404 research outputs found
Hyperfine Interactions in the Heavy Fermion CeMIn_5 Systems
The CeMIn_5 heavy fermion compounds have attracted enormous interest since
their discovery six years ago. These materials exhibit a rich spectrum of
unusual correlated electron behavior, and may be an ideal model for the high
temperature superconductors. As many of these systems are either
antiferromagnets, or lie close to an antiferromagnetic phase boundary, it is
crucial to understand the behavior of the dynamic and static magnetism. Since
neutron scattering is difficult in these materials, often the primary source of
information about the magnetic fluctuations is Nuclear Magnetic Resonance
(NMR). Therefore, it is crucial to have a detailed understanding of how the
nuclear moments interact with conduction electrons and the local moments
present in these systems. Here we present a detailed analysis of the hyperfine
coupling based on anisotropic hyperfine coupling tensors between nuclear
moments and local moments. Because the couplings are symmetric with respect to
bond axes rather than crystal lattice directions, the nuclear sites can
experience non-vanishing hyperfine fields even in high symmetry sites.Comment: 15 pages, 5 figure
Local edge modes in doped cuprates with checkerboard polaronic heterogeneity
We study a periodic polaronic system, which exhibits a nanoscale superlattice
structure, as a model for hole-doped cuprates with checkerboard-like
heterogeneity, as has been observed recently by scanning tunneling microscopy
(STM). Within this model, the electronic and phononic excitations are
investigated by applying an unrestricted Hartree-Fock and a random phase
approximation (RPA) to a multiband Peierls-Hubbard Hamiltonian in two
dimensions
Spin Fluctuations and the Pseudogap in Organic Superconductors
We show that there are strong similarities in the spin lattice relaxation of
non-magnetic organic charge transfer salts, and that these similarities can be
understood in terms of spin fluctuations. Further, we show that, in all of the
kappa-phase organic superconductors for which there is nuclear magnetic
resonance data, the energy scale for the spin fluctuations coincides with the
energy scale for the pseudogap. This suggests that the pseudogap is caused by
short-range spin correlations. In the weakly frustrated metals
k-(BEDT-TTF)_2Cu[N(CN)_2]Br, k-(BEDT-TTF)_2Cu(NCS)_2, and
k-(BEDT-TTF)_2Cu[N(CN)_2]Cl (under pressure) the pseudogap opens at the same
temperature as coherence emerges in the (intralayer) transport. We argue that
this is because the spin correlations are cut off by the loss of intralayer
coherence at high temperatures. We discuss what might happen to these two
energy scales at high pressures, where the electronic correlations are weaker.
In these weakly frustrated materials the data is well described by the chemical
pressure hypothesis (that anion substitution is equivalent to hydrostatic
pressure). However, we find important differences in the metallic state of
k-(BEDT-TTF)_2Cu_2(CN)_3, which is highly frustrated and displays a spin liquid
insulating phase. We also show that the characteristic temperature scale of the
spin fluctuations in (TMTSF)_2ClO_4 is the same as superconducting critical
temperature, which may be evidence that spin fluctuations mediate the
superconductivity in the Bechgaard salts.Comment: 7 pages, 4 figures; to appear in PR
Impurities near an Antiferromagnetic-Singlet Quantum Critical Point
Heavy fermion systems, and other strongly correlated electron materials,
often exhibit a competition between antiferromagnetic (AF) and singlet ground
states. Using exact Quantum Monte Carlo (QMC) simulations, we examine the
effect of impurities in the vicinity of such AF- singlet quantum critical
points, through an appropriately defined impurity susceptibility, .
Our key finding is a connection, within a single calculational framework,
between AF domains induced on the singlet side of the transition, and the
behavior of the nuclear magnetic resonance (NMR) relaxation rate . We
show that local NMR measurements provide a diagnostic for the location of the
QCP which agrees remarkably well with the vanishing of the AF order parameter
and large values of . We connect our results with experiments on
Cd-doped CeCoIn
Local Magnetic Inhomogeneities in Lightly Doped BaFeAs
We report As NMR measurements in BaFeAs doped with Ni. Like
Co, Ni doping suppresses the antiferromagnetic and structural phase transitions
and gives rise to superconductivity for sufficiently large Ni doping. The spin
lattice relaxation rate diverges at , with a critical exponent consistent
with 3D ordering of local moments. In the ordered state the spectra quickly
broaden inhomogeneously with doping. We extract the average size of the ordered
moment as a function of doping, and show that a model in which the order
remains commensurate but with local amplitude variations in the vicinity of the
dopant fully explains our observations.Comment: 4 pages, 4 figure
A predictive standard model for heavy electron systems
We propose a predictive standard model for heavy electron systems based on a
detailed phenomenological two-fluid description of existing experimental data.
It leads to a new phase diagram that replaces the Doniach picture, describes
the emergent anomalous scaling behavior of the heavy electron (Kondo) liquid
measured below the lattice coherence temperature, T*, seen by many different
experimental probes, that marks the onset of collective hybridization, and
enables one to obtain important information on quantum criticality and the
superconducting/antiferromagnetic states at low temperatures. Because T* is
~J^2\rho/2, the nearest neighbor RKKY interaction, a knowledge of the
single-ion Kondo coupling, J, to the background conduction electron density of
states, \rho, makes it possible to predict Kondo liquid behavior, and to
estimate its maximum superconducting transition temperature in both existing
and newly discovered heavy electron families.Comment: 4 pages, 2 figures, submitted to J. Phys.: Conf. Ser. for SCES 201
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