1,851 research outputs found
Critical temperature and Ginzburg region near a quantum critical point in two-dimensional metals
We compute the transition temperature and the Ginzburg temperature
above near a quantum critical point at the boundary of an
ordered phase with a broken discrete symmetry in a two-dimensional metallic
electron system. Our calculation is based on a renormalization group analysis
of the Hertz action with a scalar order parameter. We provide analytic
expressions for and as a function of the non-thermal control
parameter for the quantum phase transition, including logarithmic corrections.
The Ginzburg regime between and occupies a sizable part of
the phase diagram.Comment: 5 pages, 1 figur
Turning a First Order Quantum Phase Transition Continuous by Fluctuations: General Flow Equations and Application to d-Wave Pomeranchuk Instability
We derive renormalization group equations which allow us to treat order
parameter fluctuations near quantum phase transitions in cases where an
expansion in powers of the order parameter is not possible. As a prototypical
application, we analyze the nematic transition driven by a d-wave Pomeranchuk
instability in a two-dimensional electron system. We find that order parameter
fluctuations suppress the first order character of the nematic transition
obtained at low temperatures in mean-field theory, so that a continuous
transition leading to quantum criticality can emerge
Fermion loops, loop cancellation and density correlations in two dimensional Fermi systems
We derive explicit results for fermion loops with an arbitrary number of
density vertices in two dimensions at zero temperature. The 3-loop is an
elementary function of the three external momenta and frequencies, and the
N-loop can be expressed as a linear combination of 3-loops with coefficients
that are rational functions of momenta and frequencies. We show that the
divergencies of single loops for low energy and small momenta cancel each other
when loops with permuted external variables are summed. The symmetrized N-loop,
i.e. the connected N-point density correlation function of the Fermi gas, does
not diverge for low energies and small momenta. In the dynamical limit, where
momenta scale to zero at fixed finite energy variables, the symmetrized N-loop
vanishes as the (2N-2)-th power of the scale parameter.Comment: 24 pages (including 3 EPS figures), LaTeX2e; submitted to Phys. Rev.
Correlated hopping of electrons: Effect on the Brinkman-Rice transition and the stability of metallic ferromagnetism
We study the Hubbard model with bond-charge interaction (`correlated
hopping') in terms of the Gutzwiller wave function. We show how to express the
Gutzwiller expectation value of the bond-charge interaction in terms of the
correlated momentum-space occupation. This relation is valid in all spatial
dimensions. We find that in infinite dimensions, where the Gutzwiller
approximation becomes exact, the bond-charge interaction lowers the critical
Hubbard interaction for the Brinkman-Rice metal-insulator transition. The
bond-charge interaction also favors ferromagnetic transitions, especially if
the density of states is not symmetric and has a large spectral weight below
the Fermi energy.Comment: 5 pages, 3 figures; minor changes, published versio
What are spin currents in Heisenberg magnets?
We discuss the proper definition of the spin current operator in Heisenberg
magnets subject to inhomogeneous magnetic fields. We argue that only the
component of the naive "current operator" J_ij S_i x S_j in the plane spanned
by the local order parameters and is related to real transport of
magnetization. Within a mean field approximation or in the classical ground
state the spin current therefore vanishes. Thus, finite spin currents are a
direct manifestation of quantum correlations in the system.Comment: 4 pages, 1 figure, published versio
Soft Fermi Surfaces and Breakdown of Fermi Liquid Behavior
Electron-electron interactions can induce Fermi surface deformations which
break the point-group symmetry of the lattice structure of the system. In the
vicinity of such a "Pomeranchuk instability" the Fermi surface is easily
deformed by anisotropic perturbations, and exhibits enhanced collective
fluctuations. We show that critical Fermi surface fluctuations near a d-wave
Pomeranchuk instability in two dimensions lead to large anisotropic decay rates
for single-particle excitations, which destroy Fermi liquid behavior over the
whole surface except at the Brillouin zone diagonal.Comment: 12 pages, 2 figures, revised version as publishe
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