arXiv subjects

M. B. Maple

Publications and source records attributed to M. B. Maple.

At least 37 records · Page 2Linked to original sources

arXiv2018

Time Reversal Symmetry Breaking in the B Phase of the Heavy Fermion Superconductor PrOs$_4$Sb$_{12}$

We present polar Kerr effect measurements of the filled skutterudite superconductor PrOs$_4$Sb$_{12}$. Simultaneous AC susceptibility measurements allow us to observe the superconducting transition under the influence of heating from the optical beam. A nonzero Kerr angle $θ_K$ develops below the superconducting transition, saturating at $\sim 300$ nrad at low temperatures. This result is repeated across several measurements of multiple samples. By extrapolating the measured $θ_K(T)$ to zero optical power, we are able to show that the Kerr angle onset temperature in one set of measurements is consistent with the transition to the B phase at $T_{C2}$. We discuss the possible explanations for this result and its impact on the understanding of multi-phase and inhomogeneous superconductivity in PrOs$_4$Sb$_{12}$.

cond-mat.supr-con↗

arXiv2018

Search for Multipolar Instability in URu$_2$Si$_2$ Studied by Ultrasonic Measurements under Pulsed Magnetic Field

The elastic properties of URu$_2$Si$_2$ in the high-magnetic field region above 40 T, over a wide temperature range from 1.5 to 120 K, were systematically investigated by means of high-frequency ultrasonic measurements. The investigation was performed at high magnetic fields to better investigate the innate bare 5$f$-electron properties, since the unidentified electronic thermodynamic phase of unknown origin, so called `hidden order'(HO) and associated hybridization of conduction and $f$-electron ($c$-$f$ hybridization) are suppressed at high magnetic fields. From the three different transverse modes we find contrasting results; both the $Γ_4$(B$_{\rm 2g}$) and $Γ_5$(E$_{\rm g}$) symmetry modes $C_{66}$ and $C_{44}$ show elastic softening that is enhanced above 30 T, while the characteristic softening of the $Γ_3$(B$_{\rm 1g}$) symmetry mode $(C_{11}-C_{12})/2$ is suppressed in high magnetic fields. These results underscore the presence of a hybridization-driven $Γ_3$(B$_{\rm 1g}$) lattice instability in URu$_2$Si$_2$. However, the results from this work cannot be explained by using existing crystalline-electric field (CEF) schemes applied to the quadrupolar susceptibility in a local $5f^2$ configuration. Instead, we present an analysis based on a band Jahn-Teller effect.

cond-mat.str-el↗

arXiv2018

Observation of transition from semiconducting to metallic ground state in high-quality single crystalline FeSi

We report anomalous physical properties of single-crystalline FeSi over a wide temperature range 1.8-400 K. X-ray diffraction, specific heat, and magnetization measurements indicate that the FeSi crystals synthesized in this study are of high quality with a very low concentration of magnetic impurities ($\sim$0.01$\%$). The electrical resistivity $ρ$($T$) can be described by activated behavior with an energy gap $Δ$ = 57 meV between 67 K and 150 K. At temperatures below 67 K, $ρ$($T$) is significantly lower than an extrapolation of the activated behavior, and the Hall coefficient and magneto-resistivity undergo a sign change in this region. At $\sim$19 K, a transition from semiconducting to metallic-like behavior is observed with deceasing temperature. Whereas the transition temperature is very robust in a magnetic field, the magnitude of the resistivity below $\sim$30 K is very sensitive to magnetic field. There is no indication of a bulk phase transition or onset of magnetic order in the vicinity of either 67 K or 19 K from specific heat and magnetic susceptibility measurements. These measurements provide evidence for a conducting surface state in FeSi at low temperatures.

cond-mat.mtrl-sci↗

arXiv2018

Measurements of the NMR Knight shift tensor and nonlinear magnetization in URu$_2$Si$_2$

URu$_2$Si$_2$ exhibits an anomalous peak in the nonlinear magnetic susceptibility at the hidden order transition. In order to investigate this anomaly, we conducted direct magnetization measurements and investigated the detailed angular dependence of the $^{29}$Si nuclear magnetic resonance Knight shift tensor. We find that the nonlinear magnetization is smaller than previously reported, and the analogous nonlinear Knight shift tensor is below the detection limit. Our results suggest that the magnitude of the anomalous peak is sample dependent.

cond-mat.str-el↗

arXiv2018

The effective increase in atomic scale disorder by doping and superconductivity in Ca$_3$Rh$_4$Sn$_{13}$

The comprehensive research of the electronic structure, thermodynamic and electrical transport properties reveals the existence of inhomogeneous superconductivity due to structural disorder in Ca$_3$Rh$_4$Sn$_{13}$ doped with La (Ca$_{3-x}$La$_x$Rh$_4$Sn$_{13}$) or Ce (Ca$_{3-x}$Ce$_x$Rh$_4$Sn$_{13}$) with superconducting critical temperatures $T_c^{\star}$ higher than those ($T_c$) observed in the parent compounds. The $T-x$ diagrams and the entropy $S(x)_T$ isotherms well document the relation between degree of an atomic disorder and separation of the {\it high-temperature} $T_c^{\star}$ and $T_c$-bulk phases. In these dirty superconductors with the mean free path much smaller than the coherence length, the Werthamer-Helfand-Hohenber theoretical model does not well fits the $H_{c2}(T)$ data. We suggest that this can result from two-band superconductivity or from the presence of strong inhomogeneity in these systems. The multiband model very well describes the $H-T$ dependencies, but the present results as well as our previous studies give arguments for the scenario based on the presence of nanoscopic inhomogeneity of the superconducting state. We also revisited the nature of structural phase transition at $T^{\star}\sim 130-170$ K and documented that there might be another precursor transition at higher temperatures. The impact of the magnetic Ce-Ce correlations on the increase of $T_c$ in respect to the critical temperatures of Ca$_{3-x}$La$_x$Rh$_4$Sn$_{13}$ is also discussed.

cond-mat.supr-con↗

arXiv2018

High pressure effects on non-fluorinated BiS2-based superconductors La$_{1-x}$M$_x$OBiS$_2$ (M = Ti and Th)

Layered \textit{Ln}OBiS$_2$ compounds with \textit{Ln} = La, Ce, Pr, Nd, and Yb can be rendered conducting and superconducting via two routes, substitution of F for O or the tetravalent ions Ti, Zr, Hf, and Th for trivalent \textit{Ln} ions. Electrical resistivity measurements on non-fluorinated La$_{0.80}$Ti$_{0.20}$OBiS$_2$ and La$_{0.85}$Th$_{0.15}$OBiS$_2$ superconductors were performed between $\sim$1.5 K and 300 K and under pressure up to 2.4 GPa. For both compounds, the superconducting transition temperature $T_c$, which is $\sim$2.9 K at ambient pressure, gradually increases with pressure to 3.2-3.7 K at $\sim$1 GPa, above which it is suppressed and the superconducting transitions become very broad. Measurements of the normal state electrical resistivity of the two compounds reveal discontinuous changes of the resistivity as a function of pressure at $\sim$0.6 GPa. Surprisingly, above 1.3 GPa, semiconducting-like behavior reappears in La$_{0.80}$Ti$_{0.20}$OBiS$_2$. This study reveals a new high-pressure phase of La$_{1-x}$$M$$_x$OBiS$_2$ containing the tetravalent ions $M$ = Ti, Th which does not favor superconductivity. In contrast, application of pressure to fluorinated LaO$_0.5$F$_0.5$BiS$_2$ produces an abrupt tetragonal-monoclinic transition to a metallic phase with an enhanced $T_c$. These results demonstrate that the response of the normal and superconducting properties of LaOBiS$_2$-based compounds depends strongly on the atomic site where the electron donor ions are substituted.

cond-mat.supr-con↗

arXiv2017

Evolution of the Kondo lattice electronic structure above the transport coherence temperature

The temperature-dependent evolution of the Kondo lattice is a long-standing topic of theoretical and experimental investigation and yet it lacks a truly microscopic description of the relation of the basic $f$-$d$ hybridization processes to the fundamental temperature scales of Kondo screening and Fermi-liquid lattice coherence. Here, the temperature-dependence of $f$-$d$ hybridized band dispersions and Fermi-energy $f$ spectral weight in the Kondo lattice system CeCoIn$_5$ is investigated using $f$-resonant angle-resolved photoemission (ARPES) with sufficient detail to allow direct comparison to first principles dynamical mean field theory (DMFT) calculations containing full realism of crystalline electric field states. The ARPES results, for two orthogonal (001) and (100) cleaved surfaces and three different $f$-$d$ hybridization scenarios, with additional microscopic insight provided by DMFT, reveal $f$ participation in the Fermi surface at temperatures much higher than the lattice coherence temperature, $T^*\approx$ 45 K, commonly believed to be the onset for such behavior. The identification of a $T$-dependent crystalline electric field degeneracy crossover in the DMFT theory $below$ $T^*$ is specifically highlighted.

cond-mat.str-el↗

arXiv2016

Quantum critical scaling in the disordered itinerant ferromagnet UCo$_{1-x}$Fe$_x$Ge

Belitz-Kirkpatrick-Vojta (BKV) theory shows in excellent agreement with experiment that ferromagnetic quantum phase transitions (QPTs) in clean metals are generally first-order due to the coupling of the magnetization to electronic soft modes, in contrast to the classical analogue that is an archetypical second-order phase transition. For disordered metals BKV theory predicts that the second order nature of the QPT is restored because the electronic soft modes change their nature from ballistic to diffusive. Our low-temperature magnetization study identifies the ferromagnetic QPT in the disordered metal UCo$_{1-x}$Fe$_x$Ge as the first clear example that exhibits the associated critical exponents predicted by BKV theory.

cond-mat.str-el↗

arXiv2016

Analogy between the "Hidden Order" and the Orbital Antiferromagnetism in URu$_{2-x}$Fe$_x$Si$_2$

We study URu$_{2-x}$Fe$_x$Si$_2$, in which two types of staggered phases compete at low temperature as the iron concentration $x$ is varied: the nonmagnetic "hidden order" (HO) phase below the critical concentration $x_c$, and unconventional antiferromagnetic (AF) phase above $x_c$. By using polarization resolved Raman spectroscopy, we detect a collective mode of pseudovector-like $A_{2g}$ symmetry whose energy continuously evolves with increasing $x$; it monotonically decreases in the HO phase until it vanishes at $x=x_c$, and then reappears with increasing energy in the AF phase. The mode's evolution provides direct evidence for unified order parameter for both nonmagnetic and magnetic phases arising from the orbital degrees-of-freedom of the uranium-5$f$ electrons.

cond-mat.str-el↗

arXiv2016

Superconducting gap structure in ambient-pressure-grown $\mathrm{LaO_{0.5}F_{0.5}BiS_{2}}$

We have performed transverse-field muon spin relaxation (TF-$μ$SR) measurements on ambient-pressure-grown polycrystalline $\mathrm{LaO_{0.5}F_{0.5}BiS_{2}}$. From these measurements, no signature of magnetic order is found down to 25 mK. The value of the magnetic penetration depth extrapolated to 0 K is 0.89 (5) $μ$m. The temperature dependence of superconducting penetration depth is best described by either a multigap s + s-wave model with $Δ_{1}$ = 0.947 (7) meV and $Δ_{2}$ = 0.22 (4) meV or the ansiotropic s-wave model with $Δ(0)$ = 0.776 meV and anisotropic gap amplitude ratio $Δ_{min}/Δ_{max}$ = 0.34. Comparisons with other potentially multigap $\mathrm{BiS_{2}}$-based superconductors are discussed. We find that these $\mathrm{BiS_{2}}$-based superconductors, including $\mathrm{Bi_{4}O_{4}S_3}$ and the high-pressure synthesized $\mathrm{LaO_{0.5}F_{0.5}BiS_{2}}$, generally conform to the Uemura relation.

cond-mat.supr-con↗

arXiv2016

Upper critical magnetic field of $Ln$O$_{0.5}$F$_{0.5}$BiS$_2$ ($Ln$ = La, Nd) superconductors at ambient and high pressure

The upper critical field $H_{c2}$ of polycrystalline samples of $Ln$O$_{0.5}$F$_{0.5}$BiS$_{2}$ ($Ln$ = La, Nd) at ambient pressure (tetragonal structure) and high pressure (HP) (monoclinic structure) have been investigated via electrical resistivity measurements at various magnetic fields up to 8.5 T. The $H_{c2}$($T$) curves for all the samples show an uncharacteristic concave upward curvature at temperatures below $T_c$, which cannot be described by the conventional one-band Werthamer-Helfand-Hohenberg theory. For the LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ sample under HP, as temperature is decreased, the upper critical field $H_{onset}$, estimated from the onset of the superconducting transitions, increases slowly between 4.9 and 5.8 T compared with the slope of $H_{onset}$($T$) below 4.9 T and above 5.8 T. This anomalous behavior reveals a remarkable similarity in superconductivity between LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ samples measured under HP and synthesized under HP, although the crystal structures of the two samples were reported to be different. The experimental results support the idea that local atomic environment, which can be tuned by applying external pressure and can be quenched to ambient pressure via high temperature-pressure annealing, is possibly more essential to the enhancement of $T_c$ for BiS$_2$-based superconductors than the structural phase transition. On the other hand, such anomalous behavior is very subtle in the case of NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ under HP, suggesting that the anisotropy of the upper critical field in the $ab$-plane and the possible lattice deformation induced by external pressure is weak. This explains why the pressure-induced enhancement of $T_c$ for NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ is not as large as that for LaO$_{0.5}$F$_{0.5}$BiS$_{2}$.

cond-mat.supr-con↗

arXiv2016

Study of Localized Character of 4f Electrons and Ultrasonic Dispersions in SmOs4Sb12 by High-Pressure High-Frequency Ultrasonic Measurements

We present high-frequency ultrasonic measurements on the filled skutterudite SmOs$_4$Sb$_{12}$ under hydrostatic pressure. The results clarify that the 4$f$ electrons in this compound transform from delocalized at ambient pressure to localized at high pressures with a crossover pressure of approximately 0.7 GPa. This drastic change in the 4$f$ electrons under pressure is apparently related to the non-Fermi liquid state, which appears in an intermediate-pressure range of 0.5-1.5 GPa. The results or our analysis strongly suggest that the ferro-octupolar interaction becomes dominant at high pressure. Moreover, we report the pressure dependence of the ultrasonic dispersion, which is due to rattling, over a wide range of ultrasonic frequencies up to 323 MHz. The drastic change in the ultrasonic dispersions and the frequency dependent elastic anomaly in the $C_{11}$ mode at lower temperatures imply a possible coupling between rattling phonons and 4$f$ electrons.

cond-mat.str-el↗

arXiv2016

Robust Upward Dispersion of the Neutron Spin Resonance in the Heavy Fermion Superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$

The neutron spin resonance is a collective magnetic excitation that appears in copper oxide, iron pnictide, and heavy fermion unconventional superconductors. Although the resonance is commonly associated with a spin-exciton due to the $d$($s^{\pm}$)-wave symmetry of the superconducting order parameter, it has also been proposed to be a magnon-like excitation appearing in the superconducting state. Here we use inelastic neutron scattering to demonstrate that the resonance in the heavy fermion superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$ with $x=0,0.05,0.3$ has a ring-like upward dispersion that is robust against Yb-doping. By comparing our experimental data with random phase approximation calculation using the electronic structure and the momentum dependence of the $d_{x^2-y^2}$-wave superconducting gap determined from scanning tunneling microscopy for CeCoIn$_5$, we conclude the robust upward dispersing resonance mode in Ce$_{1-x}$Yb$_{x}$CoIn$_5$ is inconsistent with the downward dispersion predicted within the spin-exciton scenario.

cond-mat.supr-con↗

arXiv2016

Multiband superconductivity in the correlated electron filled skutterudite system Pr(1-x)Ce(x)Pt4Ge12

Studies of superconductivity in multiband correlated electronic systems has become one of the central topics in condensed matter/materials physics. In this paper, we present the results of thermodynamic measurements on the superconducting filled skutterudite system Pr$_{1-x}$Ce$_x$Pt$_4$Ge$_{12}$ ($ 0 \leq x \leq 0.2$) to investigate how substitution of Ce at Pr sites affects superconductivity. We find that an increase in Ce concentration leads to a suppression of the superconducting transition temperature from $T_{c}\sim 7.9$ K for $x=0$ to $T_c\sim 0.6$ K for $x=0.14$. Our analysis of the specific heat data for $x\leq 0.07$ reveals that superconductivity must develop in at least two bands: the superconducting order parameter has nodes on one Fermi pocket and remains fully gapped on the other. Both the nodal and nodeless gap values decrease, with the nodal gap being suppressed more strongly, with Ce substitution. Ultimately, the higher Ce concentration samples ($x>0.07$) display a nodeless gap only.

cond-mat.str-el↗

arXiv2016

Effect of atomic disorder and Ce doping on superconductivity of Ca3Rh4Sn13: Electric transport properties under high pressure

We report the observation of a superconducting state below 8K coexistent with a spin-glass state caused by atomic disorder in Ce substituted Ca3Rh4Sn13. Measurements of specific heat, resistivity, and magnetism reveal the existence of inhomogeneous superconductivity in samples doped with Ce with superconducting critical temperatures Tc higher than those observed in the parent compound. For Ca3Rh4Sn13, the negative value of the change in resistivity with pressure P, correlates well with the calculated decrease in the density of states at the Fermi energy with P. Based on band structure calculations performed under pressure, we demonstrate how the change in DOS would affect Tc of Ca3Rh4Sn13 under negative lattice pressure in samples that are strongly defected by quenching.

cond-mat.supr-con↗

arXiv2016

Phase diagram and thermal expansion measurements on the system of URu_{2-x}Fe_xSi_2

Thermal expansion, electrical resistivity, magnetization, and specific heat measurements were performed on URu$_{2-x}$Fe$_{x}$Si$_{2}$ single crystals for various values of the Fe concentration $x$ in both the hidden order (HO) and large moment antiferromagnetic (LMAFM) regions of the phase diagram. Our results show that the paramagnetic (PM) to HO and LMAFM phase transitions are manifested differently in the thermal expansion coefficient. For Fe concentrations near the boundary between the HO and LMAFM phases at $x_c$ ~ 0.1, we observe two features in the thermal expansion upon cooling, one that appears to be associated with the transition from the PM to the HO phase and another one at lower temperature that may be due to the transition from the HO to the LMAFM phase. These two features have not been observed in other measurements such as specific heat or neutron scattering. In addition, the uniaxial pressure derivative of the transition temperature, based on a calculation using thermal expansion and specific heat data, changes dramatically when crossing from the HO to the LMAFM phase.

cond-mat.str-el↗

arXiv2016

Investigation of superconducting and normal-state properties of the filled-skutterudite system PrPt$_{4}$Ge$_{12-x}$Sb$_{x}$

We report a study of the superconducting and normal-state properties of the filled-skutterudite system PrPt$_{4}$Ge$_{12-x}$Sb$_x$. Polycrystalline samples with Sb concentrations up to $x =$ 5 were synthesized and investigated by means of x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements. We observed a suppression of superconductivity with increasing Sb substitution up to $x =$ 4, above which, no signature of superconductivity was observed down to 140 mK. The Sommerfeld coefficient, $γ$, of superconducting specimens decreases with increasing $x$ up to $x =$ 3, suggesting that superconductivity may depend on the density of electronic states in this system. The specific heat for $x =$ 0.5 exhibits an exponential temperature dependence in the superconducting state, reminiscent of a nodeless superconducting energy gap. We observed evidence for a weak "rattling" mode associated with the Pr ions, characterized by an Einstein temperature $Θ_{\mathrm{E}} \sim$ 60 K for 0 $\leq x \leq$ 5; however, the rattling mode may not play any role in suppressing superconductivity.

cond-mat.supr-con↗

arXiv2016

Chemical Substitution and High Pressure Effects on Superconductors in the $Ln$OBiS$_{2}$ ($Ln$ = La-Nd) System

A large number of compounds which contain BiS$_{2}$ layers exhibit enhanced superconductivity upon electron doping. Much interest and research effort has been focused on BiS$_{2}$-based compounds which provide new opportunities for exploring the nature of superconductivity. Important to the study of BiS$_{2}$-based superconductors is the relation between structure and superconductivity. By modifying either the superconducting BiS$_2$ 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. In this article, we will review some of the recent progress on research of the effects of chemical substitution in BiS$_{2}$-based compounds, with special attention given to the compounds in the $Ln$OBiS$_{2}$ ($Ln$ = La-Nd) system. Strategies which are reported to be essential in optimizing superconductivity of these materials will also be discussed.

cond-mat.supr-con↗