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J. L. Sarrao

Publications and source records attributed to J. L. Sarrao.

At least 109 records · Page 6Linked to original sources
arXiv2003

Fulde-Ferrell-Larkin-Ovchinnikov Superconducting State in CeCoIn5

We report specific heat measurements of the heavy fermion superconductor CeCoIn5 in the vicinity of the superconducting critical field H_{c2}, with magnetic field in the [110], [100], and [001] directions, and at temperatures down to 50 mK. The superconducting phase transition changes from second to first order for field above 10 T for H || [110] and H || [100]. 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. We obtain similar results for H || [001], with FFLO state occupying a smaller part of the phase diagram.

cond-mat.str-el↗
arXiv2003

Small-polaron hopping conductivity in bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

We report anisotropic resistivity measurements on a La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ single crystal over a temperature $T$ range from 2 to 400 K and in magnetic fields $H$ up to 14 T. For $T\geq 218$ K, the temperature dependence of the zero-field in-plane $ρ_{ab}(T)$ resistivity obeys the adiabatic small polaron hopping mechanism, while the out-of-plane $ρ_{c}(T)$ resistivity can be ascribed by an Arrhenius law with the same activation energy. Considering the magnetic character of the polarons and the close correlation between the resistivity and magnetization, we developed a model which allows the determination of $ρ_{ab,c}(H,T)$. The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites.

cond-mat.str-el↗
arXiv2003

Avoided Antiferromagnetic Order and Quantum Critical Point in CeCoIn$_5$

We measured specific heat and resistivity of heavy fermion CeCoIn5 between the superconducting critical field $H_{c2} = 5 T$ and 9 T, with field in the [001] direction, and at temperatures down to 50mK. At 5T the data show Non Fermi Liquid behavior down to the lowest temperatures. At field above 8T the data exhibit crossover from the Fermi liquid to a Non Fermi Liquid behavior. We analyzed the scaling properties of the specific heat, and compared both 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 (AF) in CeCoIn5, with the quantum critical point in the vicinity of the $H_{c2}$. Below $H_{c2}$ the AF phase which competes with the paramagnetic ground state is superseded by the superconducting transition.

cond-mat.str-el↗
arXiv2003

Magnetism and superconductivity in CeRh_{1-x}Ir_xIn_5 heavy fermion materials

We report on zero-field muon spin relaxation studies of cerium based heavy-fermion materials CeRh_{1-x}Ir_xIn_5. In the superconducting x=0.75 and 1 compositions muon spin relaxation functions were found to be temperature independent across T_c; no evidence for the presence of electronic magnetic moments was observed. The x=0.5 material is antiferromagnetic below T_N=3.75 K and superconducting below T_c=0.8 K. Muon spin realxation spectra show the gradual onset of damped coherent oscillations characteristic of magnetic order below T_N. At 1.65 K the total oscillating amplitude accounts for at least 85% of the sample volume. No change in muon precession frequency or amplitude is detected on cooling below T_c, indicating the microscopic coexistence of magnetism and superconductivity in this material.

cond-mat.supr-con↗
arXiv2003

Magnetism and superconductivity in Ce_2RhIn_8

We report the discovery of pressure-induced superconductivity, with T_c=2 K, in the heavy-fermion antiferromagnet Ce_2RhIn_8, where superconductivity and magnetic order coexist over an extended pressure interval. A T-linear resistivity in the normal state, accessed by an applied magnetic field, does not appear to derive from the existence of a 2D quantum-critical spin-density wave.

cond-mat.str-el↗
arXiv2003

Low Frequency Spin Dynamics in the CeMIn_5 Materials

We measure the spin lattice relaxation of the In(1) nuclei in the CeMIn_5 materials, extract quantitative information about the low energy spin dynamics of the lattice of Ce moments in both CeRhIn_5 and CeCoIn_5, and identify a crossover in the normal state. Above a temperature T* the Ce lattice exhibits "Kondo gas" behavior characterized by local fluctuations of independently screened moments; below T* both systems exhibit a "Kondo liquid" regime in which interactions between the local moments contribute to the spin dynamics. Both the antiferromagnetic and superconducting ground states in these systems emerge from the "Kondo liquid" regime. Our analysis provides strong evidence for quantum criticality in CeCoIn_5.

cond-mat.str-el↗
arXiv2002

Superconductivity and Quantum Criticality in CeCoIn_5

Electrical resistivity measurements on a single crystal of the heavy-fermion superconductor CeCoIn_5 at pressures to 4.2 GPa reveal a strong crossover in transport properties near P^* \approx 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.

cond-mat.str-el↗
arXiv2002

Bulk Band Gaps in Divalent Hexaborides

Complementary angle-resolved photoemission and bulk-sensitive k-resolved resonant inelastic x-ray scattering of divalent hexaborides reveal a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. This semiconducting gap implies that carriers detected in transport measurements arise from defects, and the measured location of the bulk Fermi level at the bottom of the conduction band implicates boron vacancies as the origin of the excess electrons. The measured band structure and X-point gap in CaB_6 additionally provide a stringent test case for proper inclusion of many-body effects in quasi-particle band calculations.

cond-mat.str-el↗
arXiv2002

de Haas - van Alphen measurements on CeRhIn5 under pressure

Measurements of the de Haas - van Alphen effect have been carried out on the heavy fermion anti-ferromagnet CeRhIn5 at temperatures between 25 mK and 500 mK under pressure. We present some preliminary results of our measurements to track the evolution of the Fermi surface as the pressure induced superconducting transition is approached.

cond-mat.str-el↗
arXiv2002

Non-Locality and Strong Coupling in the Heavy Fermion Superconductor CeCoIn$_{5}$: A Penetration Depth Study

We report measurements of the magnetic penetration depth $λ$ in single crystals of CeCoIn$_{5}$ down to $\sim$0.14 K using a tunnel-diode based, self-inductive technique at 28 MHz. While the in-plane penetration depth tends to follow a power law, $λ_{//} \sim {\it T}^{3/2}$, the data are better described as a crossover between linear ({\it T} $\gg $ ${\it T}^\ast $) and quadratic ({\it T} $\ll {\it T}^\ast $) behavior, with ${\it T}^\ast $ the crossover temperature in the strong-coupling limit. The {\it c}-axis penetration depth $λ_{\perp} $ is linear in {\it T}, providing evidence that CeCoIn$_{5}$ is a {\it d}-wave superconductor with line nodes along the {\it c}-axis. The different temperature dependences of $λ_{//} $ and $λ_{\perp} $ rule out impurity effects as the source of ${\it T}^{\ast} $.

cond-mat.supr-con↗
arXiv2002

Magnetic structure of antiferromagnetic NdRhIn5

The magnetic structure of antiferromagnetic NdRhIn5 has been determined using neutron diffraction. It has a commensurate antiferromagnetic structure with a magnetic wave vector (1/2,0,1/2) below T_N = 11K. The staggered Nd moment at 1.6K is 2.6mu_B aligned along the c-axis. We find the magnetic structure to be closely related to that of its cubic parent compound NdIn3 below 4.6K. The enhanced T_N and the absence of additional transitions below T_N for NdRhIn5 are interpreted in terms of an improved matching of the crystalline-electric-field (CEF), magnetocrystalline, and exchange interaction anisotropies. In comparison, the role of these competing anisotropies on the magnetic properties of the structurally related compound CeRhIn5 is discussed.

cond-mat.str-el↗
arXiv2002

Bandgap recovery and electron doping on cleaved [100] surfaces of divalent semi-metal hexaborides

This paper has been withdrawn by the authors due to new theoretical evidence and experimental proof that the semiconducting bandgap reported in this paper and ascribed to a surface region is in fact a bulk property of divalent hexaborides. As reported in J. D. Denlinger et al., cond-mat/0107429, which supercedes this paper, bulk-sensitive boron K-edge soft x-ray emission provides a complementary confirmation of the X-point band gap identified by angle-resolved photoemission.

cond-mat.str-el↗
arXiv2002

CeCoIn5 - a quantum critical superfluid

We have made the first complete measurements of the London penetration depth $λ(T)$ of CeCoIn5, a quantum-critical metal where superconductivity arises from a non-Fermi-liquid normal state. Using a novel tunnel diode oscillator designed to avoid spurious contributions to $λ(T)$, we have established the existence of intrinsic and anomalous power-law behaviour at low temperature. A systematic analysis raises the possibility that the unusual observations are due to an extension of quantum criticality into the superconducting state.

cond-mat.supr-con↗
arXiv2002

Intersite coupling effects in a Kondo lattice

The La dilution of the Kondo lattice CeCoIn_5 is studied. The scaling laws found for the magnetic susceptibility and the specific heat reveal two well-separated energy scales, corresponding to the single impurity Kondo temperature T_K and an intersite spin-liquid temperature T^*. The Ce-dilute alloy has the expected Fermi liquid ground state, while the specific heat and resistivity in the dense Kondo regime exhibit non-Fermi-liquid behavior, which scales with T^*. These observations indicate that the screening of the magnetic moments in the lattice involves antiferromagnetic intersite correlations with a larger energy scale in comparison with the Kondo impurity case.

cond-mat.str-el↗
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

Spin dynamics in a hole-doped S=1/2 Heisenberg antiferromagnet with a disordered ground state

Only 3% hole doping by Li is sufficient to suppress the long-range antiferromagnetic order in La2CuO4. Spin dynamics in such a disordered state was investigated with measurements of the dynamic magnetic structure factor S(omega,q), using cold neutron spectroscopy, for La2(Cu0.94Li0.06)O4. The S(omega,q) is found to sharply peak at (pi,pi), and its dynamics to be relaxational. Confirming theoretical expectation for the quantum disordered 2D S=1/2 Heisenberg antiferromagnet, the energy scale saturates at a finite value at low temperatures. Possible connection to the ``pseudo spin gap'' phenomenon observed in the NMR/NQR studies on underdoped cuprates is discussed.

cond-mat.supr-con↗