Non-conventional superconductivity in magnetic In and Sn nanoparticles.

We report on experimental evidence of non-conversional pairing in In and Sn nanoparticle assemblies. Spontaneous magnetizations are observed, through extremely weak-field magnetization and neutron-diffraction measurements, to develop when the nanoparticles enter the superconducting state. The superconducting transition temperature TC shifts to a noticeably higher temperature when an external magnetic field or magnetic Ni nanoparticles are introduced into the vicinity of the superconducting In or Sn nanoparticles. There is a critical magnetic field and a critical Ni composition that must be reached before the magnetic environment will suppress the superconductivity. The observations may be understood when assuming development of spin-parallel superconducting pairs on the surfaces and spin-antiparallel superconducting pairs in the core of the nanoparticles.

Charge transfer enhanced magnetic correlations in type-II multiferroic Co3TeO6.

Magnetic structure of the Co ions in monoclinic Co3TeO6 in the antiferroelectric state at 16 K has been determined by neutron powder together with single-crystal diffractions. The indices of the magnetic reflections that appear at the incommensurate positions were determined by diffractions from a single crystal, which allow to uniquely identify the magnetic modulation vector. There are two crystallographically distinct Co layers. Magnetic incommensurability appears in the Co spins in the layers comprising zig-zag chains, with a magnetic modulation vector of (0.357, 0.103, 0.121) at 3 K but changes to (0.4439, 0, 0.137) at 16 K, while the Co ions in the honeycomb webs form a collinear antiferromagnetic structure. Thermal reduction rate of the Co moments in the honeycomb webs was found to be much smaller than those in the zigzag chains. Shifting of large amounts of electronic charge into the Co─O bonds in the honeycomb webs on warming is used to understand the behavior.

Enhanced lattice perfection by low temperature thermal annealing in photoelectric (CH3NH3)PbBr3.

The coupling between the organic CH3NH3+ cations and inorganic perovskite PbBr3- framework in a large single crystal of (CH3NH3)PbBr3 weighting 13 g was studied using neutron diffraction and inelastic neutron scattering. Two lattice incommensurate (ICM) phases were found, one at higher temperatures, marked ICMHT, which appeared between 147 and 135 K. The second one, marked ICMLT, developed below 143 K and remained at 75 K. The transition from the ICMLT to ICMHT phase upon warming gave rise to extremely large lattice shrinking, followed by extremely large lattice expansion of the tetragonal basal plane of the PbBr3 lattice. There was a progressive decrease in the width of the Bragg peaks from the PbBr3 lattice upon warming, which can be described using a critical exponent for each type of Bragg peak to show complete ordering of the atoms into a (CH3NH3)PbBr3 lattice at 194 K. (CH3NH3)PbBr3 exhibits six definitive acoustic-like phonon branches at 75 K. The six branches renormalizes into two at 200 K, with the frequencies of both the transverse and longitudinal modes greatly enhanced. The asymmetric structure of the CH3NH3 ions helps to understand the observed behaviors.

Largely Enhanced Ferromagnetism in Bare CuO Nanoparticles by a Small Size Effect.

Magnetic properties of fully oxygenated bare CuO nanoparticles have been investigated using magnetization, X-ray diffraction, neutron diffraction, and Raman scattering measurements. The Langevin field profile is clearly revealed in the isothermal magnetization of 8.8 nm CuO nanoparticle assembly even at 300 K, revealing a 172 times enhancement of the ferromagnetic responses over that of bulk CuO. Surface magnetization of 8.8 nm CuO reaches 18% of the core magnetization. The Cu spins in 8.8 nm CuO order below 400 K, which is 1.7 times higher than the 231 K observed in bulk CuO. A relatively simple magnetic structure that may be indexed using a modulation vector of (0.2, 0, 0.2) was found for the 8.8 nm CuO, but no magnetic incommensurability was observed in bulk CuO. The Cu spins in 8.8 nm CuO form spin density waves with length scales of 5 chemical unit cells long along the crystallographic a- and c-axis directions. Considerable amounts of electronic charge shift from around the Cu lattice sites toward the interconnecting regions of two neighboring Cu-Cu ions, resulting in a stronger ferromagnetic direct exchange interaction for the neighboring Cu spins in 8.8 nm CuO.

Evidence of High-Temperature Superconductivity at 18 K in Nanosized Rhombohedral Bi Enhanced by Ni-Doping.

Superconductivity in bulk rhombohedral Bi has recently been detected to appear below 0.53 mK and 5.2 µT. Here, we unambiguously demonstrate that superconductivity in rhombohedral Bi can be greatly enhanced by incorporating Ni ions onto the Bi sites and reducing the size to the nanometer scale. The superconducting transition temperature T C of 12 nm rhombohedral Bi nanoparticles (NPs) reaches 4 K at ambient pressure. T C is significantly enhanced to reach 7, 12, and 18 K in 6, 8, and 10% Ni-doped Bi NPs, respectively, where superconductivity is found to coexist with ferromagnetism. Ni-doping causes a significant amount of electronic charges to shift toward the interconnecting regions between neighboring Bi ions. First-principles calculations reveal that the Ni ions serve as charge and spin suppliers for the developments of superconductivity and ferromagnetism.

Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe.

Single-crystalline SnSe has attracted much attention because of its record high figure-of-merit ZT ≈ 2.6; however, this high ZT has been associated with the low mass density of samples which leaves the intrinsic ZT of fully dense pristine SnSe in question. To this end, we prepared high-quality fully dense SnSe single crystals and performed detailed structural, electrical, and thermal transport measurements over a wide temperature range along the major crystallographic directions. Our single crystals were fully dense and of high purity as confirmed via high statistics 119Sn Mössbauer spectroscopy that revealed <0.35 at. % Sn(IV) in pristine SnSe. The temperature-dependent heat capacity (C p) provided evidence for the displacive second-order phase transition from Pnma to Cmcm phase at T c ≈ 800 K and a small but finite Sommerfeld coefficient γ0 which implied the presence of a finite Fermi surface. Interestingly, despite its strongly temperature-dependent band gap inferred from density functional theory calculations, SnSe behaves like a low-carrier-concentration multiband metal below 600 K, above which it exhibits a semiconducting behavior. Notably, our high-quality single-crystalline SnSe exhibits a thermoelectric figure-of-merit ZT ∼1.0, ∼0.8, and ∼0.25 at 850 K along the b, c, and a directions, respectively.

Fault Dynamics of the 1999 Chi-Chi earthquake: clues from nanometric geochemical analysis of fault gouges.

The 1999 Chi-Chi, Taiwan earthquake (Mw 7.6) produced surface ruptures for about 90 km along the north-south trending Chelungpu fault, with surface displacements of up to 12 m. Based on the combination of nanoscopic investigation and geochemistry analysis of core samples from a 450 m long inclined borehole drilled through the slip zone, we suggest the dynamical processes that likely occurred in the northern portion of the Chelungpu fault during the faulting. Our analysis revealed that the frictional heating could have reached 1200 °C, which would cause most of the siderite in the fault gouge to evaporate, resulting in a large amount of nano-size siderite grains with a mean diameter 20 nm. These nano grains could have acted as a mechanical lubricant to reduce the dynamic frictional resistance during sliding, giving rise to the large but smooth type of slipping seen in the north.

Complex magnetic incommensurability and electronic charge transfer through the ferroelectric transition in multiferroic Co3TeO6.

Polarized and unpolarized neutron diffractions have been carried out to investigate the nature of the magnetic structures and transitions in monoclinic Co3TeO6. As the temperature is lowered below 26 K long range order develops, which is fully incommensurate (ICM) in all three crystallographic directions. Below 19.5 K additional commensurate magnetic peaks develop, consistent with the Γ4 irreducible representation, along with a splitting of the ICM peaks along the h direction which indicates that there are two separate sets of magnetic modulation vectors. Below 18 K, this small additional magnetic incommensurability disappears, ferroelectricity develops, an additional commensurate magnetic structure consistent with Γ3 irreducible representation appears, and the k component of the ICM wave vector disappears. Synchrotron x-ray diffraction measurements demonstrate that there is a significant shift of the electronic charge distribution from the Te ions at the crystallographic 8 f sites to the neighboring Co and O ions. These results, together with the unusually small electric polarization, its strong magnetic field dependence, and the negative thermal expansion in all three lattice parameters, suggest this material is an antiferroelectric. Below15 K the k component of the ICM structure reappears, along with second-order ICM Bragg peaks, which polarized neutron data demonstrate are magnetic in origin.

Photoenhanced Ferromagnetism in High-K+-Containing K-Ni-Cr Prussian Blue Analogues Coated on Rb-Co-Fe Nanocubes.

A large enhancement of the Ni and Cr ferromagnetic moments under UV-light irradiation has been detected in 55 nm thick K0.98Ni[Cr(CN)6]0.70[(H2O)6]0.30·0.11H2O Prussian blue analogues coated on 240 nm Rb0.76Co[Fe(CN)6]0.74[(H2O)6]0.26·0.56H2O nanocubes. Two separate magnetic transitions were found. The one at 72 K marks the magnetic ordering of the Ni and Cr ions on the shell. A higher degree of electronic connection along the Ni-N-C-Cr-C-N-Ni chains was achieved by the incorporation of a larger amount of K+ ions into the voids enclosed by the NiN6 and CrC6 octahedra, which was used to understand the appearance of photoenhanced ferromagnetism in the K-Ni-Cr network. A weak moment developed in the core below 10 K, corresponding to separate ordering of the Co and Fe ions in the Rb-Co-Fe network. Photoinduced ferromagnetism of the Co and Fe ions in the Rb-Co-Fe was also detected.

Engineering and Scaling the Spontaneous Magnetization Reversal of Faraday Induced Magnetic Relaxation in Nano-Sized Amorphous Ni Coated on Crystalline Au.

We report on the generation of large inverse remanent magnetizations in nano-sized core/shell structure of Au/Ni by turning off the applied magnetic field. The remanent magnetization is very sensitive to the field reduction rate as well as to the thermal and field processes before the switching off of the magnetic field. Spontaneous reversal in direction and increase in magnitude of the remanent magnetization in subsequent relaxations over time were found. All of the various types of temporal relaxation curves of the remanent magnetizations are successfully scaled by a stretched exponential decay profile, characterized by two pairs of relaxation times and dynamic exponents. The relaxation time is used to describe the reduction rate, while the dynamic exponent describes the dynamical slowing down of the relaxation through time evolution. The key to these effects is to have the induced eddy current running beneath the amorphous Ni shells through Faraday induction.

Development of Ferromagnetic Superspins in Bare Cu Nanoparticles by Electronic Charge Redistribution.

We report on the results of investigating the ferromagnetic properties of bare Cu nanoparticles. Three sets of bare Cu nanoparticle assemblies with mean particle diameters of 6.6, 8.1, and 11.1 nm were fabricated, employing the gas condensation method. Curie-Weiss paramagnetic responses to a weak driving magnetic field were detected, showing the appearance of particle superspins that overcomes the diamagnetic responses from the inner core. The isothermal magnetization displays a Langevin field profile together with magnetic hysteresis appearing even at 300 K, demonstrating the existence of ferromagnetic superspins in the Cu nanoparticles. Shifting of a noticeable amount of electronic charge from being distributed near the lattice sites in bulk form toward their neighboring ions in nanoparticles was found. The extended 3d and 4s band mixture are the main sources for the development of localized 3d holes for the development of ferromagnetic particle superspins in Cu nanoparticles.

Huge Inverse Magnetization Generated by Faraday Induction in Nano-Sized Au@Ni Core@Shell Nanoparticles.

We report on the design and observation of huge inverse magnetizations pointing in the direction opposite to the applied magnetic field, induced in nano-sized amorphous Ni shells deposited on crystalline Au nanoparticles by turning the applied magnetic field off. The magnitude of the induced inverse magnetization is very sensitive to the field reduction rate as well as to the thermal and field processes before turning the magnetic field off, and can be as high as 54% of the magnetization prior to cutting off the applied magnetic field. Memory effect of the induced inverse magnetization is clearly revealed in the relaxation measurements. The relaxation of the inverse magnetization can be described by an exponential decay profile, with a critical exponent that can be effectively tuned by the wait time right after reaching the designated temperature and before the applied magnetic field is turned off. The key to these effects is to have the induced eddy current running beneath the amorphous Ni shells through Faraday induction.

Development of a ferromagnetic component in the superconducting state of Fe-excess Fe1.12Te(1-x)Sex by electronic charge redistribution.

The general picture established so far for the links between superconductivity and magnetic ordering in iron chalcogenide Fe1+y(Te(1-x)Sex) is that the substitution of Se for Te directly drives the system from the antiferromagnetic end into the superconducting regime. Here, we report on the observation of a ferromagnetic component that developed together with the superconducting transition in Fe-excess Fe1.12Te(1-x)Sex crystals using neutron and x-ray diffractions, resistivity, magnetic susceptibility and magnetization measurements. The superconducting transition is accompanied by a negative thermal expansion of the crystalline unit cell and an electronic charge redistribution, where a small portion of the electronic charge flows from around the Fe sites toward the Te/Se sites. First-principles calculations show consistent results, revealing that the excess Fe ions play a more significant role in affecting the magnetic property in the superconducting state than in the normal state and the occurrence of an electronic charge redistribution through the superconducting transition.

Fine structures of self-assembled beta-cyclodextrin/Pluronic in dilute and dense systems: a small angle X-ray scattering study.

The evolution of the fine structures of self-assembled polypseudorotaxane (PPR) in Pluronic (PL F108) solutions containing dilute to dense beta-cyclodextrin (ß-CD) was illustrated for the first time by small angle X-ray scattering (SAXS). Dense ß-CD (∼19 w/v%) was found feasible to be dispersed in 24% citric acid solution. 5% of PL F108 formed cylindrical micelles of 1 nm in radius and 8 nm in length in the presence of 24% citric acid through the dehydration of citric acid and citrate. PPR was formed through host-guest interaction between PL F108 and ß-CD. In dilute ß-CD system (1%), the single chains of PPR with separated ß-CD stacks on PL F108 were formed. The numbers of ß-CD in each stack increased from 1 to 4 on increasing ß-CD concentration to 9%. In a dense ß-CD system, PPR condensed to correlated structures majorly composed of two unit blocks through the hydrogen bonds between PPRs. Two distinguishable correlated domains with correlation lengths of 50 nm (marked α-phase) and 46 nm (marked ß-phase) along the chains, but without fine periodic structure within each individual domain, were identified in the 10% ß-CD solution. Periodic stacking of ß-CD in the domains developed in the 12% solution. As ß-CD concentration increased from 12 to 19%, the correlated heights of α and ß phases reduced from 41 and 32 nm to 30 and 10 nm, respectively. There were 48 ß-CDs that stabilized on each PL F108 chain in the 19% ß-CD system, which is in good agreement with stoichiometry.

Correlation of thermostability and conformational changes of catechol 2, 3-dioxygenases from two disparate micro-organisms.

We have investigated the structure of recombinant catechol 2, 3-dioxygenase (C23O) purified from two species in which the enzyme has evolved to function at different temperature. The two species are mesophilic bacterium Pseudomonas putida strain mt-2 and thermophilic archaea Sulfolobus acidocaldariusDSM639. Using the primary sequence analysis, we show that both C23Os have only 30% identity and 48% similarity but contain conserved amino acid residues forming an active site area around the iron ion. The corresponding differences in homology, but structural similarity in active area residues, appear to provide completely different responses to heating the two enzymes. We confirm this by small angle X-ray scattering and demonstrate that the overall structure of C23O from P. putida is slightly different from its crystalline form whereas the solution scattering of C23O from S. acidocaldarius at temperatures between 4 and 85°C ideally fits the calculated scattering from the single crystal structure. The thermostability of C23O from S. acidocaldarius correlates well with conformation in solution during thermal treatment. The similarity of the two enzymes in primary and tertiary structure may be taken as a confirmation that two enzymes have evolved from a common ancestor.

Large magnetoresistance and charge transfer between the conduction and magnetic electrons in layered oxyselenide BiOCu(0.96)Se.

The electrical and magnetic properties of slightly Cu-deficient BiOCu(0.96)Se have been investigated using neutron and X-ray diffraction, ac magnetic susceptibility, magnetization and electric resistivity measurements. The layered BiOCu(0.96)Se crystallizes into a tetragonal lattice with a P4/nmm symmetry. Thermal profiles of the electrical resistivity reveal a semiconductor type behavior, but depart from its course at low temperatures when antiferromagnetic coupling becomes thermally loosened at 140 K. Positive magnetoresistances are obtained at all temperatures studied. With an applied magnetic field of 0.5 kOe, the magnetoresistance reaches 235% at 2 K. It decreases with increasing temperature, but stabilizes to 70% above 60 K. Both ferromagnetic and antiferromagnetic coupling are detected between the Cu spins in the SeCu4 pyramidal blocks, which results in a non-collinear spin arrangement at low temperatures. The antiferromagnetic component becomes disordered above T(N) = 140 K, whereas the ferromagnetic moment persists up to T(C) = 300 K. Interlayer charge transfer between the conduction and magnetic electrons gives rise to an anomaly in the magnetic order parameter.

Spin polarization and quantum spins in Au nanoparticles.

The present study focuses on investigating the magnetic properties and the critical particle size for developing sizable spontaneous magnetic moment of bare Au nanoparticles. Seven sets of bare Au nanoparticle assemblies, with diameters from 3.5 to 17.5 nm, were fabricated with the gas condensation method. Line profiles of the X-ray diffraction peaks were used to determine the mean particle diameters and size distributions of the nanoparticle assemblies. The magnetization curves M(H(a)) reveal Langevin field profiles. Magnetic hysteresis was clearly revealed in the low field regime even at 300 K. Contributions to the magnetization from different size particles in the nanoparticle assemblies were considered when analyzing the M(H(a)) curves. The results show that the maximum particle moment will appear in 2.4 nm Au particles. A similar result of the maximum saturation magnetization appearing in 2.3 nm Au particles is also concluded through analysis of the dependency of the saturation magnetization M(P) on particle size. The M(P)(d) curve departs significantly from the 1/d dependence, but can be described by a log-normal function. Magnetization can be barely detected for Au particles larger than 27 nm. Magnetic field induced Zeeman magnetization from the quantum confined Kubo gap opening appears in Au nanoparticles smaller than 9.5 nm in diameter.

Spin, charge and lattice couplings in Cu-deficient oxysulphide BiOCu0.94S.

The electrical and magnetic properties of slightly Cu-deficient BiOCu(0.94)S are investigated using neutron diffraction, ac magnetic susceptibility, magnetization and electrical resistivity measurements. The Cu spins order in a ferromagnetic arrangement below T(C) = 250 K. An antiferromagnetic component develops below 180 K when the crystalline unit cell experiences a sharp thermal contraction upon cooling, resulting in a canted ferromagnetic spin arrangement at low temperatures. In the magnetically ordered state the electrical transport can be described using three-dimensional variable range hopping conduction. An applied magnetic field can effectively reduce the hopping barrier. Spin-charge couplings are clearly revealed when the resistivity departs from the hopping conduction and begins to increase with increasing temperatures above 250 K where the Cu spins become disordered.

Interplay between the crystalline and magnetic structures in lightly Cr-doped Bi(0.37)Ca(0.63)Mn(0.96)Cr(0.04)O(2.99).

The interplay between the crystalline and magnetic structures of a 4% Cr-doped Bi(0.37)Ca(0.63)Mn(0.96)Cr(0.04)O(2.99) has been investigated by alternating current (ac) magnetic susceptibility, electrical resistivity, and neutron diffraction measurements. The compound crystallizes into a monoclinic P2(1)/m symmetry. A Jahn-Teller distortion occurs at 280 K. The thermal behavior of charge transport may be described by a three-dimensional variable range hopping conduction. Strong interplay between the localized magnetic electrons and the itinerant electrons are clearly revealed as the localization length increases by 20% when the Mn spins become ordered below 85 K. Short range magnetic correlations persist up to 160 K. The collinear magnetic structure can be viewed as consisting of ferromagnetic spin-trimers antiferromagnetically embedded in a ferromagnetic environment. Cr-doping reduces the charge ordering temperature and the magnetic ordering temperature. It nevertheless introduces long-range ferromagnetism.

Short range magnetic correlations induced by La substitution in Ho(1-x)La(x)Mn(2)O(5).

Magnetic susceptibility, x-ray diffraction, neutron diffraction and Raman scattering measurements are employed to study the effects of La substitution on the magnetic properties of multiferroic HoMn(2)O(5). 9% and 18% La-substituted compounds crystallize into the same orthorhombic Pbam symmetry as the parent compound. The magnetic responses to an ac driving magnetic field between 40 and 140 K are greatly enhanced by 18% La substitution. The neutron magnetic diffraction patterns reveal the development of short range magnetic correlations below 140 K. In addition, two Raman peaks and a series of new x-ray diffraction peaks suddenly develop below this temperature. Incommensurate long range antiferromagnetic order appears below 38 K. Magnetic frustration could be the main mechanism governing the present observations.