Showing papers by "M. B. Maple published in 2013"

Superconductivity of F-substituted LnOBiS2 (Ln=La, Ce, Pr, Nd, Yb) compounds

Abstract: Polycrystalline samples of F-substituted LnOBiS2 (Ln = La, Ce, Pr, Nd, Yb) compounds were synthesized by solid-state reaction. The samples were characterized by X-ray diffraction measurements and found to have the ZrCuSiAs crystal structure. Electrical resistivity and magnetic susceptibility measurements were performed on all of the samples and specific heat measurements were made on those with Ln = La, Ce, and Yb. All of these compounds exhibit superconductivity in the range 1.9 K–5.4 K, which has not previously been reported for the compounds based on Ce, Pr, and Yb. The YbO0.5F0.5BiS2 compound was also found to exhibit magnetic order at ∼2.7 K that apparently coexists with superconductivity below 5.4 K.

Pressure-induced unconventional superconducting phase in the topological insulator Bi2Se3.

Abstract: Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi2Se3 under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature Tc and upper critical field Hc2 that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, Tc remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where Tc remains constant. Together with a quasilinear temperature dependence of Hc2 that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc points to an unconventional pressure-induced pairing state in Bi2Se3 that is unique among the superconducting topological insulators.

Superconductivity induced by electron doping in the system La1-xMxOBiS2 (M = Ti, Zr, Hf, Th)

Abstract: We report a strategy to induce superconductivity in the BiS$_2$-based compound LaOBiS$_2$ Instead of substituting F for O, we increase the charge-carrier density (electron dope) via substitution of tetravalent Th$^{+4}$, Hf$^{+4}$, Zr$^{+4}$, and Ti$^{+4}$ for trivalent La$^{+3}$ It is found that both the LaOBiS$_2$ and ThOBiS$_2$ parent compounds are bad metals and that superconductivity is induced by electron doping with \emph{T$_c$} values of up to 285 K The superconducting and normal states were characterized by electrical resistivity, magnetic susceptibility, and heat capacity measurements We also demonstrate that reducing the charge-carrier density (hole doping) via substitution of divalent Sr$^{+2}$ for La$^{+3}$ does not induce superconductivity

Superconductivity induced by electron doping in La 1-x M x OBiS 2 (M= Ti, Zr, Hf, Th)

Abstract: We report a strategy to induce superconductivity in the BiS${}_{2}$-based compound LaOBiS${}_{2}$. Instead of substituting F for O, we increase the charge-carrier density (electron dope) via substitution of tetravalent Th${}^{+4}$, Hf${}^{+4}$, Zr${}^{+4}$, and Ti${}^{+4}$ for trivalent La${}^{+3}$. It is found that both the LaOBiS${}_{2}$ and ThOBiS${}_{2}$ parent compounds are bad metals and that superconductivity is induced by electron doping with T${}_{c}$ values of up to 2.85 K. The superconducting and normal states were characterized by electrical resistivity, magnetic susceptibility, and heat capacity measurements. We also demonstrate that reducing the charge-carrier density (hole doping) via substitution of divalent Sr${}^{+2}$ for La${}^{+3}$ does not induce superconductivity.

Pressure-induced enhancement of superconductivity and suppression of semiconducting behavior in L n O 0.5 F 0.5 BiS 2 ( L n = La ,Ce) compounds

Abstract: Electrical resistivity measurements as a function of temperature between 1 and 300 K were performed at various pressures up to 3 GPa on the superconducting layered compounds $\mathit{Ln}$O${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ ($\mathit{Ln}$ = La, Ce). At atmospheric pressure, LaO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ and CeO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ have superconducting critical temperatures ${T}_{c}$ of 3.3 and 2.3 K, respectively. For both compounds, the superconducting critical temperature ${T}_{c}$ initially increases, reaches a maximum value of 10.1 K for LaO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ and 6.7 K for CeO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$, and then gradually decreases with increasing pressure. Both samples also exhibit transient behavior in the region between the lower ${T}_{c}$ phase near atmospheric pressure and the higher ${T}_{c}$ phase at higher pressures. This region is characterized by a broadening of the superconducting transition, in which ${T}_{c}$ and the transition width $\ensuremath{\Delta}$${T}_{c}$ are reversible with increasing and decreasing pressure. There is also an appreciable pressure-induced and hysteretic suppression of semiconducting behavior up to the pressure at which the maximum value of ${T}_{c}$ is found. At pressures above the value at which the maximum in ${T}_{c}$ occurs, there is a gradual decrease of ${T}_{c}$ and further suppression of the semiconducting behavior with pressure, both of which are reversible.

Enhancement of superconductivity near the pressure-induced semiconductor-metal transition in BiS2-based compounds LnO(0.5)F(0.5)BiS2 (Ln = La, Ce, Pr, Nd)

Abstract: Measurements of electrical resistivity were performed between 3 and 300 K at various pressures up to 2.8 GPa on the BiS2-based superconductors LnO0.5F0.5BiS2 (Ln = Pr, Nd). At lower pressures, PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2 exhibit superconductivity with critical temperatures Tc of 3.5 and3.9 K, respectively. As pressure is increased, both compounds undergo a transition at a pressure Pt from a low Tc superconducting phase to a high Tc superconducting phase in which Tc reaches maximum values of 7.6 and 6.4 K for PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2, respectively. The pressure-induced transition is characterized by a rapid increase in Tc within a small range in pressure of ~0.3 GPa for both compounds. In the normal state of PrO0.5F0.5BiS2, the transition pressure Pt correlates with the pressure where the suppression of semiconducting behaviour saturates. In the normal state of NdO0.5F0.5BiS2, Pt is coincident with a semiconductor-metal transition. This behaviour is similar to the results recently reported for the LnO0.5F0.5BiS2 (Ln = La, Ce) compounds. We observe that Pt and the size of the jump in Tc between the two superconducting phases both scale with the lanthanide element in LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd).

Enhancement of superconductivity near the pressure-induced semiconductor–metal transition in the BiS2-based superconductors LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd)

Abstract: Measurements of electrical resistivity were performed between 3 and 300 K at various pressures up to 2.8 GPa on the BiS2-based superconductors LnO0.5F0.5BiS2 (Ln=Pr, Nd). At lower pressures, PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2 exhibit superconductivity with critical temperatures Tc of 3.5 and 3.9 K, respectively. As pressure is increased, both compounds undergo a transition at a pressure Pt from a low Tc superconducting phase to a high Tc superconducting phase in which Tc reaches maximum values of 7.6 and 6.4 K for PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2, respectively. The pressure-induced transition is characterized by a rapid increase in Tc within a small range in pressure of ∼0.3 GPa for both compounds. In the normal state of PrO0.5F0.5BiS2, the transition pressure Pt correlates with the pressure where the suppression of semiconducting behaviour saturates. In the normal state of NdO0.5F0.5BiS2, Pt is coincident with a semiconductor–metal transition. This behaviour is similar to the results recently reported for the LnO0.5F0.5BiS2 (Ln=La, Ce) compounds. We observe that Pt and the size of the jump in Tc between the two superconducting phases both scale with the lanthanide element in LnO0.5F0.5BiS2 (Ln=La, Ce, Pr, Nd).

Yb valence change in Ce 1 − x Yb x CoIn 5 from spectroscopy and bulk properties

Abstract: The electronic structure of Ce${}_{1\ensuremath{-}x}$Yb${}_{x}$CoIn${}_{5}$ has been studied by a combination of photoemission, x-ray absorption, and bulk property measurements. Previous findings of a Ce valence near 3$+$ for all $x$ and of an Yb valence near 2.3$+$ for $x\ensuremath{\geqslant}0.3$ were confirmed. One new result of this study is that the Yb valence for $x\ensuremath{\leqslant}0.2$ increases rapidly with decreasing $x$ from 2.3 toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around $x=0.2$. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of $\ensuremath{\approx}$50$%$ cross-sectional area reductions of the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ sheets for $x=1$ compared to $x=0$, and a smaller, essentially proportionate, size change of the $\ensuremath{\alpha}$ sheet for $x=0.2$. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing $x$ in this alloy system are discussed.

Non-Fermi liquid regimes with and without quantum criticality in Ce1−xYbxCoIn5

Abstract: One of the greatest challenges to Landau’s Fermi liquid theory—the standard theory of metals—is presented by complex materials with strong electronic correlations. In these materials, non-Fermi liquid transport and thermodynamic properties are often explained by the presence of a continuous quantum phase transition that happens at a quantum critical point (QCP). A QCP can be revealed by applying pressure, magnetic field, or changing the chemical composition. In the heavy-fermion compound CeCoIn5, the QCP is assumed to play a decisive role in defining the microscopic structure of both normal and superconducting states. However, the question of whether a QCP must be present in the material’s phase diagram to induce non-Fermi liquid behavior and trigger superconductivity remains open. Here, we show that the full suppression of the field-induced QCP in CeCoIn5 by doping with Yb has surprisingly little impact on both unconventional superconductivity and non-Fermi liquid behavior. This implies that the non-Fermi liquid metallic behavior could be a new state of matter in its own right rather than a consequence of the underlying quantum phase transition.

Ferromagnetic quantum critical point in UCo 1-x Fe x Ge

Abstract: We have carried out a comprehensive study of the UCo1-xFexGe series across the entire range of compositions 0 <= x <= 1, and report the results of x-ray diffraction, magnetization, specific heat, and electrical resistivity to uncover the T-x phase diagram. Substitution of Fe into UCoGe initially results in an increase in the Curie temperature and a rapid destruction of the superconductivity. Near x = 0.22, the ferromagnetic transition is suppressed to zero temperature at an apparent ferromagnetic itinerant electron quantum critical point, where the temperature dependence of the electrical resistivity and specific heat in this region reveal non-Fermi liquid behavior.

Hybridization-driven orthorhombic lattice instability in URu_{2}Si_{2}

Abstract: We have measured the elastic constant (C11-C12)/2 in URu2Si2 by means of high-frequency ultrasonic measurements in pulsed magnetic fields H || [001] up to 61.8 T in a wide temperature range from 1.5 to 116 K. We found a reduction of (C11-C12)/2 that appears only in the temperature and magnetic field region in which URu2Si2 exhibits a heavy-electron state and hidden-order. This change in (C11-C12)/2 appears to be a response of the 5f-electrons to an orthorhombic and volume conservative strain field \epsilon_xx-\epsilon_yy with {\Gamma}3-symmetry. This lattice instability is likely related to a symmetry-breaking band instability that arises due to the hybridization of the localized f electrons with the conduction electrons, and is probably linked to the hidden-order parameter of this compound.

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.

Nuclear magnetic resonance studies of pseudospin fluctuations in URu2Si2

Abstract: Here, we report 29Si nuclear magnetic resonance measurements in single crystals and aligned powders of URu2Si2 in the hidden order and paramagnetic phases. The spin-lattice relaxation data reveal evidence of pseudospin fluctuations of U moments in the paramagnetic phase. We find evidence for partial suppression of the density of states below 30 K and analyze the data in terms of a two-component spin-fermion model. We propose that this behavior is a realization of a pseudogap between the hidden-order transition THO and 30 K. This behavior is then compared to other materials that demonstrate precursor fluctuations in a pseudogap regime above a ground state with long-range order.

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 \URuSi\ 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 $S_\parallel$ 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_\perp ~ \approx~0.01-0.04 $\mu_B$/U. In order to separate $S_\parallel$ and $S_\perp$ 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 $S_\parallel$. Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is $S^{\rm{max}}_\parallel ~ \leq~1\cdot 10^{-3} ~\mu_B$/U.

Superconductivity of F-substituted LnOBiS2 (Ln = La, Ce, Pr, Nd, Yb) compounds

Abstract: Polycrystalline samples of F-substituted LnOBiS2 (Ln = La, Ce, Pr, Nd, Yb) compounds were synthesized by solid-state reaction. The samples were characterized by x-ray diffraction measurements and found to have the ZrCuSiAs crystal structure. Electrical resistivity and magnetic susceptibility measurements were performed on all of the samples and specific heat measurements were made on those with Ln = La, Ce, and Yb. All of these compounds exhibit superconductivity in the range 1.9 K - 5.4 K, which has not previously been reported for the compounds based on Ce, Pr, and Yb. The YbO0.5F0.5BiS2 compounds was also found to exhibit magnetic order at ~2.7 K that apparently coexists with superconductivity below 5.4 K.

Magnetotransport properties of single-crystalline LaFeAsO

Abstract: Measurements of magnetization, specific heat, electrical resistivity, Hall effect, and magnetoresistance on single crystalline samples of LaFeAsO grown in a NaAs flux are reported. While this material is known to be a semimetal, the temperature dependence of the electrical resistivity data presented herein is reminiscent of semiconducting behavior and exhibits distinct features associated with a structural transition and spin density wave (SDW) order. Low-temperature x-ray diffraction measurements have confirmed that the structural transition in these samples occurs near 140 K, compared to a transition temperature of 156 K observed in polycrystalline samples. Magnetoresistance and Hall coefficient measurements were performed in magnetic fields up to 9 T applied perpendicular to the basal plane using a van der Pauw configuration. The charge carrier density and mobility indicate that electrons are the majority charge carriers and exhibit features indicative of the structural transition and SDW formation. Isotherms of magnetoresistivity measured as a function of magnetic field can be scaled onto a single curve.

Dependence of the superconducting transition temperature of the filled skutterudite compound PrPt4Ge12 on hydrostatic pressure

Abstract: The temperature-dependent ac susceptibility of the filled skutterudite superconductor PrPt 4 Ge 12 has been measured under hydrostatic He-gas pressure to 0.58 GPa. The superconducting transition temperature T c decreases linearly with pressure P from 7.91 K at ambient pressure to 7.83 K at 0.58 GPa, giving the rate dT c / dP = − 0.19 ± 0.03 K/GPa. Evidence is presented that suggests that the value of T c in this compound is slightly reduced due to magnetic pair-breaking effects from the Pr 3+ cations.

Thermoelectric Properties of Correlated Electron Systems Ln3Pt4Ge6and LnPt4Ge12(Ln = Ce, Pr) and Non-centrosymmetric X2T12P7(X = Yb, Hf and T = Fe, Co)

Abstract: We report measurements of the thermoelectric power Sand electrical resistivity ρ for correlated electron systems Ln 3Pt4Ge6and LnPt4Ge12(Ln= Ce, Pr) and X 2 T 12P7(X= Yb, Hf and T= Fe, Co). The thermoelectric power factor S 2∕ ρ is utilized as a means to assess the potential viability of these materials for thermoelectric applications. Sis observed to be sensitive to Lnin Ln 3Pt4Ge6and LnPt4Ge12with Ce-based compounds providing a much larger Sand S 2∕ ρ than Pr-based materials. The character of Sfor the Ce-based compounds is consistent with an intermediate Ce valence. In the case of X 2 T 12P7compounds reported herein, it appears that it is possible to tune the magnitude of Smore effectively by varying Trather than X; the magnitude of Sis significantly larger with T= Fe than when T= Co.

Local distortions about Nd in NdOs4Sb12

Abstract: We present an extended X-ray absorption fine structure (EXAFS) study of the NdOs4Sb12 system at the Nd and Os LIII edges. Nd is expected to be a “rattler” atom in this skutterudite crystal and we obtain a low Einstein temperature (61 K) as expected. However, although ultrasonic measurements have suggested a large off-center displacement of Nd, the EXAFS data are inconsistent with such a model. An analysis of the first two Os-Sb peaks in the Os LIII data also shows no evidence for unusual disorder. In contrast the Nd-Os peak in the Nd LIII data and the Os-Os peak in the Os LIII data both show significant disorder at 4 K, with the largest disorder found for the Nd-Os pair. This suggests that a distortion of the structure that involves mainly the Os atoms is present; it is likely that this distortion - which may also be dynamic - also causes the anomalies in the ultrasonic data (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.