585 research outputs found
Phonon-mediated tuning of instabilities in the Hubbard model at half-filling
We obtain the phase diagram of the half-filled two-dimensional Hubbard model
on a square lattice in the presence of Einstein phonons. We find that the
interplay between the instantaneous electron-electron repulsion and
electron-phonon interaction leads to new phases. In particular, a
d-wave superconducting phase emerges when both anisotropic phonons
and repulsive Hubbard interaction are present. For large electron-phonon
couplings, charge-density-wave and s-wave superconducting regions also appear
in the phase diagram, and the widths of these regions are strongly dependent on
the phonon frequency, indicating that retardation effects play an important
role. Since at half-filling the Fermi surface is nested, spin-density-wave is
recovered when the repulsive interaction dominates. We employ a functional
multiscale renormalization-group method that includes both electron-electron
and electron-phonon interactions, and take retardation effects fully into
account.Comment: 8 pages, 5 figure
Skyrmion Lattice in a Chiral Magnet
Skyrmions represent topologically stable field configurations with
particle-like properties. We used neutron scattering to observe the spontaneous
formation of a two-dimensional lattice of skyrmion lines, a type of magnetic
vortices, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice
stabilizes at the border between paramagnetism and long-range helimagnetic
order perpendicular to a small applied magnetic field regardless of the
direction of the magnetic field relative to the atomic lattice. Our study
experimentally establishes magnetic materials lacking inversion symmetry as an
arena for new forms of crystalline order composed of topologically stable spin
states
spl(2,1) dynamical supersymmetry and suppression of ferromagnetism in flat band double-exchange models
The low energy spectrum of the ferromagnetic Kondo lattice model on a N-site
complete graph extended with on-site repulsion is obtained from the underlying
spl(2,1) algebra properties in the strong coupling limit. The ferromagnetic
ground state is realized for 1 and N+1 electrons only. We identify the large
density of states to be responsible for the suppression of the ferromagnetic
state and argue that a similar situation is encountered in the Kagome,
pyrochlore, and other lattices with flat bands in their one-particle density of
states.Comment: 7 pages, 1 figur
Topological Hall effect in the A-phase of MnSi
Recent small angle neutron scattering suggests, that the spin structure in
the A-phase of MnSi is a so-called triple- state, i.e., a superposition of
three helices under 120 degrees. Model calculations suggest that this structure
in fact is a lattice of so-called skyrmions, i.e., a lattice of topologically
stable knots in the spin structure. We report a distinct additional
contribution to the Hall effect in the temperature and magnetic field range of
the proposed skyrmion lattice, where such a contribution is neither seen nor
expected for a normal helical state. Our Hall effect measurements constitute a
direct observation of a topologically quantized Berry phase that identifies the
spin structure seen in neutron scattering as the proposed skyrmion lattice
Solution of the infinite range t-J model
The t-J model with constant t and J between any pair of sites is studied by
exploiting the symmetry of the Hamiltonian with respect to site permutations.
For a given number of electrons and a given total spin the exchange term simply
yields an additive constant. Therefore the real problem is to diagonalize the
"t- model", or equivalently the infinite U Hubbard Hamiltonian. Using
extensively the properties of the permutation group, we are able to find
explicitly both the energy eigenvalues and eigenstates, labeled according to
spin quantum numbers and Young diagrams. As a corollary we also obtain the
degenerate ground states of the finite Hubbard model with infinite range
hopping -t>0.Comment: 15 pages, 2 figure
Van Hove singularity and spontaneous Fermi surface symmetry breaking in Sr3Ru2O7
The most salient features observed around a metamagnetic transition in
Sr3Ru2O7 are well captured in a simple model for spontaneous Fermi surface
symmetry breaking under a magnetic field, without invoking a putative quantum
critical point. The Fermi surface symmetry breaking happens in both a majority
and a minority spin band but with a different magnitude of the order parameter,
when either band is tuned close to van Hove filling by the magnetic field. The
transition is second order for high temperature T and changes into first order
for low T. The first order transition is accompanied by a metamagnetic
transition. The uniform magnetic susceptibility and the specific heat
coefficient show strong T dependence, especially a log T divergence at van Hove
filling. The Fermi surface instability then cuts off such non-Fermi liquid
behavior and gives rise to a cusp in the susceptibility and a specific heat
jump at the transition temperature.Comment: 11 pages, 4 figure
On a global differential geometric approach to the rational mechanics of deformable media
In the past the rational mechanics of deformable media was largely concerned with materials governed by linear constitutive equations. In recent years, the theory has expanded considerably towards covering materials for which the constitutive equations are inherently nonlinear, and/or whose mechanical properties resemble in some respects those of a fluid and in others those of a solid. In the present article we formulate a satisfactory global mathematical theory of moving deformable media, which includes all these aspects
The Reconstruction Problem and Weak Quantum Values
Quantum Mechanical weak values are an interference effect measured by the
cross-Wigner transform W({\phi},{\psi}) of the post-and preselected states,
leading to a complex quasi-distribution {\rho}_{{\phi},{\psi}}(x,p) on phase
space. We show that the knowledge of {\rho}_{{\phi},{\psi}}(z) and of one of
the two functions {\phi},{\psi} unambiguously determines the other, thus
generalizing a recent reconstruction result of Lundeen and his collaborators.Comment: To appear in J.Phys.: Math. Theo
Exact integral equation for the renormalized Fermi surface
The true Fermi surface of a fermionic many-body system can be viewed as a
fixed point manifold of the renormalization group (RG). Within the framework of
the exact functional RG we show that the fixed point condition implies an exact
integral equation for the counterterm which is needed for a self-consistent
calculation of the Fermi surface. In the simplest approximation, our integral
equation reduces to the self-consistent Hartree-Fock equation for the
counterterm.Comment: 5 pages, 1 figur
Strong Longitudinal Magnetic Fluctuations near Critical End Point in UCoAl: A ^59Co-NMR Study
We report ^59Co-NMR measurements in UCoAl where a metamagnetism occurs due to
enhancement of ferromagnetism by magnetic field. The metamagnetic transition
from a paramagnetic (PM) state to a ferromagnetic state is a first order
transition at low temperatures, but it changes to a crossover at high
temperatures on crossing the critical end pint (CEP) at T_CEP ~ 12 K. The
contrasting behavior between the relaxation rates 1/T_1 and 1/T_2 suggests that
the longitudinal magnetic fluctuation of U moment is strongly enhanced
especially near the CEP. A wide diffusion of the fluctuation from the CEP can
be confirmed even in the PM state where the magnetic transition does not occur.Comment: 5pages, 6 figures, to be published in J. Phys. Soc. Jp
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