Syntheses, crystal structure and magnetic properties of Tl9RETe6 (RE = Ce, Sm, Gd)

The three compounds Tl9RETe6 with RE = Ce, Sm, Gd were synthesized from the elements at 1020 K. Their isostructural crystal structures are ordered derivatives of the Tl5Te3 type with rare-earth metal and thallium occupying different Wyckoff positions. The structures can be understood as charge-ordered in accordance with the Zintl-Klemm concept: 9 Tl+ + RE3+ + 6 Te2-. DFT calculations for Tl9GdTe6, however, result in a low, but finite density of states at the Fermi level. Magnetic data confirm trivalent Gd, but indicate a small amount of Ce4+ in Tl9CeTe6; no indications for long-range magnetic order was found down to T = 2 K

Generation of Sub-nanosecond H Atom Pulses for Scattering from Single-Crystal Epitaxial Graphene

Pulsed molecular beams allow high-density gas samples to be cooled to low internal temperatures and to produce narrow speed distributions. They are particularly useful in combination with pulsed-laser-based detection schemes and have also been used as pump pulses in pump–probe experiments with neutral matter. The mechanical response of pulsed valves and chopper wheels limits the duration of these pulses typically to about 10–100 μs. Bunch compression photolysis has been proposed as a means to produce atomic pulses shorter than 1 ns─an experimental capability that would allow new measurements to be made on chemical systems. This technique employs a spatially chirped femtosecond duration photolysis pulse that produced an ensemble of H atom photoproducts that rebunches into a short pulse downstream. To date, this technique could not produce strong enough beams to allow new experiments to be carried out. In this paper, we report production of pulsed H atom beams consistent with a 700 ps pulse duration and with sufficient intensity to carry out differentially resolved inelastic H scattering experiments from a graphene surface. We observe surprisingly narrow angular distributions for H atoms incident normal to the surface. At low incidence energies quasi-elastic scattering dominates, and at high incidence energy we observe a strongly inelastic scattering channel. These results provide the basis for future experiments where the H atoms synchronously collide with a pulsed-laser-excited surface

Physical realization of a quantum spin liquid based on a novel frustration mechanism

Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. Such phenomena is usually sought in simple lattices where antiferromagnetic interactions and/or anisotropies that favor specific alignments of the magnetic moments are "frustrated" by lattice geometries incompatible with such order e.g. triangular structures. Despite an extensive search among such compounds, experimental realizations remain very few. Here we describe the investigation of a novel, unexplored magnetic system consisting of strong ferromagnetic and weaker antiferromagnetic isotropic interactions as realized by the compound Ca$_{10}$Cr$_7$O$_{28}$. Despite its exotic structure we show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid including coherent spin dynamics in the ground state and the complete absence of static magnetism.Comment: Modified version accepted in Nature Physic

Critical exponents and intrinsic broadening of the field-induced transition in NiCl2_2⋅\cdot4SC(NH2_2)2_2

The field-induced ordering transition in the quantum spin system NiCl$_2$$\cdot$4SC(NH$_2$)$_2$ is studied by means of neutron diffraction, AC magnetometry and relaxation calorimetry. The interpretation of the data is strongly influenced by a finite distribution of transition fields in the samples, which was present but disregarded in previous studies. Taking this effect into account, we find that the order-parameter critical exponent is inconsistent with the BEC universality class even at temperatures below 100 mK. All results are discussed in comparison with previous measurements and with recent similar studies of disordered Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$

The Ernst equation and ergosurfaces

We show that analytic solutions \mcE of the Ernst equation with non-empty zero-level-set of \Re \mcE lead to smooth ergosurfaces in space-time. In fact, the space-time metric is smooth near a "Ernst ergosurface" $E_f$ if and only if \mcE is smooth near $E_f$ and does not have zeros of infinite order there.Comment: 23 pages, 4 figures; misprints correcte

Anomalous metamagnetism in the low carrier density Kondo lattice YbRh3Si7

We report complex metamagnetic transitions in single crystals of the new low carrier Kondo antiferromagnet YbRh3Si7. Electrical transport, magnetization, and specific heat measurements reveal antiferromagnetic order at T_N = 7.5 K. Neutron diffraction measurements show that the magnetic ground state of YbRh3Si7 is a collinear antiferromagnet where the moments are aligned in the ab plane. With such an ordered state, no metamagnetic transitions are expected when a magnetic field is applied along the c axis. It is therefore surprising that high field magnetization, torque, and resistivity measurements with H||c reveal two metamagnetic transitions at mu_0H_1 = 6.7 T and mu_0H_2 = 21 T. When the field is tilted away from the c axis, towards the ab plane, both metamagnetic transitions are shifted to higher fields. The first metamagnetic transition leads to an abrupt increase in the electrical resistivity, while the second transition is accompanied by a dramatic reduction in the electrical resistivity. Thus, the magnetic and electronic degrees of freedom in YbRh3Si7 are strongly coupled. We discuss the origin of the anomalous metamagnetism and conclude that it is related to competition between crystal electric field anisotropy and anisotropic exchange interactions.Comment: 23 pages and 4 figures in the main text. 7 pages and 5 figures in the supplementary materia

Factor analysis for gene regulatory networks and transcription factor activity profiles

BACKGROUND: Most existing algorithms for the inference of the structure of gene regulatory networks from gene expression data assume that the activity levels of transcription factors (TFs) are proportional to their mRNA levels. This assumption is invalid for most biological systems. However, one might be able to reconstruct unobserved activity profiles of TFs from the expression profiles of target genes. A simple model is a two-layer network with unobserved TF variables in the first layer and observed gene expression variables in the second layer. TFs are connected to regulated genes by weighted edges. The weights, known as factor loadings, indicate the strength and direction of regulation. Of particular interest are methods that produce sparse networks, networks with few edges, since it is known that most genes are regulated by only a small number of TFs, and most TFs regulate only a small number of genes. RESULTS: In this paper, we explore the performance of five factor analysis algorithms, Bayesian as well as classical, on problems with biological context using both simulated and real data. Factor analysis (FA) models are used in order to describe a larger number of observed variables by a smaller number of unobserved variables, the factors, whereby all correlation between observed variables is explained by common factors. Bayesian FA methods allow one to infer sparse networks by enforcing sparsity through priors. In contrast, in the classical FA, matrix rotation methods are used to enforce sparsity and thus to increase the interpretability of the inferred factor loadings matrix. However, we also show that Bayesian FA models that do not impose sparsity through the priors can still be used for the reconstruction of a gene regulatory network if applied in conjunction with matrix rotation methods. Finally, we show the added advantage of merging the information derived from all algorithms in order to obtain a combined result. CONCLUSION: Most of the algorithms tested are successful in reconstructing the connectivity structure as well as the TF profiles. Moreover, we demonstrate that if the underlying network is sparse it is still possible to reconstruct hidden activity profiles of TFs to some degree without prior connectivity information

Bulk fermi surface of the Weyl type-II semimetallic candidate γ−MoTe2

The electronic structure of WTe$_2$ and orthorhombic $\gamma-$MoTe$_2$, are claimed to contain pairs of Weyl type-II points. A series of ARPES experiments claim a broad agreement with these predictions. We synthesized single-crystals of MoTe$_2$ through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) \emph{via} quantum oscillatory phenomena. We find that the superconducting transition temperature of $\gamma-$MoTe$_2$ depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to WTe$_2$, the magnetoresistivity of $\gamma-$MoTe$_2$ does not saturate at high magnetic fields and can easily surpass $10^{6}$ \%. Remarkably, the analysis of the de Haas-van Alphen (dHvA) signal superimposed onto the magnetic torque, indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman-effect precluding the extraction of the Berry-phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level $\varepsilon_F$. Here, we show that a shift of the DFT valence bands relative to $\varepsilon_F$, in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron- and hole-bands eliminates their crossings and, therefore, the Weyl type-II points predicted for $\gamma-$MoTe$_2$.Comment: 13 pages, 7 figures, supplementary file not included (in press

Real-time monitoring shows substantial excess all-cause mortality during second wave of COVID-19 in Europe, October to December 2020.

The European monitoring of excess mortality for public health action (EuroMOMO) network monitors weekly excess all-cause mortality in 27 European countries or subnational areas. During the first wave of the coronavirus disease (COVID-19) pandemic in Europe in spring 2020, several countries experienced extraordinarily high levels of excess mortality. Europe is currently seeing another upsurge in COVID-19 cases, and EuroMOMO is again witnessing a substantial excess all-cause mortality attributable to COVID-19.Funding statement: The EuroMOMO network hub at Statens Serum Institut receives funding from European Centre for Disease Prevention and Control, Solna, Sweden, through a framework contract 2017-2020.S