Session L24: Mesoscopic Materials and Devices I

Manipulating edge transport in quantum anomalous Hall insulators

The quantum anomalous Hall (QAH) effect provides a path to obtain dissipation-less, one-dimensional (1D) edge states at zero magnetic field. It’s recent experimental realization in magnetic topological insulator thin films lies at the overlap of several areas of condensed matter physics: dilute magnetic semiconductors, low dimensional electron transport and topologically non-trivial material systems. In this talk, we demonstrate how careful compositional and electrical tuning of epitaxial films of Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ enables access to a robust zero-field quantized Hall effect, despite sample roughness [1] and low carrier mobility. In samples that show near-dissipation-less transport, we manipulate the intermixing between edge states and dissipative channels via a tilted-field crossover from ballistic 1D edge transport to diffusive transport [2]. This crossover manifests in a gate-tunable giant anisotropic magneto-resistance effect that we use as a quantitative probe of dissipation in our systems. Finally, we discuss experiments with mesoscopic channels of QAH insulator thin films, and discuss the effect of their modified magnetic anisotropy on edge transport. This work was carried out in collaboration with A. Richardella, C-X Liu, M. Liu, W. Wang, N. P. Ong, and N. Samarth. [1] A. Richardella, A. Kandala et. al APL Materials 3 (8), 083303 (2015) [2] A. Kandala, A. Richardella et. al. Nature Commun. 6:7434 (2015)   

Spin-orbit Josephson $\phi_{\mathrm{0}}$-junction in nanowire quantum dots

The Josephson effect describes supercurrent flowing through a junction connecting two superconducting leads by a thin barrier[1]. This current is driven by a superconducting phase difference $\phi $ between the leads and it is strictly zero when $\phi $ vanishes, due to the chiral and time reversal symmetry of the Cooper pair tunneling process[2]. Only if these underlying symmetries are broken the supercurrent for $\phi \quad =$ 0 may be finite[3]. This corresponds to a ground state of the junction being offset by a phase $\phi_{\mathrm{0}}$. Here, for the first time, we report such Josephson $\phi_{\mathrm{0}}$-junction. Our realization is based on a nanowire quantum dot. We use a quantum interferometer device in order to investigate phase offsets and demonstrate that $\phi_{\mathrm{0}}$ can be controlled by electrostatic gating. Our results have possible far reaching implications for superconducting flux and phase defined quantum bits as well as for exploring topological superconductivity in quantum dot systems. 1. Josephson, \textit{Phys. Lett.} \textbf{1,} 251--253 (1962). 2. Yip, S.-K., De Alcantara Bonfim, O. F. {\&} Kumar, P., \textit{Phys. Rev. B} \textbf{41,} 11214--11228 (1990). 3. Zazunov, A., Egger, R., Jonckheere, T. {\&} Martin, T.,\textit{Phys. Rev. Lett.} \textbf{103,} 147004 (2009).   

Controlling the magnetic state of a carbon nanotube Josephson junction with the superconducting phase.

The Kondo effect is a many-body phenomenon that screens the magnetic moment of an impurity in a metal. The associated singlet state can be probed in a single impurity by electronic transport in a quantum dot (QD), here made of a carbon nanotube (CNT), which provides a localized electron between the two contacts. Using superconducting leads, one can investigate the competition between the Kondo effect and the superconductivity induced in the CNT. The superconductivity can destroy the Kondo singlet in favor of a magnetic doublet, leading to a sign reversal of the supercurrent in the S-CNT-S junction. This singlet-doublet transition depends on the Kondo temperature and the superconducting gap, as well as the position of the impurity level. We demonstrate experimentally that the superconducting phase difference across the QD can also control this magnetic transition. We use the measurement of the relation between the supercurrent and this superconducting phase as a tool to probe the transition. We show that it has a distinctly anharmonic behavior, that reveals the phase-mediated singlet to doublet transition, in good agreement with finite temperature quantum Monte Carlo calculations. We extract as well a phase diagram of the phase-controlled quantum transition at zero temperature.   

Majorana fermions in topological Yu-Shiba-Rusinov chains and lattices with or without spin-orbit interaction.

Recent spin polarized scanning tunneling microscopy (SPSTM) experiments in magnetic chains (S. Nadj-Perge et al., Science 2014) opened new routes for detecting the elusive Majorana fermions (MFs). Within the deep Yu-Shiba-Rusinov (YSR) limit we calculate [1] the spatially resolved tunneling conductance of topological ferromagnetic chains [2] measured by means of SPSTM. Our analysis reveals novel signatures of MFs arising from the interplay of their strongly anisotropic spin-polarization and the magnetization content of the tip. We investigate the occurrence and evolution of zero/finite bias peaks for a single or two coupled chains forming a Josephson junction, when a preexisting chiral symmetry controlling the number of MFs per chain edge is preserved or weakly broken. We also reveal alternative pathways for engineering MFs without spin-orbit interaction (SOI). On one hand, we highlight that antiferromagnetic YSR chains become topological by inducing an artificial SOI using external fields [3], while on the other, we pursue mechanisms for stabilizing magnetic textures and topological YSR lattices [4] following the self-organization principle for topological spiral chains [5]. [1] P. Kotetes et al., Physica E 74, 614 (2015), [2] A. Heimes, D. Mendler, and P. Kotetes, New J. Phys. 17 023051 (2015), [3] A. Heimes, P. Kotetes, and G. Sch\"{o}n,PRB 90, 060507(R) (2014), [4] M. Schecter, P. Kotetes, K. Flensberg, and J. Paaske, [5] M. Schecter et al., arXiv:1509.07399.   

Suppressed Conductance From Spin Selection Rules in F-CNT-F Quantum Dots

Conductance through a quantum dot can be suppressed due to spin selection rules governing the hoping of an additional electron onto an already-occupied quantum dot. Measurements of this effect in a carbon nanotube quantum dot with ferromagnetic contacts will be presented. Suppressed conductance peaks are observed in the Coulomb diamond plots at zero field and explained using spin selection rules. The pattern of suppressed peaks is observed to change with applied magnetic field as the spin ground state of the occupied quantum dot changes.   

Hybrid Quantum Point Contact-Superconductor Devices Using InSb Nanowires

Recent experiments using hybrid nanowire (NW)-superconductor (SC) devices have provided evidence for Majorana quasiparticles in tunneling experiments [1,2]. However, these tunneling experiments are marked by a soft superconducting gap, which likely originates from disorder at the NW-SC interface [3]. Hence, clean NW-SC interfaces are important for future Majorana studies. By carefully processing the NW-SC interface, we have realized quantized conductance steps in quantum point contacts fabricated from InSb NWs and superconducting contacts. We study the length dependence of ballistic behavior and the induced superconductivity in InSb NWs by quantum point contact spectroscopy. Additionally, we discuss how the transport in InSb NW-SC quantum point contacts evolves in magnetic field. \newline \newline References: 1. V. Mourik et al, Science 336, 1003 (2012). 2. A. Das et al, Nature Phys. 8, 887 (2012). 3. S. Takei et al, Phys. Rev. Lett. 110, 186803 (2013).   

Flat band of midgap rotating surface states in 3D Dirac and Weyl semimetals under circularly polarized radiation

We report the investigation of novel surface states which develop when 3D Dirac or Weyl semimetals are placed under circularly polarized electromagnetic radiation. We find that a gap opens up from the hybridization between Floquet side bands, which leads to the appearance of midgap surface states in the form of evanescent waves decaying from the surface exposed to the radiation. We observe a phenomenon reminiscent of Landau quantization by which the midgap surface states get a large degeneracy proportional to the radiation flux traversing the surface of the semimetal. We show that all these surface states carry angular current, leading to a modulation of their charge that rotates with the same frequency of the radiation, which should manifest in the observation of a macroscopic chiral current in the irradiated surface.   

Quantum coherence of mesoscopic stadia and wires coupled to the environment

Quantum phase coherence was measured in quasi-1D wires and in mesoscopic stadia connected to wide sample regions by wire-like necks, to investigate the effects of environmental and interdevice coupling on decoherence as contrasted to intrinsic materials properties. Measurements of quantum phase coherence lengths used weak-antilocalization on nanolithographic InGaAs/InAlAs structures at 390 mK. For quantum wires, experiments show that longer wire lengths result in longer phase coherence lengths. The result is understood from the observation that longer wires average out decoherence introduced at the end sections by environmental coupling. For stadia with quantum-wire-like necks, stadium-wire coupling dominates decoherence in the stadia, rather than environmental coupling. Stadia with wider and shorter necks show longer phase coherence lengths. The result is understood from the observation that wider and shorter wires are geometrically similar to stadia, implying a stronger wave function hybridization between stadia and connecting necks and thus weaker decoherence from stadium-wire coupling. The work shows that geometry has to be taken into account in measured mesoscopic coherence.   

Spin-polarized conductance anomalies in one-dimensional channels

We explain the emergence of conductance anomalies ($\sim0.3G_0$ and $\sim0.7G_0$, $G_0=2e^2/h$) in one-dimensional channels by using an unrestricted Hartree-Fock approach with a three-dimensional Coulomb interaction. The latter predicts the onset of a pair of degenerate spin-polarized configurations (or channels), with specific conductance above a concentration-dependent threshold. The $0.3G_0$ anomaly is a consequence of the 1D nature of the carrier density of states at the conductance onset, which weakens with temperature. Meanwhile, the second anomaly manifests itself as shoulders in the quantum conductance at the onset of concentration-dependent spin polarization, and becomes more pronounced as the temperature increases above 0 K, in agreement with experimental results. Our model also explains the dependence of the anomalies on drain and gate biases, longitudinal magnetic field, and channel length.   

An interacting adiabatic quantum motor

We consider the effect of electron-electron interactions on the performance of an adiabatic quantum motor based on a Thouless pump operating in reverse. We model such a device by electrons in a 1d wire coupled to a slowly moving periodic potential associated with the classical mechanical degree of freedom of the motor. This periodic degree of freedom is set into motion by a bias voltage applied to the 1d electron channel. We investigate the Thouless motor with interacting leads modeled as Luttinger liquids. We show that interactions enhance the energy gap opened by the periodic potential and thus the robustness of the Thouless motor against variations in the chemical potential. We show that the motor degree of freedom can be described as a mobile impurity in a Luttinger liquid obeying Langevin dynamics with renormalized coefficients due to interactions, for which we give explicit expressions.   

The Quantum Pinch Effect in Semiconducting Quantum Wires

We investigate a two-component, cylindrical, quasi-one-dimensional quantum plasma subjected to a {\em radial} confining harmonic potential and an applied magnetic field in the symmetric gauge. It is demonstrated that such a system as can be realized in semiconducting quantum wires offers an excellent medium for observing the quantum pinch effect at low temperatures. An exact analytical solution of the problem allows us to make significant observations: surprisingly, in contrast to the classical pinch effect, the particle density as well as the current density display a {\em determinable} maximum before attaining a minimum at the surface of the quantum wire. The effect will persist as long as the equilibrium pair density is sustained. Therefore, the technological promise that emerges is the route to the precise electronic devices that will control the particle beams at the nanoscale$^{1}$. 1. M.S. Kushwaha, Appl. Phys. Lett. {\bf 103}, 173116 (2013).   

Control of Nanofilament Structure and Observations of Quantum Point Contact Behavior in Ni/NiO Nanowire Junctions

Resistive switching is ideal for use in non-volatile memory where information is stored in a metallic or insulating state. Nanowire junctions formed at the intersection of two Ni/NiO core/shell nanowires have emerged as a leading candidate structure where resistive switching occurs due to the formation and destruction of conducting filaments. However, significant knowledge gaps remain regarding the conduction mechanisms as measurements are typically only performed at room temperature. Here, we combine temperature-dependent current-voltage (IV) measurements from 15 - 300 K with magnetoresistance studies and achieve new insight into the nature of the conducting filaments. We identify a novel semiconducting state that behaves as a quantum point contact and find evidence for a possible electric-field driven phase transition. The insulating state exhibits unexpectedly complex IV characteristics that highlight the disordered nature of the ruptured filament while we find clear signs of anisotropic magnetoresistance in the metallic state. Our results expose previously unobserved behaviors in nanowire resistive switching devices and pave the way for future applications where both electrical and magnetic switching can be achieved in a single device.   

Puzzling electron behavior analogous to the Braess paradox in~a mesoscopic network

A counterintuitive behavior analogous to the Braess paradox is encountered in a two-terminal mesoscopic network patterned in a two-dimensional electron system (2DES) [1]. Decreasing locally the electron density of one channel in the network paradoxically leads to an increased network conductance. Our scanning gate microscopy experiments reveals this puzzling conductance variation, thanks to tip-induced localized modifications of electron flow throughout the network's channels at low temperature, in the ballistic and coherent regime of transport. We compare the amplitude of the measured anomalous conductance variation with~conductance changes induced by other mechanisms at play in the mesoscopic network,~such as interference phenomena between different paths, and Coulomb blockade due to disorder-induced localized states. The robustness of this puzzling behavior is inspected by varying the global 2DES density, magnetic field and temperature. [1] M. G. Pala~\textit{et al.}, Phys. Rev. Lett. 108, 076802 (2012).