Paul C. Canfield

Detection of orbital fluctuations above the structural transition temperature in the iron pnictides and chalcogenides

We use point-contact spectroscopy (PCS) to probe AEFe2As2 (AE=Ca, Sr, Ba) and Fe1+yTe. For AE=Sr,Ba we detect orbital fluctuations above TS while for AE=Ca these fluctuations start below TS. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where an in-plane resistive anisotropy is known to exist. We predict an in-plane resistive anisotropy of Fe1+yTe above TS. Our data are examined in light of the recent work by Lee and Phillips (arXiv:1110.5917v2). We also study how joule heating in the PCS junctions impacts the spectra. Spectroscopic information is only obtained from those PCS junctions that are free of heating effects while those PCS junctions that are in the thermal regime display bulk resistivity phenomena.

A single-solenoid pulsed-magnet system for single-crystal scattering studies

We present a pulsed-magnet system that enables x-ray single-crystal diffraction in addition to powder and spectroscopic studies with the magnetic field applied on or close to the scattering plane. The apparatus consists of a single large-bore solenoid, cooled by liquid nitrogen. A second independent closed-cycle cryostat is used for cooling samples near liquid helium temperatures. Pulsed magnetic fields close to ~30 T with a zero-to-peak-field rise time of ~2.9 ms are generated by discharging a 40 kJ capacitor bank into the magnet coil. The unique characteristic of this instrument is the preservation of maximum scattering angle (~23.6°) on the entrance and exit sides of the magnet bore by virtue of a novel double-funnel insert. This instrument will facilitate x-ray diffraction and spectroscopic studies that are impractical, if not impossible, to perform using split-pair and narrow-opening solenoid magnets. Furthermore, it offers a practical solution for preserving optical access in future higher-field pulsed magnets.

Development of viable solutions for the synthesis of sulfur bearing single crystals

The discovery of high temperature superconductivity in FeAs and FeSe based compounds has once again focused the condensed matter community on the need to systematically explore compounds containing chalcogens and pnictogens. Whereas some solution growth techniques have been developed to handle P and As, and Sb and Bi are versatile solvents in their own right, S has remained a problematic element to incorporate into conventional solution growth. To a large extent its low boiling point, combined with its polymeric nature in a molten state have made S an uninviting solvent. In this paper we present our development of a range of binary sulfur bearing solutions (some even sulfur rich) and demonstrate how we have been able to use these as useful starting points for the growth of a wide range of transition metal–sulfur–X ternary compounds. We present growth details and basic characterization data for Ni3Bi2S2, Co3Sn2S2, Fe2GeS4, CoSSb, and CePd3S4. In addition we present a remarkably simple method for the growth of single crystalline Co with crystallization taking place below the Curie temperature.

A novel approach for x-ray scattering experiments in magnetic fields utilizing trapped flux in type-II superconductors

We introduce a novel approach to x-ray scattering studies in applied magnetic fields by exploiting vortices in superconductors. This method is based on trapping magnetic flux in a small disk-shaped superconductor (known as a trapped field magnet, TFM) with a single-crystal sample mounted on or at close proximity to its surface. This opens an unrestricted optical access to the sample and allows magnetic fields to be applied precisely along the x-ray momentum transfer, facilitating polarization-sensitive experiments that have been impractical or impossible to perform to date. The TFMs used in our study remain stable and provide practically uniform magnetic fields for days, which are sufficient for comprehensive x-ray diffraction experiments, specifically x-ray resonance exchange scattering (XRES) to study field-induced phenomena at a modern synchrotron source. The TFM instrument has been used in a proof-of-principle XRES study of a meta-magnetic phase in a rare-earth compound, TbNi(2)Ge(2), in order to demonstrate its potential.

Growing intermetallic single crystals using in situ decanting

High temperature metallic solution growth is one of the most successful and versatile methods for single crystal growth, and is particularly suited for exploratory synthesis. The method commonly utilizes a centrifuge at room temperature and is very successful for the synthesis of single crystal phases that can be decanted from the liquid below the melting point of the silica ampoule. In this paper, we demonstrate the extension of this method that enables single crystal growth and flux decanting inside the furnace at temperatures above 1200°C. This not only extends the number of available metallic solvents that can be used in exploratory crystal growth but also can be particularly well suited for crystals that have a rather narrow exposed solidification surface in the equilibrium alloy phase diagram.

Agreement between local and global measurements of the London penetration depth

Abstract Recent measurements of the superconducting penetration depth in Ba(Fe 1− x Co x ) 2 As 2 appeared to disagree on the magnitude and curvature of Δ λ ab ( T ), even near optimal doping. These measurements were carried out on different samples grown by different groups. To understand the discrepancy, we use scanning SQUID susceptometry and a tunnel diode resonator to measure the penetration depth in a single sample. The penetration depth observed by the two techniques is identical with no adjustments. We conclude that any discrepancies arise from differences between samples, either in growth or crystal preparation.

Design, discovery and growth of novel materials

This editorial introduces the special issue on design, discovery, and growth of novel materials. The papers of this special issue review and reveal technical details of of how specific growths are developed and implemented.

Frequency dependence of the spin glass freezing temperatures in icosahedral R–Mg–Zn (R = rare earth) quasicrystals

We present ac susceptibility measurements with the frequency spanning three orders of magnitude on single grain, icosahedral R–Mg–Zn (R = rare earth) quasicrystals. The freezing temperature in Gd-based, Heisenberg spin glasses in this family increases by ∼2% with a frequency increase from 10u2009Hz to 10u2009kHz, whereas the freezing temperature in the non-Heisenberg members of the family is significantly more responsive to the frequency change (by 16–22%), suggesting that an additional magnetic anisotropy distribution in the non-Heisenberg spin glasses causes changes in the low frequency magnetic dynamics.

Order-disorder transition and magnetic quantum oscillations in the vortex state of strong type-II superconductors

We present results of μSR, dHvA and SQUID magnetization measurements on borocarbide superconductors, which show a remarkable correlation between an order-disorder transition of the vortex lattice, observed in the μSR measurements and enhanced additional damping of dHvA oscillations in the peak-effect region. It is, therefore, concluded that an important mechanism of additional damping of dHvA oscillations in the superconducting state should be associated with enhanced scattering of quasi particles by the pair potential in disordered vortex lattices.