arXiv subjects

N. E. Phillips

Publications and source records attributed to N. E. Phillips.

9 recordsLinked to original sources

arXiv2015

Specific heat of Ba$_{0.59}$K$_{0.41}$Fe$_{2}$As$_{2}$, and a new method for identifying the electron contribution: two electron bands with different energy gaps in the superconducting state

We report measurements of the specific heat of Ba$_{0.59}$K$_{0.41}$Fe$_{2}$As$_{2}$, an Fe-pnictide superconductor with $T_c$ = 36.9 K, for which there are suggestions of an unusual electron pairing mechanism. We use a new method of analysis of the data to derive the parameters characteristic of the electron contribution. It is based on comparisons of $α$-model expressions for the electron contribution with the total measured specific heat, which give the electron contribution directly. It obviates the need in the conventional analyses for an independent, necessarily approximate, determination of the lattice contribution, which is subtracted from the total specific heat to obtain the electron contribution. It eliminates the uncertainties and errors in the electron contribution that follow from the approximations in the determination of the lattice contribution. Our values of the parameters characteristic of the electron contribution differ significantly from those obtained in conventional analyses of specific-heat data for five similar hole-doped BaFe$_{2}$As$_{2}$ superconductors, which also differ significantly among themselves. They show that the electron density of states is comprised of contributions from two electron bands with superconducting-state energy gaps that differ by a factor 3.8, with 77$\%$ coming from the band with the larger gap. The variation of the specific heat with magnetic field is consistent with extended $s$-wave pairing, one of the theoretical predictions. The relation between the densities of states and the energy gaps in the two bands is not consistent with a theoretical model based on interband interactions alone. Comparison of the normal-state density of states with band-structure calculations shows an extraordinarily large effective mass enhancement, for which there is no precedent in similar materials and no theoretical explanation.

cond-mat.supr-con↗

arXiv2010

Heat capacity study of BaFe$_{2}$As$_{2}$: effects of annealing

Heat-capacity, X-ray diffraction, and resistivity measurements on a high-quality BaFe$_{2}$As$_{2}$ sample show an evolution of the magneto-structural transition with successive annealing periods. After a 30-day anneal the resistivity in the (ab) plane decreases by more than an order of magnitude, to 12 $μΩ$cm, with a residual resistance ratio $\sim$36; the heat-capacity anomaly at the transition sharpens, to an overall width of less than K, and shifts from 135.4 to 140.2 K. The heat-capacity anomaly in both the as-grown sample and after the 30-day anneal shows a hysteresis of $\sim$0.15 K, and is unchanged in a magnetic field $μ_{0}$H = 14 T. The X-ray and heat-capacity data combined suggest that there is a first order jump in the structural order parameter. The entropy of the transition is reported.

cond-mat.supr-con↗

arXiv2006

Specific Heat of Na0.35CoO2,1.3H2O: Effects of Sample Age; Non-Magnetic Pair Breaking, Two Energy Gaps, and Strong Fluctuations in the Superconducting State

The specific heats of three samples of Na0.35CoO2,1.3H2O show an evolution of the superconductivity, and its ultimate disappearance, with increasing sample age. An overall increase in pair-breaking action, which occurs preferentially in an electron band with a small energy gap, produces a shift in the relative contributions of two electron bands to the superconducting condensation. The similarity of the time scale for these changes to that recently reported for structural changes in the CoO2 layers and the formation of O vacancies suggests a relation between the two effects and an explanation for the strong sample dependence of the properties of this material more generally. The onset of the transition to the vortex state is independent of magnetic field, suggesting the presence of unusually strong fluctuation effects.

cond-mat.supr-con↗

arXiv2004

Heat Capacity of Na$_{0.3}$CoO$_{2}$$\cdot$1.3H$_{2}$O, a New Two-Gap Superconductor: Comparison with the Heat Capacity of MgB$_2$

The superconducting-state heat capacity of Na$_{0.3}$CoO$_{2}$$\cdot$1.3H$_{2}$O shows unusual, marked deviations from BCS theory, at all temperatures. At low temperatures the heat capacity has the $T^2$ dependence characteristic of line nodes in the energy gap, rather than the exponential temperature dependence of a fully gapped, conventional superconductor. At temperatures of the order of one fifth of the critical temperature and above, the deviations are strikingly similar to those of MgB$_2$, which are known to be a consequence of the existence of substantially different energy gaps on different sheets of the Fermi surface. A two-gap fit to the Na$_{0.3}$CoO$_{2}$$\cdot$1.3H${_2}$O data gives gap amplitudes of 45% and 125% of the BCS value, on parts of the Fermi surface that contribute, respectively, 45% and 55% to the normal-state density of states. The temperature of the onset of the transition to the vortex state is independent of magnetic field, which shows the presence of unusually strong fluctuations.

cond-mat.supr-con↗

arXiv2001

Phenomenological two-gap model for the specific heat of MgB_2

We show that the specific heat of the superconductor MgB_2 (MgB2) 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_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_2 is a volume effect.

cond-mat.supr-con↗

arXiv2001

Specific Heat of CeRhIn5: Pressure-Driven Evolution of the Ground State from Antiferromagnetism to Superconductivity

Measurements of the specific heat of antiferromagnetic CeRhIn5, 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.

cond-mat.str-el↗

arXiv2001

Identification and Characterization of Two Energy Gaps in Superconducting MgB2 by Specific-Heat Measurements

This paper reports specific-heat measurements on superconducting MgB2 in magnetic fields to 9 T. In zero magnetic field the data can be fitted to a two-gap model, which is a generalization of a semi-empirical model for strong-coupled, single-gap superconductors (the alpha-model), to within the precision of the data. Both gaps close at Tc, with one gap larger and one smaller than the BCS weak-coupling-limit. (The two gaps have a ratio ~ 4:1.) Each gap accounts for ~ 50% of the normal electron density of states. The parameters characterizing the fits agree well with those from recent spectroscopic measurements and theory. In magnetic fields the superconductong anomaly is broadened as expected for an anisotropic, randomly-oriented superconductor.

cond-mat.supr-con↗

arXiv2001

Specific Heat of Mg ^{11}B_2

Measurements of the specific heat of Mg ^{11}B_2 (MgB2), from 1 to 50 K, in magnetic fields to 9 T, give the Debye temperature, $Θ$ = 1050 K, the coefficient of the normal-state electron contribution, $γ_n$ = 2.6 mJ mol^{-1} K^{-2}, and a discontinuity in the zero-field specific heat of 133 mJ mol^{-1} K^{-1} at $T_c$ = 38.7 K. The estimated value of the electron-phonon coupling parameter, $λ$ = 0.62, could account for the observed $T_c$ only if the important phonon frequencies are unusually high relative to $Θ$. At low $T$, there is a strongly field-dependent feature that suggests the existence of a second energy gap, about four times smaller than the major gap.

cond-mat.supr-con↗

arXiv2000

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 700 mJ/mole Ce-K2. Crystal 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 n-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 heavy-fermion superconductivity prefers to develop in one structure type and not another.

cond-mat.str-el↗