NMR investigations of dynamical processes in orientationally ordered and disordered NaCN,RbCN and CsCN

The nuclear spin lattice relaxation timeT ₁ of the²³Na,⁸⁵Rb,⁸⁷Rb,¹³³Cs,¹⁴N nuclei is measured in NaCN, RbCN and CsCN as a function of temperature below and above the ferroelastic phase transition temperatureT _c. BelowT _c the behaviour ofT ₁ of the alkali nuclei renders possible to determine the flip frequency of the CN molecules and its temperature dependence. AboveT _c from the¹⁴NT ₁ the correlation time τ_c of the rotational motions of the CN molecules and its temperature dependence is determined. An empirical rule is verified demonstrating that atT _c the correlation times take nearly the same values for all cyanides. For the high and low temperature phases one obtains atT _c about τ_c=5·10^?13s and τ_c=5·10^?11s, respectively. The results are discussed with respect to the mechanism of the phase transition.     

Perturbation calculations for the eight-vertex model around the Ising point for T → Tc+. II

The dispersion expansion for the two-point electric correlation function in the eight-vertex model is calculated to first order in the four-spin coupling in the scaling limit in the high-temperature regime. It is found that there is a close relation between the T_c⁻ and T_c⁺ spin-spin-energy density correlation function in the Ising model. This is used to simplify tremendously the calculation in the T_c⁺ case; the result can be obtained from the T_c⁻ result by deleting two variables in the expansion.     

Unusual behavior of nuclear relaxation in CeCu2Si2 “possible evidence for triplet superconductivity”

Nuclear relaxation of ⁶³Cu in the superconducting state of the Kondo-lattice system CeCu₂Si₂ has been studied with the use of the ⁶³Cu nuclear quadrupole resonance technique under zero field and down to 65mK. The nuclear spin-lattice relaxation rate (1/T₁) decreases drastically just below T_c=0.67 K down to 0.5T_c without the apparent enchanced behavior and then is found to be almost temperature independent below 0.3T_c. These results suggest that the superconductivity in CeCu₂Si₂ is not in the usual BCS regime. The analysis based upon the existing triplet pairing model with an anisotropic energy gap describes well the behavior from T_c down to 0.5T_c, while the temperature independence below 0.3T_c remains unexplained.     

Temperature dependence of the diffuse streak system in K2SnCl6

We have investigated the system of diffuse streaks in K₂SnCl₆ single crystals in the temperature interval from 4 to 640 K by the photographic registration of the scattered neutron intensity. The streak intensity decreases from T_{c¹ = 261 K} with increasing as well as with In the region of the phase transition at T_{c¹ = 261 K} and T_{c² = 255 K} the intensity decreases with falling temperature to one third. The high temperature behaviour is explained by a soft A_2g librational mode. Below T_{c² = 255 K} the intensity is proportional to ΔT. The streaks are detectable down to 230 K.     

The zero-distribution of the great partition function in the complex fugacity plane of gases of point particles with logarithmic interaction

The equation of state for gases of point particles with logarithmic interaction is derived. The system exhibits a phase transition at a critical temperature T_c. The critical temperature is a function of the dimension of the system. A hard core must be added below T_c to prevent the system from collapsing. The specific heat diverges on both sides as |T_c - T|^-2 in any dimension.For T≤T_c there are no zeros of the grand partition function in the complex fugacity plane, for T >T_c all zeros occupy the whole negative real axis. The density of zeros will be calculated.     

Electrical resistivity of iron-vanadium alloys between 78 and 1200 K

Electrical resistivity (?) of FeV alloys containing 0.5, 0.9, 2.7, and 6.1 at% V has been measured as a function of temperature (T) between 78 and 1200 K. The ? vs. T curves exhibit a change in the slope at the ferromagnetic Currie temperature (T_c). The d?/dT vs. T curves in the neibhorhood of T_c are similar to the corresponding plot for pure Fe. Our studies confirm the previously observed anomalous effect of V on T_c of Fe, i.e., that T_c increase with small additions of V to Fe. The critical index λ⁺ associated with the power law of d?/dT just above T_c has been determined as a function of V concentration.     

Das kritische magnetische Verhalten von EuO

Mössbauer effect and magnetization measurements were employed in order to study the static and especially the dynamic magnetic properties of the nearly Heisenberg ferromagnet EuO near its Curie temperature,T _c=69.2 °K. The critical exponent β of the spontaneous magnetization was determined to be β=0.34±0.02. It was shown that critical slowing down of spin fluctuations takes place nearT _c with spin relaxation times between 7×10^?11 sec (T=1.01T _c) and 1.5×10^?1 sec (T=1.03T _c). The experimental values of the relaxation time were found to be in satisfactory agreement with theoretically computed ones. Just belowT _c the Mössbauer spectra exhibit relaxation effects, which are characteristic for the occurence of critical super-paramagnetism. Investigations of several samples indicated quantitatively, that critical superparamagnetism has its origin in the non ideal composition of the real crystal.     

Coherent state and superconductivity in the free carrier-bipolaron interacting system

In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T¹ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T_c ∼ ω_c, where ω_c is the Debye frequency related with local electron-phonon coupling. In general we have T¹ > T_c ⪢ T_c0 (T_c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high-T_c superconductor.     

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.     

Ising ferromagnet with biquadratic exchange

A spin one Ising system with biquadratic exchange, is investigated, using Green's function technique in random phase approximation (RPA). Transition temperature T_c and <(S^z)²> at T_c, are found to increase with biquadratic exchange parameter α for sc, bcc and fcc lattices. The variation of <(S^z)²> at T_c with α is found to be the same for the above lattices.     

Charge symmetrical pairing correlations for Even-Even nuclei withN=Z

The temperature dependence of the critical heat currentQ _c in He II has been measured in the temperature region 3 · 10^?5 (?K)<T _λ?T<1.2 · 10^?2 (?K). The result Q_c∫ (T _λ?T)^1.07±0.01is consistent with a divergent mutual friction nearT _λ proposed recently byAhlers.     

Precursor diamagnetism in superconducting Al films

The precursor diamagnetism is studied in a stack of Al films whose thickness is much smaller than the coherence length. The magnetization shows the (T ? T_c)^?1 behavior in a finite range of temperature above T_c. At T_c, in low magnetic fields constant magnetization independent of the field strength is observed. The results are in agreement with the theories of the two-dimensional superconductor.     

Doping dependence of the superconducting gap by Andreev reflection in Au/La2−xSrxCuO4 point-contact junctions

We studied the doping dependence of the superconducting gap in La_2−xSr_xCuO₄ (LSCO) by means of Andreev reflection measurements in Au/LSCO point-contact junctions. The Andreev reflection features were found to disappear at T_c^A close to the bulk T_c. The fit of the conductance curves with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The low-temperature dominant isotropic gap component Δ_s follows very well the T_c vs. x curve, while the gap-like features observed by angle-resolved photoemission spectroscopy and tunneling scale with T^∗. This confirms the different origin of these two energy scales at T<T_c.     

Anisotropy of microwave conductivity in the superconducting and normal states of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>−<Emphasis Type="Italic">x</Emphasis>: 3D-2D crossover

The imaginary parts of microwave conductivity σ″(T<T_c) and resistivity ρ (_T)=1/σ(T>T_c) along (σ _ab ^″ and ρ_ab) and across and (σ _c ^″ and ρ_c the cuprate ab planes of a YBa₂Cu₃O_7?x crystal with the oxygen doping level x varying from 0.07 to 0.47 were measured in the temperature range 5≤ T≤200 K. In the superconducting state, the σ _ab ^″ (T)/σ _ab ^″ (0) and σ _c ^″ (T/σ _c ^″ (0) curves coincide for an optimally doped (x=0.07) crystal, but, with an increase in x, the slopes of the σ _c ^″ (T)/σ _c ^″ (0) curves decrease noticeably at T<T_c/3, on the background of small changes happening to the σ _ab ^″ (T/σ _ab ^″ (0) curves. The two-dimensional (2D) transport along the ab planes in the normal state of YBa₂Cu₃O_7?x is always metallic, but there is a crossover (at x=0.07) from the Drude to hopping (at x>0.07) conductivity along the c axis. This is confirmed both by the estimates of the lowest metallic and the highest tunneling conductivities along the c axis and by quantitative comparison of the measured ρ_c(T) curves with the curves calculated in the polaron model of quasiparticle transport along the c axis.     

Li- and V-NMR study of LiVS2

⁷Li- and ⁵¹V-NMR have been measured to make clear the electronic state in a two-dimensional triangular lattice LiVS₂. Knight shift of both ⁷Li- and ⁵¹V-NMR is almost independent of temperature below the phase transition temperature T_c of about 310 K from the paramagnetic state to non-magnetic state. The ⁵¹V- spin-lattice relaxation rate 1/T₁ reveals an exponential temperature dependence below T_c, indicating a gap structure of electronic state. These results are consistent with a non-magnetic state with a trimer singlet of V³⁺ spins below T_c.     

The speed of sound in hadronic matter

We calculate the speed of sound c _s in an ideal gas of resonances, whose mass spectrum is assumed to have the Hagedorn form ρ(m)～m ^?a exp?{bm}, which leads to singular behavior at the critical temperature T _c =1/b. With a=4 the pressure and the energy density remain finite at T _c , while the specific heat diverges there. As a function of the temperature, the corresponding speed of sound initially increases similarly to that of an ideal pion gas, until near T _c resonance effects dominate, which causes c _s to vanish as (T _c ?T)^1/4. In order to compare this result to the physical resonance gas models, we introduce an upper cut-off M in the resonance mass integration. Although the truncated form still decreases somewhat in the region around T _c , the actual critical behavior in these models is no longer present.     

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB₂) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB₂ with T _c =5.5 K, while NbB₂ and TaB₂ have been observed to be nonsuperconducting up to 0.37K. H _c2(T) is linear in temperature below T _c , leading to a rather low H _c2(0)=0.1 T. At T close to T _c , H _c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB₂ samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB₂, NbB₂, and MgB₂, respectively, rather than T ² and T ³, as previously reported for MgB₂.     

Br NQR relaxation of the mixed-valence compound (NH4)4SbIIISbVBr12

⁸¹Br NQR measurements of the mixed-valence compound (NH₄)₄Sb^III Sb^VBr₁₂, which has Sb^VBr₆ ^? and Sb^IIIBr₆ ^3? octahedra in different oxidation states in the tetragonal pseudo-K₂PtCl₆ structure, were carried out by pulse method in the temperature range between 80 and 300 K. The phase transition temperature of T _c = 212 K was redetermined. The T ₁ values are quite different between Sb^VBr₆ ^? and Sb^IIIBr₆ ^3? octahedra. For all resonance lines T ₁ minima were observed at T _c. The T ₁ behavior at T _c was explained by a softening of the rotary lattice mode around a principal axis of each octahedron along the c axis of the crystal. The distinctive feature in the temperature dependence of both ⁸¹Br NQR frequencies and T ₁ values for each anion indicates that the static rotation may occur for Sb^VBr₆ ^? but not for Sb^IIIBr₆ ^3? at T _c in the low temperature phase.     

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.     

High-resolution heat capacity of SnCl2·2H2O single crystal

The heat capacity of SnCl₂·2H₂O single crystal was measured in the close vicinity of the phase transition temperature, T_c = 217.994 ± 0.01 K. Its anomalous part ΔC could be expressed as ΔC = A_± ⊥ (T ? T_c)/T_c⊥^-α_±, where α₊ = 0.492 ± 0.02, α_- = 0.492 ± 0.02, A₊ = 1.148 JK^?mol^?, and A_- = 1.155 JK^?mol^?. A quasi-isothermal absorption of the enthalpy amounting to 34 J mol^? was observed at T_c.