G. A. Samara

Sandia National Laboratories, Albuquerque, New Mexico, United States

Are you G. A. Samara?

Claim your profile

Publications (111)218.96 Total impact

  • MRS Online Proceeding Library 01/2011; 358.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Highly crystalline nanoclusters of MoSâ were synthesized and their optical absorption and photoluminescence spectra were investigated. Key results include: (1) strong quantum confinement effects with decreasing size; (2) preservation of the quasiparticle (or excitonic) nature of the optical response for clusters down to â¼ 2.5 nm in size which are only two unit cells thick; (3) demonstration that 3-D confinement produces energy shifts which are over an order of magnitude larger than those due to 1-D confinement; (4) observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size; and (5) observation of photoluminescence due to both direct and surface recombination. Application is to photocatalysts for solar fuel production and detoxification of chemical waste.
    MRS Online Proceeding Library 01/2011; 452.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: High-pressure Brillouin and Raman scattering spectroscopy and x-ray diffraction measurements were carried out on disordered Pb ( Sc <sub>1/2</sub> Nb <sub>1/2</sub>) O <sub>3</sub> , considered to be a model system for phase transitions in relaxor ferroelectrics and related materials. The observed pressure-dependent Raman spectra are unusual, with the relaxor state distinguished by broad Raman bands. Raman spectra as a function of pressure reveal a new peak at 370 cm <sup>-1</sup> , with two peaks near 550 cm <sup>-1</sup> merge above 2–3 GPa, indicating a structural phase transition in this pressure range consistent with earlier dielectric measurements. A significant softening in the longitudinal acoustic mode is observed by Brillouin scattering. Both the temperature and pressure dependencies of the linewidth reveal that the longitudinal acoustic mode softening arises from electrostrictive coupling between polar nanoregions and acoustic modes. X-ray diffraction indicates that the pressure-volume compression curve changes near 2 GPa.
    Journal of Applied Physics 05/2010; · 2.19 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Neutron scattering experiments using triple axis spectrometers have been performed for the relaxor ferroelectric materials K1-xLixTaO3 (x=0.05, 0.10) in order to study the behavior of the zone-center (ZC) transverse-optic (TO) phonon mode (ferroelectric mode). A major contrast between the x=0.05 and 0.10 samples is the ferroelectric transition-observed only for the latter material at TC=115 K on warming and as detected by dielectric measurements and neutron diffraction. The ZC TO mode for x=0.05 shows monotonic softening with decreasing temperature down to 10 K, whereas the x=0.10 sample shows a phonon component below TC which hardens with decreasing temperature in addition to a phonon mode which behaves similarly to that of the x=0.05 sample. This suggests a phase separation of the x=0.10 sample into ferroelectric and relaxor states below TC, possibly originating from a percolative nature of the ferroelectric state.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 02/2009; 600:254-256. · 1.32 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ceramic samples of Pb0.99La0.01(Zr0.91Ti0.09)O3 were studied by dielectric and time-of-flight neutron diffraction measurements at 300 and 250 K versus pressure. Isothermal dielectric data (300/250 K) suggest structural transitions with onsets near 0.35/0.37 GPa, respectively, for increasing pressure. On pressure release, only the 300K transition occurs (0.10 GPa; none indicated at 250 K). Diffraction data at 300 K show the sample has the R3c structure, remaining in that phase cooling to 250 K. Pressure increase (either 300 or 250 K) above 0.3 GPa yields a Pnma–like (AO) phase (two other prominent peaks in the spectra suggest a possible incommensurate cell). Temperature/pressure excursions show considerable phase hysteresis.
    Ferroelectrics 12/2008; 377(1):120-136. · 0.38 Impact Factor
  • G Samara, E Venturini, B Morosin
    [Show abstract] [Hide abstract]
    ABSTRACT: Zr-rich, La-doped lead zirconate titanate ceramic samples with composition near Pb0.99La0.01(Zr0.91Ti0.09)O3 (hereafter PLZT) were studied by time-of-flight neutron diffraction and dielectric measurements at 295 K and 1 bar and at 250 K versus increasing and decreasing pressure to 0.55 GPa. The diffraction data at 295 K show that the sample has the rhombohedral ferroelectric R3c structure (FR(LT)) and remains in that phase upon cooling to 250 K. As pressure is increased at 250 K, a transition is observed above 0.3 GPa to essentially the antiferroelectric CaTiO3 orthorhombic Pnma (AO) phase. There appear two other peaks in the neutron spectra, suggesting a possible incommensurate cell for this AO. After the initial drop of the FR(LT) content, the transformation remains incomplete to 0.55 GPa with ~10% of the sample retaining the FR(LT) low pressure phase. On pressure release, 91% of the FR(LT) phase is recovered with the remainder remaining in the AO phase. Isothermal dielectric data at 250 K suggest a structural transition with an onset near 0.37 GPa for increasing pressure, consistent with the diffraction results.
    Journal of Physics Conference Series 07/2008; 121(2):022024.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present high-energy x-ray (67 keV) and neutron-scattering measurements on a single crystal of K1−xLixTaO3 for which the Li content (x=0.02) is less than xc=0.022, the critical value below which no structural phase transitions have been reported in zero field. While the crystal lattice does remain cubic down to T=10 K under both zero-field and field-cooled (E≤4 kV/cm) conditions, the Bragg peak intensity changes significantly at TC=63 K. A strong and frequency-dependent dielectric permittivity is observed at ambient pressure, a defining characteristic of relaxors. However an extensive search for static polar nanoregions, which is also widely associated with relaxor materials, detected no evidence of elastic neutron diffuse scattering between 300 and 10 K. Neutron inelastic scattering methods were used to characterize the transverse acoustic and optic phonons (TA and TO modes) near the (200) and (002) Bragg peaks. The zone-center TO mode softens monotonically with cooling but never reaches zero energy in either zero field or in external electric fields of up to 4 kV/cm. These results are consistent with the behavior expected for a dipolar glass in which the local polar moments are frozen and exhibit no long-range order at low temperatures.
    Physical Review B 07/2008; 78(14):144202. · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Increasing concern surrounding the use of lead in consumer products has stimulated research to identify candidates to replace lead-based materials used in many commercial applications. To be integrated into commercial products; however, a lead-free replacement must be fabricated using common industrial techniques while maintaining dielectric properties equivalent to the current lead-based systems. Texturing has been shown to dramatically enhance the dielectric properties of lead-free materials such that several potential systems are now being considered as replacements for the current lead-based materials. In this work, a large degree of texturing has been introduced to bismuth titanium oxide bulk samples through the process of screen printing large, plate-like seeds in a matrix of equi-axial powder. The degree of texturing achieved gives rise to a high probability of excellent dielectric properties, making textured bismuth titanate a viable replacement for commercial lead-based dielectrics.
    01/2008; 3:1-3.
  • Dale L. Huber, Jess P. Wilcoxon, George A. Samara
    Applied Physics Letters 01/2008; 92(2):9902-. · 3.52 Impact Factor
  • Physical review. B, Condensed matter 11/2007; 76(19). · 3.66 Impact Factor
  • Physical review. B, Condensed matter 11/2007; 76(19). · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The effects of hydrostatic pressure on the dielectric response of single crystal Cd <sub>2</sub> Nb <sub>2</sub> O <sub>7</sub> (CNO) were investigated over the temperature range of 80–450 K . The main pressure effects are (1) large suppression of the magnitude of the dielectric constant ε<sup>′</sup> over most of the temperature range and (2) very weak pressure dependence of the ferroelastic and ferroelectric (FE) transition temperatures and of the relaxational response near 200 K , as reported earlier for ceramic CNO. In view of result (2) the emphasis of the present paper is on the dielectric response above the transition region, i.e., in the high temperature paraelectric phase. The ε′(T) response in this phase can be explained by the T dependence of the uncoupled soft FE mode frequency associated with the Nb O <sub>6</sub> octahedra, and the large decrease in ε<sup>′</sup> with pressure follows from the expected increase of this frequency which can be estimated from the dielectric data. In this phase ε<sup>′</sup> is very well represented by a Curie-Weiss law ε<sup>′</sup>=C/(T-T<sub>0</sub>) , where C=1.16×10<sup>5</sup> K and T<sub>0</sub>=174 K at 1 bar . Remarkably, the logarithmic pressure derivatives of C and T<sub>0</sub> are found to be essentially identical to those of Ba Ti O <sub>3</sub> implicating the crucial role of the B <- roman>O <sub>6</sub> octahedra in the soft mode character of the pyrochlore (CNO) structure, as in the case of the A B O <sub>3</sub> perovskite structure. Pressure had a relatively weak influence on the various overlapping dielectric relaxations below 200 K , but the results revealed a dipolar relaxation in the 350–400 K region. It is suggested that this relaxation is associated with the motion of a defect complex involving the oxygen vacancy, a common feature in the related A B O <sub>3</sub> oxides.
    Journal of Applied Physics 11/2006; · 2.19 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Zr-rich, Nb-doped lead zirconate titanate ceramic and powder samples with composition near Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 [PZT95/5(2Nb)] have been studied in the range of hydrostatic pressure 0–6.2 kbar and temperature 12–295 K by time-of-flight neutron powder diffraction and dielectric measurements. The combination of the two techniques has led to further insights into the properties and pressure-induced ferroelectric rhombohedral R3c (FR(LT)) to antiferroelectric orthorhombic Pbam (AO) phase transition in this material, and the diffraction results have provided a detailed view of the ionic displacements induced by changes in pressure and temperature as well as the displacements accompanying the transition. At 295 K the diffraction results revealed a sharp transition at 2.1 kbar; at 200 K this transition occurs at 1.1 kbar. The transformation is incomplete: after the initial sharp drop in the FR(LT) content at the transition, 20 wt % of the sample remains in the low-pressure FR(LT) phase. Above the transition, the fraction of FR(LT), which exists as a minority phase in the high-pressure AO phase, continues to decrease, but even at our highest pressure of 6.2 kbar, ∼8 wt % of the sample remains in the FR(LT) phase. The volume contraction at the FR(LT)-to-AO transition unexpectedly results in the retained minority FR(LT) being anisotropically ”clamped,” with its a axis slightly expanded and c axis contracted at the transition. On pressure release to 1 bar at 295 K, only 26% of the FR(LT) phase is recovered, and this remains in the clamped state because of the surrounding majority AO phase. Heating the sample above 350 K at 1 bar followed by cooling to room temperature results in full recovery of the FR(LT) phase. The spontaneous polarization (PS) of the FR(LT) phase and its pressure and temperature dependences were determined from the ionic displacements. At 295 K, PS=38 μC∕cm2—a value greater than the 31–32 μC∕cm2 commonly observed on ceramic PZT95/5(2Nb) samples. The difference is undoubtedly related to residual porosity in ceramic samples as well as the inability of the poling electric field to align all the polar domains. PS increases monotonically with decreasing temperature, reaching a value of ∼44 μC∕cm2 at 12 K.
    Physical Review B 04/2006; 73(14). · 3.66 Impact Factor
  • G. A. Samara, C. E. Barnes
    [Show abstract] [Hide abstract]
    ABSTRACT: The influence of hydrostatic pressure on the structural bistability and electronic properties of the processing-induced MFe center in Fe-doped n-type InP was investigated. Earlier work has shown that, when occupied by electrons, the center can be reversibly placed in either of two configurations, termed A and B, by the proper choice of electric biasing conditions and temperature. Pressure strongly modifies the energetics and kinetics of the various electronic transitions and of the transformations associated with the center. The activation volumes (ΔV*) for these processes were determined. In the absence of barriers to electron capture, or for small barriers, ΔV* can be interpreted as the breathing mode relaxation associated with electron emission or capture. At pressures ⩾8 kbar, the center exists only in the A configuration regardless of bias conditions, because at these pressures the energetics and kinetics of the various processes have changed so much as to always favor the A configuration. It is also shown that, whereas the A⇄B transformations are charge state controlled at 1 bar, this is not the case at high pressure where the transformations can be brought about without electron emission or hole capture. Earlier tentative atomic models for the center are discussed, and it is shown that some features of one of the models including the signs of the breathing mode relaxations associated with the various electron emissions are consistent with the experimental results, but issues remain. The results are also found to be generally consistent with first-principles calculations on defects in InP, but it is emphasized that whereas these calculations are for simple defects, the defects associated with the MFe center are more complex.
    Physical Review B 04/2006; 73(15). · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The temperature dependence of the dielectric response of KLT-3 shows no evidence of a thermodynamic phase transition, but reveals two prominent relaxational features associated with the off-center Li^+ ion, one attributed to the hopping of the Li^+ dipole and the other with the reorientation of Li^+ - Li^+ ion pair. Both relaxations are Debye-like and follow Arrhenius kinetics, the energy barriers decreasing with pressure. While pressure favors the relaxational behavior, a biasing dc electric field favors long-range order of the dipolar system. The interplay between pressure and field provides additional insight. Pressure suppresses the magnitude of the ε'(T) response over the whole temperature range The results allow evaluation of the contributions of the soft mode and of the Li^+ dipoles to the measured ε'(T) response.
    03/2006;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Zr-rich, Nb-doped lead zirconate titanate ceramic and powder samples with composition near Pb{sub 0.99}(Zr{sub 0.95}Ti{sub 0.05}){sub 0.98}Nb{sub 0.02}Oâ [PZT95/5(2Nb)] have been studied in the range of hydrostatic pressure 0-6.2 kbar and temperature 12-295 K by time-of-flight neutron powder diffraction and dielectric measurements. The combination of the two techniques has led to further insights into the properties and pressure-induced ferroelectric rhombohedral R3c (F{sub R(LT)}) to antiferroelectric orthorhombic Pbam (A{sub O}) phase transition in this material, and the diffraction results have provided a detailed view of the ionic displacements induced by changes in pressure and temperature as well as the displacements accompanying the transition. At 295 K the diffraction results revealed a sharp transition at 2.1 kbar; at 200 K this transition occurs at 1.1 kbar. The transformation is incomplete: after the initial sharp drop in the F{sub R(LT)} content at the transition, 20 wt % of the sample remains in the low-pressure F{sub R(LT)} phase. Above the transition, the fraction of F{sub R(LT)}, which exists as a minority phase in the high-pressure A{sub O} phase, continues to decrease, but even at our highest pressure of 6.2 kbar, â8 wt % of the sample remains in the F{sub R(LT)} phase. The volume contraction at the F{sub R(LT)}-to-A{sub O} transition unexpectedly results in the retained minority F{sub R(LT)} being anisotropically ''clamped,'' with its a axis slightly expanded and c axis contracted at the transition. On pressure release to 1 bar at 295 K, only 26% of the F{sub R(LT)} phase is recovered, and this remains in the clamped state because of the surrounding majority A{sub O} phase. Heating the sample above 350 K at 1 bar followed by cooling to room temperature results in full recovery of the F{sub R(LT)} phase. The spontaneous polarization (P{sub S}) of the F{sub R(LT)} phase and its pressure and temperature dependences were determined from the ionic displacements. At 295 K, P{sub S}=38 μC/cm²--a value greater than the 31-32 μC/cm² commonly observed on ceramic PZT95/5(2Nb) samples. The difference is undoubtedly related to residual porosity in ceramic samples as well as the inability of the poling electric field to align all the polar domains. P{sub S} increases monotonically with decreasing temperature, reaching a value of â44 μC/cm² at 12 K.
    Physical Review B - PHYS REV B. 01/2006; 73(6):064105-064105.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Relaxor behavior and lattice dynamics have been studied by employing dielectric measurements and neutron-scattering methods for a single crystal of K{sub 1-x}Li{sub x}TaO{sub 3} (x=0.05), where a small amount of a Ca impurity ({approx}15 ppm) was incorporated during the single-crystal growth procedure. The dielectric constant {epsilon}({omega},T) shows qualitatively similar behavior to that of Ca-free KLT with x=0.043 with both compositions exhibiting relaxational properties with no evidence for a ferroelectric transition. The absolute value of {epsilon}({omega},T=0) for the present crystal is larger by an order of magnitude than that of the Ca-free sample due to charge carriers induced by the Ca doping. This large value is shown to be due to a Maxwell-Wagner relaxation process associated with the low temperature (<8 K) activation of frozen electronic carriers. The dielectric loss tangent tan {delta} reveals three Debye-type relaxations with Arrhenius activation energies of 80, 135, and 240 meV that are assigned to Li{sup +} dipoles, Ca{sup 2+}-related relaxation, and the Li{sup +}-Li{sup +} dipolar pairs, respectively. In the neutron scattering results, diffuse scattering ridges appear around the nuclear Bragg peaks along the [100] direction below {approx}150 K and phonon line broadening features start to appear at even higher temperatures suggesting that polar nanoregions (PNRs) start to form at these temperatures. These results are supported by the dielectric data that reveal relaxor behavior starting at {approx}200 K on cooling. From analyses of the diffuse intensities at different zones, we have derived atomic displacements in the PNRs. The results suggest that the displacements include a uniform phase shift of all of the atoms in addition to the atomic displacements corresponding to a polarization vector of the transverse-optic soft-ferroelectric-mode, a finding that is analogous to that in the prototypical relaxor material Pb(Mg{sub 13}Nb{sub 23})O{sub 3}.
    Physical Review B 01/2006; 74(5). · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The effects of hydrostatic pressure and biasing dc electric field on the relaxor dielectric response of samples of Pb[(Mg1/3Ta2/3)0.95Zr0.05]O3 with 12%, 15%, and 90% B-site cationic order were investigated. Qualitatively similar decreases in the amplitudes of the real part of the dielectric constant (ε') at both the peak temperatures (Tm) of the V'(T,omega) response in the high temperature phase, i.e., above Tm, are observed on increasing the three variables: pressure, biasing field, and B-site order - effects that are interpreted in terms of stiffening of the underlying soft ferroelectric mode of the lattice. Strong deviation of the frequency-independent V'(T) from the Curie-Weiss law above Tm, attributed to correlations among polar nanodomains, gives way to adherence to this law above the Burns temperature Td. This is the temperature where polar nanodomains first make their presence known. The evolution with decreasing temperature below Td of short-range order in the nanodomains is estimated from the V'(T) response and shows essentially no dependence on the degree of B-site order. The correlation length for the interaction among the polar nanodomains was also estimated from the dielectric data and found to exhibit strong increase (decrease) with temperature (pressure) as T approaches Tm from above - characteristics of perovskite relaxors. A high temperature dielectric relaxation with an activation energy of 1.27 eV is observed for the 90% ordered sample, but not for the 12% and 15% ordered samples. This relaxation is attributed to increased oxygen vacancies in the 90% sample that form during the long (64 h) annealing time at 1350 °C to achieve this high level of ordering.
    Journal of Applied Physics 01/2006; 100. · 2.19 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Terahertz THz radiation from a 100 oriented InAs surfaces is dominated by the photo-Dember effect. The strength of the radiation is influenced by screening the radiation with doped carriers. When irradiated by femtosecond pulses, the wafer with the lowest doping concentration radiates THz power nearly two orders higher than the wafer with highest doping concentration. With identical optical excitation and same doping concentration, a p-type InAs generates stronger THz waves than an n-type InAs due to the weaker screening effect. The low doping p-type InAs 1 10 16 cm −3 sample is the strongest THz wave emitter among all the unbiased semiconductors we have ever tested with a Ti:sapphire laser oscillator. The drift-diffusion equation DDE is used in the study of carrier drift and diffusion as well as subsequent THz radiation from InAs wafers. The calculation explains well the experimental observation of the relationship between a THz electric field and the doping properties of InAs. The physical pictures of the carrier drift and diffusion characteristics in InAs surfaces are also clearly provided in this report.
    Physical Review B - PHYS REV B. 01/2006; 73(14):149902-149902.
  • Physical Review B. 01/2006; 73(14):-.