32 research outputs found
Potential ring of Dirac nodes in a new polymorph of CaP
We report the crystal structure of a new polymorph of CaP, and an
analysis of its electronic structure. The crystal structure was determined
through Rietveld refinements of powder synchrotron x-ray diffraction data.
CaP is found to be a variant of the MnSi structure type, with a
Ca ion deficiency compared to the ideal 5:3 stoichiometry to yield a
charge-balanced compound. We also report the observation of a secondary phase,
CaPH, in which the Ca and P sites are fully occupied and the presence
of interstitial hydride ions creates a closed-shell electron-precise compound.
We show via electronic structure calculations of CaP that the compound
is stabilized by a gap in the density of states compared to the hypothetical
compound CaP. Moreover, the calculated band structure of CaP
indicates that it should be a three-dimensional Dirac semimetal with a highly
unusual ring of Dirac nodes at the Fermi level. The Dirac states are protected
against gap opening by a mirror plane in a manner analogous to graphene. The
results suggest that further study of the electronic properties of CaP
will be of interest
Z topology and superconductivity from symmetry lowering of a 3D Dirac Metal AuPb
3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons
and exhibit exotic physical properties It has been suggested that structurally
distorting a DSM can create a Topological Insulator (TI), but this has not yet
been experimentally verified. Furthermore, quasiparticle excitations known as
Majorana Fermions have been theoretically proposed to exist in materials that
exhibit superconductivity and topological surface states. Here we show that the
cubic Laves phase AuPb has a bulk Dirac cone above 100 K that gaps out upon
cooling at a structural phase transition to create a topologically non trivial
phase that superconducts below 1.2 K. The nontrivial Z = -1 invariant in
the low temperature phase indicates that AuPb in its superconducting state
must have topological surface states. These characteristics make AuPb a
unique platform for studying the transition between bulk Dirac electrons and
topological surface states as well as studying the interaction of
superconductivity with topological surface states
Comparative Electronic Structures of the Chiral Helimagnets Cr1/3NbS2 and Cr1/3TaS2
Magnetic materials with noncollinear spin textures are promising for
spintronic applications. To realize practical devices, control over the length
and energy scales of such spin textures is imperative. The chiral helimagnets
Cr1/3NbS2 and Cr1/3TaS2 exhibit analogous magnetic phase diagrams with
different real-space periodicities and field dependence, positioning them as
model systems for studying the relative strengths of the microscopic mechanisms
giving rise to exotic spin textures. Here, we carry out a comparative study of
the electronic structures of Cr1/3NbS2 and Cr1/3TaS2 using angle-resolved
photoemission spectroscopy and density functional theory. We show that bands in
Cr1/3TaS2 are more dispersive than their counterparts in Cr1/3NbS2 and connect
this result to bonding and orbital overlap in these materials. We also
unambiguously distinguish exchange splitting from surface termination effects
by studying the dependence of their photoemission spectra on polarization,
temperature, and beam size. We find strong evidence that hybridization between
intercalant and host lattice electronic states mediates the magnetic exchange
interactions in these materials, suggesting that band engineering is a route
toward tuning their spin textures. Overall, these results underscore how the
modular nature of intercalated transition metal dichalcogenides translates
variation in composition and electronic structure to complex magnetism.Comment: 46 pages, 18 figures, 5 table
Parp1 Localizes within the Dnmt1 Promoter and Protects Its Unmethylated State by Its Enzymatic Activity
Aberrant hypermethylation of CpG islands in housekeeping gene promoters and widespread genome hypomethylation are typical events occurring in cancer cells. The molecular mechanisms behind these cancer-related changes in DNA methylation patterns are not well understood. Two questions are particularly important: (i) how are CpG islands protected from methylation in normal cells, and how is this protection compromised in cancer cells, and (ii) how does the genome-wide demethylation in cancer cells occur. The latter question is especially intriguing since so far no DNA demethylase enzyme has been found.Our data show that the absence of ADP-ribose polymers (PARs), caused by ectopic over-expression of poly(ADP-ribose) glycohydrolase (PARG) in L929 mouse fibroblast cells leads to aberrant methylation of the CpG island in the promoter of the Dnmt1 gene, which in turn shuts down its transcription. The transcriptional silencing of Dnmt1 may be responsible for the widespread passive hypomethylation of genomic DNA which we detect on the example of pericentromeric repeat sequences. Chromatin immunoprecipitation results show that in normal cells the Dnmt1 promoter is occupied by poly(ADP-ribosyl)ated Parp1, suggesting that PARylated Parp1 plays a role in protecting the promoter from methylation.In conclusion, the genome methylation pattern following PARG over-expression mirrors the pattern characteristic of cancer cells, supporting our idea that the right balance between Parp/Parg activities maintains the DNA methylation patterns in normal cells. The finding that in normal cells Parp1 and ADP-ribose polymers localize on the Dnmt1 promoter raises the possibility that PARylated Parp1 marks those sequences in the genome that must remain unmethylated and protects them from methylation, thus playing a role in the epigenetic regulation of gene expression
The 2021 room-temperature superconductivity roadmap.
Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms.In memoriam, to Neil Ashcroft, who inspired us all
Electrically Conductive Metal–Organic Frameworks
Metal–organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in MOFs, yet it allows for diverse applications in electrocatalysis, charge storage, and chemiresistive sensing, among others. In this Review, we discuss the efforts undertaken so far to achieve efficient charge transport in MOFs. We focus on four common strategies that have been harnessed toward high conductivities. In the “through-bond” approach, continuous chains of coordination bonds between the metal centers and ligands’ functional groups create charge transport pathways. In the “extended conjugation” approach, the metals and entire ligands form large delocalized systems. The “through-space” approach harnesses the π–π stacking interactions between organic moieties. The “guest-promoted” approach utilizes the inherent porosity of MOFs and host–guest interactions. Studies utilizing less defined transport pathways are also evaluated. For each approach, we give a systematic overview of the structures and transport properties of relevant materials. We consider the benefits and limitations of strategies developed thus far and provide an overview of outstanding challenges in conductive MOFs.National Science Foundation (Grant 1122374
Consequences and control of multi-scale (dis)order in chiral magnetic textures
Transition metal-intercalated transition metal dichalcogenides (TMDs) are
promising platforms for next-generation spintronic devices based on their wide
range of electronic and magnetic phases, which can be tuned by varying the host
lattice or the identity of the intercalant, along with its stoichiometry and
spatial order. Some of these compounds host a chiral magnetic phase in which
the helical winding of magnetic moments propagates along a high-symmetry
crystalline axis. Previous studies have demonstrated that variation in
intercalant concentrations can have a dramatic impact on the formation of
chiral domains and ensemble magnetic properties. However, a systematic and
comprehensive study of how atomic-scale order and disorder impacts collective
magnetic behavior are so far lacking. Here, we leverage a combination of
imaging modes in the (scanning) transmission electron microscope (S/TEM) to
directly probe (dis)order across multiple length scales and show how subtle
changes in the atomic lattice can be leveraged to tune the mesoscale spin
textures and bulk magnetic response, with direct implications for the
fundamental understanding and technological implementation of such compounds
A Three‐Dimensional Porous Organic Semiconductor Based on Fully sp 2 ‐Hybridized Graphitic Polymer
Dimensionality plays an important role in the charge transport properties of organic semiconductors. Although three-dimensional semiconductors, such as Si, are common in inorganic materials, imparting electrical conductivity to covalent three-dimensional organic polymers is challenging. Now, the synthesis of a three-dimensional π-conjugated porous organic polymer (3D p-POP) using catalyst-free Diels–Alder cycloaddition polymerization followed by acid-promoted aromatization is presented. With a surface area of 801 m2 g−1, full conjugation throughout the carbon backbone, and an electrical conductivity of 6(2)×10−4 S cm−1 upon treatment with I2 vapor, the 3D p-POP is the first member of a new class of permanently porous 3D organic semiconductors.U.S. Department of Energy, Office of Basic Energy Sciences (Grant DESC0018235)National Science Foundation (Award 1122374