984 research outputs found
Metamagnetism of itinerant electrons in multi-layer ruthenates
The problem of quantum criticality in the context of itinerant ferro- or
metamagnetism has received considerable attention [S. A. Grigera et. al.,
Science 294, 329 (2001); C. Pfleiderer et. al., Nature, 414, 427 (2001)]. It
has been proposed that a new kind of quantum criticality is realised in
materials such as MnSi or Sr_3Ru_2O_7. We show based on a mean-field theory
that the low-temperature behaviour of the n-layer ruthenates
Sr_{n+1}Ru_nO_{3n+1} can be understood as a result of a Van Hove singularity
(VHS). We consider a single band whose Fermi energy, E_F, is close to the VHS
and deduce a complex phase diagram for the magnetism as a function of
temperature, magnetic field and E_F. The location of E_F with respect to the
VHS depends on the number of layers or can be tuned by pressure. We find that
the ferromagnetic quantum phase transition in this case is not of second but of
first order, with a metamagnetic quantum critical endpoint at high magnetic
field. Despite its simplicity this model describes well the properties of the
uniform magnetism in the single, double and triple layer ruthenates. We would
like to emphasise that the origin of this behaviour lies in the band structure.Comment: 7 pages, 3 figures, typos corrected and acknowledgement added, to
appear in the Europhysics Letter
Thermodynamics of itinerant metamagnetic transitions
Theoretical studies of the metamagnetism and anomalous phase of Sr3Ru2O7 have
focused on the role of van Hove singularities, although much experimental
evidence points towards quantum criticality having a large effect. We
investigate the magnetic and thermodynamic properties of systems where magnetic
field tunes through such a peak in the electronic density of states. We study
the generic case of a van Hove singularity in 2D. We see that in combination
with the requirement of number conservation and interaction effects the peak in
the density of states produces several interesting phenomena including raising
the critical field of the transition above naive estimates, altering the
relationship between temperature and field scales and creating a distinctive
double-peak structure in the electronic specific heat. We show that this
apparent non-Fermi liquid behaviour can be caused at mean-field level by a peak
in the density of states.Comment: 6 pages, 4 figure
Magnetic domain formation in itinerant metamagnets
We examine the effects of long-range dipolar forces on metamagnetic
transitions and generalize the theory of Condon domains to the case of an
itinerant electron system undergoing a first-order metamagnetic transition. We
demonstrate that within a finite range of the applied field, dipolar
interactions induce a spatial modulation of the magnetization in the form of
stripes or bubbles. Our findings are consistent with recent observations in the
bilayer ruthenate SrRuO.Comment: 4 pages, 3 figures, minor changes, references adde
Breakdown of the Fermi-liquid regime in the 2D Hubbard model from a two-loop field-theoretical renormalization group approach
We analyze the particle-hole symmetric two-dimensional Hubbard model on a
square lattice starting from weak-to-moderate couplings by means of the
field-theoretical renormalization group (RG) approach up to two-loop order.
This method is essential in order to evaluate the effect of the
momentum-resolved anomalous dimension which arises in the
normal phase of this model on the corresponding low-energy single-particle
excitations. As a result, we find important indications pointing to the
existence of a non-Fermi liquid (NFL) regime at temperature displaying
a truncated Fermi surface (FS) for a doping range exactly in between the
well-known antiferromagnetic insulating and the -wave singlet
superconducting phases. This NFL evolves as a function of doping into a
correlated metal with a large FS before the -wave pairing
susceptibility finally produces the dominant instability in the low-energy
limit.Comment: 9 pages, 9 figures; published in Phys. Rev.
On the Evolution of Simple Material Structures
The evolution of a distribution of material inhomogeneities is investigated by analyzing the evolution of the corresponding material connections. Some general geometric relations governing such evolutions are derived. These relations are then analyzed by looking at the restrictions imposed by the material symmetry group
Interaction flow method for many-fermion systems
We propose an interaction flow scheme that sums up the perturbation expansion
of many-particle systems by successively increasing the interaction strength.
It combines the unbiasedness of renormalization group methods with the
simplicity of straight-forward perturbation theory. Applying the scheme to
fermions in one dimension and to the two-dimensional Hubbard model we find that
at one-loop level and low temperatures there is ample agreement with previous
one-loop renormalization group approaches. We furthermore present results for
the momentum-dependence of spin, charge and pairing interactions in the
two-dimensional Hubbard model.Comment: 14 pages, 14 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
Variational ground states of the two-dimensional Hubbard model
Recent refinements of analytical and numerical methods have improved our
understanding of the ground-state phase diagram of the two-dimensional (2D)
Hubbard model. Here we focus on variational approaches, but comparisons with
both Quantum Cluster and Gaussian Monte Carlo methods are also made. Our own
ansatz leads to an antiferromagnetic ground state at half filling with a
slightly reduced staggered order parameter (as compared to simple mean-field
theory). Away from half filling, we find d-wave superconductivity, but confined
to densities where the Fermi surface passes through the antiferromagnetic zone
boundary (if hopping between both nearest-neighbour and next-nearest-neighbour
sites is considered). Our results agree surprisingly well with recent numerical
studies using the Quantum Cluster method. An interesting trend is found by
comparing gap parameters (antiferromagnetic or superconducting) obtained with
different variational wave functions. They vary by an order of magnitude and
thus cannot be taken as a characteristic energy scale. In contrast, the order
parameter is much less sensitive to the degree of sophistication of the
variational schemes, at least at and near half filling.Comment: 18 pages, 4 figures, to be published in New J. Phy
The Acquisition of Physical Knowledge in Generative Neural Networks
As children grow older, they develop an intuitive understanding of the
physical processes around them. Their physical understanding develops in
stages, moving along developmental trajectories which have been mapped out
extensively in previous empirical research. Here, we investigate how the
learning trajectories of deep generative neural networks compare to children's
developmental trajectories using physical understanding as a testbed. We
outline an approach that allows us to examine two distinct hypotheses of human
development - stochastic optimization and complexity increase. We find that
while our models are able to accurately predict a number of physical processes,
their learning trajectories under both hypotheses do not follow the
developmental trajectories of children.Comment: Published as a conference paper at ICML 202
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
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