The nature of the tetragonal-to-orthorhombic structural transition at Ts ≈ 90 K in single crystalline FeSe is studied using shear-modulus, heat-capacity, magnetization and NMR measurements. The transition is shown to be accompanied by a large shear-modulus softening, which is practically identical to that of underdoped Ba(Fe,Co)2As2, suggesting very similar strength of the electronlattice coupling. On the other hand, a spin-fluctuation contribution to the spin-lattice relaxation rate is only observed below Ts. This indicates that the structural, or "nematic", phase transition in FeSe is not driven by magnetic fluctuations.PACS numbers: 74.70. Xa, 74.25.Bt, 74.25.Ld, 74.25.nj One of the most intriguing questions in the study of iron-based superconductors concerns the relation between structure, magnetism and superconductivity [1][2][3][4][5][6][7][8][9][10]. Stripe-type antiferromagnetic order often occurs at the same or at a slightly lower temperature than the tetragonal-to-orthorhombic structural distortion and the two types of order are closely related by symmetry. They break the four-fold rotational symmetry of the high-temperature phase, which can be associated with a nematic degree of freedom [4,6]. Superconductivity typically is strongest around the point where the structural transition (T s ) and the antiferromagnetic transition (T N ) are suppressed by pressure or chemical substitution. Whether the magnetic or the structural instability is the primary one, is still under intense debate [10], also because of its relevance to the pairing mechanism [5,6]. Recently, scaling relations between the shear modulus related to the structural distortion, C 66 , and the spinlattice relaxation time T 1 as a measure of the strength of spin fluctuations, have been proposed [7,8] in order to address the above question. They were found to be well satisfied in the Ba(Fe,Co) 2 As 2 system [7,8], where T s and T N are in close proximity to each other, suggesting a magnetically-driven structural transition [7]. Clearly, it is of great interest to see if a relation between shear modulus and spin fluctuations is universally observed in other iron-based materials.FeSe is structurally the simplest iron-based superconductor and has attracted a lot of attention because of a nearly four-fold increase of its T c ≈ 8 K under pressure [11]. Moreover, this system is particularly interesting with respect to the relation of structure and magnetism, since it undergoes a tetragonal-to-orthorhombic structural phase transition at T s ∼ 90 K, similar to that found in the 1111-and 122-type parent compounds [2], but does not order magnetically at ambient pressure [12,13]. Spin fluctuations at low temperatures were, however, observed in nuclear magnetic resonance (NMR) measurements [14]. Surprisingly, the orthorhombic distortion of FeSe is not reduced upon entering the superconducting state [9] in strong contrast to underdoped BaFe 2 As 2 [3,15], indicating different couplings between structure and superconductivity. This strongly motivates fu...

Quasicrystals are metallic alloys that possess long-range, aperiodic structures with diffraction symmetries forbidden to conventional crystals. Since the discovery of quasicrystals by Schechtman et al. in 1984, there has been considerable progress in resolving their geometric structure. For example, it is well known that the golden ratio of mathematics and art occurs over and over again in their crystal structure. However, the characteristic properties of the electronic states--whether they are extended as in periodic crystals or localized as in amorphous materials--are still unresolved. Here we report the first observation of quantum (T = 0) critical phenomena of the Au-Al-Yb quasicrystal--the magnetic susceptibility and the electronic specific heat coefficient arising from strongly correlated 4f electrons of the Yb atoms diverge as T→0. Furthermore, we observe that this quantum critical phenomenon is robust against hydrostatic pressure. By contrast, there is no such divergence in a crystalline approximant, a phase whose composition is close to that of the quasicrystal and whose unit cell has atomic decorations (that is, icosahedral clusters of atoms) that look like the quasicrystal. These results clearly indicate that the quantum criticality is associated with the unique electronic state of the quasicrystal, that is, a spatially confined critical state. Finally we discuss the possibility that there is a general law underlying the conventional crystals and the quasicrystals.

The organization and allelic recombination of the merozoite surface protein-1 gene of Plasmodium vivax (PvMsp-1), the most widely prevalent human malaria parasite, were evaluated in complete nucleotide sequences of 40 isolates from various geographic areas. Alignment of 31 distinct alleles revealed the mosaic organization of PvMsp-1, consisting of seven interallele conserved blocks flanked by six variable blocks. The variable blocks showed extensive variation in repeats and nonrepeat unique sequences. Numerous recombination sites were distributed throughout PvMsp-1, in both conserved blocks and variable block unique sequences, and the distribution was not uniform. Heterozygosity of PvMsp-1 alleles was higher in Asia (0.953 ؎ 0.009) than in Brazil (0.813 ؎ 0.047). No identical alleles were shared between Asia and Brazil, whereas all but one variable block nonrepeat sequence found in Brazil occurred in Asia. These observations suggest that P. vivax populations in Asia are ancestral to Brazilian populations, and that PvMsp-1 has heterogeneity in frequency of allelic recombination events. Recurrent origins of new PvMsp-1 alleles by repeated recombination events were supported by a rapid decline in linkage disequilibrium between pairs of synonymous sites with increasing nucleotide distance, with little linkage disequilibrium at a distance of over 3 kb in a P. vivax population from Thailand, evidence for an effectively high recombination rate of the parasite. Meanwhile, highly reduced nucleotide diversity was noted in a region encoding the 19-kDa C-terminal epidermal growth factorlike domain of merozoite surface protein-1, a vaccine candidate. T he human malaria parasite Plasmodium vivax is prevalent worldwide, and accounts for 70-80 million cases annually, mostly in Asia and Latin America (1). Growing resistance of P. vivax strains to chloroquine is spurring the development of a vaccine against P. vivax malaria. One current vaccine candidate is merozoite surface protein-1 (MSP-1), a 200-kDa protein expressed on the surface of the P. vivax merozoite (2). MSP-1 of Plasmodium species is synthesized as a high-molecular-weight precursor and then processed into several fragments (3). At the time of red cell invasion by the merozoite, only the 19-kDa C-terminal fragment (MSP-1 19 ), which contains two epidermal growth factor-like domains, remains on the surface. Antibodies against MSP-1 19 inhibit merozoite entry into red cells (4), and immunization with MSP-1 19 protects monkeys from challenging infections (5, 6). Hence, MSP-1 19 is considered a promising vaccine candidate.Importantly, there is extensive allelic diversity of MSP-1 among isolates (7), and this polymorphism may hamper development of effective vaccines. In Plasmodium falciparum, the most virulent malaria parasite, polymorphism in PfMsp-1 is well characterized. PfMsp-1 consists of several interallele variable blocks flanked by conserved or semiconserved blocks. Variation in this gene is basically dimorphic; i.e., one or the other of two different residues (8, ...