M. F. Hundley

A new heavy-fermion superconductor CeIrIn5 : A relative of the cuprates?

CeIrIn5 is a member of a new family of heavy-fermion compounds and has a Sommerfeld specific-heat coefficient of 720 mJ/molK2. It exhibits a bulk, thermodynamic transition to a superconducting state at Tc = 0.40 K, below which the specific heat decreases as T2 to a small residual T-linear value. Surprisingly, the electrical resistivity drops below instrumental resolution at a much higher temperature T0 = 1.2 K. These behaviors are highly reproducible and field-dependent studies indicate that T0 and Tc arise from the same underlying electronic structure. The layered crystal structure of CeIrIn5 suggests a possible analogy to the cuprates in which spin/charge pair correlations develop well above Tc.

Critical examination of heat capacity measurements made on a Quantum Design physical property measurement system

Abstract We examine the operation and performance of an automated heat-capacity measurement system manufactured by Quantum Design (QD). QD’s physical properties measurement system (PPMS) employs a thermal-relaxation calorimeter that operates in the temperature range of 1.8–395 K. The accuracy of the PPMS specific-heat data is determined here by comparing data measured on copper and synthetic sapphire samples with standard literature values. The system exhibits an overall accuracy of better than 1% for temperatures between 100 and 300 K, while the accuracy diminishes at lower temperatures. These data confirm that the system operates within the ±5% accuracy specified by QD. Measurements on gold samples with masses of 4.5 and 88 mg indicate that accuracy of ±3% or better can be achieved below 4 K by using samples with heat capacities that are half or greater than the calorimeter addenda heat capacity. The ability of a PPMS calorimeter to accurately measure sharp features in Cp(T) near phase transitions is determined by measuring the specific heat in the vicinity of the first-order antiferromagnetic transition in Sm2IrIn8 (T0=14 K) and the second-order hidden order (HO) transition in URu2Si2 (TN=17 K). While the PPMS measures Cp(T) near the second-order transition accurately, it is unable to do so in the vicinity of the first-order transition. We show that the specific heat near a first-order transition can be determined from the PPMS-measured decay curves by using an alternate analytical approach. This correction is required because the latent heat liberated/absorbed at the transition results in temperature–decay curves that cannot be described by a single relaxation time constant. Lastly, we test the ability of the PPMS to measure the specific heat of Mg11B2, a superconductor of current interest to many research groups, that has an unusually strong field-dependent specific heat in the mixed state. At the critical temperature the discontinuity in the specific heat is nearly 15% lower than measurements made on the same sample using a semi-adiabatic calorimeter at Lawrence Berkeley National Laboratory.

Transport‐magnetism correlations in the ferromagnetic oxide La0.7Ca0.3MnO3

We present results of temperature and magnetic field dependent resistivity ρ(H,T) and bulk magnetization M(H,T) measurements on post‐annealed La0.7Ca0.3MnO3 thin films that were grown via pulsed‐laser deposition. Both the resistivity and the anomalously large negative magnetoresistance peak near the ferromagnetic ordering temperature (Tc=250 K), with Δρ/ρ0=−85% at 50 kOe. A clear correlation is found between ρ and M that is described by the phenomenological expression ρ(H,T)∝exp[−M(H,T)/M0]. This correlation reflects the important interplay between transport and magnetism in this system, and suggests that the transport below Tc involves polaron hopping.

Epitaxial nanotwinned Cu films with high strength and high conductivity

We report on the synthesis of epitaxial (single-crystal-like), nanotwinned Cu films via magnetron sputtering. Increasing the deposition rate from 1 to 4 nm/s decreased the average twin lamellae spacing from 16 to 7 nm. These epitaxial nanotwinned Cu films exhibit significantly higher ratio of hardness to room temperature electrical resistivity than columnar grain (nanocrystalline), textured, nanotwinned Cu films.

Specific heat and anisotropic superconducting and normal-state magnetization of HoNi2B2C

Abstract The magnetization of single-crystal HoNi 2 B 2 C has been measured as a function of applied field ( H ) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi 2 B 2 C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c -axis and with a much weaker temperature dependence for H applied parallel to the c -axis, indicating that the Ho +3 magnetic moments lie predominately in the tetragonal a − b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T N =5.0 K. Remarkably, in 20 Oe applied field both superconductivity ( T c =8.0 K) and antiferromagnetism ( T N =5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H − T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H c2 with a deep minimum at T N =5 K. The high-field magnetization data for H applied perpendicular to the c -axis also reveal a cascade of three phase transitions for T H H versus T phase diagram for HoNi 2 B 2 C at low temperatures.

Magnetism and heavy fermion-like behavior in the RBiPt series

Members of the RBiPt (R=Ce–Lu with the exceptions of Pm and Eu) series have been grown as single crystals. Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on representatives. The high temperature resistance uniformly changes from that of a small‐gap semiconductor or semimetal seen in NdBiPt to that of a heavy‐fermion metal seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K2 mole. Further, the lighter rare earth members show an unusually sharp increase in their resistance associated with antiferromagnetic ordering at low temperatures.

Coexistence of magnetism and superconductivity in CeRh 1 − x Ir x In 5

We report a thermodynamic and transport study of the phase diagram of

Superconductivity and magnetism in a new class of heavy-fermion materials

{\mathrm{CeRh}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{In}}_{5}.

Specific heat of single-crystal YNi2B2C and TmNi2B2C superconductors

Bulk superconductivity is observed over a broad range of doping,

Specific heat and anisotropic magnetic susceptibility of Pr2CuO4, Nd2CuO4 and Sm2CuO4 crystals

0.3lxl~1,