2,767 research outputs found
Truncated unity functional renormalization group for multiband systems with spin-orbit coupling
Although the functional renormalization group (fRG) is by now a
well-established method for investigating correlated electron systems, it is
still undergoing significant technical and conceptual improvements. In
particular, the motivation to optimally exploit the parallelism of modern
computing platforms has recently led to the development of the
"truncated-unity" functional renormalization group (TU-fRG). Here, we review
this fRG variant, and we provide its extension to multiband systems with
spin-orbit coupling. Furthermore, we discuss some aspects of the implementation
and outline opportunities and challenges ahead for predicting the ground-state
ordering and emergent energy scales for a wide class of quantum materials.Comment: consistent with published version in Frontiers in Physics (2018
Pressure dependence of diffusion in simple glasses and supercooled liquids
Using molecular dynamics simulation, we have calculated the pressure
dependence of the diffusion constant in a binary Lennard-Jones Glass. We
observe four temperature regimes. The apparent activation volume drops from
high values in the hot liquid to a plateau value. Near the critical temperature
of the mode coupling theory it rises steeply, but in the glassy state we find
again small values, similar to the ones in the liquid. The peak of the
activation volume at the critical temperature is in agreement with the
prediction of mode coupling theory
Laminar and turbulent dynamos in chiral magnetohydrodynamics-I: Theory
The magnetohydrodynamic (MHD) description of plasmas with relativistic
particles necessarily includes an additional new field, the chiral chemical
potential associated with the axial charge (i.e., the number difference between
right- and left-handed relativistic fermions). This chiral chemical potential
gives rise to a contribution to the electric current density of the plasma
(\emph{chiral magnetic effect}). We present a self-consistent treatment of the
\emph{chiral MHD equations}, which include the back-reaction of the magnetic
field on a chiral chemical potential and its interaction with the plasma
velocity field. A number of novel phenomena are exhibited. First, we show that
the chiral magnetic effect decreases the frequency of the Alfv\'{e}n wave for
incompressible flows, increases the frequencies of the Alfv\'{e}n wave and of
the fast magnetosonic wave for compressible flows, and decreases the frequency
of the slow magnetosonic wave. Second, we show that, in addition to the
well-known laminar chiral dynamo effect, which is not related to fluid motions,
there is a dynamo caused by the joint action of velocity shear and chiral
magnetic effect. In the presence of turbulence with vanishing mean kinetic
helicity, the derived mean-field chiral MHD equations describe turbulent
large-scale dynamos caused by the chiral alpha effect, which is dominant for
large fluid and magnetic Reynolds numbers. The chiral alpha effect is due to an
interaction of the chiral magnetic effect and fluctuations of the small-scale
current produced by tangling magnetic fluctuations (which are generated by
tangling of the large-scale magnetic field by sheared velocity fluctuations).
These dynamo effects may have interesting consequences in the dynamics of the
early universe, neutron stars, and the quark--gluon plasma.Comment: 23 pages, 4 figure
Vibrational States of Glassy and Crystalline Orthotherphenyl
Low-frequency vibrations of glassy and crystalline orthoterphenyl are studied
by means of neutron scattering. Phonon dispersions are measured along the main
axes of a single crystal, and the corresponding longitudinal and transversal
sound velocities are obtained. For glassy and polycrystalline samples, a
density of vibrational states is determined and cross-checked against other
dynamic observables. In the crystal, low-lying zone-boundary modes lead to an
excess over the Debye density of states. In the glass, the boson peak is
located at even lower frequencies. With increasing temperature, both glass and
crystal show anharmonicity.Comment: 7 pages of LaTeX (svjour), 2 tables, 10 figures accepted in Eur.
Phys. J.
Diffusion and jump-length distribution in liquid and amorphous CuZr
Using molecular dynamics simulation, we calculate the distribution of atomic
jum ps in CuZr in the liquid and glassy states. In both states
the distribution of jump lengths can be described by a temperature independent
exponential of the length and an effective activation energy plus a
contribution of elastic displacements at short distances. Upon cooling the
contribution of shorter jumps dominates. No indication of an enhanced
probability to jump over a nearest neighbor distance was found. We find a
smooth transition from flow in the liquid to jumps in the g lass. The
correlation factor of the diffusion constant decreases with decreasing
temperature, causing a drop of diffusion below the Arrhenius value, despite an
apparent Arrhenius law for the jump probability
Dissipative magnetic structures and scales in small-scale dynamos
Small-scale dynamos play important roles in modern astrophysics, especially
on Galactic and extragalactic scales. Owing to dynamo action, purely
hydrodynamic Kolmogorov turbulence hardly exists and is often replaced by
hydromagnetic turbulence. Understanding the size of dissipative magnetic
structures is important in estimating the time scale of Galactic scintillation
and other observational and theoretical aspects of interstellar and
intergalactic small-scale dynamos. Here we show that the thickness of magnetic
flux tubes decreases more rapidly with increasing magnetic Prandtl number than
previously expected. Also the theoretical scale based on the dynamo growth rate
and the magnetic diffusivity decrease faster than expected. However, the scale
based on the cutoff of the magnetic energy spectra scales as expected for large
magnetic Prandtl numbers, but continues in the same way also for moderately
small values - contrary to what is expected. For a critical magnetic Prandtl
number of about 0.27, the dissipative and resistive cutoffs are found to occur
at the same wavenumber. For large magnetic Prandtl numbers, our simulations
show that the peak of the magnetic energy spectrum occurs at a wavenumber that
is twice as large as previously predicted.Comment: 6 pages, 8 figures, 2 tables, submitted to MNRA
Low frequency excitations of C60 chains inserted inside single-walled carbon nanotubes
The low frequency excitations of C60 chains inserted inside single-walled
carbon nanotubes (SWNTs) have been studied by inelastic neutron scattering
(INS) on a high quality sample of peapods. The comparison of the
neutron-derived generalized phonon density of states (GDOS) of the peapods
sample with that of a raw SWNTs allows the vibrational properties of the C60
chains encapsulated in the hollow core of the SWNTs to be probed. Lattice
dynamical models are used to calculate the GDOS of chains of monomers, dimers
and polymers inserted into SWNTs, which are compared to the experimental data.
The presence of strong interactions between C60 cages inside the nanotube is
clearly demonstrated by an excess of mode density in the frequency range around
10 meV. However, the presence of a quasi-elastic signal indicates that some of
the C60\'s undergo rotational motion. This suggests that peapods are made from
a mixture of C60 monomers and C60 n-mer (dimer, trimer ... polymer) structures
Harmonic behavior of metallic glasses up to the metastable melt
In two amorphous alloys ZrTiCuNiBe and ZrAlNiCu coherent neutron scattering has been measured over five decades in energy, including measurements in the metastable melt of a metallic alloy more than 80 K above Tg. In the vibrational spectra a pronounced "boson" peak is found: Even in crystallized samples the density of states exceeds the Debye ω2 model, and in the amorphous state low-frequency vibrations are further enhanced. The peak position shows no dispersion in q, while intensities are strongly correlated with the static structure factor. Over the full energy range the temperature dependence is strictly harmonic. From high-energy resolution measurements we establish lower bounds for the temperatures at which structural α and fast β relaxation become observable
- …