Creating superconductivity in WB2 through pressure-induced metastable planar defects.

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, Tcof 17 K at 91 GPa. Upon further compression up to 187 GPa, the Tcgradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2 (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials.

Ancient DNA confirmation of lepromatous leprosy in a skeleton with concurrent osteosarcoma, excavated from the leprosarium of St. Mary Magdalen in Winchester, Hants., UK.

To establish a biological profile and disease aetiologies for one of four burials recovered during a Time Team dig at the St. Mary Magdalen leprosarium, Winchester, UK in AD 2000. Osteological techniques were applied to estimate age at death, biological sex, stature and pathology. Visual assessment of the material was supplemented by radiographic examination. Evidence for leprosy DNA was sought using ancient DNA (aDNA) analysis. The remains are those of a male individual excavated from a west-east aligned grave. The skeleton shows signs of two pathologies. Remodelling of the rhino-maxillary area and degenerative changes to small bones of the feet and reactive bone on the distal lower limbs suggest a multibacillary form of leprosy, whereas the right tibia and fibula show the presence of a primary neoplasm identified as an osteosarcoma. The aDNA study confirmed presence of Mycobacterium leprae in several skeletal elements, and the strain was genotyped to the 3I lineage, one of two main SNP types present in mediaeval Britain and ancestral to extant strains in America. This is a rare documentation of leprosy in association with a primary neoplasm.

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.

A case of childhood tuberculosis from late mediaeval Somerset, England.

BACKGROUND: The remains of a 3-5 year-old child from the late mediaeval cemetery serving the Priory of St. Peter and St. Paul, Taunton, Somerset, UK was the subject of an aDNA study. OBJECTIVE: The aim was to distinguish between two differential diagnoses suggested by earlier osteological examination of the remains; either tuberculosis or Langerhans cell histiocytosis. FINDINGS: The remains tested positive for MTB complex markers, corroborating this diagnosis reached on osteological grounds. Based on positivity for the mtp40 element and a deletion in the pks15/1 locus, we conclude that infection was due to a strain of the human pathogen M.tuberculosis belonging to lineage 4. Although DNA recovered from the case was heavily fragmented, sex determination by amelogenin PCR suggested these are the remains of a young male child. The findings are discussed considering additions to the literature since the original report. CONCLUSIONS: Descriptions of tuberculosis in children from this period are rare and burial Sk2077 represents the first UK example of a pre-adolescent individual to have a molecular diagnosis combined with osteological pathology. This provides an important reference of childhood tuberculosis and insight into the likely presence of tuberculosis in the mediaeval adult population served by this cemetery.

A15 Nb3Si: a 'high'Tcsuperconductor synthesized at a pressure of one megabar and metastable at ambient conditions.

A15 Nb3Si is, until now, the only 'high' temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb3Si and its production from tetragonal Nb3Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb3Si material to traceTcas a function of pressure.Tcis suppressed to ∼5.2 K at 88 GPa. Then, using theseTc(P) data for A15 Nb3Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb3Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 Nb3Si. This is in contrast to the explosive compression (up toP∼ 110 GPa) of tetragonal Nb3Si, which produced 50%-70% A15 material,Tc= 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 Nb3Si structural transformation. Our theoretical calculations show that A15 Nb3Si has an enthalpy vs the tetragonal structure that is 70 meV atom-1smallerat 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom-1. The fact that 'annealing' the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom-1.

Superconductivity in a unique type of copper oxide.

The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics. High-T c cuprates crystallize into a layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high-T c cuprates are elongated along the c axis, leading to a 3dx 2-y 2 orbital at the top of the band structure wherein the doped holes reside. This scenario gives rise to 2D characteristics in high-T c cuprates that favor d-wave pairing symmetry. Here, we report superconductivity in a cuprate Ba2CuO4-y , wherein the local octahedron is in a very exceptional compressed version. The Ba2CuO4-y compound was synthesized at high pressure at high temperatures and shows bulk superconductivity with critical temperature (T c ) above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the T c for the isostructural counterparts based on classical La2CuO4 X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron, the 3d3z 2-r 2 orbital will be lifted above the 3dx 2-y 2 orbital, leading to significant 3D nature in addition to the conventional 3dx 2-y 2 orbital. This work sheds important light on advancing our comprehensive understanding of the superconducting mechanism of high T c in cuprate materials.

Unusual effects of Be doping in the iron-based superconductor FeSe.

Recent superconducting transition temperatures (T c) over 100 K for monolayer FeSe on SrTiO3 have renewed interest in the bulk parent compound. In KCl:AlCl3 flux-transport-grown crystals of FeSe0.94Be0.06, FeSe0.97Be0.03 and, for comparison, FeSe, this work reports doping of FeSe using Be-among the smallest of possible dopants, corresponding to an effective 'chemical pressure'. According to lattice parameter measurements, 6% Be doping shrank the tetragonal FeSe lattice equivalent to a physical pressure of 0.75 GPa. Using this flux-transport method of sample preparation, 6% of Be was the maximum amount of dopant achievable. At this maximal composition of FeSe0.94Be0.06, the lattice unit cell shrinks by 2.4%, T c-measured in the bulk via specific heat-increases by almost 10%, the T c versus pressure behavior shifts its peak [Formula: see text] downwards by ~1 GPa, the high temperature structural transition around T S = 89 K increases by 1.9 K (in contrast to other dopants in FeSe which uniformly depress T S), and the low temperature specific heat γ increases by 10% compared to pure FeSe. Also, upon doping by 6% Be the residual resistivity ratio, ρ(300 K)/ρ(T → 0), increases by almost a factor of four, while ρ(300 K)/ρ([Formula: see text]) increases by 50%.

Bi-2212/1T-TaS2 Van der Waals junctions: Interplay of proximity induced high-T c superconductivity and CDW order.

Understanding the coexistence, competition and/or cooperation between superconductivity and charge density waves (CDWs) in the transition metal dichalcogenides (TMDs) is an elusive goal which, when realized, promises to reveal fundamental information on this important class of materials. Here, we use four-terminal current-voltage measurements to study the Van der Waals interface between freshly exfoliated flakes of the high-T c superconductor, Bi-2212, and the CDW-dominated TMD layered material, 1T-TaS2. For highly transparent barriers, there is a pronounced Andreev reflection feature providing evidence for proximity-induced high-T c superconductivity in 1T-TaS2 with a surprisingly large energy gap (~20 meV) equal to half that of intrinsic Bi-2212 (~40 meV). Our systematic study using conductance spectroscopy of junctions with different transparencies also reveals the presence of two separate boson modes, each associated with a "dip-hump" structure. We infer that the proximity-induced high-T c superconductivity in the 1T-TaS2 is driven by coupling to the metastable metallic phase coexisting within the Mott commensurate CDW (CCDW) phase and associated with a concomitant change of the CCDW order parameter in the interfacial region.

Superconducting properties of the s ±-wave state: Fe-based superconductors.

Although the pairing mechanism of Fe-based superconductors (FeSCs) has not yet been settled with consensus with regard to the pairing symmetry and the superconducting (SC) gap function, the vast majority of experiments support the existence of spin-singlet sign-changing s-wave SC gaps on multi-bands ([Formula: see text]-wave state). This multi-band [Formula: see text]-wave state is a very unique gap state per se and displays numerous unexpected novel SC properties, such as a strong reduction of the coherence peak, non-trivial impurity effects, nodal-gap-like nuclear magnetic resonance signals, various Volovik effects in the specific heat (SH) and thermal conductivity, and anomalous scaling behaviors with a SH jump and condensation energy versus T c, etc. In particular, many of these non-trivial SC properties can easily be mistaken as evidence for a nodal-gap state such as a d-wave gap. In this review, we provide detailed explanations of the theoretical principles for the various non-trivial SC properties of the [Formula: see text]-wave pairing state, and then critically compare the theoretical predictions with experiments on FeSCs. This will provide a pedagogical overview of to what extent we can coherently understand the wide range of different experiments on FeSCs within the [Formula: see text]-wave gap model.

Pressure-induced superconductivity in the giant Rashba system BiTeI.

At ambient pressure, BiTeI exhibits a giant Rashba splitting of the bulk electronic bands. At low pressures, BiTeI undergoes a transition from trivial insulator to topological insulator. At still higher pressures, two structural transitions are known to occur. We have carried out a series of electrical resistivity and AC magnetic susceptibility measurements on BiTeI at pressure up to ∼40 GPa in an effort to characterize the properties of the high-pressure phases. A previous calculation found that the high-pressure orthorhombic P4/nmm structure BiTeI is a metal. We find that this structure is superconducting with T c values as high as 6 K. AC magnetic susceptibility measurements support the bulk nature of the superconductivity. Using electronic structure and phonon calculations, we compute T c and find that our data is consistent with phonon-mediated superconductivity.

Mycobacterium tuberculosis ESAT-6 is a leukocidin causing Ca2+ influx, necrosis and neutrophil extracellular trap formation.

Mycobacterium tuberculosis infection generates pulmonary granulomas that consist of a caseous, necrotic core surrounded by an ordered arrangement of macrophages, neutrophils and T cells. This inflammatory pathology is essential for disease transmission and M. tuberculosis has evolved to stimulate inflammatory granuloma development while simultaneously avoiding destruction by the attracted phagocytes. The most abundant phagocyte in active necrotic granulomas is the neutrophil. Here we show that the ESAT-6 protein secreted by the ESX-1 type VII secretion system causes necrosis of the neutrophils. ESAT-6 induced an intracellular Ca(2+) overload followed by necrosis of phosphatidylserine externalised neutrophils. This necrosis was dependent upon the Ca(2+) activated protease calpain, as pharmacologic inhibition prevented this secondary necrosis. We also observed that the ESAT-6 induced increase in intracellular Ca(2+), stimulated the production of neutrophil extracellular traps characterised by extruded DNA and myeloperoxidase. Thus we conclude that ESAT-6 has a leukocidin function, which may facilitate bacterial avoidance of the antimicrobial action of the neutrophil while contributing to the maintenance of inflammation and necrotic pathology necessary for granuloma formation and TB transmission.

The specific heat of the electron-doped La-1038 compound (Ca0.85La0.15)10(FeAs)10(Pt3As8).

The specific heat of polycrystalline (Ca0.85La0.15)10(FeAs)10(Pt3As8), an electron-doped iron-based superconductor (T(c)(onset) = 34.6 K) with Ca/La ions and Pt3As8 separating the FeAs layers, was measured between 0.4 and 48 K.This compound has been recently reported to represent an electron-doped variant of the non-superconducting 10-3-8 phase, featuring a superconducting transition in the range of that of the 10-4-8 phase. This family of compounds is unique among the iron pnictide superconductors discovered to date due to the second metal pnictide layer, Pt3As8, present in the structure competing with the familiar FeAs layer for the electron from the Ca/La. This superconductor is further unusual in that it has a rather low crystalline symmetry (triclinic) for such a high superconducting transition temperature. The specific heat γ is found to be approximately 26 mJ/(Ca/La mol)K(2), comparable to 122 iron-based superconductors electron-doped on the Fe sites and a factor of two smaller than 122 compounds hole-doped on the cation site, e.g., Ba(1-x)K(x)Fe2As2. The present work also investigates the discontinuity in the specific heat at T(c), ΔC, to compare with the global trend, established by Bud'ko, Ni and Canfield (BNC), of ΔC/T(c) versus T(c) found for essentially all iron-based superconductors. The result is a value lower than the BNC trend by a factor of ten, consistent with a severely broadened superconducting transition.

d-f hybridization and quantum criticality in weakly-itinerant ferromagnets.

We investigate the unusual magnetic, thermodynamic and transport properties of nearly-critical, weakly-itinerant ferromagnets with the general formula UTX, where T=Rh, Co and X=Ge, Si. As a unique feature of these systems, we show how changes in the V(df) hybridization, which controls their proximity to a ferromagnetic instability, determine the evolution of the ground state magnetization, M(0), the Curie temperature, T(C), the density of states at the Fermi level, N(E(F)), the T(2) resistivity coefficient, A, and the specific heat coefficient, γ. The universal aspect of our findings comes from the dependence on only two parameters: the transition metal T(d) bandwidth, W(d), and the distance between the T(d) and U(f) band centers, C(T(d)) - C(U(f)). We discuss our results in connection to data for URh(1-x)Co(x)Ge.

Specific heat versus field for LiFe(1-x)Cu(x)As.

LiFeAs is one of the new class of iron superconductors with a bulk [Formula: see text] in the 15-17 K range. We report on the specific heat characterization of single crystal material prepared by self-flux growth techniques with significantly improved properties, including a much decreased residual gamma, γ(r) (≡C/T as T → 0), in the superconducting state. Thus, in contrast to previous explanations of a finite γ(r) in LiFeAs being due to intrinsic states in the superconducting gap, the present work shows that such a finite residual γ in LiFeAs is instead a function of sample quality. Further, since LiFeAs has been characterized as nodeless with multiple superconducting gaps, we report here on its specific heat properties in zero and applied magnetic fields, to compare to similar results on nodal iron superconductors. For comparison, we also investigate LiFe(0.98)Cu(0.02)As, which has the reduced T(c) of ≈9 K and an H(c2) of 15 T. Interestingly, although presumably both LiFeAs and LiFe(0.98)Cu(0.02)As are nodeless, they clearly show a non-linear dependence of the electronic density of states (is proportional to specific heat γ) at the Fermi energy in the mixed state with the applied field, similar to the Volovik effect for nodal superconductors. However, rather than indicating nodal behavior, the satisfactory comparison with a recent theory for γ(H) for a superconductor with two isotropic gaps in the presence of impurities argues for nodeless behavior. Thus, in terms of specific heat in a magnetic field, LiFeAs can serve as the prototypical multiband, nodeless iron superconductor.

Specific heat discontinuity, ΔC, at Tc in BaFe2(As0.7P0.3)2-consistent with unconventional superconductivity.

We report the specific heat discontinuity, ΔC/T(c), at T(c) = 28.2 K of a collage of single crystals of BaFe(2)(As(0.7)P(0.3))(2) and compare the measured value of 38.5 mJ mol(-1) K(-2) with other iron pnictide and iron chalcogenide (FePn/Ch) superconductors. This value agrees well with the trend established by Bud'ko, Ni and Canfield, who found that ΔC/T(c) is proportional to aT(c)(2) for 14 examples of doped Ba(1 - x)K(x)Fe(2)As(2) and BaFe(2 - x)TM(x)As(2), where the transition metal TM = Co and Ni. We extend their analysis to include all the FePn/Ch superconductors for which ΔC/T(c) is currently known and find ΔC/T(c) is proportional to aT(c)(1.9) and a = 0.083 mJ mol(-1) K(-4). A comparison with the elemental superconductors with T(c) > 1 K and with A-15 superconductors shows that, contrary to the FePn/Ch superconductors, electron-phonon-coupled conventional superconductors exhibit a significantly different dependence of ΔC on T(c), namely ΔC/T(c) is proportional to aT(c)(1.9). However ΔC/γT(c) appears to be comparable in all three classes (FePn/Ch, elemental and A-15) of superconductors with, for example, ΔC/γT(c) = 2.4 for BaFe(2)(As(0.7)P(0.3))(2). A discussion of the possible implications of these phenomenological comparisons for the unconventional superconductivity believed to exist in the FePn/Ch is given.

Evidence for coexistence of superconductivity and magnetism in single crystals of Co-doped SrFe(2)As(2).

In order to investigate whether magnetism and superconductivity coexist in Co-doped SrFe(2)As(2), we have prepared single crystals of SrFe(2-x)Co(x)As(2), x = 0 and 0.4, and characterized them via x-ray diffraction, electrical resistivity in zero and applied field up to 9 T as well as at ambient and applied pressure up to 1.6 GPa, and magnetic susceptibility. At x = 0.4, there is both magnetic and resistive evidence for a spin density wave transition at 120 K, while T(c) = 19.5 K-indicating coexistent magnetism and superconductivity. A discussion of how these results compare with reported results, both in SrFe(2-x)Co(x)As(2) and in other doped 122 compounds, is given.

Specific heat anomalies for [Formula: see text] in superconducting single crystal doped BaFe(2)As(2): comparison of different flux growth methods.

One way to address the nature of the superconductivity in the new iron pnictides is to measure the low temperature specific heat in the superconducting state, where the temperature, field, and angular dependences of the specific heat each give important information. We report on an initial study of the specific heat down to 0.4 K in single crystals of Ba(0.6)K(0.4)Fe(2)As(2), T(c) = 32 K, prepared via Sn-flux and In-flux methods and compare to literature data for samples prepared using the self-flux method. We also report on the specific heat in zero and 1 T applied magnetic fields of Ba(Fe(0.926)Co(0.074))(2)As(2), T(c) = 22 K, prepared via the In-flux method. All samples show upturns in the specific heat divided by temperature below 2 K, with the upturn in the Sn-flux sample starting already at 4 K. These upturns, which are strongly dependent on the preparation method, impede determination of the intrinsic properties.

Superconductivity and disorder in PrOs(4)Sb(12).

The specific heat, and dc and ac magnetic susceptibility are reported for a large single crystal of PrOs(4)Sb(12) and, after grinding, its powder. The physical properties of the crystal are typical of the majority of reported PrOs(4)Sb(12) samples. The room temperature effective paramagnetic moment of the crystal was consistent with the Pr(3+) ionic configuration and full (or nearly full) occupancy of the Pr sublattice. The crystal showed two distinct anomalies in the specific heat and an overall discontinuity in C/T of approximately 1000 mJ K(-2) mol(-1). The upper transition (at T(c1)) was characteristically rounded. The anomaly at T(c2) was very sharp, consistent with a good quality for the crystal. We observed a shoulder in χ(') and two peaks in χ('') below T(c1). However, there were no signatures in χ(') of the lower temperature transition. Grinding to powder size smaller than 50 µm completely suppresses the upper superconducting transition in both the specific heat and magnetic susceptibility. It also strongly reduces ΔC/T(c) at T(c2). Stress annealing brings back some of this lost ΔC/T(c) but does not restore the upper temperature transition. Possible explanations of the existence of two superconducting specific heat anomalies for single crystals are discussed.

Superconductivity in undoped single crystals of BaFe(2)As(2): field and current dependence.

In previous work on undoped MFe(2)As(2), partial drops in the resistivity indicative of traces of superconductivity have been observed for some samples with M =  Ba (T(c)∼20 K, up to 25% drop in ρ) and M =  Ca (T(c)∼10 K, up to 45% drop in ρ). A complete drop in the resistivity to ρ = 0, along with a finite fraction of Meissner flux expulsion, has been observed for M  = Sr, T(c) = 22 K. Using In-flux grown single crystal samples of undoped BaFe(2)As(2), we find a complete drop in the resistivity to 0 for most samples beginning at T(c)(onset) = 22.5 K. However-in contrast to the SrFe(2)As(2) results-there is no measurable Meissner effect and no suppression of the resistive superconducting transition with annealing. The current sensitivity of the superconducting resistive transition in our samples of BaFe(2)As(2) is quite strong, with an increase in the current density of a factor of 15 to ∼1.5 A cm(-2) not changing T(c)(onset) but broadening the transition significantly and causing ρ to remain finite as [Formula: see text]. To investigate whether this unusually low critical current is indicative of filamentary conduction lacking the apparent anisotropy seen in the critical magnetic field, H(c2), measurements for, e.g., the bulk superconductor Co-doped BaFe(2)As(2), H(c2) was measured in both crystalline directions. These BaFe(2)As(2) samples show H(c2)(T) values in the ab-plane and along the c-axis comparable to those seen for BaFe(2-x)Co(x)As(2), which has a similar T(c). Since the lack of T(c) suppression after annealing argues against strain-induced superconductivity as proposed for the other undoped MFe(2)As(2) materials, another possible cause for the superconductivity in BaFe(2)As(2) is discussed.