3D Conduction Research Articles

A new quasi-one-dimensional compound Ba3TiTe5 and superconductivity induced by pressure

We report systematic studies of a new quasi-one-dimensional (quasi-1D) compound, Ba3TiTe5, and the high-pressure induced superconductivity therein. Ba3TiTe5 was synthesized at high pressure and high temperature. It crystallizes into a hexagonal structure (P63/mcm), which consists of infinite face-sharing octahedral TiTe6 chains and Te chains along the c axis, exhibiting a strong 1D characteristic structure. The first-principles calculations demonstrate that Ba3TiTe5 is a well-defined 1D conductor; thus, it can be considered a starting point to explore the exotic physics induced by pressure by enhancing the interchain hopping to move the 1D conductor to a high-dimensional metal. For Ba3TiTe5, high-pressure techniques were employed to study the emerging physics dependent on interchain hopping, such as the Umklapp scattering effect, spin/charge density wave (SDW/CDW), superconductivity and non-Fermi liquid behavior. Finally, a complete phase diagram was plotted. The superconductivity emerges at 8.8 GPa, near which the Umklapp gap is mostly suppressed. Tc is enhanced and reaches a maximum of ~6 K at ~36.7 GPa, where the SDW/CDW is completely suppressed, and a non-Fermi liquid behavior appears. Our results suggest that the appearance of superconductivity is associated with the fluctuation due to the suppression of the Umklapp gap and that the enhancement of the Tc is related to the fluctuation of the SDW/CDW.

Cross-line elements for free element method in thermal and mechanical analyses of functionally gradient materials

In this paper, a new type of finite elements, called as Cross-Line Elements (CLEs), are constructed, which have fewest nodes to interpolate physical variables in both two-dimensional (2D) and three-dimensional (3D) problems. These CLEs are then used in a new mesh free method, the Free Element Method (FREM), for solving general 2D and 3D boundary value problems of partial differential equations. FREM is a strong-form element collocation method, combining the advantages of the finite element method and mesh free method in the aspects of setting up shape functions and generating computational meshes through node by node. The distinct feature of FREM is that only one independent element is needed for each collocation node and the element can be freely formed by the nodes surrounding the collocation node. A few numerical examples for 2D and 3D heat conduction and solid mechanics problems will be given to validate the correctness and demonstrate the potential of the constructed elements and proposed numerical method.

Scaling-up Atomically Thin Coplanar Semiconductor-Metal Circuitry via Phase Engineered Chemical Assembly.

Two-dimensional (2D) layered semiconductors, with their ultimate atomic thickness, have shown promise to scale down transistors for modern integrated circuitry. However, the electrical contacts that connect these materials with external bulky metals are usually unsatisfactory, which limits the transistor performance. Recently, contacting 2D semiconductors using coplanar 2D conductors has shown promise in reducing the problematic high contact resistance. However, many of these methods are not ideal for scaled production. Here, we report on the large-scale, spatially controlled chemical assembly of the integrated 2H-MoTe2 field-effect transistors (FETs) with coplanar metallic 1T'-MoTe2 contacts via phase engineered approaches. We demonstrate that the heterophase FETs exhibit ohmic contact behavior with low contact resistance, resulting from the coplanar seamless contact between 2H and 1T'-MoTe2 confirmed by transmission electron microscopy characterizations. The average mobility of the heterophase FETs was measured to be as high as 23 cm2 V-1 s-1 (comparable with those of exfoliated single crystals), due to the large 2H-MoTe2 single-crystalline domain size (486 ± 187 μm). By developing a patterned growth method, we realize the 1T'-MoTe2 gated heterophase FET array whose components of the channel, gate, and contacts are all 2D materials. Finally, we transfer the heterophase device array onto a flexible substrate and demonstrate the near-infrared photoresponse with high photoresponsivity (∼1.02 A/W). Our study provides a basis for the large-scale application of phase-engineered coplanar MoTe2 semiconductor-metal structure in advanced electronics and optoelectronics.

Weak antilocalization in Sb2Te3 nano-crystalline topological insulator

Microwave-assisted solvothermal route has been followed to synthesize Sb2Te3 nanomaterial. The morphology and topography of the material have been studied in FESEM and AFM measurements respectively. The optical band gap of the material is found to be ~ 0.25 eV as measured in IR spectroscopy. Temperature dependent resistivity of the nanostructure shows thermal activation behavior in the high temperature region as well as it follows 3D Variable Range Hopping mechanism in the moderate temperature region. Electron-electron interaction and quantum interference dominated upturn in the resistivity is observed at T < 24 K. Low field magnetoconductivity (MC) shows disorder induced weak localization (WL) which makes over to weak antilocalization (WAL) at T ≥ 4 K. MC after post annealing of the sample shows WAL behavior at T < 4 K also. Quantum correction to the low field magnetoconductivity of the material follows Hikami-Larkin-Nagaoka (HLN) equation. The obtained parameters of the HLN equation indicate that the system follows the 2D conduction mechanism and presence of a single conduction channel at low temperature. High field MR exhibits linear and positive magnetoresistance (LPMR) which suggests the survival of robust topological surface state conduction in the nanostructured bulk Sb2Te3 topological insulator.

Conductive Oxide Interfaces for Field Effect Devices

AbstractThe discovery of 2D conductivity at the interface of insulating oxides uncovered a trove of rich correlated‐electron physics, whose origins are still being debated 15 years later. In parallel to the massive research thrust into the fundamentals of these phenomena, the past decade has seen dramatic progress in harnessing this system toward practical devices. This report summarizes the physical background of the interface phenomena, and presents the recent evolution of field effect devices based on conductive oxide interfaces. The discussion is built on progressive examples of device concepts, covering a wide range of oxide material systems. The role of defects is critically interwoven throughout the narrative. The promises and hurdles toward practical large‐scale implementation of these devices are highlighted at the conclusion. This report is intended to bridge between the physics of conductive oxide interfaces and the practicalities of developing them into device technology. By presenting an overview of the state of the art, it is hoped to inspire new applications for these diverse new capabilities. The journey of conductive oxide interfaces from fundamentals to technology may be viewed as a prototype for the future maturation of other systems of emergent physics and novel materials from labs to devices.

Interlayer fractional quantum Hall effect in a coupled graphene double layer

In two-dimensional (2D) electron systems under strong magnetic fields, interactions can cause fractional quantum Hall (FQH) effects. Bringing two 2D conductors to proximity, a new set of correlated states can emerge due to interactions between electrons in the same and opposite layers. Here we report interlayer correlated FQH states in a system of two parallel graphene layers separated by a thin insulator. Current flow in one layer generates different quantized Hall signals in the two layers. This result is interpreted by composite fermion (CF) theory with different intralayer and interlayer Chern-Simons gauge-field coupling. We observe FQH states corresponding to integer values of CF Landau level (LL) filling in both layers, as well as "semi-quantized" states, where a full CF LL couples to a continuously varying partially filled CF LL. Remarkably, we also recognize a quantized state between two coupled half-filled CF LLs, attributable to an interlayer CF exciton condensate.

Empirical Electronic Polarizabilities: Deviations from the Additivity Rule. II. Structures Exhibiting Ion Conductivity

AbstractIon conductivity in minerals and compounds is associated with continuous migration paths and leads to negative total electronic polarizability deviations [Δ = (αobs−αcalc)/αobs)] up to 13%. Such deviations can be related to continuous migration paths determined in an earlier Voronoi–Dirichlet Partition (VDP) analysis of fast‐ion conductors, that is, compounds such as LiB3O5 with 1D conductivity, Li4SiO4 with 2D conductivity, and Li2SO4 with 3D conductivity. Using the Voronoi–Dirichlet (VD) method in this study, the migration paths and the dimensionality of many more compounds with negative polarizability deviations are identified. Continuous migration paths are found for the confirmed Li‐ion conductors LiFePO4 and Li3VO4 and for the potential Li‐ion conductors LiGeBO4, CsLiB6O10, Li2Ti3O7, LiMn0.8Fe0.17PO4, Li2NaPO4, and LiNaSO4. Similarly, continuous migration paths are found for the confirmed Na‐ion conductors Na4P2O7, Na2SO4, and Na6Mg(SO4)4, and for the potential Na‐ion conductors Na6Mg2(CO3)4SO4, Na2TiSiO5, NaPO3, Na3SO4F, Na6(SO4)2FCl, Na15(SO4)5F4Cl, Na21(SO4)7F6Cl, Na22K(CO3)2(SO4)9Cl, and Na2SeO4. Potassium migration paths are found for the confirmed ion conductors KVO3 and K2SO4. In contrast to the application of VD partitioning of cation conductors, VD partitioning of anion conductors leads to migration paths in (Zr,Y)O8 polyhedra in Zr0.894Y0.095Hf0.011O1.95 and (Gd,Hf)O6 and (Gd,Hf)O8 polyhedra in Gd2Hf2O7 where O2− ions are mobile.

Mechanically Flexible Conductors for Stretchable and Wearable E-Skin and E-Textile Devices.

Considerable progress in materials development and device integration for mechanically bendable and stretchable optoelectronics will broaden the application of "Internet-of-Things" concepts to a myriad of new applications. When addressing the needs associated with the human body, such as the detection of mechanical functions, monitoring of health parameters, and integration with human tissues, optoelectronic devices, interconnects/circuits enabling their functions, and the core passive components from which the whole system is built must sustain different degrees of mechanical stresses. Herein, the basic characteristics and performance of several of these devices are reported, particularly focusing on the conducting element constituting them. Among these devices, strain sensors of different types, energy storage elements, and power/energy storage and generators are included. Specifically, the advances during the past 3 years are reported, wherein mechanically flexible conducting elements are fabricated from (0D, 1D, and 2D) conducting nanomaterials from metals (e.g., Au nanoparticles, Ag flakes, Cu nanowires), carbon nanotubes/nanofibers, 2D conductors (e.g., graphene, MoS2 ), metal oxides (e.g., Zn nanorods), and conducting polymers (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate), polyaniline) in combination with passive fibrotic and elastomeric materials enabling, after integration, the so-called electronic skins and electronic textiles.

On the sources placement in the method of fundamental solutions for time-dependent heat conduction problems

The method of fundamental solutions is a more and more popular meshless method for solving boundary or initial–boundary value problems. The most important issue in this method is the determination of the positions of the source points. The accuracy of the method depends strongly on the distribution of the source points. In this paper placement of these points for transient heat conduction problems is studied. The problems are initial–boundary value problems and they are considered in a time-space domain. Because of that, the placement of the source points differs from the classical distribution of the source points for boundary values problems. In the paper, four different possible sources distributions are considered for 1D, 2D and 3D transient heat conduction problems. The results show very good accuracy in case of the source points placed in a space much bigger than the considered region, additionally with the negative time coordinate.

A Regularization Scheme for 2D Conductivity Imaging by the D-Bar Method

This article deals with the D-bar method in order to determine the conductivity distribution inside a body from the electrical measurements at the boundary. We introduce a new algorithm called regularization which allows us to get a high order error estimates. Then we prove the stability of the corresponding method and we treat the case of nonsmooth conductivities.

Structural and Electrochemical Analyses on the Transformation of CaFe2O4-Type LiMn2O4 from Spinel-Type LiMn2O4.

Lithium manganese oxides have received much attention as positive electrode materials for lithium-ion batteries. In this study, a post-spinel material, CaFe2O4-type LiMn2O4 (CF-LMO), was synthesized at high pressures above 6 GPa, and its crystal structure and electrochemical properties were examined. CF-LMO exhibits a one-dimensional (1D) conduction pathway for Li ions, which is predicted to be superior to the three-dimensional conduction pathway for these ions. The stoichiometric LiMn2O4 spinel (SP-LMO) was decomposed into three phases of Li2MnO3, MnO2, and Mn2O3 at 600 °C and then started to transform into the CF-LMO structure above 800 °C. The rechargeable capacity (Qrecha) of the sample synthesized at 1000 °C was limited to ∼40 mA h·g–1 in the voltage range between 1.5 and 5.3 V because of the presence of a small amount of Li2MnO3 phase in the sample (=9.1 wt %). In addition, the Li-rich spinels, Li[LixMn2–x]O4 with x = 0.1, 0.2, and 0.333, were also employed for the synthesis of CF-LMO. The sample prepared from x = 0.2 exhibited a Qrecha value exceeding 120 mA h·g–1 with a stable cycling performance, despite the presence of large amounts of the phases Li2MnO3, MnO2, and Mn2O3. Details of the structural transformation from SP-LMO to CF-LMO and the effect of Mn ions on the 1D conduction pathway are discussed.

Tomonaga-Luttinger Liquid in a Box: Electrons Confined within MoS2 Mirror-Twin Boundaries

Scanning tunneling microscope observations reveal for the first time the discrete energy spectrum of a truly 1D conductor, providing a crucial tool for testing the limits of the Tomonaga-Luttinger liquid theory that describes interacting electrons.

Fundamental insights to topological quantum materials: A real-space view of 13 cases by supersymmetry of valence bonds approach

We present a real-space view of one-dimensional (1D) to three-dimensional (3D) topological materials with 13 representative samples selected from each class, including 1D trans-polyacetylene, two-dimensional (2D) graphene, and 3D topological insulators, Dirac semimetals, Weyl semimetals, and nodal-line semimetals. This review is not intended to present a complete up-to-date list of publications on topological materials, nor to provide a progress report on the theoretical concepts and experimental advances, but rather to focus on an analysis based on the valence-bond model to help the readers gain a more balanced view of the real-space bonding electron characteristics at the molecular level versus the reciprocal-space band picture of topological materials. Starting from a brief review of low-dimensional magnetism with “toy models” for a 1D Heisenberg antiferromagnetic chain, 1D trans-polyacetylene and 2D graphene are found to have similar conjugated π-bond systems, and the Dirac cone is correlated with their unconventional 1D and 2D conduction mechanisms. Strain-driven and symmetry-protected topological insulators are introduced from the perspective of material preparation and valence-electron sharing in the valence-bond model analysis. The valence-bond models for the newly developed Dirac semimetals, Weyl semimetals, and nodal line semimetals are examined with more emphasis on the bond length and electron sharing, which is found to be consistent with the band picture. The real-space valence-bond analysis of topological materials with a conjugated π-bond system suggests that these topological materials must be classified with concepts borrowed from group theory and topology, so that a supersymmetry may absorb the fluctuating broken symmetry. Restoration of a thermodynamic system with higher entropy (i.e., the lower Gibbs free energy) is more appropriate to describe such topological materials instead of the traditional material classification with the lowest enthalpy for the presumed rigid crystal structure.

Transition to the quantum hall regime in InAs nanowire cross-junctions

We present a low-temperature electrical transport study on four-terminal ballistic InAs nanowire cross-junctions in magnetic fields aligned perpendicular to the cross-plane. Two-terminal longitudinal conductance measurements between opposing contact terminals reveal typical 1D conductance quantization at zero magnetic field. As the magnetic field is applied, the 1D bands evolve into hybrid magneto-electric sub-levels that eventually transform into Landau levels for the widest nanowire devices investigated (width = 100 nm). Hall measurements in a four-terminal configuration on these devices show plateaus in the transverse Hall resistance at high magnetic fields that scale with (ve2/h)−1. e is the elementary charge, h denotes Planck’s constant and v is an integer that coincides with the Landau level index determined from the longitudinal conductance measurements. While the 1D conductance quantization in zero magnetic field is fragile against disorder at the NW surface, the plateaus in the Hall resistance at high fields remain robust as expected for a topologically protected Quantum Hall phase.

Magnetic‐TEGFET: Transistor Without a Gate

Low‐temperature current–voltage characteristics of n‐type GaAs/GaAlAs quantum wells delta‐doped in GaAs channel with Be acceptors are studied in the presence of a magnetic field. Negatively charged acceptor ions localize 2D conduction electrons by a combined effect of a quantum well and magnetic field parallel to the growth direction. In acceptor‐doped samples, the Hall electric field plays the role of the gate voltage. It is shown that at magnetic fields as weak as 1.5 T (or higher), the drain current reaches a constant value independent of the drain voltage. This phenomenon is due to the electron localization resulting in the decrease of conducting electron density in the crossed‐field configuration. The above special behavior of acceptor‐doped GaAs/GaAlAs heterostructures is exploited to realize a device called Magnetic‐TEGFET (Magnetic Two‐dimensional Electron Gas Field Effect Transistor) operating at low temperatures. The elimination of the gate in the studied transistor suppresses the gate‐to‐drain leakage current, which, in the standard TEGFETs, results in the electronic shot noise.

Non-Born effects in scattering of electrons in a conducting tube with a low concentration of impurities

We extend the theory of non-Born effects in resistivity $\rho$ of clean conducting tubes (developed in our previous work arXiv:1810.00426) to ``strips'' -- quasi-one-dimensional structures in 2D conductors. Here also an original Van Hove singularity in dependence of $\rho$ on the position of chemical potential $\varepsilon$ is asymmetrically split in two peaks for attracting impurities. However, since amplitudes of scattering at impurities depend on their positions, these peaks are inhomogeneously broadened. Strongest broadening occurs in the left peak, arising, for attracting impurities, due to scattering at quasistationary levels. In contrast with the case of tube these levels form not a unique sharp line, but a relatively broad impurity band with a weak quasi-Van Hove feature on its lower edge. Different parts of $\rho(\varepsilon)$ are dominated by different groups of impurities: close to the minimum the most effective scatterers, paradoxically are the ``weakest'' impurities -- those, located close to nodes of the electronic wave-function, so that the bare scattering matrix elements are suppressed. The quasi-Van Hove feature at left maximum is dominated by strongest impurites, located close to antinodes.

Synthesis, Structure, and Conductivity of Alluaudite‐Related Phases in the Na2MoO4–Cs2MoO4–CoMoO4 System

Phase formation study of the Na2MoO4–Cs2MoO4–CoMoO4 system resulted in new cesium‐containing alluaudite‐related phases. The solid solution Na4–2x‐yCsyCo1+x(MoO4)3 (0 ≤ x, y ≤ 0.30), based on the alluaudite‐type Na4–2xCo1+x(MoO4)3, and triple molybdate Na10(Cs4‐xNax)Co5(MoO4)12 (0 ≤ x ≤ 0.30) were found, and their structures were solved. In the structure of Na3.21Cs0.37Co1.21(MoO4)3 (a = 13.0917(8) Å, b = 13.5443(8) Å, c = 7.1217(4) Å, space group C2/c, β = 112.331(2), Z = 4), the cesium ions partially substitute the Na+ in the channels running along the c‐axis. The structure of Na10(Cs3.77Na0.23)Co5(MoO4)12 (a = 13.6572(3) Å, b = 11.5063(3) Å, c = 27.9898(5) Å, space group Pbca, Z = 4) was proved to be the aristotype for the pseudo orthorhombic Na25Cs8R5(MoO4)24 (R = Fe, Sc, In). The compounds contain alluaudite‐like layers of MoO4 tetrahedra and pairs of edge‐shared (Co, Na)O6 or (R, Na)O6 and NaO6 octahedra, which are connected by bridging MoO4 tetrahedra to form 3D frameworks differing from the alluaudite type. The frameworks contain channels along the c‐axis filled by Cs+ and Na+ ions. Bond valence sum (BVS) maps show that the alluaudite‐related molybdates can have a 2D sodium‐ion conductivity at elevated temperatures in contrast to the alluaudite‐type cathode material Na2+2xFe2‐x(SO4)3 with a 1D conductivity. The measured ionic conductivity of Na4–2xCo1+x(MoO4)3, Na4–2x‐yCsyCo1+x(MoO4)3, and Na10Cs4Co5(MoO4)12 reaches 10–3–10–2 S cm–1 at 500 °C.

Extremely Light Carrier‐Effective Mass in a Distorted Simple Metal Oxide

AbstractExotic electron transport properties such as quasi 1D conductivity are useful to realize advanced electronic devices showing unique properties. Anisotropic electron transport properties are often found in complex metal oxides due to their complicated crystal structures. Although simple metal oxides with distorted crystal structures could also be expected to show anisotropic electron transport properties, it is rarely studied most likely due to the lack of their high‐quality epitaxial films. Here anisotropic electron transport properties, showing “fast electron transport path,” in a simple distorted metal oxide, NbO2, is reported. High‐quality NbO2 epitaxial films with different crystallographic orientations on (0001) and (102) α‐Al2O3 single crystal substrates are fabricated, and the electron transport properties at room temperature are measured. Both the resistivity and absolute value of the thermopower along the [11] NbO2 is pretty small as compared with other directions. Experimentally obtained electron carrier effective mass in the [11] direction is surprisingly small, only 0.051 me, which is similar to that of high‐mobility GaAs. Since simple metal oxides have several advantages against complex oxides in view of easy fabrication, the present results will be beneficial for realizing advanced electronic devices using simple metal oxides.

Spatially and Conductivity Log Constrained AEM Inversion

We have developed an algorithm and released open-source code for 1D inversion of airborne electromagnetic data incorporating spatial and conductivity log constraints. The deterministic gradient based inversion algorithm uses an all-at-once approach, in which whole datasets or flight lines are inverted simultaneously. This allows spatial constraints to be imposed while also ensuring the inversion model closely matches any downhole conductivity logs that are near to the flight lines. The intent of the algorithm is to improve consistency along and across flight lines by taking advantage of the assumed coherency of the geology. Instead of roughness constraints, ‘sameness’ constraints are used. To implement these the regularization penalizes differences between the conductivity of 1D model/layer pairs and the weighted average conductivity of every other neighbouring 1D model within a user selected radius of their position. The neighbour averages are computed with inverse distance to a power weighting. The comparisons can be made over equivalent elevations. Downhole conductivity log constraints are imposed in a similar fashion, by penalizing the differences between conductivity logs, averaged over selected intervals, with their respective neighbouring 1D models. Overall the regularization encourages the final 1D conductivity models to be as similar as possible to their neighbours and to conductivity logs. It is demonstrated with an example that the method enhances geological interpretation by improving the model’s continuity along and between flight lines, and its match to conductivity logs.

Making the world from topological order.

Topology used to be a term confined to a branch of pure mathematics, where it referred to an invariant property of shape. The classic example was the way objects containing a single hole, like a torus and a coffee cup with handle, can be smoothly moulded into one another without tearing. But topological considerations have long played a role in the physics of matter, where for example they might dictate particular arrangements of component parts that can’t be erased from the system. The classic example here is the fact that a ‘hairy ball’ can’t be combed flat without having at least two pointy tufts. Such ‘defects’ in organization can be considered ‘topologically protected’, since they are robust against any recombing of the hair. They are universal features that don’t depend on the material specifics of the system: topological defects in liquid crystals are analogous to defects in spacetime called cosmic strings. In the past several decades in particular, properties of matter arising from topological considerations have become a major theme, reflected for example in the award of the 1985 and 1998 Nobel Prizes in Physics for discoveries involving the quantum Hall effect. Here the ‘Hall conductance’, quantifying the passage of electrical current in a 2D conductor in the presence of a transverse magnetic field, takes precise integral or fractional multiples of a particular quantized value related to the electron charge. This behaviour persists regardless of how we modify the material, for example by adding impurities. Topological phases and transitions were also a feature of the work that won the 2016 Nobel Prize. It has become recognized that the topological properties of the quantum-mechanical electronic structures of certain materials can give them unusual and perhaps useful properties. Some researchers think, for example, that ‘topological matter’ might supply quantum bits for quantum computation that resist the randomizing effects of noise. Xiao-Gang Wen of the Massachusetts Institute of Technology has been developing ideas about ‘topological order’ in fundamental physics for several decades. His notions of how topology in the underlying structure of spacetime might give rise to fundamental particles and forces make a connection to the topological phases recognized in condensed matter, revealing a new unifying principle in physics. National Science Review spoke to him about his work.