Altshuler Aronov Spivak Oscillations Research Articles

Multiple andreev reflections in topological insulator nanoribbons

Superconducting proximity junctions made of topological insulator (TI) nanoribbons (NRs) provide a useful platform for studying topological superconductivity. We report on the fabrication and measurement of Josephson junctions (JJs) using Sb-doped Bi2Se3 NRs in contact with Al electrodes. Aharonov–Bohm and Altshuler–Aronov–Spivak oscillations of the axial magneto-conductance of TI NR were observed, indicating the existence of metallic surface states along the circumference of the TI NR. We observed the supercurrent in the TI NR JJ and subharmonic gap structures of the differential conductance due to multiple Andreev reflections. The interface transparency of the TI NR JJs estimated based on the excess current reaches τ = 0.83, which is among the highest values reported for TI JJs. The temperature dependence of critical current is consistent with the short and ballistic junction model confirming the formation of highly transparent superconducting contacts on the TI NR. Our observations would be useful for exploring topological Josephson effects in TI NRs.

Observation of Aharonov-Bohm and Al'tshuler-Aronov-Spivak oscillations in the background of universal conductance fluctuations in silicon nanowires

Magnetoresistance (MR) oscillations of multiple periodicities are recorded in singly connected silicon nanowires of diameter . At 100 K we observe oscillations of periodicity and 0.444 T corresponding to h/e and Aharonov-Bohm (AB) oscillations, whereas at 10 K we record periodicities of 0.98 T, 0.49 T and 0.25 T corresponding to h/e, (Al'tshuler-Aronov-Spivak (AAS)) and oscillations. At 2.5 K we find magnetoresistance oscillations with multiple periodicities of 1.3 T, 0.52 T, and 0.325 T corresponding to AB and AAS oscillations. The and peaks can be attributed to the interference of time-reversed paths originating from the core orbits that scatter coherently on the surface of the nanowires multiple times. We also observed 20 mT and 60 mT oscillations of small amplitude superimposed on a quasi-periodic background which we attribute to the quantum interference of special surface states associated with skipping orbits that propagate quasi-ballistically. The aperiodic fluctuations in the MR at all temperatures are universal conductance fluctuations (UCF) originating from randomly spaced impurity scattering in the core of the nanowire.

Surface-Sensitive Two-Dimensional Magneto-Fingerprint in Mesoscopic Bi2Se3 Channels

Periodic Aharonov–Bohm and Altshuler–Aronov–Spivak oscillations have traditionally been observed in lateral transport through patterned mesoscopic loops of diffusive conductors. However, our studies of perpendicular-to-plane magnetotransport in straight-channel, diffusive devices of epitaxial Bi2Se3 surprisingly reveal signatures of Aharonov–Bohm orbits, periodic conductance fluctuation magneto-fingerprints, even though the devices are not explicitly patterned into loops. We show that the length scale of these orbits corresponds to the typical perimeter of triangular terraces found on the surface of these thin film devices, strongly suggesting that the periodic magneto-fingerprint arises from coherent scattering of electron waves from the step-edges. Our interpretation is bolstered by control measurements in devices without such surface morphology that only show a conventional, aperiodic magneto-fingerprint. These results show that lithographically patterned Bi2Se3 devices provide a novel class of mesoscopic physical systems for systematic studies of coherent surface sensitive transport.

Observing Altshuler–Aronov–Spivak Oscillation in a Hexagonal Antidot Array of Monolayer Graphene

We show that hexagonal antidot lattices of monolayer graphene exhibited the Altshuler–Aronov–Spivak (AAS) effect in low field magnetoresistance. In higher magnetic fields, Aharonov–Bohm-type oscillations were visible. The phase of AAS oscillation indicated that the chirality effect of graphene is suppressed because of inter-valley scattering due to boundary scatterings.

Oscillations of Low-Field Magnetoresistivity of Two-Dimensional Electron Gases in Al0.22Ga0.78N/GaN Heterostructures in a Weak Localization Region

Low-field magnetotransport properties of two-dimensional electron gases (2DEGs) are investigated in Al0.22Ga0.78N/GaN heterostructures. By means of a tilting magnetic field, unexpected oscillations of magnetoresistivity are observed in a weak localization region. Qualitative understanding based on Altshuler—Aronov—Spivak oscillations is proposed for the case of interface disorder in Al0.22Ga0.78N/GaN heterostructures.

Competition between magnetic field dependent band structure and coherent backscattering in multiwall carbon nanotubes

Magnetotransport measurements in large diameter multiwall carbon nanotubes (20–40 nm) demonstrate the competition of a magnetic-field dependent bandstructure and Altshuler–Aronov–Spivak oscillations. By means of an efficient capacitive coupling to a backgate electrode, the magnetoconductance oscillations are explored as a function of Fermi level shift. Changing the magnetic field orientation with respect to the tube axis and by ensemble averaging, allows the contributions of different Aharonov–Bohm phases to be identified. The results are in qualitative agreement with numerical calculations of the band structure and the conductance.

Quantum interference in multiwall carbon nanotubes

Recent low temperature conductance measurements on multiwall carbon nanotubes in perpendicular and parallel magnetic fields are reported. An efficient gating technique allows for considerable tuning of the nanotube doping level. This enables us to study extensively the effect of the nanotube bandstructure on electron quantum interference effects such as weak localization, universal conductance fluctuations and the Aharonov–Bohm effect. We show that the magnetoresistance in the perpendicular magnetic field is strongly suppressed at certain gate voltages Ugate which can be linked with the bottoms of one-dimensional electronic subbands. This assignment allows a detailed comparison of theoretical calculations with the experimental data. In agreement with the theory, a pronounced energy dependence of the elastic mean free path with a strong enhancement close to the charge neutrality point is observed. In the large parallel magnetic field, we observe a superposition of h/2e-periodic Altshuler–Aronov–Spivak oscillations and an additional h/e-periodic contribution to the conductivity. The latter contribution shows a diamond-like pattern in the B − Ugate-plane, which reflects the magnetic field dependence of the density of states of the nanotube's outermost shell.

Numerical simulations of a ballistic spin interferometer with Rashba spin-orbital interaction

We numerically investigate the transport behavior of a quasi-one-dimensional (1D) square loop device containing the Rashba spin-orbital interaction in the presence of a magnetic flux. The conductance versus the magnetic field shows the Al'tshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB) oscillations. We focus on the oscillatory amplitudes, and find that both of them are strongly dependent on the spin precession angle (i.e., the strength of the spin-orbit interaction) and exhibit no periodic oscillations, in good agreement with a recent experiment by Koga et al. [cond-mat/0504743 (unpublished)]. However, our numerical results for the ideal 1D square loop device for the node positions of the amplitudes of the AB and AAS oscillations are found to show some discrepancies with the results for quasi-1D square loops with a finite width. In the presence of disorder and taking the disorder ensemble average, the AB oscillation in the conductance disappears, while the time-reversal symmetric AAS oscillation still remains. Furthermore, the node positions of the AAS oscillatory amplitude remain the same.

Compressive deformation of a bulk Ce-based metallic glass

We numerically investigate the transport behavior of a quasi-one-dimensional (1D) square loop device containing the Rashba spin-orbital interaction in the presence of a magnetic flux. The conductance versus the magnetic field shows the Al'tshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB) oscillations. We focus on the oscillatory amplitudes, and find that both of them are strongly dependent on the spin precession angle (i.e., the strength of the spin-orbit interaction) and exhibit no periodic oscillations, in good agreement with a recent experiment by Koga [cond-mat/0504743 (unpublished)]. However, our numerical results for the ideal 1D square loop device for the node positions of the amplitudes of the AB and AAS oscillations are found to show some discrepancies with the results for quasi-1D square loops with a finite width. In the presence of disorder and taking the disorder ensemble average, the AB oscillation in the conductance disappears, while the time-reversal symmetric AAS oscillation still remains. Furthermore, the node positions of the AAS oscillatory amplitude remain the same.

Aharonov–Bohm-type effects in different arrays of antidots

We have investigated Aharonov–Bohm-type oscillation in the quantum Hall plateau transition region in three types of square arrays of antidots; a large ( 50 × 160 antidots) array, a small ( 5 × 10 antidots) array, and the sample with antidots placed only near the side edges. The temperature dependence of the amplitude confirmed that the oscillation originates from the fine structure in the density of single particle states circumnavigating around each antidot. In addition, we have also observed Altshuler–Aronov–Spivak oscillation near zero magnetic field in square arrays of antidots.

Quantum oscillation and decoherence in triangular antidot lattice

Quantum oscillation phenomena in triangular antidot lattice have been investigated. Altshuler–Aronov–Spivak oscillations and Aharonov–Bohm (AB)-type oscillations are observed at low magnetic field, and AB-type oscillations due to edge channels are observed in the quantum Hall regime. Measurements of the temperature dependence of these oscillations furnish information on the mechanism of decoherence in the antidot lattice, which is compared with the single ring case.

Anti localization due to spin–orbit interaction in metal-doped carbon nanotubes

Electrode atoms are diffused, with only about 5% volume ratio, into the top end of multi-walled carbon nanotubes (MWNTs), standing in nanopores of porous alumina membranes. We find that diffusion of heavy materials leads to an anti-localization in the Altshuler–Aronov–Spivak oscillations in the MWNT bulk. This effect is only observable when electrons are injected through the diffusion region, and undergo a π-phase shift in their electron waves, caused by polarized injection of spin-flipped electrons due to spin-orbit interaction in the diffusion region.

Anti-localization caused by small doping of heavy-mass impurity-atoms in carbon nanotubes and a novel spintronics device

Multi-walled carbon nanotubes (MWNTs), standing in nanoporous alumina membranes, are doped at one end by impurity atoms from electrode materials. Doping of the light-mass materials (carbon and aluminum) leads to weak localization in Altshuler–Aronov–Spivak oscillation, consistent with past reports. In contrast, we find that doping of heavy-mass materials (gold and platinum) at a volume ratio of only about 5% changes this weak localization to anti-localization. It is understood by a drastic change of the phase interference, caused by the polarized injection of spin-flipped electrons due to spin–orbit interaction in the diffusion region, in the bulk of the MWNTs. We also propose a novel spintronics (electron-wave phase switching) circuit using this effect.

Transport signatures for correlated disorder in self-assembled InAs quantum dots on GaAs

We report electronic transport measurements on two-dimensional electron gases in a Ga[Al]As heterostructure with an embedded layer of InAs self-assembled quantum dots. At high InAs dot densities, Altshuler–Aronov–Spivak oscillations are observed. The presence of these oscillations correlates with the observation of a metal–insulator transition, and with the existence of a maximum in the electron mobility as a function of the electron density. These results indicate hexagonal short-range ordering of the charged InAs dots.

Drastic change of phase interference by small diffusion of heavy-mass electrode atoms in carbon nanotubes and phase switching device

We slightly diffuse atoms of electrode materials into one end of multiwalled carbon nanotubes (MWNTs), grown using nanoporous alumina membranes. Diffusion of the light-mass materials (carbon and aluminum) lead to weak localization in Altshuler–Aronov–Spivak oscillation, consistent with past reports. In contrast, we find that diffusion of heavy-mass materials (gold and platinum) at the volume ratio of only about 5% change this weak localization to antilocalization. It is understood by a drastic change of the phase interference caused by the injection of spin-flipped electrons due to spin–orbit interaction in the diffusion region, in the entire part of the MWNTs. We also propose an electron-wave phase switching circuit using this effect.

Vol. 30, nos. 1-4 author index

Quantum oscillation phenomena in triangular antidot lattice have been investigated. Altshuler–Aronov–Spivak oscillations and Aharonov–Bohm (AB)-type oscillations are observed at low magnetic field, and AB-type oscillations due to edge channels are observed in the quantum Hall regime. Measurements of the temperature dependence of these oscillations furnish information on the mechanism of decoherence in the antidot lattice, which is compared with the single ring case.