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.     

Pyroelectric properties of a ferroelectric single crystal [NH2(CH3)2]3Sb2Cl9 (DMACA)

The pyroelectric properties of DMACA single crystals have been measured in the range 135–293 K, revealing the existence of ferroelectric second order phase transition at T_c = 243 K. The saturation value of spontaneous polarization P_s along a-axis amounts to 6.8 × 10⁻³ Cm⁻² at about 203 K. Critical exponent β = 0.5 has been found in the region 0.5–10 K away from T_c.     

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).     

Effect of doping with iron on the charge ordering in La0.33Ca0.67Mn1−y Fe y O3 (y = 0, 0.05) manganites

The formation and specific features of the superstructure in La_0.33Ca_0.67Mn_1?y Fe _y O₃ (y = 0, 0.05) manganites doped with iron are investigated using transmission electron microscopy. The electron diffraction patterns of the manganites are studied in the temperature range 90–300 K, and the high-resolution electron microscope images recorded at temperatures of 91–92 K are analyzed. In both manganites, the structural transition that is accompanied by the formation of the superstructure and which is directly observed from the appearance of additional peaks in the electron diffraction patterns occurs at a temperature that is in close agreement with the charge ordering temperature T _CO determined from the temperature dependences of the magnetization M(T). In the temperature range 90 < T < 200 K, the undoped compound has a commensurate superstructure characterized by the vector q = 1/3a* and triple the unit cell «3a × b × c» (where a ≈ b ≈ √2a _c , c ≈ 2a _c , and a _c ～ 3.9 Å is the lattice parameter of a simple perovskite). The doping with iron (5 at. %) brings about a decrease in the charge ordering temperature T _CO by 50 K and the formation of an incommensurate structure for which the magnitude of the vector q is smaller by approximately 15%. The unit cell of the superstructure in the iron-doped compound is not triple the unit cell but involves defects of ordering, such as quadrupling of the unit cell, numerous translations by a _c √2 along the a direction, and dislocation-type defects in the stripe structure of the charge ordering. These pseudoperiodic defects lead to a decrease in the magnitude of the vector q and are responsible for the incommensurability of the structure. A decrease in the charge ordering temperature T _CO due to the doping with iron and the incommensurability of the superstructure correlate with the change in the concentration of Mn³⁺ Jahn-Teller ions as a result of their replacement by Fe³⁺ non-Jahn-Teller ions.     

Central peak for a scalar ϕ 4-model: mode coupling approach revisited

We reinvestigate the mode coupling approach to the central peak which occurs in the vicinity of a structural phase transition at T _c. For a scalar ? ⁴-model it is shown that the use of renormalized vertices leads to quite different results compared to recent calculations with bare vertices. Particularly, we prove that the latter are obtained in leading order of the anharmonicity constant of the on-site potential from a perturbational treatment of the renormalized vertices. Again, this mode coupling approach may yield a dynamical transition at a temperature T _c'(≥ T _c) at which the dynamics becomes nonergodic, i.e. a central peak occurs. For a ? ⁴- model with infinite range interactions our theoretical predictions are consistent with numerical results. Furthermore, if the fluctuations in the vicinity of Tc are Gaussian, no dynamical transition occurs above Tc. Therefore the temperature T ₀'obtained from the Ginzburg criterion sets an upper bound for T _c'. If a dynamical transition occurs, it is shown that the nonergodicity parameter as function of wave vector q and temperature T follows from an universal master function.     

Reflexivity and Correlation of Magnetic States of Nanostructures in the Nb(70 nm)/Ni<Subscript>0.65</Subscript>Cu<Subscript>0.35</Subscript>(6.5 nm)/Si Ferromagnet–Superconductor Heterostructure

We have studied the Nb(70 nm)/Ni_0.65Cu_0.35(6.5 nm)/Si layered structure in the temperature range T = 1.5–10 K using polarized neutron reflectometry. The correlation of the states of magnetic structures is observed at temperature T = 9 K, which is slightly higher than the superconducting transition temperature T_c = 8.5 K of the structure. At temperature T = 4 K, which is lower than T_c, the effect of reflexivity of magnetic states existing at T = 9 K was observed.     

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.     

High-resolution spectroscopy of HoFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a study of phase transitions

The transmission spectra of HoFe₃(BO₃) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm^–1 with a resolution up to 0.1 cm^–1. A new first-order structural phase transition close to the second-order transition is recorded at T_c = 360 K by the appearance of a new phonon mode at 976 cm^–1. The reasons for considerable differences in T_c for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho³⁺ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at T_N = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at T_SR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to T_SR = 4.0 K.     

Manifestation of an intermediate phase in a ferroelastic K3Na(CrO4)2:MnO 4 2− in raman and IR absorption spectra

In a broad temperature range of 4–300 K, we have performed a complex combined investigation of phase transitions in crystals of a ferroelastic K₃Na(CrO₄)₂:MnO ₄ ^2? using methods of Raman light scattering and IR light absorption. Considerable changes that we have observed in both Raman and IR spectra in the range of T ≈ 150 K testify to the occurrence of another phase transition that has not been observed before at this temperature. We have performed a group-theoretical analysis and compared its results with experimental spectra, which has allowed us to conclude that there are two phase transitions in this crystal, \(P\bar 3m1 \to C2/m \to C2/c\) , which occur at temperatures T _c1 ≈ 230 K and T _c2 ≈ 150 K, respectively.     

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.     

Specific heat of SbSI

The temperature dependence of the specific heat of vapor and flux grown single crystal samples of ferroelectric antimony sulfur iodide (SbSI) was measured over a temperature range from 230K to 400K. C_p^E at T_c of the electrical flux and vapor grown samples are 0.078 cal/gmK and 0.076 cal/gmK respectively. ΔS and ΔQ at T_c are evaluated. The samples measured were grown by two different methods[1,2]. Differences are observed between the measurements made on the samples, both in the magnitude of the specific heat and the temperature at which the ferroelectric transition takes place. These differences are caused by the different levels of oxygen impurities in the samples.     

Anisotropy of magnetic properties of a twin-free EuBaCo2O5.5 single crystal

The magnetic characteristics of a twin-free EuBaCo₂O_5.5 single crystal were studied. It is shown that Co ions exhibit an Ising-type behavior with an easy axis directed along the a axis in the temperature range from 2 K to T _c (250 K). As the temperature decreases from 300 K, a number of magnetic phase transitions occur under magnetic fields H ∥ a. In the range where antiferromagnetic ordering of Co³⁺ ions exists, metamagnetic transitions were observed to occur in fields whose values were dependent on temperature. The strengths of competing interactions in the crystal are estimated.     

Magnetic properties of the compounds N(CH3)4MnIIMIII(CN)6 ·8 H2O(MIII = Mn,Fe)

In the isostructural cyanobridged chain compounds N(CH₃)₄Mn^IIM^III(CN)₆ · 8H₂O high spin Mn(II) ions couple antiferromagnetically to low spin Mn(III) of Fe(III) ions. The Mn^II–Mn^III compound orders ferrimagnetically below T_N = 28.5 ± 1 K. The tetragonal a and b axes are easy ones for the magnetic moments. In the Mn^II–Fe^III compound antiferromagnetic order occurs below T_N = 9.3 K, with spins aligned along the tetragonal c axis. The compound undergoes a meta-magnetic transition from the antiferromagnetic to a ferrimagnetic phase. This occurs at 2 K for a field H_crit ≈ 1.2 T. The temperature dependence of H_crit, which vanishes at T_N, is followed. The tricritical temperature T¹ is ～ 5 K.     

A Mössbauer study of antiferroelectric transition in Fe: NH4H2PO4

Mössbauer studies in ⁵⁷Fe³⁺: NH₄H₂PO₄ (ADP) were conducted over a range of temperatures covering the antiferroelectric transition at 148 K, using both powder and single crystal absorbers. No anomaly was observed in the temperature dependence of chemical isomershift and normalised resonance intensity at the transition point T_c. However, a sudden increase in the line width around T_c was observed, consistent with the expected increase in electric field gradient created by the onset of antiferroelectric ordering. These experiments clearly show that the impurity Fe atoms do not play any role in the soft mode responsible for the spontaneous polarization in ADP.     

Electron-phonon interaction and coulomb pseudopotential in AuGa alloys from resistivity and superconductivity measurements

Measurements of the electrical resistivity as a function of temperature from 250 to 270 K for Au and f.c.c. phase AuGa alloys are presented. The linear dependence of the resistivity with temperature and previous results of the superconducting transition temperature of the alloys are used to obtain a value for T_c of pure Au. From these results a new method is introduced to estimate the Coulomb pseudopotential μ¹ for gold.     

Phase mixing in the critical region of the ferroelectric phase transition in KD2PO4

A novel neutron high resolution double crystal arrangement has been used to measure changes in d-spacing, d-spread and mosaicity through the first order ferroelectric phase transition of DKDP. Coexistence of both para and ferroelectric phases was directly observed in the range T_c to T_c — 3 K. No hysteresis could be detected within the precision of the temperature measurement (10^-3K). The origin of the overshoot effect has been elucidated.     

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.     

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.     

57Fe Mössbauer study of Lu2Fe3Si5 iron silicide superconductor

With the advent of Fe–As based superconductivity it has become important to study how superconductivity manifests itself in details of ⁵⁷Fe Mössbauer spectroscopy of conventional, Fe-bearing superconductors. To this end, the iron-based superconductor Lu₂Fe₃Si₅ has been studied by ⁵⁷Fe Mössbauer spectroscopy over the temperature range from 4.4 K to room temperature with particular attention to the region close to the superconducting transition temperature (T_c=6.1 K). Consistent with the two crystallographic sites for Fe in this structure, the observed spectra appear to have a pattern consisting of two doublets over the whole temperature range. The value of Debye temperature was estimated from temperature dependence of the isomer shift and the total spectral area and compared with the specific heat capacity data. Neither abnormal behavior of the hyperfine parameters at or near T_c, nor phonon softening were observed.     

Low temperature elastic properties of a superconducting disordered metal

We have measured the elastic properties of a glass at audiofrequencies (0.2–1.5 kHz) in the temperature interval 0.01 to 10 K. In the superducting glassy metal Cu₆₀Zr₄₀ (T_c=0.31 K) both the sound velocity and attenuation are similar to that of an insulating glass below 0.05 K. Above that temperature (T_c>T>0.05 K) the relaxation process is mainly governed by electrons which are thermally excited above the BCS gap. Above the superconducting transition we observed new features in the sound velocity which were not observed in high frequency measurements.