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J. C. Lashley

Publications and source records attributed to J. C. Lashley.

32 records · Page 2Linked to original sources
arXiv2006

Valence-Band UPS, 6$p$ Core-Level XPS Photoemission Spectroscopy, and Low-Energy Electron Diffraction of a Uranium (001) Single Crystal

Valence-band ultraviolet photoemission spectroscopy (UPS) at 173K and 6p core-level X-ray photoemission spectroscopy (XPS) at room temperature were performed on a high quality uranium single crystal. Significant agreement is found with first-principles electronic band-structure calculations, using a generalized gradient approximation (GGA). In addition, using Low Energy Electron Diffraction (LEED) for the (001) surface, we find a well-ordered orthorhombic crystallographic structure representative of the bulk material.

cond-mat.other↗
arXiv2006

Pinning frequencies of the collective modes in $α$-uranium

Uranium is the only known element that features a charge-density wave (CDW) and superconductivity. We report a comparison of the specific heat of single-crystal and polycrystalline $α$-uranium. \red{Away from the the phase transition the specific heat of the polycrystal is larger than that of the single crystal, and the aim of this paper is to explain this difference.} In the single crystal we find excess contributions to the heat capacity at 41 K, 38 K, and 23 K, with a Debye temperature, $Θ_D$ = 256 K. In the polycrystalline sample the heat capacity curve is thermally broadened ($Θ_D$ = 184 K), but no excess heat capacity was observed. The excess heat capacity, $C_ϕ$ (taken as the difference between the single crystal and polycrystal heat capacities) is well described in terms of collective-mode excitations above their respective pinning frequencies. This attribution is represented by a modified Debye spectrum with two cutoff frequencies, a pinning frequency, $ν_o$, for the pinned CDW (due to grain boundaries in the polycrystal), and a normal Debye acoustic frequency occurring in the single crystal. We explain the 50-year-old difference in Debye temperatures between heat capacity and ultrasonic measurements.

cond-mat.other↗
arXiv2005

Inversion symmetry breaking in noncollinear magnetic phase of a triangular lattice antiferromagnet CuFeO2

Magnetoelectric and magnetoelastic phenomena correlated with a phase transition into noncollinear magnetic phase have been investigated for single crystals of CuFeO2 with a frustrated triangular lattice. CuFeO2 exhibits several long-wavelength magnetic structures related to the spin frustration, and it is found that finite electric polarization, namely inversion symmetry breaking, occurs with noncollinear but not at collinear magnetic phases. This result demonstrates that the noncollinear spin structure is a key role to induce electric polarization, and suggests that frustrated magnets which often favor noncollinear configurations can be plausible candidates for magnetoelectrics with strong magnetoelectric interaction.

cond-mat.str-el↗
arXiv2005

Strong electron-phonon coupling in delta-phase stabilized Pu

Heat capacity measurements of the delta-phase stabilized alloy Pu-Al suggest that strong electron-phonon coupling is required to explain the moderate renormalization of the electronic density of states near the Fermi energy. We calculate the heat capacity contributions from the lattice and electronic degrees of freedom as well as from the electron-lattice coupling term and find good overall agreement between experiment and theory assuming a dimensionless electron-phonon coupling parameter of order unity, lambda ~ 0.8. This large electron-phonon coupling parameter is comparable to reported values in other superconducting metals with face-centered cubic crystal structure, for example, Pd (lambda ~ 0.7) and Pb (lambda ~ 1.5). Further, our analysis shows evidence of a sizable residual low-temperature entropy contribution, S_{res} ~ 0.4 k_B (per atom). We can fit the residual specific heat to a two-level system. Therefore, we speculate that the observed residual entropy originates from crystal-electric field effects of the Pu atoms or from self-irradiation induced defects frozen in at low temperatures.

cond-mat.mtrl-sci↗
arXiv2005

Catastrophic Fermi surface reconstruction in the shape-memory alloy AuZn

AuZn undergoes a shape-memory transition at 67 K. The de Haas van Alphen effect persists to 100 K enabling the observation of a change in the quantum oscillation spectrum indicative of a catastrophic Fermi surface reconstruction at the transition. Coexistence of both Fermi surfaces at low temperatures is suggestive of an intrinsic phase separation in the bulk of the material. In addition, a Dingle analysis reveals a sharp change in the scattering mechanism at a threshold cyclotron radius, which we suggest to be related to the underlying microstructure that drives the shape-memory effect.

cond-mat.str-el↗
arXiv2005

Fermi Surface as a Driver for the Shape-Memory Effect in AuZn

Martensites are materials that undergo diffusionless, solid-state transitions. The martensitic transition yields properties that depend on the history of the material and may allow it to recover its previous shape after plastic deformation. This is known as the shape-memory effect (SME). We have succeeded in identifying the primary electronic mechanism responsible for the martensitic transition in the shape-memory alloy AuZn by using Fermi-surface measurements (de Haas-van Alphen oscillations) and band-structure calculations. This strongly suggests that electronic band structure is an important consideration in the design of future SME alloys.

cond-mat.mtrl-sci↗
arXiv2004

Absence of magnetic moments in plutonium

Many theories published in the last decade propose that either ordered or disordered local moments are present in elemental plutonium at low temperatures. We present new experimental data and review previous experimental results. None of the experiments provide any evidence for ordered or disordered magnetic moments (either static or dynamic) in plutonium at low temperatures, in either the alpha- or delta-phases. The experiments presented and discussed are magnetic susceptibility,electrical resistivity, NMR, specific heat, and both elastic and inelastic neutronscattering. Many recent calculations correctly predict experimentally observed atomic volumes, including that of delta-Pu. These calculations achieve observed densities by the localization of electrons, which then give rise to magnetic moments. However, localized magnetic moments have never been observed experimentally in Pu. A theory is needed that is in agreement with all the experimental observations. Two theories are discussed that might provide understanding of the ensemble of unusual properties of Pu, including the absence of experimental evidence for localized magnetic moments; an issue that has persisted for over 50 years.

cond-mat.mtrl-sci↗
arXiv2004

Tunable Thermal Expansion Behavior in the Intermetallic YbGaGe

We investigate the effects of carbon and boron doping on the thermal expansion in the hexagonal (P63/mmc) intermetallic YbGaGe. X-ray powder diffraction was used to measure the lattice constants on pure and doped (C or B at nominal levels of 0.5 %) samples from T~10 K to T~300 K. Also measured were resistivity, specific-heat, and magnetic susceptibility. While the pure YbGaGe samples exhibit positive thermal volume expansion, (V300K-V10K)/V300K = 0.94%, the volume expansion in the lightly C and B-doped samples, contract and tend towards zero volume expansion. Such a strong response with such light doping suggests that the underlying mechanism for the reported zero volume expansion is substitutional disorder, and not the previously proposed valence fluctuations.

cond-mat.mtrl-sci↗
arXiv2004

Suppression of the $γ-α$ structural phase transition in $Ce_{0.8} La_{0.1} Th_{0.1}$ by large magnetic fields

The $γ-α$ transition in Ce$_{0.8}$La$_{0.1}$Th$_{0.1}$ is measured as a function of applied magnetic field using both resistivity and magnetization. The $γ- α$ transition temperature decreases with increasing magnetic field, reaching zero temperature at around 56 T. The magnetic-field dependence of the transition temperature may be fitted using a model that invokes the field and temperature dependence of the entropy of the $4f$-electron moments of the $γ$ phase, suggesting that the volume collapse in Ce and its alloys is primarily driven by entropic considerations.

cond-mat.str-el↗
arXiv2004

Low temperature field-effect in crystalline organic material

Molecular organic materials offer the promise of novel electronic devices but also present challenges for understanding charge transport in narrow band systems. Low temperature studies elucidate fundamental transport processes. We report the lowest temperature field effect transport results on a crystalline oligomeric organic material, rubrene. We find field effect switching with on-off ratio up to 10^7 at temperatures down to 10 K. Gated transport shows a factor of ~10 suppression of the thermal activation energy in 10-50 K range and nearly temperature independent resistivity below 10 K.

cond-mat.mtrl-sci↗
arXiv2003

Domain State Occurring in the de Haas-van Alphen Effect in Silver

Hysteresis has been observed in de Haas-van Alphen measurements of the Condon domains in silver, and it shows the first-order nature of the transition to the domain state. The hysteresis, and thus the first-order nature, is manifested in a nonlinear effect where a double-valued response of the amplitude with the applied external field is observed.

cond-mat.mtrl-sci↗
arXiv2003

Experimental electronic heat capacities of $α-$ and $δ-$Plutonium; heavy-fermion physics in an element

We have measured the heat capacities of $δ-$Pu$_{0.95}$Al$_{0.05}$ and $α-$Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduction of the electronic contribution to the heat capacity of $δ-$Pu$_{0.95}$Al$_{0.05}$; we find $γ= 64 \pm 3$ mJK$^{-2}$mol$^{-1}$ as $T \to 0$. This is a factor $\sim 4$ larger than that of any element, and large enough for $δ-$Pu$_{0.95}$Al$_{0.05}$ to be classed as a heavy-fermion system. By contrast, $γ= 17 \pm 1$ mJK$^{-2}$mol$^{-1}$ in $α-$Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of $δ-$Pu$_{0.95}$Al$_{0.05}$, one or both of which may be associated with the formation of the $α'-$ martensitic phase. We suggest that the large $γ$-value of $δ-$Pu$_{0.95}$Al$_{0.05}$ may be caused by proximity to a quantum-critical point.

cond-mat.str-el↗
arXiv2001

Elastic and thermodynamic properties of the shape-memory alloy AuZn

The current work reports on the elastic shear moduli, internal friction, and the specific heat of the B2 cubic ordered alloy AuZn as a function of temperature. Measurements were made on single-crystal and polycrystalline samples using Resonant Ultrasound Spectroscopy (RUS), semi-adiabatic calorimetry and stress-strain measurements. Our results confirm that this alloy exhibits the shape-memory effect and a phase transition at 64.75 K that appears to be continuous (second-order) from the specific heat data. It is argued that the combination of equiatomic composition and a low transformation temperature constrain the chemical potential and its derivatives to exhibit behavior that lies at the borderline between that of a first-order (discontinuous) and a continuous phase transition. The acoustic dissipation does not peak at the transtion temperature as expected, but shows a maximum well into the low-temperature phase. The Debye temeprature value of 219 K, obtained from the low-temperature specific heat data is in favorable agreement with that determined from the acoustic data (207 K) above the transition.

cond-mat.mtrl-sci↗