R. A. Fisher

Lawrence Berkeley National Laboratory, Berkeley, California, United States

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Publications (269)512.27 Total impact

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    ABSTRACT: We report measurements of the specific heat of two samples of carbon-doped MgB2, Mg(B1−xCx)2, x = 0.05 and 0.1, in magnetic fields to μ0H = 9 T and at temperatures from ∼1 K to somewhat above the critical temperature for superconductivity for each sample. The carbon doping reduced the critical temperature from 39 K for MgB2 to 31.4 K and 19.7 K for the x = 0.05 and 0.1 samples, respectively. The results give the electron–phonon coupling and the electron density of states, including the individual contributions of the π and σ bands. These quantities are compared with theoretical calculations. The results also give the energy gaps on the π and σ bands, which are compared with other experimental determinations, and also with theoretical calculations that include predictions of the “merging” of the two gaps as a consequence of the band filling and increased interband scattering associated with doping.
    Physica C Superconductivity 02/2013; 485:168–176. · 0.72 Impact Factor
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    ABSTRACT: GeZn2O4 synthesized at ambient pressure adopts the rhombohedral phenacite crystal structure, whereas cubic or tetragonal inverse spinel phases are formed at high pressures. We have measured the thermal expansion for all three forms of GeZn2O4 at temperatures from 10 K to 400 K (or higher) using synchrotron x-ray powder diffraction. The phenacite form exhibits negative thermal expansion below 300 K, changing to positive thermal expansion above that temperature. In contrast to this behavior, the cubic and tetragonal inverse spinel phases exhibit positive thermal expansion below room temperature. Characterization of these materials using x-ray and neutron diffraction, as well as heat capacity and Raman spectroscopy, will be described. Possible structural reasons for the different thermal expansion behaviors of the phenacite and spinel forms of GeZn2O4 will be discussed.
    03/2011;
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    ABSTRACT: Materials exhibiting geometrical magnetic frustration have been very topical in condensed matter physics due to their large ground state degeneracy usually leading to a great variety of behavior. GCO and GNO are spinels where the Co/Ni ions form a sublattice identical to the pyrochlore. This topology naturally leads to magnetic frustration which can be relieved to permit the appearance of longrange magnetic order. GCO has a N'eel temperature of 21 K, below which a tetragonal distortion is observed. However, our heat capacity measurements show only about half of the entropy expected when integrated up to 75 K. This strongly suggests the existence of frustration which is evidenced by our neutron data where strong diffuse scattering (DF) is observed. GNO also exhibits strong DF at the same q-position but with different shape. We will present the neutron data alongside a Monte-Carlo analysis of the DF and the implications on the nature and strength of the different interactions in these two systems.
    03/2011;
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    ABSTRACT: We report resonant ultrasound spectroscopy (RUS), dilatometry/magnetostriction, magnetotransport, magnetization, specific heat, and $^{119}$Sn M\"ossbauer spectroscopy measurements on SnTe and Sn$_{0.995}$Cr$_{0.005}$Te. Hall measurements at $T=77$ K indicate that our Bridgman-grown single crystals have a $p$-type carrier concentration of $3.4 \times 10^{19}$ cm$^{-3}$ and that our Cr-doped crystals have an $n$-type concentration of $5.8 \times 10^{22}$ cm$^{-3}$. Although our SnTe crystals are diamagnetic over the temperature range $2\, \text{K} \leq T \leq 1100\, \text{K}$, the Cr-doped crystals are room temperature ferromagnets with a Curie temperature of 294 K. For each sample type, three-terminal capacitive dilatometry measurements detect a subtle 0.5 micron distortion at $T_c \approx 85$ K. Whereas our RUS measurements on SnTe show elastic hardening near the structural transition, pointing to co-elastic behavior, similar measurements on Sn$_{0.995}$Cr$_{0.005}$Te show a pronounced softening, pointing to ferroelastic behavior. Effective Debye temperature, $\theta_D$, values of SnTe obtained from $^{119}$Sn M\"ossbauer studies show a hardening of phonons in the range 60--115K ($\theta_D$ = 162K) as compared with the 100--300K range ($\theta_D$ = 150K). In addition, a precursor softening extending over approximately 100 K anticipates this collapse at the critical temperature, and quantitative analysis over three decades of its reduced modulus finds $\Delta C_{44}/C_{44}=A|(T-T_0)/T_0|^{-\kappa}$ with $\kappa = 0.50 \pm 0.02 $, a value indicating a three-dimensional softening of phonon branches at a temperature $T_0 \sim 75$ K, considerably below $T_c$. We suggest that the differences in these two types of elastic behaviors lie in the absence of elastic domain wall motion in the one case and their nucleation in the other.
    Physical review. B, Condensed matter 11/2010;
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    ABSTRACT: An extensive calorimetric study of the normal- and superconducting-state properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x > 0.06), which illustrates the strong competition between magnetism and superconductivity to monopolize the Fermi surface in the underdoped region and the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All superconducting samples exhibit a residual electronic density of states of unknown origin in the zero-temperature limit, which is minimal at optimal doping but increases to the normal-state value in the strongly under- and over-doped regions. The remaining specific heat in the superconducting state is well described using a two-band model with isotropic s-wave superconducting gaps. Comment: Submitted to Europhysics Letters
    EPL (Europhysics Letters) 07/2010; · 2.26 Impact Factor
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    ABSTRACT: The cubic spinel GeNi2O4, in which the spin-1 Ni^2+ ions are located at the vertices of a lattice of corner-sharing tetrahedra, exhibits interesting magnetic properties. For example, GeNi2O4 has two N'eel transitions (TN1 = 12.1 K and TN2 = 11.4 K) that are not associated with any obvious structural phase transition. In the past we have reported the results of magnetic susceptibility, heat capacity, synchrotron x-ray powder diffraction, and neutron powder diffraction measurements for this material. In this talk we will describe the results of diffuse magnetic scattering measurements of the magnetic correlations in GeNi2O4 made at temperatures above and below TN1 and TN2. The samples were single crystals grown by the traveling solvent floating zone technique at AIST. The measurements were made using the PRISMA spectrometer at ISIS and a triple-axis spectrometer at NIST.
    03/2010;
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    ABSTRACT: We propose a model which suggests that structural martensitic transitions are related to significant changes in the electronic structure, and are effected by high-magnetic fields. The magnetic field dependence is considered unusual as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons. We provide a theoretical framework which can be used to describe the effect of high magnetic field on the transition and lattice dynamics in which the field dependence originates from the dielectric constant. The model is compared with some recent experimental results.
    Journal of Physics Conference Series 02/2010; 200(3):032062.
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    ABSTRACT: We report on the determination of the electronic heat capacity of a slightly overdoped (x=0.075) Ba(Fe1−xCox)2As2 single crystal with a Tc of 21.4 K. Our analysis of the temperature dependence of the superconducting-state specific heat provides strong evidence for a two-band s-wave order parameter with gap amplitudes 2Δ1(0)/kBTc=1.9 and 2Δ2(0)/kBTc=4.4.
    Physical review. B, Condensed matter 01/2010; 81(6).
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    ABSTRACT: Ferroelectric ceramic materials have a wide range of applications because of their piezoelectric and pyroelectric properties. One of their most important physical properties is the specific heat. In this study, the specific heats of a series of lead-zirconate-titanate (PZT) compositions in the vicinity of the morphotropic phase boundary (MPB) were measured. The temperature range was from 1.8 to 300 K. It is believed that these are the lowest temperature measurements ever made on PZT. Differences between the specific heats of the different compositions were very small. However, the calculated Debye temperatures were slightly different. The results are useful in computing design parameters for technical devices.
    01/2010;
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 32(37).
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    ABSTRACT: Three samples of Na0.3CoO2⋅1.3H2O that differed in sample age showed different ordering: after three and five days, superconductivity with T c∼4.5 K but different superconducting condensates; after 40 days, no superconductivity but charge-density wave order at T CDW∼7 K. A pair-breaking action that progresses with sample age and acts preferentially in one of two electron bands produces the changes in the superconducting condensate, and ultimately destroys the superconductivity. Theoretical calculations predicted the charge-density wave transition.
    Journal of Superconductivity and Novel Magnetism 03/2009; 22(3):295-298. · 0.70 Impact Factor
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    ABSTRACT: Na0.3CoO2 1.3H2O progressively changes its properties with ambient temperature ageing from two-gap, layered superconductivity to charge-density-wave order as the number of O vacancies in the layers increase. The charge-density-wave order was theoretically predicted on the basis of band-structure calculations, independently of knowledge of the specific-heat measurements.
    Journal of Physics Conference Series 03/2009; 150(5):052210.
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    ABSTRACT: We report ambient-pressure magnetization, heat capacity, and thermal-expansion measurements of the ferromagnetic superconductor UGe2 in high magnetic fields. An analysis of the magnetic heat capacity derived from both magnetization and specific-heat data shows that UGe2 is well described in the framework of the molecular-field theory. Our heat-capacity and thermal-expansion results reveal a clear crossover regime, a feature that illustrates the proximity to the quantum critical end point of a first-order boundary between two different ferromagnetic phases. Furthermore, we show that the ferromagnetic contribution to these thermodynamic quantities can be split into two terms with distinct Grüneisen parameters.
    Physical review. B, Condensed matter 01/2009; 80(17).
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    ABSTRACT: Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au0.52Zn0.48. Elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q =(1.33,0.67,0) at temperatures corresponding to each sample's transition temperature (TM = 64 and 45 K, respectively). Although the new Bragg peaks appear in a discontinuous manner in the Au0.52Zn0.48 sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near TM are in favorable accord with a continuous transition. A fit to the pressure dependence of TM suggests the presence of a critical end point in the AuZn phase diagram located at TM* = 2.7 K and p* = 3.1 GPa.
    Physical Review Letters 10/2008; 101(13):135703. · 7.94 Impact Factor
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    ABSTRACT: The specific heat of Na0.3CoO21.3H2O gives evidence of two electron bands with comparable densities of states and different superconducting-state energy gaps. A non-magnetic pair-breaking action, which acts preferentially in the band with the smaller gap, progresses with sample age. The two bands and non-magnetic pair breaking have implications for possible pairing mechanisms; the effects of the pair breaking constitute the "sample dependence" of the properties of this material. In concert with structural changes reported by others, the specific heat suggests dual roles for O vacancies —tuning the carrier concentration to favor superconductivity, and pair breaking to destroy it.
    EPL (Europhysics Letters) 05/2008; 82(4):47011. · 2.26 Impact Factor
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    ABSTRACT: Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity, and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni2MnGa over the temperature range 100<T<250 K. All measurements detect clear signatures of the premartensitic transition (T(PM) approximately 247 K) and the martensitic transition (T(M) approximately 196 K). Temperature-dependent UPS shows a dramatic depletion of states (pseudogap) at T(PM) located 0.3 eV below the Fermi energy. First-principles electronic structure calculations show that the peak observed at 0.3 eV in the UPS spectra for T>T(PM) is due to the Ni d minority-spin electrons. Below T(M) this peak disappears, resulting in an enhanced density of states at energies around 0.8 eV. This enhancement reflects Ni d and Mn d electronic contributions to the majority-spin density of states.
    Physical Review Letters 04/2008; 100(16):165703. · 7.94 Impact Factor
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    ABSTRACT: The pairing interaction leading to the formation of the Cooper pairs remains unidentified in the ferromagnetic superconductor UGe2. Nevertheless, there is strong experimental evidence that superconductivity is not mediated by the magnetic fluctuations that drive TCurie (p) to zero; it rather appears closely related to another phase boundary Tx (p) that occurs at lower pressure. Theoretical works suggested that this additional phase boundary could arise either from a coupling between SDW and CDW orderings or from a peak in the electronic density of states. Although the existence of this anomaly is experimentally incontestable between 0.6 and 1.2 GPa, the situation at ambient pressure remains ambiguous. We discuss the aforementioned scenarios in the light of recent high-resolution thermal expansion and calorimetric measurements realized under high magnetic fields at ambient pressure.
    03/2008;
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    ABSTRACT: Specific-heat and magnetic-susceptibility measurements are reported for the polycrystalline spinel compounds GeNi2O4 and GeCo2O4 in magnetic fields up to 14 T and 0.5K
    Physical review. B, Condensed matter 01/2008; 78(10).
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    ABSTRACT: For most superconducting samples of Na0.35CoO2.1.3H2O the transition to the superconducting state occurs near 4.5 K. For some non-superconducting samples a different transition, near 7 K, shows the existence of another ordering that competes with the electron pairing of the superconducting state. Specific-heat measurements in zero field and 9 T show that the 7-K transition is essentially independent of field, which is suggestive of a CDW transition. The specific-heat anomaly is consistent with a CDW on 1/4 of the Fermi surface and an order parameter with a temperature dependence similar to that of the BCS transition. A theoretical study using a band-structure fit to ARPES data for Na0.3CoO2 supports the presence of CDW order. Under renormalization group flow an onsite plus nearest-neighbor Hubbard interaction leads to an effective low-energy electron-electron interaction containing scattering processes that favor a CDW with waves of period 3^.5ex1-.1em/ -.15em.25ex2 a, where a is the lattice constant. A mean-field analysis confirms that this effective low-energy interaction can lead to real-space density modulations with period 3^.5ex1-.1em/ -.15em.25ex2 a.
    03/2007;
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    ABSTRACT: Specific-heat measurements on three samples of Na0.35CoO2.1.3H2O show an evolution of the superconductivity and its eventual disappearance with increasing sample age. The results, in combination with other recent work [1], provide a basis for understanding the extreme ``sample dependence'' of the properties of this material. Samples of different age are in effect samples of slightly different materials: A non-magnetic pair-breaking action produces a residual density of electron states that increases with sample age. It occurs preferentially in the electron band (one of two with different energy gaps) with the smaller gap, producing a change in the nature of the superconducting condensation. It also weakens the overall electron pairing of the superconducting state until it gives way to a competing ordering, possibly a CDW. The same combinations of features in the specific heat have been seen in measurements on other individual samples, showing that they are ``intrinsic''. The changes in the specific heat are evidently related to structural and electronic changes that occur on a similar time scale [1], and include an increasing concentration of O vacancies, which could be the pair-breaking scattering centers. [1] P. W. Barnes et al., Phys. Rev. B 72, 134515 (2005).
    03/2007;

Publication Stats

606 Citations
512.27 Total Impact Points

Institutions

  • 1987–2013
    • Lawrence Berkeley National Laboratory
      • Geochemistry Department
      Berkeley, California, United States
    • University of Geneva
      • Department of Condensed Matter Physics
      Genève, GE, Switzerland
  • 2008–2010
    • Los Alamos National Laboratory
      • Materials Science and Technology Division
      Los Alamos, California, United States
  • 1962–2009
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, CA, United States
  • 2002
    • Florida State University
      • Department of Physics
      Tallahassee, FL, United States
    • University of California, Davis
      • Department of Geology
      Davis, California, United States
  • 1988–2002
    • Amherst College
      • Department of Physics
      Amherst Center, Massachusetts, United States
  • 2001
    • CSU Mentor
      Long Beach, California, United States
  • 1991
    • French National Centre for Scientific Research
      • Institut Néel
      Lutetia Parisorum, Île-de-France, France