arXiv subjects

R. L. Meng

Publications and source records attributed to R. L. Meng.

At least 19 recordsLinked to original sources

arXiv2007

Pressure dependence of the superconducting transition and electron correlations in Na_xCoO_2 \cdot 1.3H_2O

We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the decrease of T_c is accompanied by a weakening of the spin correlations at a finite wave vector and a reduction of the density of states (DOS) at the Fermi level. At high pressures above 1.93 GPa, however, the decrease of T_c is mainly due to a reduction of the DOS. These results indicate that the electronic/magnetic state of Co is primarily responsible for the superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling evidence for the presence of line nodes in the superconducting gap function.

cond-mat.supr-con↗

arXiv2006

Na content dependence of superconductivity and the spin correlations in Na_{x}CoO_{2}\cdot 1.3H_{2}O

We report systematic measurements using the ^{59}Co nuclear quadrupole resonance(NQR) technique on the cobalt oxide superconductors Na_{x}CoO_{2}\cdot 1.3H_{2}O over a wide Na content range x=0.25\sim 0.34. We find that T_c increases with decreasing x but reaches to a plateau for x \leq0.28. In the sample with x \sim 0.26, the spin-lattice relaxation rate 1/T_1 shows a T^3 variation below T_c and down to T\sim T_c/6, which unambiguously indicates the presence of line nodes in the superconducting (SC) gap function. However, for larger or smaller x, 1/T_1 deviates from the T^3 variation below T\sim 2 K even though the T_c (\sim 4.7 K) is similar, which suggests an unusual evolution of the SC state. In the normal state, the spin correlations at a finite wave vector become stronger upon decreasing x, and the density of states at the Fermi level increases with decreasing x, which can be understood in terms of a single-orbital picture suggested on the basis of LDA calculation.

cond-mat.supr-con↗

arXiv2005

Enhanced critical current density of YBa2Cu3Ox films grown on Nd1/3Eu1/3Gd1/3Ba2Cu3Ox with nano-undulated surface morphology

We report a simple and easily controllable method where a nano-undulated surface morphology of Nd1/3Eu1/3Gd1/3Ba2Cu3Ox (NEG) films leads to a substantial increase in the critical current density in superconducting YBa2Cu3Ox (YBCO) films deposited by pulsed laser deposition on such NEG layers. The enhancement is observed over a wide range of fields and temperatures. Transmission electron microscopy shows that such YBCO films possess a high density of localized areas, typically 20 x 20 nm2 in size, where distortion of atomic planes give rotational (2 to 5 degrees) moire patterns. Their distribution is random and uniform, and expected to be the origin of the enhanced flux pinning. Magneto-optical imaging shows that these films have excellent macroscopic magnetic uniformity.

cond-mat.supr-con↗

arXiv2004

Comparative Raman Studies of Sr2RuO4, Sr3Ru2O7 and Sr4Ru3O10

The polarized Raman spectra of layered ruthenates of the Srn+1RunO3n+1 (n=1,2,3) Ruddlesden-Popper series were measured between 10 and 300 K. The phonon spectra of Sr3Ru2O7 and Sr4Ru3O10 confirmed earlier reports for correlated rotations of neighboring RuO6 octahedra within double or triple perovskite blocks. The observed Raman lines of Ag or B1g symmetry were assigned to particular atomic vibrations by considering the Raman modes in simplified structures with only one double or triple RuO6 layer per unit cell and by comparison to the predictions of lattice dynamical calculations for the real Pban and Pbam structures. Along with discrete phonon lines, a continuum scattering, presumably of electronic origin, is present in the zz, xx and xy, but not in the x'y' and zx spectra. Its interference with phonons results in Fano shape for some of the lines in the xx and xy spectra. The temperature dependencies of phonon parameters of Sr3Ru2O7 exhibit no anomaly between 10 and 300 K where no magnetic transition occurs. In contrast, two B1g lines in the spectra of Sr4Ru3O10, corresponding to oxygen vibrations modulating the Ru-O-Ru bond angle, show noticeable hardening with ferromagnetic ordering at 105 K, thus indicating strong spin-phonon interaction.

cond-mat.mtrl-sci↗

arXiv2004

Thermodynamic properties in the normal and superconducting states of Na(x)CoO(2)*yH(2)O powder measured by heat capacity experiments

The heat capacity of superconducting Na(x)CoO(2)*yH(2)O was measured and the data are discussed based on three different models: The thermodynamic Ginzburg-Landau model, the BCS theory, and a model including the effects of line nodes in the superconducting gap function. The electronic heat capacity is separated from the lattice contribution in a thermodynamically consistent way maintaining the entropy balance of superconducting and normal states at the critical temperature. It is shown that for a fully gapped superconductor the data can only be explained by a reduced (about 55 %) superconducting volume fraction. The data are compatible with 100 % superconductivity in the case where line nodes are present in the superconducting gap function.

cond-mat.supr-con↗

arXiv2003

Phonons and Magnetic Excitations in Mott-Insulator LaTiO$_3$

The polarized Raman spectra of stoichiometric LaTiO$_3$ (T$_N = 150$ K) were measured between 6 and 300 K. In contrast to earlier report on half-metallic LaTiO$_{3.02}$, neither strong background scattering, nor Fano shape of the Raman lines was observed. The high frequency phonon line at 655 cm$^{-1}$ exhibits anomalous softening below T$_N$: a signature for structural rearrangement. The assignment of the Raman lines was done by comparison to the calculations of lattice dynamics and the nature of structural changes upon magnetic ordering are discussed. The broad Raman band, which appears in the antiferromagnetic phase, is assigned to two-magnon scattering. The estimated superexchange constant $J = 15.4\pm0.5$ meV is in excellent agreement with the result of neutron scattering studies.

physics.optics↗

arXiv2003

Raman Phonons and Ageing-Related Surface Disorder in NaxCoO2

The polarized Raman spectra from ab and ac surfaces of single crystal NaxCoO2 (x~0.7), parent compound of recently discovered superconductor NaxCoO2.yH2O, are reported and discussed. The crystals were hexagon platelets of typical size 3x3x0.1 mm. Three of the five (A1g+E1g+3E2g) Raman active phonons were unambiguously identified at 458 (E1g), 494(E2g) and 574 (A1g) cm-1. The spectra from ab and ac surfaces differ significantly and provide evidence that within hours after preparation the ac surface, unlike the ab one, is strongly disordered. Within several days the disorder extends over the ab surface too.

cond-mat.supr-con↗

arXiv2003

Effect of Hydrostatic Pressure on the Superconductivity in NaxCoO2.yH2O

The effect of hydrostatic pressure on the superconducting transition temperature of Na{0.35}CoO{2}.yH{2}O was investigated by ac susceptibility measurements up to 1.6 GPa. The pressure coefficient of T{c} is negative and the dependence T{c}(p) is nonlinear over the pressure range investigated. The magnitude of the average dlnT{c}/dp=-0.07 GPa^{-1} is comparable to the pressure coefficient of electron-doped high-T{c} copper oxide superconductors with a similar value of T{c}. Our results provide support to the assumption of two-dimensional superconductivity in Na{0.35}CoO{2}.yH{2}O, which is similar to the cuprate systems, and suggest that intercalation of larger molecules may lead to an enhancement of T{c}.

cond-mat.supr-con↗

arXiv2003

Water Content and Superconductivity in Na0.3CoO2*yH2O

We report here the correlation between the water content and superconductivity in Na0.3CoO2*yH2O under the influences of elevated temperature and cold compression. The x-ray diffraction of the sample annealed at elevated temperatures indicates that intergrowths exist in the compound at equilibrium when 0.6 < y < 1.4. Its low-temperature diamagnetization varies linearly with y, but is insensitive to the intergrowth, indicative of quasi-2D superconductivity. The Tc-onset, especially, shifts only slightly with y. Our data from cold compressed samples, on the other hand, show that the water-loss non-proportionally suppresses the diamagnetization, which is suggestive of weak links.

cond-mat.supr-con↗

arXiv2003

Thermodynamic Properties and Pressure Effect on the Superconductivity in CaAlSi and SrAlSi

Superconductivity in the C32 compounds CaAlSi and SrAlSi is investigated by heat capacity (Cp) measurements as well as high-pressure ac-susceptibility experiments. The heat capacity in the superconducting state is well explained by the BCS theory in the weak-coupling limit. The jump of Cp at the superconducting transition (compared to the normal state electronic heat capacity) is 1.62 and 1.21 for CaAlSi and SrAlSi, respectively, and agrees well with the BCS value of 1.43. The magnitudes of the superconducting gaps are estimated as 2.7 meV (CaAlSi) and 1.4 meV (SrAlSi). The pressure effect on the superconducting transition temperature, dlnTc/dp, is positive in CaAlSi (+0.026 GPa-1) but negative in SrAlSi (-0.024 GPa-1). The opposite sign of the pressure coefficients of Tc indicates distinct differences in the electronic structure and the density of states at the Fermi energy of the two compounds in agreement with recent band structure calculations.

cond-mat.supr-con↗

arXiv2003

Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na$_{0.35}$CoO$_{2}$$\cdot y$H$_{2}$O

We report a precise $^{59}$Co nuclear quadrupolar resonance (NQR) measurement on the recently discovered cobalt oxyhydrate Na$_{0.35}$CoO$_{2}$$\cdot y$H$_{2}$O superconductor from $T$=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate $1/T_1$ follows a Curie-Weiss type temperature ($T$) variation, $1/T_1T=C/(T-θ)$, with $θ$=-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below $T_c$=3.9 K, $1/T_1$ decreases with no coherence peak and follows a $T^n$ dependence with $n\simeq$2.2 down to $\sim$2.0 K but crosses over to a $1/T_1\propto T$ variation below $T$=1.4 K, which suggests non s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.

cond-mat.supr-con↗

arXiv2003

Separation Between Antiferromagnetic and Ferromagnetic Transitions in Ru_1-xCu_xSr_2EuCu_2O_8+d

The macroscopic magnetizations of Ru_1-xCu_xSr_2EuCu_2O_8+d with x between 0 and 0.15 were investigated. A ferromagnet-like transition as well as an antiferromagnet-like transition appear around T_M in the low-field magnetization and around T_AM in the high-field differential susceptibility, respectively. The separation between them, which is accompanied by a flat plateau in the magnetic C_p, increases with x. Superparamagnetic M(H) and slow spin dynamics, i.e. characteristics of nanomagnetic clusters, were observed far above T_M. A comparison with RuSr_2(Eu_1-yCe_y)Cu_2O_10+d and some manganites further suggests that a phase separation occurs, which can describe well the conflicting magnetic-superconductivity data previously reported.

cond-mat.supr-con↗

arXiv2002

Superconductivity in the C32 Intermetallic Compounds AAl(2-x)Si(x), with A=Ca and Sr, and 0.6<x<1.2

The intermetallic compounds AAl2-xSix, where A = Ca, Sr or Ba, crystallize in the C32 structure, same as the recently discovered MgB2 with a high superconducting transition temperature of 39 K. For x = 1, superconductivity has been observed in AAlSi with A = Ca and Sr, but not with A = Ba. The transition temperatures are 7.8 and 5.1 K, respectively for CaAlSi and SrAlSi. The CaAl2-xSix compound system display a Tc-peak at x = 1, a possible x-induced electronic transition at x ~ 0.75 and a possible miscibility gap near x ~ 1.1 which results in a very broad superconducting transition. The Seebeck coefficients of AAlSi indicate that their carriers are predominantly electrons in nature, in contrast to the holes in MgB2.

cond-mat.supr-con↗

arXiv2002

Pressure Effect on the Superconducting and Magnetic Transitions of the Superconducting Ferromagnet RuSr2GdCu2O8

The superconducting ferromagnet RuSr2GdCu2O8 was investigated at high pressure. The intra-grain superconducting transition temperature, Tc, is resolved in ac-susceptibility as well as resistivity measurements. It is shown that the pressure shift of Tc is much smaller than that of other high-Tc compounds in a similar doping state. In contrast, the ferromagnetic transition temperature, Tm, increases with pressure at a relative rate that is about twice as large as that of Tc. The high-pressure data indicate a possible competition of the ferromagnetic and superconducting states in RuSr2GdCu2O8.

cond-mat.supr-con↗

arXiv2001

Superconductivity in pure and electron doped MgB2: Transport properties and pressure effects

The normal state and superconducting properties of MgB2 and Mg1-xAlxB2 are discussed based on structural, transport, and high pressure experiments. The positive Seebeck coefficient and its linear temperature dependence for Tc<T<160 K provide evidence that the low-temperature transport in MgB2 is due to hole-like metallic carriers. Structural and transport data show the important role of defects as indicated by the correlation of Tc, the residual resistance ratio, and the microstrain extracted from x-ray spectra. The decrease of Tc with hydrostatic pressure is well explained by the strong-coupling BCS theory. The large scatter of the pressure coefficients of Tc for different MgB2 samples, however, cannot be explained within this theory. We speculate that pressure may increase the defect density, particularly in samples with large initial defect concentration.

cond-mat.supr-con↗

arXiv2001

Correlation between the residual resistance ratio and magnetoresistance in MgB2

The resistivity and magnetoresistance in the normal state for bulk and thin-film MgB2 with different nominal compositions have been studied systematically. These samples show different temperature dependences of normal state resistivity and residual resistance ratios although their superconducting transition temperatures are nearly the same, except for the thin-film sample. The correlation between the residual resistance ratio (RRR) and the power law dependence of the low temperature resistivity, rho vs. T^c, indicates that the electron-phonon interaction is important. It is found that the magnetoresistance (MR) in the normal state scales well with the RRR, a0(MR) proportional to (RRR)^2.2 +/- 0.1 at 50 K. This accounts for the large difference in magnetoresistance reported by various groups, due to different defect scatterings in the samples.

cond-mat.supr-con↗

arXiv2001

Unusual Field-Dependence of the Intragrain Superconductive Transition in RuSr2EuCu2O8

A narrow intragrain phase-lock transition was observed in RuSr2EuCu2O8 under a magnetic field H up to a few Tesla. The corresponding transition temperature, T2, decreases rapidly (about 100 K/T at low fields) with H indicating that the grains of RuSr2EuCu2O8 behave like a Josephson-junction-array instead of a homogeneous bulk superconductor. Our data suggest that the bulk superconducting transition may occur on a length scale well below the grain size of 2 to 6 micrometer.

cond-mat.supr-con↗

arXiv2001

Penetration Depth and Anisotropy in MgB2

The penetration depth lambda of MgB2 was deduced from both the ac susceptibility chi and the magnetization M(H) of sorted powders. The good agreement between the two sets of data without geometric correction for the grain orientation suggests that MgB2 is an isotropic superconductor.

cond-mat.supr-con↗