Jahn-Teller transitions in Yb<Emphasis Type="Italic">X</Emphasis>O<Subscript>4</Subscript> (<Emphasis Type="Italic">X</Emphasis>=V,P) stimulated by a strong magnetic field

The effect of an external magnetic field directed along various symmetry axes of a crystal on Jahn-Teller-type structural phase transitions (quadrupole ordering) is studied in YbPO₄ and YbVO₄ crystals with zircon structure. In the absence of a magnetic field, the crystals are in a precritical state and do not exhibit a spontaneous quadrupole ordering. It is shown that, in a field H ∥ [110], the strain susceptibility χ_γ increases with the field and, at a sufficiently high field strength, an orthorhombic lattice deformation along the [100] axis arises in the crystals under study; i.e., a stimulated Jahn-Teller phase transition of γ symmetry occurs. Using interaction constants determined from independent experiments, we calculated phase diagrams and anomalies in the magnetic and magnetoelastic properties of the YbPO₄ and YbVO₄ crystals near the stimulated phase transitions, investigated the effect of various pairwise interactions on them, and analyzed possible experimental observations of the predicted effects.     

The possibility of the “giant” isotope effect for ultrasound absorption in crystals

The Zeeman effect, magnetization M(H), and differential magnetic susceptibility dM/dH of ErVO₄ crystals in a pulsed magnetic field have been experimentally and theoretically studied. In magnetic fields H ∥ [001] and H ∥ [100], the energy levels of Er³⁺ ions exhibit mutual approach and crossing (the crossover effect), which results in the peaks in dM/dH and the jumps in M(H) curves at low temperatures. The anomalies in the magnetic properties related to the crossover in ErVO₄ for H ∥ [001] are highly sensitive to the electronic structure of Er³⁺ ion, which allows this effect to be used for refining the crystal field parameters. The influence of the temperature, field misorientation from the symmetry axis, parameters of pair interactions, and other factors on the magnitude and character of magnetic anomalies in ErVO₄ crystals is considered.     

On a soluble model of an antiferromagnetic chain with Dzyaloshinsky interactions. I

The one-dimensional anisotropic XY model with antisymmetric nearest-neighbour interactions (Dzyaloshinsky interactions) in the presence of a magnetic field B in the z direction is investigated. Special attention is given to the critical behaviour of the system. In the ground state, the susceptibility χ^zz has a logarithmic singularity as B → 1 provided that the strength Δ of the Dzyaloshinsky interaction obeys the inequality |Δ| ≤ |γ|, where γ is the anisotropy parameter. However, if |Δ| #62; |γ|, χ^zz diverges with a power -$\text{1}\text{2}$ singularity as B ↑ (1 + Δ² - γ²)^{$\text{1}\text{2}$}. The specific heat is studied for low temperatures. Furthermore for a few special values of γ and B the static correlation functions 〈S^α_jS^β_k〉 are evaluated in the low-temperature limit, for α, β = x, y, z and |j - k| = 1, 2. Finally we consider the chain with Dzyaloshinsky interactions only.     

Magnetic and magnetoelastic properties of GdZn single crystals

The magnetic and magnetoelastic properties of GdZn have been investigated by various ways: magnetization, magnetic susceptibility, magnetocaloric effect, pressure dependence of the Curie temperature, spontaneous magnetostriction, parastriction and sound velocity measurements. The γ-mode strain is large for an S-state ion, and two orders of magnitude larger than the ?-one. Both the first-order B^γ,2 and the three isotropic second-order magnetoelastic coefficients exhibit a m²(1 ? 0.5m²) thermal variation (where m(T, H) is the reduced magnetization), which differs from the classical behaviour. All the results suggest a noticeable contribution of the conduction band to the magnetism of GdZn.     

Description of the nuclear surface by moments

The parameters proposed by W. D. Myers to describe the radial shape dependence of the nuclear surface are shown to be as useful as they are natural. For spherical nuclei, the central radiusC, the charge radiusR, the quadratic radiusQ are redefined, and it is shown how they are interrelated by Myers' surface widthb, flair γ₃, crookedness γ₄, and the higher shape parameter γ₅. All these quantities are calculated for some special charge distribution functions. (For symmetric distributions, possessing a symmetry center atC, the odd surface momentsb ³γ₃ andb ⁵γ₅ vanish.) The connection of the surface moments Г_μ =b ^μ γ _μ with the volume momentsF _k that have been extracted by K. W. Ford and J. G. Wills from muonic atoms, is indicated.     

Spatial anisotropy of xenon-defect configurations in cubic single crytals

Ion-implanted tungsten and copper single crystals are investigated by the perturbed angular correlation technique, using an allowed β?γ cascade in ¹³³Xe decay. The spatial distribution function describing the orientation of the quadrupole hyperfine interaction does not show up the cubic symmetry of the lattice in the case of xenon-induced radiation damage. This conclusion is interpreted in terms of xenon-vacancy loop interaction.     

Magnetoelastic effects in rare-earth vanadates YbVO4 and HoVO4

Magnetoelastic anomalies in the thermal expansion and Young modulus, as well as the ΔE-effect in rare-earth vanadates RVO₄ (R = Ho, Yb), are investigated experimentally and theoretically. A considerable softening of the Young modulus is observed for HoVO₄ and YbVO₄ at T < 70 K and T < 150 K, respectively; this effect is adequately described in the framework of the generalized susceptibility formalism. It is shown that the field dependences of the ΔE-effect and their temperature variation in YbVO₄ can also be described using this approach. To compare with experiment, the magnetoelastic contributions to the Young modulus of an isotropic polycrystal from various elastic modes have been averaged. For the Yb vanadate, considerable magnetoelastic anomalies in the thermal expansion along the tetragonal a and c axes have been discovered. The magnetoelastic contributions are used for determining completely symmetric magnetoelastic coefficients; the role of the completely symmetric quadrupole constant for magnetoelastic effects is analyzed.     

Anomalies in thermal expansion of DyVO4 induced by quadrupolar ordering

The thermal expansion of the DyVO₄ crystal has been experimentally and theoretically investigated in the range of the Jahn-Teller structural phase transition. The manifestation of totally symmetric magnetoelastic interactions upon this transition has been studied for the first time. It is found that the temperature dependences of the unit-cell and thermal expansion parameters along the nonactive Jahn-Teller direction in the basal plane for the DyVO₄ crystal exhibit characteristic magnetoelastic anomalies at T<T _c due to the ordering of quadrupole moments of Dy³⁺ ions. The magnetoelastic contributions of the totally symmetric ε^α1 and ε^α2 and symmetry-lowering ε^γ modes to the thermal expansion are calculated within the general crystal-field formalism. The total quadrupolar coefficient G ^γ and magnetoelastic coefficient B ^γ are determined from the spectroscopic and spontaneous deformation data. It is demonstrated that the thermal expansion of the DyVO₄ crystal in the tetragonal and orthorhombic phases is well described in the framework of the unified model using a common set of interaction parameters for both phases.     

Temperature dependence of magnetooptic effects in rare-earth magnetic semiconductor γ-Dy<Subscript>2</Subscript>S<Subscript>3</Subscript>

Temperature dependences of the Faraday effect (FE), which is linear in a magnetic field B; of the nonreciprocal linear birefringence (NB) associated with magnetic field-induced spatial dispersion; and of the Cotton-Mouton effect (CME), which is quadratic in a magnetic field B, have been studied in the transmission region of the γ-Dy₂S₃ cubic magnetic semiconductor (T _d symmetry class) at wavelength λ = 633 nm in the temperature range T = 25–294 K. As the temperature is lowered, the magnitudes of the FE and of the two main NB components, α₀₀₁ and α₀₁₁, increase in proportion to the magnetic susceptibility χ. This behavior implies that the magnitude of these effects is determined by the magnetic moment m of the Dy³⁺ ion induced by the magnetic field B. The CME component \(\beta _{001} (k||[1\bar 10],B||[001])\) grows in proportion to the magnetic susceptibility squared, χ²; i.e., β₀₀₁ ～ m². By contrast, the component \(\beta _{111} (k||[1\bar 10],B||[111]))\) exhibits a weaker temperature dependence, which indicates the manifestation of microscopic mechanisms in the CME component β₁₁₁ that differ from those for β₀₀₁.     

Interaction of transverse magnetoresistance effects in extrinsic and intrinsic InSb

In extrinsic and intrinsic n-InSb the dependence of the resistivity on transverse magnetic fields is investigated. Four contributions are distinguished: the geometrical, the inhomogeneity, the mobility, and the ambipolar effect. The interactions of these effects are studied, not only from the resistivity, but also from the dependence of the electric field in the middle section of rectangular samples on the magnetic field. All theoretical derivations are verified by own experiments and compared to measurements reported in the literature.The interaction of the various magnetoresistance effects is here discussed for the first time, and new assertions on the physical magnetoresistivity, i.e. on the dependence of the carrier mobilities μ^± on the transverse magnetic field B can be made. For electrons μ^? = μ₀^?/(1 + γ^?B) is found, where γ^? is an empirical magnetoresistivity coefficient and μ₀^? the electron mobility with B = 0. Further, it is shown that theories presently existing on the physical magnetoresistivity are not able to explain these results.     

Study of hyperfine interactions in the intermetallic compound CePd2Si2 using PAC technique with 111Cd as probe nuclei

Perturbed γ???γ angular correlation spectroscopy (PAC) has been used to investigate the hyperfine interactions in the intermetallic compound CePd₂Si₂ using ¹¹¹In→¹¹¹Cd probe nuclei. Samples of CePd₂Si₂ were prepared by melting constituent elements in an arc furnace under pure argon atmosphere. Carrier-free ¹¹¹In nuclei were introduced into the samples by thermal diffusion at 800°C in vacuum during 12 h. The measurements were performed in the temperature range of 4.2–300 K. Above the magnetic transition temperature (T _N ?=?10 K), the results show two distinct and well defined quadrupole interactions that were assigned to probe nuclei occupying Ce and Si sites in the compound. The quadrupole frequencies were found to decrease linearly with increasing temperature. The PAC spectra taken below 10 K were analyzed with a model including combined electric quadrupole plus magnetic dipole interactions, from which the hyperfine magnetic field was determined.     

The crossover behaviour of the susceptibility of the Ising model on anisotropic lattices

The behaviour of the reduced susceptibility χ′ of a d-dimensional Ising system in the limit t_d = tanhβJ_d → 0 is investigated. The critical exponent γ_n of (∂ⁿχ′/∂tⁿ_d)_td = 0 is, for n = 2,3, found to satisfy γ_n ? (n + 1)γ₀, and hence, by inequalities found by Liu and Stanley, γ_n = (n + 1) γ₀. For d = 2, the latter identify is valid for all n.     

Induced quadrupole effects near a crossover in a tetragonal TbLiF4 sheelite in a strong magnetic field up to 50 T

The anomalies of magnetic properties of TbLiF₄ caused by the interaction of the energy levels of a rare-earth ion in a strong magnetic field up to 50 T directed along the [100] and [110] axes are studied experimentally and theoretically. The jumps in magnetization M(H) and the maxima of the differential magnetic susceptibility dM(H)/dH are found in critical fields H _c = 28 and 31 T, where the lower component of the excited doublet approaches the ground-state singlet of a Tb³⁺ ion. Based on the crystal-field model with known interaction parameters, we calculated the Zeeman effect and the magnetization and magnetic susceptibility curves for the TbLiF₄ crystal, which adequately describe magnetic anomalies and critical parameters of a crossover. It is shown that the jumpwise change in the α- and γ-symmetry quadrupole interactions in TbLiF₄ caused by changes in the corresponding quadrupole moments during the crossing of energy levels leads, in accordance with experiments, to a decrease in the critical field H _c by approximately 4 T and an increase in the maximum of the differential susceptibility dM(H)/dH near the crossover more than twofold. This behavior can be considered as an analog of the induced quadrupole transition caused by a change of the ground state of the rare-earth ion during crossover.     

The potts glass in uniform and random fields: a monte carlo investigation

As a simple model of an anisotropic orientational glass with short range forces, the 3-state Potts model on the simple cubic lattice with nearest neighbor interactions drawn from a Gaussian distribution is considered. With Monte Carlo methods we study the response of the system to a uniform “field” which favors one of the states. This is motivated by experiments which apply stress that favors one molecular orientation of the quadrupolar glass. The responsem to that fieldh=H/k _BT is analyzed in terms of an expansionm= χ₁ h+χ₁ h ²+χ₁ h ³+..., where χ₁ is the linear susceptibility, and χ₂,χ₁₃ are nonlinear susceptibilities. Unlike the case of spin glasses, where the spin inversion symmetry of the system in the absence of fields implies χ₂≡0,χ₂ is nonzero here and diverges to ?∞ at the zero temperature transition of the model, while χ₃ diverges to +∞ as in spin glasses. At inifinite temperature, however, χ₁=1/3, χ₂=1/18 and χ₃=-1/54, i.e. the nonlinear susceptibilities have a different sign as at low temperature. In contrast, a random field does not induce a uniform order parameterm but only a glass order parameterq. The temperature dependence of this glass order parameterq(T) shows for intermediate field strength order parameterq(T) shows for intermediate field strength a maximum of the slopedq(T)/dT very similar to corresponding experiments.     

Level crossing in hyperfine interactions studied by perturbed γ-γ angular correlations

The technique of differential γ-γ angular correlation measurements has been applied to an investigation of the hyperfine interactions in the 482 keV level of¹⁸¹Ta. The activity was embedded in the lattice of a hafnium single crystal. The investigation of the quadrupole interaction gave for the electric interaction frequencyω ₀=(313±4) MHz. The electric field gradient was found to be axially symmetric, the asymmetry parameter beingη<0.1. Furthermore the combined collinear magnetic dipole and electric quadrupole interaction was studied. The angular correlation was investigated as a function of the strength of the external magnetic field by integral as well as time differential measurements. The integral anisotropy as function of the magnetic field has the shape of a resonance curve. The maximum was observed at a magnetic field ofB _res=(24.2±0.5)kG.     

Magnetic dipole moments of excited states of105Pd and other spectroscopic investigations in the decay of105g Ag

The IPAC-method has been used with the external magnetic field of 9.0 T of a split coil superconducting magnet and with the internal field of PdFe for a measurement of magnetic moments of¹⁰⁵Pd states populated in the decay of 41.29d¹⁰⁵Ag. The results are:μ _7/2 (645 keV)=?1.49(9) nmμ _5/2 (319 keV)=+0.95(20) nmμ _3/2 (280keV)=?0.074(13) nm. The half-life of the 645 keV state has been redetermined by use of cooled thallium free Nal detectors asT _1/2 (645 keV)=126(2) ps. γ-γ directional correlations were measured with a system of three large Ge planar diodes for the cascades 443–645 keV, 332–319 keV, 393–280 keV, and 64–280 keV and analyzed in terms of dipole quadrupole mixing ratios of theγ-lines involved. The spin of the 673 keV state was derived as l/2⁺.     

How indirect (p,t) processes populate the γ-band of 168Yb

Direct (p, t) transfer to the γ-band of ¹⁶⁸Yb is found to be less important than indirect processes involving collective quadrupole excitation and correlated l=0 pair transfer. The 2⁺_γ cross section can be explained by such processes but the 3⁺_γ and 4⁺_γ can not. Hexadecapole excitation to the 4⁺_γ and the role of the unnatural parity of the 3⁺_γ are discussed.     

Nonlinear magnetoelastic effects in ultrathin epitaxial FCC Co(0 0 1) films:: an ab initio study

Those linear and nonlinear magnetoelastic coupling coefficients which determine the magnetostrictive stress and the strain-induced out-of-plane magnetic anisotropy in epitaxially grown FCC Co(0 0 1) films are calculated by the ab initio density functional electron theory. The nonlinear couplings have a strong effect on the change Δσ₁^m of the in-plane magnetostrictive stress resulting from a change of the magnetization direction from [0 1 0] to [1 0 0], but a negligibly small effect on the out-of-plane anisotropy e_MCA. The calculations confirm the experimental result that the measured out-of-plane anisotropy cannot be totally attributed to volume magnetoelastic effects. Estimates are given for the nonlinear magnetoelastic coupling coefficients m₁^γ,2 and m₂^γ,2.