50 research outputs found
Second-order topological insulators and superconductors with an order-two crystalline symmetry
Second-order topological insulators and superconductors have a gapped excitation spectrum in bulk and along boundaries, but protected zero modes at corners of a two-dimensional crystal or protected gapless modes at hinges of a three-dimensional crystal. A second-order topological phase can be induced by the presence of a bulk crystalline symmetry. Building on Shiozaki and Sato's complete classification of bulk crystalline phases with an order-two crystalline symmetry [Phys. Rev. B 90, 165114 (2014)], such as mirror reflection, twofold rotation, or inversion symmetry, we classify all corresponding second-order topological insulators and superconductors. The classification also includes antiunitary symmetries and antisymmetries
Conductance matrix symmetries of multiterminal semiconductor-superconductor devices
Nonlocal tunneling spectroscopy of multiterminal semiconductor-superconductor
hybrid devices is a powerful tool to investigate the Andreev bound states below
the parent superconducting gap. We examine how to exploit both microscopic and
geometrical symmetries of the system to extract information on the normal and
Andreev transmission probabilities from the multiterminal electric or
thermoelectric differential conductance matrix under the assumption of an
electrostatic potential landscape independent of the bias voltages, as well as
the absence of leakage currents. These assumptions lead to several symmetry
relations on the conductance matrix. Next, by considering a numerical model of
a proximitized semiconductor wire with spin-orbit coupling and two normal
contacts at its ends, we show how such symmetries can be used to identify the
direction and relative strength of Rashba versus Dresselhaus spin-orbit
coupling. Finally, we study how a voltage-bias-dependent electrostatic
potential as well as quasiparticle leakage break the derived symmetry relations
and investigate characteristic signatures of these two contributions.Comment: 16 pages, 10 figure
Symmetry-based indicators for topological Bogoliubov–de Gennes Hamiltonians
We develop a systematic approach for constructing symmetry-based indicators of a topological classification for superconducting systems. The topological invariants constructed in this work form a complete set of symmetry-based indicators that can be computed from knowledge of the Bogoliubov–de Gennes Hamiltonian on high-symmetry points in the Brillouin zone. After excluding topological invariants corresponding to the phases without boundary signatures, we arrive at a natural generalization of symmetry-based indicators [H. C. Po, A. Vishwanath, and H. Watanabe, Nat. Commun. 8, 50 (2017)] to Hamiltonians of Bogoliubov–de Gennes type
Higher-order topological superconductivity from repulsive interactions in kagome and honeycomb systems
We discuss a pairing mechanism in interacting two-dimensional multipartite lattices that intrinsically leads to a second order topological superconducting state with a spatially modulated gap. When the chemical potential is close to Dirac points, oppositely moving electrons on the Fermi surface undergo an interference phenomenon in which the Berry phase converts a repulsive electron–electron interaction into an effective attraction. The topology of the superconducting phase manifests as gapped edge modes in the quasiparticle spectrum and Majorana Kramers pairs at the corners. We present symmetry arguments which constrain the possible form of the electron–electron interactions in these systems and classify the possible superconducting phases which result. Exact diagonalization of the Bogoliubov-de Gennes Hamiltonian confirms the existence of gapped edge states and Majorana corner states, which strongly depend on the spatial structure of the gap. Possible applications to vanadium-based superconducting kagome metals AV3Sb5 (A = K, Rb, Cs) are discussed
Thermodynamic transitions and topology of spin-triplet superconductivity: Application to UTe
The discovery of unconventional superconductivity in the heavy-fermion
material UTe has reinvigorated research of spin-triplet superconductivity.
We perform a theoretical study of coupled two-component spin-triplet
superconducting order parameters and their thermodynamic transitions into the
superconducting state. With focus on the behavior of the temperature dependence
of the specific heat capacity, we find that two-component time-reversal
symmetry breaking superconducting order may feature vanishing or even negative
secondary specific heat anomalies. The origin of this unusual specific heat
behavior is tied to the non-unitarity of the composite order parameter.
Additionally, we supply an analysis of the topological surface states
associated with the different possible spin-triplet orders: single-component
orders host Dirac Majorana surface states in addition to possible bulk nodes. A
second component breaking time-reversal symmetry gaps these surfaces states
producing chiral Majorana hinge modes. DFT+ band-structure calculations
support that these topological phases are realized in UTe when introducing
weak superconducting pairing. Our topological analysis suggests measurable
signatures for surface-probe experiments to acquire further evidence of the
superconducting pairing symmetry.Comment: 20 pages, 8 figure
Phase Asymmetry of Andreev Spectra From Cooper-Pair Momentum
In analogy to conventional semiconductor diodes, the Josephson diode exhibits
superconducting properties that are asymmetric in applied bias. The effect has
been investigated in number of systems recently, and requires a combination of
broken time-reversal and inversion symmetries. We demonstrate a dual of the
usual Josephson diode effect, a nonreciprocal response of Andreev bound states
to a superconducting phase difference across the normal region of a
superconductor-normal-superconductor Josephson junction, fabricated using an
epitaxial InAs/Al heterostructure. Phase asymmetry of the subgap Andreev
spectrum is absent in the absence of in-plane magnetic field and reaches a
maximum at 0.15 T applied in the plane of the junction transverse to the
current direction. We interpret the phase diode effect in this system as
resulting from finite-momentum Cooper pairing due to orbital coupling to the
in-plane magnetic field, without invoking Zeeman or spin-orbit coupling
The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs
We report on the spectroscopic confirmation of 68 new bright (
mag) and blue (pre-)white dwarfs (WDs). This finding has allowed us to almost
double the number of the hottest (kK) known WDs
brighter than mag. We increased the number of known ultra-high
excitation (UHE) WDs by 20%, found one unambiguous close binary system
consisting of one DA WD with an irradiated low-mass companion, one DAO, and one
DOA WD that are likely in their transformation phase of becoming pure DA WDs,
one rare, naked O(H) star, two DA and two DAO WDs with
possibly in excess of 100kK, three new DOZ WDs, and three of our targets are
central stars of (possible) planetary nebulae.
Using non-local thermodynamic equilibrium models, we derived the atmospheric
parameters of these stars and by fitting their spectral energy distribution we
derived their radii, luminosities, and gravity masses. In addition, we derived
their masses in the Kiel and Hertzsprung-Russell diagram (HRD). We find that
Kiel, HRD, and gravity mass agree only in half of the cases. This is not
unexpected and we attribute this to the neglect of metal opacities, possibly
stratified atmospheres, as well as possible uncertainties of the parallax zero
point determination.
Furthermore, we carried out a search for photometric variability in our
targets using archival data, finding that 26% of our targets are variable. This
includes 15 new variable stars, with only one of them being clearly an
irradiation effect system. Strikingly, the majority of the variable stars
exhibit non-sinusoidal light-curve shapes, which are unlikely explained in
terms of close binary systems. We propose that a significant fraction of all
(not just UHE) WDs develop spots when entering the WD cooling phase. We suggest
that this could be related to the on-set of weak magnetic fields and possibly
diffusion.Comment: Accepted for publication in A&