arXiv subjects

P. G. Pagliuso

Publications and source records attributed to P. G. Pagliuso.

At least 109 records · Page 6Linked to original sources

arXiv2002

Neutron scattering study of cyrstal fields in CeRhIn5

Neutron scattering results for the tetragonal compound CeRhIn5 give evidence for two crystal field (CF) excitations at 6.9 and 23.6 meV. The scattering can be fit assuming a set of CF parameters B$^{0}_{2}$ = -1.03 meV, B$^{0}_{4}$ = 0.044 meV and B$^{4}_{4}$ = 0.122 meV. To compare our results to previous work, we calculate the susceptibility and specific heat for this CF scheme, including a molecular field term $λ= $35 mol/emu to account for the Kondo effect. We also include a calculation based on these CF parameters that uses the non-crossing approximation to the Anderson model to estimate the effect of Kondo physics on the susceptibility, specific heat and neutron linewidths.

cond-mat.str-el↗

arXiv2002

Field distribution and flux-line depinning in the mixed-state of MgB2 probed by Conduction Electron Spin Resonance

We report the first observation of the internal magnetic-field distribution and flux-line depinning in the vortex-state of a type-II superconductor probed by conduction electrons spin resonance (CESR) technique. The CESR measurements were performed in the recently discovered MgB2 type-II superconductor compound with transition temperature Tc ~ 39 K using microwave sources at 4.1 (S-band) and 9.5 GHz (X-band) corresponding to resonance fields of Hr ~ 1455 and 3390 Oe for g ~ 2.00 in the normal state, respectively. From the distortion of the CESR line in the superconducting state, the field distribution function, n(H), in the vortex-state was inferred, and from the broadening of the line a direct estimate of the standard deviation, sigma ~ 14 Oe, was obtained at ~ 28 K and 7 K for S-band and X-band, respectively. Furthermore, our experiments allowed the determination of the flux-line lattice depinning temperature for both employed microwave frequencies.

cond-mat.supr-con↗

arXiv2002

First order superconducting phase transition in CeCoIn$_5$

We investigated the magnetic field dependence of the superconducting phase transition in heavy fermion CeCoIn_5 (T_c = 2.3 K) using specific heat, magneto-caloric effect, and thermal expansion measurements. The superconducting transition becomes first order when the magnetic field is oriented along the 001 crystallographic direction with a strength greater that 4.7 T, and transition temperature below T_0 ~ 0.31 T_c. The change from second order at lower fields is reflected in strong sharpening of both specific heat and thermal expansion anomalies associated with the phase transition, a strong magnetocaloric effect, and a step-like change in the sample volume. The first order superconducting phase transition in CeCoIn_5 is caused by Pauli limiting in type-II superconductors, and was predicted theoretically in the mid 1960s. We do not see evidence for the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state (predicted by an alternative theory also dating back to mid-60's) in CeCoIn_5 with field H || [001].

cond-mat.supr-con↗

arXiv2002

Comparison of the Crystal Structure of the Heavy-Fermion Materials CeCoIn5, CeRhIn5 and CeIrIn5

The crystal structure of the recently discovered heavy-fermion (HF) superconductor CeCoIn5 (Tc = 2.3 K) has been determined by high-resolution neutron powder diffraction. It is tetragonal (space group P4/mmm), with lattice parameters a = 4.61292(9) A and c = 7.5513(2) A at ambient conditions. Whereas CeCoIn5 is isostructural with the HF aniferromagnet CeRhIn5 and the HF superconductor CeIrIn5, its cell constants and its only variable positional parameter, zIn2, differ significantly from the corresponding ones of CeRhIn5 and CeIrIn5. As a result, the distortions of the cuboctahedron [CeIn3], which is the key structural unit in all three materials, are different in CeCoIn5 from the ones in CeRhIn5 and CeIrIn5. The compounds CeCoIn5 and CeIrIn5, which contain the most distorted (in one or another way) [CeIn3] cuboctahedra exhibit superconductivity at ambient pressure below 2.3 K and 0.4 K respectively. On the other hand, CeRhIn5, in which [CeIn3] cuboctahedra are the less distorted, and the parent compound of the materials CeTIn5 i.e. the cubic HF CeIn3 are antiferromagnets at ambient pressure with TN = 3.8 K and 10 K correspondingly.

cond-mat.str-el↗

arXiv2002

Neutron-diffraction study of field-induced transitions in the heavy-fermion compound Ce2RhIn8

We present neutron diffraction measurements in high magnetic fields (0 to 14.5 T) and at low temperatures (2.5, 2.3, 0.77 and 0.068 K) on single crystals of the tetragonal heavy fermion antiferromagnet Ce2RhIn8. For B//[110] the field dependence of selected magnetic and nuclear reflections reveals that the material undergoes several transitions, the temperature dependence of which suggests a complex B-T phase diagram. We present the detailed evolution of the integrated intensities of selected reflections and discuss the associated field-induced transitions.

cond-mat.str-el↗

arXiv2002

Effect of pressure on magnetic structure in heavy fermion CeRhIn5

The effect of hydrostatic pressure on the incommensurate antiferromagnetic structure of CeRhIn5 is investigated with neutron diffraction using a He pressure cell. At 3.8 kbar, the staggered magnetic moment is 0.37(4) Bohr magneton per Ce at 1.6 K, which is the same as the ambient-pressure value. The Neel temperature T_N=3.8(1) K is also the same as the ambient-pressure one, although the curve of order parameter has changed by pressure. The incommensurability "delta" of the magnetic wave vector q_M=(1/2,1/2,"delta") has reduced from "delta"=0.297 at ambient pressure to "delta"=0.294(1) at 3.8 kbar.

cond-mat.str-el↗

arXiv2002

Magnetic Properties of Heavy Fermion Superconductors CeRhIn5 and Ce2RhIn8

Some recent neutron scattering works on CeRhIn5 and Ce2RhIn8, together with related resistivity and specific heat measurements, are summarized. In spite of its layered crystal structure, CeRhIn5 is shown to be 3-dimensional both magnetically and in transport. We also find that the Fisher-Langer behavior is closely followed in CeRhIn5. This may circumvent the Kondo lattice model and support applying established Fermi-liquid superconductivity theory to heavy fermion superconductors.

cond-mat.str-el↗

arXiv2002

Magnetocrystalline Anisotropy in a Single Crystal of CeNiGe2

We report measurements on single crystals of orthorhombic CeNiGe2, which is found to exhibit highly anisotropic magnetic and transport properties. The magnetization ratio M(H//b)/M(H^b) at 2 K is observed to be about 18 at 4 T and the electrical resistivity ratio r//b/r^b is about 70 at room temperature. It is confirmed that CeNiGe2 undergoes two-step antiferromagnetic transition at 4 and 3 K, as reported for polycrystalline samples. The application of magnetic field along the b axis (the easy magnetization axis) stabilizes a ferromagnetic correlation between the Ce ions and enhances the hopping of carriers. This results in large negative magnetoresistance along the b axis.

cond-mat.str-el↗

arXiv2001

Synthesis, characterization and ageing of MgB2

We studied the influence of sample preparation and defects in the superconducting properties samples using atomic ratios of Mg:B=1:1 and Mg:B=1:2. Samples were characterized by SEM, and XRD, and the magnetization properties were examined in a SQUID magnetometer. The presence of Mg vacancies was determined by Rietveld analysis. Most of the samples exhibited sharp superconducting transitions with Tcs between 37- 39 K. We found a strong correlation between the crystal strain and the Tc. This strain was related to the presence of Mg vacancies. In addition, results showed that some samples degraded with time when exposed to ambient conditions. In these samples the Tc did not change with time, but the superconducting transition became broader and the Meissner fraction decreased. This effect was only present in samples with poor connectivity between grains and smaller grain sizes. The degradation was related to a surface decomposition as observed by X-ray Photoelectron Spectroscopy. No correlation was found between this effect and the presence of Mg vacancies.

cond-mat.mtrl-sci↗

arXiv2001

Response of the Heavy-Fermion Superconductor CeCoIn$_5$ to Pressure: Roles of Dimensionality and Proximity to a Quantum-Critical Point

We report measurements of the pressure-dependent superconducting transition temperature $T_c$ and electrical resistivity of the heavy-fermion compound CeCoIn$_5$. Pressure moves CeCoIn$_5$ away from its proximity to a quantum-critical point at atmospheric pressure. Experimental results are qualitatively consistent with theoretical predictions for strong-coupled, d-wave superconductivity in an anisotropic 3D superconductor.

cond-mat.str-el↗

arXiv2001

Effect of La doping on magnetic structure in heavy fermion CeRhIn5

The magnetic structure of Ce0.9La0.1RhIn5 is measured using neutron diffraction. It is identical to the incommensurate transverse spiral for CeRhIn5, with a magnetic wave vector q_M=(1/2,1/2,0.297), a staggered moment of 0.38(2)Bohr magneton per Ce at 1.4K and a reduced Neel temperature of 2.7 K.

cond-mat.str-el↗

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

Anomalous Superconducting Properties and Field Induced Magnetism in CeCoIn5

In the heavy fermion superconductor CeCoIn5 (Tc=2.3K) the critical field is large, anisotropic and displays hysteresis. The magnitude of the critical-field anisotropy in the a-c plane can be as large as 70 kOe and depends on orientation. Critical field measurements in the (110) plane suggest 2D superconductivity, whereas conventional effective mass anisotropy is observed in the (100) plane. Two distinct field-induced magnetic phases are observed: Ha appears deep in the superconducting phase, while Hb intersects Hc2 at T=1.4 K and extends well above Tc. These observations suggest the possible realization of a direct transition from ferromagnetism to Fulde-Ferrel-Larkin-Ovchinnikov superconductivity in CeCoIn5.

cond-mat.str-el↗

arXiv2001

Magnetotransport of CeRhIn5

We report measurements of the temperature-dependent anisotropic resistivity and in-plane magnetoresistance on single crystals of the tetragonal heavy-fermion antiferromagnet (TN = 3.8 K) CeRhIn5. The measurements are reported in the temperature range 1.4 K to 300 K and in magnetic fields to 18 tesla. The resistivity is moderately anisotropic, with a room-temperature c-axis to in-plane resistivity ratio rho_c/rho_a(300 K) = 1.7. rho(T) measurements on the non-magnetic analog LaRhIn5 indicate that the anisotropy in the CeRhIn5 resistivity stems predominately from anisotropy in Kondo-derived magnetic scattering. In the magnetically ordered regime an applied field H reduces TN only slightly due to the small ordered moment (0.37mu_B) and magnetic anisotropy. The magnetoresistance (MR) below TN is positive and varies linearly with H. In the paramagnetic state a positive MR is present below 7.5 K, while a high-field negative contribution is evident at higher temperatures. The positive contribution decreases in magnitude with increasing temperature. Above 40 K the positive contribution is no longer observable, and the MR is negative. The low-T positive MR results from interactions with the Kondo-coherent state, while the high-T negative MR stems from single-impurity effects. The H and T-dependent magnetotransport reflects the magnetic anisotropy and Kondo interactions at play in CeRhIn5.

cond-mat.str-el↗

arXiv2001

On the origin of the zero-resistance anomaly in heavy fermion superconducting Ir: a clue from magnetic field and Rh-doping studies

We present the results of the specific heat and AC magnetic susceptibility measurements of $CeIr_{1-x}Rh_xIn_5$ for x from 0 to 0.5. As x is increased from 0 both quantities reflect the competition between two effects. The first is a suppression of superconductivity below the bulk transition temperature of T$_c = 0.4$ K, which is due to the pair breaking effect of Rh impurities. The second is an increase in the volume fraction of the superconducting regions above T$_c$, which we attribute to defect-induced strain. Analysis of the H-T phase diagram for CeIrIn$_5$obtained from the bulk probes and resistance measurements points to the filamentary origin of the inhomogeneous superconductivity at T$_ρ\approx 1.2$ K, where the resistance drops to zero. The identical anisotropies in the magnetic field dependence of the specific heat and the resistance anomalies in CeIrIn$_5$ indicate that the filamentary superconductivity is intrinsic, involving electrons from the part of the Fermi surface responsible for bulk superconductivity.

cond-mat.str-el↗

arXiv2001

Two energy scales and slow crossover in YbAl3

Experimental results for the susceptibility, specific heat, 4f occupation number, Hall effect and magnetoresistance for single crystals of YbAl$_{3}$ show that, in addition to the Kondo energy scale $k_{B}T_{K}$ $% \sim $ 670K, there is a low temperature scale $T_{coh}<50$K for the onset of coherence. Furthermore the crossover from the low temperature Fermi liquid regime to the high temperature local moment regime is slower than predicted by the Anderson impurity model. These effects may reflect the behavior of the Anderson Lattice in the limit of low conduction electron density.

cond-mat.str-el↗

arXiv2001

Different Gd3+ sites in CaB6: an ESR study

The local environment of Gd3+ (4f7 S=7/2) ions in single crystals of Ca1-xGdxB6 (0.0001 < x < 0.01) is studied by means of Electron Spin Resonance (ESR). The spectra for low concentration samples (x < 0.001) show a split spectrum due to cubic crystal field effects(CFE). The lineshape of each fine structure line is lorentzian, indicating an insulating environment for the Gd3+ ions. For higher concentrations (0.003 < x < 0.01), the spectra show a single resonance (g=1.992(4), DH1/2 ~ 30-60 Oe) with no CFE and dysonian lineshape indicating metallic environment for the Gd3+ ions. For intermediate concentrations, a coexistence of spectra corresponding to insulating and metallic regions is observed. Most of the measured samples show the weak ferromagnetism (WF) as reported for Ca1-xLaxB6 (x ~ 0.005), but, surprisingly, this WF has no effect in our ESR spectra either for metallic or insulating environments. This result suggests that the ferromagnetism in these systems might be isolated in clusters (defect-rich regions) and its relationship with metallicity is nontrivial.

cond-mat.str-el↗

arXiv2001

Multiple phase transitions in Ce(Rh,Ir,Co)In5 heavy-fermion superconductors

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(HFS). CeRh1-xIrxIn5 displays superconductivity(SC) (Tc < 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 TN ~ 4 K. For CeCo1-xRhxIn5, the superconducting state (Tc ~ 2.3 K for x = 0) becomes a magnetic state (TN ~ 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.

cond-mat.str-el↗