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Noncentrosymmetric triangular magnets offer a unique platform for realizing strong quantum fluctuations. However, designing these quantum materials remains an open challenge attributable to a knowledge gap in the tunability of competing exchange interactions at the atomic level. Here, we create a new noncentrosymmetric triangular S = 3/2 magnet CaMnTeO$_6$ based on careful chemical and physical considerations. The model material displays competing magnetic interactions and features nonlinear optical responses with the capability of generating coherent photons. The incommensurate magnetic ground state of CaMnTeO$_6$ with an unusually large spin rotation angle of 127 deg.(1) indicates that the anisotropic interlayer exchange is strong and competing with the isotropic interlayer Heisenberg interaction. The moment of 1.39(1) $\mu$B, extracted from low-temperature heat capacity and neutron diffraction measurements, is only 46% of the expected value of the static moment 3 $\mu$B. This reduction indicates the presence of strong quantum fluctuations in the half-integer spin S = 3/2 CaMnTeO$_6$ magnet, which is rare. By comparing the spin-polarized band structure, chemical bonding, and physical properties of AMnTeO$_6$ (A = Ca, Sr, Pb), we demonstrate how quantum-chemical interpretation can illuminate insights into the fundamentals of magnetic exchange interactions, providing a powerful tool for modulating spin dynamics with atomically precise control.

Significant efforts have focused on the magnetic excitations of relativistic Mott insulators, predicted to realize the Kitaev quantum spin liquid (QSL). This exactly solvable model involves a highly entangled state resulting from bond-dependent Ising interactions that produce excitations which are non-local in terms of spin flips. A key challenge in real materials is identifying the relative size of the non-Kitaev terms and their role in the emergence or suppression of fractional excitations. Here, we identify the energy and temperature boundaries of non-Kitaev interactions by direct comparison of the Raman susceptibility of RuCl3 with quantum Monte Carlo (QMC) results for the Kitaev QSLs. Moreover, we further confirm the fractional nature of the magnetic excitations, which is given by creating a pair of fermionic quasiparticles. Interestingly, this fermionic response remains valid in the non-Kitaev range. Our results and focus on the use of the Raman susceptibility provide a stringent new test for future theoretical and experimental studies of QSLs., Comment: https://www.nature.com/articles/s41535-020-0216-6. arXiv admin note: substantial text overlap with arXiv:1809.07782

We report $\beta$-detected nuclear magnetic resonance ($\beta$-NMR) measurements in Bi$_{2}$Se$_{3}$:Ca (BSC) and Bi$_{2}$Te$_{3}$:Mn (BTM) single crystals using $^{8}$Li$^{+}$ implanted to depths on the order of 100 nm. Above $\sim 200$ K, spin-lattice relaxation (SLR) reveals diffusion of $^{8}$Li$^{+}$, with activation energies of $\sim 0.4$ eV ($\sim 0.2$ eV) in BSC (BTM). At lower temperatures, the nuclear magnetic resonance (NMR) properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift in BSC. From this, we make a detailed comparison with the isostructural tetradymite Bi$_{2}$Te$_{2}$Se (BTS) [McFadden et al., Phys Rev. B 99, 125201 (2019)]. In the magnetic BTM, the effects of the dilute Mn moments predominate, but remarkably the $^{8}$Li signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the SLR that is suppressed in a high applied field. This detailed characterization of the $^{8}$Li NMR response is an important step towards using depth-resolved $\beta$-NMR to study the low-energy properties of the chiral topological surface state (TSS). With the bulk NMR response now established in several Bi$_{2}$Ch$_{3}$ tetradymite topological insulators (TIs), the prospect of directly probing their chiral TSS using the depth resolution afforded by $\beta$-NMR remains strong., Comment: 14 pages, 10 figures

Symmetry and topology play key roles in the identification of phases of matter and their properties. Both concepts are central to understanding quantum Hall ferromagnets (QHFMs), two-dimensional electronic phases with spontaneously broken spin or pseudospin symmetry whose wavefunctions also have topological properties. Domain walls between distinct broken symmetry QHFM phases are predicted to host gapless one-dimensional (1D) modes that emerge due to a topological change of the underlying electronic wavefunctions at such interfaces. Although a variety of QHFMs have been identified in different materials, probing interacting electronic modes at these domain walls has not yet been accomplished. Here we use a scanning tunneling microscope (STM) to directly visualize the spontaneous formation of boundary modes, within a sign-changing topological gap, at domain walls between different valley-polarized quantum Hall phases on the surface of bismuth. By changing the valley occupation and the corresponding number of modes at the domain wall, we can realize different regimes where the valley-polarized channels are either metallic or develop a spectroscopic gap. This behavior is a consequence of Coulomb interactions constrained by the symmetry-breaking valley flavor, which determines whether electrons in the topological modes can backscatter, making these channels a unique class of interacting Luttinger liquids.

We report measurements on the high temperature ionic and low temperature electronic properties of the 3D topological insulator Bi$_2$Te$_2$Se using ion-implanted $^8$Li $\beta$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range 5-28 keV, the probes penetrate beyond the expected range of the topological surface state, but are still within 250 nm of the surface. At temperatures above ~150 K, spin-lattice relaxation measurements reveal isolated $^8$Li$^{+}$ diffusion with an activation energy $E_{A} = 0.185(8)$ eV and attempt frequency $\tau_{0}^{-1} = 8(3) \times 10^{11}$ s$^{-1}$ for atomic site-to-site hopping. At lower temperature, we find a linear Korringa-like relaxation mechanism with a field dependent slope and intercept, which is accompanied by an anomalous field dependence to the resonance shift. We suggest that these may be related to a strong contribution from orbital currents or the magnetic freezeout of charge carriers in this heavily compensated semiconductor, but that conventional theories are unable to account for the extent of the field dependence. Conventional NMR of the stable host nuclei may help elucidate their origin., Comment: 17 pages, 12 figures, submitted to Phys. Rev. B

A key characteristic of quantum spin liquids(QSL) is the presence of fractional excitations related to their entanglement properties, yet experimental verification of their statistics is missing. For example, in the potential Kitaev spin liquid, RuCl3 experiments uncovered signs of fractional particles, though not their Fermi statistics. Here we employ Raman scattering to reveal the true nature of the magnetic excitations, using improved experimental methods and analysis to remove the influence of laser heating and thermal fluctuations. Via the energy loss and gain spectra, we extract the energy and temperature dependence of the Raman susceptibility to prove RuCl3's magnetic response is given by pair creation of fermionic excitations. Furthermore, by comparing with quantum Monte Carlo (QMC) results for the exact Kitaev limit, we are able to discern the energy and temperature range where additional interaction terms are important. Our results open new directions in QSL research by providing a new way to investigate fractional excitations and the importance of terms causing spinon confinement., Comment: 29 pages, 4 figures

Crystal structures play a vital role in determining materials properties. In Li-ion cathodes, the crystal structure defines the dimensionality and connectivity of interstitial sites, thus determining Li-ion diffusion kinetics. While a perfect crystal has infinite structural coherence, a class of recently discovered high-capacity cathodes, Li-excess cation-disordered rocksalts, falls on the other end of the spectrum: Their cation sublattices are assumed to be randomly populated by Li and transition metal ions with zero configurational coherence based on conventional X-ray diffraction, such that the Li transport is purely determined by statistical effects. In contrast to this prevailing view, we reveal that cation short-range order, hidden in diffraction, is ubiquitous in these long-range disordered materials and controls the local and macroscopic environments for Li-ion transport. Our work not only discovers a crucial property that has previously been overlooked, but also provides new guidelines for designing and engineering disordered rocksalts cathode materials.

Novel broken symmetry states can spontaneously form due to Coulomb interactions in electronic systems with multiple internal degrees of freedom. Multi-valley materials offer an especially rich setting for the emergence of such states, which have potential electronic and optical applications. To date, identification of these broken symmetry phases has mostly relied on the examination of macroscopic transport or optical properties. Here we demonstrate a powerful direct approach by visualizing the wave functions of bismuth surface states with a scanning tunneling microscope. Strong spin-orbit coupling on the surface of bismuth leads to six degenerate teardrop-shaped hole pockets. Our spectroscopic measurements reveal that this degeneracy is fully lifted at high magnetic field as a result of exchange interactions, and we are able to determine the nature of valley ordering by imaging the broken symmetry Landau level (LL) wave functions. The remarkable spatial features of singly degenerate LL wave functions near isolated defects contain unique signatures of interference between spin-textured valleys, which identify the electronic ground state as a quantum Hall ferroelectric. Our observations confirm the recent prediction that interactions in strongly anisotropic valley systems favor the occupation of a single valley, giving rise to emergent ferroelectricity in the surface state of bismuth., Comment: Accepted to Nature Physics, available through Advance Online Publication

The electronic properties of the heavy metal superconductor LaIr3 are reported. The estimated superconducting parameters obtained from physical properties measurements indicate that LaIr3 is a BCS-type superconductor. Electronic band structure calculations show that Ir d- states dominate the Fermi level. A comparison of electronic band structures of LaIr3 and LaRh3 shows that the Ir-compound has a strong spin-orbit-coupling effect, which creates a complex Fermi surface., Comment: 6 pages and 5 figures

It has been long hoped that the realization of long-range ferromagnetic order in two-dimensional (2D) van der Waals (vdW) crystals, combined with their rich electronic and optical properties, would open up new possibilities for magnetic, magnetoelectric and magneto-optic applications. However, in 2D systems, the long-range magnetic order is strongly hampered by thermal fluctuations which may be counteracted by magnetic anisotropy, according to the Mermin-Wagner theorem. Prior efforts via defect and composition engineering, and proximity effect only locally or extrinsically introduce magnetic responses. Here we report the first experimental discovery of intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers by scanning magneto-optic Kerr microscopy. In such a 2D vdW soft ferromagnet, for the first time, an unprecedented control of transition temperature of ~ 35% - 57% enhancement is realized via surprisingly small fields (<= 0.3 Tesla in this work), in stark contrast to the stiffness of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field enables an effective anisotropy far surpassing the tiny magnetocrystalline anisotropy, opening up a sizable spin wave excitation gap. Confirmed by renormalized spin wave theory, we explain the phenomenon and conclude that the unusual field dependence of transition temperature constitutes a hallmark of 2D soft ferromagnetic vdW crystals. Our discovery of 2D soft ferromagnetic Cr2Ge2Te6 presents a close-to-ideal 2D Heisenberg ferromagnet for studying fundamental spin behaviors, and opens the door for exploring new applications such as ultra-compact spintronics., Comment: to appear in Nature

Two-dimensional materials have significant potential for the development of new devices. Here we report the electronic and structural properties of $\beta$-GeSe, a previously unreported polymorph of GeSe, with a unique crystal structure that displays strong two-dimensional structural features. $\beta$-GeSe is made at high pressure and temperature and is stable under ambient conditions. We compare it to its structural and electronic relatives $\alpha$-GeSe and black phosphorus. The $\beta$ form of GeSe displays a boat conformation for its Ge-Se six-ring, while the previously known $\alpha$ form, and black phosphorus, display the more common chair conformation for their six-rings. Electronic structure calculations indicate that $\beta$-GeSe is a semiconductor, with an approximate bulk band gap of $\Delta~\approx$ 0.5 eV, and, in its monolayer form, $\Delta~\approx$ 0.9 eV. These values fall between those of $\alpha$-GeSe and black phosphorus, making $\beta$-GeSe a promising candidate for future applications. The resistivity of our $\beta$-GeSe crystals measured in-plane is on the order of $\rho \approx$ 1 $\Omega$cm, while being essentially temperature independent.

Nematic quantum fluids with wavefunctions that break the underlying crystalline symmetry can form in interacting electronic systems. We examine the quantum Hall states that arise in high magnetic fields from anisotropic hole pockets on the Bi(111) surface. Spectroscopy performed with a scanning tunneling microscope shows that a combination of local strain and many-body Coulomb interactions lift the six-fold Landau level (LL) degeneracy to form three valley-polarized quantum Hall states. We image the resulting anisotropic LL wavefunctions and show that they have a different orientation for each broken-symmetry state. The wavefunctions correspond precisely to those expected from pairs of hole valleys and provide a direct spatial signature of a nematic electronic phase.

Cr2Ge2Te6 has been of interest for decades, as it is one of only a few naturally forming ferromagnetic semiconductors. Recently, this material has been revisited due to its potential as a 2 dimensional semiconducting ferromagnet and a substrate to induce anomalous quantum Hall states in topological insulators. However, many relevant properties of Cr2Ge2Te6 still remain poorly understood, especially the spin-phonon coupling crucial to spintronic, multiferrioc, thermal conductivity, magnetic proximity and the establishment of long range order on the nanoscale. We explore the interplay between the lattice and magnetism through high resolution micro-Raman scattering measurements over the temperature range from 10 K to 325 K. Strong spin-phonon coupling effects are confirmed from multiple aspects: two low energy modes splits in the ferromagnetic phase, magnetic quasielastic scattering in paramagnetic phase, the phonon energies of three modes show clear upturn below Tc, and the phonon linewidths change dramatically below Tc as well. Our results provide the first demonstration of spin-phonon coupling in a potential 2 dimensional atomic crystal.

We report a new, cleavable, strong-topological-metal, Zr2Te2P, which has the same tetradymite-type crystal structure as the topological insulator Bi2Te2Se. Instead of being a semiconductor, however, Zr2Te2P is metallic with a pseudogap between 0.2 and 0.7 eV above the fermi energy (EF). Inside this pseudogap, two Dirac dispersions are predicted: one is a surface-originated Dirac cone protected by time-reversal symmetry (TRS), while the other is a bulk-originated and slightly gapped Dirac cone with a largely linear dispersion over a 2 eV energy range. A third surface TRS-protected Dirac cone is predicted, and observed using ARPES, making Zr2Te2P the first system to realize TRS-protected Dirac cones at M points. The high anisotropy of this Dirac cone is similar to the one in the hypothetical Dirac semimetal BiO2. We propose that if EF can be tuned into the pseudogap where the Dirac dispersions exist, it may be possible to observe ultrahigh carrier mobility and large magnetoresistance in this material.

A long-standing issue in topological insulator research has been to find a material that provides an ideal platform for characterizing topological surface states without interference from bulk electronic states and can reliably be fabricated as bulk crystals. This material would be a bulk insulator, have a surface state Dirac point energy well isolated from the bulk valence and conduction bands, have high surface state electronic mobility, and be growable as large, high quality bulk single crystals. Here we show that this major materials obstacle in the field is overcome by crystals of lightly Sn-doped Bi1.1Sb0.9Te2S (Sn-BSTS) grown by the Vertical Bridgeman method, which we characterize here via angle-resolved photoemission spectroscopy, scanning tunneling microscopy, transport studies of the bulk and surface states, and X-ray diffraction and Raman scattering. We present this new material as a bulk topological insulator that can be reliably grown and studied in many laboratories around the world., Comment: 21 pages, 5 figures

A three-dimensional strong-topological-insulator or -semimetal hosts topological surface states which are often said to be gapless so long as time-reversal symmetry is preserved. This narrative can be mistaken when surface state degeneracies occur away from time-reversal-invariant momenta. The mirror-invariance of the system then becomes essential in protecting the existence of a surface Fermi surface. Here we show that such a case exists in the strong-topological-semimetal Bi$_4$Se$_3$. Angle-resolved photoemission spectroscopy and \textit{ab initio} calculations reveal partial gapping of surface bands on the Bi$_2$Se$_3$-termination of Bi$_4$Se$_3$(111), where an 85 meV gap along $\bar{\Gamma}\bar{K}$ closes to zero toward the mirror-invariant $\bar{\Gamma}\bar{M}$ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin-helicity., Comment: 5 pages, 3 figures

Cr2Ge2Te6 has been of interest for decades, as it is one of only a few ferromagnetic insulators. Recently, this material has been revisited due to its potential as a substrate for Bi2Te3, a topological insulator. This enables the possibility of studying the anomalous quantum Hall effect in topological insulators, and a route to novel spintronic devices. To probe the compatibility of these two materials, we use polarized variable temperature Raman microscopy to study Bi2Te3-Cr2Ge2Te6 heterostructure as well as the phonon dynamics of Cr2Ge2Te6. We found the temperature dependence of the Cr2Ge2Te6 phonons results primarily from anharmonicity, though a small magneto-elastic coupling is also observed. Our results confirm the potential of Cr2Ge2Te6 as a substrate for topological insulators., Comment: 4 pages, 5 figures

The influence of magnetic dopants on the electronic and chemical environments in topological insulators (TIs) is a key factor when considering possible spintronic applications based on topological surface state properties. Here we provide spectroscopic evidence for the presence of distinct chemical and electronic behavior for surface and bulk magnetic doping of Bi2Te3. The inclusion of Mn in the bulk of Bi2Te3 induces a genuine dilute ferromagnetic state, with reduction of the bulk band gap as the Mn content is increased. Deposition of Fe on the Bi2Te3 surface, on the other hand, favors the formation of iron telluride already at coverages as low as 0.07 monolayer, as a consequence of the reactivity of the Te-rich surface. Our results identify the factors that need to be controlled in the realization of magnetic nanosystems and interfaces based on TIs.

The hallmark of a time-reversal symmetry protected topologically insulating state of matter in two-dimensions (2D) is the existence of chiral edge modes propagating along the perimeter of the system. To date, evidence for such electronic modes has come from experiments on semiconducting heterostructures in the topological phase which showed approximately quantized values of the overall conductance as well as edge-dominated current flow. However, there have not been any spectroscopic measurements to demonstrate the one-dimensional (1D) nature of the edge modes. Among the first systems predicted to be a 2D topological insulator are bilayers of bismuth (Bi) and there have been recent experimental indications of possible topological boundary states at their edges. However, the experiments on such bilayers suffered from irregular structure of their edges or the coupling of the edge states to substrate's bulk states. Here we report scanning tunneling microscopy (STM) experiments which show that a subset of the predicted Bi-bilayers' edge states are decoupled from states of Bi substrate and provide direct spectroscopic evidence of their 1D nature. Moreover, by visualizing the quantum interference of edge mode quasi-particles in confined geometries, we demonstrate their remarkable coherent propagation along the edge with scattering properties that are consistent with strong suppression of backscattering as predicted for the propagating topological edge states., Comment: 15 pages, 5 figures, and supplementary material

The synthesis, structure, and elementary magnetic and electronic properties are reported for layered compounds of the type Na3-xMIr2O6 and Na3-xM2IrO6, where M is a transition metal from the 3d series (M=Zn, Cu, Ni, Co, Fe and Mn). The rhombohedral structures, in space group R-3m, were determined by refinement of neutron and synchrotron powder diffraction data. No clear evidence for long range 2:1 or 1:2 honeycomb-like M/Ir ordering was found in the neutron powder diffraction patterns except in the case of M = Zn, thus in general the compounds are best designated as sodium deficient {\alpha}-NaFeO2-type phases with formulas Na1-xM1/3Ir2/3O2 or Na1-xM2/3Ir1/3O2. Synchrotron powder diffraction patterns indicate that several of the compounds likely have honeycomb in-plane metal-iridium ordering with disordered stacking of the layers. All the compounds are sodium deficient under our synthetic conditions and are black and insulating. Weiss constants derived from magnetic susceptibility measurements indicate that Na0.62Mn0.61Ir0.39O2, Na0.80Fe2/3Ir1/3O2, Na0.92Ni1/3Ir2/3O2, Na0.86Cu1/3Ir2/3O2, and Na0.89Zn1/3Ir2/3O2 display dominant antiferromagnetic interactions. For Na0.90Co1/3Ir2/3O2 the dominant magnetic interactions at low temperature are ferromagnetic while at high temperatures they are antiferromagnetic; there is also a change in the effective moment. Low temperature specific heat measurements (to 2 K) on Na0.92Ni1/3Ir2/3O2 indicate the presence of a broad magnetic ordering transition. X-ray absorption spectroscopy shows that iridium is at or close to the 4+ oxidation state in all compounds. 23Na nuclear magnetic resonance measurements comparing Na2IrO3 to Na0.92Ni1/3Ir2/3O2 and Na0.89Zn1/3Ir2/3O2 provide strong indications that the electron spins are short-range ordered in the latter two materials. All of the compounds are spin glasses., Comment: Journal of Solid State Chemistry to be published, 43 pages, 16 figures

We describe the topological surface states of Bi$_4$Se$_3$, a compound in the infinitely adaptive Bi$_2$-Bi$_2$Se$_3$ natural superlattice phase series, determined by a combination of experimental and theoretical methods. Two observable cleavage surfaces, terminating at Bi or Se, are characterized by angle resolved photoelectron spectroscopy and scanning tunneling microscopy, and modeled by ab-initio density functional theory calculations. Topological surface states are observed on both surfaces, but with markedly different dispersions and Kramers point energies. Bi$_4$Se$_3$ therefore represents the only known compound with different topological states on differently terminated surfaces., Comment: 5 figures references added Published in PRB: http://link.aps.org/doi/10.1103/PhysRevB.88.081108

Topological surface states, a new kind of electronic state of matter, have recently been observed on the cleaved surfaces of crystals of a handful of small band gap semiconductors. The underlying chemical factors that enable these states are crystal symmetry, the presence of strong spin orbit coupling, and an inversion of the energies of the bulk electronic states that normally contribute to the valence and conduction bands. The goals of this review are to briefly introduce the physics of topological insulators to a chemical audience and to describe the chemistry, defect chemistry, and crystal structures of the compounds in this emergent field., Comment: Submitted to Journal of Materials Chemistry, 47 double spaced pages, 9 figures

We report spin- and angle-resolved photoemission studies of a topological insulator from the infinitely adaptive series between elemental Bi and Bi$_2$Se$_3$. The compound, based on Bi$_4$Se$_3$, is a 1:1 natural superlattice of alternating Bi$_2$ layers and Bi$_2$Se$_3$ layers; the inclusion of S allows the growth of large crystals, with the formula Bi$_4$Se$_{2.6}$S$_{0.4}$. The crystals cleave along the interfaces between the Bi$_2$ and Bi$_2$Se$_3$ layers, with the surfaces obtained having alternating Bi or Se termination. The resulting terraces, observed by photoemission electron microscopy, create avenues suitable for the study of one-dimensional topological physics. The electronic structure, determined by spin- and angle- resolved photoemission spectroscopy, shows the existence of a surface state that forms a large, hexagonally shaped Fermi surface around the $\Gamma$ point of the surface Brillouin zone, with the spin structure indicating that this material is a topological insulator., Comment: published version, 5 pages, 4 figures

We describe the crystal growth, crystal structure, and basic electrical properties of Bi2Te1.6S1.4, which incorporates both S and Te in its Tetradymite quintuple layers in the motif -[Te0.8S0.2]-Bi-S-Bi-[Te0.8S0.2]-. This material differs from other Tetradymites studied as topological insulators due to the increased ionic character that arises from its significant S content. Bi2Te1.6S1.4 forms high quality crystals from the melt and is the S-rich limit of the ternary Bi-Te-S {\gamma}-Tetradymite phase at the melting point. The native material is n-type with a low resistivity; Sb substitution, with adjustment of the Te to S ratio, results in a crossover to p-type and resistive behavior at low temperatures. Angle resolved photoemission study shows that topological surface states are present, with the Dirac point more exposed than it is in Bi2Te3 and similar to that seen in Bi2Te2Se. Single crystal structure determination indicates that the S in the outer chalcogen layers is closer to the Bi than the Te, and therefore that the layers supporting the surface states are corrugated on the atomic scale., Comment: To be published in Physical Review B Rapid Communications 16 douuble spaced pages. 4 figures 1 table

We demonstrate that the layered room temperature ferromagnet Fe7Se8 and the topological insulator Bi2Se3 form crystallographically oriented bulk composite intergrowth crystals. The morphology of the intergrowth in real space and reciprocal space is described. Critically, the basal planes of Bi2Se3 and Fe7Se8 are parallel and hence the good cleavage inherent in the bulk phases is retained. The intergrowth is on the micron scale. Both phases in the intergrowth crystals display their intrinsic bulk properties: the ferromagnetism of the Fe7Se8 is anisotropic, with magnetization easy axis in the plane of the crystals, and ARPES characterization shows that the topological surface states remain present on the Bi2Se3. Analogous behavior is found for what has been called "Fe-doped Bi2Se3."

We report the characterization of Bi$_2$Te$_2$Se crystals obtained by the modified Bridgman and Bridgman-Stockbarger crystal growth techniques. X-ray diffraction study confirms an ordered Se-Te distribution in the inner and outer chalcogen layers, respectively, with a small amount of mixing. The crystals displaying high resistivity ($> 1 \mathrm{\Omega cm}$) and low carrier concentration ($\sim 5\times 10^{16}$/cm$^3$) at 4 K were found in the central region of the long Bridgman-Stockbarger crystal, which we attribute to very small differences in defect density along the length of the crystal rod. Analysis of the temperature dependent resistivities and Hall coefficients reveals the possible underlying origins of the donors and acceptors in this phase., Comment: 16 pages, 5 figures, accepted by PRB

The structure and electronic order at the cleaved (001) surfaces of the newly-discovered pnictide superconductors BaFe$_{2-x}$Co$_{x}$As$_{2}$ with x ranging from 0 to 0.32 are systematically investigated by scanning tunneling microscopy. A $\sqrt{2}\times\sqrt{2}$ surface structure is revealed for all the compounds, and is identified to be Ba layer with half Ba atoms lifted-off by combination with theoretical simulation. A universal short-range charge order is observed at this $\sqrt{2}\times\sqrt{2}$ surface associated with an energy gap of about 30 meV for all the compounds.