low-T Region Research Articles

Rare-earth pyrocoboltates [formula omitted]Co2O7 (R = Ce, Yb): Effective spin-[formula omitted] antiferromagnetic insulators with strong geometrical frustration

The two new rare-earth pyrocoboltates R2Co2O7 (R = Ce, Yb) adopt the layered monoclinic Ca2Nb2O7-type structure with the space group P21(No. 4). Both compounds have been investigated by DC magnetic susceptibility χdc(T), AC magnetic susceptibility χac(T) and isothermal magnetization M(B) measurements. The observed effective magnetic moment of Ce2Co2O7 indicates a contribution from the Co sublattice while that of Yb2Co2O7 is mainly from the localized 4f13 electronic configuration of Yb3+ ion. A magnetic ordering of the antiferromagnetic-type is observed in Ce2Co2O7 at TN≃ 34 K. This ordering is accompanied by the opening of a magnetic spectral gap, Δ=57.7 K (~ 5.0 meV) in the spin-wave spectrum and it survives in an external magnetic field of 7 T. In addition to the transition at TN,χac(T) measurements reveal a dome-like feature centered at ≃ 20 K in high frequency which indicates the presence of possible short-range correlation. In Yb2Co2O7, a magnetic phase transition is observed below TN=2.94 K, which is suppressed below 1.9 K in an external magnetic field of 3 T. In the low-T region where the crystal electric field effects can be neglected, an effective spin-1/2 state is realized in Yb2Co2O7, with a Landé-g factor of 3.76 which is comparable to values observed from electron spin resonance (ESR) measurements on related systems. In addition, M(B) of Yb2Co2O7 shows a metamagnetic-like feature at a critical field of HC≈2.5 T. Signatures from both compounds reveal the presence of strong magnetic frustrations with frustration index f of 9.17 and 15.12 for Ce2Co2O7 and Yb2Co2O7, respectively.

Experimental and kinetic modeling study of methyl heptanoate low-temperature oxidation in a jet-stirred reactor

Methyl heptanoate (MHP) is a potential surrogate component for fatty acid methyl esters found in biodiesel. The carbon chain length of MHP is long enough to enable low temperature (low-T) reactivity and negative temperature coefficient oxidation behavior during the combustion experiments, similar to the real biodiesel fuels. This paper investigated the low-T oxidation of MHP at 780 Torr and equivalence ratios of 0.5, 1.0 and 1.5 in a jet-stirred reactor. Detailed speciation profiles of fuel, intermediates and products were obtained using synchrotron vacuum ultraviolet photoionization mass spectrometry. A comprehensive kinetic model with 779 species and 3594 reactions was developed and validated against the new experimental data. Model analysis indicated that the dominant reaction pathways for MHP consumption were H-abstraction reactions by radicals of OH, HO2 to produce MHP radicals under all experimental conditions. In this low-T oxidation region, O2 addition reactions were responsible for the consumption of MHP radicals. The formation pathways of unsaturated methyl esters were strongly related to the decomposition of cyclic ethers. Furthermore, the formations of CH3OOH and C2H5OOH were closely linked to the reactions of CH3O2 and C2H5O2 as well as the radicals of CH2O and HO2. This work provides detailed information relevant to low-T biodiesel oxidation chemistry and guidance for the application of biodiesel in internal combustion engines.

Influence of interchain interactions on the tumbling of chains in a polymer melt during shear flow

Polymer chains in both dilute solutions and melts undergo cyclic rotation and retraction, which is known as tumbling, under steady shear flow. However, it is still not known how the individual molecules in melts rotate freely under the constraints caused by surrounding chains. In this work, a Brownian dynamics simulation is used to investigate the influences of the interchain interactions on the polymer chain motions in both dilute solutions and melts under steady shear flow. Compared with previous simulation studies, a greater number of similarities and differences between tumbling in dilute solutions and melts are addressed, and the results explicitly suggest the critical role of the entanglements in melts during shear flow. Three components of the gyration radius in different directions [flow direction (⟨Rgx2⟩), gradient direction (⟨Rgy2⟩), and vorticity direction (⟨Rgz2⟩)] are shown to exhibit different dependencies on the shear rate depending on whether dilute solutions or melts are being examined. However, the characteristic tumbling times τr in both cases are proportional to γ˙−2/3. The distributions P(T) of time T that the chains spend in each tumbling cycle show that both states exhibit an exponential decay of P(T/τr) in the high-T region. In the low-T region, P(T/τr) in the melts with variable shear rates are coincident with each other, while P(T/τr) in dilute solutions show different shapes. With respect to the distributions of chain orientation, both cases show the same scaling relationships for shear rates and chain lengths. Based on these findings, main conclusions are as follows. The entanglements still restrict the evolutions of polymer chain configurations despite the number of entanglements decreasing with increased flow strength. The tumbling motion in melts can occur inside the tube, and the chain behaviors inside the confining tubes are rather similar to those in dilute solutions.

Y3Fe5O12 Nanoparticles as Efficient Pinning Centers and Temperature Dependence of the Critical Current in YBa2Cu3O7-δ Nanoparticles

Magnetic measurements have been reported in nanosized YBa2Cu3O7-δ (YBCO) cylindrical tablets doped with different proportions of Y3Fe5O12 (YIG) nanoparticles. X-ray diffraction and magnetization measurements have been achieved to characterize samples and study the influence of temperature T, applied field H, and added YIG on the intergranular critical current JC. The field has been applied perpendicular to the tablet. Temperature exponential decay of JC in YBCO nanoparticles has been observed and analyzed in the collective and correlated model framework. A two-current model has been proposed: the low-T region behavior has been attributed to the trapping effectiveness of ferromagnetic and superconducting nanoarea interfaces. In the high-T region, there are many ferromagnetic traps to ensnare a very small number of vortices leading to a flexing of JC decrease vs. T and to very low values of JC. Field dependence of JC of many tablets of YBCO with different YIG quantities has shown an optimum enhancement of JC for 0.5% of YIG in weight.

Low-temperature Pd/FER NOx adsorbers: Operando FT-IR spectroscopy and performance analysis

A Pd-doped FER zeolite sample is synthesized and investigated for potential use in the low-temperature NOx adsorption. The catalyst is characterized by BET, XRD, TEM, NaCl titration and by in-situ FT-IR spectroscopy following adsorption of CO, NH3 and NO for the characterization of the Pd sites. Over the calcined sample, Pd is present mainly as PdOx clusters/particles; near 13 % of Pd is present in the form of isolated ions.The NOx adsorption / desorption performances at low temperature are investigated by microreactor studies and operando FT-IR upon NO/O2 adsorption followed by TPD under relevant operating conditions, i.e. in the presence of water and carbon dioxide. It is found that NO adsorbs in the form of Pd nitrosyls (hydrated and anhydrous) and nitrates; near 59 μmol/gcat of NOx could be adsorbed, corresponding to a NO/Pd ratio near 0.6. Upon heating, nitrosyls decompose and transform into anhydrous species leading to NO desorption in the low-T region (below 200 °C) and at higher temperature, with maximum near 370 °C. The presence of CO negatively affects the NOx storage capacity, due to a competition of CO and NO over the Pd storage sites.

Abnormal dependence of multiferroicity on high-temperature electro-poling in GdMn2O5

Magnetically induced ferroelectric polarization in rare-earth RMn2O5 manganites is believed to originate from the symmetric exchange striction associated with a specific antiferromagnetic phase in the low temperature (T) region and would be irrelevant with electropoling in the high-T paramagnetic-paraelectric phase region. In this work, we demonstrate that low-T pyroelectric polarization of GdMn2O5 single crystals along the b axis in the antiferromagnetic phase exhibits remarkable dependence on the electropoling history imposed in the high-T paramagnetic-paraelectric phase. In particular, the high-T electropoling results in a reversal of ferroelectric polarization in the low-T region, which can be flopped back by the electropoling being sustained in the low-T ferroelectric region. The existence of an electrically polarizable magnetic cluster state in the high-T paramagnetic-paraelectric region is proposed based on a combination of experimental observations and first-principles calculations. An intrinsic correlation between the low-T antiferromagnetic ordering and the high-T polarizable state is discussed. The present experiments unveil the emergent phenomena on multiferroicity of RMn2O5 and suggest an alternative scenario for electrocontrol of magnetism.

Bulk and confined n - alkanes: n - undecane in unmodified vs. modified silica gels by positron annihilation lifetime spectroscopy and electron spin resonance techniques

A joint investigation of n-undecane (n-UND) in the bulk and confined states in irregular silica gel (SG) matrices differing in their surface composition via two extrinsic probes using positron annihilation lifetime spectroscopy (PALS) and electron spin resonance (ESR) as well as DSC technique is presented. Ortho-positronium (o-Ps) exhibits a pronounced change in the annihilation behavior as detected by the o-Ps lifetime,τ3, as a function of temperature due to the enhanced amorphization of the n-UND after confinement with respect to the bulk solid n-UND. In addition, with an exception of the low-T region it exhibits rather small alterations in the confined state in the unmodified (virgin) SG100Å-SIL compared to modified (silanized) SG100Å-C4. On the other side, the stable free radical of nitroxide type 2,2,6,6-tetramethylpiperidinyl-1-oxy(TEMPO) shows the essential differences in the spectral properties with temperature not only in the confined n-UND with respect to the bulk sample, but especially in the confined state in the modified SG-C4 matrix compared to the unmodified SG100Å-SIL one. These differences are attributed to the presence and absence of the preferential interaction of the polar spin probe TEMPO with the polar surface of the virgin SG matrix or the apolar surface of the silanized SG100Å-C4 one, respectively. The coinciding relationships between the characteristic PALS and ESR temperatures reflecting the changes in annihilation and rotation dynamics of the respective extrinsic probes are related to the dynamic and phase transitions as found by wide-T DSC.

Development of half metallicity within mixed magnetic phase of Cu1−xCoxMnSb alloy

Cubic half-Heusler Cu1−xCoxMnSb () compounds have been investigated both experimentally and theoretically for their magnetic, transport and electronic properties in search of possible half metallic antiferromagnetism. The systems (Cu,Co)MnSb are of particular interest as the end member alloys CuMnSb and CoMnSb are semi metallic (SM) antiferromagnetic (AFM) and half metallic (HM) ferromagnetic (FM), respectively. Clearly, Co-doping at the Cu-site of CuMnSb introduces changes in the carrier concentration at the Fermi level that may lead to half metallic ground state but there remains a persistent controversy whether the AFM to FM transition occurs simultaneously. Our experimental results reveal that the AFM to FM magnetic transition occurs through a percolation mechanism where Co-substitution gradually suppresses the AFM phase and forces FM polarization around every dopant cobalt. As a result a mixed magnetic phase is realized within this composition range while a nearly HM band structure is developed already at the 10% Co-doping. Absence of T2 dependence in the resistivity variation at low T-region serves as an indirect proof of opening up an energy gap at the Fermi surface in one of the spin channels. This is further corroborated by the ab initio electronic structure calculations that suggests that a nearly ferromagnetic half-metallic ground state is stabilized by Sb-p holes produced upon Co doping.

Molecular Orientation of a Terbium(III)-Phthalocyaninato Double-Decker Complex for Effective Suppression of Quantum Tunneling of the Magnetization.

Single-molecule magnet (SMM) properties of crystals of a terbium(III)-phthalocyaninato double-decker complex with different molecular packings (1: TbPc2, 2: TbPc2·CH2Cl2) were studied to elucidate the relationship between the molecular packing and SMM properties. From single crystal X-ray analyses, the high symmetry of the coordination environment of 2 suggested that the SMM properties were improved. Furthermore, the shorter intermolecular Tb–Tb distance and relative collinear alignment of the magnetic dipole in 2 indicated that the magnetic dipole–dipole interactions were stronger than those in 1. This was confirmed by using direct current magnetic measurements. From alternating current magnetic measurements, the activation energy for spin reversal for 1 and 2 were similar. However, the relaxation time for 2 is three orders of magnitude slower than that for 1 in the low-T region due to effective suppression of the quantum tunneling of the magnetization. These results suggest that the SMM properties of TbPc2 highly depend on the molecular packing.

Free energy calculations for rings and chains formed by dipolar hard spheres

We employ a method based on Monte Carlo grand-canonical simulations to precisely calculate partition functions of non-interacting chains and rings formed by dipolar hard spheres (DHS) at low temperature. The extended low temperature region offered by such cluster calculations, compared to what had been previously achieved with standard simulations, opens up the possibility of exploring a part of the DHS phase diagram which was inaccessible before. The reported results offer the unique opportunity of verifying well-established theoretical models based on the ideal gas of cluster approximation in order to clarify their range of validity. They also provide the basis for future studies in which cluster-cluster interactions will be included.

Thermal conductivity of the quark matter for the SU(2) light-flavor sector

We investigate the thermal conductivity (κ) of the quark matter at finite quark chemical potential (μ) and temperature (T), employing the Green–Kubo formula, for the SU(2) light-flavor sector with the finite current-quark mass m = 5 MeV . As a theoretical framework, we construct an effective thermodynamic potential from the (μ, T)-modified liquid-instanton model (mLIM). Note that all the relevant model parameters are designated as functions of T, using the trivial-holonomy caloron solution. By solving the self-consistent equation of mLIM, we acquire the constituent-quark mass M0 as a function of T and μ, satisfying the universal-class patterns of the chiral phase transition. From the numerical results for κ, we observe that there emerges a peak at μ≈200 MeV for the low-T region, i.e. T≲100 MeV . As T increase over T≈100 MeV , the curve for κ is almost saturated as a function of T in the order of ~ 10-1 GeV 2, and grows with respect to μ smoothly. At the normal nuclear-matter density ρ0 = 0.17 fm -3, κ shows its maximum 6.22 GeV 2 at T≈10 MeV , then decreases exponentially down to κ≈0.2 GeV 2. We also compute the ratio of κ and the entropy density, i.e. κ/s as a function of (μ, T) which is a monotonically decreasing function for a wide range of T, then approaches a lower bound at very high T: κ/s min ≳0.3 GeV -1 in the vicinity of μ = 0.

Thermodynamic Study of Nodal Structure and Multiband Superconductivity of KFe2As2

The temperature, field, and field-orientation dependences of the electronic specific heat Ce of the ironpnictide superconductor KFe2As2 have been investigated. Thermodynamic evidence of the presence of line nodes is obtained from the T and $\sqrt{H}$ linear dependences of Ce/T in the low-T and low-H region. Under a magnetic field rotated within the tetragonal ab plane, a fourfold oscillation is observed in Ce with a sign change at 0.08Tc. On the basis of the Doppler-shift analysis, the observed Ce minima in H // [100] at low T indicate the presence of line nodes somewhere on the Fermi surface where the Fermi velocity is parallel to the [100] direction; this is consistent with the octet-line-node scenario proposed recently by a photoemission experiment. In addition, the low-T Ce/T exhibits an unusual upturn on cooling at moderate fields only for H // ab, which is understood in terms of the strong Pauli paramagnetic effect on multiband superconductivity.

Isotope effects and temperature dependences in the action of the glucose dehydrogenase of the mesophilic bacterium Bacillus megaterium

The glucose dehydrogenase of the mesophilic bacterium Bacillus megaterium (optimal growth around 35 °C) exhibits non-linear Eyring temperature dependences from 25 to 55 °C in its catalysis of the oxidation by hydride-transfer to NAD+ of the β-anomers of 1-h-D-glucose and 1-d-D-glucose (rate constant kcat/KMβ). A break around 300K separates a high-T region from a low-T region. In the high-T region, isotopic enthalpies of activation within a considerable experimental error are equal to zero. In the low-T region, the enthalpies of activation are roughly equal for the isotopic substrates but are different from zero. An alternative treatment with Eyring plots taken as effectively linear produces enthalpies of activation having the unusual feature of being larger for the H-substrate (26 kJ/mol) than for the D-substrate (21 kJ/mol). Compensation of the enthalpic effect by a more positive entropy for the H-substrate then reproduces the isotope effects. For oxidation by NADP+ of the same pair of isotopic glucose substrates, catalysis by the glucose dehydrogenase of Thermoplasma acidophilum, a thermophilic archaeon, leads to temperature dependences characterized by a high-T region and a low-T region separated by a gentle thermal transition (K. Anandarajah, K.B. Schowen, and R.L. Schowen, Z. phys. Chem. 2008, 222, 1333-1347). Tentative approaches to a mechanistic interpretation of both cases rely on models featuring configurational searches of the enzyme for tunneling states, followed by hydrogen-transfer tunneling, although explanations can be constructed also on the basis of simple transition-state stabilization without tunnelling.

Phase transitions in heavy-quark QCD from an effective theory

With combined hopping parameter and strong coupling expansions, we calculate a dimensionally reduced Polyakov-loop effective theory valid for heavy quarks at nonzero temperature and arbitrary chemical potential. We numerically compute the critical endpoint of the deconfinement transition as a function of quark masses and number of flavours. We also investigate the applicability of the model to the low-T and high density region, specifically in terms of baryon condensation phenomena.

Molecular dynamics simulations of nucleation from vapor to solid composed of Lennard-Jones molecules

We performed molecular dynamics (MD) simulations of nucleation from vapor at temperatures below the triple point for systems consisting of 10(4)-10(5) Lennard-Jones (L-J) type molecules in order to test nucleation theories at relatively low temperatures. Simulations are performed for a wide range of initial supersaturation ratio (S(0) ≃ 10-10(8)) and temperature (kT = 0.2-0.6ε), where ε and k are the depth of the L-J potential and the Boltzmann constant, respectively. Clusters are nucleated as supercooled liquid droplets because of their small size. Crystallization of the supercooled liquid nuclei is observed after their growth slows. The classical nucleation theory (CNT) significantly underestimates the nucleation rates (or the number density of critical clusters) in the low-T region. The semi-phenomenological (SP) model, which corrects the CNT prediction of the formation energy of clusters using the second virial coefficient of a vapor, reproduces the nucleation rate and the cluster size distributions with good accuracy in the low-T region, as well as in the higher-T cases considered in our previous study. The sticking probability of vapor molecules onto the clusters is also obtained in the present MD simulations. Using the obtained values of sticking probability in the SP model, we can further refine the accuracy of the SP model.

Temperature scaling of dynamic hysteresis in Zr-rich PbZr1−xTixO3 ceramics

The temperature scaling relations of the dynamic hysteresis were investigated in Zr-rich lead zirconate titanate (PZT) bulk ceramics. At each relation curve, there is a discontinuous district corresponding to the low- and high-temperature phase transitions. The temperatures are divided into low-T region and high-T region. At each temperature region, three power-law scaling relations were obtained for hysteresis area ⟨A⟩, coercivity Ec, and remnant polarization Pr with T, respectively. The six temperature scaling relations are field dependent and the simplified forms are different from the results obtained from the soft and hard PZT ceramics.

Single Crystal Magnetic Susceptibility of 1-D [PM·Cu(NO3)2·(H2O)2

Single crystal studies of the magnetic susceptibility of the chain compound [PM·Cu(NO3)2·(H2O)2]n (PM = pyrimidine) show that it is a good example of a uniform S=1/2 antiferromagnetic Heisenberg chain, characterised by a single exchange parameter |2J|/kB= 36 K. The Cu g-tensor principal values are g= 2.34(2) and g| =2.05(2). In the low-T region a divergent contribution to the susceptibility (Curie tail) is observed that exhibits a pronounced anisotropy. This Curie tail cannot be reconciled assuming chain defects or impurities as its origin. It rather appears intrinsic to the chains and associated with the relative canting of the local g-tensors of neighbouring Cu ions and the Dzialoshinski-Moriya interaction. Our study constitutes evidence for a staggered contribution to the susceptibility which has been predicted theoretically recently.

Interplane charge transport in YBa2Cu3O7-y: Spin-gap effect on in-plane and out-of-plane resistivity.

We have investigated the anisotropic resistivities of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ using detwinned crystals with various oxygen contents (6.68 \ensuremath{\le}7-y\ensuremath{\le} 6.93). The out-of-plane resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ shows a crossover from high-T metallic to low-T semiconducting behavior while the in-plane resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{a}}$ deviates in the low-T region from T-linear dependence. We find that the crossover in ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ is linked with the onset of nonlinearity in ${\mathrm{\ensuremath{\rho}}}_{\mathit{a}}$, which seems to be associated with the ``spin gap'' suggested by neutron and NMR studies.

Low-temperature specific heat of icosahedral and amorphous Pd−U−Si alloys

Icosahedral (I) Pd0.588U0.206Si0.206 can be obtained from melt-spun amorphous (A) ribbons by annealing. The specific heatC (measured betweenT=0.1 K and 20 K) shows very similar behavior for both phases. The main features ofC are as follows. (i) The vibrational heat capacityCph dominatesC at highT.Cph is almost identical in both phases, in agreement with recent inelastic neutron scattering data. (ii) Shallow maxima in (C−Cph)/T vs.T are found at 5.4 and 4.3 K forI andA phases, respectively, associated with magnetic order. These maxima are suppressed by ≈20% in an applied magnetic field of 6 T. (iii) A large quasi-linear contribution is observed with a low-T coefficient γ=165 mJ/mole U K2 for theI phase and γ=120 mJ/mole U K2 for theA phase. In the low-T region,C is hardly affected by a field of 6 T. This hints at the formation of a narrow 5f band with a comparable density of states for bothI andA phases.

THERMODYNAMIC PROPERTIES OF THE ONE-DIMENSIONAL EXTENDED HUBBARD MODEL (Ⅱ)——U<0

Entropy, internal energy, specific heat, and magnetic susceptibility of the one-dimensional negative-U extended Hubbard model are computed. An analytical solution of magnetic susceptibility in the strong-coupling (|U|→∞) limit is obtained. Therma-dynamic properties of the system can be discussed in two temperature regions. The high T-region corresponds to decrease in number of double occupied states. The low T-region corresponds to the distribution of electron charge. The effect of W on the-rmodynamic properties is obvious.