Neutron scattering study of the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr2CuO2Cl2

We have carried out a neutron scattering investigation of the static structure factorS(q _2D ) (q _2D is the in-plane wave vector) in the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr₂CuO₂Cl₂. For the spin correlation length we find quantitative agreement with Monte Carlo results over a wide range of temperature. The combined Sr₂CuO₂Cl₂-Monte Carlo data, which cover the length scale from 1 to 200 lattice constants, are predicted without adjustable parameteres by renormalized classical theory for the quantum nonlinear sigma model. For the structure factor peakS(0), on the other hand, we findS(0) ² for the reduced temperature range 0.16<T/2 _s <0.36, whereas current theories predict that at low temperaturesS(0)T ^{2 2}. This discrepancy has important implications for the interpretation of many derivative quantities such as NMR relaxation rates. In the ordered phase, we have measured the temperature dependence of the out-of-plane spin-wave gap. Its low-temperature value of 5.0 meV corresponds to an XY anisotropyJ _XY /J=1.4×10^–4. From measurements of the sublattice mangetization we obtain =0.22±0.01 for the order parameter exponent. This may either reflect tricricality as in La₂CuO₄, or it may indicate finite-size two-dimensional XY behavior as suggested by Bramwell and Holdsworth. As in theS=1 system K₂NiF₄, the gap energy in Sr₂CuO₂Cl₂ scales linearly with the order parameter up to the Néel temperature. We also reanalyze static structure factor data for K₂NiF₄ using the exact low temperature result for the correlation length of Hasenfratz and Niedermayer and including the Ising anisotropy explicitly. Excellent agreement between experiment and theory is obtained for the correlation length, albeit with the spin-stiffness _s reduced by 20% from the spin-wave value. As in Sr₂CuO₂Cl₂ we find thatS(0) ² for the reduced temperature range 0.22<T/2 _s <0.47.     

Chemical pressure effects on the Kondo behaviour of CePt2Ge2

The compound, CePt₂Ge₂, recently reported to crystallize in a monoclinic modification of the CaBe₂Ge₂ structure, has been identified to be an antiferromagnetic Kondo lattice (T _N =2.2 K). The influence of positive and negative chemical pressure on the Kondo behavior is investigated by measurements of magnetic susceptibility and electrical resistivity () on the solid solutions, Ce_1–x La _x Pt₂Ge₂ and Ce_1–z Y _z Pt₂Ge₂. Y and La substituents cause nearly similar effects on the magnetic and electrical resistivity behaviour of CePt₂Ge₂, though the unit-cell volumes vary in opposite directions. We propose that the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction may contribute to decide the temperature at which attains a maximum at low temperatures (5 K) in CePt₂Ge₂.     

Laser-induced reactions of NO2 in the visible region

The reaction of NO₂ with isobutane, induced by 488 nm laser radiation, to form 2-nitro-2-methylpropane has been investigated and the results computer-modeled according to two possible reaction mechanisms. The first scheme involves the direct abstraction of H from isobutane by vibronically excited NO₂ (NO ₂ ^* ), and the second, abstraction by an intermediate NO₃ radial produced by NO ₂ ^* +NO₂. The modeling results strongly support the NO ₂ ^* scheme as the dominant reaction mechanism.     

Energy-band structure of semiconductors having the chalcopyrite lattice II. MgSiP2, ZnGeP2, ZnSiAs2, and CdSiP2

The pseudopotential method is used to calculate the band structure of MgSiP₂, ZnGeP₂, ZnSiAs₂, and CdSiP₂. The valence band of these compounds is shown to consist of several allowed subbands separated by energy gaps. There are two levels at the top of the valence band, slightly separated in ZnGeP₂ and ZnSiAs₂ and well-separated in MgSiP₂ and CdSiP₂ — in which there is significant tetragonal compression of the lattice. The effective electron masses at the bottom of the conduction band turn out to be anisotropic, with m/m 7–10.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 95–100, June, 1970.     

Spectroscopic study of a TEA CO2 laser of the CO2/X,X=H2; N2/H2 type

The spectral output of a TEA CO₂ laser utilizing nonconventional CO₂/H₂ and CO₂/N₂/H₂ gas mixtures was analysed. A simultaneous multiline output in P-branch of the 00°110°0 transition was recorded. A domination of the P(18) line is typical for the first, and that of the P(20) line for the second gas mixture. The enhancement of the P(18) line intensity in spectra (different from P(20) line domination-CO₂/N₂/H₂ mixture) is explained primarily by a smaller contribution of the R(23) line gain (01¹111¹0 CO₂ band), to P(20) line (00°110°0 band). All measurements were conducted with a nondispersive optical cavity.     

Energy band structure of CdSnAs2 and CdGeAs2

The energy band structure of CdSnA₂ and CdGeAs₂2 is calculated by the empirical-pseudopotential method, taking account of spin-orbital interaction. The calculated parameters – E_g = 0.25, _cr = 0.05, _s.o = 0.44 for CdSnAs₂ and E_g = 0.55, _cr = 0.20, and _s.o = 0.33 for CdGeAs₂ (all in eV) — are in good agreement with experiment. The conclusion drawn in a series of experimental works that there are additional minima close to the bottom of the CdSnAs₂ conduction band is not confirmed in the calculation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 78–82, December, 1981.     

Experimental study of cw carbon monoxide flame chemical laser of the CS2/O2/CO2 type

The performance of a continuous-wave (cw) CO flame chemical laser (FCL) of the CS₂/O₂/CO₂ type is presented. The laser gives up to 0.7 W cw output power on a number ofP _v (J) lines corresponding to 1110, ..., 76 vibrational bands of CO molecule. The measured values of chemical efficiency based on the reaction O+CSCO^*(v)+S and the specific power are 0.1% and 0.7J/g, respectively. The spectral composition of the CO FCL of the CS₂/O₂/CO₂ type shows lasing in the region from 5.194 to 5.573 m. All experimental measurements are conducted with a nondispersive optical cavity.     

Tunneling conductance of a Bi2Sr2CaCu2O8-SnO2 junction along the c-axis

fine structure was observed in the conductance curve of a tunneling junction composed of a single crystalline Bi2212 and an evaporated SnO₂ film. It is similar to those of Bi2212-GaAs mechanical junctions and there is a certain correspondence between the structure and the phonon density of states. Thus the previous conclusion that the structure is due to phonons has been complemented by this work. The energy gap 2 was 57 meV at 13 K and T _c was 78 K. 2(0)/k _B T _c is then 8.3. (T) showed the BCS-like temperature dependence.     

Spectral composition of the light from plasmas created by electron beams in nitrogen and He-N2 and Ne-N2 mixtures

Plasmas created by electron beams in N₂ and He-N₂ and Ne-N₂ mixtures at high pressures are studied experimentally. It is shown that N ₂ ⁺ ions in the B² _u ⁺ state are produced during charge exchange of Ne ₂ ⁺ ions with N₂ molecules. A comparison of the intensity of light from the B² _u ⁺ X² _g ⁺ transition of the N ₂ ⁺ ion in He-N₂ and Ne-N₂ mixtures shows that under these experimental conditions charge exchange of Net ions with N₂ is less effective than the analogous process involving He ₂ ⁺ ions. It is found that in an Ne-N₂ mixture the intensity of emission on transitions of the 2^t system of nitrogen is greater than in N₂.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 57–61, December, 1977.     

Measurement of the F5/2 and H(2)9/2 manifold lifetime in Nd:YLiF4

We present direct measurements of the lifetime of the ⁴F_5/2 and ²H(2)_9/2 manifold in Nd³⁺:YLiF₄, using a fluorescence pump-probe technique. The technique populates the ⁴F_5/2 and ²H(2)_9/2 manifold directly with a pump pulse. Via excited state absorption from this excited manifold, the ²F(2)_5/2 manifold of Nd³⁺ is populated with a delayed probe pulse. The population in the ⁴F_5/2 and ²H(2)_9/2 manifold is monitored as a function of time by observing the change in integrated UV fluorescence from the ²F(2)_5/2 manifold for each time delay between pump and probe pulses. The pump and probe beams come from the fundamental and second harmonic wavelengths of a femtosecond Ti:sapphire regenerative amplifier. The measured lifetime agrees well with the energy gap law, based on other nonradiative lifetime measurements from the literature for Nd³⁺:YLiF₄.     

Transport properties of LaCu2Si2 and CeCu2Si2 between 1.5K and 300K

We report measurements of the electrical resistivity, the thermal conductivityk and the thermoelectric powerS between 1.5K and 300K on the anomalous CeCu₂Si₂ compound and on LaCu₂Si₂ as reference compound. For LaCu₂Si₂ the temperature dependences of andS are in accord with those found in otherd band metals. For CeCu₂Si₂ the observed resistivity (220 µ cm at 200K) leads to a very short electronic mean free path which is of the order of the Ce-Ce spacing. Correspondingly,k is almost identical with the phonon contributionk _p . Below 20K, resistivity and thermoelectric power strongly suggest Fermi liquid behavior with a degeneracy temperature between 20K and 40K. Above 200K, both andS decrease proportionally to –ln(T/1 K).Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Jülich/KölnPart of this work will be presented at the Int. Conf. on Rare Earths in the Metallic State, St. Pierre de Chartreuse, Sept. 1978     

Laser-induced reactions of NO2 in the visible region

The 488 nm laser-induced reaction of NO₂ with CO has been investigated and the results computer-modeled. Two reaction mechanisms were considered:1) the direct reaction of vibronically excited NO₂ (NO ₂ ^* ) with CO to form CO₂, and 2), the reaction of an intermediate NO₃ radical formed by the reaction of NO ₂ ^* with NO₂ followed by NO₃+CONO₂+CO₂. The modeling results strongly support the former as the dominant mechanism.Federal Junior Fellow (1980–1984)     

No Zero Energy States for the Supersymmetric x 2 y 2 Potential

We show that the positive supersymmetric matrix-valued differential operator H = p _x ² + p _y ² + x ² y ² + x₃ + y₁ has no zero modes, i.e., H = 0 implies = 0. The result depends on a virial type argument for the corresponding supercharge. The model may be regarded as a simple relative of dimensional reductions of supersymmetric Yang–Mills theories.     

Microscopic model of the coexistence of superconductivity and ferromagnetism in the hybrid ruthenate-cuprate oxide RuSr2GdCu2O8

A microscopic t-J-I model with competing antiferromagnetic (J) and ferromagnetic (I) exchange interactions is proposed for strongly correlated electrons in RuSr₂GdCu₂O₈. It is assumed that for CuO₂ layers and for RuO₂ layers. A superconducting solution of $d_{x^2 - y^2 }$ symmetry was obtained for the CuO₂ layers while competition between ferromagnetism and p-type triplet superconducting pairing is obtained for RuO₂ layers. It is shown that the RuO₂ layers have a lower carrier concentration in the Hubbard subband formed by coupled ((d _xy -p)-π) orbitals compared with a bulk Sr₂RuO₄ crystal, which leads to stabilization of the ferromagnetic state in the RuO₂ layer.