Thermal changes in the optical properties of SnSxSe2−x crystals

Optical measurements on crystals in the series SnS_xSe_2?x for 0 ? x ? 2, have yielded information on the changes in the ordinary refractive index $\text{Δn}\text{ΔT}$ and the energy gap $\text{ΔEg}\text{ΔT}$ in the temperature range 125–425 K. The coefficient $\text{Δn}\text{ΔT}$ has values +40 to +160 × 10^?6K^?1 and this confirms that covalent bonding predominantly exists in these materials. The coefficient $\text{ΔEg}\text{ΔT}$ remains fairly consistent for all values of x with an average value of -8.0×10^-4eV K^-1.     

The cooperative Jahn-Teller effect in uranium (4+) hydroxy-sulphate

The magnetic susceptibility measurements of orthorhombic U(OH)₂SO₄ within the temperature range 4.2–300 K have revealed a magnetic anomaly at T_D = 21 K associated with crystallographic transition induced by the cooperative Jahn-Teller effect. Above 21 K the magnetic susceptibility of the uranium (4+) ion corresponds to the electronic ground doublet ${}_{\mathrm{¡}}\text{M}{}_{\mathrm{J}}\text{= ± 2〉}$ confirming thus the antiprismatic symmetry (D_4d) of the crystal field at the uranium site. Below T_D the system of two singlets ($\text{1}\text{√2}$)|2〉 ± ($\text{1}\text{√2}$)|$\text{2}$〉 separated by δ(T) is the ground state of the uranium ion.     

Electrical resistance and magnetoresistance of amorphous alloys Gd67Co33 and Pd80Si20

In search for structural contributions to the low temperature anomaly we report high resolution resistance and magnetoresistance measurements ($\text{0.02}\text{K}\text{? T ? 20}\text{K}$) of amorphous splats of Gd₆₇Co₃₃ and Pd₈₀Si₂₀. For both alloys, the resistivity ?(H = 0, T) has a minimum at T ～ 10 K and increases with decreasing T. The ferromagnetic Gd₆₇Co₃₃ shows a strong negative field dependence of $\text{Δ?}\text{?(0)}$, saturating at H ～ 2T for T = 4.2 K but no measurable change in $\text{??}\text{?}\text{T}$ below 10 K is observed.The diamagnetic Pd₈₀Si₂₀ exhibits a positive field dependent magnetoresistance $\text{[Δ?}\text{?(0)]}\text{(}\text{H}\text{)}$ at low temperatures. Additionally, a field dependent part in $\text{??}\text{?}\text{T}$ is found which is probably due to paramagnetic impurities (～ 1 ppm Fe). However, there is also a field independent contribution in the amorphous state of Pd₈₀Si₂₀, which vanishes after crystallization. We attribute this to non-magnetic scattering induced by the disordered structure.     

The superconducting transition temperature of bulk samples of the niobium-germanium A15-phase after implantation of Ge,Si, O and Ar ions

Bulk material of Nb₃ (Ge_0.8Nb_0.2) with A15 structure and a superconducting transition temperature T_c of 6.5 K has been implanted with Ge, Si, Ar and O ions and subsequently annealed at high temperatures. After annealing between 700 and 750°C the Ge implanted samples showed a strong increase in T_c up to 16.2 K. With Si ions only a T_c of 13 K was obtained, with Ar and O ions T_c remained below 9 K. From X-ray measurements carried out on high T_c Ge implanted samples it could be concluded that the implanted surface layer grows up to a high degree epitaxially on the single crystallites of the bulk material. The lattice constant $\text{a}$₀ of the implanted film was reduced by 0.02 Å with respect to the bulk material. This reduction in $\text{a}$₀ is stronger than expected from the transition temperature of the implanted surface layer.     

Analysis of the high-temperature spin-glass susceptibility: Determination of the local magnetic exchange

New measurements are reported of the magnetic susceptibility above the freezing temperature T_f for noble-metal spin-glasses with $\text{1}\text{2}$ to 6 at.% Fe or Mn. The susceptibility for T_f<T?5 T_f is not Curie-Weiss, but local magnetic correlations manifest themselves and provide a key for the determination of the exchange interactions in spin-glasses. The exchange parameters J_n are resolved up to 5 neighbors for $\text{Au}$Fe, $\text{Cu}$Mn, $\text{Au}$Mn and $\text{Pt}$Mn through a configuration ensemble calculation which includes atomic short range order.     

Ferroelectric lithium tantalate—III. Temperature dependence of the structure in the ferroelectric phase and the para-electric structure at 940°K

The neutron scattering from a single crystal of LiTaO₃ (Curie temperature 907°K) has been measured at 760, 820, 885 and 940°K, and has been remeasured at 298°K. Least squares structural refinement of the five data sets shows that as the temperature approaches T_c the oxygen atom approaches the position $\text{x,}\text{1}\text{3}\text{,}\text{1}\text{12}$, with respect to Ta at the origin, in space group R3c. The temperature variation of the oxygen y- and z-coordinates is very similar to that of the spontaneous polarization. The lithium atom position below T_c remains essentially invariant as a function of temperature. At T_c, the oxygen atom occupies the position $\text{x,}\text{1}\text{3}\text{,}\text{1}\text{12}$, the lithium atom becomes disordered and distributed over the positions $\text{00z}\text{and}\text{0, 0,}\text{1}\text{2}\text{-z}$, and the tan alum atom becomes located at an inversion center, in space group $\text{R}\text{3}\text{c}$. The lithium atom sites above T_c lie 0.374 Å on either side of the oxygen atom plane at $\text{z =}\text{1}\text{4}$.     

Study by brillouin scattering of the C33 constant of thiourea submitted to an electric field

The C₃₃ constant is discontinuous for the lock-in transition at T₁ = 169 K. The variation ΔT₁ of the temperature of transition is a linear function of the applied electric field E and we find $\text{d}\text{T}{}_1\text{d}\text{E}\text{=0.82 deg.cm kV}{}^{\mathrm{?1}}$.Above a mean field E = 10 kV cm^?1 the transition observed for a first heating spreads on several degrees because damages appear in the incommensurate phase and the electric field becomes inhomogeneous.The results obtained at low fields are in very good accordance with the value of $\text{d}\text{T}{}_1\text{d}\text{E}$ calculated from the Clapeyron formula.Taking into account of the incommensurability of the phase above T₁, it can be shown that

$\text{dT}{}_1\text{dE}\text{=}\text{C}\text{2πP}{}_{\mathrm{o}}\text{3}\text{3}\text{?}\text{2}$

The knowledge of the spontaneous polarisation P₀ gives for the Curie constant C = 2.1 × 10³ K in qualitative agreement with the value deduced from measurements of the dielectric constant in the ferroelectric direction.     

Thermal and magnetic studies of the nearly one-dimensional antiferromagnetic system CsNiBr3

Magnetic susceptibility, specific heat and ¹³³Cs magnetic resonance measurements in a single crystal of CsNiBr₃ are reported. The data reveal two magnetic transitions separating the paramagnetic phase from the antiferromagnetic ground state. At the higher transition temperature T_N2 = (14.25 ± 0.05)K a net magnetic moment is observed only along the hexagonal c-axis, while only below the lower transition temperature T_N1 = (11.75 ± 0.05)K a perpendicular component of the magnetic moment appears also. Above T_N2 CsNiBr₃ can be described as a one-dimensional antiferromagnet with intrachain exchange interaction $\text{J}\text{k}{}_{\mathrm{B}}\text{= ?(17.0 ± 0.2)}\text{K}$ and single-ion anisotropy constant $\text{D}\text{k}{}_{\mathrm{B}}\text{? ?1.5}\text{K}$. Below T_N1, the data are consistent with the non-colinear triangular structure of the Ni²⁺ moments proposed previously for the isomorphic crystal CsNiCl₃. A reduced value of the zero-temperature susceptibility over the classical value is found and atrributed to the zero point deviations.     

Magnetic phase transitions in the 1D ising antiferromagnet CsCoCl3

We report on Raman scattering from the magnons in CsCoCl₃ with particular emphasis on two magnetic phase transitions occurring at T_N = 20.82 K and at 8 K. The T → 0 magnon spectrum is fitted by a $\text{S =}\text{1}\text{2}$ anisotropic Heisenberg model. With a simple cluster model we can explain the temperature behaviour of the magnon frequencies and intensities. The physics of the 8 K phase transition is also discussed.     

Study of the local susceptibility in the Fe: Au system

Mössbauer effect measurements of the Fe: $\text{Au}$ system are reported which yield the local susceptibility of the Fe ions. At high temperatures (T ? 10°K) a Curie Weiss law with θ₁ = 9.8°K is observed, whereas at low temperatures the Curie Weiss temperature is θ₂ = 0.45°K. In both temperature regimes the local susceptibility is a linear function of the total susceptibility. In addition below 4°K the local magnetization is a linear function of the total magnetization up to external fields of 60 kG.     

NMR spin lattice relaxation in dilute CuFe alloys with impurity interactions

Previous measurements by Wilkening and Hesse have shown, that the excess relaxation rate ΔT^-1₁ of the matrix nuclei in $\text{CuF}$e dilute alloys can be explained in terms of the LD-model with rapid spin diffusion. Measurements reported in this paper confirm the existence of an electric quadrupole diffusion barrier. It could be shown that the influence of the quadrupole barrier is coupled to large clusters within the alloy. The electron spin lattice relaxation time τ₁ behaves temperature independent in the range 30 K ? T ? 300 K. This can be understood if an effective correlation time τ is introduced, which results from a distrubution of temperature dependent times τ^cl(T) belonging to clusters of different size.     

N2 Monolayers on graphite: Specific heat and vapor pressure measurements — thermodynamics of size effects and steric factors

Calorimetric and vapor pressure measurements of N₂ monolayers on Grafoil have been made in the regions recently explored by neutron scattering. The phase transition at T = 48 K involves approximately symmetric specific heat anomalies of 2&#x0303; K FWHM, and the peak position shifts about 1 K from $\text{1}\text{3}$ to $\text{2}\text{3}$ of one monolayer. In spite of the peak width and temperature shift the transition is intrinsically first order, due to melting of the √3 registered structure into a disordered phase in the presence of 2D vapor. The shift and broadening are shown to be a consequence of size effects primarily due to large edge-to-area ratios of the ordered phase domains. A formula relating the domain size to the shift in transition temperature gives r = 50' A for the mean radius of the domains. At film densities above $\text{2}\text{3}$ monolayer the peak broadens and shifts to a higher temperature until a critical region is reached near T= 85 K. The combined heat capacity and vapor pressure data indicate that there is no two phase region at coverages near monolayer completion and that the transition near T = 85 K is higher than first order. Effects of molecular orientation and surface-normal vibration appear in certain regions of temperature and density. A phase diagram is constructed which is consistent with both calorimetric and neutron scattering results.     

Nonexponential proton relaxation for tunneling of NH+4 in the rotating frame

Proton spin-lattice relaxation times T₁ and T_1θ have been measured in NH₄IO₄ between 150 K and 50 K. The relaxation in the rotating frame was strongly nonexponential between 70 K and 53 K, which supports the tunneling assisted relaxation model of NH⁺₄ in the rotating frame. The tunneling frequency was determined to be $\text{Λ}{}_0\text{2Π}\text{=1.5 MHz}$.     

Critical slowing down of fluctuations in squaric acid (H2C4O4) at the phase transition observed by 13C spin-lattice relaxation

Spin lattice relaxation T₁ of naturally abundant ¹³C nuclei in squaric acid was measured close to the antiferroelectric-paraelectric phase transition temperature T_c = 373 K. A rapid increase in $\text{1}\text{T}{}_1$ is observed close to T_c coming from above, which follows the power law $\text{1}\text{T}{}_1\text{～ ε}{}^{\mathrm{?1.4}}$ where $\text{ε =}\text{(T ? T}{}_{\mathrm{c}}\text{)}\text{T}{}_{\mathrm{c}}$. This behaviour is explained on the basis of the two-dimensional character of the fluctuations.     

The impurity resistivity and resistivity minima in AlCr and AlMn alloys

The impurity resistivity of $\text{Al}$Cr between 1.5 and 50°K was determined with a characteristic temperature for the T² variation θ₁=960±40°K. The behaviour of the resistivity minimum both in $\text{Al}$Cr and $\text{Al}$Mn alloys with impurity concentration provides evidence that a T³ phonon resistivity is found also in aluminium with anomalous impurity resistivity.     

NMR study of one-dimensional ionic conductor with hollandite-type structure. II. Frequency dependence of spin-lattice relaxation time of 27Al

Frequency dependence of spin-lattice relaxation time T₁ of ²⁷Al in one-dimensional K⁺ ion conductor, K-Al-priderite, was measured at 45 K in the frequency range from 10.1 MHz to 55 MHz. It is found that T₁ is proportional to ω^1.49±0.05 and agrres well with the $\text{ω}{}^{\mathrm{<mtext>3</mtext><mtext>4</mtext>}}$ dependence derived by the continuum diffusion model. The intrinsic activation energy is determined to be 0.058 eV by doubling the slope E_NMR=0.029 eV of the d(ln T₁)/dT curve in the low temperature region. The frequency dependence of T₁ in the high temperature region measured in the frequency range from 11.5 MHz to 20.8 MHz shows a tendency that the frequency dependence becomes smaller than the $\text{ω}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence as temperature is raised above 450 K.     

Specific heats and phase transition in TMMC

The specific heats of the nearly one-dimensional antiferromagnet TMMC showed a sharp discontinuity in the temperature range ?(≡∥1?T/T_N∥) ≈ 2 × 10^-3 at T_N =0.835 ± 0.010 K. The remaining magnetic entropy below T_N is about 1% of its total for S = $\text{5}\text{2}$.     

The low temperature electrical properties of 1T-TaS2

Measurements of the electrical resistivity of 1T-TaS₂ to 0.03 K show that the increase in resistivity below ～ 50 K is extrinsic.Below 2 K the resistivity is described by ? = ?₀ exp $\text{T}{}_0\text{/T)}\text{1}\text{3}$. Because of this fractional power law behavior, we conclude that the increase is due to Anderson localization by random impurity and/or defect potentials. Other difficulties in understanding the properties of 1T-TaS₂ are also pointed out.     

One-dimensional magnetism in N2H6FeF5,

A new Fe(III) fluoride N₂H₆FeF₅ has been investigated by magnetic susceptibility measurements and Mössbauer resonance. It has a one-dimensional magnetic behaviour. The intrachain exchange integral ($\text{J}\text{k}\text{= -10.2}\text{K}$) has been determined by fitting the χ^-1 = f(T) curve and the ratio between the inter- and intrachain exchange integrals evaluated with Oguchi's formula. Below T_N = 9 K, N₂H₆FeF₅ shows a long range three-dimensional anti-ferromagnetic ordering.