Anderson–Hubbard model(Chiesa et al 2008 Phys. Rev. Lett. 101 086401). Here the temperature dependence ofthe density of states of this ensemble is examined. In the atomic limit, there isno zero-bias anomaly at zero temperature, but one develops at small nonzerotemperatures. With hopping, small temperatures augment the zero-temperaturekinetic-energy-driven zero-bias anomaly, while at larger temperatures the anomaly is filledin.

Coexistence of Coulomb Blockade and Zero Bias Anomaly in a Strongly Coupled Nanodot

The current-voltage characteristics through a metallic nanoparticle which is well coupled to a metallic lead are measured. It is shown that the I-V curves are composed of two contributions. One is a suppression of the tunneling conductivity at the Fermi level, and the second is an oscillating feature which shifts with gate voltage. The results indicate that zero-bias anomaly and Coulomb blockade phenomena coexist in an asymmetric strongly coupled zero-dimensional system.

Local interlayer tunneling between two-dimensional electron systems in the ballistic regime

We study a theoretical model of virtual scanning tunneling microscopy (VSTM), a proposed application of interlayer tunneling in a bilayer system to locally probe a two-dimensional electron system (2DES) in a semiconductor heterostructure. We consider tunneling for the case where transport in the 2DESs is ballistic, and show that the zero-bias anomaly is suppressed by extremely efficient screening. Since such an anomaly would complicate the interpretation of data from a VSTM, this result is encouraging for efforts to implement such a microscopy technique.

Zero-Bias Anomalies in Narrow Tunnel Junctions in the Quantum Hall Regime

We report on the study of cleaved-edge-overgrown line junctions with a serendipitously created narrow opening in an otherwise thin, precise line barrier. Two sets of zero-bias anomalies are observed with an enhanced conductance for filling factors ν>1 and a strongly suppressed conductance for ν<1. A transition between the two behaviors is found near ν≈1. The zero-bias anomaly (ZBA) line shapes find explanation in Luttinger liquid models of tunneling between quantum Hall edge states. The ZBA for ν<1 occurs from strong backscattering induced by suppression of quasiparticle tunneling between the edge channels for the n=0 Landau levels. The ZBA for ν>1 arises from weak tunneling of quasiparticles between the n=1 edge channels.

Inelastic electron tunneling spectrum from surface magnon and magnetic impurity scatterings in magnetic tunnel junctions

Analytic expressions for contributions to the inelastic electron tunneling spectrum (IETS) from surface magnon scattering and magnetic impurity scattering are obtained. It is shown that surface magnon scattering alone does not lead to peaks in the IETS. The peaks at small bias often observed in the IETS of magnetic junctions are due to magnetic impurity scattering, in agreement with the traditional model for zero-bias anomaly. These impurity resonance peaks can sometimes split due to the impurities' magnetic coupling to the electrodes. Measurements of AlO and MgO barrier junctions yield excellent agreement to the theory. The experiment further shows that the magnetic impurities in MgO barriers are strongly coupled to the electrodes but those in AlO barriers are not magnetically coupled to the electrodes.

Suppression of Kondo-assisted cotunneling in a spin-1 quantum dot with spin-orbit interaction

Kondo-type zero-bias anomalies have been frequently observed in quantum dots occupied by two electrons and attributed to a spin-triplet configuration that may become stable under particular circumstances. Conversely, zero-bias anomalies have been so far quite elusive when quantum dots are occupied by an even number of electrons greater than two, even though a spin-triplet configuration is more likely to be stabilized there than for two electrons. We propose as an origin of this phenomenon the spin-orbit interaction, and we show how it profoundly alters the conventional Kondo screening scenario in the simple case of a laterally confined quantum dot with four electrons.

Effect of impurity scattering on the linear and nonlinear conductances of quasi-one-dimensionaldisordered quantum wires by asymmetrically lateral confinement

We have studied the linear conductance and source–drain bias spectroscopies of clean anddisordered quantum wires (QWs) against thermal cycling and lateral shifting, which changethe impurity configuration. Conductance quantization and the zero bias anomaly (ZBA)are robust in clean QWs. In contrast, disordered QWs show complexities in the ways ofconductance resonance, peak splitting and trace crossing in source–drain biasspectroscopies. The experimental results and theoretical predictions are in congruence.Moreover, the resonant state arising from the impurities results in either a single peak ordouble-splitting peaks in the spectroscopies from the detailed impurity configurations. Theresonant splitting peaks are found to influence the ZBA, indicating that a cleanQW is crucial for investigating the intrinsic characteristics of the ZBA of QWs.

Effect of strong correlations on the disorder-induced zero bias anomaly in the two-site Anderson-Hubbard model

Several recent exact-diagonalization calculations have established that the Anderson-Hubbard model has a disorder-induced zero bias anomaly (ZBA) (also called a disorder-induced pseudogap) in the density of states. In order to understand the physics of the ZBA, we study a simplified problem---an ensemble of two-site molecules with random site energies---for which analytical results are possible. For this ensemble, we examine how the ZBA forms in both the weakly correlated (mean field) and strongly correlated limits. In the weakly correlated case, the ZBA can be understood as the result of level repulsion between bonding and antibonding molecular orbitals. A similar level repulsion occurs in the strongly correlated case too but a larger contribution to the ZBA comes from the suppression of a triplet excitation mode. This inherently many-body mechanism does not have a counterpart in mean-field models.

Physical mechanism for a kinetic energy driven zero-bias anomaly in the Anderson-Hubbard model

The combined effects of strong disorder, strong correlations, and hopping in the Anderson-Hubbard model have been shown to produce a zero-bias anomaly which has an energy scale proportional to the hopping and minimal dependence on interaction strength, disorder strength, and doping. Disorder-induced suppression of the density of states for a purely local interaction is inconsistent with both the Efros-Shklovskii Coulomb gap and the Altshuler-Aronov anomaly, and moreover the energy scale of this anomaly is inconsistent with the standard energy scales of both weak- and strong-coupling pictures. We demonstrate that a density of states anomaly with similar features arises in an ensemble of two-site systems, and we argue that the energy scale $t$ emerges in strongly correlated systems with disorder due to the mixing of lower and upper Hubbard orbitals on neighboring sites.

Impact of electron-electron interactions induced by disorder at interfaces on spin-dependent tunneling in Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions

In this study, we demonstrate that, beyond the standard magnon excitations, the electron-electron interactions in presence of disorder in CoFeB electrodes play a significant role on the zero-bias anomaly measured in CoFeB/MgO/CoFeB junctions. The low-temperature dependence of the tunneling conductance presents cusplike dip at low voltage varying as the square root of the bias voltage. The amplitude of this zero-bias anomaly decreases with the annealing temperature, indicating interface crystallization and related increase in the tunnel magnetoresistance.

Effect of scattering on zero-bias anomaly and conductance reduction in quasi-one-dimensional quantum wires

We present a systematic study on the zero-bias conductance peak and its dependences on the carrier density and structural geometry in quasi-one-dimensional quantum wires (QWs). This zero-bias anomaly (ZBA) is suppressed by either decreasing the carrier density or increasing the QW length. The differential conductance at zero bias decreases with increasing temperature in accordance with a thermal-activation model up to a well-defined cut-off temperature. We demonstrate that the activation energy, cut-off temperature, and width of the ZBA are correlated, and suggest that these features are controlled by electron scattering in QWs.

Interaction corrections to tunneling conductance in ballistic superconductors

It is known that in the two-dimensional disordered superconductors electron-electron interactions in the Cooper channel lead to the negative logarithmic in temperature correction to the tunneling conductance above the critical temperature. Physically this result appears due to the density of states suppression by superconductive fluctuations near the Fermi level. It is interesting that the other correction, which accounts for the Maki-Thompson-type interaction of fluctuations, is positive and exhibits strong power law, which dominates the logarithmic term in the immediate vicinity of the critical temperature. An interplay between these two contributions determines the zero-bias anomaly in fluctuating superconductors. This paper is devoted to the fate of such interaction corrections in the ballistic superconductors. It turns out that ballistic dynamic fluctuations perturb single-particle density of states near the Fermi level at the energy scale which is different from that in the diffusive case. As the consequence, fluctuation region becomes much broader. In this regime we confirm that correction to the tunneling conductance remains negative and logarithmic not too close to the critical temperature while in the immediate vicinity of the transition we find novel power law for the Maki-Thompson contribution. It is suggested that peculiar non-monotonous temperature dependence of the tunneling conductance may be probed via magneto-tunnel experiments.

Study of Kondo effect in MgO-based magnetic tunnel junctions by electron tunnelling spectroscopy

To investigate the Kondo effect in MgO-based magnetic tunnel junctions (MTJs) electron tunnelling spectroscopy was carried out in Co60Fe20B20/textured MgO(001)/Co70Fe30magnetic tunnel junctions at temperatures between 2 K and 300 K. The zero-bias anomaly (ZBA) or the logarithmic temperature-dependent peak was observed in dynamic resistance. This behaviour can explain consistently the reduced TMR ratio of MgO-based MTJs compared with the theoretical expectation at low temperature. The properties of ZBAs were also investigated for both single crystal MTJs and the MTJs with the boron-doped top ferromagnetic electrode in order to find out the origin of the Kondo effect.

The zero-bias anomaly of point contacts with ferromagnetic Ni and with the heavy-fermion antiferromagnet CeAl2

We have investigated spear-anvil type point-contacts between ferromagnetic nickel as well as the heavy-fermion antiferromagnet CeAl2 and various simple metals (Cu, Ta, Nb). Contacts with small resistance usually showed electron-phonon scattering, Andreev reflection in case of superconducting counter-electrodes, as well as anomalies due to magnetic ordering. With increasing contact resistance (decreasing contact size) a zero-bias anomaly appeared in both Ni and CeAl2 contacts. It is conventionally attributed to resonant scattering at two-level systems or at magnetic impurities (Kondo effect). At contacts of ~ 1 nm diameter it suppressed completely all other spectral features. We discuss whether those mechanisms are relevant here and what alternatives there might be.

Benefits of using undoped GaAs/AlGaAs heterostructures: A case study of the zero-bias bias anomaly in quantum wires

We use quantum wires fabricated on undoped GaAs/AlGaAs heterostructures in which the average impurity separation is greater than the device size to compare the behaviour of the zero-bias anomaly against predictions from Kondo and spin polarisation models. Cleanliness and reproducibility are significantly improved in our undoped devices when compared to our doped devices.

An alternative view at the zero-bias anomaly of metallic point contacts

We have studied the zero-bias anomaly of point contacts as function of contact size for a wide range of materials from simple normal metals, superconductors, and magnets to highly correlated heavy-fermion compounds. The size δR of the zero-bias anomaly varied in a universal manner proportional to the square of the contact resistance R between 1Ω and 10kΩ, that is for contact radii from about 15nm down to 0.2nm. Magnetic impurities, two-level systems, or a tunneling barrier are unlikely to be the main source of this anomaly. We suggest instead Kondo-type scattering of the conduction electrons due to a spontaneous electron spin polarization at the point contact.

Electron-electron interaction and tunnel density of states at the Fermi level in high-density two-dimensional electron system

The zero bias anomaly (ZBA) of the tunnel conductivity are investigated in the Al/δ – GaAs tunnel structure with 2D electron concentration in δ-layer to be equal to 3.6*1012 cm−2. We find that the dip (ZBA) in the tunnel density of states near the Fermi level EF reveals logarithmic dependence on the energy e in the range kT < ϵ< ℏ/r. Here ϵ is the energy relative to Ef and r is the elastic relaxation time of 2D electrons. The depth of the ZBA is proportional to ln(T/T0) in the range T = 0.1 – 20 K. These results are in agreement with Aronov-Altshuler theory in which the correction to the 2D density of states is due to the effects of the electron-electron interaction in diffusion channel. The unusual behavior of the tunnel magnetoconductivity is observed.

Effects of strong correlations on the disorder-induced zero-bias anomaly in the extendedAnderson–Hubbard model

We study the effect of strong correlations on the zero-bias anomaly (ZBA) in disorderedinteracting systems. We focus on the two-dimensional extended Anderson–Hubbard model(EAHM) on a square lattice. The EAHM has both on-site and nearest-neighbourinteractions and randomly chosen site energies. We use a mean-field theory thatincorporates strong correlations and treats the disorder potential exactly. We use asimplified atomic-limit approximation for the diagonal inelastic self-energy that becomesexact in the large-disorder limit, and the off-diagonal self-energy is treated within theHartree–Fock approximation. The validity of these approximations is discussedin detail. We find that strong correlations have a significant effect on the ZBAat half-filling, and enhance the ZBA gap when the interaction is finite ranged.

Inelastic electron tunneling spectra of MgO-based magnetic tunnel junctions with different electrode designs

MgO-based magnetic tunnel junctions with up to 230% tunnel magnetoresistance ratio at room temperature and up to 345% at 13 K are prepared. The lower electrode is either exchange-biased or free, while the top electrode is free or an exchanged-biased artificial ferrimagnet, respectively. Additionally, a pseudo-spin-valve (hard-soft switching) design with two unpinned electrodes is used. Inelastic electron-tunneling spectra for each of these systems show a strong variation in the zero-bias anomaly with a reduced peak for some of the junctions. At voltages around 200 mV additional structures are found, which are not known from junctions with lower magneto resistance, such as alumina-based junctions. We discuss the spectra for the different electrode types and compare our findings with respect to barrier material and magnetoresistance ratio.