Thus far, a detection of the Dirac or Weyl fermions in topological semimetals remains often elusive, since in these materials conventional charge carriers exist as well. Here, measuring a field-induced length change of the prototype Weyl semimetal TaAs at low temperatures, we find that its c-axis magnetostriction amounts to relatively large values whereas the a-axis magnetostriction exhibits strong variations with changing the orientation of the applied magnetic field. It is discovered that at magnetic fields above the ultra-quantum limit, the magnetostriction of TaAs contains a linear-in-field term, which, as we show, is a hallmark of the Weyl fermions in a material. Developing a theory for the magnetostriction of noncentrosymmetric topological semimetals and applying it to TaAs, we additionally find several parameters characterizing the interaction between the relativistic fermions and elastic degrees of freedom in this semimetal. Our study shows how dilatometry can be used to unveil Weyl fermions in candidate topological semimetals.
The temperature dependencies of the lower critical field H_{c1}(T) of several filled-skutterudite superconductors were investigated by local magnetization measurements. While LaOs_{4}As_{12} and PrRu_{4}As_{12} exhibit the H_{c1}(T) dependencies consistent with the single-band BCS prediction, for LaRu_{4}As_{12} (the superconducting temperature T_{c}=10.4 K) with a similar three-dimensional Fermi surface, we observe a sudden increase in H_{c1}(T) deep in a superconducting state below about 0.32T_{c}. Remarkably, a rapid rise of H_{c1}(T) at approximately the same reduced temperature 0.27T_{c} is also found for the heavy-fermion compound PrOs_{4}Sb_{12} (T_{c}≃1.78 K), in fair accord with the earlier macroscopic study. We attribute the unusual H_{c1}(T) dependencies of LaRu_{4}As_{12} and PrOs_{4}Sb_{12} to a kink structure in their superfluid densities due to different contributions from two nearly decoupled bands. Whereas LaRu_{4}As_{12} is established as a two-band isotropic s-wave superconductor, nonsaturating behavior of H_{c1}(T) is observed for PrOs_{4}Sb_{12}, indicative of an anisotropic structure of a smaller gap. For this superconductor with broken time-reversal symmetry, our findings suggest a superconducting state with multiple symmetries of the order parameters.
The magnetic-field-dependent spin ordering of strained BiFeO3 films is determined using nuclear resonant scattering and Raman spectroscopy. The critical field required to destroy the cycloidal modulation of the Fe spins is found to be significantly lower than in the bulk, with appealing implications for field-controlled spintronic and magnonic devices.
Recently, strain engineering has been shown to be a powerful and flexible means of tailoring the properties of ABO3 perovskite thin films. The effect of epitaxial strain on the structure of the perovskite unit cell can induce a host of interesting effects, these arising from either polar cation shifts or rotation of the oxygen octahedra, or both. In the multi-ferroic perovskite bismuth ferrite (BiFeO3-BFO), both degrees of freedom exist, and thus a complex behaviour may be expected as one plays with epitaxial strain. In this paper, we review our results on the role of strain on the ferroic transition temperatures and ferroic order parameters. We find that, while the Néel temperature is almost unchanged by strain, the ferroelectric Curie temperature strongly decreases as strain increases in both the tensile and compressive ranges. Also unexpected is the very weak influence of strain on the ferroelectric polarization value. Using effective Hamiltonian calculations, we show that these peculiar behaviours arise from the competition between antiferrodistortive and polar instabilities. Finally, we present results on the magnetic order: while the cycloidal spin modulation present in the bulk survives in weakly strained films, it is destroyed at large strain and replaced by pseudo-collinear antiferromagnetic ordering. We discuss the origin of this effect and give perspectives for devices based on strain-engineered BiFeO3.
Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property--antiferromagnetism--has not been investigated extensively, especially in thin films. Here we bring insight into the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to tune the exchange bias and giant-magnetoresistive response of spin valves.
Numerically predicting rate constants of protein folding and other relevant biological events is still a significant challenge. We show that the combination of partial path transition interface sampling with the optimal interfaces and free-energy profiles provided by path collective variables makes the rate calculation for practical biological applications feasible and efficient. This methodology can reproduce the experimental rate constant of Trp-cage miniprotein folding with the same level of accuracy as transition path sampling at a fraction of the cost.
Models, Chemical , Peptides/chemistry , Hydrophobic and Hydrophilic Interactions , Kinetics , Models, Molecular , Protein Folding , Protein Structure, Secondary , Thermodynamics
A spin reorientation accompanying the temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition is reported in strained epitaxial FeRh thin films. (57)Fe conversion electron Mössbauer spectrometry showed that the Fe moments have different orientations in FeRh grown on thick single-crystalline MgO and in FeRh grown on ion-beam-assist-deposited (IBAD) MgO. It was also observed, in both samples, that the Fe moments switch orientations at the AFM to FM phase transition. Perpendicular anisotropy was evidenced in the AFM phase of the film grown on IBAD MgO and in the FM phase of that grown on regular MgO. Density-functional theory calculations enabled this spin-reorientation transition to be accurately reproduced for both FeRh films across the AFM-FM phase transition and show that these results are due to differences in strain.
In multiferroic BiFeO(3) thin films grown on highly mismatched LaAlO(3) substrates, we reveal the coexistence of two differently distorted polymorphs that leads to striking features in the temperature dependence of the structural and multiferroic properties. Notably, the highly distorted phase quasiconcomitantly presents an abrupt structural change, transforms from a standard to a nonconventional ferroelectric, and transitions from antiferromagnetic to paramagnetic at 360±20 K. These coupled ferroic transitions just above room temperature hold promises of giant piezoelectric, magnetoelectric, and piezomagnetic responses, with potential in many applications fields.
We report the influence of epitaxial strain on the multiferroic phase transitions of BiFeO3 films. Using advanced characterization techniques and calculations we show that while the magnetic Néel temperature hardly varies, the ferroelectric Curie temperature TC decreases dramatically with strain. This is in contrast with the behavior of standard ferroelectrics where strain enhances the polar cation shifts and thus TC. We argue that this is caused by an interplay of polar and oxygen tilting instabilities and that strain can drive both transitions close together to yield increased magnetoelectric responses.
We propose a system allowing the characterization of thin magnetic multilayer structures that combine conversion electron Mossbauer spectrometry (CEMS) under applied magnetic field with the magneto-optical Kerr effect (MOKE) technique. Measured hysteresis loops obtained from the MOKE part are used for investigation of sample surface magnetic properties. The CEMS part of such a system is suitable for studying the spatial spin distribution during magnetization reversal under applied magnetic field, whose values are established from the measured MOKE loop. The combined technique is demonstrated on the results obtained at 300 K on an exchange-coupled ferrimagnetic amorphous GdFe/TbFe bilayer, where the center of the GdFe layer is enriched in (57)Fe. Both techniques confirm in-plane uniaxial anisotropy. The spin structure at the position of the probe layer is analyzed for several values of the external magnetic field applied in the hard magnetization axis direction.
We investigate the kinetic pathways of folding and unfolding of the designed miniprotein Trp- cage in explicit solvent. Straightforward molecular dynamics and replica exchange methods both have severe convergence problems, whereas transition path sampling allows us to sample unbiased dynamical pathways between folded and unfolded states and leads to deeper understanding of the mechanisms of (un)folding. In contrast to previous predictions employing an implicit solvent, we find that Trp-cage folds primarily (80% of the paths) via a pathway forming the tertiary contacts and the salt bridge, before helix formation. The remaining 20% of the paths occur in the opposite order, by first forming the helix. The transition states of the rate-limiting steps are solvated native-like structures. Water expulsion is found to be the last step upon folding for each route. Committor analysis suggests that the dynamics of the solvent is not part of the reaction coordinate. Nevertheless, during the transition, specific water molecules are strongly bound and can play a structural role in the folding.
Protein Folding , Proteins/chemistry , Proteins/metabolism , Tryptophan/metabolism , Computer Simulation , Models, Molecular , Nuclear Magnetic Resonance, Biomolecular , Protein Denaturation , Protein Structure, Tertiary , Solvents , Water
BACKGROUND: The purpose of this study was to evaluate the accuracy of repetitions-to-fatigue (RTF) using an absolute load of 102.3 kg (225 lbs) to estimate one-repetition maximum (1-RM) bench press performance in college football players using various prediction equations. EXPERIMENTAL DESIGN: a prospective study on the association between muscular endurance and muscular strength. PARTICIPANTS: 260 players from NCAA Division IA (n=43), IAA (n=63), II (n=129), and red-shirts (n=25) were evaluated at the conclusion of a minimum of eight weeks of heavy-resistance training during the off-season. MEASURES: all subjects performed a 1-RM bench press and RTF using an absolute load of 102.3 kg. RESULTS: The Mayhew et al. NFL-225 equation nonsignificantly overestimated 1-RM from RTF by 0.5 kg, while the Chapman et al. NFL-225 equation significantly underpredicted by 3.2 kg, although both equations were comparable in the number of players predicted within +/-4.5 kg of actual 1-RM (52% vs 51%, respectively). Only two of nine RTF equations currently in use produced predicted 1-RM values that were not significantly different from actual 1-RM performance. CONCLUSIONS: Specific NFL-225 equations are more accurate in estimating 1-RM bench press from absolute muscle endurance in college football players than previous published RTF equations. The accuracy of prediction decreases at higher repetitions.
Exercise Test/methods , Physical Endurance/physiology , Weight Lifting/physiology , Adolescent , Adult , Football/physiology , Humans , Male , Mathematics , Muscle Fatigue/physiology , Muscle, Skeletal/physiology , Prospective Studies , Weight-Bearing/physiology
The changes in the levels of total protein and four globulin fractions were followed up throughout the entire course of complications caused by Gram-negative facultative pathogens in 37 acute cases of respiratory insufficiency accompanying different underlying illnesses and in 9 chronic, bedridden patients given artificial ventilation. At the onset of the infectious complications, in the first place in septic shock, the levels of various globulin fractions showed a decrease corresponding to a half-life of 2 to 4 days. Neither the increased catabolism, nor the protein losses by the urine and tracheal secretions offer a sufficient explanation for the escape of globulins of this extent from the plasma. It seems that this is a consequence of the increase in capillary permeability due to the effect of antigen-antibody reactions and that of endotoxin. As a result, in the critical phase of the infectious complications, at the point of culmination, e.g. in septic shock, diminished amount of different globulins is transported to the site of utilization, that is, to the inflammatory area.
Bacterial Infections/blood , Blood Proteins/metabolism , Serum Globulins/metabolism , Adult , Alpha-Globulins/metabolism , Beta-Globulins/metabolism , Female , Humans , Male , Middle Aged , Respiratory Insufficiency/blood , Shock, Septic/blood , gamma-Globulins/metabolism
