arXiv subjects

M. B. Maple

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

At least 163 records · Page 9Linked to original sources

arXiv2003

Superconductivity and the high field ordered phase in the heavy fermion compound PrOs$_4$Sb$_{12}$

Superconductivity is observed in the filled skutterudite compound \PrOsSb{} below a critical temperature temperature $T_\mathrm{c} = 1.85$ K and appears to develop out of a nonmagnetic heavy Fermi liquid with an effective mass $m^{*} \approx 50 m_\mathrm{e}$, where $m_\mathrm{e}$ is the free electron mass. Features associated with a cubic crystalline electric field are present in magnetic susceptibility, specific heat, electrical resistivity, and inelastic neutron scattering measurements, yielding a Pr$^{3+}$ energy level scheme consisting of a $Γ_{3}$ nonmagnetic doublet ground state, a low lying $Γ_{5}$ triplet excitied state at $\sim 10$ K, and much higher temperature $Γ_{4}$ triplet and $Γ_{1}$ singlet excited states. Measurements also indicate that the superconducting state is unconventional and consists of two distinct superconducting phases. At high fields and low temperatures, an ordered phase of magnetic or quadrupolar origin is observed, suggesting that the superconductivity may occur in the vicinity of a magnetic or quadrupolar quantum critical point.

cond-mat.supr-con↗

arXiv2003

Magnetic Relaxation in the Peak Effect Region of CeRu$_2$

The different pinning strengths of the flux line lattice in the peak effect (PE) region of a polycrystalline sample of CeRu$_2$ with a superconducting transition temperature {$T_c = 6.1$ K} have been probed by means of magnetization measurements with a SQUID magnetometer as the temperature $T$ and the magnetic field $H$ are varied. Magnetic relaxation measurements were used to monitor the flux line dynamics in the PE region. For {$T < 4.5$ K} and $H < H_P$, where $H_P$ is the field where the magnetization reaches a maximum in the PE region, the relaxation rate was found to be significantly larger in the descending-field branch of the PE than it is in other sections of the PE region. For {$T \geq 4.5$ K}, the relaxation rate in the entire PE region is so large that the magnetization reached a stable (equilibrium) value within {$10^4$ s}. This experimentally determined stable state appears as an increase of the magnetization in the PE region and has a dome shape superimposed on a linear interpolation through the PE region. It was also found that the PE in CeRu$_2$ can be suppressed by rapid thermal cycling of the sample between {10 K} and {300 K} four times. The reversible magnetization after the PE has been suppressed coincides with the linear interpolation through the PE region, in contrast to the behavior of the equilibrium magnetization when the PE is present. PACS number: 74.25.Qt, 74.70.Ad

cond-mat.supr-con↗

arXiv2002

muSR linewidth and isotropic pairing in superconducting PrOs4Sb12

Transverse-field muon spin rotation measurements in the vortex-lattice mixed state of the heavy-fermion (HF) superconductor PrOs4Sb12 yield a temperature dependence of the penetration depth indicative of an isotropic or nearly isotropic energy gap. This is not seen to date in any other HF superconductor and is a signature of isotropic pairing symmetry, possibly related to a novel nonmagnetic "quadrupolar Kondo" HF mechanism in PrOs4Sb12. The T = 0 relaxation rate σ_s(0) = 0.91(1) μs^-1 yields an estimated penetration depth λ(0) = 3440(20) Å, which is considerably shorter than in other HF superconductors.

cond-mat.str-el↗

arXiv2002

Low temperature specific heat of the heavy fermion superconductor PrOs$_4$Sb$_{12}$

We report the magnetic field dependence of the low temperature specific heat of single crystals of the first Pr-based heavy fermion superconductor PrOs$_4$Sb$_{12}$. The low temperature specific heat and the magnetic phase diagram inferred from specific heat, resistivity and magnetisation provide compelling evidence of a doublet ground state and hence superconductivity mediated by quadrupolar fluctuations. This establishes PrOs$_4$Sb$_{12}$ as a very strong contender of superconductive pairing that is neither electron-phonon nor magnetically mediated.

cond-mat.supr-con↗

arXiv2002

Effects of lattice disorder in the UCu_(5-x)Pd_(x) system

The UCu(5-x)Pd(x) system exhibits non-Fermi liquid (NFL) in the thermodynamic and transport properties at low temperatures for Pd concentrations 0.9 <~ x <~ 1.5. The local structure around the U, Cu, and Pd atoms has been measured for 0 <= x <= 1.5 using the X-ray Absorption Fine Structure (XAFS) technique in order to quantify the effects of lattice disorder on the NFL properties. A model which allows a percentage of the Pd atoms to occupy nominal Cu (16e) sites, s, was used to fit the Pd and Cu K edge and U L(III) edge data. Pd/Cu site interchange was found to occur in all samples (x != 0), reaching a minimum value of s ~0.17 at x = 0.7 and increasing monotonically to s ~ 0.4 at x=1.5. These data also determine the static disorder the nearest neighbor U--Cu pairs. The results indicate that the measured U--Cu static disorder is not sufficient to explain the NFL behavior of the magnetic susceptibility within the single-ion Kondo disorder model and casts doubt on the applicability of this model to UCu(5-x)Pd(x).

cond-mat.str-el↗

arXiv2002

Coexistence of superconductivity and ferromagnetism in the graphite-sulfur system

Superconducting characteristics such as the Meissner-Ochsenfeld state, screening supercurrents and hysteresis loops of type-II superconductors were observed from the temperature and magnetic field dependences of the magnetic moment, m(T, H), in graphite powders reacted with sulfur for temperatures below 9.0 K. The temperature dependence of the lower critical field Hc1(T) was determined and the zero-temperature penetration depth, lambda(0), was estimated (lambda (0) = 227 nm). The superconductivity was observed to be highly anisotropic and to coexist with a ferromagnetic state that has a Curie temperature well above room temperature. A continuous transition from the superconducting state to the ferromagnetic state could be achieved by simply increasing the applied magnetic field.

cond-mat.supr-con↗

arXiv2002

Electronic Structure and Charge Dynamics of Huesler Alloy Fe2TiSn Probed by Infrared and Optical Spectroscopy

We report on the electrodynamics of a Heusler alloy Fe2TiSn probed over four decades in energy: from the far infrared to the ultraviolet. Our results do not support the suggestion of Kondo-lattice behavior inferred from specific heat measurements. Instead, we find a conventional Drude-like response of free carriers, with two additional absorption bands centered at around 0.1 and 0.87 eV. The latter feature can be interpreted as excitations across a pseudogap, in accord with band structure calculations.

cond-mat.mtrl-sci↗

arXiv2001

Slow Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5

Low-temperature muon spin-lattice relaxation measurements in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, indicate inhomogeneously distributed f-electron spin fluctuation rates, and exhibit a time-field scaling of the muon relaxation function indicative of long-lived spin correlations. In UCu_4Pd the scaling exponent γis small and temperature independent. In UCu_{3.5}Pd_{1.5} γvaries with temperature, increasing with decreasing temperature similar to spin-glass AgMn. This suggests that the spin-glass state found for x \gtrsim 2 in UCu_{5-x}Pd_x modifies the low-frequency spin dynamics in UCu_{3.5}Pd_{1.5}.

cond-mat.str-el↗

arXiv2001

Hot isostatic pressing of bulk magnesium diboride: mechanical and superconducting properties

Two different hot isostatic pressing cycles (HIPing) were investigated to synthesize bulk MgB2 samples: a "standard" cycle where a low vessel pressure is maintained while heating to the process temperature with a subsequent simultaneous pressure and temperature decrease and a new method - dense material cooling under pressure (DMCUP). The latter method allowed the synthesis of dense samples with diameters up to 20 mm and thicknesses up to 10 mm from commercial MgB2 powder. Optimal conditions for the DMCUP method with glass encapsulation (maximum pressure 200 MPa, maximum temperature 1000 C over 200 min, and cooling under pressure) resulted in a dense material with a sharp superconducting transition at 38.5 K. This method employs a pressure which is one order of magnitude less than previously reported for pressure assisted sintering of dense material and can be scaled to larger sample sizes and complex shapes. The data for density, microhardness, fracture toughness and sound speed as well as superconducting properties for bulk magnesium diboride are presented. Ball milling the powder enhances sintering and results in a more homogeneous final microstructure.

cond-mat.supr-con↗

arXiv2001

Direct experimental verification of applicability of single-site model for angle integrated photoemission of small $T_{K}$ concentrated Ce compounds

Bulk-sensitive high-resolution Ce 4f spectra have been obtained from 3d $\to$ 4f resonance photoemission measurements on La$_{1-x}$Ce$_x$Al$_2$ and La$_{1-x}$Ce$_x$Ru$_2$ for $x = 0.0, 0.04, 1.0$. The 4f spectra of low-Kondo-temperature ($T_{K}$) (La,Ce)Al$_2$ are essentially identical except for a slight increase of the Kondo peak with $x$, which is consistent with a known increase of $T_{K}$ with $x$. In contrast, the 4f spectra of high-$T_{K}$ (La,Ce)Ru$_2$ show a Kondo-like peak and also a 0.5 eV structure which increases strongly with $x$. The resonance photon-energy dependences of the two contributions are different and the origin of the 0.5 eV structure is still uncertain.

cond-mat.str-el↗

arXiv2001

Improved superconducting properties of MgB2

We present electrical transport, magnetization, and specific heat measurements on bulk MgB2 samples (T_{c} = 38.5 K) synthesized under 200 MPa pressure using a process based on hot isostatic pressing with cooling under pressure. The samples are fully dense and display excellent superconducting properties, including a narrow superconducting transition width (ΔT_{c} = 0.75 K), a high upper critical field H_{c2} (H_{c2}(0) ~ 155 kOe), and a critical current density J_{c} that is the largest yet measured for bulk samples of MgB2 (J_{c}(0) ~ 1.4 MA/cm^{2}). Specific heat measurements yielded a jump ΔC at T_{c} of 92 mJ/mol K. These superconducting properties are comparable to those obtained with techniques that are not so well suited to industrial scale fabrication.

cond-mat.supr-con↗

arXiv2001

Glassy Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) = Gbar(t/H^γ) expected from glassy dynamics. At 0.05 K γ(x=1.0) = 0.35 \pm 0.1, but γ(x=1.5) = 0.7 \pm 0.1. This is in contrast to inelastic neutron scattering results, which yield γ= 0.33 for both concentrations. There is no sign of static magnetism \gtrsim 10^{-3} μ_B/U ion in either material above 0.05 K. Our results strongy suggest that both alloys are quantum spin glasses.

cond-mat.str-el↗

arXiv2001

Interplane Transport and Superfluid Density in Layered Superconductors

We report on generic trends in the behavior of the interlayer penetration depth $λ_c$ of several different classes of quasi two-dimensional superconductors including cuprates, Sr$_2$RuO$_4$, transition metal dichalcogenides and organic materials of the $(BEDT-TTF)_2X$-series. Analysis of these trends reveals two distinct patterns in the scaling between the values of $λ_c$ and the magnitude of the DC conductivity: one realized in the systems with a Fermi liquid (FL) ground state and the other seen in systems with a marked deviation from the FL response. The latter pattern is found primarily in under-doped cuprates and indicates a dramatic enhancement (factor $\simeq 10^2$) of the energy scale $Ω_C$ associated with the formation of the condensate compared to the data for the FL materials. We discuss implications of these results for the understanding of pairing in high-$T_c$ cuprates.

cond-mat.supr-con↗

arXiv2000

^{27}Al Impurity-Satellite NMR and Non-Fermi-Liquid Behavior in U_{1-x}Th_xPd_2Al_3

Non-Fermi-liquid (NFL) behavior in the f-sublattice-diluted alloy system U_{1-x}Th_xPd_2Al_3 has been studied using ^{27}Al nuclear magnetic resonance (NMR). Impurity satellites due to specific U near-neighbor configurations to ^{27}Al sites are clearly resolved in both random and field-aligned powder samples. The spatial mean Kbar and rms spread delta K of impurity satellite shifts, which are related to the mean chibar and rms spread delta chi of the inhomogeneous susceptibility, have been measured in field-aligned powders with the crystalline c axis both perpendicular and parallel to the external field. The relatively narrow lines observed at low temperatures suggest that disorder- induced inhomogeneity of the f-ion--conduction-electron hybridization is not the cause of NFL behavior in these alloys: at low temperatures the experimental values of delta chi(T)/chibar(T) are much smaller than required by disorder- driven models. This is in contrast to results in at least some alloys with disordered non-f-ion nearest neighbors to f ions ("ligand disorder"), where disorder-driven theories give good accounts of NFL behavior. Our results suggest that f-ion dilution does not produce as much inhomogeneity of the hybridization strength as substitution on ligand sites.

cond-mat.str-el↗

arXiv1999

Dynamics of Flux Creep in Underdoped Single Crystals of Y_1-xPr_xBa_2Cu_3O_7-d

Transport as well as magnetic relaxation properties of the mixed state were studied on strongly underdoped Y_1-xPr_xBa_2Cu_3O_7-d crystals. We observed two correlated phenomena - a coupling transition and a transition to quantum creep. The distribution of transport current below the coupling transition is highly nonuniform, which facilitates quantum creep. We speculate that in the mixed state below the coupling transition, where dissipation is nonohmic, the current distribution may be unstable with respect to self-channeling resulting in the formation of very thin current-carrying layers.

cond-mat.supr-con↗

arXiv1999

Relationship Between Conductivity and Phase Coherence Length in Cuprates

The large ($10^2 - 10^5$) and strongly temperature dependent resistive anisotropy $η= (σ_{ab}/σ_c)^{1/2}$ of cuprates perhaps holds the key to understanding their normal state in-plane $σ_{ab}$ and out-of-plane $σ_{c}$ conductivities. It can be shown that $η$ is determined by the ratio of the phase coherence lengths $\ell_i$ in the respective directions: $σ_{ab}/σ_c = \ell_{ab}^2/\ell_{c}^2$. In layered crystals in which the out-of-plane transport is incoherent, $\ell_{c}$ is fixed, equal to the interlayer spacing. As a result, the T-dependence of $η$ is determined by that of $\ell_{ab}$, and vice versa, the in-plane phase coherence length can be obtained directly by measuring the resistive anisotropy. We present data for hole-doped $YBa_2Cu_3O_y$ ($6.3 < y < 6.9$) and $Y_{1-x}Pr_xBa_2Cu_3O_{7-δ}$ ($0 < x \leq 0.55$) and show that $σ_{ab}$ of crystals with different doping levels can be well described by a two parameter universal function of the in-plane phase coherence length. In the electron-doped $Nd_{2-x}Ce_{x}CuO_{4-y}$, the dependence $σ_{ab}(η)$ indicates a crossover from incoherent to coherent transport in the c-direction.

cond-mat.supr-con↗

arXiv1999

Optical Properties of MFe_4P_12 filled skutterudites

Infrared reflectance spectroscopy measurements were made on four members of the MFe_4P_12 family of filled skutterudites, with M=La, Th, Ce and U. In progressing from M=La to U the system undergoes a metal-insulator transition. It is shown that, although the filling atom induces such dramatic changes in the transport properties of the system, it has only a small effect on lattice dynamics. We discuss this property of the compounds in the context of their possible thermoelectric applications.

cond-mat.str-el↗

arXiv1999

Quantum Creep in Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7}$ Crystals: Magnetic Relaxation and Transport

We report transport and magnetic relaxation measurements in the mixed state of strongly underdoped Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7} crystals. A transition from thermally activated flux creep to temperature independent quantum flux creep is observed in both transport and magnetic relaxation at temperatures T * 5 K. Flux transformer measurements indicate that the crossover to quantum creep is preceded by a coupling transition. Based on these observations we argue that below the coupling transition the current is confined within a very narrow layer beneath the current contacts.

cond-mat.supr-con↗