Investigation of LiFeAs by means of “break-junction” technique

In our tunneling investigation using Andreev superconductor-normal metal-superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap ??_L = (2.5?C3.4) meV and the small gap ??_S = (0.9?C1) meV at T = 4.2 K for the T _C ^local ?? (10.5?C14) K (the contact area critical temperature which deviation causes the variation of ??_L). The BCS-ratio is found to be 2??_L/k _B T _C = 4.6?C5.6, whereas 2??_S/k _B T _C ? 3.52 results from induced superconductivity in the bands with the small gap.     

Tunneling spectroscopy of layered superconductors: intercalated Li0.48(C4H8O)xHfNCl and De-intercalated HfNCl0.7

Tunneling measurements have been carried out on layered superconductors of the β(SmSI)-type – Li_0.48(THF)_xHfNCl (THF?=?C₄H₈O) and HfNCl_0.7 – by means of break-junction and scanning tunneling spectroscopy. Break-junction technique reveals Bardeen-Cooper-Schrieffer (BCS) – like gap structures with typical gap values of 2Δ (4.2 K) = 11–12 meV for Li_0.48(THF)_xHfNCl with the highest T_c = 25.5 K. Some of our measurements revealed multiple gaps and dip-hump structures, the largest gap 2Δ (4.2 K) ≈ 17–20 meV closing at T_c. This was shown both by break-junction and scanning-tunneling spectroscopy. From these experiments it stems that the highest obtained gap ratio 2Δ/k_BT_c ~ 8 substantially exceeds the BCS weak-coupling limiting values: ≈3.5 and ≈4.3 for s-wave and d-wave order parameter symmetry, respectively. Such large 2Δ/k_BT_c ratios are rather unusual for conventional superconductors but quite common to high-T_c cuprates, as well as to organic superconductors. Our studies allowed to collect much more evidence concerning the huge pairing energy in those materials and to investigate in detail the complexity of their superconducting gap spectra. An origin of the observed phenomena still remains to be clarified.     

Charge-order-driven spin-peierls transition in alpha(')-NaxV2O5

We conclude from 23Na and 51V NMR measurements in alpha(')-NaxV2O5 (x = 0.996) (i) a charge-ordering transition starting at T相似文献   

Radioactive decay speedup at T=5 K: electron-capture decay rate of (7)Be encapsulated in C(60)

The electron-capture (EC) decay rate of (7)Be in C(60) at the temperature of liquid helium (T=5 K) was measured and compared with the rate in Be metal at T=293 K. We found that the half-life of (7)Be in endohedral C(60) ((7)Be@C(60)) at a temperature close to T=5 K is 52.47+/-0.04 d, a value that is 0.34% faster than that at T=293 K. In this environment, the half-life of (7)Be is nearly 1.5% faster than that inside Be metal at room temperature (T=293 K). We then interpreted our observations in terms of calculations of the electron density at the (7)Be nucleus position inside the C(60); further, we estimate theoretically the temperature dependence (at T=0 K and 293 K) of the electron density at the Be nucleus position in the stable center inside C(60). The theoretical estimates were almost in agreement with the experimental observations.     

Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO_0.88F_0.12FeAs samples with bulk critical temperature T _c = (52.5 ± 1) K using break-junction technique. The data obtained can- not be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2 K are Δ _L = 10.5 ± 2 meV and Δ _s = 2.3 ± 0.4 meV, respectively.     

Raman studies and optical properties of some (PbO)x(Bi2O3)0.2(B2O3)0.8−x glasses

Five (PbO)_x(Bi₂O₃)_0.2(B₂O₃)_0.8−x glasses, where x = 0, 0.2, 0.3, 0.4 and 0.6, were prepared. The dilatometric glass transition temperature (T_g) was found in the region 470 (x = 0)≥ T_g ( °C) ≥ 347 (x = 0.6), and the density (ρ) varied within 4.57 (x = 0) ≤ ρ (g/cm³) ≤ 8.31 (x = 0.6). Raman spectra indicated the conversion of BO₃ to BO₄ entities for low x values but for x > 0.3, namely, for x → 0.6, back‐conversion occurred, most probably. From the measurements of the optical transmission on very thin bulk samples, the room temperature optical gap values (E_g) were determined to be in the range 4.03 (x = 0)≥ E_g (eV) ≥ 3.08 (x = 0.6). The temperature (T) dependence of the optical gap (E_g(T)) in the region 300 ≤ T(K) ≤ 600 was examined and approximated by a linear relationship of the form of E_g(T) = E_g(0)− γT, where γ × 10⁻⁴(eV/K) varied from 5.1 to 6.8. The non‐linear refractive index (n₂) was estimated from the optical gap values and it was found to correspond to the n₂ values calculated from the experimental third‐order non‐linear optical susceptibility taken from the literature. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

Magnetic and electric studies of a new Co(II) perovskite-like material

Structural phase transitions in the perovskite-like material [(CH₄)₁₂(NH₃)₂]CoCl₄ have been observed using differential thermal scanning. The material shows an order-disorder transition at T ₁ = 396 ± 5 K with entropy, (ΔS ₁) = 12.8 J/mole/K. A "chain melting" transition with a major endothermic peak at T ₂ = 337 ± 3 K and a minor one at T ′ = 316 ± 2 K, has total entropy ΔS = 28 J/mole/K. At low temperatures, the transitions at T ₃ = 288 ± 3 K and at T ₄ = 188 ± 3 K, have entropies of ΔS ₃ = 14.4 J/mole/K and ΔS ₄ = 2.6 J/mole/K respectively. AC magnetic susceptibility in the temperature range 78-290 K, in a magnetic field of 160 A/m and at a frequency of 320 Hz is presented. The results indicate changes in symmetry at 188 K. Dielectric permittivity has been studied as a function of temperature in the range 300-430 K and frequency range (60 Hz-100 kHz), confirming the observed transitions. The dielectric permittivity reflects rotational and conformational transition for the material. The variation of the real part of the conductivity with temperature is thermally activated with different activation energies in the range of ionic hopping. The temperature dependence of the dc conductivity and that of the ions hopping rate have indicated that the concentration of mobile ions is independent of temperature. The dependence of the conductivity on frequency follows the universal power law, <artwork name="GPHT31040ei1"> in the temperature range 340 K<T<390 K. Values 0 <s ₁ <1 dominate at low frequency and correspond to translational hopping motion and values 1<s ₂<2 dominate at high frequencies and correspond to well localized hopping and/or reorientational motion. For T > 396 K, the AC conductivity was fitted to <artwork name="GPHT31040ei2"> with 0<s<1. Comparison with the corresponding Cu-containing material is discussed.     

Magnetic spin ladder (C5H12N)2CuBr4: high-field magnetization and scaling near quantum criticality

The magnetization, M(H< or =30 T,0.7< or =T< or =300 K), of (C5H12N)2CuBr4 has been used to identify this system as an S = 1/2 Heisenberg two-leg ladder in the strong-coupling limit, J( perpendicular) = 13.3 K and J( parallel) = 3.8 K, with H(c1) = 6.6 T and H(c2) = 14.6 T. An inflection point in M(H,T = 0.7 K) at half saturation, M(s)/2, is described by an effective XXZ chain. The data exhibit universal scaling behavior in the vicinity of H(c1) and H(c2), indicating that the system is near a quantum critical point.     

Far-infrared and mid-infrared absorption in (Bi,Pb)-Sr-(Ca,Y)-Cu-O

The reflectivities of seven films belonging to the Bi-Sr-Ca-Cu-O family, with T_c ranging from 60 to 95 K, and of a single crystal of insulating Bi-Sr-Y-Cu-O, have been measured at room temperature in the normal phase. The optical conductivity has been analyzed in terms of Drude (D), mid-infrared (MIR), and charge-transfer (CT) contributions. As T_c increases, the spectral weight within the charge-transfer gap (D+MIR) increases. Meanwhile, the peak frequency of the MIR band moves towards the far-infrared and the optical conductivity becomes more similar to that of a normal metal.     

Spin gap in the zigzag spin-1/2 chain cuprate Sr(0.9)Ca(0.1)CuO(2)

We report a comparative study of (63)Cu nuclear magnetic resonance spin lattice relaxation rates T(1)(-1) on undoped SrCuO(2) and Ca-doped Sr(0.9)Ca(0.1)CuO(2) spin chain compounds. A temperature independent T(1)(-1) is observed for SrCuO(2) as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T(1)(-1) for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J(1)-J(2) Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J(2)-exchange coupling as a possible cause of the spin gap.     

Rotational and Vibrational Temperature Measurements in a High‐Pressure Cylindrical Dielectric Barrier Discharge (C‐DBD)

The rotational (T_R) and vibrational (T_v) temperatures of N₂ molecules were measured in a high‐pressure cylindrical dielectric barrier discharge (C‐DBD) source in Ne with trace amounts (0.02 %) of N₂ and dry air excited by radio‐frequency (rf) power. Both T_R and T_v of the N₂ molecules in the C ³Π_u state were determined from an emission spectroscopic analysis the 2^nd positive system (C ³Π_u → B³Π_g). Gas temperatures were inferred from the measured rotational temperatures. As a function of pressure, the rotational temperature is essentially constant at about 360 K in the range from 200 Torr to 600 Torr (at 30W rf power) and increases slightly with increasing rf power at constant pressure. As one would expect, vibrational temperature measurements revealed significantly higher temperatures. The vibrational temperature decreases with pressure from 3030 K at 200 Torr to 2270 K at 600 Torr (at 30 W rf power). As a function of rf power, the vibrational temperature increases from 2520 K at 20 W to 2940 K at 60 W (at 400 Torr). Both T_R and T_v also show a dependence on the excitation frequency at the two frequencies that we studied, 400 kHz and 13.56 MHz. Adding trace amounts of air instead of N₂ to the Ne in the discharge resulted in higher T_R and T_v values and in a different pressure dependence of the rotational and vibrational temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Antiferromagnetic ordering driven by the molecular orbital order of C(60) in alpha'-tetra-kis-(Dimethylamino)-ethylene-C(60)

We have studied the ground state of a fullerene-based magnet, the alpha;{'}-phase tetra-kis-(dimethylamino)-ethylene-C60 (alpha'-TDAE-C(60)), by electron spin resonance and magnetic torque measurements. Below T(N) = 7 K, nonparamagnetic field dependent resonances with a finite excitation gap (1.7 GHz) are observed along the a axis. Strong enhancement in their intensity as temperature is decreased is inconsistent with excitation from a singlet state, which had been proposed for the alpha'-phase ground state. Below T(N), nonquadratic field dependence of the magnetic torque signal is also observed in contrast to quadratic field dependence in the paramagnetic phase. The angle-dependent torque signals below T(N) indicate the existence of an anisotropy of the bulk magnetization. From both experiments, we propose an antiferromagnetic ground state driven by the cooperative orientational ordering of C(60) in the alpha'-TDAE-C(60).     

Electron spin-lattice relaxation in noncommon metals YBa2Cu3Ox and Rb1C60

Using an original modulation technique, the electron spin-lattice relaxation have been investigated in two noncommon metals: YBa₂Cu₃O_x, high-T_c material doped with 1% Gd, and Rb₁C₆₀, linear polymer phase fulleride. In the first case, the Korringa-like temperature dependence of the Gd³⁺ longitudinal relaxation time T₁, is found forx = 6.59 in a wide temperature range 4.2 <T < 200 K, both above and below T_c = 56 K. Atx = 6.95 (T_c = 90 K), the T₁ behavior within 50 <T < 200 K is evidently affected by spin gap opening with the gap value of about 240 K. At 200 K, an unexpected acceleration of the relaxation rate takes place, suggesting some change in the relaxation mechanism. The data are discussed in terms of the Barnes-Plefke theory with allowance made for microscopic separation of the normal and superconducting phases. In Rb₁C₆₀, the evolution of the ESR line and relaxation rates have been studied accurately in the range of the metal-insulator transition (below 50 K). Interpretation is suggested which takes into account breaking down the relaxation bottleneck due to opening of the energy gap near the Fermi surface. The gap value of about 100 cm^?1 is estimated from the analysis of relaxation rates, lineshape and spin susceptibilities.     

Spin gaps and spin dynamics in superconducting cuprates

Spin gap effects, consisting of a declining uniform susceptibility and spin paramagnetic NMR shift at low temperaturesin the normal state and associatedT ₁ behavior, are discussed and documented in several cuprate superconductors. Dynamic spin magnetism in these systems is further reviewed in the light of mean-field models, where we note that detailed results from the model by Millis, Monien, and Pines are not borne out in recent neutron data on YBa₂Cu₃O_6.92.T ₁ data on¹⁷O in La_1.85Sr_0.15CuO₄ are presented, showing consistency with neutron dynamic susceptibility data forT≧80 K, but exhibiting a strong spin gap character below 80 K which is not present in the neutron data. Data for Zn-doped YBCO withT _c≈60 K are also presented, showing strong RKKY broadening from localized moments in the planes, but no spin gap effect such as that found in theT _c=60 K oxygen-deficient phase.     

Splitting of the electronic states near EF in A4C60 compounds (A = alkali metal)

The electronic structure of A₄C6₀ compounds (A=Na, K, Cs) has been studied using electron energy-loss spectroscopy. Both the low energy loss functions and Cls core excitation measurements reveal a strong splitting of the electronic states near the Fermi level in comparison with K₃C₆₀ or K₆C₆₀. This splitting indicates a lowering of the high degeneracy of the C₆₀ molecular orbitals due to a broken symmetry ground state and/or the opening of a correlation gap in these systems.     

Manifestation of quantum statistics in vibrational dynamics of poly(ethylene) crystals

The temperature dependences of the transverse expansion ?_⊥(T) and the longitudinal contraction ?_‖(T) (with respect to the axes of chain molecules) in large-sized poly(ethylene) (PE) crystal grains (100×60×60 nm) are measured using x-ray diffraction in the temperature range 5–380 K. The temperature dependence of the elongation of the molecular skeleton ?_C(T) is obtained by Raman spectroscopy. It is found that the dependences ?_⊥(T), ?_‖(T), and ?_C(T) exhibit a similar specific nonlinear behavior. Analysis of these dependences indicates that the nonlinearity is associated with the quantum statistics of transverse vibrations. The energies and amplitudes of zero-point (at T=0) transverse (torsional and bending) vibrations and the relevant zero-point components ?_‖(0) and ?_C(0) are estimated. It is revealed that the zero-point components make a considerable contribution to the dynamics of the PE crystal up to the melting temperature (～400 K).     

Gap at the Fermi level in the intermetallic vacancy system RBiSn(R=Ti,Zr,Hf)

The new class of intermetallic compounds RNiSn(R=Ti,Zr,Hf) may be characterised by the presence of an ordered sublattice of Ni atom vacancies in comparison with normal metals RNi₂Sn with no Ni vacancies. We report unusual transport and optical properties of the RNiSn system. The electrical resistivity of RNiSn is very high (3<p<100) mOhm*cm; the temperature coefficient of resistivity (TCR) is negative and strongly dependent on the annealing conditions. For some samples ZrNiSn and for a single crystal of TiNiSn the resistivity can be described by the Mott's law at temperatures 0.1<T<20 K. A phase transition nearT=100 K without change of crystal structure was deduced from Hall effect data and the temperature dependence of the lattice constant. Preliminary data on transport phenomena in RPtSn and RPdSn(R=Ti,Zr,Hf) compounds are also reported. The unusual properties of RNiSn system might be related to a gap of the electron spectrum near the Fermi energy.     

The adsorption of cyclic hydrocarbons on Ru(001): II. Cyclohexane

The adsorption of cyclohexane on Ru(001) at 90 K has been investigated by thermal desorption mass spectrometry, EELS, UV photoemission and LEED. Thermal desorption indicates the adsorption of the undissociated molecule first in a chemisorbed monolayer (T_d = 200 K) with subsequent formation of multilayers (T_d = 165 K) at higher exposures. The vibrational spectrum obtained by EELS is characterized by a frequency shift of the C-H stretching mode from 2920 cm^?1 (multilayer) to 2560 cm^?1 for the chemisorbed monolayer. Off-specular EELS data indicate two different electron scattering mechanisms for the C-H stretching mode. Whereas for the C-H stretching mode of the multilayer, large angle electron impact scattering is observed, the C-H soft-mode of the monolayer is largely due to small angle dipolar scattering. The He I photoelectron spectra of cyclohexane multilayers are characteristic of the undissociated molecule. A new assignment of C(2s) and the lowest C(2p) level, based on a comparison with benzene, shows that the chemisorbed monolayer is characterized by the absence of emission or broadening of the 2a_1u level. This is attributed to C_3v symmetry of the chemisorbed layer and to a possible interaction of the 2a_Iu orbital with the metal surface.     

The pressure dependence of the superconducting transition temperature of LaT4P12(T = Fe,Ru, Os)

The superconducting transition temperature, T_c, of the LaT₄P₁₂ compounds with T = Fe, Ru, or Os has been measured under hydrostatic pressure P up to 1.8 GPa. The T = Fe compound exhibits a substantial increase of T_c from T_c (P = 0) = 4.1 K at a rate (dT_c/dP)_P=0= +7.2 x 10^-1 K/GPa. In contrast, the Ru and Os compounds exhibit only weak decreases of T_c from T_c (P = 0) = 7.2 K and 1.8 K with (dT_c/dP)_P=0= -1.6 x 10^-1 K/GPa and -9.5 x 10^-2 K/GPa, respectively. An analysis of this strikingly divergent behavior of T_c(P) in terms of the structural characteristics of the RT₄X₁₂ class of compounds where R = rare earth element, T = Fe, Ru, or Os, and X = P, As, or Sb suggests that T_c(P) for these materials consists of two competing contributions: a depression of T_c due to the compression of the lattice (i.e., decrease in volume), and an enhancement of T_c due to the effect of pressure on La itself.