62 research outputs found
Topological superconductivity in artificial heterostructures
In this thesis we will design an artificial heterostructure between a conventional s-wave superconductor with strong spin-orbit coupling and non-collinear magnets. We evaluate under which conditions Majorana edge states can form and obtain the topological phase diagram. For two-dimensional interfaces we additionally extract the spontaneous surface current resulting from the chiral edge states. We investigate these properties using both analytical methods as well as numerical tight-binding calculations
Superconductivity and Mottness in Organic Charge Transfer Materials
The phase diagrams of quasi two-dimensional organic superconductors display a
plethora of fundamental phenomena associated with strong electron correlations,
such as unconventional superconductivity, metal-insulator transitions,
frustrated magnetism and spin liquid behavior. We analyze a minimal model for
these compounds, the Hubbard model on an anisotropic triangular lattice, using
cutting-edge quantum embedding methods respecting the lattice symmetry. We
demonstrate the existence of unconventional superconductivity by directly
entering the symmetry-broken phase. We show that the crossover from the Fermi
liquid metal to the Mott insulator is associated with the formation of a
pseudogap. The predicted momentum-selective destruction of the Fermi surface
into hot and cold regions provides motivation for further spectroscopic
studies. Our results are in remarkable agreement with experimental phase
diagrams of -BEDT organics.Comment: 9 pages, 3 figures; Supplemental Material: 8 pages, 10 figure
Mott transition and pseudogap of the square-lattice Hubbard model: results from center-focused cellular dynamical mean-field theory
The recently proposed center-focused post-processing procedure [Phys. Rev.
Research 2, 033476 (2020)] of cellular dynamical mean-field theory suggests
that central sites of large impurity clusters are closer to the exact solution
of the Hubbard model than the edge sites. In this paper, we systematically
investigate results in the spirit of this center-focused scheme for several
cluster sizes up to in and out of particle-hole symmetry. First we
analyze the metal-insulator crossovers and transitions of the half-filled
Hubbard model on a simple square lattice. We find that the critical interaction
of the crossover is reduced with increasing cluster sizes and the critical
temperature abruptly drops for the cluster. Second, for this
cluster size, we apply the center-focused scheme to a system with more
realistic tight-binding parameters, investigating its pseudogap regime as a
function of temperature and doping, where we find doping dependent
metal-insulator crossovers, Lifshitz transitions and a strongly renormalized
Fermi-liquid regime. Additionally to diagnosing the real space origin of the
suppressed antinodal spectral weight in the pseudogap regime, we can infer
hints towards underlying charge ordering tendencies.Comment: 29 pages, 15 figure
Single- and two-particle observables in the Emery model: a dynamical mean-field perspective
We compare the dynamical mean-field descriptions of the single-band Hubbard
model and the three-band Emery model at the one- and two-particle level for
parameters relevant to high-Tc superconductors. We show that even within
dynamical mean-field theory, accounting solely for temporal fluctuations, the
intrinsic multi-orbital nature of the Emery model introduces effective
non-local correlations. These lead to a non-Curie-like temperature-dependence
of the magnetic susceptibility, also seen in nuclear magnetic resonance
experiments in the pseudogap regime by M. Avramovska, et al. [Journal of
Superconductivity and Novel Magnetism 33, 2621 (2020)]. We demonstrate the
agreement of our results with these experiments for a large range of dopings
and trace back the effective non-local correlations to an emerging
oxygen-copper singlet by analyzing a minimal finite-size cluster model. Despite
this correct description of the hallmark of the pseudogap at the two-particle
level, i.e., the drop in the Knight shift of nuclear magnetic resonance,
dynamical mean-field theory fails to properly describe the spectral properties
of the pseudogap.Comment: 7 pages, 7 figure
Stabilizing Even-Parity Chiral Superconductivity in SrRuO
Strontium ruthenate (SrRuO) has long been thought to host a
spin-triplet chiral -wave superconducting state. However, the singletlike
response observed in recent spin-susceptibility measurements casts serious
doubts on this pairing state. Together with the evidence for broken
time-reversal symmetry and a jump in the shear modulus at the
superconducting transition temperature, the available experiments point towards
an even-parity chiral superconductor with -like
symmetry, which has consistently been dismissed based on the
quasi-two-dimensional electronic structure of SrRuO. Here, we show how
the orbital degree of freedom can encode the two-component nature of the
order parameter, allowing for a local orbital-antisymmetric spin-triplet state
that can be stabilized by on-site Hund's coupling. We find that this exotic
state can be energetically stable once a complete, realistic
three-dimensional model is considered, within which momentum-dependent
spin-orbit coupling terms are key. This state naturally gives rise to
Bogoliubov Fermi surfaces.Comment: 6+10 pages, 5 figure
Lattice-Boltzmann hydrodynamics of anisotropic active matter
A plethora of active matter models exist that describe the behavior of
self-propelled particles (or swimmers), both with and without hydrodynamics.
However, there are few studies that consider shape-anisotropic swimmers and
include hydrodynamic interactions. Here, we introduce a simple method to
simulate self-propelled colloids interacting hydrodynamically in a viscous
medium using the lattice-Boltzmann technique. Our model is based on
raspberry-type viscous coupling and a force/counter-force formalism which
ensures that the system is force free. We consider several anisotropic shapes
and characterize their hydrodynamic multipolar flow field. We demonstrate that
shape-anisotropy can lead to the presence of a strong quadrupole and octupole
moments, in addition to the principle dipole moment. The ability to simulate
and characterize these higher-order moments will prove crucial for
understanding the behavior of model swimmers in confining geometries.Comment: 11 pages, 3 figures, 3 table
Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector
A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements
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