M. B. Salamon

High-temperature thermopower in La2/3Ca1/3MnO3 films: Evidence for polaronic transport.

Thermoelectric power, electrical resistivity, and magnetization experiments, performed in the paramagnetic phase of La{sub 2/3}Ca{sub 1/3}MnO{sub 3}, provide evidence for polaron-dominated conduction in colossal magnetoresistance materials. At high temperatures, a large, nearly-field-independent difference between the activation energies for resistivity {rho} and thermopower {ital S}, a characteristic of Holstein polarons, is observed, and ln{rho} ceases to scale with the magnetization. On approaching {ital T}{sub {ital c}}, both energies become field dependent, indicating that the polarons are magnetically polarized. Below {ital T}{sub {ital c}}, the thermopower follows a law {ital S}({ital H}){approximately}1/{rho}({ital H}) as in nonsaturated ferromagnetic metals. {copyright} {ital 1996 The American Physical Society.}

High-temperature thermopower in La{sub {bold 2/3}}Ca{sub {bold 1/3}}MnO{sub {bold 3}} films: Evidence for polaronic transport

Thermoelectric power, electrical resistivity, and magnetization experiments, performed in the paramagnetic phase of La{sub 2/3}Ca{sub 1/3}MnO{sub 3}, provide evidence for polaron-dominated conduction in colossal magnetoresistance materials. At high temperatures, a large, nearly-field-independent difference between the activation energies for resistivity {rho} and thermopower {ital S}, a characteristic of Holstein polarons, is observed, and ln{rho} ceases to scale with the magnetization. On approaching {ital T}{sub {ital c}}, both energies become field dependent, indicating that the polarons are magnetically polarized. Below {ital T}{sub {ital c}}, the thermopower follows a law {ital S}({ital H}){approximately}1/{rho}({ital H}) as in nonsaturated ferromagnetic metals. {copyright} {ital 1996 The American Physical Society.}

Hidden magnetism and quantum criticality in the heavy fermion superconductor CeRhIn5

With only a few exceptions that are well understood, conventional superconductivity does not coexist with long-range magnetic order (for example, ref. 1). Unconventional superconductivity, on the other hand, develops near a phase boundary separating magnetically ordered and magnetically disordered phases. A maximum in the superconducting transition temperature Tc develops where this boundary extrapolates to zero Kelvin, suggesting that fluctuations associated with this magnetic quantum-critical point are essential for unconventional superconductivity. Invariably, though, unconventional superconductivity masks the magnetic phase boundary when T < Tc, preventing proof of a magnetic quantum-critical point. Here we report specific-heat measurements of the pressure-tuned unconventional superconductor CeRhIn5 in which we find a line of quantum–phase transitions induced inside the superconducting state by an applied magnetic field. This quantum-critical line separates a phase of coexisting antiferromagnetism and superconductivity from a purely unconventional superconducting phase, and terminates at a quantum tetracritical point where the magnetic field completely suppresses superconductivity. The T → 0 K magnetic field–pressure phase diagram of CeRhIn5 is well described with a theoretical model developed to explain field-induced magnetism in the high-Tc copper oxides, but in which a clear delineation of quantum–phase boundaries has not been possible. These experiments establish a common relationship among hidden magnetism, quantum criticality and unconventional superconductivity in copper oxides and heavy-electron systems such as CeRhIn5.

S-wave spin-triplet order in superconductors without inversion symmetry: Li2Pd3B and Li2Pt3B.

We investigate the order parameter of noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B via the behavior of the penetration depth lambda(T). The low-temperature penetration depth shows BCS-like behavior in Li2Pd3B, while in Li2Pt3B it follows a linear temperature dependence. We propose that broken inversion symmetry and the accompanying antisymmetric spin-orbit coupling, which admix spin-singlet and spin-triplet pairing, are responsible for this behavior. The triplet contribution is weak in Li2Pd3B, leading to a wholly open but anisotropic gap. The significantly larger spin-orbit coupling in Li2Pt3B allows the spin-triplet component to be larger in Li2Pt3B, producing line nodes in the energy gap as evidenced by the linear temperature dependence of lambda(T). The experimental data are in quantitative agreement with theory.

Coexistence of localized and itinerant carriers near TC in calcium-doped manganites

We explore the possibility that polaronic distortions in the paramagnetic phase of La{sub 0.67}Ca{sub 0.33}MnO{sub 3} manganites persist in the ferromagnetic phase by considering the observed electrical resistivity to arise from coexisting field- and temperature-dependent polaronic and band-electron fractions. We use an effective medium approach to compute the total resistivity of the two-component system, and find that a limit with low percolation threshold explains the data rather well. To test the validity of this model, we apply it to the thermoelectric coefficient. We propose a plausible mean-field model that reproduces the essential features of a microscopic model and provides a comparison with the experimental mixing fraction, as well as the magnetization and magnetic susceptibility. {copyright} {ital 1999} {ital The American Physical Society}

Probing the superconducting gap symmetry of PrOs4Sb12: a penetration depth study.

We report measurements of the magnetic penetration depth lambda in single crystals of PrOs4Sb12 down to 0.1 K, with the ac field applied along the a, b, and c directions. In all three field orientations, lambda approximately T2 and superfluid density rho(s) approximately T2 for T<0.3T(c). Data are best fit by the 3He A-phase-like gap with multidomains, each having two point nodes along a cube axis, and parameter Delta(0)(0)/k(B)T(c)=2.6, suggesting that PrOs4Sb12 is a strong-coupling superconductor with two point nodes on the Fermi surface. We also confirm the double transitions at 1.75 and 1.85 K seen in other measurements.

Occurrence of long‐range helical spin ordering in Dy‐Y multilayers (invited)

The magnetic ordering of highly perfect single‐crystal multilayer films of alternate layers of magnetic Dy and nonmagnetic Y prepared by molecular beam epitaxy has been studied by neutron diffraction. Results on a series of films with Dy thicknesses of approximately 16 atomic planes (≊45 A) and Y thicknesses ranging from 10 to 22 planes have confirmed the existence of long‐range helimagnetic ordering of the Dy 4f spins which is propagated through the intervening Y layers in phase coherence. The propagation vectors in both Dy and Y layers have been calculated from the wave vector of the magnetic satellites and the intensity of the bilayer harmonics. The propagation vector for Dy decreases continuously with temperature, while that in the Y is temperature independent and equal to 0.31 A−1. The nature of the ordering and the noninteger multiple of π for the phase change of the propagation wave vector across the Y, suggests that the mechanism of long‐range coupling is a conduction band spin‐density wave in bot...

Evidence for Nodal Quasiparticles in the Nonmagnetic Superconductor YNi2B2C via Field-Angle-Dependent Heat Capacity

Field-angle dependent heat capacity of the non-magnetic borocarbide superconductor YNi2B2C reveals a clear fourfold oscillation, the first observation of its kind. The observed angular variations were analyzed as a function of magnetic field angle, field intensity and temperature to provide its origin. The quantitative agreement between experiment and theory strongly suggests that we are directly observing nodal quasiparticles generated along<100>by the Doppler effect. The results demonstrate that field-angle heat capacity can be a powerful tool in probing the momentum-space gap structure in unconventional superconductors such as high Tc cuprates, heavy fermion superconductors, etc.

Neutron scattering studies of rare earth magnetic multilayers

Abstract Single crystal artificial superlattices prepared by molecular beam epitaxy and consisting of Dy, Er, and Gd layers alternated with Y all exhibit long-range magnetic order that is propagated through intervening non-magnetic Y layers as shown by neutron diffraction studies. In the case of Dy systems, the phase and chirality of the helical order are preserved from one Dy layer to another. The phase shift across the Y layer is proportional to the Y layer thickness, and so is in general a non-integral multiple of π, suggesting that the coupling is propagated by a spin-density-wave mechanism. The magnetic coherence range, obtained from a series of samples of constant Dy thickness and varying number of Y layers, shows a linear variation with the inverse of the Y layer thickness.

Magnetocaloric effect and temperature coefficient of resistance of La2/3(Ca,Pb)1/3MnO3

The magnetocaloric effect of a La2/3(Ca,Pb)1/3MnO3 single crystal was examined. From the measurements of isothermal magnetization and heat capacity around the Curie temperature, we have calculated the maximum isothermal magnetic entropy change as ∼7.5 J/kg K and the adiabatic temperature changes as ∼5.6 K, near room temperature for a 7 T magnetic field variation. Besides, the sharp drop of resistivity at the insulator–metal transition associated with the magnetic transition gives rise to a large temperature coefficient of resistance (TCR). For the La0.7Ca0.3MnO3 and La2/3(Ca,Pb)1/3MnO3 single crystal, the peak of TCR is ∼50%/K at 218 K and ∼10%/K at 290 K, respectively.