The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

Collisional broadening of the J = 1 ← 0 transition of HC15N by nitrogen,oxygen, helium,and air

Nitrogen-broadened linewidths have been measured for the J = 1 ← 0 rotational transition in the fundamental vibrational state of the hydrogen cyanide isotopic species ¹H¹²C¹⁵N, located at 88 055.01 MHz. The data have been recorded for four temperature levels in the range 234–295 K. The collisional half-widths derived from these measurements allowed us to determine, to our knowledge for the first time, the temperature dependence of the foreign gas broadening parameter. Linewidth measurements at room temperature with oxygen, air, and helium as broadening gases are also reported.     

The density of states of b.c.c. Nb3Au: A comparison with A-15 Nb3Au

When Nb₃Au is converted from the A-15 structure to the b.c.c. allotrope, a sharp decrease in the superconducting transition temperature, T_c, occurs (11.1 to 1.5 K). To study possible causes for this sharp decrease, specific heat measurements have been made on the b.c.c. form of Nb₃Au yielding values of γ=19.2 mJ/mole-K², β=0.274 mJ/mole-K⁴ and θ_D=305 K. These parameters compared with published values for the A-15 Nb₃Au indicate that a sharp drop in the density of states at the Fermi level, N(0), occurs when the A-15 structure is converted to the b.c.c. form.     

Multiple photon excitation of the v2 + v6 combination band of SF6 in a bulk and in a pulsed free jet

Multiple photon excitation of the v₂ + v₆ combination band of SF₆ in a bulk at T ≈ 295 K and cooled in a pulsed free jet up to T_V ≈ 160 K and T_R ≈ 40 K by a pulsed TEA CO₂ laser has been investigated. Obtained results are compared with the data on the v₃ vibration excitation. At exciting energy fluences ø = 0.1?2.5 J cm^-2 the levels in the region of the discrete vibrational states (v=3?5) are found mainly to be excited. Multiphoton absorption spectra at room temperature have a sharp resonant structure. The fraction of interacting molecules is considerably (3–7 times) less compared than that for the case of v₃ vibration excitation. Multiphoton absorption of the v₂ + v₆ and v₃ vibrations of SF₆ is shown to be proportional to the dipole moments of the corresponding transitions.     

Raman-gain,linewidth, and effectiveg-value with spin-flip-Raman scattering in inSb

In this paper we report on measurements of spin-flip-Raman gain inn-InSb as a function of the magnetic field. The measurements were carried out at temperatures of 1.8 K and 4.2 K and at a carrier concentration of 1.35×10¹⁵ cm^?3. The Raman cross sections obtained from these results, e.g. 1.25×10^?20 cm²/sr at a magnetic field of 10 kG and a pump frequency of 1884.35 cm^?1, agree very well with those theoretically predicted by Wherrett and Wolland. Furthermore, these measurements yield line shapes and linewidths of the spontaneous scattering (100–1500 MHz) and allow the determination of the effectiveg-value with an accuracy known from ESR-investigations. These results are discussed in terms of already published theoretical investigations.     

Electron scattering in CdxHg1−xTe

Experimental studies of the electron mobility in Cdinx,Hgin1?xTe/0x/0.33, 10¹⁵ cm^?3n10¹⁸ cm^?3, 4.2 K ?T ?300 K/and of the thermoelectric power of intrinsic HgTe from 300 K down to 5 K are reported. These results are interpreted in terms of calculations based on the variational solution of the Boltzmann equation. Analysis shows that in pure samples at low temperatures, the electron mobility is limited by ionized donors, heavy holes, and, in some cases, unresolved defect scattering. In the doped samples with x0̆.1, disorder scattering also becomes significant. Polar-optical phonon scattering is dominant at high temperatures. The sharp decrease of mobility in the region 20–40 K, which occurs for pure samples with x0̌.14, is explained by interband optical phonon scattering. The thermoelectric power of intrinsic HgTe is strongly affected by phonon-drag of holes at low temperatures and by electron-electron scattering at high temperatures.     

Hall effect and magnetoresistance of (SN)x films

Hall effect and electrical conductivity have been investigated between 77 K and 300 K and the magnetoresistance at 4.2 K for a number of (SN)_x films deposited at substrate temperatures between — 10 and 50°C. The small magnitude of the Hall mobility (? 1 cm² Vsec^?1 at 300 K) and its activated temperature dependence are interpreted in terms of a heterogenous model for (SN)_x films with thin depletion layers separating highly conductive islands. The hole concentration in these islands (p ≈ 10²¹ cm^?3, the microscopic mobility (μ ≈ 500 cm² Vsec^?1 at 4.2 K) and the temperatures dependence of μ are found to be close to values for (SN)_x crystals.     

Magnon Raman scattering in the quasi 1-D antiferromagnet CsCoBr3

Raman scattering from magnons has been observed in the three magnetic phases of CsCoBr₃. In the 1-D Ising phase T > 28K a broad band at 96 cm^?1 is observed. This band grows in intensity but shows little renormalisation (100.5 cm^?1 at 14K) in the partially disordered antiferromagnetic phase 14K < T < 28K. For T < 14K additional structure at 111.5, 123.5, 133, and 141 cm^?1 is attributed to magnon-magnon combination bands. Two extra magnon branches are expected for this ferrimagnetic phase. One of these has an energy of ≈ 11 cm^?1.     

Anomalies in the temperature dependence of the critical field in antiferromagnetic single crystal dysprosium

An intensive study of the temperature variation of the critical field, H_c, in antiferromagnetic dysprosium, reveals small, abrupt changes in the value of H_c at particular temperatures between the Néel temperature, T_N, and Curie temperature T_c. These measurements made on two single crystals of different quality, show that anomalies occur at ≈99, 114, 127, 131, 138, 146 and 153 K, and the jump in the critical field is ≈ 10^-2 T on each occasion. The anomalies are attributed to a distortion of the helical antiferromagnetic structure whenever the periodicity of the helix is commensurate with that of the hexagonal crystal lattice. Accompanying changes in the helical turn angle, Δω, are estimated and found to increase as the temperature approaches T_c.     

Magnetic-field and temperature effects on the optical properties of dicyano-2,2′-bipyridyl-platinum(II) single crystals

Studies of the polarized emission of [Pt(CN)₂(bipy)] single crystals as function of temperature (1.9 K ⩽ T ⩽ 295 K) and homogeneous magnetic fields (0 ⩽ H ⩽ 6 T), and the temperature dependence of the polarized absorption spectrum are reported. Raising the temperature from 1.9 to 7 K or increasing the magnetic field from 0 to 1 T results in a blue shift of ≈175 cm^-1 in the E ⊥ a polarized emission (E: electric field vector, a: crystallographic a axis). Between 1.9 and 295 K at H = 0) and between 0 and 6 T (at T = 1.9 K), the emission lifetime decreases by factors of ≈10³ and ≈10², respectively. The results are explained within the C'_2v symmetry of the single complex assuming a coupling between neighboring central ions.     

Spatially modulated magnetic structure of AgFeO2: Mössbauer study on 57Fe nuclei

The results of the Mössbauer study of ferrite AgFeO₂ manifesting multiferroic properties (at T ≤ T _N2) have been presented. The hyperfine interaction parameters of ⁵⁷Fe nuclei have been analyzed in a wide temperature range including the points of two magnetic phase transitions (T _N2 ≈ 7–9 K and T _N1 ≈ 15–16 K). It has been shown that the Mössbauer spectra of the ⁵⁷Fe nuclei are sensitive to the variations of the character of the magnetic ordering of Fe³⁺ ions in the studied ferrite. The results of the model identification of a series of spectra (4.7 K ≤ T ≤ T _N2) under the assumption of the cycloid magnetic structure of ferrite AgFeO₂ have been presented. The analysis of the results has been performed in comparison with the literature data for other oxide multiferroics.     

Optical absorption and track studies of gamma-irradiated ZnP glass

The optical absorption spectra of a specially prepared zinc phosphate glass (ZnP) were studied as a function of gamma dose. The optical absorption spectra in the wavelength region of 200–800 nm were recorded for pristine ZnP glass sample and a number of samples exposed to different γ-doses in the range 10⁰–10⁶ Gy. The optical energy gap (E_g) was determined using Tauc's plot. It was found to be independent of γ-irradiation up to the dose of 10³ Gy followed by a slow decrease of E_g up to 10⁵ Gy. However, a sharp decrease in optical energy band gap was observed at 10⁶ Gy. The bulk-etch rates for different γ-doses were also determined and found to be constant up to the dose of 10⁵ Gy. On the other hand, there is a 12% increase in the bulk-etch rate for the dose of 10⁶ Gy.     

Spectroscopic evidence for superfluidity in liquid He droplets

A beam of liquid He _N droplets (N≈5000) is prepared by a cryogenic free jet expansion, doped with single molecules and studied via laser spectroscopy. The spectra consist of sharp lines (Δν?0.003–0.1 cm^?1) and show rotational structure (for SF₆), providing the first information on the droplet temperature, which is 0.37 K for⁴He _N and 0.14 K for³He _N . The rotational constants give information on the microscopic interaction of the molecule with the He environment. Each of the sharp vibronic lines of the elecronic S₁←S₀ transition in glyoxal (C₂O₂H₂) is accompanied by a phonon sideband, showing a distinct gap followed by a sharp peak at the roton energy (8.1 K) and a second weaker maxon peak followed by a broad multiphonon wing. These provide the first evidence for superfluidity in⁴He _N predicted theoretically.     

Nuclear spin relaxation,hybridization, and low-temperature 4f spin fluctuations in intermediate-valent SmB6

¹¹B spin-lattice relaxation measurements have been carried out in SmB₆ samples with large low-temperature resistivities. Above 15 K the relaxation is activated, with approximately the same gap (≈6 meV) as found previously in transport and optical measurements. 4f spin fluctuations apparently dominate the relaxation, so that these results give strong evidence for 4f-conduction band hybridization at the gap edge. An anomalous peak in the relaxation rate was observed at ≈5 K, which is tentatively attributed to fluctuations of “remagnetized” Sm³⁺ ions near Sm-site vacancies.     

Internal friction studies of the effect of hydrogen on the oxygen snoek peaks in V and Nb

The effect of hydrogen in solid solution on the oxygen Snoek peak activation parameters in V and Nb have been investigated. Using an analysis, proposed by Welter and coworkers, the following activation parameters were obtained. For V: ΔH = 1.16 ± 0.03 eV and τ₀ ≈ 27 × 10^-15 s; for Nb: ΔH = 1.12 ± 0.03 eV and τ₀ ≈ 7 × 10^-15 s. The values of these parameters are independent of the hydrogen concentration in the alloys, indicating that there is no interaction of hydrogen and oxygen at the temperatures where the oxygen Snoek peaks occur.     

Novel magnetic behaviour of the system ZnxCa1−xFe2O4

New magnetic system Zn_xCa_1−xFe₂O₄ (x = 0.4 to 0.6) is reported having extremely unusual properties like small ordered moments ≈ 0.13 ∼ 0.23 μ_B, large ordering temperatures T_c ≈ 700–750 K and surprising lack of hyperfine field (HF) at Fe for temperatures greater than T¹(≈ 0.25 T_c) in spite of bulk magnetization in the system. The HF appears below T¹ implying evolution of local moments far below T_c. Sharp susceptibility maxima resembling those of spin-glasses are observed for x = 0.4 and 0.5, which may be attributed to the single domain-superparamagnetic (SD-SP) transition and the overall behaviour shows a cluster spin-glass type of ordering.     

Predissociation linewidths for the Schumann-Runge bands of 16O18O

We have measured predissociation linewidths for rotational lines from the (2-0)–(15-0) Schumann-Runge bands of ¹⁶O¹⁸O. The behaviour of the vibrational widths is intermidiate between those observed previously for ¹⁶O₂ and ¹⁸O₂, with predissociation maxima at ν' = 4, 7, 10, (15). Our experimental linewidths are found to exhibit systematic variation with rotation for some of the bands studied. A model of the predissociation, including the interactions of the B³Σ^-_u state with repulsive ⁵Π_u, ³Π _u, ¹Π_u and ³Σ⁺_u states, and with molecular interaction parameters determined by a least-squares fit to previous ¹⁶O₂ linewidths, is found to predict accurately the observed vibrational and rotational dependences of linewidths for ¹⁶O¹⁸O.     

An investigation of the spin glass behaviour in iron antimonate by iron-57 and antimony-121 Mössbauer spectroscopy

Iron antimonate, which contains a superlattice composed of an ordered array of cations in the rutile-type structure, has been shown to undergo a spin glass transition atca. 20K which is induced by antisite atomic ordering. The iron-57 Mössbauer spectra recorded at 298 and 77K provide information on the relaxation times of short range magnetically ordered clusters identified by magnetic susceptibility measurements. The spectrum of 4K is tentatively interpreted in terms of the existence, at temperatures below the spin glass transition temperature, of a hyperfine magnetic field distribution at Fe³⁺ ions with a mean value of 487 kOe and which contains a supertransferred contribution from the nearest neighbour Fe³⁺ ions. The¹²¹Sb Mössbauer spectra are characteristic of Sb⁵⁺ and the increasing linewidths at lower temperatures are consistent with the presence of a supertransferred hyperfine magnetic field at the Sb⁵⁺ species ofca. 16^kOe.     

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 = $\text{5}\text{2}$.     

Chemisorption of carbon monoxide on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-adsorption infrared spectroscopy has been combined with thermal desorption and surface stoichiometry measurements to study the structure of CO chemisorbed on a {111}- oriented platinum ribbon under uhv conditions. Desorption spectra show a single peak at coverages > 10¹⁴ molecules cm^?2, with the desorption energy decreasing with increasing coverage up to 0.4 of a monolayer, and then remaining constant at ≈135 kJ mol^?1 until saturation. The “saturation” coverage at 300 K is 7 × 10¹⁴ molecules cm^?2, and no new low temperatures state is formed after adsorption at 120 K. Infrared spectra show a single very intense, sharp band over the spectral range investigated (1500 to 2100 cm^?1), which first appears at low coverages at 2065 cm^?1 and shifts continuously with increasing coverage to 2101 cm^?1 at 7 × 10¹⁴ molecules cm^?2. The halfwidth of the band at 2101 cm^?1 is 9.0 cm^?1, independent of temperature and only slightly dependent on coverage. The band intensity does not increase uniformly with increasing coverage, and hysteresis is observed between adsorption and desorption sequences in the variation of both the band intensity and frequency as a function of coverage. The frequency shift and the virtual invariance of the absorption band halfwidt with increasing coverage (Jespite recent LEED evidence for overlayer compression in this system) are attributed to strong dipole-dipole coupling in the overlayer.