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Register a new userZero-bias transport anomaly in metallic nanobridges: Magnetic field dependence and universal conductance fluctuations
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We present data of transport measurements through a metallic nanobridge exhibiting diffusive electron transport. A logarithmic temperature dependence and a zero-bias anomaly in the differential conductance are observed, independent of magnetic field. The data can be described by a single scaling law. The theory of electron-electron interaction in disordered systems, adapted to the case of finite-size systems in non-equilibrium, yields quantitative agreement with experiment. Measurements of universal conductance functuations support the assumptions of the theory about the electronic phase coherence.
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We have observed interaction effects in the differential conductance $G$ of short, disordered metal bridges in a well-controlled non-equilibrium situation, where the distribution function has a double Fermi step. A logarithmic scaling law is found both for the temperature and for the voltage dependence of $G$ in all samples. The absence of magnetic field dependence and the low dimensionality of our samples allow us to distinguish between several possible interaction effects, proposed recently in nanoscopic samples. The universal scaling curve is explained quantitatively by the theory of electron-electron interaction in diffusive metals, adapted to the present case, where the sample size is smaller than the thermal diffusion length.
A one-dimensional semiconductor nanowire proximitized by a nearby superconductor may become a topological superconductor hosting localized Majorana zero modes at the two wire ends in the presence of spin-orbit coupling and Zeeman spin splitting (arising from an external magnetic field). The hallmark of the presence of such Majorana zero modes is the appearance of a zero-temperature quantized zero-bias conductance peak in the tunneling spectroscopy of the Majorana nanowire. We theoretically study the temperature and the tunnel coupling dependence of the tunneling conductance in such nanowires to understand possible intrinsic deviations from the predicted conductance quantization. We find that the full temperature and the tunneling transmission dependence of the tunnel conductance does not obey any simple scaling relation, and estimating the zero-temperature conductance from finite-temperature and finite-tunnel-broadening tunneling data is difficult in general. A scaling relation, however, does hold at the extreme weak-tunneling low-temperature limit where the conductance depends only on the dimensionless ratio of the temperature and tunnel broadening. We also consider the tunneling contributions from nontopological Andreev bound states which may produce almost-zero-bias conductance peaks, which are not easy to distinguish from the Majorana-induced zero-bias peaks, finding that the nontopological almost-zero-modes associated with Andreev bound states manifest similar temperature and transmission dependence as the topological Majorana modes. We comment on the Zeeman splitting dependence of the zero-bias conductance peak for finite temperature and tunnel coupling.
The zero-bias conductance peak in d-wave superconductors splits in an applied magnetic field. In this work, the experimentally observed universal relation delta ~ B0^(1/2) for strip-shaped samples is derived analytically based on the long-ranged current contributions from Abrikosov vortices inside the sample. The result is in full agreement with observed key properties, and features such as hysteresis effects are made accessible. Employing a magnetically induced additional order parameter is not necessary for the physical explanation of the universal relation.
Magnetic resonance is a widely-established phenomenon that probes magnetic properties such as magnetic damping and anisotropy. Even though the typical resonance frequency of a magnet ranges from gigahertz to terahertz, experiments also report the resonance near zero frequency in a large class of magnets. Here we revisit this phenomenon by analyzing the symmetry of the system and find that the resonance frequency ($omega$) follows a universal power law $omega varpropto |H-H_c|^p$, where $H_c$ is the critical field at which the resonance frequency is zero. When the magnet preserves the rotational symmetry around the external field ($H$), $p = 1$. Otherwise, $p=1/2$. The magnon excitations are gapped above $H_c$, gapless at $H_c$ and gapped again below $H_c$. The zero frequency is often accompanied by a reorientation transition in the magnetization. For the case that $p=1/2$, this transition is described by a Landau theory for second-order phase transitions. We further show that the spin current driven by thermal gradient and spin-orbit effects can be significantly enhanced when the resonance frequency is close to zero, which can be measured electrically by converting the spin current into electric signals. This may provide an experimentally accessible way to characterize the critical field. Our findings provide a unified understanding of the magnetization dynamics near the critical field, and may, furthermore, inspire the study of magnon transport near magnetic transitions.
Graphene provides a fascinating testbed for new physics and exciting opportunities for future applications based on quantum phenomena. To understand the coherent flow of electrons through a graphene device, we employ a nanoscale probe that can access the relevant length scales - the tip of a liquid-He-cooled scanning probe microscope (SPM) capacitively couples to the graphene device below, creating a movable scatterer for electron waves. At sufficiently low temperatures and small size scales, the diffusive transport of electrons through graphene becomes coherent, leading to universal conductance fluctuations (UCF). By scanning the tip over a device, we map these conductance fluctuations textit{vs.} scatterer position. We find that the conductance is highly sensitive to the tip position, producing $delta G sim e^2/h$ fluctuations when the tip is displaced by a distance comparable to half the Fermi wavelength. These measurements are in good agreement with detailed quantum simulations of the imaging experiment, and demonstrate the value of a cooled SPM for probing coherent transport in graphene.
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