Norman E. Phillips

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.

Phenomenological two-gap model for the specific heat of MgB2

The authors show that the specific heat of the superconductor MgB{sub 2} in zero field, for which significant non-BCS features have been reported, can be fitted, essentially within experimental error, over the entire range of temperature to T{sub c} by a phenomenological two-gap model. The resulting gap parameters agree with previous determinations from band-structure calculations, and from various spectroscopic experiments. The determination from specific heat, a bulk property, shows that the presence of two superconducting gaps in MgB{sub 2} is a volume effect.

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

We report a new family of Ce-based heavy-fermion compounds whose electronic specific heat coefficients range from about 400 to over 700mJ/mol-Ce K2. Crystals in this family form as CenTmIn3n-2m, where T = Rh or Ir, n = 1 or 2, and m = 1, with a tetragonal structure that can be viewed as m-layers of CeIn3 units stacked sequentially along the c-axis with intervening m-layers of TIn2. Ambient and high-pressure studies show that the quasi-2D layers of CeIn3 produce unconventional superconducting and magnetic ground states. This family should enable new understanding of the relationship between magnetism and superconductivity in heavy-fermion materials and more generally of why heavyfermion superconductivity prefers to develop in one structure type and not another.

The magnetic instability in the heavy fermion compounds Ce1?x La x Ru2Si2

The magnetization and the specific heat of Ce1−xLaxRu2Si2 with x≤0.13 are reported with special attention to the effect of magnetic field and the role of lanthanum doping. Evidence is given of differences between the undoped (x=0) and the solid solution (x≠0) cases. A common feature is the occurrence of well-defined anomalies in the magnetization at the “metamagnetic” fieldHM independently of whether the ground state is one of long-range order or Pauli paramagnetism. Forx=0, the ground state appears to be a Pauli paramagnet for any strength of the magnetic field; quantum fluctuations or deviations from an ideal lattice may prevent the occurrence of a true static magnetic transition.

Specific heat of CeRhIn5: Pressure-driven evolution of the ground state from antiferromagnetism to superconductivity

Measurements of the specific heat of antiferromagnetic CeRhIn{sub 5}, to 21 kbar, and for 21 kbar to 70 kOe, show a discontinuous change from an antiferromagnetic ground state below 15 kbar to a superconducting ground state above, and suggest that it is accompanied by a weak thermodynamic first-order transition. Bulk superconductivity appears, apparently with d-wave electron pairing, at the critical pressure, 15 kbar; with further increase in pressure a residual temperature-proportional term in the specific heat disappears.

Influence of sample quality on the magnetic properties of URU2Si2

Abstract The influence of the sample quality on the magnetic properties of the heavy-fermion superconductor URu 2 Si 2 has been studied by neutron scattering, specific heat, electrical resistivity, and magnetic susceptibility measurements. Two single crystals prepared under identical conditions received different heat treatments. One crystal was annealed, the other was used as-grown. While the macroscopic properties and the magnetic excitations are essentially the same for the two crystals, the magnetic Bragg peak intensity have completely different temperature dependences. Despite this, the low-temperature magnetic moment is identical for the two samples, showing that the small moment of 0.023(3) μ B is intrinsic. The finite correlation length (∼ 500 A) appears to be related to defects. We discuss the relevance of itinerant versus localized behavior of the 5f electrons responsible for the magnetism, and the possibility of two successive phase transitions.

MAGNETISM AND SUPERCONDUCTIVITY IN HEAVY-FERMION SYSTEMS

We discuss some consequences of the interplay between magnetism and superconductivity in the two heavy fermion systems URu2Si2 and UPt3, notably on the temperature dependence of the specific heat, on possible observation of Larkin-OvchinnikovFulde-Ferrel phase, and on the anisotropy of the upper critical field. We demonstrate that in UPt3, a clear double steep superconducting transition can be obtained reversibly.

Response of CeRh2Si2 to pressure

Abstract Under atmospheric pressure, CeRh2Si2 orders antiferromagnetically at TN = 35 K, with magnetic entropy S = Rln2 associated with the ordered groundstate. Application of modest pressure (≈ 9 kbar) to CeRh2Si2 suppresses TN to near zero Kelvin, increases its Sommerfeld coefficient of specific heat by nearly a factor of 3.5 and induces some form of superconductivity below 400 mK which is depressed by a magnetic field at a rate of − 80 mK/kG.

Pressure dependences of the specific heats of UPt3, UBe13 and CeAl3

The specific heats of UPt3, UBe,3 and CeAl3 have been measured under pressures to ≈9 kbar. The densities of electronic states decrease sharply with increasing pressure. For UPt3 the spin fluctuation terms decrease with increasing pressure, suggesting a positive correlation with superconductivity. For CeAl3 there is a qualitative change in the temperature dependence of the specific heat.

Low-temperature heat capacity of. cap alpha. -uranium and its relation to the pressure dependence of T/subc/

The heat capacities of five samples of ..cap alpha..-uranium, including one single crystal, have been measured between approximately 0.1 and 2 K, at zero pressure. The four polycrystalline samples showed broad bulk superconducting transitions. The single crystal, for which susceptibility measurements showed a transition near 0.3 K, was not completely superconducting at the lowest temperature, and an estimate T/subc/ less than or equal to 0.1 K was made. The shapes of the heat-capacity anomalies associated with the transitions to the superconducting state were those of broadened BCS transitions, thus showing that local moments and pair-breaking mechanisms are not involved in limiting the values of T/subc/. The value of ..gamma.., the coefficient of the electronic heat capacity, were significantly higher for the polycrystalline samples than for the single crystal, and there is evidence from other work of a similar trend in the lattice heat capacity. It is suggested that these trends are related to the pressure dependence of the same parameters, and that these pressure dependences and that of T/subc/ are all produced by an unusual and strong pressure dependence of the phonon spectrum. (AIP)