Towards understanding two-level-systems in amorphous solids -- Insights from quantum circuits

Amorphous solids show surprisingly universal behaviour at low temperatures. The prevailing wisdom is that this can be explained by the existence of two-state defects within the material. The so-called standard tunneling model has become the established framework to explain these results, yet it still leaves the central question essentially unanswered -- what are these two-level defects? This question has recently taken on a new urgency with the rise of superconducting circuits in quantum computing, circuit quantum electrodynamics, magnetometry, electrometry and metrology. Superconducting circuits made from aluminium or niobium are fundamentally limited by losses due to two-level defects within the amorphous oxide layers encasing them. On the other hand, these circuits also provide a novel and effective method for studying the very defects which limit their operation. We can now go beyond ensemble measurements and probe individual defects -- observing the quantum nature of their dynamics and studying their formation, their behaviour as a function of applied field, strain, temperature and other properties. This article reviews the plethora of recent experimental results in this area and discusses the various theoretical models which have been used to describe the observations. In doing so, it summarises the current approaches to solving this fundamentally important problem in solid-state physics.Comment: 34 pages, 7 figures, 1 tabl

Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published versio

Quantum sensors for microscopic tunneling systems

The anomalous low-temperature properties of glasses arise from intrinsic excitable entities, so-called tunneling Two-Level-Systems (TLS), whose microscopic nature has been baffling solid-state physicists for decades. TLS have become particularly important for micro-fabricated quantum devices such as superconducting qubits, where they are a major source of decoherence. Here, we present a method to characterize individual TLS in virtually arbitrary materials deposited as thin-films. The material is used as the dielectric in a capacitor that shunts the Josephson junction of a superconducting qubit. In such a hybrid quantum system the qubit serves as an interface to detect and control individual TLS. We demonstrate spectroscopic measurements of TLS resonances, evaluate their coupling to applied strain and DC-electric fields, and find evidence of strong interaction between coherent TLS in the sample material. Our approach opens avenues for quantum material spectroscopy to investigate the structure of tunneling defects and to develop low-loss dielectrics that are urgently required for the advancement of superconducting quantum computers

Non-equilibrium Phonon Generation and Detection in Microstructure Devices

We demonstrate a method to excite locally a controllable, non-thermal distribution of acoustic phonon modes ranging from 0 to ∼200 GHz in a silicon microstructure, by decay of excited quasiparticle states in an attached superconducting tunnel junction (STJ). The phonons transiting the structure ballistically are detected by a second STJ, allowing comparison of direct with indirect transport pathways. This method may be applied to study how different phonon modes contribute to the thermal conductivity of nanostructuresThe authors thank R. B. Van Dover, J. Blakely, S. Baker, K. Schwab, and Cornell LASSP for loan of key equipment, and L. Spietz for photolithography recipes. We thank R. B. Van Dover, K. Schwab, E. Smith, J. Parpia, D. Ralph, B. Plourde, M. Blencowe, D. Westly, R. Pohl, P. Berberich, and C. Mellor for helpful discussions and thank D. Toledo, J. Chang and A. Lin for help with apparatus. The authors acknowledge funding from the National Science Foundation (NSF) (DMR 0520404) and Department of Energy (DOE) (DE-SC0001086). This publication is based on work supported in part by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). This work was performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECS-0335765

A Room-Temperature Ferroelectric Resonant Tunneling Diode

Resonant tunneling is a quantum-mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum-well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room-temperature ferroelectric-modulated resonant tunneling and negative differential resistance (NDR) behaviors in all-perovskite-oxide BaTiO3/SrRuO3/BaTiO3 QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO3 ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2 × 104. The observed NDR effect is explained an energy bandgap between Ru-t2g and Ru-eg orbitals driven by electron–electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric-based quantum-tunneling devices in future oxide electronics

Magnetoresistive and Thermoresistive Scanning Probe Microscopy with Applications in Micro- and Nanotechnology

This work presents approaches to extend limits of scanning probe microscopy techniques towards more versatile instruments using integrated sensor concepts. For structural surface analysis, magnetoresistive sensing is introduced and thermoresistive sensing is applied to study nanoscale phonon transport in chain-like molecules. Investigating with these techniques the properties of shape memory polymers, a fabrication method to design application-inspired micro- and nanostructures is introduced

Superconductor

This book contains a collection of works intended to study theoretical and experimental aspects of superconductivity. Here you will find interesting reports on low-Tc superconductors (materials with Tc 30 K). Certainly this book will be useful to encourage further experimental and theoretical researches in superconducting materials

Electrical detection of spin state switching in electromigrated nanogap devices

Spin crossover is an effect shown in some transition metal complexes where the spin state of the molecule undergoes a transition from a low spin to a high spin state via the application of light, pressure or a change in temperature. This behaviour makes these complexes an attractive candidate to form electronic molecular-scale switches as the electrical resistance of the compound differs between the two spin states. Although the spin crossover effect is commonly studied in its bulk form, the integration of a single molecule into a solid-state device while maintaining the magnetic bi-stability is highly desirable, but remains challenging. This is not only due to difficulties in capturing a single molecule between electrodes and making electrical connections but it is also due to the strong coupling effects imparted on the molecule by the high-density metallic states of the electrodes that can prevent the spin transition from occurring.In recent years there have been many attempts at studying spin crossover complexes at a single molecule level. Many of these have used scanning tunneling microscopy or break junction techniques. While these studies have highlighted the unique and promising electronic properties of these compounds, these techniques are unsuitable for real world devices. This thesis demonstrates a means to make electrical contact to single or small numbers of molecules between gold electrodes fabricated using a bilayer nanoimprint lithography and a feedback controlled electromigration method. This method, enabling high throughput and low-cost fabrication is potentially suitable for scaling to large area planar devices and as such may be used for commercially producing molecular devices.To validate the quality of the nanogaps, devices containing self-assembled monolayers of benzenethiol were first studied. The shape and magnitude of I-V curves measured on nanogap devices containing the benzenethiol monolayers are in good agreement with previously published work using similar molecules in mechanically controlled break junctions. The resulting I-V characteristics were analyzed using the single level resonant tunneling model as well as transition voltage spectroscopy and are consistent with transport through molecular junctions in which the benzenethiol molecules are - stacked. These highly conducting molecular junctions may have potential uses for “soft” coupling to sensitive target molecules.Following validation of the molecular nanojunction fabrication and measurement process, the experimental work shifted to studying electronic transport through spin crossover complexes with a focus on Schiff-base compounds that are specifically tailored for surface deposition. In the case of measurements made on the bulk compound, a sharp spin transition centered at a temperature around 80 K was observed, while a shift to lower temperatures was found for thin films of the complex. In contrast, nanojunction devices containing single molecules displayed very different behaviour, with distinct and reproducible telegraphic-like switching between two resistance states when cooled below 160 K. These two states are attributed to the two different spin states of the complex. The presence of these two resistive states indicates that the spin crossover is preserved at the single molecule level and that a spin-state dependent tunneling process is taking place. Interestingly, in some cases a multi-level switching behaviour is detected with four possible conductance states. This behaviour is attributed to the presence of two spin crossover molecules in the nanogap

Towards Oxide Electronics:a Roadmap

At the end of a rush lasting over half a century, in which CMOS technology has been experiencing a constant and breathtaking increase of device speed and density, Moore's law is approaching the insurmountable barrier given by the ultimate atomic nature of matter. A major challenge for 21st century scientists is finding novel strategies, concepts and materials for replacing silicon-based CMOS semiconductor technologies and guaranteeing a continued and steady technological progress in next decades. Among the materials classes candidate to contribute to this momentous challenge, oxide films and heterostructures are a particularly appealing hunting ground. The vastity, intended in pure chemical terms, of this class of compounds, the complexity of their correlated behaviour, and the wealth of functional properties they display, has already made these systems the subject of choice, worldwide, of a strongly networked, dynamic and interdisciplinary research community. Oxide science and technology has been the target of a wide four-year project, named Towards Oxide-Based Electronics (TO-BE), that has been recently running in Europe and has involved as participants several hundred scientists from 29 EU countries. In this review and perspective paper, published as a final deliverable of the TO-BE Action, the opportunities of oxides as future electronic materials for Information and Communication Technologies ICT and Energy are discussed. The paper is organized as a set of contributions, all selected and ordered as individual building blocks of a wider general scheme. After a brief preface by the editors and an introductory contribution, two sections follow. The first is mainly devoted to providing a perspective on the latest theoretical and experimental methods that are employed to investigate oxides and to produce oxide-based films, heterostructures and devices. In the second, all contributions are dedicated to different specific fields of applications of oxide thin films and heterostructures, in sectors as data storage and computing, optics and plasmonics, magnonics, energy conversion and harvesting, and power electronics