1,553 research outputs found
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 PrCeCuO
We have investigated the doping dependence of the penetration depth vs.
temperature in electron doped PrCeCuO using a model
which assumes the uniform coexistence of (mean-field) antiferromagnetism and
superconductivity. Despite the presence of a pairing gap in the
underlying spectrum, we find nodeless behavior of the low- penetration depth
in underdoped case, in accord with experimental results. As doping increases, a
linear-in- 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 =1/2
(=), 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
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