Hysteresis effects at a cooperative high-spin (5T2) ⇌ low-spin (1A1) transition in dithiocyanatobis (4, 7-dimethyl-1, 10-phenanthroline) iron (II)

Hysteresis has been observed at the cooperative high-spin (⁵T₂) ? low-spin (¹A₁) transition in Fe (4, 7-(CH₃)₂-phen)₂(NCS)₂ where phen = 1, 10-phenanthroline. The transition is centered on T_c> = 121.7 K for rising and on T_c< = 118.6 K for lowering of temperature. The observations are in only qualitative agreement with the thermodynamic theory of Slichter and Drickamer.     

Variable temperature EPR study of Fe2+ spin transition in Cu2+ doped [Fe(trz)(Htrz)2](BF4)

The temperature dependence of the EPR spectrum of Cu²⁺ in the range 293–393 K exhibits a low-spin (S=0) to high-spin (S=2) transition of the Fe²⁺ ions, with hysteresis (T _c↑=363 K,T _c↓=343 K). At 103 K, the principal values of theg and hyperfine tensors of Cu²⁺ ions are revealed by hyperfine structure.     

Fe2+ spin transition in [Fe(trz)(Htrz)2](BF4) as evidenced by a variable temperature EPR study

The EPR of Fe³⁺ ions has been used for the first time to evidence a low-spin (S=0) to high-spin (S=2) transition of Fe²⁺ ions in an octahedral ferrous complex [Fe(trz)(Htrz)₂](BF₄). The temperature dependence of the intensity of the Fe³⁺ EPR line atg=4.3 reveals a spin transition which occurs for the Fe²⁺ ions, with hysteresis. The transition temperatures areT _c↑=374 K in the warming mode andT _c↓=345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.     

Low energy electron diffraction beam profiles and phase transition at Si(111)-7 X 7 surface

Angular profiles of low energy electron diffraction (LEED) beams from Si(111)-7 × 7 are measured for various crystal temperatures T near the phase transition with apparent critical temperature T_c ≈ 1140 K. From analyses of the profiles it is concluded that (1) long range superstructure order persists for T up to at least 50 K above T_c and (2) with increasing T the correlation length characterizing the short-range order peaks for T ≈ T_c ? 100 K and decreases rapidly for T >T_c. Conclusion (1) is discussed with reference to a dislocation network model of Si(111)-7 × 7 reconstruction.     

Effect of structural relaxation on low energy excitations in amorphous Zr x Cu1−x

We report on an investigation of the liquid-quenched metallic glass Zr _x Cu_1?x (0.6≦x≦0.74) subjected to heat treatments below the glass transition temperatureT _g. Annealing temperatures up to 200°C (<0.8T _g) were chosen as to achieve topological relaxation only. The superconducting transition temperaturesT _c are lowered, as already observed for other metallic glasses. Low temperature measurements of the thermal conductivity (0.5 K≦T≦15 K) and of the specific heat (0.1 K≦T≦3 K) were carried out in order to determine the effect of structural relaxation on the low energy configurational excitations characteristic of the amorphous state. The annealed samples show no detectable (<20%) change in the specific heat forT?T _c, but an increase of the thermal conductivity by a factor of 2 forT?T _c is observed. Within the tunneling model of two level systems (TLS) for the low energy excitations, this behavior can be qualitatively understood in terms of a change of the TLS relaxation time distribution upon annealing. This distribution differs from that of the commonly used standard tunneling model. The change of the phonon scattering by TLS directly observed forT?T_c is largely responsible for the enhancement of the thermal conductivity found also aboveT _c.     

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

Polarization properties of Li2−x Na x Ge4O9 (0.2 ≤ x ≤ 0.3) crystals

The polarization switching in sinusoidal fields and the pyroelectric properties of Li_2?x Na _x Ge₄O₉ (0.2 ≤ x ≤ 0.3) crystals are measured in the temperature range T _c ?T ≤ 40 K. The behavior of the P?E hysteresis loops with variations in temperature is investigated for crystals with phase transition temperatures T _c < 300 K and T _c > 300 K. It is shown that, for crystals with phase transition temperatures T _c < 300 K, the temperature dependence of the hysteresis loop exhibits a behavior typical of crystals with second-order phase transitions. The crystals with phase transition temperatures T _c > 300 K are characterized by double hysteresis loops in the temperature range T _c ?T ₁ ≈ 30 K. The correlation between the polarization properties and possible structural transformations of the Li_2?x Na _x Ge₄O₉ crystals due to the change in the concentration ratio of Na and Li ions is discussed.     

Spin crossover in the magnetic phase transition in YBa2(Cu1−x Fex)3O7±δ superconductors

All iron ions in the Cu1 and Cu2 local lattice sites of the YBa₂(Cu_0.9 ⁵⁷Fe_0.1)₃O_7.01 superconductor with T _c=31 K experienced magnetic ordering below T _m=22 K. Therefore, at T < T _m, magnetic ordering coexisted with superconductivity. According to the Mössbauer spectroscopy data, iron ions in Cu2 (Fe2) sites were in the low-spin state at T < T _m(S= 3/2 or 1/2), whereas an equal number of iron ions in Cu1 (Fe1) sites were in the high-spin Fe³⁺ state (S=5/2). The magnetic transition near T _m changed iron ion spin states-low-spin ions turned into high-spin ions, and vice versa. This preserved the spin balance between iron ions in the Cu1 and Cu2 layers. Control measurements on other samples of the YBa₂(Cu_{1? x} Fe_x)₃O_7±δ series substantiated these conclusions.     

A thermal and Raman investigation of the phase transitions above room temperature in anhydrous potassium acetate

Results using thermal and Raman techniques are presented on the phase transition in potassium acetate occurring above room temperature. Two phase transitions are observed at T_c1 = 349K and T_c2 = 413K. The magnitude of the entropy change at T_c1 and the Raman results suggest that this transition is orientational in nature and is due to rotation of the acetate ion about the c-c axis. The T_c2 transition is displacive in nature, and the Raman results show a marked softening of the lowest frequency mode as T_c2 is approached from below.     

Metal-nonmetal transition and superconductivity in frozen tin-hydrogen films

We have studied the d.c.-conductivity σ and the superconducting transition temperature T_c in frozen mixtures of Sn and molecular H₂ (Sn-H₂) or atomic H(Sn-H). The samples were prepared by co-deposition of Sn and H₂ (H) onto a sapphire substrate held at a temperature of about 5 K. Both systems show a quite different behaviour. While the Sn-H₂ system exhibits a metal-insulator transition at c_H2 = 0.6 which agrees well with percolation theory, the Sn-H samples show a metal-semiconductor transition already at c_H?0.32. Both systems have an increased T_c in the metallic region. H₂ seems to be mobile in Sn-H₂ films even down to 5 K after annealing of the samples at 10 K. Atomic H, on the other hand, is bound up to T?200 K.     

Observation of a transition from BCS to HTSC-like superconductivity in Ba1 − x KxBiO3 single crystals

The temperature dependences of the upper critical field B _c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba_{1 ? x} K_xBiO₃ single crystals with transition temperatures 6 ≤ T _c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B _c2(T) curves of the crystals with T _c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T _c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T _c ≈ 32 K and T _c ≈ 11 K in the temperature range T ? T _c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.     

Superconductivity in the Zr-D system under pressure

The superconducting transition temperature T _c of the ZrD_0.48 alloy is measured in the pressure range up to 41.5 GPa. The measurements are carried out in a high-pressure chamber with diamond anvils by the inductometric method. It is found that T _c(P) increases to 3.1 K at a pressure below 30 GPa, exhibits a sharp increase up to 8 K near 30 GPa, and then smoothly decreases to ～6.5 K at 41.5 GPa. A similar dependence T _c(P) is obtained for pure Zr. The similarity of the T _c(P) curves suggests that the dependence T _c(P) observed for ZrD_0.48 is due to the presence of ω-phase in this alloy at pressures P<30 GPa and the ω-β transition at P≈30 GPa, which leads to the establishment of new ratios between the phases in the Zr-D system. In the pressure range studied, no indications are observed for new superconducting phases similar to the phases of intermediate composition in the Ti-H(D) system, which are formed by the hydrogen transfer from tetrahedral to octahedral interstitials.     

23Na nuclear magnetic resonance investigation of the lattice distortion in α-NaxV2O5

The full temperature dependence of the electric field gradient tensor at the Na sites has been determined by nuclear magnetic resonance (NMR) in the temperature range 8–330 K in α-Na_{x 2}O₅ (x = 0.996). Above the spin-Peierls transition (T _c = 34.7 K), only a single Na site is observed in agreement with the Pmmn space group proposed to describe this compound as the first example of a 1/4-filled ladder system. Below T_c, eight distinct quadrupolar²³Na sites are observed according to the distortion wave vector k_c = (1/2, 1/2, 1/4) previously reported. In addition, the opening of a spin gap is evidenced by a rapid drop of the magnetic hyperfine shift²³K at T_c. The results are discussed in the context of a charge-order-driven spin-Peierls transition.     

Temperature variation of phonon frequency distribution in the fast ion conductor lead fluoride

A weighted phonon frequency distribution has been measured in PbF₂ at temperatures 10, 302, 660 and 910 K, using a neutron scattering technique. At 10 K good agreement is found between the measured distribution and the phonon density-of-states calculated from the low temperature dispersion relation of PbF₂. At the higher temperatures, near the ionic conductivity transition temperature, T_c ～ 700 K, the optic modes are observed to broaden into a high energy tail consistent with strong anharmonicity or extensive disorder. A low energy peak arising from transverse acoustic modes remains well defined even at temperatures above T_c.     

The Nernst-Ettingshausen coefficient in hole-doped manganites

A systematic study of the Nernst-Ettingshausen coefficient (NEC) in hole-doped manganites of the LaMnO₃ and SmMnO₃ systems was carried out at temperatures both above and below the point of transition to the magnetically ordered state (T _c). The results obtained for T > T _c suggest that conduction is mediated here by small-radius polarons. For all the compositions studied, the Nernst mobilities at T = 300 K are small (of the order of 0.1–2 cm²/V s) and the carrier relaxation time at T > T _c increases with carrier energy. At temperatures below T _c, the NEC exhibits an anomalous behavior. The giant NEC effect was observed, which consists in a strong dependence of the NEC on applied magnetic field and the presence of a peak in the temperature dependence of the NEC at T ≈ T _c. Near the transition to the magnetically ordered state, the NEC follows a behavior similar to that of colossal magnetoresistance and giant magnetothermoelectric power. A possible origin of the anomalous NEC behavior at T < T _c is discussed.     

Ferroelastic phase transition and anomalous elastic and thermal properties of betaine borate (CH3)3NCH2COO·H3BO3

The temperature and pressure derivatives of the elastic constants of orthorhombic betaine borate, (CH₃)₃NCH₂COO·H₃BO₃, have been determined by measuring temperature and stress induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 140 and 300 K and 0 and 3 kbar. The elastic ‘shear’ resistance c₄₄ exhibits a value as low as 0.0492×10¹⁰Nm^-2at 293 K. With decreasing temperature c₄₄ approaches zero at ca. 142.5 K, indicating an acoustic soft mode behaviour connected with a ferroelastic phase transition. The softening of c₄₄ is described in a good approximation by c₄₄(T)_p=0 =alogT/T₀ with a=0.0663×10¹⁰Nm^-2 and T₀ = 139.5 K. Further, c₄₄ decreases with increasing pressure according to the linear relation c₄₄(p)_{T=293 K} = 0.0492?0.184×10^-4p (p in bar, c₄₄ in 10¹⁰ Nm^-2). All other elastic constants show a quite normal temperature and pressure dependence. At 293 K the transition is induced by a pressure of 2.65 kbar. The transition temperature T_c depends linearly on pressure according to T_c = 142.5+0.0568 p (pinbar, T_cinK). Passing through the transition no discontinuous change of the lattice constants is observed. The three principal coefficients of thermal expansion and the pressure derivatives of the dielectric constants exhibit discontinuities at the transition. The transition is of strongly second order.     

Dynamic phenomena in layer superconducting cuprate and organic metals

Dynamic electron spin resonance (ESR) and extended x-ray absorption edge fine structure (XAFS) measurements suggest that layered organic metals and cuprate superconductors behave similarly. The response to microwave radiation in a modulated external magnetic field indicates that: (i) triplet state, T * ESR is observed below T_c for both; (ii) the condensation of free spin doublet D to T* occurs above the transition temperature to superconductivity T_c (10 ± 1 K for the organic metal (BEDT-TTF)₃Ta₂F₁₁ and 92 to 12 K for YBa₂Cu₃O_7-δ and its rare earth derivatives); (iii) antiferomagnetic (AF) resonance is detected above T_c for the organic metal. Here the exchange field between the aligned AF domains: J_AF(150 K) = 130.7 mT (153 mK) is greater than the exchange term J(150 K) ≈ 15 mT (20 mK) between free spins (S = 1/2) leading to T* states; the lifetime of AF domains τ_AF decreases below 150 K and resonance is not detected below 44 K (i.e. τ_AF < 10^-10 s) allowing a superconducting transition to appear below 10 K; (iv) the relaxation time τ₁ for the half field, triplet state ESR absorption increases fourfold near 10 K for the organic metal and, (v) the onset of superconductivity is detected in all superconductors by the appearance of an energy loss at exactly H=0 and, magnetization oscillations observed versus H below T_c when the samples are cooled in a non-zero field H. The spin-lattice relaxation time for the organic metal triplet state, half field ESR near 10 K is interpreted using the Gorter phenomenological relation τ₁ = C_H /α_H, C_H and α_H are respectively the heat capacity and the thermal contact coefficient to the lattice by the spin system, at constant field H . Complementary changes in x-ray edge widths near T_c are correlated to electron-phonon interactions.     

Probing the phase transition of methane with Mössbauer effect in 57Fe impurities

Mossbauer studies were performed in ⁵⁷Fe impurities (0.3% at.) in solid methane, in order to investigate the properties of the order-disorder phase transition taking place at T_c=20.4K. From the temperature dependence of the relative values of the recoil-free-fraction - f - it was found that near T_c a change occurs in the slope of f(T²) without discontinuity in the value of f. A consistent model is proposed by which immediately below T_c both phases, I and II coexist with a gradual evolution of the ordered state (II) on the account of the high temperature plastic state (I). The Debye temperature of phase II (θ_II = 101±7K) increases with respect to that of phase I (θ_I = 72±3K), indicating a hardening in the phonon spectrum and precluding the onset of rotational modes of the disordered molecules.     

Evidence for a three Tc behavior of the Kondo superconductor (La,Y)Ce

The dependence of the superconducting transition temperature T_c on Ce impurity concentration is reported for the (La_0.80Y_0.20)Ce system. Susceptibility and resistivity measurements on samples with about 0.85 at. % Ce show unique behavior which must be interpreted as a superposition of three transition curves: a first transition at 0.55 K from the normal to the superconducting state, a second one at 0.27 K back to the normal state, and finally a third one at about 0.05 K again to the superconducting state. The experimental evidence of this third transition (T_c3) is of special importance for the decision about the relevant physical mechanism of pair-breaking at low temperature (T ? T_K).     

The low field magnetisation of Y4Co3

The onset of magnetic order at 6 K and a superconducting transition at 2 K was previously observed in Y₄Co₃. In this paper we investigate the magnetisation in very low fields, in the vicinity of the magnetic and superconducting transition (T_c). Below T_c the M-H curves are characterised by broad hysteresis loops. This type of the behaviour indicates that the sample consists of different regions. Some of these regions are superconducting below T_c and others are magnetically ordered below 6 K.