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Nickel-rich layered cathode materials and their correlation with the electronic structure are crucial to understanding the functionality of Li-ion batteries in the commercial deployment of electric vehicles. In this study,...

We report on the synthesis and electronic properties of epitaxial perovskite ${\mathrm{La}}_{1\text{\ensuremath{-}}x}{\mathrm{Sr}}_{x}\mathrm{Rh}{\mathrm{O}}_{3}$ thin films. Thin films with a Sr content ranging from $x=0$ to 0.5 have been grown using molecular beam epitaxy. Transport and x-ray photoemission spectroscopy data reveal an insulator-metal-insulator transition, accompanied by a $p$- to $n$-type carrier change observed in Hall measurements. Combined with theoretical calculations, we find that the addition of Sr does not directly dope carriers into the conduction band, but rather induces localized Rh $4d$ states within the $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_{3}$ band gap. The bandwidth of the impurity band increases with Sr content, eventually causing the valence band (VB) and the localized Rh $4d$ band to overlap, which explains the first insulator-to-metal transition occurring at $x=0.35$. For Sr content $xg0.4$, possible cation ordering results in an increase of the gap between the VB and the Rh $4d$ band, leading to the second metal-to-insulator transition. We map out the electronic phase diagram of the Sr-doped $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_{3}$ system and suggest strategies to engineer the electronic states in rhodate systems via delocalizing the ${\mathrm{Rh}}^{3+}$ states.

The unequal electronic occupation of localized orbitals (orbital polarization), and associated lowering of symmetry and degeneracy, play an important role in the properties of transition metal oxides. Here, we examine systematically the underlying origin of orbital polarization, taking as exemplar the $3d$ manifold of ${\mathrm{Co}}^{2+}$ in a variety of spin, orbital, and structural phases in the double perovskite ${\mathrm{La}}_{2}{\mathrm{CoTiO}}_{6}$ and the (001) superlattice ${({\mathrm{LaCoO}}_{3})}_{1}+{({\mathrm{LaTiO}}_{3})}_{1}$ systems. Superlattices are of specific interest due to the large experimentally observed orbital polarization of their Co cations. Based on first principles calculations, we find that robust and observable orbital polarization requires symmetry reduction through the lattice structure; the role of local electronic interactions is to greatly enhance the orbital polarization.

${\mathrm{LaCoO}}_{3}$ provides a poignant example of a transition metal oxide where the cobalt cations display multiple spin states and spin transitions and which continues to garner substantial attention. In this work, we describe first principles studies, based on $\mathrm{DFT}+U$ theory, of superlattices containing ${\mathrm{LaCoO}}_{3}$, specifically ${({\mathrm{LaCoO}}_{3})}_{n}+{({\mathrm{LaTiO}}_{3})}_{n}$ for $n=1,2$. The superlattices show strong electron transfer from Ti to Co resulting in ${\mathrm{Co}}^{2+}$, significant structural distortions and a robust orbital polarization of the ${\mathrm{Co}}^{2+}$. We predict high-spin ${\mathrm{Co}}^{2+}$ and a checkerboard (G-type) antiferromagnetic ground state. We provide a detailed analysis of the magnetic interactions and phases in the superlattices. We predict that ferromagnetic order on the ${\mathrm{Co}}^{2+}$ can be stabilized by hole doping (e.g., replacing La by Sr), which is rather unusual for ${\mathrm{Co}}^{2+}$ cations.

Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from

Through a combination of experimental measurements and theoretical modeling, we describe a strongly orbital-polarized insulating ground state in an ${({\mathrm{LaTiO}}_{3})}_{2}/{({\mathrm{LaCoO}}_{3})}_{2}$ oxide heterostructure. X-ray absorption spectra and ab initio calculations show that an electron is transferred from the titanate to the cobaltate layers. The charge transfer, accompanied by a large octahedral distortion, induces a substantial orbital polarization in the cobaltate layer of a size unattainable via epitaxial strain alone. The asymmetry between in-plane and out-of-plane orbital occupancies in the high-spin cobaltate layer is predicted by theory and observed through x-ray linear dichroism experiments. Manipulating orbital configurations using interfacial coupling within heterostructures promises exciting ground-state engineering for realizing new emergent electronic phases in metal oxide superlattices.

The original version of this article omitted the following from the Acknowledgements: “CAM and AL were supported by the NSF MRSEC program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634). Additionally, this research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under ‘Contract No. DE-AC02-05CH11231’.” This has now been corrected in both the PDF and HTML versions of the article.

In its orthorhombic T d polymorph, MoTe2 is a type-II Weyl semimetal, where the Weyl fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance and superconductivity were also observed in T d-MoTe2. Understanding the superconductivity in T d-MoTe2, which was proposed to be topologically non-trivial, is of eminent interest. Here, we report high-pressure muon-spin rotation experiments probing the temperature-dependent magnetic penetration depth in T d-MoTe2. A substantial increase of the superfluid density and a linear scaling with the superconducting critical temperature T c is observed under pressure. Moreover, the superconducting order parameter in T d-MoTe2 is determined to have 2-gap s-wave symmetry. We also exclude time-reversal symmetry breaking in the superconducting state with zero-field μSR experiments. Considering the strong suppression of T c in MoTe2 by disorder, we suggest that topologically non-trivial s +− state is more likely to be realized in MoTe2 than the topologically trivial s ++ state., Understanding the superconductivity in topological materials is of eminent interest. Here, Guguchia et al. report temperature-dependent magnetic penetration depth in the superconducting state of T d-MoTe2 under pressure, suggesting a topologically nontrivial s +− order parameter.

Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and $e\text{\ensuremath{-}}\text{ph}$ interaction with interfacial phonons in an iron-based superconductor ${\mathrm{Sr}}_{2}{\mathrm{VO}}_{3}\mathrm{FeAs}$ (${T}_{c}\ensuremath{\approx}33\text{ }\text{ }\mathrm{K}$) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the $e\text{\ensuremath{-}}\text{ph}$ coupling locally modulated by O vacancies in the ${\mathrm{VO}}_{2}$ layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.

We have explored a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single crystal Sr$_2$VO$_3$FeAs shows that a spin-polarized tunneling current can switch the Fe-layer magnetism into a non-trivial $C_4$ (2$\times$2) order, not achievable by thermal excitation with unpolarized current. Our tunneling spectroscopy study shows that the induced $C_4$ (2$\times$2) order has characteristics of plaquette antiferromagnetic order in Fe layer and strongly suppressed superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases the $C_4$ state. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors., Comment: 33 pages, 16 figures

Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial phonons in an iron-based superconductor Sr_{2}VO_{3}FeAs (T_{c}≈33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in the VO_{2} layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.

Re-based double perovskites (DPs) exhibit a complex interplay of structural and metal-insulator transitions. Here we systematically study the ground state electronic and structural properties for a family of Re-based DPs $A_2B$ReO$_6$ ($A$=Sr, Ca and $B$=Cr, Fe), which are related by a common low energy Hamiltonian, using density functional theory + $U$ calculations. We show that the on-site interaction $U$ of Re induces orbital ordering (denoted COO), with each Re site having an occupied $d_{xy}$ orbital and a C-type alternation among $d_{xz}/d_{yz}$, resulting in an insulating state consistent with experimentally determined insulators Sr$_2$CrReO$_6$, Ca$_2$CrReO$_6$, and Ca$_2$FeReO$_6$. The threshold value of $U_{Re}$ for orbital ordering is reduced by inducing $E_g$ octahedral distortions of the same C-type wavelength (denoted COD), which serves as a structural signature of the orbital ordering; octahedral tilting also reduces the threshold. The COO, and the concomitant COD, are a spontaneously broken symmetry for the Sr based materials (i.e. $a^0a^0c^-$ tilt pattern), while not for the Ca based systems (i.e. $a^-a^-b^+$ tilt pattern). Spin-orbit coupling does not qualitatively change the physics of the COO/COD, but can induce relevant quantitative changes. We prove that a single set of $U_{Cr}$,$U_{Fe}$,$U_{Re}$ capture the experimentally observed metallic state in Sr$_2$FeReO$_6$ and insulating states in the other three systems. We predict that the COO is the origin of the insulating state in Sr$_2$CrReO$_6$, and that the concomitant COD may be experimentally observed at sufficiently low temperatures (ie. space group P$4_2/m$). Additionally, given our prescribed values of $U$, we show that the COO induced insulating state in Ca$_2$CrReO$_6$ will survive even if the COD amplitude is suppressed (e.g. due to thermal fluctuations)., Comment: 29 pages, 20 figures, 7 tables

We investigate the effects of vacancy defects on the electronic and transport properties of carbon nanotubes through density functional calculations. In both cases, where vacancies aggregate into larger clusters and are disordered, conductance changes from metallic to insulating regime, while their origins are different. For small vacancy clusters, the suppression of conductance is led by the defect states associated with π-topological and σ-dangling bond defects, while the local gap opening plays a role for large vacancy clusters. In disordered tubes with various types of vacancy defects, conductance decreases exponentially due to the Anderson localization. The localization length not only depends on the type of vacancy defects but also the tube chirality.

We investigate a magnetic proximity effect on the Dirac surface states of a topological insulator (TI) induced by a ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}/\mathrm{EuS}$ interface, using density-functional theory (DFT) and a low-energy effective model, motivated by a recent experimental realization of the interface. We consider a thin ferromagnetic insulator EuS film stacked on top of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$(111) slabs of three or five quintuple layers (QLs) with the magnetization of EuS normal to the interface ($z$ axis), which breaks time-reversal symmetry. It is found that a charge transfer and surface relaxation makes the Dirac cones electron doped. For both three and five QLs, the top-surface Dirac cone has an energy gap of 9 meV, while the bottom-surface Dirac cone remains gapless. This feature is due to the short-ranged induced magnetic moment of the EuS film. For the five QLs, an additional Dirac cone with an energy gap of 2 meV is formed right below the bottom-surface Dirac point, while for three QLs, there is no additional Dirac cone. We also examine the spin-orbital texture of the Dirac surface states with broken time-reversal symmetry, using DFT and the effective model. We find that the ${p}_{z}$ orbital is coupled to the $z$ component of the spin moment in the opposite sign to the ${p}_{x}$ and ${p}_{y}$ orbitals. The ${p}_{z}$ and radial $p$ orbitals are coupled to the in-plane spin texture in the opposite handedness to the tangential $p$ orbital. The result obtained from the effective model agrees with our DFT calculations. The calculated spin-orbital texture may be tested from spin-polarized angle-resolved photoemission spectroscopy.

The first-principles density functional theory calculations have been performed to understand LaTiO${}_{3}$/LaAlO${}_{3}$ superlattice. By taking into account of the structural distortions, $U$ dependence, and the exchange correlation functional dependence, we show that the ferromagnetic spin and antiferro-orbital ordering is stabilized in the wide range of strains, which is notably different from the previous reports on the titanate systems. The ground-state spin and orbital configurations critically depend on the structural properties. Our results suggest a possible strain control of the magnetic property in transition-metal oxide heterostructures.

Using first-principles density functional theory calculations, we investigated the electronic structure and magnetic properties of (LaMnO${}_{3}$)${}_{m}$/(LaNiO${}_{3}$)${}_{n}$ superlattices stacked along the (001) direction. The electrons are transferred from Mn to Ni, and the magnetic moments are induced at Ni sites that are paramagnetic in bulk and other types of superlattices. The size of the induced moment is linearly proportional to the amount of transferred electrons, but it is larger than the net charge transfer. The charge transfer and magnetic properties of the $(m,n)$ superlattice can be controlled by changing the $m/n$ ratio. Considering the ferromagnetic couplings between Mn and Ni spins and the charge-transfer characteristic, we propose the (2,1) superlattice as the largest moment superlattice. carrying $\ensuremath{\sim}$8${\ensuremath{\mu}}_{B}$ per formula unit.

We investigate the Schottky barrier and effective work function (EWF) at TiN/HfO${}_{2}$ interface through density functional calculations. For different interfaces that consist of either Ti-O or N-Hf interface bonds, the intrinsic metal-induced gap states are nearly independent of the interface structure, with similar decay lengths into the oxide. Due to the weak Fermi-level pinning, the EWF is more sensitive to the extrinsic effect of interface bonding. As N-rich interface bonds are replaced by O-rich bonds, the EWF decreases by up to 0.36 eV, which is attributed to the formation of opposing interface dipoles. To improve the band gap and EWF, we perform both hybrid functional and quasiparticle (QP) calculations. In the $G{W}_{0}$ approximation, in which the Green's function is self-consistently calculated by updating only QP energies and the full frequency-dependent dielectric function is used, the agreement of the EWF with experiment is greatly improved, while QP calculations at the ${G}_{0}{W}_{0}$ level or using the plasmon-pole dielectric function tend to overestimate the EWF. In the self-consistent $GW$ approach, in which both QP energies and wave functions are updated in iterations, the band gap is overestimated, resulting in the lower EWF. On the other hand, the EWF is severely underestimated with the hybrid functional because of the larger shift of the valence band edge level of HfO${}_{2}$.

We report the results of first-principles calculations for the atomic, electronic, and magnetic properties of the Co atoms embedded in divacancy defects in graphene nanoribbons with zigzag-shaped edges. We find that the edges play an important role in the stabilization of a Co-divacancy complex near the edge, where the edge C atoms undergo large relaxations, resulting in a tilted-edge structure. When Co is positioned in the middle of the ribbon, the edge C atoms generally remain on the sheet, forming a flat-edge structure due to the small edge effect. The spin polarization of the edges is determined by two bipartite lattices separated by the Co atom. When a Co-divacancy pair is formed near the same edge, the edge structure is significantly modified. The magnetic interaction between the Co atoms is either ferromagnetic or antiferromagnetic, depending on the relative positions of the Co atoms, and its magnetic ordering can be described by the combined effect of the bipartite lattices formed around individual Co atoms.

Graphene magnetism has been proposed but based on thermodynamically unstable zigzag edges and dangling electrons with broken sublattice symmetry. From results of first-principles calculations, we propose a way to realize thermodynamically stable graphene ferromagnetism by seamlessly incorporating transition metals into the graphene honeycomb network. An Fe atom substituting a carbon-carbon dimer of graphene can result in nearly square-planar covalent bonding between the spin-polarized Fe 3$d$ orbitals and graphene dangling bond states. Dangling bond passivation of the divacancy pore with N and O strongly affects the Fe incorporation into the graphene network in terms of energetics and electronic structure. The Fe-N${}_{4}$ or Fe-C${}_{4}$ incorporated graphene is predicted to show long-range ferromagnetism particularly due to carrier mediation when electron doped.

The title nanowires are synthesized for the first time by a chemical vapor transport process without any catalyst using a 1:1 Ge/C powder mixture and FeI2 as precursors (650870 °C, 10 min).

Single-crystalline free-standing hexagonal Fe(1.3)Ge nanowires (NWs) are synthesized for the first time using a chemical vapor transport process without using any catalyst. Interestingly, Fe(1.3)Ge NWs are found to be ferromagnetic at room temperature, while bulk Fe(1.3)Ge has the lower critical temperature of 200 K. We perform first-principles density functional calculations and suggest that the observed strong ferromagnetism is attributed to the reduced distances between Fe atoms, increased number of Fe-Fe bonds, and the enhanced Fe magnetic moments. Both experimental and theoretical studies show that the magnetic moments are enhanced in the NWs, as compared to bulk Fe(1.3)Ge. We also modulate the composition ratio of as-grown iron germanide NWs by adjusting experimental conditions. It is shown that uniaxial strain on the hexagonal plane also enhances the ferromagnetic stability.

Vacancies in carbon nanotubes usually aggregate into larger vacancies. Using first-principles and tight-binding calculations, we investigate the alignment of missing atoms and the movement of pentagon-heptagon defects that are formed by reconstructions in large vacancy clusters ${V}_{n}$ $(n\ensuremath{\le}36)$, where $n$ is the number of missing atoms. In nanotubes with small diameters, missing atoms have a tendency to form a serial network rather than a large hole due to the existence of large curvatures. It is generally found that the parallel alignment of missing atoms along the tube axis is energetically more favorable than the spiral alignment. Thus, the removal of atoms leads to the longitudinal movement of a pentagon-heptagon defect on the tube wall, which is in good agreement with the kink motion observed during superplastic deformation of single-wall nanotubes. The preference of the longitudinal motion of the pentagon-heptagon defect is more prominent in armchair tubes compared with other chiral tubes.

Chemical protein synthesis is important for dissecting the molecular basis of protein function. Here we advance its scope by demonstrating the significant improvement of the multifaceted pharmaceutical profile of small proteins exclusively via a chemical-based approach. The focus of this work centered on CCL-5 (RANTES) derivatives with potent anti-HIV activity. The overall chemical strategy involved a combination of coded and noncoded amino acid mutagenesis, peptide backbone engineering, and site-specific polymer attachment. The ability to alter specific protein residues, as well as precise control of the position and type of polymer attachment, allows for the exploration of specific molecular designs and resulted in novel CCL-5 analogues with significant differences in their respective biochemical and pharmaceutical properties. Using this approach, the complex-interplay of variables contributing to the noncovalent self-association (aggregation) state, CCR-5 specificity, in vivo elimination half-life, and anti-HIV activity of CCL-5-based protein analogues could be empirically evaluated via total chemical synthesis. This work has led to the identification of potent (sub-nanomolar) anti-HIV proteins with significantly improved pharmaceutical profiles, and illustrates the increasing value of protein chemical synthesis in contemporary therapeutic discovery. These antiviral molecules provide a novel mechanism of action for the development of a new generation of anti-HIV therapeutics which are still desperately needed.

We report the results of action-derived molecular dynamics simulations for the migration and coalescence processes of monovacancies in graphene and carbon nanotubes with different chiralities. In carbon nanotubes, the migration pathways and barriers of a monovacancy depend on the tube chirality, while there is no preferential pathway in graphene due to the lattice symmetry and the absence of the curvature effect. The probable pathway changes from the axial to circumferential direction as the chirality varies from armchair to zigzag. The chirality dependence is attributed to the preferential orientation of the reconstructed bond formed around each vacancy site. It is energetically more favourable for two monovacancies to coalesce into a divacancy via alternative movements rather than simultaneous movements. The energy barriers for coalescence are generally determined by the migration barrier for the monovacancy, although there are some variations due to interactions between two diffusing vacancies. In graphene and armchair nanotubes, two monovacancies prefer to migrate along different zigzag atomic chains rather than a single atomic chain connecting these vacancies. On the other hand, in zigzag tubes, the energy barrier for coalescence increases significantly unless monovacancies lie on the same circumference.