Session P05: Nanostructures, Heterostructures and other Artificially Structured Materials

The Electronic Transport Properties of Silicon Meta-lattice Made by High Pressure CVD

A nanoscale 3D superlattice, called meta-lattice, can be synthesized by infiltrating a template of close-packed nanometer-scale silica spheres with Si by high pressure chemical vapor deposition. Their structures can be controlled by using different size spheres; accordingly, their electronic transport properties are geometry-dependent, and both localized and extended electronic states in three dimensions may exist, hence both band conduction and variable range hoping (VRH) may occur simultaneously. Metalattices offer a platform to continuously change properties of electronic transport by changing their geometries. We employ a tight-binding method to quantitatively look into the impact of geometry. The resultant electronic band structure is also used in the case of VRH conductance.
  

Observation of Exposed and Hidden Au Quantum Well States in Au/Ag Heterostructures

In this work, atomically uniform Au films are grown on a Ag(111) thin film. The Au quantum well states (QWS) are clearly observed by using Angle-resolved photoemission spectroscopy (ARPES). The Au QWSs hybridize with the Ag states and thus, show an energy dependence on the thickness of the supporting Ag film. Furthermore, a sandwich structure of Ag/Au/Ag is fabricated with various thicknesses of each constituent layer. The Au QWSs, though buried under the Ag overlayer, are still observable through hybridization with the Ag states. Interestingly, the hybrid quantum well states with Au d orbitals states possess a large effective mass as a result of hybridization with partially localized d states. Detailed first-principles simulations are performed to illustrate the orbital components of hybrid QWSs and provide an insight into the band dispersion of hybrid states.   

On the formation of suspended double-stranded atomic chains

The smallest structures with higher stability connecting two metallic electrodes can be identified by the evolution of the conductance in the process of the controlled breakage of the contact between two metals. Here we report on the identification of double-stranded atomic chains formed on Au using a Scanning Tunneling Microscope. Molecular dynamics simulations and density functional theory electron transport calculations help to unveil the richness of geometric structures possible with double-stranded chains, their stability and how these can evolve to form monoatomic chains.   

Universal Conductance Fluctuation in Sierpinski Carpet

We report a theoretical investigation on conductance fluctuation of disordered two-terminal device in Sierpinski carpet. We find that the conductance fluctuation does not display a universal feature in circular orthogonal ensemble. But in the presence of magnetic field or spin-obit coupling, a universal conductance fluctuation (UCF) appears. More specify, in the presence of magnetic field, the UCF value is about 0.74 ± 0.01 (e2/h), while when applied spin-orbit coupling the UCF value exists also around 0.74 ± 0.01 (e2/h). By further investigating the conductance distribution in the critical disorder strength, we find that both of the symmetries share the same distribution function. Our work provide a better understanding of the transport properties of regular fractal structure and shed light on more transport studies of fractals.   

Experimental demonstration of nanoelectronics below 1 mK electron temperature

Extending the experimental temperature domain of nanoelectronic devices into the microkelvin regime would make novel quantum states of matter accessible with great potential for quantum technologies. Cooling bulk metals to microkelvin temperatures by adiabatic demagnetization of nuclear spins has been an established technique for decades, however reaching electron temperatures below 1 mK in nanofabricated devices remained an unsolved challenge up until now. This is due to the thermal decoupling of electrons in micro- and nanostructures from the cold substrate in combination with high frequency electronic noise directly heating the electron system. We demonstrate the first successful nuclear magnetic cooling of electrons in a nanofabricated device to microkelvin temperatures, achieved by integrating indium as nuclear refrigerant onto the metallic islands of a Coulomb blockade thermometer with the device leads attached to bulk indium nuclear cooling stages. By performing the nuclear demagnetization process, we demonstrate electron temperatures below 500 μK by primary electron thermometry, and show that the device stays below 1 mK for several days.   

Trapped Ion Heating from Correlated Motion of Electrode Adsorbates

Trapped ion qubits are plagued by a phenomenon called “anomalous heating,” where motional modes of the ion are excited by the environment causing decoherence of the encoded quantum information. Evidence suggests the origin of this heating is associated with the surface of the trapping electrodes. In this work, we combine first-principles electronic structure calculations, molecular dynamics simulations, and master equations to explore the role of high densities of adsorbed molecules on the electrode surface as a potential source of trapped ion heating. Weakly adsorbed molecules interact with the metallic surface and acquire an induced dipole, which can couple to the ion, causing electric field noise. When a large density of adsorbates are present, on the order of a monolayer, their collective vibrational motion can create non-trivial frequency-dependent electric field noise and thus ion heating. We analyze the coverage dependence of this effect for different possible electrode adsorbates.   

Toward 2D Fermi-Hubbard Quantum Simulation with and Oxide Nanoelectronic Platform

The interface of LaAlO₃/SrTiO₃ supports a 2D electron gas [1] that can be further reconfigured into nanostructures, using conductive AFM lithography [2]. The density of nanostructures (~2 nm) is comparable to the mean electron separation, giving rise to the idea that this platform could be used for quantum simulation of 2D Fermi-Hubbard problems. Here we describe efforts to create various 2D lattice structures, and investigate their properties at low temperatures and high magnetic fields.

[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004).
[2] C. Cen, et al., Nature Materials 7, 298 (2008).   

Charge transfer, orbital polarization, and magnetism of (LaCoO3)n+(LaTiO3)n (n=1,2) superlattice

The perovskite LaCoO₃ has earned much attention because of the many relevant spin states of the Co³⁺ cation (ranging from S=0 to S=2 and their combinations). When LaCoO₃ forms a superlattice with other transition metal oxides, the combination of charge transfer, quantum confinement, strain and structural distortions can lead to interesting changes of its properties. We have studied systematically the electronic and magnetic properties of LaCoO₃+LaTiO₃ (LCO+LTO) superlattices using density functional theory + U calculations. We find that one electron (formal charge) is transferred from Ti to Co, resulting in Co²⁺ (d⁷) in the LCO+LTO superlattice. There are two relevant spin states of the Co²⁺: (i) Low-spin (LS) S=0.5, and (ii) high-spin (HS) S=1.5; and mixtures of HS and LS across the different Co²⁺ cations can give other effective spin states such as S=1. Unlike Co³⁺ in LaCoO₃ or Co²⁺ in CoO, both the HS and LS in the superlattice have strong orbital polarization. We will describe the energetic stability of HS versus LS and its U dependence, the magnetic ordering of the HS and LS ground states, how antiferromagnetic and ferromagnetic superexchange interactions underlie the computed orderings.
  

Doping dependence of phonon anomalies in (Bi,Pb)2(Sr,La)2CuO6 revealed by inelastic X-ray scattering

While charge modulations are ubiquitous in high-temperature superconducting cuprates, understanding their coupling to phonons can provide crucial insight into the role of electron-phonon interactions in high-temperature superconductivity. Here, we have used high-resolution inelastic X-ray scattering to study the doping dependence of the low-energy phonons in the monolayer cuprate (Bi,Pb)₂(Sr,La)₂CuO_6+d, from heavily underdoped (p=0.03) to overdoped (p=0.21) samples. We observe that the longitudinal acoustic phonon along the Cu-O bond direction exhibits a line-shape broadening near the wave vector ~ 0.25 rlu in underdoped samples even when charge order is suppressed, while no broadening occurs in the overdoped samples. Surprisingly, an additional low energy mode around 4 meV is observed in the underdoped samples and is absent in overdoped materials. The origin of the longitudinal phonon broadening and the new low energy mode will be discussed especially in relation to charge order.   

High Tc Superconductor-Half metallic ferromagnet planar devices

Superconducting spintronics is an emergent field where the interfacial interactions between superconductors (S) and ferromagnets (F) are crucial. These interactions can give rise to equal-spin triplet Cooper pairs, thereby opening the door to coherent, dissipationless spin transport. In this scenario, the case of high-temperature d-wave superconductors (e.g. YBa₂Cu₃O₇) combined with half-metal ferromagnets (e.g. La_0.7Sr_0.3MnO₃) is especially interesting. One of the challenges specific to those materials is the fabrication of nanoscale planar devices in which multiple S/F interfaces can be concatenated and electrically sensed individually. The difficulty comes from the fact that complex-oxides are often incompatible with many of the bottom-up nanofabrication approaches that are standard with metals. We will describe here various top-bottom approaches we have developed ad hoc to fabricate oxide planar S/F nanodevices, which are a combination of Pulsed Laser Deposition, Electron Beam Lithography and Ion Beam Etching. We will also present temperature-dependent magnetotransport measurements performed to characterize their superconducting and magnetic properties.   

The Formation of Wannier Mott-Frenkel Hybrid Excitonic Polariton In Different Heterostructures

It has been suggested theoretically and realized experimentally that combining organic material and inorganic semiconductors in one heterostructure would result in resonant interactions between the Frenkel excitons in the organic material and the Wannier-Mott excitons in the semiconductors, leading to the formation of an exciton hybridization state. The new materials, possessing the complimentary characteristics of both exciton types, such as large exciton radius, enormous oscillator strength and room-temperature operation properties, would enhance optical nonlinearities and promise to have useful applications in both the field of Bose-Einstein condensation of polaritons and polariton lasers. In this work, we consider a strong coupling of the hybrid excitons and photons near excitonic resonance analytically with the purpose of determining the electronic structure, energy, and dispersion relation of the hybrid exciton-polariton. We study different confinement parameters for various nano-scale heterostructures, and in doing so, we discuss the conditions necessary for their formation. Our ab initio approach moves us a step closer to realizing new, novel optoelectrical materials that exhibit the strengths of each constituent.   

X-ray Optics Fabrication Using Unorthodox Approaches

X-ray microscopes are unique tools for studying buried features of biological and magnetic systems with high spatiotemporal resolution. The limitations of the standard e-beam lithography method for fabricating these delicate nano-photonic devices can be overcome by using unconventional lithography methods such as direct-write and gray-scale ion beam lithography, ion beam implantation lithography, a combination of atomic layer deposition and focused ion beam micromachining and last but not least by using two-photon photopolymerization (2PP). For instance, bottom-up growth of atomic layer deposition allows atomic scale control over the zone width while top-down micro-machining using focused ion beams allow freely selectable aspect ratios and tilt angles. In another approach, we took advantage of 2PP and fabricated high aspect ratio diffractive/refractive kinoforms out of low-loss polymeric materials for the first time. We will discuss the benefits of these methods over the conventional fabrication routes based on theoretical coupled wave theory calculations, direct imaging experiments as well as ptychographic coherent diffractive imaging.
  

Observation of Bulk Polarization Transitions and Second-Order Acoustic Corner States Protected by Generalized Chiral Symmetry

A new class of topological lattice characterized by bulk polarization has been introduced recently, such system has been shown to host Wannier-type higher order states. Here, we introduce and measure topological bulk polarization in 3D printed two-dimensional acoustic meta-structures, and observe topological transitions as the design parameters are tuned. We also demonstrate that our topological meta-structure hosts both 1D edge and Wannier-type second-order corner states with unique acoustic properties. The edge states have the angular momentum that reverses for opposite propagation direction, thus supporting directional excitation. We observe the second order topological states protected by the generalized chiral symmetry of the meta-structure, which are localized at the corners and are pinned to ‘zero energy’. Interestingly, unlike the corner states protected by the conventional chiral symmetry, the generalized chiral symmetry of our three-atom sublattice enables their spectral overlap with the continuum of bulk states without leakage. The confinement and inherent robustness of the corner states is theoretically analyzed and experimentally confirmed by deliberately introducing disorder. Our findings open new directions for advanced sound propagation and manipulation.