Anomalous magnetic properties of the complex (ET)2C60

Investigations of the temperature dependences of the magnetic permeability and dielectric permittivity in the temperature range 4.2 K⩽T ⩽300 K and the field dependence of the magnetization M(B) in fields B⩽50 T show that the magnetic properties of the complex (ET)₂C₆₀ cannot be described on the basis of the standard model, which assumes that the paramagnetic oxygen impurity makes the dominant contribution. It is found that the magnetism in (ET)₂C₆₀ is due to the diamagnetic properties of the C₆₀ and ET molecules and to specific paramagnetic centers of the type C ₆₀ ⁻ , which possess an anomalously low g factor |g|≈0.14. An experimentum crucis is proposed for checking the oxygen paramagnetic center model for pure C₆₀ films. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 10, 733–738 (25 May 1999)     

Spin–lattice relaxation phenomena in the magnetic state of a suggested Weyl semimetal CeAlGe

ABSTRACT

In this work, we report the results of DC susceptibility, AC susceptibility and related technique, resistivity, transverse and longitudinal magnetoresistance and heat capacity on polycrystalline magnetic semimetal CeAlGe. This compound undergoes antiferromagnetic type ordering around 5.2 K (T¹). Under the application of external magnetic fields, parallel alignment of magnetic moments is favoured above 0.5?T. At low field and temperature, frequency and AC field amplitude response of AC susceptibility indicate the presence of spin–lattice relaxation phenomena. The observation of spin–lattice interaction suggests the presence of the Rashba–Dresselhaus spin–orbit interaction which is associated with inversion and time-reversal symmetry breaking. Additionally, the presence of negative and asymmetric longitudinal magnetoresistance indicates anomalous velocity contribution to the magnetoresistance due to the Rashba–Dresselhaus spin–orbit interaction which is further studied by heat capacity.     

Quantum effects in submillimetre hot hole semiconductor lasers

The hole dynamics and emission processes in degenerate band semiconductors in strong crossedE B fields were studied both theoretically and experimentally. The Luttinger effective mass Hamiltonian was used to study the Landau level energy spectrum in anisotropic valence bands of Ge. The dependence of the energy spectrum onE,H fields orientation is analysed. The role of quantum effects, such as interaction and mixing of light and heavy hole states in the scattering process and Landau level population is studied. The results of experimental studies of stimulated emission spectra for intersub-band and cyclotron transitions as well as their dependence onE,H field orientation are presented, the experimental data being in good agreement with the quantum model calculations.     

Quantum corrections to the conductance of AlGaAs/GaAs-based quasiballistic quantum wires

Quantum conductance corrections (QCCs) due to weak localization and interaction effects of quantum quasiballistic wires are investigated for the first time. At temperatures in the range 2 K<T<12 K a crossover of these corrections from one-dimensional behavior to zero-dimensional behavior is observed. It is shown that the phase coherence length in the wires studied is less than the length L _T =(ℏD/kT)^1/2 at all temperatures. It is found that the conventional theory of QCCs describes correctly the experimental temperature dependence of the QCCs but gives a much lower value than the experimental one. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 973–976 (25 June 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Magnetic fields at 181Ta nuclei in Laves phases of RFe2 (R=Nd, Pr, Sm, Gd, Dy, Yb, Lu)

By studying the perturbed angular correlations of γ-rays emitted during the decay of ¹⁸¹Hf impurities in the Laves phases of PrFe₂, DyFe₂, and YbFe₂, we have investigated the magnetic hyperfine interaction between these compounds and its daughter nucleus ¹⁸¹Ta, and have determined the temperature dependence of the magnetic hyperfine fields. At room temperature we obtained the following values of these magnetic hyperfine fields B _hf:B _hf(PrFe₂)=7.6(1) T, B _hf(DyFe₂)=15.5(5) T, and B _hf(YbFe₂)=18.8(3) T. When taken together with data obtained previously, the results of our experiments show that for Ta nuclei in the RFe₂ Laves phases the values of B _hf depend strongly on whether R is a light or a heavy rare-earth element, which allows us to conclude that in these phases the value of the magnetic moment induced at the impurity Ta nuclei depends on the interatomic distance. Zh. éksp. Teor. Fiz. 111, 1085–1091 (March 1997)     

Temperature dependence of the hyperfine field of Tc in Fe

Using BaF₂ scintillation counters we made a series of DPAC measurements on⁹⁹TcFe in the temperature range 77 < T < 1020 K. Our results at room temperature are consistent with earlier measurements of Raghavan and Raghavan. Contrary to earlier suppositions, H_hf(T) does not have an anomalous temperature dependence like that of OsFe.     

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb₆X₁₂]²⁺ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable B_o field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of ⁹⁵Mo nuclei in structurally related compounds containing [Mo₆Br₈]⁴⁺ cluster cores. Experimentally determined δ_iso(⁹³Nb) values are in the range from 2400 to 3000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (V_zz and δ₃₃) coinciding with the molecular four-fold axis of the [Nb₆X₁₂]²⁺ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (V_zz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (V_yy) and the largest CS principal axis (δ₁₁) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by ^79/81Br NQR and ³⁵Cl solid-state NMR measurements.     

Magnetism-Related Properties of CdSb Revealed by the Zeeman 121Sb NQR Spectra

The ¹²¹Sb NQR spectra of CdSb single crystal were measured in the presence of weak (up to 500 Oe) external magnetic fields. As the analysis of the ¹²¹Sb EFG symmetry showed, two magnetically nonequivalent Sb sites exist in the CdSb crystal lattice. An upper limit of the local magnetic field, which may exist in CdSb characterized with the observed ¹²¹Sb NQR spectra, was estimated by modeling the Zeeman perturbed patterns. This amounted to H _loc ∼10 G, hence being much less than earlier found local fields H _loc ∼30–200 G in bismuth-based compounds.     

TDPAC measurements on phase transition and hydrogen up-take in C15 laves phases

Intermetallic compounds with C15 structure are studied by TDPAC in the search for changes in the electric field gradient (EFG) on¹⁸¹Ta probes due to martensitic phase transition and hydrogen absorption. The residual low EFG of the cubic phase of HfV₂, ZrV₂ and Hf₅Zr₅V₂ is distinctly increased in the low temperature modification. Hydrogen does not affect the distribution of probe sites, but influences the features of the martensitic transition. In TaV₂ the sites with low EFG are split into an unperturbed and a strongly perturbed fraction.     

119Sn Mössbauer spectroscopy of U x Th1−x CoSn and UCoAl1−x Sn x

Magnetization and¹¹⁹Sn Mössbauer spectroscopy studies were performed on two pseudoternary systems, namely U _x Th_1?x CoSn and UCoAl_1?x Sn _x . Magnetic hyperfine fields on¹¹⁹Sn nuclei were found to be due to local surrounding of U atoms which carry magnetic moments. In pure UCoSn the valuesB _hf=8.66 T and μ_∪ satisfy the relationB _hf=6.75 μ_B found for a number of UTX compounds studied. Local surrounding effects are indicated by multicomponent spectra obtained for the two pseudoternary series.     

Hyperfine fields for119Sn in laves phases Rt2 (R=rare earth: T=Fe,Co, Ni)

The hyperfine fields acting on¹¹⁹Sn nuclei in the RT₂ (R=Sm?Lu; T=Fe, Co, Ni) have been measured by Mössbauer spectroscopy. It has been found that the hyperfine fields acting on¹¹⁹Sn nuclei in the RT₂ compounds are changed at some electronic occupation of the 4f shell of the R-component. The occupation of 4f shell is varied with the T-component. The sharp change of the hyperfine fields are connected with the change of the electronic band structure.     

The spin-isospin decomposition of the nuclear symmetry energy from low to high density

The energy per particle BA in nuclear matter is calculated up to high baryon density in the whole isospin asymmetry range from symmetric matter to pure neutron matter.The results,obtained in the framework of the Brueckner-Hartree-Fock approximation with two-and three-body forces,confirm the well-known parabolic dependence on the asymmetry parameterβ=(N?Z)/A(β^2 law)that is valid in a wide density range.To investigate the extent to which this behavior can be traced back to the properties of the underlying interaction,aside from the mean field approximation,the spin-isospin decomposition of BA is performed.Theoretical indications suggest that theβ^2 law could be violated at higher densities as a consequence of the three-body forces.This raises the problem that the symmetry energy,calculated according to theβ^2 law as a difference between BA in pure neutron matter and symmetric nuclear matter,cannot be applied to neutron stars.One should return to the proper definition of the nuclear symmetry energy as a response of the nuclear system to small isospin imbalance from the Z=N nuclei and pure neutron matter.     

Doping and temperature dependence of inversion symmetry breaking in La2−xSrxCuO4

The doping dependence of the Raman spectra of high quality La_2−xSr_xCu^16,18O₄ polycrystalline compounds has been investigated at low temperatures. It is shown that symmetry forbidden bands peaked at ∼150 cm⁻¹, ∼280 cm⁻¹, and ∼370 cm⁻¹ are activated in the (xx/yy) polarization Raman spectra due to the local breaking of the inversion symmetry mainly at low temperatures and for doping concentrations for which the compound is superconducting. The apparent A₁-character of the activated modes in the symmetry reduced phase indicates a reduction from the D_2h to C_2v or D₂ crystal symmetries, which associates the observed modes to specific IR-active phonons with eigenvectors mainly along the c-axis. The temperature and doping dependence of this inversion symmetry breaking and the superconducting transition temperature are very similar, though the symmetry reduction occurs at significantly higher temperatures.     

Measurements of the hyperfine magnetic fields at Ta and Fe in the Laves compounds (Zrx Hf1−x)Fe2 for 0≤x≤1

The hyperfine magnetic fields at¹⁸¹Ta and⁵⁷Fe in the ferromagnetic Laves intermetallic compounds (Zr_xHf_1?x)Fe₂ (0≤x≤1) have been measured by the methods of TDPAC and Mössbauer effect, respectively, and shown to be practically independent of x at x≥0.4. An average value B_hf (Ta)=?6.52 T at 300 K was obtained for samples with x≥0.4, and ?14.2 T for pure HfFe₂ in the hexagonal C14 modification. For 0hf(Ta) in (Zr_0.9 Hf_0.1)Fe₂ from that for B_hf (Fe) and the bulk magnetization was confirmed, studied in detail, and shown to exist for all x≥0.4. The temperature dependence of B_hf(Ta) in HfFe₂ was close to that of B_hf (Fe).     

Magnetoplastic effect in LiF crystals and longitudinal spin relaxation

New features of the dependence of the average travel distance l of dislocations on the magnetic field B have been found in an investigation of the magnetostimulated dislocation mobility in LiF crystals: A transition has been found from ordinary proportionality l∝B ² to saturation l≈ const in high fields B. It is shown that the experimental points can be described satisfactorily by the theoretical dependence l∝ [(B ₀/B)²+1]⁻¹ (B ₀≈0.8 T), typical for the mechanism of longitudinal spin relaxation in a system of radical pairs, which are supposedly formed when dislocation nuclei interact with paramagnetic impurity centers. According to the theory, the level of the field B ₀ is determined by the characteristic frequency of the oscillations of the internal fields in the lattice, which for B ₀∼1 T is of the order of 10¹¹s⁻¹, which corresponds to the typical frequency of characteristic oscillations of dislocation segments between pinning centers, which, naturally, does not depend on temperature. This in turn explains the fact that the measured values of B ₀ are the same at 293 K and 77 K. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 749–753 (10 December 1999)     

Experimental and ab initio study of the nuclear quadrupole interaction of 181 Ta-probes in an α-Fe2O3 single crystal

We report perturbed-angular-correlation (PAC) experiments on ¹⁸¹Hf (→¹⁸¹Ta)-implanted corundum α-Fe₂O₃ single crystal in order to determine the magnitude, symmetry and orientation of the electric-field-gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbital (FP-APW+lo) calculations. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.     

The EFG of the In-O-Bond In2O3, AgO and Ag2O

PAC measurements on¹¹¹In in In₂O₃, AgO and Ag₂O were performed to study the influence of different oxygen coordinations to the EFG.¹¹¹In was implanted at 500keV; in addition, AgO and Ag₂O were irradiated in the cyclotron to produce¹¹¹In via the (α, 2n) reaction. Using the point charge model and the known lattice structure of the oxides, the antishielding factor was calculated; it shows a strong dependence on the In-O bondlength.     

Shubnikov-de Haas effect and effective mass inn-InP in dependence on carrier concentration

Shubnikov-de Haas investigations onn-InP are presented and the effective mass as a function of carrier concentration is determined. The experiments are carried out with bulk and liquid phase epitactically grown material and carrier concentrations betweenn=10¹⁷ and 10¹⁹cm^–3. Temperatures employed ranged fromT=2 to 77 K and magnetic fields were applied up toB=22 T. Supplementing Kane's theory by introducing both temperature and electron concentration dependence, the nonparabolicity of the effective mass for temperatures betweenT=0 K and 300 K is calculated. The result fits quite well to the experimental results.Dedicated to Prof. Dr. Günter Lautz on the occasion of his 70th birthday     

Discrepancies in NMRON Knight Shifts of impurity nuclei in ferromagnets

Precision NMRON field shift studies have been carried out up to 8T on a⁵⁴MnNi single crystal along a hard [100] direction and on a¹²⁵SbFe single crystal along a hard [110] direction. For both systems, high field (B _app>-1T) and low field (0.3T<B _app<0.8T) data sets are obtained. The analysis reveals a significant discrepancy between the apparent Knight Shift dependent on whether the low field or high field data set is utilised. For both systems, consideration of the high field data sets yield a zero Knight Shift K(⁵⁴MnNi)=+0.0(0.2)% and K(¹²⁵SbFe)=+0.2(1.4)%, whereas the low field data sets yield K(⁵⁴MnNi)=+7.5(3.9)% and K(¹²⁵SbFe)=−5.4(3.3)% respectively. The field range dependence of K suggests that only Knight shifts measured in large fields (>-1T) are meaningful for establishing systematics. This casts some doubt on the greater bulk of the literature’s NMRON Knight shift studies, where predominantly low fields have been used to determine K.