L. E. DeLong

The pressure dependence of the superconducting transition temperature of LaT4P12(T = Fe, Ru, Os)

Abstract The superconducting transition temperature, Tc, of the LaT4P12 compounds with T = Fe, Ru, or Os has been measured under hydrostatic pressure P up to 1.8 GPa. The T = Fe compound exhibits a substantial increase of Tc from Tc (P = 0) = 4.1 K at a rate (dTc/dP)P=0= +7.2 x 10-1 K/GPa. In contrast, the Ru and Os compounds exhibit only weak decreases of Tc from Tc (P = 0) = 7.2 K and 1.8 K with (dTc/dP)P=0= -1.6 x 10-1 K/GPa and -9.5 x 10-2 K/GPa, respectively. An analysis of this strikingly divergent behavior of Tc(P) in terms of the structural characteristics of the RT4X12 class of compounds where R = rare earth element, T = Fe, Ru, or Os, and X = P, As, or Sb suggests that Tc(P) for these materials consists of two competing contributions: a depression of Tc due to the compression of the lattice (i.e., decrease in volume), and an enhancement of Tc due to the effect of pressure on La itself.

Tuning the J eff = 1 2 insulating state via electron doping and pressure in the double-layered iridate Sr 3 Ir 2 O 7

Sr3Ir2O7 exhibits a novel J(eff) = 1/2 insulating state that features a splitting between Jeff = 1/2 and 3/2 bands due to spin-orbit interaction. We report a metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+ for Sr2+) or application of high pressure up to 35 GPa. Our study of single-crystal Sr3Ir2O7 and (Sr1-xLax)(3)Ir2O7 reveals that application of high hydrostatic pressure P leads to a drastic reduction in the electrical resistivity by as much as six orders of magnitude at a critical pressure P-C = 13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35 GPa produces no fully metallic state at low temperatures, possibly as a consequence of localization due to a narrow distribution of bonding angles theta. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7 readily induces a robust metallic state in the resistivity at low temperatures; the magnetic ordering temperature is significantly suppressed but remains finite for (Sr0.95La0.05)(3)Ir2O7 where the metallic state occurs. The results are discussed along with comparisons drawn with Sr2IrO4, a prototype of the J(eff) = 1/2 insulator.

Non-Fermi-liquid behavior in nearly ferromagnetic SrIrO3 single crystals

We report transport and thermodynamic properties of single-crystal SrIrO3 as a function of temperature T and applied magnetic field H. We find that SrIrO3 is a non-Fermi-liquid metal near a ferromagnetic instability, as characterized by the following properties: (1) small ordered moment but no evidence for long-range order down to 1.7 K; (2) strongly enhanced magnetic susceptibility that diverges as T or T1/2 at low temperatures, depending on the applied field; (3) heat capacity C(T,H) ~ -Tlog T that is readily amplified by low applied fields; (4) a strikingly large Wilson ratio at T< 4K; and (5) a T3/2-dependence of electrical resistivity over the range 1.7 < T < 120 K. A phase diagram based on the data implies SrIrO3 is a rare example of a stoichiometric oxide compound that exhibits non-Fermi-liquid behavior near a quantum critical point (T = 0 and H = 0.23 T).

Microscopic study of magnetostatic spin waves

A relatively new method is developed to numerically calculate the spin-wave-related properties of a magnetic body of arbitrary shape. Starting with a discrete dipole approximation and the linearized Landau–Lifshitz equation, the resonant frequencies and the associated amplitudes of the individual moments are obtained for all modes; from this information we are able to calculate the energy absorbed by the various modes excited by a position- and time-dependent external magnetic field. The method has been demonstrated for a number of cases including thin disks and rings and for equilibrium configurations ranging from the saturated high-field limit to the vortex states at low fields.

Strongly localized magnetization modes in permalloy antidot lattices

Antidot lattices (ADLs) patterned into soft magnetic thin films exhibit rich ferromagnetic resonance (FMR) spectra corresponding to many different magnetization modes. One of the predicted modes is highly localized at the edges of the antidots; this mode is difficult to detect experimentally. Here we present FMR data for a permalloy thin film patterned into a square array of square antidots. Comparison of these data with micromagnetic simulations permits identification of several edge modes. Our simulations also reveal the effect of the antidot shape on the mode dispersion.

First-order spin reorientation transition and specific-heat anomaly in CeFeO3

We report the successful synthesis of single-phase polycrystalline CeFeO3, and a thorough study of its magnetic and thermal properties. An abrupt spin reorientation transition occurs at T = 240 K, indicating a first-order phase transition from Γ4 (Gx, Ay, Fz) to Γ1 (Ax, Gy, Cz). A distinct thermal anomaly in the specific heat due to spin reorientation has been detected, with the onset and completion of the spin reorientation at 240 K and 220 K, respectively. The low-temperature specific heat exhibits a Schottky anomaly caused by paramagnetic Ce3+. The very-low-temperature specific heat increases with increasing the applied magnetic field, suggesting an antiferromagnetic transition of Ce3+ exists below T = 1.8 K.

Evidence for an extended critical region near the metamagnetic transition of UCoAl

Electrical resistivity and ac magnetic susceptibility studies of the 5f-metamagnet UCoAl reveal the presence of strong spin fluctuation effects for temperatures below 30 K and magnetic fields below the critical metamagnetic field μ0Hc≈1 T, where a non-Fermi liquid term ∼T3/2 is prominent in the electrical resistivity. A step-wise increase of the residual resistivity and an increasingly prominent Fermi liquid T2 term of the resistivity is observed as the field is increased though the metamagnetic transition.

Generating wave vector specific Damon-Eshbach spin waves in Py using a diffraction grating

A patterned square silver antidot lattice on a thin uniform permalloy film facilitates direct coupling of a quasi-uniform microwave field to short wavelength magnetic modes. The resulting modes are studied as a function of both the magnitude and orientation (relative to the symmetry axes of the array) of an in-plane, external DC magnetic field. The observed modes are identified as surface spin waves with wavelengths matching the Fourier components of the silver array.

Pressure-induced insulating state in Ba1-xRxIrO3 (R = Gd, Eu) single crystals

BaIrO3 is a novel insulator with coexistent weak ferromagnetism, charge and spin density wave. Dilute RE doping for Ba induces a metallic state, whereas application of modest pressure readily restores an insulating state characterized by a three-order-of-magnitude increase of resistivity. Since pressure generally increases orbital overlap and broadens energy bands, a pressure-induced insulating state is not commonplace. The profoundly dissimilar responses of the ground state to light doping and low hydrostatic pressures signal an unusual, delicate interplay between structural and electronic degrees of freedom in BaIrO3.

Normal and superconducting state properties of U6Fe at low temperatures and high magnetic fields

Abstract High purity U 6 Fe polycrystals with a resistance ratio rrr =9 at T =4 K are found to exhibit a record onset temperature (T 0 > 4.0 K) to superconductivity for U materials. Our measured values of the electronic coefficient of heat capacity γ ∗ =(150±3) mJ/mol · K 2 , the mean Debye temperature Θ ( T →0)=166 K and the normalized jump in heat capacity at T c , Δ / γT c =2.3±0.1, are all in good agreement with previous work † , and suggest that U 6 Fe is a strong-coupled superconductor. Resistive upper critical field measurements on high purity samples revealed a pronounced S-shaped curvature of H c2 ( T ) and evidence for anisotropy, similar to effects seen in UPt 3 ‡ , but not observed in lower purity U 6 Fe samples † . We find H c2 (0) > 10T and transition widths ΔT c ∝H 1 2 at moderate fields, followed by an abrupt increase in Δ T c for H>8T. Our H c2 data cannot be explained by existing theoretical models. Preliminary heat capacity measurements in magnetic fields 0⩽H⩽10T suggest that γ ∗ may decrease by ≈10% at H =10 T . These results and electrical resistivity data will be discussed in terms of a potential charge density wave instability in U 6 Fe at T⪅110 K.