Showing papers by "M. B. Maple published in 2006"

Specific heat and ac susceptibility studies of the superconducting phase diagram of Pr Os 4 Sb 12

Abstract: We report on ac-susceptibility and heat-capacity measurements of the superconductor $\mathrm{Pr}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ in magnetic fields. The resulting phase diagram reveals two distinguishable superconducting phases with transitions at the upper critical field ${B}_{c2}$ and a slightly lower field ${B}_{c2}^{*}$. Between ${B}_{c2}^{*}$ and $0.7\ifmmode\times\else\texttimes\fi{}{B}_{c2}^{*}$ the ac-susceptibility data shows a region with enhanced pinning properties characterized by an extended peak effect. The heat-capacity data reveal an extremely strongly coupled superconductivity with a considerable contribution of heavy quasiparticles. This unusual strong-coupling behavior originates in a sharp increase of the superfluid density at ${T}_{c}$. The decrease of the discontinuity of the specific heat at ${T}_{c}$ and the corresponding pronounced increase of the Ginzburg-Landau-Maki parameter ${\ensuremath{\kappa}}_{2}$ indicate that the superconductivity is most probably not Pauli limited in a large temperature range.

Multiple ordered phases in the filled skutterudite compound PrOs4As12

Abstract: Magnetization, specific heat, and electrical resistivity measurements were made on single crystals of the filled skutterudite compound PrOs{sub 4}As{sub 12}. Specific heat measurements indicate an electronic specific heat coefficient {gamma} {approx} 50-200 mJ/mol K{sup 2} at temperatures 10 K {le} T {le} 18 K, and {approx} 1 J/mol K{sup 2} for t {le} 1.6 K. Magnetization, specific heat, and electrical resistivity measurements reveal the presence of two, or possibly three, ordered phases at temperatures below {approx} 2.3 K and in fields below {approx} 3 T. The low temperature phase displays antiferromagnetic characteristics, while the nature of the ordering in the other phase(s) has yet to be determined.

Field-dependent ordered phases and Kondo phenomena in the filled skutterudite compound PrOs4As12

Abstract: Electrical resistivity, specific heat, and magnetization measurements to temperatures as low as 80 mK and magnetic fields up to 16 T were made on the filled skutterudite compound PrOs4As12. The measurements reveal the presence of two ordered phases at temperatures below approximately 2.3 K and in fields below approximately 3 T. Neutron-scattering experiments in zero field establish an antiferromagnetic ground state <2.28 K. In the antiferromagnetically ordered state, the electronic-specific heat coefficient γ ≈ 1 J/mol·K2 below 1.6 K and 0 ≤ H ≤ 1.25 T. The temperature and magnetic-field dependence of the electrical resistivity and specific heat in the paramagnetic state are consistent with single-ion Kondo behavior with a low Kondo temperature on the order of 1 K. The electronic-specific heat in the paramagnetic state can be described by the resonance-level model with a large zero-temperature electronic-specific heat coefficient that decreases with increasing magnetic field from approximately 1 J/mol·K2 at 3 T to approximately 0.2 J/mol·K2 at 16 T.

Heavy fermion fluid in high magnetic fields: an infrared study of CeRu4Sb12.

Abstract: We report a comprehensive infrared magnetospectroscopy study of a CeRu4Sb12 compound revealing quasiparticles with a heavy effective mass m*, with a detailed analysis of optical constants in fields up to 17 T. We find that the applied magnetic field strongly affects the low-energy excitations in the system. In particular, the magnitude of m* approximately = 70 m(b) (m(b) is the quasiparticle band mass) at 10 K is suppressed by as much as 25% at 17 T. This effect is in quantitative agreement with the mean-field solution of the periodic Anderson model augmented with a Zeeman term.

Structure-dependent ferromagnetism in Au 4 V studied under high pressure

Abstract: At ambient pressure, ordered ${\mathrm{Au}}_{4}\mathrm{V}$ is metallic and displays ferromagnetism at ${T}_{C}=45\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The electrical resistivity has been measured as a function of temperature from $1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and as a function of pressure up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ using various high-pressure devices. A kink, which is associated with the reduction of scattering due to the onset of ferromagnetic order, is seen in the electrical resistance as a function of temperature. With applied pressure, this kink is found to broaden and increase in temperature at a rate of approximately $2.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{GPa}$. Above $\ensuremath{\sim}18\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the broadening of the kink prohibits accurate determination of ${T}_{C}$; however, upon reducing the pressure, no signatures of ferromagnetism were evident in the electrical resistivity. Both energy-dispersive $(P\ensuremath{\le}61\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ and angle-dispersive $(P\ensuremath{\le}27\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ x-ray diffraction measurements at room temperature show a gradual transition from the body-centered-tetragonal phase of ${\mathrm{Au}}_{4}\mathrm{V}$ to a disordered face-centered-cubic structure. This transition is irreversible and continuous, and the data have been fit to a Birch-Murnaghan equation of state. In order to investigate the origin of the magnetic interactions in ${\mathrm{Au}}_{4}\mathrm{V}$, we have also measured the electrical resistivity of ${\mathrm{Au}}_{1\ensuremath{-}x}{\mathrm{V}}_{x}$ alloys and determined their Kondo temperatures ${T}_{K}$ for $xl1%$. The pressure dependence of ${T}_{K}$ for $x=0.5%$ was measured up to $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, for which the Kondo temperature increases at a rate of $d{T}_{K}∕dP=6.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{GPa}$. Using the volume dependence of the exchange interaction between magnetic vanadium ions, we find that the pressure-induced increase of the Curie temperature in ${\mathrm{Au}}_{4}\mathrm{V}$ can be explained by an increase in the exchange interaction parameter and the number of magnetic nearest neighbors.

Angular-dependent torque measurements on CeCoIn 5 single crystals

Abstract: Angular-dependent torque measurements were performed on single crystals of ${\mathrm{CeCoIn}}_{5}$, a heavy fermion superconductor $({T}_{c0}=2.3\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, in the temperature, $T$, range $1.9\phantom{\rule{0.3em}{0ex}}\mathrm{K}\ensuremath{\le}T\ensuremath{\le}20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and magnetic fields $H$ up to $14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. A large paramagnetic effect is found in the normal state. Torque measurements in the mixed state were also performed. The torque curves show sharp hysteresis peaks at $\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}$ ($\ensuremath{\theta}$ is the angle between $H$ and the $c$-axis of the crystal), a result of intrinsic pinning of vortices. The anisotropy $\ensuremath{\gamma}\ensuremath{\equiv}\sqrt{{m}_{c}∕{m}_{a}}$ in the mixed state was determined from the reversible part of the vortex contribution to the torque signal using Kogan's model [Phys. Rev. B 38, 7049 (1988)]. The anisotropy $\ensuremath{\gamma}$ decreases with increasing magnetic field and temperature. The fact that $\ensuremath{\gamma}$ is not a constant points towards a multiband scenario in this heavy fermion material.

Nonlinear susceptibility: Evidence for antiferroquadrupolar fluctuations and a nonmagnetic Γ 1 ground state in the heavy fermion superconductor PrOs 4 Sb 12

Abstract: Nonlinear susceptibility $({\ensuremath{\chi}}_{3})$ measurements were performed on single crystals of the heavy fermion superconductor ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ in order to probe the nature of the ${\mathrm{Pr}}^{3+}$ ground state in the crystalline electric field. Calculations of ${\ensuremath{\chi}}_{3}(T)$ have been carried out (i) within a mean field model of intersite magnetic and quadrupolar interactions for an ionic (zero hybridization) crystal field ${\ensuremath{\Gamma}}_{1}$ singlet ground state and a first excited state magnetic triplet, and (ii) within a two-channel Anderson impurity model assuming a ${\ensuremath{\Gamma}}_{23}$ doublet ground state. The experimental ${\ensuremath{\chi}}_{3}$ results are in best agreement with the nonmagnetic ${\ensuremath{\Gamma}}_{1}$ ground state in the tetrahedral crystalline electric field of the ${\mathrm{Pr}}^{3+}$ ions with antiferroquadrupolar intersite coupling and weak antiferromagnetic intersite coupling.

Scaling behavior of angular-dependent resistivity in Ce Co In 5 : Possible evidence for d -wave density waves

Abstract: In-plane angular-dependent resistivity (ADR) was measured in the non-Fermi liquid regime of $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$ single crystals at temperatures $T\ensuremath{\leqslant}20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in magnetic fields $H$ up to $14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Two scaling behaviors were identified in the low-field region where resistivity shows a linear $T$ dependence, separated by a critical angle ${\ensuremath{\theta}}_{c}$, which is determined by the anisotropy of $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$; i.e., ADR depends only on the perpendicular (parallel) field component below (above) ${\ensuremath{\theta}}_{c}$. These scaling behaviors and other salient features of ADR are consistent with $d$-wave density waves.

Interplay between spin-glass and non-Fermi-liquid behavior in Y1-xUxPd3

Abstract: We report the results of a study of the ac and dc magnetic susceptibilities of Y1�xUxPd3. The magnetic properties of the system gradually evolve from the spin-glass SG behavior observed for x = 0.4 to the nonFermi-liquid NFL ground state that develops at x = 0.2. For x = 0.4, the SG properties are remarkably similar to those found in canonical spin glasses. We have been able to analyze our data in terms of critical slowing down when T approaches the critical temperature associated with a spin-glass freezing phase transition. The values obtained for the dynamic and static critical exponents z =1 0±1, = 0.91, = 1.82 support the existence of a true spin-glass phase transition in Y0.6U0.4Pd3. On the other hand, the evolution of magnetic properties with a decreasing U concentration suggests an interplay between the SG and NFL ground states. The ground state that finally appears for x = 0.2 displays a temperature dependence of its low-field ac susceptibility consistent with the prediction of the Griffith’s phase model of NFL behavior. The role played by compositional disorder in the development of the NFL state is discussed.

Signature of the magnetic transitions in Y 0.2 Pr 0.8 Ba 2 Cu 3 O 7-δ in high field angular magnetoresistivity

Abstract: In-plane (ab) and out-of-plane (c-axis) magnetoresistivity display different symmetry crossovers and/or transitions in 14 T magnetic field applied parallel to the CuO 2 planes. The in-plane magnetoresistivity crosses over from four-fold symmetry below 6 K to two-fold symmetry at higher temperatures, which becomes dominant at temperatures higher than 40 K. The out-of-plane magnetoresistivity changes at 17 K from four fold symmetry to ordinary sin 2 θ at higher temperatures. The behaviour of the c-axis magnetoresistivity can be ascribed to the antiferromagnetic ordering of the Pr spins whereas the symmetry change of the in-plane magnetoresistivity at 6 K might be attributed to commensurate to incommensurate crossovers of the spin subsystems. The antiferromagnetic order of the Cu(2) sublattice seems to have only a week effect on the magnetoresistivity.

Point-contact spectroscopy on heavy-fermion superconductors

Abstract: Hints at unconventional superconductivity are present in many heavy-fermion superconductors. Among other methods, point-contact spectroscopy is an important tool to study the symmetry and the nodal structure of the energy gap of unconventional superconductors. Andreev reflection of charge carriers at the interface between a normal metal and a superconductor is the main mechanism for a current at low bias through a nanosized constriction, leading to maxima in the differential conductance d I / d V vs. V. The position of the maxima depends not only on the size of the gap along given directions, but also on the order-parameter symmetry, the microscopic details of the contact and the barrier strength. In addition, if the order parameter exhibits a sign change, an Andreev surface bound state leads to a conductance anomaly at zero bias. We will review recent developments in this field and focus particularly on investigations of CeCoIn 5 , a possible d-wave superconductor.

Pulsed laser deposition of PrFe4Sb12 thin films

Abstract: Results from a growth and characterization study of PrFe4Sb12 thin films synthesized by pulsed laser deposition (PLD) are reported. Films were grown under a variety of conditions where substrate temperature, annealing schedule, and laser power were optimized to produce single phase material. Various substrates were used for film growth. X-ray diffraction data indicate that the films are primarily single phase with very small amounts of antimony inclusions. The high quality of the films is reflected in electrical resistivity vs temperature data which are consistent with previous results for bulk polycrystalline and single crystal specimens.

Penetration depth study in the heavy-fermion superconductor PrOs4Sb12

Abstract: The penetration depth λ in the filled-skutterudite heavy-fermion superconductor PrOs4Sb12 has been measured using transverse-field muon spin rotation (TF- μ SR ). It is found to be temperature-independent at low temperatures, consistent with a nonzero gap for quasiparticle excitations. In contrast, radiofrequency (RF) measurements yield a stronger temperature dependence of λ ( T ) , indicative of point nodes in the gap. A ∼ 10 % discrepancy is found at low temperatures. This may be due to mechanisms that modify the vortex-state field distribution, or to the surface scattering which breaks pairs in an odd-parity superconductor. Alternatively, it may be a matter of field orientation of nodal gap structure in the μ SR measurements.

Structure-dependent ferromagnetism in Au4V studied under high pressure

Abstract: to 30 GPa using various high-pressure devices. A kink, which is associated with the reduction of scattering due to the onset of ferromagnetic order, is seen in the electrical resistance as a function of temperature. With applied pressure, this kink is found to broaden and increase in temperature at a rate of approximately 2.7 K/GPa. Above 18 GPa, the broadening of the kink prohibits accurate determination of TC; however, upon reducing the pressure, no signatures of ferromagnetism were evident in the electrical resistivity. Both energy-dispersive P61 GPa and angle-dispersive P27 GPa x-ray diffraction measurements at room temperature show a gradual transition from the body-centered-tetragonal phase of Au4V to a disordered face-centered-cubic structure. This transition is irreversible and continuous, and the data have been fit to a Birch-Murnaghan equation of state. In order to investigate the origin of the magnetic interactions in Au4V, we have also measured the electrical resistivity of Au1xVx alloys and determined their Kondo temperatures TK for x1%. The pressure dependence of TK for x=0.5% was measured up to 2.8 GPa, for which the Kondo temperature increases at a rate of dTK/dP=6.5 K/GPa. Using the volume dependence of the exchange interaction between magnetic vanadium ions, we find that the pressure-induced increase of the Curie temperature in Au4V can be explained by an increase in the exchange interaction parameter and the number of magnetic nearest neighbors.

Physical properties of Lu1-xYbxNi2B2C

Abstract: The Lu1−x Yb x Ni2B2 system exhibits superconductivity at low Yb concentrations (0≤x≤ 0.1), Kondo behaviour at low and moderate Yb concentrations (0≤x≤ 0.34), and heavy fermion behaviour with a non-magnetic ground state at high Yb concentrations. In order to characterize the evolution of these phenomena with x, we have performed dc magnetic susceptibility, electrical resistivity, thermoelectric power, and specific heat measurements on single crystals of Lu1−x Yb x Ni2B2 with various values of x between 0 and 1. The enhanced suppression of T c by Yb substitution, compared to other rare earth substitutions, is consistent with the behaviour of superconducting Kondo systems with T K≫T c when T K decreases with increasing x. The electronic specific heat coefficient γ increases from ∼11 mJ/mol K2 for x=0 to ∼530 mJ/mol K2 for x=1. The curve of the reduced specific heat jump ΔC/ΔC 0 versus T c/T c0, where ΔC 0 and T c0 refer to the LuNi2B2C matrix, does not conform to the BCS theory, but is consistent with the m...

Non-Fermi liquid behavior and quantum criticality in Sc1-xUxPd3 and URu2-xRexSi2

Abstract: The compounds Sc 1 - x U x Pd 3 and URu 2 - x Re x Si 2 exhibit non-Fermi liquid behavior in the vicinity of zero-temperature magnetic phase transitions accessed by chemical substitution. Notably, the quantum phase transitions in these pseudobinary systems are associated with spin glass and ferromagnetic ordering, respectively. Recent studies have uncovered new details of the unconventional physical phenomena occurring in these materials.

High temperature mixed state c -axis dissipation in low carrier density Y 0.54 Pr 0.46 Ba 2 Cu 3 O 7-δ

Abstract: The nature of the out-of-plane dissipation was investigated in underdoped $Y_{0.54}Pr_{0.46}Ba_{2}Cu_{3}O_{7-\delta}$ single crystals at temperatures close to the critical temperature. For this goal, temperature and angle dependent out-of-plane resistivity measurements were carried out both below and above the critical temperature. We found that the Ambegaokar-Halperin relationship [V. Ambegaokar, and B. I. Halperin, Phys. Rev. Lett. \textbf{22}, 1364 (1969)] depicts very well the angular magnetoresistivity in the investigated range of field and temperature. The main finding is that the in-plane phase fluctuations decouple the layers above the critical temperature and the charge transport is governed only by the quasiparticles. We also have calculated the interlayer Josephson critical current density, which was found to be much smaller than the one predicted by the theory of layered superconductors. This discrepancy could be a result of the d-wave symmetry of the order parameter and/or of the non BCS temperature dependence of the c-axis penetration length.

Magnetic response of Y 0.47 Pr 0.53 Ba 2 Cu 3 O 7-δ : Superconductivity, glassiness, and paramagnetism

Abstract: The magnetic response of the strongly underdoped ${\mathrm{Y}}_{047}{\mathrm{Pr}}_{053}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ was investigated We found the presence of superconducting and magnetic orders deep into the paramagnetic state, up to $200\phantom{\rule{03em}{0ex}}\mathrm{K}$, which manifest as diamagneticlike response and hysteresis, respectively We propose that the main source of irreversibility in this $T$ range is the softening and melting of the glassy state into a viscous liquid of entities that behave like superparamagnetic particles with antiferromagnetic cores

Temperature dependence of the low-energy crystal field excitation in PrOs4Sb12: effect of the energy gap

Abstract: We have performed inelastic neutron scattering measurements of the singlet-triplet crystal-field (CF) excitation in the filled skutterudite compound PrOs4Sb12 in the vicinity of the superconducting transition. The CF linewidth decreases sharply below Tc. The decrease is consistent with an isotropic energy gap of 2Δ/kBTc=6.6±0.8, which is in good agreement with NQR measurements [H. Kotegawa et al., Phys. Rev. Lett. 90, (2003) 027001]. We observe additional scattering that may be due to low-energy vibrational (rattling) modes that may be relevant to the superconductivity.

Effect of annealing on glassy dynamics and non-Fermi liquid behavior in UCu4Pd

Abstract: Longitudinal-field muon spin relaxation (LF- μ SR) experiments have been performed in unannealed and annealed samples of the heavy-fermion compound UCu 4 Pd to study the effect of disorder on non-Fermi liquid behavior in this material. The muon spin relaxation functions G ( t , H ) obey the time-field scaling relation G ( t , H ) = G ( t / H γ ) previously observed in this compound. The observed scaling exponent γ = 0.3 ± 0.1 , independent of annealing. Fits of the stretched- exponential relaxation function G ( t ) = exp [ - ( Λ t ) K ] to the data yielded stretching exponentials K 1 for all samples. Annealed samples exhibited a reduction of the relaxation rate at low temperatures, indicating that annealing shifts fluctuation noise power to higher frequencies. There was no tendency of the inhomogeneous spread in rates to decrease with annealing, which modifies but does not eliminate the glassy spin dynamics reported previously in this compound. The correlation with residual resistivity previously observed for a number of NFL heavy-electron materials is also found in the present work.

Growth via pulsed laser deposition and characterization of thin films of the electron-doped superconductor Sm2−xCexCuO4−y

Abstract: We report on the growth of thin films of the electron-doped superconductor Sm 2− x Ce x CuO 4− y (SCCO) by pulsed laser deposition (PLD) Electrical resistivity ρ ( H , T ) measurements on the thin films were made at temperatures T between 20 mK and 300 K in magnetic fields H up to 18 T The superconducting transition temperature is as high as T c = 178 K, with a transition width Δ T c = 093 K, for an optimally doped film The ρ ( H , T ) measurements on this thin film are compared to similar measurements made on single crystals A vortex-glass scaling analysis was applied to the ρ ( H , T ) data, yielding values of the vortex-glass melting temperature T g and critical exponent ν ( z − 1) In addition, ρ ( H , T ) data are presented for an underdoped sample with x = 0125, also grown via PLD

Magnetic penetration depth measurements of the heavy-fermion superconductor PrOs4Sb12

Abstract: Previous studies of the magnetic penetration depth in the heavy-fermion superconductor PrOs 4 Sb 12 reported contradictory results about its pairing state. While muon spin rotation measurements report a fully gapped quasiparticle spectrum, inductive measurements at radio frequencies suggest the existence of point nodes in the energy gap. Here, we report on new measurements of the in-plane magnetic penetration depth of PrOs 4 Sb 12 . The measurements were performed using a tunnel diode oscillator that exhibits excellent frequency stability and low-power dissipation. We report the first ever measurements of the penetration depth of this material at temperatures as low as 20 mK. At about 450 mK, we observe the onset of a superconducting-transition-like drop in the magnetic penetration depth.

Electrical, magnetic, and thermal properties of UCu5−xPtx

Abstract: We report measurements of the structural and electronic properties of UCu5 − x Pt x , a Pt-based analogue of the nFL system UCu5 − x Pd x , in the concentration range 0≤ x ≤ 5. Forx ≤ 2.5 and x ≥ 4.5, the UCu5 − x Pt x system crystallizes in the AuBe5 structure, but is mixed phase in the range 2.5 < x < 4.5. We observe a rapid suppression of long range antiferromagnetic order for small concentrations of Pt. The electrical resistivity, DC magnetic susceptibility, and specific heat of UCu5 − x Pt x in the concentration range 0.75 ≤ x ≤ 1 have temperature dependencies at low temperature (T ≤ 10 K) consistent with the non-Fermi liquid behaviour found in UCu5 − x Pd x . Above x = 1, there is a transition from non-Fermi liquid behaviour to Fermi liquid behaviour with no observation of any spin glass behaviour.

Susceptibility inhomogeneity and non-Fermi liquid behavior in UCu5-xPtx

Abstract: Transverse-field μ SR shifts and relaxation rates have been measured in the non-Fermi liquid (NFL) alloy system UCu 5 - x Pt x , x = 1.0 , 1.5, and 2.5. At low temperatures the fractional spread in Knight shifts δ K / K ≈ δ χ / χ is ≳ 2 for x = 1 , but is only half this value for x = 1.5 and 2.5. In a disorder-driven scenario where the NFL behavior is due to a broadly distributed (Kondo or Griffiths-phase cluster) characteristic energy E , our results indicate that δ E / E av ≈ ( δ K / K ) T → 0 is similar for UCu 5 - x Pd x ( x = 1 and 1.5) and UCu 4 Pt, but is reduced for UCu 5 - x Pt x , x = 1.5 and 2.5. This reduction is due to a marked increase of E with increasing x ; the spread δ E is found to be roughly independent of x. Our results correlate with the observed suppression of other NFL anomalies for x > 1 in UCu 5 - x Pt x but not in UCu 5 - x Pd x , and are further evidence for the importance of disorder in the NFL behavior of both these alloy systems.