Magnetism-Induced Band-Edge Shift as the Mechanism for Magnetoconductance in CrPS4 Transistors

Transistors realized on the 2D antiferromagnetic semiconductor CrPS4 exhibit large magnetoconductance due to magnetic-field-induced changes in the magnetic state. The microscopic mechanism coupling the conductance and magnetic state is not understood. We identify it by analyzing the evolution of the parameters determining the transistor behavior─carrier mobility and threshold voltage─with temperature and magnetic field. For temperatures T near the Néel temperature TN, the magnetoconductance originates from a mobility increase due to the applied magnetic field that reduces spin fluctuation induced disorder. For T ≪ TN, instead, what changes is the threshold voltage, so that increasing the field at fixed gate voltage increases the density of accumulated electrons. The phenomenon is explained by a conduction band-edge shift correctly predicted by the ab initio calculations. Our results demonstrate that the band structure of CrPS4 depends on its magnetic state and reveal a mechanism for magnetoconductance that had not been identified earlier.

Multiple antiferromagnetic phases and magnetic anisotropy in exfoliated CrBr3 multilayers

In twisted two-dimensional (2D) magnets, the stacking dependence of the magnetic exchange interaction can lead to regions of ferromagnetic and antiferromagnetic interlayer order, separated by non-collinear, skyrmion-like spin textures. Recent experimental searches for these textures have focused on CrI3, known to exhibit either ferromagnetic or antiferromagnetic interlayer order, depending on layer stacking. However, the very strong uniaxial anisotropy of CrI3 disfavors smooth non-collinear phases in twisted bilayers. Here, we report the experimental observation of three distinct magnetic phases-one ferromagnetic and two antiferromagnetic-in exfoliated CrBr3 multilayers, and reveal that the uniaxial anisotropy is significantly smaller than in CrI3. These results are obtained by magnetoconductance measurements on CrBr3 tunnel barriers and Raman spectroscopy, in conjunction with density functional theory calculations, which enable us to identify the stackings responsible for the different interlayer magnetic couplings. The detection of all locally stable magnetic states predicted to exist in CrBr3 and the excellent agreement found between theory and experiments, provide complete information on the stacking-dependent interlayer exchange energy and establish twisted bilayer CrBr3 as an ideal system to deterministically create non-collinear magnetic phases.

Electronic Structure of Few-Layer Black Phosphorus from μ-ARPES

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based microfocus angle resolved photoemission (μ-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters, which provide an accurate description of the electronic structure for all thicknesses.

Expansion of the Materials Cloud 2D Database

Two-dimensional (2D) materials are among the most promising candidates for beyond-silicon electronic, optoelectronic, and quantum computing applications. Recently, their recognized importance sparked a push to discover and characterize novel 2D materials. Within a few years, the number of experimentally exfoliated or synthesized 2D materials went from a few to more than a hundred, with the number of theoretically predicted compounds reaching a few thousand. In 2018 we first contributed to this effort with the identification of 1825 compounds that are either easily (1036) or potentially (789) exfoliable from experimentally known 3D compounds. Here, we report on a major expansion of this 2D portfolio thanks to the extension of the screening protocol to an additional experimental database (MPDS) as well as the updated versions of the two databases (ICSD and COD) used in our previous work. This expansion leads to the discovery of an additional 1252 monolayers, bringing the total to 3077 compounds and, notably, almost doubling the number of easily exfoliable materials to 2004. We optimize the structural properties of all these monolayers and explore their electronic structure with a particular emphasis on those rare large-bandgap 2D materials that could be precious in isolating 2D field-effect-transistor channels. Finally, for each material containing up to 6 atoms per unit cell, we identify the best candidates to form commensurate heterostructures, balancing requirements on supercell size and minimal strain.

Gate-Controlled Magnetotransport and Electrostatic Modulation of Magnetism in 2D Magnetic Semiconductor CrPS4

Using field-effect transistors (FETs) to explore atomically thin magnetic semiconductors with transport measurements is difficult, because the very narrow bands of most 2D magnetic semiconductors cause carrier localization, preventing transistor operation. Here, it is shown that exfoliated layers of CrPS₄ -a 2D layered antiferromagnetic semiconductor whose bandwidth approaches 1 eV-allow the realization of FETs that operate properly down to cryogenic temperature. Using these devices, conductance measurements as a function of temperature and magnetic field are performed to determine the full magnetic phase diagram, which includes a spin-flop and a spin-flip phase. The magnetoconductance, which depends strongly on gate voltage, is determined. reaching values as high as 5000% near the threshold for electron conduction. The gate voltage also allows the magnetic states to be tuned, despite the relatively large thickness of the CrPS₄ multilayers employed in the study. The results show the need to employ 2D magnetic semiconductors with sufficiently large bandwidth to realize properly functioning transistors, and identify a candidate material to realize a fully gate-tunable half-metallic conductor.

Accurate Prediction of Hall Mobilities in Two-Dimensional Materials through Gauge-Covariant Quadrupolar Contributions

Despite considerable efforts, accurate computations of electron-phonon and carrier transport properties of low-dimensional materials from first principles have remained elusive. By building on recent advances in the description of long-range electrostatics, we develop a general approach to the calculation of electron-phonon couplings in two-dimensional materials. We show that the nonanalytic behavior of the electron-phonon matrix elements depends on the Wannier gauge, but that a missing Berry connection restores invariance to quadrupolar order. We showcase these contributions in a MoS_{2} monolayer, calculating intrinsic drift and Hall mobilities with precise Wannier interpolations. We also find that the contributions of dynamical quadrupoles to the scattering potential are essential, and that their neglect leads to errors of 23% and 76% in the room-temperature electron and hole Hall mobilities, respectively.

Quenching the bandgap of two-dimensional semiconductors with a perpendicular electric field

Perpendicular electric fields can tune the electronic band structure of atomically thin semiconductors. In bilayer graphene, which is an intrinsic zero-gap semiconductor, a perpendicular electric field opens a finite bandgap. So far, however, the same principle could not be applied to control the properties of a broader class of 2D materials because the required electric fields are beyond reach in current devices. To overcome this limitation, we design double ionic gated transistors that enable the application of large electric fields of up to 3 V nm^-1. Using such devices, we continuously suppress the bandgap of few-layer semiconducting transition metal dichalcogenides (that is, bilayer to heptalayer WSe₂) from 1.6 V to zero. Our results illustrate an excellent level of control of the band structure of 2D semiconductors.

Quasi-1D Electronic Transport in a 2D Magnetic Semiconductor

Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qualitative and quantitative differences of all quantities measured along the in-plane a and b crystallographic directions is found. In particular, a qualitatively different dependence of the conductivities σ_a and σ_b on temperature and gate voltage, accompanied by orders of magnitude differences in their values (σ_b /σ_a  ≈ 3 × 10²  to 10⁵ at low temperature and negative gate voltage) are observed, together with a different behavior of the longitudinal magnetoresistance in the two directions and the complete absence of the Hall effect in transverse resistance measurements. These observations appear not to be compatible with a description in terms of conventional band transport of a 2D doped semiconductor. The observed phenomenology-and unambiguous signatures of a 1D van Hove singularity detected in energy-resolved photocurrent measurements-indicate that electronic transport through CrSBr multilayers is better interpreted by considering the system as formed by weakly and incoherently coupled 1D wires, than by conventional 2D band transport. It is concluded that CrSBr is the first 2D semiconductor to show distinctly quasi-1D electronic transport properties.

Magnetization dependent tunneling conductance of ferromagnetic barriers

Recent experiments on van der Waals antiferromagnets have shown that measuring the temperature (T) and magnetic field (H) dependence of the conductance allows their magnetic phase diagram to be mapped. Similarly, experiments on ferromagnetic CrBr₃ barriers enabled the Curie temperature to be determined at H = 0, but a precise interpretation of the magnetoconductance data at H ≠ 0 is conceptually more complex, because at finite H there is no well-defined phase boundary. Here we perform systematic transport measurements on CrBr₃ barriers and show that the tunneling magnetoconductance depends on H and T exclusively through the magnetization M(H, T) over the entire temperature range investigated. The phenomenon is reproduced by the spin-dependent Fowler–Nordheim model for tunneling, and is a direct manifestation of the spin splitting of the CrBr₃ conduction band. Our analysis unveils a new approach to probe quantitatively different properties of atomically thin ferromagnetic insulators related to their magnetization by performing simple conductance measurements.

Many standard techniques for investigating magnetic properties in the bulk are ill suited to atomically thin van der Waals materials. Here, Wang et al take a prototypical van der Waals ferromagnet, Chromium Bromide, and show how tunneling conductance can elucidate the material magnetic properties.

Shear and Breathing Modes of Layered Materials

Layered materials (LMs), such as graphite, hexagonal boron nitride, and transition-metal dichalcogenides, are at the center of an ever-increasing research effort, due to their scientific and technological relevance. Raman and infrared spectroscopies are accurate, non-destructive approaches to determine a wide range of properties, including the number of layers, N, and the strength of the interlayer interactions. We present a general approach to predict the complete spectroscopic fan diagrams, i.e., the relations between frequencies and N for the optically active shear and layer-breathing modes of any multilayer comprising N ≥ 2 identical layers. In order to achieve this, we combine a description of the normal modes in terms of a one-dimensional mechanical model, with symmetry arguments that describe the evolution of the point group as a function of N. Group theory is then used to identify which modes are Raman- and/or infrared-active, and to provide diagrams of the optically active modes for any stack composed of identical layers. We implement the method and algorithms in an open-source tool to assist researchers in the prediction and interpretation of such diagrams. Our work will underpin future efforts on Raman and infrared characterization of known, and yet not investigated, LMs.

Wannier90 as a community code: new features and applications

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.

Persistence of Magnetism in Atomically Thin MnPS3 Crystals

The magnetic state of atomically thin semiconducting layered antiferromagnets such as CrI₃ and CrCl₃ can be probed by forming tunnel barriers and measuring their resistance as a function of magnetic field (H) and temperature (T). This is possible because the spins within each individual layer are ferromagnetically aligned and the tunneling magnetoresistance depends on the relative orientation of the magnetization in adjacent layers. The situation is different for systems that are antiferromagnetic within the layers in which case it is unclear whether magnetoresistance measurements can provide information about the magnetic state. Here, we address this issue by investigating tunnel transport through atomically thin crystals of MnPS₃, a van der Waals semiconductor that in the bulk exhibits easy-axis antiferromagnetic order within the layers. For thick multilayers below T ∼ 78 K, a T-dependent magnetoresistance sets in at μ₀H ∼ 5 T and is found to track the boundary between the antiferromagnetic and the spin-flop phases known from bulk measurements. We show that the magnetoresistance persists as thickness is reduced with nearly unchanged characteristic temperature and magnetic field scales, albeit with a different dependence on H, indicating the persistence of magnetism in the ultimate limit of individual monolayers.

Bulk and Surface Electronic Structure of the Dual-Topology Semimetal Pt_{2}HgSe_{3}

We report high-resolution angle-resolved photoemission measurements on single crystals of Pt_{2}HgSe_{3} grown by high-pressure synthesis. Our data reveal a gapped Dirac nodal line whose (001) projection separates the surface Brillouin zone in topological and trivial areas. In the nontrivial k-space range, we find surface states with multiple saddle points in the dispersion, resulting in two van Hove singularities in the surface density of states. Based on density-functional theory calculations, we identify these surface states as signatures of a topological crystalline state, which coexists with a weak topological phase.

Relative Abundance of [Formula: see text] Topological Order in Exfoliable Two-Dimensional Insulators

Quantum spin Hall insulators make up a class of two-dimensional materials with a finite electronic band gap in the bulk and gapless helical edge states. In the presence of time-reversal symmetry, [Formula: see text] topological order distinguishes the topological phase from the ordinary insulating one. Some of the phenomena that can be hosted in these materials, from one-dimensional low-dissipation electronic transport to spin filtering, could be promising for many technological applications in the fields of electronics, spintronics, and topological quantum computing. Nevertheless, the rarity of two-dimensional materials that can exhibit nontrivial [Formula: see text] topological order at room temperature hinders development. Here, we screen a comprehensive database we recently identified of 1825 monolayers that can be exfoliated from experimentally known compounds to search for novel quantum spin Hall insulators. Using density-functional and many-body perturbation theory simulations, we identify 13 monolayers that are candidates for quantum spin Hall insulators including high-performing materials such as AsCuLi₂ and (platinum) jacutingaite (Pt₂HgSe₃). We also identify monolayer Pd₂HgSe₃ (palladium jacutingaite) as a novel Kane-Mele quantum spin Hall insulator and compare it with platinum jacutingaite. A handful of promising materials are mechanically stable and exhibit [Formula: see text] topological order, either unperturbed or driven by small amounts of strain. Such screening highlights a relative abundance of [Formula: see text] topological order of around 1% and provides an optimal set of candidates for experimental efforts.

Determining the phase diagram of atomically thin layered antiferromagnet CrCl3

Changes in the spin configuration of atomically thin, magnetic van der Waals multilayers can cause drastic modifications in their opto-electronic properties. Conversely, the opto-electronic response of these systems provides information about the magnetic state, which is very difficult to obtain otherwise. Here, we show that in CrCl₃ multilayers, the dependence of the tunnelling conductance on applied magnetic field, temperature and number of layers tracks the evolution of the magnetic state, enabling the magnetic phase diagram to be determined experimentally. Besides a high-field spin-flip transition occurring for all thicknesses, the in-plane magnetoconductance exhibits an even-odd effect due to a low-field spin-flop transition. Through a quantitative analysis of the phenomena, we determine the interlayer exchange coupling as well as the layer magnetization and show that in CrCl₃ shape anisotropy dominates. Our results reveal the rich behaviour of atomically thin layered antiferromagnets with weak magnetic anisotropy.

Valley-Engineering Mobilities in Two-Dimensional Materials

Two-dimensional materials are emerging as a promising platform for ultrathin channels in field-effect transistors. To this aim, novel high-mobility semiconductors need to be found or engineered. Although extrinsic mechanisms can in general be minimized by improving fabrication processes, the suppression of intrinsic scattering (driven, for example, by electron-phonon interactions) requires modification of the electronic or vibrational properties of the material. Because intervalley scattering critically affects mobilities, a powerful approach to enhance transport performance relies on engineering the valley structure. We show here the power of this strategy using uniaxial strain to lift degeneracies and suppress scattering into entire valleys, dramatically improving performance. This is shown in detail for arsenene, where a 2% strain stops scattering into four of the six valleys and leads to a 600% increase in mobility. The mechanism is general and can be applied to many other materials, including in particular the isostructural antimonene and blue phosphorene.

Magnetic 2D materials and heterostructures

The family of two-dimensional (2D) materials grows day by day, hugely expanding the scope of possible phenomena to be explored in two dimensions, as well as the possible van der Waals (vdW) heterostructures that one can create. Such 2D materials currently cover a vast range of properties. Until recently, this family has been missing one crucial member: 2D magnets. The situation has changed over the past 2 years with the introduction of a variety of atomically thin magnetic crystals. Here we will discuss the difference between magnetic states in 2D materials and in bulk crystals and present an overview of the 2D magnets that have been explored recently. We will focus on the case of the two most studied systems-semiconducting CrI₃ and metallic Fe₃GeTe₂-and illustrate the physical phenomena that have been observed. Special attention will be given to the range of new van der Waals heterostructures that became possible with the appearance of 2D magnets, offering new perspectives in this rapidly expanding field.

Probing magnetism in 2D materials at the nanoscale with single-spin microscopy

The discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has generated widespread interest. Making further progress in this area requires quantitative knowledge of the magnetic properties of vdW magnets at the nanoscale. We used scanning single-spin magnetometry based on diamond nitrogen-vacancy centers to image the magnetization, localized defects, and magnetic domains of atomically thin crystals of the vdW magnet chromium(III) iodide (CrI₃). We determined the magnetization of CrI₃ monolayers to be ≈16 Bohr magnetons per square nanometer, with comparable values in samples with odd numbers of layers; however, the magnetization vanishes when the number of layers is even. We also found that structural modifications can induce switching between ferromagnetic and antiferromagnetic interlayer ordering. These results demonstrate the benefit of using single-spin scanning magnetometry to study the magnetism of 2D vdW magnets.

Microfocus Laser-Angle-Resolved Photoemission on Encapsulated Mono-, Bi-, and Few-Layer 1T'-WTe2

Two-dimensional crystals of semi-metallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few-layer 1T'-WTe₂ and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiments about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T'-WTe₂ by laser-based microfocus angle-resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe₂ comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T'-WTe₂ and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.

Graphene Materials Strengthen Aqueous Polyurethane Adhesives

Carboxyl-functionalized graphene platelets (GP) and graphene oxide (GO) sheets were added to a commercial aqueous adhesive dispersion of thermoplastic polyurethane (TP) (Idrotex 200 from FacGB s.r.l.). For both additives, the weight percentage was of industrial interest, 0.01 and 0.1 wt %. The addition of GP/GO was carried out in a simple and scalable-up process that can be applied to other materials and additives. Mechanical, peel tests were applied on polyurethane strips (75 mm long, 15 mm wide, and 1.5 mm thick) prepared cutting extruded sheets obtained using Estane 58091, a 70D aromatic polyester-based TP. The tests with 0.01 wt % of GP showed statistically significant higher forces at first failure and maximum forces with respect to the pristine adhesive. Sample characterization was carried out with scanning electron microscopy, infrared spectroscopy, X-ray diffraction, and thermal analysis. A mechanism is suggested for the improved performance of the low-dose GP adhesive.

Very large tunneling magnetoresistance in layered magnetic semiconductor CrI3

Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report magneto-transport measurements on exfoliated CrI3 crystals. We find that tunneling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10,000%. The evolution of the magnetoresistance with magnetic field and temperature reveals that the phenomenon originates from multiple transitions to different magnetic states, whose possible microscopic nature is discussed on the basis of all existing experimental observations. This observed dependence of the conductance of a tunnel barrier on its magnetic state is a phenomenon that demonstrates the presence of a strong coupling between transport and magnetism in magnetic van der Waals semiconductors.

Prediction of a Large-Gap and Switchable Kane-Mele Quantum Spin Hall Insulator

Fundamental research and technological applications of topological insulators are hindered by the rarity of materials exhibiting a robust topologically nontrivial phase, especially in two dimensions. Here, by means of extensive first-principles calculations, we propose a novel quantum spin Hall insulator with a sizable band gap of ∼0.5  eV that is a monolayer of jacutingaite, a naturally occurring layered mineral first discovered in 2008 in Brazil and recently synthesized. This system realizes the paradigmatic Kane-Mele model for quantum spin Hall insulators in a potentially exfoliable two-dimensional monolayer, with helical edge states that are robust and that can be manipulated exploiting a unique strong interplay between spin-orbit coupling, crystal-symmetry breaking, and dielectric response.

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds

Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.

Breakdown of Optical Phonons' Splitting in Two-Dimensional Materials

We investigate the long-wavelength dispersion of longitudinal and transverse optical phonon modes in polar two-dimensional materials, multilayers, and their heterostructures. Using analytical models and density-functional perturbation theory in a two-dimensional framework, we show that at variance with the three-dimensional case these modes are degenerate at the zone center but the macroscopic electric field associated with the longitudinal-optical modes gives rise to a finite slope at the zone center in their corresponding phonon dispersions. This slope increases linearly with the number of layers and it is determined solely by the Born effective charges of the material and the dielectric properties of the surrounding media. Screening from the environment can greatly reduce the slope splitting between the longitudinal and transverse optical modes and can be seen in the experimentally relevant case of boron nitride-graphene heterostructures. As the phonon momentum increases, the intrinsic screening properties of the two-dimensional material dictate the transition to a momentum-independent splitting similar to that of three-dimensional materials. These considerations are essential to understand electrical transport and optical coupling in two-dimensional systems.

Performance of arsenene and antimonene double-gate MOSFETs from first principles

In the race towards high-performance ultra-scaled devices, two-dimensional materials offer an alternative paradigm thanks to their atomic thickness suppressing short-channel effects. It is thus urgent to study the most promising candidates in realistic configurations, and here we present detailed multiscale simulations of field-effect transistors based on arsenene and antimonene monolayers as channels. The accuracy of first-principles approaches in describing electronic properties is combined with the efficiency of tight-binding Hamiltonians based on maximally localized Wannier functions to compute the transport properties of the devices. These simulations provide for the first time estimates on the upper limits for the electron and hole mobilities in the Takagi's approximation, including spin-orbit and multi-valley effects, and demonstrate that ultra-scaled devices in the sub-10-nm scale show a performance that is compliant with industry requirements.

Emergence of One-Dimensional Wires of Free Carriers in Transition-Metal-Dichalcogenide Nanostructures

We highlight the emergence of metallic states in two-dimensional transition-metal-dichalcogenide nanostructures-nanoribbons, islands, and inversion domain boundaries-as a widespread and universal phenomenon driven by the polar discontinuities occurring at their edges or boundaries. We show that such metallic states form one-dimensional wires of electrons or holes, with a free charge density that increases with the system size, up to complete screening of the polarization charge, and can also be controlled by the specific edge or boundary configurations, e.g., through chemisorption of hydrogen or sulfur atoms at the edges. For triangular islands, local polar discontinuities occur even in the absence of a total dipole moment for the island and lead to an accumulation of free carriers close to the edges, providing a consistent explanation of previous experimental observations. To further stress the universal character of these mechanisms, we show that polar discontinuities give rise to metallic states also at inversion domain boundaries. These findings underscore the potential of engineering transition-metal-dichalcogenide nanostructures for manifold applications in nano- and optoelectronics, spintronics, catalysis, and solar-energy harvesting.

Large-Area Epitaxial Monolayer MoS2

Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth methods are needed to open the way to applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to electrical, optical, and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the growth of high-quality monolayer MoS2 with control over lattice orientation. We show that the monolayer film is composed of coalescing single islands with limited numbers of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment via van der Waals interaction, we can easily transfer the grown material and fabricate devices. Local potential mapping along channels in field-effect transistors shows that the single-crystal MoS2 grains in our film are well connected, with interfaces that do not degrade the electrical conductivity. This is also confirmed by the relatively large and length-independent mobility in devices with a channel length reaching 80 μm.

Band-like electron transport with record-high mobility in the TCNQ family

The occurrence of extremely pronounced band-like transport with very high electron mobility in fluorinated tetracyanoquinodimethane (F2 -TCNQ) single-crystal field-effect transistors is discovered. This finding identifies the Fx -TCNQ family as a paradigm to investigate the fundamental aspects of electronic transport in organic crystals.

Role of magnetic resonance cholangiography in biliary complications of orthotopic liver transplantation

PURPOSE

The aim of this study was to evaluate the role of magnetic resonance cholangiography (MRC) in the detection of biliary complications following orthotopic liver transplantation (OLT).

MATERIALS AND METHODS

Seventy-eight transplant patients with clinically suspected biliary complications were evaluated with 1.5-T magnetic resonance imaging (MRI) using a surface coil. All patients were imaged with the following sequences: axial T1-weighted and axial and coronal T2-weighted, 2D spin echo (SE) breath-hold radial cholangiography, and coronal 3D single-shot turbo spin echo (SS-TSE) with respiratory triggering. Patients with negative MRI underwent clinical and sonographic followup. When biliary complications were present, diagnostic confirmation was obtained by endoscopic retrograde cholangiopancreatography (ERCP) (n=13), percutaneous transhepatic cholangiography (PTC) (n=20), ultrasonography (n=10) or computed tomography (CT) (n=2). In 11 cases, surgical confirmation was also obtained.

RESULTS

MRC detected biliary complications in 44/78 patients, in particular, 42 biliary strictures (37 anastomotic and five intrahepatic), 40 of which were confirmed by other imaging modalities. In 25/37 cases of anastomotic stricture, preanastomotic dilatation of the biliary tract was also demonstrated. Other MRC-detected biliary complications were biliary sludge (n=4), biloma (n=5), and biliary stones (n=3). In four cases, PTC revealed biliary complications that had not been detected with MRC (false negative results). In two cases, MRC showed unconfirmed strictures of the intrahepatic ducts and biliodigestive anastomosis (false positive results). The sensitivity, specificity, positive (PPV) and negative (NPV) predictive values and diagnostic accuracy of MRC were 93.5%, 94.4%, 96.7%, 89.5% and 93.9%, respectively.

CONCLUSIONS

Our results confirm that MRC is a reliable technique for depicting biliary anastomoses and detecting biliary complications after OLT. The high diagnostic accuracy of MRC indicates that this examination should be routinely employed in all OLT patients with clinically suspected biliary complications.

Self-report of circadian type reflects the phase of the melatonin rhythm

This study examined the relationship between circadian rhythm characteristics of the pineal hormone melatonin and individual differences in circadian type and mood. 95 healthy young men and 22 women were assessed each hour (00:00-07:00 h) for blood levels of melatonin throughout one night in the laboratory. Each subject was assessed for circadian type (morning, afternoon, or evening type) and morning mood (PANAS). Circadian type was strongly related to the melatonin acrophase but not to amplitude or time of year of assessment. Also, morning types evidenced a more rapid decline in melatonin levels after the peak than did evening types. Evening types were younger than were morning types. Female morning types reported more positive affect upon waking than did female afternoon or evening types. Males showed no such discrimination. Age was related to both melatonin acrophase and circadian type but did not explain the relationship between them. The results replicate and extend findings on circadian type and psychological and physiological variables.

Cytokines and cognitive behavior

Proinflammatory cytokines help orchestrate host responses to infection and are a major communication link between peripheral immunity and the CNS. These cytokines initiate a number of CNS events that culminate in both physiological and behavioral changes. Peripheral IL-1beta also affects information processing. A series of experiments examining the effect of learning intensity, motivation, and cytokine dose are reported. Using a well-established Morris water maze (MWM) system with female C57BL/6 mice, we report that (1) IL-1 (100 ng/mouse, i.p.) has no effect on MWM learning when mice are subjected to a spaced as opposed to a massed learning protocol; (2) water temperature is critical to the IL-1 effect on learning insofar as IL-1 interferes with learning in a warm-water but not a cold-water maze, and (3) higher doses (1,000 ng/mouse, i.p.) of IL-1 in experimental systems known to produce the IL-1-induced learning deficit with lower doses (100 ng/mouse, i.p.) show consistent facilitation, not impairment, of learning.

Human behavioral assessment in neurotoxicology: producing appropriate test performance with written and shaping instructions

Neurotoxic effects are of such breadth and complexity that functional biomarkers (behavioral tests) that integrate many areas of the nervous system predominate in human neurotoxicology research. The increasing distribution of chemical and other manufacturing throughout the world, particularly in developing nations, suggests the acute need to develop biomarkers for chemical exposures and effects that can be employed internationally. A language-free method for training performance on behavioral tests is described, which holds promise for international research that circumvents the vagaries of translation. Four behavioral tests were administered to 74-114 adult US subjects. Procedures, collectively termed shaping, produced effective performance on three tests [Symbol Digit, Vigilant Attention Test (VAT), Digit Span Forward and Backward], and produced appropriate but unacceptably slow performance in initial testing on the Simple Reaction Time test. Effective performance on the Symbol-Digit test also was produced by shaping instruction, without assistance from examiners, in small groups of residents of Taipei (Taiwan) and US children between the ages of 5 and 16.

Selective approaches to basic neurobehavioral testing of children in environmental health studies

To identify neurotoxic effects in children living near hazardous waste sites, the Agency for Toxic Substances and Disease Registry (ATSDR) has designed a basic Pediatric Environmental Neurobehavioral Test Battery (PENTB) for children 1 through 16 years of age. It emphasizes tests appropriate to the stages of a child's development. These stages were fundamental factors in selecting tests for the PENTB, which includes both informant- and performance-based assessment procedures. Assessment of children under 4 years of age is restricted to four informant-based instruments, to evaluate as many functions as possible while minimizing testing time and the professional expertise needed in the test setting. The assessment of children 4 through 16 years of age includes 10 performance-based tests to evaluate key functions within the cognitive, motor, and sensory domains analogous to functions affected by neurotoxic chemicals in adults. In all age groups, it is crucial to also assess family, cultural, economic, and other potentially confounding variables.

Legionella pneumophila-induced visual learning impairment reversed by anti-interleukin-1 beta

Infecting mice with the opportunistic intracellular pathogen Legionella pneumophila markedly inhibited place learning of infected C57BL/6 mice as determined by the Morris water maze test. Mice infected with L. pneumophila evinced much less ability to learn the position of a hidden platform than did normal noninfected mice, which quickly learned the location of the hidden platform and escaped from the cool water of the pool with increasing efficiency. However, infected mice treated with anti-interleukin-1 (anti-IL-1) neutralizing antibody learned the task with about the same efficiency as the controls. When the animals were tested 1 week after learning, control animals remembered the task well and were able to escape with near maximal efficacy. On the other hand, L. pneumophila-infected mice performed as poorly after the 1 week rest as during the training period, indicating that infection blocked learning and not merely performance. Mice infected with L. pneumophila and given the antibody treatment were found to be indistinguishable from controls in that they remembered the task and escaped with good efficiency. Thus, the results of this study suggest that the pro-inflammatory cytokine, IL-1 beta, is involved, at least partly, in the attenuation of spatial navigational learning in mice infected acutely with a sublethal concentration of L. pneumophila. These results, therefore, suggest that cognitive impairment of L. pneumophila-infected mice may be related to the cytokine IL-1 beta and, furthermore, that cytokines may be related to learning and memory changes experienced by individuals suffering acute bacterial infections.

Spatial learning impairment in mice infected with Legionella pneumophila or administered exogenous interleukin-1-beta

The effect of interleukin-1 beta (IL 1 beta) on spatial learning was examined. In one experiment, C57BL/6 mice were given daily injections (100 ng/mouse) of recombinant murine IL1 beta prior to training on the Morris water maze. In another experiment, mice were infected with a sublethal dose of a gram-negative bacterium (Legionella pneumophila; Lp). Mice rendered ill by the infection were given either anti-IL1 beta antibodies (100 micrograms/mouse) or saline and then trained on the water maze. Results indicated that (1) exogenous IL1 beta blocked acquisition of spatial learning, (2) Lp infection attenuated learning on this task, and (3) neutralizing circulating IL1 beta in Lp-infected mice normalized learning despite the continuation of the illness. The data indicate that cognitive impairment may be a component of cytokine-mediated sickness behavior.

IL-1 beta and TNF alpha modulate delta 9-tetrahydrocannabinol-induced catalepsy in mice

The role of the proinflammatory cytokines interleukin-1 alpha (IL-1 alpha), interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF alpha) in THC-induced catalepsy in mice was examined. Recombinant IL-1 beta (400 ng/mouse, IV) and TNF alpha (500 ng/mouse, IV) were effective in potentiating the cataleptic effect of low-dose THC (10 micrograms/mouse, IV). Recombinant IL-1 alpha and IL-6 did not potentiate catalepsy at any dose tested. Anti-IL-1 beta and anti-TNF alpha antibodies were effective in attenuating high-dose (75 micrograms/mouse) THC-induced catalepsy. Antibodies to IL-1 alpha and IL-6 had no effect on catalepsy. Early onset catalepsy (10 min postinjection) was potentiated by exogenous recombinant IL-1 beta and TNF alpha but only later catalepsy (2 h postinjection) was attenuated by antibodies to endogenous IL-1 beta or TNF alpha. This divergence of the cytokine effect suggests that these substances regulate, by different mechanisms, the early and late THC-induced cataleptic response.

Influence of physician communication on newly diagnosed breast patients' psychologic adjustment and decision-making

BACKGROUND

Physician-patient communication is of critical importance when a breast cancer diagnosis is made, because the emotionally overwhelmed patient must be educated about her disease and available treatments so she can participate in decisions about her care. A research study addressed the hypothesis that patients whose surgeons used psychotherapeutic techniques during the cancer diagnostic interview would have better psychologic adjustment to their cancer.

METHODS

One hundred women surveyed 6 months after surgery completed the Cancer Diagnostic Interview Scale (CDIS) and the SCL-90-R, a measure of psychologic well being.

RESULTS

Factor analysis of the CDIS revealed that the physician's caring attitude was perceived by the women as most important, with information-giving as a much weaker component. Multiple regression analysis supported the hypothesis that psychologic adjustment was predicted by physician behavior during the cancer diagnostic interview. Other significant predictors of adjustment were a history of psychiatric problems and premorbid life stressors.

CONCLUSIONS

Provision of information needed for decision-making appears to be valued largely within the context of a caring physician-patient relationship. Specific surgeons' behaviors believed to facilitate patient adjustment include expressing empathy, allowing sufficient time for patients to absorb the cancer diagnosis, providing information, and engaging the patient in treatment decision-making.

Peer support group for adolescents with chronic illness

This study assessed the effects of a monthly peer support group for adolescents with cancer and other hematological diseases. These adolescents shared activities and experiences with nondisabled high school students. At the group's conclusion, the adolescents reported that the group helped them cope with their illness and improved the quality of their daily lives. Nondisabled students reported that the group favorably affected their attitudes about, and intended behavior toward, peers with chronic illnesses. These results suggest that such groups can provide important benefits for individuals with chronic illnesses as well as for their nondisabled peers.

Psychosocial aspects of melanoma

Seventy-five patients with melanoma were surveyed for recent history of major stress, coping styles, and psychiatric disturbance. Recurrence of disease was strongly related to biological variables (stage and Breslow depths) but not to psychological measures. Major life stress was not related to stage, Breslow, Clark level, or estimates of lymphocytic infiltration of tumor. Coping styles were paradoxically related to major life stress such that history of major stresses was associated with greater confrontation of the melanoma diagnosis, greater will to fight the disease, and less avoidance of its frightening aspects. Experience with fewer major life stresses was associated with a defeatist attitude characterized by an expectation of a poor prognosis and little control over outcomes. More than 50% of the sample had experienced at least one major life stress in the past 5 years. This figure is consistent with prior work and indicative of a higher than normal rate of major life stress in the years before diagnosis.

Impact of a hospital smoking ban: changes in tobacco use and employee attitudes

The authors investigated the impact of a complete smoking ban on 349 employees of a cancer treatment center. A questionnaire administered approximately 4 months after the ban was initiated queried smokers on the impact of the ban on their smoking habits, their experience of withdrawal symptoms during the workday, and changes in work habits. A separate questionnaire asked nonsmokers about changes in the work environment. Results showed that few smokers quit while a majority decreased their consumption. Withdrawal symptoms were a problem in less than half the smokers, but those reporting signs of physical dependency on nicotine tended to smoke more before and after work and reported negative changes in work performance. Nonsmokers in general reported positive effects on the work environment.

Active-duty and veteran alcoholics: differences in psychopathology presentation

The present study examines the differences in the presentation of psychopathology between active duty military and veteran patients enrolled in alcohol rehabilitation programs. The Millon Clinical Multiaxial Inventory was used to assess personality disorders, clinical syndromes, and substance abuse. Most veterans were seen as having avoidant and dependent personality disorders as well as a great deal of anxiety and depression. Forty-seven percent were above cutoff scores for alcohol abuse. Air Force patients were predominantly narcissistic and antisocial with much less distress. Only 9% were classified psychometrically as abusers. The results indicate that not only are veteran alcoholics more chronic but that active-duty alcohol abusers are underreporting abuse and care must be used in their assessment.

Factor-based special scales for the MCMI

Factor analytic work with the Million Clinical Multiaxial Inventory (MCMI) has shown a remarkably stable factor structure. The eight Basic Personality scales have a three-factor structure: Aloof/Social, Submissive/Aggressive, and Labile/Restrained. These dimensions appear to be the same as those suggested by the DSM-III-R advisory committee on personality disorders. The MCMI as a whole has a five-factor structure: Detached, Submissive, Suspicious, High Social Energy, and General Distress. The present work operationalizes these two sets of factors into scales for use by clinicians and researchers. The resultant scales show excellent reliabilities across three subject samples (N = 253, N = 185, N = 184) and demonstrate appropriate convergent and divergent validity estimates against the MCMI itself.

Objective psychological testing of U.S. Air Force officers in pilot training

Clinical psychologists are increasingly assisting flight surgeons in the assessment of students in pilot training. However, some psychological tests reported in the literature are ill-suited to efficient clinical evaluation of aviators. Recent advances in clinical psychometrics offer improvements in reliability, personality theory, and norms. We administered the Multidimensional Aptitude Battery, the Personality Research Form, and the Millon Clinical Multiaxial Inventory to 350 Air Force officers undergoing Undergraduate Pilot Training. We present normative data for use by practitioners assessing similar populations.

Factor structure of the MCMI basic personality scales and common-item artifact

The eight basic personality scales of the Millon Clinical Multiaxial Inventory (MCMI) were derived from Millon's theory of personality, but the adequacy of the MCMI for measuring Millon's personality constructs has never been assessed. One major problem with using factor analysis to illuminate the structure of the MCMI personality scales is that artifactual structure may result from item overlap among the scales. To analyze this, item-overlap coefficients were factored and compared to the factor structures of five subject samples. For the eight basic personality scales, three factors emerged for the overlap matrix and each of the five sample matrices: Aloof-Social, Aggressive-Submissive, and Lability-Restraint. It was concluded that these three factors are inconsistent with Millon's theory and that they will be found artifactually across a wide variety of populations due to overlapping items.

Air Force Pilot Personality: Hard Data on the "Right Stuff"

Three-hundred and fifty Air Force pilots undergoing Undergraduate Pilot Training were administered the Personality Research Form (PRF) and the Millon Clinical Multiaxial Inventory (MCMI) within the first four weeks of training. Pilots were significantly different from non-flying college students on several PRF scales. Cluster analyses indicated that three very distinct personality types exist in the data. These types were supported through multiple cross-validations. Descriptions of pilot personality types are given in terms of PRF and MCMI personality variables. The accuracy of prevailing stereotypes is examined in light of the data. Implications for future research and pilot selection criteria are discussed.

The operating characteristics of the Millon Clinical Multiaxial Inventory

The operating characteristics of the 20 scales of the Millon Clinical Multiaxial Inventory (MCMI) were analyzed with respect to the construction sample data as presented in the test manual. Sensitivity, specificity, positive predictive power, negative predictive power, and overall diagnostic power of each scale were derived. Results indicated that eight scales show excellent characteristics, nine were classified as fair, and three were determined to have poor positive predictive power for identifying the presence of a syndrome in an individual patient. Five scales had good positive predictive power for identifying the most prominent syndrome in a patient's clinical picture, eleven scales were classified as fair, and four were seen as poor on this dimension. We suggest a method for determining the utility of individual scales for different clinical populations and discuss implications of this type of analysis of the MCMI for diagnosis of the individual case.