Maple Group

Research Highlights

Superconductivity in layered BiS2-based Compounds

A novel family of superconductors based on BiS2-based superconducting layers were discovered in 2012. In short order, other BiS2-based superconductors with the same or related crystal structures were discovered with superconducting critical temperatures Tc of up to 10 K. Many experimental and theoretical studies have been carried out with the goal of establishing the basic properties of these new materials and understanding the underlying mechanism for superconductivity. In this selective review of the literature, we distill the central discoveries from this extensive body of work, and discuss the results from different types of experiments on these materials within the context of theoretical concepts and models. Read More

Weak hybridization and isolated localized magnetic moments in the compounds CeT2Cd20

The CeT2Cd20 (T = Ni, Pd) systems have a large separation between Ce ions, limiting the Ruderman-Kittel-Kasuya-Yosida interaction between the localized moments of these ions. However, these compounds were not found to order magnetically down to 0.138 K and under applied pressure of up to 2.5 GPa. This fragile magnetic order may have potential applications as a material for performing adiabatic demagnetization. Read More

Nodal to Nodeless Superconducting Energy-Gap Structure Change Concomitant with Fermi-Surface Reconstruction in the Heavy-Fermion Compound CeCoIn5

London penetration depth measurements were made on the system Ce1-xRxCoIn5 (R = La, Nd, Yb) down to T ≈ 50 mK. The power law behavior of the London penetration depth is Δλ(T) ~ Tn describes the Ce1-xRxCoIn5 system, where the value of n corresponds to the structure of the superconducting gap. In the case of CeCoIn5, n = 1, consistent with line nodes in the SC gap. As the rare earths (La, Nd, Yb) are doped into this system, TC is suppressed and n evolves. For La and Nd, n increases from 1 and saturates at about 2, which shows a change to a "dirty", nodal superconductor. However, for Yb, n increases above 3, indicating an evolution to a nodeless SC gap. These SC gap changes occur at about the same doping range as the reported Fermi surface reconstruction, suggesting the two phenomena are closely related. Read More

Enhancement of superconductivity in La1-xSmxO0.5F0.5BiS2

The superconducting transition temperature Tc and superconducting volume fraction were found to increase with x in the La1-xSmxO0.5F0.5BiS2 samples. The solubility limit of Sm has a large value of x ~ 0.8 in La1-xSmxO0.5F0.5BiS2, and a continuous decrease in the a-axis and increase in the c-axis is observed with increasing x. Bulk superconductivity was observed in the samples according to magnetic susceptibility and specific heat measurements. No evidence for a structural phase transition was found in this study. The results demonstrate that the superconducting critical temperature Tc of tetragonal BiS2-based compounds is correlated with the lattice parameters and can be significantly enhanced by Sm substitution. This gives a promising way to further increase the Tc of BiS2-based superconductors by modifying the blocking layers through the substitution of heavier Ln lanthanides (Ln = Eu - Tm) or synthesizing the parent LnO1-xFxBiS2 compounds. Read More

Electrodynamics of the antiferromagnetic phase of URu2Si2

In this work, two ordered phases of URu2Si2 have been studied spectroscopically: the hidden order phase and the antiferromagnetic phase, which is induced by Fe and Os substitution. The two phases show few differences other than an overall smooth increase in the gap and the transition temperature with substitution of Fe and Os. In the ordered states the gap and the transfer of spectral weight are characteristic of density waves and consistent with a partial gapping scenario. The normal states are also very similar: they are Fermi-liquid-like with a scaling factor b ≈ 1.0 characteristic of a Fermi liquid dominated by resonant impurity scattering. Read More

Pressure studies of the quantum critical alloy Ce0.93Yb0.07CoIn5

Heavy-fermion compound Ce0.93Yb0.07CoIn5 was further studied through electrical transport measurements under pressure due to having a well-established magnetic field-induced quantum critical point. Key findings include the observation of a suppression of the antiferromagnetic ordering quantum fluctuations as pressure is increased demonstrated by the behavior of the linear T term in resistivity corresponding to a heavier quasiparticle; in contrast, the contribution to the √T resistivity term remains fairly stable with pressure, suggesting that the inelastic scattering from separate, light effective mass quasiparticles is unaffected by applied pressure. Read More

Pressure-induced phase transition in La1-xSmxO0.5F0.5BiS2

Electrical resistivity measurements on polycrystalline samples of the BiS2-based superconductors La1-xSmxO0.5F0.5BiS2 (x = 0.1, 0.3, 0.6, 0.8) were performed from 2 K to room temperature under applied pressures. In the normal state, semiconducting-like behavior is suppressed with increasing pressure. A reversible low-Tc to high-Tc superconductor phase transition was observed in all of the samples at a pressure that is proportional to the Sm concentration. With increasing Sm concentration, Tc is suppressed, and a larger pressure is necessary to induce the transition from the SC1 phase to the SC2 phase. It is also found that an optimal Tc could be tuned by decreasing the a lattice parameter in the SC1 phase at ambient pressure or by increasing a in the SC2 phase under pressure. These results indicate that the high-pressure behavior of Sm-substituted LaO0.5F0.5BiS2 is largely determined by Sm concentration or the lattice parameter a at ambient pressure. Therefore, the evolution of Tc under pressure for the parent compound SmO0.5F0.5BiS2 can be estimated, and we find that the SC1 and SC2 phases exhibit almost indistinguishable Tc values; this result suggests that applied pressure may not induce a phase transition in SmO0.5F0.5BiS2. Read More

Investigation of superconducting and normal-state properties of the filled-skutterudite system PrPt4Ge12-xSbx

We have performed experiments on the pseudoternary alloy system PrPt4Ge12-xSbx to probe the unconventional superconductivity in PrPt4Ge12. We observed a suppression of Tc with x to x = 4. The electronic specific heat coefficient γ decreases with x in the superconducting region, indicating that the density of states may be an important factor in determining Tc. A Pr "rattling" mode with an Einstein temperature ϴE ≈ 60 K for 0 ≤ x ≤ 5 was derived from Rietveld refinement of XRD data and fits of an Einstein model to specific heat data; however, no correlation between Tc and ϴE was observed. The exponential T-dependence of the specific heat in the superconducting state for a sample with x = 0.5 suggests a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity with increasing x. Read More

Broken time-reversal symmetry in superconducting Pr1-xCexPt4Ge12

We report results of zero-field muon spin relaxation experiments on the filled-skutterudite superconductors Pr1-xCexPt4Ge12 to investigate the effect of Ce doping on broken time-reversal symmetry (TRS) in the superconducting state. In these alloys broken TRS is signaled by the onset of a spontaneous static local magnetic field Bs below the superconducting transition temperature. We find that Bs decreases linearly with x →0 at x≈0.4, close to the concentration above which superconductivity is no longer observed. The (Pr,Ce)Pt4Ge12 and isostructural (Pr,La)Os4Sb12 alloy series both exhibit superconductivity with broken TRS, and in both the decrease of Bs is proportional to the decrease of Pr concentration. This suggests that Pr-Pr intersite interactions are responsible for the broken TRS. The two alloy series differ in that the La-doped alloys are superconducting for all La concentrations, suggesting that in (Pr,Ce)Pt4Ge12 pair-breaking by Ce doping suppresses superconductivity. Read More

Quantum criticality and superconducting pairing in Ce1-xYbxCoIn5 alloys

Electrical transport measurements were made on the heavy-fermion system Ce1-xYbxCoIn5 under high fields and pressures. One particular observation was that the field-induced quantum critical point was completely suppressed at xnominal = 0.2. Also the superconducting critical temperature Tc and Kondo lattice coherence temperature Tcoh were both suppressed as x was increased. Of particular interest was the evolution of Tc and Tcoh as pressure was applied; what we observed was both parameters were both enhanced as pressure was increased. This is apparently conflicting with the theory for composite pairing where both parameters should behave opposite each other as pressure is applied. This particular question is very intriguing for further study (ref: Pressure studies of the quantum critical alloy Ce0.93Yb0.07CoIn5). Read More

Chemical Substitution and High Pressure Effects on Superconductivity in the LnOBiS2 (Ln = La-Nd) System

A large number of compounds which contain BiS2 layers exhibit enhanced superconductivity upon electron doping. Much interest and research effort has been focused on BiS2-based compounds which provide new opportunities for exploring the nature of superconductivity. Important to the study of BiS2-based superconductors is the relation between structure and superconductivity. By modifying either the superconducting BiS2 layers or the blocking layers in these layered compounds, one can effectively tune the lattice parameters, local atomic environment, electronic structure, and other physical properties of these materials. Some of the recent progress on research of the effects of chemical substitution in BiS2-based compounds, with special attention given to the compounds in the LnOBiS2 (Ln = La-Nd) system, are reviewed in this article. Strategies which are reported to be essential in optimizing superconductivity of these materials will also be discussed. Read More

Structure and physical properties of RT2Cd20 (R = rare earth, T = Ni, Pd) compounds with the CeCr2Al20-type structure

Eleven new RNi2Cd20 (R = Y, La-Nd, Sm, Gd, Tb) and RPd2Cd20 (R = Ce, Pr, Sm) were grown in high temperature, cadmium-rich solutions. These compounds formed as single crystals in the CeCr2Al20-type structure. Electrical resistivity, magnetization, and specific heat measurements were performed and the results discussed. These new X=Cd compounds allow for further investigations of correlated electron behavior in the RT2X20 family, which may not be present in the X=Al, Zn series of compounds. Read More

Investigation of magnetic order in SmTr2Zn20 (Tr = Fe, Co, Ru) and SmTr2Cd20 (Tr = Ni, Pd)

In a study of the caged 1-2-20 compounds SmTr2Zn20 (Tr = Fe, Co, Ru) and SmTr2Cd20 (Tr = Ni, Pd), the low temperature magnetic orders of these compounds is explored. Enhanced Sommerfeld coefficients and quadratic temperature dependence of low temperature resistivity suggest that SmRu2Zn20 and SmPd2Cd20 experience enhanced quasiparticle mass due to hybridization between localized 4ƒ and conduction electrons. It is suggested that SmRu2Zn20 is a member of a rare class of Sm-based heavy fermion ferromagnets. Read More

Chemical pressure tuning of URu2Si2 via isoelectronic substitution of Ru with Fe

Specific heat and neutron diffraction measurements are used to support the conclusion that chemical substitution of Ru with Fe acts as a "chemical pressure" in the URu2Si2 system. This chemical pressure effect of adding iron seems to mimic the effects of pressure upon the URu2Si2 system, providing a powerful way of measuring the pressure effects on the system at ambient pressures. Thus, this provides a means for exploring the mysterious "hidden order" of the URu2Si2 system. Read More

Conventional Magnetic Superconductors

Long-range magnetic order typically acts to destroy superconductivity. However, there are a host of "conventional magnetic superconductors" in which the interplay between superconductivity and magnetism give rise to some exotic and interesting behavior including reentrant superconductivity, magnetic field induced superconductivity, and the coexistence of antiferromagnetic order and superconductivity. This article addresses the effect of magnetic impurities on Tc in binary materials that led to the idea of a magnetic exchange interaction between the localized spins of the magnetic impurity ions and the spins of the conduction electrons. This is followed by a discussion of several classes of superconducting materials including ternary and quaternary compounds. Read More

Pressure-induced enhancement of superconductivity and suppression of semiconducting behavior in LnO0.5F0.5BiS2 (Ln = La, Ce) compounds

The superconducting transition temperature Tc observed for some of the high-Tc cuprates are known to respond dramatically to pressure. We found that the BiS2-based superconducting materials, which form in a layered crystal structure similar to the cuprate materials also exhibit large jumps in their values of Tc. For the LaO0.5F0.5BiS2 and CeO0.5F0.5BiS2 compounds we found that by applying pressure, we could induce a phase transition from a low-Tc phase to a high-Tc phase. In LaO0.5F0.5BiS2, there is a threefold enhancement of superconductivity as Tc rapidly jumps from 3.3 K at ambient pressure to 10 K at a critical pressure of PT ~ 1 GPa. Read More

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

This is a follow up study to our original investigation of the BiS2-based layered superconducting materials under pressure in which we observed a pressure-induced phase transition and an associated enhancement of superconductivity in the two LnO0.5F0.5BiS2 (Ln = La, Ce) compounds. We performed similar measurements of electrical resistivity for the LnO0.5F0.5BiS2 (Ln = Pr, Nd) compounds under pressure in which we observed a similar pressure dependence of Tc characterized by a rapid increase in Tc within a small range of pressure ~ 0.3 GPa from a low-Tc phase to a high-Tc phase. We observed that for the four LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd) compounds, both the critical pressure PT and the size of the jump in Tc both scale with the lanthanide element in LnO0.5F0.5BiS2. Read More

Evolution of critical pressure with increasing Fe substitution in the heavy-fermion system URu2-xFexSi2

Recently, it has been shown that the isoelectronic substitution of smaller ions of Fe for Ru in the heavy-fermion compound URu2Si2 acts as a chemical pressure Pch in which the evolution of T0 with Fe concentration x is remarkably consistent with the pressure dependence of T0. In this article, we report on the combined effect of Fe substitution and the application of pressure in the URu2-xFexSi2 system with (x = 0.025, 0.05, 0.10, 0.15 and 0.20). We found that the critical pressure was reduced from Pc = 1.2 GPa at x = 0.025 to Pc = 0 GPa at x = 0.15. Furthermore, by converting the concentration of Fe to a chemical pressure, Pch(x), we consistently found that the induced HO to LMAFM phase transition occurred at various combinations of x and P such that Pch(x) + P = 1.5 GPa. Read More