Zero-bias anomalies in electrochemically fabricated nanojunctions

A streamlined technique for the electrochemical fabrication of metal nanojunctions (MNJs) between lithographically defined electrodes is presented. The first low-temperature transport measurements in such structures reveal suppression of the conductance near zero-bias. The size of the zero-bias anomaly (ZBA) depends strongly on the fabrication electrochemistry and the dimensions of the resulting MNJ. We present evidence that the nonperturbative ZBA in atomic-scale junctions is due to a density of states suppression in the leads.Comment: 4 pages, 4 figure

Anomalous Aharonov-Bohm conductance oscillations from topological insulator surface states

We study transport properties of a topological insulator nanowire when a magnetic field is applied along its length. We predict that with strong surface disorder, a characteristic signature of the band topology is revealed in Aharonov Bohm (AB) oscillations of the conductance. These oscillations have a component with anomalous period $\Phi_0=hc/e$, and with conductance maxima at odd multiples of $\frac12\Phi_0$, i.e. when the AB phase for surface electrons is $\pi$. This is intimately connected to the band topology and a surface curvature induced Berry phase, special to topological insulator surfaces. We discuss similarities and differences from recent experiments on Bi$_2$Se$_3$ nanoribbons, and optimal conditions for observing this effect.Comment: 7 pages, 2 figure

Dynamical mean field theory for strongly correlated inhomogeneous multilayered nanostructures

Dynamical mean field theory is employed to calculate the properties of multilayered inhomogeneous devices composed of semi-infinite metallic lead layers coupled via barrier planes that are made from a strongly correlated material (and can be tuned through the metal-insulator Mott transition). We find that the Friedel oscillations in the metallic leads are immediately frozen in and don't change as the thickness of the barrier increases from one to eighty planes. We also identify a generalization of the Thouless energy that describes the crossover from tunneling to incoherent Ohmic transport in the insulating barrier. We qualitatively compare the results of these self-consistent many-body calculations with the assumptions of non-self-consistent Landauer-based approaches to shed light on when such approaches are likely to yield good results for the transport.Comment: 15 pages, 12 figures, submitted to Phys. Rev.

Coexistence of a triplet nodal order-parameter and a singlet order-parameter at the interfaces of ferromagnet-superconductor Co/CoO/In junctions

We present differential conductance measurements of Cobalt / Cobalt-Oxide / Indium planar junctions, 500nm x 500nm in size. The junctions span a wide range of barriers, from very low to a tunnel barrier. The characteristic conductance of all the junctions show a V-shape structure at low bias instead of the U-shape characteristic of a s-wave order parameter. The bias of the conductance peaks is, for all junctions, larger than the gap of indium. Both properties exclude pure s-wave pairing. The data is well fitted by a model that assumes the coexistence of s-wave singlet and equal spin p-wave triplet fluids. We find that the values of the s-wave and p-wave gaps follow the BCS temperature dependance and that the amplitude of the s-wave fluid increases with the barrier strength.Comment: 5 pages, Accepted to Phys. Rev.

Spin-polarized electron transport in ferromagnet/semiconductor heterostructures: Unification of ballistic and diffusive transport

A theory of spin-polarized electron transport in ferromagnet/semiconductor heterostructures, based on a unified semiclassical description of ballistic and diffusive transport in semiconductor structures, is developed. The aim is to provide a framework for studying the interplay of spin relaxation and transport mechanism in spintronic devices. A key element of the unified description of transport inside a (nondegenerate) semiconductor is the thermoballistic current consisting of electrons which move ballistically in the electric field arising from internal and external electrostatic potentials, and which are thermalized at randomly distributed equilibration points. The ballistic component in the unified description gives rise to discontinuities in the chemical potential at the boundaries of the semiconductor, which are related to the Sharvin interface conductance. By allowing spin relaxation to occur during the ballistic motion between the equilibration points, a thermoballistic spin-polarized current and density are constructed in terms of a spin transport function. An integral equation for this function is derived for arbitrary values of the momentum and spin relaxation lengths. For field-driven transport in a homogeneous semiconductor, the integral equation can be converted into a second-order differential equation that generalizes the standard spin drift-diffusion equation. The spin polarization in ferromagnet/semiconductor heterostructures is obtained by invoking continuity of the current spin polarization and matching the spin-resolved chemical potentials on the ferromagnet sides of the interfaces. Allowance is made for spin-selective interface resistances. Examples are considered which illustrate the effects of transport mechanism and electric field.Comment: 23 pages, 8 figures, REVTEX 4; minor corrections introduced; to appear in Phys. Rev.

Dynamics of stripe patterns in type-I superconductors subject to a rotating field

The evolution of stripe patterns in type-I superconductors subject to a rotating in-plane magnetic field is investigated magneto-optically. The experimental results reveal a very rich and interesting behavior of the patterns. For small rotation angles, a small parallel displacement of the main part of the stripes and a co-rotation of their very ends is observed. For larger angles, small sideward protrusions develop, which then generate a zigzag instability, ultimately leading to a breaking of stripes into smaller segments. The short segments then start to co-rotate with the applied field although they lag behind by approximately $10^\circ$. Very interestingly, if the rotation is continued, also reconnection of segments into longer stripes takes place. These observations demonstrate the importance of pinning in type-I superconductors.Comment: To appear in Phys. Rev.

Chaotic scattering through coupled cavities

We study the chaotic scattering through an Aharonov-Bohm ring containing two cavities. One of the cavities has well-separated resonant levels while the other is chaotic, and is treated by random matrix theory. The conductance through the ring is calculated analytically using the supersymmetry method and the quantum fluctuation effects are numerically investigated in detail. We find that the conductance is determined by the competition between the mean and fluctuation parts. The dephasing effect acts on the fluctuation part only. The Breit-Wigner resonant peak is changed to an antiresonance by increasing the ratio of the level broadening to the mean level spacing of the random cavity, and the asymmetric Fano form turns into a symmetric one. For the orthogonal and symplectic ensembles, the period of the Aharonov-Bohm oscillations is half of that for regular systems. The conductance distribution function becomes independent of the ensembles at the resonant point, which can be understood by the mode-locking mechanism. We also discuss the relation of our results to the random walk problem.Comment: 13 pages, 9 figures; minor change

Large diamagnetic persistent currents

In multichannel rings, evanescent modes will always co-exist with propagating modes. The evanescent modes can carry a very large diamagnetic persistent current that can oscillate with energy and are very sensitive to impurity scattering. This provides a natural explanation for the large diamagnetic persistent currents observed in experiments.Comment: 5 figure

Spin separation in cyclotron motion

Charged carriers with different spin states are spatially separated in a two-dimensional hole gas. Due to strong spin-orbit interaction holes at the Fermi energy have different momenta for two possible spin states travelling in the same direction and, correspondingly, different cyclotron orbits in a weak magnetic field. Two point contacts, acting as a monochromatic source of ballistic holes and a narrow detector in the magnetic focusing geometry are demonstrated to work as a tunable spin filter.Comment: 4 pages, 2 figure

Sign Reversals of ac Magnetoconductance in Isolated Quantum Dots

We have measured the electromagnetic response of micron-size isolated mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them to an electromagnetic micro-resonator. Both dissipative and non dissipative responses exhibit a large magnetic field dependent quantum correction, with a characteristic flux scale which corresponds to a flux quantum in a dot. The real (dissipative) magnetoconductance changes sign as a function of frequency for low enough density of electrons. The signal observed at frequency below the mean level spacing corresponds to a negative magnetoconductance, which is opposite to the weak localization seen in connected systems, and becomes positive at higher frequency. We propose an interpretation of this phenomenon in relation to fundamental properties of energy level spacing statistics in the dots.Comment: 4 pages, 4 eps figure