Showing papers by "M. B. Maple published in 2014"

Persistent non-metallic behavior in Sr2IrO4 and Sr3Ir2O7 at high pressures.

Abstract: Iridium-based 5d transition-metal oxides are attractive candidates for the study of correlated electronic states due to the interplay of enhanced crystal-field, Coulomb and spin-orbit interaction energies. At ambient pressure, these conditions promote a novel Jeff = 1/2 Mott-insulating state, characterized by a gap of the order of ~0.1 eV. We present high-pressure electrical resistivity measurements of single crystals of Sr2IrO4 and Sr3Ir2O7. While no indications of a pressure-induced metallic state up to 55 GPa were found in Sr2IrO4, a strong decrease of the gap energy and of the resistance of Sr3Ir2O7 between ambient pressure and 104 GPa confirm that this compound is in the proximity of a metal-insulator transition.

Superconductivity of La 3 Co 4 Sn 13 and La 3 Rh 4 Sn 13 : A comparative study

Abstract: We report the electric transport and thermodynamic properties of the skutterudite-related ${\mathrm{La}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ and ${\mathrm{La}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ superconductors. Applying an external pressure to ${\mathrm{La}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$, the resistive superconducting critical temperature ${T}_{c}$ decreases, while the critical temperature of ${\mathrm{La}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ is enhanced with increasing pressure. The positive pressure coefficient $d{T}_{c}/dP$ correlates with a subtle structural transition in ${\mathrm{La}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ and is discussed in the context of lattice instabilities. Specific-heat data show that both compounds are typical BCS superconductors. However, ${\mathrm{La}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ also exhibits a second superconducting phase at higher temperatures, which is characteristic of inhomogeneous superconductors. We calculate the specific heat for an inhomogeneous superconducting phase, which agrees well with experimental $C(T)$ data for ${\mathrm{La}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$. We also found that an applied pressure reduces this second superconducting phase.

Probing the superconductivity of PrPt4Ge 12 through Ce substitution

Abstract: We report measurements of electrical resistivity, magnetic susceptibility, specific heat, and thermoelectric power on the system Pr1-xCexPt4Ge12 Superconductivity is suppressed with increasing Ce concentration up to x = 05, above which there is no evidence for superconductivity down to 11 K The Sommerfeld coefficient {\gamma} increases with increasing x from 48 mJ/mol K^2 up to 120 mJ/mol K^2 at x = 05, indicating an increase in strength of electronic correlations The temperature dependence of the specific heat at low temperatures evolves from roughly T^3 for x = 0 to e^(-\Delta /T) behavior for x = 005 and above, suggesting a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity Fermi-liquid behavior is observed throughout the series in low-temperature magnetization, specific heat, and electrical resistivity measurements

Effect of yttrium substitution on the superconducting properties ofLa1-xYxO0.5F0.5BiS2

Abstract: We present the effect of yttrium substitution on superconductivity in the ${\mathrm{La}}_{1\ensuremath{-}\mathit{x}}{\mathrm{Y}}_{\mathit{x}}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ system. Polycrystalline samples with nominal Y concentrations up to 40% were synthesized and characterized via electrical resistivity, magnetic susceptibility, and specific-heat measurements. Y substitution reduces the lattice parameter $a$ and unit-cell volume V, and a correlation between the lattice parameter $c$, the La-O-La bond angle, and the superconducting critical temperature ${T}_{c}$ is observed. The chemical pressure induced by Y substitution for La produces neither the high-${T}_{c}$ superconducting phase nor the structural phase transition seen in ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ under externally applied pressure.

Investigation of magnetic order in Sm T r 2 Zn 20 ( T r = Fe , Co , Ru ) and Sm T r 2 Cd 20 ( T r = Ni , Pd )

Abstract: Single crystals of the ``cage compounds'' $\mathrm{Sm}{\mathit{Tr}}_{2}{\mathrm{Zn}}_{20}$ ($Tr=\mathrm{Fe}$, Co, Ru) and $\mathrm{Sm}T{r}_{2}{\mathrm{Cd}}_{20}$ ($Tr=\mathrm{Ni}$, Pd) have been investigated by means of electrical resistivity, magnetization, and specific-heat measurements. The compounds ${\mathrm{SmFe}}_{2}{\mathrm{Zn}}_{20},{\mathrm{SmRu}}_{2}{\mathrm{Zn}}_{20}$, and ${\mathrm{SmNi}}_{2}{\mathrm{Cd}}_{20}$ exhibit ferromagnetic order with Curie temperatures of ${T}_{C}=47.4$, 7.6, and 7.5 K, respectively, whereas ${\mathrm{SmPd}}_{2}{\mathrm{Cd}}_{20}$ is an antiferromagnet with a N\'eel temperature of ${T}_{N}=3.4$ K. No evidence for magnetic order is observed in ${\mathrm{SmCo}}_{2}{\mathrm{Zn}}_{20}$ down to 110 mK. The Sommerfeld coefficients $\ensuremath{\gamma}$ are found to be $57 {\mathrm{mJ}/\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{K}}^{2}$ for ${\mathrm{SmFe}}_{2}{\mathrm{Zn}}_{20},79.5 {\mathrm{mJ}/\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{K}}^{2}$ for ${\mathrm{SmCo}}_{2}{\mathrm{Zn}}_{20},258 {\mathrm{mJ}/\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{K}}^{2}$ for ${\mathrm{SmRu}}_{2}{\mathrm{Zn}}_{20},165 {\mathrm{mJ}/\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{K}}^{2}$ for ${\mathrm{SmNi}}_{2}{\mathrm{Cd}}_{20}$, and $208 {\mathrm{mJ}/\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{K}}^{2}$ for ${\mathrm{SmPd}}_{2}{\mathrm{Cd}}_{20}$. Enhanced values of $\ensuremath{\gamma}$ and a quadratic temperature dependence of the electrical resistivity at low temperature for ${\mathrm{SmRu}}_{2}{\mathrm{Zn}}_{20}$ and ${\mathrm{SmPd}}_{2}{\mathrm{Cd}}_{20}$ suggest an enhancement of the quasiparticle masses due to hybridization between localized $4f$ and conduction electron states.

Absence of a static in-plane magnetic moment in the `hidden-order' phase of URu$_2$Si$_2$

Abstract: We have carried out a careful magnetic neutron scattering study of the heavy fermion compound URu2Si2 to probe the possible existence of a small magnetic moment parallel to tetragonal basal plane in the ‘hidden-order’ phase. This small in-plane component of the magnetic moment on the uranium sites S∥ has been postulated by two recent models (rank-5 superspin/hastatic order) aiming to explain the hidden-order phase, in addition to the well-known out-of-plane component S⊥ ≈ 0.01–0.04 μB/U. In order to separate S∥ and S⊥, we take advantage of the condition that for magnetic neutron scattering only the components of the magnetic structure that are perpendicular to the scattering vector Q contribute to the magnetic scattering. We find no evidence for an in-plane magnetic moment S∥. Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is Smax∥ ⩽ 1 × 10−3 μB/U.

Symmetry and correlations underlying Hidden Order in URu2Si2

Abstract: In this paper, we experimentally investigate the symmetry in the hidden order (HO) phase of intermetallic URu2Si2 by mapping the lattice and magnetic excitations via inelastic neutron and x-ray scattering measurements in the HO and high-temperature paramagnetic phases. At all temperatures, the excitations respect the zone edges of the body-centered tetragonal paramagnetic phase, showing no signs of reduced spatial symmetry, even in the HO phase. The magnetic excitations originate from transitions between hybridized bands and track the Fermi surface, whose features are corroborated by the phonon measurements. Due to a large hybridization energy scale, a full uranium moment persists in the HO phase, consistent with a lack of observed crystal-field-split states. Our results are inconsistent with local order-parameter models and the behavior of typical density waves. Finally, we suggest that an order parameter that does not break spatial symmetry would naturally explain these characteristics.

Resolution of the discrepancy between the variation of the physical properties of Ce1-xYbxCoIn5 single crystals and thin films with Yb composition

Abstract: Extraordinary electronic phenomena including an Yb valence transition, a change in Fermi surface topology, and suppression of the heavy fermion quantum critical field at a nominal concentration 0.2 have been found in the system. These phenomena have no discernable effect on the unconventional superconductivity and normal-state non-Fermi liquid behaviour that occur over a broad range of up to 0.8. However, the variation of the coherence temperature and the superconducting critical temperature with nominal Yb concentration for bulk single crystals is much weaker than that of thin films. To determine whether differences in the actual Yb concentration of bulk single crystals and thin film samples might be responsible for these discrepancies, we employed Vegard’s law and the spectroscopically determined values of the valences of Ce and Yb as a function of to determine the actual composition of bulk single crystals. This analysis is supported by energy-dispersive X-ray spectroscopy, wavelength-dispersive X-ray ...

Universality in the magnetic response of metamagnetic metals.

Abstract: We report measurements of the nonlinear susceptibility ${\ensuremath{\chi}}_{3}(T)$ in the metamagnetic heavy fermion (HF) compound ${\mathrm{UPt}}_{3}$. At high temperatures, ${\ensuremath{\chi}}_{3}(T)l0$ and is small. It turns positive for $T\ensuremath{\le}35$ K, forms a peak at $T\ensuremath{\cong}10$ K, and then decreases to zero with further decreasing temperature. The peak in $\ensuremath{\chi}{}_{3}$ occurs at a temperature ${T}_{3}$, which is roughly half of ${T}_{1}$, the temperature of the maximum in the linear susceptibility. We present results on ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ and ${\mathrm{UPd}}_{2}{\mathrm{Al}}_{3}$ to show that this feature is common to other HF materials. A two-level model to describe the metamagnetic transition, with separation between the levels being the only energy scale, captures all experimentally observed features.

Enhancement of the hidden order/large moment antiferromagnetic transition temperature in the URu2−xOsxSi2 system

Abstract: Electrical resistivity, specific heat, and magnetization measurements on the URuOsSi system suggest a phase transition from the ‘hidden order’ phase to another unidentified phase that is likely to be a large moment antiferromagnetic phase. It is noteworthy that the hidden order/large moment antiferromagnetic phase boundary is enhanced from 17.5 K at = 0 to 50 K at = 1. However, as increases, the gap opening in the Fermi surface due to the hidden order phase transition, deduced from electrical resistivity and specific heat measurements, decreases. This study reveals that both Fe and Os isoelectronic substitutions for Ru in URuSi yield an enhancement of . In contrast to the URuFeSi system, where the unit cell volume decreases with , in the URuOsSi system, the unit cell volume increases with . Thus the enhancement of the hidden order/large moment antiferromagnetic transition temperature cannot be solely due to an increase in chemical pressure.

From local moment to mixed-valence regime in Ce 1 − x Yb x CoIn 5 alloys

Abstract: We investigated the onset of the many-body coherence in the $f$-orbital single crystalline alloys ${\text{Ce}}_{1\ensuremath{-}x}$${\text{Yb}}_{x}$${\text{CoIn}}_{5}$ ($0\ensuremath{\le}x\ensuremath{\le}0775$) through thermodynamic and magnetotransport measurements Our study shows the evolution of the many-body electronic state as the Kondo lattice of Ce moments is transformed into an array of Ce impurities Specifically, we observe a crossover from the predominantly localized Ce moment regime to the predominantly itinerant Yb $f$-electronic states regime for about 60$%$ of Yb doping Our analysis of the residual resistivity data unveils the presence of correlations between Yb ions, while from our analysis of specific heat data we conclude that for $065\ensuremath{\le}x\ensuremath{\le}0775$, Yb $f$ electrons strongly interact with the conduction electrons while the Ce moments remain completely decoupled The sublinear temperature dependence of resistivity across the whole range of Yb concentrations suggest the presence of a unconventional scattering mechanism for the conduction electrons

Non-Fermi Liquid Behavior Close to a Quantum Critical Point in a Ferromagnetic State without Local Moments

Abstract: A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc_{3.1}In. Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid (NFL) behavior, manifested in the logarithmic divergence of the specific heat both in the ferro- and the paramagnetic states. Sc_{3.1}In displays critical scaling and NFL behavior in the ferromagnetic state, akin to what had been observed only in f-electron, local moment systems. With doping, critical scaling is observed close to the QCP, as the critical exponents, and delta, gamma and beta have weak composition dependence, with delta nearly twice, and beta almost half of their respective mean-field values. The unusually large paramagnetic moment mu_PM~1.3 mu_B/F.U. is nearly composition-independent. Evidence for strong spin fluctuations, accompanying the QCP at x_c = 0.035 +- 0.005, may be ascribed to the reduced dimensionality of Sc_{3.1}In, associated with the nearly one-dimensional Sc-In chains.

Epitaxial stabilization of ultra thin films of electron doped manganites

Abstract: Ultra-thin films of the electron doped manganite La0.8Ce0.2MnO3 were grown in a layer-by-layer growth mode on SrTiO3 (001) substrates by pulsed laser interval deposition. High structural quality and surface morphology were confirmed by a combination of synchrotron based x-ray diffraction and atomic force microscopy. Resonant X-ray absorption spectroscopy measurements confirm the presence of Ce4+ and Mn2+ ions. In addition, the electron doping signature was corroborated by Hall effect measurements. All grown films show a ferromagnetic ground state as revealed by both dc magnetization and x-ray magnetic circular dichroism measurements and remain insulating contrary to earlier reports of a metal-insulator transition. Our results hint at the possibility of electron-hole asymmetry in the colossal magnetoresistive manganite phase diagram akin to the high-Tc cuprates.

Muon spin rotation and relaxation in Pr 1 − x Nd x Os 4 Sb 12 : Magnetic and superconducting ground states

Abstract: Muon spin rotation and relaxation ($\ensuremath{\mu}$SR) experiments have been carried out to characterize magnetic and superconducting ground states in the ${\text{Pr}}_{1\ensuremath{-}x}$${\text{Nd}}_{x}$${\text{Os}}_{4}$${\text{Sb}}_{12}$ alloy series. In the ferromagnetic end compound ${\text{NdOs}}_{4}$${\text{Sb}}_{12}$ the spontaneous local field at positive-muon (${\ensuremath{\mu}}^{+}$) sites below the ordering temperature ${T}_{C}$ is greater than expected from dipolar coupling to ferromagnetically aligned Nd${}^{3+}$ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For $0.45\ensuremath{\le}x\ensuremath{\le}0.75$, ${\ensuremath{\mu}}^{+}$ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at ${\ensuremath{\mu}}^{+}$ sites below ${T}_{C}$ are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd${}^{3+}$ moments, and speculate that the Nd${}^{3+}$-${\ensuremath{\mu}}^{+}$ interaction is suppressed and disordered by Pr doping. In an $x=0.25$ sample, which is superconducting below ${T}_{c}=1.3$ K, there is no sign of ``spin freezing'' (static Nd${}^{3+}$ magnetism), ordered or disordered, down to 25 mK. Dynamic ${\ensuremath{\mu}}^{+}$ spin relaxation is strong, indicating significant Nd-moment fluctuations. The ${\ensuremath{\mu}}^{+}$ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below ${T}_{c}$, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For $0.25\ensuremath{\le}x\ensuremath{\le}0.75$, the $\ensuremath{\mu}$SR data provide evidence against phase separation; superconductivity and Nd${}^{3+}$ magnetism coexist on the atomic scale.

Landau renormalizations of superfluid density in the heavy-fermion superconductor CeCoIn5.

Abstract: The formation of heavy-fermion bands can occur by means of the conversion of a periodic array of local moments into itinerant electrons via the Kondo effect and the huge consequent Fermi-liquid renormalizations. Leggett predicted for liquid (3)He that Fermi-liquid renormalizations change in the superconducting state, leading to a temperature dependence of the London penetration depth Λ quite different from that in BCS theory. Using Leggett's theory, as modified for heavy fermions, it is possible to extract from the measured temperature dependence of Λ in high quality samples both Landau parameters F(0)(s) and F(1)(s); this has never been accomplished before. A modification of the temperature dependence of the electronic specific heat C(el), related to that of Λ, is also expected. We have carefully determined the magnitude and temperature dependence of Λ in CeCoIn(5) by muon spin relaxation rate measurements to obtain F(0)(s) = 36 ± 1 and F(1)(s) = 1.2 ± 0.3, and we find a consistent change in the temperature dependence of C(el). This, the first determination of F(1)(s) with a value ≪ F(0)(s) in a heavy-fermion compound, tests the basic assumption of the theory of heavy fermions, that the frequency dependence of the self-energy is much more important than its momentum dependence.

Quantum criticality and superconducting pairing in Ce(1-x)Yb(x)CoIn5 alloys

Abstract: In this paper we review some of our recent experimental and theoretical results on transport and thermodynamic properties of heavy-fermion alloys Ce(1-x)Yb(x)CoIn5. Charge transport measurements under magnetic field and pressure on these single crystalline alloys revealed that: (i) relatively small Yb substitution suppresses the field induced quantum critical point, with a complete suppression for nominal Yb doping x>0.20; (ii) the superconducting transition temperature Tc and Kondo lattice coherence temperature T* decrease with x, yet they remain finite over the wide range of Yb concentrations; (iii) both Tc and T* increase with pressure; (iv) there are two contributions to resistivity, which show different temperature and pressure dependences, implying that both heavy and light quasiparticles contribute to inelastic scattering. We also analyzed theoretically the pressure dependence of both T* and Tc within the composite pairing theory. In the purely static limit, when we ignore the lattice dynamics, we find that the composite pairing mechanism necessarily causes opposite behaviors of T* and Tc with pressure: if T* grows with pressure, Tc must decrease with pressure and vice versa.

Crossover between Fermi liquid and non-Fermi liquid behavior in the non-centrosymmetric compound Yb2Ni12P7.

Abstract: A crossover from a non-Fermi liquid to a Fermi liquid phase in Yb2Ni12P7 is observed by analyzing electrical resistivity ρ(T), magnetic susceptibility χ(T), specific heat C(T), and thermoelectric power S(T) measurements. The electronic contribution to specific heat, Ce(T), behaves as Ce(T)/T ~ −ln(T) for 5 K 4 K. A crossover between Fermi-liquid and non-Fermi liquid behavior suggests that Yb2Ni12P7 is in close proximity to a quantum critical point, in agreement with results from recent measurements of this compound under applied pressure.

Electrical resistivity of single crystals of LaFeAsO under applied pressure

Abstract: Measurements of electrical resistivity under applied pressure were performed on single-crystalline samples of LaFeAsO grown in a molten NaAs flux. We observe a smooth suppression of spin-density wave order under nearly hydrostatic applied pressures up to 2.6 GPa and in quasihydrostatic pressures up to 14.7 GPa. Similar to some of the other reports on single and polycrystalline samples of LaFeAsO, these crystals exhibit a resistivity that increases as temperature is lowered. By fitting an Arrhenius law to the the semiconducting-like temperature dependence of the electrical resistivity, we extract an energy gap that is suppressed with pressure and vanishes near 10 GPa. This is accompanied by the emergence of a metallic temperature dependence of the electrical resistivity. A similar behavior is also observed in diamond anvil cell experiments carried out to $\ensuremath{\sim}37\phantom{\rule{4pt}{0ex}}\mathrm{GPa}$. Complete transitions to a bulk superconducting phase are not observed in any of the experiments.

Crossover between Fermi liquid and non-Fermi liquid behavior in the non-centrosymmetric compound Yb$_2$Ni$_{12}$P$_7$

Abstract: A crossover from a non-Fermi liquid to a Fermi liquid phase in Yb$_2$Ni$_{12}$P$_7$ is observed by analyzing electrical resistivity $\rho(T)$, magnetic susceptibility $\chi(T)$, specific heat $C(T)$, and thermoelectric power $S(T)$ measurements The electronic contribution to specific heat, $C_{e}(T)$, behaves as $C_{e}(T)/T \sim -\ln(T)$ for 5 K $ $ 4 K A crossover between Fermi-liquid and non-Fermi liquid behavior suggests that Yb$_2$Ni$_{12}$P$_7$ is in close proximity to a quantum critical point, in agreement with results from recent measurements of this compound under applied pressure

Tuning the Band Gap of PbCrO4 Through High‐Pressure: Evidence of Wide‐to‐Narrow Semiconductor Transitions.

Abstract: Commercial polycrystalline and cleaved platelets from natural PbCrO4 are studied in a diamond anvil cell at ≤ 21 GPa.

Magnetism of Pd 1 − x Ni x alloys near the critical concentration for ferromagnetism

Abstract: Close to the critical composition for this alloy, muon spin rotation and relaxation show spins clustered with short-range order, not isolated.

Resolution of the discrepancy between the variation of the physical properties of Ce1-xYbxCoIn5 single crystals and thin films with Yb composition

Abstract: Measurements of physical properties show that Yb enters the single crystals systematically and in registry with the nominal Yb concentration x of the starting material dissolved in the molten indium flux.

Muon spin rotation and relaxation in Pr 1-x Nd x Os 4 Sb 12 : Paramagnetic states

Abstract: Positive-muon (${\ensuremath{\mu}}^{+}$) Knight shifts have been measured in the paramagnetic states of ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Nd}}_{x}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ alloys, where $x=0$, 0.25, 0.45, 0.50, 0.55, 0.75, and 1.00. In Pr-substituted ${\mathrm{NdOs}}_{4}{\mathrm{Sb}}_{12}$ ($x\ensuremath{\le}0.75$), but not in ${\mathrm{NdOs}}_{4}{\mathrm{Sb}}_{12}$, Clogston-Jaccarino plots of ${\ensuremath{\mu}}^{+}$ Knight shift $K$ versus magnetic susceptibility $\ensuremath{\chi}$ exhibit an anomalous saturation of $K(\ensuremath{\chi})$ at $\ensuremath{\sim}\ensuremath{-}0.5$% for large susceptibilities (low temperatures), indicating a reduction of the coupling strength between ${\ensuremath{\mu}}^{+}$ spins and $4f$ paramagnetism for temperatures $\ensuremath{\lesssim}15$ K. We speculate that itinerant ${\mathrm{Pr}}^{3+}$ quadrupolar excitations, invoked to mediate the superconducting Cooper-pair interaction, might modify the ${\ensuremath{\mu}}^{+}$-$4f$ ion indirect spin-spin interaction.

Structure and Physical Properties of RT2Cd20 (R = Rare Earth, T: Ni, Pd) Compounds with the CeCr2Al20-Type Structure.

Abstract: Single crystals of the new compounds LnNi2Cd20 (Ln: Y, La—Nd, Sm, Gd, Tb) and LnPd2Cd20 (Ln: Ce, Pr, Sm) are prepared from mixtures of the elements (alumina crucibles, 800—900 °C, 36—96 h).