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We report the influence of two-band superconductivity on the flux creep and the critical current densities of a MgB2 thin film. The small magnetic penetration depth of λ=50±10 nm at T=4 K is related to a clean π-band. We find a high self-field critical current density Jc, which is strongly reduced with applied magnetic field, and attribute this to suppression of the superconductivity in the π-band. The temperature dependence of the creep rate S (T) at low magnetic field can be explained by a simple Anderson–Kim mechanism. The system shows high pinning energies at low field that are strongly suppressed by high field.

We report the intrinsic superconducting parameters in a heavily overdoped Ba(Fe1-xCox)2As2 (x=0.14) single crystal and their influence in the resulting vortex dynamics. We find a bulk superconducting critical temperature of 9.8 K, magnetic penetration depth λab (0)=660 ± 50 nm, coherence length ξab (0)=6.4 ± 0.2 nm, and the upper critical field anisotropy γT→Tc ≈ 3.7. The vortex phase diagram, in comparison with the optimally doped compound, presents a narrow collective creep regime. The intrinsic pinning energy plays an important role in the resulting vortex dynamics as compared with similar pinning landscape and comparable intrinsic thermal fluctuations. Fil: Kim, Jeehoon. Pohang University Of Science And Technology; Corea del Sur Fil: Haberkorn, Nestor Fabian. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Gofryk, K.. Oak Ridge National Laboratory; Estados Unidos Fil: Graf, M.J.. Oak Ridge National Laboratory; Estados Unidos Fil: Ronning, F.. Oak Ridge National Laboratory; Estados Unidos Fil: Sefat, A.S.. Oak Ridge National Laboratory; Estados Unidos Fil: Movshovich, R.. Oak Ridge National Laboratory; Estados Unidos Fil: Civale, L.. Oak Ridge National Laboratory; Estados Unidos

The heavy-fermion antiferromagnet CeRhIn 5 exhibits a complex interplay between magnetism and unconventional superconductivity (SC) as a function of applied pressure and magnetic field. This interplay leads to a line of magnetic quantum-critical points within the superconducting state. A comparison of nuclear spin-relaxation measurements on CeRhIn 5 to those made on CeCoIn 5 , PuCoGa 5, and PuRhGa 5 suggests that SC and magnetism also may be related closely in these isostructural superconductors.

We report specific heat and thermal conductivity measurements of the heavy fermion superconductor CeCoIn 5 in the vicinity of the superconducting critical field H c 2 , with magnetic field in the plane of this quasi-2D compound and at temperatures down to 50 mK. The superconducting phase transition changes from second to first order for field above 10 T, as evident from a sharp peak in specific heat and a jump in thermal conductivity, indicating the importance of the Pauli limiting effect in CeCoIn 5 . In the same range of magnetic field, we observe a second specific heat anomaly within the superconducting state. We interpret this anomaly as a signature of a Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) inhomogeneous superconducting state. In addition, the thermal conductivity data as a function of field display a kink at a field H k below the superconducting critical field, which closely coincides with the low-temperature anomaly in specific heat, tentatively identified with the appearance of the FFLO superconducting state. Our results indicate that the thermal conductivity is enhanced within the FFLO state, and call for further theoretical investigations of the real-space structure of the order parameter (and in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state.

We report observation of the specific heat anomaly within the superconducting state of the heavy fermion CeCoIn5. It appears in the vicinity of the superconducting critical field Hc2, where the superconducting transition changes from second to first order, above 10 T for H || [1 1 0] and H || [1 0 0] , and above 4.7 T for H || [0 0 1] , and at temperatures on the order of 0.1Tc. We interpret the anomaly within the superconducting state as a signature of a Fulde–Ferrell–Larkin–Ovchinnikov FFLO inhomogeneous superconducting state.

Following our recent discovery of superconductivity at ambient pressure in heavy fermion CeIrIn 5 and CeCoIn 5 , we studied the nature of the superconducting states in these compounds with specific heat, thermal conductivity, and NMR measurements. The low temperature specific heat in both CeIrIn 5 and CeCoIn 5 includes T 2 terms, consistent with the presence of nodes in the superconducting energy gap, indicating spin-singlet pairing. The thermal conductivity data present a T -linear term consistent with the universal limit (CeIrIn 5 ) and a low temperature T 3 variation in the clean limit (CeCoIn 5 ), also in accord with predictions for an unconventional superconductor with lines of nodes. An NMR study of CeCoIn 5 reveals suppressed spin susceptibility in the superconducting state, and the low temperature H c2 in CeCoIn 5 is Pauli limited, both indicating spin-singlet pairing. Taken together, our measurements point to d-wave superconductivity in this family of compounds.

We report results of magnetic and transport measurements on single crystals of a new rare earth series RIr2Ga (R=La, Pr, Nd, Sm, and Gd through Tm). These compounds appear to crystallize in the triple-hexagonal Na3As structure with space group P63cm. Magnetic ordering temperatures show deviation from De Gennes scaling. These compounds are an interesting example of a quasi-2D structure in which there are anisotropic magnetic interactions.

We report possible mixed valence behavior in the new ternary compounds CeIr 2 Ga and YbIr 2 Ga. These materials crystallize in the hexagonal Na 3 As structure of space group P6 3 cm. Magnetization, specific heat and transport measurements show reduced magnetic moments and the absence of magnetic order above 0.04 K. The quasi-2D structure of these compounds offers the possibility for investigating the role of spatial dimensionality on mixed valence phenomena.

Kondo insulator materials such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3, and SmB6, are 3d, 4f and 5f intermetallic compounds. At high temperatures they behave like metals but a gap Δ in the conduction band opens at the Fermi energy as the temperature is reduced. It has been proposed that the formation of the low-temperature gap is a consequence of the hybridization between the conduction band and the f-electron levels. If this is true, Kondo metal physics should be recovered when the gap is closed at high magnetic fields. We report here specific heat results of Ce3Bi4Pt3 in DC and pulsed magnetic fields up to 60 T. We see evidence for the reduction of the gap in 18 T and a rapid increase of the Sommerfeld coefficient C H /T T→0 in 30 T> H >40 T . Numerical results and the analysis of the data with the Coqblin–Schrieffer model prove a field-induced Kondo insulator-to-Kondo metal crossover.

Superconductivity can be induced at high temperatures in Pb 2 Sr 2 RCu 3 O 8 (R, rare earth) by partially doping Ca 2+ for R 3+ . In order to understand the interplay between magnetism and superconductivity, the magnetic properties of the parent compounds, Pb 2 Sr 2 RCu 3 O 8, have been studied. The work presented here includes magnetic susceptibility and specific heat measurements on R=Dy and extends our previous studies on R=Ce, Pr, Tb, Ho and Er. Specific heat experiments suggest that the Dy ions order magnetically with an ordering temperature of 1.3 K. The magnetic susceptibility data are in good agreement with the susceptibility calculated using crystal-field parameters that are extrapolated from previous modeling of the R=Er and Ho analogs of this series.

Competing interactions in the presence of coupled spin, charge, and lattice degrees of freedom in heavy-fermion materials lead to a near degeneracy of ground states in some systems. A small perturbation in unit-cell volume or composition subsequently can produce a qualitative change in the ground state, for example, from magnetically ordered to superconducting, with, in some instances, the appearance of a non-Fermi-liquid (NFL) state near their boundary. We have studied two heavy-fermion materials, CeCu 2 Si 2 and CeRh 2 Si 2 , which illustrate these behaviors. Measurements of the atomic structure of CeCu 2+ x Si 2 as a function of x suggest that structural inhomogeneity may influence the preferred ground state and the existence of NFL behavior in this material. Thermal expansion measurements on CeRh 2 Si 2 as a function of pressure reveal the evolution of spin-lattice coupling as the balance between RKKY and Kondo interactions is tuned by small volume changes.

CeRhIn 5 is a prototypical heavy-fermion antiferromagnet where the localized 4f electron of Ce hybridizes weakly with ligand electrons. Applying pressure suppresses the antiferromagnetism and induces unconventional superconductivity when the same Ce 4f electrons become delocalized and participate in the formation of Cooper pairs. The emergent superconducting phase unavoidably hides the existence of quantum-critical point, preventing direct access to it. Measurements of the heat capacity and electrical resistivity under magnetic field and pressure reveal unusual normal state properties at a projected quantum-critical point ( P 2 ) : divergence in C / T and sublinear temperature dependence in the resistivity.

We investigated the magnetoresistance and de-Haas-van-Alphen effect (dHvA) in CeIrIn 5 for magnetic fields up to 45 T and in the temperature range 0.03–1 K. A concomitant drop in the resistivity and in the amplitude of the dHvA oscillations mark the onset of the metamagnetic transition at 28 T for field parallel to [0 0 1]. Both features are gradually suppressed when the field is rotated away from [0 0 1]. There is no significant change in the dHvA frequencies and the cyclotron effective masses, at odds with other known metamagnetic systems.

A 50 nm thick Permalloy film has been studied using magnetic resonance force microscopy (MRFM). The ferromagnetic resonance signal has been mechanically detected utilizing a cantilever with a Nd 2 Fe 14 B tip. The measurements were performed in the temperature range between 10 and 70 K and a DC field applied perpendicular to the surface of the film. The microwave field was in the plane. The measurements indicate a decrease of the ferromagnetic resonance field with increasing temperature which may be attributed to temperature-dependent changes of the saturation magnetization. The measurements demonstrate the capability of MRFM to study temperature-dependent phenomena.

We investigated geometrically frustrated YbIn 1 - x Rh x Cu 4 in order to search for quantum criticality associated with ferromagnetic correlations. We report low-temperature specific heat down to 50 mK of YbIn 1 - x Rh x Cu 4 with x = 0.5 and 0.7. For both x = 0.5 and 0.7 C / T ( T ) increases rapidly with decreasing temperature, and displays a broad maximum at about T = 0.25 K , reaching values of up to ∼ 5 J / mol K 2 .

We report on the discovery of a low-temperature phase transition in Bi 2 Sr 2 Ca(Ni x Cu 1− x ) 2 0 8 high temperature superconductor. This transition manifests itself as a sharp reduction of the thermal conductivity of the samples at a temperature T ∗ c ≈200 mK . The temperature dependence of the thermal conductivity changes dramatically from nearly quadratic above the transition to T -linear behavior below the transition. Application of a small magnetic field suppresses the low-temperature phase. Such behavior is consistent with appearance of a second bulk low-temperature superconducting state at T ∗ c .

Measurements of specific heat C ( T ) and electrical resistivity ρ ( T ) of the Sn-substituted heavy fermion superconductor CeCoIn 5 are reported. Superconductivity in CeCoIn 5 - x Sn x is rapidly suppressed at a rate d T c / d x = - 0.6 K / at % Sn. Both the non-Fermi liquid T-linear behavior of ρ ( T ) and the logarithmic divergence of C / T at low-T in CeCoIn 5 - x Sn x suggest proximity to a quantum critical point. Substitution of (larger) Sn for (smaller) In results in an increase in the f-electron/conduction-electron hybridization rather than the expected negative chemical pressure effect and may, at least in part, be the reason for the rapid destruction of superconductivity in CeCoIn 5 .

We measured the specific heat and resistivity of heavy fermion CeCoIn 5 between the superconducting critical field H c 2 = 4.95 and 9 T, with the field in the [0 0 1] direction, and at temperatures down to 50 mK. These results show that this compound has a quantum critical point (QCP) with the magnetic field as the tuning parameter. For a field of 5 T just above H c 2 the temperature dependence of both specific heat C p ( T ) and resistivity ρ ( T ) show non-Fermi liquid (NFL) behavior down to the lowest temperatures with C p ( T ) ∝ - log ( T ) and ρ ( T ) - ρ ( T = 0 ) ∝ T . For fields above 8 T the data exhibit a crossover from a NFL to a Fermi liquid behavior. Specific heat and resistivity data show behavior predicted by spin-fluctuation theory, suggesting that the NFL behavior is due to incipient antiferromagnetism (AFM) in CeCoIn 5 with the quantum critical point in the vicinity of H c 2 . For fields below H c 2 the AFM phase separated by a QCP from the paramagnetic ground state is not observed, as the system becomes first superconducting.

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.

A growing number of chemically substituted intermetallic compounds of Ce and U exhibit non-Fermi-liquid (NFL) behavior in their low temperature physical properties and apparently constitute a new class of strongly correlated f-electron materials. In this paper, we update the experimental situation for the archetypal NFL f-electron system Y1−xUxPd3 and briefly describe recent experiments on the related systems M1−xUxPd3 (M  Sc, La, Pr, Zr, Th).

Certain chemically substituted f-electron compounds exhibit non-Fermi liquid behavior in their low-temperature physical properties and appear to constitute a new class of strongly correlated f-electron materials. In this paper, we update the experimental situation for the archetypal Y 1− x U x Pd 3 system and briefly describe recent experiments on the U 1− x Th x Pd 2 Al 3 system.

We have measured the specific heat of single crystals of superconducting YNi 2 B 2 C and TmNi 2 B 2 C. Analysis of YNi 2 B 2 C data in zero and 5 T magnetic fields indicates modestly strong electron-phonon coupling and a superconducting energy gap Δ ( O ) = 29 K . Superconductivity coexists with antiferromagnetic order below 1.5 K in TmNi 2 B 2 C. An interpretation of very low temperature specific heat indicates substantial magnetic anisotropy, with ferromagnetic planes weakly coupled antiferromagnetically to each other. Two-dimensional ferromagnetic spin waves produce a large linear-in-temperature contribution to the specific heat. We find no compelling evidence for an unconventional pairing mechanism in these systems.

Heat capacity measurements were carried out on Ce 2 T 1-x Co x In 8 (T = Rh, Ir), bilayer variants of the single-layer CeTIn 5 heavy fermions superconductors. We report the evolution of the ground state properties of Ce 2 RhIn 8 and Ce 2 IrIn 8 with Co substitution. We have determined from specific heat data a temperature-composition phase diagram and predict Ce 2 CoIn 8 to be an antiferromagnet at ambient pressure with T N 1.6 K, based on simple extrapolation up to x = 1. The role of the reduced dimensionality and layering effects is discussed.

Magnetic susceptibility, electrical resistivity and heat capacity data for single crystals of Ce(Rh,Ir)1−x(Co,Ir)xIn5 (0⩽x⩽1) have allowed us to construct a detailed phase diagram for this new family of heavy-fermion superconductors. CeRh1−xIrxIn5 displays superconductivity(SC) (T c ≲1 K ) over a wide range of composition, which develops out of and coexists (0.30≲x≲0.5) with a magnetically ordered state, with T N ∼4 K . For CeCo1−xRhxIn5, the superconducting state ( T c ∼2.3 K for x=0) becomes a magnetic state ( T N ∼4 K , for x=1) with two phase transitions observed for 0.40≲x≲0.25. CeCo1−xIrxIn5 also shows two transitions for 0.30≲x≲0.75. For those alloys in which SC is found, a roughly linear relationship between Tc and the lattice parameter ratio c/a, was found, with composition as the implicit parameter. The interplay between magnetism and SC for CeRh1−x(Ir,Co)xIn5 and the possibility of two distinct superconducting states in CeCo1−xIrxIn5 are discussed.

Thermal conductivity of Bi2Sr2Ca(Cu1−xNix)2O8 high-temperature superconductor display a low-temperature anomaly. It manifests itself as a sharp reduction of the thermal conductivity of the samples at a temperature T ∗ c ≈200 mK. We interpret this anomaly as an indication of a marginal stability of the dx2−y2 due to interaction with the spins of magnetic impurities. We suggest that dx2−y2+idxy state forms below T ∗ c .

Specific heat, magnetic susceptibility, and electrical resistivity were measured as a function of temperature in polycrystalline PrInAg2. A broad peak, similar to a conventional Kondo anomaly, occurs in the specific heat at 400 mK. Below the peak, an enhanced linear contribution is observed with a Sommerfeld coefficient of ∼ 6.5 J/mol K2. This is the first Pr-based compound to display heavy-fermion behavior, and it has a Kondo temperature of ∼ 1 K. Inelastic -neutron-scattering experiments by Galera et al. found that the non-Kramers doublet (Γ3) in PrInAg2 is the ground state with a first excited level at 5.9 meV. Since the ground state does not have a magnetic moment, our results point to the possibility of a new type of interaction between the conduction electrons and the quadrupolar moments of the localized 4f electrons. The magnetic susceptibility follows a Curie-Weiss law above ∼ 50 K with an effective moment of 3.67 μB. At low temperatures, the susceptibility falls below simple Curie-Weiss behavior, and it has a value close to that estimated for a Van Vleck contribution with the crystal electric field levels observed in neutron scattering. The electrical resistivity monotonically decreases with temperature and then saturates below ∼ 10 K. A weak decrease is observed in the resistivity around 1 K which is consistent with the small Kondo temperature.

We report results of resistivity measurements at ambient and hydrostatic pressures of up to ≈ 6 kbar. Large resistivity anisotropy sets in below the phase-transition temperature of 0.4 K. In some directions the resistivity increases below 0.4 K, which we interpret as partial gapping of the Fermi surface. Hydrostatic pressure of less than 1 kbar suppresses the low-temperature phase entirely.

We deposited thin films of CeCoIn 5 on various substrates with pulsed laser deposition (PLD) from elemental targets and targets of several compositions. The best samples so far were obtained with a target with excess In composition and at growth temperatures around 400 °C. The laser repetition rate appears to play a minor role. The films are predominantly c -axis oriented but also show other orientations as well as In-rich phases. We observed superconductivity with a T c of 1.95 K in small, well oriented crystallites of CeCoIn 5 . Although the films produced thus far are not phase pure, this work indicates that it is possible to grow epitaxial CeCoIn 5 films.

We report specific heat of CeCo ( In 0.975 Cd 0.025 ) 5 in magnetic field along basal plane. The superconducting transition is not sharp first order-like in high fields and the specific heat at field 12 T displays a sharp peak anomaly at 0.14 K inside the superconducting state. The anomaly is larger and sharper than that for the proposed Fulde–Ferell–Larkin–Ovchinnikov state in pure CeCoIn 5 .

We have performed magnetization and magnetic relaxation measurements on high-quality single crystals of CeCoIn5 at temperatures down to 28 mK and fields up to 17 T. The experiments were carried out by means of a force magnetometer operating in a plastic dilution refrigerator in a 18 T magnet. Data were collected with magnetic field applied both parallel and perpendicular to the ab-plane. For both orientations, pure magnetization signal reveals subtle changes in behavior in a very narrow field range near Hc2. The possible formation of the FFLO state in CeCoIn5 and how the vortices’ dynamics evolve when crossing such state is discussed on the basis of our results.

We present specific heat and susceptibility data on Sm ( La , Sr ) CuO 4 in magnetic fields up to 9 T and temperatures down to 100 mK. We find a broad peak in specific heat which is insensitive to magnetic field at a temperature of 1.5 K with a value of 2.65 J/mol K. The magnetic susceptibility at 5 T continues to increase down to 2 K, the lowest temperature measured. The data suggest that the Sm spin system may be an ideal realization of the frustrated Heisenberg antiferromagnet on the square lattice.

Measurements of magnetic susceptibility, specific heat, and electrical resistivity have been performed on single crystals of CeRhIn 5 - x Sn x . The antiferromagnetic order at T N = 3.8 K that occurs in the heavy-fermion pressure-induced superconductor CeRhIn 5 is suppressed by Sn substitution at a critical concentration x c ∼ 0.35 . Non-Fermi liquid behavior is observed in the vicinity of x c suggesting an antiferromagnetic quantum critical point. The evolution of the antiferromagnetic order, a Schottky anomaly centered around 3 K attributed to short-range magnetic correlations, and non-Fermi liquid behavior in CeRhIn 5 - x Sn x indicates a localized/itinerant crossover of the Ce f electrons, similar to the one that occurs in CeRhIn 5 at applied pressure.

Metallic YbInCu4 undergoes a first-order valence transition at 42 K , where the specific volume increases by 0.5% upon cooling. It is believed that the valence transition is driven by band structure effects that help quench Yb localized moments via a Kondo mechanism. The known phase diagram indicates that the transition can be suppressed with external magnetic fields. We used an adiabatic calorimeter to measure the specific heat in the high-field phase of the material. Data obtained in magnetic fields up to 50 T show an enhanced Sommerfeld coefficient and anomalies in the specific heat that are compatible with a much depressed Kondo temperature. Our data indicate quite different energy scales in the low-field and high-field phases of YbInCu4.

The specific heat of Ni-doped Bi2Sr2CaCu2O8 crystals has been measured between 0.035 and 2 K in magnetic fields up to 9 T. Both pure and doped samples display identical low-temperature nuclear Schottky anomaly tails at T mK . The difference between the specific heat of the Ni-doped sample and that of the pure one is a broad peak centered at about 1 K. Applied magnetic fields move the peak to higher temperatures, which is consistent with spin glass-like behavior of the impurity atoms. Freezing of magnetic impurities may be related to the observation of a second low-temperature transition in several of the Ni-doped samples.

The low-temperature heat capacity C ( T ) of the Kondo insulator Ce 3 Bi 4 Pt 3 has been measured in the new 60 T long pulse magnet at NHMFL-LANL. A large increase in the zero temperature Sommerfeld coefficient γ =( C / T ) T →0 is observed in a magnetic field of 40 T. The high magnetic field value γ 40T =47 mJ / mol K 2 is consistent with a magnetic field induced Kondo insulator to heavy fermion metal phase transition, observed for the first time with a thermodynamic probe.

Low frequency magnetic susceptibility measurements have been performed on YbBiPt from 3 to 800 mK. In addition to the known peak at 400 mK, an anomaly exists near 120 mK. The lowest temperature feature arises from a surface phase (e.g. Bi 2 Pt) and is not a property of the main material. No additional features in the susceptibility are attributed to the bulk material down to the lowest temperature.