arXiv subjects

R. J. Cava

Publications and source records attributed to R. J. Cava.

At least 343 records · Page 19Linked to original sources

arXiv2002

Structure and Superconductivity in Zr-Stabilized, Nonstoichiometric Molybdenum Diboride

The structure and physical properties of the Zr-stabilized, nonstoichiometric molybdenum diboride superconductor are reported. Good quality material of the diboride structure type can only be obtained by partial substitution of Zr for Mo, and the quenching of melts. The phase is best made with boron in excess of the ideal 2:1 boron to metal ratio. Powder neutron diffraction measurements show that the non-stoichiometry is accommodated by atom deficiency in the metal layers. The diboride structure type exists for (Mo.96Zr.04)xB2 for x between 0.85 and 1.0. Electron diffraction shows that the stoichiometric material, x=1, has a significant number of stacking faults. Tc increases from 5.9 to 8.2K with the introduction of metal vacancies. Resistivity measurements indicate that (Mo.96Zr.04).88B2 is a bad metal, and specific heat measurements show that gamma= 4.4 mJ/mol K2, and that deltaC/gammaTc = 1.19. Preliminary boron isotope effect measurements indicate an exponent 0.11(5). Analysis of the data in terms of the electronic structure is reported, allowing an estimate of the electron-phonon coupling constant, lamda = 0.1-0.3, making these weak-coupling superconductors. Preliminary characterizations of the superconductivity in the related phases NbxB2 and (Mo.96X.04).85B2 for X=Ti, and Hf are reported.

cond-mat.supr-con↗

arXiv2002

Robust paramagnetism in Bi2-xMxRu2O7 (M=Mn,Fe,Co,Ni,Cu) pyrochlore

We report physical property characterization of Bi2-xMxRu2O7 pyrochlores, including magnetic suseptibility, resistivity, and Seebeck coefficients. The solid solution exists up to x=0.5 for (M=Cu,Ni,Co) and up to x=0.1 for (M=Fe,Mn). None of the doped materials exhibit ferromagnetism or any localized ruthenium moment behavior. Instead we find the Ru-O and Bi-O sublattices to be essentially independent, with any magnetism resulting from the unpaired transition metal dopant spins. Cobalt substitution for bismuth results in localized Co{2+}, and low temperature spin-glass transitions in several cases. Nickel moments on the pyrochlore lattice display properties intermediate to localized and itinerant. Finally, copper doping results in only an enhancement of the Pauli metallic density of states.

cond-mat.supr-con↗

arXiv2002

Dirty Spin Ice: The Effect of Dilution on Spin Freezing in Dy2Ti2O7

We have studied spin freezing in the diluted spin ice compound Dy2-xYxTi2O7 where the non-magnetic Y ions replace the magnetic Dy ions on the frustrated pyrochlore lattice. Magnetic a.c. and d.c. susceptibility data are presented with an analysis of relaxation times for dilutions of x = 0, 0.1, 0.2, and 0.4. Site dilution apparently decreases the relative number of spins participating in the ice-like freezing near 16 K while leaving the freezing temperature unchanged. Correspondingly the distribution of relaxation times associated with the freezing is broadened only slightly with increasing dilution, suggesting that the freezing process observed near T = 16 K involves local correlations among the spins.

cond-mat.mtrl-sci↗

arXiv2002

Coherence-Incoherence and Dimensional Crossover in Layered Strongly Correlated Metals

Correlations between electrons and the effective dimensionality are crucial factors that shape the properties of an interacting electron system. For example, the onsite Coulomb repulsion, U, may inhibit, or completely block the intersite electron hopping, t, and depending on the ratio U/t, a material may be a metal or an insulator. The correlation effects increase as the number of allowed dimensions decreases. In 3D systems, the low energy electronic states behave as quasiparticles (QP), while in 1D systems, even weak interactions break the quasiparticles into collective excitations. Dimensionality is particularly important for a class of new exotic low-dimensional materials where 1D or 2D building blocks are loosely connected into a 3D whole. Small interactions between the blocks may induce a whole variety of unusual transitions. Here, we examine layered systems that in the direction perpendicular to the layers display a crossover from insulating-like, at high temperatures, to metallic-like character at low temperatures, while being metallic over the whole temperature range within the layers. We show that this change in effective dimensionality correlates with the existence or non-existence of coherent quasiparticles within the layers.

cond-mat.str-el↗

arXiv2002

Experimental determination of superconducting parameters for the intermetallic perovskite superconductor ${\text {MgCNi}}_3

We have measured upper-critical-field $H_{\text c2}$, specific heat C, and tunneling spectra of the intermetallic perovskite superconductor MgCNi${}_3$ with a superconducting transition temperature $T_{\text c}\approx 7.6$ K. Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field $H_{\text c}$(0), coherence length $ξ$(0), penetration depth $λ$(0), lower-critical-field $H_{\text c1}$(0), and Ginsberg-Landau parameter $κ$(0). From the specific heat, we obtain the Debye temperature $\it Θ_{\text D} \approx$ 280 K. We find a jump of $ΔC/γT_{\text c}$=2.3 at $T_{\text c}$ (where $\it γ$ is the normal state electronic specific coefficient), which is much larger than the weak coupling BCS value of 1.43. Our tunneling measurements revealed a gap feature in the tunneling spectra at $\it Δ$ with $2\it Δ/{\text k}_{\text B}T_{\text c}\approx$ 4.6, again larger than the weak-coupling value of 3.53. Both findings indicate that MgCNi$_3$ is a strong-coupling superconductor. In addition, we observed a pronounced zero-bias conductance peak (ZBCP) in the tunneling spectra. We discuss the possible physical origins of the observed ZBCP, especially in the context of the pairing symmetry of the material.

cond-mat.supr-con↗

arXiv2001

Multiple Field-Induced Phase Transitions in a Geometrically-Frustrated Dipolar Magnet - Gd2Ti2O7

Field-driven phase transitions generally arise from competition between Zeeman energy and exchange or crystal-field anisotropy. Here we present the phase diagram of a frustrated pyrochlore magnet Gd2Ti2O7, where crystal field splitting is small compared to the dipolar energy. We find good agreement between zero-temperature critical fields and those obtained from a mean-field model. Here, dipolar interactions couple real-space and spin-space, so the transitions in Gd2Ti2O7 arise from field-induced "cooperative anisotropy" reflecting the broken spatial symmetries of the pyrochlore lattice.

cond-mat.stat-mech↗

arXiv2001

(13)C NMR investigation of the superconductor MgCNi_3 up to 800K

We report (13)C NMR characterization of the new superconductor MgCNi_3 (He et al., Nature (411), 54 (2001)). We found that both the uniform spin susceptibility and the spin fluctuations show a strong enhancement with decreasing temperature, and saturate below ~50K and ~20K respectively. The nuclear spin-lattice relaxation rate 1/(13)T_1T exhibits typical behaviour for isotropic s-wave superconductivity with a coherence peak below Tc=7.0K that grows with decreasing magnetic field.

cond-mat.supr-con↗

arXiv2001

Carbon Concentration Dependence of the Superconducting Transition Temperature and Structure of MgCxNi3

The crystal structure of the superconductor MgCxNi3 is reported as a function of carbon concentration determined by powder neutron diffraction. The single-phase perovskite structure was found in only a narrow range of carbon content, 0.88 < x < 1.0. The superconducting transition temperature was found to decrease systematically with decreasing carbon concentration. The introduction of carbon vacancies has a significant effect on the positions of the Ni atoms. No evidence for long range magnetic ordering was seen by neutron diffraction for carbon stoichiometries within the perovskite phase stability range.

cond-mat.supr-con↗

arXiv2001

Transport, magnetic, thermodynamic and optical properties in Ti-doped Sr_2RuO_4

We report on electrical resistivity, magnetic susceptibility and magnetization, on heat capacity and optical experiments in single crystals of Sr_2Ru_(1-x)Ti_xO_4. Samples with x=0.1 and 0.2 reveal purely semiconducting resistivity behavior along c and the charge transport is close to localization within the ab-plane. A strong anisotropy in the magnetic susceptibility appears at temperatures below 100 K. Moreover magnetic ordering in c-direction with a moment of order 0.01 mu_B/f.u. occurs at low temperatures. On doping the low-temperature linear term of the heat capacity becomes reduced significantly and probably is dominated by spin fluctuations. Finally, the optical conductivity reveals the anisotropic character of the dc resistance, with the in-plane conductance roughly following a Drude-type behavior and an insulating response along c.

cond-mat.supr-con↗

arXiv2001

The Reactivity of MgB2 with Common Substrate and Electronic Materials

The reactivity of MgB2 with powdered forms of common substrate and electronic materials is reported. Reaction temperatures between 600 C and 800 C, encompassing the range commonly employed in thin-film fabrication, were studied. The materials tested for reactivity were ZrO2, yttria stabilized zirconia (YSZ), MgO, Al2O3, SiO2, SrTiO3, TiN, TaN, AlN, Si, and SiC. At 600 C, MgB2 reacted only with SiO2 and Si. At 800 C, however, reactions were observed for MgB2 with Al2O3, SiO2, Si, SiC, and SrTiO3. The Tc of MgB2 decreased in the reactions with SiC and Al2O3.

cond-mat.supr-con↗

arXiv2001

The complex superstructure in Mg1-xAlxB2 at x~0.5

Electron diffraction and high resolution microscopy have been performed on Mg1-xAlxB2 with x~0.5. This composition displays a superstructure with a repeat period of exactly 2c along the c axis and about 10 nm in the a-b plane. The superstructure results in ring-shaped superreflections in the diffraction pattern. Irradiation by a strong electron beam results in a loss of the superstructure and a decrease of about 1% in the c lattice parameter. In-situ heating and cooling on the other hand showed that the superstructure is stable from 100 K to 700 K. Possible origins for the superstructure are proposed.

cond-mat↗

arXiv2001

Core pinning by intragranular nanoprecipitates in polycrystalline MgCNi_3

The nanostructure and magnetic properties of polycrystalline MgCNi_3 were studied by x-ray diffraction, electron microscopy, and vibrating sample magnetometry. While the bulk flux-pinning force curve F_p(H) indicates the expected grain-boundary pinning mechanism just below T_c = 7.2 K, a systematic change to pinning by a nanometer-scale distribution of core pinning sites is indicated by a shift of F_p(H) with decreasing temperature. The lack of scaling of F_p(H) suggests the presence of 10 to 20% of nonsuperconducting regions inside the grains, which are smaller than the diameter of fluxon cores 2xi at high temperature and become effective with decreasing temperature when xi(T) approaches the nanostructural scale. Transmission electron microscopy revealed cubic and graphite nanoprecipitates with 2 to 5 nm size, consistent with the above hypothesis since xi(0) = 6 nm. High critical current densities, more than 10^6 A/cm^2 at 1 T and 4.2 K, were obtained for grain colonies separated by carbon. Dirty-limit behavior seen in previous studies may be tied to electron scattering by the precipitates, indicating the possibility that strong core pinning might be combined with a technologically useful upper critical field if versions of MgCNi_3 with higher T_c can be found.

cond-mat.supr-con↗

arXiv2001

Low Temperature Fabrication of MgB2

We report the fabrication of MgB2 with bulk superconducting properties by conventional solid state methods at temperatures as low as 550 C. Mg deficiencies of the type Mg1-xB2 were tested. Tc was found to decrease by about 1K at large x, though the amount of non-stoichiometry, if any, is likely to be very small. For specific processing conditions, indications of the 25-30 K transition often seen in thin films were seen in the bulk materials. The lower temperature transition may be associated with the grain boundaries. These results indicate that it should be possible to fabricate MgB2 with bulk properties in in-situ thin films at temperatures of 600 C or less.

cond-mat.supr-con↗

arXiv2001

Absence of a structural transition up to 40 Gpa in MgB2 and the relevance of magnesium non-stoichiometry

We report measurements on MgB2 up to ~40GPa. Increasing pressure yields a monotonous decrease of the lattice parameters and of the c/a ratio, but no structural transition down to parameters smaller than those of AlB2. The transition superconducting temperature also decreases with temperature in a sample dependent way. The results are explained by an increase of the filling of the 2D pxy bands with pressure, the Mg stoichiometry determining the starting position of the Fermi level. Our measurements indicate that these hole bands are the relevant ones for superconductivity.

cond-mat.supr-con↗

arXiv2001

Temperature dependence of the structural parameters of the non-oxide perovskite superconductor MgCNi3

We report the structural parameters of superconducting MgCxNi3 (x=0.96, TC=7.3 K) as a function of temperature, from 2 K to 295 K, determined by neutron powder diffraction profile refinement. The compound has the perovskite structure over the whole temperature range, with symmetry Pm3m and a=3.81221(5) A at 295 K: no structural or long range magnetic ordering transitions were observed. The lattice parameter a and the Debye-Waller factors for the individual atoms decrease smoothly with decreasing temperature. There are no unusual changes of the structural parameters near TC.

cond-mat.supr-con↗

arXiv2001

The suppression of superconductivity in MgCNi3 by Ni-site doping

The effects of partial substitution of Cu and Co for Ni in the intermetallic perovskite superconductor MgCNi3 are reported. Calculation of the expected electronic density of states suggests that electron (Cu) and hole (Co) doping should have different effects. For MgCNi3-xCux, solubility of Cu is limited to approximately 3% (x = 0.1), and Tc decreases systematically from 7K to 6K. For MgCNi3-xCox, solubility of Co is much more extensive, but bulk superconductivity disappears for Co doping of 1% (x = 0.03). No signature of long range magnetic ordering is observed in the magnetic susceptibility of the Co doped material.

cond-mat.supr-con↗

arXiv2001

Giant anharmonicity and non-linear electron-phonon coupling in MgB$_{2}$; A combined first-principles calculations and neutron scattering study

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB$_{2}$. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E$_{2g}$ symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B $σ$ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to explaining the observed high T$_{c}$ and boron isotope effect in MgB$_{2}$

cond-mat.mtrl-sci↗

arXiv2001

Optical studies of charge dynamics in the optimally-doped Bi2212 and the non-superconducting cobaltate single crystals

An analysis of the frequency-dependent scattering rate reveals signatures of the superconducting gap as well as the pseudogap in high-T_c cuprates. These features can be identified in the temperature-dependent spectral function W(omega), inverted from the optical data of the optimally-doped Bi2212 using an experimentally unambiguous method that shows the behavior of both the maxima and minima in the spectral function.

cond-mat.supr-con↗