Showing papers by "M. B. Maple published in 1991"

Basic similarities among cuprate, bismuthate, organic, Chevrel-phase, and heavy-fermion superconductors shown by penetration-depth measurements.

Abstract: Muon-spin-relaxation and bulk measurements of the magnetic-field penetration depth suggest that the cuprate high-${\mathit{T}}_{\mathit{c}}$, bismuthate, organic Chevrel-phase, and heavy-fermion systems possibly belong to a unique group of superconductors characterized by high transition temperatures ${\mathit{T}}_{\mathit{c}}$ relative to the values of ${\mathit{n}}_{\mathit{s}}$/${\mathit{m}}^{\mathrm{*}}$ (carrier density/effective mass). This feature distinguishes these exotic superconductors from ordinary BCS superconductors.

Evidence for non-Fermi liquid behavior in the Kondo alloy Y1-xUxPd3.

Abstract: We present measurements of electrical resitivity \ensuremath{\rho}(T), specific heat C(T), entropy S(T), and magnetic susceptibility \ensuremath{\chi}(T) as functions of temperature T for the alloy system ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{U}}_{\mathit{x}}$${\mathrm{Pd}}_{3}$. For x=0.2, \ensuremath{\rho}(T)/\ensuremath{\rho}(0) is almost linear in T for 0.2\ensuremath{\le}T\ensuremath{\le}20 K, and C(T)/T\ensuremath{\sim}-ln(\ensuremath{\alpha}T) for 0.6T16 K. Also, the added molar entropy per U, \ensuremath{\Delta}S(T)-\ensuremath{\Delta}S(0) appears to saturate to (R/2)ln(2), suggesting that \ensuremath{\Delta}S(0)\ensuremath{\approxeq}(R/2)ln(2). We argue that our data provide comprehensive evidence in a dilute alloy for the two-channel quadrupolar Kondo effect (${\mathit{T}}_{\mathit{K}}$\ensuremath{\approxeq}42 K) with concomitant ``marginal Fermi liquid'' phenomenology.

Massive electron state in YbBiPt.

Abstract: Measurements of the electrical resistivity, magnetoresistance, ac and dc magnetic susceptibility and specific heat on YbBiPt indicate this compound to be a IveryP heavy-electron system. The low-temperature Sommerfeld coefficient of 8 J/K2 (mole Yb) is not affected by a phase transition at 0.4 K. We suggest that the heavy-mass state in YbBiPt is unconventional in that it develops from Bloch states in an electronic subsystem with low carrier concentration. © 1991 The American Physical Society.

Electronic structure studies on the n-type doped superconductors R 2-x M x CuO 4-δ (R=Pr,Nd,Sm; M=Ce,Th) and Nd 2 CuO 4-x F x by electron-energy-loss spectroscopy

Abstract: The electronic structure of the high-${\mathit{T}}_{\mathit{c}}$ superconductors ${\mathit{R}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (R=Pr, Nd, Sm; M=Ce, Th) and ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{38}$${\mathrm{F}}_{02}$ has been studied by high-energy electron-energy-loss spectroscopy in transmission Core-level spectroscopy, particularly on O 1s and Cu 2p levels yields information on the partial unoccupied density of states near the Fermi level, while that on R and Ce 3d and 4d levels may serve as a valence monitor for these constituents A pre-edge structure in the O 1s spectrum is polarized in the ${\mathrm{CuO}}_{2}$ plane and assigned to transitions into a two-dimensional conduction band consisting of Cu 3d hybridized to O 2p states In the Cu 3${\mathit{d}}^{9}$ configuration we found a slight admixture (\ensuremath{\sim}10%) of Cu 3${\mathit{d}}_{3\mathit{z}}^{2}$-${\mathit{r}}^{2}$ to the Cu 3${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ hole states similar to the p-type doped superconductors Upon doping we recognized a reduction (\ensuremath{\sim}14%) of the excitonic Cu 2${\mathit{p}}_{3/2}$ line, indicating that the introduced electrons go onto the Cu sites, thus producing charge carriers having predominantly Cu 3d character

Superconductivity and magnetism in Eu 1 − x Pr x Ba 2 Cu 3 O 7 − δ

Abstract: Measurements of the electrical resistivity \ensuremath{\rho} as a function of temperature T, and low-temperature specific heat C as a function of T and magnetic field H, have been performed for Pr concentrations 0\ensuremath{\le}x\ensuremath{\le}1 in the ${\mathrm{Eu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ system (\ensuremath{\delta}\ensuremath{\approxeq}0.05). The superconducting critical temperature ${\mathit{T}}_{\mathit{c}}$(x) in the metallic phase (0\ensuremath{\le}x\ensuremath{\lesssim}${\mathit{x}}_{\mathit{c}}$\ensuremath{\approxeq}0.5) was determined from the resistivity \ensuremath{\rho}(x,T), while the N\'eel temperature ${\mathit{T}}_{\mathit{N}}$(x) in the insulating phase (${\mathit{x}}_{\mathit{c}}$\ensuremath{\lesssim}x\ensuremath{\le}1) was inferred from specific-heat anomalies due to antiferromagnetic (AFM) ordering of the Pr ions. The effect of ``chemical'' pressure on ${\mathit{T}}_{\mathit{c}}$(x) in the ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ system is opposite to that of applied hydrostatic pressure. The Pr contributions to the specific heat in the insulating phase have the form C(T)=${\mathit{MT}}^{3}$ for T${\mathit{T}}_{\mathit{N}}$, characteristic of three-dimensional AFM magnons. In contrast, the Pr contribution to C(T) in the metallic phase takes the form of a broad anomaly that can be described by a spin-1/2 Kondo model. The entropy associated with the Pr specific-heat anomalies in both the metallic and insulating phases is close to R ln2, indicative of a doublet ground state for Pr in the crystalline electric field and a Pr valence close to 4+.

Low-temperature specific heat of the Y1-xPrxBa2Cu3O7- delta system.

Abstract: Low-temperature specific-heat {ital C} measurements as a function of temperature {ital T} between 0.5 and 30 K are reported for Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} compounds with {ital x}=0, 0.2, 0.3, 0.4, 0.6, 0.8, and 1.0. The Pr contribution to the specific heat of all of the samples can be described as the sum of a Pr nuclear Schottky anomaly of the form {ital C}{sub {ital N}}({ital T})={ital AT}{sup {minus}2}, a linear term {ital C}{sub {ital L}}({ital T})={gamma}{ital T}, and a Pr magnetic anomaly. For compounds with {ital x}{le}0.6, which are metallic and superconducting, the Pr magnetic specific-heat anomaly has the same temperature dependence as a Kondo anomaly. For the compounds with {ital x}{ge}0.8, which are insulating, the magnetic specific-heat anomaly is consistent with antiferromagnetic ordering of the Pr ions with Neel temperatures {ital T}{sub {ital N}} of 10.9 and 15.7 K for {ital x}=0.8 and 1.0, respectively. For {ital T}{lt}{ital T}{sub {ital N}}, the antiferromagnetic specific-heat anomaly has the form {ital C}{sub {ital M}}({ital T})={ital MT}{sup 3}, characteristic of three-dimensional antiferromagnetic magnons.

Evidence for a time-dependent crossover from surfacelike to bulklike flux relaxation in YBa 2 Cu 3 O 7 − δ

Abstract: dc flux-relaxation experiments were performed on single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ at low magnetic fields. A sharp, time-dependent crossover in the logarithmic magnetization decay rate was observed in two different crystals. Consideration of the size dependence of the relaxation rate in one of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals suggests that the crossover in the relaxation rate is due to a transition from surface-controlled to bulk-controlled relaxation behavior. Our observations seem to indicate that, over a certain range of temperatures and magnetic fields, surface pinning may play a significant role in the flux-relaxation behavior of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.

Anomalies in the irreversibility line and upper critical field of the electron-doped superconductor Sm1.85Ce0.15CuO4−y

Abstract: We report the observation of anomalies in the irreversibility line H ( T ∗ ) and resistive upper critical field H c2 ( T ) for two different samples of the electron-doped superconductor Sm 1.85 Ce 0.15 CuO 4− y , a specimen consisting of magnetically aligned single-crystal grains and a single crystal. The irreversibility line conforms to a power a law dependence in the H-T plane with an exponent that changes at T ∗ T c ≅0.5 and H ≅1 kOe m ≅1.8 in low fields to m ≅4.2 in high fields, for applied magnetic fields parallel to the c -axis. These observations may indicate two different temperature and/or field dependences for the pinning potential. A single value of m =1.6 is observed for the exponent of the irreversibility line for H ⊥ c in the field and temperature range studied. The resistively determined H c2 ( T ) data taken with H ‖ c show the same power law behavior, with a change in the value of the exponent at T T c ≅0.5 and H ≅10 kOe from m =1.6 in low fields to m =2.4 in high fields. For H ⊥ c , the H c2 ( T ) data are characterized by a single exponent m =1.4 in the field and temperature range studied.

“Irreversibility Line” and magnetic relaxation in a Sm1.85Ce0.15CuO4−y single crystal

Abstract: We report on the static and dynamic magnetic behavior of a Sm 1.85 Ce 0.15 CuO 4− y single crystal for fields ranging from 20 Oe to 6 kOe. The irreversibility line, extracted from static magnetization versus temperature measurements in different magnetic fields H , shows power law behavior with an exponent that changes at ∼ 1 koe from 1.7 for fields H ⩽1 kOe to 3.5 for H ⩾ 1 kOe. The magnetic relaxation study shows that for values of the applied field greater than 600 Oe, an extrapolation of the normalized relaxation rate S n to zero temperature gives a non-zero value. Thismay be indicative of nonthermally activated relaxation processes. For 1 kOe, S n versus temperature exhibits two peaks, a possible indication of the presence of two pinning mechanisms. A distribution of activation energies for the pinning barriers was inferred to explain the experimental observations. The distribution functions were calculated from magnetic relaxation data taken in different applied magnetic fields.

The effect of strong flux line correlations on the irreversibility line of high temperature superconductors

Abstract: A phenomenological model is proposed to account for a recently observed crossover in the temperature dependence of the irreversibility line of high temperature superconductors from a (1 - T/Tc) 3 2 to a (1 - T/Tc)3 dependence. The model, which distinguishes between weak correlations and strong correlations in the flux line lattice, indicates that the crossover should occur at magnetic fields corresponding to a flux line spacing of about λ/4, λ being the penetration depth transverse to the field.

Flux pinning in Y1−xPrxBa2Cu3O7−δ high Tc superconductors

Abstract: Magnetic relaxation measurements have been made on two sets of samples of Y1−xPrxBa2Cu3O7−δ high Tc superconductors with 0≤x≤0.10. The samples were prepared as fibers using a sol‐gel process and as polycrystalline pellets using a standard solid‐state reaction technique. In an applied field of 5 kOe at 30 K, the fluxoid pinning potential U was observed to increase approximately linearly with Pr concentration x at the rate dU/dx≊1 eV. A mechanism for the enhancement of U may be the suppression of the superconducting order parameter in the vicinity of the Pr ions due to magnetic pair breaking.

Anisotropic superconducting and normal state properties of the electron-doped Ln2-xCexCuO4-y single crystals (Ln = Nd and Sm)

Abstract: Although previous resistive measurements of the superconducting transition under applied magnetic field on a Sm1.85Ce0.15CuO4-y single crystal did not show much dissipation effects, magnetic relaxation data on another superconducting single crystal yield a small flux pinning energy, of the order of 20 meV at T = 6 K when H⌈c, similar to hole-doped superconductors. In the electron-doped compounds, the normal-state resistivity, which exhibits substantial anisotropy, does not follow a linear temperature dependence, but rather a T2 law up to about 200 K, consistent with Fermi-liquid theory for electron-electron scattering. In superconducting Nd2-xCexCuO4-y single crystals, both in-plane and out-of- plane resistivities exhibit metallic behavior with T2 dependence.

Upper critical magnetic field of the heavy-electron superconductors U1-xThxBe13 (x=0 and 2.9%) doped with paramagnetic Gd and other rare-earth ions.

Abstract: The temperature T dependence of the upper critical magnetic field ${\mathit{H}}_{\mathit{c}2}$ of the heavy-electron superconductors ${\mathrm{U}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Th}}_{\mathit{x}}$${\mathrm{Be}}_{13}$ (x=0 and 2.9%) doped with various concentrations of Gd has been determined from low-frequency ac magnetic susceptibility measurements in magnetic fields up to 60 kOe. The ${\mathit{H}}_{\mathit{c}2}$(T) curves for ${\mathrm{U}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Gd}}_{\mathit{x}}$${\mathrm{Be}}_{13}$ samples deviate from the ${\mathit{H}}_{\mathit{c}2}$(T) curves of ${\mathrm{UBe}}_{13}$ near ${\mathit{T}}_{\mathit{c}}$, which is consistant with the depairing of superconducting electrons via the Zeeman interaction between the spins of the superconducting electrons and the exchange field associated with the Gd spins. This suggests that ${\mathrm{UBe}}_{13}$ exhibits singlet superconductivity. In contrast, the ${\mathit{H}}_{\mathit{c}2}$(T) curves for (${\mathrm{U}}_{0.97}$${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$ doped with Gd scale with ${\mathit{H}}_{\mathit{c}2}$(T) of pure (${\mathrm{U}}_{0.97}$${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$ and do not reflect superconducting depairing by the Gd ions. These results are consistent with either strong spin-orbit scattering due to the presence of Th in ${\mathrm{UBe}}_{13}$, or to a qualitatively different type of superconductivity involving triplet spin pairing in (${\mathrm{U}}_{0.97}$${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$. Measurements of the temperature dependence of ${\mathit{H}}_{\mathit{c}2}$ for ${\mathrm{U}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{R}}_{\mathit{x}}$${\mathrm{Be}}_{13}$ compounds where R=La, Lu, and Ce for various compositions x as well as ${\mathrm{U}}_{0.985}$${\mathit{R}}_{0.015}$${\mathrm{Be}}_{13}$ compounds for R=Tb, Dy, Ho, and Er are also presented and compared with the Gd-doped ${\mathrm{UBe}}_{13}$ system. The results of low-temperature specific-heat measurements of ${\mathrm{UBe}}_{13}$ doped with various concentrations of Gd are also discussed.

Shock consolidation of crystallographically aligned Bi2Sr2CaCu2O8 powders

Abstract: Bulk samples of crystallographically aligned Bi 2 Sr 2 CaCu 2 O 8 were fabricated by shock-consolidation of tapped micaceous powders. Basal plane alignment was verified by means of optical microscopy, X-ray diffraction and magnetic measurements. We have explored the dependence of this crystallographic alignment as a function of both shock pressure (from 35 kbar to 135 kbar) and powder particle size (from

Magnetic and structural phase transitions in UAu2Si2

Abstract: We report magnetic susceptibility, neutron, and x‐ray diffraction measurements on polycrystalline UAu2Si2. Magnetic susceptibility data show the existence of two phases below 50 K. Since no superlattice neutron peaks were observed within the sensitivity of the apparatus we conclude that the system is not a canted ferromagnet, as was suggested earlier. On the basis of the temperature dependence of the neutron and x‐ray intensity we discuss a possibility that UAu2Si2 undergoes two structural phase transitions, at 48 and at 18 K and that these transitions are correlated with the magnetic state of the material.

μSR studies of heavy fermion systems

Abstract: We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe13, URu2Si2 and U6Fe. The magnetic field penetration depth in the superconducting state of UBe13 is greater than 10000 A, as no increase in the transverse field relaxation rate is observed belowTc. A sharp increase in the precession frequency is seen, starting atTc. This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U6Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atTc=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.

Changes in the Pr-induced TC depression of 123 compounds by chemical pressure

Abstract: We present structural data refined from X-ray spectra for R 1−x Pr x Ba 2 Cu 3 O 7−δ (R=Er, Gd, Eu, Sm, Nd) systems with x=0, 0.1, and 0.4. The evolution of T c with the ionic radius of the rare earth for different values of x can be related to the Cu2O4 distance. The effect of chemical pressure on T c in R 1−x Pr x Ba 2 Cu 3 O 7−δ systems is opp osite to that of hydrostatic pressure on the Y 1−x Pr x Ba 2 Cu 3 O 7−δ system.

Evidence for unconventional superconductivity in single crystals of the antiferromagnetic heavy-electron compound URu2Si2

Abstract: Specific heat C(T) and upper critical field Hc2(T) measurements have been performed on two single crystal specimens (denoted A and B) of the antiferromagnetic heavy-electron superconductor URu2Si2. Specific heat measurements on both single crystals reveal two distinct jumps, indicative of two superconducting phases. This is reminiscent of the antiferromagnetic heavy-electron compound UPt3 in which multiple superconducting transitions have been observed and attributed to coupled antiferromagnetic and multicomponent superconducting order parameters, although two superconducting phases associated with two different states of the crystal cannot be ruled out. The relative magnitudes of the two specific heat jumps observed in crystal B suggest that the two superconducting phases occupy nearly equal volume fractions of the crystal. At temperatures below the lower jump, the specific heat of both crystals can be described by C3(T) = γs(0)T+AT3 with γ3(0) ≈ 0.5γn(0), where γn(0) is the value of the normal-state electronic specific heat coefficient γn(T), extrapolated to T = 0. The two critical temperatures inferred from the specific heat jumps in crystal B, measured in magnetic fields H between 0 and 15kOe applied parallel to the c-axis, have a similar dependence on H. Resistive measurements of Hc2(T) on specimens from crystal A between 0 and 60 kOe reveal a kink near 2 kOe for H⌈c and strong positive curvature in Hc2(T) below ∼3 kOe for H⌈a.

Correlations between T c and n s / m * (carrier density/ effective mass) in high- T c and organic superconductors

Abstract: We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenT c and the relaxation rate σ αn s/m * (carrier density/effective mass) of hole-doped high-T c cuprate superconductors (La2, Sr3)CuO4, YBa2Cu3O7 (Y1−xPrx)Ba2Cu3O7, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)2Cu(NCS)2.

Oscillatory Oxidation of CO Over Pt at Pressures from 10 to 760 Torr

Abstract: Oscillations in the rate of CO oxidation over polycrystalline Pt have been measured at total pressures of 760 and 10 Torr. Despite the pressure change of two orders of magnitude, oscillations occurred between similar values of high and low reactivity, and over roughly the same experimental parameters. The lack of dependence of the reaction kinetics on the pressure is not predicted by the Langmuir-Hinshelwood equations that provide the rate multiplicities underlying the oscillations in a number of recent models. This lack of agreement suggests that the current understanding of CO oxidation and oscillatory reaction kinetics remains incomplete. The primary difference between the oscillations at the two pressures occurred in the waveforms, which were highly structured and fractal-like at P = 760 Torr, and became simple square waves on the evolved catalyst surface at P = 10 Torr. Differences in the catalyst surface structure and in coupling between parts of the catalyst through concentration gradients in the gas may account for these changes in the complexity of the waveforms.

Recent Topics of μSR Studies on High- T c Systems

Abstract: History of the muon spin relaxation (μSR) measurements dates back to the late 1950’s and 60’s following the discovery of parity violation The application of μSR to condensed matter physics has steadily developed1 in the 70′s and early 80′s; mainly in the study of magnetic properties of ferro- or antiferromagnets and spin glasses Extensive μSR studies on high-Tc systems2–4 have increased the recognition of this technique as one of the very powerful experimental methods in the study of magnetism and superconductivity

Magnetism in electron-doped copper oxides

Abstract: We report muon spin relaxation/rotation measurements on sintered powder samples of Nd2−x Ce x CuO4−y and a large single crystal of Nd2CuO4−y We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La2−x Sr x CuO4−y , although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins Static magnetic order in Nd2CuO4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials

Anisotropic pressure dependence of the critical temperature of REBa2Cu3O7−δ

Abstract: The hydrostatic pressure dependence of the superconducting critical temperature, T c , was measured in highly crystalline oriented REBa 2 Cu 3 O 7−δ (RE=Yb, Dy, Y, Gd) thin films grown on different substrates. The anisotropic strain field induced by the substrate and the different crystalline orientations of the films allows the three strain derivatives of the critical temperature to be estimated. A large anisotropy is found in the ab plane with T c increasing on compression parallel to the CuO chains and decreasing on compression perpendicular to them. The results on c textured films indicate a non linear relation between T c and the microscopic parameter which determines its pressure dependence.

Muon spin relaxation in the heavy fermion system UPt3

Abstract: We report muon spin rotation/relaxation (μSR) measurements of the heavy fermion superconductor UPt3 in external fields Hext∥c. We find that the muon Knight shift is unchanged in the superconducting state, consistent with odd‐parity pairing (such as p wave). The transverse field relaxation is observed to be strongly field dependent, decreasing with increasing field. Below Tc the increase is barely detectable in an applied field of 4 kG∥c. On the basis of the high field measurements, we estimate the low temperature penetration depth to be λ(T→0)≥ (R18)11 000 A.