266 research outputs found
Torsional Newton-Cartan Geometry and Lifshitz Holography
We obtain the Lifshitz UV completion in a specific model for z=2 Lifshitz
geometries. We use a vielbein formalism which enables identification of all the
sources as leading components of well-chosen bulk fields. We show that the
geometry induced from the bulk onto the boundary is a novel extension of
Newton-Cartan geometry with a specific torsion tensor. We explicitly compute
all the vevs including the boundary stress-energy tensor and their Ward
identities. After using local symmetries/Ward identities the system exhibits
6+6 sources and vevs. The FG expansion exhibits, however, an additional free
function which is related to an irrelevant operator whose source has been
turned off. We show that this is related to a second UV completion.Comment: v2: 5 pages, matches version published in PR
Role of multiorbital effects in the magnetic phase diagram of iron-pnictides
We elucidate the pivotal role of the bandstructure's orbital content in
deciding the type of commensurate magnetic order stabilized within the
itinerant scenario of iron-pnictides. Recent experimental findings in the
tetragonal magnetic phase attest to the existence of the so-called charge and
spin ordered density wave over the spin-vortex crystal phase, the latter of
which tends to be favored in simplified band models of itinerant magnetism.
Here we show that employing a multiorbital itinerant Landau approach based on
realistic bandstructures can account for the experimentally observed magnetic
phase, and thus shed light on the importance of the orbital content in deciding
the magnetic order. In addition, we remark that the presence of a hole pocket
centered at the Brillouin zone's -point favors a magnetic stripe
rather than a tetragonal magnetic phase. For inferring the symmetry properties
of the different magnetic phases, we formulate our theory in terms of magnetic
order parameters transforming according to irreducible representations of the
ensuing D point group. The latter method not only provides
transparent understanding of the symmetry breaking schemes but also reveals
that the leading instabilities always belong to the subset
of irreducible representations, independent of their C or C nature.Comment: 11 pages, 6 figure
Unravelling Incommensurate Magnetism and Its Emergence in Iron-Based Superconductors
We focus on a broad class of tetragonal itinerant systems sharing a tendency
towards the spontaneous formation of incommensurate magnetism with ordering
wavevectors or
. Employing a Landau approach, we
obtain the generic magnetic phase diagram and identify the leading
instabilities near the paramagnetic-magnetic transition. Nine distinct magnetic
phases exist that either preserve or violate the assumed -symmetry of the
paramagnetic phase. These are single- and double- phases consisting
of magnetic stripes, helices and whirls, either in an individual or coexisting
manner. These nine phases can be experimentally distinguished by polarized
neutron scattering, or, for example, by combining measurements of the induced
charge order and magnetoelectric coupling. Within two representative
five-orbital models, suitable for BaFeAs and LaFeAsO, we find that the
incommensurate magnetic phases discussed here are accessible in iron-based
superconductors. Our investigation unveils a set of potential candidates for
the unidentified -symmetric magnetic phase that was recently observed in
BaNaFeAs. Among the phases stabilized we find a
spin-whirl crystal, which is a textured magnetic -symmetric phase. The
possible experimental observation of textured magnetic orders in iron-based
superconductor, opens new directions for realizing intrinsic topological
superconductors.Comment: 18 pages, 6 figures + supplementary materia
Hot-lines topology and the fate of the spin resonance mode in three-dimensional unconventional superconductors
In the quasi-two-dimensional (quasi-2D) copper- and iron-based
superconductors, the onset of superconductivity is accompanied by a prominent
peak in the magnetic spectrum at momenta close to the wave-vector of the nearby
antiferromagnetic state. Such a peak is well described in terms of a spin
resonance mode, i.e., a spin-1 exciton theoretically predicted for quasi-2D
superconductors with a sign-changing gap. The same theories, however, indicate
that such a resonance mode should be absent in a three-dimensional (3D) system
with a spherical Fermi surface. This raises the question of the fate of the
spin resonance mode in layered unconventional superconductors that are not
strongly anisotropic, such as certain heavy-fermion compounds and potentially
the newly discovered nickelate superconductor NdNiO. Here, we use the
random-phase-approximation to calculate the dynamical spin susceptibility of 3D
superconductors with a -wave gap symmetry and corrugated
cylindrical-like Fermi surfaces. By varying the out-of-plane hopping anisotropy
, we demonstrate that the appearance of a spin resonance mode is
determined by the topology of the hot lines -- i.e. lines on the Fermi surface
that are connected by the magnetic wave-vector. For an in-plane
antiferromagnetic wave-vector, the hot lines undergo a topological transition
from open lines to closed loops at a critical value. The closed hot
lines cross the nodal superconducting lines, making the spin resonance mode
overdamped and incoherent. In contrast, for an out-of-plane antiferromagnetic
wave-vector, the hot lines remain open and the spin resonance mode remains
sharp. We discuss the experimental implications of our results for the
out-of-plane dispersion of the spin resonance mode and, more generally, for
inelastic neutron scattering experiments on unconventional superconductors.Comment: 12 pages, 11 figure
Strong-coupling expansion of multi-band interacting models: mapping onto the transverse-field - Ising model
We investigate a class of two-dimensional two-band microscopic models in
which the inter-band repulsive interactions play the dominant role. We first
demonstrate three different schemes of constraining the ratios between the
three types of inter-band interactions -- density-density, spin exchange, and
pair-hopping -- that render the model free of the fermionic sign-problem for
any filling and, consequently, amenable to efficient Quantum Monte Carlo
simulations. We then study the behavior of these sign-problem-free models in
the strong-coupling regime. In the cases where spin-rotational invariance is
preserved or lowered to a planar symmetry, the strong-coupling ground state is
a quantum paramagnet. However, in the case where there is only a residual Ising
symmetry, the strong-coupling expansion maps onto the transverse-field
- Ising model, whose pseudospins are associated with local inter-band
magnetic order. We show that by varying the band structure parameters within a
reasonable range of values, a variety of ground states and quantum critical
points can be accessed in the strong-coupling regime, some of which are not
realized in the weak-coupling regime. We compare these results with the case of
the single-band Hubbard model, where only intra-band repulsion is present, and
whose strong-coupling behavior is captured by a simple Heisenberg model.Comment: Contribution to the Philip W. Anderson Memorial Special Issue of
Annals of Physic
Unconventional superconductivity protected from disorder on the kagome lattice
Motivated by the recent discovery of superconductivity in the kagome
VSb (: K, Rb, Cs) metals, we perform a theoretical study of the
symmetry-allowed superconducting orders on the two-dimensional kagome lattice
with focus on their response to disorder. We uncover a qualitative difference
between the robustness of intraband spin-singlet (even-parity) and spin-triplet
(odd-parity) unconventional superconductivity to atomic-scale nonmagnetic
disorder. Due to the particular sublattice character of the electronic states
on the kagome lattice, disorder in spin-singlet superconducting phases becomes
non-pair-breaking despite the fact that the gap structure features sign
changes. By contrast, spin-triplet condensates remain fragile to disorder on
the kagome lattice. We demonstrate these effects in terms of the absence of
impurity bound states and an associated weak disorder-induced -suppression
for spin-singlet order. We also discuss the consequences for quasi-particle
interference and their inherent tendency for momentum-space anisotropy due to
sublattice effects on the kagome lattice. For unconventional kagome
superconductors, our results imply that any allowed spin-singlet order,
including for example -wave superconductivity, exhibits a
disorder-response qualitatively similar to standard conventional -wave
superconductors.Comment: 16 pages, 12 figure
Transglycosylating β-d-galactosidase and α-l-fucosidase from Paenibacillus sp. 3179 from a hot spring in East Greenland
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