2,451 research outputs found
The Missing Link: Magnetism and Superconductivity
The effect of magnetic moments on superconductivity has long been a
controversial subject in condensed matter physics. While Matthias and
collaborators experimentally demonstrated the destruction of superconductivity
in La by the addition of magnetic moments (Gd), it has since been suggested
that magnetic fluctuations are in fact responsible for the development of
superconducting order in other systems. Currently this debate is focused on
several families of unconventional superconductors including high-Tc cuprates,
borocarbides as well as heavy fermion systems where magnetism and
superconductivity are known to coexist. Here we report a novel aspect of
competition and coexistence of these two competing orders in an interesting
class of heavy fermion compounds, namely the 1-1-5 series: CeTIn5 where T=Co,
Ir, or Rh. Our optical experiments indicate the existence of regions in
momentum space where local moments remain unscreened. The extent of these
regions in momentum space appears to control both the normal and
superconducting state properties in the 1-1-5 family of heavy fermion (HF)
superconductors.Comment: 6 pages, 2 figure
Quantum transport of two-dimensional Dirac fermions in SrMnBi2
We report two-dimensional quantum transport in SrMnBi single crystals.
The linear energy dispersion leads to the unusual nonsaturated linear
magnetoresistance since all Dirac fermions occupy the lowest Landau level in
the quantum limit. The transverse magnetoresistance exhibits a crossover at a
critical field from semiclassical weak-field dependence to the
high-field linear-field dependence. With increase in the temperature, the
critical field increases and the temperature dependence of
satisfies quadratic behavior which is attributed to the Landau level splitting
of the linear energy dispersion. The effective magnetoresistant mobility
cm/Vs is derived. Angular dependent magnetoresistance
and quantum oscillations suggest dominant two-dimensional (2D) Fermi surfaces.
Our results illustrate the dominant 2D Dirac fermion states in SrMnBi and
imply that bulk crystals with Bi square nets can be used to study low
dimensional electronic transport commonly found in 2D materials like graphene.Comment: 5 papges, 4 figure
β models for random hypergraphs with a given degree sequence
We introduce the beta model for random hypergraphs in order to represent
the occurrence of multi-way interactions among agents in a social network. This model
builds upon and generalizes the well-studied beta model for random graphs, which instead only considers pairwise interactions. We provide two algorithms for fitting the
model parameters, IPS (iterative proportional scaling) and fixed point algorithm, prove
that both algorithms converge if maximum likelihood estimator (MLE) exists, and provide algorithmic and geometric ways of dealing the issue of MLE existence
Coexistence of magnetism and superconductivity in CeRh1-xIrxIn5
We report a thermodynamic and transport study of the phase diagram of
CeRh1-xIrxIn5. Superconductivity is observed over a broad range of doping, 0.3
< x < 1, including a substantial range of concentration (0.3 < x <0.6) over
which it coexists with magnetic order (which is observed for 0 < x < 0.6). The
anomalous transition to zero resistance that is observed in CeIrIn5 is robust
against Rh substitution. In fact, the observed bulk Tc in CeRh0.5Ir0.5In5 is
more than double that of CeIrIn5, whereas the zero-resistance transition
temperature is relatively unchanged for 0.5 < x < 1
Two dimensional Dirac fermions and quantum magnetoresistance in CaMnBi
We report two dimensional Dirac fermions and quantum magnetoresistance in
single crystals of CaMnBi. The non-zero Berry's phase, small cyclotron
resonant mass and first-principle band structure suggest the existence of the
Dirac fermions in the Bi square nets. The in-plane transverse magnetoresistance
exhibits a crossover at a critical field from semiclassical weak-field
dependence to the high-field unsaturated linear magnetoresistance ( in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The
temperature dependence of satisfies quadratic behavior, which is
attributed to the splitting of linear energy dispersion in high field. Our
results demonstrate the existence of two dimensional Dirac fermions in
CaMnBi with Bi square nets.Comment: 5 pages, 4 figure
Multiband effects on beta-FeSe single crystals
We present the upper critical fields Hc2(T) and Hall effect in beta-FeSe
single crystals. The Hc2(T) increases as the temperature is lowered for field
applied parallel and perpendicular to (101), the natural growth facet of the
crystal. The Hc2(T) for both field directions and the anisotropy at low
temperature increase under pressure. Hole carriers are dominant at high
magnetic fields. However, the contribution of electron-type carriers is
significant at low fields and low temperature. Our results show that multiband
effects dominate Hc2(T) and electronic transport in the normal state
Superconducting MgB2 thin films by pulsed laser deposition
Growth of MgB2 thin films by pulsed laser deposition is examined under ex
situ and in situ processing conditions. For the ex situ process, Boron films
grown by PLD were annealed at 900 C with excess Mg. For the in situ process,
different approaches involving ablation from a stoichiometric target under
different growth conditions, as well as multilayer deposition involving
interposed Mg layers were examined and analyzed. Magnetic measurements on ex
situ processed films show TC of ~39 K, while the current best in situ films
show a susceptibility transition at ~ 22 K.Comment: 3 pages, PD
Intrinsic Structural Disorder and the Magnetic Ground State in Bulk EuTiO3
The magnetic properties of single-crystal EuTiO3 are suggestive of nanoscale
disorder below its cubic-tetragonal phase transition. We demonstrate that
electric field cooling acts to restore monocrystallinity, thus confirming that
emergent structural disorder is an intrinsic low-temperature property of this
material. Using torque magnetometry, we deduce that tetragonal EuTiO3 enters an
easy-axis antiferromagnetic phase at 5.6 K, with a first-order transition to an
easy-plane ground state below 3 K. Our data is reproduced by a 3D anisotropic
Heisenberg spin model.Comment: 5 pages, 4 figure
Magnetic-field dependence of antiferromagnetic structure in CeRh1-xCoxIn5
We investigated effects of magnetic field H on antiferromagnetic (AF)
structures in CeRh_{1-x}Co_xIn_5 by performing the elastic neutron scattering
measurements. By applying H along the [1,-1,0] direction, the incommensurate AF
state with the propagation vector of q_{h1}=(1/2,1/2,0.297) observed at H=0 is
replaced by the commensurate AF state with the q_{c2} = (1/2, 1/2, 1/4)
modulation above 2 T for x=0.23, while the AF states with the
q_{c1}=(1/2,1/2,1/2) and q_{h2}=(1/2,1/2,0.42) modulations seen at H=0 change
into a single q_{c1}-AF state above ~1.6 T for x=0.7. These results suggest the
different types of AF correlation for Co concentrations of 0.23 and 0.7 in an
applied magnetic field H.Comment: 4 pages, 2 figures, to appear in the proceedings of ICM2009
(Karlsruhe, Germany
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