Unconventional superconductivity in CeIrIn5 and CeCoIn5: specific heat and thermal conductivity studies

Low temperature specific heat and thermal conductivity measurements on the ambient pressure heavy fermion superconductors CeIrIn5 and CeCoIn5 reveal power law temperature dependences of these quantities below T(c). The low temperature specific heat in both CeIrIn5 and CeCoIn5 includes T2 terms, consistent with the presence of nodes in the superconducting energy gap. The thermal conductivity data present a T-linear term consistent with the universal limit (CeIrIn5), and a low temperature T3 variation in the clean limit (CeCoIn5), also in accord with prediction for an unconventional superconductor with lines of nodes.     

Superconductivity and quantum criticality in CeCoIn5

Electrical resistivity measurements on a single crystal of the heavy-fermion superconductor CeCoIn5 at pressures to 4.2 GPa reveal a strong crossover in transport properties near P(*) approximately 1.6 GPa, where T(c) is a maximum. The temperature-pressure phase diagram constructed from these data provides a natural connection to cuprate physics, including the possible existence of a pseudogap.     

Anomalous thermal expansion of InSe layered semiconductors in the low-temperature region

The temperature dependence of the linear thermal expansion coefficient (TEC) of an InSe single crystal in the (001) plane is measured in the temperature range 7–50 K. A peak in the thermal expansion is detected near T = 10 K, after which the sample shrinks upon heating. The effect of an external magnetic field of up to 6 T, which is parallel to the (001) plane, on the TEC is investigated. The observed partial suppression of the peak and crystal compression by the field indicates the relation of these anomalies to possible electron ordering in InSe layers.     

Metamagnetic quantum criticality in Sr3Ru2O7 studied by thermal expansion

We report low-temperature thermal expansion measurements on the bilayer ruthenate Sr3Ru2O7 as a function of magnetic field applied perpendicular to the ruthenium-oxide planes. The field dependence of the c-axis expansion coefficient indicates the accumulation of entropy close to 8 T, related to an underlying quantum critical point. The latter is masked by two first-order metamagnetic transitions which bound a regime of enhanced entropy. Outside this region the singular thermal expansion behavior is compatible with the predictions of the itinerant theory for a two-dimensional metamagnetic quantum critical end point.     

Deposition of superconducting CeCoIn5 thin films by co-sputtering and evaporation

Thin superconducting films of CeCoIn₅ were prepared in situ by simultaneous thermal evaporation of indium and dc magnetic field assisted sputtering of planar metallic Ce and Co targets. To achieve an effective sputtering of the magnetic Co target a special geometry with two facing planar targets (Ce and Co) and magnetic field perpendicular to the targets was used. The stoichiometric (0 0 1)-oriented CeCoIn₅ films were grown on r-cut sapphire substrates with a high-rate of 100 nm/min. The temperature dependence of the electrical resistivity revealed the characteristic heavy-fermion behavior and a superconducting transition at about 2 K in agreement with the literature data for CeCoIn₅ bulk material and thin films.     

Perturbing the superconducting planes in CeCoIn5 by Sn substitution

In contrast to substitution on the Co or Ce site, Sn substitution has a remarkably strong effect on superconductivity in CeCoIn5-xSnx, with Tc-->0 beyond only 3.6% Sn. Instead of being randomly distributed on in-plane and out-of-plane In sites, extended x-ray absorption fine structure measurements show the Sn atoms preferentially substitute within the Ce-In plane. This result highlights the importance of the In1 site to impurity scattering and clearly demonstrates the two-dimensional nature of superconductivity in CeCoIn5.     

Characteristics of CeCoIn5/Al/AlOx/Nb and CeCoIn5/Al/AlOx/Al tunnel junctions

We report characteristics of CeCoIn₅/Al/AlO_x/Nb and CeCoIn₅/Al/AlO_x/Al tunnel junctions fabricated on the (0 0 1) surface of CeCoIn₅ crystal platelets. The main result of this work is the observation of a low Josephson current (as compared with that expected from the Ambegaokar–Baratoff formula), which is consistent with idea that the order parameter in the heavy-fermion superconductor CeCoIn₅ has unconventional pairing symmetry.     

Hexagonal and square flux line lattices in CeCoIn5

Using small-angle neutron scattering, we have imaged the magnetic flux line lattice (FLL) in the d-wave heavy-fermion superconductor CeCoIn5. At low fields we find a hexagonal FLL. Around 0.6 T this undergoes what is most likely a first-order transition to square symmetry, with the nearest neighbors oriented along the gap node directions. This orientation of the square FLL is consistent with theoretical predictions based on the d-wave order parameter symmetry.     

Avoided antiferromagnetic order and quantum critical point in CeCoIn5

We measured the specific heat and resistivity of heavy fermion CeCoIn5 between the superconducting critical field H(c2)=5 T and 9 T, with the field in the [001] direction, and at temperatures down to 50 mK. At 5 T the data show a non-Fermi liquid (NFL) behavior down to the lowest temperatures. At the field above 8 T the data exhibit a crossover from the Fermi liquid to a non-Fermi liquid behavior. We analyzed the scaling properties of the specific heat and compared both the resistivity and the specific heat with the predictions of a spin-fluctuation theory. Our analysis leads us to suggest that the NFL behavior is due to incipient antiferromagnetism (AFM) in CeCoIn5 with the quantum critical point in the vicinity of H(c2). Below H(c2) the AFM phase which competes with the paramagnetic ground state is superseded by the superconducting transition.     

Effect of pressure on the thermal expansion of MgO up to 200~GPa

Constant temperature and pressure molecular dynamics (MD) simulations are performed to investigate the thermal expansivity of MgO at high pressure, by using effective pair-wise potentials which consist of Coulomb, dispersion, and repulsion interactions that include polarization effects through the shell model (SM). In order to take into account non-central forces in crystals, the breathing shell model (BSM) is also introduced into the MD simulation. We present a comparison between the volume thermal expansion coefficient α dependences of pressure P at 300 and 2000~K that are obtained from the SM and BSM potentials and those derived from other experimental and theoretical methods in the case of MgO. Compared with the results obtained by using the SM potentials, the MD results obtained by using BSM potentials are more compressible. In an extended pressure and temperature range, the α value is also predicted. The properties of MgO in a pressure range of 0--200~GPa at temperatures up to 3500~K are summarized.     

Anharmonicity and quantum effects in thermal expansion of an Invar alloy

We have investigated the anharmonicity and quantum effects in the Invar alloy Fe(64.6)Ni(35.4) that shows anomalously small thermal expansion. We have performed Fe and Ni K-edge extended x-ray-absorption fine-structure spectroscopic measurements and the computational simulations based on the path-integral effective-classical-potential theory. The first nearest-neighbor (NN) shells around Fe show almost no thermal expansion, while those around Ni exhibit meaningful but smaller expansion than that of fcc Ni. At low temperature, the quantum effect is found to play an essentially important role, which is confirmed by comparing the quantum-mechanical simulations to the classical ones. The anharmonicity (asymmetric distribution) clearly exists for all the first NN shells as in normal thermal expansion systems, implying the breakdown of the direct correspondence between thermal expansion and anharmonicity.     

Anisotropic effect of Cd and Hg doping on the Pauli limited superconductor CeCoIn5

We studied the effect of impurity on the first order superconducting (SC) transition and the high field-low temperature (HFLT) SC state of CeCoIn5 by measuring the specific heat of CeCo(In1-xCdx)_{5} with x=0.0011, 0.0022, and 0.0033 and CeCo(In1-xHgx)_{5} with x=0.000 16, 0.000 32, and 0.000 48 at temperatures down to 0.1 K and fields up to 14 T. Cd substitution rapidly suppresses the crossover temperature T0, where the SC transition changes from second to first order, to T=0 K with x=0.0022 for H parallel[100], while it remains roughly constant up to x=0.0033 for H parallel[001]. The associated anomaly of the proposed FFLO state in Hg-doped samples is washed out by x=0.000 48, while remaining at the same temperature, indicating high sensitivity of that state to impurities. We interpret these results as supporting the nonmagnetic, possibly FFLO, origin of the HFLT state in CeCoIn5.     

Texture in the superconducting order parameter of CeCoIn5 revealed by nuclear magnetic resonance

We present a 115In NMR study of the quasi-two-dimensional heavy-fermion superconductor CeCoIn5 believed to host a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. In the vicinity of the upper critical field and with a magnetic field applied parallel to the ab plane, the NMR spectrum exhibits a dramatic change below T*(H) which well coincides with the position of reported anomalies in specific heat and ultrasound velocity. We argue that our results provide the first microscopic evidence for the occurrence of a spatially modulated superconducting order parameter expected in a FFLO state. The NMR spectrum also implies an anomalous electronic structure of vortex cores.     

Radiation damage in single crystal L-Histidine as studied by low-temperature thermoluminescence techniques

X-irradiation of single crystal L-Histidine at 10 K produces TL glow peaks at 38, 72, 84, 122, 162, 204, and 245 K. The 84 K peak is the most intense one and is characterized by a thermal activation energy of 0.073 eV and frequency factor of 1.1×10³s^-1. Moreover, it is readily photobleachable, whereas the other glow peaks are not, and is tentatively correlated with the thermal decay of a carboxyl anion radical. Computer simulation of the Randall-Wilkins first-order TL expression provided a check on the experimentally derived parameters characterizing the 38 and 84 K peaks. The initial-rise method did not produce accurate parameters for the 38 K peak; however, computer simulation yielded an activation energy of 0.022 eV and a frequency factor of 20 s^-1 which were in agreement with the experimental shape of the glow curve. This TL peak is attributed to the thermal destruction of an imidazole cation radical. Emission spectra measurements of the 84 K luminescence (other peaks were of insufficient intensity) indicated that TL results from thermal release of electrons and their subsequent de-excitation to the ground state via the singlet and triplet manifolds. At sufficiently high temperatures (～78 K) one only observes singlet state emission due to intersystem crossing.     

Andreev reflection in heavy-fermion superconductors and order parameter symmetry in CeCoIn5

Differential conductance spectra are obtained from nanoscale junctions on the heavy-fermion superconductor CeCoIn5 along three major crystallographic orientations. Consistency and reproducibility of characteristic features among the junctions ensure their spectroscopic nature. All junctions show a similar conductance asymmetry and Andreev reflectionlike conductance with a reduced signal ( approximately 10%-13%), both commonly observed in heavy-fermion superconductor junctions. Analysis using the extended Blonder-Tinkham-Klapwijk model indicates that our data provide the first spectroscopic evidence for d_{x;{2}-y;{2}} symmetry. To quantify our conductance spectra, we propose a model by considering the general phenomenology in heavy fermions, the two-fluid behavior, and an energy-dependent density of states. Our model fits to the experimental data remarkably well and should invigorate further investigations.     

Feedback spin resonance in superconducting CeCu2Si2 and CeCoIn5

We show that the recently observed spin resonance modes in heavy-fermion superconductors CeCoIn5 and CeCu2Si2 are magnetic excitons originating from superconducting quasiparticles. The wave vector Q of the resonance state leads to a powerful criterion for the symmetry and node positions of the unconventional gap function. The detailed analysis of the superconducting feedback on magnetic excitations reveals that the symmetry of the superconducting gap corresponds to a singlet d_{x;{2}-y;{2}} state symmetry in both compounds. In particular this resolves the long-standing ambiguity of the gap symmetry in CeCoIn5. We demonstrate that in both superconductors the resonance peak shows a significant dispersion away from Q which can be checked experimentally. Our analysis reveals the similar origin of the resonance peaks in the two heavy-fermion superconductors and in layered cuprates.     

Nuclear quantum effects on the thermal expansion coefficient of hexagonal boron nitride monolayer

This work examines the importance of vibrational delocalization on a basicthermomechanical property of a hexagonal boron nitride monolayer, namely its thermalexpansion coefficient (TEC). Using a recently parametrized bond-order potential of theTersoff type, the TEC was theoretically obtained from the thermal variations of thelattice parameter a(T) calculated using threedifferent methods: (i) the quasiharmonic approximation; (ii) its anharmonic improvementbased on self-consistent phonons; (iii) fully anharmonic Monte Carlo simulations possiblyenhanced within the path-integral framework to account for nuclear quantum effects. Theresults obtained with the three methods are generally consistent with one another and withother recently published data, and indicate that the TEC is negative at least up to ca.700 K, quantum mechanical effects leading to a significant expansion by about 50% relativeto the classical result. Comparison with experimental data on bulk hexagonal BN suggestssignificant differences, which originate from possible inaccuracies in the model that tendto underestimate the lattice parameter itself, and most likely from the 2D nature of themonolayer and the key contribution of out-of-plane modes. The effects of isotopic purityin the natural abundances of boron are found to be insignificant.     

Counterflow diffusion flames: effects of thermal expansion and non-unity Lewis numbers

In this work we re-examine the counterflow diffusion flame problem focusing in particular on the flame–flow interactions due to thermal expansion and its influence on various flame properties such as flame location, flame temperature, reactant leakage and extinction conditions. The analysis follows two different procedures: an asymptotic approximation for large activation energy chemical reactions, and a direct numerical approach. The asymptotic treatment follows the general theory of Cheatham and Matalon, which consists of a free-boundary problem with jump conditions across the surface representing the reaction sheet, and is well suited for variable-density flows and for mixtures with non-unity and distinct Lewis numbers for the fuel and oxidiser. Due to density variations, the species and energy transport equations are coupled to the Navier–Stokes equations and the problem does not possess an analytical solution. We thus propose and implement a methodology for solving the free-boundary problem numerically. Results based on the asymptotic approximation are then verified against those obtained from the ‘exact’ numerical integration of the governing equations, comparing predictions of the various flame properties.     

Anomalous thermal expansion at phase transitions of Ag4RbI5

High resolution capacitance dilatometry has been used to verify the first-order nature of the 208 K phase transition in Ag RbI and to measure the volume change at the 122 K first-order phase transition. The latter is in accord with prediction using the Clausius-Clapeyron equation. Problems relating to the chemical stability of the compound are discussed.