Ground state of Hubbard model on 2 × 2 × 2 lattice

The energy and the total spin of the ground state of Hubbard model on 2 × 2 × 2 lattice are calculated for various values of the intra-atomic Coulomb repulsion I. The total spin S of the ground state assumes the smallest value when the electron number N is 2, 3 or 6, irrespective of the value of I. For N = 5, S is $\text{3}\text{2}$ and is independent of I, too. For N = 4 or 7, S assumes the smallest value when I is not very large, and the ground state becomes completely ferromagnetic when I exceed 39 or 200 times the inter atomic transfer energy, respectively.     

Investigation of 4f-magnetism in CeNi2P2, EuNi2P2 and YbNi2P2 by susceptibility and NMR studies

The results of ³¹P NMR Knight shift (KS) and spin-lattice relaxation time (T₁) as well as magnetic susceptibility (_ξ) measurements in the temperature interval 4.2–300K are reported for the compounds RENi₂P₂(RE = Ce, EuandYb) in order to understand the nature of the 41 magnetism in these compounds. The KS results conclusively establish that all these compounds exhibit non-magnetic ground states. There is a low-temperature upturn in _ξ for CeNi₂P₂ and YbNi₂P₂ presumably due to magnetic impurities. The estimated spin fluctuation temperatures (T_sf) for EuNi₂P₂ above 50K agree qualitatively with those obtained earlier from Mössbauer and _ξ data employing ionic interconfigurational fluctuation model, but seem to disagree at low temperatures. Implications of this result are discussed.     

Current-voltage characteristics of voltage-biased Tl2Ba2CaCu2O8 intrinsic Josephson junctions

The voltage-biased current-voltage (I–V) characteristics of intrinsic Josephson junctions (IJJs), which are fabricated with misaligned high temperature superconducting Tl₂Ba₂CaCu₂O₈ (Tl-2212) thin film, are investigated experimentally. Three characteristic regions in the I–V curve are observed at 47 K. In the low voltage part, the current firstly increases and then decreases slowly with increasing the biased voltage, which is shown as a bump. In the next region, the current slightly increases with increasing the biased voltage until a sudden decrease of the current appears. Thereafter, branch structure forms with increasing the voltage on the I–V characteristic. The influence of the self-heating on the I–V characteristics is investigated and the temperature dependence of the I–V characteristics is measured to explore these characteristics in detail.     

Structural properties and band structure of heavy fermion systems: CeCu2Si2 and LaCu2Si2

The structure of CeCu₂Si₂, isotypic with ThCr₂Si₂, has been refined in a single-crystal study. The atomic parameters were used in self-consistent LMTO band structure calculations for CeCu₂Si₂ and isostructural LaCu₂Si₂. The results are analysed in terms of energy levels, charge distribution and in particular Fermi surface properties. The Ce-4f levels are situated mainly aboveE _F , except near the Γ-point. The density-of-states atE _F is large and heavily concentrated around the Ce-4f band. This makes local Ce 4f fluctuations possible, while interaction with the rest of the band structure is small. The comparison with LaCu₂Si₂ shows that the additional Cef electron is partly promoted into the non-f bands. Large enhancement factors are required to bring band results into agreement with observed specific heat and magnetic critical field. Strong Fermi surface anisotropies are pointed out for both compounds suggesting new experiments on single crystals.     

Critical magnetic field H c2 and electron scattering in MgB2

The correlation between the upper critical field H _c2 and the residual resistivity ρ of the MgB₂ compound is studied in a wide range of ρ values. The slope ?dH _c2/dT of the temperature dependence of H _c2 near T _c is found to increase steadily with increasing residual resistivity to 100 μΩ cm. Over the range from 0 to 50 μΩ cm, the dependence of ?dH _c2/dT on ρ is fitted well by a linear function, which is typical of a single-band superconductor. The fit is performed using the electronic parameters of the two bands (π and σ) that form the Fermi surface. The field H _c2 is assumed to depend on the electronic parameters of the σ band only. Using this approximation, the following quantities are estimated: the relative contribution of σ electrons to the total conductivity along the boron planes (which turns out to be about 1/2), the ratio of the mean free paths of electrons in the σ and π bands l _σ/l _π ≈ 1.5, and the ratios between some other parameters describing electron scattering.     

Oscillator strengths and transition probabilities of the Be(I) isoelectronic sequence for 2p2-2s2p and 2s2p-2s2 transitions

Oscillator strengths and transition probabilities are obtained from an intermediate coupling scheme. The initial basis of the eigenstates is formed inLS-coupling. To obtain the resulting energy matrix,Z^-1 perturbation theory is applied. The values of oscillator strengths and transition probabilities for 1s²2p²?1s²2s2p and 1s²2s2p?1s²2s² transitions are calculated and the comparison is made with some recent results. The nuclear charges (Z) involved varied fromZ=4(Be(I)) toZ=35(Br(XXXII)).     

Spin-forbidden transitions between 2p 2 and 2s2p states in the beryllium isoelectronic sequence

Fine-structure splittings, wavelengths and transition probabilities are evaluated for transitions 2p ^{2 1} S ₀→2s2p ³ P ₁ ⁰, 2p ^{2 1} D ₂→2s2p ³ P _J ⁰ (J=1, 2) and 2p ^{2 3} P _J →2s2p ¹ P ₁ ⁰ (J =0, 1, 2) in beryllium-like ions using configuration interaction wavefunctions. Some significant differences with previous calculations are obtained. At present there are no experimental results. However, from calculations reported here it should clearly be worthwhile to search for these lines in laboratory and astrophysical spectra. The decay of the 2p ^{2 1} D ₂ level is discussed.     

Europium valency in EuCu2−x Fe x Si2, EuCu2Si2−x Ge x,EuCu2−x Si2+x and EuNi2−x Si2+x

Mössbauer spectroscopy, X-ray photoemission spectroscopy and magnetization studies of EuCu_2?x Fe _x Si₂, EuCu₂Si_2?x Ge _x , EuCu_2?x Si_2+x , and EuNi_2?x Si_2+x have been performed. In EuFe₂Si₂ and EuNi₂Si₂ the Eu ion is trivalent, in EuCu₂Si₂ it is of intermediate valency. In all systems withx=0, many inequivalent Eu sites of intermediate valency are formed. In EuCu_2?x Fe _x Si₂, the average valency of Eu moves nonmonotonically toward Eu³⁺. In EuCu₂Si_2?x Ge _x , EuCu_2?x Si_2+x and EuNi_2?x Si_2+x , the Eu average valency moves quickly towards Eu²⁺. In all three systems, whenx=1, all Eu ions are already stable divalent. The systems EuCuSi₃ and EuNiSi₃ order magnetically at 40 K and 34 K, and exhibit hyperfine fields of 323 kOe and 458 kOe, respectively. XPS and Mössbauer isomer shift determinations of the average Eu valence as a function of temperature in EuCu_1.5Fe_0.5Si₂ are in good agreement. The experimental observations concerning the Eu valencies can not be explained solely in terms of the local volume available to the Eu ion; the nature of chemical environment plays a dominant role.     

Natural radiative lifetimes in the2S1/2 and2D5/2,3/2 sequences of aluminum

Natural radiative lifetimes have been measured for the 3s ² ns ² S _1/2 (n≦10) and 3s ² nd ² D _5/2,3/2 (n≦11) sequences of aluminum using pulsed laser excitation of an atomic beam. The investigated states were populated from the ground configuration using UV pulses of wavelength down to 210 nm produced by a Nd: YAG- or an excimer-pumped dye-laser system. TheS-state lifetimes increase regularily with increasing principal quantum number but this is not the case for theD sequence.     

Theory of a single Oxygen hole propagating in Sr2CuO2Cl2: the spin of the quasiparticles in CuO2 planes

Recent photoemission experiments have measured E vs. k for a single hole propagating in antiferromagnetically aligned Sr₂CuO₂Cl₂. Comparisons with (i) the t - t′ - J model, for which the carrier is a spinless vacancy, and (ii) a strong-coupling version of the three-band Emery model, for which the carrier is a S = 1/2 hole moving on the Oxygen sublattice, have demonstrated that if one wishes to describe the quasiparticle throughout the entire first Brillouin zone the three-band model is superior. Here we present a new variational wave function for a single Oxygen hole in the three-band model: it utilizes a classical representation of the antiferromagnetically ordered Cuspin background but explicitly includes the quantum fluctuations of the lowest energy doublet of the Cu-O-Cu bond containing the Oxygen hole. We find that this wave function leads to a quasiparticle dispersion for physical exchange and hopping parameters that is in excellent agreement with the measured ARPES data. We also obtain the average spin of the Oxygen hole, and thus deduce that this spin is only quenched to zero at certain wave vectors, helping to explain the inadequacy of the t - t′ - J model to match the experimentally observed dispersion relation everywhere in the first Brillouin zone.     

Electromechanical properties of 2-2 cement based piezoelectric composite

A 2-2 type cement based piezoelectric composites were fabricated by dice-and-fill technique. The influences of lead magnesium niobate-lead zirconate-lead titanate (PMN) ceramic volume fraction on the electromechanical properties of the composite were investigated. The results indicate that the planar electromechanical coupling factor K_p of the composite is hardly influenced by the PMN volume fraction, which fluctuates between 35% and 37%, while both the thickness electromechanical coupling factor K_t and the mechanical quality factor Q_m exhibit the trend of increase. When PMN volume fraction is 69.2%, K_t could reach to 49.8%, which is larger than that of the PMN ceramic (K_t = 46%).When PMN volume fraction is 41.7%, Q_m is 4.5, which is also much less than that of the PMN ceramic (Q_m = 70). This means that the composite has wider frequency band and higher sensitivity to be used as transducers. With the increase of PMN volume fraction, the acoustic impedance of the composite also exhibits the trend of increase.     

Hyperfine structure of the 8p 2 P 3/2 and 8p 2 P 1/2 levels of115In

The hyperfine structure of the 8p ² P _3/2 and² P _1/2 levels of¹¹⁵In was measured using high-resolution laser spectroscopy on a collimated indium atomic beam. Step-wise excitations from the metastable 5² P _3/2 state via the shortlived 6s ² S _1/2 state were employed. For the magnetic-dipole and electric-quadrupole interaction constantsa andb, respectively, the following results were obtained:a(8p ² P _3/2)=16.3 (3) MHz,b(8p ² P _3/2)=11 (3) MHz anda(8p ² P _1/2)=44.0 (5) MHz. The magnetic hyperfine structure is shown to be strongly influenced by core polarization. For the electric-quadrupole moment of¹¹⁵In the valueQ=0.79 (20) barn is deduced.     

Resistivity anisotropy and charge density wave in 2H - NbSe2 and 2H - TaSe2

The in-plane and out-of-plane resistivities of both 2H-TaSe₂ and 2H-NbSe₂ were determined down to 10 K. For both compounds, the resistivity anisotropy shows notably a slope change at temperatures where a CDW transition is expected to occur. On the other hand, for both compounds the resistivity anisotropy at the lowest temperature of measurement is much greater than expected by the Lawrence–Doniach model, which relates the critical magnetic field anisotropy to the normal state resistivity anisotropy for 3D-anisotropic superconductors.     

Elastic constants of 2H-MoS2 and 2H-NbSe2 extracted from measured dispersion curves and linear compressibilities

Recent neutron data on the dispersion curves and X-ray measurements of the linear compressibilities of the 2H polytypes of MoS₂ and NbSe₂ have been used to obtain approximate values of the five independent elastic constants of these materials. In the case of NbSe₂ sufficient information is available to over-determine the elastic constants and the results are self consistent within estimated uncertainties, although the uncertainties are especially large for c₃₃ and c₁₁. Additional related considerations such as Debye temperatures and model calculations of c₃₃, and c₄₄ are also made. It is found that there is significant and unexplained disagreement between the value of the low temperature specific heat Debye temperature of NbSe₂ and the value determined on the basis of the elastic constants, but that the model predictions of c₃₃ and c₄₄ are in satisfactory agreement with the values extracted from the neutron data for both MoS₂, and NbSe₂.     

Dielectric relaxation and ferroelectricity in Cd2Nb2O7 pyrochlore

The dielectric dispersion of Cd₂Nb₂O₇ pyrochlore in a weak electric field was studied in a broad frequency range (100 Hz to 13 MHz) using the crystal samples slowly cooled (0.5 K/min) in the temperature interval from 300 to 80 K. As the temperature decreased down to T _c=196 K and T _max～190 K, the dielectric permittivity exhibited deviation from the Curie-Weiss law. It is suggested that this behavior is related to the development of a short-range correlation between microscopic polar regions formed at T→T _max ⁺ . The local order parameter q(T) ～ 〈P _i P _j 〉^1/2 was calculated using the permittivity ε′(T) measured at various frequencies. The variation of this parameter is compared to that of the spontaneous polarization P _s (T) determined from the measurements of a pyroelectric current in the external electric field E _dc =0.95 kV/cm. In the frequency range from 100 Hz to 13 MHz, the dispersion of the dielectric response in the temperature region of 180–192 K is characteristic of a relaxator ferroelectric featuring a glasslike behavior. The parameters of this state were determined, including the activation energy of the polarization fluctuations (E _a ≈0.01 eV), the relaxation rate at T → ∞ (f ₀=1.9×10¹² Hz), and the polarization fluctuation freezing temperature (T _f =183 K). In Cd₂Nb₂O₇ pyrochlore, in contrast to the known relaxator ferroelectrics of the PMN type studied previously, this state coexists with the normal ferroelectric state appearing at T _c.     

Low temperature heat capacities and thermal properties of DyAl2, ErAl2 and LuAl2

The heat capacities of the compounds DyAl₂, ErAl₂ and LuAl₂ were measured in an adiabatic calorimeter from approximately 5 to 300 K. The compounds DyAl₂ and ErAl₂ show C_P anomalies at 58.0 and 10.2 K, respectively, which are attributed to the destruction of magnetic order. In order to separate the crystal field and magnetic contributions from the measured heat capacities, it was necessary to evaluate the lattice heat capacity. The lattice term, C_L was obtained from the C_P data of LuAl₂ by a method of interpolation which gave values of C_L for an arbitrary R Al₂ compound. Using this “interpolated lattice blank”, excess entropies associated with the crystal field and magnetic terms were computed throughout the series. These values are quite close to R In (2J + 1). The results also indicate that, for the compounds studied, the degeneracy of the lowest ground state is completely lifted. In addition, the magnetic contribution to the heat capacity of the magnetically ordered R A1₂ phases was found to exhibit an exponential dependence below the temperature corresponding to the spin wave energy gap and a T^{$\text{3}\text{2}$} dependence above this temperature.Detailed calculations were performed to characterize the influence of cubic crystal field in ErAl₂ on the ⁴I_{$\text{15}\text{2}$} ground state multiplet of the Er³⁺ ion. It is concluded that the magnetic ordering in ErAl₂ takes place within the Γ₈³ quartet state. Smoothed values of heat capacity, entropy and related thermodynamic functions are tabulated.     

Low-field irreversibility line and its anisotropy of textured (Bi,Pb)2Sr2Ca2Cu3Oy silver-clad tapes

The irreversibility line H_irr(T) of textured (Bi,Pb)₂Sr₂Ca₂Cu₃O_y silver-clad tapes was obtained by ac susceptibility (ACS) and magnetoresistivity (MR) below 1500 G. It is found that, in the framework of flux motion, the H_irr(T) is determined by the thermally activated flux flow (TAFF) rather than the so-called giant flux creep, and the experimentally obtained H_irr(T) line is related to a certain resistivity criterion in both ACS and resistivity measurements. H_irr(T) is found to be significantly depressed by the amplitude of applied driving AC field and generally lower than that measured from the resistivity. The anisotropy in H_irr(T) is found to follow a scaling behaviour as described by the anisotropic effective mass model.     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

The magnetic structure of HoCo2Si2

We have carried out neutron diffraction on a HoCo₂Si₂ powder sample at 4.2 K. The magnetic structure of this compound is collinear antiferromagnetic with the holmium magnetic moments parallel to the c-axis of the crystal. The magnetic moment value of holmium is 9.85 μ_B. The magnetic space group is I4/mm'm' (Sh⁴¹⁰₁₂₈) k = 000 The ordering temperature is t_n = 12(1) K.