Hyperfine spectroscopic study of the metal-insulator transition in V2O3

The metal-insulator (M-I) transition in vanadium sesquioxide V₂O₃ has been investigated by time differential perturbed angular correlation measurements of the electric fieldgradient (EFG) and the magnetic hyperfine field at dilute¹¹¹Cd impurities. The EFG undergoes a first-order change at the M-I transition at T_t=160 K, but does not reflect the high temperature resistivity anomaly. The increase of the EFG with temperature in the metallic phase can be attributed to thermal variations of the oxygen sublattice. The temperature dependence of the magnetic hyperfine field in the insulating phase follows a Brioullin function with a saturation value of H_hf(O)=15 KOe and an extrapolated Neel temperature, which, depending on the impurity concentration, varies between 188 and 230 K.     

Study of highT c superconductivity in Y1Ba2Cu3O7−x by PAC

HighT _c superconductivity in the YBaCuO superconductor has been investigated through the quadrupole interaction of the probe nuclei⁹⁹Tc. The quadrupole interactions were measured by the TDPAC method from 77 to 296 K. The probe nuclei⁹⁹Tc were introduced into the YBaCuO superconducting specimen by diffusion. The derived electric quadrupole interaction parameters show that the probe nuclei are subject to a unique EFG interaction and occupy a substitutional lattice site in the YBaCuO superconductor. A strong EFG of 10¹⁹ V/cm² was observed. The temperature dependence of the EFG exhibits a linear decrease with temperature increase. Anomalies of both EFG and ν were found in the superconducting transition temperature region. The role of the oxygen vacancies in the Cu−O chains is discussed.     

The internal electric fieldgradient in antiferroelectric PbZrO3 studied by perturbed angular correlations

The internal electric fieldgradient (EFG) at the Zr-site in antiferroelectric polycristalline PbZrO₃ has been investigated as a function of temperature by time differential angular correlation measurements. For the application of this technique PbZrO₃ was doped with small amounts of radioactive¹⁸¹Hf. In the antiferroelectric phase of PbZrO₃ the EFG decreases continously with increasing temperature and exhibits a sharp discontinuity at 230 °C, the Curie point of the compound. In the cubic phase a small remaining EFG is observed which is probably caused by lattice imperfections in the source. The antiferroelectric distortion of the lattice decreases slightly when the temperature approaches the Curie point, as shown by the temperature dependence of the asymmetry parameter of the EFG. The measured EFG at room temperature is compared with the result of a lattice sum calculation.     

Critical behavior of electric field gradient in MnSi studied by muon level-crossing resonance

The electric quadrupole interaction of⁵⁵Mn nuclei was studied in the weakly ferromagnetic system MnSi using muon level-crossing resonance (LCR) technique. The temperature dependence of the electric field gradient (EFG) shows a critical behavior near the ferromagnetic transition temperature, indicating that the EFG due to the conduction electron is strongly correlated with the magnetic susceptibility in the itinerant electron magnetism. The temperature dependence of EFG is in reasonable agreement with the self-consistent renormalization theory developed by Moriya and coworkers.We gratefully acknowledge helpful discussion with Dr. N. Nishida. We also wish to thank Keith Hoyle and Curtis Ballard for technical support.     

Direct evidence for charge ordering and electronic phase separation in BixSr1−xMnO3 at room temperature

We present a study of charge ordering and electronic phase separation (EPS) phenomenon in Bi_xSr_1−xMnO₃, for an exhaustive range of x (0.25x0.75), by STM/STS at room temperature (RT) and specific heat measurements at high temperatures (350–650 K). Atomically resolved STM images of the samples, in real space, show the presence of stripe-like charge-ordered (CO) phase coexisting with charge-disordered (CD) phase. The STM images further reveal that the fraction of CO phase increases with an increase in x. The conductance spectra of these phases measured at nano level by STS are discussed. The transition to CO phase above RT is corroborated by specific heat measurements in all samples, giving a T_CO(x) phase diagram for this system.     

Colossal magnetostriction effect in HoMn2O5

We have investigated the detailed field and temperature dependence of the dielectric constant, electric polarization, magnetization and magnetostriction in orthorhombic HoMn₂O₅ single crystals. HoMn₂O₅ displays incommensurate antiferromagnetic ordering below 39 K, becoming commensurate on further cooling. The commensurate-incommensurate transition takes place at low temperatures. The inherent magnetic frustration in this material is lifted by a small lattice distortion, primarily involving shifts of the Mn³⁺ cations and giving rise to a canted antiferroelectric phase. Colossal magnetostriction effect was observed and a novel phase transition diagram was build.     

Pac studies of ion implanted silicon

The annealing behaviour of radiation induced defects in ion implanted silicon is studied by the perturbed angular correlation method (PAC). Between 700 K and 1000 K the trapping and detrapping of vacancy-oxygen complexes is observed. In annealed p-Si a well defined, axially symmetric electric field gradient (EFG) appears at low temperatures. This EFG is oriented to the surface and not to any crystallographic direction. The size of the EFG depends strongly on the surface charge.     

Composition Dependence of Electrocaloric Effect in (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 Single Crystals

Composition dependence of electrocaloric effect is investigated in (1-x)Pb(Mg_1/3Nb_2/3)O_3-xPbTiO₃ single crystals by using an eighth-order Landau—Devonshire theory. The applied electric field along [001] direction reduces the ferroelectric-ferroelectric phase transition temperatures, but increases the Curie temperatures. The electrocaloric coefficients of tetragonal phase are much larger than that of rhombohedral and monoclinic phase. A negative electrocaloric effect is observed near the M_C-T phase transition in 0.69 Pb(Mg_1/3Nb_2/3)O₃-0.31 PbTiO₃ single crystal. The application of a strong enough electric field results in a high adiabatic temperature change over a broad range of temperature. Therefore, it would be useful to construct a solid state cooling cycle over a broad temperature range for practical applications.     

Mössbauer studies of FexNbS2 (x = 0.25, 0.33 and 0.5)

Mössbauer studies of Fe_xNbS₂ (x = 0.25, 0.33 and 0.5) have been carried out for temperatures from 4.2K to about 715K. The iron exists in high spin divalent state for all the compositions. The temperature dependence of quadrupole splitting and center shift shows a reversible phase transition at about 600K and a possible disordering of Fe vacancies beyond this temperature for Fe_0.25NbS₂ and a reversible phase transition at 490K for Fe_0.5NbS₂. The hyperfine magnetic fields have been evaluated from the magnetically ordered spectra. The observed temperature dependent line intensities of the quadrupole doublet are attributable to the temperature dependence of the difference in the meansquare amplitude of vibrations parallel and perpendicular to the EFG axis.     

A naive model for the EFG in hep metals and numerical results for zinc

A naive model for the charge distribution in hep metals suggests that the conduction electron charge shift, which is related to the deviation ofc/a from (8/3)^1/2, is the essential source for the electric field gradient (EFG). This charge shift is derived approximately from the elastic coefficientss _ik of the host by application of simple electrostatics. The EFG is obtained from lattice sum calculations involving the ions and the conduction electrons in the hexagonal planes as well as between the hexagonal planes. The result for the EFG is in agreement with the universal correlation proposed by Raghavan et al. The anisotropy of the probe ion vibration, which averages the EFG over a finite volume, is found to contribute a dominant term to the observed temperature dependence of the EFG. Numerical results for the hep-metal Zn are in good agreement with the available experimental data. The trend of the probe deptendence of the EFG in Zn is reproduced if the effect of the probe valence on the surrounding conduction electron charge is taken into account.     

Electric field gradients in semiconducting alloys

The results of recent perturbed angular correlation (PAC) experiments in semiconducting alloys are discussed. The observed temperature dependence of the electric field gradients (EFG) differs drastically from that of methals. The variation of the conduction electron density with temperature certainly influences the EFG but can not explain in general the data. A model developed for In₂Te₅ based on the change of the local electron density is discussed in detail.     

Mössbauer study of the magnetic and electric hyperfine interaction of dilute57Fe impurities in rare earth metal hosts

The hyperfine interaction of dilute⁵⁷Fe in the rare earth (RE)metals Gd to Lu was investigated by Mössbauer measurements with⁵⁷Co doped RE sources. In all hosts well split, 2-lines spectra were observed at room temperature, with slight asymmetries of the line intensities in some cases. The quadrupole splitting eQV_zz/2 increases from 0.29 mm/sec for Gd to 0.50 mm/sec for Tb, and decreases by less than 10 % between Tb and Lu. Only about 10 % of the corresponding electric fieldgradient (EFG) can be accounted for by the ionic EFG on a substitutional RE site. The temperature dependence of the EFG was measured in the case of Tb. No variation within 3 percent was found between 300 K and 700 K. Measurements of the magnetic hyperfine interaction at low temperatures were carried out in Tb. The saturation field of⁵⁷Fe in this host is H_hf(FeTb;4.2 K)=25(2) KOe. The temperature dependence of the magnetic hyperfine field does not follow the host magnetization (T_c=220K) but vanishes at about 80 K. Similar anomalies of H_hf(T) have previously been observed for other transition element impurities in the RE ferromagnets.     

Angular correlation studies on 172Lu(172Yb) in GaN and measurements at low temperatures

For optoelectronic devices semiconductors with large band gap doped with rare earth are used. Doping is generally performed during growth but for more structured doping the ion implantation technique is preferable. The perturbed γγ-angular correlation technique is an ideal tool to study the behavior of semiconductors after implantation. An adequate rare earth isotope for such investigations of semiconductors is ¹⁷²Yb. The temperature dependence of the hyperfine fields for ¹⁷²Lu(¹⁷²Yb) in GaN has been analysed. The total electric field gradient (EFG) at the site of this probe is a superposition of the lattice EFG due to the GaN wurtzite structure and the EFG due to the 4f shell of the rare earth probe itself. The latter is strongly temperature dependent and opposed to the lattice EFG which in contrast is nearly constant since the lattice parameters change only negligibly with temperature. At elevated temperatures the sublevels of the 4f shell, split by the crystal electric field, are equally populated. But at low temperatures the lowest level is occupied preferentially. Sign and magnitude of the EFG caused by the 4f shell can be calculated (Tomala et al, J Magn Magn Mater 89(1–2):143, 1990) and the result compared to the experimental data.     

EFG in hcp Metals from a phenomenological approach

Variation of the electric field gradient at Cd nuclei in disordered Mg_xCd_1-x with x at two constant temperatures and the temperature dependence of the EFG in Tl were theoretically investigated in the framework of a naive model of the EFG in hcp metals developed by Bodenstedt and Perscheid.     

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.     

Temperature dependence of the electric field gradient at Ta nuclei in hafnium pyrovanadate

The electric field gradient EFG in hafnium pyrovanadate has been measured at Hf sites at different temperatures with the time-differential perturbed angular correlation method. The results obtained show the existence of a phase transition around 110°C.     

Thermodynamics of chiral symmetry at low densities

The phase diagram of two-color QCD as a function of temperature and baryon chemical potential is considered. Using a low-energy chiral Lagrangian based on the symmetries of the microscopic theory, we determine, at the one-loop level, the temperature dependence of the critical chemical potential for diquark condensation and the temperature dependence of the diquark condensate and baryon density. The prediction for the temperature dependence of the critical chemical potential is consistent with the one obtained for a dilute Bose gas. The associated phase transition is shown to be of second order for low temperatures and first order at higher temperatures. The tricritical point at which the second order phase transition ends is determined. The results are carried over to QCD with quarks in the adjoint representation and to ordinary QCD at a non-zero chemical potential for isospin.     

Evolution of the optical properties of Pb<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>Sc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (PBSN-6) solid solution single crystals caused by a static electric field

The electric-field-induced variation of the optical properties (small-angle light scattering, birefringence) of PBSN-6 solid solutions was studied. It was found that in the absence of an electric field, the cubic nonpolar matrix contains, at temperatures below the dielectric permittivity maximum, spontaneously polarized regions of the ferroelectric phase not less than 10⁴ Å in size. It was shown that a weak electric field (～0.4 kV/cm) is capable of inducing a kinetic phase transition to the ferroelectric state, with the temperature of this transformation depending on the sample heating rate. The destruction of the induced state was accompanied by a sharp peak in the temperature dependence of the small-angle light scattering intensity (indicating the percolation nature of the transition) and was independent of the sample heating rate. The boundaries of stability of the induced state in various modes of application of an external electric field were determined, and the E-T phase diagram was constructed.     

Calculated Electric Field Gradients and Electronic Properties of Acceptors in CdTe

Linearised augmented plane wave (LAPW) calculations are performed in order to determine the electric field gradients (EFG) induced by group V and group Ib acceptors in CdTe at the nearest neighbour (NN) atomic site. Experimentally, the EFG are measured by the perturbed γγ-angular correlation spectroscopy using the radioactive probe isotopes ¹¹¹In and ⁷⁷Br. Besides the identification of the experimentally observed defects, the LAPW calculations provide information about fundamental properties of the EFG. For the group Ib acceptors, the dependence of the EFG on the NN distance is opposite to the expectation from the simple point charge model. This revised version was published online in September 2006 with corrections to the Cover Date.     

Mott transition from a spin liquid to a Fermi liquid in the spin-frustrated organic conductor kappa-(ET)2Cu2(CN)3

The pressure-temperature phase diagram of the organic Mott insulator kappa-(ET)2Cu2(CN)3, a model system of the spin liquid on triangular lattice, has been investigated by 1H NMR and resistivity measurements. The spin-liquid phase is persistent before the Mott transition to the metal or superconducting phase under pressure. At the Mott transition, the spin fluctuations are rapidly suppressed and the Fermi-liquid features are observed in the temperature dependence of the spin-lattice relaxation rate and resistivity. The characteristic curvature of the Mott boundary in the phase diagram highlights a crucial effect of the spin frustration on the Mott transition.