Theory of magnetic phases of hexagonal rare earth manganites

The magnetic phases of hexagonal perovskites RMnO_3 (R=Ho, Er, Tm, Yb, Sc, Y) are analysed using group theory and the Landau theory of phase transitions. The competition between various magnetic order parameters is discussed in the context of antiferromagnetic interactions. A phenomenological model based on four one-dimensional magnetic order parameters is developed and studied numerically. It is shown that coupling of the various order parameters leads to a complex magnetic field-temperature phase diagram and the results are compared to experiment.Comment: 11 pages, 14 figures. Manuscript with higher quality figures can be obtained here: http://www.physics.mun.ca/~curnoe/papers/RMnO3.submit.pd

Quantum spin configurations in Tb2Ti2O7

Low energy collective angular momentum states of the Tb3+ ions in Tb2Ti2O7 are classified according to the irreducible representations of the octahedral point group. Degeneracy lifting due to the exchange interaction is discussed. Diffuse neutron scattering intensity patterns are calculated for each collective angular momentum state and the ground state is inferred by comparing to experiment.Comment: 5 pages, 1 colour figure. Slight corrections and additions to text and figur

Focused-ion-beam-induced deposition of superconducting nanowires

Superconducting nanowires, with a critical temperature of 5.2 K, have been synthesized using an ion-beam-induced deposition, with a Gallium focused ion beam and Tungsten Carboxyl, W(CO)6, as precursor. The films are amorphous, with atomic concentrations of about 40, 40, and 20 % for W, C, and Ga, respectively. Zero Kelvin values of the upper critical field and coherence length of 9.5 T and 5.9 nm, respectively, are deduced from the resistivity data at different applied magnetic fields. The critical current density is Jc= 1.5 10^5 A/cm2 at 3 K. This technique can be used as a template-free fabrication method for superconducting devices.Comment: Accepted for publication in Applied Physics Letter

d-Wave superconductivity on the checkerboard Hubbard model at weak and strong coupling

It has been argued that inhomogeneity generally can enhance superconductivity in the cuprate high-Tc materials. To check the validity of this claim, we study d-wave superconductivity on the checkerboard Hubbard model on a square lattice using the Cellular Dynamical Mean Field theory method with an exact diagonalization solver at zero temperature. The d-wave order parameter is computed for various inhomogeneity levels over the entire doping range of interest in both strong and weak coupling regimes. At a given doping, the size of the d-wave order parameter manifests itself directly in the height of the coherence peaks and hence is an appropriate measure of the strength of superconductivity. The weak coupling results reveal a suppression of the order parameter in the presence of inhomogeneity for small to intermediate hole dopings, while it is enhanced for large dopings. In contrast, for strong coupling there is a monotonic decrease in the maximum amplitude of the superconducting order parameter with inhomogeneity over the entire doping range of interest. Furthermore, at moderately high inhomogeneity, the system undergoes a first-order transition from the superconducting to the normal state in the underdoped regime. In the overdoped regime, the change in the value of the superconducting order parameter correlates with the height of the lowest energy peak in the spectral weight of antiferromagnetic spin fluctuations, confirming the connection between antiferromagnetic fluctuations and d-wave superconductivity found in earlier studies on the homogeneous case. Our results are benchmarked by comparisons with numerically exact results on the checkerboard Hubbard ladder.Comment: Expanded version includes results on checkerboard Hubbard ladder: 10 pages, 12 figure

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2−x_{2-x}Cex_xCuO4−δ_{4-\delta}

We have investigated the doping dependence of the penetration depth vs. temperature in electron doped Pr$_{2-x}$Ce$_x$CuO$_{4-\delta}$ using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a $d_{x^2-y^2}$ pairing gap in the underlying spectrum, we find nodeless behavior of the low-$T$ penetration depth in underdoped case, in accord with experimental results. As doping increases, a linear-in-$T$ behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.Comment: Accepted in PRL for publicatio

Type-I superconductivity in ScGa3 and LuGa3 single crystals

We present evidence of type-I superconductivity in single crystals of ScGa3 and LuGa3, from magnetization, specific heat, and resistivity measurements: low critical temperatures Tc = 2.1-2.2 K; field-induced secondto first-order phase transition in the specific heat, critical fields less than 240 Oe; and low Ginzburg-Landau coefficients {\kappa} approx 0.23 and 0.30 for ScGa3 and LuGa3, respectively, are all traits of a type-I superconducting ground state. These observations render ScGa3 and LuGa3 two of only several type-I superconducting compounds, with most other superconductors being type II (compounds and alloys) or type I (elemental metals and metaloids).Comment: 5 pages, 6 figure

Spherical agglomeration of superconducting and normal microparticles with and without applied electric field

It was reported by R. Tao and coworkers that in the presence of a strong electric field superconducting microparticles assemble into balls of macroscopic dimensions. Such a finding has potentially important implications for the understanding of the fundamental physics of superconductors. However, we report here the results of experimental studies showing that (i) ball formation also occurs in the absence of an applied electric field, (ii) the phenomenon also occurs at temperatures above the superconducting transition temperature, and (iii) it can also occur for non-superconducting materials. Possible origins of the phenomenon are discussed.Comment: Small changes in response to referee's comments. To be published in Phys. Rev.

Current and Shot Noise Measurements in a Carbon Nanotube-Based Spin Diode

Low-temperature measurements of asymmetric carbon nanotube (CNT) quantum dots are reported. The CNTs are end-contacted with one ferromagnetic and one normal-metal electrode. The measurements show a spin-dependent rectification of the current caused by the asymmetry of the device. This rectification occurs for gate voltages for which the normal-metal lead is resonant with a level of the quantum dot. At the gate voltages at which the current is at the maximum current, a significant decrease in the current shot noise is observed

Running-phase state in a Josephson washboard potential

We investigate the dynamics of the phase variable of an ideal underdamped Josephson junction in switching current experiments. These experiments have provided the first evidence for macroscopic quantum tunneling in large Josephson junctions and are currently used for state read-out of superconducting qubits. We calculate the shape of the resulting macroscopic wavepacket and find that the propagation of the wavepacket long enough after a switching event leads to an average voltage increasing linearly with time.Comment: 6 pages, 3 figure

Rocking ratchets in 2D Josephson networks: collective effects and current reversal

A detailed numerical study on the directed motion of ac-driven vortices and antivortices in 2D Josephson junction arrays (JJA) with an asymmetric periodic pinning potential is reported. Dc-voltage rectification shows a strong dependence on vortex density as well as an inversion of the vortex flow direction with ac amplitude for a wide range of vortex density around $f$=1/2 ($f$=$Ha^2 / \Phi_0$), in good agreement with recent experiments by Shal\'om and Pastoriza [Phys. Rev. Lett. {\bf 94}, 177001 (2005)]. The study of vortex structures, spatial and temporal correlations, and vortex-antivortex pairs formation gives insight into a purely collective mechanism behind the current reversal effect.Comment: 4 pages, 5 figures. Accepted for publication in Phys. Rev. Let