arXiv subjects
Publications and source records attributed to C. W. Chu.
The review is devoted to a discussion of the effects of high pressure imposed on superconducting materials. Low-temperature superconductors, high-temperature superconducting cuprates, and some unconventional superconducting compounds are investigated. Experimental as well as theoretical results regarding the pressure effects on Tc and other interesting properties are summarized.
The novel field-induced re-entrant phase in multiferroic hexagonal HoMnO3 is investigated to lower temperatures by dc magnetization, ac susceptibility, and specific heat measurements at various magnetic fields. Two new phases have been unambiguously identified below the Neel transition temperature, TN=76 K, for magnetic fields up to 50 kOe. The existence of an intermediate phase between the P[6]_3[c]m and P[6]_3c[m] magnetic structures (previously predicted from dielectric measurements) was confirmed and the magnetic properties of this phase have been investigated. At low temperatures (T<5 K) a dome shaped phase boundary characterized by a magnetization jump and a narrow heat capacity peak was detected between the magnetic fields of 5 kOe and 18 kOe. The transition across this phase boundary is of first order and the magnetization and entropy jumps obey the magnetic analogue of the Clausius-Clapeyron relation. Four of the five low-temperature phases coexist at a tetracritical point at 2 K and 18 kOe. The complex magnetic phase diagram so derived provides an informative basis for unraveling the underlying driving forces for the occurrence of the various phases and the coupling between the different orders.
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.
Orthorhombic Y$_{1-x}$Ca$_x$MnO$_3$ ($0 \leq x \leq 0.5$) was prepared under high pressure and the variations with $x$ of its structural, magnetic, electrical properties and the polarized Raman spectra were investigated. The lattice parameters change systematically with $x$. Although there are strong indications for increasing disorder above $x = 0.20$, the average structure remains orthorhombic in the whole substitutional range. Ca doping increases conductivity, but temperature dependence of resistivity $ρ$(T) remains semiconducting for all $x$. The average magnetic exchange interaction changes from antiferromagnetic for $x < 0.08$ to ferromagnetic for $x > 0.08$. The evolution with $x$ of the Raman spectra provides evidence for increasingly disordered oxygen sublattice at $x \geq 0.10$, presumably due to quasistatic and/or dynamical Jahn-Teller distortions.
We report an infrared reflection spectroscopy study of La$_{1/2}$Ca$_{1/2}$MnO$_3$ over a broad frequency range and temperature interval which covers the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. The structural phase transition, accompanied by a ferromagnetic ordering at T$_C$=234 K, leads to enrichment of the phonon spectrum. A charge ordered antiferromagnetic insulating ground state develops below the Néel transition temperature T$_N$=163 K. This is evidenced by the formation of charge density waves and opening of a gap with the magnitude of 2$Δ_0$ = (320 $\pm$ 15) cm$^{-1}$ in the excitation spectrum. Several of the infrared active phonons are found to exhibit anomalous frequency softening. The experimental data suggest coexistence of ferromagnetic and antiferromangetic phases at low temperatures.
A re-entrant novel phase has been observed in the hexagonal ferroelectric HoMnO3 in the presence of magnetic fields, in the temperature ranges defined by the plateau of the dielectric constant anomaly. The dielectric plateau evolves with fields from a narrow sharp dielectric peak at the Mn-spin rotation transition at 32.8 K in zero magnetic field. Such a field-induced dielectric plateau anomaly appears both in the temperature sweep at a constant field and in the field sweep at a constant temperature without detectable hysteresis. This is attributed to the indirect coupling between the ferroelectric and antiferromagnetic orders, arising from an antiferromagnetic domain wall effect, where the magnetic order parameter of the Mn subsystem has to change sign across the ferroelectric domain wall in the compound, that influences the ferroelectric domains via a local magnetostrictive effect.
We investigate the polarity-dependent field-induced resistive switching phenomenon driven by electric pulses in perovskite oxides. Our data show that the switching is a common occurrence restricted to an interfacial layer between a deposited metal electrode and the oxide. We determine through impedance spectroscopy that the interfacial layer is no thicker than 10 nm and that the switch is accompanied by a small capacitance increase associated with charge accumulation. Based on interfacial I-V characterization and measurement of the temperature dependence of the resistance, we propose that a field-created crystalline defect mechanism, which is controllable for devices, drives the switch.
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.
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.
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}.
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.
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.
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.
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.
We have studied the doping dependence of the in-plane and out-of-plane superfluid density, ρ^s(0), of two monolayer high-Tc superconductors, HgBa_2CuO_{4+δ} and La_{2-x}Sr_xCuO_4, using the low frequency ac-susceptibility and the muon spin relaxation techniques. For both superconductors, ρ^s(0) increases rapidly with doping in the under- and optimally doped regime and becomes nearly doping independent above a critical doping, p_c = 0.20.
The hysteretic and reversible polarity-dependent resistive switch driven by electric pulses is studied in both Ag/Pr0.7Ca0.3MnO3/YBa2Cu3O7 sandwiches and single-layer Pr0.7Ca0.3MnO3 strips. The data demonstrate that the switch takes place at the Ag-Pr0.7Ca0.3MnO3 interface. A model, which describes the data well, is proposed. We further suggest that electrochemical diffusion is the cause for the switch.
The properties of the superconducting ferromagnets RuSr2(Gd/Eu)Cu2O8 and RuSr2(Ln/Ce)2Cu2O10 are reviewed. The problem of coexistence and competition between magnetic and superconducting states is discussed. A model of phase separation into ferromagnetic and anti-ferromagnetic structures is proposed.
The macroscopic magnetizations of a RuSr2(Eu0.7Ce0.3)2Cu2O10+d sample were investigated. A ferromagnet-like transition occurs around T_M in the low-field magnetization. Highly nonlinear M(H), non-Curie-Weiss susceptibility, and slow spin-dynamics, however, were observed up to T_1 approx 2-3 T_M. In addition, an antiferromagnet-like differential-susceptibility maximum of Ru appears around a separate temperature T_AM between T_1 and T_M. The data are therefore consistent with a phase-separation model: superparamagnetic clusters (or short-range spin-orders) are first precipitated from the paramagnetic matrix below T_1, followed by an antiferromagnetic transition of the matrix at T_AM and an apparent ferromagnetic (FM) transition around T_M, where the long-range spin-order is established in the FM species imbedded in the matrix.