Quantum critical behaviour in doped Y bB12 studied by ab initio calculations

The Kondo insulator Y bB₁₂ is known to undergo a transition to the metallic state with doping or under an external magnetic field. Within the virtual crystal approximation (VCA), we calculated the occupation of the Yb 4f and 5d shells, and , as a function of doping of Y bB₁₂ with the rare earths Tm and Lu. We found that exhibits an anomalous change at the critical concentration of the dopant, in agreement with experiment ( for Y b_1−xLu_xB₁₂ and for Y b_1−xTm_xB₁₂). We suggest that the critical behaviour seems to be strictly connected with the change of and in consequence the change of the Yb valency.     

Multiple superconducting gap and anisotropic spin fluctuations in iron arsenides: Comparison with nickel analog

We present extensive ⁷⁵As NMR and NQR data on the superconducting arsenides PrFeAs_0.89F_0.11, LaFeAsO_0.92F_0.08, LiFeAs and Ba_0.72K_0.28Fe₂As₂ single crystal, and compare with the nickel analog LaNiAsO_0.9F_0.1. In contrast to LaNiAsO_0.9F_0.1 where the superconducting gap is shown to be isotropic, the spin lattice relaxation rate 1/T₁ in the Fe-arsenides decreases below T_c with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T_c and shows a step-wise T variation as well. These results indicate spin-singlet superconductivity with multiple gaps in the Fe-arsenides. The Fe antiferromagnetic spin fluctuations are anisotropic and weaker compared to underdoped copper-oxides or cobalt-oxide superconductors, while there is no significant electron correlations in LaNiAsO_0.9F_0.1. We will discuss the implications of these results and highlight the importance of the Fermi surface topology.     

Possible coexistence of superconductivity and magnetism in intermetallic NiBi3

NiBi₃ polycrystals were synthesized via a solid state method. X-ray diffraction analysis shows that the main phase present in the sample corresponds to NiBi₃ in a weight fraction of 96.82 % according to the refinement of the crystalline structure. SEM - EDS and XPS analysis reveal a homogeneous composition of NiBi₃, without Ni traces. The powder superconducting samples were studied by performing magnetic measurements. The superconducting transition temperature and critical magnetic fields were determined as , Oe and Oe. The superconducting parameters were , , and κ=5.136. Isothermal measurements below the transition temperature show an anomalous behavior. Above the superconducting transition the compound presents ferromagnetic characteristics up to 750 K, well above the Ni Curie temperature.     

Low-temperature measurements of magnetic susceptibility and specific heat of Eu2Ti2O7—An XY pyrochlore

Magnetic susceptibility obtained from magnetization measurement (for fields H=0.1 and 1.0 T) of polycrystalline Eu₂Ti₂O₇ shows two distinct features. Firstly, increases on cooling below 300 K and attains a temperature-independent constant value at 68 K (T_max). Secondly, shows an antiferromagnetic increase below 4.9±0.1 K. The former behavior is explained by crystal field (CF) theory. CF levels and wave functions of ground and excited states are determined accurately from analyses of and earlier reported Mössbauer and optical spectra. Analysis of vs. 1/T curve at low temperatures gives the classical nearest-neighbor exchange interaction J^cl=−0.76 K and a weak dipolar interaction D_nn=0.0056 K. C_P of polycrystalline sample of Eu₂Ti₂O₇ and Y₂Ti₂O₇ are measured between 1.8-35 and 1.8-120 K respectively and θ_D vs. T (K) curves are calculated. At 4 K, θ_D of Eu₂Ti₂O₇ shows a kink and dC_P/dT curve show a maximum. Optical results show energy exchange between Eu³⁺ ions at intrinsic and extrinsic (defect) sites via super-exchange interaction at low temperature which may account for the observed anomalous behavior of and C_P.     

Ground state electronic and magnetic properties of RCrSb3 (R=La, Ce, Nd, Gd and Dy): A first principles study

The electronic density of states (DOS) and magnetic moments of rare-earth antimonides (RCrSb₃) have been studied by the first principles full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). For the exchange-correlation potential, the LSDA+U method is used. The effective moments of LaCrSb₃, CeCrSb₃, NdCrSb₃, GdCrSb₃, and DyCrSb₃ were found to be , , , and respectively. The exchange-splittings of Cr-3d state electrons and 4f-states of rare earth elements were analyzed to explain the magnetic nature of these systems. The Cr atom plays a significant role on the magnetic properties due to the hybridization between Cr-3d and Sb-5p state orbitals. The results obtained are compared and found to be in close agreement with the available data.     

Thermal and magnetothermal properties of La0.5Pr0.5Mn2Si2

The thermal and magnetothermal properties of La_0.5Pr_0.5Mn₂Si₂ and isostructural LaFe₂Si₂ intermetallic compounds in the temperature range 4.5-303 K are reported with and without applied magnetic field. The electronic, lattice, and magnetic contributions to the heat capacity of La_0.5Pr_0.5Mn₂Si₂ are determined and analyzed. We have determined and from heat capacity experiments; the values are in line with those from the magnetization measurements. We conclude that in order to observe the anomaly in the heat capacity data around in the system, the transition around should occur in a narrow temperature interval.     

Ac magnetotransport in La0.7Sr0.3Mn0.95Fe0.05O3 at low dc magnetic fields

We report the ac electrical response of La_0.7Sr_0.3Mn_1−xFe_xO₃(x=0.05) as a function of temperature, magnetic field (H) and frequency of radio frequency (rf) current (). The ac impedance (Z) was measured while rf current directly passes through the sample as well as in a coil surrounding the sample. It is found that with increasing frequency of the rf current, Z(T) shows an abrupt increase accompanied by a peak at the ferromagnetic Curie temperature. The peak decreases in magnitude and shifts down with increasing value of H. We find a magnetoimpedance of for at around room temperature when the rf current flows directly through the sample and when the rf current flows through a coil surrounding the sample. It is suggested that the magnetoimpedance observed is a consequence of suppression of transverse permeability which enhances skin depth for current flow. Our results indicate that the magnetic field control of high frequency impedance of manganites is more useful than direct current magnetoresistance for low-field applications.     

The effect of the Bi source on optical properties of Bi2Te3 nanostructures

nanostructures were synthesized by using different Bi sources via a simple solvothermal process, in which and BiCl₃ were used as the Bi sources. Optical properties of nanostructures prepared with and BiCl₃ as the Bi sources were investigated by micro-Raman spectroscopy. The Raman scattering spectrum of hexagonal nanoplates prepared by using as the Bi source shows that the infrared (IR) active mode A_1u, which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shows up clearly in the Raman scattering spectrum. We attribute the appearance of the infrared active A_1u mode in the Raman spectrum to crystal symmetry breaking of hexagonal nanoplates. However, the Raman scattering spectrum of nanostructures with irregular shape prepared by using as the Bi source only exhibits the two characteristic Raman modes of crystals. Micro-Raman measurements on nanostructures with different morphologies offer us a potential way to tailor optical properties of nanostructures by controlling the morphologies of the nanostructures, which is very important for practical applications of nanostructures in thermoelectric devices.     

Low field magnetocaloric effect and heat capacity of A-site ordered NdBaMn2O6 manganite

The magnetocaloric effect (MCE) in the A-site ordered manganite NdBaMn₂O₆ is studied. The MCE in this compound has an anomalous behavior. In low magnetic fields, the abrupt transitions between the direct and inverse magnetocaloric effects are observed. In a relatively strong magnetic field , the direct and inverse effects are observed only at cooling, while the heating mode reveals only an inverse MCE. The value of the MCE ( and for  kOe) does not reach high values, but the proximity of the effects occurring at room temperatures expects the use of both effects in the magnetic cooling technology.     

Combined neutron and synchrotron X-ray diffraction study of Sr/Mg-doped lanthanum gallates up to high temperatures

Combined neutron diffraction and high-resolution synchrotron X-ray powder diffraction methods have been used to examine the crystal structures of two sample sets of Sr/Mg-doped Lanthanum gallate with the compositions La_0.9Sr_0.1Ga_1−yMg_yO_{3−0.5(0.1+y)} (y=0, 0.1, 0.2) and La_0.8Sr_0.2Ga_1−yMg_yO_{3−0.5(0.2+y)} (y=0.15, 0.2) up to 900 °C. At room temperature all samples of the first series exhibit orthorhombic structures with space group Imma: La_0.9Sr_0.1GaO_2.95: , , ; La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9: , , ; La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85: , , . The samples of the second series have the cubic perovskite structure with space group at room temperature: La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825: ; La_0.8Sr_0.2Ga_0.8Mg_0.20O_2.80: . Samples of the first series transform from the orthorhombic to a rhombohedral (Imma→) structure at ∼170 °C for La_0.9Sr_0.1GaO_2.95, at ∼430 °C for La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9, and between 600 and 700 °C for La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85. Both La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825 and La_0.8Sr_0.2Ga_0.8Mg_0.2 show no structural deviations from the cubic aristotype over the whole temperature range. The room temperature Imma structures of the first series are justified by a domain model and are rationalized in terms of static disorder increasing with Mg content, thus driving the phase transition temperatures to higher values in agreement with tolerance factor considerations. The distortion of the rhombohedral high-temperature phases (octahedra tilting and compression) and the effect of phase transitions on the ionic conductivity are discussed.     

Optical phonon modes and structure of ZnGa2Se4 and ZnGa2S4

We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa₂Se₄ and ZnGa₂S₄. Most of the compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry (defect stannite), has been reported. For ZnGa₂Se₄ we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group for the investigated crystals of ZnGa₂Se₄.Regarding ZnGa₂S₄ we have found three modes exclusively showing Raman activity (2A⊕1B₁), and only eight modes showing infrared as well as Raman activity (3B₂⊕5E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa₂S₄ most likely crystallizes in space group .     

Shape-dependent local internal stress of α-Cr2O3 nanocrystal fabricated by pulsed laser ablation

The α-Cr₂O₃ single-crystal nanocondensates were fabricated by pulsed laser ablation in air and characterized by analytical electron microscopy regarding shape-dependent local internal stress of the anisotropic crystal. The nanocondensates formed predominantly as rhombohedra with well-developed surfaces and occasionally hexagonal plate with thin edges and blunt corners. Such nanocondensates showed Raman shift for the CrO₆ polyhedra, indicating a local compressive stress up to ca. 4 GPa on the average. Careful analysis of the lattice fringes revealed a local compressive stress (0.5% strain) at the thin edge of the hexagonal plates and a local tensile stress (0.3–1.0% strain) near the relaxed , , and (0 0 0 1) surfaces of truncated rhombohedra. The combined effects of nanosize, capillarity force at sharp edge, and specific surface relaxation account for the retention of a local internal compressive stress built up in an anisotropic crystal during a very rapid heating–cooling process.     

Isothermal magnetic entropy behavior in Tb5Si3 : Sign reversal and non-monotonic variation with temperature, and implications

The magnetic entropy change (), a measure of the magnetocaloric effect, in Tb₅Si₃, a compound exhibiting unusual positive magnetoresistance following a magnetic-field-induced transition below the magnetic transition temperature (∼69 K), has been investigated. We found that is negative in the paramagnetic state. At the magnetic transition temperature, shows a sign reversal from a negative value (in the paramagnetic state) to a positive value (in the magnetically ordered state). The high-field state, which is interestingly the high resistive state, is found to be associated with higher entropy, i.e., large positive , behaving like a paramagnet. On the basis of this observation, we conclude that the magnetic field induces magnetic fluctuations in the system resulting in positive magnetoresistance, thereby rendering support to the idea of inverse metamagnetism in this compound. In addition, we note that the Arrott plots present an interesting scenario.     

Model calculations of the energy band structures of double stranded DNA in the presence of water and Na ions

Using the ab initio Hartree-Fock crystal orbital method in its linear combination of atomic orbitals form we have calculated the band structures of poly(-) and poly(-). Here, besides the nucleotide bases, the sugar and phosphate parts of the nucleotide were also taken into account together with their first water shell and Na⁺ ions. We use the notation with a tilde above the abbreviation of a base for the whole nucleotide; for instance poly() means a guanine base with the deoxyribose and PO₄⁻ groups to which it is bound. The obtained band structures were compared with previous single chain calculations as well as with the earlier poly(-) and poly(-) calculation without water but in the presence of counterions. One finds that all the bands of poly(-) and poly(-) are shifted considerably upwards as compared to the previous single chain results (poly(), poly(), poly() and poly()). This effect is explained by the ∼0.2e charge transfer from the sugars of both chains to the nucleotide bases. The fundamental gaps between the nucleotide base-type highest filled and lowest unfilled bands are decreased in both cases by 1-3 eV, because the valence bands are purine-type and the conduction bands pyrimidine-type, respectively, while in the case of single homopolynucleotides they belong to the same base. We also pointed out that the LUMO is mainly Na⁺-like in both investigated cases and several unoccupied bands (belonging to the Na⁺ ions, the phosphate group and the water molecules) can be found between this and the first unoccupied pyrimidine-like empty band.     

Giant magnetoimpedance and colossal ac magnetoresistance of a Cu coil wound on La0.67Sr0.33MnO3

We observed the giant magnetoimpedance and colossal ac magnetoresistance for a Cu coil wound on La_0.67Sr_0.33MnO₃ under low dc magnetic fields. Even though the dc magnetoresistance for La_0.67Sr_0.33MnO₃ plate is only −2.4% under H=12 kOe, a Cu coil wound on La_0.67Sr_0.33MnO₃ plate exhibits a colossal ac magnetoresistance of −93% at 10 MHz and a giant magnetoimpedance of −59% in a wide frequency range of 500 kHz-10 MHz under a longitudinal field . The transverse magnetoimpedance is weaker than the longitudinal one. The giant magnetoimpedance and colossal ac magnetoresistance for a Cu coil wound on La_0.67Sr_0.33MnO₃ are connected with the variation of permeability induced by dc magnetic field.     

Nanophases in mechanochemically synthesized AgI-CuI system: structure, phase stability and phase transitions

Nanoscale crystallites of Ag-rich (Ag_1−xCu_xI, x=0.05, 0.10, 0.15 and 0.25), Cu-rich (Cu_1-yAg_yI, y=0.05, 0.10, 0.15 and 0.25) and intermediate Ag_1-xCu_xI (x=0.50) solid solutions and end members AgI, CuI with sizes in the range of 46-13 nm were synthesized by attrition at ambient temperature in a soft mechanochemical reaction (MCR) of Ag, Cu and I. Monophasic γ-AgI (zincblende, ) with disordered Ag⁺ sublattice and the crystallite size of about ∼31 nm was realized in the case of Ag_0.75Cu_0.25I (x=0.25) composition. Lattice parameter decreases linearly from 649 to 604 pm with increasing Cu concentration in the AgI-CuI system validating Vegard's law. Smallest size (∼13 nm) agglomerated nanocrystals were realized in the Cu-rich composition Cu_0.75Ag_0.25I (), while unagglomerated uniform-sized (∼17 nm) and spherical shape nanocrystallites of Ag_0.50Cu_0.50I () with maximum strain were synthesized for sensor applications using MCR. Differential scanning calorimetry study shows the systematic changes in the phase transition temperature with Cu substitution. Ag-rich composition posses less enthalpy (ΔH (x or Cu=0.05, 0.10, 0.15, 0.25)=6.0, 6.11, 6.6, 6.3 in kJ/mol) and entropy (ΔS (y or Ag=0.05, 0.10, 0.15, 0.25)=14.15, 14.1, 15.03, 13.6 in J/mol K) when compared to undoped AgI () implying greater thermal stability of γ-phase due to Cu-strengthened Ag-I bond. Enhanced entropy () in Cu_0.75Ag_0.25I (Cu-rich) solid solutions relative to CuI () indicates Ag-induced cation disorder. Fifteen percent Ag-doped CuI (Cu_0.85Ag_0.15I) nanocrystals apparently behave like microscopic p-n junctions with currents in the range of 10⁻⁶-10⁻⁸ A characterized by a non-linear I-V curve.     

High pressure and temperature study of hydrogen storage material BH3NH3 from ab initio calculations

We report on BH₃NH₃, which is material considered promising to use as hydrogen storage, using density functional theory with generalized gradient approximation (GGA). We study the phase transition of BH₃NH₃ at high pressure and temperature. Our observed phase transition of BH₃NH₃ from body-centered tetragonal to orthorhombic at supports the recent and earlier studies. We observe the phase transformation of BH₃NH₃ at , which is in good agreement with experimental value. Specifically, we predict the phase transition at to be orthorhombic to body-centered tetragonal on the basis of our first principles calculations.     

First-principles study on the formation energies of intrinsic defects in LiNbO3

First-principles plane-wave ultrasoft pseudopotential method within local density approach (LDA) has been used to study three possible vacancy-defect models for non-stoichiometric lithium niobate (LiNbO₃): (1) the oxygen-vacancy model , (2) the niobium-vacancy model , and (3) the lithium-vacancy model . The corresponding formation energies are obtained via energy minimization of a supercell. In Nb-rich environment, the calculated defect formation energies, both under oxidation and reduction conditions, show little effect on the intrinsic defect structures. We find that the lithium vacancy model has the most stable configuration in the non-stoichiometric lithium niobate crystals. Our calculations also show that the formation of any type of neutral defects and Frenkel pairs in a Nb-rich environment is difficult.     

Local magnetization study of the first-order superconducting transition in CeCoIn5

We report local magnetization measurements on the heavy fermion superconductor CeCoIn₅ using a micro-Hall probe. Below a critical point T₀ (), the local magnetization shows a clear jump at the superconducting transition temperatures for both H∥a and H∥c, indicating that the phase transition at the upper critical field H_c2 becomes a first-order phase transition. In addition, we observed an undershoot behavior of magnetization jump above (H∥c), which suggests a rapid change of textured superconducting structure with vortices and the nodal planes expected in the Fulde-Ferrell-Larkin-Ovchinnikov state.