Showing papers on "Ballistic conduction published in 2019"
TL;DR: It is demonstrated that the nearly perfectly hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport with enhanced electron self-focusing effects, suggesting a novel class of ballistic electronic devices exploiting the unique transport characteristics of strongly faceted Fermani surfaces.
Abstract: Geometric electron optics may be implemented in solids when electron transport is ballistic on the length scale of a device. Currently, this is realized mainly in 2D materials characterized by circular Fermi surfaces. Here we demonstrate that the nearly perfectly hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport. We probe this directional ballistic regime in a single crystal of PdCoO2 by use of focused ion beam (FIB) micro-machining, defining crystalline ballistic circuits with features as small as 250 nm. The peculiar hexagonal Fermi surface naturally leads to enhanced electron self-focusing effects in a magnetic field compared to circular Fermi surfaces. This super-geometric focusing can be quantitatively predicted for arbitrary device geometry, based on the hexagonal cyclotron orbits appearing in this material. These results suggest a novel class of ballistic electronic devices exploiting the unique transport characteristics of strongly faceted Fermi surfaces. Ballistic electron beams in clean metals can be focused by passing currents through well designed contraptions, which is mostly done in isotropic materials described by a circular Fermi surface. Here, the authors demonstrate that the almost hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport with enhanced electron self-focusing effects.
123 citations
TL;DR: It is shown that the transport properties of BP device under high electric field can be improved greatly by the interface engineering of high-quality HfLaO dielectrics and transport orientation and by designing the device channels along the lower effective mass armchair direction.
Abstract: As a strong candidate for future electronics, atomically thin black phosphorus (BP) has attracted great attention in recent years because of its tunable bandgap and high carrier mobility. Here, we show that the transport properties of BP device under high electric field can be improved greatly by the interface engineering of high-quality HfLaO dielectrics and transport orientation. By designing the device channels along the lower effective mass armchair direction, a record-high drive current up to 1.2 mA/μm at 300 K and 1.6 mA/μm at 20 K can be achieved in a 100-nm back-gated BP transistor, surpassing any two-dimensional semiconductor transistors reported to date. The highest hole saturation velocity of 1.5 × 107 cm/s is also achieved at room temperature. Ballistic transport shows a record-high 36 and 79% ballistic efficiency at room temperature and 20 K, respectively, which is also further verified by theoretical simulations.
63 citations
TL;DR: An analytic family of quasilocal conservation laws that break the spin-reversal symmetry and compute a lower bound on the spin Drude weight, which is found to be a fractal function of the anisotropy parameter.
Abstract: We demonstrate ballistic spin transport of an integrable unitary quantum circuit, which can be understood either as a paradigm of an integrable periodically driven (Floquet) spin chain, or as a Trotterized anisotropic (XXZ) Heisenberg spin-1/2 model. We construct an analytic family of quasilocal conservation laws that break the spin-reversal symmetry and compute a lower bound on the spin Drude weight, which is found to be a fractal function of the anisotropy parameter. Extensive numerical simulations of spin transport suggest that this fractal lower bound is in fact tight.
48 citations
TL;DR: In this article, the magnetic field dependence of the supercurrent follows a clear Fraunhofer-like pattern, and Shapiro steps appear upon microwave irradiation, and multiple Andreev reflections give rise to conductance enhancement.
Abstract: We fabricate Josephson field-effect transistors in germanium quantum wells contacted by superconducting aluminum and demonstrate supercurrents carried by holes that extend over junction lengths of several micrometers. In superconducting quantum point contacts we observe discretization of supercurrent, as well as Fabry-Perot resonances, demonstrating ballistic transport. The magnetic field dependence of the supercurrent follows a clear Fraunhofer-like pattern, and Shapiro steps appear upon microwave irradiation. Multiple Andreev reflections give rise to conductance enhancement and evidence a transparent interface, confirmed by analyzing the excess current. These demonstrations of ballistic superconducting transport are promising for hybrid quantum technology in germanium. A© 2019 American Physical Society.
31 citations
TL;DR: Transport between two semi-infinite solvable models is studied and it is shown that a slowly-relaxing region forms around the integrability-breaking junction.
Abstract: Transport phenomena are central to physics, and transport in the many-body and fully-quantum regime is attracting an increasing amount of attention. It has been recently revealed that some quantum spin chains support ballistic transport of excitations at all energies. However, when joining two semi-infinite ballistic parts, such as the XX and XXZ spin-1/2 models, our understanding suddenly becomes less established. Employing a matrix-product-state ansatz of the wavefunction, we study the relaxation dynamics in this latter case. Here we show that it takes place inside a light cone, within which two qualitatively different regions coexist: an inner one with a strong tendency towards thermalization, and an outer one supporting ballistic transport. We comment on the possibility that even at infinite time the system supports stationary currents and displays a non-zero Kapitza boundary resistance. Our study paves the way to the analysis of the interplay between transport, integrability, and local defects.
29 citations
TL;DR: A Metal-Sown Selective Area Growth (MS SAG) technique which allows decoupling selective deposition and nucleation growth conditions by temporarily isolating these stages and demonstrates that complex InSb nanowire networks of high crystal and electrical quality can be achieved this way.
Abstract: Selective area growth is a promising technique to realize semiconductor-superconductor hybrid nanowire networks, potentially hosting topologically protected Majorana-based qubits. In some cases, however, such as the molecular beam epitaxy of InSb on InP or GaAs substrates, nucleation and selective growth conditions do not necessarily overlap. To overcome this challenge, we propose a metal-sown selective area growth (MS SAG) technique, which allows decoupling selective deposition and nucleation growth conditions by temporarily isolating these stages. It consists of three steps: (i) selective deposition of In droplets only inside the mask openings at relatively high temperatures favoring selectivity, (ii) nucleation of InSb under Sb flux from In droplets, which act as a reservoir of group III adatoms, done at relatively low temperatures, favoring nucleation of InSb, and (iii) homoepitaxy of InSb on top of the formed nucleation layer under a simultaneous supply of In and Sb fluxes at conditions favoring selectivity and high crystal quality. We demonstrate that complex InSb nanowire networks of high crystal and electrical quality can be achieved this way. We extract mobility values of 10000-25000 cm2 V-1 s-1 consistently from field-effect and Hall mobility measurements across single nanowire segments as well as wires with junctions. Moreover, we demonstrate ballistic transport in a 440 nm long channel in a single nanowire under a magnetic field below 1 T. We also extract a phase-coherent length of ∼8 μm at 50 mK in mesoscopic rings.
28 citations
TL;DR: An optical method is used to probe heat conduction at submicrometer length scales, allowing for direct observation of thermal phonons with mean free paths up to 200 nm in semicrystalline polyethylene films using transient grating spectroscopy, yielding insights into the microscopic origins of their high thermal conductivity.
Abstract: Thermally conductive polymer crystals are of both fundamental and practical interest for their high thermal conductivity that exceeds that of many metals In particular, polyethylene fibers and oriented films with uniaxial thermal conductivity exceeding 50 W ⋅ m − 1 ⋅ K − 1 have been reported recently, stimulating interest into the underlying microscopic thermal transport processes While ab initio calculations have provided insight into microscopic phonon properties for perfect crystals, such properties of actual samples have remained experimentally inaccessible Here, we report the direct observation of thermal phonons with mean free paths up to 200 nm in semicrystalline polyethylene films using transient grating spectroscopy Many of the mean free paths substantially exceed the crystalline domain sizes measured using small-angle X-ray scattering, indicating that thermal phonons propagate ballistically within and across the nanocrystalline domains; those transmitting across domain boundaries contribute nearly one-third of the thermal conductivity Our work provides a direct determination of thermal phonon propagation lengths in molecular solids, yielding insights into the microscopic origins of their high thermal conductivity
27 citations
TL;DR: In this paper, a generalized hydrodynamic theory for dynamics in random integrable systems, including diffusive corrections due to disorder, was derived and used to study nonequilibrium energy and spin transport.
Abstract: We study the nonequilibrium dynamics of random spin chains that remain integrable (i.e., solvable via Bethe ansatz): because of correlations in the disorder, these systems escape localization and feature ballistically spreading quasiparticles. We derive a generalized hydrodynamic theory for dynamics in such random integrable systems, including diffusive corrections due to disorder, and use it to study nonequilibrium energy and spin transport. We show that diffusive corrections to the ballistic propagation of quasiparticles can arise even in noninteracting settings, in sharp contrast to clean integrable systems. This implies that operator fronts broaden diffusively in random integrable systems. By tuning parameters in the disorder distribution, one can drive this model through an unusual phase transition, between a phase where all wave functions are delocalized and a phase in which low-energy wave functions are quasilocalized (in a sense we specify). Both phases have ballistic transport; however, in the quasilocalized phase, local autocorrelation functions decay with an anomalous power law, and the density of states diverges at low energy.
25 citations
TL;DR: In this article, the authors demonstrate that the nearly perfectly hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport, which can be quantitatively predicted for arbitrary device geometry, based on the hexagonal cyclotron orbits appearing in this material.
Abstract: Geometric electron optics may be implemented in solid state when transport is ballistic on the length scale of a device. Currently, this is realized mainly in 2D materials characterized by circular Fermi surfaces. Here we demonstrate that the nearly perfectly hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport. We probe this directional ballistic regime in a single crystal of PdCoO2 by use of focused ion beam (FIB) micro-machining, defining crystalline ballistic circuits with features as small as 250nm. The peculiar hexagonal Fermi surface naturally leads to electron self-focusing effects in a magnetic field, well below the geometric limit associated with a circular Fermi surface. This super-geometric focusing can be quantitatively predicted for arbitrary device geometry, based on the hexagonal cyclotron orbits appearing in this material. These results suggest a novel class of ballistic electronic devices exploiting the unique transport characteristics of strongly faceted Fermi surfaces.
23 citations
TL;DR: In this article, the authors showed that the thermal resistance of graphene on SiO2 is increased by one order of magnitude by the addition of a top layer of MoS2, over the temperature range of 150-300 K, providing pathways for increasing the efficiency of thermoelectric applications using van der Waals materials.
Abstract: Nanoscale scanning thermal microscopy (SThM) transport measurements from cryogenic to room temperature on 2D structures with sub 30 nm resolution are reported. This novel cryogenic operation of SThM, extending the temperature range of the sample down to 150 K, yields a clear insight into the nanothermal properties of the 2D nanostructures and supports the model of ballistic transport contribution at the edge of the detached areas of exfoliated graphene which leads to a size-dependent thermal resistance of the detached material. The thermal resistance of graphene on SiO2 is increased by one order of magnitude by the addition of a top layer of MoS2, over the temperature range of 150–300 K, providing pathways for increasing the efficiency of thermoelectric applications using van der Waals (vdW) materials. Density functional theory calculations demonstrate that this increase originates from the phonon transport filtering in the weak vdW coupling between the layers and the vibrational mismatch between MoS2 and graphene layers.
21 citations
TL;DR: In this article, the trajectory of the out-of-equilibrium electron population in the low-bandgap semiconductor InSb was followed using reflective cross-polarized 2D THz time-domain spectroscopy in the range of 1-12 THz, showing that the nonlinear response in the first picoseconds is dominated by coherent ballistic motion of the electrons.
Abstract: Using reflective cross-polarized 2D THz time-domain spectroscopy in the range of 1-12 THz, we follow the trajectory of the out-of-equilibrium electron population in the low-bandgap semiconductor InSb. The 2D THz spectra show a set of distinct features at combinations of the plasma-edge and vibration frequencies. Using finite difference time domain simulations combined with a tight binding model of the band structure, we assign these features to electronic nonlinearities and show that the nonlinear response in the first picoseconds is dominated by coherent ballistic motion of the electrons. We demonstrate that this technique can be used to investigate the landscape of the band curvature near the Γ-point, as illustrated by the observation of anisotropy in the (100)-plane.
TL;DR: In this article, the authors show that by adding only two fitting parameters to a purely ballistic transport model, they can accurately characterize the currentvoltage characteristics of nanoscale MOSFETs.
Abstract: We show that by adding only two fitting parameters to a purely ballistic transport model, we can accurately characterize the current-voltage characteristics of nanoscale MOSFETs. The model is an extension of Natori’s model and includes transmission probability and drain-channel coupling parameter. The latter parameter gives rise to a theoretical RON that is significantly larger than those predicted previously. To validate our model, we fabricated n-channel MOSFETs with varying channel lengths. We show the length dependence of these parameters to support a quasi-ballistic description of our devices.
TL;DR: In this article, a thermally induced exchange reaction of single-crystalline Ge/Si core/shell nanowires and lithographically defined Al contact pads was used to construct a self-aligned quasi-one-dimensional crystalline Al leads with contact transparencies greater than 96%.
Abstract: Semiconductor-superconductor hybrid systems have outstanding potential for emerging high-performance nanoelectronics and quantum devices. However, critical to their successful application is the fabrication of high-quality and reproducible semiconductor-superconductor interfaces. Here, we realize and measure axial Al-Ge-Al nanowire heterostructures with atomically precise interfaces, enwrapped by an ultrathin epitaxial Si layer further denoted as Al-Ge/Si-Al nanowire heterostructures. The heterostructures were synthesized by a thermally induced exchange reaction of single-crystalline Ge/Si core/shell nanowires and lithographically defined Al contact pads. Applying this heterostructure formation scheme enables self-aligned quasi one-dimensional crystalline Al leads contacting ultrascaled Ge/Si segments with contact transparencies greater than 96%. Integration into back-gated field-effect devices and continuous scaling beyond lithographic limitations allows us to exploit the full potential of the highly transparent contacts to the 1D hole gas at the Ge-Si interface. This leads to the observation of ballistic transport as well as quantum confinement effects up to temperatures of 150 K. Low-temperature measurements reveal proximity-induced superconductivity in the Ge/Si core/shell nanowires. The realization of a Josephson field-effect transistor allows us to study the subgap structure caused by multiple Andreev reflections. Most importantly, the absence of a quantum dot regime indicates a hard superconducting gap originating from the highly transparent contacts to the 1D hole gas, which is potentially interesting for the study of Majorana zero modes. Moreover, underlining the importance of the proposed thermally induced Al-Ge/Si-Al heterostructure formation technique, our system could contribute to the development of key components of quantum computing such as gatemon or transmon qubits.
TL;DR: In this paper, the authors explored metal-based air-channel transistors with coplanar gates with different air channel distances and showed that the devices can exhibit field-enhanced features, but will eventually be field-lowered with increasing gate potential.
Abstract: Air-channel transistors can exhibit ballistic transport and immunity from carrier scattering if the channel distance is smaller than the mean free path (MFP) of electrons in an ambient environment. This study explored metal-based air-channel transistors with coplanar gates with different air-channel distances. The results indicate that the devices can exhibit field-enhanced features, but will eventually be field-lowered with increasing gate potential because the coplanar gates attract and intercept a portion of electron emission. The air-channel distance determines the device conductivity with or without a gate electric field, whereas the gate-to-gate distance determines the interception of electron emission. These devices may be utilized as inverters for vacuum transistors.
TL;DR: In this paper, the authors investigated the ballistic transport in normal metal/graphene/superconductor junctions in edge contact geometry and showed that the transmission probabilities follow the normal state conductance highlighting the interplay between the Andreev processes and the electronic interferometer.
Abstract: We report the study of ballistic transport in normal metal/graphene/superconductor junctions in edge-contact geometry. While in the normal state, we have observed Fabry-P\'erot resonances suggesting that charge carriers travel ballistically, the superconducting state shows that the Andreev reflection at the graphene/superconductor interface is affected by these interferences. Our experimental results in the superconducting state have been analyzed and explained with a modified Octavio-Tinkham-Blonder-Klapwijk model taking into account the magnetic pair-breaking effects and the two different interface transparencies, i.e., between the normal metal and graphene, and between graphene and the superconductor. We show that the transparency of the normal metal/graphene interface strongly varies with doping at large scale, while it undergoes weaker changes at the graphene/superconductor interface. When a cavity is formed by the charge transfer occurring in the vicinity of the contacts, we see that the transmission probabilities follow the normal state conductance highlighting the interplay between the Andreev processes and the electronic interferometer.
TL;DR: In this article, the authors performed 19 electromechanical tests of platinum nanocontacts with in situ transmission electron microscopy to measure contact size and conductance, and they found that the ballistic transport equations under-predict the contact size by more than an order of magnitude.
Abstract: Conductive modes of atomic force microscopy are widely used to characterize the electronic properties of materials, and in such measurements, contact size is typically determined from current flow. Conversely, in nanodevice applications, the current flow is predicted from the estimated contact size. In both cases, it is very common to relate the contact size and current flow using well-established ballistic electron transport theory. Here we performed 19 electromechanical tests of platinum nanocontacts with in situ transmission electron microscopy to measure contact size and conductance. We also used molecular dynamics simulations of matched nanocontacts to investigate the nature of contact on the atomic scale. Together, these tests show that the ballistic transport equations under-predict the contact size by more than an order of magnitude. The measurements suggest that the low conductance of the contact cannot be explained by the scattering of electrons at defects nor by patchy contact due to surface roughness; instead, the lower-than-expected contact conductance is attributed to approximately a monolayer of insulating surface species on the platinum. Surprisingly, the low conductance persists throughout loading and even after significant sliding of the contact in vacuum. We apply tunneling theory and extract best-fit barrier parameters that describe the properties of this surface layer. The implications of this investigation are that electron transport in device-relevant platinum nanocontacts can be significantly limited by the presence and persistence of surface species, resulting in current flow that is better described by tunneling theory than ballistic electron transport, even for cleaned pure-platinum surfaces and even after loading and sliding in vacuum.
TL;DR: increasing further the magnetic field allows us to probe the Landau level spectrum in the constriction and unveil distortions due to the combination of confinement and deconfinement of Landau levels in a saddle potential.
Abstract: We report on the evolution of the coherent electronic transport through a gate-defined constriction in a high-mobility graphene device from ballistic transport to quantum Hall regime upon increasing the magnetic field At a low field, the conductance exhibits Fabry–Perot resonances resulting from the npn cavities formed beneath the top-gated regions Above a critical field B* corresponding to the cyclotron radius equal to the npn cavity length, Fabry–Perot resonances vanish, and snake trajectories are guided through the constriction with a characteristic set of conductance oscillations Increasing further the magnetic field allows us to probe the Landau level spectrum in the constriction and unveil distortions due to the combination of confinement and deconfinement of Landau levels in a saddle potential These observations are confirmed by numerical calculations
TL;DR: In this article, it was shown that the SiC polytype of a diamond-edge graphene ribbons does not determine the ballistic conductivity of the ribbon's edge states, but it is assumed that the edge states are associated with asymmetric edge terminations.
Abstract: Zigzag-edge graphene sidewall ribbons grown on $6H$-SiC ${11\overline{2}n}$ facet walls are ballistic conductors. It is assumed that graphene sidewall ribbons grown on $4H$-SiC ${11\overline{2}n}$ facets would also be ballistic. In this work, we show that SiC polytype indeed matters: ballistic sidewall graphene ribbons only grow on $6H$-SiC facets. $4H$ and $4H$-passivated sidewall graphene ribbons are diffusive conductors. Detailed photoemission and microscopy studies show that $6H$-SiC sidewall zigzag ribbons are metallic with a pair of $n$-doped edge states associated with asymmetric edge terminations. In contrast, $4H$-SiC zigzag ribbons are strongly bonded to the SiC, severely distorting the ribbon's $\ensuremath{\pi}$ bands. ${\text{H}}_{2}$ passivation of the $4H$ ribbons returns them to a metallic state but they show no evidence of edge states in their photoemission-derived band structure.
TL;DR: It is demonstrated that nanowires are an ideal platform for studies of the Aharonov-Bohm effect of neutral and charged excitons, as they facilitate the controlled fabrication of nearly ideal quantum rings by combining all-binary radial heterostructures with axial crystal-phase quantum structures.
Abstract: Phase coherence in nanostructures is at the heart of a wide range of quantum effects such as Josephson oscillations between exciton-polariton condensates in microcavities, conductance quantization in 1D ballistic transport, or the optical (excitonic) Aharonov-Bohm effect in semiconductor quantum rings. These effects only occur in structures of the highest perfection. The 2D semiconductor heterostructures required for the observation of Aharonov-Bohm oscillations have proved to be particularly demanding, since interface roughness or alloy fluctuations cause a loss of the spatial phase coherence of excitons, and ultimately induce exciton localization. Experimental work in this field has so far relied on either self-assembled ring structures with very limited control of shape and dimension or on lithographically defined nanorings that suffer from the detrimental effects of free surfaces. Here, it is demonstrated that nanowires are an ideal platform for studies of the Aharonov-Bohm effect of neutral and charged excitons, as they facilitate the controlled fabrication of nearly ideal quantum rings by combining all-binary radial heterostructures with axial crystal-phase quantum structures. Thanks to the atomically flat interfaces and the absence of alloy disorder, excitonic phase coherence is preserved even in rings with circumferences as large as 200 nm.
TL;DR: In this article, a comprehensive analytical theory of one-dimensional Kondo lattices with isotropic exchange interaction between electrons and KI was proposed and compared with previously obtained numerical results and discussed possible experiments where the theory could be tested.
Abstract: We study one-dimensional Kondo Lattices (KL) which consist of itinerant electrons interacting with Kondo impurities (KI) - localized quantum magnetic moments. We focus on KL with isotropic exchange interaction between electrons and KI and with a high KI density. The latter determines the principal role of the indirect interaction between KI for the low energy physics. Namely, the Kondo physics becomes suppressed and all properties are governed by spin ordering. We present a first-ever comprehensive analytical theory of such KL at an arbitrary doping and predict a variety of regimes with different electronic phases. They range from commensurate insulators (at filling factors 1/2, 1/4 and 3/4) to metals with strongly interacting conduction electrons (close to these three special cases) to an exotic phase of a helical metal. The helical metals can provide a unique platform for realization of an emergent protection of ballistic transport in quantum wires. We compare out theory with previously obtained numerical results and discuss possible experiments where the theory could be tested.
TL;DR: In this article, a novel implementation of the cryo-etching method, which enabled us to fabricate graphene nanoconstrictions encapsulated between hexagonal boron nitride thin films with unprecedented control of the structure edges, was reported.
Abstract: More than a decade after the discovery of graphene, ballistic transport in nanostructures based on this intriguing material still represents a challenging field of research in two-dimensional electronics. The presence of rough edges in nanostructures based on this material prevents the appearance of truly ballistic electron transport as theo\-re\-tically predicted and, therefore, not well-developed plateaus of conductance have been revealed to date. In this work we report on a novel implementation of the cryo-etching method, which enabled us to fabricate graphene nanoconstrictions encapsulated between hexagonal boron nitride thin films with unprecedented control of the structure edges. High quality smooth nanometer-rough edges are characterized by atomic force microscopy and a clear correlation between low roughness and the existence of well-developed quantized conductance steps with the concomitant occurrence of ballistic transport is found at low temperature. In par\-ti\-cu\-lar, we come upon exact 2$e^{2}/h$ quantization steps of conductance at zero magnetic field due to size quantization, as it has been theoretically predicted for truly ballistic electron transport through graphene nanoconstrictions.
TL;DR: A physically valid quasi-ballistic drain current model applicable for nanoscale symmetric Double Gate (SDG) MOSFETs and shown to be continuous in terms of terminal charges and capacitances in all regions of operation.
Abstract: This paper presents a physically valid quasi-ballistic drain current model applicable for nanoscale symmetric Double Gate (SDG) MOSFETs. The proposed drain current model includes both diffusive and ballistic transport phenomena. The model considers the important positional carrier scattering dependency effect near the source region described in terms of transmission and reflection co-efficients related to the scattering theory. The significance of carrier transport near the bottleneck source region is illustrated where the carriers diffuse into the channel at a relatively lower velocity before accelerating ballistically. The results obtained demonstrate carrier scattering dependency at the critical layer defined near the low field source region on the drain current characteristics. The proposed model partly evolves from Natori’s ballistic bulk MOSFET model that is modified accordingly to be valid for a symmetric Double Gate MOSFET in the nanoscale regime. Carrier degeneracy and Fermi–Dirac statistics are included in the work so as to justify the complete physicality of the model. The model is further extended and is shown to be continuous in terms of terminal charges and capacitances in all regions of operation. A comparative analysis is also done between the proposed quasi-ballistic model and a hypothetical complete ballistic device.
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TL;DR: In this paper, an artificial Kronig-Penney-like superlattice potential can be imposed on a few-mode electron waveguides to deform the electronic subbands into a manifold of new subbands with magnetically-tunable fractional conductance.
Abstract: The paradigm of electrons interacting with a periodic lattice potential is central to solid-state physics. Semiconductor heterostructures and ultracold neutral atomic lattices capture many of the essential properties of 1D electronic systems. However, fully one-dimensional superlattices are highly challenging to fabricate in the solid state due to the inherently small length scales involved. Conductive atomic-force microscope (c-AFM) lithography has recently been demonstrated to create ballistic few-mode electron waveguides with highly quantized conductance and strongly attractive electron-electron interactions. Here we show that artificial Kronig-Penney-like superlattice potentials can be imposed on such waveguides, introducing a new superlattice spacing that can be made comparable to the mean separation between electrons. The imposed superlattice potential "fractures" the electronic subbands into a manifold of new subbands with magnetically-tunable fractional conductance (in units of $e^2/h$). The lowest $G=2e^2/h$ plateau, associated with ballistic transport of spin-singlet electron pairs, is stable against de-pairing up to the highest magnetic fields explored ($|B|=16$ T). A 1D model of the system suggests that an engineered spin-orbit interaction in the superlattice contributes to the enhanced pairing observed in the devices. These findings represent an important advance in the ability to design new families of quantum materials with emergent properties, and mark a milestone in the development of a solid-state 1D quantum simulation platform.
TL;DR: In this article, the authors reported theoretical investigations of ballistic quantum transport properties of smoothly bent semiconducting single-walled carbon nanotubes (SWCNTs) and showed that the smooth bent induces a small electron redistribution on the SWCNT arc, which leads to rich and major transport differences between the NxN and PxP tubes.
TL;DR: Ohtomo et al. as mentioned in this paper demonstrated that Dirac-cone-like bands emerge at the 1D zigzag interface of a ZnO/MoS2 lateral heterostructure (LHS), creating a highly mobile 1D transport channel with a high Fermi velocity of 4.5×105m/s.
Abstract: The discovery of high-mobility electron gas at the two-dimensional (2D) LaAlO3/SrTiO3 heterointerface [A. Ohtomo and H. Hwang, Nature (London) 427, 423 (2004)10.1038/nature02308] has attracted great research interest due to the ballistic transport of spatially confined electrons. When such kind of valence discontinuity extends to the interface of a well-chosen in-planar 2D heterostructure, a one-dimensional (1D) confined interface can be generated. Herein, through first-principles modeling, we demonstrate that Dirac-cone-like bands emerge at the 1D zigzag interface of a ZnO/MoS2 lateral heterostructure (LHS), creating a highly mobile 1D transport channel with a high Fermi velocity of 4.5×105m/s. The metallic state at the 1D heterointerface is attributed to the polar discontinuity that introduces excess charge carriers. Nontrivial polarization (excess carriers appear at boundaries) results in a large built-in electric field within the ZnO and MoS2 monolayers. Remarkably, the ultrafast 1D conducting channel is independent of the arrangements of width (m and n) in the (ZnO)m(MoS2)n LHS, which are further illustrated by tight-binding-based orbital and polarization analysis.
TL;DR: In this article, a ballistic transport experiment using high-quality graphene that revealed Fermi surface anisotropy in the magnetoresistance was described. But the results were limited to two-dimensional (2D) materials.
Abstract: Monolayer graphene and bilayer graphene have strikingly different properties. One such difference is the shape of the Fermi surface. Although anisotropic band structures can be detected in optical measurements, they have so far been difficult to detect in transport experiments on twodimensional materials. Here we describe a ballistic transport experiment using high-quality graphene that revealed Fermi surface anisotropy in the magnetoresistance. The shape of the Fermi surface is closely related with the cyclotron orbit in real space. Electron trajectories in samples with triangular lattices of holes depend on the anisotropy of the Fermi surface. We found that this results in the magnetoresistance which are dependent on crystallographic orientation of the antidot lattice, which indicates the anisotropic Fermi surface of bilayer graphene which is a trigonally-warped circle in shape. While in monolayer, shape of magnetoresistance was approximately independent of the orientation of antidot lattice, which indicates that the Fermi surface is a circle in shape. The ballistic transport experiment is a new method of detecting anisotropic electronic band structures in two-dimensional electron systems.
TL;DR: In this paper, the fabrication of both antidot lattices and unidirectional stripe patterns upon molecular beam epitaxy grown MgZnO and ZnO heterostructures was reported.
Abstract: We report the fabrication of both antidot lattices and unidirectional stripe patterns upon molecular beam epitaxy grown MgZnO/ZnO heterostructures. The magnetoresistance of these high mobility devices exhibits commensurability oscillations associated with ballistic transport of carriers executing orbital motion within the geometry of the imposed modulation.
TL;DR: In this paper, a topology optimization method for the volume-to-point (VP) heat conduction problem is proposed, which is based on phonon Boltzmann transport equation (BTE) and solid isotropic material with penalization (SIMP) method.
Abstract: The volume-to-point (VP) heat conduction problem is one of the fundamental problems of cooling for electronic devices. The existed reports about the VP problem are mainly based on the Fourier’s law which works well at the macroscopic scale. However, the length scale of modern electronic devices has reduced to micro- and nano-scale, at which optimization methods that are capable of dealing with the non-Fourier heat conduction are desired now. In this paper, phonon Boltzmann transport equation (BTE) and solid isotropic material with penalization (SIMP) method are coupled to develop a topology optimization method for ballistic-diffusive heat conduction. Phonon BTE is transformed into equation of phonon radiative transport, which is solved by the discrete ordinate method. To realize the topology optimization, SIMP method is adopted to penalize the phonon extinction coefficient, which equals to the reciprocal of phonon mean-free-path, and an explicit constraint on the global gradient of the nominal material density is used to ensure the solutions being well-posed and mesh-independent. By using the developed topology optimization method, it is found that the optimal material distributions for the VP problem in ballistic-diffusive heat conduction significantly deviate from the traditional tree-like structure obtained in diffusive heat conduction, and the results vary with the Knudsen number (Kn). This is related to the different coefficient interpolation ways in the SIMP method and phonon ballistic transport. When Kn → 0, instead of converging to the conventional tree-like structure which fully stretches into the interior zone, the new method gradually produces the result obtained by the topology optimization which interpolates the reciprocal of the thermal conductivity in diffusive heat conduction. As Kn increases, the high thermal-conductive filling materials show a trend to gather around the low-temperature boundary, and there are more thick and strong trunk structures, less tiny and thin branch structures in the optimized material distributions. In addition, the ratio of the optimized average temperature to the value of the uniform material distribution \begin{document}$\left( {T_{{\rm{ave}},{\rm{opt}}}^{\rm{*}}/T_{{\rm{ave}},{\rm{uni}}}^{\rm{*}}} \right)$\end{document} also increases. The dependence of the topology optimization results on Kn can be attributed to the size effect of the thermal conductivity caused by phonon ballistic transport. In the diffusive heat conduction, filling materials with different length scales have the same efficiency to build high thermal-conductive channels. However, with ballistic effect enhancing, size effect makes the effective thermal conductivities of the branch structure lower than those of the trunk structure, as the former is smaller than the latter. As a result, the branch structures are less efficient compared with the trunk structures in terms of building high thermal-conductive channels, and the optimal material distributions have more trunk structures and fewer branch structures. When the ballistic effect becomes significant enough, say at Kn = 0.1, the topology optimization gets a dough-like material distribution in which branches merge into trunks. The proposed topology optimization method have the potential to provide guidance in designing nanoscale electronic devices for improving the heat dissipation capability.
TL;DR: In this article, the authors proposed a theory that explains the observed activation nature of that transport, its exponential non-ohmicity, variations between nominally identical structures, and failure trends.
Abstract: In spite of extensive research, the physics of electric transport in the OFF (high resistance) state of resistive random access memory remains poorly understood. Here, we propose a theory that explains the observed activation nature of that transport, its exponential non-ohmicity, variations between nominally identical structures, and failure trends. Our theory proceeds from the finding that the diffusion transport mechanisms must be ruled out as leading to nonphysically high temperatures ( $T\sim 4000-5000$ K) well above melting. The Schottky emission remains acceptable due to its underlying ballistic transport by ~1 eV electrons spreading the Joule heat over much larger distances without melting. Taking into account, the random fields of charged defects the Schottky emission enables one to describe the device variability and failures.