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.     

Effects of composition on the upper critical field of V-Ga

Measurements of the temperature dependence of the upper critical field, H_c2(T), for a series of V_100?xGa_x materials are presented for 20.5 ≤ × ≤ 29.6. Fits of the data to conventional theory for a paramagnetically limited, dirty, type II superconductor show: 1) a maximum in T_c and H_c2(0) for x ? 25; 2) a constant ( for x ≤ 25; 3) a slowly increasing value of λ_so with increasing x up to x ～ 25; and 4) good agreement with stoichiometric ordered and thermally disordered V₃Ga. Above $\text{x ? 25}$ broader transitions are observed. For x = 25, T_c = 15.3 K, (, λ_so = 0.3 and H_c2(0) = 23.4 tesla. The effects of inclusion of strong-coupling in the theory are discussed briefly.     

Metal-nonmetal transition in solid argon-lanthanum mixtures

A continuous metal-nonmetal transition is observed in solid ArLa mixtures. The d.c. conductivity σ starts at a La atomic fraction c_La ? 0.15 with $\text{σ = 2 × 10}{}^{-2}\text{[Ω-cm]}{}^{-1}$ and increases exponentially with c_La until σ reaches the minimum metallic conductivity $\text{σ}{}_{\min }\text{= 300 ± 100[Ω-cm]}{}^{-1}$ at c_La ? 0.4. The temperature dependence of σ shows variable range hopping between localized states for c>_La ? 0.4 and metallic behaviour for $\text{c}{}_{\mathrm{<mtext>La</mtext>}}\text{? 0.4.}$     

The concentration dependence of the ferroelectric transition temperature of the solid solution of TSCC and TSCB

The concentration dependence of the ferroelectric transition temperature T_c of single crystals of the solid solution tris-sarcosine calcium chloride_1?x bromide_x is studied experimentally by measurement of the static dielectric constant ?_b and described by the empirical relation $\text{T}{}_{\mathrm{c}}\text{(X) = T}{}_{\mathrm{c}}\text{(0) [1?(X/X}{}_{\mathrm{c}}\text{)]}\text{1}\text{2}$. The Ising model with transverse field and the model of coupled anharmonic oscillators are used to explain the concentration dependence of the transition temperature T_c.     

A small parameter in the 1Nc expansion and narrowness of hadronic resonances

The dynamical basis for the validity of the $\text{1}\text{N}{}_{\mathrm{c}}$ expansion is investigated in the context of QCD in 1 + 1 dimensions. This is carried out by studying the first non-leading corrections in $\text{1}\text{N}{}_{\mathrm{c}}$ to the mass operator in the space of physical states. The correction to the real part of the mass operator has a direct implication for the convergence of the $\text{1}\text{N}{}_{\mathrm{c}}$ expansion, since a small effective parameter is identified, where its smallness depends on the dynamical circumstances in a known way. The generated imaginary part of the mass operator provides us with an insight concerning the question of the narrowness of hadronic resonances. In order to have a more realistic contact with our world, we include also effects due to the flavor symmetry group SU(N_f). This allows us to understand better the validity and usefulness of the notions of resonance dominance and (smooth) Regge behavior. We also discuss the expansion with $\text{N}{}_{\mathrm{f}}\text{N}{}_{\mathrm{c}}$ fixed and compare the results with those obtained from Dual Resonance Model. It is remarked that a nonuniformity exists between the limits N_c → ∞, N_f = fixed and N_c → ∞, $\text{N}{}_{\mathrm{f}}\text{N}{}_{\mathrm{c}}\text{=}\text{fixed}$, which may affect physical quantities.     

A Mössbauer study of amorphous Fe40Ni40B20

Amorphous Fe₄₀Ni₄₀B₂₀ (VITROVAC 0040) alloy has been investigated using ⁵⁷Fe Mössbauer Spectroscopy. The Curie temperature T_c is found to be well defined and is 695 ± 1 K. The quadrupole splitting just above T_c is 0.64 mm sec^?1. The crystallization temperature is 698 ± 2 K, close to but definitely above T_c. The average hyperfine field H_eff(T) of the glassy state shows a temperature dependence of $\text{H}{}_{\mathrm{<mtext>eff</mtext>}}\text{(0)[1 ? B}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{(T/T}{}_{\mathrm{c}}\text{)}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{? C}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{(T/T}{}_{\mathrm{c}}\text{)}{}^{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{? …]}$ indicative of the existence of spin wave excitations. The values of $\text{B}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{C}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ are found to be 0.40 and 0.06, respectively, for T/T_c ? 0.72. At temperatures close to T_c, H_eff(T) varies as (1 ? T/T_c)^β where β is one of the critical exponents and its value is found to be 0.29 ± 0.02.     

Functional derivative δTc/δα2 (Ω)F(Ω) for weak coupling superconductors

We present approximate analytic calculation of the functional derivative $\text{δT}{}_{\mathrm{c}}\text{δα}{}^2\text{(Ω)F(Ω)}$, where T_c is the superconducting critical temperature and $\text{α}{}^2\text{(Ω)F(Ω)}$ is the electron-phonon spectral function, within the “square-well model” for the phonon mediated electron-electron interaction and weak coupling limit $\text{ω}{}_{\mathrm{D}}\text{(2πT}{}_{\mathrm{c}}\text{)? 1 (ω}{}_{\mathrm{D}}$ is the Debye energy). It is found that $\text{δT}{}_{\mathrm{c}}\text{δα}{}^2\text{(Ω)F(Ω) = (1 + λ)}{}^{-1}\text{G(Ω)}$ where λ is the familiar electron-phonon coupling parameter and $\text{G(Ω)}$ is a universal function of the reduced frequency $\text{Ω}\text{=}\text{Ω}\text{T}{}_{\mathrm{c}}$. We compare this formula with accurate numerical results for several weak coupling superconductors. The overall agreement is good     

Infrared study of lattice and free carrier effects in p-type CuInTe2 single crystals

Infrared reflectivity spectra of p-type CuInTe₂ single crystals were measured at room temperature for the polarization direction E ⊥ c and E | c in the wavenumber range from 55 to 4000 cm^?1. Fits to a dielectric function including contributions from lattice oscillators and free carriers gave the first determination of the effective hole masses of $\text{m}{}_{\mathrm{p\perp }}\text{m}{}_0\text{= 0.85}$ and $\text{m}{}_{\mathrm{p|}}\text{m}{}_0\text{= 0.66}$. Two optical phonon modes were found for both polarization directions.     

The high-temperature behaviour of lattice QCD with fermions

By Monte Carlo simulation on the lattice, we calculate the high-temperature behaviour of the energy density ? in SU(2) and SU(3) QCD with Wilson fermions. From the leading term of the hopping parameter expansion, we find that ? converges very rapidly to the Stefan-Boltzmann limit of an asymptotically free quark-gluon gas. The behaviour in the non-asymptotic region indicates that chiral symmetry restoration occurs at a higher temperature (T_ch) than colour deconfinement (T_c), with $\text{T}{}_{\mathrm{<mtext>ch</mtext>}}\text{T}{}_{\mathrm{<mtext>c</mtext>}}\text{≈ 1.3}$.     

Critical field and specific heat of the anisotropic superconductor Nb3S4

We report on the first measurements of the critical field B_c2 and the specific heat of Nb₃S₄, which is a linear structured compound with a superconducting transition temperature of 3.65 K. The angular dependence of B_c2 is well described by the effective-mass model. The ratio of the critical fields parallel and perpendicular to the c-axis gives a value of 4.6. The small value of the specific heat jump at $\text{T}{}_{\mathrm{c}}\text{(}\text{Δc}\text{γT}{}_{\mathrm{c}}\text{= 0.95)}$ can be explained with an anisotropic gap function.     

Dielectric susceptibility and correlation length of one-dimensional CDW with impurties. II. weak disorder

Dependence of static dielectric susceptibility and correlation length of charge density waves (CDW) with weak defects on parameter of incommensurability with lattice is investigated. In almost commensurate phase (h?h_ch_c), $\text{χ ～ (h?h}{}_{\mathrm{c}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{In}{}^{\mathrm{<mtext>-4</mtext><mtext>3</mtext>}}\text{h}{}_{\mathrm{c}}\text{/h?h}{}_{\mathrm{c}}$ and R_c ～ (h ? h_c)^{$\text{2}\text{3}$}. In${}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{h}{}_{\mathrm{c}}\text{/h ? h}{}_{\mathrm{c}}$. Far from commensurability $\text{(h?h}{}_{\mathrm{c}}\text{) χ～ (a+h}{}^2{}_{\mathrm{c}}\text{/h}{}^2\text{)}{}^{\mathrm{<mtext>-2</mtext><mtext>3</mtext>}}$, $\text{R}{}_{\mathrm{c}}\text{～ (a + h}{}^2{}_{\mathrm{c}}\text{/h}{}^2\text{)}{}^{\mathrm{<mtext>-2</mtext><mtext>3</mtext>}}$, where a is the dimensionless ratio of random potential intensities, corresponding to backward and forward scattering impurities.     

Attenuation of longitudinal ultrasonic wave near the diffuse phase transition in CdF2

The attenuation of a longitudinal ultrasonic wave propagating in the [111] direction in CdF₂ is studied as function of temperature from 300K to 1030K. An ultrasonic attenuation peak has been observed for the first time near 983K. This peak is used to define the diffuse transition temperature (T_c = 983K) in CdF₂ which is well below its melting temperature of 1372K. The Arrhenius activation energy of anion motion above T_c was obtained from the temperature dependence of the attenuation and the theory of the dynamics of the coupled crystalline-cage-charged-liquid fluctuations. The elastic constant, $\text{C}{}_{11}\text{+2}\text{C}{}_{12}\text{+4}\text{C}{}_{44}\text{)}\text{3}$, measured simulataneously with the ultrasonic attenuation displays a large decrease near 983K in addition to the nearly linear decrease in the elastic constant with temperature.     

Gravitational collapse in quantum mechanics with relativistic kinetic energy

It is proved that the quantum mechanical Hamiltonian $\text{H = Σ}{}_{\mathrm{i=1}}{}^{\mathrm{N}}\text{(p}{}^2\text{+ m}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? κ Σ}{}_{\mathrm{i>j}}\text{|x}{}_{\mathrm{i}}\text{? x}{}_{\mathrm{j}}\text{|}{}^{\mathrm{?1}}$ for bosons (resp, fermions) is bounded from below if N ≤ c_bκ^?1 (resp. $\text{N ≤ c}{}_{\mathrm{f}}\text{κ}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$). H is unbounded from below if N ≥ c_b^lκ^?1 (resp. $\text{N ≥ c}{}_{\mathrm{f}}{}^{\mathrm{l}}\text{κ}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$). The constants c_b and c_b^l (resp. c_f and c_f^l) differ by about a factor 2 (resp. 4).     

Critical small-angle X-ray scattering of the liquid sodium-lithium system

Small-angle X-ray scattering of the liquid sodium-37 w/o lithium system in the range $\text{1.034 × 10}{}^{-4}\text{? (T?T}{}_{\mathrm{<mtext>c</mtext>}}\text{)/T}{}_{\mathrm{<mtext>c</mtext>}}\text{?}\text{1.503 × 10}{}^{-2}$ yields the critical exponents v = 0.655 ± 0.030 and λ = 1.296 ± 0.061 for the temperature dependence of the correlation length η and of the coefficient (?c/?μ)_{T, P} respectively.     

Structure magnetique en champ applique nul de l'acetate de manganese tetrahydrate Mn(CH3COO)2, 4H2O

The magnetic structure of manganous acetate Mn(CH₃COO)₂, 4H₂O has been solved by neutron diffraction. Manganous acetate crystallizes in the space group $\text{P2}{}_1\text{c}$ with Z = 6. Manganese atoms (in position 2a and 4e) are located in (100) planes. Below T_N = 3.18 K this compound is antiferromagnetic in a zero applied field with the k vector [$\text{1}\text{2}$ 00]. The plane (100) is ferromagnetic. The magnetic group is $\text{P}{}_{\mathrm{2a}}\text{2}{}_1\text{c}$.     

Charge transport in layer semiconductors

Electron and hole-drift velocity is measured in the layer semiconductors HgI₂, GaSe, PbI₂ and GaS, mainly on the direction parallel to the c-axis between 80 and 400 K. In the electric field range in question, electron and hole-drift velocity is proportional to the field except in the case of GaS, where a superohmic behaviour is observed. At 300 K the mobility parallel to the c-axis is $\text{μ}{}_{\mathrm{e}}\text{, = 100}\text{cm}{}^2\text{Vsec}$, $\text{μ}{}_{\mathrm{h}}\text{= 4}\text{cm}{}^2\text{Vsec}$ for HgI₂; $\text{μ}{}_{\mathrm{e}}\text{= 80}\text{cm}{}^2\text{Vsec}$, $\text{μ}{}_{\mathrm{h}}\text{= 210}\text{cm}{}^2\text{Vsec}$ for GaSe; $\text{μ}{}_{\mathrm{e}}\text{, = 8}\text{cm}{}^2\text{Vsec}$, $\text{μ}{}_{\mathrm{h}}\text{= 2}\text{cm}{}^2\text{Vsec}$ for PbI₂. The highest hole mobility observed in GaS is $\text{μ}{}_{\mathrm{h}}\text{= 80}\text{cm}{}^2\text{Vsec}$.Where it is possible to compare these data with mobility values perpendicular to the c-axis, the three-dimensional character of the energy bands near the fundamental gap is proved.For HgI₂ and GaSe we find no evidence for the large anisotropy of charge-carrier transport properties usually attributed to layered semiconductors. The mobility-temperature dependences found are interpreted on the basis of polar and non-polar optical phonon scattering mechanisms, except in the case of GaS, where a trapping model is used. The effective masses of electrons and holes are reported for PbI₂ and, for the first time, for HgI₂.     

Surface cyclotron resonance in Si under uniaxial stress

The cyclotron resonance of inversion-layer electrons on (100)p-type Si is found to depend sensitively on an externally applied compressive stress. At low temperatures (T ? 10 K) we observe a considerable increase of the cyclotron mass m¹_c with stress S along the [001] direction. The effect is most strongly observed at low electron densities n_s. For $\text{S～1.5 × 10}{}^9\text{dyne}\text{cm}{}^2$ and n_s～2 × 10¹¹cm^-2 we obtain $\text{m}{}^1{}_{\mathrm{c}}\text{～0.4 m}{}_0$ instead of the expected 0.2m₀. Along with this change of $\text{m}{}^1{}_{\mathrm{c}}$ a strong narrowing of the resonance is noted. Raising the temperature gives an additional n_s- dependent increase of $\text{m}{}^1{}_{\mathrm{c}}$.     

Magnetostrictive strain and thermal expansion in NdCo5 in the spin orientation temperature region

Magnetostrictive strain and thermal expansion measurements in an oriented assembly of NdCo₅ particles along the c-axis were carried out between 77 and 330 K. The measurement strains in the c-derection and transverse to the c-direction exhibited anomalous behavior at 285 and 245 K, which are assoiciated with the spin reorientation process in this temperature range. The estimated magnetostrictive energy $\text{1}\text{2}\text{E(λ}{}_{\mathrm{c}}\text{- λ⊥}{}_{\mathrm{c}}\text{)}{}^2$ is comparable to the anisotropy energy at 285 K. There is also a pronouced anomaly in ther thermal expansion in the spin reorientation temperature region.