29 research outputs found
Correlated Fermions on a Checkerboard Lattice
A model of strongly correlated spinless fermions hopping on a checkerboard
lattice is mapped onto a quantum fully-packed loop model. We identify a large
number of fluctuationless states specific to the fermionic case. We also show
that for a class of fluctuating states, the fermionic sign problem can be
gauged away. This claim is supported by numerically evaluating the energies of
the low-lying states. Furthermore, we analyze in detail the excitations at the
Rokhsar-Kivelson point of this model thereby using the relation to the height
model and the single-mode approximation.Comment: 4 Pages, 3 Figures; v4: updated version published in Phys. Rev.
Lett.; one reference adde
Engineering higher order Van Hove singularities in two dimensions: the example of the surface layer of SrRuO
The properties of correlated electron materials are often intricately linked
to Van Hove singularities (VHs) in the vicinity of the Fermi energy. The class
of these VHs is of great importance, with higher order ones -- with power-law
divergence in the density of states -- leaving frequently distinct signatures
in physical properties. We use a new theoretical method to detect and analyse
higher order Van Hove singularities (HOVHs) in two-dimensional materials and
apply it to the electronic structure of the surface layer of SrRuO. We
then constrain a low energy model of the VHs of the surface layer of
SrRuO against angle-resolved photoemission spectroscopy and
quasiparticle interference data to analyse the VHs near the Fermi level. We
show how these VHs can be engineered into HOVHs.Comment: 8 pages including Supplemental Material, 5 figure
Distinct Magnetic Phase Transition at the Surface of an Antiferromagnet
In the majority of magnetic systems the surface is required to order at the same temperature as the bulk. In the present Letter, we report a distinct and unexpected surface magnetic phase transition at a lower temperature than the Néel temperature. Employing grazing incidence x-ray resonant magnetic scattering, we have observed the near-surface behavior of uranium dioxide. UO2 is a noncollinear, triple-q, antiferromagnet with the U ions on a face-centered cubic lattice. Theoretical investigations establish that at the surface the energy increase—due to the lost bonds—is reduced when the spins near the surface rotate, gradually losing their component normal to the surface. At the surface the lowest-energy spin configuration has a double-q (planar) structure. With increasing temperature, thermal fluctuations saturate the in-plane crystal field anisotropy at the surface, leading to soft excitations that have ferromagnetic XY character and are decoupled from the bulk. The structure factor of a finite two-dimensional XY model fits the experimental data well for several orders of magnitude of the scattered intensity. Our results support a distinct magnetic transition at the surface in the Kosterlitz-Thouless universality class
Material-specific gap function in the high-temperature superconductors
We present theoretical arguments and experimental support for the idea that
high-Tc superconductivity can occur with s-wave, d-wave, or mixed-wave pairing
in the context of a magnetic mechanism. The size and shape of the gap is
different for different materials. The theoretical arguments are based on the
t-J model as derived from the Hubbard model so that it necessarily includes
three-site terms. We argue that this should be the basic minimal model for
high-Tc systems. We analyze this model starting with the dilute limit which can
be solved exactly, passing then to the Cooper problem which is numerically
tractable, then ending with a mean field approach. It is found that the
relative stability of s-wave and d-wave depends on the size and the shape of
the Fermi surface. We identify three striking trends. First, materials with
large next-nearest-neighbor hopping (such as YBa(2)Cu(3)O(7-x)) are nearly pure
d-wave, whereas nearest-neighbor materials (such as La(2-x)Sr(x)CuO(4)) tend to
be more s-wave-like. Second, low hole doping materials tend to be pure d-wave,
but high hole doping leads to s-wave. Finally, the optimum hole doping level
increases as the next-nearest-neighbor hopping increases. We examine the
experimental evidence and find support for this idea that gap function in the
high-temperature superconductors is material-specific.Comment: 20 pages; requires revtex.sty v3.0, epsf.sty; includes 6 EPS figures;
Postscript version also available at
http://lifshitz.physics.wisc.edu/www/koltenbah/papers/gapfunc2.ps . This
version contains an extensive amount of new work including theoretical
background, an additional mean field treatment with new figures, and a more
thorough experimental surve
Instability of Anisotropic Fermi Surfaces in Two Dimensions
The effect of strong anisotropy on the Fermi line of a system of correlated
electrons is studied in two space dimensions, using renormalization group
techniques. Inflection points change the scaling exponents of the couplings,
enhancing the instabilities of the system. They increase the critical dimension
for non Fermi liquid behavior, from 1 to 3/2. Assuming that, in the absence of
nesting, the dominant instability is towards a superconducting ground state,
simple rules to discern between d-wave and extended s-wave symmetry of the
order parameter are given.Comment: 5 pages, revte
Microscopic mechanisms of spin-dependent electric polarization in 3d oxides
We present a short critical overview of different microscopic models for
nonrelativistic and relativistic magnetoelectric coupling including the
so-called "spin current scenario", ab-initio calculations, and several recent
microscopic approaches to a spin-dependent electric polarization in 3d oxides.Comment: 8 pages, 3 figure
Critical Ising modes in low-dimensional Kondo insulators
We present an Ising-like intermediate phase for one-dimensional Kondo
insulator systems. Resulting from a spinon splitting, its low-energy
excitations are critical Ising modes, whereas the triplet sector has a spectral
gap. It should occur as long as the RKKY oscillation amplitude dominates over
any direct exchange between localized spins. The chiral fixed point, however,
becomes unstable in the far Infra-Red limit due to prevalent fluctuations among
localized spins which induce gapless triplet excitations in the spectrum. Based
on previous numerical results, we obtain a paramagnetic disordered state ruled
by the correlation length of the single impurity Kondo model.Comment: 7 pages, RevTeX; last version: to be published in Physical Review
Transport Properties of the One Dimensional Ferromagnetic Kondo Lattice Model : A Qualitative Approach to Oxide Manganites
The transport properties of the ferromagnetic Kondo lattice model in one
dimension are studied via bosonization methods. The antiferromagnetic
fluctuations, which normally appear because of the RKKY interactions, are
explicitly taken into account as a direct exchange between the ``core'' spins.
It is shown that in the paramagnetic regime with the local antiferromagnetic
fluctuations, the resistivity decays exponentially as the temperature increases
while in the ferromagnetic regime the system is an almost perfect conductor. %A
non-perturbative description of localized spin polarons %in the paramagnetic
region is obtained.
The effect of a weak applied field is discussed to be reduced to the case of
the ferromagnetic state leading to band splitting. The qualitative relevance of
the results for the problem of the Oxide Manganites is emphasized.Comment: 4 pages, REVTe
Systematics of two-component superconductivity in from microwave measurements of high quality single crystals
Systematic microwave surface impedance measurements of YBCO single crystals
grown in crucibles reveal new properties that are not directly seen
in similar measurements of other YBCO samples. Two key observations obtained
from complex conductivity are: a new normal conductivity peak at around 80K and
additional pairing below 65K. High pressure oxygenation of one of the crystals
still yields the same results ruling out any effect of macroscopic segregation
of O-deficient regions. A single complex order parameter cannot describe these
data, and the results suggest at least two superconducting components.
Comparisons with model calculations done for various decoupled two-component
scenarios (i.e. s+d, d+d) are presented. Systematics of three single crystals
show that the 80K quasiparticle peak is correlated with the normal state
inelastic scattering rate. Close to Tc, the data follow a mean-field behavior.
Overall, our results strongly suggest the presence of multiple pairing
temperature and energy scales in .Comment: 14 pages, 2-column, Revtex, 5 embedded postscript figures, uses
graphicx. Postscript version also available at
http://sagar.physics.neu.edu/preprints.htm
Ginzburg-Landau theory of vortices in a multi-gap superconductor
The Ginzburg-Landau functional for a two-gap superconductor is derived within
the weak-coupling BCS model. The two-gap Ginzburg-Landau theory is, then,
applied to investigate various magnetic properties of MgB2 including an upturn
temperature dependence of the transverse upper critical field and a core
structure of an isolated vortex. Orientation of vortex lattice relative to
crystallographic axes is studied for magnetic fields parallel to the c-axis. A
peculiar 30-degree rotation of the vortex lattice with increasing strength of
an applied field observed by neutron scattering is attributed to the multi-gap
nature of superconductivity in MgB2.Comment: 11 page